Coastal landslide monitoring at Aldbrough East Riding ofYorkshire UK
P R N Hobbs L D Jones M P Kirkham C V L Pennington D J R Morgan ampC DashwoodBritish Geological Survey Keyworth Nottingham NG12 5GG UK
P R NH 0000-0001-7225-5852 C V LP 0000-0002-3560-9030Correspondence prnhbgsacuk
Abstract The paper describes results to date of a continuing monitoring study of coastal lsquosoft cliffrsquo recession at the BritishGeological Surveyrsquos (BGSrsquos) Coastal Landslide Observatory (CLO) on the east coast of England at Aldbrough East Riding ofYorkshire The cliffed site part of the 50 km long Holderness coast consists of glacial deposits and is one of the most rapidlyeroding coastlines in Europe This rapid rate of erosion provides an ideal opportunity for observation and process understandingbecause it facilitates the collection of data over periods of time encompassing significant new landslide events at the samelocation The results of two approaches are reported first terrestrial Light Detection and Ranging (LiDAR) surveying (TLS)second the installation of instrumented boreholes The aim of the research is to combine these to investigate the role oflandslides and their pre-conditioning factors and the influence of geology geotechnics topography and environmental factorson cliff recession To date an average recession rate of 18 m aminus1 and a maximum rate of 34 m aminus1 have been recorded for thesite The establishment of the CLO and its conceptual geologicalndashgeotechnical model are described in a related paper
Received 18 December 2018 revised 24 April 2019 accepted 12 May 2019
This study follows that of Dixon amp Bromhead (2002) whichmonitored deep-seated rotational landsliding in the London ClayFormation at Warden Point Isle of Sheppey Kent in particulartheir observations (since 1971) of first-time movements and theextensive use of piezometers More recently the value of theapplication of terrestrial-based Light Detection and Ranging(LiDAR) techniques to monitoring cliff recession is now widelyrecognized (Hobbs et al 2002 2013 Rosser et al 2005 Poultonet al 2006 Young amp Ashford 2006 Quinn et al 2010) TheHolderness coast has been the subject of intensive study for manydecades (Valentin 1971 Pringle 1985 Butcher 1991 Pethick 1996Prandle et al 1996 Lee amp Clark 2002 Newsham et al 2002Brown 2008 Lee 2008 2011 Quinn et al 2009 2010) and recentlylsquoprocess-responsersquo modellers have focused on this coastline(Walkden amp Dickson 2008 Ashton et al 2011 Walkden amp Hall2011 Castedo et al 2012 2015) Holderness is reported to have thefastest receding coastline in Europe at 2 m aminus1 overall (Bird 2008Castedo et al 2015) The British Geological Surveyrsquos (BGSrsquos)Coastal Landslide Observatory (CLO) is situated 10 km SE ofHornsea (Fig 1) and 2 km SE of the Building ResearchEstablishmentrsquos (BRE) Cowden lsquolowland clay tillrsquo geotechnicalresearch site (Marsland amp Powell 1985)
That part of the study described here is based on the conceptualmodel outlined by Hobbs et al (2019) and seeks to refine it furtherThe nature of landsliding at the CLO has been observed to beprimarily deep-seated rotational with secondary toppling andmudflow Deep-seated landslides occur episodically within estab-lished embayments topples occur frequently both within land-slipped masses and on unslipped promontories whereas mudflowsoccur less frequently on the peripheries of landslide masses Thetills are jointed and there is evidence of stress relief in the tillsforming the cliff causing fresh discontinuities and opening ofexisting ones Erosion at the cliff toe is virtually continuousthroughout the year but is affected by the presence (or otherwise) ofa sandy beach the thickness content and location (on the platform)
of which vary throughout the seasons The precise morphology ofthe rotational landsliding is influenced by the complex dispositionof the various glacial deposits forming the cliff and the results ofseveral stages of glacial advance and retreat (Evans 2017)Fortuitously a fresh landslide event (14 February 2017) occupyingthe greater part of the central embayment was observed by groundstaff at the leisure park and reported to the authors This was arcuatein plan with an initial vertical displacement of 50 mm andoccupying about 80 of the embaymentrsquos length
The Aldbrough CLO encompasses three landslide embaymentswhich have persisted throughout the monitoring period Surveyshave been carried out at 3 or 6 month intervals although the preciseinterval has varied from 2001 to the present The boreholes arealigned with and landward of the central embayment and areperpendicular to the coast Details of the projectrsquos survey andmonitoring (2001ndash2013) have been given by Hobbs et al (2013)drilling and instrumentation (to 2015) by Hobbs et al (2015a) andgeotechnical laboratory testing by Hobbs et al (2015b)
Engineering geology
The lithostratigraphy at the site is summarized in Figure 2 a fullerdescription has been provided by Hobbs et al (2019) From anengineering geology viewpoint the major till units belonging to theWithernsea Skipsea Till and Bridlington Members of theHolderness Formation represent fissured lightly to heavily over-consolidated materials having similar geotechnical properties thatare in general agreement with descriptions by Bell amp Forster (1991)Bell (2002) and Powell amp Butcher (2003) However the laminatedsilty clayey (and glacitectonized) Dimlington Bed between theSkipsea Till and Bridlington members at around 15 m depthcombines low strength and high average permeability with highplasticity and compressibility Evidence from exposure on the cliffwhen compared with the borehole logs suggests that theDimlington Bed is prone to liquefaction and may have undergone
copy 2019 UKRI The British Geological Survey This is an Open Access article distributed under the terms of the Creative Commons Attribution 40 License(httpcreativecommonsorglicensesby40) Published by The Geological Society of London Publishing disclaimer wwwgeolsocorgukpub_ethics
Research article Quarterly Journal of Engineering Geology and Hydrogeology
Published Online First httpsdoiorg101144qjegh2018-210
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extrusion and thus thinning of the bed at the cliff It is unclearwhether or not minor observed (pre-landslide event) subsidenceclose to the cliff edge can be attributed to this
Geotechnical laboratory testing described in detail by Hobbset al (2019) has revealed the following at the Aldbrough CLO(1) the Dimlington Bed has lsquohighrsquo plasticity higher clay and siltcontents and higher water content and shrinkage limit than the tillsand is weaker more permeable and more compressible than the tills(2) the tills have a lsquolowrsquo to lsquointermediatersquo plasticity well-graded
particle-size distribution and lsquomediumrsquo shrinkage limit (3) theupper part of the Withernsea Member shows features attributable toweathering (4) the effective peak shear strength behaviour anddensities of the tills are similar (5) residual strengths for the tillmembers are very similar whereas that for the Dimlington Bed ismuch lower (6) residual friction angles for the till members are highowing to significant sand content (the strength sensitivity of the tillsis likely to be small (Reeves et al 2006) although specimen sizeand preparation need to be taken into account) (7) geotechnical
Fig 1 Map showing location of Aldbrough Coastal Landslide Observatory CLO (box) (BGScopyUKRI) Contains Ordnance Survey data copy Crown Copyrightand database rights 2018
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differences between tills are minor and are in general agreementwith published values
The landslide and hydrogeological processes within the CLOcliff slope are in dynamic equilibrium in the sense of lsquocompetingrsquoprocesses tending to cancel each other out but only temporarilythus resulting in episodic activity (Chandler amp Brunsden 1995) Forexample the zone of partial saturation increases in depth towardsthe cliff as evidenced by established negative piezometric porepressures these marginally reduce vertical stress and increase intacteffective strength in that part of the cliff at the same time stress reliefreduces mass strength Significant groundwater has been observedbeing discharged from the Mill Hill Bed to the cliff and a muchsmaller amount by the silt laminae within the Dimlington BedHowever the dynamic equilibrium is affected by the initiation andprogress of fresh rotational landslides on the cliff as groundwaterpathways are partially blocked under certain conditions of landslidedisplacement Overall pore pressures measured in the boreholepiezometer arrays are much lower than hydrostatic There isevidence of increased saturation and consequent softening of thetills immediately adjacent to the Mill Hill and Dimlington BedsFactors in the hydrological regime are a possible lack of continuityof some strata and the presence of stress-relief fissures close to thecliff Although all formations are believed to persist throughout theCLO poor borehole core recovery has not allowed this to beconfirmed
A further factor in the hydrogeology regime is the spacing andpersistence of joints within the tills and the consequent increase inthe mass permeability of the formations Trial drilling in Phase 1using air-flush suggested the presence of persistent open joints
within the Withernsea Member although their influence may havebeen exaggerated by near-surface desiccation cracks Examinationof Phase 2 borehole core initially 28 m landward (in 2015) of thecliff revealed few joints and fissures compared with those exposedon the cliff It is likely that close to the cliff vertical transmission ofgroundwater is greatly enhanced by stress relief fissures
Cliff recession monitoring
Since 2001 cliff monitoring has been maintained at the Aldbroughtest site using Terrestrial LiDAR Surveying (TLS) (Hobbs et al2002 2013 2015a Poulton et al 2006 Miller et al 2007 Buckleyet al 2008) more recently augmented by unmanned aerial vehicle(drone) photogrammetry The data from the TLS are used toconstruct digital elevation models (DEMs) examples of which areshown in Figure 3 These were compared and used to characterizelandslide processes and to calculate volume changes betweensurveys Up to November 2017 38 surveys had been carried out atAldbrough 27 of which have been used in volume calculations Thedata obtained from the monitoring surveys have allowed geomor-phological assessments and multiple cross-sections for slopestability analyses to be derived Volumes lost from the cliff havebeen calculated directly from the TLS models for the periodSeptember 2001 to November 2017 (Table 1 Fig 4) theserepresenting a potentially useful calibration dataset for coastalprocess modelling (Pethick 1996 Walkden amp Dickson 2008) Thedata shown have been extracted from the central 100 m of the studysite retreating along the line of recorded migration of the centralembayment
Fig 2 Schematic stratigraphy withphotograph of central (100 m) embayment(not to scale) (BGScopyUKRI) M Member
Fig 3 Selected biennial digital elevationmodels (DEMs) for CLO cliff (centralembayment) (BGScopyUKRI) IntermediateDEMs are omitted for clarity Width ofDEM is 100 m
Coastal landslide monitoring
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The cumulative volume loss for the study period to date(1 September 2001 to 23 November 2017 ie 162 years) was53 000 m3 per 100 m along coast giving an estimated gross weightof 111 300 tonnes This translates to a total of c 27 m linearrecession Twelve month volumetric losses from the cliff (Table 1)in the central embayment of the CLO range from 12 to 63 m3 mminus1
(along coast) the average being 33 m3 mminus1 These figures equate tosediment yield if sediment retained on the beach is included(Prandle et al 1996 Newsham et al 2002) The average equivalentcliff recession of the study site over the monitoring period derivedfrom TLS is 19 m aminus1 This agrees with historical averagerecession rates for Holderness as a whole obtained from pointdata of 080ndash20 m aminus1 determined by Pethick (1996) and Castedoet al (2015) but greater than the 13 m aminus1 average of Quinn et al(2010)
Landslide processes
The conceptual geologicalndashgeotechnical model for the CLO isshown in Figure 5 This has been derived from data described byHobbs et al (2019)
The primary type of landsliding at the CLO is observed to bedeep-seated rotational These landslides daylight 1ndash3 m above thecliff toe a position largely determined by the undulating boundarybetween the Bridlington Member and the Dimlington Bed Thiscompares with a deeper-seated landslide at nearby Cowden cliff
described by Butcher (1991) as having a lsquocompoundrsquo slip surfacerather than a simple rotational one extending to several metresbelow sea-level Indeed such landslides have been observed to thesouth of the CLO daylighting beneath beach level and also on thenorth Norfolk coast at Sidestrand (Hobbs et al 2008) Dixon ampBromhead (2002) in their study of London Clay landslides atWarden Point Isle of Sheppey Kent noted that bedding-relatedfeatures lsquocontrolled the location of the basal part of the slip surfacersquoand that this was normal in stiff plastic clays Major rotationallandslide events at the CLO result in cliff-top recessions of up to 7 mat mid-embayment (Fig 6a and d) with near-vertical backscarpsfully exposing the Hornsea Member Pickwell (1878) emphasizedthe role of the lsquoboulder clayrsquo (Bridlington Member) in providing thebase of the landslips and a lsquorevetmentrsquo against erosion of theoverlying deposits and landslide debris depending on its elevationlocally He also illustrated types of rotational and compositelandslides on the Holderness coast including at lsquoAldboroughrsquo(Pickwell 1878 fig 6) and gave detailed accounts of losses of landfrom the period He added that lsquoAlmost the whole length of the cliffin this parish [Aldbrough] may be considered as one huge landslipfrom end to endrsquo
Secondary landslides tend to occur within the slipped mass(Fig 6b and c) although toppling of Withernsea Member blocks(typical volume 3ndash5 m3) also occurs from the over-steepenedpromontories separating embayments Toppling has also featuredbut on a much larger scale at Warden Point (Dixon amp Bromhead
Table 1 Table of cliff recession derived from selected TLS surveys September 2001 to November 2017 (BGScopyUKRI)
Increm Cumul Increm Cumul Cumul Increm Cumul Cumul 12 mnth Meantime time loss loss lossm loss loss lossm incr loss recess
Period 100 m 100 m 100 m 100 m 100 m rate
Date start end (days) (days) (m3) (m3) (m3) (tonnes) (tonnes) (tonnes) (m3) (myr)
TLS data are unavailable for 2008 Mean recession rate is here calculated from incremental volumetric loss for each monitoring period by normalizing the monitoring period to12 months and dividing by cliff height (17 m) and test section width (100 m) Whilst being an approximation in terms of cliff height this is not a cliff-top recession rate
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2002) At the CLO toppling has been observed either where avertical rotational landslide back-scarp undergoes degradation andsecondary movement or where the upper part of a vertical (or near-vertical) inter-embayment promontory partially collapses Topplingmay also be promoted by minor pre-failure subsidence of the clifftop resulting in a seaward tilt or where a graben-like feature hasdeveloped at the rear of an already rotated slip mass Mudflows tendto occur on the peripheries of rotated slip blocks in response to theamount of surface water on the slope resulting from seepage andordirect rainfall and where the slipped masses have had time todegrade sufficiently
Examination of TLS-derived cross-sections revealed an overallminimum slope angle of 45deg and a maximum of 66deg athoughsteeper and temporarily near-vertical slopes have been observed atthe site particularly at promontories There have been manyinstances where slope stability analyses have returned factors ofsafety less than unity for lsquostablersquo cliff slopes probably owing atleast in part to long-term partial saturation within the main bodies ofthe tills and the Hornsea Member close to the cliff face (Butcher1991 Hobbs et al 2013)
The monitoring has shown that the cycle of major landslideevents at the CLO is every 6ndash7 years this being based on threeevents identified since September 2001 (ie August 2004 March
2010 and February 2017 Figs 4 and 7) This compares with a cycleof around 30ndash40 years for the 40 m high London Clay cliffs atWarden Point Isle of Sheppey Kent (DixonampBromhead 2002) Aswas the case at Warden Point the embayments at the CLO haveretreated along the same heading (due west in this case) andmaintained their dimensions over the monitoring period This is atodds with the suggestion of Pethick (1996) that embaymentsmigrated southward at Holderness Pickwell (1878) observed3ndash4 year cycles of landslide activity at Tunstall (10 km SSE ofAldbrough)
The relationship between incremental volume loss and totalrainfall is also shown in Figure 7 (total rainfall refers to that since theprevious survey) This shows a broad-scale agreement betweenrainfall and volume loss from the cliff (calculated from TLS) withpeaks in volume loss following the major landslide events (It shouldbe noted that pre-2012 rainfall data are averages taken from threeEast Riding of Yorkshire stations within a 23 km radius of the CLO)
Oceanographic and meteorological factors
Coastal processes such as storms and the energy provided by highwaves at the coast play a major role in coastal erosion around Britainand these are found to be particularly enduring on the east coast north
Fig 4 Plot of date v cumulative volumeloss from cliff (per 100 m) at CLO(September 2001 datum) (BGScopyUKRI)Dates of major fresh landslide events incentral embayment are shown as dashedred lines with arrows
Fig 5 Conceptual geologicalndashgeotechnical model for the CLO(BGScopyUKRI)
Coastal landslide monitoring
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of the Humber (median duration gt13 h) with the likelihood ofspanning High Water (Dhoop amp Mason 2018) for example ananticyclonic storm on 18 December 2009 lasted 195 h at Hornsea Astorm surge described by the Environment Agency as the mostserious for 60 years hit the east coast of England on 5 December2013 causing severe coastal flooding and erosion most notably inEast Anglia During this event the high tide levels (predicted) atBridlington and Spurn Point were 615 m at 1754 h and 725 m at1853 h respectively This compares with estimated mean high-waterspring (MHWS) and mean low-water spring (MLWS) of 644 m and114 m respectively at Aldbrough Wave height recorded by theChannel Coastal Observatory (CCO) for the lsquoHornsearsquo buoybelonging to the East Riding of Yorkshire Regional CoastalMonitoring Programme (ERYRCMP 1995) peaked in the earlyhours of 6 December with waves in excess of 6 m accompanied by amaximum wind speed of 208 m sminus1 (Force 8ndash9) recorded at thenearby BGS weather station However in terms of wave energyalone higher peakswere recorded on 24March and 10October 2013a maximumwave height of 74 m having been recorded on 23MarchAnother notable lsquostormrsquo year (since June 2008) was 2010 Wavedirection was predominantly and consistently from the NNE and NElsquoOnshorersquo waves (defined here as derived from compass pointsN340deg to N140deg) represent gt80 of the total This highlights thevulnerability of the Holderness coast to the erosive wave energywhich predominantly emanates from an average angle of incidence425deg to the current average coastline at Aldbrough (CCO 2017)Notable rainfall events included 12ndash15 and 24ndash25 June 2007 this
month had over four times the average rainfall equivalent to a200 year return event at Holderness (Hanna et al 2008)
The occurrence of storms as defined by CCO (2017) andrecorded at the Hornsea buoy from 2009 is plotted againstincremental volume loss for 100 m of cliff (calculated from TLS) inFigure 8 It should be noted that a storm event is indicated using thelsquopeaks-over-thresholdrsquomethod (CCO 2017) where the lsquowave heightthresholdrsquo was variously defined over the monitoring period from300 to 375 m and based on 025 year return periods A comparablelong-term trend is shown but the number of storms does not appearto have a causal effect on cliff volume change However a closeragreement is evident after 2013 where TLS surveys are morefrequent
A plot of average wave-climate energy v incremental volumeloss for 100 m of cliff (calculated from TLS) is shown in Figure 9the wave data for which were provided by the Hornsea buoy (CCO2017) The lsquowave-climatersquo energy P was calculated here as follows(Dexawave httpwwwdexawavecom)
P frac14 057 (HS)2 TP
where P is wave energy (kW mminus1) HS is significant wave height(m) (half-hourly data) and TP is time period between each wavecrest (s)
The plot shows similar trends with time for cliff volume loss andwave energy particularly when taken over several years However acausal effect is not indicated
Fig 6 Examples of different stages of deep-seated rotational landsliding in the central embayment of the CLO (a) August 2004 (b) February 2011(c) November 2011 (d) November 2017 All cliff access was carried out with a full risk assessment (BGScopyUKRI)
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The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
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Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
extrusion and thus thinning of the bed at the cliff It is unclearwhether or not minor observed (pre-landslide event) subsidenceclose to the cliff edge can be attributed to this
Geotechnical laboratory testing described in detail by Hobbset al (2019) has revealed the following at the Aldbrough CLO(1) the Dimlington Bed has lsquohighrsquo plasticity higher clay and siltcontents and higher water content and shrinkage limit than the tillsand is weaker more permeable and more compressible than the tills(2) the tills have a lsquolowrsquo to lsquointermediatersquo plasticity well-graded
particle-size distribution and lsquomediumrsquo shrinkage limit (3) theupper part of the Withernsea Member shows features attributable toweathering (4) the effective peak shear strength behaviour anddensities of the tills are similar (5) residual strengths for the tillmembers are very similar whereas that for the Dimlington Bed ismuch lower (6) residual friction angles for the till members are highowing to significant sand content (the strength sensitivity of the tillsis likely to be small (Reeves et al 2006) although specimen sizeand preparation need to be taken into account) (7) geotechnical
Fig 1 Map showing location of Aldbrough Coastal Landslide Observatory CLO (box) (BGScopyUKRI) Contains Ordnance Survey data copy Crown Copyrightand database rights 2018
P R N Hobbs et al
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differences between tills are minor and are in general agreementwith published values
The landslide and hydrogeological processes within the CLOcliff slope are in dynamic equilibrium in the sense of lsquocompetingrsquoprocesses tending to cancel each other out but only temporarilythus resulting in episodic activity (Chandler amp Brunsden 1995) Forexample the zone of partial saturation increases in depth towardsthe cliff as evidenced by established negative piezometric porepressures these marginally reduce vertical stress and increase intacteffective strength in that part of the cliff at the same time stress reliefreduces mass strength Significant groundwater has been observedbeing discharged from the Mill Hill Bed to the cliff and a muchsmaller amount by the silt laminae within the Dimlington BedHowever the dynamic equilibrium is affected by the initiation andprogress of fresh rotational landslides on the cliff as groundwaterpathways are partially blocked under certain conditions of landslidedisplacement Overall pore pressures measured in the boreholepiezometer arrays are much lower than hydrostatic There isevidence of increased saturation and consequent softening of thetills immediately adjacent to the Mill Hill and Dimlington BedsFactors in the hydrological regime are a possible lack of continuityof some strata and the presence of stress-relief fissures close to thecliff Although all formations are believed to persist throughout theCLO poor borehole core recovery has not allowed this to beconfirmed
A further factor in the hydrogeology regime is the spacing andpersistence of joints within the tills and the consequent increase inthe mass permeability of the formations Trial drilling in Phase 1using air-flush suggested the presence of persistent open joints
within the Withernsea Member although their influence may havebeen exaggerated by near-surface desiccation cracks Examinationof Phase 2 borehole core initially 28 m landward (in 2015) of thecliff revealed few joints and fissures compared with those exposedon the cliff It is likely that close to the cliff vertical transmission ofgroundwater is greatly enhanced by stress relief fissures
Cliff recession monitoring
Since 2001 cliff monitoring has been maintained at the Aldbroughtest site using Terrestrial LiDAR Surveying (TLS) (Hobbs et al2002 2013 2015a Poulton et al 2006 Miller et al 2007 Buckleyet al 2008) more recently augmented by unmanned aerial vehicle(drone) photogrammetry The data from the TLS are used toconstruct digital elevation models (DEMs) examples of which areshown in Figure 3 These were compared and used to characterizelandslide processes and to calculate volume changes betweensurveys Up to November 2017 38 surveys had been carried out atAldbrough 27 of which have been used in volume calculations Thedata obtained from the monitoring surveys have allowed geomor-phological assessments and multiple cross-sections for slopestability analyses to be derived Volumes lost from the cliff havebeen calculated directly from the TLS models for the periodSeptember 2001 to November 2017 (Table 1 Fig 4) theserepresenting a potentially useful calibration dataset for coastalprocess modelling (Pethick 1996 Walkden amp Dickson 2008) Thedata shown have been extracted from the central 100 m of the studysite retreating along the line of recorded migration of the centralembayment
Fig 2 Schematic stratigraphy withphotograph of central (100 m) embayment(not to scale) (BGScopyUKRI) M Member
Fig 3 Selected biennial digital elevationmodels (DEMs) for CLO cliff (centralembayment) (BGScopyUKRI) IntermediateDEMs are omitted for clarity Width ofDEM is 100 m
Coastal landslide monitoring
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The cumulative volume loss for the study period to date(1 September 2001 to 23 November 2017 ie 162 years) was53 000 m3 per 100 m along coast giving an estimated gross weightof 111 300 tonnes This translates to a total of c 27 m linearrecession Twelve month volumetric losses from the cliff (Table 1)in the central embayment of the CLO range from 12 to 63 m3 mminus1
(along coast) the average being 33 m3 mminus1 These figures equate tosediment yield if sediment retained on the beach is included(Prandle et al 1996 Newsham et al 2002) The average equivalentcliff recession of the study site over the monitoring period derivedfrom TLS is 19 m aminus1 This agrees with historical averagerecession rates for Holderness as a whole obtained from pointdata of 080ndash20 m aminus1 determined by Pethick (1996) and Castedoet al (2015) but greater than the 13 m aminus1 average of Quinn et al(2010)
Landslide processes
The conceptual geologicalndashgeotechnical model for the CLO isshown in Figure 5 This has been derived from data described byHobbs et al (2019)
The primary type of landsliding at the CLO is observed to bedeep-seated rotational These landslides daylight 1ndash3 m above thecliff toe a position largely determined by the undulating boundarybetween the Bridlington Member and the Dimlington Bed Thiscompares with a deeper-seated landslide at nearby Cowden cliff
described by Butcher (1991) as having a lsquocompoundrsquo slip surfacerather than a simple rotational one extending to several metresbelow sea-level Indeed such landslides have been observed to thesouth of the CLO daylighting beneath beach level and also on thenorth Norfolk coast at Sidestrand (Hobbs et al 2008) Dixon ampBromhead (2002) in their study of London Clay landslides atWarden Point Isle of Sheppey Kent noted that bedding-relatedfeatures lsquocontrolled the location of the basal part of the slip surfacersquoand that this was normal in stiff plastic clays Major rotationallandslide events at the CLO result in cliff-top recessions of up to 7 mat mid-embayment (Fig 6a and d) with near-vertical backscarpsfully exposing the Hornsea Member Pickwell (1878) emphasizedthe role of the lsquoboulder clayrsquo (Bridlington Member) in providing thebase of the landslips and a lsquorevetmentrsquo against erosion of theoverlying deposits and landslide debris depending on its elevationlocally He also illustrated types of rotational and compositelandslides on the Holderness coast including at lsquoAldboroughrsquo(Pickwell 1878 fig 6) and gave detailed accounts of losses of landfrom the period He added that lsquoAlmost the whole length of the cliffin this parish [Aldbrough] may be considered as one huge landslipfrom end to endrsquo
Secondary landslides tend to occur within the slipped mass(Fig 6b and c) although toppling of Withernsea Member blocks(typical volume 3ndash5 m3) also occurs from the over-steepenedpromontories separating embayments Toppling has also featuredbut on a much larger scale at Warden Point (Dixon amp Bromhead
Table 1 Table of cliff recession derived from selected TLS surveys September 2001 to November 2017 (BGScopyUKRI)
Increm Cumul Increm Cumul Cumul Increm Cumul Cumul 12 mnth Meantime time loss loss lossm loss loss lossm incr loss recess
Period 100 m 100 m 100 m 100 m 100 m rate
Date start end (days) (days) (m3) (m3) (m3) (tonnes) (tonnes) (tonnes) (m3) (myr)
TLS data are unavailable for 2008 Mean recession rate is here calculated from incremental volumetric loss for each monitoring period by normalizing the monitoring period to12 months and dividing by cliff height (17 m) and test section width (100 m) Whilst being an approximation in terms of cliff height this is not a cliff-top recession rate
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2002) At the CLO toppling has been observed either where avertical rotational landslide back-scarp undergoes degradation andsecondary movement or where the upper part of a vertical (or near-vertical) inter-embayment promontory partially collapses Topplingmay also be promoted by minor pre-failure subsidence of the clifftop resulting in a seaward tilt or where a graben-like feature hasdeveloped at the rear of an already rotated slip mass Mudflows tendto occur on the peripheries of rotated slip blocks in response to theamount of surface water on the slope resulting from seepage andordirect rainfall and where the slipped masses have had time todegrade sufficiently
Examination of TLS-derived cross-sections revealed an overallminimum slope angle of 45deg and a maximum of 66deg athoughsteeper and temporarily near-vertical slopes have been observed atthe site particularly at promontories There have been manyinstances where slope stability analyses have returned factors ofsafety less than unity for lsquostablersquo cliff slopes probably owing atleast in part to long-term partial saturation within the main bodies ofthe tills and the Hornsea Member close to the cliff face (Butcher1991 Hobbs et al 2013)
The monitoring has shown that the cycle of major landslideevents at the CLO is every 6ndash7 years this being based on threeevents identified since September 2001 (ie August 2004 March
2010 and February 2017 Figs 4 and 7) This compares with a cycleof around 30ndash40 years for the 40 m high London Clay cliffs atWarden Point Isle of Sheppey Kent (DixonampBromhead 2002) Aswas the case at Warden Point the embayments at the CLO haveretreated along the same heading (due west in this case) andmaintained their dimensions over the monitoring period This is atodds with the suggestion of Pethick (1996) that embaymentsmigrated southward at Holderness Pickwell (1878) observed3ndash4 year cycles of landslide activity at Tunstall (10 km SSE ofAldbrough)
The relationship between incremental volume loss and totalrainfall is also shown in Figure 7 (total rainfall refers to that since theprevious survey) This shows a broad-scale agreement betweenrainfall and volume loss from the cliff (calculated from TLS) withpeaks in volume loss following the major landslide events (It shouldbe noted that pre-2012 rainfall data are averages taken from threeEast Riding of Yorkshire stations within a 23 km radius of the CLO)
Oceanographic and meteorological factors
Coastal processes such as storms and the energy provided by highwaves at the coast play a major role in coastal erosion around Britainand these are found to be particularly enduring on the east coast north
Fig 4 Plot of date v cumulative volumeloss from cliff (per 100 m) at CLO(September 2001 datum) (BGScopyUKRI)Dates of major fresh landslide events incentral embayment are shown as dashedred lines with arrows
Fig 5 Conceptual geologicalndashgeotechnical model for the CLO(BGScopyUKRI)
Coastal landslide monitoring
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of the Humber (median duration gt13 h) with the likelihood ofspanning High Water (Dhoop amp Mason 2018) for example ananticyclonic storm on 18 December 2009 lasted 195 h at Hornsea Astorm surge described by the Environment Agency as the mostserious for 60 years hit the east coast of England on 5 December2013 causing severe coastal flooding and erosion most notably inEast Anglia During this event the high tide levels (predicted) atBridlington and Spurn Point were 615 m at 1754 h and 725 m at1853 h respectively This compares with estimated mean high-waterspring (MHWS) and mean low-water spring (MLWS) of 644 m and114 m respectively at Aldbrough Wave height recorded by theChannel Coastal Observatory (CCO) for the lsquoHornsearsquo buoybelonging to the East Riding of Yorkshire Regional CoastalMonitoring Programme (ERYRCMP 1995) peaked in the earlyhours of 6 December with waves in excess of 6 m accompanied by amaximum wind speed of 208 m sminus1 (Force 8ndash9) recorded at thenearby BGS weather station However in terms of wave energyalone higher peakswere recorded on 24March and 10October 2013a maximumwave height of 74 m having been recorded on 23MarchAnother notable lsquostormrsquo year (since June 2008) was 2010 Wavedirection was predominantly and consistently from the NNE and NElsquoOnshorersquo waves (defined here as derived from compass pointsN340deg to N140deg) represent gt80 of the total This highlights thevulnerability of the Holderness coast to the erosive wave energywhich predominantly emanates from an average angle of incidence425deg to the current average coastline at Aldbrough (CCO 2017)Notable rainfall events included 12ndash15 and 24ndash25 June 2007 this
month had over four times the average rainfall equivalent to a200 year return event at Holderness (Hanna et al 2008)
The occurrence of storms as defined by CCO (2017) andrecorded at the Hornsea buoy from 2009 is plotted againstincremental volume loss for 100 m of cliff (calculated from TLS) inFigure 8 It should be noted that a storm event is indicated using thelsquopeaks-over-thresholdrsquomethod (CCO 2017) where the lsquowave heightthresholdrsquo was variously defined over the monitoring period from300 to 375 m and based on 025 year return periods A comparablelong-term trend is shown but the number of storms does not appearto have a causal effect on cliff volume change However a closeragreement is evident after 2013 where TLS surveys are morefrequent
A plot of average wave-climate energy v incremental volumeloss for 100 m of cliff (calculated from TLS) is shown in Figure 9the wave data for which were provided by the Hornsea buoy (CCO2017) The lsquowave-climatersquo energy P was calculated here as follows(Dexawave httpwwwdexawavecom)
P frac14 057 (HS)2 TP
where P is wave energy (kW mminus1) HS is significant wave height(m) (half-hourly data) and TP is time period between each wavecrest (s)
The plot shows similar trends with time for cliff volume loss andwave energy particularly when taken over several years However acausal effect is not indicated
Fig 6 Examples of different stages of deep-seated rotational landsliding in the central embayment of the CLO (a) August 2004 (b) February 2011(c) November 2011 (d) November 2017 All cliff access was carried out with a full risk assessment (BGScopyUKRI)
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The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
differences between tills are minor and are in general agreementwith published values
The landslide and hydrogeological processes within the CLOcliff slope are in dynamic equilibrium in the sense of lsquocompetingrsquoprocesses tending to cancel each other out but only temporarilythus resulting in episodic activity (Chandler amp Brunsden 1995) Forexample the zone of partial saturation increases in depth towardsthe cliff as evidenced by established negative piezometric porepressures these marginally reduce vertical stress and increase intacteffective strength in that part of the cliff at the same time stress reliefreduces mass strength Significant groundwater has been observedbeing discharged from the Mill Hill Bed to the cliff and a muchsmaller amount by the silt laminae within the Dimlington BedHowever the dynamic equilibrium is affected by the initiation andprogress of fresh rotational landslides on the cliff as groundwaterpathways are partially blocked under certain conditions of landslidedisplacement Overall pore pressures measured in the boreholepiezometer arrays are much lower than hydrostatic There isevidence of increased saturation and consequent softening of thetills immediately adjacent to the Mill Hill and Dimlington BedsFactors in the hydrological regime are a possible lack of continuityof some strata and the presence of stress-relief fissures close to thecliff Although all formations are believed to persist throughout theCLO poor borehole core recovery has not allowed this to beconfirmed
A further factor in the hydrogeology regime is the spacing andpersistence of joints within the tills and the consequent increase inthe mass permeability of the formations Trial drilling in Phase 1using air-flush suggested the presence of persistent open joints
within the Withernsea Member although their influence may havebeen exaggerated by near-surface desiccation cracks Examinationof Phase 2 borehole core initially 28 m landward (in 2015) of thecliff revealed few joints and fissures compared with those exposedon the cliff It is likely that close to the cliff vertical transmission ofgroundwater is greatly enhanced by stress relief fissures
Cliff recession monitoring
Since 2001 cliff monitoring has been maintained at the Aldbroughtest site using Terrestrial LiDAR Surveying (TLS) (Hobbs et al2002 2013 2015a Poulton et al 2006 Miller et al 2007 Buckleyet al 2008) more recently augmented by unmanned aerial vehicle(drone) photogrammetry The data from the TLS are used toconstruct digital elevation models (DEMs) examples of which areshown in Figure 3 These were compared and used to characterizelandslide processes and to calculate volume changes betweensurveys Up to November 2017 38 surveys had been carried out atAldbrough 27 of which have been used in volume calculations Thedata obtained from the monitoring surveys have allowed geomor-phological assessments and multiple cross-sections for slopestability analyses to be derived Volumes lost from the cliff havebeen calculated directly from the TLS models for the periodSeptember 2001 to November 2017 (Table 1 Fig 4) theserepresenting a potentially useful calibration dataset for coastalprocess modelling (Pethick 1996 Walkden amp Dickson 2008) Thedata shown have been extracted from the central 100 m of the studysite retreating along the line of recorded migration of the centralembayment
Fig 2 Schematic stratigraphy withphotograph of central (100 m) embayment(not to scale) (BGScopyUKRI) M Member
Fig 3 Selected biennial digital elevationmodels (DEMs) for CLO cliff (centralembayment) (BGScopyUKRI) IntermediateDEMs are omitted for clarity Width ofDEM is 100 m
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The cumulative volume loss for the study period to date(1 September 2001 to 23 November 2017 ie 162 years) was53 000 m3 per 100 m along coast giving an estimated gross weightof 111 300 tonnes This translates to a total of c 27 m linearrecession Twelve month volumetric losses from the cliff (Table 1)in the central embayment of the CLO range from 12 to 63 m3 mminus1
(along coast) the average being 33 m3 mminus1 These figures equate tosediment yield if sediment retained on the beach is included(Prandle et al 1996 Newsham et al 2002) The average equivalentcliff recession of the study site over the monitoring period derivedfrom TLS is 19 m aminus1 This agrees with historical averagerecession rates for Holderness as a whole obtained from pointdata of 080ndash20 m aminus1 determined by Pethick (1996) and Castedoet al (2015) but greater than the 13 m aminus1 average of Quinn et al(2010)
Landslide processes
The conceptual geologicalndashgeotechnical model for the CLO isshown in Figure 5 This has been derived from data described byHobbs et al (2019)
The primary type of landsliding at the CLO is observed to bedeep-seated rotational These landslides daylight 1ndash3 m above thecliff toe a position largely determined by the undulating boundarybetween the Bridlington Member and the Dimlington Bed Thiscompares with a deeper-seated landslide at nearby Cowden cliff
described by Butcher (1991) as having a lsquocompoundrsquo slip surfacerather than a simple rotational one extending to several metresbelow sea-level Indeed such landslides have been observed to thesouth of the CLO daylighting beneath beach level and also on thenorth Norfolk coast at Sidestrand (Hobbs et al 2008) Dixon ampBromhead (2002) in their study of London Clay landslides atWarden Point Isle of Sheppey Kent noted that bedding-relatedfeatures lsquocontrolled the location of the basal part of the slip surfacersquoand that this was normal in stiff plastic clays Major rotationallandslide events at the CLO result in cliff-top recessions of up to 7 mat mid-embayment (Fig 6a and d) with near-vertical backscarpsfully exposing the Hornsea Member Pickwell (1878) emphasizedthe role of the lsquoboulder clayrsquo (Bridlington Member) in providing thebase of the landslips and a lsquorevetmentrsquo against erosion of theoverlying deposits and landslide debris depending on its elevationlocally He also illustrated types of rotational and compositelandslides on the Holderness coast including at lsquoAldboroughrsquo(Pickwell 1878 fig 6) and gave detailed accounts of losses of landfrom the period He added that lsquoAlmost the whole length of the cliffin this parish [Aldbrough] may be considered as one huge landslipfrom end to endrsquo
Secondary landslides tend to occur within the slipped mass(Fig 6b and c) although toppling of Withernsea Member blocks(typical volume 3ndash5 m3) also occurs from the over-steepenedpromontories separating embayments Toppling has also featuredbut on a much larger scale at Warden Point (Dixon amp Bromhead
Table 1 Table of cliff recession derived from selected TLS surveys September 2001 to November 2017 (BGScopyUKRI)
Increm Cumul Increm Cumul Cumul Increm Cumul Cumul 12 mnth Meantime time loss loss lossm loss loss lossm incr loss recess
Period 100 m 100 m 100 m 100 m 100 m rate
Date start end (days) (days) (m3) (m3) (m3) (tonnes) (tonnes) (tonnes) (m3) (myr)
TLS data are unavailable for 2008 Mean recession rate is here calculated from incremental volumetric loss for each monitoring period by normalizing the monitoring period to12 months and dividing by cliff height (17 m) and test section width (100 m) Whilst being an approximation in terms of cliff height this is not a cliff-top recession rate
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2002) At the CLO toppling has been observed either where avertical rotational landslide back-scarp undergoes degradation andsecondary movement or where the upper part of a vertical (or near-vertical) inter-embayment promontory partially collapses Topplingmay also be promoted by minor pre-failure subsidence of the clifftop resulting in a seaward tilt or where a graben-like feature hasdeveloped at the rear of an already rotated slip mass Mudflows tendto occur on the peripheries of rotated slip blocks in response to theamount of surface water on the slope resulting from seepage andordirect rainfall and where the slipped masses have had time todegrade sufficiently
Examination of TLS-derived cross-sections revealed an overallminimum slope angle of 45deg and a maximum of 66deg athoughsteeper and temporarily near-vertical slopes have been observed atthe site particularly at promontories There have been manyinstances where slope stability analyses have returned factors ofsafety less than unity for lsquostablersquo cliff slopes probably owing atleast in part to long-term partial saturation within the main bodies ofthe tills and the Hornsea Member close to the cliff face (Butcher1991 Hobbs et al 2013)
The monitoring has shown that the cycle of major landslideevents at the CLO is every 6ndash7 years this being based on threeevents identified since September 2001 (ie August 2004 March
2010 and February 2017 Figs 4 and 7) This compares with a cycleof around 30ndash40 years for the 40 m high London Clay cliffs atWarden Point Isle of Sheppey Kent (DixonampBromhead 2002) Aswas the case at Warden Point the embayments at the CLO haveretreated along the same heading (due west in this case) andmaintained their dimensions over the monitoring period This is atodds with the suggestion of Pethick (1996) that embaymentsmigrated southward at Holderness Pickwell (1878) observed3ndash4 year cycles of landslide activity at Tunstall (10 km SSE ofAldbrough)
The relationship between incremental volume loss and totalrainfall is also shown in Figure 7 (total rainfall refers to that since theprevious survey) This shows a broad-scale agreement betweenrainfall and volume loss from the cliff (calculated from TLS) withpeaks in volume loss following the major landslide events (It shouldbe noted that pre-2012 rainfall data are averages taken from threeEast Riding of Yorkshire stations within a 23 km radius of the CLO)
Oceanographic and meteorological factors
Coastal processes such as storms and the energy provided by highwaves at the coast play a major role in coastal erosion around Britainand these are found to be particularly enduring on the east coast north
Fig 4 Plot of date v cumulative volumeloss from cliff (per 100 m) at CLO(September 2001 datum) (BGScopyUKRI)Dates of major fresh landslide events incentral embayment are shown as dashedred lines with arrows
Fig 5 Conceptual geologicalndashgeotechnical model for the CLO(BGScopyUKRI)
Coastal landslide monitoring
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of the Humber (median duration gt13 h) with the likelihood ofspanning High Water (Dhoop amp Mason 2018) for example ananticyclonic storm on 18 December 2009 lasted 195 h at Hornsea Astorm surge described by the Environment Agency as the mostserious for 60 years hit the east coast of England on 5 December2013 causing severe coastal flooding and erosion most notably inEast Anglia During this event the high tide levels (predicted) atBridlington and Spurn Point were 615 m at 1754 h and 725 m at1853 h respectively This compares with estimated mean high-waterspring (MHWS) and mean low-water spring (MLWS) of 644 m and114 m respectively at Aldbrough Wave height recorded by theChannel Coastal Observatory (CCO) for the lsquoHornsearsquo buoybelonging to the East Riding of Yorkshire Regional CoastalMonitoring Programme (ERYRCMP 1995) peaked in the earlyhours of 6 December with waves in excess of 6 m accompanied by amaximum wind speed of 208 m sminus1 (Force 8ndash9) recorded at thenearby BGS weather station However in terms of wave energyalone higher peakswere recorded on 24March and 10October 2013a maximumwave height of 74 m having been recorded on 23MarchAnother notable lsquostormrsquo year (since June 2008) was 2010 Wavedirection was predominantly and consistently from the NNE and NElsquoOnshorersquo waves (defined here as derived from compass pointsN340deg to N140deg) represent gt80 of the total This highlights thevulnerability of the Holderness coast to the erosive wave energywhich predominantly emanates from an average angle of incidence425deg to the current average coastline at Aldbrough (CCO 2017)Notable rainfall events included 12ndash15 and 24ndash25 June 2007 this
month had over four times the average rainfall equivalent to a200 year return event at Holderness (Hanna et al 2008)
The occurrence of storms as defined by CCO (2017) andrecorded at the Hornsea buoy from 2009 is plotted againstincremental volume loss for 100 m of cliff (calculated from TLS) inFigure 8 It should be noted that a storm event is indicated using thelsquopeaks-over-thresholdrsquomethod (CCO 2017) where the lsquowave heightthresholdrsquo was variously defined over the monitoring period from300 to 375 m and based on 025 year return periods A comparablelong-term trend is shown but the number of storms does not appearto have a causal effect on cliff volume change However a closeragreement is evident after 2013 where TLS surveys are morefrequent
A plot of average wave-climate energy v incremental volumeloss for 100 m of cliff (calculated from TLS) is shown in Figure 9the wave data for which were provided by the Hornsea buoy (CCO2017) The lsquowave-climatersquo energy P was calculated here as follows(Dexawave httpwwwdexawavecom)
P frac14 057 (HS)2 TP
where P is wave energy (kW mminus1) HS is significant wave height(m) (half-hourly data) and TP is time period between each wavecrest (s)
The plot shows similar trends with time for cliff volume loss andwave energy particularly when taken over several years However acausal effect is not indicated
Fig 6 Examples of different stages of deep-seated rotational landsliding in the central embayment of the CLO (a) August 2004 (b) February 2011(c) November 2011 (d) November 2017 All cliff access was carried out with a full risk assessment (BGScopyUKRI)
P R N Hobbs et al
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The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
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Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The cumulative volume loss for the study period to date(1 September 2001 to 23 November 2017 ie 162 years) was53 000 m3 per 100 m along coast giving an estimated gross weightof 111 300 tonnes This translates to a total of c 27 m linearrecession Twelve month volumetric losses from the cliff (Table 1)in the central embayment of the CLO range from 12 to 63 m3 mminus1
(along coast) the average being 33 m3 mminus1 These figures equate tosediment yield if sediment retained on the beach is included(Prandle et al 1996 Newsham et al 2002) The average equivalentcliff recession of the study site over the monitoring period derivedfrom TLS is 19 m aminus1 This agrees with historical averagerecession rates for Holderness as a whole obtained from pointdata of 080ndash20 m aminus1 determined by Pethick (1996) and Castedoet al (2015) but greater than the 13 m aminus1 average of Quinn et al(2010)
Landslide processes
The conceptual geologicalndashgeotechnical model for the CLO isshown in Figure 5 This has been derived from data described byHobbs et al (2019)
The primary type of landsliding at the CLO is observed to bedeep-seated rotational These landslides daylight 1ndash3 m above thecliff toe a position largely determined by the undulating boundarybetween the Bridlington Member and the Dimlington Bed Thiscompares with a deeper-seated landslide at nearby Cowden cliff
described by Butcher (1991) as having a lsquocompoundrsquo slip surfacerather than a simple rotational one extending to several metresbelow sea-level Indeed such landslides have been observed to thesouth of the CLO daylighting beneath beach level and also on thenorth Norfolk coast at Sidestrand (Hobbs et al 2008) Dixon ampBromhead (2002) in their study of London Clay landslides atWarden Point Isle of Sheppey Kent noted that bedding-relatedfeatures lsquocontrolled the location of the basal part of the slip surfacersquoand that this was normal in stiff plastic clays Major rotationallandslide events at the CLO result in cliff-top recessions of up to 7 mat mid-embayment (Fig 6a and d) with near-vertical backscarpsfully exposing the Hornsea Member Pickwell (1878) emphasizedthe role of the lsquoboulder clayrsquo (Bridlington Member) in providing thebase of the landslips and a lsquorevetmentrsquo against erosion of theoverlying deposits and landslide debris depending on its elevationlocally He also illustrated types of rotational and compositelandslides on the Holderness coast including at lsquoAldboroughrsquo(Pickwell 1878 fig 6) and gave detailed accounts of losses of landfrom the period He added that lsquoAlmost the whole length of the cliffin this parish [Aldbrough] may be considered as one huge landslipfrom end to endrsquo
Secondary landslides tend to occur within the slipped mass(Fig 6b and c) although toppling of Withernsea Member blocks(typical volume 3ndash5 m3) also occurs from the over-steepenedpromontories separating embayments Toppling has also featuredbut on a much larger scale at Warden Point (Dixon amp Bromhead
Table 1 Table of cliff recession derived from selected TLS surveys September 2001 to November 2017 (BGScopyUKRI)
Increm Cumul Increm Cumul Cumul Increm Cumul Cumul 12 mnth Meantime time loss loss lossm loss loss lossm incr loss recess
Period 100 m 100 m 100 m 100 m 100 m rate
Date start end (days) (days) (m3) (m3) (m3) (tonnes) (tonnes) (tonnes) (m3) (myr)
TLS data are unavailable for 2008 Mean recession rate is here calculated from incremental volumetric loss for each monitoring period by normalizing the monitoring period to12 months and dividing by cliff height (17 m) and test section width (100 m) Whilst being an approximation in terms of cliff height this is not a cliff-top recession rate
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2002) At the CLO toppling has been observed either where avertical rotational landslide back-scarp undergoes degradation andsecondary movement or where the upper part of a vertical (or near-vertical) inter-embayment promontory partially collapses Topplingmay also be promoted by minor pre-failure subsidence of the clifftop resulting in a seaward tilt or where a graben-like feature hasdeveloped at the rear of an already rotated slip mass Mudflows tendto occur on the peripheries of rotated slip blocks in response to theamount of surface water on the slope resulting from seepage andordirect rainfall and where the slipped masses have had time todegrade sufficiently
Examination of TLS-derived cross-sections revealed an overallminimum slope angle of 45deg and a maximum of 66deg athoughsteeper and temporarily near-vertical slopes have been observed atthe site particularly at promontories There have been manyinstances where slope stability analyses have returned factors ofsafety less than unity for lsquostablersquo cliff slopes probably owing atleast in part to long-term partial saturation within the main bodies ofthe tills and the Hornsea Member close to the cliff face (Butcher1991 Hobbs et al 2013)
The monitoring has shown that the cycle of major landslideevents at the CLO is every 6ndash7 years this being based on threeevents identified since September 2001 (ie August 2004 March
2010 and February 2017 Figs 4 and 7) This compares with a cycleof around 30ndash40 years for the 40 m high London Clay cliffs atWarden Point Isle of Sheppey Kent (DixonampBromhead 2002) Aswas the case at Warden Point the embayments at the CLO haveretreated along the same heading (due west in this case) andmaintained their dimensions over the monitoring period This is atodds with the suggestion of Pethick (1996) that embaymentsmigrated southward at Holderness Pickwell (1878) observed3ndash4 year cycles of landslide activity at Tunstall (10 km SSE ofAldbrough)
The relationship between incremental volume loss and totalrainfall is also shown in Figure 7 (total rainfall refers to that since theprevious survey) This shows a broad-scale agreement betweenrainfall and volume loss from the cliff (calculated from TLS) withpeaks in volume loss following the major landslide events (It shouldbe noted that pre-2012 rainfall data are averages taken from threeEast Riding of Yorkshire stations within a 23 km radius of the CLO)
Oceanographic and meteorological factors
Coastal processes such as storms and the energy provided by highwaves at the coast play a major role in coastal erosion around Britainand these are found to be particularly enduring on the east coast north
Fig 4 Plot of date v cumulative volumeloss from cliff (per 100 m) at CLO(September 2001 datum) (BGScopyUKRI)Dates of major fresh landslide events incentral embayment are shown as dashedred lines with arrows
Fig 5 Conceptual geologicalndashgeotechnical model for the CLO(BGScopyUKRI)
Coastal landslide monitoring
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of the Humber (median duration gt13 h) with the likelihood ofspanning High Water (Dhoop amp Mason 2018) for example ananticyclonic storm on 18 December 2009 lasted 195 h at Hornsea Astorm surge described by the Environment Agency as the mostserious for 60 years hit the east coast of England on 5 December2013 causing severe coastal flooding and erosion most notably inEast Anglia During this event the high tide levels (predicted) atBridlington and Spurn Point were 615 m at 1754 h and 725 m at1853 h respectively This compares with estimated mean high-waterspring (MHWS) and mean low-water spring (MLWS) of 644 m and114 m respectively at Aldbrough Wave height recorded by theChannel Coastal Observatory (CCO) for the lsquoHornsearsquo buoybelonging to the East Riding of Yorkshire Regional CoastalMonitoring Programme (ERYRCMP 1995) peaked in the earlyhours of 6 December with waves in excess of 6 m accompanied by amaximum wind speed of 208 m sminus1 (Force 8ndash9) recorded at thenearby BGS weather station However in terms of wave energyalone higher peakswere recorded on 24March and 10October 2013a maximumwave height of 74 m having been recorded on 23MarchAnother notable lsquostormrsquo year (since June 2008) was 2010 Wavedirection was predominantly and consistently from the NNE and NElsquoOnshorersquo waves (defined here as derived from compass pointsN340deg to N140deg) represent gt80 of the total This highlights thevulnerability of the Holderness coast to the erosive wave energywhich predominantly emanates from an average angle of incidence425deg to the current average coastline at Aldbrough (CCO 2017)Notable rainfall events included 12ndash15 and 24ndash25 June 2007 this
month had over four times the average rainfall equivalent to a200 year return event at Holderness (Hanna et al 2008)
The occurrence of storms as defined by CCO (2017) andrecorded at the Hornsea buoy from 2009 is plotted againstincremental volume loss for 100 m of cliff (calculated from TLS) inFigure 8 It should be noted that a storm event is indicated using thelsquopeaks-over-thresholdrsquomethod (CCO 2017) where the lsquowave heightthresholdrsquo was variously defined over the monitoring period from300 to 375 m and based on 025 year return periods A comparablelong-term trend is shown but the number of storms does not appearto have a causal effect on cliff volume change However a closeragreement is evident after 2013 where TLS surveys are morefrequent
A plot of average wave-climate energy v incremental volumeloss for 100 m of cliff (calculated from TLS) is shown in Figure 9the wave data for which were provided by the Hornsea buoy (CCO2017) The lsquowave-climatersquo energy P was calculated here as follows(Dexawave httpwwwdexawavecom)
P frac14 057 (HS)2 TP
where P is wave energy (kW mminus1) HS is significant wave height(m) (half-hourly data) and TP is time period between each wavecrest (s)
The plot shows similar trends with time for cliff volume loss andwave energy particularly when taken over several years However acausal effect is not indicated
Fig 6 Examples of different stages of deep-seated rotational landsliding in the central embayment of the CLO (a) August 2004 (b) February 2011(c) November 2011 (d) November 2017 All cliff access was carried out with a full risk assessment (BGScopyUKRI)
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The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
2002) At the CLO toppling has been observed either where avertical rotational landslide back-scarp undergoes degradation andsecondary movement or where the upper part of a vertical (or near-vertical) inter-embayment promontory partially collapses Topplingmay also be promoted by minor pre-failure subsidence of the clifftop resulting in a seaward tilt or where a graben-like feature hasdeveloped at the rear of an already rotated slip mass Mudflows tendto occur on the peripheries of rotated slip blocks in response to theamount of surface water on the slope resulting from seepage andordirect rainfall and where the slipped masses have had time todegrade sufficiently
Examination of TLS-derived cross-sections revealed an overallminimum slope angle of 45deg and a maximum of 66deg athoughsteeper and temporarily near-vertical slopes have been observed atthe site particularly at promontories There have been manyinstances where slope stability analyses have returned factors ofsafety less than unity for lsquostablersquo cliff slopes probably owing atleast in part to long-term partial saturation within the main bodies ofthe tills and the Hornsea Member close to the cliff face (Butcher1991 Hobbs et al 2013)
The monitoring has shown that the cycle of major landslideevents at the CLO is every 6ndash7 years this being based on threeevents identified since September 2001 (ie August 2004 March
2010 and February 2017 Figs 4 and 7) This compares with a cycleof around 30ndash40 years for the 40 m high London Clay cliffs atWarden Point Isle of Sheppey Kent (DixonampBromhead 2002) Aswas the case at Warden Point the embayments at the CLO haveretreated along the same heading (due west in this case) andmaintained their dimensions over the monitoring period This is atodds with the suggestion of Pethick (1996) that embaymentsmigrated southward at Holderness Pickwell (1878) observed3ndash4 year cycles of landslide activity at Tunstall (10 km SSE ofAldbrough)
The relationship between incremental volume loss and totalrainfall is also shown in Figure 7 (total rainfall refers to that since theprevious survey) This shows a broad-scale agreement betweenrainfall and volume loss from the cliff (calculated from TLS) withpeaks in volume loss following the major landslide events (It shouldbe noted that pre-2012 rainfall data are averages taken from threeEast Riding of Yorkshire stations within a 23 km radius of the CLO)
Oceanographic and meteorological factors
Coastal processes such as storms and the energy provided by highwaves at the coast play a major role in coastal erosion around Britainand these are found to be particularly enduring on the east coast north
Fig 4 Plot of date v cumulative volumeloss from cliff (per 100 m) at CLO(September 2001 datum) (BGScopyUKRI)Dates of major fresh landslide events incentral embayment are shown as dashedred lines with arrows
Fig 5 Conceptual geologicalndashgeotechnical model for the CLO(BGScopyUKRI)
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of the Humber (median duration gt13 h) with the likelihood ofspanning High Water (Dhoop amp Mason 2018) for example ananticyclonic storm on 18 December 2009 lasted 195 h at Hornsea Astorm surge described by the Environment Agency as the mostserious for 60 years hit the east coast of England on 5 December2013 causing severe coastal flooding and erosion most notably inEast Anglia During this event the high tide levels (predicted) atBridlington and Spurn Point were 615 m at 1754 h and 725 m at1853 h respectively This compares with estimated mean high-waterspring (MHWS) and mean low-water spring (MLWS) of 644 m and114 m respectively at Aldbrough Wave height recorded by theChannel Coastal Observatory (CCO) for the lsquoHornsearsquo buoybelonging to the East Riding of Yorkshire Regional CoastalMonitoring Programme (ERYRCMP 1995) peaked in the earlyhours of 6 December with waves in excess of 6 m accompanied by amaximum wind speed of 208 m sminus1 (Force 8ndash9) recorded at thenearby BGS weather station However in terms of wave energyalone higher peakswere recorded on 24March and 10October 2013a maximumwave height of 74 m having been recorded on 23MarchAnother notable lsquostormrsquo year (since June 2008) was 2010 Wavedirection was predominantly and consistently from the NNE and NElsquoOnshorersquo waves (defined here as derived from compass pointsN340deg to N140deg) represent gt80 of the total This highlights thevulnerability of the Holderness coast to the erosive wave energywhich predominantly emanates from an average angle of incidence425deg to the current average coastline at Aldbrough (CCO 2017)Notable rainfall events included 12ndash15 and 24ndash25 June 2007 this
month had over four times the average rainfall equivalent to a200 year return event at Holderness (Hanna et al 2008)
The occurrence of storms as defined by CCO (2017) andrecorded at the Hornsea buoy from 2009 is plotted againstincremental volume loss for 100 m of cliff (calculated from TLS) inFigure 8 It should be noted that a storm event is indicated using thelsquopeaks-over-thresholdrsquomethod (CCO 2017) where the lsquowave heightthresholdrsquo was variously defined over the monitoring period from300 to 375 m and based on 025 year return periods A comparablelong-term trend is shown but the number of storms does not appearto have a causal effect on cliff volume change However a closeragreement is evident after 2013 where TLS surveys are morefrequent
A plot of average wave-climate energy v incremental volumeloss for 100 m of cliff (calculated from TLS) is shown in Figure 9the wave data for which were provided by the Hornsea buoy (CCO2017) The lsquowave-climatersquo energy P was calculated here as follows(Dexawave httpwwwdexawavecom)
P frac14 057 (HS)2 TP
where P is wave energy (kW mminus1) HS is significant wave height(m) (half-hourly data) and TP is time period between each wavecrest (s)
The plot shows similar trends with time for cliff volume loss andwave energy particularly when taken over several years However acausal effect is not indicated
Fig 6 Examples of different stages of deep-seated rotational landsliding in the central embayment of the CLO (a) August 2004 (b) February 2011(c) November 2011 (d) November 2017 All cliff access was carried out with a full risk assessment (BGScopyUKRI)
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The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
P R N Hobbs et al
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
of the Humber (median duration gt13 h) with the likelihood ofspanning High Water (Dhoop amp Mason 2018) for example ananticyclonic storm on 18 December 2009 lasted 195 h at Hornsea Astorm surge described by the Environment Agency as the mostserious for 60 years hit the east coast of England on 5 December2013 causing severe coastal flooding and erosion most notably inEast Anglia During this event the high tide levels (predicted) atBridlington and Spurn Point were 615 m at 1754 h and 725 m at1853 h respectively This compares with estimated mean high-waterspring (MHWS) and mean low-water spring (MLWS) of 644 m and114 m respectively at Aldbrough Wave height recorded by theChannel Coastal Observatory (CCO) for the lsquoHornsearsquo buoybelonging to the East Riding of Yorkshire Regional CoastalMonitoring Programme (ERYRCMP 1995) peaked in the earlyhours of 6 December with waves in excess of 6 m accompanied by amaximum wind speed of 208 m sminus1 (Force 8ndash9) recorded at thenearby BGS weather station However in terms of wave energyalone higher peakswere recorded on 24March and 10October 2013a maximumwave height of 74 m having been recorded on 23MarchAnother notable lsquostormrsquo year (since June 2008) was 2010 Wavedirection was predominantly and consistently from the NNE and NElsquoOnshorersquo waves (defined here as derived from compass pointsN340deg to N140deg) represent gt80 of the total This highlights thevulnerability of the Holderness coast to the erosive wave energywhich predominantly emanates from an average angle of incidence425deg to the current average coastline at Aldbrough (CCO 2017)Notable rainfall events included 12ndash15 and 24ndash25 June 2007 this
month had over four times the average rainfall equivalent to a200 year return event at Holderness (Hanna et al 2008)
The occurrence of storms as defined by CCO (2017) andrecorded at the Hornsea buoy from 2009 is plotted againstincremental volume loss for 100 m of cliff (calculated from TLS) inFigure 8 It should be noted that a storm event is indicated using thelsquopeaks-over-thresholdrsquomethod (CCO 2017) where the lsquowave heightthresholdrsquo was variously defined over the monitoring period from300 to 375 m and based on 025 year return periods A comparablelong-term trend is shown but the number of storms does not appearto have a causal effect on cliff volume change However a closeragreement is evident after 2013 where TLS surveys are morefrequent
A plot of average wave-climate energy v incremental volumeloss for 100 m of cliff (calculated from TLS) is shown in Figure 9the wave data for which were provided by the Hornsea buoy (CCO2017) The lsquowave-climatersquo energy P was calculated here as follows(Dexawave httpwwwdexawavecom)
P frac14 057 (HS)2 TP
where P is wave energy (kW mminus1) HS is significant wave height(m) (half-hourly data) and TP is time period between each wavecrest (s)
The plot shows similar trends with time for cliff volume loss andwave energy particularly when taken over several years However acausal effect is not indicated
Fig 6 Examples of different stages of deep-seated rotational landsliding in the central embayment of the CLO (a) August 2004 (b) February 2011(c) November 2011 (d) November 2017 All cliff access was carried out with a full risk assessment (BGScopyUKRI)
P R N Hobbs et al
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The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Coastal landslide monitoring
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
P R N Hobbs et al
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
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Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The lsquoHolderness Experimentrsquo carried out between 1993 and 1996monitored the processes of sediment transport along the rapidlyretreating Holderness coastline which provides the largest singlecoastal source of sediments to the North Sea (Prandle et al 1996)Various processes have an impact on sediment transport includingtides storm surges and waves Breaking waves in particular have animportant impact on the beach and the nearshore zone (Wolf 1998)Pethick amp Leggett (1993) indicated that high-energy waves withlong return periods (eg 8ndash15 months) are responsible for almost allthe net southerly sediment transport and that these are alsoresponsible for offshore bar development A detailed account of
available wave and wind data for the CLO (to 2013) has beengivenby Hobbs et al (2013)
The Phase 1 borehole installation post-dated a major landslideevent in March 2010 More recently a fresh event occurred on14 February 2017 at the same embayment in line with the boreholesand was lsquocapturedrsquo by the borehole instrumentation on 1 March2017 primarily in the form of significantly accelerated boreholedisplacement of up to 30 mm (cumulative) The timing of this latestevent heralding the start of a new landslide cycle allowed thepiezometers to equilibrate and a substantial precursory inclinometerdataset to be established
Fig 7 Plot of date v total rainfall and incremental volume loss (per 100 m) from cliff Includes Met Office data (2001ndash2011) (BGScopyUKRI) lsquoTotalrainfallrsquo refers to total rainfall since last survey Dates of major fresh landslide events in central embayment are shown by dotted red lines with arrowsnotable storms are shown by dotted blue lines
Fig 8 Plot of date v incremental volumeloss from cliff (per 100 m) and number ofstorms since last survey (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Coastal landslide monitoring
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Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Instrumentation results
Inclinometers
Boreholes 1b 2b and 3b (Figs 10ndash12) contain inclinometer casingto full depth which is lsquodippedrsquo using a digital probe The results areshown in the form of a cumulative plot of the northerly and easterlycomponents of lateral displacement for each dated survey wherethe temporal datum is the dataset from that boreholersquos first survey(the plotrsquos centreline) and the displacement datum is at 20 m depththat is the bottom of the borehole is assumed to be stable Forcomparison the plots are at the same scale The inclinometermethod and detailed analysis of data have been described by Hobbset al (2015a)
The inclinometer results from Borehole 1b (up to August 2017)are shown in Figure 10 as cumulative lateral displacement wherethe left-hand plot (Axis A) represents northward displacementand the right-hand plot (Axis B) eastward displacement The scaleon the x-axes (positive) is 0ndash30 mm (displacement) and on the y-axes is 0ndash20 m (depth) The plots show positive lateral
displacements from a depth of 175 m upward within theBridlington Member although significant displacements occuronly above 125 m within the Skipsea Till Member and overlyingdeposits including the major one recorded between October 2016and August 2017 and attributed to the landslide event of 14February 2017 Displacements have consistently increased upholein an overall linear trend reaching a maximum eastward component(to August 2017) of 28 mm at a depth of 05 m in the HornseaMember Also displacements have accumulated in a positivedirection throughout The equivalent maximum northerly displace-ment is 18 mm
Prior to the event of February 2017 there has been a precursory(positive) trend starting between September 2015 and January 2016and accelerating during 2016 This suggests a lsquolagrsquo of between 13and 17 months on the B-axis (east) between initiation of perceptibleaccelerated movement and the landslide event itself although theA-axis (north) movement has a shorter lag of between 4 and7 months It should be noted that this dataset has a July 2012baseline
Fig 9 Plot of date v incremental volumeloss from cliff (per 100 m) and averagewave-climate energy (Hornsea buoy)ERYRCMP (1995) Includes data fromCCO (BGScopyUKRI)
Fig 10 Cumulative displacements for BH1b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east July 2012 baseline Blue arrowsindicate pre-event displacements of winter 2016ndash2017
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Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Equivalent data for Borehole 2b (up to August 2017) areshown in Figure 11 to the same scale Here displacements weregreatly reduced compared with BH1b a maximum cumulativenortherly displacement of 7 mm having been reached at a depthof 15 m in the Hornsea Member Lateral displacements occurredabove a depth of 16 m and increased gradually uphole It is notedthat here the A-axis displacement albeit small exceeded that ofthe B-axis when compared with BH1b possibly indicatingsome form of stress rotation This dataset has an April 2012baseline
Equivalent data but over a shorter period for Borehole 3b (up toAugust 2017) are shown in Figure 12 to the same scale Here onlyvery small displacements were seen initiating above 135 m depthand peaking at 3 mm at a depth of 50 m within the Withernsea
Member Displacements on both axes were comparable in amountThis dataset has a March 2015 baseline
An example of a lsquotimersquo plot (BH1a 4 m depth) is shown inFigure 13 This sigmoidal curve clearly shows detection of theprecursory build-up in displacement towards the cliff leading to thelandslide event of 14 February 2017 and subsequent decrease indisplacement after the event This inclinometer closest to the cliffhas thus lsquopredictedrsquo the landslide event by more than a year Resultsfor other depths are similar but reduce proportionately in magnitudeto a depth of 12 m below which there is no response to the landslideevent (Fig 10)
The overall picture is one of consistency in displacementdirection throughout the monitoring period and of proportionalityin displacement response that is a reduction proportionate with
Fig 11 Cumulative displacements for BH2b up to November 2017 (BGScopyUKRI) Axis A is north Axis B is east April 2012 baseline
Fig 12 Cumulative displacements for BH3b up to August 2017 (BGScopyUKRI) Axis A is north Axis B is east March 2015 baseline
Coastal landslide monitoring
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increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
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The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
increasing distance from the cliff (ie from BH1b to BH3b) Thereare no precursory rainfall or storm events suggestive of a link to thisspecific event
Stress relief
In passive stress relief laboratory tests on London Clay dilation wasmeasured by Fourie amp Potts (1991) as a consequence of a reductionin mean effective stress and was found to be due to both swelling ofclay minerals and passive shearing Fourie amp Potts concluded thatthis process rather than being linear accelerated with increased
stress relief It is likely that the cliffs at the CLO are subject to thesame processes as described above although the time scalesinvolved in clay swelling and the role of pre-existing discontinuitiesin the field are difficult to lsquoscale uprsquo from laboratory tests Thepresence of pre-existing shear surfaces within the tills is aconsideration (Winter et al 2017) and has been observed withinunslipped cliff sections although not in Phase 2 borehole core
When lateral restraint is removed from a soil body a condition oflsquoactive earth pressurersquo prevails and Ka is used to represent theapplicable lsquocoefficient of active earth pressurersquo Typical pre-failuredisplacements are quoted for lsquostiff claysrsquo as 001H where H is soil
Fig 13 Example plot of time v displacement (cumulative) for BH1b inclinometer 35 m depth (BGScopyUKRI) Axis A is north Axis B is east April 2012baseline Dashed red line is landslide event
Fig 14 lsquoTimersquo plot of month v piezometric pressure and total effective daily rainfall (BHs 1a 2a and 3a) April 2012 to November 2017 (BGScopyUKRI)Landslide event (14 February 2017) is marked by red dashed line and arrow Borehole 3a record starts February 2015
P R N Hobbs et al
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body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
P R N Hobbs et al
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
body height (Azizi 2000) this gives a pre-failure displacement ofaround 170 mm at the CLO Coastal landslides in London Clay atWarden Point Isle of Sheppey were analysed by Dixon ampBromhead (2002) In a modelling study of cuttings in stiffweathered London Clay Ellis amp OrsquoBrien (2007) noted that slopestability in cuttings was reduced by the initial earth pressurecoefficient and the pre-yield stiffness and rate of post-peak strain-softening an increase in the last promoted progressive failure Theyalso described a lsquofine balancersquo between horizontal stress decreaseand a tendency for pore pressure increase (in cuttings) Tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses were small they peaked in near-vertical saturatedCalifornian cliffs of weakly lithified sediment resulting in smallshallow but frequent block failures
Piezometers
Five fully grouted vibrating-wire piezometer sensors were installedin boreholes 1a and 2a and six in borehole 3a The results are shownin the form of a lsquotimersquo plot (Fig 14) of pore pressure readings at
intervals of 6 h from shortly after the Phase 1 installations and theequivalent lsquoprofilersquo plots (Fig 15) The results show extendedperiods of pore pressure equilibration for most sensors plus widevariation in the long-term stability of individual sensors the latterbeing possibly influenced by temperature variation at the shallowestsensors The piezometer installation and a detailed analysis ofresults have been described by Hobbs et al (2015a)
Although pore pressure values tend to be low overall thelandslide event of February 2017 triggered distinct increases at the4 m 8 m and 12 m sensors in BH2a (Fig 14) Somewhatunexpectedly there was no response to this event in BH1a whichis closest to the cliff This is currently unexplained although loss ofcontact between sensors and formation is suspected possibly owingto clay shrinkage andor stress relief There was also no response inBH3a which is furthest from the cliff With respect to effectiverainfall (Fig 14) there is no discernible correlation with porepressure at any depth in the boreholes Overall there is a trend ofeither steady decreases of pore pressures with time or maintenanceof constant values with the exceptions of the February 2017landslide event in BH2a the BH3a sensor at 20 m and the BH2a
Fig 15 Piezometer lsquoprofilersquo plots (a) BHs 1a and 2a (b) BH3a (BGScopyUKRI) No sensors were present in BHs 1a and 2a at 2 m depth Purple dashedlines represent idealized hydrostatic profiles
Coastal landslide monitoring
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sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
P R N Hobbs et al
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respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
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events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
sensor at 4 m However apart from the anomalies described abovethe results overall agree with those for nearby Cowden described byPowell amp Butcher (2003) where observed pore pressure values didnot exceed 60 kPa to 20 m depth
Plots of piezometer borehole profiles for each site visit are shown inFigure 15 These emphasize the markedly sub-hydrostatic nature forBoreholes (BHs) 1a and 3a but less markedly sub-hydrostatic forBH2a down to 12 m Similar lsquodepressedrsquo pore pressures weredescribed for the London Clay Formation cliffs at Warden Point(Dixon amp Bromhead 2002) The Dimlington Bed at least in BHs 1aand 2a acts as a minor source of water to the lower cliff whereas theSkipsea Till Member in BH2a does not The Dimlington Bed alsosaturates the upper parts of the underlying Bridlington MemberObservations of the cliff slope indicate that theMill Hill Bed is amajorsource of water to the mid- and lower cliff and hence to landslidedeposits on the cliff Pickwell (1878) described lsquoland springsrsquo as thechief cause of the landslides at Aldbrough Any mechanism wherebydrainage from the Mill Hill and Dimlington Beds is blocked bylandslide deposits on the cliff slope is likely to be transient and difficultto observe and quantify The data indicate that hydraulic continuitywith the cliff and between boreholes is in an enhanced state comparedwith the situation further inland owing mainly to the presence ofpersistent joints within the till members and their lsquoopening outrsquo withtime as a result of progressive stress relief as discussed above
The pore pressures at 20 m depth within the boreholes diminishtoward the cliff from 75 to 10 kPa and readings are relativelyconstant when compared with the shallower sensors This isprobably because at this depth the formation is at constanttemperature and beyond the direct influence of stress relief andjoint opening caused by cliff recession Negative (suction) pressureshave been recorded at 4 m depth in BH1a and at 2 4 and 8 m depthin BH3a a maximum suction of minus12 kPa having been recorded (toNovember 2017) in BH1a at 4 m depth These suctions do notappear to respond to seasonal influences It is possible thatinfiltration from the surface is simply inadequate to influence thesensors at 4 m depth and below It was noted by Powell amp Butcher(2003) that at Cowden to the north of Aldbrough suctions ofminus20 kPa were required for slope stability analyses to emulateobserved landslide behaviour
The effect of cliff slope and formation saturation on tensile stressrelease was explored by Hampton (2002) who noted that althoughtensile stresses are small they peak in near-vertical saturated cliffsDixon amp Bromhead (2002) noted that lsquoa zone of depressed porepressures was carried inlandrsquo as the (largely unweathered) LondonClay cliff at Warden Point retreated Unlike the porewater regime at
the CLO the cliffs at Warden Point exhibited sub-hydrostatic orhydrostatic behaviour However likeWarden Point the rate of stressrelief at the CLO does not allow a steady-state seepage regime todevelop Dixon amp Bromhead (2002) concluded that lsquolateral stressreduction has an important role in modifying pore pressures inheavily over-consolidated cohesive soilsrsquo This probably applies atthe CLO but to a lesser extent owing to the greater permeability ofsome lithostratigraphic units
Slope stability analysis
Cross-sections have been constructed from TLS surveys (Fig 16) soas to be aligned with the lsquobrsquo boreholes These were then used tocreate the ground surface profiles for 2D slope stability analysesThe inputs to slope stability analysis also included stratigraphiclayers strength and density data and hydrological data obtainedfrom the conceptual model (Fig 5) the last of these probably beingthe most problematic Geotechnical data were taken from the testingprogramme (Hobbs et al 2015b 2019)
To investigate the engineering stability of the cliff sectionsderived from TLS surveys landslides were modelled in lsquoGalenarsquo(limit equilibrium) software (v710) supported by lsquoFLACslopersquo(finite-element) software (v 70) This approach has the advantagethat these two independent methods may be combined to refine eachmodel A key difference in their use is that the Galena model allowsthe input of a slip surface whereas the FLACslope model is capableof lsquosuggestingrsquo one Examples are taken from the October 2016 TLSsurvey (Figs 17 and 18) the last before the 14 January 2017landslide event The FLACslope model suggests a flattening (no toeuplift) of the slip plane close to the cliff within or close to the level ofthe Dimlington Bed thus providing more of a lsquocompositersquo style oflandslide (Varnes 1978 UNESCO Working Party on WorldLandslide Inventory 1993) although field evidence suggests thismay be exaggerated and reflect the simplified nature of the CLOmodel in terms of 3D bedding morphology The Galena exampleuses multiple piezometric levels for the major lithostratigraphicunits and the FLACslope example a single phreatic surface Detailsof slope stability methods and results have been given by Hobbset al (2013) and Parkes (2015)
It will be noted that the examples do not agree regarding Factor ofSafety (FoS) but both are significantly less than unity (ie unstable)The latter is due in part to enhanced suctions close to the cliff faceand depressed pore pressures overall but mainly to the use (in thiscase) of residual strength data The equivalent FoSs for Galena andFLACslope using lsquopeakrsquo strength data were 126 and 092
Fig 16 Selected cross-sections aligned with lsquobrsquo boreholes derived from TLS (September 2001 to August 2017 left to right) (BGScopyUKRI) Location ofBH1b (installed March 2012) March and August 2017 profiles do not illustrate 14 February 2017 landslide event owing to concentration of displacement insouthern part of the embayment at this time Cliff-top displacement between February 2011 and March 2012 profiles was due to subsidence and destructionof concrete roadway
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
respectively (ie at or close to a stable condition) Drainage andunder-drainage pathways are complex and time-dependent as theyare prone to disruption by landslide activity on the cliff The slipsurface input to lsquoGalenarsquo (Fig 17) is based on observation TLS dataand the FLACslope model and is considered representative of thesubsequent major fresh event (February 2017) which daylighted atthe cliff within the Dimlington Bed and with a cliff-top recession of3 m on the lsquobrsquo borehole alignment Although the FLACslopeexample shown (Fig 18) reflects the observed landslide profile ofFebruary 2017 the model is very sensitive to the position of thephreatic surface small changes result in wide differences in thepattern of displacement and hence lsquosuggestedrsquo landslides with verydifferent scales and mechanisms
It is unclear to what extent the presence of suctions near-surfaceenhances slope stability by increasing effective strength in thosestrata affected as described by Butcher (1991) Dixon amp Bromhead(2002) and Parkes (2015) Although a fully undrained condition is
unlikely (Quinn et al 2010) major till members tend to subdivideinto blocks separated by pre-existing joints and stress-relief inducedfissures (close to the cliff ) within which transient undrainedunloading conditions can occur at depth Such conditions may nothave been detected owing to the lsquooff-sliprsquo location of the piezometerinstallations at the CLO and are probably therefore not wellmodelled by the slope stability analysis software employed here
Discussion
The monitoring study at Aldbrough reported between September2001 and November 2017 has demonstrated that at a specific sitewith a 16ndash17 m high cliff in glacial deposits typical of theHolderness coast deep-seated rotational landslides are the dominantagent of cliff recession although possibly with modificationtowards a composite mode near the toe A potential cyclicity of6ndash7 years has been shown covering three major rotational landslide
Fig 17 Example of Galena output Profile aligned with lsquobrsquo boreholes October 2016 using residual strength data (BGScopyUKRI) Dashed horizontal lines arepiezometric levels continuous red line is slip surface (estimated)
Fig 18 Example of FLACslope outputprofile aligned with lsquobrsquo boreholesOctober 2016 using residual strength dataFoS is 044 (BGScopyUKRI) Coloursindicate shear strain rate contours arrowsindicate displacement vectors
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
events at the study site occupying virtually the full width of thesame embayment the most recent dated 14 February 2017Relationships between cliff recession and landsliding and environ-mental factors such as rainfall storms and wave height are complexbut matching trends over several years have been demonstratedHowever causal relationships have been tentative partial or notdemonstrated
The establishment of an array of six boreholes landward of thecliff and the use of a digital inclinometer probe in particular hasbeen very successful in resolving small displacements that are hereattributed to stress relief Precursory enhanced cliffward displace-ments have been demonstrated many months prior to the February2017 landslide event These displacements have also been shown tobe consistent and proportional to depth and distance from the cliff(although it is recognized that the terminations of the boreholeswould ideally have been deeper than 20 m) This therefore providesan early warning method for the types of geological materials foundat Holderness The hydrological factors such as drainage to the clifffrom the Mill Hill and Dimlington Beds and an established partialsaturation or suction regime within the near-surface Withernsea Tillclose to the cliff have been shown to affect slope stability Thepiezometer array in BH2a has responded directly (at 4 8 and 12 m)to the February 2017 event whereas the closest array to the cliff(BH1a) has not Loss of contact between the sensors and theformation possibly owing to a combination of stress relief and clayshrinkage is suspected although this cannot be confirmed atpresent
The mechanism for deep-seated rotational landsliding at the CLOhas been established The role of the Dimlington Bed here is criticalas it provides a weak saturated and (in part) permeable horizon thathas been subjected to shear deformation both during formation andas a result of landsliding The bed is assumed to provide the basalshear medium for the landslides and as the elevation is undulatingthis affects the precise landslide morphology locally There is alsoevidence at its outcrop on the cliff that the bed may have beensubject to extrusion possibly in response to liquefaction Natural as-formed or post-deformational variations in thickness are unknownat this scale In addition the Mill Hill Bed has been shown to act asan aquifer supplying significant amounts of groundwater to the cliffslope and the landslide masses on it
Although the TLS monitoring programme has been affected bysome irregularities in the timing of surveys and technical problemssince 2001 mainly associated with global positioning it hasdemonstrated that the technique is capable of tracking grossmorphological changes in the cliff slope more accurately thanpreviously possible At the same time it is recognized that it has notbeen able to monitor minor landslide activity (eg rock falls topplesor mudflows) occurring between major deep-seated eventsNevertheless determination of cliff volumes lost to instability anderosion has been possible and these data should be valuable incalibrating coastal modelling preparing coastal engineeringassessments and comparing with Holderness-wide erosion calcula-tions Although observations have been made of the beachthroughout the monitoring a quantitative assessment has yet to bemade The fact that the deep-seated landslides did not penetrate tobeach or platform level at this site to some extent justified thisClearly erosion of the cliff by waves and particularly by stormsprovides the conditions for the dynamics of the geotechnicalprocesses to persist
For future work it is proposed to study in more detail therelationship between landsliding and the wave and storm regime(where available) to continue the observations of landslide cyclicityand ultimately the progressive interception of the cliff with thedownhole instruments Preliminary results of the recent ProactiveInfrastructure Monitoring and Evaluation (Electrical ResistivityTomography) (PRIME (ERT)) installations will also be reported
Conclusions
Cliff recession monitoring (since 2001) has revealed the followingat the BGSrsquos Aldbrough Coastal Landslide Observatory (CLO)
(1) Landslide processes are the major factors in cliff recession(2) Primary landslide type is deep-seated rotational with
secondary topples rock fall and mudflows(3) Major rotational landslides daylight at 1ndash2 m above platform
level (15ndash16 m below cliff-top)(4) Major rotational landslides utilize the undulose Dimlington
Bed as the seaward or basal part of the slip surface(5) The Dimlington Bed is subject to liquefaction and possibly
extrusion at outcrop on the cliff This may be a contributory factor toobserved landsliding and cliff edge subsidence
(6) Major co-located rotational landslide events follow a 6 or7 year cycle
(7) Landslide activity is related to antecedent rainfall and tostorm frequency and wave-climate energy The new PRIME (ERT)installations will be monitored and any relationships with rainfallexamined
(8) Landslide embayments are formed by individual landslidesthat subsequently degrade on the cliff slope
(9) Established landslide embayments have consistentlymigrated westward
(10) Till strata have pervasive but widely spaced jointsWidespread additional fissures develop in proximity to the cliffand are thought to be a result of stress relief
(11) Volumetric losses from the cliff range from 1200 to6300 m3 per 100 m per annum
(12) Average equivalent cliff recession of the CLO derived fromTLS is 18 m aminus1
Drilling instrumentation and testing have revealed the following atthe CLO
(1) Borehole displacements (derived from inclinometers) haveincreased progressively towards the cliff as the cliff lsquoapproachesrsquo theboreholes
(2) Significant borehole displacements are founded at around12 m below ground level (ie within the Skipsea Till Member) andincrease uphole
(3) Borehole displacements have undergone a period ofsignificant acceleration owing to the landslide event of February2017
(4) Piezometric pressures are below hydrostatic throughout allboreholes
(5) Piezometric pressures reduce (at the same level) towards thecliff persistent permeable layers drain to the cliff
(6) Piezometric pressures have continued to reduce afterexpected equilibration times following installation presumablyowing to the reducing distance to the cliff with time
(7) Piezometric pressures increased in BH2a at 4 8 and 12 m inresponse to the February 2017 landslide event No responses wererecorded in BH1a and BH3a
(8) Small suctions (negative pore pressures) exist in theuppermost 4 m in BH1a closest to the cliff
(9) The residual strength of the Dimlington Bed is 60 lowerthan the average for the tills
(10) Applying residual rather than peak strength data to mostslope stability models reduces the factor of safety against sliding towell below 10 indicating a condition of instability
(11) Geotechnical properties of the tills agree with publisheddata Differences between tills were found to be small older tillstended to be only slightly stronger and stiffer than younger tills
(12) High-quality core recovery in weak and heterogeneousglacial deposits requires specialist drilling techniques
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
The project has demonstrated the need for geological andgeotechnical information in coastal landslide analysis and model-ling The project has also demonstrated the usefulness of rapidlyeroding lsquosoft clayrsquo cliffs in the study of landslide processes inparticular their pre- and post-event behaviour in terms ofgeomorphology and subsurface behaviour Although this studyhas concentrated on the cliffs at the CLO data from the beach andplatform will also be analysed and reported in due course
Acknowledgements The authors would like to acknowledge thecontribution of many present and former colleagues at BGS who have lenttheir support and expertise to the lsquoSlope Dynamicsrsquo team in particular R LawleyK Freeborough S Holyoake D Gunn S Pearson A Gibson G JenkinsC Jordan H Jordan P Balson R Swift P Meldrum and C Inauen and that ofthe many students who have participated in the project Thanks also go toT Mason of the Channel Coastal Observatory (CCO) and to East Riding ofYorkshire Council for oceanographic data and to D J Hutchinson of QueenrsquosUniversity Kingston Ontario Canada The authors would also like to extendspecial thanks to P Allison of Shorewood Leisure Group without whose full co-operation this study would not have been possible This paper is published withthe permission of the Executive Director of the British Geological SurveyBGScopyUKRI (2018)
Funding The Natural Environment Research Council (NERC) supported thisresearch
Scientific editing by Joel Smethurst Mark Lee
ReferencesAshton AD Walkden M amp Dickson M 2011 Equilibrium responses of
cliffed coasts to changes in the rate of sea level rise Marine Geology 284217ndash229 httpsdoiorg101016jmargeo201101007
Azizi F 2000 Applied Analyses in Geotechnics Spon LondonBell FG 2002 The geotechnical properties of some till deposits occurring along
the coastal areas of Eastern England Engineering Geology 63 49ndash68httpsdoiorg101016S0013-7952(01)00068-0
Bell FG amp Forster A 1991 Geotechnical characteristics of the till deposits ofHolderness In Forster A Culshaw MG Cripps JC Little JA amp MoonCF (eds) Quaternary Engineering Geology Proceedings of the 25th AnnualConference of the Engineering Group of the Geological Society HeriotndashWattUniversity Edinburgh 10ndash14 September 1989 Geological Society LondonEngineering Geology Special Publications 7 111ndash118 httpsdoiorg101144GSLENG19910070107
Bird E 2008 Coastal Geomorphology an Introduction 2nd edn WileyNew York
Brown S 2008 Soft cliff retreat adjacent to coastal defences with particularreference to Holderness and Christchurch Bay UK Doctoral thesisUniversity of Southampton
Buckley SJ Howell JA Enge HD amp Kurz TH 2008 Terrestrial laserscanning in geology data acquisition processing and accuracy considerationsJournal of the Geological Society London 165 625ndash638 httpsdoiorg1011440016-76492007-100
Butcher AP 1991 The observation and analysis of a failure in a cliff of glacialclay till at Cowden Holderness In Chandler RJ (ed) Slope StabilityEngineering Thomas Telford London 271ndash276
Castedo R Paredes C Fernaacutendez M amp de la Vega R 2012 Modelo proceso-respuesta de recesioacuten de acantilados por variacioacuten del nivel del marAplicacioacuten en la Costa de Holderness (Reino Unido) Boletiacuten Geoloacutegico yMinero 123 109ndash126
Castedo R de la Vega-Panizo R Fernandez-Hernandez M amp Paredes C2015 Measurement of historical cliff-top changes and estimation of futuretrends using GIS data between Bridlington and Hornsea ndash Holderness Coast(UK) Geomorphology 230 146ndash160 httpsdoiorg101016jgeomorph201411013
CCO 2017 Channel Coastal Observatory website httpswwwchannelcoastorgChandler JH amp Brunsden D 1995 Steady state behaviour of the Black Ven
mudslide The application of archival analytical photogrammetry to studies oflandform change Earth Surface Processes and Landforms 20 255ndash275httpsdoiorg101002esp3290200307
Dhoop T ampMason T 2018 Spatial characteristics and duration of extremewaveevents around the English coastline Journal of Marine Science andEngineering 6 14 httpsdoiorg103390jmse6010014
Dixon N amp Bromhead EN 2002 Landsliding in London Clay coastal cliffsQuarterly Journal of Engineering Geology and Hydrogeology 35 327ndash343httpsdoiorg1011441470-92362000-53
ERYRCMP 1995 East Riding of Yorkshire Regional Coastal MonitoringProgramme East Riding of Yorkshire Council Beverley
Ellis EA amp OrsquoBrien AS 2007 Effect of height on delayed collapse of cuttingsin stiff clay Proceedings of the Institution of Civil Engineers GeotechnicalEngineering 160 73ndash84 httpsdoiorg101680geng2007160273
Evans DJA 2017 Conceptual glacial ground models British and Irish casestudies In Griffiths JS amp Martin CJ (eds) Engineering Geology andGeomorphology of Glaciated and Periglaciated Terrains ndash EngineeringGroup Working Party Report Geological Society London EngineeringGeology Special Publications 28 369ndash500 httpsdoiorg101144EGSP284
Fourie AB amp Potts DM 1991 A numerical and experimental study of LondonClay subjected to passive stress reliefGeacuteotechnique 41 1ndash15 httpsdoiorg101680geot19914111
Hampton MA 2002 Gravitational failure of sea cliffs in weakly lithifiedsediment Environmental and Engineering Geoscience 8 175ndash191httpsdoiorg10211383175
Hanna E Mayes J Beswick M Prior J ampWood L 2008 An analysis of theextreme rainfall in Yorkshire June 2007 and its rarityWeather 53 253ndash260httpsdoiorg101002wea319
Hobbs PRN Humphreys B et al 2002Monitoring the role of landslides in lsquosoftcliffrsquo coastal recession In McInnes RG amp Jakeways J (eds) Proceedings ofthe International Conference on Instability Planning and Management May2002 Ventnor Isle of Wight Thomas Telford London 589ndash600
Hobbs PRN Pennington CVL Pearson SG Jones LD Foster C LeeJR amp Gibson A 2008 Slope Dynamics Project Report the Norfolk Coast(2000ndash2006) British Geological Survey Open Report OR08018
Hobbs PRN Jones LD et al 2013 Slope Dynamics Project ReportHolderness Coast ndash Aldbrough Survey amp Monitoring 2001ndash2013 BritishGeological Survey Open Report OR11063
Hobbs PRN Kirkham MP amp Morgan DJR 2015b Geotechnicallaboratory testing of glacial deposits from Aldbrough Phase 2 boreholesBritish Geological Survey Internal Report OR15056
Hobbs PRN Jones LD et al 2019 Establishment of a coastal landslideobservatory at Aldbrough East Riding of Yorkshire UKQuarterly Journal ofEngineering Geology and Hydrogeology httpsdoiorg101144qjegh2018-209
Lee EM 2008 Coastal cliff behaviour Observations on the relationshipbetween beach levels and recession rates Geomorphology 101 558ndash571httpsdoiorg101016jgeomorph200802010
Lee EM 2011 Reflections on the decadal-scale response of coastal cliffs to sea-level rise Quarterly Journal of Engineering Geology and Hydrogeology 44481ndash489 httpsdoiorg1011441470-923610-063
Lee EM ampClark AR 2002 Investigation andManagement of Soft Rock CliffsThomas Telford London
Marsland A amp Powell JJM 1985 Field and Laboratory Investigations of theClay Tills at the Building Research Establishment Test Site at CowdenHolderness In FordeMC (ed)Proceedings of the International Conferenceon Construction in Glacial Tills and Boulder Clays Edinburgh TechnicsPress Edinburgh 147ndash168
Miller PE Mills JP Edwards SJ Bryan PG Marsh SH Hobbs P ampMitchell H 2007 A robust surface matching technique for integratedmonitoring of coastal geohazards Marine Geology 30 109ndash123 httpsdoiorg10108001490410701296598
Newsham R Balson PS Tragheim DG amp Denniss AM 2002Determination and prediction of sediment yields from recession of theHolderness Coast Journal of Coastal Conservation 8 49ndash54 httpsdoiorg1016521400-0350(2002)008[0049DAPOSY]20CO2
Parkes MT 2015 Back analysis of slope instability at an active coastal site inAldbrough East Ridings of Yorkshire through limit equilibrium and finitedifference software approaches to assess driving mechanisms and differencesbetween approaches MSc thesis University of Portsmouth
Pethick J 1996 Coastal slope development temporal and spatial periodicity inthe Holderness cliff recession In Anderson MG amp Brooks SM (eds)Advances in Hillslope Processes 2 Wiley Chichester 897ndash917
Pethick J amp Leggett D 1993 The morphology of the Anglian coast In HillenR amp Verhagen HJ (eds) Coastlines of the Southern North Sea AmericanSociety of Civil Engineers New York 52ndash64
Pickwell R 1878 The encroachments of the sea from Spurn Point toFlamborough Head and the works executed to prevent the loss of landMinutes and Proceedings of the Institute of Civil Engineers 51 191ndash212httpsdoiorg101680imotp187822489
Poulton CVL Lee JR Hobbs PRN Jones LD amp Hall M 2006Preliminary investigation into monitoring coastal erosion using terrestrial laserscanning case study at Happisburgh Norfolk Bulletin of the GeologicalSociety of Norfolk 56 45ndash64
Powell JJM amp Butcher AP 2003 Characterisation of a glacial clay till atCowden Humberside In Tan TS Phoon KK et al (ed) Characterisationand Engineering Properties of Natural Soils Swets amp Zeitlinger Lisse983ndash1020
Prandle D Ballard G et al 1996 The Holderness Coastal Experimentrsquo93ndashrsquo96 Proudman Oceanographic Laboratory Report 44
Pringle AW 1985 Holderness coastal erosion and the significance of ordsEarth Surface Processes and Landforms 10 107ndash124 httpsdoiorg101002esp3290100204
Quinn JD Philip LK ampMurphy W 2009 Understanding the recession of theHolderness Coast East Yorkshire UK a new presentation of temporal and
Coastal landslide monitoring
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
Quinn JD Rosser NJ Murphy W amp Lawrence JA 2010 Identifying thebehavioural characteristics of clay cliffs using intensive monitoring andgeotechnical numerical modelling Geomorphology 120 107ndash122 httpsdoiorg101016jgeomorph201003004
Reeves GM Sims I amp Cripps JC (eds) 2006 Clay Materials Used inConstruction Geological Society London Engineering Geology SpecialPublications 21
Rosser NJ Petley DN Lim M Dunning SA amp Allison RJ 2005Terrestrial laser scanning for monitoring the process of hard rock coastal clifferosion Quarterly Journal of Engineering Geology and Hydrogeology 38363ndash375 httpsdoiorg1011441470-923605-008
UNESCO Working Party on World Landslide Inventory 1993 A suggestedmethod for describing the activity of a landslide Bulletin of the InternationalAssociation of Engineering Geology 47 53ndash57
Valentin H 1971 Land loss at Holderness In Steers JA (ed) Applied CoastalGeomorphology Macmillan London 116ndash137
Varnes DJ 1978 Slope movement types and processes In Schuster RL ampKrizek RJ (eds) Landslides analysis and control Transportation Research
Board Special Report 176 National Academy of Sciences Washington DC11ndash33
Walkden MJA amp Dickson M 2008 Equilibrium erosion of soft rock shoreswith a shallow or absent beach under increased sea level riseMarine Geology251 75ndash84 httpsdoiorg101016jmargeo200802003
Walkden MJA amp Hall JW 2011 A mesoscale predictive model of theevolution and management of a soft-rock coast Journal of Coastal Research27 529ndash543 httpsdoiorg102112JCOASTRES-D-10-000991
Winter MG Troughton V Bayliss R Golightly C Spasic-Gril L HobbsPRN amp Privett KD 2017 Design and construction considerations InGriffiths JS amp Martin CJ (eds) Engineering Geology and Geomorphologyof Glaciated and Periglaciated Terrains Geological Society LondonEngineering Geology Special Publications 28 831ndash890 httpsdoiorg101144EGSP288
Wolf J 1998 Waves at Holderness Results from in-situ measurements InProceedings of Oceanology International 98 lsquoThe Global Oceanrsquo Society forUnderwater Technology Spearhead Exhibitions Ltd Brighton 3 387ndash398
Young AP amp Ashford SA 2006 Application of airborne LiDAR for seacliffvolumetric change and beach-sediment budget contributions Journal ofCoastal Research 22 307ndash318 httpsdoiorg10211205-05481
P R N Hobbs et al
by guest on August 23 2020httpqjeghlyellcollectionorgDownloaded from
