Cefas contract report 00AB2D
Shoreline variability in the vicinity of the
Great Yarmouth Outer Harbour
Authors: Tony Dolphin, Jon Rees
Issue date: 08 July 2011
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page i
Cefas Document Control
Title:
Submitted to: Marine Management Organisation
Date submitted: 08 July, 2011
Project Manager: Jon Rees
Report compiled by: Dr Tony Dolphin
Quality control by: Dr John Bacon
Approved by & date: Dave Carlin
Version: 1.0
Version Control History
Author Date Comment Version
Tony Dolphin 06 July, 2011 Draft for review 0.2
Tony Dolphin 08 July, 2011 Review and QA comments
incorporated
1.0
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page ii
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page iii
Shoreline variability in the vicinity of the
Great Yarmouth Outer Harbour
Authors:
Tony Dolphin, Jon Rees
Issue date: 8 July, 2011
Head office
Centre for Environment, Fisheries & Aquaculture Science
Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK
Tel +44 (0) 1502 56 2244 Fax +44 (0) 1502 51 3865
www.cefas.co.uk
Cefas is an executive agency of Defra
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page iv
Executive Summary
1.1. The Great Yarmouth Outer Harbour (GYOH) was constructed in 2007/8. This short report
examines readily available data on the shorelines neighbouring the GYOH to give an initial
assessment of their natural variability and their response to the development to date.
1.2. The GYOH lies toward the centre of the Caister Ness to Lowestoft Ness coastal cell. Beaches in
this area are sheltered from North Sea waves by a 30‐km long sand bank complex known as
the Great Yarmouth Banks. Despite the reduction in inshore wave energy caused by wave
breaking and bottom friction on the banks, the shorelines there exhibit greater variability
(higher rates of change and high spatial variability in rates of change) than adjacent beaches
which are not in the lee of coastal banks (e.g., north Norfolk).
1.3. Shoreline change was investigated by analysing beach positions extracted from the
Environment Agency (EA) beach profiles and aerial photographs spanning almost 20 years. The
sea floor bathymetry surrounding the GYOH has also been analysed using soundings gathered
as part of the GYOH monitoring program. These bathymetry data do not extend out to the
Great Yarmouth Banks. At the time of writing no data on changes in the banks (which
influences inshore wave climate and the direction and rate of longshore sediment transport)
had been received.
1.4. Beaches to the north and south of the mouth of the Yare and the GYOH show the same spatial
pattern; in each area beaches accrete in the north and erode in the south. This pattern is a
reasonably persistent feature of the c. 20 year record and indicates shoreline rotation.
1.5. It was considered by HR Wallingford (2010) that if the GYOH had a detectable impact on
sediment transport and shoreline position, it would manifest as a blockage to the net
southerly directed longshore sediment transport. In this simplistic scenario the blockage
would cause a build‐up of sediment to the north of the GYOH and, as a result of reduced
sediment supply, erosion to the south. At the time of writing there was no evidence showing
build‐up (accretion) to the north and erosion to the south. Instead, our analysis shows that the
opposite occurs with accretion against the southern harbour wall at Gorleston and slight
erosion immediately north of the GYOH. Potential causes include natural coastline rotation
(presumably driven by inshore wave climate), an assumed recent phase of northerly longshore
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page v
sediment transport acting counter to the long‐term transport direction (investigation
required) and complex circulation patterns including eddies.
1.6. On the assumption that the GYOH could result in detectable impacts other than that proposed
by HR Wallingford (2010), selected shorelines to the north and south were analysed for
changes in position before and after the development. These results show that:
Persistent erosion has been occurring in the Hopton area for the last 20 years and possibly
for longer. There is no evidence at the time of writing this report that the shoreline
behaviour has changed following construction of the GYOH.
The shoreline at the Gorleston Golf Course experiences no persistent trend and had short
phases (2‐5 years) of erosion and accretion. On average the beach is slowly accreting. Recent
phases of erosion are well within the natural envelope of variability on this beach, have
occurred previously and are not likely to be impacts of the GYOH construction. Future
analysis (in 2 – 3 years time) will help to clarify the situation.
At Gorleston, there is a strong trend of shoreline advance averaging 4.4 – 4.9 m/yr over the
20 year period. Following construction of the GYOH there was step‐change in accretion rate
from 3.4 – 15.1 m/yr . Although the timing of the step‐change in accretion rate coincides
with the GYOH development, cause and effect must be determined before impact can be
attributed.
At one kilometre north of the GYOH there are no persistent erosion or accretion trends, and
no strong evidence of the GYOH trapping longshore drift sediments moving south. The
pre/post‐construction rate shows a very minor change from 0.78 m/yr to 1.13 m/yr.
1.7. Changes in the elevation of the sea floor bathymetry following the GYOH construction are
localised with no detectable impacts offshore. Following construction the sea floor accreted to
the north, indicating that there has been a minor blockage to longshore sediment transport
for around 500 m. The build up of sediment is slowing with time suggesting bypassing
continues. A similar deposit is found at Gorleston. Further analysis of the bathymetry time‐
series is required to distinguish if these deposits occur at different times (indicating longshore
transport blockages) or at the same time (indicating cross‐shore transport with no inference
for GYOH impacts).
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Table of contents
1 Introduction ................................................................................................................................... 1
2 Data and methods ......................................................................................................................... 2
2.1 Data ......................................................................................................................................... 2
2.2 Methods .................................................................................................................................. 2
2.2.1 Shoreline position from beach profiles – Excursion Distance Analysis (EDA) ................. 2
2.2.2 Shoreline position from aerial photographs ................................................................... 4
2.2.3 Shoreline position statistics from EDA and aerial photographs ..................................... 5
2.2.4 GYOH Bathymetry ........................................................................................................... 7
2.2.5 MCA/UKHO bathymetry .................................................................................................. 8
3 Results and discussion .............................................................................................................. 10
3.1 Overview ............................................................................................................................... 10
3.2 Shoreline behaviour .............................................................................................................. 11
3.2.1 Northern beaches: Great Yarmouth to Caister ............................................................. 11
3.2.2 Southern beaches: Gorleston ‐ Corton ......................................................................... 13
3.3 Pre and post GYOH construction shoreline behaviour ......................................................... 16
3.4 Bathymetry ........................................................................................................................... 21
4 Conclusions and recommendations ......................................................................................... 23
4.1 Summary of conclusions ....................................................................................................... 23
4.2 Conclusions ........................................................................................................................... 23
4.3 Recommendations for further investigation ........................................................................ 25
5 References ................................................................................................................................... 27
6 Acknowledgements .................................................................................................................... 28
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1 Introduction
This report has been commissioned by the Marine Management Organisation. Its purpose is to
provide an independent assessment of the shoreline variability in the Great Yarmouth area and to
investigate the detectable impacts of the Great Yarmouth Outer Harbour (GYOH) which was
constructed in 2007/8. The expected spatial and temporal scales over which GYOH impacts to the
coastal system occur are not known. This report has been produced in a very short time scale and
therefore some analyses/results could not be included. Therefore this report should be considered
as an interim document – see Section 4.3 for recommendations on future work.
The study area is the coastal cell bounded by the sedimentary headlands (locally called nesses) in the
north at Caister Ness and the south at Lowestoft Ness (Figure 1.2). The shorelines in this coastal cell
are sheltered by the Great Yarmouth Banks, a complex of sand banks that are 1 – 9 km offshore,
shelter 30 km of coastline and run approximately parallel to the coast. In the lee of the Great
Yarmouth banks, despite having a reduced wave‐energy climate (as a result of bottom‐friction and
wave breaking on bank crests) the shorelines have greater rates of change than beaches not
protected by coastal banks (Figure 1.1). Additionally the demarcation between areas of eroding and
accreting coasts are sharp and long‐term records show that the coastal sections analysed to date
have highly variable erosion/accretion patterns through space and time (unpublished research by
the authors).
Figure 1.1: Shoreline trends along the Norfolk – Suffolk coastline for 1991 – 2007 (adapted from
Environment Agency Shoreline Management Group, 2007).
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2 Data and methods
2.1 Data
Several data sources were investigated and requested (see Table 3.1).
Given the short lead time to gather data and produce this report, not
all data requested had arrived at the time of writing. The data types
sought were chosen primarily to quantify variability in shorelines. The
shoreline data acquired represent the medium‐term (years – decades)
beach behaviour. As local beach response is also controlled by the
wave climate and position, and by the elevation and shape of sand
banks, we also considered bathymetric and wave data. Bathymetric
data covering the sand banks had not arrived at the time of writing.
There are no long‐term wave records inshore of the bank, but wave
records are available from the West Gabbard wavebuoy in the
southern North Sea. An assessment of inshore wave climate and
longshore drift rates, which can control the erosion/accretion
behaviour of a beach, was not undertaken as this requires numerical
modelling which was not possible in the reporting time scale. The
required modelling could be undertaken at Cefas or one of a number
of other organisations that have wave/tide/sediment transport models
in the area (e.g., University of East Anglia, Tyndall Centre).
2.2 Methods
This sections briefly describes the methods used to examine variability
in the shoreline and nearshore environments.
2.2.1 Shoreline position from beach profiles – Excursion Distance
Analysis (EDA)
Excursion Distance Analysis (EDA) is a useful technique for extracting
shoreline position data, at a given elevation, from beach profiles. The
essence of EDA is to measure the distance from the beach profile
survey marker out to the point where the beach falls to a specified
elevation such as the Mean High Water Springs (MHWS) or Mean Sea
Figure 1.2: Location map
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Level (MSL) datums. The excursion distances were calculated by the Environment Agency using the
SANDS (Shoreline And Nearshore Data System) software developed by Halcrow Group PLC. Excursion
distances were appended to the results of previous analyses (1991 ‐ 2007; Environment Agency
Shoreline Management Group, 2007) to give a record of c. 15 years prior to GYOH construction and
c. 3 years post construction (2008 ‐ 2010/11). The following elevation datums were used in the
analysis:
HAT (1.39 m ODN)
MHWS (1.02 m ODN)
MHWN (0.6 m ODN)
MSL (0.1 m ODN)
MLWN (‐0.37 m ODN)
MLWS (‐0.79 m ODN)
LAT (‐1.48 m ODN)
where ODN is Ordnance Datum Newlyn (approximately mean sea level).
Other outputs of the SANDS analysis, such as beach steepness, are not used in this report but could
be considered if a more detailed account is required.
Data type Data source Status
Beach profiles Environment Agency Received
Beach profiles Environment Agency on behalf
of East Port
Received
Aerial photography Environment Agency Received
Sea floor bathymetry HR Wallingford on behalf of
East Port
Received
LIDAR Environment Agency Requested. Dispatched and
awaiting delivery.
Recommended use in next
version of this report
Sea floor bathymetry (including
coastal sandbanks)
Maritime and Coastguard
Agency/ UK Hydrographic
Office
Requested. Awaiting response.
Recommended use in next
version of this report
Table 2.1: Data requested and status at time of writing this report.
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Figure 2.1: Beach profile example indicating the shoreline positions (125 m at time A and 84 m at time B)
calculated using EDA. The shoreline change is a retreat in this example as indicated by the black arrows.
Adapted from Environment Agency Shoreline Management Group (2007).
2.2.2 Shoreline position from aerial photographs
The Environment Agency commission annual (late summer) low‐tide aerial photography for
monitoring the coastline and the state of coastal defences. From 2005 the annual aerial photography
was commissioned as an ortho‐rectified product (i.e., referenced to a geographic coordinate
system). The hardcopy plates for selected years prior to 2005 (1992, 1994, 1997, 2001) were
separately scanned and geo‐referenced by the Environment Agency and are also used here.
The aerial photographs from the study area were imported into Arc‐GIS and the low‐tide shoreline
from each image was digitised. An offset correction for variability in tidal level, such as that used by
Dolphin et al. (submitted), has not been applied here due to the short time‐scale for reporting. In
most cases the aerial photo data are in agreement with beach profiles, at least in a relative sense,
however an offset correction would be needed if the results were to be used in isolation.
The analysis of shoreline variability from low‐tide shorelines follows the method of Theiler et al.
(2009). The underlying dataset is a time‐series of shoreline positions (distances) measured along a
series of transects out to the intersection points of each transect and shoreline (Figure 2.2). In this
analysis the low‐tide shoreline locations were measured at transects spaced every 50 m along the
Shoreline retreat (at MHW)
Profile at time A
Profile at time B
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18 km length of coastline between Caister Ness to Lowestoft Ness. The time‐series of shoreline
positions at each of the 367 transects was then used to statistically characterise the behaviour of the
coastline. The statistics were mapped so that the spatial variability in shorelines response could be
examined. As the transects are spaced every 50 m, the primary utility of the aerial photograph
analysis is its much higher spatial resolution (compared to the EDA on beach profiles).
Figure 2.2: Example map of shorelines (coloured lines) and their intersection (measurement point) with each
transect. The measurement distances to each intersection are then statistically analysed and mapped to
characterise the spatial variability in shoreline behaviour. Adapted from: Theiler et al. (2009).
2.2.3 Shoreline position statistics from EDA and aerial photographs
To investigate the spatial variability in beach behaviour four statistics were calculated to describe
the shoreline behaviour as determined from shoreline positions derived from EDA on beach profiles
and at the transects used in the aerial photograph analysis.
LRR is the Linear Regression Rate‐of‐change statistic (negative values indicate erosion)
LRR R2 is the R2‐value associated with the LRR
EPR is the End Point Rate‐of‐change (negative values indicate erosion)
SCE is the Shoreline Change Envelope (this is a distance not a rate)
The LRR was determined by fitting a least squares regression to all of the shoreline positions
(determined at beach profiles for EDA and at each transect for aerial photos). An example of the LRR
calculation is shown in Figure 2.3 (top panel). The LRR is the slope of the regression line determined
for shoreline positions on each transect. LRR uses all of the shoreline positions to estimate the rate
of change. It tends to under‐estimate the rate of change in comparison to other statistics such as the
EPR. As the R2‐value gives an indication of the closeness of fit to the curve, it can also be used to
determine if the erosion/accretion is a persistent trend (high R2).
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See next page for figure caption
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Figure 2.3: Graphical representations of the Linear Regression Rate, R2 (top panel), End Point Rate (middle
panel) and Shoreline Change Envelope (bottom panel) statistics. Source: Theiler et al., 2009.
The End Point Rate‐of change (EPR) was determined by dividing the distance of shoreline movement
by the time‐interval between the oldest and youngest (end points) shorelines. A disadvantage of the
EPR is that it ignores all intervening shoreline measurements and therefore can be unrepresentative
of the general shoreline change rate if either or both of the end points are anomalies.
The Shoreline Change Envelope (SCE) is a distance, not a rate. It is the distance between the closest
and farthest shorelines along each transect and indicates, as a range, the distance extents over
which the beach varies.
There is an unaccounted for bias in the aerial photograph analysis as conducted here because of the
time‐distribution of the data (photographs). That is, there are more data in the last 6 years of the
record (annual) than in the previous 14 years. The bias affects the LRR and SCE statistics described
below (Section 2.2.3), but not the EPR statistic. This bias can be easily removed by geo‐referencing
and digitising the other hardcopy plates so that the analysis can utilise the full aerial photograph
dataset. Due to the short reporting time scale the remaining aerials have not been prepared or
included in this analysis, however this could be done as required in any future edition of this report.
The bias described is not present in the EDA of beach profile data as the measurements are generally
regular, with the exception of EDA conducted at lower elevation datums (e.g., LAT). As a result of
missing data at lower elevations, EDA is presented for MHWS, MSL and MLWN, where there are few
data gaps.
2.2.4 GYOH Bathymetry
The GYOH bathymetric survey datasets were supplied by HR Wallingford as XYZ files. These data
have not been presented elsewhere in detail, as far as the authors are aware. Surveys were
conducted using a single‐beam echo‐sounder. The survey extents are highly variable and do not
consistently follow the same lines as recommended for bathymetric monitoring associated with
marine licences. These data do not appear to have undergone a thorough quality analysis; for
example, Figure 2.4 shows an artefact in the data which is probably due to squat. Squat can occur at
the beginning of a transect line (see Figure 2.4, black rectangle, for example) when the vessel (and
sounder) elevation change as a result of a change in vessel speed. The data are used here
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nonetheless, but they should be subjected to an appropriate quality analysis before being used in
any further reporting.
Due to the limited reporting time only a selection of the bathymetric data provided were analysed.
These were the surveys from 2006, 2008 and 2009, representing pre and post construction surveys.
Bathymetric surfaces were produced as TINs (Triangulated Irregular Networks) and the 2008 and
2009 TINs were subtracted from the 2006 TIN to give a residual map, also known as an
erosion/accretion map. This analysis was conducted in Arc‐GIS using the 3D Analyst tool kit.
Figure 2.4: 2007 bathymetry showing an artefact (marked by a black rectangle) in the data probably due to
vessel squat. White lines mark the survey tracks.
2.2.5 MCA/UKHO bathymetry
Requests were made for the MCA and UKHO bathymetric datasets because the GYHO bathymetric
datasets are spatially limiting and do not include the margins of the adjacent sand banks. Without
some understanding of the variability in the local sandbanks the impacts of the GYOH may be
masked by changes in the banks, inshore wave climate and longshore sediment transport. Equally as
likely, changes in the bank system may alter medium‐term (years – decades) longshore transport
patterns that could be falsely attributed to the GYOH. The current monitoring program is weak in
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this regard and provides insufficient evidence for identifying GYOH impacts on the surrounding
environment. If a precautionary stance were taken it is possible that changes in shoreline behaviour
could be attributed to the GYOH. Use of existing data from the MCA/UKHO is useful in this regard,
although the extents are very much linked to channels and may not always span the relevant areas.
To address these deficiencies an amended monitoring program that includes sections of the
adjacent sand banks is needed.
The full MCA and UKHO dataset has been requested under the MEMORANDUM OF
UNDERSTANDING ON THE EXCHANGE OF MARINE SURVEY DATA AND PLANNING OF FUTURE
SURVEYS that exists between the government organisations including Cefas, DEFRA, JNCC, MCA,
Natural England, British Geological Survey and the UK Hydrographic Office. These data are yet to be
delivered and therefore are not included in this version of the report.
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3 Results and discussion
3.1 Overview
The general pattern of shoreline behaviour in the study area over the past 18 – 20 years can be
divided into two zones that exhibit similar shoreline trends (see Figure 3.1 – 3.3).
Northern beaches. North of the GYOH shorelines show a medium‐term accretion trend
(Wellington and Brittania Piers). To the south and at the location of the GYOH, shorelines
are stable or eroding. EA beach profiling ceased at the GYOH area in 2007 prior to
construction.
Southern beaches (Gorleston – Corton). Shorelines in the Gorleston area (Gorleston Golf
course to Great Yarmouth) are accreting in the medium term at a rate that increases with
distance to the north. To the south, shorelines are eroding in the medium term. At Corton
(south of Hopton), the beach is depleted with no intertidal‐elevation sediments seaward of
the rock revetments. As a result the shoreline there cannot presently retreat further.
Comparison of the results for EDA at different levels generally produces the same patterns (see
Figure 3.2 and Figure 3.3). The same spatial patterns are also evident when comparing the EDA
beach profile results with the low‐tide aerial photograph analysis (see Figure 3.1 and Figure 3.2). This
consistency adds confidence to the results and indicates that the same broad patterns of shoreline
change are evident regardless of the intertidal elevation used.
The following sections describe the results in the two zones (Section 3.2), followed by a comparison
of shoreline response and bathymetry before and after the GYOH construction (Sections 3.3 and 3.4
respectively).
Shorelines can also be generated from historical maps to identify the longer‐term beach behaviour
(including any long term cycles) and give context to the medium term patterns shown here.
However, as historical maps are not readily available, the longer‐term analysis has not been
conducted and any conclusions drawn in this report are valid only in the c. 20 year period (1991 –
2011). Long‐term cycles of erosion and accretion (several decades) may occur on some beaches in
connection with changes in the coastal sand banks (e.g., Park and Vincent, 2007 and Dolphin et al.,
2007), for example, but cannot be registered in this analysis.
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3.2 Shoreline behaviour
3.2.1 Northern beaches: Great Yarmouth to Caister
The LRR, R2 and SCE are presented in Figure 3.1, and, along with Figure 3.2 and Figure 3.3 (red
arrows), highlight the similar spatial trends in the two zones. North of the GYOH the trend is beach
advance at a rate of 2.3 – 3.7 m/yr. The Shoreline Change Envelope is wide with the beach position
varying between 46 m and 68 m during the 1991 – 2010 period. Beaches around the Wellington and
Britannia Piers have a steady accretion rate as seen in the dark blue LRR and R2 areas on Figure 3.1
(marked by a ). The SCE is also high in this area (yellow – orange) as a result of persistent advance.
Toward the south the accretion rate decreases with the area just to the north of the GYOH exhibiting
persistent erosion (negative LRR and high R2; Figure 3.1 ). The closest beach profile to this area
(N4A5, South Beach, Figure 3.3) lies just to the north () and experiences low accretion rates (< 0.8
m/yr; Figure 3.2) and has a low R2 indicating that there is no erosion/accretion trend there. The
southward accretion to erosion trend is well‐illustrated in Figure 3.3 (LRR and EPR).
The GYOH is marked by a pink box (in all of the shoreline change figures) indicating its location and
the sections of the beach that were profiled up until construction began in 2007. These profiles are
no longer monitored but were eroding at ‐1.3 – ‐3.8 m/yr (Figure 3.2). Note that in Figure 3.1 the
breakwater walls were digitised and hence the aerial photo analysis indicates ‘shoreline’ advance.
This section of Figure 3.1 should be ignored when considering sandy shoreline change.
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Figure 3.1: Shoreline change statistics based on the Environment Agency low‐tide aerial photographs. The
pink boxes mark transects N4A6 and N4A7 whose shoreline position is advanced (high accretion rates) due
to the construction of the Great Yarmouth Outer Harbour in 2007. Circled numbers are described in the text.
SCE LRR R^2
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3.2.2 Southern beaches: Gorleston ‐ Corton
The shorelines from Gorleston to Corton follow a broad pattern of accretion in the north and erosion
in the south. The pivot point between the eroding and accreting coasts is just north of the Gorleston
Golf Course in the aerial photograph analysis (in Figure 3.1; ), however the EDA of beach profiles
suggests the pivot point is further south (in Figure 3.1; ), just to the north of Hopton where the
rate‐of‐change becomes negative (Figure 3.2, LRR = ‐0.4 m/yr at MSL). Although the spatial
resolution is higher for the photo‐derived shorelines, the absence of a correction for tidal level and
the bias due to missing data (Section 2.2.3) may explain the discrepancy with the beach profile EDA.
In the absence of a refined analysis of the aerial photographs (Section 4.3), the pivot point between
accretion in the north and erosion in the south is interpreted to be just north of Hopton at EA
transect SWG4 (Figure 3.3). The alongshore trend of accretion in the north and erosion in the south
is similar to the northern beaches, suggesting a medium‐term shoreline rotation (Figure 3.3).
The accretion rate at Gorleston is around 2.5 m/yr (aerial photographs) to just under 5 m/yr (EDA).
The shape of the shoreline and the build up of sediment against the harbour wall indicates trapping
of longshore drift sediments. The relatively high R2‐values show a reasonably consistent trend of
accretion which, in the light of net annual southerly longshore drift, suggests that sediments are
trapped during opposing northerly transport events and/or there is a circulation mechanism that
feeds sediments from the north into the Gorleston area where they are trapped. One possible
mechanism is a persistent eddy formed in the lee of the protruding harbour walls on each flood tide
delivering bypass sediments to the Gorleston area. Field data and/or numerical modelling are
required to test this mechanism.
Hopton and south of Hopton
The highest erosion rates observed (other than pre‐2007 at the GYOH location) are at south end of
Hopton (EA profile SWF1; Figure 3.3) where the shoreline is retreating at a rate of ‐1.7 m, ‐1.7 m and
‐1.9 m (MHWS, MSL and MLWN). The aerial photo evidence is in agreement, showing the highest
rate of retreat is just south of Hopton, between Hopton and the Broadland Sands Holiday Park (,
Figure 3.1 and Figure 3.2), where there is a band of relatively high retreat rates (‐1.7 – ‐2.7 m/yr).
The high R2 values in this area suggest that the erosion has been persistent over the past c. 20 years.
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Figure 3.2: Shoreline change statistics based on the Environment Agency 1‐km series beach profiles. LRR, EPR and SCE are based on a time‐series of shoreline positions at the MHWS, MSL and MLWN elevations conducted using SANDS. The pink boxes
mark transects N4A6 and N4A7, which were not monitored beyond 2007 due to the construction of the Great Yarmouth Outer Harbour.
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Figure 3.3: SCE, LRR and EPR bar charts with EA transect labels and location descriptions. The pink box marks the two profiles that were discontinued in 2007 prior to the GYOH construction.
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Figure 3.4: Time‐series of shoreline position at the EA beach profile transect SWF2.
South of Hopton, in the Corton area, the beaches are severely depleted and in the surveys prior to
1994 and in 2004 – 2010 there was no beach above the MSL level. As a result the sea is acting
directly on the coastal defences (rock revetment) and the shoreline retreat rates are low, as
expected. It is important to note that the ‘shorelines’ there are hard defences and not beaches. For
example, the EPR at SWF2 is 0 m (Figure 3.2 and Figure 3.3) as highlighted in the beach profiles EDA
results (Figure 3.4).
3.3 Pre and post GYOH construction shoreline behaviour
Four key beach profiles were selected to examine the shoreline response for the periods before and
after the GYOH construction. A more detailed analysis could be conducted by considering all beach
profiles and by investigating the time‐series data from aerial photographs (see Recommendations,
Section 4.3). Pre‐construction is taken to be before 2008 and post‐construction after 2008. The
January 2008 data were included as post‐construction data, despite the GYOH being under
construction at that time, because the development is likely to have already been influencing
sediment transport patterns and because it increases the number of samples used to calculate post‐
construction statistics. Experimental exclusion of the January 2008 data had negligible effect on the
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resultant statistics. It is important to note that the post‐construction data cover a significantly
shorter time‐period and it is likely that the wider system has not yet fully adjusted to the presence of
the GYOH. Of particular relevance is the variability in net longshore sediment transport and its
interaction with the GYOH. For example, if the period since GYOH construction has a net longshore
transport to the north we would expect to observe a different shoreline response to that from a
phase of transport to the south.
Table 2.1 presents the pre and post‐construction LRR rate of change statistics based on EDA at the
MSL elevation. Only MSL statistics are presented in this section. It is important to bear in mind that
the post‐construction data are based on a small number of samples. Figure 3.5 and Figure 3.6
present the EDA results at all seven levels and give detail to the shoreline position through time.
Note that the y‐axes do not have the same scale.
Profile N4A5 is the first EA 1‐km series profile to the north of the GYOH. Its importance is as an
indicator of build up of longshore drift sediments during phases of net southerly sediment transport.
HR Wallingford (2010) believes that the only detectable indicator that the GYOH is having an impact
on sediment transport is during phases of net southerly longshore transport in which there is a build
up of sediment north of the GYOH and erosion on the southern beaches.
Profile N4A5 does not have a strong trend of erosion or accretion. Shorelines accreted in 1992 –
1997 and 2001 – 2004, eroded in 1997 – 2001 and were stable in 2004 – 2010 (Figure 3.5; left panel).
There is no significant change in the shoreline position over the 2003 – 2010 period, which includes
the GYOH construction. At the time of writing there is no evidence of accretion to the north of the
port, although further confidence could be given to this statement by including the GY series beach
profiles and the aerial photograph shorelines.
Transect Pre‐GYOH period Post‐GYOH period Pre‐GYOH
LRR (m/yr)
Post‐GYOH
LRR (m/yr)
N4A5 08/91 – 01/07 01/08 – 03/11 0.78 (26) 1.13 (5)
SWG1 02/93 – 01/07 01/08 – 07/10 3.37 (24) 15.11 (6)
SWG3 08/92 – 01/07 01/08 – 07/10 1.50 (25) ‐1.0 (6)
SWF1 08/92 – 01/07 01/08 – 07/10 ‐1.62 (24) ‐1.39 (6)
Table 3.1: Pre and post‐construction Linear Regression Rate‐of‐change statistics. The number of samples for
each rate‐of‐change calculation is given in brackets.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 18 of 29
Profile SWG1 is just south of the GYOH and the mouth of the Yare, and is also an important indicator
of GYOH impacts, according to HR Wallingford (2010), due to anticipated erosion if the GYOH is
interrupting the supply of sediments from the north. At some time during the period 1993 – 1997,
the upper parts (above MLWN) of the beach have eroded. Since that time (1997 – 2010), the
shoreline has been steadily advancing at a rate of 3.37 m/yr prior to construction until a step‐change
coincident with GYOH construction of 15.11 m/yr. As noted above, the time elapsed since
construction is short and the high post‐construction accretion rate at SWG1 is likely to be short‐
lived. An hypothesis as well as cause and effect evidence is required to determine whether this
result is coincidence or an impact. If it is an impact HR Wallingford’s (2010) single impact hypothesis,
which operates only under transport to the south, will require revision.
SWG3 is located adjacent to the Gorleston Golf course and experiences phases of erosion and
accretion without any persistent trend. The record is divided into four sections with no significant
change in shoreline position from 1992 – 2000, followed by accretion in 2000 – 2003, no significant
change/slow retreat in 2004 – 2008, and slow retreat during 2008 – 2010 (Figure 3.6, left panel). The
2008 – 2010 erosive period begins around 1 – 1.5 years after GYOH construction began and is
distinguished from the pre‐construction record by a switch from an accretion rate of 1.5 m/yr to
erosion of ‐1 m/yr (Table 3.1). Although this change appears quite marked, the pre‐construction
accretion rate is dominated by shoreline advanced of c. 15 m (MSL) in the summer of 2001 (Figure
3.6).
The SWF1 profiles near Hopton have the smallest LRR difference (Table 3.1), meaning that the
shoreline response has not changed significantly since the construction of the GYOH. The erosion
rate is slightly lower post‐construction (‐1.39 m/yr) than pre‐construction (‐1.62 m/yr). Although the
beach at SWF1 erodes fairly consistently, it does have a strong seasonal signal (alternating ups and
downs on Figure 3.6) and accreted over 10 m (MSL) and in 2004/5. Whilst there is currently no
evidence of a changing erosion rate or an GYOH impact on erosion rates at Hopton, potential
impacts may take several years to develop, particularly if the net longshore transport has been
primarily to the north in the post‐construction period to‐date.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 19 of 29
Figure 3.5: Time‐series of shoreline position at the EA beach profile transects N4A5 and SWG1. The vertical black line is the nominal division of pre and post‐construction used in Table 3.1.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 20 of 29
20
40
60
80
100
120
Figure 3.6: Time‐series of shoreline position at the EA beach profile transects SWG3 and SWF1. The vertical black line is the nominal division of pre and post‐construction used in Table 3.1.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 21 of 29
3.4 Bathymetry
The sea floor bathymetry data supplied by HR Wallingford on behalf of East Port Ltd is a series of
single‐beam echo sounder surveys from June 2005 until September 2010 at half yearly intervals. The
data extents are highly variable, which is unusual for survey undertaken as part of a monitoring
agreement as consistency in survey is usually a condition of monitoring. In general the survey
extents enlarge with time; in June 2005 the survey area was 1.08 km2, in October 2005 it was 6.2
km2 and in September 2010 it was 17.3 km2. Although a detailed inspection has not been undertaken
here, a visual inspection of the data reveals that some of the records may not be fit for purpose (e.g.
Figure 2.4, Section 2.2.4). The 2010 surveys also use a different coordinate system (WGS84 UTM
Zone 31N) to all previous surveys (British National Grid). Survey tracks are not the same (repeat
lines) although the spacing is reasonably consistent.
Selected bathymetry datasets (2006, 2008, 2009) were chosen to permit a basic comparison
between the pre and post‐construction bathymetry. Previous analysis showed little change in the
position of bathymetric contours (HR Wallingford, 2010), however the contour analysis is not
revealing of the changes between contours and it is not possible to determine the changes in bed
elevation. The residual (erosion/accretion) maps produced here compare elevations across the
common area of two maps: 2006 – 2009 for pre minus post construction, and 2008 – 2009 for
changes in the first year post‐construction.
The erosion/accretion maps (Figure 3.7) in both cases show that most of the area surveyed is
unchanging as marked in grey (+/‐ 0.25 m change = within survey accuracy). In the nearshore zone of
the 2006‐2009 map (left panel) there is an accumulation of sediments on both sides of the GYOH
following construction and (up to 1.4 m) some bed lowering near the GYOH entrance probably
associated with dredging during construction. The nearshore accumulation to the north has a limited
alongshore extent (c. 500 m) and thus would appear to be due to trapping of longshore drift
sediments rather than a cross‐shore transport episode. There is also accumulation to the south,
which could be a cross‐shore or alongshore transport deposit. Closer inspection of the timing of
deposition is required to reveal the cause.
In the first year after construction there are no strong patterns of erosion or accretion. The survey
area is dominated by areas of no change (grey) and patches of slight minor accretion (c. 0.5 m). The
entrance to the GYOH has deepened by up to 2.8 m, presumably as a result of dredging.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 22 of 29
Figure 3.7: Erosion/accretion maps for 2006 – 2009 (left) and 2008 – 2009 (right).
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 23 of 29
4 Conclusions and recommendations
4.1 Summary of conclusions
Based on the analysis undertaken it is our opinion that:
there is a build‐up of sediment to the north of the GYOH (seen in bathymetry data and
expected to be confirmed in as yet unanalysed beach profiles) that could be due to the
presence of the GYOH;
the change in accretion rate at Gorleston is coincident with port construction and requires
further investigation to determine cause and effect; and
the erosion in the Hopton area is persistent over the last 20 years, shows no change
following GYOH construction, and is therefore not an impact of the GYOH.
A caveat to the above summary points is that further analysis is needed to refine and give
confidence to our conclusions and the evidence base.
4.2 Conclusions
Shoreline change statistics in the Great Yarmouth area show that the beaches to the north and south
of the GYOH exhibit the same spatial pattern: accretion in the northern beaches and erosion in the
southern beaches (Figure 3.3 as highlighted by red arrows). This pattern suggests that the coastlines
on both sides of the Yare mouth have rotated toward the south over the course of the past c. 20
years, probably in response to changes in the inshore wave climate. The assessment of coastal
change as an impact due to the GYOH (or any other construction) must take into account the natural
variability of the shoreline (and coastal processes), which in this case the broader pattern of recent
beach rotation. An important limiting factor here is that no data have been gathered on changes in
adjacent banks and the inshore wave climate (see Section 4.3), which are important drivers of
sediment transport and shoreline change.
HR Wallingford (2010) suggest that the only impact the GYOH could have on the neighbouring
shorelines is an accumulation of southward travelling nearshore (longshore drift) sediments and a
subsequent disruption of sediment supply (erosion) on beaches to the south (e.g., Gorleston to
Hopton). There is some evidence in support of accumulation to the north (bathymetry; further
beach profile analysis required) but no evidence of erosion to the south. Instead, our observations
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 24 of 29
show that accretion occurs in the south (Gorleston) and aerial photos suggest there is some
shoreline retreat close to the GYOH (Figure 3.1, Figure 3.2, Figure 3.3), which contrasts with the
bathymetric accretion in the same area (Figure 3.7).
The concept of a blockage to the longshore transport system that results in accretion to the north
and erosion to the south of the GYOH can only occur when the net drift is to the south. Therefore,
this scenario (proposed by HR Wallingford, 2010) gives evidence of an impact but is only valid for
periods of net southerly transport. It does not consider other possibilities such as phases of net
longshore transport to the north and/or complex circulation patterns. Although the shoreline data
(profiles and aerial photographs) presented do not presently support accumulation, bathymetric
data do show accumulation 0 – 500 m updrift of the GYOH that may be due to a sediment transport
blockage. Inspection of the GY series of beach profiles and other bathymetric datasets is required to
determine whether this deposit, as well as the similar feature at Gorleston, is related to the
presence of the GYOH. We also note that only 3 years have elapsed since construction and, on a
coast with variable longshore transport direction, impacts may not be identifiable within this time
period.
Changes in the elevation of the sea floor bathymetry following the GYOH construction are localised
with no detectable impacts offshore. The build up of sediment north of the GYOH appears to be
slowing with time (smaller accretion area in right panel of Figure 3.7) suggesting bypassing
continues, but it is not possible to say whether the sediment does or does not re‐enter the beach
system. The subtidal accretion at Gorleston (Figure 3.7). Further analysis of the bathymetry time‐
series is required to distinguish if these deposits occur at different times (indicating longshore
transport blockages) or at the same time (indicating cross‐shore transport with no inference for
GYOH impacts).
Some of the shoreline change patterns close to the GYOH show step changes that occur around the
time of construction, namely the increase in shoreline advance at Gorleston and the switch from
accretion to erosion (1 – 1.5 years post‐construction) at the Gorleston Golf Course (Table 3.1, SWG1
and SWG3 respectively). Hypotheses need to be formulated and cause and effect determined before
these changes can be ascribed as impacts or coincidences. For cause and effect to be investigated
estimates of longshore drift magnitude and direction, along with general circulation patterns, needs
to be investigated. Both require field data and numerical modelling. Longshore transport rates
require the wave heights, periods and directions of the inshore wave climate, which in turn requires
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 25 of 29
hindcast modelling of waves over the Great Yarmouth sandbanks. Models of this area are already in
operation (e.g., Thurston et al., 2010; Stansby et al., 2006).
South of the Gorleston Golf Course the shoreline has been subject to persistent erosion for the last
c. 20 years. Of the profiles presented here, Hopton (SWF1) has the smallest difference in LRR before
and after the GYOH construction. There is no evidence that the erosion rate has changed
significantly nor is there evidence of any changes in erosion rate around the time of GYOH
construction. As Hopton is some distance from the GYOH, and given the potential variability in
longshore sediment transport rate and direction, any impact on sediment supply may not yet be
apparent there. Questions remain over the time‐ and space‐scales of potential impact. A continued
and appropriate monitoring plan is likely to quantify address these.
4.3 Recommendations for further investigation
This report was commissioned and conducted over a short time scale of around a month. Several
datasets were identified but were not delivered at the time of writing. Additionally there was not
sufficient time for a comprehensive analysis of the available/requested data. As a result the
following work (listed in order of priority) is recommended to be conducted and incorporated in a
revision of this report or as a separate document:
Beach volumes. The time‐series of beach volumes (or cross‐sectional areas) needs to be
carefully calculated and presented alongside shoreline change statistics. Volume
measurements are needed because beaches can change shape and position without
necessarily changing beach volume. That is shoreline movements may occur with an
unchanging beach volume, which signifies shoreline retreat but is not beach erosion per se.
The volume data presented in the HR Wallingford (2010) report were not supplied for use in
this report. The calculation method used there is also unclear; cross‐sectional areas were
calculated down to LAT, however very few profiles extend to LAT and an appropriate
method must be utilised to ensure that areas are not partly determined by the profile
length, which varies from survey to survey. It is unclear whether those volume estimates are
fit for purpose.
Shoreline positions and bathymetry. Only a selection of key profiles and bathymetric data
have been used in this report. Analysis of the GY series of beach profiles and remaining
bathymetric data will significantly increase the spatial resolution of the results and assist in
determining if there is/has been a longshore sediment transport blockage. Additionally, the
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 26 of 29
aerial photographs from missing period could easily be geo‐referenced to remove bias from
the low‐tide shorelines analysis. An adjustment for tidal levels is also recommended.
Inshore wave climate and longshore drift. An understanding of longshore sediment
transport magnitudes and directions is needed to quantify natural variability in sediment
transport and where/when impacts due to the GYOH may be expected to appear. For
example, if sediment transport has been primarily to the north since the construction then
no impact will be expected on beaches as far south as Hopton. Equally, a change in transport
direction could disrupt the sediment supply from the north and result in beach depletion
(although there is no evidence of this to date). The lack of inshore wave data is a significant
obstacle to identifying and impacts the GYOH may have (in the past and for the future). This
could be redressed using numerical wave models (e.g., Thurston et al., 2010) and/or making
directional wave measurements inshore of the banks.
Tidal circulation. Patterns of sediment transport may be more complicated than the simple
model of the GYOH as a barrier to longshore transport in which sediment builds up on one
side and erodes on the other. For example, sediments may bypass the barrier but not return
to the nearshore if they are directed off shore; or persistent tidal eddies may develop that
alter the downstream sediment transport pathways giving rise to areas of high and low
sediment supply. A tidal current model capable of resolving flows around structures could be
used to investigate the flow and sediment transport patterns, and to formulate hypotheses
regarding potential impacts that can then be examined in the shoreline response data.
Historical shoreline mapping. Historical maps and navigation charts can be used to plot
shorelines for decades to hundreds of years. An analysis of the longer‐term shoreline
behaviour is useful to identify natural variability in the position of the coast, including areas
that may be subject to cycles of erosion and accretion.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 27 of 29
5 References
Dolphin, T.J., Vincent, C.E., Bacon, J.C., Dumont, E., and Terentjeva, A. Medium-term
impacts of a segmented, shore-parallel breakwater system. Submitted to Coastal
Engineering.
Dolphin, T.J., Vincent, C.E., Coughlan, C.E. and Rees, J.M., 2007. Variability in Sandbank
Behaviour at Decadal and Annual Time-Scales and Implications for Adjacent
Beaches. Journal of Coastal Research, SI50, 731 – 737.
Environment Agency Shoreline Management Group, 2007. Great Yarmouth Coastal
Monitoring: Coastal Trends Analysis.
HR Wallingford, 2010. Great Yarmouth Beach and Nearshore Monitoring Report. Report
EX6469 Release 2, February 2011.
Park, H.–B. and Vincent, C.E., 2007. Evolution of Scroby Sands in the East Anglian coast,
UK. . Journal of Coastal Research, SI50,868 – 873.
SANDS software by Halcrow Group PLC. (http://www.halcrow.com/sands)
Stansby, P., Kuang, C., Laurece, D. and Launder, B., 2006. Sandbanks for coastal
protection: implications of sea-level rise. Part 1: application to East Anglia.
Tyndall Centre Working Paper 86.
Thieler, E.R., Himmelstoss, E.A., Zichichi, J.L., and Ergul, A., 2009. Digital Shoreline
Analysis System (DSAS) version 4.0—An ArcGIS extension for calculating
shoreline change. U.S. Geological Survey Open-File Report 2008-1278.
Available online at http://pubs.usgs.gov/of/2008/1278/ .
http://woodshole.er.usgs.gov/project-pages/DSAS/index.htm
Thurston, K.J., Vincent, C.E., and Dolphin, T.J., 2010. The Influence of Storm Surges on
Sandbank Evolution: The Great Yarmouth Sandbanks, UK. In Proceedings of the
15th Physics of Estuaries and Coastal Seas (PECS) Conference, Colombo, Sri
Lanka, 14-17 September 2010.
Shoreline Variability in the vicinity of the Great Yarmouth Outer Harbour Page 28 of 29
6 Acknowledgements
The authors would like to acknowledge the other government agencies within the DEFRA
family that have provided data for this report: the Environment Agency, the Maritime and
Coastguard Agency and the UK Hydrographic Office. HR Wallingford also provided beach
profile and bathymetric data gathered under contract to East Port Ltd. In particular we would
like to thank Phillip Staley, Gary Watson and Will Riggs of the Environment Agency for
making data available at short notice and updating the EDA results.
© Crown copyright 2011
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