CHANGES IN BEACH SURFACE SEDIMENT COMPOSITION Compiled by Tamsin Watt. Edited by Rendel Williams and Cherith Moses 1 Aims .................................................................................................................................. 2 2 Introduction ....................................................................................................................... 2 3 Sites .................................................................................................................................. 2 4 Survey methodology ......................................................................................................... 2 5 Analysis procedure............................................................................................................ 3 5.1 Case study 1: 23rd to 24 th June 2004 ........................................................................ 7 5.1.1 Discussion .......................................................................................................... 9 5.2 Case study 2: 14 th to 15 th September 2004.............................................................. 10 5.2.1 Discussion ........................................................................................................ 13 5.3 Case study 3 - 10 th to 12 th March 2003 .................................................................... 13 5.3.1 Discussion ........................................................................................................ 15 5.4 Conclusions ............................................................................................................. 16 6 Outlook for BAR phase 2 ................................................................................................ 16 BAR Phase I, February 2003 – January 2005 Science Report: Changes in beach surface sediment composition
Transcript
CHANGES IN BEACH SURFACE SEDIMENT COMPOSITION
Compiled by Tamsin Watt. Edited by Rendel Williams and Cherith Moses
5.1 Case study 1: 23rd to 24th June 2004 ........................................................................7 5.1.1 Discussion ..........................................................................................................9
5.2 Case study 2: 14th to 15th September 2004..............................................................10 5.2.1 Discussion ........................................................................................................13
5.3 Case study 3 - 10th to 12th March 2003 ....................................................................13 5.3.1 Discussion ........................................................................................................15
5.4 Conclusions .............................................................................................................16 6 Outlook for BAR phase 2 ................................................................................................16
BAR Phase I, February 2003 – January 2005 Science Report: Changes in beach surface sediment composition
1 Aims • To measure spatial variations in surface sediment with time in response to wave forcing
2 Introduction The surface layer of a shingle beach is a product of short term sorting processes that preferentially select different grain sizes from the core depending upon the forcing conditions. Shingle beaches are known to be highly reactive and surface sediments can respond even to individual waves in a train (Powell, 1990). This results in a surface layer that is highly unstable with lateral changes occurring within a few metres and pronounced temporal changes occurring over as little as one tidal event. Although sediment transport processes on sand beaches are well documented, research on shingle and mixed beaches is at present underdeveloped. With coastal protection measures turning more towards soft engineering solutions, we need to increase our understanding of surface sediment changes in order to aid the development and management of coastal renourishment schemes. BAR aims to address some of these issues through the development and analysis of a new GPS surveying technique.
3 Sites The no maintenance and Renourishment Sites at Pevensey Bay have been used to investigate short term changes in beach surface sediment in order to give a direct comparison. Beach sections of 100m have been surveyed, between groyne bays 41 and 43 at the No Maintenance Site and 42 and 44 at the renourished site.
4 Survey methodology BAR has developed a new survey methodology to incorporate measurement of beach surface grain size variation during profile surveys. The basic principle of the surveying procedure is the same (See report 2004-12 Changes in beach topography) but in addition material grain sizes are recorded using unique values derived from an existing classification procedure see Figure 1, A, B, C and D.
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) 1-20mm (unique value 5)
C) 20 – 50mm (unique value 35)
Figure 1: Grain size classifications, grain sizes le1 and pebbles larger than 50mm are given the un
Surveys are carried out as cross-shore profile survindicated that beach surface cross-shore variationvariations. Survey points are recorded every 0.3m30m. At Pevensey Bay surveys are conducted withhigher spatial resolution of between 5 – 10m. Beacaccount and additional profile lines may be surveyis captured. Using dual channel RTK GPS, systemTogether with data interpolation the surface is usu5cm.
5 Analysis procedure Arcview GIS (Geographical Information System) isunique codes attached to the X, Y and Z point datademonstrate sediment sorting. Repeat survey datacalculate changes in sediment size that have occuUsing the calculated codes a legend can be const‘coarsening’ or ‘fining’ under the specified wave cosurface volume changes can also be constructed wwith the beach surface sediment maps (Figure 4). one that incorporates all of the possible outcomes
BAR Phase I, February 200Science Report: Changes in beach su
A
B) 10 – 20mm (unique value 15)
D) Sand and shingle (unique value 4) ss than 1mm are given the unique value ique value 75.
eys since preliminary investigations s are more pronounced than shore parallel and profile line spacing is between 20-in pre-defined beach sections and at a h surface topography is also taken into
ed to ensure that any longshore variation point accuracy at capture is usually <2cm. ally determined to a vertical accuracy of <±
used to perform the data analysis. The are used to build surface maps that i.e. pre- and post-storm are then used to rred across the beach surface (Figure 2). ructed that demonstrates surface sediment nditions (Figure 3). Maps that demonstrate ithin Arcview and these are combined
A new legend may then be constructed, , for example it may be possible for a
beach to show coarsening in combination with either material loss or material gain and this must be demonstrated within the legend (Figure 5). In this way it is possible to identify patterns between beach surface coarsening and/or fining and surface losses and gains, improving understanding of mixed beach transport processes and response.
Figure 2: Within Arcview the unique codes in the survey data are used to construct grain size maps that demonstrate size variation across the beach surface.
BAR Phase I, February 2003 – January 2005 Science Report: Changes in beach surface sediment composition
Figure 3: Changes in sediment size across the beach surface can be calculated from repeat surveys i.e. pre- and post-storm. Calculations performed on the unique codes enables the construction of a legend that demonstrates surface sediment ‘coarsening’ or ‘fining’ under the specified wave conditions.
atDa
-100100No
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g: 1234
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: and shingle toSandSmall pebblesMedium pLarCo
gb
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Figure 4: Surface sediment change maps can then be combined with surface volume change maps in order to identify patterns between beach surface coarsening and/or fining and surface volume losses or gains. This example is for surveys recorded on 23-24 June 2004. -100 (red) refers to areas of erosion and 100 (blue) refers to areas of accretion. See Figure 5 below.
BAR Phase I, February 2003 – January 2005 Science Report: Changes in beach surface sediment composition
Figure 5: Surface volume changes are compared with surface sediment grain size changes and a legend is constructed that demonstrates all possible outcomes. For example, areas in which sand and shingle has been replaced with large pebbles may now have either a negative (-) or positive (+) outcome, which refers to material erosion or accretion respectively.
Using this technique the two beach sites were investigated and analysed during three different wave climate events.
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5.1 Case study 1: 23rd to 24th June 2004 Both sites were surveyed on the morning of the 23rd and 24th June 2004. Conditions at this time were stormy with maximum wave heights of 5.9m. Predominant wave direction was from the southwest (Figure 6).
Wave height (m)Maximum wave height (m)Wave direction
Wave Conditions 22nd to 27th June 2004
Figure 6: Graph of wave conditions during surveys. Pr-e and post-storm surveys are demonstrated with a red dashed line and red full line respectively.
Survey data were used to construct surface change maps for both sites demonstrated in Figure 7 and Figure 8. The positive (+) and negative (-) extensions in the legend are representative of material gains and losses in combination with grain size changes.
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Figure 7: Surface sediment change map, No Maintenance Site.
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5.1.1 Discussion Under storm conditions the No Maintenance Site (Figure 7), demonstrates exposure of the sand and shingle matrix and localised exposure of sand across the beach crest, which is indicative of crest erosion and retreat. Across the foreshore positive coarsening of beach sediments has occurred, i.e. coarsening has occurred in combination with accretion. In this case coarser sediments have been deposited on top of what was previously sand and shingle, due to material draw-down from the eroded crest. The beach toe in contrast shows negative coarsening i.e. beach material has been removed yet the beach sediments have become coarser. This is most likely due to removal or winnowing out of the sand sized fraction from the surface layer, which leaves behind the coarser, heavier shingle sediments as a lag deposit. It is immediately noticeable at the Renourishment Site (Figure 8) that a large area of beach shows ‘no change’ in the surface sediment. In contrast to the No Maintenance Site, it demonstrates ‘no change’ over almost 50% of its surface, with sediment changes occurring in strips along the beach crest and the beach toe. This may be due to the renourishment activity that occurs within the beach section and the position or orientation of the section within Pevensey Bay. The renourishment material that is deposited is a mix of unsorted sand and shingle and this is pushed up against the beach crest. Under normal wave action this deposit becomes sorted into the beach material but under storm waves the crest is eroded and the sand and shingle mix is drawn down the beach. In addition this site is more exposed to waves, lying near the centre of Pevensey Bay and therefore it experiences the highest levels of erosion (hence the necessity for renourishment directly at this site). For much of the year surface sediments in this section are observed to be of unique value 4 (shingle on top of sand). For these reasons this site may show little change under normal wave action and changes that are centred along the beach crest under storm waves. Erosion of the beach crest and draw-down of material occurs at both sites but at the Renourishment Site the draw-down material is of finer size fractions, with sand and shingle and finer sediments being deposited across the beach toe and a thin band of coarser sediments being deposited just seawards of these. In general this would seem to indicate that the renourished site contains larger amounts of fine sediment within the crest. Again this may be a result of the renourishment activity, which leads to the formation of cliffs at the beach crest that are more easily eroded. In addition these cliffs often contain lenses of pure sand that form as the renourishment material settles out. Under storm waves erosion of the cliff occurs and the sand is removed and eventually deposited across the beach surface. In general under storm conditions both sites demonstrate a fining along the beach crest and exposure of the sand and shingle matrix which is indicative of erosion. However the No Maintenance Site shows deposition of coarser sediment across the foreshore and exposure of coarser sediment across the beach toe. In contrast the Renourishment Site demonstrates large areas of ‘no change’ and deposition of finer sediments across both the foreshore and the beach toe.
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5.2 Case study 2: 14th to 15th September 2004 Both sites were surveyed on the morning of the 14th and the 15th September 2004. Conditions at this time were post storm following maximum wave heights of 4.5m and the beach can be considered to be under recovery. Predominant wave direction was from the south west (Figure 9).
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5.2.1 Discussion During post storm conditions the No Maintenance Site (Figure 11), demonstrates deposition of coarser sediment across the foreshore and towards the western side of the beach crest. In contrast deposition of finer sediment occurs across the beach toe and towards the eastern side of the beach crest. In general such patterns of material deposition are indicative of accretionary waves pushing material up the beach. For example, previously exposed patches of sand and shingle have become covered by coarser sediments whereas across the beach toe previously exposed larger pebbles are again covered with sand or a mix of sand and shingle. The patterns of fining and coarsening across the beach crest may be evidence of longshore transport as under the accretionary conditions the south westerly waves deposit the coarser (heavier) material into the western side of the beach section, whereas the finer (lighter) sediments are transported further to be deposited into the eastern ide of the beach section. s
The Renourishment Site (Figure 12) demonstrates a higher level of coarsening across the beach crest as material is pushed up the beach and the exposed sand and shingle becomes covered with larger pebbles. Referring back to the previous storm in June, this may be expected as erosion is more severe on the Renourishment Site due to the cliffing which occurs. Across the foreshore sand and shingle has been deposited and across the beach toe in areas coarser material has been deposited, which again is indicative of accretionary wave action. However across the beach toe material losses are observed and coarser sediment is revealed. This may be due to material being pushed up the beach from the beach toe leaving the heavier pebbles behind as a lag deposit and demonstrating a material loss. It is also possible that the 2.5m waves experienced between the 14th and the 15th may be strong enough to continue a level of erosion across the beach toe. In general both sites show material deposition patterns indicative of accretionary wave action and beach recovery.
5.3 Case study 3 - 10th to 12th March 2003 Both sites were surveyed on the morning of the 10th and 12th March 2004. Conditions at this time were mildly stormy with maximum wave heights of 3.4m. During these surveys wave direction was east/south east (Figure 13).
Figure 14 and Figure 15 demonstrate sand and shingle exposure across the beach crest at both sites and a coarsening of sediment across the foreshore which is indicative of crest erosion and material draw-down. Again erosion is more pronounced at the Renourishment Site. However much of the coarsening across the foreshore and beach toe is negative on both sites, i.e. volume losses have occurred with material coarsening. This is in contrast to the storm of June 23rd to 24th and is most likely a result of loss of sand and fine sediments with shingle left as a lag deposit. This may be a feature of lower energy storm waves which are not strong enough to draw down the larger heavier shingle size fractions. Interestingly on the no maintenance beach section, an area of coarser material has been deposited in the eastern half and pronounced exposure of sand and shingle has occurred in the western half. This would seem to indicate some element of longshore sediment sorting. However, wave action is predominantly from the southeast and therefore patterns of erosion and deposition would be expected to have occurred the other way around. In general, sediment movement patterns are in agreement with those demonstrated during the storm of the 23rd to 24th June. However due to the lower energy storm waves erosion is less aggressive and much of the surface demonstrates what is most likely loss of sand rather than sever draw-down of larger sized sediments.
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5.4 Conclusions The preliminary results presented indicate that the GPS surveying technique can be used to monitor short term surface changes on shingle beaches. Also, patterns of surface sediment movement, as well as material losses and gains, can be related to specific wave conditions.
6 Outlook for BAR phase 2 The results reported here require further analysis and further steps for the BAR project will involve, analysis at different beach sites including closed beach systems and natural or unmanaged beach sites. Initial investigations are being made to devise a probability matrix for the different grain size transitions, which would provide some predictive or modelling capability relating input wave conditions to surface changes and beach material response.
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References This report is partly based on the following reviews, protocols and reports: 2004-02-19-Report_first cliff retreat rates for Kent.doc 2003-05-14-Report_Beach sediment sampling 01.doc 2003-06-12-Review_Sediment sampling Literature.doc 2004-07-12-Protocol_Beach material sampling.doc 2003-11-30-Report_Beach sediment sampling at Pevensey 01.doc 2004-02-01-Protocol_Material size classes for beach surveys.doc 2004-04-01-Report_Beach sediment sampling at Pevensey 02-Beachlands.doc 2004-10-22-Report_Beach sediment sampling at Pevensey 03-Sandcastle.doc
Powell, K.A., 1990. Predicting short-term profile response for shingle beaches. Report SR 219, HR Wallingford, Wallingford.
BAR Phase I, February 2003 – January 2005 Science Report: Changes in beach surface sediment composition
