Changes of shingle volume
The aim of this part of the BERM project is to monitor long term beach volume changes.
While the input of shingle into the East Sussex coastal area is approached by calculating cliff retreat and estimating the flint content of the Chalk, the output needs to be assessed with regard to:
§ Long shore movement
§ Offshore movement
§ Flint attrition
All three processes may result in changes of the beach volume.
The general long shore movement of the East Sussex coast is towards the east so that the longshore transport of flint may be detected by for example a build up of flint to the west of features inhibiting this transport (such as the Seaford breakwater) or the decrease of flint east of such features.
A general decrease of flint volume along the East Sussex coast may indicate loss to the offshore zone either in the form of pebbles or their attrition products. Because the amount of loss due to attrition may be estimated from the attrition experiments an estimation of 'whole pebble' loss to the offshore zone may be possible from comparing the total loss with the attrition loss.
Changes of beach volume can be estimated from shore profile data provided by the Environmental Agency. Nearly 300 beach profiles along the East Sussex coast have been surveyed annually since 1973. Whilst being informative some difficulties have been encountered in interpreting the data. The photogrammetrical survey has changed format over the year so that the profile data is of different quality. From 1975 to 1976 height measurements where recorded at equal distances of 5m. This changed to variable distances in 1977 and since 1990 a description of the shore surface is also provided, allowing for the identification of the beach and its composition.
Other difficulties with the data involve the lateral displacement of the profiles, their vertical accuracy and the displacement of profile starting point. Initial analysis of the profiles has given results somewhat at odds with natural processes. These difficulties are currently being addressed in consultation with the Environment Agency.
Figure 1 shows that a rock platform was present during the last years. The contour lines (e.g. the 0m contour line) are located shoreward of their previous positions for which the only explanation with regard to a shore platform is rapid (in the order of several decimetres) uplift which is not a process observed on the East Sussex coastline. Therefore the tendency shown by that profile must be due to the problems mentioned above.
Figures 1 and 2: Graphs showing the surface elevation (contour lines) of survey profiles; profile years on the x-axis (1973-1995, the 0 behind the years is a necessary artefact of the graphic program), distance offshore from the profile starting point on the y-axis. Letters on the profiles from 1990 onwards denote surface cover (SH= shingle, SA= sand, RO= rock, CO= concrete, UN= undifferentiated) Seaward movement of a contour line indicates accumulation of material, landward movement of the contour line indicates erosion. Positive heights well seaward in the profiles are an artefact of the graphic program.
Figure 1: Graphs showing the surface elevation of profile G515 in front of the Belle Tout lighthouse for the years 1975 to 1995
Figure 2 shows a profile just west of the Newhaven breakwater (profile G468) and therefore the whole profile is likely to represent the shingle beach (SH signature for the last years). This profile seems to record an increase in beach volume though the interannual variation can be considerable (see 1991)
Reference:
Thorburn, A. 1977: Report on the problems of coastal erosion. East Sussex County Council, Lewes
U. Dornbusch 29-01-2001