Aim of this part of the BERM project is it to determine the rate of cliff retreat along the whole of the East Sussex coastline. The retreat data is needed to calculate the amount of flint that is injected annually into the coastal zone.
The measurements are done using ArcView GIS. The present day cliff top data from the Ordnance Survey Digital Database (Map feature 375) is kindly provided by East Sussex County Council. Older cliff top lines are digitised from old OS maps such as the first edition of the Ordnance Survey ‘6 inch’ map which were surveyed in the mid 1870s. The area between the most recent and the old cliff line is calculated and the area then divided by the length of this coastal stretch. This value is then divided by the time interval between the two surveys, providing a mean annual retreat rate.
The figure below shows an example for the cliffs east at Birling Gap. Including buildings or roads helps in determining the accuracy of the overlay (see Birling Gap Hotel), but in most places along the coastline there are only a few features that can be found both in the new and in the old maps.
The cliff erosion next to the steps at Birling Gap measured from the figure above is 89m (for the period 1873-74 to 1997). This amounts to a mean annual retreat rate of 71cm which is slightly more than the 69cm reported in Cleeve and Williams (1987) for the period 1873-1987 but much less than the 91cm (1875-1961) reported by May (1971).
Cleeve J. and R.B.G: Williams 1987: Cliff erosion in East Sussex. GEMS, University of Sussex: Brighton. 68p
May, V.J. 1971: The retreat of chalk cliffs. Geographical Journal, 137: 203-206