- Credit: Archant
SIR - Re: East Cliff Erosion. I doubt very much that replacement of the former drainage system above the East Cliff as detailed in the letter by Julia Creeke to the Sidmouth Herald Feb 1, 2013, by itself, will be sufficient to slow the cliff slumping.
I believe it would need to be done in conjunction with re-establishing the East Beach to something resembling its historic self.
The fact that the eroding cliff face is essentially vertical suggests that the sea level portion of the cliff face is not as resistant to erosion as the sandstone of the West Cliff.
If I recall correctly from the Promenade Jurassic Coast Information Kiosk there is a discontinuity in the geology between the east and west cliffs, which may mean that the more erosion resistant geology is below sea level along the East Beach.
Since there is little or no beach below the cliffs, there is little to reduce the force of the waves on the cliff base. In stormy weather, this impact will cause vibrations (however small) at the base that will eventually increase the instability of the soil/rock structure above.
Thus, the first form of action should be to either re-establish the East Beach or the installation of some other barrier to arrest the force of wave action on the cliff base.
The re-establishment of the cliff top drainage system to reduce the weight of water in the profile should then be considered. This would also reduce the potential for surface flow over the edge of the cliff.
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It should be noted that while the suggested drainage system will reduce the amount of water flowing into the upper profile it will not reduce the lateral flow from the area outside the drainage system boundaries, and thus may not significantly reduce the volume of water saturating the profile immediately above the greensand or any other impermeable layer within the profile.
R W Borden, London