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The nature of sea cliff retreat
The geological structure is the key determinant of the rate at which cliffs
erode. Granite and other hard, resistant cliffs are the slowest to erode and
may appear not to erode at all. Sandy, glacial cliffs are amongst the most rapid
and average rates of 1.8 m/year are recorded from the Holderness coast (Lee
1995). Though the unconsolidated cliffs, for example, of Covehithe in
Suffolk can reach over 5 m/year (Lee et al. 2001).
Two principle types of erosion occur; falling/toppling which tend to be associated
with harder rock cliffs and slumping and sliding which occur in softer material.
Erosion may be a relatively straight forward process involving rock falls and
rotational slippages respectively. Where simple geological structures are present
the rate of erosion is a product of the cohesion of the cliff itself. The process
can be simply defined. More complex cliffs with weak superficial deposits overlying
harder rocks, cliffs with sequences of stiff clays and weak sandy material or
with a hard rock may show more complex patterns of recession, involving the
development of terracing and 'relict' cliffs divorced from the sea (Lee
et al. 2001).
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Guidance: Cliff
erosion occurs intermittently at many sites. Even in softer
rock types slopes may remain stable for many years. However, major landslips
can occur and in a very short space of time as happened on the Axmouth
to Lyme Regis undercliffes in Devon. The rate of retreat can
also be linked to the degree of exposure of the cliff. On the Dorset coast
the table shows that exposed west facing cliffs have eroded at a rate
of 0.37 m/year whilst the less exposed east facing slopes the erosion
has been 0.03 m/year. The sandy cliffs of the Ukraine on the Black Sea
coast show a similar pattern with rate of between 2-4 m/year on exposed
coasts and 0.83 m/year on sheltered sites (Shuisky
1996).
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References
Lee, E.M.
1995.
Coastal cliff recession in Great Britain: the significance for sustainable development.
In: Directions in European Coastal management, ed.Healy, M.G. & Doody, J.P.Samara Publishing Ltd. Cardigan, 185-193.
Abstract...
Lee, E.M., Brunsden, D., Roberts, H., Jewell, S. & McInnes, R.
2001.
Restoring biodiversity to soft cliffs.
English Nature Research Report, 398
English Nature.
Shuisky, Y.D.
1996.
Shoreline monitoring on the Ukrainian coast of the Black Sea.
In: Coastal Management and Habitat Conservation, eds.Salman, A.H.P.M., Langeveld, M.J. & Bonazountas, M.EUCC, Leiden, The Netherlands, 377-388.
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