Sunday, February 17, 2013
SIR - As a civil engineer it is with mounting disquiet that I read recent letters in the Herald recommending a drainage scheme to address cliff erosion to the east of Sidmouth. Clearly this suggestion is well-intentioned but I fear it demonstrates a lack of understanding of the mechanics of coastal erosion.
Coastal erosion is a process driven by marine action. Thus it proceeds from the bottom up, not vice-versa. In other words, the root cause of the instability is the destruction of the cliff face at beach level. That is what is determining the rate of erosion. What is happening in the soft deposits overlying the rock is a side issue. It is a process which would be self-limiting if the supporting rock wasn’t continually being removed by the sea.
If public funds are used to address the problem they must be directed at mitigating the effects of the destructive marine action occurring at the foot of the cliffs. Needless to say, solutions must be based on sound engineering principles.
Incidentally, regarding the estimate of the current erosion rate quoted by P Griew (Opinion, February 8), I would suggest that extrapolating from 1.5 metres per year to 500 feet per 100 years is highly questionable as it assumes constant conditions.
Clearly many factors could be influencing the rate. A crucial one for example is the natural variability in the erosion resistance of the rock. The quality of the rock that is presently under attack is poor. It exhibits not just the usual ‘horizontal’ planes of weakness associated with sedimentary rock, but also frequent vertical fractures. Thus, currently the sea is not so much eroding the rock but plucking out large blocks.
P Griew implies that an apparent fifteen-fold increase in erosion rate has been caused by installation of the breakwaters. It is difficult to envisage how they could produce such a massive effect. Have other potential explanations, such as varying quality of the rock, and therefore the nature of the erosion process, been considered?
1, Western Field