Cliff fall defence
- Credit: Archant
SIR - There are probably few, if any, still in post at East Devon District Council (EDDC) who remember and were involved in the Sidmouth Sea Defence Scheme. Similarly, many who now live in Sidmouth have come here since the completion of the scheme and know nothing of how it came into being, nor the conditions that preceded its building.
The conditions then prevailing were very wet, windy winters, which, as today, resulted in serious cliff falls, following on a long period of relative stability. There is one proven fact that needs to be remembered - Sidmouth’s sandstone cliffs fall at an average rate of 60 feet per 100 years. This rate may be erratic in the short term going up and down as conditions vary, but the fact is there is an average rate.
This is not some figure which is a guess, but came from a world renowned authority on coastal erosion and shingle movement, who was Emeritus Professor of Geography at King’s College, London. I have withheld his name on purpose because I do not have his consent to reveal it, but I can tell you he lives not very far away and has known Lyme Bay over a long period.
When the Sea Defence Scheme was proposed there was very serious concerns that it might produce adverse effects elsewhere along the coast and this was a serious concern to the National Trust who own long stretches of cliff land close to Sidmouth, and it was they who brought in the professor to assess the scheme and give an opinion. I had been on the committee which helped the trust to buy much of the land close to Sidmouth in 1986 and by the time of the Sea Defence Scheme was a member of the National Trust Regional Committee for Devon and Cornwall, so was in a position to know exactly what the professor said and recommendations made.
The professor very much regretted that no research had been done to actually track where and how Sidmouth’s shingle moves, something which had just become possible as a result of work at Southampton University.
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The trust raised the question of coastal erosion and cliff falls affecting their land. This led the trust to adopt a policy of managed natural retreat, but they asked the direct question “could anything be done to slow the rate of cliff fall taking the land”.
The professor set out the causes of the problem and a possible partial solution, which I believe needs to be brought into the public domain because it could well offer something other than a lot more very expensive hard engineering.
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Our cliffs are composed of layers of relatively soft permeable sandstone and impervious greensand. When it rains the topsoil absorbs the water and it soaks down into the sandstone until it reaches the greensand where it can go no further. This results in a huge increase in the weight of the cliffs. Just think of the weight of a litre bottle of milk and there are literally thousands of those absorbed into the sandstone, vastly increasing the weight of the cliffs which are inherently unstable. Then there is the affect of frost and sun and add to this the vibrations of the sea at the base of the cliff and here is the recipe for cliff falls. The cliffs do not fall from the bottom, the sea causes only relatively slow erosion of the sandstone, they fall from the top and the greatest weight of water is in those top layers. Where the water can find its way downhill it drains down to the lowest level causing problems of exacerbated falls there.
The main necessity is to get the water out of the cliffs to reduce their weight and the effects of frost and sun. The simplest way to achieve this is by means of a straight forward land drainage scheme utilising perforated plastic pipe laid in the top soil at regular intervals and following the contour, the water picked up at the landward end and piped away for disposal. The scheme would apparently have to extend landward for two to three fields and would have to include gardens too since they absorb rain also. The result would be a much reduced rate of cliff fall, possibly as much as 50 per cent. If the figure was applied to the rate of fall, 60 feet per 100 years, it could possibly reduce it to just 30 feet, or half the present rate! Because of the friable nature of the cliffs, falls could never be eliminated, hence the trust’s adoption of ‘managed retreat’.
This, compared to other schemes, is relatively cheap to implement and in view of the current problems would seem to be well worth putting in place.
Incidentally, something which EDDC seem conveniently to have forgotten is that one of the conditions imposed in the grant of Government funding for the 1994 Sea Defence Scheme was that EDDC were required under that contract to top up Sidmouth beach with fresh supplies of shingle every six years! To my knowledge not a pebble has been added to the beach by EDDC in the 16 years since the completion of the Sea Defence Scheme in December 1995.
Peak Hill Road, Sidmouth