Pennington Point: letters page

For months, the letters pages of the Sidmouth Herald have been buzzing with 'what to do' about cliff erosion east of Port Royal:

CLIFF FALL DEFENCE

3rd February

 

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.

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.

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.

Julia Creeke

Cliff fall defence - Letters - Sidmouth Herald

A young film-maker has the same opinion: record heavy rains in 2012 saturated the cliff tops:
x
Sidmouth Cliff falls by adr films - YouTube

But others disagree - and so the debate in the letters pages continues:

STABILITY NEEDED

10th February

 

D Pedder (letters, January 25) asks whether we, as householders on Cliff Road, would be prepared to donate the necessary length of our gardens to allow the cliff top to be angled to stop further falls.

In 2002, when the council was proposing a rock revetment at the base of the cliffs and re-profiling at the top, the residents unanimously agreed to give up the necessary 15 metres of garden to allow this to be done.

Since then we have all lost about that length anyway – I personally have lost 15 metres since 2007, a fact verifiable from professional surveys.

Whilst we might have qualms about giving up a further similar amount, I have no doubt that if this was to lead to stability then we would be prepared to do so.

Julia Creek (letters, February 1) states that the cliff only falls from the top, the sea causing relatively little erosion.

If that were the case then the cliffs would not be vertical but would slope back.

There is erosion from both the top and the bottom.

If her proposal for a land drainage scheme could help prevent erosion from the top, then we would of course be very interested.

However, Sidmouth Town Council removed the drainage from Alma Field, which was helping prevent erosion from our gardens, in the 1990s and it has not been replaced.

And I am not sure where her estimate of a current erosion rate of 60 feet in 100 years originates – at present we can show by professional survey that it is at least 1.5 metres a year or about 500 feet in 100 years – up from 10 cms a year between 1880 and 1990 (source: Royal Haskoning Report 2009).

This is an increase of about 15 times since the groynes to protect the Sidmouth frontage were installed in 1995.

Her estimate of a current erosion rate of less than a foot a year would be very heaven indeed.

Paul Griew

Leader

Cliff Road Action Group

Stability needed - Letters - Sidmouth Herald

A SENSIBLE IDEA

10th February

  

In reply to Julia Creeke’s letter in the Herald last week, I would like to support her suggestion of drainage at the top of the cliffs at Pennington Point.

Her knowledge and history of the Sea Defence Scheme and her information from the professor of geology make this scheme common sense.

We all realise that the cliffs are falling from the top and the cause that has speeded all this up has been the enormous amount of rain we have had over the last few months.

We also realise that we have to expect a certain amount of natural erosion.

We also know that something has to be done at Pennington Point to avoid the possibility of erosion that in a massive storm could put the town at risk of flooding.

However, what we need now is action, not years of studies.

I hope the district council will take on board Miss Creeke’s ideas so that this very common sense suggestion is not overlooked by the Beach Management Board.

Councillor Maggie Baldwin

A sensible idea - Letters - Sidmouth Herald

FACTORS INFLUENCING CLIFF EROSION!

17th February

 

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?

David McCluskey

Factors influencing cliff erosion! - Letters - Sidmouth Herald

Meanwhile, another film-maker shows the power of the waves:

High Tide @ Sidmouth near alma bridge - YouTube 
And another notices that all the beach has disappeared under the eastern cliffs - perhaps caused by the 'new' groynes...
 

Pennington Point, Sidmouth - the erosion continues - YouTube

CLIFF PROBLEM

10th March

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.

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

Cliff problem - Letters - Sidmouth Herald

And finally, a round-up of recent opinion:

CLIFF REMEDIES

10th March

  

SIR - In my letter to the Herald of February 15, I, in effect, argued that the overriding issue is not the steepness of the slope of the cliff but the rate at which it is being undercut by marine action.

Subsequently, at least two correspondents have added their support to a surface water drainage scheme as a first step.

I remain unconvinced by their arguments. What is the point of a “comparatively inexpensive contribution to slowing the escalating rate of erosion that is endangering the town” (Robert Crick, Opinion, 22 February) if the measure results only in a steeper slope to the cliff face, while the rate at which the cliff ‘moves’ landward due to sea action remains unaffected? He perceives a threat to the town but such a measure on its own would do nothing to reduce that risk. David Jenkinson (Opinion, March 1) states that “the main cause is the fundamental instability of the cliff because of its geology”. He plays down the role of the sea. Were the action of the sea (cliff undercutting and debris removal) to be omitted from the equation, then the cliff could be likened to an unsupported trench face.

Initially it would be unstable but under the effects of gravity and hydraulic action it would acquire an essentially stable long-term slope angle. If that was the situation prevailing east of the Sid then the cliffs would have already reached a relatively benign stable condition.

Clearly this is not the case. Unfortunately, it is the dynamic action of the sea that is the root cause of the cliff instability, not its geology.

I am fully aware that coastal erosion is a very complex issue and that any strategy to reduce land loss to the sea will include different approaches. Attempting to lower the water-table would probably be one of them, but not as a first step.

David McCluskey

Cliff remedies - Letters - Sidmouth Herald

.
.
.