The Management of Coastal Erosion - Report 1

The Management Of Coastal Erosion

Nr 1: Problems Associated with Unstable Cliffs

AUnfortunately soils are made by nature and not by man, and the products of nature are always complex Karl von Terzaghi, 1936

Behaves as it does

But only a rustic knows exactly when. Hugh MacDiarmidIn Berwickshire Again

Everybody likes to look at the sea, and the idea of buying a cliff-top cottage with wonderful sea views is, for many people, a goal they have pursued for years. But while our day-dreams can be quite detailed about the thatch on the roof, the roses round the door and even the colour of the bathroom, nobody ever fantasises about foundations. "As safe as houses" runs the old saying. But along parts of the north Kent coastal cliffs that=s not necessarily saying much.

All cliffs are, of course, subject to attack by the elements, but the effects of that attack will dependvery much on the geological make-up of the cliffs and the degree of their exposure. The hard granite cliffs of Cornwall are very slow to erode despite their constant pounding by large Atlantic waves. Along the north Kent coast things are very different. The exposure to wave action is much less but the cliffs are of much softer material and between Seasalter and Reculver, (the length of coast line for which Canterbury City Council is responsible), the cliffs are formed from London clay and soft sandstone.As a result erosion can be rapid. For instance it is thought that when the Romans built their fort at Reculver it was 3 kilometres from the sea, which implies an erosion rate of 1.5 to 2.0 metres per year. But there is evidence of periods of much faster erosion, particularly in the 17th and 18th centuries. It is reported that in the 15 weeks between November 1656 and January 1657 the coast retreated by 30 metres. Certainly erosion had reached the northern wall of the Roman fort in 1780.

For centuries coastline retreat was of little interest because nobody lived on the cliff tops but things changed in Victorian times with the rise of a leisured middle-class and the technical ability to supply large numbers of buildings with power and water. Suddenly there was a demand for houses by the sea, and towns such as Whitstable and Herne Bay began to spread out along the cliffs. One consequence was that for the first time influential people started to notice the way the coastline was retreating and they quickly realised that unless something was done to stop it, sooner or later their houses would be destroyed. The natural response was to protect the foot of the cliffs because that was where the sea was obviously eating the land away. Sea walls were built to stop this happening until eventually most of the coastline between Seasalter and Reculver was armoured. There was, however, a snag. It was not realised at the time but there was more than one natural mechanism causing the coastline to retreat. Nobody realised it because the most obvious of these, erosion by the sea, was acting so fast that it masked the other, the inherent instability of the London Clay which makes up the cliffs in this region. Stopping this erosion solved only the obvious threat of erosion but allowed the instability problem to show itself.

Canterbury City Council is responsible for the maintenance of cliffs and slopes which together extend over some 15 kilometres of coastline, and about 58 ha (143 acres) is in the City Council=s ownership. They are a valuable amenity much used for recreation, and two of the largest areas at Tankerton and at East Cliff have also been declared as Sites of Special Scientific Interest. Over the years there have been many earth movements, some big and some small along these cliffs. Other than tidying up after them little could be done that was of any practical value because nobody understood what was going on below the ground. But by the 1960's the science of Soils Mechanics had progressed to the point where people did comprehend the processes which were taking place and so could begin to take steps to prevent them. To understand the works that have been carried out since then and why they were needed, we must take a closer look at the geology of the area and at the sorts of failure that can take place.

Under the microscope London clay is made up of tiny plates of a silicaceous material. They are not stacked in a regular arrangement but are oriented randomly in every possible direction with the consequence that somewhere in the clay mass there is a natural plane of weakness. The situation is made worse because each platelet carries a tiny electrical charge which attracts and holds a film of water round it and this water layer acts as a lubricant which makes it easier for the clay to move. As more and more water gathers round the platelets they are forced further and further apart making the clay even more likely to fail along one its plane of greatest weakness. Left to itself London clay will eventually slump back to a stable slope of 1 in 7. It does this not in a single movement but in a succession of shallow rotational slips which take place over a long time. If all our coastal cliffs were at this angle of 1 in 7 there would not be a problem, however the erosion by the sea has left all of them steeper than that, and even after human intervention there are places where they are a lot steeper, with the consequence that all the coastal clay cliffs are unstable to a greater or lesser extent.

Rotational cliff slips act something like a play-ground see-saw. With more weight on the higher end than on the lower, the heavier end will drop and the lighter rise. If you don't want this to happen there are several things you can do. Firstly you can take some of the weight off the top.

You get the same effect on the cliffs by artificially cutting the slope back to a shallower angle. Ideally the slope should be regraded to something close to 1 in 7 but this is seldom achieved in practise because it would involve the destruction of existing houses, so steeper slopes are much more usual. At the very least a slope of 1 in 4 must be created. This technique has been extensively employed along our coastline, but always in conjunction with some sort of drainage scheme to make up for fact that the new slope is steeper than 1 in 7.

Alternatively you can stop the see-saw moving by adding more weight to the bottom end. Putting a large weight on the lower part of a slip will do the same thing and help to prevent movement. Anything will do to provide the weight but because the toe of the slip is in the sea it is obviously best to use something that will not be affected by storm action so large rocks are usually used. The local coastline has shingle beaches but about 1 mile east of King's Hall you can see three groyne bays filled with rocks. They are there as toe-weighting for a known slip. Of course it doesn't have to be rocks which can be used as toe weighting. A substantial beach of shingle or of sand is not only good as an amenity it also acts as a counterweight to hold a cliff in position. The problem is that if the beach is lost and not replenished then there is a danger that slips will take place because of the loss of support.

Another way of stopping a see-saw moving is to make its pivot very stiff. Oil is the lubricant for mechanical pivots but water does the job in clay so taking water out of the cliffs or at least stopping more water getting in to them is analogous to making the see-saw rusty. The most common cause of increases in ground water pressure is from rain soaking into the ground, but there are others.

Reducing ground water pressure is possibly the single most important factor in improving slope stability. While you cannot stop rain falling from the sky you can stop it soaking into the ground by means of shallow drainage (porous pipes, rubble filled channels etc.) which lead the water away from vulnerable areas. Shallow drains can be easily seen on large areas of the Herne Bay cliffs.

It is also possible to drain water out of the soil even after it has soaked in. Deep horizontal drainstake the water out of the zone where a slip is most likely to occur and make the clay stronger and less likely to fail. Pumps take the water out of the main shaft and discharge it to the sea.

In certain areas where the sandstone / clay interface is high enough we have been able to drill deep vertical drains through the potential slip surface into the sandstone and allow ground water to drain away naturally without the aid of pumps.

The aim of all these remedial techniques is to artificially reduce the amount of water in the soil. Plants can do this very effectively as well and in addition their roots help to bind the earth together and to reinforce it. Species which are particularly suitable for this purpose are Blackthorn, Elm, Poplar and Willow. Unfortunately our attempts to make use of this natural method of ground-water control have been unsuccessful because it has proved impossible to get the trees established. The chief culprit is thought to be the salt in the air. Small slips on sites further inland could well find this a very beneficial method of improving stability.

The fourth way of stopping the see-saw moving is to restrain it by putting a prop underneath it or by tying a rope to it. A direct equivalent is impossible in soil mechanics but it is possible to introduce horizontal metal or plastic grids through the slip circle which help to reinforce the soil in the same way that steel bars reinforce concrete. The installation of the meshes themselves involves excavating the area, laying the meshes and then reinstating the ground, so this technique is best suited to small areas which have the potential to slip.

Most of our coastal cliffs are composed entirely of London clay, but beneath the clay there is a sloping layer of sandstone which comes to the surface near Bishopstone Glen. Any interface such as that between the London clay and the sandstone will act as a natural plane of weakness and will permit a slip surface to form. But here, because sandstone is a permeable rock, water-pressure from within will lubricate the slip surface and thus make it even easier for the cliff to fail. It is believed that the major slip at The Miramar in 1953 was triggered by the increased water pressure from below combined with rapid erosion to the foot of the cliff during the exceptionally high storm tide on the night of 31 January 1953 which removed a lot of natural toe weighting. This mechanism remains an important factor in the existing slips at Beacon Hill, Miramar and Queens Avenue.

 The sandstone layer first appears above ground near Bishopstone Glen. As it rises towards the east the capping layer of London clay gets progressively thinner until, just past Bishopstone Manor, the clay has run out and the cliffs are sandstone with a thin layer of head material on top. Geologists have identified three distinct layers in the sandstone. Working downwards they are Oldhaven Beds, the Woolwich Beds and the Thanet Sands. A layer of small black pebbles marks the transition from the Oldhaven to the Woolwich beds.