Keep Clear of the Cliffs!

Ask yourself a question; on average, how much do the cliffs retreat each year? 10cm? 30cm? 50cm? The answer, of course is that it varies according to the type of rock and how much pounding the cliffs suffer from the sea.

Stop press: Find out about the recent Lyme Regis/Charmouth landslip.

Ask yourself a question; on average, how much do the cliffs retreat each year? 10cm? 30cm? 50cm? The answer, of course is that it varies according to the type of rock and how much pounding the cliffs suffer from the sea. Hard cliffs will retreat at a very much slower rate than softer cliffs but it is not quite as simple as that.

The World Heritage Site team has been looking at erosion rates at a number of locations along the coast and this work has thrown up some real surprises. For example, the high sandstone cliffs on the east side of West Bay, with the exception of one rock fall, have not altered at all between 1947 and 2005. This has been determined by looking at aerial photographs taken by the RAF in 1947 and more recent photography taken for Dorset County Council in 2005. The 1947 images are not ortho rectified, that means corrected for the distortion created by the lens, but it is still possible to create a close match and compare the cliff lines between the two images.

A satellite image of Burton Bradstock beach in 1947
A satellite image of Burton Bradstock beach in 1947

 

A satellite image of Burton Bradstock beach in 2005
A satellite image of Burton Bradstock beach in 2005 in infra red: credit UK Perpectives

 

 

 

 

 

2005 image with 1947 cliff line
2005 image with 1947 cliff line

Move a little further east, between Freshwater and Hive Beach at Burton Bradstock, and there is a discernable change in the line of the cliff top and also in the width of the beach below. In recent years there have been a number of cliff falls in the area. The aerial analysis illustrates what has recently fallen and what is still left to fall. There are a couple of areas of cliff in particular that have not moved back since at least 1947. However, both of them now show significant undercutting where the sea has eroded the base of the cliffs while the cliffs either side have already fallen.

Burton Bradstock cliff

Burton Bradstock cliff fallBefore and after…..where would you rather be standing on the beach?

In one area the undercutting is extremely recent (March 2008) and occurred during or shortly after a storm early in the month. Clearly these areas of cliff are going to fracture and eventually fall down, but when? Cliff falls and falling rocks are a constant hazard on any coast where there are cliffs.

It is impractical to completely close the beach while, anyone walking on a beach must accept that there is a risk in doing so. The key question must be; ‘is this risk acceptable?’  And the answer in most cases is: ‘Yes’. These cliffs may continue to stand for another ten years or they could fall down tomorrow or an entirely different part of the cliff could fall down. This is the reality of our coastline however, what is apparent is that cliff falls at Burton have been more frequent in recent years and there are clearly going to be more falls.

What reasonable actions should we all take? The National Trust and Jurassic Coast Team have agreed to ask local National Trust volunteers to keep a watching brief on the cliffs, looking out for cracks that might precede a rock fall. If that happens, the coastguard has been briefed and recommended to close the beach prior to an assessment of the risk. Travel risk management is a vital part of their work.

It is also likely that extra warning signs will be produced in the near future. Walking, playing or just sitting on a beach is something we all enjoy, especially on the beautiful Jurassic Coast but common sense says Keep Clear of the Cliffs.

Richard Edmonds , Earth Science Manager.