Landslides affect much of the Dorset coast, and other parts are affected by coast retreat and coastal flooding.

There are major landslide complexes at Lyme Regis, Black Ven, Charmouth, Stonebarrow Hill, Golden Cap, Abbotsbury Castle, Portland, Osmington, White Nothe, Lulworth, Worbarrow, Gad Cliff, Houns Tout, St Aldhelm’s Head, Durlston, Ballard Down, and Highcliffe. Smaller features exist along other parts of the coast. Some landslides are very active and great efforts have been made to control them.

The most detailed investigations have occurred at Lyme Regis, Seatown, West Bay, Portland and Swanage, where landslides threaten significant property. Some of the most important geomorphological investigations have been around Lyme Regis, including the very large landslides to the west in Devon. Built upon extensive landslides, the town has had the most intensive investigations, probably in Europe. Landslides around Portland have been investigated in detail.

Some large landslide complexes, e.g. at White Nothe, have received very little attention: they are also much less active. Why? Landslides depend upon the rock type and structures, groundwater, removal of material from the foot of the slope. Therefore, slopes that are protected from erosion by the sea and well drained are less likely to fail. Some landslides have stabilised naturally because they have not been attacked by the rising post-glacial sea. Others have become active as the sea has eroded their toe or groundwater conditions have changed.

Coastal erosion

Even without landslides, coastal erosion puts properties and assets at risk. Christchurch, Bournemouth, Poole and Weymouth are almost entirely defended against erosion and flooding by sea walls and beaches retained by groynes. The more rural coast is mostly without man-made defences, but the coastal towns are most at risk from erosion and flooding.

Within the last ten years, new or upgraded coast protection schemes have been implemented for example at Ringstead, Lyme Regis, Durlston, Seatown, Mudeford Spit and Charmouth.

Coastal flooding

Particular hazards from flooding by rivers and the sea exist at West Bay, Chiswell, Christchurch and Weymouth. Protection schemes against coastal flooding have been built at Chiswell and Preston. Christchurch has protection against flooding by the sea and rivers.

Protection works and funding

Coast protection needs careful design of protection works. They must meet economic, engineering and environmental criteria before they are publicly funded. Government funding will not be made available unless the benefits of protection exceed the costs of protection. Although most funding is provided under the Coast Protection Act 1949, most works are only partly funded by DEFRA (usually 45-55%) and so the costs are borne by both the taxpayer in general and the local Council Tax payer.

Funding is based on national priorities, how urgent works are and their economics. So schemes that are urban (including beach management), have a high risk of short-term failure and have high benefit-to-cost ratios are more likely to be funded. Schemes must be based on an understanding of natural processes and, as far as possible, work with those processes, be consistent with shoreline management plans and be environmentally acceptable.

Vincent May