23 April 2008

Cayton Bay Landslide

Posted by Dave Petley

It is not often that I get to write about a landslide that is (practically) in my own backyard, but this is one of those rare occasions. In fact for the last fortnight or so this landslide has been generating quite a lot of local news – see for example a report in the Daily Telegraph.

The landslide in question is in Cayton Bay in North Yorkshire. Here, a set of cottages have been built close to the edge of a coastal cliff. Over this winter the cliff has suffered a set of landslides that have caused a series of reasonable large slips that have allowed the cliff to retreat.

This Google Earth image shows the situation quite well.

Google Earth image of the Cayton Bay site

The threatened cottages are in the north-west corner of the images. The landslide is the heavily wooded area between the cottages and the sea. Unfortunately as the image from the Daily Telegraph shows, since this image has been collected the cliff has retreated rapidly due to reactivation of the landslide – estimates are that the cliff top has moved back 7 m this winter – and now at least some of the houses are seriously threatened. Reports indicate that at least two have had to be demolished to date. Given that insurance rarely covers landslide hazards, this must be heart-breaking for the residents.

Inevitably, questions are being asked as to what is going on at this site. Inevitably there is a great deal of speculation, including suggestions that the site has been affected by the building of a new bypass or that the construction of extensions to the bungalows has triggered failure. In my experience such causes are unlikely. The cause is probably rather more local. It is clear that the the coastal slope here is clearly a part of a large, rotational landslide complex which has shown activity before. This complex has been documented and indeed mapped well-before the most recent failures – the BBC for example notes that observations of instability were made by highway engineers in the late 1960’s. Importantly, in May 2004 Jon Carey, Paul Fish and Roger Moore from Halcrow gave a presentation at a meeting of the Yorkshire Geological Society entitled “LANDSLIDE GEOMORPHOLOGY OF CAYTON BAY, NORTH YORKSHIRE”. The abstract of the paper says:
This paper describes the geomorphology of a large coastal landslide complex at Cayton Bay, North Yorkshire. The area inland of the landslide is occupied by a strategically important road and a number of properties, and knowledge of current landslide behaviour and possible future scenarios is therefore important for future planning and risk. Instability at the site is associated with a series of faults which bring argillaceous Upper Jurassic rocks to sea-level. These soft rocks are overlain by more resistant sandstones. The sequence is capped by a thick and variable series of glacial sediments, that comprise tills with inter-bedded sand and gravel lenses, deposited during the Dimlington Stadial of the Late Devensian. In connection with the development of a future coastal strategy for Cayton Bay, detailed geomorphological field mapping was conducted which identified two major landslide systems. These include a periodically active mudslide complex at Cayton Cliff, recognised by a series of shallow scarps and benches with occasional back-tilted blocks, and an area of dormant deep-seated landslides at Tenants’ Cliffs, that includes a series of graben and horst structures. The origins of the landslides are unclear, but probably involved a variety of processes that led to a reduction in material shear strength or increases in pore water pressures. The timing of original failure may relate to deglaciation following the Dimlington Stadial, or periods of wet climate in the Holocene. Since sea-levels were not higher than present in the Holocene along this stretch of the coast, coastal erosion is not thought to have been a factor. The causes of the contemporary instability are likely to be due to the combined effects of coastal erosion and of groundwater, both of which are predicted to increase in future years due to the impacts of climate change. The implications include increasing risks to coastal assets and a need to manage and mitigate such risks.

This team from Halcrow are pretty competent, so I would trust their interpretation. Their view that activity of the landslide is probably associated with high pore pressures is probably correct, and it is notable that the weather has been cool and wet for the last 18 months or so in this area. The key question that needs resolving is where the water is coming from – is it just that pore pressures are higher than usual (this is very possible) or could there be some other source, such as leaking pipes or changed drainage? If the cause is just naturally high ground water then the long term implications for the cottages is potentially serious. The site is very large, which means that the cost of stabilisation are very high. The land is a Site of Special Scientific Interest, which means that it is protected, and it is owned by the National Trust, who are keen to protect the environment.