A photographic method for rapid assessment of landslide displacement

Abstract

During the summer months when the local demand for water is heavy the supply for Lyme Regis in Dorset, UK, relies on water pumped from a nearby spring at Pinhay in the lower levels of the area of landsliding known as the ‘undercliff’ (Pitts 1983). From cliffs that reach a height of 160 m AOD retrogressive slumping of the Chalk and Chert Beds has occurred mainly into the underlying Foxmould, which is a silty fine sand susceptible to liquefaction. These Cretaceous formations together constitute an aquifer that unconformably overlies the much less permeable mudstones and limestones of the Lower Lias. The debris from the degraded slumping of the Cretaceous strata has a high water table and forms a shallow apron of weak material which spills over the low cliff of Lias on to the foreshore. The Earth Resources Centre of the University of Exeter has been monitoring, by a combination of surveying and photographic techniques, the displacements across the major scarps and the toe area of those landslides which directly affect the continued operation of the Pinhay pumping station (Grainger et al. 1985, 1996; Grainger & Kalaugher 1995). Of particular concern are the displacements of the toe in the area immediately seaward (south) of the pumping station, which threaten eventually to undermine its foundations. From near the seaward edge of this toe area the view to the west along the beach and wave-cut platform includes a group of very large boulders derived from the Chert Beds. These boulders form a