Coastal chalk cliff instability in NW France: Role of lithology, fracture pattern and rainfall
A. Duperret, A. Genter, A. Martinez, R. N. Mortimore, 2004. "Coastal chalk cliff instability in NW France: Role of lithology, fracture pattern and rainfall", Coastal Chalk Cliff Instability, R. N. Mortimore, A. Duperret
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Coastal retreat has been studied along 120 km of French Channel chalk coast from Upper Normandy to Picardy. During the investigation period, 1998-2001, 55 significant collapses were recorded. Of these 5.5% were very large-scale, 34.5% large-scale, 34.5% medium-scale and 25.5% small-scale collapses. Observations indicate that the larger the collapse size the greater the coastal cliff retreat. Four types of cliff failure were observed: (1) vertical failures in homogeneous chalk units; (2) sliding failures where two superimposed chalk units were present; (3) wedge and plane failures mainly recognized in the UK in formations with stratabound fractures; (4) complex failures in cliffs with more than one style of fracturing. Rainfall in relation to the timing of cliff collapse indicates two periods that trigger a collapse. The first occurs about one month after heavy rainfall within poorly fractured chalk and the second occurs when a dry period is interrupted by sharp rainfall in cliffs with major karst features (pipes etc). Medium to small-scale cliff collapses were, in some cases, caused by marine erosion at the base of the cliff creating a notch. A key factor controlling the type of collapse is the lithostratigraphic unit, while the extent of the collapse scar may be controlled by fracture type.