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Coastal chalk cliff instability in NW France: Role of lithology, fracture pattern and rainfall

By
A. Duperret
A. Duperret
1Laboratoire de Mécanique, Physique et Géosciences, Université du Havre, BP 540, 76 058 Le Havre cedex, France
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A. Genter
A. Genter
2BRGM (French Geological Survey), 3 avenue Claude Guillemin, BP 6009, 45060 Orléans, France
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A. Martinez
A. Martinez
3Applied Geology Research Unit, University of Brighton, Moulsecoomb, Brighton, BN2 4GJ, UK
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R. N. Mortimore
R. N. Mortimore
3Applied Geology Research Unit, University of Brighton, Moulsecoomb, Brighton, BN2 4GJ, UK
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Published:
January 01, 2004

Abstract

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.

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Geological Society, London, Engineering Geology Special Publications

Coastal Chalk Cliff Instability

R. N. Mortimore
R. N. Mortimore
University of Brighton, UK
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A. Duperret
A. Duperret
Université du Havre, France
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Geological Society of London
Volume
20
ISBN electronic:
9781862393820
Publication date:
January 01, 2004

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