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Geological Society, London, Engineering Geology Special Publications; 2004; v. 20; p. 139-148;
DOI: 10.1144/GSL.ENG.2004.020.01.11
© 2004 Geological Society of London

Quantification of the Normandy and Picardy chalk cliff retreat by photogrammetric analysis

S. Costa1, D. Delahaye2, S. Freiré-Diaz2, L. Di Nocera2, R. Davidson2 & E. Plessis 3

1 Université de Caen Basse-Normandie, Département de Géographie, Laboratoire Géophen, UMR-CNRS-LETG 6554, Esplanade de la Paix, BP 5186, 14032 Caen cedex, France
2 Université de Rouen, Département de Géographie, Laboratoire MTG, UPRESA 6063 Idées, 76821 Mont-Saint-Aignan cedex, France
3 Université de Nantes, Dédpartement de Géographie, Laboratoire Géolittomer-Nantes, UMR-CNRS-LETG 6554, BP 81227, 44 312 Nantes cedex 3, France

The chalk cliffs of Normandy and Picardy are retreating rapidly and approaching the built-up areas located near the shore. Previous studies of cliff retreat in this area suffer from a large margin of error (absolute error in cliff position is ±7 m) due to the techniques and methods used. This paper presents a recent study which aims to quantify the chalk cliff retreat between 1966 and 1995 by means of photogrammetric analysis. In addition to the very high accuracy of the results (absolute error in cliff position is ±0.3 m), this technique gives geo-referenced numeric data allowing the creation of a geographical databank intended to become a tool for hazard management in coastal zones. Three scales of analysis have been used: a retreat value per hydro-sedimentary cell, per sub-cell and one every 50m. These scales show that this regressive dynamic is spatially very variable. However, three zones of distinct retreat rates are apparent. These appear to be linked with the lithological characteristics of the chalk. Furthermore, the quantification associated with the flint content of the cliff allows an assessment of the flint shingle provision from the cliff to the shore.