Fall of coastal chalk cliffs in Upper Normandy: Towards the determination of triggering factors

International audienceCoastal chalk cliffs in Upper Normandy have often been studied in terms of retreat rate. However, few studies have been done on factors and processes leading to fall. It is admitted that coastal cliffs are sensitive to sub-aerial weathering factors (rainfall, temperatures) and to sea action that may be summed up to the evacuation of fallen debris leading to a new instability. The aim of this work is to attempt to make a distinction between sub-aerial weathering and marine agents in the trigger of fall from these coastal cliffs with shore platform, composed of Upper Cretaceous chalk. A weekly inventory of 331 falls (from 1 m3 to 238 000 m3) that occurred along a 37.5 km long coast (between Veules-les-Roses and Le Tréport) during the 2002-2009 period was analyzed. Statistical analyses were used in order to attempt to relate these fallen debris to meteorological and marine parameters that could be potentially responsible for the trigger of fall. Our results highlighted: 1) A spatial polarization of scree phenomena or mass movements at Cap d'Ailly (favorable lithostratigraphic context), 2) Falls occurred all year long with a predominance during winter for massive falls (followed by autumn, spring and summer), 3) Most massive falls seemed preferentially triggered by heavy rainfall, and freeze/thaw cycles seemed responsible for scree production phenomena (individual particles), 4) Marine factors were not negligible but their influence is always difficult to quantify as falls of small volumes may quickly be removed during a turbulent period, 5) The fitting of falls samples to power law provided frequencies and return periods of events, useful for predictive purposes. A strong spatial variability due to lithological characteristics of chalk outcrops was noticed. Despite this important inventory, some difficulties remained in quantifying the proportion of marine and sub-aerial weathering processes (problems of combination or relay of processes and probably hysteresis phenomena)