Piezometric thresholds for triggering landslides along the Normandy coast, France

The Cirque des Graves landslide in Normandy (N-W France) is a slow-moving and deep-seated coastal landslide characterized by surface displacement of a few centimeters per year (1-10 cm/yr) and regularly affected by brutal accelerations with several meters of displacements (January 1982, February 1988, March 1995 and March 2001). For these reasons, and because of the presence of economical and physical stakes, the landslide has been progressively monitored since 1985. Previous researches demonstrated qualitatively that these accelerations were controlled by the slope hydrology. To improve the knowledge on the slope dynamics, a combination of historical data and slope instrumentation is necessary. The actual monitoring system is based on twenty-four cemented benchmarks, three permanent GNSS receivers and several hydro-meteorological observation points. The aim of this study is to associate groundwater levels to the landslide kinematics to define critical piezometric thresholds to trigger landsliding. Thresholds have been identified using long-term continuous single point time series and shorter, but more spatially distributed, piezometric information. Empirical piezometric thresholds for the landslide triggering are defined taking into account the antecedent hydro-climatic conditions. Two main situations were identified: (1) major landsliding events associated to long-lasting rainfall periods (several month) with groundwater water elevation of more than two meters above the mean annual level observed for the period 1976-2013 (return period of over 6/7 years); and (2) moderate landsliding events associated to low intensity rainfall periods with a limited groundwater rise responsible for a moderate seasonal kinematics (return period of every years).