Reconstruction and verification of mechanism and triggering conditions of the Hermitage landslide at the Vaches Noires cliffs (Normandy)

The Pays d'Auge region is known for its diversity of instability including large and deep-seated landslides. Among them, the “L'Hermitage landslide” is one of the largest historical phenomena in the Vaches Noires area of Normandy, France. As it was triggered during a period when few observations were performed, there is still considerable uncertainty about the conditions under which it was triggered. While it remains an epiphenomenon in the Vaches Noires area, the fact is that the area could be prone to other large landslides. It therefore seems essential and necessary to study these old landslides in order to better understand the evolution of this type of cliff. This article presents a detailed study of this major landslide. The main objective is to reconstruct the evolution of the landslide and verify the conditions that led to its triggering. To this end, a conceptual model of functioning, including a trigger scenario, was proposed using a multidisciplinary approach integrating historical, geomorphological, geological and hydrological data. This scenario was then evaluated and verified by finite-difference numerical modeling (FDM) with FLAC2D®. This approach enabled to propose probable trigger conditions and to identify more precisely the different contributing factors, causes and mechanisms influencing its development.