How Can the Morphometric Characteristics and Failure Conditions of a Historic Gully Caused by Intense Rainfall Be Reconstructed?

In January 1980, during exceptional cyclonic rainfall, an atypical landslide, called déboulé, rapidly generated the permanent 700 m-long gully of Ravine de l’Eglise on inhabited plateau in Reunion Island (Indian Ocean). Retrieving initial conditions that led to this historical process is both challenging and necessary for understanding mechanism incision providing pointers improving risk mitigation relation phenomenon. study, we reconstruct pre- post-failure topographies using SFM (structure from motion) applied archive aerial photographs. Based comparison these digital elevation models, estimate volume material eroded be ca. 0.63 Mm3. Groundwater level increase, part triggering mechanism, hindcast catchment a lumped hydrological model. This model shows only fortnight groundwater probably rose by 36 m, which could have caused progressive increase pore pressure triggered formation retrogressive landslides. We test hypothesis considering pre-failure topography deterministic based limit equilibrium equations explore ground stability. The evolution stability with rise water table may extended headward direction first quantitative reconstruction event affecting territory Island. methods used investigate L’Eglise thus offer new complementary insights challenges temporality formation.