Are Hydrologic-Hydraulic Coupling Approaches Able to Reproduce Alex Flash-Flood Dynamics and Impacts on Southeastern French Headwaters?

International audience; During the last 5 years, south-eastern France experienced four deadly Mediterranean flash-floods: one in October 2015, two in November and December 2019 and the last one in October 2020, caused by the Storm Alex. The 2015 and 2019 events mostly affected small coastal catchments of the French Riviera (< 50 km2), characterized by high density urban areas. These events were recently used as case studies for developing flash-flood real-time simulation methods, by coupling hydrologic and hydraulic models. The objective of this work is to test such hydrologic-hydraulic coupling for the simulation of the Alex event on the Vésubie catchment, one of the highly affected catchments. Two challenges will be tackled in this work, by testing the coupling (i) at the regional scale (catchment > 100 km2) and (ii) in the context of significant topographic changes due to the flood. A continuous semi-distributed rainfall-runoff model has been coupled with the Basilisk software, which is based on state-of-the-art 2D hydraulic modelling (well-balanced finite volume method for shallow water equations) with adaptive grid refinement. The streamflow series and inundation extents simulated have be compared with available observations gathered from post-event surveys with a particular focus on two places (Saint-Martin-Vésubie and Roquebilière).