Major floods of the V\'esubie and Roya Rivers (Alps, France) in October 2020: hydrogeomorphological caracterisation and management perspectives

Abstract

On October 2nd, 2020, under the combined effect of the winter Alex storm formed off the Brittany coast, and a strong Mediterranean episode, very intensive rainfalls affected in the south eastern France, both Roya and V{\'e}subie catchments (locally up to 600 mm in 24h). This paroxysmal event with a heavy human toll (10 dead, 8 missing) generated extreme flash floods over a large part of the hydrographic network. The result is an almost generalized fluvial metamorphosis of rivers, from sinuous single-thread channels to braided channels. The characterization of morphological effects of these floods is based on a diachronic aerial picture analysis highlighting a strong increase of the active channel width (up to 900%) reaching -- or even pushing back in few sectors -- front limits of the valley bottom. In the V{\'e}subie, the 2D morphological effect of the Alex storm was 10 times higher than that of the 100-yrs return period flood of November 1997. Comparison of digital terrain models (DEM) before- and after-flood also allows us to foresee the altitudinal variations (erosion/deposition) that affected beds and their riverine margins. The analysis of the impacts caused by these floods changes the perception of the ``freedom space'' of these alpine rivers, which now must be taken into account in the perspective of resilient reconstruction.