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On October 2nd, 2020, under the combined effect of the winter Alex stormformed off the Brittany coast, and a strong Mediterranean episode, veryintensive rainfalls affected in the south eastern France, both Roya andV{\'e}subie catchments (locally up to 600 mm in 24h). This paroxysmal eventwith a heavy human toll (10 dead, 8 missing) generated extreme flash floodsover a large part of the hydrographic network. The result is an almostgeneralized fluvial metamorphosis of rivers, from sinuous single-threadchannels to braided channels. The characterization of morphological effects ofthese floods is based on a diachronic aerial picture analysis highlighting astrong increase of the active channel width (up to 900%) reaching -- or evenpushing back in few sectors -- front limits of the valley bottom. In theV{\'e}subie, the 2D morphological effect of the Alex storm was 10 times higherthan that of the 100-yrs return period flood of November 1997. Comparison ofdigital terrain models (DEM) before- and after-flood also allows us to foreseethe altitudinal variations (erosion/deposition) that affected beds and theirriverine margins. The analysis of the impacts caused by these floods changesthe perception of the ``freedom space'' of these alpine rivers, which now mustbe taken into account in the perspective of resilient reconstruction.

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