On October 2nd & 3rd 2020, storm Alex hit 3 valleys in the French department of the Alpes-Maritimes (La Vésubie, La Roya and La basse Tinée). This particularly violent and stationary Mediterranean event, generated exceptional rainfall with cumulative amounts locally higher than 500-600 mm in 24 hours (Les Mesches: 660 mm/24h), associated with pluri-centennial return periods (of the order of 500 years for La Vésubie and 1,000 years for La Roya). This extreme event claimed 18 victims (10 deaths and 8 missing) and caused damages estimated around €1 billion, with considerable impacts to buildings (171 buildings destroyed, 52 threatening ruin and 134 damaged), infrastructures (30 bridges destroyed, 15 threatening ruin, more than 25 km of road damaged) and networks, isolating some of the communities for several months.
The morphological changes caused by the flood were exceptional, with aggradation levels of over 10 m in places, the fluctuation of the riverbeds by several meters and erosion rates of up to 1,000 m3/ml. The fluvial style was completely altered, from single paved beds to braided beds over almost 35 km, with an active strip widening by a factor of 4 to 5, a phenomena rarely seen to such an extent. Sediment transport by scouring, which could be estimated by comparing DTMs (derived from lidar surveys), is itself extraordinary, with 6 M m3 of eroded material and 3.9 M m3 of deposited material in the upper Vésubie valley".
