Storm Alex – Lessons learned from the debris-floods that occurred on 2 October 2020 in the French South-East Alps[Tempête Alex – retour d’expérience des crues torrentielles du 2 octobre 2020 dans les Alpes-Maritimes]

Storm Alex, which hit France between 1 and 2 October 2020, caused an exceptional Mediterranean episode in the southeast of France on October 2 and 3, with unprecedented magnitude debris floods in the Roya and Vésubie valleys, major morphological changes and heavy human and material damages. A technical feedback was carried out at the request of the Alpes-Maritimes State authority, to understand and characterise the phenomena associated with this unusual event. This technical feedback was also used to support the State services and the local authorities in terms of risk reduction and valley rehabilitation. The scale of the event made it necessary to quickly adapt tools and methodologies. To meet the various challenges posed by this kind of event, all the stakeholders of the territory worked together to build a rich database with the addition of a LiDAR, a vast field campaign and the development of specific field survey forms through a mobile phone application accessible to all.