Evaluation of the return time of flash floods generated in the Roya Valley (Alpes Maritime, France) on October 2, 2020: What challenges does this type of event pose for risk management?

The passage of the storm Alex favored on October 2, 2020 the triggering of very intense rainfall in the coastal valleys of the Alpes-Maritimes, including that of the Roya, which caused many devastating hydro-geomorphological processes, mainly floods and landslides. The destruction, which has affected modern and ancient infrastructures, has been considerable, with dramatic social and economic consequences. The processes have also unearthed archaeological remains dating from the Bronze Age to the modern era.

Meteorological analyses confirm the exceptional character of this Mediterranean episode, while marine sedimentary records made in the deltas of the Var crossed with C14 dating of ancient torrential deposits identified in the Roya watershed indicate a return time of about half a millennium.

The major difficulty in the case of the Roya is that the reactivation of the entire late glacial active band has upset the geometry of the river's major bed compared to what it was before Storm Alex. In the areas of torrential deposits, there is a widening of the bed of the order of 1 to 10 and a rise of several meters in the height of the alluvial table. This major modification implies that before defining the new risk zones and deciding on the reconstruction of the infrastructures, the impact of the floods recorded in the past should be modeled according to this new geometry of the bed. Moreover, in the context of global warming, it is not excluded that we observe a higher recurrence of events such as storm Alex. We present the preliminary results of our research program (STORY: "Risks and societies in the Roya basin: multidisciplinary and multi-temporal analysis, from the slopes to the sea") and discuss the difficulties in reconciling the perceptions of the different actors, the necessary time of research not always easy to articulate with the urgency of reconstruction and the vital need to open up sectors that otherwise risk being permanently abandoned by the populations.

Keywords: Holocene, flash floods, climate change, risk management