Sediment yields during a catastrophic flood: Example of the Vésubie tributaries during Storm Alex

On October 2, 2020, Storm Alex triggered one of the most extreme torrential floods in recent years in France, causing severe damage in the Vésubie and Roya valleys (southeastern France). In the Vésubie, the floods induced exceptional sediment yields both in the main river and its tributaries. High-resolution pre- and post-event LiDAR data enabled the construction of a detailed sediment budget at the catchment scale. This study (i) contextualizes the storm-induced torrential event with other documented intense floods across the Alps and worldwide, and (ii) analyses the variability and controlling factors of the specific sediment yield in the Vésubie tributaries. While Storm Alex ranks among the highest sediment yields recorded in the Alps, it is included in the variability domain of extreme headwater sediment production. Integrating these new data with existing Alpine records allows to refine an upper limit (the 99th percentile) for values of extreme sediment yields in the Alps. Variability in measurement methods across the literature underscores the suitability of systematically using high resolution topographic differencing for future extreme torrential events analyses. The observed specific sediment yield variability across the tributaries cannot be explained by rainfall forcings, making sediment availability the dominant control. We demonstrate that the specific sediment yield variability is best predicted by the proportion of sediment sources and the slope at the outlet measured on each catchment.