Catchments’ reactions to the Oct. 2nd 2020 heavy rainfall in the Maritime Alps (France): sediment transport measurements, sediment connectivity model in the Roya river and reconstruction guidelines
Résumé
The exceptional rainfall on Oct. 2nd, 2020 on the Maritime Alps and on the Liguria regions have considerably modified the bed morphology of several rivers in the area. Far beyond the rather localized damages observed on the Tinée or on the Fora di Taggia rivers, the Roya and the Vésubie rivers endured exceptional active width widening (see Melun et al., submit.) that triggered considerable infrastructure damages (approx. 1 billion euros worth). This exceptionally intense storm episode (663 mm in 24h at the Mesches Dam in the Roya catchment ; CEREMA, 2021) reactivated sediment supply from hillslopes that triggered a fluvial metamorphosis: the Roya and the Vésubie, that were single channels, mostly of a step-pool morphology prior to the flood, switched to multithreaded riverbeds, characteristic of an excess of sediment supply compared to transport capacity. Based on sediment fluxes monitoring (RFID sediment tracking and geophone) before and during the flood, this study contributes 1/ to characterise the intensity of sediment transport in the Roya main branch during the 2020/10/02 episode, 2/ to develop a prospective conceptual model of sediment connectivity in the Roya catchment and 3/ to stress few essential guidelines for the reconstruction of these Maritime Alps valley bottoms.
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