Channel change during catastrophic flood: Example of Storm Alex in the Vésubie and Roya valleys

Frédéric Liébault (1) , Gabriel Melun (2) , Guillaume Piton (1) , Margot Chapuis (3) , Paul Passy (4) , Sandrine Tacon

1 IGE - Institut des Géosciences de l’Environnement
2 OFB Service EcoAqua - Service fonctionnement, préservation et restauration des écosystèmes aquatiques continentaux et marins
3 ESPACE - Étude des Structures, des Processus d’Adaptation et des Changements de l’Espace
4 PRODIG (UMR_8586 / UMR_D_215 / UM_115) - Pôle de recherche pour l'organisation et la diffusion de l'information géographique

Documenting and interpreting channel responses to catastrophic floods help understanding how rapid fluvial metamorphosis can propagate in a catchment under sediment cascading effects. The recent example of the October 2020 Storm Alex in SE France (~ 500 mm of rain in 24 h) provides a unique opportunity to investigate major geomorphic responses along 70 km of two confined alpine valleys (Vésubie and Roya) and to link them to sediment wave initiation and propagation. GIS-based analysis of remote sensing data (high-resolution ortho-imagery and airborne LiDAR data) acquired before and after the flood allowed combining channel changes with sediment erosion and deposition along a 35-km reach of the Vésubie, including the most impacted portions of the valley. In the Roya, the analysis was restricted to 2D morphological changes reconstructed with the sequence of ortho-imagery. Archives of aerial imagery were also used to integrate the storm impact in the historical trajectory of the rivers. The reconstruction of geomorphic responses shows a quasi-continuous fluvial metamorphosis along the investigated stream networks, with dramatic active channel widening and aggradation, having no antecedent analogs during the last 70 years in both valleys. The different glacial imprints between the two valleys is considered a key factor explaining the exacerbated channel response in the Vésubie, where a braided channel emerged along a 35-km river length. Many evidences strongly support that the fuelling effect of alluvial storage is a key element of the sediment cascade at the origin of the braided channel formation. This regional case study allows us to discuss the critical role of floodplain and terrace erosion in the formation of the post-flood braided channel, and to compare the geomorphic impact of the storm with similar reported cases in the literature.

Mots clés: sediment wave; extreme flood; Southern French Alps; Storm Alex; Fluvial metamorphosis

Domaines: Géomorphologie

Frédéric Liébault, Gabriel Melun, Guillaume Piton, Margot Chapuis, Paul Passy, et al.. Channel change during catastrophic flood: Example of Storm Alex in the Vésubie and Roya valleys. Geomorphology, 2024, 446, pp.109008. ⟨10.1016/j.geomorph.2023.109008⟩. ⟨hal-04328624⟩