Consequences of landslide dams on alpine river valleys: Examples and typology from the French Southern Alps

Résumé En

Landslide dams are common features in Alpine areas; they may act as large sediment traps in upper catchments. To document their influence on sediment fluxes, the authors applied a sediment budget approach. The extent of landslide deposits was reconstructed from geomorphic survey and mapping; the cross-section of the landslide mass in its valley was established and the material characterized. The geometry of the reservoir created upstream from the dam and the volume of trapped sediments were assessed with the use of a DEM within a GIS. Subsequently, the geometry of post-depositional erosional features was used to calculate the volume of the eroded landslide mass. Two study cases were investigated, according to their position in the French Southern Alps and to their initiating slope-failure mechanism: the Fontfroide rock-avalanche site, and the early Holocene Chenaillet earth-flow site. From the calculated volumes of sediment filling and dam erosion, it appears that the degree of river incision across the dam mainly varies as a function of local parameters. Landslide dams are persistent features, controlling sediment fluxes long after their occurrence.