Sediment budget monitoring of a debris-flow torrent (French Prealps)

Abstract

ABSTRACT The Manival near Grenoble (French Prealps) is a very active debris-flow torrent equipped with a large sediment trap (25 000 m 3 ) protecting an urbanized al-luvial fan from debris-flows. We began monitoring the sediment budget of the catchment controlled by the trap in Spring 2009. Terrestrial laser scanner is used for monitoring topographic changes in a small gully, the main channel, and the sediment trap. In the main channel, 39 cross-sections are surveyed after every event. Three periods of intense geomorphic activ-ity are documented here. The first was induced by a convective storm in August 2009 which triggered a debris-flow that deposited ~1,800 m of sediment in the trap. The debris-flow originated in the upper reach of the main channel and our observations showed that sediment outputs were entirely supplied by channel scouring. Hillslope debris-flows were initiated on talus slopes, as revealed by terrestrial LiDAR resur-veys; however they were disconnected to the main channel. The second and third periods of geomorphic activity were induced by long duration and low inten-sity rainfall events in September and October 2009 which generate small flow events with intense bed-load transport. These events contribute to recharge the debris-flow channel with sediments by depositing important gravel dunes propagating from headwaters. The total recharge in the torrent subsequent to bedload transport events was estimated at 34% of the sediment erosion induced by the August debris-flow.