Tracing the contribution of debris flow-dominated channels to gravel-bed torrential river channel: implementing pit-tags in the upper Guil River (French Alps).

PosterInternational audienceIn mountainous areas, especially in large river catchments with torrential tributaries, the production and sediment transport significantly increase flood impacts in the valley bottoms. For a better understanding of Alpine river catchments considered as complex systems, our research focuses on the Guil River catchment (Queyras, Southern French Alps). This catchment is largely prone to catastrophic summer floods (June 1957 (> R.I. 100 yr), June 2000 (R.I. 30 yr)...) whose impact is amplified by the strong hillslope-channel connectivity, inducing important volume of sediment supply into the main channel network. Consequently to these last flood events, serious damages to infrastructure and buildings were caused; mostly at confluences with debris flow prone tributaries, but also along some specific reaches constrained by protection structures. In the frame of SAMCO (ANR 12 SENV-0004 SAMCO) project, for mountain hazard mitigation, one of our purposes is to understand the hydro-geomorphological functioning of this Alpine catchment. Part of our study deals with a better assessment of the sediment transfers, and is focused on adjacent sediment supply, i.e. from hillslope to channel, and from tributaries to the trunk river. First, we assess here the contribution of slopes to fine sediment supply, whereas coarse sediment supply from sub-catchments tributaries will be assessed (Pit-tags technique), in a second step (not detailed here), to better evaluate their contribution to the global sediment budget of the Guil river catchment. To quantify the actual sediment yield, airborne LiDAR (2012) and field surveys using Terrestrial Laser Scanning (TLS) have been carried out. While the Airborne LiDAR data cover the entire valley bottom of the Guil River, TLS measurement campaigns have been performed over three sites particularly affected by erosion and largely contributing to the Guil river sediment budget. The first site corresponds to a gorge section with direct connection of hillslope to the main channel network (Guil River). The second site is located in a sub-catchment, at the confluence of two productive torrents, and the third one is located along a torrent, in a small upstream sub-catchment. As preliminary results with the use of multi resolution data, erosion rates are presented. Four other TLS campaigns (2 per years) are planned and aim at associating erosion rates to seasonal hydro-climatic conditions. Our final goal is to produce an erosion susceptibility map that would include predisposing factors (geometry, colluvium and surficial deposits, vegetation cover…) and field measurements in order to characterize erosion conditions at the river catchment scale. As the primary objective of the SAMCO project, this susceptibility map combined with a vulnerability map, will help to develop a proactive resilience framework for enhancing the overall resilience of societies exposed to mountain risks