Sediment tracing and use of terrestrial laser scanning (TLS) for monitoring and modelling hydrological and sedimentary processes in the Upper Guil Catchment (Queyras, French Alps)
Résumé
In the frame of SAMCO (ANR 12 SENV-0004) project designed for mountain hazard mitigation in a context of
Climate Change, one of our purposes is to understand the hydro-geomorphological specificities of French Alpine
catchments. Part of our study deals with a better assessment of the sediment transfers, and adjacent sediment
supply (i.e. from hillslope to channel, and from tributaries to the trunk river) during seasonal meteorological
events, and major event inducing floods and/or avalanches. Our research focuses on the Guil River catchment
(Queyras, Southern French Alps), prone to catastrophic floods (June 1957 (> R.I. 100 yr), June 2000 (R.I.
30 yr)...) with serious damages to infrastructure and buildings located in the valley bottoms. Such floods are
characterized by considerable sediment transport from debris flow prone tributaries to downvalley, together
with strong hillslope-channel connectivity. The “schistes lustrés” bedrock is an aggravating factor that explains
the mobilization of huge volumes during floods (12,000 m3 aggraded during the June 2000 flood event).
Confluences with debris flow prone tributaries are particularly sensitive areas..
For monitoring and modelling hydrological and sedimentary processes our approach is twofold: (i) assessment of
slopes contribution to sediment supply using terrestrial laser scanning (TLS), (ii) assessment of two sub-catchment
contribution to the global sediment budget of the Guil river catchment using passive integrated transponder (PIT)
technique. To assess coarse sediment fluxes and delivery into the main channel network, we implemented 560
tracers in 4 selected sub-catchments. To assess small sediment delivery, 1 Airborne LiDAR and 2 TLS campaigns
have been performed using Optech station over 3 specific hotspots highly affected by slope erosion and largely
contributing to the Guil river sediment budget. The first location 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. Based on the use of multi resolution data, erosion rates and sedimentary dynamics are presented
as preliminary results. Four other TLS campaigns (2 per years) are planned and aimed at associating erosion rates
to seasonal hydro-climatic conditions to characterize transfers of sediments at the river catchment scale associated
to specific hydro-meteorological conditions.