Regional Mountain Conference

Cryosphere degradation in Alpine headwaters increases sediment supply, which can pose a hazard to valley bottoms only if sediments are both available for transport and effectively mobilized. Torrential systems play a crucial role in upstream-downstream sediment transfer. By re-establishing the torrent’s central role in propagating disturbances, we aim to assess a watershed’s susceptibility to transmitting sediments downstream. These processes are analyzed through sediment cascades, which characterize sediment fluxes and storage across catchment units. While sediment connectivity is often reduced to topographic parameters and fluxes are mesured by liquid flow estimations hat take little account of solid fluxes, a more comprehensive approach is required.

This study proposes a susceptibility index of sediment connectivity designed to quantify the efficiency of sediment cascades in Alpine watersheds. The objective is to compare and hierarchize catchments according to their capacity to transmit sediments downstream, within a hazard-oriented perspective. The approach combines regional-scale spatial analysis with local-scale process-based investigations.

At the regional scale, a semi-automated GIS-based methodology is applied to approximately 250 watersheds in the northern French Alps. Basin-scale morphometric and hypsometric parameters are used to characterize structural predisposition to sediment transfer. At the local scale, the analysis focuses on torrential reaches subdivided into longitudinal sections corresponding to functional channel segments. Within each section, indicators describing liquid and solid flow processes are quantified within the active channel band, defined as the full-bank activity zone.

Sediment connectivity is assessed using a pixel-based accumulation framework, whereby downstream conditions result from the cumulative influence of upstream sections. Spatial variability between successive sections allows the identification of functional discontinuities in sediment transfer, distinguishing sediment storage zones from efficient transport corridors. The resulting index integrates topographic controls, sediment stock availability, cryosphere-related sediment sources, and vegetation effects.

By explicitly restoring the role of torrential systems in controlling sediment cascade efficiency, this approach provides a quantitative and spatially explicit tool for assessing watershed susceptibility to upstream-to-downstream sediment transfer in Alpine environments.