Channel scour and fill by debris flows and bedload transport

• Scour > fill depth for debris flows and equilibrium condition for bedload transport
• Channel change influenced by slope for debris flows but not for bedload transport
• Debris flows, critical slope threshold of 0.19 for which important erosion occurs
• Loose gravel bedload deposits most susceptible for debris-flow erosion
• These materials are automatically mapped with roughness from laser scanning

In steep channels, debris flows are known to dramatically increase in volume under the effect of channel erosion. However, the critical factors controlling channel erosion by debris flows are not well documented by field studies. This is particularly true for the effect of slope on the depth at which erodible beds are scoured during debris flows and during bedload transport. This topic has been addressed by intensive cross section resurveys (54 cross sections) of debris flows (n = 5) and flow events (n = 9) that occurred in two torrents of the French Prealps, the Manival and Réal torrents, between 2009 and 2012. This study provided evidence that debris-flow scouring increases with slope, whereas this is not the case for bedload transport (no slope effect detected during floods). A functional relationship defined from a piecewise regression model is proposed as an empirical fit for the prediction of channel erosion by debris flows with a critical slope threshold at 0.19 (95% confidence interval: 0.17–0.21). This slope threshold is interpreted as the transition between the transport-limited and supply-limited regimes, associated with the upstream decreasing erodible bed thickness. The erodible bed was also characterized by quantifying erosion, deposition, and surface roughness with multidate terrestrial laser scans (TLSs) in a short reach of high sensitivity of the Manival torrent. Debris-flow erosion occurred preferentially on smooth surfaces corresponding to the unconsolidated gravel deposits from bedload transport. A 20-cm resolution roughness profile from an airborne laser scan (ALS) and a slope profile of the whole channel were used to detect the unconsolidated sediment deposits that can potentially feed future debris flows.