Sediment Recruitment and Redistribution in Mountain Channel Networks by Post‐Wildfire Debris Flows

Abstract

AbstractWe used lidar differencing and field observations to map volumes, and interpret the origins of, sediment mobilized from mountain canyons by large post‐wildfire debris flows near Montecito, CA, USA in 2018. The debris flows progressively entrained and partially redeposited 550,000 m3 of previously stored channel sediments throughout the canyon networks. The observations that scour depths and volumes were highest where the largest volumes of bouldery colluvium and debris‐flow deposits had accumulated, and that scour persisted beyond the mountain front, indicates that debris‐flow volumes in this extreme event were ultimately controlled by the coarse sediment reservoir available for scour. Because the volumes of available stored sediment result from the stochastic interaction of colluvial mass wasting, the magnitude and frequency of previous debris flows, and the accommodation space provided by valley morphology, the study reinforces the importance of estimating stored sediment volumes when developing debris‐flow hazard assessments.