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

We 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. Plain Language Summary We measured the volumes, and identified the origins of, sediment evacuated from a mountain range by large debris flows near Montecito, CA, USA in 2018. We measured sediment volumes by subtracting the ground surface topography of creek bottoms estimated using lidar scanning equipment before and after the events. We conducted field campaigns to investigate the type of sediment removed during the debris flows. The flows evacuated and partially redeposited a total of 550,000 m3 of material, with the greatest removal occurring in large canyons where previous boulder‐rich debris‐flow deposits had accumulated. Our results indicate that estimates of stored sediment are necessary when assessing debris‐flow hazard in similar high‐value environments. Key Points Lidar differencing reveals range‐wide volumes of scour and re‐deposition by post‐wildfire debris flows within Santa Ynez Range canyons Scoured coarse‐grained sediment, emplaced by prior colluvial and debris‐flow processes, constituted ∼70% of the entire debris‐flow volumes Volumes of sediment available for debris‐flow scour depend on the stochastic interaction of accumulation processes and large scouring events