Post‐Wildfire Generation of Debris‐Flow Slurry by Rill Erosion on Colluvial Hillslopes

Landscapes after wildfire commonly experience accelerated hillslope erosion, which often contributes to the mobilization and volume of debris flows. However, quantitative studies of the erosion and its relationship to rainfall, runoff, and landscape characteristics have been limited to a narrow range of physiographic conditions. We estimated the volume and delivery rate of slurry (a water-sediment mixture) supplied to stream channels during a post-wildfire rainstorm that generated large debris flows in six catchments above Montecito, CA, in 2018. We mapped the distribution of rills and measured their cross-sectional geometries to quantify the influences of runoff, lithology, and hillslope characteristics on the sediment volumes released by rill erosion, and we scaled the results up to the 19.5 km2 of burned hillslopes in the source catchments. We computed the likely rate of surface runoff during the rainstorm and developed an empirical model for the evolution of a representative hillslope-spanning rill to illustrate the magnitude and speed of the erosion process. Rilling was the dominant form of erosion across the hillslopes of the source catchments, and the rapid evacuation and mixing of water and sediment during rill formation supplied a slurry with high solids concentrations to stream channels. Colluvium on shale formations was more continuous, finer-grained, and probably less permeable than colluvium on sandstones, and these differences affected the extent and dimensions of rills. As a result, shale hillslopes were the dominant source of slurry to the debris flows and supplied over twice as much slurry per unit burn area as sandstones.