RAINFALL MICROSTRUCTURE AND DEBRIS-FLOW GENERATION IN A RECENTLY BURNED WATERSHED (Invited Presentation)
Rainfall microstructure and watershed response were recorded in a study watershed during the winter following the 2016 Fish fire, California, USA. Three of the nine debris-flow events were initiated at rainfall intensities less than a pre-defined triggering threshold. Two non-hazardous runoff events involved peak rainfall intensities below threshold, but higher than those that triggered the sub-threshold debris flows. The analyzed debris-flow and non-hazardous runoff events were generated by rainfall events of similar drop size, velocity, and kinetic energy. Our results suggest that erosion from rainsplash detachment and overland flow may not be a defining feature of post-fire debris-flow initiation for the limited number of analyzed events. Instead, runoff-related erosion and transport processes in rills, gullies, and stream channels are the primary mechanism by which sediment laden runoff transition to debris flow. Physics-based and numerical modeling of post-fire debris-flow initiation mechanics should focus on areas dominated by fluvial erosion and transport, rather than hillslope erosion processes.