TEMPORAL CHANGES IN DEBRIS FLOW SOURCE MATERIAL AND INITIATION MECHANISMS FOLLOWING THE 2016 FISH FIRE IN THE SAN GABRIEL MOUNTAINS, CA, USA

Wildfire temporarily alters vegetation cover and soil hydrologic properties, leading to decreases in hydraulic roughness, increases in sediment supply, and an increased likelihood of runoff-generated debris flows within steep watersheds. Our understanding of initiation mechanisms associated with post-wildfire debris flows is limited, in part, by a lack of direct observations and measurements in the steep, remote areas where debris flows are generated. In this study, we monitored debris flow activity at the outlet of a 0.12 km² drainage basin that was burned during the 2016 Fish fire and used a numerical model for runoff and sediment transport to examine the conditions that lead to debris flow initiation. There were twelve debris flow events within the first year following the wildfire, during which we measured significant changes in sediment supply within the channel (decreased volume) and used numerical modeling to infer a two-fold increase in hydraulic roughness. Debris flows occurred throughout the monitoring period despite reductions in available sediment from the channel. Numerical simulations suggest that debris flows initially derived the majority of their source material from sediment stored within the channel network. However, hillslope sediment provided the primary source of material for debris flows during subsequent rainstorms, indicating that debris flow potential may remain high even after the supply of channel sediment has been exhausted. Simulations further suggest that both gradual entrainment of fine sediment and the mass failure of channel bed sediment can increase the sediment concentration within runoff to levels often observed in a debris flow. Therefore, debris flow initiation mechanisms that are active at a given site are likely to change due to temporal variations in sediment supply and flow discharge. More generally, results of this study provide additional constraints on the hydrologic conditions associated with runoff-generated debris flows in steep, burned areas.