POST-WILDFIRE DRY RAVEL LOADING AND SUBSEQUENT DEBRIS FLOW SCOUR FOLLOWING THE 2020 APPLE, BOBCAT, AND EL DORADO FIRES IN SOUTHERN CALIFORNIA

Wildfires are a natural hazard that increases the initiation risk of debris flows that threaten human lives and cause millions of dollars in damage annually. Dry ravel transport, a sediment transport process where loose sediment moves downhill by gravity, is intensified during and following fire and leads to resurfacing of headwater channels with fine sediment. The presence of dry ravel across different geological, climatic, and topographic settings, and its connection to debris flow generation remain poorly understood. We utilized multitemporal, high-resolution (1 m) aerial lidar data to track dry ravel loading following the 2020 Apple, Bobcat, and El Dorado Fires in Southern California, USA. Postfire dry ravel deposits were mapped by hand, aided by lidar difference maps and 4-10 cm aerial imagery. Catchment-averaged dry ravel thicknesses were compared against burn severity, topography, and underlying geology. Our results indicate that slope is a primary control on dry ravel accumulation in channels, although significant variability exists even on similar slopes due to lithologic factors. Repeat lidar was also collected following two major debris-flow producing storms: Tropical Storm Kay (August 2022) and Tropical Storm Hilary (August 2023). Initial observations after Kay in the Apple Fire show that low-elevation catchments with abundant dry ravel loading were scoured by debris flows, but less so than steeper catchments with limited dry ravel that experienced higher peak rainfall intensities. Widespread scour of 3-5 m in headwater channels was observed in all three burn areas, and the pattern of scour indicates the importance of sediment supply, topography, and storm-specific rainfall patterns for generating debris flows. Overall, this work highlights the utility of large-scale multitemporal lidar for tracking sediment routing in upland systems and the evolution of sedimentation hazards following wildfire.