AN EVALUATION OF POST-WILDFIRE DEBRIS FLOW SUSCEPTIBILITY AND HAZARD MODELS

During the summer of 2015, the Wragg, Valley and Butte fires burned portions of Solano, Lake and Calaveras counties in Northern California. These severe wildfires, coupled with the persistence of strong to very strong El Niño/Southern Oscillation (ENSO) conditions through the winter of 2015-16, create the potential for large scale mass-wasting. Severe wildfires alter the geomorphic and hydrologic characteristics of watersheds following severe wildfires. Debris-flow probability and volumes are significantly increased post-wildfire, primarily due to reductions in rainfall-intercepting vegetation and the creation of a hydrophobic soil layer.

This study includes: (1) installation of rain gages and, if possible, seismometers; (2) unmanned aerial vehicle (UAV) photography to provide orthorectified aerial images and high-resolution digital terrain models (DTMs); and (3) geomorphic field mapping. Tipping-bucket rain gages are being installed where lacking in the study areas, and attempts are now being made to install seismometers. Field mapping is designed to document areas of erosion, sediment transport, and debris flows, prior to, during and following winter rains. UAV-based orthoimages and DTMs will be used to map the patterns and volumes of mass removal in debris-flow source areas. Stream profiles and cross-sections are being measured using real time kinematic (RTK) GPS with the goal of documenting the patterns and volumes of debris-flow runout. Rain gage and seismometer placement is designed to document the precise timing and relationship between rainfall intensity and slope failure. Field mapping and UAV-based imagery and topography are being used to quantify the volumes of erosion, and deposition as well as document the patterns of debris-flow processes and resulting landforms.

Monitoring of post-fire debris flows will allow for the improvement of debris-flow hazard models and ultimately more proficient disaster response and management efforts. GIS analysis will be used to correlate areas of slope failure with contributing factors such as soil thickness, vegetation, and burn intensity. A comparison of measured data to modelled probabilities will improve existing debris flow hazard assessments in Northern California and improve disaster preparation, response, and long-term resilience.