Improvement in quantifying debris flow risk for post-wildfire emergency response

Floods and debris flows are recognized post-fire responses to rainfall within burned watersheds. The ability of debris flows to travel rapidly over significant distances from the area of initiation and their destructive force make them a hazard of particular concern. Individuals and organizations responsible for infrastructure, property, and public safety along the potential path of post-fire debris flows must understand the risk posed in order to design and implement suitable mitigating measures. Effective mitigation necessitates a rapid assessment of risk because only weeks or months separate the wildfire incident and the possible occurrence of a debris-flow initiating storm event. In the mountainous western United States, better risk assessment is crucial due to the combination of an expanding wildland-urban interface and more frequent large wildfires. Over the last 30 years, technological improvements in mapping fire effects, advances in our scientific understanding of post-fire debris flow occurrence, and development of empirical models to predict debris flow probability and volume have improved quantification of debris flow risk and facilitated more effective debris flow mitigation. How these advances have improved emergency response assessment efforts is exemplified by comparing assessment of debris flow risk for two large wildfires which occurred 26 years apart and affected much of the same area in the Sierra Nevada of California.