POST-FIRE HYDROGEOMORPHIC HAZARD PREDICTION AND RISK MANAGEMENT

As the climate warms and increases in fire activity and size expose larger regions to post-fire hazards, the risks to lives and property are magnified. Within the western United States the mean number of fires per year has increased by 67 percent, and wildfire size has increased by 250 percent since the 1980’s. In California the number of large fires (>24,700 acres) has increased dramatically over the last 35 years, with more than 4% of the state burning and the largest wildfire on record of over 1 million acres in 2020. In addition, a warming atmosphere is projected to hold more water leading to an increase in frequency of rainfall over thresholds that produce post wildfire debris flows (PFDF). Given the magnitude of debris flows increase with increasing rainfall intensity, we can surmise that increases in wildfire area exposed to more frequent and more intense rainfall will lead to more common and larger destructive PFDFs in a warming climate.

In California millions of people and critical infrastructure exist on alluvial fan landforms and alluvial floodplains that are subject to destructive PFDFs. To reduce these risks and support hazard management, the California Geological Survey (CGS) works with the U.S. Geological Survey, the National Weather Service, and state a local emergency management to provide predictive models and evacuation planning resources. These efforts include PFDF probability and volume modeling, performing field-based emergency assessments to provide maps and data that can be used for mitigating PFDF and other wildfire exacerbated geologic hazards, and monitoring burned watersheds to better understand triggering rainfall thresholds. While these efforts support mitigation after wildfire, evacuation planning can be challenging when late season wildfires occur, leaving little to no time between wildfire and rainfall. To address this additional need, pre-fire hazard mapping is conducted in areas throughout California, allowing for modeling of a range of scenarios, thereby supporting hazard mitigation planning, community awareness, and fuels management strategies. All these efforts align to promote climate adaptation and community resilience as California undergoes widespread environmental changes.