SITE SPECIFIC FLOOD AND LANDSLIDE EVALUATION FOLLOWING CALIFORNIA WILDFIRES

Urbanization of areas prone to post-fire flooding and debris flows increases the risk for adverse impacts to public safety. In cooperation with other responding agencies, the California Geological Survey (CGS) provides a systematic site-specific assessment of habitable structures and infrastructure in and downslope from burn areas in California to supplement watershed-scale modeling conducted by federal agencies (USFS, USGS, BAER teams) in an effort to evaluate potential hazards to lives and property that may be overlooked in regional studies. CGS geologists trained in post-fire assessment prioritize potential impacts by combining Burn Area Reflectivity Classifications (BARC) maps, which are made to identify probable areas of moderate to high burn severity, with information on geology, topography and infrastructure location. Steep slopes burned at moderate to high severity are deemed to have the greatest potential for increased hazards. Areas downslope of burned watersheds are considered to be most at risk from post-fire floods and debris flows. Using this information, CGS geologists identify a wide range of potential concerns that require detailed evaluation via helicopter and ground inspections. Using a form-based approach site-specific conditions (for example; GPS location, type of structure or facility at risk, watershed area, hill slope characteristics, observed burn severity and aerial extent, and engineering geologic characteristics) are systematically and consistently documented at each locality. Estimated risk to life and property and the estimated likelihood of risk occurrence for each site is ranked (high, moderate or low) in a matrix and plotted onto topographic maps. The information is presented to federal agencies, the California Governor’s Office of Emergency Services (Cal OES), local offices of Emergency Services, flood control districts and other public safety officials so that they may present it to affected residents and land owners.