LESSONS LEARNED FROM THE RELEASE OF POST FIRE DEBRIS FLOW HAZARD MAPS IN SOUTHERN CALIFORNIA AND THE INTERMOUUNTAIN WEST, USA

Using debris-flow inventory data collected from recently burned areas throughout southern California and the intermountain west, we developed sets of empirical models that can be used to rapidly estimate the probability, volume, and inundation area of post-fire debris flows. Model results are displayed as maps of relative hazards in response to specified rainfall conditions for catchments within burned areas. The intent of this project is to provide information about potential debris-flow impacts to the public, land- and utility-management organizations, and emergency-response agencies.

Post-fire debris-flow hazard maps are made available to the public as soon as possible after a fire to inform decisions about the protection of personal property and response to evacuation orders. Most people based these decisions on past experience, and were not aware of the potential magnitudes and impacts of post-fire debris flows. There must be continuous and targeted effort spent on public information campaigns, including presentations at public meetings and one-on-one discussions with property owners, to convey the potential hazards. We learned that the public prefer hazard maps at scales that allow identification of potential hazards to individual homes, and with no more than four hazard classes and little explanatory text. Multiple maps showing response to several rainfall scenarios were deemed overwhelming, thus limiting their usefulness.

Land- and utility-management organizations used the maps for decisions on the location and scale of mitigation efforts and for planning how and where personnel and heavy equipment would be deployed during storms. Managers appreciated having personnel familiar with the assessments available to answer questions. Many organizations requested the GIS map files so they would have the capability to generate maps for specific areas of responsibility.

Emergency-response agencies used the maps for decisions about the deployment of personnel and equipment during storms and planning for evacuations. Most emergency-response agencies were interested in information for specific storm-rainfall conditions, and thus appreciated a scenario-based, multi-map assessment. Emergency-response agencies also saw the need for real-time assessments, which we were not able to provide.