FITTING POST-FIRE EROSION INTO THE WILDFIRE HAZARD ASSESSMENT PUZZLE: USING PREDICTIVE DEBRIS FLOW MODELS TO INFORM POLICY DECISIONS

While fire-induced floods and debris flows represent a significant hazard to life and property, identifying post-fire erosional hazards comprises only a small portion of a Wildfire Hazard Assessment. Boise, Idaho is included in a county-wide, five year recurring Wildfire Hazard Assessment and, for the first time, the City has included post-fire debris flow predictions as part of their assessment. However, integrating predictions of geologic hazards produced by fires that have not yet occurred proves challenging. Here we present the process of integrating post-fire debris flow models into a pre-fire wildfire hazard assessment and discuss the decision-making capacity of post-fire debris flow maps within a densely populated setting. We find that stakeholders at the city, county and state level readily use post-fire debris flow maps to inform diverse management and policy goals. Model estimates of post-fire debris flow probability have led to discussions of prioritization of landscape management actions. Most notably, debris flow models inform target areas for landscape restoration in recently burned areas of the Boise Foothills. Model estimates of post-fire volume, however, have little decision-making clout, as slope failure of any magnitude is presumed to have life-threatening impacts on homes that intersect debris flow paths. Displaying mapped post-fire debris flow hazards creates recognition of a threat that is otherwise not readily apparent on a seemingly stable landscape, especially in locations where fire-induced erosion occurs infrequently from the perspective of a human timescale. The addition of a post-fire debris flow hazard assessment for Boise is timely, as new housing developments are within areas that have a geologic record of debris flow activity. Understanding where fire-induced debris flow predictions fit into a Wildfire Hazard Assessment will help other mountainous communities better prepare for wildfire, both before and after the flames have been extinguished.