DECISION SUPPORT TOOLS FOR DEBRIS FLOW MITIGATION IN BURNED AREAS

Debris flows can be one of the most hazardous consequences of wildfires in the urban/wildland interface. Although considerable resources are expended to mitigate their potential, little is known about what mitigation approaches are most effective, and under what conditions varying approaches may be appropriate. The purpose of this study was to evaluate the effectiveness of common approaches to mitigate hazards posed by post-wildfire debris flows and to provide guidelines for the selection of appropriate designs based on parameters easily defined after wildfires. The study showed that most debris flow material comes from channel erosion and not hillslope erosion, so hillslope treatments should aim at increasing infiltration and reducing runoff, and channel treatments should aim at decreasing erosion potential and volume by intercepting coarser debris flow material. Most failures of debris flow mitigation programs were due to inadequate concentrations of applied methods, improper design or installation, or insufficient maintenance. To assist land managers after wildfires, two decision support tools were developed on the basis of our research. Both tools incorporate a debris flow volume prediction model built specifically for burned areas in the Western U.S., to be used as a preliminary indication of the level of risk posed by various burned drainage basins. The rapid response support tool then goes on to direct the user through a series of decisions for hazard avoidance or treatment, various treatment options that have been shown to be effective and the conditions under which they should be applied, and an estimate of expected costs for different treatment options. The long term response support tool has less specific information, as long term response will involve more technical design, but users are led through the pros and cons of engineered treatment options such as debris basins, debris racks, and deflection berms.