AN APPROACH FOR RAPIDLY ASSESSING RELATIVE POST-WILDFIRE DEBRIS-FLOW AND FLOOD HAZARDS FROM RECENTLY BURNED BASINS

Wildfires can increase the susceptibility of burned drainage basins to debris-flow and sediment-laden streamflow hazards. The recent occurrence of extensive wildfires throughout the western U.S., and the enhanced probability of future fires, highlights the need for rapid assessment methodologies for assessing the potential for fire-related debris flow and flooding. Such tools can be useful both in pre-fire planning and post-fire response. Applying models before the occurrence of wildfires can be used to identify sensitive basins, and serve to direct protection efforts during a fire incident. Hazard assessment models can also guide and prioritize post-fire data collection and field mitigation efforts. By combining GIS-platform information with field observations, we have developed a method to quickly rank the relative hazard posed by sediment-laden streamflow and debris flows from burned basins. The methodology relies on data readily available before, during, or immediately after a wildfire, and includes basin area, basin gradient, and extent and severity of the fire within the basin. The basin area and gradient are extracted from DEMs, and the area of each basin burned at varying severities (high, moderate, and low) is obtained either from maps of fire potential or burn severity. A ranking matrix, based on the finding that larger, steeper, more extensively burned basins will produce larger events than smaller, gentler, less extensively burned basins, is used to classify the relative hazards for each basin. Measures of soil erodibility can also be incorporated into the ranking matrix if such data are available. Field observations are used to identify facilities at risk (e.g., buildings, bridges, wells), and other factors that may exacerbate the hazard (e.g., accumulations of mine tailings or other materials in channels). Continuing work focuses on the collection of rainfall and peak-discharge data from recently burned basins to define quantitative relations that can be used to predict peak discharges as a function of rainfall rates and basin characteristics.