PROGRESS IN QUANTIFYING DEBRIS FLOW RISK FOR POST-WILDFIRE EMERGENCY RESPONSE

Floods, rockfalls, and debris flows can be postfire responses to storms in burned watersheds. The ability of debris flows to travel rapidly over significant distances from their point of initiation and their destructive force are a special concern. Individuals and organizations responsible for infrastructure, property, and public safety along the potential path of postfire debris flows must understand the risk posed in order to take effective mitigating measures. In the mountainous western US, better risk assessment is crucial because of the expanding wildland-urban interface and more frequent large wildfires. So it is fortunate that during the last 30 years, advances in our scientific understanding of the physical process, technological improvements in mapping fire effects, and development of empirical models to assess probability, volume, and inundation areas have improved quantification of debris flow risk. This progress is exemplified by comparing the debris flow hazard assessment for two wildfires in the Sierra Nevada of central California. During assessment of the 1987 Stanislaus Complex fire in the Tuolumne River canyon, potential debris flow paths were identified based on professional judgment. Assessment focused on locations up gradient from values-at-risk (i.e., structures, roads, etc.) where evidence of past infiltration-initiated debris flows coincided with high soil burn severity. Visual delineation of polygons during helicopter overflight with later field testing for soil hydrophobicity produced the soil burn severity map. For the 2011 Motor fire in the Merced River canyon, an empirical model was applied to all the affected watersheds to identify the probability and likely volume of postfire debris flows. The model reflects the understanding that postfire debris flows are nearly always due to runoff-initiation. Consequently, predictions are based on a design storm and multiple physical watershed characteristics including soil burn severity. Interpretation of pre- and post-fire spectral reflectance in satellite imagery provided a detailed soil burn severity map. Polygons were verified or adjusted based on field testing of soil hydrophobicity. The watersheds with higher probabilities were compared to the location of values-at-risk to identify potential mitigation needs.