RAINFALL THRESHOLDS FOR POST-FIRE DEBRIS FLOW GENERATION, WESTERN SIERRA NEVADA, CA

Wildfires have become more frequent, intense, and widespread in the western United States in the era of global climate change. Concurrently, development along wildland-urban interfaces has increased. Thus, quantifying post-fire hazards, such as debris flows and their triggering rainfall conditions is critical for effective emergency response. We monitored the hydrologic and geomorphic response of 10 recently burned watersheds in the area impacted by the 2018 Ferguson fire and the 2019 Briceburg fires in the lower Merced River Canyon near Yosemite National Park. During our monitoring, which spanned the period between November 2018 and May 2020, our study basins produced 26 debris flows in response to 60 rainstorms. The corresponding peak rainfall intensity data were used to establish empirically-derived rainfall-intensity-duration thresholds for debris-flow initiation for these burned areas. First-year data reveals objectively-defined 15-min-duration rainfall-intensity thresholds of approximately 31 and 35 mm/h for the Ferguson and Briceburg fires respectively. Given the small sample size and a bimodal distribution of 15-min-duration peak rainfall intensities observed, the objectively-defined threshold may be only partially constrained. When compared to modeled estimates of spatially-explicit thresholds, the objectively-defined debris flow thresholds in our analysis were higher. When compared to empirically-derived thresholds from other regions our empirical values fell in the middle of the range. This study suggests that model refinement or development of a regionally-specific model may be warranted, to provide more accurate information for use in emergency management decisions, potentially reducing unnecessary impacts to the public.