RAINFALL TRIGGERING OF POST-FIRE DEBRIS FLOWS OVER A 28-YEAR PERIOD NEAR EL PORTAL, CALIFORNIA, USA

Wildfires frequently affect the steep hillslopes in the vicinity of El Portal, California, USA, a small community established during the California Gold Rush in the mid-1800s. In addition to El Portal’s historical significance, California State Route (SR) 140 connects El Portal to the San Joaquin Valley and is a major transportation and economic corridor to Yosemite National Park (YNP). In 2019, an estimated 4.5 million tourists visited and accessed YNP via SR140. In the years following wildfires, the burned watersheds above the valley produced debris flows during periods of intense rainfall. These debris flows impacted the community of El Portal and motorists traveling on SR 140 and the local road network. The steep slopes and confinement of the valley limits potential options for mitigating the risk of debris flows to travelers and local homes and businesses. Because of the physical setting, emergency management is left with evacuation orders or temporary road closures as the best option for risk reduction. The long-term effectiveness of evacuation and temporary road closure is highly dependent on establishing a reliable local rainfall intensity-duration threshold that officials can used to guide emergency response actions and timing. Here, we present an analysis of the rainfall conditions that were associated with initiation of nine post-fire debris-flow events in the vicinity of El Portal from 1990 to 2018. Our results highlight the modest rainfall rates that triggered debris flows in the steep watersheds near El Portal, with recurrence intervals typically less than 1-year. Furthermore, radar data from more recent events (2012 – 2018) highlight the spatial variability of intense rainfall in the area, complicating weather forecasting and monitoring for early warning purposes. Further research and expanded rainfall monitoring are needed to provide a robust rainfall threshold that will effectively mitigate risk for residents and motorists, while minimizing the impact of road closures and evacuations. NOTE: this work will be presented by the co-authors in memory of Jerome (Jerry) De Graff, who passed away on 24 March 2020.