Estimating the annual probability of post-fire debris flow in southern California
Abstract
Southern California has an elevated risk from post-fire debris flows due to the combination of high fire frequency, steep topography, dense population, and recurrent atmospheric rivers delivering intense rainfall. As a step towards quantifying this risk, we address the question: What is the annual probability of post-fire debris flow in southern California? We ask this question for the region (as a whole) and for 1 km cells throughout the area to characterize the spatial distribution of susceptibility. Additionally, we evaluate the probability of large magnitude debris-flow events, such as the one on 9 January 2018 in Montecito, California, which killed 23 people and damaged or destroyed over 400 homes. For a given drainage, the annual probability of post-fire debris flow can be determined by the intersection of two events: the drainage basin burns, and the rainfall intensity (I) after the fire exceeds the triggering threshold (T) for debris flow. We evaluate the probability of fire across southern California using a historical database of fire perimeters from 1950 to 2018. We evaluate the probability of threshold-exceeding rainfall using a two-step process. First, we estimate the local rainfall intensity threshold using an established empirical model for debris-flow likelihood. This model is a function of basin slope (derived from a 10 m digital elevation model), soil characteristics (obtained from a general soil map of the U.S.), and burn severity (estimated from historical data on burn severity for the local vegetation types). We then use NOAA precipitation-frequency Atlas 14 to evaluate the probability that I > T and I > 3T, with the later criteria representative of the triggering conditions for large magnitude events. Preliminary results suggest that at least small debris flows can be expected nearly every year because of frequent fires and low thresholds. Results depicting the spatial distribution of debris-flow probability across southern California can be used to identify priority areas for more detailed pre-fire hazard assessment and/or mitigation. Additional work is needed to extend the analysis to examine the sensitivity of debris-flow susceptibility to possible future changes in fire size/severity and precipitation frequency.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMNH33E0968K
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 4306 Multihazards;
- NATURAL HAZARDS;
- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY