THE RECURRENCE INTERVAL OF POST-FIRE DEBRIS-FLOW GENERATING RAINFALL IN THE SOUTHWESTERN UNITED STATES

In the southwestern United States, post-fire debris flows commonly initiate during short bursts of intense rainfall. To date, the return period of these rainfall rates has not been quantified. Here, we combine an existing database of debris-flow occurrences and corresponding peak storm intensities with a geospatial library of rainfall recurrence interval (RI) information and climate type to determine the distribution of the estimated return period of the rainfall associated with 316 observed post-fire debris flows in the southwestern United States. Our results indicate that a majority (77%) of the observed debris flows were triggered by rainfall intensities with a RI less than 2 years. Climatic and geographic differences in RI were evident in our analysis. Debris flows in six of the analyzed climates within California, Colorado, and New Mexico were primarily associated with 1-year or less RI intensities (76%), whereas 89% of the observed debris flows in arid cold steppe climate type of Arizona, Colorado, and Utah were generated during storms greater than 2-year RI. Event consequence, as defined by the impact on downstream communities and infrastructure, was not directly related to RI, as very destructive debris flows were initiated at a wide range of RI intensities (0.9 - > 1000 years). Further research is needed into the hydrologic, vegetation, and geomorphic differences between the arid steppe climate to the other six types included in this analysis to identify any causal mechanisms to explain the potential need for higher rainfall rates required to initiate post-fire debris flows. Our results also highlight that post-fire debris-flow initiation can be expected during common rainfall intensities in the southwestern United States; therefore, emergency management plans and risk mitigation efforts must focus on both extreme and frequent rainstorms.