RECENT ADVANCES IN THE IDENTIFICATION OF RAINFALL CONDITIONS THAT RESULT IN POST-FIRE DEBRIS FLOWS

The combination of population growth into wildfire-susceptible terrain and increased wildfire occurrence and size throughout the intermountain western USA and southern California leads to ever-increasing risk from post-fire floods and debris flows. Analyses of monitoring data from recently burned areas are used to improve methods for defining the rainfall conditions that result in post-fire debris flows and increase the accuracy and precision of these definitions.

The simplest description of rainfall conditions that initiate post-fire debris flows are storm rainfall totals. These totals are often measured over 12- or 24- hour periods, thus reflecting a lack of precision. Observations that debris flows occur during periods of high rainfall, coupled with the ability to deploy multiple tipping bucket rain gages through recently burned areas, led to evaluation of the effects of periods of peak rainfall on debris-flow occurrence. This effort defined rainfall intensity-duration thresholds for debris-flow occurrence for durations between 1 and 20 hours. Further, recent development of affordable and resilient instrumentation to document the timing of debris-flow passage during storms has advanced our understanding of triggering rainfall conditions. Comparison of debris-flow timing and rain gage data indicates that debris flow occurrence rarely correspond to measures of peak storm rainfall with durations greater than 60 minutes. Instead, these rainfall intensity peaks occur well after the passage of the flow. These findings indicate that the accuracy of rainfall thresholds can be improved if measures of rainfall within the 60 minutes preceding debris flows are used in their definition. In addition, the recognition that brief (<30 min), intense rainfall triggers most post-fire debris flows indicates that such measures of storm rainfall can thus be used to increase the precision of definitions of the rainfall conditions that will lead to post-fire debris flows. The instrumentation and analyses described above can provide accurate and precise definitions of triggering rainfall conditions in different settings and that should be used in the generation of post-fire debris flow hazard maps, and can be used to examine changes in triggering rainfall conditions over time.