RAINFALL INTENSITY-DURATION THRESHOLDS FOR POST-WILDFIRE DEBRIS FLOWS IN ARIZONA

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in runoff. On steep slopes, this can promote the formation of runoff-generated debris flows. The initiation of runoff-generated debris flows has been closely linked to rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows in Arizona, which differs from both a climatic and hydro-geomorphic perspective from many other areas in the western U.S. where rainfall ID thresholds are more established. To help establish more precise rainfall ID thresholds in Arizona, 5 drainage basins burned during the 2017 Pinal fire were instrumented with rain gauges and pressure sensors at the channel outlets to monitor rainfall intensity and debris flow activity. Eight debris flows occurred during the subsequent monsoon season and a number of rainstorms led to runoff with no clear signs of debris flow activity. For each of the eight debris flow events, the rainfall intensity required to trigger the debris flow was estimated as the intensity (averaged over a given duration) at the time of peak recorded pressure within the channel. A receiver operator characteristic analysis suggests that rainfall intensities of more than 56 mm/hr over a 15-minute interval tend to lead to debris flow initiation. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, about 20 mm/hr lower than ID thresholds derived under the assumption that the triggering rainfall intensity is equal to the maximum rainfall intensity recorded during a rainstorm. A rainfall ID threshold based on a duration of 15 minutes performed best in predicting debris flow initiation, which is consistent with other studies in the western U.S., suggesting that they are the best predictors for post-fire debris flow occurrence in Arizona. In Arizona, wildfire season is usually immediately followed by intense monsoonal rains, requiring rapid debris-flow hazard assessments. Results of this study provide improved estimates of the rainfall intensities and durations required to trigger post-wildfire debris flows in Arizona, which will help improve future hazard assessments throughout Arizona and in similar geologic settings.