CAN WE REDUCE THE IMPACTS OF POST-WILDFIRE FLOODS TO PROTECT DOWNSTREAM COMMUNITIES? EVALUATING THE EFFECTIVENESS OF A NEW MITIGATION STRATEGY ON THE MUSEUM FIRE FLOODPLAIN, FLAGSTAFF, ARIZONA

As wildfires continue to increase in size and severity, communities downstream of burn scars grapple with the increased risk of flooding and debris flows. The 2019 Museum Fire burned the steep slopes of the Spruce Wash Watershed, which drains directly into the downstream community of Flagstaff, AZ. The 2019 and 2020 monsoons following the fire were the driest on record, resulting in minimal post-fire runoff events. The 2021 monsoon, however, brought substantial rainfall to the burn scar, resulting in debris-laden floods that caused extensive damage to the downstream community. Three years of geomorphic monitoring data from the burned headwaters of this watershed show slow soil hydrologic recovery, indicating a continued risk for post-fire floods. In response to the 2021 flow events and a persistent flooding threat, the Coconino County Flood Control District (FCD) and Natural Channel Designs, Inc. employed novel flood mitigation techniques to alluvial surfaces downstream of the burn scar to promote infiltration and sediment deposition on fan surfaces, aimed at reducing flow and the volume of sediment entering the downstream community. The FCD employed the same approach following the 2010 Schultz Fire, however, no data were collected to determine the efficacy of the mitigation effort. To assess the efficacy of this mitigation strategy, we instrumented two treated, and one untreated alluvial surface downstream of our headwaters monitoring sites. Over the 2022 monsoon, we recorded flow discharge and conducted repeat surface-topography measurements after storms to analyze differences in 1) peak flow discharge, 2) suspended sediment load, 3) sediment deposits, and 4) zones of erosion between the treated and untreated fans to assess the performance of the treated fans. Preliminary results from the first storms of the season indicate flood flows fully infiltrated on the treated fans following small (< 25mm) rainstorms. We report on our results from the 2022 monsoon within the context of overall watershed recovery and discuss differences between treated and untreated fan surfaces. Ultimately, our findings may be used by land managers for post-fire flood mitigation on future burn scars.