BEFORE THE WILDFIRE: ASSESSING POST-FIRE DEBRIS FLOW PROBABILITIES AND POTENTIAL INUNDATIONS ZONES FOR PRE-FIRE MITIGATION EFFORTS, A CASE STUDY FROM COCONINO COUNTY, ARIZONA

Hotter and larger wildfires across the western U.S. are adversely impacting downstream communities. The Schultz Fire severely burned 6,100 ha of U.S. Forest Service lands in Coconino County, Arizona, in June 2010. Rainfall from an above-average monsoon resulted in debris flows on forest land and sediment-laden floods downstream causing extensive damage to homes and infrastructure, one fatality, and an economic impact estimated at between $133 million and $147 million. In 2016, Coconino County Public Works partnered with FEMA, JE Fuller Hydrology and Geomorphology, Inc., and the Arizona Geological Survey (AZGS) to assess post-fire debris-flow and flooding risks from a reasonable scenario wildfire under current and treated forest conditions in two pilot study areas, Fort Valley and the City of Williams (AZGS OFR-17-06). The goal of the study was to identify at-risk areas and potential mitigation measures to reduce risks. Here, we report on the debris-flow modeling component of the project.

Areas potentially impacted by post-wildfire debris flows were assessed through reconnaissance field investigations and debris flow modeling. Three models were employed: (1) a 2016 U.S. Geological Survey (USGS) regression equation of debris-flow probability for the western U.S.; (2) a 2014 USGS debris-flow volume model; and (3) the USGS Laharz inundation model, modified for Arizona. While the 2016 probability model was developed with data from Arizona, the debris-flow volume model and Laharz required an evaluation of predicted results using geomorphic data of field-mapped Schultz Fire flow deposits. The debris-flow volume model over estimated volumes of smaller mapped deposits but estimated larger deposits well. Laharz predicted inundation zones that reasonably reflected mapped debris-flow deposits in the Schultz Fire burned area. The pilot study areas were then assessed using all three models.

The probabilities of post-fire debris flow occurrence were high and moderately-high under current and treated forest conditions. Modeled inundation zones were limited to undeveloped areas in Fort Valley but encroached into developed areas within the City of Williams. Results from this assessment allows time for Coconino County to select and implement mitigation measures in a more measured and cost-effective manner.