Yavapai County Pre-Wildfire Assessment of Potential Post-Fire Flows, A FEMA PDM Advanced Assistance Program Study

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Author
Youberg, Ann
Loverich, Joseph
Adamaitis, Chandler
Scragg, Matthew
Affiliation
Arizona Geological Survey
JE Fuller Hydrology and Geomorphology
Issue Date
2025-07
Keywords
Wildfires -- Environmental aspects -- Arizona.
Yavapai County
wildland-urban interface
Flood control -- Arizona.
post-wildfire flood
debris flows
burn severity

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Citation
Youberg, A., Loverich, J. Adamaitis, C., and Scragg, M., 2025. Yavapai County Pre-Wildfire Assessment of Potential Post-Fire Flows, A FEMA PDM Advanced Assistance Program Study. Arizona Geological Survey, Open File Report OFR-25-6, 35 p.
Publisher
Arizona Geological Survey (Tucson, AZ)
Description
As wildfires continue to increase in size and severity across the western U.S. and development continues into the wildland-urban interface, local agencies such as flood control districts and public works departments need information to plan for and reduce the impacts from wildfires and the aftermaths of fires. To help identify and reduce the risks from impacts of post-wildfire flooding and debris flows in Yavapai County (the County), we conducted a multi-phase study to identify, prior to a wildfire, potential post-wildfire flooding and debris-flow hazard areas. The purpose of the project is to help the County and municipalities understand which areas are potentially at risk from the impacts of post-fire flood and debris flows and provide data for identifying and planning long-term flood control measures and mitigation needs, forest health initiatives, emergency management planning, and community planning efforts. The goals of this overall study were to 1) conduct a countywide reconnaissance-level assessment to identify vulnerable areas at greater risks of impacts from post-wildfire flooding and debris flows, 2) quantify those risks in two smaller study areas, and 3) identify targeted pre-disaster mitigation opportunities and strategies to reduce risks from post-fire floods and debris flows. The countywide GIS analysis (Task 1, Appendix A) identified 33 hydrologic unit code (HUC 12) watersheds with at least one built asset class (critical facilities, dams, buildings, and transportation infrastructure) as potentially threatened by the impacts of post-fire flows. Of these 33 watersheds, eight vulnerable areas (VA) were delineated from portions of six of those 33 HUC 12 watersheds, based on development within or below steep forested slopes and a working knowledge of the locations of previous flood studies. Two study areas, Lynx Creek and Cottonwood, were selected for additional analyses to assess potential post-fire flooding and debris-flow hazards (Tasks 2 and 3). Simulated soil burn severity data that represents a given fire scenario (e.g., high frequency, moderate severity or low frequency, high severity) is needed to evaluate potential post-fire debris-flow and flood hazards. The Prescott National Forest modeled a high-severity fire scenario across the two study areas, and these data were used to generate simulated soil burn severity maps. Flood hazards were evaluated by estimating discharges for the 10- and 100-year design storms under current conditions and then under burned conditions using the simulated soil burn severity data. Results show that during the 10- year storm, the post-fire discharges are estimated to be approximately 4 times larger than the pre-fire discharges in the Lynx Creek area and approximately 2 times larger in the Cottonwood area. During the 100-year storm, the post-fire discharges are estimated to be 2-3 times larger in the Lynx Creek area and about 2 times larger in the Cottonwood area. Additionally, the 10-year post-fire discharge results are generally similar to the 100-year, pre-fire results in both modeling domains. Post-fire debris-flow likelihood and volumes were also estimated using the simulated soil burn severity data for the 1-, 2- and 5-year design storms. Modeled debris-flow volumes were then used to estimate probabilities of debris-flow inundation for peak-flow depths (maximum thickness of flow) of 10 cm and 50 cm for the 2- and 5-year design storms. Results show that there is a high likelihood of debris flows in all watersheds within the two study areas for all design storms, given the simulated soil burn severity data. The variability of estimated debris-flow volumes from different watersheds in each study, however, provides information with which to assess relative hazards between basins. Results from the debris-flow routing and inundation modeling indicate that the 10 cm and 50 cm peak-flow depths are iii likely limited in extent, and they also indicate where flows may travel which, when used in conjunction with the volume data, can help to identify and prioritize areas for mitigation efforts. Finally, specific mitigation alternatives were developed for the unincorporated Lynx Creek study area and data that can be used to update the County hazard mitigation plan (HMP) were identified (Task 5). This pre-fire assessment is not intended to identify probabilities, magnitudes, or extents of any hydrogeologic event but rather to identify areas vulnerable to post-fire flooding and debris flows. We used the best available regional data at the time the analysis was conducted for this project. The data and results presented here are static information and provide a snapshot in time of conditions across the County based on those best-available data.
URI
http://hdl.handle.net/10150/677764
Additional Links
https://library.azgs.arizona.edu/
Language
en
Series/Report no.
Arizona Geological Survey Open File Report
Rights
Arizona Geological Survey. All rights reserved.
Collection Information
Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
North Bounding Coordinate
35.523
South Bounding Coordinate
33.856
West Bounding Coordinate
-113.356
East Bounding Coordinate
-111.445
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