Project period: 8 21 23-8 20 25PROJECT title: forecasting postfire debris-flow hazards in the southwestern uswildfires are becoming larger and more severe throughout the southwestern US

This Cooperative Agreement award from the U.S. Geological Survey (USGS) under the Research and Data Collection program (CFDA 15.808) provides $140,000 to the University of Arizona from August 21, 2023 to August 20, 2025. The purpose is to improve forecasting of post-wildfire debris flow hazards in the Southwestern U.S., especially in Arizona. Key activities include:

• Collecting data from a recent post-fire debris flow event in Arizona to improve numerical models for forecasting debris flow hazards, such as debris flow size and potential impacts.
• Developing the ability to rapidly assess debris flow hazards across burned watersheds, delineate inundation zones, and provide rainfall thresholds for debris flow initiation to support National Weather Service warning systems.
• Organizing a workshop to promote collaboration and share expertise among federal scientists, academic researchers, and practitioners on post-fire debris flow hazards and resource management decisions.

The results will be disseminated through academic conferences and scientific publications. This work aims to provide timely and reliable information to resource managers as wildfire activity increases across the Western U.S.

Severe wildfires decrease vegetation cover and reduce the ability of soil to infiltrate water. In steep landscapes, the resulting increases in runoff and erosion can promote the formation of debris flows, or rapidly moving slurries of water, MUD, and rock fragments. Debris flows can travel for many kilometers downstream of burned areas and exert substantial impact forces on objects in their path. Burned landscapes are most susceptible to debris flows in the first several months following fire, but an increase in debris flow hazard may persist for several years. A primary goal of this project is to improve our ability to forecast debris flow hazards following fire, especially in arizona, including delineating zones downstream of burned areas that are likely to be inundated if a debris flow occurs. We will collect data from a recent postfire debris flow event in arizona to improve numerical models that are critical for forecasting debris flow hazards, including the size of potential debris flows and what they might impact. Data collection will focus, in particular, on information needed to improve and test debris flow runout models. This project will improve the ability to rapidly assess debris flow hazards across numerous watersheds within a burned area, delineate zones downstream of a burned area that are likely to be to inundated if a debris flow occurs, and provide estimates of rainfall thresholds for debris-flow initiation that the national weather service can use for issuing warnings. This project will also promote collaboration and sharing of expertise among federal scientists, academic researchers, and practitioners by organizing a workshop focused on postfire debris flow hazards. One goal of the workshop will be to identify questions that are most critical for helping to inform postfire resource management decisions. Providing timely and reliable information about postfire debris flow hazards to resource managers is becoming increasingly important throughout the western US as many regions experience increases in fire activity. Results of the project will be disseminated through presentations at academic conferences and publications in scientific journals.