After a wildfire, how intense does rainfall need to be to cause a debris flow?

Ann Youberg (UArizona Arizona Geological Survey) and Luke McGuire (UArizona Dept of Geosciences), along with their partner Francis Rengers (US Geological Survey), study how wildfires throughout the Southwest, such as the Bighorn Fire in the Santa Catalina Mountains, Tucson, Arizona, impacts geologic hazards, specifically post-wildfire debris flows.

Debris flows are mixtures of water, soil, and rock (think flowing concrete) that behave different from floods. Because debris flows are a thick slurry and can carry large boulders and trees, they can generate high impact forces which may damage buildings and infrastructure and, more importantly, pose significant threat to human life and safety. In recently burned landscapes, there is frequently a rapid switch from floods to debris flows once a critical rainfall intensity is exceeded. One of the of questions our research addresses is how intense does rainfall need to be to cause a debris flow in the Southwest?

To answer that question, we monitor rainfall rates and debris flow activity within recent burn scars throughout the Southwest. Most recently, we set up a monitoring site at the Tadpole Fire burn scar (Figure 1) in the Gila National Forest north of Silver City, NM.

During the second week of July, we installed equipment in four watersheds on the north side of Tadpole Ridge (Figure 1) with in the Tadpole Fire burn scar. We installed non-vented pressure transducers in four bedrock channels (Figure 2A); these allow us to capture the timing and type of flow (flood/debris flow). In a different watershed, we installed a USGS geophone (Figure 2B) that allows us to not only capture timing and type of flow, but also allows us to calculate flow velocity.

Above the pressure transducer in one of the watersheds, we also installed a rain-triggered camera (Figure 3) to capture the flows on video. Finally, in addition to the rain gauge on the geophone and on the camera, we installed a third rain gauge much higher in the watershed to capture rainfall information in areas where debris flows are likely to start. Throughout the monsoon, we will return to the study site after significant rain events to download our data and to assess deposits to field verify if a flood or a debris flow occurred.

We also have active debris flow monitoring sites in the Superstition Mountains, the Pinal Mountains, and the Tularosa Mountains. Findings from these studies will help us understand how fires like the Bighorn Fire in the Santa Catalina Mountains will respond to monsoon rainstorms. Results will inform USGS post-fire debris-flow models (https://landslides.usgs.gov/hazards/postfire_debrisflow/), and provide information for warning thresholds to the National Weather Service (NWS) and partner state and local agencies such as county flood control districts.

Acknowledgments. Support for this work is provided by the Federal Emergency Management Agency (FEMA) and the Arizona Department of Emergency and Military Affairs (DEMA) through the Hazard Mitigation Grant Program (HMGP), and by the National Oceanic and Atmospheric Association (NOAA) National Integrated Drought Information System (NIDIS) program. We thank the Gila National Forest and the Silver City Ranger District for their assistance in setting up the Tadpole Fire monitoring site.

Contributed by Ann Youberg (UArizona Arizona Geological Survey) and Luke McGuire (UArizona Dept. of Geosciences)

Cover figure. Perimeter of Tadpole Fire as of 29 June 2020 from infrared data (red line).