How Often Do Rainstorms Cause Debris Flows in Burned Areas of the Southwestern U.S.?

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Debris flows, sometimes referred to as mudslides, mudflows, lahars, or debris avalanches, are common types of fast-moving landslides. They usually start on steep hillsides as a result of shallow landslides, or from runoff and erosion that liquefy and accelerate to speeds in excess of 35 mi/h. The consistency of debris flows ranges from thin, watery to thick, rocky mud that can carry large items such as boulders, trees, and cars. They are among the most numerous types of landslides in the world and are particularly dangerous to life and property because of their high speeds and the sheer destructive force of their flow.

mud, large boulders, and overturned FTE next to house

Debris flow damage following the 2012 Waldo Canyon fire, Colorado, U.S. (Credit: Dennis Staley, U.S. Geological Survey. Public domain.)

In the southwestern U.S., wildfires and short periods of intense rainfall are both common occurrences. In unburned areas, debris flows are more commonly caused by long-lasting rainstorms or rainfall in areas that are already soaked by previous storms. In burned areas, short bursts of heavy rain over steep terrain can produce debris flows more so than in unburned areas due to changes in ground surface properties from the intense heat of fires. Issuing debris flow warnings to potentially impacted areas “downstream” from burned areas is especially difficult because:

  1. there is often little time between the start of the rain and the initiation of the debris flows,
  2. when a rainstorm occurs soon after a wildfire, time may be too short to establish and implement plans to reduce the risk, and
  3. predicting the time, location and intensity of brief intense rainfalls is very difficult.
graph with line

Best linear (straight-line) fit used to calculate the 3.9-year recurrence interval (RI) of the observed rainfall intensity for the storm that produced a debris flow on 10 July 2013 in the area burned by the 2012 Waldo Canyon fire, Colorado, U.S. (modified from Staley et al., Public domain.)

“Despite nearly two decades of research expanding our understanding of post-fire debris-flow mechanics and triggering conditions and improving upon our methods of forecasting and early warning, the emergency management community and public are often unprepared to address the risk-reduction challenges unique to this type of hazard,” wrote USGS authors Staley, Kean, and Rengers in the paper summarized here. “Improved knowledge of the recurrence interval (RI) of debris-flow-producing rainstorms would help emergency managers assess the likelihood of impacts to downstream communities; however, to date, there has not been a thorough analysis of the RI of the rainfall events that correspond with post-fire debris-flow initiation.”

In their study, Staley, Kean, and Rengers determined the recurrence interval (RI, the time between occurrences) of the brief heavy rainstorms that triggered debris flows in previously burned areas. The scientists looked at 316 debris flows that occurred in 18 burn areas across the southwestern U.S. during 2000-2014. These locations were throughout 5 states (California, Arizona, New Mexico, Colorado, and Utah) and in 7 different climate types. For each debris flow they used the measured amount of rainfall at the nearest rain gage to obtain the rainfall intensity for the 3 durations (15-, 30-, and 60 min). Using a National Oceanic and Atmospheric Administration (NOAA) RI database, the scientists constructed graphs of RI versus rainfall intensity at each location. They then used the graphs to estimate each site’s RI by tracing where the observed rainfall intensity values measured for the 3 durations intersected on the graph’s line.

histograms

Recurrence interval (RI) distributions corresponding to all debris flow observations in the southwestern United States for (A) 15-, (B) 30- and (C) 60-minute rainfall intensities. The x-axis is the estimated recurrence interval (RI) between rainfalls of a particular intensity, and the y-axis is the number of debris flows that were caused by that particular rainfall intensity within (A) 15, (B) 30 and (C) 60 minutes. (Public domain.)

The results showed differences in debris flow occurrence and RI across the region’s climate types, between states, and within each state. Most importantly, the researchers found that 77% of the observed debris flows were triggered by rainfall intensities with RI’s of less than 2 years. This means that post-fire debris flows can be expected during a range of rainstorm intensities in the southwestern U.S., both rare extreme storms and more frequent heavy rainfall scenarios. The four debris flow events mentioned at the beginning of this article (2003 San Bernardino, CA; 2010 San Gabriel Mountains, CA; 2012 Manitou Springs, CO; and 2018 Montecito, CA) were caused by both short- and long-RI rainfalls, illustrating examples of impactful events in both categories. Emergency management plans and risk mitigation strategies must consider both scenarios for debris-flow hazard mitigation, and improvements are needed to anticipate potential risks to infrastructure downslope of potential debris flows.

Landslide scientists would like to be able to forecast the chances of an impactful debris flow in burned areas using weather data and weather forecasts so that emergency managers, first responders, and those potentially in harm’s way can prepare for and respond appropriately to the event. Additional research will also further investigate the physical mechanisms that initiate debris flows to better understand their growth and how far they can travel. Research in this area is more important than ever since potentially longer fire seasons and more intense rainfalls due to climate change as well as population growth into hazardous areas may make these risks more severe in the future.

- written by Lisa Wald, USGS, February 17, 2021

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The Scientists Behind the Science

person sitting under a palm tree in dry area

Dennis Staley (Public domain.)

Dennis Staley is a research physical scientist with the USGS Landslide Hazards Program. His research primarily focuses on post-fire debris-flow initiation, size, and early warning. In his spare time, he rides long distances on his bicycle and was the Bike Champion of the 2018 Iditarod Trail Invitational 130.

person on steep slope with monitoring instrument above

Jason Kean (Public domain.)

Jason Kean has worked at the USGS in Golden, CO since 2007, where he focuses on understanding debris-flow processes and hazards after wildfire. Jason's favorite part of the job is setting up monitoring stations to "catch" debris flows in action to learn how they start and how fast they travel. When he’s not working, Jason likes to ride his bike, and he tries to keep up with his kids on the ski slopes.

person with sunglasses on and snowy slope in background

Francis Rengers (Public domain.)

Francis Rengers is a geologist who has been with the USGS for 6 years.  Francis uses field observations, remote sensing (e.g., lidar and structure-from-motion photogrammetry), and modeling to help understand debris flow hazards in burn areas in the Western U.S. In his free time, Francis enjoys trail running and reading Asterix and Obelix comic books.