Portland, Ore., USA: Over the past few years, wildfires have ripped through forests around the world with no signs of slowing down. The devastation of a wildfire can be severe on its own, but there are additional hazards to consider after the flames have been extinguished.
Debris flows are fast-moving slurries of soil, rock, and water that can seemingly form out of nowhere. They race down slopes, carrying and engulfing anything in their path. Bare, fire-ravaged slopes are a perfect setting for the development of these hazardous flows.
“The biggest issue is any sort of steep area where you can essentially bring large quantities of sediment down slope,” says Eli Orland, an associate scientist at the Universities Space Research Association (USRA) in NASA Goddard Space Flight Center’s Hydrological Sciences Laboratory. Orland is a coauthor of a new study to be presented on Tuesday at the Geological Society of America’s annual meeting. “In this case, gravity is not exactly your friend,” he adds.
In the U.S., post-wildfire debris flow hazards are modeled and monitored by the U.S. Geological Survey (USGS) and affiliate organizations like the U.S. Forest Service (USFS) Burn Area Emergency Response Team (BAER). These scientists address conditions on the ground after a fire to identify areas that may be prone to additional damage from flooding—including debris flows.
“[The USGS] and USFS are the foremost organizations in the United States for post-wildfire hazard response,” says Orland. Scientists and emergency responders look to rapidly identify areas of concern for post-fire impacts, but these site surveys and field-based data collection are time consuming. Orland points out that there are many areas around the world that do not have the resources or capability to quickly assess post-wildfire conditions. The NASA team wanted to develop new information that these communities may use to change that.
The researchers are aiming to see if remote sensing data could be used identify areas of debris flow hazards in regions experiencing wildfires—all without setting boots on unstable ground. The team created a model to assess post-fire debris hazards, using freely available data that captured rainfall intensities, burn severity, and topography.
Precipitation data was collected from NASA’s Global Precipitation Measurement (GPM) mission, which incorporates infrared and passive microwave data from several satellites to provide measurements of precipitation intensity around the globe, every half hour—a process that Orland says occurs “in near real-time.” Burn severity was determined by satellite imagery from the Landsat (NASA/USGS) and Sentinel (European Space Agency) programs, and steep slopes are identified from digital elevation model data from NASA’s Shuttle Radar Topography Mission.
Using this satellite data, the team creates a model that identifies areas at high risk of debris flows after a fire. They rely on the detailed, ground-based data collected by USGS to further refine and train their model, using machine learning technologies.
“There are so many different ways that artificial intelligence and machine learning are used today to understand and approximate hydrologic processes and hazards,” said Dalia Kirschbaum, chief of the Hydrological Sciences Laboratory at NASA Goddard and a co-author of the study. “One of the things that’s actually quite limited in post-fire debris flow work is the availability of event inventories on a global scale that allow us to establish these relationships,” she explained, adding that it was critical to use the information collected by the USGS, USFS, and their partners for effective training.
When they tested their model against data from several burned areas surveyed by USGS to identify the potential for post-fire debris flows, they found that short-duration, high-intensity rainfall in the GPM record is most frequently the trigger for debris flows—an observation consistent with previous research, but the first evidence that these relationships are discernable from satellite-derived global precipitation data. “The satellite data really shows promise in being able to resolve the post-fire debris flow hazards at a larger watershed level,” she notes.
Kirschbaum says that their work currently provides a big picture overview of hazards for a region. She hopes that in future models, the team can tease out details about not only where debris flows originate, but where they might flow—an important factor in hazard mitigation.
The researchers note that work shouldn’t replace detailed, ground-based investigations. Kirschbaum says their goal is to provide near–real-time information and tools for emergency responders around the world to better prepare for potential hazards after a wildfire.
Elijah Orland, Universities Space Research Association, NASA Goddard Space Flight Center, firstname.lastname@example.org
Paper No. 148-6: Real-Time Assessment of Post-Fire Debris Flow Hazards Using Globally Available Data
Tuesday, 12 Oct., 9:50 a.m. PDT
Session 148: T134. Advances in Wildfire-Related Earth-Surface Processes I: https://gsa.confex.com/gsa/2021AM/meetingapp.cgi/Session/51581
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