In the aftermath of the recent Los Angeles wildfires, a new danger is emerging that could have devastating consequences for local communities. Debris flows, often triggered by precipitation in areas ravaged by fires, pose a significant threat. Scientists and engineers from federal and state agencies have been surveying the burned regions to assess this looming hazard. Their observations have already been fed into U.S. Geological Survey (USGS) computer programs to create hazard maps, pinpointing areas most at risk.
Unlike typical flooding, debris flows are catastrophic torrents of mud, water, and debris that can reach speeds of up to 56 kilometers per hour (35 miles per hour). These flows can travel vast distances through valleys and canyons, carrying with them sediment, burned trees, cars, and even boulders. The devastation they can bring is illustrated by the January 2018 incident in Montecito, California, where debris flows killed 23 people and destroyed more than 100 homes after the Thomas fire.
The Los Angeles wildfires were still ablaze when teams began their crucial work. By mid-January, scientists had started examining the charred landscapes left by the Palisades and Eaton fires. They sought to identify potential sites for debris flows, guided by satellite images and on-the-ground surveys. Wearing protective heat- and flame-resistant gear, these teams confirmed vegetation losses and sampled soils to better understand the damage.
Jeremy Lancaster, one of the scientists, highlighted the urgency of their work.
“It literally rained four days into our deployment,” – Jeremy Lancaster
This swift onset of rain underscores the importance of rapid response during the wet season.
The Palisades burn area features gently sloping terrain with watersheds that flow to the ocean, posing a lesser risk compared to the Eaton fire's burn scar. The latter's topography suggests a greater likelihood of debris flows. To mitigate these risks, debris basins and dams in strategic locations can provide some defense by catching debris before it reaches populated areas.
The National Weather Service (NWS) plays a critical role in alerting residents about potential debris flows. They may issue a watch alert to mobile devices 12 to 36 hours before an incoming storm if predicted rainfall could trigger such events. However, as Jayme Laber from the NWS explains, timely warnings may not always be guaranteed.
“Don’t wait for the warning because … it could be zero minutes of lead time,” – Jayme Laber
Residents are advised to take early precautions during a watch alert period.
“The watch is probably the best time to start taking your actions of being prepared.” – Jayme Laber
The unpredictability of debris flows makes them particularly dangerous. Unlike flooding, sandbags cannot prevent their progression.
“This isn’t like flooding; you don’t put up sandbags to prevent things from happening,” – Jaime Kostelnik
These hazardous flows can remain a threat for two to five years post-fire, depending on rainfall patterns. Past experiences have shown that rainfall thresholds can induce significant soil movement and erosion.
“These rainfall thresholds were developed based on lots of field work and observations of the types of rain intensities that caused or triggered soil movement and erosion, and which turned into debris flows,” – Jayme Laber
The complex interplay between fire damage and precipitation necessitates rapid deployment of scientific teams during vulnerable periods.
“We assume that it’s going to rain soon because we’re in the middle of wet season, so we deploy rapidly.” – Jeremy Lancaster
For residents impacted by the fires, the possibility of evacuation due to debris flows adds another layer of distress.
“Either you’ve lost your home or you’re just returning to your home after being evacuated from the fires, and then here you’re being told to evacuate again,” – Jayme Laber
The dual threat of fire followed by debris flows illustrates the cascading nature of natural disasters in regions like Los Angeles. As communities recover from the initial impact of wildfires, they must remain vigilant against this subsequent threat.
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