In 2020, one of Los Angeles County’s largest wildfires broke out in the San Gabriel Mountains, burning more than 115,000 acres, damaging or destroying more than 150 structures, and raining ash and smoke on weary Angelenos.
But even as exhausted firefighters finally extinguished the blaze, the Bobcat Fire — like other so-called “mega-fires” that are becoming more common due to climate change — carries the potential to cause more damage to after this. As storms flood burned areas, flooding, mudslides and debris flows can add to fire damage.
Understanding how water accumulates and monitoring the movement of runoff and streamflow in burned areas can help authorities predict when and where these events will occur after a wildfire so they can provide relief to affected residents early. flood warning and garbage movement.
A slippery slope
Common knowledge has long held that the loss of vegetation during a fire makes the soil vulnerable to erosion because the plant roots that hold the soil in place wither and die. Scientists, however, have long had a different view: that as the leaves burn, their waxy coating becomes an organic, oily substance on the earth’s surface.
This waxy coating creates a water-repellant layer on or near the surface. Scientists believe that this layer prevents the soil from absorbing water, resulting in rapid runoff similar to a “Slip ‘N Slide” that carries mud and debris.
New research published in Communication in Nature questioned that scientific theory.
A watershed search
Scientists at the USC Dornsife College of Letters, Arts and Science, in collaboration with researchers from the University of Michigan, the US Geological Survey and Rutgers University, monitored the two rainy seasons following the Bobcat Fire, from December 2020 to March 2022. The team ends. that water, in fact, is absorbed by the burnt earth with this waxy coating.
Specifically, the team studied three watersheds—areas of land that drain rain and snowmelt into streams and rivers—in the San Gabriel Mountains of Southern California. Two of the watersheds burned during the 2020 Bobcat Fire and the other was untouched.
The researchers found that post-wildfire, a large part of the water flow in all three watersheds came from water absorbed by the soil.
Joshua West, professor of Earth sciences who led the study at USC Dornsife, said it was not surprising that the flow of water and debris in the stream in the burned area was four to 10 times greater than the flow before. fire stream in the area. What he didn’t expect was that the storm water seeped into the ground in two burned watersheds.
This finding contradicts what scientists previously believed that little water was absorbed into the burned watershed due to the presence of waxy soils.
In the unburned watershed, however, the researchers found that the trees absorbed the water as expected, preventing it from reaching streams.
West and Ph.D. Candidate Abra Atwood believed that according to the popular idea, the rise of water in the rivers came from the burned areas because the burned trees and plants could not store water in their roots as usual, but not because of lack of soil to absorb water.
The research team’s finding that the water-repellent layer does not prevent water from being absorbed into the ground strengthens their hypothesis that the water in the streams comes from rain and groundwater, which leads to further flooding of the burned areas. area compared to unburnt.
Rising water poses a lasting threat
Identifying high-risk areas for debris flows and mudslides and accurately predicting debris flow rates after rainfall in burned areas depends on understanding how infiltration occurs. of groundwater in different areas and how it contributes to the flow of streams.
Also, the dynamics of water flow and how water accumulates below the surface can affect how quickly landscapes can be repaired after a wildfire. This recovery affects the stability of the hillsides and helps the forests against severe drought.
On the other hand, water accumulation can contribute to landslides up to four years after a fire as pressure builds on the ground.
“The accumulation of groundwater suggests that the potential for landslides will extend beyond two years after the fire, posing a long-term problem,” West said. “The amount of water stored in areas affected by the Bobcat Fire, for example, may serve as an indicator of future flooding concerns in the coming years.”
West is confident that the study’s findings will provide valuable information that the USGS can use to improve burn area monitoring and predict flooding and mudslides after a wildfire.
A. Atwood et al, Importance of subsurface water for hydrological response during storms in post-wildfire bedrock landscapes, Communication in Nature (2023). DOI: 10.1038/s41467-023-39095-z
Provided by the University of Southern California
Citation: What causes mudslides and floods after fires? It’s not what scientists thought (2023, July 17) retrieved on 17 July 2023 from https://phys.org/news/2023-07-mudslides-wildfires-scientists-thought.html
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