MUDSLIDESWhat Causes Mudslides and Floods After Wildfires? Hint: It’s Not What Scientists Thought
Researchers believed a waxy coating in burnt soil caused water to run over the ground’s surface. They now find that burnt ground can absorb water, findings that can help them more accurately predict flooding and mudslides after a fire.
Key points
· Scientists once assumed that flooding and mudslides after wildfires were linked to the waxy coating that builds up on charred soil, preventing water absorption.
· USC Dornsife researchers found that water flow came from absorbed water in both burnt and unburnt areas, suggesting that water was, in fact, being absorbed into burnt ground.
· The discovery provides valuable insights into where and when potential flooding and mudslides may occur and how landscapes recover after a wildfire.
In 2020, one of the largest wildfires in Los Angeles County raged across the San Gabriel Mountains, scorching more than 115,000 acres, damaging or destroying over 150 structures and raining ash and smoke down on pandemic-weary Angelenos.
But even after exhausted firefighters had finally snuffed out the flames, the Bobcat Fire — like other so-called “mega-fires” that have become more common due to climate change — carried the potential to wreak more havoc in its wake. As rainstorms deluge burnt areas, flooding, mudslides and debris flow can compound the fire’s damage.
Understanding how water accumulates and monitoring the movement of runoff and streamflow in burn areas helps authorities predict when and where these post-wildfire events might occur so they can provide affected residents with early warning of flash flooding and debris movement.
A Slippery Slope
Common knowledge has long held that loss of vegetation during a fire leaves soil vulnerable to erosion because the plant roots that hold the soil in place wither and die. Scientists, however, have long held a different view, that as leaves burn, their waxy coating forms an organic, oily substance on the soil’s surface. This waxy coating creates a water-repellant layer at or near the surface. Scientists believed this layer prevented the ground from absorbing water, resulting in rapid water runoff akin to a Slip ‘N Slide that carries mud and debris.
New research published in Nature Communications has called that scientific theory into question.
A Watershed Finding
Scientists at the USC Dornsife College of Letters, Arts and Science, in collaboration with researchers from the University of Michigan, the U.S. Geological Survey and Rutgers University, monitored two wet seasons following the Bobcat Fire, from December 2020 to March 2022. The team concluded that water was, in fact, being absorbed by the burnt ground that contained this waxy coating.
Specifically, the team studied three watersheds — areas of land that drain rainfall and snowmelt into streams and rivers — in Southern California’s San Gabriel Mountains. Two of the
