UNIVERSITY PARK, Pa. – The mudslides that follow wildfires in Southern California can be deadly and difficult to predict. New research can help officials identify areas prone to these mudslides and respond before disaster occurs, according to scientists.
Mudslides, or debris flows, can occur when rainfall washes away the buildup of sediment in mountain channels. Roughly equal parts water and sediment, debris flows are strong enough to carry large boulders downhill and threaten communities on or near the mountains. The debris flows in January 2018 that hit Montecito, California, killed 23 people and caused hundreds of millions of dollars in damage. Authorities attributed the mudslides to the wildfires that swept through the area the previous month.
Vegetation holds back the sediment in these steep landscapes, but as the vegetation burns during a wildfire, gravity transports the sediment from the hillsides down to the channel in a process called dry sediment loading, said Roman DiBiase, assistant professor of geosciences at Penn State.
In steep landscapes like those in Southern California, dry sediment loading plays a more significant role in the severity of post-wildfire mudslides than rainfall, shallow landslides and burn severity, according to the scientists. They reported their findings in the February issue of Geology.
“The channels typically are covered with cobbles and boulders that don’t move very easily or very often with storms,” DiBiase said. “The transported sediment that was trapped behind the vegetation is much finer, so you end up filling the channel with fine-grained materials that make it easier to start a debris flow in the channel itself.”
DiBiase and Michael Lamb, professor of geology at the California Institute of Technology, looked at three lidar datasets of the western San Gabriel Mountains. Lidar involves flying a laser scanner over a landscape and sensing the returning light. It allows scientists to reconstruct the topography at a high resolution.