Where and Why Mudslides Happen

6/1/2018 (Permalink)

Mudslides, also known as debris flows, are a type of landslide that can occur after a natural disaster, such as a wildfire. Debris flows often contain mud, rock and other materials.

“Human modification” of land can also make certain slopes and steep areas “vulnerable to landslides during and after heavy rains,” according to the Center for Disease Control and Prevention, which estimates that roughly 25 to 50 people die each year in the United States because of land and mudslides.

But mudslides can also occur without a wildfire preceding it, such as what happened recently in Mendocino, CA.

A few different factors contribute to debris flows in Southern California, David Peterson, a professor of forest ecology at the University of Washington, told Fox News.

The Thomas Fire, which burned more than 280,000 acres and primarily affected Ventura, Montecito, Carpinteria and Santa Barbara Counties, among others, removed “all of the living and dead vegetation that protected the soil beneath,” Peterson said. The land quickly eroded when there was no vegetation to hold it in place.

The fire was also hot enough to “cook out” the organic matter in the soil, which helps the earth to absorb water when it rains.

“When you have five or more inches of water in a day or two, it doesn’t buffer the impact of the rain on the soil,” he said, adding that the soil also became very hot from the wildfires. This results in the so-called “hydrophobic effect” which causes the soil to repel water.