US-NEWS-CALIF-MUDSLIDES-LA

The Montecito city sign along the 101 freeway on Jan. 15, 2018 in Montecito, Calif. After recent fires, California is once again under threat of mudslides. (Katie Falkenberg/Los Angeles Times/TNS)

With parts of California yet again burned by severe fires, the state is facing a new winter of mudslides.

Mudslides have always been at the heart of the peril of living close to the mountains in California. All it can take is an intense amount of rain in a short amount of time to create damaging flows of mud and debris that can kill people and destroy buildings.

Mudslides are a particular risk for people living close to mountains. California’s mountains are quite tall, and the elevation drops extremely rapidly as water drains to the sea. The situation has become more dangerous as humans came to inhabit these paths of destruction.

“There’s a competition between the growth of the mountains and the erosion from the rainstorms,” U.S. Geological Survey hydrologist Jason Kean said. “They’re in this constant battle.”

HOW DO WILDFIRES WORSEN THE MUDSLIDE RISK?

When hills are verdant and healthy, vegetation can anchor the soil in place, even during heavy storms. But when the protective blanket of vegetation is burned off, hillsides become vulnerable to erosion, and slopes can come crashing down in a torrent of mud, rocks and dead branches like white-water rapids, destroying homes.

The heat from the fire makes it harder for water to infiltrate the top soil layer, Kean said. Ash tends to clog the soil after fire, and oily substances can affect the ground’s ability to absorb water. Impact from the rain can also help seal the top layer of sediment.

As a result, the soils become repellent to water. Instead of percolating below the surface, water starts flowing on the surface downhill and begins to pick up rock and debris, Kean said.

Recommended for you