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Massive Mudslide

A mudslide devastated a Southern California community that just had the largest wildfire in history

January 12, 2018
By Cici Zhang
Justin Sullivan/Getty Images

Firefighters search for people trapped in mudslide debris on January 10, 2018 in Montecito, California.

A deadly mudslide struck Montecito, California, in the early morning of Tuesday, January 8. Rivers of mud, rocks, and trees tumbled from the hills, damaging hundreds of homes in Southern California. At least 17 people were killed. As of Friday morning, 43 were still missing.

Montecito is an affluent neighborhood with celebrity residents like Oprah Winfrey and Ellen DeGeneres. But now most of Montecito is dotted with mud-filled houses, wrecked cars, and snapped power-line poles. Parts of U.S. 101 Freeway remain closed and covered with mud and debris. Nearly 20,000 acres were affected. Tens of thousands of people were left without power. Many still don't have safe drinking water. About 500 first responders used helicopters and dogs to help trapped residents. 

FROM RAIN TO MUD

Mudslides happen when heavy rainfalls occur in a mountainous region. As the run-off water moves downhill, it accumulates into streams and rivers. Along the way, these rivers pick up mud and debris as large as truck-sized boulders and trees.

“We call it debris flow,” says Francis Rengers, a geologist at the U.S. Geological Survey. What happened in Montecito, he says, was a debris flow “that moved like a wall and ended up being very destructive.”   

Rengers and his colleagues can predict where debris flows might occur using science, math, and statistics. They consider how steep a mountain slope is, how much rainfall has caused disasters before, and each area’s soil conditions. Luckily, many were warned before the mudslides began.

Mike Eliason/Santa Barbara County Fire Department/Handout via REUTERS

Cars stuck in flooded water on the U.S. 101 Freeway after the mudslide.

MAGNIFIED BY WILDFIRES

Another important factor in the occurrence and severity of mudslides is wildfires. Montecito was one of the many cities affected by the Thomas Fire. Ignited on December 4, 2017, the Thomas Fire has burned more than 280,000 acres in California. It's the largest wildfire in California's history. It's still burning, but is 92 percent contained.

Normally water soaks into the soil when it rains. After a wildfire, however, most water runs over the surface. “It’s like you put a raincoat over the landscape,” says Rengers. In fire-scorched areas like Montecito, the soil is too dry for water to seep into. In addition, some of the types of plants that burned in the fire can coat the soil with waxy molecules, making it even more repellant to water.

After a wildfire, as little as half an inch of rainfall per hour can cause a debris flow. A total of more than 10 centimeters (4 inches) of rain fell in 24 hours in Montecito. At its heaviest, half an inch of rain fell in five minutes, which only worsened conditions.

"It's going to be worse than anyone imagined for our area," Santa Barbara County Fire Department spokesman Mike Eliason told the Los Angeles Times when weather forecasters first began predicting the heavy rainfalls. "Following our fire, this is the worst-case scenario."

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