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From fires to floods: Danger shifts as pounding rains hit Southern California

This a common pattern in the Southland, especially as the fall fire season gives way to the winter rainy season.

But the region is particularly vulnerable now after a December that included the Thomas fire, the largest ever recorded in California history.

Evacuation orders in Thomas Fire zone
Evacuation orders in Thomas fire zone Los Angeles Times

That fire burned a huge swath of Ventura and Santa Barbara counties. And overnight, there were numerous mudslides reported, closing roads including the 101 Freeway in Ventura.

Here’s an explainer about what is happening:

What causes mudflows?

Soil in a burned area can be repellent to water, creating a floodlike flow on the ground that picks up rock and debris.

In an area that has not burned, soil can become saturated. Pressure builds up underground, and soil starts moving and begins picking up mud and debris as it starts flowing downhill.

Water rushing down with only mud is called a mudflow. If the flow picks up rocks, branches and sometimes massive boulders, that’s called a debris flow.

Mud and debris flows are types of shallow landslides, generally defined as less than 15 feet deep.

Another type of shallow landslide involves a saturated hillside that collapses but does not move very far, such as one that buries a roadway with dirt and rocks from a neighboring slope. They can happen up to an hour after a burst of intense rain.

Landslides that strike in recently burned areas are the easiest to predict, as wildfires have burned away roots of trees and vegetation that had kept soil in place.

Sometimes, authorities have accurately predicted when debris flows will occur, based on forecast rainfall rates, and have called for evacuations of homes before the rivers of mud and debris begin flowing.

How much rain does it take?

In Southern California’s unburned areas, 10 inches of rainfall during the winter is needed to nearly saturate the ground. After that point, a burst of rain of just one-quarter of an inch an hour can trigger widespread shallow landslides, including debris flow, Kean said.

But for burned areas, mud and debris flows can strike with only intense rainfall, even if the ground is not saturated.

What preparations were made for this storm?

Beginning Monday, officials began evacuating thousands of residents from burn areas, including Ventura and Santa Barbara counties and well as parts of L.A. County.

The storm is expected to dump as much as 7 inches of rain in the mountains burned by the Thomas fire, a large amount that was expected to cause slides.

Less rain is forecast in the burn areas near Sylmar, Santa Clarita, Bel-Air and northern San Diego County. But officials said mudflows were possible there as well.

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