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Heavy rainfall could bring flash flooding, mud flows to Southern California

File: An umbrella is shared as rain pelts downtown Sacramento, Calif., Friday, March 4, 2016.
File: An umbrella is shared as rain pelts downtown Sacramento, Calif., Friday, March 4, 2016.
Rich Pedroncelli/AP

Southern California is in for another bout of rain and thunder, with rainfall potentially heavy enough to trigger the first mud flows of the season in recent burn areas.

The National Weather Service says the region could see downpours of a half-inch an hour or more starting late Thursday afternoon through Friday morning. It has issued flash flood watches for recent burn areas in the San Gabriel and Santa Monica mountains and the San Fernando, Santa Clarita, San Gabriel and Antelope valleys.

Heavy rainfall is expected to start after midnight in Los Angeles and Ventura counties.

County officials have gone door-to-door offering advice to residents who live below areas burned in recent wildfires, including the Sand Fire and the San Gabriel Complex Fire.

“We’ve gone home by home to residents who may be vulnerable and given them advice as to how they can protect their homes. And many times it’s structural advice — they’ll clean out culverts or other areas that transport water behind their properties,” said L.A. County Department of Public Works spokesperson Kerjon Lee.

The county has permanent structures called debris basins to capture stormwater and prevent it from getting into neighborhoods and homes, but in burn areas extra precautions are needed, Lee told KPCC.

The high intensity of a wildfire “creates almost a waxen layer that repels stormwater,” Lee said. “It doesn’t absorb into the surface as it would anyplace else in the basin, so what you have is a combination of that ash and mud and debris, but also a surface that doesn’t really readily absorb water, and so it all slides down the mountainside.”

To help protect neighborhoods, the county uses some of the same timber found in railway lines to shore up hillsides and prevent slides.

Some residents also install plywood or concrete walls to divert mud from their properties, Lee said.

The best strategy is to plan ahead. Lee said it was important to know if you live in an area susceptible to flooding or mud flows — those in the foothills or living next to a flood control channel, for instance.

You can get more information on preparing for flooding and other disasters at the county’s emergency preparedness page.