This Next Huge Rainstorm Could Bring Debris Flows

Southern California’s about to get walloped by an atmospheric river that’s expected to drop between one to six inches of precipitation across different parts of the region between Thursday and Friday.

It could be enough to make a dent in the heavy rain deficit we find ourselves in, put a good amount of snow on our mountains, but potentially cause debris flows, especially in areas that recently burned.

The U.S. Geological Survey has a map detailing the risk associated with the different burn areas and it highlights what you’d expect.

The hills that the Apple, El Dorado, Silverado, Lake and Ranch 2 fires mercilessly tore through last year are all at an elevated risk for debris flows.

Soil is free to be eroded by rain, as the vegetation that long held it in place is gone. And the waxy, water repelling layer that often forms on the surface during a fire is ready to carry boulders and any remaining trees, downhill.

That said, it’s the Bobcat fire burn area that’s of most concern, according to Jason Kean, a research hydrologist with the USGS landslide hazards program.

“This is an area that has a long history of debris flows after a wildfire,” he said. “It’s probably got the highest threat in the world for this type of hazard.”

He attributes that to a combination of steep topography, a high frequency of fires, and a large population of people living on and along the bottom of the mountains.

“A garden variety storm can cause problems,” Kean said.

Look out for rainfall that exceeds a general debris flow-triggering threshold of half an inch per hour.

“We’re always prepared for things to go crazy, but there isn’t that anticipation that this storm system will bring ... significant problems,” said Stephen Frasher, public information officer with Public Works.

“Driveways and small roads near the burn areas could experience more mud than otherwise might be the case.”

Over the next day the department will update its debris and mudflow potential forecast, which is another tool that can help residents understand how much risk they’re in.

The National Weather Service in both Oxnard and San Diego could issue flash flood warnings. And in L.A. County, if it’s bad enough, you could get one of those deafening wireless emergency alerts telling you it’s time to evacuate.

Another reminder why in disaster-prone Southern California, it’s always good to have a go bag ready just in case.

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