It seems to happen almost every year.
Our hills burn some time between late summer and early winter, and then, just as we’re recovering, a big rainstorm comes through and dumps a whole lot of water on the charred landscapes causing dangerous mudslides and debris flows.
We saw it last September in San Bernardino County, when mudslides hit the communities of Forest Falls and Oak Glen. One woman died.
Sometimes, it’s a strong river of mud that shoves around some cars and does some light damage to homes. Like what happened in Orange County during a March 2021 rain storm, months after a fire burned through the area.
Other times, the fires are so bad and the rains so intense that it’s not just a small river of sediment and stones that comes down hill. Instead, it’s a wall of impenetrable mud and water carrying with it trees and boulders, destroying peoples houses and leaving them scrambling to get to higher ground.
Like what happened in 2018 in Montecito when 23 people lost their lives in a mudslide. The record setting Thomas Fire had burned through the area just a few weeks prior.
Intense rain has always brought destructive debris flows to Southern California’s hilly communities (see “Los Angeles Against the Mountains” written by John McPhee in 1988). But wildfires make them much more likely.
When a fire scorches a hillside a few things happen.
A waxy, water repellent layer can form on the surface of the soil, making it less able to absorb the water falling from the sky. And vegetation, that’d normally hold soil in place and protect it from fast falling raindrops, is missing, leaving the soil to be battered, break loose and flow fast down hill, collecting all sorts of dangerous debris on the way down.
“You don’t have that binding matter, and the organic matter in the soil has been fried. And you’ve got this water repellent layer. That combo makes the soil easier to erode,” Jason Kean, a research hydrologist with the U.S. Geological Survey landslide hazards program, said.
It seems these conditions are likely to become more common as the climate continues to change.
Our fire season is getting longer as our temperatures rise and our rainy season potentially shrinks in length. That means more hills denuded by flames and more soil left vulnerable to rain.
Intense, short bursts of precipitation — exactly what trigger debris flows — could also become more of the norm as climate change progresses. Bad news for flood risk across the state.
“The thing I think that we can expect more often is the big ones. The ones more like Montecito. Now those aren’t going to happen every year, but they may become more frequent,” said Kean.
The increased risk associated with post-fire debris flows doesn’t completely abate for at least five years, until vegetation has had some time to reestablish.
If you’re curious about the risk for your area, the USGS has a dedicated map that might help. And if you live in L.A. County, the Department of Public Works forecasts debris and mudflow threats to different basins across the region when it rains.
To be safe, if you live in a hilly area that’s burned in the past two years, anticipate some sort of post-fire debris-flows when it rains.
And then there are the bigger, long term things you should do.
The National Oceanic and Atmospheric Administration put together this extensive guide on how to prepare. The good news is that the supplies are similar to what you should have if you’ve properly prepared for earthquakes.
