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Climate and Environment

What You Should Know About Mudslide Risks With All This Rain. Yes, There's Reason To Worry

A house covered in mud up to its windows with large boulders around it.
Debris from a mudslide covers a home on Jan. 10, 2018 in Montecito.
(Justin Sullivan
Getty Images)
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Now here's something I haven't had a chance to write in some time: The following storms could cause mudslides across Southern California because we've had too much rain in recent months.

"Usually what you need is about 10 inches of rain over the course of the winter followed by an intense storm. And that's when you get into the range when you can have landslides anywhere," said Jason Kean, research hydrologist with the U.S. Geological Survey landslide hazards program.

Steeper, mountainous areas prone to debris flows, like Ojai, have seen dozens of inches of rain so far this winter, so any storm that brings a significant amount of precipitation over a short period could very well have issues. The real number to look out for here is a 1/4 inch of rain per hour, the general threshold where debris flows become a concern.

"That doesn't mean everything's going to go when you get that. But those are the conditions we worry about and those rates are definitely on the table for this week," Kean said.

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The L.A. Department of Public Works is warning of moderate flooding and sediment deposition in La Tuna Canyon, Sunland-Tujunga, Agua Dulce, and many other spots recently impacted by wildfires.

It's difficult to predict exactly when or where debris flows will occur, though they're more likely to blast down steep hills, through recently burned areas and in places you've seen them before. Any number of spots along Pacific Coast Highway come to mind, as does Montecito in Santa Barbara County. The latter area saw 7,000 mudslides during the early January storm, according to Kean.

Why debris flows happen

Burned areas aside — which we'll get to in the next section — much of whether debris flows are possible or likely comes down to how saturated the soil is.

It’s like a sand castle. If you add a little bit of water that holds it together. Add too much water and that thing’s just going to fall apart.
— Jason Kean, USGS research hydrologist

On years like this one, prolonged, heavy rainfall has brought much of our soil close to saturation. When a big, intense burst of rain comes along, it can actually build up pressure within the soil, pushing the grains apart and keeping them from sticking together.

“It’s like a sand castle,” said Kean. "If you add a little bit of water that holds it together. Add too much water and that thing’s just going to fall apart. What we’re worried about for this storm is getting too much water.”

Enough water, and all of a sudden you have an unstoppable wall of mud that can move boulders and obliterate trees.

Generally speaking, our steeper slopes are more susceptible.

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There's a long history of debris flows across SoCal, like what occurred in La Conchita in 2005 and in Montecito in 2018 — the latter of which was the result of another major factor that can make mudflows more likely: wildfire.

Over the past decade or so when we've talked about debris flows, usually it's in areas that've recently been stripped clean by one.

Debris flows, mudslides and flooding are more likely after fires

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. 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 downhill, collecting all sorts of dangerous debris on the way.

“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,” Kean said.

A helicopter dumps water on the a wildfire burning in the San Gabriel Mountains.
A helicopter dumps water on the western edge of the Station Fire, deep inside the Angeles National Forest on the outskirts of L.A. on September 3, 2009.
(Mark Ralston
AFP via Getty Images)

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. That's 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.

Do you need to prepare your home for debris flows?

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.

  • Move your trash cans and car away from the curb and into your driveway if possible.
  • Pick up sandbags from your local fire department to help divert water away from your home.
  • Look out for debris-flow warnings from your county and your local National Weather Service office. They’re watching to see if certain rainfall thresholds are going to be hit, as intense rainfall is more likely to result in debris flows. They may issue evacuation orders if that's the case. Remember, sometimes strong cells of precipitation form over burn scars for short periods and surprise even the professionals.

And then there are the bigger, long-term things you should do.

  • Buy flood insurance.
  • Get together emergency supplies, including food, first aid, extra water, flashlights, batteries, a radio, and equipment to help you stay safe if you’re stuck, wet, or outside.
  • Keep your important documents and an itemized list of everything in your home, stored in a waterproof place.
  • Plan evacuation routes.
  • Check in with your neighbors and friends outside the area before the storm comes through. Make sure you have some sort of plan to check in with each other.

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.

What do you want to know about fires, earthquakes, climate change or any science-related topics?
Jacob Margolis helps Southern Californians understand the science shaping our imperfect paradise and gets us prepared for what’s next.

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