Look Out For Landslides As Heavy Rain Comes For Our Already Saturated Soil

Last week’s storms not only brought an immense amount of rain to California, they caused more than 240 landslides across the state, many of them in Southland.

Stories have continued to trickle in about homes and lives being threatened, as hillsides, compromised by the heavy rainfall, have continued to erode. Now, with another big storm expected to move in this weekend, experts warn that another round of heavy rainfall could cause more widespread damage.

Multi-million-dollar homes crouched on a Dana Point clifftop got a little bit closer to the edge when a landslide brought tons of rock and soil crashing into the sea on February 15, 2024.

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Patrick T. Fallon

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AFP

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Why hillsides fail

Soils have a limited water carrying capacity, and given that some spots were recently saturated with more than a year’s worth of precipitation in just a few days, the risk of landslides is very real, especially with this next storm that’s expected to bring anywhere from 2 to 8 inches of rain.

“If we get intense bursts of rain on top of saturated soils I wouldn’t be surprised by reports like we saw last week or even worse,” said Jason Kean, a research hydrologist with the U.S. Geological Survey.

Kean’s part of a research group that’s installed a number of soil probes in the San Gabriels and the mountains above Santa Barbara in an effort to study landslide activity.

“These five holes in the ground are indicating we are around 60% saturated. How representative those are for the entire listening area is difficult to say,” he said.

The probes extend from the soil surface all of the way down to the bedrock, measuring how the pockets of air between all the individual particles of soil are filling up with water. When positive pressure is detected at the end of the probe, it means there’s standing water along the bedrock and the soil is getting saturated.

A view of mudslide damage which destroyed a home as a powerful long-duration atmospheric river storm, the second in less than a week, continues to impact Southern California on February 5, 2024 in Los Angeles, California.

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Mario Tama

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Getty Images North America

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Too much water and the pressure between the grains can grow to a point where they struggle to stick together — kind of like when you add too much water to a sandcastle and it collapses.

There are many factors that can influence a landslide, including the composition of the soil (clay holds onto more water than sand), the steepness of the grade, whether soil has had time to dry out between heavy bursts of rain, and how humans have affected the movement of water with construction and drainage.

It’s unclear exactly how much water is needed to trigger a collapse, but long standing guidance estimates that it’s about 10 inches of rain followed by a quarter inch burst within an hour, the latter of which isn’t all that much.

“That's what you saw in the storms of last week,” said California state geologist Jeremy Lancaster. “That storm just sat there and chugged on Monday and Tuesday.”

It helps explain why we saw so many slides.

According to Kean, when last year's cavalcade of atmospheric rivers brought more than 15 inches of rain to our area in early January, more than 7,000 landslides were triggered in the Santa Ynez mountains. While they didn’t greatly impact public infrastructure, they did do damage to Forest Service roadways. This is one of the event's Kean and his team are looking at to better determine when and why hillsides fail.

"There are some good guesses, but nobody knows exactly what it is. More work is needed,” he said.

An aerial view of a home destroyed by a mudslide on February 5, 2024 in Los Angeles, California.

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Mario Tama

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Getty Images North America

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What to look out for

Given how saturated soils are, look out for heavy bands of rain.

If you're in a slide prone area or if the last rainstorm caused noticeable erosion around your home, stay on your toes.

If you hear creaking, see pipes break or cracks open up, a deep seated, catastrophic landslide may be imminent, and you should exit your home as fast as possible. I covered this in detail following the Palos Verdes landslide last year.

The National Weather Service is in communication with USGS about potentially hazardous rainfall totals. Make sure to log into LAist.com or to our live audio stream, where we’ll be talking about the latest hazards.