We're Past Heavy Rains And You Might Be Asking: When Will The Landslides Stop?

Rain hasn’t fallen on the Santa Monica Mountains since March 7, but hillsides are still crumbling and disrupting traffic.

In the latest incident, evacuations were ordered for residents living near a house that was destroyed by a landslide in Sherman Oaks early Wednesday morning. No injuries have been reported. Firefighters arrived to find a large tree that had fallen over and downed wires, and a large portion of a hillside that had slid down toward at least three homes.

Along Pacific Coast Highway, a few landslides over the past few day have hobbled traffic. And the bottom half of Topanga Canyon — which recently received more than a year’s worth of rain in just a few weeks — is closed off to through traffic for the foreseeable future.

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When’s the landslide risk going to abate? 

Shallow landslides, like the one seen in the video above, usually occur during high intensity rainfall, and it’s difficult to pinpoint why this particular hillside failed days after the last bout of precipitation without a geotechnical report.

“It could be that following the rain there’s additional infiltration … that compromised some deep zone in there,” said Matt Thomas, research hydrologist in the USGS Landslide Hazards Program. “It’s tough to say.”

The Mariposa pedestrian bridge damaged by a landslide in San Clemente.

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Orange County Transportation Authority

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It’s possible the soil and rocks at that location were so disturbed by the recent rains that they were taken to the precipice of failure and finally slipped.

It could be that there’s still water running through the soil, causing movement and compromising the hill’s ability to stick together.

Or maybe, when the road was created, the slope was over-steepened to the point where the area was made more susceptible to landslides regardless of precipitation.

The bottom line is that these little crumbly slides in our steep, over-saturated hills should become less common the further we get from rain events.

Multimillion-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 Feb. 15, 2024.

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Patrick T. Fallon

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AFP

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Big slides will remain a risk

On the other hand, the danger of giant, home-destroying slides that’ve plagued coastal California are an ongoing risk.

“When you have a year with above-average rainfall like last year, that water is still doing work on landslides the following year,” said Thomas. “We don’t know how many years back is important to consider.”

Deep-seated landslides, as they’re known, can occur long after rain stops, as water continues to work its way deep into the soil. It can eventually percolate to the point where it can keep two layers of earth from sticking together — leading to collapse, even on a random sunny day like we saw last year at Rolling Hills Estates on the Palos Verdes Peninsula.

An aerial view of upscale homes destroyed by a landslide on Palos Verdes Peninsula in July 2023, in Rolling Hills Estates.

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Mario Tama

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Getty Images

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If you’re in a landslide-prone area, listen for groans or creeks coming from deep within the earth. Look out for new cracks or if windows and doors suddenly fall out of plumb, or if water and gas lines start to break. All of the above could be an indication that the hill is about to give out. Call the authorities if you suspect a slide is a risk.