Frequent Heavy Rain Has Made California a Mudslide Hotspot

Los Angeles saw 592 slides in one week, a reminder that excessive precipitation events set off more than flooding.

This story originally appeared on Inside Climate News and is part of the Climate Desk collaboration.

Picture the minute hand at about 8 past the hour. That’s the slope of Viet’s backyard in southern Los Angeles County. It’s a bit too aggressive for a slip-and-slide. In fact, Viet doesn’t even let his 7-year-old daughter play on the family’s small back patio.

“I don’t need her falling down that hill,” he said.

When Viet and his wife bought their house-on-a-hill five years ago, it was a win, their piece of “the Hollywood Riviera,” as real estate agents like to call the area. (A self-employed marketer in his forties, Viet asked that his last name not be used to protect his family’s privacy.)

Viet’s street runs horizontally across a huge incline that begins the Palos Verdes Peninsula, a marvel of steep cliffs and Mediterranean-style homes at the south hook of Santa Monica Bay. If you squint, it could be the terraced hills of Tuscany or, indeed, a stretch of the Côte d’Azur. The address was a solid investment and housing insurance not a problem, even though parts of the peninsula have been known to shape-shift, cracking roads and knocking houses off foundations. But not every day. The family enjoyed some easy SoCal years on their perch with its great views and gentle, dry climate.

“Whenever it rained, we’d be happy: ‘We’re not in a severe drought anymore, yay!’” Viet said. “But after this, every time it rains, I get scared.”

“This” was the atmospheric river storms that hit LA with a one-two punch (the first, a jab, the second, a wallop) in the first week of February. The usual winter rainy season in California has been amped up this year by a parade of such storms. This week again, Santa Barbara, Ventura, and LA counties are in the midst of high-volume, road-cracking, flash-flooding, climate-amplified downpours juiced by warmer Pacific Ocean temperatures. The storms are causing an unusual amount of high-profile damage, setting everyone on edge, especially Viet.

After the initial rain burst on February 1, he noticed that the top of his backyard slope, swathed in a hand-high succulent called “ice plant,” looked odd. A patch of mushy soil seemed to be shrugging off its ground cover. He asked a gardener to try and fix it. That was a Friday. Then the monster rain cells moved in on Sunday, February 3.

“All night, all I could hear was pounding on the roof, the wind blowing sideways,” he said. “It was unsettling, so when I woke up at 7:30, the first thing I did was try to go look at the rain drains and make sure everything was doing fine.”

Viet circled his home in sneakers because he’d never had cause to buy rain boots.

“I walked around to the backyard, looked down, and I was like, ‘Ohhhhh myyyyyy goooood.’”

A 40-foot-wide river of mud, rock, and roots was in full flow down his hill, already jamming up a city road 70 feet below where Viet stood, somehow safe, on the precipice.

Viet turned around, got in his car, and took a right out of his driveway, following streets around and down the hill to the base of the slide, where he was blown away by two things: the sheer tonnage of debris already piled in the street and, later, as he attempted to wade in and understand what was happening, the viscosity of the sludge.

“It was pudding. Knee-deep right away, like quicksand.”

“Sites of Failure”

In 1975, a USGS geologist named Russell Campbell submitted a research paper addressing “Soil Slips, Debris Flows, and Rainstorms” in Southern California. He’d spent years studying the deadliest regional landslides of the ’60s, trying to figure out a pattern, some threshold of conditions that might serve as a warning device. Landslides often felt so arbitrary, set off by all kinds of unpredictable SoCal things, from earthquakes to a leak in a pool. But he knew one thing for sure: “Debris flows generated by soil slips during rainstorms present a greater risk of death and injury to southern California residents than all other kinds of slope failure combined.”  Excessive precipitation events were the big-danger zone.

Campbell came up with a formula: Watch slopes that have been hit with 10 inches of rain in just a few days. If the precipitation keeps up, and those saturated spots get an additional .25 inches per hour or more, you can expect “sites of failure.” The most dangerous slopes, in his opinion? Angles of 27 to 56 degrees.

Road crews work on the Palos Verdes Peninsula after a series of destabilizing atmospheric river storms.

 Photograph: Audrey Gray

Nearly 50 years later, Campbell’s rule has largely held up, according to California State Geologist Jeremy Lancaster. He referenced it as he watched the atmospheric river storms travel downstate and plant themselves over the hilly and cavernous neighborhoods of the Santa Monica mountains starting that first weekend in February, dropping 14 inches in some places.

“We went over 10 inches, and bam, we started seeing a lot of landslides,” he said.

Lancaster’s team, which maintains a map of reported landslides, documented 252 slides statewide after the storm. “But we’re not first responders, so we don’t have boots on the ground,” he noted, and he wasn’t shocked when LA mayor Karen Bass announced that the Department of Public Works counted 592 mudslides in the city limits alone. And the rainy season is not nearly over.

“We’re likely to see more,” Lancaster said. “As we expect the rate of rainfall to intensify in a changing climate, you’re going to perturb hills and slopes.”

Global warming researchers like Mohammed Ombadi, a climate scientist in the College of Engineering at the University of Michigan, have been sounding the same alarm over the past few years. Ombadi has studied rates and types of precipitation in mountainous regions around the world.

He found that not only are we seeing more heavy storms with every degree the Earth warms, but the type of precipitation at higher elevations is more likely to be rain instead of snow. That means unstable slopes are losing the luxury of slowly absorbing and adjusting to a seasonally paced snow melt. They’re increasingly likely to face fast and forceful deluge, a powerful trigger for dangerous debris flows.

“It’s a strong signal,” Ombadi said. “The change has already been happening.” As someone with a background in civil and environmental engineering, Ombadi can’t publish research like that without thinking about practical, on-the-ground responses to protect communities from the increased risk.

“When engineers design infrastructure, we design it for a given lifetime with certain conditions,” he said, usually a risk threshold based on the past 100 years or so. “But now the threshold is changing, and you need to account for that in design.”

A Drainage Game

The week after Viet’s backyard fell, I stood at the base of that steep, still-slippery slope next to civil engineer Joe Demers, who works for one of Southern California’s largest private landslide fixers, Alpha Structural Inc. I was under the impression I was there to watch the repair job. What I didn’t understand yet was just how complex, time-consuming, and expensive stabilizing a hill can be. The site crew said it was safe enough to make my way halfway up the slope, and Salvatore Gomez, a worker with mountain-goat perching skills lifted the edge of a giant blue cover tarp to show me the state of things. LA typically dries out fast and we’d had sunny, 60-degree days since the storm, but what I saw was trickling water still pooling anywhere the mud (we were both ankle-deep in it) had caked up. Months of desaturation work was ahead.

A crew from Alpha Structural works to cover Viet’s backyard before another atmospheric river storm hits it.

Photograph: Audrey Gray

“The more I see stuff like this, the more I think I never want to live somewhere that isn’t flat,” said Demers, who’d seen more than his share at that point. Alpha Structural usually gets around 300 inquiries a week, but that first week of February, 700 people called for help. In his eight years with the company, Demers had never seen anything like it.

“Climate change is happening and all we can do is build things that resist it,” he said. “I think we just have to plan on building things for two degrees hotter.”

What’s strangely comforting about engineers is their confidence that given the resources, most tragedies can be avoided by executing on basic principles like this one: It’s 90 percent a drainage game. The first step: “Tarp down” the area so no more rain hits the hill. (Many neighborhoods in LA currently look like a patchwork quilt of giant blue and black tarps, sewn together with wire and held in place with sandbags. It’s not fancy, but it does the job temporarily.) Then, employ one or more proven methods to make sure water doesn’t have the chance to sink deep into soil through eroded rock layers where it can pool on the top of more solid bedrock, creating a conveyor belt for everything that’s loose above it. You have to keep any excessive moisture moving down-slope and off-property into safer channels.

What’s discomforting is how much this costs. Viet had to take out a $50,000 construction loan to pay for the Alpha Structural work, from the 40-by-70-foot tarping to an eventual “pipe and board” system that will recreate the grade of his slope. The company’s engineers told him that was the most affordable way to hold back another slide. (A retaining wall would have cost more.) Payments are set at $800 per month for 10 years, though Viet said they’ll try to pay it off sooner, and he’s hoping they might be eligible for FEMA assistance.

“Home insurance will not cover anything, that’s flat out the case,” he said. “You need a special insurance they call ‘a difference in condition insurance.’” Viet had never heard that phrase before this happened. Earthquake or flood insurance, sure, but neither of those would have covered his landslide.

Local, state, and federal authorities in the US have made some attempts to warn the public about the amplified danger of landslides during extreme weather. The Department of Homeland Security has a Landslides & Debris Flow site offering Before, During, and After tips, like how to recognize warning signs. (“Listen and watch for rushing water, mud, or unusual sounds.”) California emergency response teams now coordinate with the National Weather Service to warn people on social media platforms of mudflow risk in the days preceding big rainstorms. Evacuations are often issued in neighborhoods that have suffered destabilizing wildfires—the LA County Sheriff sent out a “LEAVE NOW” emergency alert in a burn area of Topanga Canyon on February 3. Regional efforts, like this Debris and Mudflow Potential Forecast, intensified after the first significant rainfall of 2018 triggered an enormous, post-fire landslide in the communities of Montecito and Carpinteria, killing 23 people.

Seulgi Moon, a professor of geomorphology (the study of the Earth’s surface) at UCLA is part of a team of research scientists who believe people need far better geohazard warning systems … and not only in California. They’ve experimented with a “superposable neural network” model (it’s AI) loaded with more than a dozen data variables from the Eastern Himalayas, the landslide-prone high mountains above the Bay of Bengal. Moon and team fed their AI model with information about terrain, slope angles, precipitation records and the places where water flows and accumulates. It gave them back something they call a Landslide Susceptibility Map.

“We have a pretty good idea of areas where landslides are likely to happen, and of probability,” she said. “But exactly where and when, what time, that’s harder.”

Moon’s team is submitting grant proposals to continue their AI research for other parts of the world too. There’s the possibility of adding data from regional seismic networks to help build a more time-sensitive warning system. Apps like MyShake have already begun offering residents of California, Oregon, and Washington real-time earthquake alerts—there could come a day when phones light up with imminent landslide warnings, too.

For now, though, the second day of hard rain here in the third week of February, Viet is monitoring his backyard slope himself, hour by hour, hoping it holds under the temporary tarps and sandbags. Building inspectors have told him the foundation of his house is not in danger, but it’s that slippery topsoil stressing him out.

“It looks secure so far, but whew, who knows?” he said. “I don’t have much more mud to give.”

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