Residents won’t know what triggered the landslide that destroyed 12 homes in Rolling Hills Estates until a geological engineering firm completes a probe slated to begin this week. But while there’s no clear smoking gun, as of yet, history — and the natural geology of the Palos Verdes Peninsula — all point to one leading suspect: water.
Whether from heavy rainfall, or an undiscovered leaky pipe, almost all of the landslides on the Palos Verdes Peninsula have been caused by water weakening the clay layers weaving through the region’s bedrock, says El Hachemi Bouali, an assistant professor of geosciences at Nevada State University.
“This weak clay is widespread across the entire Peninsula,” said Bouali, who along with two others published a research paper in 2019 that used radar and GPS to expand maps tracking subsurface instability in the area.
A sudden burst of water, such as a water main break, could create a landslide, but so could prolonged rainfall, or a tiny leak from a damaged pool or a poorly installed irrigation system, over the course of weeks or months, he said. Clay is porous, meaning it holds water for a long time, but it isn’t very permeable, so it similarly takes a long time for water to pass through it, creating a bottleneck beneath the surface.
Over weeks and months, the water saturates and further reduces the strength of already naturally weak materials, Bouali said.
“Regardless of the source (natural or anthropogenic), my guess is on water,” he said.
This year, California was hit by 14 atmospheric rivers from December through March. The Palos Verdes Peninsula received as much as 200% the average precipitation, according to the California Department of Conservation. In response, the department issued warnings to those who live in hillside areas around the state to keep a vigilant eye out for cracks and other signs of structural shifting, such as doors or windows no longer shutting properly.
Amy Rechenmacher, a professor of civil and environmental engineering practice at USC, recalled discussing Southern California’s torrential rainfall with colleagues at a convention in March.
“Everybody said it will be a few months until we start seeing the landslides,” she said.
That’s because rainwater trapped by the clay layers builds up pressure over a long period of time, Rechenmacher said, likening it to conditions that make your ears pop at the bottom of a swimming pool.
She believes these types of disasters will likely happen more often as climate change increases the frequency of extreme weather events that used to be considered “50-year storms.”
“You can’t control Mother Nature,” she said. “When it rains, the water has to flow somewhere.”
Delayed land movements, called “deep-seated landslides,” aren’t always attributable to a single event, experts say. The heavy rainfall could have created the conditions for a landslide, only for another incident — such as a leak — to add the final straw.
The canyon next to Peartree Lane — like many of the other slopes on the Peninsula — is considered highly susceptible to a deep-seated landslide because of the weaker strength of the rocks in the region and the steepness of the slopes, according to a mapping project published by the Department of Conservation in 2010.
Leaks and a high water bill
Rumors of possible causes have circulated throughout Rolling Hills Estates since the first homes began to slide last week. That deformation tore foundations from driveways as the residences were pulled away from Peartree Lane and slid a dozen feet down into the canyon. In total, 12 homes in the hillside community of 8,000 were destroyed and five others had to be evacuated due to a sewer line break. In response, Rolling Hills Estates declared a state of emergency.
Hae Young Cho first noticed cracking in the driveway of her home at 37 Peartree Lane on July 6. The following Saturday, she spotted a pipe leaking outside in her garden, but didn’t connect it to the cracks until a neighbor called her to complain that the leak was growing larger.
She asked her daughter and son-in-law to come take a look.
“And then someone called the fire department,” Cho said. Los Angeles County firefighters showed up before long and told residents they needed to evacuate.
Since then, those impacted by the landslides have been unable to return to their homes and speculation has mounted about the potential causes. Several residents have mentioned an unknown neighbor whose water bill supposedly ran into the thousands of dollars, though the water company could not find any evidence of such a bill in recent months. Others have noted seeing leaks.
One homeowner, Randy Troy, said he began noticing cracks in his home at 10 Peartree Lane as far back as April. He recalled seeing a sprinkler leaking just a few yards from his unit that same month.
“We had water sheeting down our driveway,” he said.
The family later returned home in July to find all of their gas lines had broken and needed to be replaced, Troy said.
Yvonne Kingman, a spokesperson for California Water Service, which provides water to the neighborhood, said the last high-consumption bill for the neighborhood was in January; it was resolved and the bill returned to normal the following month, she said.
“We can confirm there has been no leak within our water system in this area in at least the last 15 years,” Kingman said. “Our operations and engineering teams have been on scene since Saturday to inspect our system and monitor water pressure. Water pressure has been holding steady for our customers in Rolling Hills Estates.”
The Los Angeles County Department of Public Works declined to comment on whether its sewers experienced any leaks before the landslide. Peter Johnson, a spokesperson for Rolling Hills Estates, denied that any leaks had been reported prior to the land movement.
A main sewer line was discovered to have suffered a break on July 11, days after the land began to shift, Johnson said. “Preliminary review indicates the sewer break was likely caused by the recent land movement,” he said.
City officials would not speculate about what caused the disaster. The geological engineering firm hired by the Rolling Hills Estates Park Villas Homeowners Association, the owners of the properties, should be onsite “within the next several days to begin their assessment work and will coordinate with Cal Water Service as part of that,” Johnson said.
Some residents on edge
Uncertainty around the cause has put some residents on edge. Many of the multimillion-dollar homes in both Rolling Hills Estates and neighboring Rancho Palos Verdes rest atop steep slopes that give them magnificent views, despite the risk of potential disaster.
Kathy Stinis was enjoying lunch with a group of friends on Thursday, July 13 at the Hawthorne Boulevard Ralphs just a couple blocks from Peartree Lane. Stinis has lived on Ella Road in Rancho Palos Verdes for 50 years. Her property is built on a hillside, overlooking a street below, she said. But she hasn’t been too worried about a potential landslide, until recently.
“I do have cracks,” Stinis said, “and my patio on one side is going down a little bit.”
Still, it’s been 50 years without any major issues, she said.
When told the Peartree Lane townhouses had remained stable for 45 years, Stinis relented.
“I’m going to go turn off my sprinklers on that side of the hill,” she said.
Natural or man-made disaster?
There’s countless examples of leaks leading to landslides, both in California and across the country. Oakland sued a local utility in 2007 over allegations that leaks from a reservoir had caused a landslide that destroyed one home and damaged several others. Families sued the city of Bellevue in Washington over a 2022 landslide, attributed to a water main break, and, in turn, Bellevue sued a local school, where the pipe was located, for placing too much weight atop it.
Sometimes, the answer is also simply “all of the above.”
In 2005, two weeks of “torrential rainfall” in La Conchita in Ventura County preceded a landslide that killed 10 people, destroyed 13 homes and severely damaged 23 others, according to the state conservation department. The victims and their families later sued a 700-acre ranch growing avocados and lemons atop the hill for failing to build adequate drainage for its orchards, the Ventura County Star reported.
The ranch’s owners paid $5 million and turned over the ranch to the families to settle the case after a judge ruled that the company’s negligence had contributed to disaster.
Landslides ‘inevitable’ on Peninsula
It shouldn’t come as a surprise to anyone that the ground on the Palos Verdes Peninsula can become unstable.
Less than a mile south of Peartree Lane, in Rancho Palos Verdes, is a phenomenon referred to as the Portuguese Bend landslide complex, a roughly 260-acre area that has been slowly sliding into the sea for hundreds of thousands of years. Development in the 1950s dramatically increased the speed of the slide, causing millions of dollars of damage and destroying as many as 140 homes over the next three decades, according to a report by the city.
Additionally, in two of the more notable land displacements on the Peninsula, the 18th hole of what would become Trump National Golf Club slid into the ocean in 1999 and a 400-foot section of the Paseo del Mar roadway into San Pedro collapsed in 2011.
“Landslides are inevitable in this region,” said Bouali, the Nevada State professor who studied the Portuguese Bend landslide complex in 2019. “Although the homes on Peartree Lane were built quite close to a relatively steep slope, many homes on the Palos Verdes Peninsula are susceptible to landslides.”
One possible deterrent for landslides is through building “dewatering wells,” which pump groundwater from slopes to prevent pressure from building up. Rancho Palos Verdes is using dewatering wells to try to slow the Portuguese Bend landslide, but the wells only benefit the immediate area around where they are installed, Bouali said.
“Thus, in order to mitigate landslides for everyone, dewatering wells would need to be installed on every slope,” since most are deemed susceptible to landslides by the California Geological Survey, he said. “This is expensive, disruptive to homeowners and the environment, and would take a long time.”
Rechenmacher, after viewing photographs of the landslide on Peartree Lane, said the foundations appeared to be too shallow for how close the homes were built to the edge of the canyon, but she stressed that she had not seen any technical reports for the properties. Still, deep foundations, or the installation telephone pole-like pillars during construction, can strengthen both the house and the hillside against future landslides, she said.
But those types of protections are expensive and most developers don’t want to foot the bill, she said.
“Nobody wants to pay for it,” she said, “but somebody is paying for it now.”
Source: Orange County Register
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