On a beautiful July day earlier this year, out of the blue, catastrophe visited residents of Rolling Hills Estates on the Palos Verdes Peninsula.
It’d been about four months since the last heavy rains, but the precipitation from one of the wettest years on record appears to have been working its way deep into the earth.
It appears that raindrops percolated through the silt and sand and past deposits of volcanic ash in the Altamira Shale, before settling on layers of montmorillonite rich clays, lifted from the ocean during the Pliocene some three million years ago.
It's possible that the particles of clay then took on the water, compromising the ability of the earth beneath the homes to stick together, leading them to separate like an unevenly stacked layer cake. The constituent parts of the neighborhood – the homes, streets and sewer – began to move, pulled by gravity down an embankment into the canyon below.
A preliminary report compiled for the city of Rolling Hills Estates by the geotechnical firm GeoKinetics said that a "high level of precipitation" was likely the main factor behind the slope failure. Though, it should be noted, other inquiries are ongoing.
If you know a bit about the area, landslides should come as little surprise, especially considering there are roughly 150 others that've have been documented there.
“Is the geology conducive to land sliding along the Palos Verdes Peninsula? The answer is yes,” said Jeremy Lancaster, state geologist and director of the California Geological Survey.
Given that another year of heavy rain might be on the way, influenced by a strong El Niño, what are the odds we’ll see even more homes sliding into canyons below?
There are two key types of landslides that you should be aware of: shallow and deep seated landslides.
The one's I'm usually on LAist screaming about are the shallow ones, particularly in hilly areas that've just burned. They're a bit more predictable because we know that after a fire, hillsides are covered in a waxy, water repelling layer and have had most of their vegetation removed. So, when rain falls from the sky, there's not much to slow it down, keep it from knocking dirt loose, picking up speed and turning into a huge destructive wall of water, like what we saw in Montecito in 2018.
When reporting on landslide risk during major storms, I'm usually on the lookout for recently burned areas anticipated to receive an inch of rain per hour, because heavy rainfall makes the shallow slides more likely.
Deep–seated landslides, like what we often see in the Palos Verdes Peninsula are a whole other beast,
We know that they typically need three things to occur: steep slopes, weak rocks and some sort of triggering mechanism, such as earthquakes or water (the peninsula has all three), to get them going.
But unless there's apparent movement on the surface, they're terribly difficult to anticipate because it's unclear what's going on deep in the ground. As soon as you're dealing with layer upon layer of material from different epochs all stacked upon each other, things get much more complex.
All it takes is one weak layer of water loving clay to give out between a whole bunch of big strong rock, for a whole hillside to come down.
Terrifyingly, a deep slide can occur months after a rainy season ends because it can take water a long time to slowly percolate through the earth to get to one of those weak layers.
"Because these landslides are at greater depths, it's really difficult geologically to have precise controls on when and how exactly that motion starts. But generally we expect that an above average rainfall seasons, particularly ones that are back to back, the probability or likelihood of motion is there," said Matthew Thomas, a research hydrologist with the USGS landslide hazards program.
It can take many cycles of wetting and drying for one of those weaker layers to become a problem.
Take for instance, bentonite clay found throughout the peninsula. According to Lancaster, while the clay is quite strong when it's dry, when it's wet it weakens and expands slightly, allowing for a slight amount of creep. Over time, cracks can grow and fill with water, further jeopardizing the hill structure. Add a whole bunch of weight and steep slopes and things can destabilize over time.
So if you can't predict a slide, what's there to look out for?
Listen for creaks and groans, look for evidence of movement, including broken water pipes and growing cracks.
It’s not always rain that’s responsible. Broken sewer or irrigation lines can also compromise layers and add weight to a hillside and cause it to give out.
El Niño is sometimes linked to wetter winters here in Southern California, which means more opportunity for water to percolate deep underground and destabilize hillsides.
There’s no guarantee that slides will follow a heavy rain year, but it does increase the risk that one will happen, especially in areas that’ve already seen slides, or have active slides ongoing.
According to the preliminary report prepared for the city, there were no earlier slides detected where the July 8 slide occurred in the Rolling Hills Estates area.
Preventing the percolation of water deep into the soil is key. Good surface drainage is an important. Concrete can be used to fill cracks that open up to prevent further infiltration. And in places like Abalone Cove, they've also installed dewatering wells, which actively pump water out.
At the behest of the city of Rolling Hills Estates, the homeowners association began winterization work in November, according to a letter provided by Assistant City Manager Alexa Davis. It includes various measures meant to clear water away from vulnerable parts of the hillside, including temporary drainage, grading and sandbags.
If you’re nervous about your property, you need to be on the lookout for growing cracks and listening for sounds of the earth moving. If your doors have suddenly fallen out of plumb and your windows aren’t opening, you may be facing a catastrophic issue.
If it’s not an emergency, but you're curious about what's going on beneath your home, hire an engineer to look at the geotechnical report relevant to your property.
