On a Monday morning, with the soil of Berkeley saturated with weeks of relentless rain, a massive stream of mud, trees, roots, and debris shot off the hillside of Olivet College and fell into the kitchen and living room of a house on Middlefield Road. Seven other homes also needed to be evacuated.
Landslides caused by rainfall are a cyclical hazard to life in the Berkeley Hills, where development on steep slopes—heavy homes, roads cut at all angles, lush landscapes—compounds natural hazards from hillsides giving way during and after floods.
A 1907 landslide near Hilgard and Euclid Streets, which the city engineer attributed to a “lack of sewage leaking in the hills”, destroyed the sidewalk and cracked the gas pipes. After the 1911 Virginia Street landslide, the Berkeley Daily Gazette reported that “a goat belonging to Bill Nye’s vertical farm fell into its owner’s hillside estate, where Nanny was promptly desiccated.” And in 1998 (during a winter when landslides caused $47 million in damage in Alameda and Contra Costa counties), a house on Cragmont Street was twisted onto its foundation and pushed several feet down, causing 78-year-old Grace Fretter to evacuate with her. . Three adult children.
More destructive landslides are possible in Berkeley, the city warns, and they’re becoming more likely as a warming climate feeds more rivers into the atmosphere and increases the potential for heavy rainfall.
Two major categories of landslides threaten the Berkeley Hills.
The first is the deep, slow-moving slides. Many are in constant motion throughout the year in Berkeley. The two largest are located north of Berkeley Rose Garden and John Hinkel Park.
Huge slow slides are common in the Bay Area, where rocky outcroppings are small (at least compared to the East Coast) and faults are active. They generally shift the ground by less than 1 inch per year, and their rate of movement is affected by rainfall and groundwater conditions.
These types of deep landslides are often more of a nuisance than an immediate threat to life, causing cracks in sidewalks and slowly destroying home foundations. “It’s kind of a slush glacier that’s slowly moving downward,” said geotechnical engineer Alan Krupp. “What happens on the Earth’s surface in general doesn’t have much effect on the large, deep slides, but this whole region is just going for a ride.”
The city says they are “caused by exceptionally long periods of monsoon rain, and sometimes do not begin to move until long after the rain has stopped. … [They] It can damage large areas and many structures, resulting in huge losses in landslides.”
There are workarounds—strong foundations, for example, can reduce risk—but most Berkeley Hills homes were built on top of such large landslides and their construction “predates current best practices and laws,” according to the city.
Krupp, who has been studying landslides in the Berkeley hills for more than half a century, said his company referred to maps of the East Bay municipal area of utilities for leaky pipes to create a map of active landslides in Berkeley. The most recent version of that map, revised as recently as 1995, was included in the city’s 2019 Local Risk Mitigation Plan. It’s still mostly accurate, Krupp said.
The other, more dangerous type of landslide is the debris flow. Fast and violent, that’s what most people think of when they hear the word landslide.
Usually caused by heavy rains or earthquakes, they have the power to kill. Monday’s slide and the 1998 slide that upset Grace Fretter’s home were debris flows.
Areas prone to debris flows are completely unpredictable. They are generally “at steep hillsides, near the drainage mouths of steep slopes, and in or around the mouths of drain canyons that drain steep terrain,” according to the city. “What happened in the past is likely to slip up again in the future,” Krupp said, though he noted that it’s difficult to predict exactly where small landslides affecting a house or two might occur.
The Hazard Mitigation Plan section that reviews more than a century of landslides in Berkeley does not mention any injuries. Luck certainly played a role. But so is the consistency of the soil.
“Will it be like toothpaste, or will it be more like running water? If it’s like toothpaste, it will slow down as soon as you give it something to slow it down,” said Brian Collins, a civil engineer researcher with the US Geological Survey’s Landslide Hazard Program.
Depending on moisture levels and density, the soil can contract or expand, changing its texture, and the debris flow slowed on Monday as it reached the flat ground, where the homeowner’s property sat. If the soil had been very fluid, Collins said, it could have lasted more than a kilometer on the slope.
This kind of disaster becomes an even greater fear in a world with more intense winter storms. The city’s risk mitigation plan states that “Increases in the intensity and frequency of winter storms due to climate change will increase the landslide exposure of structures in the Berkeley Hills.”
Collins said the USGS monitors soil moisture across the Bay Area and sends it to the National Weather Service, which tracks precipitation rates. If a storm is expected to bring high-intensity rain that exceeds an area’s threshold (determined by the amount of rain it took to trigger landslides in the same area in the past), the NWS issues a warning.
“Places like Marin and the Santa Cruz Mountains, you can get 4 inches of rain in 24 hours, but something like the Berkeley Hills, when you start getting 4 inches of rain in 24 hours, that’s a lot, and it can’t be raining,” Collins said.
Trees and their roots are essential for stabilizing hillsides and clearing of vegetation and wildfires can destabilize soils and increase landslide risks.
The city rates the overall potential severity of landslides as “moderate” rather than “catastrophic” as is the case for wildfires and earthquakes.
The worst-case scenario is an ill-timed magnitude 7.0 earthquake hitting the Bay Area right now, Collins said, after weeks of heavy rain.
“You’re likely to see all kinds of roadblocks in Claremont Canyon, Wildcat Canyon,” Collins said. “You won’t see thousands of homes in the bay or anything like that, but you will probably see a lot of destruction in isolated areas.”
- Water control It can prevent landslides. Prevent pooling of water in concentrated areas and ensure that storm drains are not clogged.
- Hire a geotechnical engineer A top home inspector to identify problem spots and weak foundations before buying a home in The Hills.
- Pay attention to warning signs Like cracks in concrete floors and sidewalks, soil moving away from foundations, tilting telephone poles. They could all be signs of landslides.
- He listens for a faint growling sound that increases in volume as the debris flow approaches.
- Do you suspect an imminent landslide? Call 911 or 311, notify affected neighbors, and evacuate. Local officials can assess the potential danger.
- Can’t get away? Curl up into a tight ball and protect your head.
You can find more landslide preparedness tools at USGS.gov