On Monday morning, with Berkeley’s soils saturated by weeks of relentless rain, a huge flow of mud, trees, roots and debris came loose from the hillside of Zaytuna College and tumbled down into the kitchen and living room of a house on Middlefield Road. Seven other homes also needed to be evacuated.
Rainfall-triggered landslides are a periodic peril of life in the Berkeley Hills, where development on steep slopes — heavy homes, roads cut at all angles and lushly watered landscaping — has exacerbated the natural risk of hillsides giving way during and after deluges.
A 1907 landslide near Hilgard and Euclid avenues, attributed by the city engineer to “a lack of drainage for seepage in the hills,” destroyed a sidewalk and fractured gas pipes. After a 1911 landslide on Virginia Street, the Berkeley Daily Gazette reported that “a goat belonging to Bill Nye’s vertical farm dropped into the estate of an owner further down the hillside, where Nanny was promptly milked dry.” And in 1998 (during a winter when landslides caused $47 million in damage in Alameda and Contra Costa counties), a home on Cragmont Avenue was twisted on its foundation and pushed several feet downhill, causing 78-year-old Grace Fretter to evacuate with her three grown children.
Far more devastating landslides are possible in Berkeley, the city warns, and they are becoming likelier as a warming climate fuels more intense atmospheric rivers and raises the probability of intense rainfall events.
Two main categories of landslides threaten 6,000 structures in the Berkeley Hills.
The first is deep-seated, slow-moving slides. Several are in continual motion year-round in Berkeley; the largest two are just north of the Berkeley Rose Garden and John Hinkel Park.
Slow, massive slides are common in the Bay Area, where bedrocks are young (at least compared to the East Coast) and faults are active. They generally shift ground by less than 1 inch per year, their rate of movement influenced by rainfall and groundwater conditions.
These types of deep-earth landslides are more often a nuisance than a direct threat to life, causing cracks in sidewalks and slowly wrecking the foundations of homes. “It’s kind of like a mud glacier moving slowly down,” geotechnical engineer Alan Kropp said. “What happens at the surface of the ground generally doesn’t have much impact on the big, deep slides, but that whole area is just going for a ride.”
The city says they are “triggered by exceptionally long periods of seasonal rainfall, and sometimes do not start moving until long after the rain has stopped. … [They] can damage large areas and many structures, resulting in extensive landslide losses.”
There are workarounds — strong foundations, for example, can reduce risks — but most Berkeley Hills homes were built atop these large landslides and their construction “predates current best practices and codes,” according to the city.
Kropp, who’s been studying landslides in the Berkeley Hills for over half a century, said his firm referenced the East Bay Municipal Utility District’s maps of leaking pipes to create a map of active Berkeley landslides. The most recent version of that map, last revised in 1995, was included in the city’s 2019 local hazard mitigation plan. Kropp said it is still mostly accurate.
The other more dangerous type of landslide common in Berkeley is the shallow landslide. Fast and violent, they’re what most people think of when they hear the word landslide.
Monday’s slide was a debris flow, a type of shallow slide. Usually triggered by intense rain or earthquakes, debris flows have the potential to kill.
Areas prone to debris flows are not completely unpredictable. They’re generally “at the base of steep hillsides, near the mouths of steep hillside drainages, and in or around the mouths of canyons that drain steep terrain,” according to the city. And “what’s slid in the past is likely to slide again in the future,” Kropp said, though he noted that it’s hard to predict exactly where small landslides affecting one or two homes might occur.
The section of the hazard mitigation plan that reviews more than a century of landslides in Berkeley doesn’t mention any injuries. Luck has surely played a role. But so has the consistency of the soil.
“Is it going to be more like toothpaste, or is it going to be more like running water? If it’s more like toothpaste, it’ll slow down as soon as you give it something to slow down upon,” said Brian Collins, a research civil engineer with the U.S. Geological Survey’s landslide hazards program.
Depending on moisture levels and density, soils can contract or dilate, changing its texture, and Monday’s debris flow slowed as it reached flat ground, where the homeowner’s property sat. If the soil had been very fluid, it could have continued more than a kilometer downhill, Collins said.
This type of disaster becomes a larger fear in a world with more intense winter storms. “Increases in the intensity and frequency of winter storms due to climate change will increase exposure to landslides for structures in the Berkeley hills,” the city’s hazard mitigation plan states.
The USGS monitors soil moisture across the Bay Area and communicates it to the National Weather Service, which keeps track of rainfall rates, Collins said. If a storm is expected to bring high-intensity rain exceeding an area’s threshold (determined by how much 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, they can take 4 inches of rain in 24 hours, but something like the Berkeley Hills, when you start to get 4 inches of rain in 24 hours, that’s a lot, and it can’t handle it,” Collins said.
Trees and their roots are essential to stabilizing hillsides and both vegetation removal and wildfires can destabilize soil and increase the risk of landslides.
The city classifies the overall potential severity of landslides as “moderate,” not “catastrophic” as it does for wildfires and earthquakes.
A worst-case scenario, Collins said, is for an ill-timed magnitude 7.0 earthquake to rattle the Bay Area right now, after weeks of heavy rain.
“You’d probably see all sorts of things blocking the road at Claremont Canyon, Wildcat Canyon,” Collins said. “You’re not going to see thousands of homes down at the Bay or anything like that, but you’d probably see a lot of damage in isolated areas.”
Defending against landslides
- Water control can prevent landslides. Prevent water from pooling up in concentrated areas, and make sure your storm drains aren’t clogged.
- Hire a geotechnical engineer on top of a home inspector to identify problem spots and weak foundations before buying a home in the hills.
- Watch for warning signs like cracks in concrete floors and pavement, soil moving away from foundations, leaning telephone poles. They can all be signs of landslides.
- Listen for a faint rumbling sound that increases in volume as the debris flow nears.
- Suspect an imminent landslide? Call 911 or 311, inform affected neighbors, and evacuate. Local officials can assess potential danger.
- Can’t get away? Curl into a tight ball and protect your head.
Find more landslide preparedness tools at USGS.gov