On Monday morning, with Berkeley soils saturated from weeks of relentless rain, a huge flow of mud, trees, roots and debris tore off the hillside of Zaytuna College and collapsed in the kitchen and sitting room of a house on Middlefield Road. Seven other houses also had to be evacuated.
Rainfall-triggered landslides are a periodic danger to life in the Berkeley Hills, where development on steep slopes—heavy homes, roads cut at all angles, and lushly irrigated gardens—has exacerbated the natural risk of slope failure. during and after the floods.
A 1907 landslide near Hilgard and Euclid avenues, attributed by the city engineer to “lack of seepage drainage in the hills,” destroyed a sidewalk and ruptured gas lines. After a 1911 mudslide on Virginia Street, the Berkeley Daily Gazette reported that “a goat belonging to Bill Nye’s vertical farm fell onto an owner’s property further down the hill, where Nanny was promptly milked.” And in 1998 (during a winter when landslides triggered $47 million in damage in Alameda and Contra Costa counties), a home on Cragmont Avenue twisted at its foundation and was pushed several feet downhill, prompting 78-year-old Grace Fretter to evacuate with her three adult children.
Much more devastating mudslides are possible in Berkeley, the city warns, and they are becoming more likely as a climate warming fuels more intense atmospheric rivers and increases the probability of heavy rain events.
Two main categories of landslides threaten the Berkeley Hills.
The first is deep, slow-moving landslides. Several are on the move year-round at Berkeley; the two largest are just north of the Berkeley Rose Garden and John Hinkel Park.
Massive, slow slides are common in the Bay Area, where bedrocks are young (at least compared to the East Coast) and fault lines are active. They typically shift soil less than 1 inch per year, their rate of movement influenced by rainfall and groundwater conditions.
These types of deep landslides are more often a nuisance than an outright threat to life, causing cracks in sidewalks and slowly destroying the foundations of houses. “It’s like a slowly descending glacier of mud,” said geotechnical engineer Alan Kropp. “What happens on the ground surface usually doesn’t have much of an impact on the big, deep slides, but that whole area is just a ride.”
The city says they are “triggered by exceptionally long periods of seasonal rain and sometimes don’t start moving until long after the rain has stopped. … [They] it can damage large areas and many structures, resulting in large losses from landslides.”
There are workarounds — strong foundations, for example, can reduce risk — but most of the homes in Berkeley Hills were built on top of these large landslides and their construction “predates current best practices and codes,” according to the city.
Kropp, who has been studying landslides in the Berkeley Hills for more than half a century, said his firm referenced maps of leaking pipes from the East Bay Municipal Utilities District to create a map of active landslides. at Berkeley. The most recent version of that map, last revised in 1995, was included in the city’s local hazard mitigation plan for 2019. Kropp said it’s still mostly accurate.
The other, more dangerous type of landslide is the debris flow. Fast and violent, they are what most people think of when they hear the word landslide.
Usually triggered by heavy rain or earthquakes, they have the potential to kill. Monday’s landslide and the 1998 landslide that disturbed Grace Fretter’s home were debris flows.
Debris flow-prone areas are not completely unpredictable. They are typically “at the base of steep slopes, near steep slope drainage mouths, and in or around canyon mouths that drain steep terrain,” according to the city. And “what collapsed 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 houses might occur.
The section of the hazard mitigation plan that looks at more than a century of Berkeley landslides does not mention any injuries. Surely luck has 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 plain water? If it’s more like toothpaste, it’ll slow down as soon as you give it something to do it with,” said Brian Collins, a research civil engineer with the US Geological Survey’s landslide hazard program.
Depending on moisture levels and density, soils can contract or swell, changing their texture, and Monday’s debris flow slowed as it reached flat ground, where the owner’s property was located. If the ground had been very fluid, it could have continued more than a kilometer downhill, Collins said.
These types of disasters become a greater fear in a world with more intense winter storms. “Increases in the intensity and frequency of winter storms due to climate change will increase the collapse exposure of structures in the Berkeley Hills,” the city’s hazard mitigation plan states.
The USGS monitors soil moisture in the Bay Area and reports it to the National Weather Service, which keeps track of rainfall rates, Collins said. If a storm is expected to bring high-intensity rainfall that exceeds an area’s threshold (determined by the amount of rainfall it took to trigger mudslides in the same area in the past), the NWS issues a warning.
“Places like Marin and the Santa Cruz Mountains, they 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 handle it,” he said. Collins.
Trees and their roots are essential to stabilize slopes and remove vegetation and forest fires it can destabilize the ground 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.
The worst-case scenario, Collins said, is for a mistimed 7.0 magnitude earthquake to hit the Bay Area right now, after weeks of heavy rain.
“You would probably see all kinds of things blocking the road in Claremont Canyon, Wildcat Canyon,” Collins said. “You’re not going to see thousands of houses in the Bay or anything like that, but you’re probably going to see a lot of damage in isolated areas.”
- water control can prevent landslides. Prevent water from pooling in concentrated areas and make sure your storm drains are not clogged.
- Hire a geotechnical engineer plus a home inspector to identify trouble spots and weak foundations before buying a home in the foothills.
- Watch for warning signs such as cracks in concrete and pavement floors, dirt moving away from the foundation, leaning telephone poles. All of them can be signs of landslides.
- Listens for a slight rumbling 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 hazard.
- You can not escape? Roll up into a tight ball and protect your head.
Find more landslide preparedness tools at USGS.gov