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Understanding Bay Area Landslide Risk

Dr. Ross Stein is scientist emeritus at the United States Geological Survey, an adjunct professor of geophysics at Stanford University and the cofounder of Temblor.net, a startup that enables users to freely access data about their natural catastrophe risk.

 With the rainy season fast approaching, we sat down with geophysicist Dr. Ross Stein to discuss how Bay Area homeowners can better understand and mediate their risk for landslides. Of course, the Bay Area is known for its beautiful scenery and temperate weather, but it’s also known for frequent seismic activity and landslides. Dr. Stein has been studying the Bay Area for more than four decades, and he says both the region’s beauty and susceptibility to disasters come courtesy of the same force: the San Andreas Fault. Just as the San Andreas Fault is to blame for the Bay Area’s notorious earthquakes, it has also created conditions that make the Bay Area especially vulnerable to landslides.

retaining wall landslide Bay Bridge

The same geophysical forces that created the Bay Area’s dramatic scenery are also responsible for its increased landslide risk. Photo, Engineered Soil Repairs, 2016

Dr. Stein says two central factors precipitate landslides: steep slopes and loosely packed soil, both of which typify the Bay Area’s geology. He also points to a third factor that increases landslide risk: rain. As we move into the cooler, wetter months, the region becomes more vulnerable. Dr. Stein identifies the Santa Cruz Mountains region—with its steep terrain, loosely packed soil, and greater exposure to rain and fog—as an area that’s especially at risk for landslides. “But even areas that are relatively flat are susceptible,” he adds. “Measurements in downtown Berkeley have identified shifting soil, which indicates the area is at risk.”

Other factors can contribute to landslide risk, too. “Earthquakes are the big wildcard,” says Dr. Stein. When landslides are triggered by earthquakes, they occur quickly (typically following a seismic event by less than a minute) and can cause great destruction. For example, most of the 80,000 deaths from 2008’s Wenchuan earthquake in central China were due to landslides. So far, the Bay Area seems fortunate. While the 1989 Loma Prieta earthquake triggered some minor landslides, Dr. Stein notes that earthquake-related events are rare for the region. Indeed, there’s little historical evidence of earlier earthquakes triggering major landslides.

Given the Bay Area’s very real exposure to earthquake and landslide risk, Dr. Stein recommends that anyone who owns or is thinking of owning a home in one of the region’s many hilly neighborhoods should do some research to understand the potential for such events. He advises homeowners to look at state survey maps to understand the potential risk for a landslide affecting their home. Anyone who owns property in an area identified as high or very high risk should assume their home will need engineering adaptations to withstand earthquake and landslide events. They should then consult with a licensed geotechnical engineer to plan for any structural changes and get a sense of their costs.

Choosing a home with a beautiful hillside view doesn’t automatically mean putting yourself in harm’s way. Even in high-risk areas, structural remedies can make homes safer. Dr. Stein explains that a geotechnical engineer will search for the bedrock around a home built on a hill. Builders will then attach pylons from the house into the bedrock, securing the home and making it less vulnerable to earthquakes and landslides. Alternately, a hillside home built on stilts may be stabilized through the addition of cables. Unfortunately, engineering solutions like these don’t come cheap. Dr. Stein notes that costs can vary due to a number of factors. For instance, in cases where the slope was cut to build the home, the structure may be closer to bedrock and consequently easier to stabilize. In all cases, Dr. Stein warns that “people should avoid going straight to general contractors,” adding that it’s crucial for homeowners to consult with someone who fully understands the geological particularities of the region.

While landslides will remain unpredictable, in the future, homeowners may have more information close at hand to understand and evaluate their risks. As GPS tracking becomes more common (it’s already a central feature of smartphones), Dr. Stein foresees that soon we may have devices that monitor soil disruptions at the foundations of our homes. Improvements in satellite imagery and monitoring technology can also make it possible for Bay Area residents to fully appreciate the risks of living in this beautiful region.

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How to Protect Your Property from a Landslide

If you think your property is at risk:

  1. Learn about the history of your property. Areas that have experienced landslides in the past are likely to experience them again. Your city may keep records of past landslides, and you can also try this landslide inventory-mapping tool (in beta) from the California Department of Conservation.
  2. Hire a professional to evaluate the drainage on your property. You may need to build channels to deflect flow around buildings. Be aware: you are responsible for damage to neighboring properties caused by water diverted from your own.
  3. Plant ground cover on slopes—the root networks can help stabilize hilly areas.
  4. Consider hiring a professional to build a retaining wall. Bear in mind that any retaining wall built to contain soil should be designed by a licensed engineering contractor and built either by the designer or an experienced contractor who’s able to follow the design precisely.
  5. Have flexible pipefittings installed to avoid gas or water leaks in the event of a landslide.

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The Causes of Landslides

By Christopher Wilhite, chief engineer and co-owner at Engineered Soil Repairs, Inc.

As we approach the rainy season, it’s time to start paying attention to what rain can mean for the Bay Area. While rain may ease our persistent drought, it can also cause issues for Bay Area residents, especially after so many dry years. One consequence of rain after a drought is the increased likelihood of landslides—a familiar problem in this beautiful but hilly region. Debris flows (a type of landslide more commonly known as “mudslides”) have caused millions of dollars worth of property damage in the Bay Area. More importantly, people can and have died as a result of these fast-moving landslides. Bay Area residents: it’s time to get informed and prepared. To start the conversation, we asked Christopher Wilhite, chief engineer at Engineered Soil Repairs, Inc., to explain precisely what causes landslides. 

Landslide repairs in Lafayette

Engineered Soil Repairs, Inc. repairs damage from a landslide in Lafayette. Photo: Engineered Soil Repairs, Inc., 2016

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