The geological assessment found that about a third of the city is vulnerable to landslides, liquefaction and settlement.
Weeks after a landslide risk study unsettled Sausalito planning commissioners, the City Council had its own chance to be troubled.
The Miller Pacific Engineering Group presented its draft report to council members at their meeting on May 7. The geological assessment found that about a third of Sausalito is vulnerable to slides, liquefaction and settlement.
“I think what we see here is rather concerning,” Councilmember Melissa Blaustein said.
The report is part of a larger effort to assess the city’s vulnerability to slides. The assessment included areas in hazard zones and recommendations.
The study concluded that approximately 20% of the city is highly susceptible to landslides, including the areas of Hurricane Gulch, Spring Street Valley and Nevada Street Valley.
Additionally, another 20% of the city is susceptible to liquefaction and settlement, including most of downtown, the ferry landing area, New Town and Caledonia Street, the Marinship area, the eastern parts of Whiskey Springs and the area of Dr. Martin Luther King Jr. Academy.
“I think we are all aware of the risk, but to see that 20% of our hillsides are significantly at risk for landslides means we have to do more and do it now,” Blaustein said.
Mike Jewett, a geologist with the engineering company, said more than just the slope angle determines susceptibility. Other factors include the soil makeup, the load it bears, drainage conditions, changes in development and the rock type.
“One thing I should point out is that it does not indicate the probability of landslides,” Jewett said. “Predicting landslides is quite frankly very challenging if not impossible, but the point is to say these are the areas where the ground conditions and the inherent geological conditions are more conducive to instability.”
Settlement happens when new loads, like fill soil and buildings, are placed on soft, compressible soils like bay mud. Liquefaction occurs when loose, sandy or gravel-like soils are saturated and become liquid when intense shaking occurs, like during an earthquake.
“Settlement is a long-term impact,” Jewett said. “The areas in Marinship that were filled back in the 40s and 50s, they’ve been settling for a long time. Some of those areas have undergone all the settlement they will undergo, but as new development is considered, if new buildings are built or new fills are placed, that will induce more new settlement.”
In Sausalito, both conditions are typically found in former marshlands filled in for waterfront development, Jewett said. He noted that for both hazard zones, older buildings built before 1979 — when building codes were first adopted in the state — are the highest risk.
Councilmember Janelle Kellman said the landslide risk is one of the primary reasons the city is struggling with insurance. Its longtime insurer dropped the city because of its high-risk profile and loss history.
Kellman said the city needs to move fast with implementation measures.
“This is such a big risk for us,” she said.
The company recommended creating a program to inspect and catalog facilities — like retaining walls and drainage systems — in the vulnerable areas in order to monitor, maintain or repair them. Another recommendation included a hazard zone development ordinance, which would require stricter engineering evaluations for new development and redevelopment of sites within the hazard zones.
Sandra Bushmaker, a member of the city’s landslide committee, said many areas of the city are in the “red zone” of landslide susceptibility and that a geological hazard assessment district is under discussion. She said it is time to follow through with recommendations.
“I think there’s some great ideas and I just hope that you’ll make this a priority in your activities,” Bushmaker said.
