Sausalito geological study outlines disaster hazard zones
About a third of Sausalito is vulnerable to landslides, liquefaction and settlement, according to a draft geological assessment.
The report by Miller Pacific Engineering Group is part of a larger effort to assess the city’s vulnerability to mudslides.
In January 2023, the City Council approved an agreement to hire the engineering group for more than $50,000. The company presented a draft report of its findings, which included areas in hazard zones and recommendations, to the Planning Commission at its April 10 meeting.
“This particular study has taken more than a year to develop,” said Kevin McGowan, public works director for the city. “It is quite detailed.”
Mike Jewett, a geologist with the engineering company, said the first step to the report was updating the city’s regional geology and slope stability mapping, which was over 40 years old. Technology in the field has improved significantly since then, such as the ability to find potential soil instabilities.
“Previously, your best available mapping was done in the mid-70s on a very old topo map that itself actually came from the 1920s,” Jewett said.
The map is now updated, digitized and in color. A second map identified the hazard zones, which included evaluating where landslides have historically occurred, the slope angles in hills and valleys, and the rock and soil stability around the city.
“This is such a critical set of information that we’ve needed for a long time,” said Nastassya Saad, a planning commissioner.
According to the draft report, 30% of Sausalito is in an area susceptible to one or more hazards. About 20% is in a high landslide susceptibility zone, which includes slopes along Highway 101 and developed areas such as Wolfback Ridge, Old Town, Hurricane Gulch, Bridgeway Promenade, Spring Street Valley and Nevada Street Valley.
The report also looked at liquefaction — when sediment is saturated by water and temporarily acts like a fluid due to the shaking often caused by an earthquake — and settlement vulnerability.
“Settlement occurs when you have new loads placed on the old marsh,” Jewett said. “The very soft, stinky bay mud, I’m sure you all are very aware of, when you put your weight on it you sink into it.”
Additionally, 10% of the city, including Marinship, New Town and the downtown and Ferry Landing districts, are in areas vulnerable to liquefaction and settlement.
“So the Marinship artificial fill was placed in order to accommodate the development, and as you all know those areas have historically settled and some of those areas are still settling,” Jewett said. “Liquefaction occurs in those very same areas. The difference is the bay mud itself is not liquefiable but the fill that was placed and not very well compacted and is saturated with groundwater and when you shake it, it essentially turns into a strength lacking liquid.”
The company is recommending the city create a program to inspect and document key facilities — like retaining walls and drainage systems — in the vulnerable areas in order to monitor, maintain or repair them. Other recommendations include partnerships with other stakeholders, like Caltrans and the National Park Service.
“We recommend specifically that retaining walls, roadways, and drainage systems in zones of “high” landslide susceptibility be prioritized for evaluation and monitoring,” the draft report states.
Jewett suggested the Planning Commission consider creating an ordinance that requires development applicants go through stricter engineering evaluation in areas that are in, or could affect, the hazard zones. He said this would not be to restrict development, but to reduce risk and unexpected conditions during construction.
“This report is badly needed here,” said Jeffery Luxenberg, a planning commissioner. “Obviously, given the hillsides and what we have, the number of slides that we’ve had and the water, this is critical.”