235-Foot-High Dam Above Silicon Valley Could Liquefy in Earthquake 

Anderson Reservoir (Tony Avelar / Associated Press)
Tony Avelar / Associated Press

The water district that manages Silicon Valley’s 235-foot-high earthen Anderson Dam, which holds 29 billion gallons of water, has acknowledged that the dam’s embankments are seismically unstable and could catastrophically liquefy in a major earthquake.

When the Leroy Anderson Dam near Morgan Hill suffered its 11th flood in 67 years in February, 14,000 San Jose residents had to be evacuated. Assistant City Manager David Sykes told the Associated Press that the estimate for damage to private property is $50 million, and public infrastructure suffered about $23 million in damage.

About 150 members of the public on March 22 attended the first after-flood board meeting of the Santa Clara Valley Water District, according to the San Jose Mercury News. District engineers told the crowd that due to a well-known 2009 study of the risks of a large earthquake causing a dam break and flooding the community with two feet of standing water, the district voluntarily capped water storage at 68 percent of capacity by limiting the maximum lake height to 45 feet below the top of the dam.

But in a surprise to many in the crowd, the water district stated that a 2011 seismic stability evaluation had found Anderson Dam and Reservoir’s “downstream and upstream embankments could become unstable during a very large magnitude earthquake and the rupture of faults underlying the dam.”

Water district managers also stated that they initiated the “Anderson Dam Seismic Retrofit Project” in 2012 to create a permanent fix to the risks identified by the seismic study.

Despite serious safety risks, water district managers have said that the estimated $400 million project is still in “design phase.” They emphasized that environmental documents would still have to be prepared to comply with federal and state regulations. Permits must also be obtained from several regulatory agencies for water diversion activities during construction, including the full dewatering of the reservoir

As an interim safety measure, the district in January 2017 agreed voluntarily to reduce the reservoir’s maximum water elevation by another 10 feet. Unfortunately, the area suffered what the Mercury News called a 100-year flood zone. The overflowing Anderson Dam caused Coyote Creek to crest at 13.6 feet, four feet over flood stage.


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