Earthquake risk sparks $3.2B rebuild of major Silicon Valley dam

After years of planning, Santa Clara County’s largest dam is finally set to be torn down and rebuilt after water officials approved a $3.2 billion project to bring the aging 240-foot-tall structure up to modern earthquake standards.

The Anderson Dam Seismic Retrofit Project is slated to begin construction at the Morgan Hill site in early 2027, Santa Clara Valley Water, the agency in charge of the project, said in a Tuesday news release. The water district also said that the project “will improve water supply reliability, support groundwater recharge, reduce flood risk, and restore recreation at Anderson Reservoir and along the Coyote Creek corridor.” 

When construction starts, crews will rebuild a majority of the dam and its spillway, install new pipelines and complete other safety upgrades, Valley Water said in the news release. Crews also plan to raise the wall height of the dam by 7 feet to help with large storm flows, according to project documents. 

The project, with a grand total of $3.2 billion, is expected to be completed by December 2033, Matt Keller, a spokesperson for Valley Water, told SFGATE via email. There’s a $25 million incentive to complete the project early, he added.

The Anderson Reservoir, which is held back by the Anderson Dam, will be at full operating capacity after the project is complete, according to the news release. Currently, the reservoir is only at 3% capacity because of the earthquake concerns, according to Valley Water’s website. 

“Completing this project will significantly increase our local water storage capacity and provide flexibility for how we manage our water,” Keller said.

The reservoir, which was built in 1950 and is held back from the Anderson Dam, holds 89,278 acre-feet of water when it’s at full capacity. It has enough capacity to hold all of the county’s other nine reservoirs combined, according to Keller.

Another part of the project, the Anderson Dam Tunnel, is already under construction and is expected to be finished by the end of the year, according to Valley Water. The tunnel will swap out a 4-foot-diameter outlet pipe with one that’s 13 feet, allowing more water to flow into Coyote Creek during construction and helping to better manage reservoir levels during future storms.

Officials won’t be able to start demolishing the dam until the tunnel is complete, Keller said. As the work progresses, the project will require a tight construction schedule, especially when the winter weather sets in. 

“Once dam removal begins, it is critical to achieve stage milestones in each construction season to meet seasonal restrictions to protect the dam and downstream communities during the winter,” he said. 

Plans to rebuild the dam have been in the works for more than a decade after independent reviews in 2008 found the structure was at risk of instability and liquefaction during a major earthquake, according to the State Water Resources Control Board. According to officials, a major earthquake on either the Calaveras or Coyote Creek fault could cause the dam to fail, releasing rushing floodwaters across cities and rural areas stretching from the southern San Francisco Bay to Monterey Bay and a large portion of Silicon Valley. 

A 2019 video produced by Santa Clara Valley Water shows a “worst-case scenario” mock-up with catastrophic flooding that, in the event of a dam failure, could affect nearby communities. Federal officials also raised the alarm in 2020, demanding that the Anderson Reservoir be drained as a safety precaution.