Anderson Dam, the largest in Santa Clara County, California, is set to be demolished and rebuilt starting early 2027.
Water officials recently awarded a $1.88 billion construction contract to harden the 235-foot high structure to withstand a large earthquake, according to a July 28 press release by Santa Clara Valley Water.
The Anderson Reservoir is expected to return to its full capacity in late 2033 as the project is completed. The water level of the reservoir has been lowered to 3 percent of capacity since mid-December 2020.
“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,” Santa Clara Valley Water stated.
The project plans to increase the dam crest height by nine feet to hold more water during large storms, and replace the concrete spillway with a seven-foot taller one to discharge large storm flows, according to a March white paper by the district.
The crews will start in January 2027 to rebuild most of the dam and spillway, install new outlet pipes, and complete other major safety upgrades, Valley Water said in the press release.
The total cost for the Anderson Dam Seismic Retrofit Project is approximately $2.8 billion.
Built in 1950, the Anderson Reservoir is the largest of the 10 reservoirs run by Valley Water, and it holds approximately 90,000 acre-feet of water in full capacity, exceeding the totality of all the other nine reservoirs.
“Today marks the next phase of one of Santa Clara County’s most important water infrastructure projects that will benefit our community for generations to come,” Valley Water Board Chair Tony Estremera said in the press release.
The agency has completed most of the precursor projects to help manage flows in Coyote Creek and to prepare the site for rebuilding Anderson. The Anderson Dam Tunnel Project will be completed later this year, according to the press release. The project replaces the dam’s old 4-foot-diameter outlet pipe with a 13-foot-diameter one, allowing more water to flow into the river during construction and better managing the reservoir’s water level to withstand future storms.
The Anderson Dam sits along the Coyote Creek Fault east of Morgan Hill, while the reservoir lies parallel to the Calaveras Fault.
Plans for the safety enhancement of the dam began after independent reviews found in 2008 that the dam was at risk of instability due to earthquakes and liquefaction, according to the State Water Resources Control Board.
The Anderson Dam Seismic Retrofit Project was initiated in 2012, and the completion time was extended as new studies revealed threats from previously unidentified fault lines, according to the white paper.
