Large new dam construction moving ahead in California

A venerable Bay Area landmark, built decades ago, but now in danger of a collapse in a major earthquake. Engineers are beating a path to restore it with a massive construction project. Those who have lived and worked in the Bay Area would immediately say: The Bay Bridge. They would be wrong.

We are talking about the effort to rebuild a vital linchpin of the Bay Area’s Hetch Hetchy water system. Environmental studies are due out today on a $409 million project to replace Calaveras Dam, a 210-foot-high structure east of Milpitas in the remote, oak-studded hills along the border between Santa Clara and Alameda counties. The project by the San Francisco Public Utilities Commission, the first new Bay Area dam in a decade, will also potentially save lives in case of a disaster and rekindle the supply of endangered steelhead trout (see “California Faces Major Decision on Dams,” 30 September 2009 HSNW; and “Old U.S. Dams Become a Multibillion-dollar Threat,” 28 August 2009 HSNW).

Oakland Tribune’s Paul Rogers writes that when it was built in 1925, Calaveras was the tallest earth-fill dam in the world, an engineering marvel that created a lake three miles long. In 2001, however,the state Division of Safety of Dams declared it unsafe for a major earthquake. If the dam collapsed during such a quake on the nearby Calaveras Fault, it would send a 30-foot-high wall of water rushing into Fremont and toward Interstate 880, later studies showed. Such an event could potentially kill thousands of people. “The original dam was built in a different era. In the 1920s, it was state of the art. But it was built with horses and wagons,” said Dan Wade, the San Francisco PUC’s manager of the Calaveras Dam project.

Because of the threat, the state ordered the reservoir drained to only 40 percent of capacity, losing enough water for 300,000 people a year.

With California struggling through the third year of a drought, the loss has come at a bad time.

South Bay impact
Rogers writes that unlike water from Hetch Hetchy Reservoir in Yosemite National Park, which flows to homes and businesses in San Francisco and northern San Mateo County, most of the Calaveras Reservoir’s water serves South Bay communities like North San Jose, Palo Alto and Fremont.

The new dam will be the same size as the old one, built several hundred yards downstream on Calaveras Creek. Construction bids are scheduled to go out