Friday, November 27, 2015

A New Earthquake-Proof Calaveras Dam

Workers with the San Francisco Public Utilities Commission inspect the New Irvington Tunnel, which opened in March 2015. Image by Owen Bissell 

By Sheraz Sadiq

Since July 2010, the San Francisco Public Utilities Commission has been hard at work on one of the biggest engineering projects in the nation, the Hetch Hetchy Water System Improvement Program. At a cost of nearly five billion dollars, the program will seismically upgrade and replace aging infrastructure that brings water from Hetchy Hetchy reservoir in Yosemite National Park, 167 miles away, to the Bay Area.

A key goal of the voter-approved program, which is scheduled to run through 2018, is to make sure that the taps can keep flowing within 24 hours of a major earthquake for the system’s 2.6 million customers who live in San Francisco, Alameda, San Mateo and Santa Clara counties.

“We have the Calaveras fault, the Hayward fault in the East Bay, and then of course the San Andreas fault on the Peninsula,” said Dan Wade, Director of the Hetch Hetchy Water System Improvement Program. “And our water system crosses all three of those major faults.”

According to the U.S. Geological Survey, there is a greater than 60 percent chance of a major earthquake taking place in the Bay Area in the next 20 years. The Hetch Hetchy water system has been operating for more than 80 years, and much of its infrastructure – including pipes, local reservoirs and a 90-year-old rock and earth-filled dam – is in need of a makeover to shield it from earthquakes.

Some of the construction projects are also intended to provide redundancy and a back-up to structures that are critical to transporting water from the Sierra Nevada watershed to the Bay Area.

For example, the New Irvington Tunnel, which opened in March 2015, transports millions of gallons of water each day alongside the original Irvington Tunnel in Sunol Valley, a few miles east of Fremont.

The older Irvington Tunnel, which has not been taken out of service since 1966, when it was last inspected, lies between the Calaveras and San Andreas faults. The new tunnel, however, is steel-lined and encased with concrete to help it withstand a magnitude 7.1 earthquake.

Both tunnels carry water not only from Hetch Hetchy but also from the nearby San Antonio and Calaveras reservoirs.

The Calaveras Reservoir, which is the largest of the system’s five local reservoirs, is also in need of a seismic makeover. Its 90-year-old earth and rock-filled dam, which forms the reservoir, is located on the Santa Clara-Alameda county line, and is located only 1,500 feet from the Calaveras fault. Since 2001, state dam regulators have only allowed the reservoir to be filled to 40 percent of its capacity because the dam is prone to liquefaction, which happens when waterlogged loose soil behaves like a liquid during the violent shaking generated by a big earthquake.

As a result, construction crews are building a new, 220-foot-tall seismically safe dam a few hundred yards downstream from the original dam in the hills southeast of Fremont. At a cost of $720 million, replacing the Calaveras Dam is the biggest, most expensive and last remaining major project under the Hetch Hetchy Water System Improvement Program.

Although it will also be made of earth and rock – roughly 10 million cubic yards’ worth – cement grouting is being sprayed between spaces within the rock to create a more water-tight foundation. The reservoir will then be able to fill to capacity – 100,000 acre-feet or 31 billion gallons – when construction on the new dam finishes in 2018.

Ten million cubic yards of earth and rock will need to be excavated for the construction of the new Calaveras Dam, located at the Alameda-Santa Clara county line. Image by Owen Bissell for KQED Science
Image by Owen Bissell 

For Dan Wade, filling Calaveras reservoir to full capacity will not only boost water storage but help the regional water system cope with multi-year droughts.

“We’re in the third year of a major drought,” he said. “We need this reservoir for drought carryover storage.”

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