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Preliminary findings indicate Anderson Dam needs seismic retrofit

Contact: Marty Grimes
Office: (408) 265-2607, ext. 2881
Mobile: (408) 202-4398

Date: Oct. 13, 2010

MORGAN HILL, CA – The Santa Clara Valley Water District has received preliminary findings from an ongoing seismic stability evaluation for Anderson Dam. The findings indicate that part of the dam could experience significant slumping if a 7.25 magnitude earthquake were to occur on the Calaveras Fault within two kilometers of the dam.

As a result, the district’s dam operators will keep the water at no higher than 57 feet below the dam crest until further analysis is completed. Currently, the water level at Anderson Reservoir is at approximately 60 feet below the dam crest, which amounts to 48,806 acre-feet. The California Division of Safety of Dams (DSOD) and the Federal Energy Regulatory Commission, which also has regulatory authority over Anderson Dam, have stated that an additional safety measure is appropriate, and are reviewing this proposed change in operation.

“Our first priority is the safety of our community. Our expert consultants agree that the level we are keeping the reservoir is conservative and prudent,” said Richard P. Santos, chairman of the water district board of directors.

Since January, 2009, Anderson Reservoir has been operating under a water level restriction of 20 feet below the spillway – 40 feet below the dam’s crest. This restriction was set by DSOD based on a review of data that existed at the time, which appeared to indicate the presence of alluvium – gravel and sand from the underlying former creek bed – under the upstream dam shell.

In January, 2009, the district hired a highly qualified geotechnical consulting firm to conduct the evaluation of the dam. Since then, the consultant has drilled borings at various locations on the downstream and upstream faces of the dam and conducted laboratory testing and engineering analyses to determine if the materials used to construct the dam are sufficient to withstand a major quake.

To date, the analysis has found loosely compacted layers of liquefiable materials in the lower portions of the dam. These materials are susceptible to a reduction in strength when subjected to severe earthquake shaking. The consultant will continue the analysis to estimate the potential for damage to the downstream slope. The final report is scheduled to be completed by May, 2011.

These initial findings indicate that remediation will be necessary to correct the deficiencies and allow for the reservoir to be safely filled to its capacity. Conceptual remediation alternatives will be developed as part of the ongoing analysis and included in the final report to be completed in May 2011.

Santos said, “We need to focus now on finalizing the study and expediting the remediation of this dam so that the reservoir can be fully utilized.”

  • On Thursday, Oct. 14 at 9:30 a.m., at 5700 Almaden Expressway in San Jose, the district’s technical team will be available to members of the media to provide further information and answer questions.

Water district technical staff will give a presentation to the water district board at its regular meeting on Oct. 26. The water district’s consultants and dam regulatory agency representatives will be on hand and available to answer media questions after the presentation.


Background:
Built in 1950, Anderson Reservoir is the largest reservoir in Santa Clara County, with a capacity of about 90,000 acre-feet of water (29 billion gallons). The reservoir is located on Cochrane Road east of Morgan Hill.

The water district plans to complete seismic safety evaluations on eight of its dams by 2013. The reservoirs are an important component of Santa Clara County’s water supply, but also provide environmental benefits and recreation. In addition, the reservoirs help provide flood protection for homes and businesses downstream.

The Santa Clara Valley Water District manages wholesale drinking water resources and provides stewardship for the county's watersheds, including 10 reservoirs, hundreds of miles of streams and groundwater basins. The district also provides flood protection throughout Santa Clara County.