The Santa Clara Valley Water District has received preliminary
findings from an ongoing seismic stability evaluation for Anderson
Dam that detail part of the dam could experience
”
significant slumping
”
if a 7.25 magnitude earthquake were to occur on the Calaveras
Fault within about a mile of the dam.
The Santa Clara Valley Water District has received preliminary findings from an ongoing seismic stability evaluation for Anderson Dam that detail part of the dam could experience “significant slumping” if a 7.25 magnitude earthquake were to occur on the Calaveras Fault within about a mile 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 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.
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 analysis 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.
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.
