By Khalida Sarwari
Two South Bay dams are at risk of significant damage if a major earthquake occurs, according to the findings of a seismic study announced this week.
The study, conducted by URS Corporation, a firm that specializes in seismic safety evaluations, was reported to the Santa Clara Valley Water District Board of Directors on Tuesday.
The findings of the study, which are preliminary at this stage, showed that the Calero and Guadalupe dams are most vulnerable to earthquakes, said Marty Grimes, a project administrator for the water district.Calero is located in South San Jose and Guadalupe is within the Almaden Quicksilver County Park just southeast of Los Gatos.
The dams would be subject to significant damage if a major earthquake were to occur within close proximity of either dam, according to Grimes. For the Guadalupe Dam, it would take a 6.9-magnitude earthquake within two kilometers of the dam on the Stanford and Monte Vista fault zones and for the Calero Dam, a 6.7-magnitude earthquake within two kilometers of the dam on the Shannon fault.
The analysis found that the Almaden Dam, on the other hand, would be minimally impacted if a major earthquake occurred. That is because the dam has a well-compacted embankment on a strong foundation, said Grimes.
Grimes said the impact of such an occurrence on the dams would be “devastating” and “catastrophic,” leading to erosion and major flooding, especially in South San Jose.
All three dams are part of five of the district’s 10 reservoirs that have been subject to storage restrictions due to seismic concerns.
“This type of thing is fairly typical across California. There are quite a number of dams that have this same issue,” Grimes said. “It’s because they were built in the 30s, 40s, 50s — at the time they were state-of-the-art, but now the science has caught up to give us a better indication of what would happen if an earthquake happened closer to these dams.”
Calero has been under a storage restriction of 20 feet below the dam’s crest since January based on data that indicated the presence of alluvium — gravel and sand from the underlying creek bed — under the downstream dam embankment. Alluvium could make the dam vulnerable to damage during a major earthquake, Grimes said.
The Guadalupe Reservoir has been under a storage restriction of 14 feet below the dam’s crest since October 2006.
Grimes said the water district has requested that, based on the new findings, the storage restrictions on both the Calero and Guadalupe reservoirs be increased to 25 feet below their dam’s crest.
The additional 5-foot restriction would reduce the available storage capacity in the Calero Dam by 1,136 acre-feet and the additional 11-foot restriction in the Guadalupe Dam would reduce its available storage capacity by 1,003 acre-feet.
For the Almaden Dam, the district has requested that the restriction be removed, which could make available an additional 326 acre-feet of water storage in the reservoir.
The new restrictions must be approved by the state Division of Safety of Dams.
The seismic studies are a part of the district’s Dam Safety Program Report, which includes the installation of modern geotechnical instrumentation and extensive seismic safety evaluations at eight dams. The final report elaborating on the seismic findings related to the Calero, Guadalupe, and Almaden dams will be available in April 2012.
The reservoirs are an important component of the county’s water supply and also provide flood protection for homes and businesses downstream.
