Calero, Anderson dams on water district’s radar

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In November, the Santa Clara Valley Water District, which owns
Anderson Dam and is responsible for its seismic upkeep, briefed its
board of directors that the dam requires seismic retrofitting and
its spillway was reduced. This week, a portion of Calero Dam that
lies along the Shannon fault in South San Jose has

inadequate seismic stability

because gravel and sand material from the underlying creek bed
could liquefy and be vulnerable to damage during a major
earthquake.
After a series of 10 earthquakes in and around Morgan Hill over the last two weeks, the susceptibility of Anderson Dam breaking and putting the town underwater grew more concerning.

In November, the Santa Clara Valley Water District, which owns Anderson Dam and is responsible for its seismic upkeep, briefed its board of directors that the dam requires seismic retrofitting and its spillway was reduced. This week, a portion of Calero Dam that lies along the Shannon fault in South San Jose has “inadequate seismic stability” because gravel and sand material from the underlying creek bed could liquefy and be vulnerable to damage during a major earthquake.

Safety is the water district’s No. 1 priority said Marty Grimes, the district’s spokesman. In response to Calero, the dam’s water supply will be kept no higher than 20 feet below the dam crest “until the full integrity of the dame can be assessed or corrective action can be completed,” the district reported in a press release.

The district’s initial analysis found loosely compacted layers of potentially liquefiable materials under much of the downstream Calero dam embankment, which can lead to a loss of stability if a violent earthquake occurs. The consultant will continue the analysis to estimate the potential for damage to the dam. The final report is scheduled to be completed by March 2012.

Calero Reservoir and dam were constructed in 1935 along Calero Creek. It can store 9,934 acre-feet of water, and compared with Anderson (51,250 acre-feet) Calero is a mid-size reservoir among the 10 major dams in Santa Clara County. One acre-foot is enough water to supply a family of five for two years.

In February, the water district is scheduled to hold a public meeting with neighbors of Anderson Dam to review plans for the seismic retrofit.

The water district has put $110 million into its Capital Improvement Plan as a placeholder for fixes to Anderson, but that could change “significantly as the process of developing plans for a retrofit project progresses,” according to the district.

District officials said in October it could cost as much as $150 million and take six years to fix the 60-year-old Anderson Dam.

The type of alluvium found to be a risk at Calero is similar to that discovered last year under Anderson Dam, which also has a storage restriction imposed by the California Division of Safety of Dams.

Engineering standards and knowledge of earthquake faults have advanced significantly since these dams were built. Many dams built in this era in California are undergoing seismic evaluations. Approximately 60 dams state-wide now have storage restrictions and some will undergo extensive seismic retrofits.

At Anderson, the district found that if a 7.25-magnitude earthquake were to occur on the Calaveras Fault about a mile away the dam could experience “significant slumping.”

Currently, water is 57 feet below the dam’s crest and the California Division of Safety of Dams and the Federal Energy Regulatory Commission have said that the safety measure is appropriate.

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