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November 15, 2011 > Two more dams vulnerable to quakes

Two more dams vulnerable to quakes

By Director Richard Santos, Santa Clara Valley Water District

Over the last couple of years, we have been receiving more and more information about the seismic stability of our local dams. A couple of weeks ago, the Santa Clara Valley Water District Board of Directors heard yet another presentation about our ongoing seismic stability studies. The preliminary findings show that two dams, Calero and Guadalupe, are susceptible to significant damage if a major earthquake were to occur in their close proximity.

The board is concerned for public safety, above all else. As a result of these new findings, our staff promptly took action to operate the reservoirs under new storage restrictions to minimize the chance of the dams failing and an uncontrollable release of the water behind them. Both dams now are operated to maintain the water level no more than 25 feet below their crest. The experts have assured us that this safety buffer is conservative and appropriate.

While we are confident that we are operating our dams safely, we are continually preparing for potential disasters. I represent the water district on the Santa Clara County Emergency Preparedness Council, which is made up of all 15 cities, the County, and the water district. Because we are part of the council, we regularly coordinate and share information with other public agencies to prepare for disasters.

The type of damage that the experts predict for these dams is a slumping of the crest (top) and cracks in the dam. If this were to happen when the reservoir was full, water could flow over the top of the dam, eroding the crest and ultimately, releasing water downstream. Both of these dams are in the Guadalupe Watershed, which means the water released would flow toward the Guadalupe River, which would be unable to contain the massive flow.

Including these two dams, we currently have five dams that are under storage restrictions. This has a significant impact on our water supply. Anderson Reservoir, the largest of our ten reservoirs, is restricted to about 68 percent of its capacity. In all, our reservoir storage capacity is currently reduced by 27 percent.

Some may wonder how this happened. Were these dams built poorly? No, they were built using the state of the science at that time. We now know much more about how earthen dams behave in earthquakes, and we know much more about the various faults that crisscross the Bay Area. These new studies are using the latest, modern field and lab investigation and analytical techniques.

We have initiated a capital project to plan for a retrofit for Anderson, and another project has been initiated to address Calero and Guadalupe dams. We do not yet know what the potential solutions are or how much they will cost. The good news is it is possible to retrofit dams with these kinds of issues.

And one more piece of good news: this study has revealed that Almaden Dam is more seismically sound than previously thought. As a result, our expert consultants and an independent technical review board have recommended that the current storage restriction on Almaden Dam be lifted. If the state Division of Safety of Dams approves, an additional 326 acre-feet of water storage will be available in the reservoir.

Over the next few years, you'll continue to hear a lot about our dams. More studies will be completed. Plans for retrofits will be developed. Construction will commence, and ultimately, we will end up with safer dams and a more reliable water supply.

As always, I am available for questions or comments as your District 3 representative for the northern areas of Sunnyvale and Santa Clara; Alviso; Milpitas; and the north San Jose and Berryessa communities. Feel free to contact me at (408) 234-7707.

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