Report shows Anderson Reservoir at risk

January 6, 2009 3:40:40 PM PST
For the next 18 months officials will keep water levels down in Santa Clara County's largest reservoir. That's because a new engineering report shows Anderson Dam at risk for collapse in a major earthquake - a scenario that could put Morgan Hill under water within minutes.

City officials in Morgan Hill say the community knows it lives in the shadow of the 240 foot tall Anderson Dam - so naturally it would fall under consideration during emergency planning. The possibility of danger is much more real now - with a new dam study out that says - in a worst case scenario of a dam failure, a 35 foot wall of water could flood downtown Morgan Hill within 14 minutes.

And that's not all. An engineering report commissioned by the water district also says a complete failure of Anderson Dam could rush water eight feet deep into San Jose within three hours.

The dam was built in 1950 and was designed to address earthquakes, but not for the strength of quakes that could realistically hit the region today. In this preliminary study of seismic stability, the study evaluated the risks, if a major earthquake were to hit the Calaveras or Coyote Creek faults.

"In order for it to actually be damaged and for water to pour out, you would need the dam to be full of water, a major earthquake hit right at the dam or within a few miles from it. So the situation isn't very likely, but it's not impossible," said Susan Siravo, Santa Clara Valley Water District.

The water district says although the risk of a collapse is small, as a precaution, it will not allow the water level to reach within 30 feet of the top of Anderson Reservoir. Keeping it no more than 87 percent full - it's currently at 63 percent capacity. This safety plan will stay in place for at least 18 months while more detailed engineering studies are done to determine whether retrofit work is needed.

The Santa Clara Valley Water District has alerted neighboring cities and county leaders of the dam safety report. At next week's board meeting on Jan. 13th, the water district plans to recommend the approval of further engineering studies. These studies are estimated to cost up to $4 million.

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