Plans released on how to drain, rebuild Anderson Reservoir dam over 10 years

The South Bay's 70-year-old Anderson Reservoir dam is such an earthquake hazard that last February federal regulators ordered that the reservoir behind it be completely drained, repaired, and replaced. 

The 10-year and half a billion-dollar-plus seismic retrofit project will replace the current dam with one that can withstand a strong quake. 

Federal regulators fear that a full Anderson Reservoir dam wall would fail, releasing massive waters that would eventually find their way to the Bay and ocean, killing thousands in the process. 

Beginning on Oct. 1,  the huge reservoir will be drained at the rate of 65 million gallons a day. 

But even if a major quake hit right now, no flood would happen. 

"Right now, we expect that the dam would slump about 20 feet, so, basically sag in the middle about 20 feet and we've got a restricted reservoir level that's lower than that," said Valley Water Operations Executive Chris Hakes.

The draining process won't start until October because the reservoir, which is currently less than a third full, will continue being used for human consumption taking it down to just 11% of capacity.

While some of the water will be released down Coyote Creek, much of it will be pumped into underground aquifers. The agency will rescue and relocate any remaining fish from the reservoir.

Then, over the next three years, contractors will build a 24 foot wide, tunnel that will protect a slew of widely placed outflow pipes. 

"During a major earthquake, the piping inside of that outlet tunnel could move up to four feet. So, it's been sized accordingly so that within that 24-foot diameter the pipes can go four foot one way or another and not be damaged and continue to operate," said Hakes. That would increase the water flow out of the reservoir whenever needed for any reason.

After that, build a new, seismically safe super dam to be completed in a decade. 

"Our spillway would be able to pass all the flows that would come through the dam, our outlet tunnel would be intact and our outlet piping would continue to work and the dam would not slump," said Hakes.

Even though the dam has survived three major earthquakes, including Loma Prieta, that is not good enough. 

"Safety is our number one priority to protect our residents in those areas, to do this work in a timely manner, efficient and protect our citizens," said Valley Water District Director John Varela.

Valley Water says it has ample storage in its other reservoirs and underground aquifers sufficient to supply the needs of humans, wildlife and the environment.