Too much water a bad thing for South Bay reservoirs

February 12, 2010 3:19:40 PM PST
Most water districts are thrilled about the rainstorms that are filling our reservoirs after three years of drought, except for the South Bay. The reason is because the Santa Clara Valley Water District is worried five dams could collapse in a major earthquake if rain fills them to the brim. They were built in the 1930s and 1950s.

Morgan Hill residents have known for a long time that they live in the potential line of a deadly tsunami of water, that is why so many are monitoring state-mandated studies which will define whether the dams need to be seismically upgraded.

Water collecting just below at the Anderson Reservoir in Morgan Hill could pose a problem. It has been assigned an earthquake-limited storage capacity because its dam may be seismically unsafe.

"We really don't want to see more water in the dam than the 74 percent threshold that the State Division of Safety of Dams has regulated," Jim Ashcraft, director of Morgan Hill Public Works said.

Ashcraft says for public safety capacity needs to remain low. A dam engineering study has shown that if, for example, a 6.6-magnitude earthquake happens on the Calaveras Fault and is centered at Anderson Reservoir, it could cause the earthen dam to fail, sending a 35-foot wall of water into downtown Morgan Hill.

"It's been pretty common knowledge down here that it is a risk," Morgan Hill resident Gary Neyer said.

"I think everyone knows it's a pretty remote possibility of a catastrophic collapse, but remote or not, at least in some people's minds? in an hour, certainly for city staff it's a critical issue," Ashcraft said.

Seismic safety studies are currently underway of the aging dams at five of 10 reservoirs the Santa Clara Valley Water District operates. These include Anderson, Coyote, Calero, Guadalupe and Almaden.

During the past three years of drought, managing water levels has not been an issue. But with so much rain this year, the district says it could have to resort to releasing billions of gallons of water into the San Francisco Bay.

"If we get another series of really powerful storms, there may come a point where we just have to do some releases," Santa Clara Valley Water District spokesperson Susan Siravo said.

"I think it's a waste of water," businessman Jed Freitas said. "I mean that's a pretty precious resource that needs to be used, utilized, stored. If it takes building dams in other places or retrofitting existing dams, I think we ought to do it."

At this point, the district water system is safe. It wants to emphasize that to residents. Until the future of these five dams is decided, the district says it will try to save all the coming rainwater possible; to do this, it has been transferring water from smaller to larger reservoirs, moving some to treatment plants, pumping it into groundwater basins, and using less imported water from the Delta.