Anderson Dam may fail during quake

January 13, 2009 6:23:51 PM PST
The Santa Clara Valley Water District is taking urgent action tonight to look at the threat Anderson Dam poses in the event of an earthquake. Anderson is the district's largest of 10 reservoirs and sits east of Highway 101 above Morgan Hill.

Anderson Dam was built in the 1950's near the Calaveras Fault. At the time, engineers did not consider how the earth filled dam might collapse during an earthquake. Now preliminary studies show a major temblor could flood Morgan Hill with a 35 foot wall of water in a matter of minutes.

Rosemary Kamei sits on the Santa Clara Valley Water District Board and lives below the dam in Morgan Hill.

"I was there during the 1989 Loma Prieta earthquake and it is very scary and I think as far as I'm concerned safety is number one," said Kamei.

Kamei's board colleagues agree and today ordered a $3.5 million comprehensive study of the dam.

The State Division of Safety of Dams also ordered the district to keep water levels at Anderson Reservoir 20 feet below the spillway as an insurance policy.

"If an earthquake was to occur and if the dam were to be damaged and start slumping we feel that this level would be a very nice cushion," said Safety of Dams Chief David Gutierrez.

The district says lowering the water level of Anderson reservoir potentially means a loss of 23,000 acre feet of water and millions of dollars each year.

Those costs were mentioned today but the board chair says money is irrelevant given the state mandate and potential loss of life.

"Safety is an issue we cannot ignore and we had to take this action," said Water Board Chair Sig Sanchez.

If heavy rains come close to filling Anderson Reservoir, the district will regulate releases into Coyote Creek.

Morgan Hill city leaders were especially pleased with Tuesday's action.

"The two things that having less water does, one' it's less risk of the dam collapsing in a major earthquake and secondarily the consequences of failure are also less," said Morgan Hill Public Works Director Jim Ashcroft.

The in depth study will take 12 to 18 months, but the district says it will take further action if there's any early evidence that indicates more needs to be done.