InfrastructureCalifornia dams plagued by seismic concerns

Published 27 January 2011

Half of Santa Clara County, California’s reservoirs cannot be filled to their full capacity due to seismic concerns; engineering tests revealed that in the event of a major earthquake the dam could slump sending a deadly tidal wave across densely populated communities; seismic retrofit costs to the county’s dam are estimated at $150 million; with the reduced capacity, the county’s dams must be maintained at 67 percent of its total capacity and cannot store more water in preparation for future droughts; the lost capacity could provide water for 280,000 people for a year

Caleros reservoir, deliberately kept below capacity // Source:

Five of Santa Clara County, California, ten reservoirs cannot be filled to their full capacity due to seismic concerns, further reducing water storage capacity in a drought prone state.

According to the San Jose Mercury News, on 18 January 2010, the Santa Clara Valley Water District announced that engineering tests revealed that it must further reduce the amount of water stored in the Calero Reservoir, located in South San Jose.

Tests revealed that the foundation beneath the ninety-eight foot earthen dam contains only sand and gravel. In the event of an earthquake, engineers fear that the foundation would liquefy and cause the dam to slump.

When a dam slumps, water spills over the top in large quantities and causes disastrous flooding. These cases are rare, but highly destructive.

In 1928 the St. Francis Dam failed sending a 125-foot tall mountain of water cascading through Santa Clarita, California, killing 450 people.

To ease the strain on the dam and reduce the likelihood that water will spill over the top during an earthquake, engineers recommended that the Calero dam be maintained at 57 percent of capacity.

The latest announcement means that Santa Clara County’s ten reservoirs will hold only 67 percent of their originally designed capacity, or 113,000 acre-feet out of 169,000 acre-feet.

An acre-foot is roughly 326,000 gallons, the amount of water it takes to flood one acre a foot deep.

The reduced storage capacity hampers the county’s ability to store water in the event of future droughts, a condition which often plagues California.

In the event of heavy rains, the county will not be able to store the additional water due to these new restrictions.

The lost capacity would have been able to provide drinking water for 280,000 people for a year.

The dams provide water for roughly 1.8 million residents including the city of San Jose, the tenth largest city in the United States with nearly a million people.

Seismic retrofitting of the county’s five dams, built between the 1930s and 1950s, could cost up to $150 million. With the poor state of California’ finances, fixing the dams would require the water district to shift money away from planned spending on other projects including a desalination plant and flood control work.

The problem is significant,” said Don Gage, the chairman of the Santa Clara County water district’s board. “You have to understand that these dams are 80 years old. The earthquake standards back then were not what they are today. We are going to have to shift dollars in our capital improvement programs from other projects to these dams.”

According to Marty Grimes, a spokesman for the water district, safety is their primary concern.

We’re taking a very conservative approach to safety. That’s the No. 1 concern,” he said.

We’ll keep the water level that low until we can establish that the dam can withstand the maximum credible earthquake on that fault.”