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"We had a recent survey done and we passed," said EBMUB Director John Coleman, whose district includes Lafayette. "After the Oroville Dam breach, the state redid all of the state dams."
The redo resulted in a "fair" rating for the reservoir, meaning that the dam can be operated safely in current conditions, but could be imperiled in events like earthquakes. The Lafayette Reservoir received the low grade due to the seismic vulnerability of its outlet tower.
According to a district advisory, because of the small size of the reservoir and its low runoff, the limited water flow through the outlet, and the planned reduction of reservoir levels, "The impact of a tower failure that affects EBMUD's ability to release water from the reservoir is expected to have limited impacts to the Lafayette community."
The reservoir was built in 1927 as a standby supply of drinking water, and rather than a spillway for drainage, the outlet tower was erected in the middle of the 1.4 billion gallon watershed. "The tower acts as a spillway for the reservoir. When the reservoir gets full, water gets released without flowing over the dam," said Jimi Yoloye, EBMUD construction division manager. "The water goes into the Lafayette creek in a controlled manner."
But Russell Bowlus, DOSD northern regional engineer, offered this assessment: "Since the outlet tower also serves as the spillway for the dam, failure of the tower could cause an uncontrolled release through the spillway or outlet conduits - or both - or could block the conduits and not allow EBMUD to control the reservoir."
In its capital projects budget, EBMUD has approved modifications to the Lafayette Reservoir that include "seismic and gate control upgrades, and modification of the tower to act as a spillway capable of handling the revised probable maximum flood," meaning, the largest flood that could conceivably occur at the reservoir location. The district estimates the probable maximum flood for the Lafayette Reservoir at a volume of approximately 4,300 acre-feet. One acre-foot of water equals 326,000 gallons, enough to cover an acre of land - a little smaller than a football field - one foot deep. The largest runoff at the reservoir over the past 10 years was 770 acre-feet.
The reservoir modifications were slated for construction in 2022, but in a June 30 letter to the district from Sharon Tapia, DOSD chief, she advised that "continued delays in addressing these issues are increasing the potential risk to public safety. Consequently, EBMUD's proposal to begin construction in five more years is unacceptable."
"It could be started by 2019," Coleman said.
Lafayette Council Member Cam Burks, the council EBMUD liaison, said he would like the district to hold a public forum to explain the technical details of the state rating and to fully outline the risks to the community. The city plans to agendize this topic at a council meeting.
Not everyone appeared worried about a potential catastrophe. "If it goes, we'll be the first to swim, but we aren't losing any sleep over it," said Ricardo Vitery, who lives in the Village Center neighborhood, the closest community below the reservoir. "We're more concerned for the victims of hurricanes Harvey and Irma."
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Lamorinda Weekly, Moraga CA