SANTA NELLA - A multi-year seismic retrofit project of B.F. Sisk Dam is moving forward, the federal Bureau of Reclamation and state Department of Water Resources recently announced.
The preferred alternative to address seismic safety concerns at the structure, commonly known as San Luis Dam, involves raising the crest of the dam by 12 feet, adding shear keys to prevent slippage and construction of downstream berms to strengthen the structure, bureau spokesperson Christie Kalkowski told Mattos Newspapers.
Work on the project is expected to begin in August 2021, she said.
That alternative, Kalkowski reported, will involve an eight- to 10-year construction duration with work seven days a week, 12 months a year. Funding availability could extend the project duration.
On any given day, the Reclamation spokesperson said, approximately 76 on-site construction shift workers are expected to be on the job. In addition to the direct construction related activities, an EIR states, the project would generate administrative, design, environmental compliance and oversight jobs, as well as need for truck drivers and equipment haulers.
The dam was built more than 50 years ago to create the San Luis Reservoir, which provides more than two million acre-feet of storage capacity for the Central Valley Project and State Water Project. The Department of Water Resources operates the Reclamation-owned dam and reservoir and is a cost share partner.
A 2006 seismic study raised concerns, project officials told Mattos Newspapers in 2017.
Mynul Chowdhury, project manager for the Bureau of Reclamation, said the study determined that the soil in the earthen dam could undergo liquefaction during an earthquake, potentially causing the dam to slump below the water line and lead to failure through over-topping. The higher crest will safeguard against over-topping by increasing the freeboard, according to environmental documents on the project. The project will not increase reservoir capacity, as the maximum fill level will remain unchanged, the documents indicate.
A second scenario was the potential for an earthquake to cause cracks in the dam, leading to failure.
Chowdhury stressed that the probability of a dam failure is very low - but said the planned retrofit is needed to bring the structure up to current seismic safety standards.
“The chances (of a dam failure) are very remote, less than 1 percent in any given year. However, if the dam fails the consequences would be severe,” Chowdhury told Mattos Newspapers at the time.
One alternative considered was a reduction in reservoir capacity, which would have reduced maximum storage from just over two million acre-feet of water to just under 1.4 million acre-feet. That option, however, would have adversely impacted water supplies to the Central Valley Project and State Water Project.
Agency officials said the crest raise alternative meets all project objectives.
“This seismic upgrade project is part of our commitment to reduce the risk to downstream communities while protecting our valuable water supply,” said Ernest Conant, regional director for Reclamation’s California - Great Basin. “The strong partnership we share with the state allows us to leverage the expertise, resources and funding to ensure a safe and successful project.”
Ted Craddock, acting state water project deputy director, echoed that sentiment.
“We will continue to move forward with the important work to modernize infrastructure to protect California’s water supply and enhance public safety,” he commented in a recent news release. “This project represents a significant investment to address seismic risk to our water infrastructure.”
The bureau has implemented additional risk reduction measures as it works to put a more permanent dam safety project in place, according to the press release. Those measures include heightened earthquake monitoring, real-time seismic monitoring, dam safety tabletop exercises with local responders, increased seismic inspection criteria and an updated dam emergency management plan.
Reclamation and the Department of Water Resources plan public meetings in the spring to further inform the public of the dam safety project.
Exploratory blasting on bureau property to identify suitable material for construction of dam stability berms and other dam safety features will occur in the next few months, according to the news release. The public will be informed of the blasting, officials said.
Blasting may be heard, the press release stated, but the work is not anticipated to impact the public or the environment.
The public will also be notified of any impact to recreational uses.