Anderson Dam will start construction in 2020 and cost $400 million
Published in the March 29 – April 11, 2017 issue of Morgan Hill Life
By Staff Report
Built in 1950, Anderson Dam needs to be rebuilt at a cost of $400 million after studies on the structure released last year showed it could fail in a 7.2 magnitude or greater earthquake on the Calaveras Fault and a 6.6 magnitude or greater quake on the Coyote Creek Fault. The project is scheduled to begin in 2020 and finished in 2024.
More than 150 residents heard Santa Clara Valley Water District officials update the Anderson Dam seismic retrofit project, clarify misinformation and address concerns about the project at a March 23 meeting at the Morgan Hill Community and Cultural Center. Deputy Operating Officer Katherine Oven said if the 89,073 acre-feet Anderson Reservoir was nearly filled, a structural failure of Anderson Dam might release a wall of water 30-feet high that would inundate Morgan Hill and cause major flooding along Coyote Creek as it travels north to San Jose.
Findings of the original seismic stability evaluation completed in 2011 on Anderson Dam indicated that the downstream and upstream embankments could become unstable during a very large magnitude earthquake and the rupture of faults underlying the dam may have adverse impact on the outlet pipe and intake structure. A storage restriction of about 55 feet below the dam crest was put in place to protect the public, reducing the allowed storage capacity to 52,553 acre-feet.
“Once we knew we started the project and restricted reservoir levels,” said Hemang Desai, dam safety program manager for the SCVWD. That was in 2012. At that time, the district began assessing the problem and designing plans to fix the dam. In addition to seismically retrofitting the dam embankment, the planning phase of the project also identified the need to replace the existing outlet pipe that runs below the dam, increase the wall height of the concrete spillway and increase the height of the dam by seven feet.
Last year, while conducting studies and investigating areas around the dam to collect data for the embankment retrofit, previously unidentified seismic deficiencies were discovered that resulted in changes to the project.
“We had started, but the concept had to be changed because of the new findings,” Desai said.
The new retrofit project, originally planned to include large upstream and downstream buttresses, was modified to nearly completely replace the current dam. In addition, a new high-level outlet will be built to allow rapid draw-down of the upper level of the reservoir in case of an emergency.
Two questions asked by the public concerned the time frame of construction and the availability of funding from the federal government. Although the original dam was built in nine months, construction regulations have become much stricter in the past 67 years, Oven said.
“In 1950, there were no environmental laws or governmental agencies to address and mitigate impacts,” she said. “The world has changed. It was built to standards at the time, but now our knowledge of seismic impacts has grown.”
Ivan Guevaro, an Anderson lakefront homeowner, wondered about the funding. “What percent of the funding comes from the federal government and could they pull funds due to sanctuary cities in the state?” he asked.
“Right now there is no federal funding,” Oven said. “Our funding comes from water rates. We’ll sell bonds and pay them pack over 30 years.”
SCVWD Board Chairman John Varela, who represents the South Valley region, said state and regional officials toured the area in mid-March.
“We have verbal commitments from them to get funding,” he said.
Additional public meetings will be held during the planning stage and again as the environmental impact is prepared.
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