The U.S. Department of Interior’s Bureau of Reclamation is preparing to begin dam safety work at Boca Dam, on the Little Truckee River in California, to better resist the impacts of potential earthquakes.
Based on extensive engineering analysis, Reclamation found that Boca Dam and Dike is at risk from structural failure during an earthquake due to the presence of sand and gravel within the dam’s foundation. During an earthquake, the dam has the potential to slump or crack, leading to overtopping or a breach. The Safety of Dams Act requires Reclamation to reduce the risk of Boca Dam structural failure within acceptable levels.
“Safety is a top priority for Reclamation,” said Terri Edwards, Lahontan Basin Area office manager. “Through our Safety of Dams program, we are correcting seismic risks at Boca Dam. The project will construct an earthen buttress on the downstream face to stabilize the dam and also modify the spillway crest structure.”
The dam is owned by Reclamation and operated by the Washoe County Water Conservation District.
Reclamation says the work will begin in May 2019 and extend to October 2020, temporarily stopping during the winter. The reservoir will operate at about half its capacity, about 28 feet lower than maximum water levels, during the summer and fall of 2019 to assist construction.
Modifications will involve removing a sand and gravel lens on the downstream side of the dam and constructing a downstream stability berm to buttress the dam. The spillway will also be seismically retrofitted and the dam crest will be 25 feet wider with a reinforced concrete road surface. Reclamation awarded a $19.7 million contract for the work to NW Construction Inc. in December 2018.
The dam was completed in 1939 and has a height of 116 feet and a crest length of 1,630 feet. It impounds Boca Reservoir, which has a capacity of about 40,000 acre-feet. The dam provides flood protection for Reno and Sparks, Nevada. The reservoir is used to regulate the Truckee River and provide water for recreation, irrigation, fish and wildlife benefits, and drought supplies for municipal and industrial users.
Boca Dam and Reservoir are the major feature of the Truckee Storage Project, which was constructed to provide a supplemental supply of irrigation water to about 29,000 acres of land in the Truckee Meadows.