A $114.4 million contract to upgrade the Folsom Dam and elevate its main structure has been awarded to a joint venture of Shimmick Construction and California Engineering Contractors.
The U.S. Army Corps of Engineers, Sacramento District. has charged the team with raising the 340-foot main dam by 3.5 ft. The contract also includes elevating the structure’s left and right wing dams and modifying its eight spillway gates by adding top seals, strengthening the gates and raising the gate piers.
According to the Corps, the project, when completed in 2027, will enable better control of water releases during large flood events and increase temporary storage capacity of the Folsom Lake Reservoir by 43,000 acre-ft. The improvements will reduce flood risk to the greater Sacramento area by allowing for more water storage in extreme flood events.
"Although there's much work left to be done, this award represents the culmination of several years of effort from our dedicated team," said Elvan Childs, Corps project manager.
The Folsom Dam, located 25 miles northeast of Sacramento, is the cornerstone of the region's flood control system on the American River. The 68-year-old concrete gravity dam regulates runoff from about 1,875 sq miles of drainage area.
The Folsom Lake Reservoir has a normal storage capacity of 975,000 acre-ft with a minimum seasonally designated flood control storage space of 400,000 acre-ft. After many years of drought, the reservoir has seen an increased water level since 2022 following heavy area rains and record snowpack in the Sierra Nevada mountains. The lake is currently 21.02 ft below its full pool of 466 feet but is predicted to fill up by mid-summer.
As far back as the late 1980s, the Corps and other agencies recognized that improvements to the dam and reservoir infrastructure were necessary to provide what flood control officials deemed a minimum 200-year level of flood protection to the Sacramento area. The 1995 failure of one of the dam’s spillway gates resulted in the uncontrolled release of nearly 40% of Folsom Lake and a flood of 40,000 cu ft per second moving down the American River.
Recent work on shoring up the reservoir's aging infrastructure and improving area flood control has been ongoing since 2009. Construction of the Folsom Dam Auxiliary Spillway was completed in 2017, and Dike 8, on the southeastern side of Folsom Lake, was raised 3.5 feet in 2020.
However, a recent survey by the Corps and U.S. Bureau of Reclamation revealed a design flaw with steel rods that open and close the auxiliary spillway’s steel gates. This has caused the rods to corrode and could potentially prevent the system from operating. The spillway gates were opened for the first time in January with no issues.
According to the Corps, the majority of the project's funding was appropriated by Congress, including the allocation of $216.5 million in supplemental funds as part of the 2018 federal budget. Local sponsors include the state's Central Valley Flood Protection Board and the Sacramento Flood Control Agency, which are also providing funds.
Follow-on work will include raising the reservoir's remaining Dikes 1-7 along with the Mormon Island Auxiliary Dam and reconfiguring the Folsom Dam's sluice gates with modernized temperature control shutters.