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American River Common Features Folsom Dam Modifications Folsom Dam Raise Folsom Dam Bridge
 

 

Dam raise reduces flood risk
Raising Folsom Dam, when added to flood reduction benefits of the Common Features Project and the modifications to Folsom Dam’s outlet, will reduce Sacramento’s flood risk to about a 1-in-213 chance of flooding in any given year.

Construction effort
The Folsom Dam work involves raising the concrete section of the dam, raising the earth embankments on each side of the dam, adding larger spillway gates, and raising the Mormon Island auxiliary dam and eight dikes around the lake approximately 7 feet. These improvements will add 95,000 acre-feet of floodwater storage capacity to the lake’s current 977,000 acre-foot capacity.

All eight spillway gates will require replacement under this plan. The new gates will be approximately 66 feet high, 16 feet taller than the current gates.

The spillway stilling basin and sidewalls will be extended approximately 60 feet to ensure adequate energy dissipation of the larger flows
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Dam, auxiliary dam, and dike raising
The methods for raising the concrete dam, the earthen Mormon Island auxiliary dam and eight dikes are very different.

Raising the concrete dam involves two different efforts. One is to raise the section of the dam containing the spillway gates, and the other is to raise the concrete sections on each side of the spillway gates.

Raising the section with the spillway requires removing the roadway crossing the dam (the road is actually on a bridge that goes over the dam), raising the piers for the road, installing larger flood control gates, and replacing the road.

Engineers will raise the concrete sections of the dam on each side of the spillway 3.5 feet, and install a 3.5-foot high concrete wall on the reservoir side of these sections. The 3.5-foot wall (called a parapet) will keep wave wash from going over the dam when the lake is full.

The Mormon Island auxiliary dam and eight dikes are earthen structures. Engineers are studying two methods to raise these structures. One is to enlarge them using soil only, and the other calls for using a combination of soil and parapet walls.


 


 
   
   
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Folsom Dam Raise illustration
   
   
  Cost and schedule
  Raising Folsom Dam and modifying L.L. Anderson Dam will cost around $257 million, which includes $27 million for environmental and habitat improvements. The federal government will pay 65% of the cost, and the State Reclamation Board and Sacramento Area Flood Control Agency will pay the rest. The Corps and its partners, the State Reclamation Board, Department of Water Resources, Sacramento Area Flood Control Agency, U.S. Bureau of Reclamation, and Placer County Water Agency, could start construction as soon as 2006 and complete it in 2021.