Dam raise reduces flood
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.
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.
Dam, auxiliary dam, and dike raising
The methods for raising the concrete dam,
the earthen Mormon Island auxiliary dam and eight dikes are very
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
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.