The U.S. Army Corps of Engineers (USACE) announced January 26 that along with its contractor, Kiewit Co., it began work on the next milestone at the Folsom Dam auxiliary spillway project in Folsom, CA. This phase involved the process of impoundment, where water is pumped from the reservoir (lake side) to the approach channel space and against the control structure.

"The placement of water against the control structure was done using eight pumps to push water through two 24" diameter pipes," says Katie Charan, senior project manager with USACE, Sacramento District. "Each pipe is capable of pumping 16,000 16,000 gallons per minute for a total of 32,000 16,000 gallons per minute. The goal was to equalize the reservoir with the approach channel side over a 24 hour period, meaning, we wanted the water on either side of the temporary construction cofferdam to be at the same level."

Charan says Kiewit completed the process in 24 hours.

"We had already projected to impound water against the Control Structure in early February, but we were able to complete the process a couple of weeks early," she says. "Kiewit placed a temporary construction cofferdam on the upstream side of the approach channel to allow them to excavate and blast in the dry." Now that water is impounded against the control structure the Kiewit will work to decommission the temporary construction cofferdam from wet excavation.

Kiewit reinforced the temporary construction cofferdam last week following the discovery of a leak that partially filled the approach channel construction area. Stabilizing the leak allowed crews to safely remove construction machinery and materials that were abandoned upon discovery of increased seepage.
The temporary construction cofferdam was put in place to help conduct construction activities under dry conditions rather than in-the-wet. Removal of the temporary structure was expected to occur by February 8, 2016; however, it was decided to begin the impoundment process and remove it sooner.

The approximately $900-million auxiliary spillway project, or Joint Federal Project, is being constructed in partnership with the U.S. Bureau of Reclamation under the Corps’ Folsom Dam Modification Project, to implement dam safety and flood risk reduction features at Folsom Dam and associated facilities to help prevent catastrophic flooding in the Sacramento region. The auxiliary spillway is on track for completion, within budget, in October 2017.

The will help the Sacramento region achieve 200-year level of protection, meaning there will be a one-in-200 chance for flooding in any given year. The project will construct an auxiliary spillway to compliment the functions of the main Folsom Dam. It will allow water to be released earlier and more safely from Folsom Lake during a high water event.

The spillway will include an 1,100-foot-long approach channel that will funnel the water from the lake into the spillway; a control structure with six submerged gates that will be controlled in coordination with the gates on the main dam to control water releases; a 3,027-foot-long spillway chute that transports the water from the control structure to the American River below; and a stilling basin that will slow the racing water back to normal flow levels that the river channel will be able to withstand.