Fontenelle Dam River Outlet Works Stilling Basin Dewatering and Inspection Project

250-foot bladder dam in place inside
stilling basin

The Fontenelle Dam and Powerplant river outlet works and stilling basin dewatering and inspection project was two years in the making. A jointly funded effort by Reclamation and the state of Wyoming, the project resulted from dive inspections that revealed cobble rock debris in the stilling basin and erosion and damage to the concrete floor, east wall, and #1 and #2 dentates (tooth-like projections that help dissipate energy of water released into stilling basin). In order to determine where the cobble rock was coming from, Reclamation’s Technical Service Center, Water Resources Research Laboratory developed a model that indicated what was happening. Rock was being drawn into the concrete stilling basin during bypass discharge flows through the outlet works by the under current created. In order to obtain an accurate estimate of how much rock, erosion, and concrete damage was taking place, it was necessary to close the stilling basin off from the river and dewater the stilling basin. A 250’ x 19’ x 8’ high baffle polyethylene (water filled) bladder dam was deployed to accomplish this instead of an earthen coffer dam to avoid increasing the debris in the stilling basin.

The standing water was removed from the stilling basin through the use of dewatering pumps operated by three 125 kilowatt generators which allowed for the necessary damage inspection and repair, and debris removal. With maintenance personnel from both Flaming Gorge Field Division and Fontenelle Dam, the task of deploying the bladder dam and dewatering the stilling basin began on April 28, 2009. It took nearly 4 ½ hours to deploy the bladder dam, which included filling it with approximately 150,000 gallons of water.

The entire dewatering process took nearly 2 ½ weeks and involved overcoming technical difficulties with several generator and electrical pump problems. Now that these problems have been resolved, it is anticipated that the future dewatering efforts will take only eight to ten days.

The project was completed on May 19, 2009. In total, the dewatering process consisted of pumping out 12-13 million gallons of water, while keeping the water level at about six inches deep as 10 cubic-feet-per-second of leakage continually entered the stilling basin. Reclamation was able to conduct a thorough inspection of the stilling basin and remove the debris, as well as collect important data for future dewatering efforts. In addition, Reclamation will soon be initiating a contract for work to repair the damaged concrete in the stilling basin.

Throughout the dewatering process, Wyoming Game and Fish worked with Reclamation staff to complete a fish rescue and salvage in the stilling basin. Over 175 fish, including German brown trout, lake trout, kokanee salmon, and small-mouth bass were collected and relocated.