Colorado’s Historical Fire Leads to Major Renovation of Cheesman Dam

Upstream guard gates were installed on outlet works tunnels of the 107-year old Cheesman Dam using the unique diving technique known as saturation diving. Denver Water’s experience with this underwater construction method will be of value to other owners of older dams that were not originally constructed with upstream guard gates and with reservoirs that cannot be drained. The most devastating fire in Colorado’s history (hundreds of thousands of acres) had many negative impacts on the South Plate River Basin above Denver Water’s Cheesman Dam and Reservoir. The fire was so intense that it “cooked” the forest floor into a hard glaze almost glass like surface. Below this surface lie millions of yards of “pea” size Pikes Peak granite gravel. The drainage basin above Denver Water’s Cheesman Reservoir is covered with 3 to 6 feet of this granite gravel. Cheesman Reservoir became a primary depository for these sediments with currently about 1.3 million cubic yards deposited. Cheesman Dam’s outlet works needed modification to accommodate the build of sediment. The outlet works consists of two separate tunnels systems with control provided downstream. This paper will discuss the design and installation of the new custom upstream slide gates. The installation of these slide gates utilized a unique diving technique known as saturation diving. Saturation in this case means that most of the oxygen and other gases in the diver’s blood are replaced with helium. The divers live and work at the pressure of the underwater construction location. Divers are “pressed down” to the pressure they are working. This diving technique allows the dive team to work about 20 hours per day at an equivalent atmosphere of 6 times greater than the reservoir surface. Saturation diving requires on one decompression at the completion of the project. A saturation dive team size can consist of two, four or six divers. The Cheesman Inlet Gate project used a four man dive team with 20 support personnel working 24 hours per day 7 days a week. The diving operation was completed in 5 months and the preliminary project cost estimate for diving is about 6 million dollars. 14 pp., 18 figures.