NorthWestern Energy retained McMillen Jacobs to completely replace the spillway gate structure and discharge chute at Hebgen Dam, near West Yellowstone Montana. The Hebgen Dam is a 700 ft. long earthen embankment dam constructed with an interior concrete core-wall. The Hebgen dam acts as flood control and reservoir storage, the 386,000 acre foot volume for Hebgen Lake provides a benefit to the 11 downstream hydroelectric plants on the Madison and Missouri Rivers in Montana. The existing gate structure was seismically deficient, and the gate structure and chute do not have the hydraulic capacity to safely route the probable maximum flood. The existing spillway was constructed in 1960 after the original spillway was severely damaged in the earthquake of 1959. The existing spillway gate structure is integral with the dam near the right abutment and consists of 6 timber vertical slide gates and a bridge across the rectangular channel section. The chute has a rectangular cross section which narrows as it traverses a horizontal curve downstream to the tailrace and Madison River below. McMillen Jacobs designed and constructed the spillway cofferdam as part of a separate contract.
The first phase of this project was the demolition of the existing concrete spillway gate structure and the entire chute from the dam to the river below and assisting NorthWestern Energy in implementing and executing a temporary dam safety monitoring program for the spillway cofferdam. The second phase of the project was to install the spillway slide gate structure ((4) 11’x11’ with screw stem activators) and discharge chute (6,000 cy of reinforced concrete), place a small portion of additional concrete construction at the intake structure across the dam from the spillway; access walkway and vehicle / pedestrian bridge; remove the spillway cofferdam; and excavate and remove the spillway cofferdam material.