Boundary Dam Spillway Retrofit

This project won the ENR Northwest Best Project award and went on to win the national competition, the ENR Best of the Best Project in the small project category.

Boundary Dam is located on the Pend Oreille River near Metaline Falls, Washington, north of Spokane. Boundary Dam consists of an arch dam, reservoir, and underground power plant. The spillways on this scenic dam are cut into rock and oriented to allow full capacity water discharges from both sides. After extensive consideration surrounding modifications made to the spillways in 2016, it was determined that the total dissolved gas (TDG) was too high, this was affecting the fish downstream. It was determined that the spillway surface should be put back to as close to original condition as possible.

IMCO Construction was contracted by Seattle City Light to complete the removal of TDG modifications previously installed on the spillway. The Boundary Dam Spillway No. 1 TDG Modifications contract included removing ramps and roughness elements embedded in the spillway concrete, decommissioning post tensioned anchors, and removing stainless-steel ramps. Crews also installed reinforced concrete infill to match the pre-TDG modification spillway surface and placed spill containment, and other environmental controls. This project provided a safer spillway design at Boundary Dam, with less mist impacting the effectiveness of sensors, and the project decreased environmental impacts downstream due to TDG.