River Diversion for Boundary Dam

Diversion of the Pend Oreille River at Boundary Arch Dam site was accomplished by constructing two rockfill cofferdams and a 42-ft horseshoe tunnel in the left abutment. Model tests determined construction procedures for the cofferdams. Excavation of the tunnel was performed from the inside of the arch dam abutment via an adit from the underground power plant. Opening of the portals and rerouting of the river was by a series of singular rock blasts. The tunnel floor was left unlined and gunite was applied to the side walls and roof. The scheme accommodated river discharges during the low flow periods only, and flood flows were discharged over the partly completed arch dam. The diversion tunnel was finally closed by lowering a counterbalanced, arched, steel gate into the flow at the tunnel portal. Concrete plugs closed the access tunnel during diversion and the diversion tunnel after its closure.