Long Lake Dam

LOCATION:  Spokane, Washington   |   OWNER:  Avista Corporation
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Long Lake Dam is a 213-foot-tall concrete gravity dam owned and operated by Avista Corporation on the Spokane River northwest of Spokane, Washington. The dam is constructed with an overflow spillway consisting of 8 bays, which channel spill waters into a plunge pool at the base of the dam. During spill events, the plunge pool can become supersaturated with total dissolved gases (TDG) due to the force and angle of the water entering the pool. As part of the overall Spokane River Project’s re-licensing efforts, Avista agreed to evaluate ways to mitigate the TDG conditions below the dam.  McMillen Jacobs was awarded a design-build contract for the Long Lake Dam TDG Project, which includes adding deflectors to the downstream face of the dam, reducing the plunge pool depth, and removing a portion of the rock outcrop below spillway bays 7 and 8 at the dam. As part of the project, McMillen Jacobs provided a temporary access road, staging/stockpiling areas, a temporary cofferdam and dewatered the plunge pool. The remainder of the project will consist of constructing deflectors, spillway toe repair, partial rock excavation, construction platform/plunge pool back fill, removal of temporary road from riverbed and shoreline, restoration, temporary construction signage and re-vegetation of the construction site and impacted areas.