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US Army Corps of Engineers, Portland District,
Elk Creek Dam Removal and Stream Restoration

Design-Build Project — 2008 ACEC of Idaho Engineering Excellence Award, Honorable Mention

Elk Creek Dam was partially constructed when a court injunction stopped work in 1988. In response, the U.S. Army Corps of Engineers Portland District (USACE) determined that it would not continue the project but operate it in an incomplete state for fish passage. The USACE later determined that removing a section of the dam (creating a “notch”) and returning Elk Creek to its original alignment and gradient was the most cost-effective and biologically sound method to provide passage over the long term. The project goals for the design-build team were to: (1) Provide a Fish Passage Corridor (FPC) through the project; (2) complete the FPC by the end of the in-water work window; (3) ensure long-term stability (geomorphically balanced) in the FPC to maintain fish passage and minimize maintenance; and (4) preserve the federal investment and minimize the life-cycle costs.

We mobilized our design-build team to implement the fast-track project, design to construction in less then 9 months. The FPC Plan was developed with a graduated shift from structural measures to vegetation and stream process based measures over time. This approach provided fish passage and stream stability at project completion with long team stability and habitat and passage enhanced with vegetation plantings and a stream channel designed to convey a range of flows and sediment loads. Working with the USACE Team we were able to produce plans and specifications and mobilize the construction team in less then 3 months. The design and construction team worked side by side in the field during construction ensuring quick on-site decisions and implementing field modifications where required to meet the scheduled milestones.

Hydraulic, Hydrologic, River Engineering and Stream Restoration
McMillen’s design provided channel planform, geometry, and stream features to meet fish passage criteria. A model was developed for approximately 2,000 feet of Elk Creek and was used to analyze critical fish passage components such as flow velocity and depth. McMillen also conducted a channel stability analysis using USACE approved processes. The channel is designed to be hydraulically stable up to the 10 year flow event.

Civil Engineering
Removal of in-place concrete (55,000 cy) and structural concrete created the required fish passageway through the existing dam. The concrete channel floor was covered with cobbles and instream structures. Other work elements consisted of site clearing and excavation of fill and in-place bedrock. Elk Creek continued to flow between 5 and 150 cfs during the construction of the project. Fish and water were diverted through a 48” diameter pipe designed to meet downstream fish passage requirements. Localized groundwater was pumped into sediment removal basins.

Structural Engineering
McMillen also provided structural engineering design for the following structures: (1) Diversion Tunnel Closures (2) Gallery Plug (3) Regulating Outlet Closure, and (4) Retaining Walls. All of the retaining wall and closures were designed to meet structural requirements as well as protect fish and wildlife.