Effects of Spillway Structural Modifications on Fish Condition and Survival
Paul G. Heisey, Dilip Mathur, John R. Skalski, Robert D. McDonald, and George Velazquez
Abstract.—Historically, spillways and sluiceways at hydroelectric dams were constructed as conduits for transporting excess river flow or debris with little focus on their potential for safe fish passage routes. In recent times, however, these conveyances are increasingly viewed as viable fish passage routes and are used to increase potential survival for the declining salmonid populations, particularly in the Pacific Northwest. However, spill is uneconomical and, with some spillway configurations, may cause potentially lethal levels of total dissolved gas (TDG) saturation in the river. Recent estimates (direct effects) of juvenile anadromous fish survival and condition after passing nonturbine exit routes at hydro dams have shown much variation (83–100% survival) depending upon site-specific hydraulic characteristics, trajectory of entrained fish, and obstructions in the flow path. Efforts are underway to modify spillways and bypasses at several hydroelectric dams to decrease TDG and eliminate and/or minimize fish injury.
To minimize TDG saturation levels and passage related injuries to juvenile salmonids at Rock Island Dam on the mid-Columbia River, spillbays were modified and evaluated over a 4-year period. A rectangular notch was cut in selected upper crest gates to pass surface-oriented salmonids. The downstream concrete sills were at various depths, and selected spillbays were equipped with flow deflectors. Flow deflectors were first evaluated at about a 5.2 m water depth and later at a 0–1 m depth. Although little fish mortality or injury was observed (0–1%) at a spillbay with or without a deep-flow deflector, TDG reduction was unacceptable. With shallowflow deflectors downstream of spillbays, both the fish mortality/injury (≤1.5%) and TDG (reduction of up to 6%) levels were within the tolerance range. As a result of the multiyear investigation, spillbays with exposed downstream concrete sills are no longer used to spill water for safe fish passage. Spillway modifications appear to have achieved the goals of both protecting fish and conserving water. A similar approach may be needed at some other hydroelectric projects to achieve acceptable fish survival.