IMPACTS OF FLOW DIVERSION FOR SMALL HYDROELECTRIC POWER PLANTS ON SEDIMENT TRANSPORT, NORTHWEST WASHINGTON

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Abstract
Hydroelectric power is a major source of electric energy in northwestern Washington, and an important part of the region's economy. In “run-of-the-river” hydroelectric projects, water is diverted from a river when the discharge exceeds a specified flow and is conveyed downhill to a powerhouse. Creeks used for hydropower generation also constitute the natural spawning habitat for anadromous cold-water fish. Because fish embryo survival rates decrease with an increase in the percentage of fine sediments in spawning gravels, studies of the potential effects of flow diversion on sediment transport are typically required when applying for a permit to build a run-of-the-river project. To provide insight into the actual impacts of flow diversion on sediment conditions, and thus fish habitat, we reproduced channel-morphology measurements and sediment texture sampling originally made for the pre-construction study of Koma Kulshan, a small run-of-the-river hydroelectric power plant in north-western Washington that has been in operation since 1990. Potential non-project changes were controlled by additional sampling upstream of the project. Flow diversion appears to have had an impact on fine-sediment transport, but has not significantly affected coarse-sediment transport in the gravel-bed streams studied. This is because flow diversion represents a small portion of the peak flows likely to move coarse sediment, but can be a significant portion of the low flows during which only fine sediment is transported.

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