A Detailed Study of the Zone of Initial Dilution to EvaluateTemperature Exposure and Fish Entrainment.Fort James Operating Company’s Wauna Mill Outfall, 2006 Engineering, Pulping, and Environmental Conference

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The Oregon Department of Environmental Quality expressed a concern about the potential exposure of migrating salmon to elevated temperatures and entrainment within the near field zone of initial dilution at Fort James Operating Company’s Wauna Mill outfall in the Columbia River. Because of the unconventional diffuser arrangement used in the Wauna Mill outfall and the difficulty of obtaining dye dilution measurements in the near field, this could not be evaluated using conventional techniques alone. A detailed mixing zone study focused on the near field region was requested by Fort James Operating Company. Therefore, in addition to plume dilution modeling, a three dimensional computational fluid dynamics(CFD) model was developed to simulate at high resolution the velocity and temperature distribution within the zone of initial dilution (ZID). The CFD and plume models were based on the latest diffuser, receiving water, and effluent conditions. The plume model was calibrated to available dye dilution data. The CFD model was calibrated using velocity data measured in the field. The ability of the CFD model to provide mixing and dilution was validated using popular plume models such as CORMIX and Visual Plumes–UM3. The results showed that the outfall diffuser provides a chronic dilution of 1:100 based on a 96 hour exposure average at the regulatory mixing zone. The use of a CFD technique allowed entrainment and temperature exposure to be assessed within the highly turbulent ZID. The CFD model results showed no potential entrainment of migrating fish. Also, potential exposure of fish to elevated temperatures(>32^°C) was shown to be considerably less than the 2 seconds criteria, which is in compliance with the Oregon State water quality standards.