Determining Columbia and Snake River Project Tailrace and Forebay Zones of Hydraulic Influence using MASS2 Modeling

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Abstract

Although fisheries biology studies are frequently performed at US Army Corps of Engineers (USACE) projects along the Columbia and Snake Rivers, there is currently no consistent definition of the ``forebay'' and ``tailrace'' regions for these studies. At this time, each study may use somewhat arbitrary lines (e.g., the Boat Restriction Zone) to define the upstream and downstream limits of the study, which may be significantly different at each project. Fisheries researchers are interested in establishing a consistent definition of project forebay and tailrace regions for the hydroelectric projects on the lower Columbia and Snake rivers. The Hydraulic Extent of a project was defined by USACE (Brad Eppard, USACE-CENWP) as follows: The river reach directly upstream (forebay) and downstream (tailrace) of a project that is influenced by the normal range of dam operations. Outside this reach, for a particular river discharge, changes in dam operations cannot be detected by hydraulic measurement. The purpose of this study was to, in consultation with USACE and regional representatives, develop and apply a consistent set of criteria for determining the hydraulic extent of each of the projects in the lower Columbia and Snake rivers. A 2D depth-averaged river model, MASS2, was applied to the Snake and Columbia Rivers. New computational meshes were developed most reaches and the underlying bathymetric data updated to the most current survey data. The computational meshes resolved each spillway bay and turbine unit at each project and extended from project to project. MASS2 was run for a range of total river flows and each flow for a range of project operations at each project. The modeled flow was analyzed to determine the range of velocity magnitude differences and the range of flow direction differences at each location in the computational mesh for each total river flow. Maps of the differences in flow direction and velocity magnitude were created. USACE fishery biologists requested data analysis to determine the project hydraulic extent based on the following criteria: 1) For areas where the mean velocities are less than 4 ft/s, the water velocity differences between operations are not greater than 0.5 ft/sec and /or the differences in water flow direction are not greater than 10 degrees, 2) If mean water velocity is 4.0 ft/second or greater the boundary is determined using the differences in water flow direction (i.e., not greater than 10 degrees). Based on these criteria, and excluding areas with a mean velocity of less than 0.1 ft/s (within the error of the model), a final set of graphics were developed that included data from all flows and all operations. Although each hydroelectric project has a different physical setting, there were some common results. The downstream hydraulic extent tended to be greater than the hydraulic extent in the forebay. The hydraulic extent of the projects tended to be larger at the mid-range flows. At higher flows, the channel geometry tends to reduce the impact of project operations.

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