Application of a Hydrodynamic Model for Assessing the Hydraulic Capacity and Flow Field at Willamette Falls Dam, Oregon

The Willamette Falls Hydroelectric Power Dam, operated by Portland General Electric (PGE), is located on the Willamette River, Oregon. The Project site consists of T.W. Sullivan Power Plant and a 2,950-ft-long spillway located on the top of the Willamette Falls Dam. As part of the effort to improve the flow field and enhance the downstream juvenile fish passage in the region just upstream of the forebay (pre-forebay) where a flow control structure at the dam was proposed. The flow in the pre-forebay of Willamette Falls Dam is affected by the complex bathymetry, geometry of the structure, powerhouse flow, fish ladder flow and the spillway around the dam. The expectation was that the flow would be sensitive to the proposed flow control structures and could be modified to enhance downstream migration. In this study, a three-dimensional, free-surface hydrodynamic model (EFDC) was developed for the pre-forebay region of Willamette Falls to evaluate the feasibility of the proposed alternative and its effect on the flow field in two different flow regimes (low and high river flow). The model was also used to assess the hydraulic capacity of flow control structures. One of the challenges in this modeling study was the proper specification of the free open boundary conditions along the 2,950-feet-long spillway. In this study, a pressure boundary condition, calculated using hydraulic rating curves, was applied to the free spillway boundary. The numerical model was calibrated with Acoustic Doppler Profiler (ADP) velocity measurements at 17 stations for the existing condition. The measurement period was in the low flow season. Good agreements between model results and measured data were obtained, indicating the successful application of pressure boundary condition on the spillway boundary. The calibrated model was applied to simulate the flow field and free surface elevation in the high flow region near the control flow structures under different alternative conditions. The model results were used to evaluate the effectiveness of flow control structure alternatives for downstream fish passage. The model was also used to estimate the hydraulic capacity based on the head loss upstream of the structures. This model application demonstrated that a free surface coastal model can be used successfully to examine free surface hydraulic problems near high velocity regions upstream of spillways at dams.