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By Pablo M. Carrica, Cagri Turan and Larry Weber, IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa
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The Wanapum Dam and spillway, showing the location of the proposed fish passage
Hydropower dams produce important benefits, including clean energy, irrigation, flood control, and water supply. However, they have also contributed to a decline in the population of anadromous fish, those that swim upstream for breeding, in the Columbia River basin. The dams have impounded most of the free flowing sections of the river and created water conditions that are harmful for species such as salmon and steelhead trout.
An instantaneous free surface at the forebay, spillway and tailrace, modeled using a simplified geometry with no gates
Engineers at IIHR-Hydroscience & Engineering at the University of Iowa have been working for several years to design and analyze mitigation measures for reducing the negative impact of hydropower installations on fish. The areas of work include juvenile fish passage design, total dissolved gas modeling, spillway deflector design, forebay and tailrace flow studies, and temperature effects. Interactive work with utilities and resource agencies has resulted in the implementation of many designs and ideas developed at IIHR in partnership with external project team members.
Free surface topology on the spillway with gate slots, but no gates; the gate slots cause water splashing and air entrainment
Free surface topology with gates installed; air is allowed into the chamber below the gates through a pipe connected to atmosphere
Juvenile fish passage facilities are often complex hydraulic structures designed to attract and safely pass fish during their downstream migration. One passage currently under consideration for construction is at Wanapum Dam on the Columbia River. The salient features of this fish bypass include a spillway with an ogee (elongated S) shape and apron, straight and inclined slots for flow control gates, and other complex geometrical features intended for safe fish passage and proper hydraulic performance. IIHR was charged by the Grant County Utility District to provide experimental and numerical studies to support both conceptual and final designs of the fish bypass. For the numerical studies, the VOF capability of FLUENT was used to model the free surface flow on the spillway section of the bypass, and to analyze the flow regimes at the tailrace, where the dam water impacts the river downstream.
Several models of increasing complexity were generated to analyze coarse features of the flow and then details at specific locations. Experimental data was used to validate the numerical simulations. The simulation with the lowest level of complexity included the forebay, the spillway without gates, and the tailrace. A large pool with an inflow and a fixed-level spillway were used to control the forebay water level. A similar method was employed to set the far-field elevation of the tailrace. This model was used to generate rating curves and predict the free-surface elevation on the spillway. In both cases, good agreement with the experimental data was achieved.
CFD predictions and experimental data of the free surface elevation at the spillway walls (above) and the forebay elevation as a function of the flow rate (below)
Encouraged by the success of the simple models, a more complex model was built including gate slots, gates, and air supply chambers. A hybrid mesh consisting of mostly hexahedral cells was used. The VOF model was again used, and was found to be stable and robust, despite the added complexities. Since the ogee crest is aerated to avoid cavitation problems, a pipe was introduced to deliver air into the low pressure region downstream of the gates. The resulting flow in this region is characterized by a strong free surface deflection that is caused by the recirculating vortex at the end of the pipe, where air is pumped into the water. While the grid in the CFD model was not adequate to accurately model the air entrainment (with individual bubbles), the water flow field was properly captured. Good agreement with experimental pressure profiles at the ogee surface was also obtained, providing confidence in the model.
Predicted and measured pressure profiles along the ogee centerline, for a forebay elevation of 575 ft
Another topic of interest in the project is the flow regime that develops when water plunges into the river at the tailrace. Large amounts of bubbles are generated during the plunging process, and these bubbles can aerate the water, increasing the total dissolved gas (TDG) content, and threatening the fish if the nitrogen exceeds certain levels. Since the gas transfer from the bubbles to the water occurs mostly at high pressures, it is important that the bubbles not penetrate the tailrace waters to excessive depths. The model showed that for a range of high tailrace elevations, a skimming flow pattern prevails, in which the jet meets the tailrace pool tangentially with little downward momentum. For low tailrace elevations, however, a plunging condition does develop that carries high momentum liquid down to the bottom of the pool, causing a potential danger of large bubble dissolution and high TDG. The final fish passage design was such that during normal operation - and most of the possible abnormal
operation modes considered - the flow remains in the skimming regime.
Flow regime at the tailrace for tailwater elevations of 492ft (top), 488, 483, 479, and 475ft (bottom); the high momentum streams, colored with x-velocity, reach deep into the tailrace for the lower elevations (plunging flow regime), while the flow is parallel to the free surface for the higher elevations (skimming flow regime)
Future work includes an analysis of bubble transport and TDG distribution using the Eulerian and Lagrangian multiphase models, coupled with VOF through user-defined functions.
The authors would like to thank the Public Utility District #2 of Grant County, Washington for their financial support.
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