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McNary Dam

The McNary Dam on the Columbia River (photo courtesy of the U.S. Army Corps of Engineers).

The McNary Dam on the Columbia River (photo courtesy of the U.S. Army Corps of Engineers).

A numerical study to support TDG management and fish passage needs in the tailrace of McNary Dam was performed using TDG-MP3. The model was validated using velocity and TDG field data collected by the U.S. Army Corps of Engineers under different plant operational conditions.

The VOF method was used to predict the free-surface characteristics and spillway jet regimes. Different flow regimes are observed in the spillway bays: plunging jets are observed in bays without deflectors, while skimming or undular flows are predicted in the other bays. In most of the bays, surface jets plunge downstream, creating an eddy with the potential to transport bubbles to depth.

 
The distribution of gas volume fraction at the free surface downstream of the dam shows that bubbles entrained during spillway releases are directed toward the northern half of the channel as a result of the water entrainment.

Bubbles entrained during spillway releases are directed toward the northern half of the channel as a result of the water entrainment.

Bubbles entrained during spillway releases are directed toward the northern half of the channel as a result of the water entrainment.

For a spread spill operation, the model predicts almost uniformly distributed bubbles near the spillway face. The bubbles shrink near the riverbed, due to the air mass transfer and high pressure. The smaller the bubble size, the stronger its tendency to dissolve. The gas dissolution region occurs mainly within 500 to 1,000 feet downstream of the spillway; afterwards the bubbles move up to regions of lower pressure and the dissolution rate decreases.

The model captures the reduction of TDG longitudinally and the lateral gradient observed in the field.

The model captures the reduction of TDG longitudinally and the lateral gradient observed in the field.

High TDG production is observed downstream of spillways without deflectors as a result of exposure of bubbles to high pressure. Substantial desorption of TDG takes place near the free surface downstream of the spillway. The TDG concentration reaches a fully developed condition approximately 2,500 feet from the spillway. The model captures the reduction of TDG longitudinally and the lateral gradient observed in the field. As measured, the highest predicted TDG occurs in the northern region of the channel, due to dilution by powerhouse flows in the southern region. The error between predicted and measured average TDG values at Transects T1, T2, and T3 are -3.2 percent, -0.5 percent, and 2.0 percent, respectively.

Simulation PS-I presented a more spread spill pattern while PS-II concentrated the spill flow in the southern spillway region.

Simulation PS-I presented a more spread spill pattern while PS-II concentrated the spill flow in the southern spillway region.

After validation, TDG-MP3 was used to simulate two different operational modifications to evaluate water entrainment and minimize TDG concentration downstream of the McNary Project. Simulation PS-I presented a more spread spill pattern, while PS-II concentrated the spill flow in the southern spillway region.The following animations of streamlines show the hydrodynamics and TDG distribution for these simulations.

For both spill configurations, most of water from turbine unit #14 entrains into the spillway region. In addition, water from the north shore fish entrance (NFE) is entrained beneath the surface jet created in spillbay #1. In the southern region, most of the water from the south powerhouse fish entrance (SFE) and the station service (SS) goes into the fishway pumps. In PS-I, the highest TDG concentrations are observed at the northern region. At that location the dilution from powerhouse flows is at a minimum. The forebay TDG is 114.8 percent, and in the first 700 feet downstream of the dam, the degasification is important. In PS-II, spillway surface jets attract water from both the powerhouse and from the northern spillway region, forming a big clockwise eddy at the northern region of the McNary tailrace.

The water entrainment was evaluated calculating the cumulative flow rate through planes perpendicular to the powerhouse wall. Numerical results indicate that concentration of spill flow in the spillway southern region results in local higher water entrainment.

The concentration of spill flow in the southern region of the spillway results in locally higher water entrainment.

The concentration of spill flow in the southern region of the spillway results in locally higher water entrainment.

 

Last modified on March 2nd, 2015
Posted on March 15th, 2012