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Hells Canyon Dam

Hells Canyon Dam on the Snake River in Idaho (Photo Courtesy of the U.S. Bureau of Land Management).

Hells Canyon Dam on the Snake River in Idaho (Photo Courtesy of the U.S. Bureau of Land Management).

TDG-MP3 was used to predict the hydrodynamics and TDG distribution within the Hells Canyon tailrace and the river downstream. The study area for the TDG simulation includes an approximately seven-mile reach of the Snake River, beginning at the dam and extending downstream to just below to Wild Sheep rapids.

TDG simulation included an approximately 7-mile reach of the Snake River.

TDG simulation included an approximately 7-mile reach of the Snake River.

Two days with fairly steady conditions and available TDG field data were simulated. The percentages of spill during these days were about 52% and 48 perc. Plunging jets are observed in all the bays. Nappe deflectors divert the flow toward the center of the spillway causing turbulent aerated flow in the spillway face. Deflectors cause instability of the spillway jets due to turbulence and change in flow direction. Some water flowing above the nappe defectors in spillbay #2 formed free jets that follow a parabolic trajectory impacting the tailwater channel at about 50 ft downstream of the base of the spillway. The left training wall separates flows near the powerhouse from the chaotic flow near the spillway.

 


The model brackets the field data with a deviation of about +/- 3%. After validation, the model was run to study the effect of the operation of sluice gates. Two operations, one assuming that the sluice flow is passing only through gate #2 and another assuming evenly distributed sluice flows were simulated. More uniform TDG concentrations are observed when both sluice gates are spilling. Vertical vortexes originated by open spill gates transport saturated waters downstream of the end sill toward the free surface improving the degasification and mixing. On the other hand, bubbles move up slower when sluice #1 is closed increasing their residence time and producing more TDG. According to the model, splitting the sluice spill flow between both gates reduces the average TDG at 7 miles downstream the dam about 1.5%.

Splitting the sluice spill flow between both gates reduces the average TDG.

Splitting the sluice spill flow between both gates reduces the average TDG.

The green TDG isosurface indicates zero TDG production. The red isosurface indicates TDG production. The water saturation at the bottom of the basin, which is at approximately 80 ft depth, is 3.4. According to the model, the water in the stilling basin is undersaturated and therefore it can take as much air as the bubbles can release. Beneath the zero TDG production isosurface there is the region of TDG production. Bubbles near the free surface are in regions of lower pressure and the dissolution rate has become negative close to the free surface (blue isosurface) promoting TDG degasification.

 

Last modified on September 26th, 2012
Posted on March 16th, 2012