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Wells Dam

Wells Dam in North Central Washington state (photo courtesy of NCWpics.com).

Wells Dam in North Central Washington state (photo courtesy of NCWpics.com).

As part of the relicensing of the Wells Project, Douglas PUD initiated a series of assessments aimed at gaining a better understanding of the effect of spill operations on the TDG distribution in the Wells Dam tailrace. The goal was to identify the best operating configuration to minimize TDG and evaluate compliance using this configuration up to 7Q10 flows. The Wells Hydroelectric Project is a “hydrocombine” dam, with the spillway situated directly above the powerhouse. The complete study is available at Politano et al. (2009d).

TDG-MP3 was calibrated and validated using field data collected under five different operational conditions in 2006. TDG sensors were deployed at 15 stations in three transects at approximately 1,000 feet, 2,500 feet, and 15,000 feet downstream of the dam. Velocities were measured in the tailrace on three transects near the dam on June 4 and June 5.

The following animations show free surface characteristics predicted on June 4 when the spill was spread across the spillway.

The following animated horizontal planes at 27 feet from the free-surface and vertical sections at the center of spillway bay 7 show the water entrainment and spillway jet regimes on June 4 and June 5, 2006. Powerhouse operation prevents spilled flow from plunging to depth within the stilling basin, reducing the exposure of bubbles to high pressure and TDG production. The draft tube deck extensions and spillway lip, tend to act as deflectors for the spill. On June 4, submerged jets are predicted in all the bays. Uniform distribution of the flow is observed close to the spillway during the spread flow operation.

On June 5, a skimming surface jet is observed downstream of bay 7 through which most of the spill was concentrated. The model captures water being drawn toward the jet region, as observed in the field.

There was good agreement between predicted (black) and observed (blue) velocity vectors. TDG-MP3 captures the counterclockwise eddy near the east bank and the almost uniform profile at the most downstream transect.

The model captures the reduction of TDG with distance downstream and the lateral gradient observed in the field. The predicted lateral gradients in transects T2 and T3 were negligible as measured in the field. 6

The model captures the reduction of TDG with distance downstream and the lateral gradient observed in the field. The predicted lateral gradients in transects T2 and T3 were negligible as measured in the field.

 

The model captures the reduction of TDG with distance downstream and the lateral gradient observed in the field. The predicted lateral gradients in transects T2 and T3 were negligible as measured in the field.

 

Analysis of the sensitivity of TDG concentration to the operation of the project.

Analysis of the sensitivity of TDG concentration to the operation of the project.

After validation and calibration, the model was used to analyze the sensitivity of TDG concentration to the operation of the project. Nine runs were completed for four river flow rates in which the spill was either spread across the spillbays or concentrated in one or more spillbays. The following figures show the distribution of gas volume fraction and TDG for spread and concentrated spill operations in cross sections at 50 and 370 ft. from the dam. For the same powerhouse and spill flow rates, the gas volume fraction, and consequently the TDG, are strongly related to the spill operation with larger TDG concentrations for the spread operation, which was the historic operation of Wells Dam. According to the model, concentrated spill operations reduce the TDG production and increase the degasification at the free surface.

According to the model, concentrated spill operations reduce the TDG production and increase the degasification at the free surface.

According to the model, concentrated spill operations reduce the TDG production and increase the degasification at the free surface.

Based on the results from the sensitivity simulations, several additional operating configurations were tested toward identification of the best operation for a 7Q10 flow. The following figure shows isosurfaces of TDG, gas volume fraction and bubble diameter for a spillway configuration that significantly reduces TDG in the Wells tailrace. The highest TDG isosurfaces are observed directly below spillbay 7 corresponding with the zone of higher gas volume fraction (aerated zone). In this area, the entrained bubbles generate high levels of TDG. However, the supersaturated water quickly degasses by mass exchange with bubbles near the free surface and mass transfer at the turbulent free surface near the spillway. Moreover, as shown in the animated streamlines, strong lateral currents caused by the surface jet on bay 7 directed water toward the center of the dam contributing further to fully mixed flow and TDG dilution.

Strong lateral currents caused by the surface jet on bay 7 directed water toward the center of the dam.

Strong lateral currents caused by the surface jet on bay 7 directed water toward the center of the dam.

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