Abstract
Fish screens at hydroelectric dams help to protect rearing and migrating fish by preventing them from passing through the turbines and directing them towards the bypass channels by means of a sweeping flow parallel to the screen. However, fish screens may actually be harmful to fish if the fish become impinged on the surface of the screen or become disoriented due to poor flow conditions near the screen. Recent modifications to the vertical barrier screens (VBS) in the gate wells at the Bonneville Dam second powerhouse (B2) were intended to increase the guidance of juvenile salmonids into the juvenile bypass system but have resulted in higher mortality and descaling rates of hatchery subyearling Chinook salmon during the 2008 juvenile salmonid passage season. To investigate the potential cause of the high mortality and descaling rates, an in situ water velocity measurement study was conducted using acoustic Doppler velocimeters in the gate well slots at turbine units 12A and 14A of B2. From the measurements collected, the average approach velocity, sweep velocity, and the root mean square value of the velocity fluctuations were calculated. The approach velocities measured across the face of the VBS were variable and typically less than 0.3 m/s, but fewer than 50% were less than or equal to 0.12 m/s. There was also large variance in sweep velocities across the face of the VBS with most measurements recorded at less than 1.5 m/s. Results of this study revealed that the approach velocities in the gate wells exceeded criteria intended to improve fish passage conditions that were recommended by National Marine Fisheries Service and the Washington State Department of Fish and Wildlife. The turbulence measured in the gate well may also result in suboptimal fish passage conditions but no established guidelines to contrast those results have been published.
Highlights
Fish screens at water diversion facilities and hydroelectric dams perform a vital function in protecting rearing and migrating fish
The approach velocities at each spatial location were used to create contour plots, which were overlaid by a vector field representing the sweep velocity, to illustrate the spatial distribution of the flow at unit 14A when turbine unit discharge was 340 m3/s with the B2CC open (Figure 6a) and closed (Figure 6b)
The contour plot of approach velocity and vector plot of sweep velocity indicate the unbalanced flow distribution through the vertical barrier screens (VBS): higher approach velocities occurred at three corners of the VBS, while the higher sweep velocities were located at the lower middle portion
Summary
Fish screens at water diversion facilities and hydroelectric dams perform a vital function in protecting rearing and migrating fish. The screen itself may be harmful if fish become impinged on the screen surface or become disoriented due to poor flow conditions in close proximity to the screen [2,3,4,5,6]. It is critical to understand the hydraulic conditions (e.g., 3-D velocity components, turbulence) that fish encounter upon entering the gate well of a dam’s powerhouse. The gate well is the space immediately upstream of the turbine entrance and the collection point for fish diverted upward and away from the primary turbine intake by the guidance structure. Characterizing flows in the gate well will allow for better understanding of the conditions fish experience in the gate well and potentially identify causes of injury and mortality resulting during passage at the dam
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