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Supersaturated total dissolved gases (TDGs) generated by dams can cause gas bubble trauma and mortality in fish in downstream waters. In this study, we evaluated air entrainment and generation of supersaturated TDG at two submerged low-level outlets (LLOs) at Hugh L. Keenleyside Dam, British Columbia, Canada. Specifically, we determined how air entrainment (less than 1% volume fraction) through the gate well, discharge level through the outlets, and geometry of the stilling basins at the south and north ends of the dam influenced supersaturated TDG generation. A mathematical formulation was developed, incorporating physical processes including air entrainment, bubble breakup, and gas transfer. Numerical modeling was also adopted to validate turbulence and flowfield downstream of the submerged low-level outlets. Despite being 8-m shallower, significantly higher TDG levels were measured in the stilling basin of the south LLO (about 120%) compared to that of the north LLO (≤110%). Results show that turbulence in the stilling basin can produce smaller bubbles and increase the mass transfer coefficient across bubbles, which will substantially enhance gas transfer and TDG generation. Higher TDG generated in the shallower LLO was therefore attributed to the water depth generating stronger turbulence flow, with more efficient gas transfer. This study improves TDG prediction and helps inform the development of operational alternatives during periods of high TDG generation to mitigate impacts on the aquatic environment. [ABSTRACT FROM AUTHOR]

Understanding the air demand of a hydraulic jump in a closed conduit is important in hydropower operations and urban sewer designs. In this study, physical experiments are described to study flow regimes and the air demand of a hydraulic jump in a closed conduit with various submerged outlet depths. Flow regimes with a submerged outlet were defined following previous studies based on outlet depth. Free-surface supercritical flow with a hydraulic jump can induce a relative air demand (air flow rate to water flow rate) of approximately 3%–14%. If the hydraulic jump is followed by pressurized pipe flow, the air demand decreases with increasing outlet depth until the return roller is fully developed in the pipe. For the partially submerged hydraulic jump, the relative air demand is significantly reduced to less than 1%. Field measurements of the air demand at the Hugh Keenleyside Dam, British Columbia, were consistent with the experimental measurements for the partially submerged hydraulic jump. The dynamics of the air pocket upstream of the hydraulic jump was studied. If the air supply was constrained by nozzles of various sizes placed on the top of the air vent, the air pressure in the closed conduit decreased and the hydraulic jump was pushed upstream. The energy loss coefficient in the air vent was also studied. [ABSTRACT FROM AUTHOR]

Hydrodynamics in dam tailraces can influence the swimming behavior and survival of fish, and yet there have been few studies that have linked the movement patterns of fish to encountered flow patterns. In this study, we examined the flow field downstream of Seton Dam in British Columbia, Canada using acoustic Doppler current profilers (ADCPs), and computational fluid dynamic (CFD) modeling of two dam water release scenarios to understand their impacts on the swim speed and behavior of upriver‐migrating adult sockeye salmon (Oncorhynchus nerka). Forty‐five sockeye salmon were tagged with acoustic transmitters and tracked to understand their movement patterns, swim speed, residence time in the dam tailrace, and postdam passage survival. The water release scenarios produced contrasting tailrace hydrodynamics and altered the behavior of migrating sockeye salmon. Tagged fish avoided areas of high velocity (>2.4 m/s or four body lengths/s, twice their critical swim speed), high Reynolds shear stress (>21 Pa), and high turbulence kinetic energy (>0.12 m2 s−2) in the dam tailrace. Excessive use of anaerobic metabolism, together with high water shear stress adjacent to and downstream of the fishway entrance, were thought to be the main cause for low postdam passage survival of sockeye salmon. Additional dam water release strategies were assessed with the CFD model to target releases that improve the migration conditions for sockeye salmon. Our study highlights the importance of pairing hydraulic and ecological information to better understand and improve the migration conditions for wild fish. Key Points: Different dam water releases can greatly impact hydrodynamics in dam tailrace and thus fish migration and their survival to spawning groundsSockeye salmon clearly avoided the areas of high velocity, high shear stress, and high turbulence kinetic energy in the dam tailraceCoupling computational fluid dynamics modeling with fish tracking is an effective way to optimize dam operation for fish migration [ABSTRACT FROM AUTHOR]

Individual fish may be involuntarily removed from hydropower reservoir populations due to dam operations, a phenomenon known as fish entrainment. Entrainment for two life stages (age-0 and age-1) of Kokanee salmon (Oncorhynchus nerka) was evaluated within the forebay of the Revelstoke Dam in the upper Columbia River, British Columbia, Canada. Intensive fish surveys using hydro-acoustics were conducted to estimate fish density in the forebay at two different times (day & night) and across three seasons (spring, summer and winter). The number of fish entrained into intakes was monitored using hydro-acoustics continuously over one year. Surveyed fish densities and entrainment numbers were combined with Computational Fluid Dynamic modeling of the velocity field around the penstock intakes to estimate the threshold velocity for entrainment where 50% of the fish within this volume of water would be entrained. Fish density decreased with increasing distance from the dam face and it also decreased with increasing water depths. Entrainment of age-0 kokanee was consistently higher compared to age-1 in all times and seasons evaluated. The threshold velocities resulting in entrainment of age-0 kokanee were consistently lower (range 0.24 to 0.59 m/s) compared to those estimated for age-1 kokanee (range 0.30 to 1.08 m/s). Additional modeled scenarios showed that fish entrainment risk increased both with increasing intake discharge and the number of intakes in operation. Results demonstrate combing computational fluid dynamics models with ecological information collected using hydro-acoustics allows age-specific entrainment risk to be evaluated for vulnerable species and warrants application to other species and hydropower facilities. • A new fish entrainment risk method was proposed for kokanee in Revelstoke Dam. • The proposed method is based on hydro-acoustics and CFD modeling. • The fluid dynamics, threshold velocities and risk volumes were estimated. • Entrainment risks for different operational scenarios were evaluated. [ABSTRACT FROM AUTHOR]