Combining hydro-acoustics and hydraulic modeling for evaluating fish entrainment risk

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.