Survival of fish passing downstream at a small hydropower facility

Abstract

Hydropower dams can negatively affect upstream and downstream migratory fish populations in many ways, such as blocking access to upstream habitats and causing injuries or mortality during downstream passage. For downstream passage at projects in the USA, federal regulators and agencies responsible for oversight of hydropower facilities typically require assessment studies and mitigation to address negative effects, with a primary goal of minimising fish impingement and turbine entrainment and mortality. So as to assess the effects of downstream passage of fish populations at a unique, small hydro project on the Mississippi River, impingement and entrainment rates, Oberymeyer gate passage, spillway gate passage, turbine survival, and total downstream passage survival were estimated. It was determined that 85% of fish passing downstream at the project would be small enough to pass through the bar spacing of the trash racks and 15% would be physically excluded. When 55% of river flow enters the turbine intake channel, the total project survival rates were estimated to be 77.3% with an Obermeyer gate bypass rate of 10 and 96.6% with a gate bypass rate of 90%. Therefore, any effects on local fish populations resulting from the operation of the project are expected to be negligible and inconsequential on the basis of expected survival rates for the range and probability of river flows occurring at the project.