Effects of Jet Entry at High-Flow Outfalls on Juvenile Pacific Salmon

Abstract

We conducted field studies and laboratory experiments to explore the relationship between direct injury and mortality rates of juvenile Pacific salmon Oncorhyncus spp. and the jet entry velocities characteristic of high-flow (>28.3 m3/s) outfalls at hydroelectric facilities. During field tests, the range of calculated mean entry velocities was 9.3–13.7 m/s for low (28.3 m3/s) and high (68.0–70.2 m3/s) outfall discharge rates and two receiving water elevations. Mortality and injury rates of balloon-tagged hatchery juvenile spring chinook salmon O. tshawytscha in the field tests were less than 1%. At a high-velocity flume in a laboratory, small (87–100 mm fork length (FL)) and large (135–150 mm FL) hatchery fall chinook salmon were exposed to velocities of 0.0–24.4 m/s in a fast-fish-to-slow-water scenario. Jet entry velocities up to 15.2 m/s provided benign passage conditions for the sizes and physiological states of juvenile salmonids tested under the particular environmental conditions present during this study. Our results of direct injury and mortality indicate that a jet entry velocity up to 15.2 m/s should safely pass juvenile salmon at high-flow outfalls. It will be necessary, however, to conduct site-specific, postconstruction verification studies of fish injury and mortality at new high-flow outfalls.