Laboratory Studies of Screens for Diverting Juvenile Salmon and Trout from Turbine Intakes

Fish-guiding screens of different porosities were tested with juvenile spring chinook salmon (Oncorhynchus tshawytscha) in a laboratory model that simulated a turbine intake and gatewell (a vertical shaft in a dam that extends from the forebay deck to the ceiling of the intake). The study was part of a program to develop methods for preventing mortality of juvenile salmon and steelhead trout (Salmo gairdneri) in Kaplan turbines of low-head dams on the Columbia and Snake Rivers. If large numbers of juvenile fish could be guided into gatewells, a method of safely bypassing them around turbines might be devised. Three types of screens (wood, and single and double layers of spiral-weave conveyor belt) were attached to the intake ceiling at an angle of 45° to the flow; their lengths were adjusted to intercept one-third or two-thirds of the total flow into the intake. The screen with the greatest porosity (constructed of a single layer of belting) gave the highest guiding efficiency; 87% of the test fish were diverted into the gatewell. We believed that water deflected under the screen carried fish with it, but our tests indicated that some fish swam upwards out of the flow and into the gatewell. Diversion of 3% of the intake flow up through a gatewell with a single opening into the intake increased the guiding efficiency of only the double-layer screen. Diversion of flow through a gatewell with two openings caused a significant percentage of the guided fish to leave the gatewell and reenter the intake.