FLOWS AND FISH BEHAVIOR - LARGE DOUBLE-ENTRY SCREENING SYSTEMS

Facilities that draw water in large quantities from natural sources ar e commonly equipped with mechanically driven barrier screens for remov ing indrawn debris and captive fish. Owing to the mistransport of debr is into the works of the facility by the cyclic action of conventional screening apparatus, many plant operators favor the refitting of inta ke systems with alternative devices called dual-flow screens, whose ma nner of operation precludes the deposition of debris downstream of the screen location. Fish-catching devices, otherwise suited to the flows and mechanics of a conventional screen, are often attached without al teration to the screen panels of a dual-flow machine in the hopes of r escuing entrapped fish. Dual-flow machines are thought to be superior to conventional intake screening systems in saving impounded fish, but the full-scale experiments reported here show why the flow patterns a nd water speeds associated with a (double-entry) dual-flow screen are actually more adverse to live fish recovery than flows through a conve ntional screen. Owing to flow separations at the entries of these devi ces and the resulting concentration of flow over a restricted portion of the screening, fluid speeds comparable to flows of 30 and 45 cm/s t hrough conventional screens increased to 90 and 140 cm/s fat free-flow regions of the screenfront. In experiments with two species of juveni le fishes, survival without injury was nil. Flow trajectories were map ped by streak photography, and details of fish behavior were recorded on videotape and 35-mm film. Equations for the flow distributions were resolved from two-component vector measurements of water velocities. Also shown is an experimental apparatus (a frontwall fairing) that eli minates the flow separations, resulting in a redistribution of the inf low across the full width of the available screening.