Why cylindrical screens in the Columbia River (USA) entrain few fish

Abstract

Cooling-water intakes often entrain large numbers of larval and juvenile fish, leading to questions why a water intake on the Columbia River, USA, with axial flow, cylindrical screens on a T-screen structure entrained very few (four fish in 4,007 x 103 m3 of water sampled over four years). This is despite being downstream of spawning areas for Chinook salmon Oncorhynchus tshawytscha releasing up to 56 million pre-smolts and having screens consisting of porous plate with 9.5-mm-diameter pores capable of passing salmon fry and other small fish (poor physical exclusion). Multiple publications of experiments and observations in hydraulics and biology are used to help explain the low entrainment, concluding that effects of the screening structure rather than the actual screen are most important. Primary mechanisms appear to be: (1) bow-wave-like hydraulics at the structure’s nose cone that deflect small fish away from the screen’s pores, and (2) fish detection and avoidance of pressure and velocity changes upstream of the structure that aid deflection. These combined hydraulic and biological-behavior effects of the structure would cause salmon fry and other small fish to bypass the screened portion of the intake. At the screens, (3) high ratio of sweeping flow to approach and through-screen velocities likely further prevents entrainment. Although site- and design-dependent, this analysis may be useful for evaluating or planning other screen installations and focusing regulatory attention on hydraulics of the structure as well as the screen.