Addition of auxiliary discharge into a fishway – A contribution to fishway design at barrages of large rivers

Abstract

Finding the entrance into a fishway might be challenging for upstream migrating fish and especially so in large rivers. Auxiliary discharge, added into the downstream section of the fishway, will improve the attraction in the tailwater, but may impede passage in the fishway itself.In the present study we investigated a best practice design of an entrance channel with auxiliary water supply through a lateral screen in an experimental flume by means of hydraulic measurements and fish tests.Two screen sizes and angles and, relating thereto, two screen design velocities were compared and performed equally well. There was no difference in passage success for nase (Chondrostoma nasus), gudgeon (Gobio gobio), spirlin (Alburnoides bipunctatus), roach (Rutilus rutilus) and brown trout (Salmo trutta).The additional attempt to guide fish through the entrance channel by use of a slot which produced a more perceptible flow towards the upstream fishway section did not improve passage times of spirlin and brown trout.With a mean finisher rate of about 85%, which is a conservative assessment since it derives from an artificial, experimental situation, our flume layout turned out to be a good basis for a design recommendation.We suggest adding auxiliary discharge through a lateral screen with a design velocity of 0.4 ms−1 and we provide the necessary design specifications for geometry, hydraulics and screen arrangements in the entrance channel.