Author(s): Robert Boes; Claudia Beck; Julian Meister; Armin Peter; Maximilian Kastinger; Ismail Albayrak

Linked Author(s): Robert Boes, Claudia Beck, Julian Meister, Armin Peter, Ismail Albayrak

Keywords: Ngled bar rack; Bypass system; Fish guidance structure; Fish protection; Guidance efficiency

Abstract: Fish guidance structures (FGS) like angled bar racks with either horizontal or vertical bars protect downstream moving fish from turbine passage and guide them to a bypass. The function of the bypass system is to safely collect, carry and reintroduce the fish to the river downstream of the movement barrier. An effective bypass design is therefore essential to ensure not only fish protection, but also guidance to the tailwater. Most importantly, a bypass must be designed according to the swimming capacity and behaviour of the target fish species. Various field and laboratory studies indicate that a gradual velocity increase into the bypass with small turbulences leads to high acceptance. Previous studies demonstrated that a poor bypass design leads to low fish guidance efficiency (FGE), whereas an optimal bypass design can significantly increase the FGE. The bypass entrance should be easily found, accepted and passed by all fish species without harm, delay, flight or exhaustion. We have systematically studied two types of bar rack bypass systems with (I) curved vertical and (II) horizontal bars of different shape in the scope of two PhD projects at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich. The projects included the investigation of the hydraulics and fish guidance efficiency of different bypass geometries, namely (i) a full-depth open channel bypass, (ii) an open channel bypass with a ramp, (iii) a bypass with a restrictor as control section, featuring both a bottom opening and an overflow section, and (iv) a circular pipe bypass. Velocity measurements and live-fish tests were conducted in a 1:1 Froude-scaled ethohydraulic test channel. The swimming behaviour of various riverine fish species at these bar rack bypass systems (except for the pipe bypass) was recorded with a fish-tracking system and the obtained data were evaluated to determine the effect of the bypass hydraulics on fish behavior and guidance. The findings indicate, amongst others, that the full-depth open channel bypass was quickly accepted by fish for mild velocity gradients at the inlet. In contrast, a pipe bypass led to backwater with low flow velocities and flow circulation above the pipe and very high local velocity gradients in front of the bypass opening, which might lead to an avoidance reaction of fish. Although most available guidelines recommend a bypass discharge as a percentage of the water intake discharge, the present results indicate that the spatial velocity gradient in front of the bypass is paramount for an efficient bypass design to be quickly accepted by fish.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022537

Year: 2022