Author(s): Yuhao Yan; David Aldven; Stephanie Muller; Luiz G. M. Silva; Patrik Andreasson; David F. Vetsch; Seline Frei; Eric Lillberg; Ana T. Silva; Robert M. Boes; Ismail Albayrak

Linked Author(s): David Aldvén, Luiz G Martins da Silva, Ismail Albayrak

Keywords: Downstream fish passage; Fish guidance rack-bypass system; Atlantic salmon smolts

Abstract: Fish guidance structures (FGS) with adjacent bypass system s are promising technologies to facilitate safe downstream passage for a range of fish species and hydraulic conditions. However, successful fish guidance, depends on the design and operation of both FGS and bypass system. Despite high guidance efficiencies of recently developed FGS, bypass systems are often still the bottleneck and require optimization. To address this, we numerically developed a novel weir geometry for a full-depth open channel bypass and tested it with Atlantic Salmon smolts (Salmo salar) in a large-scale ethohydraulic flume at Vattenfall R&D in Sweden. The weir, featured by a 9° inclination angle, an S-shape in longitudinal section and a broad crest, was placed in an open -channel bypass. The bypass was integrated into a novel FGS that creates streamwise eddies. Fish experiments were conducted under four approach flow velocities (Uo = 0.26 - 0.69 ms–1) at a constant water level of 1.2 m. The spatial velocity gradient, a key hydraulic parameter, was maintained between 0 and 0.5 s–¹ within the threshold of 1 s –1 for successful passage. Fish response data were analyzed to assess bypass passage efficiency (BPE), bypass acceptance efficiency (BAE), and time-to-event metrics. The bypass system achieved a consistent BAE of 100% across all conditions, with BPE ranging from 90% to 100%. Additionally, the time- to-pass analysis indicated reduced hesitation to enter or pass the bypass under higher inflow conditions. In summary, the innovative bypass design generated favorable hydraulic conditions, contributing to high passage and acceptance efficiencies and demonstrating strong potential for improving downstream fish passage.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.206

Year: 2026