Downstream fish passage and swimming behaviour at a bypass gate with bottom and top opening

Fish guidance structures (FGS) with adjacent bypasses can provide safe downstream passage at hydropower plants. To regulate bypass discharge, gates with partial openings are installed at bypass inlets. This study examines how flow constriction at such openings affects fish behaviour and bypass entrance efficiency (BEE). In a physical model, we quantified upstream velocities with acoustic Doppler velocimetry for gates configured with either a bottom (BO) or top opening (TO) under three approach-flow velocities Uo. Ethohydraulic tests with brown trout (Salmo trutta) and common barbel (Barbus barbus) assessed their ability to locate and enter the bypass. Hydraulic measurements showed high bypass inflow velocities Uby,in up to 2.0 m/s, velocity ratios Uby,in/Uo up to 6.7, and streamwise velocity gradients SVGx up to 21 s−1, exceeding common design recommendations. Uby,in and SVGx, linked to opening location and geometry, emerged as key parameters associated with BEE and fish behaviour. Both species frequently avoided strong currents at the openings, with significantly lower and delayed passage through the TO. Brown trout tended to enter the BO more often and earlier than barbel, indicating species-specific preferences. Overall, BEE was low (BO: 27–59%; TO: 8–21%). Findings highlight that unfavourable hydraulics can impair FGS-bypass performance. Design recommendations are provided.