Ethohydraulic Evidence that Curved Guidance Walls Boost Bypass Efficiency of a Small-bodied Leuciscid

Angled racks are widely used at water intakes, yet live-fish evidence on how approach-flow conditioning interacts with fine bar spacing to affect downstream passage of small-bodied species is scarce. We tested whether adding a curved guidance wall to an angled 10 mm fine screen improves approach hydraulics and increases bypass passage probability for the potamodromous leuciscid Alburnoides kosswigi, and whether screen angle (30° vs 45°) or fish length affects outcomes. Ethohydraulic flume experiments incorporated two screen angles and the presence or absence of walls, resulting in four distinct treatments across 120 individual trials. Three-dimensional velocities were mapped with acoustic Doppler velocimetry (ADV) to quantify approach hydraulics, including streamwise spatial velocity gradient (SVG_x). Passage success (bypass entry within 30 min) was analysed using binomial GLMs. With the implementation of the guidance wall, the bypass inlet velocity was reduced from 0.94 m/s to 0.65 m/s for α = 30°, and from 0.73 m/s to 0.65 m/s for α = 45°. The SVG_x values were reduced to approximately 0.1 s^-1 at the bypass inlet, and the normal velocities along the screen were also reduced by the guidance wall. More uniform velocity distributions towards the bypass channel were observed at α = 45° with the guidance wall. Consequently, bypass efficiency increased from 56% to 80% at 30° and from 63% to 90% at 45° with the wall. In models, the wall was the only robust predictor of success (OR 3.95, 95% CI 1.66-10.08), whereas angle and fish length were not significant. However, an exploratory threshold analysis suggested a tentative 96-mm cut-off above which success increased (β = 1.50; p = 0.0595). Overall, the results show that using a curved guidance wall in the conditioning approach flow can increase downstream passage by 25-30% at angled fine screens without increasing entrainment. This suggests that guidance-wall design should be considered in intake standards at poor approach-flow conditions for small-bodied potamodromous fish.