Downstream Guidance of Small‐Bodied Species With Angled Oppermann Fine Screens

ABSTRACT Although physical barriers are being commonly used for fish protection and guidance of larger migratory species in hydropower plants, there is a lack of information on how they perform for small‐bodied fish species. Here, we tested a small‐bodied species ( Alburnus escherichii ) based on 30 individuals across 16 hydraulic treatments. These included screen angles of 22°, 30°, 38° and 45°, as well as bed slopes of 0% and 1% without a weir. Additionally, the efficacy of crump vs. streamlined bypass weirs was examined at a bed slope of 1%. All trials were video‐recorded for analysis of bypass efficiency. For all tested conditions, we found a fish protection efficiency of 100%. Results showed that 38° and 45° screen angles produced the most favourable velocity distribution—reducing approach velocities from ~0.5 m/s (30°) to ~0.4 m/s—and increased passage efficiency. Raising the bed slope from 0% to 1% elevated approach velocities and reduced efficiencies by 10%–23% across all angles. The incorporation of a weir generated a beneficial backwater effect, yielding a 9%–15% improvement over the no‐weir condition, with the streamlined profile outperforming the crump weir. Moreover, 38° screen angle has the shortest fish passage time for all tested configurations. These findings provide clear design guidelines—namely, a 38° fine‐screen rack, minimal inlet slope and a streamlined bypass weir—to optimize downstream fish guidance and mitigate injury and mortality in run‐of‐river hydropower installations.