Live-fish tests with fine screens: effect of a guidance wall on downstream migration of potamodromous species

Although angled bar racks are increasingly used to guide fish at water intakes, their efficiency for potamodromous species during downstream migrations remains poorly quantified, particularly under varied structural configurations. In this study, the behavioral responses of fish to hydraulic conditions were experimentally investigated using the Oppermann fine screen, with and without an elliptical guidance wall. The tests were conducted at a 45° screen angle, with a bar spacing of 10 mm for two potamodromous fish species, Colchic nase (Chondrostoma colchicum) and Sakarya bleak (Alburnus escherichii). Results showed the guidance wall significantly enhanced passage success for Alburnus while improving passage trends for Chondrostoma. Total body length was found to be a highly significant predictor of success for Chondrostoma, but not for Alburnus. Consequently, including fish length in predictive models dramatically increased their classification accuracy for Chondrostoma. The guidance wall decreased downstream transition time for Chondrostoma and slightly increased it for Alburnus. Both experimental measurements and CFD model results revealed that the guidance wall reduced Reynolds shear stresses, eddies, recirculation, and stagnant areas upstream of the screen, improving downstream bypass passage. These results highlight the value of incorporating guidance walls into fish passage structures to enhance migration efficiency of potamodromous species in regulated river systems.