Development and evaluation of an empirical equation for the screening effect of bar racks

Passage through hydropower plants can cause severe injuries for downstream migrating fish. Bar racks and bypass systems can help to protect fish from turbine-induced mortality if they are designed correctly following guidelines. Currently, it is not possible to assess how effective bar racks are that do not meet design criteria for fish protection. We introduced the screening effect as a performance parameter for bar racks and developed an equation for determining the screening effect of bar racks for silver eels based on an empirical approach. We compared the equation for eels with the results of behavioural laboratory studies with European eels (Anguilla anguilla), roach (Rutilus rutilus) and Atlantic salmon smolts (Salmo salar). We showed that the equation is not applicable for roach and Atlantic salmon smolts. Using univariate statistics, we identified a significant influence of the bar rack spacing on the screening effect for eels for the guidance system with (Kruskal-Wallis test, p = 0.002) and without a bypass (Kruskal-Wallis test, p = 0.0220) and for salmon without a bypass (Kruskal-Wallis test, p = 0.001) for a significance level of 0.05. The screening effect was significantly different between the fish species tested for a bar rack without bypass and 20 mm bar rack spacing (Kruskal-Wallis test, p = 0.003) and 40 mm (Kruskal-Wallis test, p < 0.001) as well as a bar rack with a bypass and either 40 mm bar rack spacing (Kruskal-Wallis test, p = 0.014) or 60 mm (Kruskal-Wallis test, p = 0.001). The results can help to perform biological evaluations and plan retrofitting measures of non-optimal bar racks.