Design of a Downstream Migration Fish Pass for Existing Hydropower Plants

Downstream migration of fishes in anthropized rivers is key for hydraulic structures managers as well as rivers managers, in particular when threatened species are concerned. In Belgium, a dedicated program (Saumon 2000) has been initiated since 1987 to favor the reintroduction and the upstream migration of salmons (salmo salar). In parallel, the recently launched 5 years European project Life4Fish aims at designing and implementing several solutions to facilitate the downstream passage of salmon smolts and silver eels at 6 hydropower plants along the Meuse River in Belgium, such as turbine replacement, predictive models, behavioral barriers and downstream fish passes.

This paper focuses on the research and development conducted in order to design downstream fish passes at 2 pilot sites. The design was realized combining the inputs of on-site biological studies, hydrodynamic numerical and experimental scale modelling, as well as considerations about the structural feasibility in the limited spaces available on the specific existing sites.

The proposed fish ways were designed considering the local hydrodynamic characteristics and the expected fish responses at each stage (attraction, uptake, still area, transfer and release). On-site fish tracking enabled to define the presence probability when fishes arrive upstream of the HPP. In parallel, large scale hydraulic numerical modeling highlighted the correlation between hydrodynamic conditions and fishes behavior. Results from these two steps of the study have been used to define the best location for downstream fish pass. After, physical scale models of the proposed systems have been built and operated to optimize the fish ways intakes and create suited surface currents upstream. From the uptake of the fishes at fish ways entrance to the downstream release, the design has been realized according to the literature on biological behavior of salmo salar along such structures. In parallel, the technical constraints at existing plants have been challenged to define optimal technically feasible solutions. Finally, the cohabitation with existing or planned upstream fish ways have been analyzed to definitively validate the design.

All the steps of the study summarized above are presented in the paper, focusing on hydraulic problems and solutions.