Hydraulics of Horizontal Bar Racks for Fish Downstream Migration

Hydropower plants (HPPs), like many other transverse structures, interrupt the ecological continuum of rivers. Horizontal bar racks (HBRs) in combination with a bypass system are an effective measure for protecting and guiding downstream migrating fish. Such HBRs were successfully installed at a number of small- to medium-sized (Q < 100 m³/s) HPPs in Europe. Despite this, only a few studies exist to assess the hydraulic and fish-guidance performances of these state-of-the-art downstream fish passage facilities. To fill these research gaps, a systematic laboratory study has been conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zürich. The varied parameters include the bar shape, the bar spacing, the horizontal rack angle, and the application of overlays. The hydraulic losses of clean, unclogged racks with modern bar shapes therefore reduce by 27…42% as compared to conventional rectangular bars. The measurements indicate that bottom and top overlays have the most significant effect on the flow field and the resulting hydraulic losses. On the one hand, such overlays can improve the guidance capacity for fish and sediments. On the other hand, they increase the hydraulic losses and lead to asymmetric downstream flow fields, potentially affecting turbine efficiency. The current contribution compares experimental results on hydraulic losses and flow fields of HBRs for two different bar shapes: rectangular bars, which were mainly used in the past, and more modern, streamlined bars with a circular tip.