Deflecting danger: The role of screen angles in fish impingement

Water abstractions in inland watercourses have the potential to harm European eel populations (Anguilla anguilla) if they are not correctly protected, potentially contributing to further declines of this critically endangered species. Current guidance aim to prevent eel impingement and entrainment at intakes and outfalls by specifying mitigation screening techniques such as screen types, screen apertures, and maximum approach velocities to the screens. These aim to prevent eels from being injured and allow them to bypass the abstraction, but they have yet to be empirically tested. In this study, screens with 3 mm apertures of horizontal and vertical wedge-wire and a Hydrolox screen were evaluated under the current Environment Agency's screening guidance document that supports the Eels (England and Wales) Regulations 2009. We measured the hydrodynamics of the screens and then observed eel behaviour and swimming dynamics upstream of the screens. The screens had minor effects on the upstream flow fields and produced suitable velocities and turbulence levels for eel escapement. At the regulation velocities, no eels impinged on the screens, validating the current guidance but impingement started to occur at higher velocities, so the guidance velocities should not be exceeded. Screens at smaller angles to the flow caused few eel impingements and therefore are preferable. The current screen guidance for 3 mm wedge-wire screens are appropriate for eels of the size tested in this study and do not cause impingement or entrainment.