Why is screening necessary?

You will need to use physical screens to prevent fish and eels, at all life stages, from being drawn into the penstock and turbine via the intake. You may also need screens on the turbine outfalls to stop upstream migrating fish and mammals from entering them.

Screen type and aperture size

Most hydropower sites in Wales use overspill type intakes. We recommend using modular Coanda type or perforated plate intake screens. They are generally suitable for most upland sites and are simple and effective in excluding fish and debris at aperture sizes that meet fish screening requirements. They also allow passive operation of the abstraction to be incorporated into weir design.

Screen aperture is the size of the gap between the bars or the maximum diameter of perforations on an intake screen. Maximum screen apertures of 3mm are required for effective fish screening in most upland catchments. In river reaches near to the sea, maximum apertures of 1 mm will be required. The finer screens are necessary to prevent upstream-migrating glass eels and elvers from entering the intake.

Overspill screens require a plunge pool on the downstream side of a weir so that fish that pass over the intake screen fall into a suitable depth of water. This prevents harm to them and stops them being stranded. More details on this are provided in our guidance section Fish passes for hydropower weirs.

Where overspill screens cannot be used, intake screening is likely to be bespoke can be more complex to design, build and maintain. In such situations, the screen aperture will still be dependent on the distance from the sea and presence of upstream-migrating glass eels and elvers but also on other fish species present and in some cases the turbine type. In these cases, developers should apply the detailed design guidance in the Environment Agency guidance Screening for intakes and outfalls: a best practice guide and Screening at intakes and outfall: measures to protect eels.

You can download Screening at intakes and outfall: measures to protect eels from Gov.uk

Other design considerations for non-overspill fish screens include preventing impingement and entrainment of fish on screens and the need for a bywash to allow downstream migrating fish to pass into the channel below the weir.

Coanda type intake screening

The term Coanda refers to the ability of a liquid to cling to a surface, in this case transferring water from the streamflow, through the screen and into the abstraction sump. Coanda screens are made of wedge wire typically with 1 mm apertures which will screen out fish and debris.

Drop-through intake screening

‘Drop-through’ screens are constructed using steeply sloped, perforated stainless-steel plate on the downstream face of the weir. Holes in the plate should be a maximum of 3mm, subject to location within the catchment. Flow passes over plate and falls into the sump below for abstraction. Like the Coanda type screen it will prevent fish and debris from entering the abstraction.

Outfall screening

In river reaches accessible by migratory salmon and sea trout, fish may be attracted by discharge flow from hydropower scheme outfalls and try to enter them.
When discharge is onto a spillway, drop structure or boulders where it is difficult for fish to access the outfall due to lack of a clear, defined channel then outfall screening for fish is unlikely to be required.

Where the outfall discharges into a leat or submerged pipe and there is a possibility that fish may access the outfall then screening will be required. This is not necessarily to prevent their access to turbine components but to reduce the risk of upstream migrating fish from becoming confused with the outfall flow and slowing down or preventing migration.

Where an outfall screen is required, it should be located close to the bank at the point where the turbine discharge enters the river. Screens should be constructed with square or rectangular bar and have horizontal spacings of 40mm (the aperture size) to prevent access for adult Atlantic salmon or 30mm for adult sea trout. A 40mm aperture size is also sufficient to screen out juvenile otter. The cross-sectional area of the outfall channel should be large enough to slow down the discharge velocity so that it is less likely to attract fish.

Intake and outfall screening - key principles

  • Ensure that your intake is fully screened to prevent fish and eels from entering the penstock and turbine
  • Use an overspill weir design and screen type where possible
  • Use a maximum aperture of 3mm for intake screens in upland catchments
  • Use a maximum aperture of 1 mm for intake screens in tidal and river reaches close to the sea
  • Ensure that your overspill weir design includes a downstream plunge pool
  • Refer to Environment Agency documents for detailed design of non-overspill intake screens
  • Design the outfall to discharge onto boulders or into a drop structure to reduce flow velocity and make access for fish difficult
  • Where the outfall is an open channel or pipe to the river you will need to install outfall screening
  • Use vertical square bar with 30 - 40mm horizontal apertures for your outfall screen

Read about fish passes for hydropower weirs


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