Designing the outfall for a hydropower scheme
Outfalls should be sited and designed so that discharge of flow from the turbine house does not erode the river channel, lead to instability of the river bank or attract migratory fish.
The environmental design of the outfall and its construction is not normally regulated through Water Resource licences but is a requirement of any Town & Country planning consent and also Flood Risk Activity Permits or Ordinary Watercourse consents for works within or adjacent to river channel.
Siting an outfall
The outfall should be located on a straight, stable river reach where the river is actively transporting sediment. Outfalls must not be located in river reaches where there is good fish spawning habitat. Fish lay eggs in areas of coarse sands, gravels and cobbles which are found in low gradient river reaches. Even small areas of gravels can still provide good habitat for spawning.
The outfall should be set back from the bank and aligned so that the discharge enters the river at an angle no greater than 45 degrees from the direction of stream flow. This reduces the risk of erosion and prevents sediment from accumulating in the outfall structure.
Reducing discharge velocities
Outfalls should be located and designed to minimise discharge flow velocities. High velocity discharges can erode the river bed and banks as well as creating attraction flows for fish. Stilling basins, sumps, drop pipes or submerged outlets can be used to reduce outflow velocities. Drop pipes are simple structures in which flow falls within a vertical chamber, loses surplus energy and allows the water to discharge at a slower velocity. For smaller schemes the outflow may be discharged onto stable bed material such as boulders or bedrock.
Where outflow velocities are low, discharge can be via a submerged outlet or via a swale or ditch arrangement.
Wetlands and swales
For some smaller schemes the outfall could be set back from the receiving watercourse and discharged into a swale, ditch or wetland flush leading to a diffuse flow into the river or stream. This has the benefits of minimising construction on the bankside, reducing risk of erosion through lower discharge velocities and creating new wetland habitat.
Designing an outfall - key principles
- Site the outfall on straight river reaches with stable channel bed and banks
- Avoid locating the outfall in reaches with fish spawning habitat.
- Align the outfall at 45 degrees to the direction of river flow to prevent sediment ingress and reduce the risk of erosion and deposition.
- Reduce outflow velocities using a drop structure or discharge onto bedrock/boulders.
- Design the outfall to be submerged, or partially submerged (where water levels are not deep enough for full submergence).
- For small schemes consider discharging via a wetland flush
- Consider local landform or position of existing natural boulders when siting outfalls to minimise modifications to river banks, and avoid the need for hard engineering for bank stabilisation.
- Use local stone to face outfalls and minimise visual impact on the landscape.
- Use a 30- or 40-mm aperture bar screen on the outfall to prevent access to upstream migrating fish