Larinier fish pass installations on the river Dun
River Dun, Hampshire.
September - November 2017
Wessex Chalk Stream & Rivers Trust
Test Valley Borough Council, Environment Agency, Black & Veatch Limited, Holbury Lakes Trout Fishery, Lockeley Mill Riverside Accomodation
Larinier Fish pass installation at two sites on the River Dun:
- Closing the existing channel with sandbags and pumping out the water to enable working in a dry environment.
- Excavation of the new channel for the fish pass and bed level raising using 30t of gravel.
- Construction of a compact Type 1 sub-base and an A393 Mesh reinforcement to form a strengthened concrete base under the fish pass.
- Engineering bricks were layered down with site mixed mortar to form the sides of the fish bypass structure.
- Resin anchor bolts were used to secure the aluminium fish bypass onto the concrete base and to attach the eel tiles to the bricks.
- Introduced a deflector formed from brushwood faggots and seeded coir rolls, tied and staked with timber poles. This feature constitutes a channel to direct fish passage.
- A smolt screen introduced at the end of the deflector to provide a hydraulic link between inlet and receiving pool.
- In addition to the fish bypass, a new weir wall, culvert and debris screens were installed to improve the flow to the fishing lake.
- Reinstating the hatcheries channel with a wooden frame and disposing of new bridges.
- Water Vole surveys were conducted by a licensed ecologist to ensure this protected species was not impacted by the works
- Use of Five Rivers’ specialist plant with bio hydraulic oil, low ground pressure wide tracks and tilt heads reduced the impact of the works on the local environment and ecology.
- Worked in collaboration with the landowner to tailor the project to their need.
- Traditionally fish passes are manufactured from steel. However, an aluminium channel requires less material and is less time consuming to manufacture, hence, reducing our environmental impact.
- Our work is improving the connectivity between the River Dun and the Test catchment area as multiple fish species are now able to bypass the mills.
- Hydro-seeding enabled rapid colonisation of the lawn.
- Eel migration was re-established via the eel passage integrated into the Larinier bypass.
- Improving the riparian habitat through vegetation cover and foraging opportunities for aquatic species.