Construction of two Larinier fish passes and by-pass channels on the River Dunn, Hampshire. The Environment Agency and the Wessex Rivers Trust since 2015 have been working in partnership to deliver a design to improve fish passage at Holbury and Lockerley Mills on the River Dun in Hampshire.
The project involved river modelling and engineering design work which was contracted to the civil engineering company Black & Veatch Limited. Hydraulic modelling was undertaken as part of the initial project design to help ascertain the best means to enhance fish passage at both mills which had various artificial flow regulation measures in place. Five Rivers mission was to build a new off-take on the right bank of the River Dun which will provide a passage through a Larinier bypass channel. The existing mill bypass stream was widened and narrowed downstream through gravel placement. In addition, an eel passage was incorporated within the fish pass and inert stones were placed to reduce the toe erosion process at the confluence between mill bypass and the main channel.
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.