Fish pass creation

Pillhill Brook Fish pass

Replacement of a defective fixed-sill weir and fish pass.

This project on the Pillhill Brook (a perennial Winter-Bourne near Andover, Hampshire), involved the replacement of a defective fixed-sill weir and fish pass.

Originally constructed from dressed stone as part of an historic milling system, the leaking concrete bag repairs were undermining surrounding bank margins and causing an obstruction to fish passage. There was also the on-going potential for drainage of and environmental damage to the main mill leat channel in the event of a flood blow-out. The original repair had also been back-filled with builder’s rubble which was proving both unsightly and unstable so a major site clearance and tidy-up was also part of the brief.


Before: Upstream view of defective weir spillway

Before: Detail of crumbling concrete bag work

During: Stepped 'fixed-sill' weir under construction.

After: Detail looking up the stepped pool / fish pass arrangement on project completion. 

After: Project completion - Fixed-sill off-take point planted with sedges.

Following a period of detailed consultation, (during which our consultants negotiated a mutually agreeable solution with the Environment Agency and local parish council), our project site manager and his team of river technicians removed the failing structure and replaced it with a custom-designed stepped fixed-sill (rock ramp) weir created from locally sourced materials to create a new fish pass. On completion, the new structure was planted up with site-won sedges and other rhizomeiferous plants such as Meadowsweet (Filipendula spp) to stabilise the new bank margins and bio-engineered wing walls.

The project objectives (ie fish pass, stepped-sill flow controls, environmental, aesthetic, durability considerations etc) were successfully achieved to the satisfaction of all parties. No sooner was the structure complete than a 12 inch trout gave its stamp of approval as it was observed shooting up through the stepped pool/riffle system and into the main channel. Do keep a lookout on our ‘Projects Re-visited’ section for a forthcoming 1st year anniversary up-date on this project.

Read more about the history of Cain Bio-Engineering and its introduction to river restoration projects, across an impressive array of the UK’s most prestigious chalk streams and their tributaries.