Trialling New systems to Improve Fish Passage at Dysynni Culvert

This month we will be trialing new flexi-baffles on a culvert in the Dysynni catchment. Thanks to support from South East Wales Rivers Trust, we’ve been supplied with flexi-baffles—special structures made from recycled plastic that have been designed to help fish migrate upstream more easily.

Image credit: South East Wales Rivers Trust

What’s the problem with culverts?

Many culverts act as barriers to fish migration. Their flat, concrete design means gravels are washed away during high flows, leaving behind a smooth chute of fast-moving water. These high velocities demand a huge amount of energy from fish, often preventing them from reaching important spawning grounds upstream.

How do flexi-baffles help?

Flexi-baffles work a bit like speed bumps in the water. By sticking out from the culvert floor, they interrupt the smooth flow, creating slower-flowing “resting pockets” where fish can pause and conserve energy. This is especially important for weaker swimmers like juvenile salmon and trout, who can make their way through the culvert in stages rather than in one exhausting effort.

The baffles also create turbulence, breaking up the high velocity into a more varied pattern that offers different microhabitats to suit fish of different sizes and swimming strengths. Because they bend during floods, they allow water and debris to pass safely, reducing the risk of blockages.

Keep an eye out for the update once the works are completed early this month.