Last year we were working on a fish passage project on the Wandle at Trewint Street, Earlsfield.

Although there are many weirs on the Wandle which impede the migration of fish Trewint Street is one of the largest, with two weirs either side of a large concrete channel. With funding from the Environment Agency, Thames Water and Defra’s Catchment Partnership Action Fund (CPAF), we have restored passage for fish and eels, allowing movement to upstream habitats.

So how did we achieve this?

A bespoke fish pass was designed by Fishtek and installed on the weir by local contractors Amenity Water Management (AWM).

At the top of the right hand weir, a series of recycled plastic baffles were fixed onto the weir. These deepen and slow the flow of water and as you can see from the picture below, the baffles are arranged with a diagonal gap up to the top. This is the path the fish use to swim up the weir.

The baffles had to be fixed to the weir in dry conditions, and so sandbags were used to divert the water down the left hand weir, leaving the right hand side high and dry while our contractors worked. Watch the timelapse footage of AWM installing the baffles.

 

The second part of the fish pass were three large wooden pre-barrages at the bottom end of the island which were designed to slow the water down and reduce the drop in water level between the channel and the baffles.

Each barrage was notched to create a path for migrating fish.

To makes sure the pass works in low flows a huge piece of wood (7m long!) was placed at the top of the left hand weir to divert water at low flow down the right hand side and the fish pass.  This ensures the pass works over a large range of flows.

Bristles were fixed to each of the barrages to allow eel passage. Eel tiles were then fixed alongside the baffles on the concrete wall making the weir passable to both fish and European Eels.

A big thanks to our contractors AWM, landowner Mr Lammas and Thames Water, Defra and the Environment Agency for their funding.