Tag Archives: Fish passage

Fish Passage Restored!

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.

Baffles

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.

Barrages going in

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.

Deflector and Eel Bristles

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.

Contractors

Dam, where’s all the water gone?

We’ve started work on the Trewint Street Fish Passage!

Fish passage on the River Wandle is impeded by over 30 in stream structures, the majority of which are weirs left from the milling era. These weirs and structures are a barrier to the movement of fish both up and downstream and also fragments and isolates habitats.

The Weir

Trewint Street is one of the significant barriers to fish passage, with two weirs either side of a large concrete island. With funding from the Environment Agency, Thames Water and Defra’s Catchment Partnership Action Fund (CPAF), we have started our project to install baffles and a fish pass to the right hand side weir, allowing the movement of fish once more!

The pass will also benefit European Eel populations which have declined by over 98% in the last 15 years, with barriers to movement being a contributory factor.

So what are we doing?

Low cost bafflesOn the right side channel, a series of baffles will be installed to the upper section of the concrete weir. These baffles are made from recycled plastic and fixed to the weirs in rows.  They slow the flow down on the weir, deepening the water and allow fish to swim up the weir through notches cut into the baffles (Image, Fishtek).

Barrages

In the lower part of the right hand channel, three notched barrages will be created to reduce the drop in water level between the channel and baffles. This will allow fish to easily swim up through the notches and through the baffles to new habitats beyond (Image, EA).

What will you see?

You will see a lot of building work on site over the next month as our contractors (Amenity Water Management (AWM) get started. You’ll also notice that the right hand channel is a lot drier than normal…

Dam!

Amenity Water Management have created a sandbag dam to keep the channel dry allowing them to work on installing the structures. All will return to normal once work is complete.

We’ll keep you posted with updates as always, but for now Tim is just happy to have wet feet again!

Tim happy once more

A little light goes a long way…

Our restoration projects along the Wandle are all progressing well, and it is time to introduce the next site on our list: the Ravensbury Park Back Channel.

The Ravensbury Park Back Channel is a remnant of the industrial milling era along the Wandle. Back channels, or “by-pass channels” were used as a mechanism to control the water passing through a mill allowing us to increase or decrease the flow depending on how much power was required. Although no longer used for this purpose, the back channel at Ravensbury Park has become part of the park’s landscape and could offer a valuable refuge for wildlife in the Wandle; in particular fish which could use the channel to by-pass the weir.

So what’s the problem?

  1. Channelisation

Like so many urban and rural rivers, the Ravensbury Park back channel has been engineered and modified to contain the river within a predetermined channel. This removes habitat and creates a stream with little variation.

Wooden banks and channelised nature of the stream

Wooden banks and channelised nature of the stream

  1. Un-natural banks

To protect the surrounding landscape from flooding, the natural sloping banks of this stream have been replaced with concrete and wood. This creates a ‘hard-edged’ bank and prevents important natural communities of marginal and aquatic plants from establishing here. A more gradually sloping vegetated bank provides habitat for many different species of riverine wildlife. Without this habitat there is a significant loss of biodiversity.

Concrete banks

A high concrete bank removes all interaction with the stream ecosystem.

  1. Little light

The Ravensbury Park back channel suffers from low light levels due to a dense tree canopy. This canopy limits the growth of aquatic plants both around the stream as well as within the stream.

A dense vegetative canopy prevents aquatic plant growth

A dense vegetative canopy prevents aquatic plant growth

Each of these issues on their own have negative consequences, but combined they significantly impact on the ecological structure and function of this little stream.

What can we do?

Over the next few months, the Wandle Trust will be taking steps to restore the Ravensbury Park back channel. This work is being funded by Defra’s Catchment Restoration Fund, and is also part of the Heritage Lottery Funded Living Wandle Landscape Partnership.

The main aim of the restoration works will be to create new habitats both in the stream and along the margins that will support fish, invertebrates and birds. More importantly, our works will provide a favourable environment for both aquatic and marginal plants which are a fundamental source of food and shelter for some organisms; vital for a thriving ecosystem.

We will be working further on our plans for this stream so keep your eyes peeled for new updates.