PROJECT

Goat Bridge

Weirs break up rivers into isolated sections, meaning a variety of aquatic wildlife such as fish and insects cannot reach a variety of habitat to complete their life cycles.

Lowering a 1.6 metre weir at Goat Bridge will bring together disconnected sections of the River Wandle at Mitcham. The weir impounds about 500 metres of river, creating water conditions more akin to a lake than a river and leaving wildlife unable to reach a variety of habitats crucial to their survival.

Adapting the weir at this section of the River Wandle is at the heart of a project between Thames Water, the South East Rivers Trust, the Environment Agency, landowners Sutton Council and designers Mott MacDonald.

Demolition work was undertaken by the landowners in the summer of 2023 on sites adjacent to the river as part of their planning conditions.

The proposed works to lower the weir and make other improvements to the river channel were due to take place in the autumn of 2023. Current financial and delivery challenges across Thames Water’s project delivery programme means these works have been put back until after the winter’s high flow risk period. Therefore, the work has been rescheduled to start in March 2024.

The final landscaping works that are required to be undertaken by the landowner as part of their planning conditions will likely follow on after the river works are completed.

  • Project date

    2023

  • Metres of river opened up

    1400

The legacy of an industrial age

In Victorian times, the Wandle was one of the world’s most intensively exploited rivers for industrial purposes: 90 mills were constructed along its 11-mile flow from Croydon and Carshalton to where it meets the Thames at Wandsworth.

Its pure chalk stream waters attracted many industries including prestigious printers William Morris and Arthur Liberty. The river’s natural form was altered by this industrial age. It was straightened and deepened to enable development.

The river’s clean water was used to manufacture a range of products including paper, gunpowder, dyes, copper and leather. Chemical waste dispersed from various industries left a rainbow of unnatural colours in the river. In the 1960s, this globally rare chalk stream was officially declared a sewer.

Weirs left the river habitat like an enclosed canal, restricting wildlife, rather than a free-flowing river, supporting a wide variety of species.

The Goat Bridge Weir was built for an industrial age

Weirs fragment and degrade habitats

Weirs can break up rivers into isolated sections, meaning aquatic wildlife cannot reach a variety of habitat to complete their life cycles. For example, chub and trout require clean gravels to lay their eggs and eels cannot move along rivers.

Fragmented rivers are also less resilient to pollution incidents, because wildlife might not be able to escape to other sections of water.

Free-flowing rivers transport sediment such as silt and gravel continually. This process of being washed away and deposited in other areas creates the diverse range of habitats that aquatic wildlife need to thrive.

Weirs impound rivers – breaking them into sections – leaving them like slow moving ponds. Silt lies on the riverbeds, preventing fish from laying eggs. Downstream from a weir, the water is starved of sediment and gravel, making these areas less suitable for wildlife.

A Brown trout laying eggs into the clean gravels of a restored section of the River Wandle

Problems caused by Goat Bridge

Goat Bridge Weir impounds (encloses) about 500 metres of the River Wandle, close to where the Beddington Sewage Treatment Works discharge channel joins the main watercourse. The project will connect the enclosed section to its neighbouring water, meaning a total of 1.4km of river will be available for fish and aquatic wildlife to move along the watercourse.

This weir

  • leaves natural chalk stream gravels smothered by a thick layer of silt
  • allows invasive plant species such as Canadian pondweed to flourish because it acts like a lake
  • is a total barrier to fish passage, preventing them reaching restored areas of habitat further upstream. Fish surveys in the past decade show that numbers of the critically endangered European Eel are drastically lower upstream of the weir than downstream
  • reduces the river’s resilience to climate change and pollution. Fish seeking refuge from Beddington Sewage Treatment Works sewage overflows have been blocked by the weir
An unhealthy section of the River Wandle upstream of a weir

Weir lowering and rock ramps

The preferred solution to addressing the problems caused by Goat Bridge Weir is to lower the existing structure by one metre.

This will be carried out by constructing a rock ramp at the location of the weir and a series of ‘bed check weirs’ upstream. These are so called because they gradually lower the water levels of the river, to provide stepped passage for aquatic wildlife to move smoothly along the river.

This will provide the opportunity to create a more natural meandering and flowing river upstream, enhancing the river for local wildlife.

To prevent the work also lowering the water levels in the nearby Spencer Road Wetlands, the existing water level at the pipe that feeds the wetlands will be maintained.

A series of bed check weirs installed elsewhere by the South East Rivers Trust

Bringing improvements to the river

Lowering the weir would bring immediate improvements to the river and its wildlife, allowing fast-flowing water over gravels, promoting the growth of in-stream and marginal plants.

Typically, chalk streams such as the Wandle offer water that is cool, high quality and full of minerals and nutrients, creating a home for a special set of species such as brown trout, mayfly and water crowfoot.

The channel downstream of Goat Bridge weir showing chalk stream characteristics

Reducing flood risk

Extensive work has been carried out to ensure this project does not increase local flood risk. In fact, it will reduce the number of properties at risk of flooding.  Flood modelling suggests, for example, that the number of properties affected by a once in 20-year event would be cut by 95% and for a 50-year event the reduction would be 60%.

During the course of putting together the project, a chance arose to provide more flood storage space and a wetlands area on the left-hand bank by the Watercress Park footbridge. The land has been provided by the London Borough of Sutton. This will provide a new area of improved habitat but also one that can store water in a flood event.

The proposed new flood water storage area for the Goat Bridge project

Thanks to our supporters

Thames Water
Environment Agency logo
The Mott MacDonald company logo
London Borough of Sutton