River Wandle Restoration at Richmond Green

Restoration to boost biodiversity

Over many years, the River Wandle has been altered for a variety of different reasons, but all to benefit humans. The catchment has become increasingly urbanised since the industrial revolution and the subsequent expansion of London.

The legacy of these changes has simplified the river habitat, leaving little space or diversity to sustain the wildlife that was associated with it, including trout, water voles and many insect species.

Natural rivers are dynamic systems, forever moving and changing, shaping the landscape and how animals and plants interact with the river. By straightening (‘canalising’) rivers, man-made changes have interrupted their natural processes, making them poor for wildlife and flood management, which has impacts on people.

The Richmond Green restoration project, funded by the Environment Agency and the London Borough of Sutton, will reverse the effects of habitat degradation by implementing a range of tried and tested river improvement techniques to bring the river corridor back to life for both people and wildlife.

An artist's impression of how the area would look after restoration. By Lyndia Durrant https://fullcircle-design.co.uk/

Tree removal and increasing light

The project intends to thin tree cover along the river’s banks to provide a beneficial light/shade ratio for the chalk stream environment, offering better conditions for a more diverse range of aquatic plant species.

This is proposed in two phases:

1 – October to November 2022: removal of three, large, fast-growing invasive non-native Caucasian Wingnut trees, which cast deep shade over much of the river channel, west of the footbridge.

Their suckering nature (expanding through shoots from roots) is a significant management concern, regardless of river restoration, although they are also a major impediment to improving the river if they are not removed. After the trees are removed, their suckering regrowth will be monitored for a year, to ensure full removal, before the river restoration work begins.

Anyone with comments or concerns about the removal of the wingnuts can email biodiversity@sutton.gov.uk

2 – Autumn 2023: Selective thinning of self-seeded poplar and sycamore, mainly on the southern bank, to ensure the correct mix of light and shade levels.

The project will replace an amount of trees with a selection of native trees more suited to the river corridor in the 2023/2024 planting season.

All tree felling is in line with the Council’s Tree Strategy. This states that healthy trees will not be cut down except for health and safety reasons, unless there are clear biodiversity benefits (such as woodland management work or river restoration).

Caucasian Wingnut trees at Richmond Green. They grow very fast, their roots shield the river from sunlight, reducing public access to the water's edge © South East Rivers Trust

Reprofiling the river's banks and removing toeboarding

The river’s profile has been artificially modified, leaving a wide channel with steep banks and little transitional habitat between the aquatic and terrestrial zones – nature that joins the river to the land.

By making the banks of the river slope more gradually, native vegetation will be able to establish and soften the transition between water and land.

This helps to create a buffer between the river ecosystem and recreational activity further up the bank.

The channel is also constrained by wooden boards known as toeboarding. Where feasible, the boards will be removed to facilitate the reprofiling of the banks and to provide a valuable space for aquatic plants to thrive.

An example of the toeboarding at Richmond Green © South East Rivers Trust

Installing marginal berms

Left to its own devices, a river will naturally form in-stream habitat features, known as berms, as it meanders through the landscape.

These features are often absent in rivers that have been artificially modified as they lack the habitat structure and complexity for berms to establish and become stable.

To kick-start this process, the project will install a series of berms to pinch the channel and increase flow velocity.

This will provide a river channel that is better adapted to low flows in summer and will help promote clean gravels on the riverbed, which provide excellent spawning habitat for fish.

Berms are designed to pinch the channel and provide anchor points for aquatic plants © South East Rivers Trust

Adding woody material and gravels

Clean gravel is a trout’s favourite place to lay eggs. It is also favoured by many aquatic insects. However, it is a fragile habitat.

The straight, over-wide channel is prone to fine sediment, washed in from the surrounding pavements and roads, accumulating on the riverbed. Fine sediment smothers the river’s gravels, cutting off the oxygen supply to incubating fish eggs and river-fly larvae.

In a natural system, trees would fall into the river and disrupt the flow, creating new habitats in the process.

In the right locations, large woody materials can pinch the channel to increase flow. This has the effect of moving the river’s gravels and cleaning them so that they provide clean spawning beds for fish to breed and for aquatic invertebrates, such as mayflies, to thrive.

To encourage these natural processes, pieces of wood will be installed throughout the channel, to promote a self-cleansing and dynamic river, where habitat continues to change and improve long after the project is finished.

The project also intends to restore the river’s natural gravel bottom by introducing locally sourced gravels to improve habitats.

Clean gravels are an indicator of good habitat © South East Rivers Trust

Creation of a backwater

The river is limited by poor connectivity to its floodplain. This means there are few areas outside the channel for aquatic wildlife to seek refuge during flood conditions or pollution incidents, or to avoid predators.

The project will investigate the feasibility of installing a backwater because these provide ideal habitat for juvenile fish and some invertebrates more associated with slower-moving water.

An example of a backwater created by SERT, elsewhere

Investigating weir lowering

Two weirs are present at the most downstream (western) extent of the site.

They impound the river – blocking in a section of water. In this case they are responsible for increasing water levels upstream, slowing the river’s flow and preventing valuable clean gravels from moving downstream, reducing habitat quality.

They are physical barrier to the movement of fish species and other aquatic wildlife.

The project partners are currently investigating the feasibility of lowering the weirs, in order to provide a free flowing, dynamic watercourse that is better connected to areas downstream.

Biodiversity enhancement and protection of existing river wildlife

The London Borough of Sutton has adopted a Biodiversity Strategy (2020-2025) which includes a rivers and wetlands habitat action plan, which sets out how we will protect and enhance these fragile ecosystems.

The project will undertake habitat and protected species surveys before the works start. Then post-work monitoring will be carried out to ensure it is delivering the intended outcomes. The council wants all of the Wandle in Sutton to be accessible for fish passage (and therefore be better for all wildlife) by 2050.

Vegetation clearance will be undertaken outside of the bird nesting season (between March to August inclusive). Kingfishers are known to use this section of the river. If they are found building their nest or breeding nearby, restoration works will be timed to avoid disturbing them.

Restoration timescale

After the tree works in the autumn of 2022, the rest of the works are planned for the winter of 2023-24. The positive effects of many of the improvements will be instantly recognisable, but other new features will take longer to emerge.

While excavation works can look unsightly for several months, nature soon bounces back and the new habitats created can support a wide range of wildflowers and interesting wetland features. Wildlife soon moves in, making use of the rich conditions.

Near the end of the project, the community will be invited to help with the final flourishes of the restoration work, for example by adding native wildflower plants and seeds.

Richmond Green river