A year on from the completion of a three-year project on the National Trust’s Holnicote Estate in Somerset to reconnect a section of a river to its floodplain – the innovative ‘Stage 0’ – river restoration technique, first pioneered in Oregon, USA – has been heralded a success.
The ‘ctrl alt delete’ of the river was the first large-scale attempt to reset a UK river to fully reconnect its waters with the surrounding floodplain by filling in a 1.2km managed, straightened and deepened section of the River Aller to transform the area and dramatically create seven hectares of waterscapes and wetlands.
A priority habitat for nature, wetlands are extremely important to not only slow the flow of water and to hold it during times of drought, but they are also significant for their ability to store carbon and act as homes for wildlife. However, sadly over 90 per cent of UK wetland habitat has been lost in the last 100 years, and over 10 per cent of our freshwater and wetland species are threatened with extinction.
Put to the test almost immediately after the project completed, and in the midst of England’s wettest 18-month period on record, the project team at Holnicote were pleased and relieved in equal measures when the landscape both improved for wildlife and helped slow the flow of water after several heavy rainfall events, helping to protect local communities downstream.
National Trust, Project Manager Ben Eardley said, “Trying any new technique is of course challenging but we need to be bold in order to tackle the climate and nature crisis.
“We had just the worst winter you can imagine post restoration in terms of the number of storms and sheer volume of rain. But despite it being record breaking conditions with high flowing water levels, the site responded really well, increasing the ability to store water within the site and lessening downstream storm flows, demonstrating the value of the restoration in providing resilience to hydrological extremes.”
To help the National Trust understand how the site has developed, researchers from several Universities (Exeter, Loughborough, Nottingham and Umeå in Sweden) have been involved with monitoring water flow, water quality and changes to habitat.
Dr Matthew Johnson from Nottingham University and Dr Richard Mason from Umeå University have been working to understand the changes to the water table and development of the river channels and habitats.
Mason said, “Ground water levels rose dramatically across the site, in some places by over a metre. This allows the site to act as a large sponge or filter in the river catchment, helping to store winter flood water, reducing flood risk downstream and releasing cleaner water more slowly in the drier months to help alleviate drought conditions.
“The twists and turns and numerous flow paths that are now present in the floodplain as the water wends its way over the site through thick vegetation, act as a filter to capture sediment eroded from upstream.
“The development of habitat to include faster flowing riffles and gravel beds has also been key for benefitting fish and other aquatic species. The site has moved from a very simple, static system with little opportunity for wildlife to one with significant dynamic complexity which has seen an explosion in life.
“Put simply, the system has moved from a very tidy area with little for wildlife to a messy complex jumble of waterscapes and diverse habitats that is full to the brim with a huge variety of plants and animals.”
Dr Alan Puttock and Professor Richard Brazier from Exeter University said, “Using drones and in-river sensors to monitor the project, we found that the dramatic changes following the reconnection of the river with its floodplain, have resulted in an attenuation (lessening) in downstream flow regimes.
“This is demonstrated by increased lag times between rainfall and stormflow, in addition to reduced downstream flood peaks, which were on average 38 per cent lower. These results mean during storms less water is leaving the site and at a slower rate, demonstrating such river restoration projects can help downstream communities at risk of flooding.
“These results mean that for the first time in the UK, we have seen the potential of allowing rivers to reset themselves to function naturally.
Other results from the monitoring of the site reveal a reduction in levels of fine sediment further downstream of the site, improving habitat for migratory fish and reducing the impacts of diffuse pollution from agriculture.
Water quality monitoring showed a reduction of average turbidity (water cloudiness) by 41 per cent following restoration, indicating the site is now trapping and storing sediment therefore reducing potentially damaging pulses of downstream sediment and nutrients.
Mason added: “The speed with which the site developed into a diverse patchwork of habitats has been a real surprise. After only a few months the river was already adjusting, creating deep pools, fast riffles, and a mosaic of wetlands which will all support different plants and animals.
“What amazes me is the amount of wetland habitat created; the river has transformed from a pipe into a sponge. When you look at the river upstream or downstream it’s still a small stream, a few meters across. It’s the same amount of water, it’s just that it is stored for much longer and it’s great to see how quickly it’s benefiting people and nature.
The Environment Agency were also a partner in the project and funder of part of the scheme. Matt Pang, Catchment Co-ordinator at the Environment Agency said, “One year on and the environmental response to the River Aller restoration exceed expectations. A rich and diverse wildlife haven has exploded from this back-to-basics, nature-based approach and the wider environment is better for it.
“Less flood risk, more wildlife, plus some rare species – the results are mind-blowing and we can’t wait to see what the coming years bring.”
Find out more about the Riverlands project
Image: A new waterscape has been created on the Holnicote Estate in Somerset to help nature and people. Credit: James Beck and National Trust Images