“Dramatic changes” to Exmoor river a “mind blowing” success
A pioneering project to restore part of the River Aller on the National Trust’s Holnicote Estate in West Somerset is being hailed as a major success, for reducing flood risks and boosting wildlife.
The National Trust filled in 1.2km of the River Aller, so water that used to flow through a straightened, deepened and managed channel, was instead moved up and out onto its wider floodplain.
The result: seven new hectares of waterscapes and wetlands, the size of more than 10 football pitches. Text continues below image.
It’s the first large-scale initiative in the UK to apply ‘Stage 0‘ techniques of river restoration initially tried out in Oregon in the USA.
The National Trust has been leading the project as part of its multi-million pound Riverlands initiative, with funding also coming from the EU’s Interreg 2Seas Co-Adapt programme, the Environment Agency, Somerset Rivers Authority (SRA), the Green Recovery Challenge Fund and Frugi.
The project was completed in 2023, after three years’ work.
Its ambitions were tested almost immediately, through the wettest 18-months on record for England.
Verdicts after one year
The project team were pleased and relieved when their newly-created waterscapes and wetlands produced huge improvements for wildlife and, crucially, slowed the flow of water – which helps to protect communities downstream in places such as Allerford and Bossington.
“the site responded really well”
Ben Eardley, National Trust Project Manager, 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.”
“what is needed”
Councillor Mike Stanton, Chair of Somerset Rivers Authority, said: “The National Trust’s conversion of the River Aller from a narrow stream to a floodable water meadow is just what is needed for flood prevention. To similarly bring the upper reaches of other Somerset rivers back to their natural – and flood-preventing – state we will need co-operation from many landowners and farmers, which will take time and money.”
“a messy complex jumble of waterscapes and diverse habitats”
More than 90% of the UK’s wetland habitat has been lost in the last 100 years, and over 10 per cent of the country’s freshwater and wetland species are threatened with extinction.
To help the National Trust understand how the River Aller site has developed, researchers from several Universities (Exeter, Loughborough, Nottingham, and Sweden’s Umeå) have been involved with monitoring water flows, water quality and changes to habitat.
Dr Matthew Johnson from Nottingham University and Dr Richard Mason from Umeå University have been working to understand changes to the water table and the development of river channels and habitats.
Dr 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.”
“increased lag times between rainfall and stormflow”
Dr Alan Puttock and Professor Richard Brazier, both from Exeter University, commented: “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.
“Enabling water to find its own pathway across a floodplain, undoing the long history of drainage of the landscape, has embodied the very definition of a nature-based solution to river restoration.”
They concluded: “As we learn more about how to work with natural processes, the work here will become a pioneering example of how to manage floodplains in an environmentally progressive way.”
“lower levels of sediment”
Other site-monitoring results show lower levels of fine sediment further downstream, which improves the aquatic habitat for migratory fish and reduces the impacts of diffuse pollution from agriculture.
Water quality monitoring showed a reduction of average turbidity, ie 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.
“a massive 1780 per cent increase in aquatic habitat”
The amount of aquatic habitat has also increased significantly from 0.18 ha3 to 3.42 ha3, a massive 1780 per cent increase. This aquatic habitat has developed in diversity with a huge increase in slower flow areas of wetland, and an overall increase in areas of faster flowing water and ponds, resulting in a new mosaic of wetland habitat types providing opportunities and refuge for wildlife leading to greater abundance and resilience.
Commenting on the ground water and habitat monitoring, Dr Richard 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.
“Connecting rivers to their floodplains is really important for many reasons. At this site, fish now have access to a lot more space, which means more food, more shelter and the ability to move and survive if one part of the river is disturbed (e.g. in floods). Furthermore, access to connected wetland habitats is important to young fish and critically lacking in the UK landscape.”
He concluded: “It’s an exciting time for river restoration and this novel technique has real potential for transforming our riverscapes, helping animal and plant life and increasing resilience to climate change. What we learn from our detailed monitoring at this site will help river restoration techniques globally. I’m excited to see how the site continues to develop.”
“benefits to wildlife”
Alongside the various studies, the local team have also been recording the benefits to wildlife.
Jack Siviter, National Trust ranger at Holnicote, commented: “The site has gone bananas in terms of the amount of wildlife and lush vegetation growth that we’ve seen.
“There is a huge variety of plants and trees starting to shoot up across the wetlands such as Greater Tussock sedge which can grow to over two meters tall. This will provide a rich landscape for wildlife to thrive within.
“Thanks to the increased wetted area we’ve seen loads of water loving creatures like wild waterfowl, water voles, eels, lamprey, grass snake, trout, and birds such as red kite, buzzard, kestrel, sparrow hawk, swallow, swift and sand martin.
“The martins, swifts and swallows are all using the site for feeding and nest building, while the birds of prey are using the site for foraging – which is in part due to the rewetting but also the longer sward.
“The constant supply of water has also resulted in the site being very green and lush with thousands of wildflowers like wild carrot, providing food for pollinators.
“As a consequence, we’ve seen raptors such as kestrels, herons and deer and even barn owls.”
The site has seen such an increase in insect life that at times there have been so many swallows the effect has been a feeding frenzy with hundreds and hundreds of birds swarming to feed on the abundance of food.
Jack concluded: “One of the coolest species we’ve seen is a green sandpiper – which is a great indicator species for healthy, lowland wetland habitat.”
“water voles happily exploring”
Water voles are being particularly closely monitored, as their presence is a sign of a healthy wetland habitat.
In 2023, there were only limited signs of water voles and only along the edges of the restored section, while the habitat was being established. However, recent surveys carried out by Derek Gow Consultancy have revealed a dense population colonising a central area, with scattered signs of them moving throughout the new wetland, showing that they are happily exploring.
Hazel Reading, Ecologist at Derek Gow Consultancy, said: “The restoration area started as a linear ditch with patches of suitable water vole habitat along it.
“This patchiness can limit the voles’ ability to migrate and find the best seasonal food, or even each other, without risking predation.
“The new habitat is expansive, connected, and has started growing up wonderfully. With an abundance of their favourite food plants and a link to the surrounding water vole populations, they’ve ended up with a little slice of vole heaven.
“Although 12 months in we can only really say it’s a foothold for now, I wouldn’t be surprised to see a big increase in their population in coming years!”
“mind blowing”
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.”
“Food for a water adviser’s soul!”
Careful monitoring will now continue to help people further understand how the site’s topography and ecology develop.
Jo Neville, South West Water Adviser for the National Trust said: “Our rivers certainly need some more good news stories at the moment and this has been such a brilliant project to be involved with.
“Every time I go to site it has changed and there is something new to see.
“Just one year in I can now stand in the middle of a mosaic of new and evolving wetland full of pools and channels with a cacophony of wildlife all around. To then be able to go on to delve in to some of the monitoring results and see just how amazingly the site is developing for climate resilience too is so inspiring – raising water tables, storing and slowing water and trapping sediments, nutrients and carbon to benefit the wider catchment.
“The team on site have been outstanding and the results are testament to their hard work and belief, as well as the invaluable expertise, advice and input from members of the river restoration community across the world.
“We are already using the learnings and knowledge across other National Trust sites, and I’m looking forward to watching how this one keeps developing. Food for a water adviser’s soul!”
“help tackle the impacts of more extreme weather events”
Matt Hemsworth, Geomorphologist at JBA Consulting who led on the design of the project, said: “The new waterscape has already helped slow and hold the water in the landscape making the catchment more resilient to the impacts of climate change.
“Projects like this and on a much larger scale can do similar to help tackle the impacts of more extreme weather events – whether that’s heavier rainfall or drought – and the sheer diversity of wildlife that the site attracts, will help us tackle the nature crisis too.
“Waterscapes like this can also store more carbon over time. The additional trees and scrub which grow will create shade and wetter soils and help create a cooler microclimate in the restored area.”