Communities across the Exmoor National Park are now better protected from flooding after a local river was ‘reset’ to create new wetlands.

The River Aller flows from its source near Wootton Courtenay under the A39 between Minehead and Porlock, before joining the River Horner at Bossington and running into the Bristol Channel.

The National Trust’s Holnicote Estate has been working with the Somerset Rivers Authority (SRA) and local farmers since 2021, looking at ways to slow the flow of the river and re-wild large section upstream of Allerford to protect around 100 homes.

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One years on from the completion of the three-year project (the first of its kind in the UK), the work has been heralded a success, with increased biodiversity and fewer reports of localised flooding.

The project utilised the innovative ‘Stage 0’, river restoration technique pioneer in Oregon, USA, with the intention being to reconnect the river to the surrounding floodplain.

To achieve this, a 1.2-kilometre stretch of the river which had previously been straightened and deepened was filled in and around seven hectares of new wetlands created – the equivalent of more than ten football pitches.

In addition to filling in the river’s straightened course with more than 4,000 tonnes of earth, around 700 tonnes of fall Holnicote timber was part-buried or pinned to the ground to slow the flow of water through the floodplain.

Around 250kg of wildflower seeds were sown and up to 25,000 native trees – such as willow, bird cherry and black poplar – were planted on the floodplain as part of the project, which formed part of the National Trust’s multi-million-pound Riverlands initiative (with the SRA and EA also providing funding).

Preliminary analysis carried out in April 2024 indicated that the level of peak flooding flows had declined by 38 per cent, providing valuable protection to properties downstream in Allerford and Bossington and ensuring the A39 could remain open.

There has also been improvements in biodiversity, with the number of insects and other species being recorded in the floodplain rising by nearly 1,800 per cent in a single year.

These benefits are expected to increase as the new trees take root, with the flow of water down to the Bristol Channel being slowed significantly.

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.”

Dr Richard Mason from Umeå University in Sweden added: “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.

“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.”

The Holnicote project followed on from a number of smaller-scale flood management schemes in the Exmoor area which were part-funded by the SRA, with monitoring being carried out by the University of Exeter since 2019.

For more information on the Holnicote project, visit www.nationaltrust.org.uk/appeal/porlock-vale-riverlands-appeal.

Dr Alan Puttock and Professor Richard Brazier stated: “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.

“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.”

The project has also been recently extended to the tune of 125 hectares further upstream through further funding from the SRA.

Councillor Mike Stanton, chairman of the SRA, said: “The National Trust’s conversion of the River Aller from a narrow stream to a flood-able 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.”