Nature-Based Solutions at Durleigh Reservoir

Using nature-based solutions to improve water quality and reduce operational and maintenance costs of water in reservoirs.

In 2020, FiveRivers and Wessex Water designed and built two multi-benefit wetlands at Durleigh Reservoir in Somerset. The wetlands were designed to filter silt and coarse sediment and reduce levels of nutrients from input sources flowing into the reservoir, helping to improve the raw water quality as well as boosting local biodiversity.

The wetlands operate alongside an enhanced treatment process at the recently refurbished £50 million Durleigh Water Treatment Centre and ensure that the area, which previously suffered with poor and deteriorating raw water quality, enjoys the highest quality drinking water. For the last three years Wessex Water have been undertaking regular sampling, with the positive results demonstrating how embracing nature-based solutions can assist in water quality improvements.

FiveRivers is a industry-leading specialist in monitoring, design and construction of ecological and environmental nature-based solutions. With over 25 years’ working with nature, our expertise encompasses terrestrial and aquatic habitats with a specific passion for rivers and wetlands.

Wessex Water are investing in the environment, striving for a more sustainable approach towards improving water quality and the local biodiversity of our water courses, while achieving lower carbon and more cost-effective solutions for both sewage works discharges and storm overflows.

Design and build

Two multi-functional wetlands were designed and built on the same site. A naturalised wetland, with 450m of restored naturalised channel and 8000m² of wetland floodplain connection, and an engineered surface water flow wetland, with 5200m² of surface area, 2,600m2 of deeper open water habitat and 2600m² of multi-species filter beds and vegetation.

The natural wetland was designed and built to promote the natural process of course sediment and silt capture during high flow events by connecting the perched and straightened Durleigh Brook back into an undulating floodplain.

The constructed wetland was designed and built to improve the quality of raw water supply from the Bridgwater and Taunton Canal, and recycled water from the treatment works (a combined inflow of up to 15ML per day) tackling the high sediment, nutrients, and pesticide loading from these sources. The wetlands were supported by the ongoing work of the Wessex Water catchment delivery teams, reducing pesticides and silt entering the Durleigh Brook by working with landowners to substitute pesticides, putting in buffer strips, hedges, woodlands and reducing run-off down to the reservoir.

An existing wet woodland was also utilised – through the construction of small NFM features within – to act as a final natural screening process before the treated water entered the reservoir.

Monitoring and outcomes

Reverting the poor-quality cultivated land to natural grasslands, restoring the Durleigh Brook to a more natural plan form and reconnecting it to its floodplain has resulted in the biodiversity and habitat availability on site to increase by between 55 and 76 per cent, calculated using Natural England’s  Biodiversity Metric. These biodiversity improvements will only continue as the habitats grow and mature.

Regular water quality monitoring has also been carried out since construction was completed, showing reductions in the total amount of ammonia present in the water by 75 per cent, and reductions in the total phosphorus and sediment loading by a third.

The results from the wetlands at Durleigh has led Wessex Water to be more determined to choose nature-based solutions, such as wetlands, to support the quality of water where possible over ‘grey’ infrastructure solutions that cause more impact on the environment, carry significant carbon implications, cost billions of pounds and are extremely disruptive to communities.

Nature as the solution

FiveRivers continue to be a valued nature-based solutions delivery partner for water companies across the UK, delivering WINEP outcomes in the current Asset Management cycle and looking forward to future AMP cycles, using nature as a solution to improve our natural capital.

Wessex Water are continuing to invest in nature-based solutions, with a further 28 schemes in development for delivery before 2025 and significant proposed investment in their AMP8 commitments to storm overflows and sewage works discharges.