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Crumpmeadow sewage treatment works wetlands
The new freshwater wetland will expand and link habitats for priority species, such as the small pearl bordered fritillary butterfly.
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Nature-based solutions: Improving resilience to low flows

Client
Severn Trent Water
Expertise
Wetland feasibility, design and pre-construction works, biodiversity net gain, ecological surveys and advice, geotechnical investigation, contaminated land, landscape plans

Opportunity

Severn Trent Water agreed to investigate the feasibility of using nature-based solutions at Cinderford, Gloucester as part of the Water Industry National Environment Programme (WINEP). The WINEP is operated by the Environment Agency to ensure all 20 water companies in England meet their environmental obligations. The nature-based solutions at Cinderford include wetland creation, river restoration and habitat enhancement, and aim to help the Cinderford Brook waterbody achieve good ecological status under the EU Water Framework Directive. We have supported Severn Trent Water across several phases of wetland creation at Crumpmeadow sewage treatment works (STW), from feasibility and outline design to construction.

Solution

Wetland feasibility and outline design

Wetland creation would improve resilience to low flows in the Cinderford Brook as well as provide multiple co-benefits, such as the creation of recreational space and biodiversity enhancement. The feasibility assessment of integrated, nature-based solutions on the land surrounding the Crumpmeadow STW identified three potential sites.

We developed a water balance model to assess changes to Q95 flow in Cinderford Brook from the creation of wetlands. The Q95 flow is the flow in cubic metres per second that was equalled or exceeded for 95% of the flow record and is an important parameter for to assess whether the wetlands support meeting the flow obligation. Our model applied data from 2016 and was compared against observed flow data, enabling us to quantify flow improvements. The model also incorporated inputs from a water balance analysis to identify tributary flows.

Stakeholder engagement was crucial throughout feasibility stage. Key stakeholders were engaged, including the Gloucestershire Wildlife Trust and the Environment Agency. This was essential to sign off the scheme.

The feasibility and outline design phase highlighted significant risks to wetland creation, including coal mining legacy and mine shafts, anecdotal evidence of contaminated land, high groundwater levels, access difficulties and the presence of protected species, particularly great crested newts. We supported Severn Trent Water to mitigate and address these risks in the second phase of works.

Detailed design and construction

One of the three proposed wetlands was selected by Severn Trent Water to progress to detailed design and construction. We supported pre-construction works, including ground investigation and contaminated land assessments as well as ecological habitat and species surveys and mitigation. We provided detailed ecological advice throughout this phase of works, particularly for habitat retention and long-term protection, wetland design and a great created newt conservation licence and mitigation package. We also prepared and submitted a Habitat Regulations Assessment for the channel works connecting the wetland into the system.

Before our colleagues at Mott MacDonald Bentley (MMB) began construction, we worked with them to modify and enhance the wetland design by refining the hydrological modelling to ensure the scheme would meet the flow obligation as well as generate biodiversity benefits for species and habitats.

Outcome

The new freshwater wetland at Crumpmeadow STW is currently being constructed by MMB and will consist of one, large storage pond and a series of small ponds, linked by small ditches. This will provide high value habitats to enhance the ecological value of the site and the local area by expanding and linking habitats for priority species, such as the small pearl bordered fritillary butterfly. We continue to provide ecological support for amphibian and reptile trapping and translocation, habitat retention, improvement and creation, and landscaping and planting.

Opportunities to maximise the environmental and societal value of the wetland have been incorporated in our design where possible. One example is the selection of species and types of plants suitable for local school children to plant on the site, delivering valuable public engagement and education.

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