Improving water quality and flood resilience at Woodwalton Fen

The challenge

Woodwalton Fen, a designated National Nature Reserve and internationally important wetland, has historically been used as a flood storage reservoir. While this supports farmland drainage, repeated flood events have introduced nutrient-rich water into the fen, threatening its delicate ecosystem. With the current flood storage agreement expiring in 2052 and increasing climate pressures, a sustainable alternative was urgently needed.

Key issues included:

• Nutrient Loading: Floodwaters introduced high levels of nitrogen and phosphorus, degrading sensitive habitats.
• Aging Infrastructure: Some flow control structures and embankments required upgrades or decommissioning.
• Environmental Constraints: Any solution had to protect biodiversity, comply with reservoir safety regulations, and integrate with the broader Great Fen restoration project.

The solution

Haskoning led a comprehensive options appraisal, evaluating 24 potential interventions. A shortlist of 14 options was developed and tested using hydraulic modelling, economic analysis, and environmental appraisal. Key strategies included:

• Alternative Flood Storage Sites: Multiple off-site reservoir options were assessed, including Oakley’s Drove, Sawtry Fen, and Whittlesey Mere, to replace Woodwalton Fen’s flood storage function.
• Water Quality Improvements: Options aimed to reduce nutrient inflow to the fen and provide controlled water storage for drought resilience.
• Hybrid Benefits: Several options combined flood mitigation with habitat creation and water supply.

The leading options—such as Option 12: Oakley’s Drove Flood and Water Storage—offered the best balance of environmental benefit, flood risk reduction, and economic value.

The result

The appraisal delivered several key outcomes:

• Strategic Clarity: A clear roadmap for replacing Woodwalton Fen’s flood storage role while enhancing its ecological condition.
• Informed Decision-Making: Multi-criteria analysis and benefit-cost ratios guided the selection of viable options.
• Environmental Gains: Preferred options support biodiversity net gain, reduce carbon emissions, and align with the Fens 2100+ strategy.

The project now moves into detailed design and stakeholder engagement phase.