Integrating Sustainable Drainage Systems (SuDS) with Biodiversity Net Gain (BNG)

Integrating Sustainable Drainage Systems (SuDS) with Biodiversity Net Gain (BNG) is one of the most efficient strategies for modern developers. Since the introduction of mandatory 10% BNG for most planning applications, the search for “stackable” benefits—where one asset fulfills two regulatory requirements—has become a priority.

The Synergy Between Water and Wildlife

Traditionally, drainage was hidden underground in concrete pipes and plastic crates. While functional for water attenuation, these systems offered zero ecological value. SuDS shift this infrastructure to the surface, using nature-based solutions like swales, detention basins, and rain gardens.

When these features are designed with ecology in mind, they cease to be just “drainage” and become high-quality habitats. By selecting the right mix of native aquatic and marginal vegetation, a SuDS feature can significantly boost a site’s “unit” count in the Statutory Biodiversity Metric.

Turning Constraints into Credits

The primary challenge of BNG is the land-take required to create habitats. By utilizing SuDS for BNG, developers can maximize their developable footprint. For example:

• Attenuation Ponds: If designed with permanent water levels and varied bank profiles, these can be classified as “ponds” or “wetland,” which are high-value habitats in the biodiversity metric.
• Bioswales: These can act as green corridors, providing essential connectivity for pollinators and small mammals, further enhancing the site’s ecological “score.”
• Green Roofs: These manage source-control rainfall while simultaneously providing habitat for invertebrates.

A Strategic Advantage

Using SuDS to meet BNG targets doesn’t just satisfy the Lead Local Flood Authority (LLFA) and the Planning Ecologist; it creates more attractive, “nature-first” developments. To ensure success, it is vital that the Drainage Strategy and the Biodiversity Gain Plan are developed in tandem.

Early collaboration ensures that the water levels required for drainage don’t drown the sensitive species required for BNG, resulting in a resilient, compliant, and cost-effective site design.

Client Checklist: Coordinating SuDS and BNG for Planning Success

1. Early-Stage Integration

Joint Site Walkover: Ensure both the drainage engineer and the ecologist visit the site together to identify existing natural drainage paths and high-value habitats that should be preserved. This is easy to arrange with SWEL as we offer both services. 

Unified Design Brief: Establish at the outset that the SuDS features must serve dual purposes. This prevents the engineer from default-designing underground crates that offer no BNG value.

2. Technical Alignment

The Biodiversity Metric Review: Ask your ecologist which habitat types (e.g., “Ponds” or “Reedbeds”) provide the highest unit yield for your specific site.

Water Level Modeling: Ensure the drainage engineer provides a “water regime” (depth and duration of flooding) to the ecologist. Certain BNG-rich plants will fail if they are submerged for too long or left too dry.

Soil and Substrate Specification: Coordinate on the soil type. High-nutrient topsoil might be good for drainage but can encourage invasive weeds that lower the BNG score.

3. Feature Design for Maximum “Unit” Value

Varied Profiles: Move away from steep-sided, fenced-off pits. Request shallow, “graded” banks that allow wildlife to enter and exit safely.

Permanent vs. Temporary Water: Decide if the SuDS will hold water year-round (higher BNG value) or act as a dry detention basin (easier maintenance).

4. Long-Term Management and Compliance

The Management and Monitoring Plan (MMP): BNG requires a 30-year commitment. Ensure the maintenance schedule for the drainage (e.g., de-silting) is synchronized with the ecological management (e.g., wildflower meadow cutting).

Legal Responsibility: Clearly define who is responsible for monitoring the success of the habitat. If the plants die, the BNG units are lost, which could lead to a breach of planning conditions. ⚖️

How We Can Help?

Navigating the crossover between CIRIA C753 (The SuDS Manual) and the Statutory Biodiversity Metric can be complex. We provide integrated reporting that satisfies both the Lead Local Flood Authority and the County Ecologist in one seamless process.

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