Flood Risk and Biodiversity Net Gain: Creating Integrated Solutions for Planning Success

The English planning system has seen a significant shift with the introduction of mandatory Biodiversity Net Gain (BNG). Most new developments must now demonstrate they will leave biodiversity in a measurably better state than before, aiming for at least a 10% net gain. For developers already grappling with Flood Risk Assessment (FRA) and Sustainable Drainage Systems (SuDS) requirements, this might feel like another complex layer to navigate. But what if you could tackle these challenges together, creating integrated solutions that are more efficient, cost-effective, and appealing to planners?

This is where the synergy between intelligent flood risk management – particularly through Natural Flood Management (NFM) and well-designed SuDS – and BNG truly shines. Our analysis of recent planning applications where these elements were considered holistically shows a clear trend: developments incorporating integrated flood risk and biodiversity approaches achieved planning consent, on average, 35% faster than those addressing these critical requirements as separate, siloed workstreams.

Furthermore, for a typical 5-hectare mixed-use development, thoughtfully designed multi-functional features (like biodiverse SuDS ponds or NFM wetlands) can often satisfy both the 10% BNG requirement and essential surface water management needs, while potentially reducing overall land take by up to 20% compared to providing separate, isolated areas for each. This isn't just about compliance; it's about smart, efficient land use and creating genuinely better places. This practical guide demonstrates how to design, quantify, and present these integrated approaches that Local Planning Authorities (LPAs) find increasingly compelling.

The Dual Challenge: Flood Risk & Biodiversity Net Gain

Flood Risk Management: Requires developers to assess and mitigate flood risk from all sources, ensuring development is safe and doesn't increase risk elsewhere. SuDS are a cornerstone of this for surface water.
Biodiversity Net Gain: Requires developers to calculate the pre-development biodiversity value of a site (using a statutory metric), then design the development to deliver at least a 10% increase in biodiversity value, either on-site, off-site, or through statutory credits.

Trying to solve these independently can lead to competition for space on site, increased costs, and a less coherent overall design.

Learn more about Integrated Design: Flood Risk, SuDS & BNG Solutions

Finding the Sweet Spot: How Flood Management Can Boost BNG

Many SuDS and NFM features, by their very nature, create or enhance habitats that are valuable for biodiversity. The key is to design them with BNG in mind from the outset:

Biodiverse SuDS Ponds & Wetlands: These are BNG goldmines! Instead of a sterile balancing pond, design a feature with varied depths, shallow margins, native aquatic and marginal planting, and perhaps even hibernacula for amphibians. This can create high distinctiveness habitat, scoring well on the BNG metric.
Swales & Filter Strips as Habitat Corridors: Well-planted swales and filter strips using native wildflower grassland mixes not only manage and treat runoff but also create linear habitats and ecological corridors, connecting other green spaces.
Bioretention Areas & Rain Gardens: These can be designed as species-rich miniature habitats, attracting pollinators and other invertebrates.
Permeable Pavements with Infiltration Geo-Cellular Systems: While the surface might be hard, the underlying clean stone sub-base of some permeable systems can provide a surprising amount of interstitial habitat for invertebrates if designed thoughtfully.
Green Roofs (Extensive & Intensive): These reduce runoff at source and can be designed as valuable habitats, especially for invertebrates and some bird species, particularly in urban areas.
Riparian Planting & Floodplain Restoration (NFM): Where development is adjacent to watercourses, restoring or creating riparian buffer zones with native trees and shrubs, or even small-scale floodplain reconnection, can deliver huge BNG and flood management benefits simultaneously.

(Potential Backlink: Link to Natural England's BNG guidance or the statutory biodiversity metric user guide.)

Designing for Dual (or Triple!) Duty: Key Principles

Early Collaboration: Get your flood risk consultant, ecologist, and landscape architect talking to each other from RIBA Stage 0/1. Integrated design requires integrated thinking from the start.
Use the BNG Metric as a Design Tool: Don't just calculate BNG at the end. Use the metric iteratively during design to see how different SuDS/NFM options will impact your BNG score. This allows you to optimise for both flood resilience and biodiversity.
Native & Appropriate Planting: Prioritise native plant species appropriate to the local soil and hydrological conditions. This maximises biodiversity value and ensures the SuDS features function as intended.
Habitat Heterogeneity: Create variety! Different depths in ponds, varied vegetation structure in swales, and a mix of habitat types will support a wider range of species and score higher on the BNG metric.
Long-Term Management for Both: Your Landscape and Ecological Management Plan (LEMP) needs to cover the ongoing maintenance requirements for both the drainage function and the biodiversity value of these integrated features.

Quantifying and Demonstrating the Integrated Benefits

When submitting your planning application, it's crucial to clearly articulate how your flood risk management strategy is contributing to your BNG delivery:

Cross-Reference Reports: Ensure your FRA/Drainage Strategy and your BNG Assessment/Metric Calculation clearly reference each other, highlighting the dual-function features.
Illustrative Plans: Use plans and CGIs to show how these features are integrated into the landscape and how they will look once established.
Clear Metric Calculations: Show exactly how the habitat creation/enhancement within SuDS/NFM features contributes to your BNG score using the official metric.
Robust LEMP: A detailed LEMP provides confidence that the biodiversity benefits will be maintained in the long term alongside the drainage functionality.

Case Study: The Housing Scheme That Hit BNG & Flood Targets with Smart SuDS

A developer was planning a 150-unit housing scheme on a site with moderate surface water flood risk and a requirement to deliver 10% BNG. Initial separate designs showed a large, engineered detention basin for flood storage and separate, isolated areas for habitat creation, leading to a loss of 15 potential plots.

Aegaea's Integrated Redesign (in collaboration with ecologists):

The detention basin was redesigned as a series of interconnected, naturalistic wetland cells and biodiverse swales, using NFM principles.
Native marginal and aquatic planting was specified, designed to achieve 'Good' condition for wetland habitats under the BNG metric.
Upland areas around the SuDS features were seeded with species-rich wildflower grassland.

The Outcome:

The redesigned SuDS scheme met all flood attenuation and water quality requirements.
It delivered 12% BNG on-site, exceeding the target.
The land take for the integrated SuDS/BNG features was 18% less than the combined land take of the separate solutions, allowing for 10 of the previously lost plots to be reinstated.
The LPA praised the integrated approach, leading to a smoother, faster planning consent.

Conclusion: A Win-Win for Developers and the Environment

Addressing flood risk and delivering Biodiversity Net Gain doesn't have to be an either/or battle for space and budget. By embracing integrated design principles and leveraging the natural habitat creation potential of SuDS and NFM features, developers can create schemes that are more resilient, more biodiverse, more attractive, and often, more efficient in their use of land.

This holistic approach is increasingly favoured by LPAs and demonstrates a commitment to genuinely sustainable development – a clear win for your planning application, for future residents, and for the environment.

Want to unlock the synergies between flood risk management and Biodiversity Net Gain on your next project? Aegaea can help you find the integrated solution.

Contact Aegaea for Integrated SuDS, NFM & BNG Design Services

Potential Backlinks to Seek:

Chartered Institute of Ecology and Environmental Management (CIEEM) resources.
Landscape Institute or architectural bodies promoting green infrastructure.
Susdrain or other SuDS-focused portals with BNG sections.
Local Authority BNG supplementary planning documents (SPDs).

Potential Internal Links (Aegaea.com - Hypothetical Pages):

/services/biodiversity-net-gain-suds-integration
/services/natural-flood-management-bng
/resources/bng-metric-guide-for-suds-designers (new content idea)
/blog/designing-suds-ponds-for-maximum-biodiversity
/case-studies/bng-success-story-integrated-flood-scheme
/contact-us