What is ‘blue-green infrastructure’? An explainer

At face value, the Avonmouth Severnside Enterprise Area (ASEA) Ecology Mitigation and Flood Defence Project, which spans Bristol and South Gloucestershire, may seem like a conventional flood resilience scheme.

In reality, the project offers a strong example of how major infrastructure can use blue-green infrastructure to protect communities, unlock growth and deliver measurable environmental value at the same time.

The 17km flood defence scheme was developed to support growth in one of the UK’s largest regeneration areas, while integrating 80 ha of new habitat creation and ecological enhancement into a single, coordinated landscape-led solution. The result is infrastructure that does more than protect land from flooding: it creates space for nature, supports climate resilience and strengthens the long-term value of the wider regeneration area.

Rather than treating flood management and biodiversity as separate workstreams, wetland habitats, species-rich grasslands and ecological corridors were designed by Mott MacDonald’s team alongside engineered defences, forming a connected system delivering multiple outcomes from the same footprint.

Importantly, ASEA demonstrates how projects can support economic development while strengthening climate resilience and contributing to nature recovery, without treating environmental outcomes as secondary considerations. While delivered at scale, the same principles are directly relevant to building projects, particularly as sites become more constrained and expectations around environmental performance continue to increase.

Against this backdrop, blue-green infrastructure is moving rapidly from an enhancement to a fundamental component of design. But understanding what is meant by blue-green infrastructure is essential.

The term blue-green infrastructure refers to networks of natural and semi-natural features that work alongside traditional engineering systems to deliver environmental, social and economic value.

‘Blue’ infrastructure includes water-based systems such as wetlands, flood storage areas, rivers and sustainable drainage systems (SuDS), while ‘green’ infrastructure encompasses habitats, trees, grasslands and multifunctional landscape features. In contrast, ‘grey’ infrastructure refers to conventionally designed hard engineering solutions using concrete and steel.

Increasingly, blue-green elements are no longer considered in isolation, but as part of an integrated approach capable of delivering flood resilience, biodiversity, climate adaptation and wellbeing benefits simultaneously.

The need for this approach is being accelerated by several converging pressures.

Climate change is increasing the frequency and intensity of extreme weather events, placing greater strain on drainage systems, increasing flood risk and creating new resilience challenges.

At the same time, the UK continues to experience significant biodiversity decline, with habitats and species under sustained pressure from development and land-use change.

Alongside these environmental challenges, regulatory expectations are evolving rapidly.

Biodiversity net gain (BNG) is already mandatory for most developments in England. However, it will also apply to Nationally Significant Infrastructure Project (NSIP) applications made on or after 2 November 2026, requiring major infrastructure schemes to deliver at least 10% BNG.

In this context, ASEA is a useful example because it delivered measurable habitat creation and ecological enhancement before these outcomes became a mandatory requirement for NSIPs. It points to the direction of travel for major infrastructure – schemes will increasingly be expected to demonstrate how they protect, enhance and manage environmental assets over the long term.

The implication for designers is clear. The challenge is no longer simply to minimise environmental harm, but to deliver measurable improvement alongside engineering performance. Blue-green infrastructure provides one of the most practical ways of responding to this shift, allowing projects to enhance biodiversity, manage water and deliver social value within a single, integrated system.

Successful delivery, however, depends on how early these systems are considered. Blue-green infrastructure is most effective when it is embedded from concept stage, rather than introduced later once spatial and structural decisions have been fixed. Early collaboration between architects, engineers, landscape architects, ecologists and environmental specialists is critical to identifying opportunities and avoiding inefficient redesign.

A landscape-led approach often enables teams to unlock solutions that satisfy engineering and environmental requirements simultaneously, rather than forcing trade-offs between competing priorities.

A range of tools support this process, including flood modelling, hydraulic assessments, ecological surveys, GIS analysis, BNG metrics and natural capital assessments. These allow project teams to understand interdependencies between systems and design accordingly.

One of the most important lessons from successful schemes is that blue-green infrastructure should be treated as critical infrastructure rather than residual landscaping.

Long-term management, monitoring and maintenance should also be designed in from the outset, recognising that ecological performance, flood performance and operational performance are closely linked. Without this, blue-green infrastructure risks being treated as a design feature rather than a functioning asset that needs to establish, adapt and perform over time.

As land availability becomes increasingly constrained, projects are also under pressure to maximise the value delivered from every square metre.

Blue-green infrastructure provides a route to achieving this by enabling multifunctional design that delivers several outcomes from the same footprint.

A flood attenuation basin can also function as wetland habitat, while sustainable drainage systems can improve water quality and support ecological networks. Carefully designed planting can provide visual screening, habitat connectivity, carbon storage and climate resilience benefits, all while contributing to the quality of the built environment.

For project teams working at site or building scale, these principles can translate into integrated solutions, such as green-blue roofs, rain gardens, swales or connected drainage systems that form part of a wider site strategy.

For contractors and delivery teams, the key is ensuring these elements are buildable, maintainable and properly connected to the wider drainage, landscape and ecological strategy.

This is where the lessons from ASEA become particularly relevant for future major infrastructure schemes.

The project shows that blue-green infrastructure is most effective when it is treated as part of the core infrastructure strategy, rather than as mitigation added once the engineering design is largely fixed.

The project’s success did not come from any single intervention, but from the way water management, habitat creation and engineering design were planned together from the outset. The scheme demonstrates how early integration can unlock solutions that are both technically robust and environmentally beneficial, avoiding the compromises that often arise when these considerations are addressed too late.

The same principles apply at building scale. Projects that delay consideration of drainage, biodiversity and landscape integration until later stages frequently encounter constraints that limit performance and increase cost. By contrast, schemes that embed these principles early can often deliver more coherent, resilient and efficient outcomes.

Looking ahead, the extension of mandatory BNG requirements to major infrastructure projects marks a significant shift in expectations across the construction sector. Future projects will need to demonstrate measurable environmental outcomes alongside traditional engineering and commercial objectives, requiring closer collaboration across disciplines throughout design and delivery.

ASEA shows that resilience, placemaking and long-term performance outcomes are not only achievable, but that they can be delivered in a way that supports growth, enhances place and contributes to wider environmental recovery. As expectations continue to evolve, blue-green infrastructure is likely to become a standard component of project delivery rather than a specialist add-on.

Rob Playford is associate landscape architect at Mott MacDonald.