Charlotte Markey of Polypipe Civils & Green Urbanisation says rain gardens will be key to meeting the changes to water management in the new Environment Bill
While traditional rain gardens have long been an accepted element of sustainable drainage systems (SuDS), their previous limitations mean that they have not traditionally been widely specified, or considered a comprehensive solution to effective water management.
Conventionally, rain gardens tend to be confined to a small area of plants and shrubs that hold and soak away rainwater run-off and most are commonly retrospectively ‘plugged in’ to existing drainage networks, collecting water from roofs, driveways or lawns. Operating effectively as a ‘soakaway,’ traditional rain gardens rely largely on infiltration and usually only support a narrow plant profile of more drought-tolerant species, meaning within a SuDS scheme they typically take a supporting role, or are added as an afterthought.
New opportunities
When integrated as part of ‘green urbanisation,’ the scope and capabilities of rain gardens are now much greater, providing a link between stormwater collection and green asset creation.
Shallow invert geocellular sub-base replacement systems can be installed under rain gardens, as well as SuDS tree pits and engineered swales, creating a foundation for water management and re-use. The systems comprise interconnected cells with unique ‘double trapezoid’ ties; these systems form a uniform structural ‘raft’ with high load-bearing characteristics. Versatile in their application, this new breed of SuDS can be adapted to a range of settings, from a small housing plot to industrial warehousing where they can be combined harmoniously with blue-green roofs.
What sets this system apart, is its ability to safely retain water at source for re-use. The water collected immediately beneath the rain garden during a rainfall event can be drawn on by surface planting through passive irrigation as required. A hydrophilic geotextile placed above the raft helps distribute the gathered water across a large surface area through capillary action. As well as being a ‘net zero’ process, this capability brings countless benefits.
Passive irrigation, combined with evapotranspiration (evaporation from land plus transpiration from plants), ensures that there is the right amount of moisture to promote growth and prevent wilting of the local greenery, making the landscape more resilient in extended dry spells. This allows the rain garden to support a broader, richer and more versatile range of plant species, as they are less reliant on infiltration for growth, and also enables greater connection with other planting across a development as part of a wider integrated solution.
Additionally – and critically – re-using water in this way also helps remove stormwater that would otherwise go into the combined sewer network that is under increasing pressure from greater population, urban density and more extreme weather events.
A solution for the future
The capabilities of rain gardens are powerful, but they take on a whole new dimension when we consider the landmark changes of the Government’s Environment Bill, which are due to come into force next year.
While far-reaching in its remit, the Bill is ground-breaking for water management, giving legal recognition of its importance in protecting and enhancing the world around us. Among the many significant developments will be the requirement for water companies to publish a water resources management plan, as well as strategies for drought, flood and overall catchment resilience. Importantly they will also be required to consider the interdependencies of each strategy.
Specifically, in recognition of the growing burden on combined sewers, the Bill focuses on building capacity into the drainage and sewerage system to meet current and future demand, and requires annual reporting on storm overflow performance.
As well as placing increased pressure on water companies and planning authorities to reduce stormwater discharge when designing and building developments, this policy change has the power to bring about a seismic shift in how we consider water as a resource to be used and reused, as well as to realise some of the wider environmental aspects of the Bill, specifically when it comes to biodiversity.
Key within the Bill is biodiversity net gain – in the form of a requirement to achieve a minimum of 10% improvement in biodiversity on or near a new development in order to get planning consent. These changes mean landscape architects and urban planners will need to re-evaluate how to introduce green infrastructure to ensure that projects comply with water management best practices and deliver an uplift in biodiversity post-completion.
These new requirements present exciting creative opportunities for the architectural community and a chance to challenge approaches to urban landscaping that are hardwired into construction practice. For too long, the UK’s application of water management techniques has been fragmented, with siloed solutions developed around reducing our use of potable water, avoiding floods or droughts, improving water quality, or reducing environmental risk. Individually, they lack the benefits of a holistic, interdependent cycle that’s more akin to what we see in nature.
The Environment Bill, plus significant changes brought about by the new Sewer Sector Guidance (SSG) in England (that allows water and sewerage companies to adopt a wider range of sewer materials and products) expand both the scope and potential for innovation. Now it is possible to use new solutions, including the next generation of rain gardens, that go way beyond traditional SuDS to benefit individuals, communities, and the environment, and integrate greater use of recycled materials.
Ultimately, these policy shifts give the architectural and urban planning communities the opportunity for greater creativity; to look at how breakthrough water management technologies can fuel more aesthetic, sustainable urban green assets while increasing biodiversity and climate change resilience.
Charlotte Markey is green urbanisation innovation manager at Polypipe Civils & Green Urbanisation