By Mark Manning, National Specifications Manager, SDS
THE potential for Sustainable Drainage Systems to green urban infrastructure and encourage wildlife has long been recognised as one of the four ‘pillars’ of SuDS, alongside their ability to protect against flooding, treat pollution and provide community amenities.
A new requirement for housing developments to achieve a ‘net gain’ in biodiversity will become law following the enactment of the Environment Bill, which is currently making its way through Parliament. Incorporating SuDS features would seem a clear opportunity for developers to meet their new obligations to protect and enhance wildlife habitats.
Lockdown has kindled a love of nature in most of us. It has left us yearning to head out of cities in search of bigger gardens and more green space. In a YouGov survey undertaken on behalf of RSPB, 87% of people agreed living close to spaces rich in wildlife and nature would be an advantage during the Coronavirus outbreak. So, in theory, the new legislation should see no push-back from housebuilders, when a wildlife-friendly development is more attractive to customers.
However, adding green space can come at a cost, if the land-take involved reduces the number of housing units, and therefore makes a development less viable. Net gain may also seem like more red tape and planning hurdles, not to mention being at odds with the pressures to deliver on the Government’s equally forceful expectations for new homes and infrastructure.
Biodiversity: what is all the fuss about?
In October 2019, the milestone State of Nature Report painted a bleak picture of ongoing species decline since the 1970s to conclude there has been ‘no let-up in net losses of the UK’s wildlife’. With 41% of species studied in decline and 15% of species under threat of extinction, it was a wake-up call. Species familiar to our grandparents, from skylarks to house sparrows, frogs, toads, butterflies, moths, barn owls, bats and wildflowers are now rare sights. More than 100 British species have become extinct in the past 100 years, many more are threatened.
The crisis has already led some housebuilders, such as Redrow, to commit to biodiversity policies. Biodiversity Net Gain is an approach to development that mandates the need for a site to end up in a better position to support wildlife and habitats than it was before construction began. Under the Environment Bill measures, applicable developments must deliver a 10% net gain in biodiversity, using a mitigation hierarchy and metric system.
How Can SuDS contribute?
The National Housebuilding Council Foundation, with support from the RSPB and Barratt, has published a design guide to help housebuilders to turn their developments into the “wildlife-friendly communities” required by Biodiversity Net Gain. It advises considering biodiversity at the earliest stages in planning.
SuDS present an opportunity to use vegetative, landscaped features to manage surface water on a site. Best practice SuDS designs mimic natural processes and manage rainwater as close as possible to where it falls, often by attenuation or infiltration.
Bringing drainage to the surface with ponds, swales and basins, could offer a dual-purpose opportunity for developers that could limit the commercial loss of land to usable development especially in space-restricted urban sites.
Where space for above-ground features is constrained, vegetative components can be designed in combination with manufactured products, such as underground stormwater storage tanks. Treatment devices such as vortex separators can also be deployed to ensure that habitats are properly protected.
Wildlife-friendly Green Spaces
A significant section of the NHBC guidance is devoted to incorporating SuDS in the development infrastructure, included through verges and the street network, for example, to support habitats. It points to the multiple benefits of creating wildlife-friendly green spaces with integrated SuDS that manage flood risk and treat surface water runoff. SuDS can also help to mitigate against urban heat and atmospheric pollution.
Net gain should make us more thoughtful in our approach to good SuDS design. Drainage designers and their developer clients must understand the needs of the local flora and fauna and consider the whole ecosystem. A design should also deliver connectivity between habitats both on and off the development.
Designers should also consider the role of vegetative SuDS components in providing pollution control. If ponds and wetlands are deployed as sacrificial devices to treat pollution, then their ability to support wildlife could be compromised. Using a hydrodynamic vortex separator, such as SDS Aqua-SwirlTM, at the inlet to the pond to remove sediment and pollutants in the runoff is highly advisable. Designing a ‘hybrid’ SuDS system and combining a basin, pond or swale with a manufactured device provides assurance of measured, repeatable and effective pollutant removal.
We should also never forget that water reuse is the first priority of the SuDS design hierarchy, yet this is all too often overlooked. Capturing surface water for reuse by means of rainwater harvesting systems is a legitimate sustainable drainage function, that also saves water in drought prone and water scarce areas. If saving water is a priority, then the biodiversity benefits may be better delivered appropriately, elsewhere on the site. Captured rainwater could even be reused to water the habitats we create, helping the flora and fauna to thrive.
In our enthusiasm to deliver biodiversity, adapt to climate change and develop more homes and better infrastructure, conflicts of interest can emerge, where one aim is in danger of thwarting another. Sustainable Drainage Systems provide an excellent opportunity to enhance our natural environment. Used appropriately alongside other stormwater management or smart technologies, they can help deliver holistic designs that truly connect and restore biodiversity.
What happens next?
The Environment Bill is currently progressing through Parliament. Once the Bill receives royal assent, a two-year transition period is planned before a mandatory, audited system of biodiversity net gain becomes law as part of the English planning system.