Michael McIvor-New at Tobermore explains how Sustainable Drainage System (SuDS) schemes are a viable and cost-effective way to achieve climate resilience and biodiversity net gain on both modern housing schemes and commercial projects.
According to the 2023 State of Nature report, 86% of habitats in the UK are not in good condition, and one in six species is at risk of extinction. To ensure new projects don’t further damage our fragile ecosystems, Biodiversity Net Gain (BNG) is now mandatory in England under Schedule 7A of the Town and Country Planning Act 1990 (as inserted by Schedule 14 of the Environment Act 2021).
All projects (barring exemptions) must deliver a BNG of 10%, providing more and/or a better quality natural habitat than before construction started. This must be maintained for a minimum of 30 years.
As well as driving habitat loss, climate change has altered the British weather. Winter rainfall is now 16% higher than it was between 1960 and 1990, and rainfall on the stormiest days increased by approximately 20%. At the same time, the desire for clean lines and practicality has led to the use of hard surfaces in many urban environments. Rainwater cannot be absorbed onsite, leading traditional drainage systems to quickly become overwhelmed, exacerbating flash and surface flooding. In England and Scotland, the National Planning Policy Framework states that SuDS must be used on developments with 10 dwellings or more (or the non-residential equivalent), unless it is inappropriate.
Although Schedule 3 remains unlikely to come into force in England, Lead Local Flood Authorities (LLFAs) are pushing SuDS through as part of planning requirements. This does, however, vary between the different authorities. For example, Westminster, Haringey and Essex, amongst others, incorporate SuDS as a requirement for planning approval. Meanwhile, over in Wales, Schedule 3 has been adopted, making SuDS a mandatory design component on all schemes. The legislation is backed by 22 SuDS Approval Bodies (SAB), which ensure projects meet national standards.
Legislative compliance with SuDS
When designing SuDS, it’s best practice to ensure that the first five millimetres of rain, essentially normal rainfall, never leaves the site. On a well-designed SuDS scheme, 40% of the water volume can be accommodated by soil alone.
For effective, compliant SuDS schemes, there are four design principles that should be followed:
- Managing quantity of rainwater runoff
- Managing water quality and preventing pollution
- Create amenities for people
- Improve biodiversity
These principles are all interlinked, and most SuDS schemes will include a variety of solutions for maximum effect.
Taking a holistic approach to SuDS design allows architects to employ the best solution for the site. This means they may choose to use a combination of SuDS devices such as permeable paving, rain gardens, tree pits and blue/green roofs to create a series of small ‘sub catchments.’ This can often better manage the water volume than one device alone, as well as enhancing the quality of the water and contributing to biodiversity.
However, that’s not always possible. Permeable paving can be used across projects of all types as a standalone solution for managing water run-off and creating amenity spaces; it also filters water by trapping and breaking down harmful pollutants within the stone. This means it supports three out of the four SuDS principles directly.
Permeable paving can also prove to be a good solution if the local planning authority requests that architects provide a ‘betterment’ of SuDS and water management. That’s because the scheme’s hard surfaces, such as asphalt roads, footpaths, roof space, and car parking, etc, can all be switched to permeable paving relatively easily.
Addressing concerns about costs
There’s a misconception that SuDS can be too expensive for all but medium to large projects. Certainly, making late design changes to add SuDS to a scheme will increase project costs. Planning SuDS into the scheme from its inception will not only help with planning compliance, but also help ensure they are more cost-effective than standard drainage; they may not require the extensive excavation, pipework, manhole covers or gullies required etc.
Traditional gully drainage systems also require regular cleaning to prevent the drainage system from becoming blocked by debris; otherwise, flooding can occur even during normal rainfall. In contrast, maintenance for SuDS schemes can be incorporated into the normal landscaping budget – permeable paving, for example, requires minimal maintenance.
Michael McIvor-New is permeable paving specialist at Tobermore