Far from splashing around in the hot tub, David Morrish examines Sustainable Drainage Systems, an increasingly important element of urban design and an important tool in the battle against climate-change events
When training as a civil engineer, I helped supervise drainage works at Telford new town, where all fresh development was on a plateau 100 metres above the River Severn.
Before a house or a factory was built, a network of surface-water sewers and balancing lakes or storage systems had to be built to ensure that during storm conditions the flow of surface-water run-off into the Severn Gorge would be controlled. Even with such control measures in place, the final culvert taking water down to the discharge was two metres wide – enough for me to drive my dumper truck through.
My interest in drainage was recently rekindled by some highway and environmental works incorporating a Sustainable Drainage System (SuDS) carried out near our home in Westgate by Kent Highways with the support of local groups. The result has been the transformation of a barren dog-walking field into an oasis of greenery.
I have recently embarked on a personal journey into how drainage design and sustainability is, or should be, approached in an era of increasing climate-change events.
Often the willingness to integrate a sustainable system into a design has been held up by site constraints or ‘stakeholder pressure’, ie developer reluctance. Yet there are simple ways to approach a drainage design differently and provide many benefits without increasing financial cost or build complexity.
A rainwater pipe, for example, can discharge into a small, vegetated planter, with clean stone and a partial pipe on the outfall. This will now provide advantages on all four principles enshrined in The SuDS manual (published in 2007 by the Construction Industry Research and Information Association) with relatively little space used and minimal construction and maintenance costs.
A conventional system of pipes and chambers at the side of an access road could be enhanced with swales (shallow, wide, vegetated hollows that store or carry run-off and remove pollutants), filter strips or tree pits. A swale and headwall system can be run shallow and flat, solving problems on sites with tight vertical levels and providing a range of benefits.
Green roofs have advanced significantly and can now be installed as intensive or extensive systems on pitched roofs and provide significant biodiverse living spaces as well as perform their standard function of slowing and filtering rainwater. With the addition of blue roof systems, which provide initial temporary rainfall storage before gradually releasing it, they can become a complete surface-water solution.
Space does not need to be at a premium when considering a green or blue solution. Rain gardens have progressed to compact engineered trenches that can be fitted along the back of footways for water conveyance. Tree planters are a more advanced variant of this system and can offer additional benefits of urban cooling and water absorption and retention as well as those normally offered by a planted rain garden.
The 2012 version of the National Planning Policy Framework created a policy requirement for SuDS and this was strengthened in 2018, with the consequence that all Local Plans must have a SuDS policy – indeed, it is a requirement for all major development via the NPPF regardless of the Local Plan policy position. Some 90 per cent of all developments now incorporate SuDS and that figure is increasing.
Kent County Council is the Lead Local Flood Authority (LLFA) here and as such the statutory consultee in the planning process to require and oversee the provision of SuDS for major development.
KCC prepared a Drainage and Planning Policy Statement in September 2015, containing guidance on how to integrate SuDs into the masterplans of large and small developments.
Surface-water drainage design should be developed in line with KCC Drainage and Planning Policy Statement (June 2017), while it should use a 40 per cent climate-change allowance as required by the Environment Agency.
Despite the legislation and volumes of advice, there often remains the vexed issue of maintaining systems that involve multi-authority support but not necessarily funding.
The UK has not, historically, put alot of focus on sustainable design. Underground infrastructure and existing buildings would often be deemed unsuitable had they been built today under current regulations. Even if all new developments were fully sustainable, they would only account for some 20 per cent of the UK’s surface water from developed areas. Retrofitting sustainable drainage may become inevitable if we are to meet future environmental targets, in the same way that we will have to install new heating systems in existing houses.
Starting from the example of small housing estates and in response to the experience of flash flooding of the past 20 years, the whole process of surface-water drainage is being rethought to address the problems of run-off caused by high-intensity storms and lack of capacity in conventional surface-water sewers.
SuDS principles are not reliant on specific local conditions but rather are part of an interconnecting system where water flows slowly from where it falls to a soakage area or discharge point through a series of features that help to treat, store, re-use, convey and actually celebrate water.
An important concept for the SuDS designer to follow is the ‘treatment train’. By passing water through several stages of treatment, sediment and other pollutants should be removed more effectively and maintenance costs reduced as this minimises the risk of downstream drainage features becoming clogged.
Designers are also extending that treatment train to create green corridors and links, add opportunities for engagement and education and to match delivery of SuDS to phasing of development.
Some politicians and the Association of SuDS Authorities now recognise that a substantial change is needed in flood-risk management, utilising natural flood management and starting from small-scale interventions, including tree-planting (roots allow more water infiltration into the ground and evaporation through the leaves) and enhancement with swales, filter strips or tree pits.
This should also reduce the role of hard engineering.
The policies of KCC in the past six years have meant that more land must be provided to accommodate run-off on new development sites; consequently, developers are being encouraged to create more green space as part of their masterplanning. A recent proposal for 450 houses near Margate included almost a third of the 20-hectare site dedicated to a combination of green play space and retained tree cover with sustainable drainage, retaining all surface water on-site.
Greater recognition of SUDS principles in planning will mean that in new developments a greater proportion of land will have to be allocated for blue green infrastructure.
Green and blue infrastructure strategy
The four important ‘pillars’ of sustainable drainage are set out in the 400 pages of The SuDS manual.
More importantly, greater attention should be placed on the principle that designers of new sites should aim to “create and sustain better places for people and nature” as part of green and blue infrastructure is the green space and water environment essential to the quality of our lives and ecosystem.
It is referred to as ‘infrastructure’ as it is as important as other types of infrastructure such as roads, schools and hospitals. It is taken to mean all green space and water of public and natural value.
Looking deeper into the benefits of sustainable drainage, systems promoting green or blue infrastructure can also provide other significant advantages. Green planning-based SuDS are widely considered to offer:
- Enhanced biodiversity (with targets for biodiversity net gain)
- Increased amenity value and creation of a pleasant and interactive space
- Pollution control
- Carbon reduction, embodied energy and footprint offset (looking forward to net-zero 2050)
- Life-cycle cost savings
- Nutrient and water demand balance in areas with sensitive soils and aquifers
- Additional resilience against climate change
- Physical and mental-health benefits for occupants
- Reduced impact on an area in the event of system failure
More local authorities, as part of their Local Plan preparation, are addressing the concept of designing around the need to manage water and encourage more green areas, using the SuDS treatment train requirements and creating green corridors and links, adding opportunities for engagement and education and matching delivery of SuDS to phasing of development.
By doing so, they are facing up to climate-change priorities by focusing on four priority action areas:
1. Effective water management and flood-risk reduction – developing natural flood-management programmes and drainage solutions
2. Building green and blue infrastructure into physical development and housing – creating vibrant, healthy and inspiring places where people want to live, work and invest
3. Enhancing green and blue corridors and networks – improving air quality, reducing carbon emissions and creating a greener, even more attractive localities
4. Recognition that green and blue infrastructure is the green space and water environment essential to the quality of our lives and ecosystems
I have concluded that SuDS principles are important design tools that might not initially have been obvious to non-professionals. Further, green and blue strategies arising from SuDS might be key to ensuring local planning authorities carry out their planning and drainage roles to deliver ambitious, climate-friendly and sustainable development and play a part in accelerating climate action to meet the UK’s and UN’s sustainable development goals.
Moreover, green and blue strategies can help enhance biodiversity and nature’s recovery by providing fit-for-purpose contributions towards nature in all developments.
It is to be hoped that CPRE members and branches can play their part, firstly by engaging in consultation on masterplans to encourage a more enlightened approach to climate change as an essential part of planning to be considered at every stage of Local Plan development.
Secondly, we should be encouraging recognition of green and blue infrastructure as just as important as other types of public infrastructure. This should include all green space and water of public and natural value, with recognition of SuDS principles meaning that in new developments more land will have to be allocated for it.
Finally, in the longer term, such engagement might help encourage more volunteers to get involved in the creation and maintenance of such systems that are so vital for us all.
What is sustainable drainage all about?
SuDS are drainage systems that are environmentally beneficial, causing minimal or no long-term detrimental damage. They are often regarded as a sequence of management practices, control structures and strategies designed to drain surface-water efficiently and sustainably while minimising pollution and managing the impact on the quality of local water bodies.
The Association of SuDS Authorities defines the purpose of sustainable drainage planning as the delivery of systems that contribute to sustainable development and improvement of the places and spaces in which we live, work and play.
Although the principles have been recognised for many years, pressure to deal with climate change and the increasing incidence of flash flooding, with all the economic, social and political concerns it entails, has caused drainage authorities to rethink their approach to disposal of surface-water run-off rather than simply passing it downstream.
Detailed techniques to manage surface-water that take account of water quantity (flooding), water quality (pollution), biodiversity (wildlife) and amenity are collectively referred to as Sustainable Drainage Systems (SuDS).
SuDS are intended to mimic nature and typically manage rainfall close to where it falls. Systems can be designed to transport surface-water and slow run-off, ideally attenuating it even before it enters watercourses.
They provide areas to store water in natural contours and can be used to allow water to soak (infiltrate) into the ground or evaporate from surface-water or be lost or transpired from vegetation.
The Construction Industry Research and Information Association (CIRIA) has produced a SuDS manual, which is used throughout the UK. There are four main categories covered by SuDS design, referred to as ‘the four pillars of SuDS’:
- Water quality
- Water quantity
Monday, January 3, 2022