Conventional stormwater management has focused on quantity (flow) management, by collecting runoff and channeling it to the closest watercourse. This has had a significant impact on the environment through the erosion of natural channels, siltation of water bodies and pollution resulting environmental degradation. It is clear an alternative approach is required. Sustainable Drainage Systems (SuDS) offer such an alternative.
SuDS offer an opportunity to protect and promote a healthy environment through designing for water quantity management; water quality treatment; enhanced amenity; and enhance biodiversity.
The problem with Conventional Drainage
As a result of increasing impervious areas and the efficiency of conventional drainage practices: peak flows increased; flow volume increase; base flows decrease. As shown below:
Figure 1: Impacts of conventional stormwater management
This has significant effect on the receiving water bodies including erosion and siltation. Additionally the runoff from conventional systems is often highly polluted.
SuDS – the alternative
SuDS aims to mimic the natural hydrological cycles. The key objectives of the SuDS approach are the effective management of stormwater runoff quantity, quality, associated amenity, and biodiversity of the urban drainage system. The relationship has historically been described using a Venn diagram. This approach has a number of problems including the joining of amenity and biodiversity which are not mutually dependant. Additionally it implied it would be possible to have a design which compromised or did not fully consider quantity. Therefore the key objectives of SuDS is more correctly described as a hierarchy (Figure 2) where each level accomplished results in a improved, more sustainable drainage system. Simply put there is not point focussing on biodiversity if life and property has not be protected
Figure 2: Relationship between the different SuDS elements
Designing a SuDS system
It is important to understand that SuDS generally embrace a number of options that are arranged in a treatment train. This helps to improve the efficiency and resilience of the system. In other words, stormwater is managed through a series of unit process, collectively known as a treatment trains. There are three key stages in the treatment train, each having slightly different combinations of SuDS options to control the stormwater:
i) Source Controls manage stormwater runoff as close to its source as possible, typically on site. Typical SuDS options include: green roofs, rainwater harvesting, permeable pavements and soakaways.
ii) Local Controls manage stormwater runoff in the local area, typically within the road reserves. Typical SuDS options include: bio-retention areas, filter strips, infiltration trenches, sand filters and swales.
iii) Regional Controls manage the combined stormwater runoff from several developments. Typical SuDS options include: constructed wetlands, detention ponds and retention ponds.
As the treatment train progresses, the number of interventions decrease, but their individual size increases. For example the source controls could be each house having a rainwater tank, the local control may be 5 houses ‘share’ a wet swale, and the regional control may be that 50 houses ‘share’ a wetland. The treatment train is shown below:
Figure 3: SuDS Treatment Train
Clearly the selection of SuDS options will be dependant on the sites current and future land use, geological conditions and many other local considerations.
SuDS offer a range of ecosystem services which may benefit society. These include:
i) Regulated climate
ii) Water and air purification
iii) Regulated water supply
iv) Erosion and sediment control
v) Habitat functions
vi) Waste treatment
Human health, well-being and cultural benefits