Sustainable drainage systems (SuDS) manage surface water run-off close to where rain falls: they slow it down, store it, clean it and return it to the ground, a watercourse or a sewer at a controlled rate. SuDS replace the conventional approach of collecting rainwater and piping it away as quickly as possible, and they are now the expected drainage strategy for new development across the UK.
How SuDS work: the management train
A SuDS scheme is designed as a management train — a sequence of components that surface water passes through on its way from the roof or road surface to the point of discharge. Source control features such as permeable paving, green roofs and rainwater harvesting reduce the volume of run-off where it is generated. Conveyance features such as swales and filter strips move the remaining water slowly across the site while sediment settles and pollutants are filtered out. Site-level storage — balancing ponds, wetlands and below-ground attenuation tanks — holds the peak volume and releases it through a flow control. Because each stage does part of the work, the scheme stays effective even when one component is overloaded or poorly maintained.
The four pillars of SuDS design
The CIRIA SuDS Manual (C753), the reference design guide used throughout the UK, defines four objectives that a scheme should deliver together. Water quantity: controlling the rate and volume of run-off to reduce flood risk on and off site. Water quality: removing silt, hydrocarbons, metals and nutrients through sedimentation, filtration and biological treatment. Amenity: creating drainage features that are safe, attractive and useful as public space. Biodiversity: providing habitat and connecting green and blue corridors. A pipe-and-tank system can meet the first objective; a SuDS scheme is expected to address all four.
Types of SuDS components
| Component | Main function | Typical use |
|---|---|---|
| Permeable paving | Infiltration or storage in the sub-base, filtration | Car parks, access roads, driveways |
| Green and blue roofs | Interception and evapotranspiration | Flat-roofed commercial and residential buildings |
| Rainwater harvesting | Volume reduction through reuse | WC flushing, irrigation, wash-down |
| Swales and filter strips | Conveyance, settlement, filtration | Roadside verges, landscaped edges |
| Bioretention and rain gardens | Filtration and biological treatment | Streetscapes, small catchments |
| Filter drains and soakaways | Conveyance, infiltration | Highways, plots with permeable ground |
| Ponds, wetlands and detention basins | Storage, treatment, amenity, habitat | Larger developments with open space |
| Attenuation tanks and geocellular storage | Below-ground storage with controlled discharge | Dense or constrained sites, under car parks and roads |
| Silt traps and separators | Pre-treatment, protection of downstream storage | Upstream of tanks, infiltration and outfalls |
SuDS design: discharge hierarchy and run-off rates
Surface water should be discharged in order of preference: reuse on site first, then infiltration to the ground, then discharge to a surface water body, then to a surface water sewer, and only as a last resort to a combined sewer. Infiltration depends on permeability testing and groundwater level, so many urban sites end up with a controlled discharge to a watercourse or sewer. Where water is discharged, peak rates are limited — on greenfield sites normally to the greenfield run-off rate, on brownfield sites to an agreed reduction on the existing rate — and the storage needed to hold back the difference is sized for the design storm with allowances for climate change and urban creep. The first few millimetres of most rainfall events should be intercepted and not discharged at all, because that first flush carries the highest pollutant load.
SuDS regulation in England, Wales and Scotland
England: Defra published the National Standards for Sustainable Drainage Systems in 2025, replacing the 2015 non-statutory technical standards. They are applied through the planning system rather than a separate approval body: national planning policy expects drainage for new development to be designed in accordance with the standards, and the lead local flood authority is a statutory consultee on major applications. Schedule 3 of the Flood and Water Management Act 2010 has not been commenced in England. Wales: Schedule 3 has been in force since January 2019 — drainage for most new development must meet the statutory Welsh SuDS standards and be approved by a SuDS Approving Body (SAB) before construction begins. Scotland: SuDS have been required for surface water discharges from new development for many years under the Water Environment (Controlled Activities) Regulations, with Scottish Water adopting qualifying SuDS assets.
SuDS vs traditional drainage
Traditional drainage collects rainwater and pipes it away as fast as possible, which concentrates flow downstream, increases flood risk and carries pollutants straight into rivers. SuDS take the opposite approach — slowing, storing and treating water at every stage. The result is lower peak flows, fewer combined sewer overflow spills, cleaner watercourses and drainage that keeps working as rainfall intensity increases with climate change.
Below-ground SuDS on constrained sites
On dense urban sites, highway schemes and commercial developments there is often no space for ponds or wide swales. Here the storage element of the management train moves underground, typically as an attenuation tank beneath a car park or road, with pre-treatment upstream and a flow control at the outlet. Large-diameter corrugated steel tanks are used where storage volumes are high, cover depths are significant or heavy traffic loading applies, because they combine high structural capacity with man-entry access for inspection and maintenance.
Related ViaCon solutions
ViaCon designs and supplies prefabricated corrugated steel attenuation tanks, pre-treatment through ViaCon Treat Solutions and rainwater reuse systems as part of our stormwater solutions range. Related glossary entries: stormwater attenuation, balancing pond, swale and permeable paving.
Frequently asked questions about SuDS
What does SuDS stand for?
SuDS stands for sustainable drainage systems. The term covers any combination of components — permeable paving, swales, ponds, filter drains, attenuation tanks — that manage surface water run-off close to where rain falls instead of piping it away as quickly as possible.
What are examples of SuDS?
Common examples are permeable paving, green roofs, rainwater harvesting, swales, filter strips, bioretention areas, balancing and detention ponds, wetlands, and below-ground attenuation tanks or geocellular storage. Most schemes combine several of them in a management train.
Are SuDS mandatory in England?
Schedule 3 of the Flood and Water Management Act 2010 has not been commenced in England, so there is no separate SuDS approval regime. Instead, SuDS are required through the planning system: national planning policy expects drainage to be designed in line with the National Standards for Sustainable Drainage Systems published by Defra in 2025, and the lead local flood authority is consulted on major development. In Wales, Schedule 3 has applied since 2019 and SuDS Approving Bodies must approve drainage before construction starts.
What is the SuDS management train?
The management train is the sequence of SuDS components that surface water passes through from source to discharge — for example permeable paving, then a swale, then a pond or attenuation tank. Each stage reduces volume, slows flow or removes pollutants, so the scheme does not rely on a single feature.
Is an attenuation tank a SuDS component?
Yes. An underground attenuation tank is a SuDS storage component that controls the rate of discharge. On its own it provides little water quality treatment, amenity or biodiversity value, so it is normally combined with upstream source control and treatment such as a silt trap, permeable paving or a swale.