SUDS: Sustainable Urban Draining Systems

Provided by Robert Barker: co-director of Baca Architects


What, Why, When, How, Extras


What is it?

Sustainable Urban Draining Systems (SUDS) such as rain gardens and green roofs are an engineered means to manage rainwater run off such that it emulates natural drainage systems. SUDS employ various techniques to effectively manage drainage which includes green and brown roofs, dry ditches (swales), detention/attenuation ponds, permeable paving, underground storage tanks and many other measures, all of which aim to detain run-off and release it slowly into watercourses or to ground.


Why use it?

Rainwater is typically collected in gutters and gullies from roofs and roads, and then discharged into drains and then sewers. In many built up areas rainwater and foul water sewers are combined. Most sewer systems are designed to cope with big rainstorms (1 in 30 years) but would be overwhelmed in more extreme events, resulting in flooding. These more extreme events are also expected to become more common with climate change. SUDS can reduce the rate of rainwater run off and therefore reduce the risk of flooding. SUDS can reduce pollution by trapping and breaking down pollutants before they enter the watercourse. SUDS can help to recharge groundwater and enhance the environment.


When to use it?

SUDS should always be used in preference to conventional drainage when possible. If SUDS were to be incorporated into existing developments peak run off rates could be reduced and therefore flood-risk overall could be reduced. Different measures may be more or less appropriate depending on location and conditions, such as use, existing pollution, geology, area available and whether a site is at risk of flooding. SUDS are particularly beneficial during extreme weather where conventional drains would be overwhelmed. SUDS are becoming more widespread in the UK, benefiting water quality and flood management.


How to use it?

Key Points:


SUDS measures

Sustainable drainage measures and water recycling. (Click image to enlarge)

  • In most cases it is beneficial to use a combination of techniques.
  • Gravity systems require natural falls across the site.
  • Brownfield sites may contain contaminants.
  • Presence of pollutants either prior or potential (this includes run off from roads and parking areas).
  • The disposal of roofwater. For example, it may be possible to discharge roofwater onto a garden area, where it can soak into the ground.
  • Maintenance and upkeep, particularly of surface level measures such as swales.


Design procedure:


  • Step 1: For Greenfield sites SUDS are designed to mimic Greenfield run off rates (ie such that the site acts as though it were a natural ‘green’ site). For Brownfield sites SUDS must be designed to mimic the existing run of rates, including allowing for climate change, as a minimum but ideally to mimic green field rates.
  • Step 2: Existing run-off rates or green-field run-off rates must be calculated based on the site area, area of impermeable surfaces and geology. Calculation of SUDS can be complex as different storm durations and scale (up to a 1% probability rain storm) must be considered as well as the permeability of the ground conditions the height of the water table and the impact of climate change.
  • Step 3: Where possible SUDS should allow water to filter into the ground (infiltration), mimicking natural processes. However, if the site has been used for industry it may contain pollutants therefore making infiltration inappropriate. Equally if the ground is predominantly clay or there is a high water table the run off rate may be so low that permeable solutions may be ineffective.


SUDS green roofs and green walls

Green roofs and walls. (Click image to enlarge)

SUDS permeable paving

Permeable paving. (Click image to enlarge)


  • Step 4: Hard surfaces such as roofs and roads result in increased run-off rates over green surfaces. Green roofs and permeable pavements will help to reduce the run-off rates closer to green-field rates. Systems that slow the rainfall before it gets to the ground, such as green or brown roofs and green walls, are also universally beneficial and will continue to reduce run-off rates even in flood-prone areas.
  • Step 5: If it is not possible to use infiltration devices, and/or permeable surfaces are insufficient to reduce run-off rates sufficiently then flood storage devices may need to be considered, these can include dry ponds swales, basins but also underground storage (typically pumped). Communal rainwater harvesting tanks may be considered, however they are often far smaller than required for large rainstorms.
  • Step 6: Ground level SUDS such as swales or rain gardens can achieve water quality and environmental improvements, through habitat creation or amenity provision. When interspersed between parking areas and buildings they can also create visual interest. However, maintenance and protection of use must be considered.
SUDS public swales allotments

Swales integrated with public realm and allotments. (Click image to enlarge)




SUDS rain garden swale

Rain gardens and swales. (Click image to enlarge)

SUDS filter strip parking

Filter strip divides parking bays. (Click image to enlarge)


Related strategies

Micro-climate Rainwater catchment Flood-risk: building placement
Flood resilience Flood resistance Green roofs
Green walls

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