Frequently asked questions (FAQs)



Q) What are Sustainable Drainage Systems (SuDS)

Drainage systems can contribute to sustainable development and improve urban design, by balancing the different issues that influence the development of communities. Sustainable Drainage Systems (SuDS) manages surface water runoff to manage water quantity (localised flooding), water quality (pollution) and improve local amenity/biodiversity. It's not new (there are plenty of case studies), it's just taking some time to become more common place.


SuDS mimic nature and typically manage rainfall close to where it falls before discharging to streams, rivers, other watercourses or sewers. SuDS can be designed to slow water down (attenuate) before it is discharged, they provide areas to store water in natural contours and can be used to allow water to soak (infiltrate) into the ground or evaporated from surface water and lost or transpired from vegetation (known as evapotranspiration).


SuDS is an approach, rather than a specific feature or component. SuDS components can range from smaller rain gardens, swales through to permeable block paving and larger components that provide storage like ponds and underground storage tanks. SuDS components can either be at the surface or underground, and can be vegetated or more engineered products. The design of the SuDS scheme and selection of the component will depend on the opportunities and challenges posed by a site. Normally, there are greater benefits by managing runoff at or near the surface and this should be exploited first. However in some circumstances like retrofitting SuDS in dense urban environments managing water on the surface can be particularly challenging - but if it can be achieved it can make an amazing contribution to the quality of places and spaces.


For more information visit SuDS background, SuDS principles and SuDS components or see the video


Q) Why use SuDS?

SuDS (sustainable drainage systems) provide a flexible approach to mimic as closely as possible the natural drainage from a site before development, to manage flood risk and treat runoff to remove pollutants. Adopting a holistic approach towards surface water drainage provides the benefits of combined water quality and quantity control, increased amenity and biodiversity value as well as a range of other benefits. This is accomplished by managing the increased flows and pollution from surface water runoff that can arise from development, ideally utilising a management train to achieve an equal balance of quantity, quality and amenity.


For more information visit SuDS benefits and SuDS performance & monitoring or see the video


Q) Are SuDS land hungry - do they take up much space?

No, they don't need to be.

Often the need for higher density developments and the land take required for some SuDS components can be considered a challenge and a barrier to the use of SuDS. With creativity and careful planning this can be overcome and great places and spaces can be delivered. The SuDS scheme at Springhill, Stroud shows how careful design and consideration of sustainable drainage issues ensures that SuDS can be incorporated into the built environment landscaping and some harder components can form an integral part of the site drainage.


The SuDS management train approach with source control components upstream of regional controls, reduces the need for larger SuDS components such as retention ponds and wetlands to be situated on site. Also a series of smaller ponds can be used upstream on the catchment limiting the volumes that need to be controlled downstream. This can then provide visually interesting features and enhance biodiversity within the development.


For new developments there may be a planning requirement to incorporate public open space and car parking, providing an ideal opportunity to simultaneously integrate SuDS and offer multifunctional benefits. When planning sustainable drainage in high density developments both innovative design and specific SuDS components are fundamental to success, and can include a range of soft and hard components depending on the opportunities and constraints of a site.


Green roofs and rainwater harvesting allow rainwater to be controlled as close to source as possible, attenuating the flow of runoff and providing other benefits. Permeable paving or other permeable surfaces can replace standard impermeable tarmac to reduce the amount of runoff generated by a development (as well as obviously providing a road surface or car parking). In some cases, depending on ground conditions, infiltration components can help provide SuDS in high density areas. Bioretention facilities (underdrained landscaped areas) offer good opportunities for water quality improvements, storage, and for amenity. For example, bioretention areas could be used as landscaped features in car parks, and in areas where green space would be required anyway.


Traffic calming, street scenes and home zones can incorporate SuDS, eg using rain gardens, to remove road gulleys, and runoff into the sewerage system, by intercepting highway runoff at source and controlling discharge into the underlying ground. Examples of this system have been successfully included in Portland, Oregon, where roadside rain gardens are planted with native species to reduce the width of the road (and the speed of vehicles) and remove runoff from the drainage network.


By careful consideration, and sometimes using innovative solutions, runoff from urban areas can be removed at source, while providing supplementary benefits. Similarly, roof runoff can be intercepted and diverted into rain gardens or rainwater harvesting systems for toilet flushing and irrigation use.


Good engagement and good design should ensure that no space is wasted, that land take is minimised while delivering a good SuDS scheme. Integration of vegetated/landscaped and proprietary/more engineered components should provide good schemes that reduce land take, (see the Riverside Place case study) Even where there is some land take, there should also be greater consideration of the benefits provided by the SuDS schemes as this should provide an opportunity for a premium on property prices.


Q) Are SuDS expensive?

No, in the majority of cases with early engagement and good design they can be cheaper, (but it would be useful for more research on the long term costs and benefits of SuDS).

In general a well designed SuDS scheme (where good designers are engaged early) should not cost more than a traditional drainage system. There is strong evidence suggesting that the construction and operational costs of SuDS, particularly multifunctional landscaped components, are less expensive than traditional drainage. This is because SuDS do not involve deep excavation or expensive materials.

In selecting a design for drainage the overall scheme costs and benefits should be considered, it may be useful to consider the functions in terms of Green Infrastructure as well as flood risk management, water quality and amenity and biodiversity. An increase in drainage costs may be offset when the drainage and landscape budgets are considered. The increase in cost for one item (such as paving) may be offset elsewhere (in ease of construction and lack of gully pots for permeable surfaces).


At the Ravenswood development in Ipswich, using SuDS meant a £600,000 saving in construction costs. Lamb Drove, Cambridgeshire, had a 10% saving on design and capital costs compared to traditional drainage systems, a saving that may have been higher if consultation had taken place before the layout of the development was decided.


For more information visit the collated evidence that includes information on some comparative costing exercises where SuDS schemes were compared to traditional drainage. 


Q) Are SuDS safe?

Yes, well designed schemes and  components are safe.

There are many ways to reduce the chances of accidents in and around sustainable drainage schemes. With appropriate design these risks can be minimised particularly if components are visible and successfully integrated into the public realm. It is recommended that open water components incorporate barrier planting (usually densely planted marginal vegetation and/or thorny plants to restrict access) and gently inclined side slopes.


Options include the design of banks for ponds and swales with a maximum of a 1 in 3 slope and the depths of ponds and wetlands should be kept to a minimum, with the maximum depth of water being located away from the edges. Often, information signs are used demonstrating the benefits of sustainable drainage. Educating the public how the SuDS scheme works and the potential risks is a vital part of involving the community.


For more information download the Health and safety framework


Q) Can SuDS be used in areas which have little or no infiltration?


Although many SuDS components using infiltration are highly effective, there are sites where infiltration is not possible, due to impermeable ground conditions, contamination or a high water table. This does not prevent the use of the SuDS approach, but requires careful thought to be given to how water can be treated to improve quality and attenuated to reduce peak flows.


Rainwater harvesting, green roofs, permeable surfaces, swales, ponds and wetlands can all operate without infiltration. Permeable surfaces, used for car parks and drives are very effective, even where infiltration is not possible. The M40 motorway services at Wheatley is a good example of the use of SuDS where infiltration is not possible. At this site infiltration was not used due to the risk of mobilising contamination in the underlying soil. Here the car park uses permeable paving for the parking bays, offering treatment and attenuation of the run off.


Q) Can SuDS be used on previously developed or contaminated sites?

Yes, if designed appropriately.

A common misconception is that SuDS cannot be used on contaminated land that is proposed for redevelopment. Up to 40% of sites developed using SuDS in Scotland have been brownfield sites. When considering sustainable drainage some components may not be appropriate for contaminated sites, such as those using infiltration, which may re-mobilise pollutants in the ground. However, components that store or convey water on the surface are likely to be more suitable. Ground investigation information will indicate that components are applicable to the local conditions, and it might be that certain parts of the development can use infiltration if there is no risk of pollution. Click here for information that SEPA has produced on using SuDS on previously developed land.


Using liners to prevent infiltration into the underlying ground will enable the use of swales, wetlands, ponds and permeable paving on sites. As SuDS tend to be shallow there will be less disruption to any contaminated ground as can happen when installing traditional piped drainage, eg where a contaminated site has been capped, trenches for piped systems can break through this layer, potentially re-mobilising contaminants. Also, traditional systems can provide a pathway for contaminants via the pipe surround. SuDS may provide a pathway, but above ground systems will quickly show if this is occurring and SuDS may offer some water quality improvements. Traditional drainage systems will not.


SuDS can reduce the effect on watercourses, which may have been historically polluted, by improving the quality and reducing the rate and volume of runoff. SuDS can provide biodiversity and habitat opportunities on contaminated sites through careful and considerate design.


SuDS can be used during construction to trap and remove contaminants from development. For example, when a site has been cleared runoff can be rapid and may contain high levels of silts, sediments and polluted material.


Q) Do SuDS pose a greater risk of groundwater contamination?

No, if designed appropriately.

SuDS should not increase groundwater contamination risks if designed appropriately for the site. All drainage systems have to be designed for exceedance so they are safe when overloaded. Certain measures can be taken to protect more sensitive areas by considerably reducing or prohibiting infiltration. In marginal areas, where polluted water may have an impact on the groundwater, the runoff can pass through one or more treatment stages, depending on the possible level of pollution and the hydro-geological conditions. If all infiltration was prohibited it is likely that a SuDS scheme would still represent an improvement over a traditional system drained using pipes. The SuDS scheme could still attenuate flow from the site and improve the surface water runoff quality.


The SuDS scheme at the M40 motorway services at Wheatley is a good example of the use of SuDS where infiltration is not possible as there is naturally occurring arsenic in the ground.


Q) Can SuDS be retrofitted?


The Dings example in Bristol demonstrates how retrofit projects can be used to address water quality and quantity issues, but can be used to improve the quality of life in an area. The case study demonstrates that opportunities exist in regeneration areas to deliver the multiple benefits of SuDS and that the issue of adoption can be resolved.


Careful consideration to retrofitting SuDS may be needed to ensure that they are incorporated into the existing urban form, this has been successfully achieved in Malmo, Sweden. There are examples of SuDS retrofits into highways to form traffic calming, by reducing the road width and removing existing gulleys (for example in Portland, USA and Auckland, New Zealand). Permeable paving can be installed in car parks and low trafficked areas and runoff from individual buildings can be removed from the public sewerage system by disconnecting downspouts from piped drainage systems and diverting runoff into rain gardens or soakaways.

For more information vist Retrofitting SuDS or see the video


Q) Would using SuDS help Combined Sewer Overflow (CSO) management and improve water quality in rivers?


Within London and other major conurbations much of the sewer system is used to convey foul water and storm water away from properties together in a single combined sewer. During periods of intense rainfall the drainage system can be inundated with water and the sewer system is designed to discharge water into watercourses through CSOs to help manage the flooding risk. The principle behind SuDS of treating and managing water close to its source should reduce the amount of water entering into the drains and sewers and reduce CSO spills and potentially improve water quality in watercourses.


Q) What happens when the capacity of the SuDS is exceeded or it fails?

All drainage systems should be designed to incorporate the provision for flows above the design capacity, to be conveyed off site with the minimum impact. The design of SuDS should mean that less damage is done when their design capacity is exceeded or if it fails, than with conventional systems. The SuDS design philosophy, unlike traditional systems, is to use a train of SuDS components. For example, once the soakaway has reached its capacity, the overland flow can be stored in a pond or wetland or underground storage. Flooding, should it occur, can also be managed to reduce the impact, for example managing water on the surface where water levels are visible, as well as careful planning and design that can ensure that areas such as playing fields should be flooded before roads and that houses are positioned so they are less likely to be inundated.


For more information visit Drainage exceedance  


Q) Do I need to use an oil separator for car park drainage?

Not necessarily.

You might not need an oil separator if you use SuDS. The SuDS approach should be used on all sites to minimise the impact of the development on the environment. Components that control pollution close to the source, such as permeable surfaces or infiltration trenches, might offer suitable means of treatment for run-off from low risk areas such as roofs, car parks, and non-operational areas. In higher risk areas, you might need other SuDS facilities such as constructed ponds, wetlands or swales.

Where there is a high risk of oil contamination, such as a fuelling point, it may be appropriate to use an oil separator as part of the SuDS scheme.


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