A Brief Overview of SuDS

The term SuDS is an acronym for Sustainable Urban Drainage Systems, although it has now been introduced into the latest building standards for both urban and rural developments. Therefore, the word Urban is often dropped from the title to reflect this and the “u” in SuDS is lower case for the same reason. Sustainable Drainage Systems were first established in the United States and originally known as Best Management Practice (BMP). When the system was adopted in Scotland the name was changed to better reflect the process.

SuDS were developed to solve the problem of diffuse pollution. Pollution that has no specific point of discharge as distinctive from point source pollution, such as end of pipe discharges. In a natural environment after heavy rainfall most of the surface water either is taken up by plants, infiltrates into the ground, or evaporates leaving only a small percentage to runoff across the ground surface and enter watercourses such as burns, rivers or lochs. In built up urban areas more of the natural ground surface is covered by impermeable surfaces such as buildings, roads and footpaths reducing the amount of rainfall that can infiltrate through the ground and increasing surface water runoff significantly. When rainwater runs off impermeable surfaces the water picks up pollutants that are on these surfaces and transports them into the watercourses causing diffuse pollution of rivers, lochs and groundwater.

Pollutants consist of many different things. These range from tiny pieces of solid or liquid matter emitted during vehicle engine combustion. Tiny pieces of zinc, copper, lead, nickel and cadmium are also produced due to wear on moving parts of vehicles. There are also other sources of pollutants that are released into the air such as soot particles from chimneys, fungal spores and dust. During rainfall these air pollutants are washed onto impermeable surfaces such as roofs. Other pollutants such as pet faeces, oils, petrol and soap or detergents from car washing can settle onto roads and driveways. As the surface water runs off into the sewers it transports all these pollutants into the nearest water course producing one form of diffuse pollution.

Conventional sewers simply collect and transport the surface water and have no way of removing most of these microscopic pollutants. The only treatment that conventional pipework sewers can provide is silt traps to collect larger pollutants. In Scotland it was realised in 1990s that diffuse pollution was a problem and if river quality was to improve then a more effective method of cleaning up surface water runoff had to be found. SuDS attempts to provide this treatment.

Unlike conventional drainage systems, SuDS adopt the earth’s natural processes to provide treatment of rainfall runoff. These include infiltration, evaporation, evapotranspiration, settlement, biological digestion and adsorption. The concept of SuDS is a radical new way of thinking about drainage systems and is based on a holistic approach with three objectives that make up the SuDS triangle.  

three principles of suds - water quality, water quantity and amenity
The SuDS Triangle

These are Water Quality to prevent pollution, Water Quantity to provide flood prevention and the third objective of Amenity to provide landscaping and improve biodiversity.

In order to rationalise SuDS at various scales the concept of a treatment train has been developed. The treatment train starts with reducing sources of pollution. This is followed by providing SuDS source control devices as close to the pollution as possible. This is followed by site control SuDS devices and finally regional control devices.

treatment train - reduce pollution - source control - site control - regional control
The SuDS Treatment Train 




There are many different SuDS devices that can be used. Source control devices include soakaways, swales, filter strips, water butts, green roofs and rain gardens. Filter drains and swales are often used along roadsides to collect the runoff from roads. Surface water can also be transported using filter drains and swales. Site and regional control devices can range from retention ponds, detention basins or if space allows wetlands.

photo of suds swale along side of public road
A typical roadside swale used for collection, treatment and transportation

All these devices use natural materials such as vegetation, structural landform or gravels to provide SuDS devices and are often called soft solutions. If these devices are well designed, they can also provide the third objective of SuDS which is to add amenity value to the area.

There are also manmade components that are classed as hard solutions. These include permeable surfaces which are often used as source control to allow water to infiltrate back into the ground. Site control devices can be provided by underground storage tanks and Stormcells. Both of these can provide attenuation and are often used for flood management. They only provide a limited amount of treatment due to settlement or biological digestion and only improve the amenity value due to being hidden below the ground.


This Video gives a brief overview of Sustainable Urban Drainage Systems (SuDS) to help explain why SuDS has been developed and explain the process of designing SuDS.





Further reading

Susdrain - resources for those involved in delivering sustainable drainage systems

The SuDS Manual C753 - CIRIA

Scottish Environment Protection Agency - Regulatory Method (WAT-RM-08)

Centre for Ecology & Hydrology - Flood Estimation Handbook Web Service




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Web page last updated  27 January 2019

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