Sustainable Drainage Systems (SuDS)

In the past drainage was hidden underground, with pipes taking water away to soakaways or the sewers.  Nowadays there’s a move towards creating more sustainable development and this means surface water (rainwater) drainage is being intercepted and kept above ground or at a much shallower level below ground.  

Keeping rainwater drainage above ground can help to manage flooding and pollution whilst also increasing biodiversity (wildlife and plants) and amenity for an area.                                    

This new way of managing surface water is known as Sustainable Drainage Systems (SuDS).  

On land without any development (known as Greenfield land) rainwater largely soaks into the ground and in very heavy rain will slowly flow across the land into ditches or streams. Some water is also absorbed by plants, and some lost via evaporation.  

The aim of SuDS is to make drainage behave as naturally as possible by following six key principles:  

• Storing water and releasing it slowly (known as attenuation) 
• Harvesting and using the rain close to where it falls
• Allowing water to soak into the ground (known as infiltration)
• Slowly transporting (conveying) water on the surface
• Filtering out pollutants and 
• Allowing sediments to settle out by controlling the flow of water.  

Sustainable Drainage Systems are not just single drainage features, like a soakaway, but are a collection of components working together to manage surface water drainage on a site. The larger a site and more complex the development, the more components are needed to create a Sustainable Drainage System that works.

Biodiversity Net Gain is an approach to development that leaves biodiversity in a better state than before development. It encourages developers to provide an increase in habitats and ecology features. Defra has recently consulted on making biodiversity net gains a mandatory element of English planning. 

An appropriate use of green SuDS components can contribute to the biodiversity value of an area and could help provide Biodiversity Net Gain.

The components used in SuDS can be grouped into seven categories and the table below provides examples of each of these. Clicking on each component will take you to the susdrain website which will provide detailed information on each component. The susdrain website has been created by the Construction Industry Research and Information Association (CIRIA) in partnership with the Environment Agency and provides information, news and resources related to SuDS. 

When determining how a development should utilise SuDS the following drainage hierarchy should always be applied.

Infiltration is always the preferred method for surface water drainage and should be investigated thoroughly. Only if infiltration is found not to be suitable should discharge to watercourse be reviewed and so on down the hierarchy. Any SuDS that utilise a discharge point will need to provide storage for the water on site so that water can be released at a controlled and slowed rate.