What are SuDS and how do they work?

In natural environments, rain falls on permeable surfaces and soaks into the ground; a process called through infiltration.

In urban areas where many surfaces are sealed by buildings and paving, natural infiltration is limited. Instead, drainage networks consisting of pipes and culverts, divert surface water to local watercourses. In some cases, this has resulted in downstream flooding and deterioration in river water quality caused when foul sewers are overwhelmed by surface water leading to a release of dirty water into rivers.

Sustainable drainage systems aim to alleviate these problems by storing or re-using surface water at source, by decreasing flow rates to watercourses and by improving water quality.

Illustration showing flow of water through sustainable drainage systems into the subsurface. During a storm event, surface water flows through swales and filter trenches that remove entrained pollutants (1). The peak river discharge is delayed and reduced by: storage of water for re-use (2), storage in ponds (3), or infiltration of water to the ground through infiltration basins and soakaways (4). This process improves the quality of water in rivers and decreases peak river discharge (5).


How do SuDS work?

Sustainable drainage systems use a sequence of techniques that together form a management train. As surface water flows through the system, flow velocity is controlled and pollutants are removed. The management train may include the following stages:

  • source control methods decrease the volume of water entering the drainage/river network by intercepting run-off water on roofs for subsequent re-use (e.g. for irrigation) or for storage and subsequent evapotranspiration (e.g. green roofs).
  • pre-treatment steps, such as vegetated swales (ditches) or filter trenches, remove pollutants from surface water prior to discharge to watercourses or aquifers.
  • retention systems delay the discharge of surface water to watercourses by providing storage within ponds, retention basins and wetlands for example.
  • infiltration systems, such as infiltration trenches and soakaways mimic natural recharge, allowing water to soak into the ground.

Our expertise lies in understanding the infiltration capacity of the ground and the potential impacts on ground stability and water quality.

An example pre-treatment system (infiltration trench)


An example infiltration system (infiltration basin)

We are able to design and install a range of systems to suit each situation or location.