Sustainable drainage systems (SuDS)

Sustainable drainage systems (SuDS) are drainage solutions that provide an alternative to the direct channelling of surface water through networks of pipes and sewers to nearby watercourses.

By mimicking natural drainage regimes, SuDS aim to reduce surface water flooding, improve water quality and enhance the amenity and biodiversity value of the environment. SuDS achieve this by lowering flow rates, increasing water storage capacity and reducing the transport of pollution to the water environment. See this section of the British Geological Survey Site.

The need for alternative drainage such as SuDS is likely to increase to meet environmental challenges such as climate change and population growth. Provision for SuDS and the national standards required for their design, construction, maintenance and operation is included in the Flood and Water Management Act 2010 and also in the National Planning Guidance.

A recent DTI and industry funded research project to investigate the economic incentives, social impacts and ecological benefits of sustainable drainage systems produced a number of Information Sheets, published by HR Wallingford. One of those reports is titled “Operation and Maintenance of Sustainable Drainage Infrastructure”.

This report expresses concerns about both design and maintenance of such systems and makes clear that, without proper provision for maintenance, they are liable to fail to perform as required. It points out that considerations that affect the design of SuDS structures, methods and components should include:
• The drainage and water quality functions they are required to perform;
• The maintenance required to ensure they continue to work as intended;
• An assessment of the future repair or replacement requirements.

Furthermore it states that the cost of maintenance is often significant compared with capital construction costs of sustainable drainage systems. It is therefore vital that the cost of implementing long-term management agreements is accounted for during the planning stages.

What are SuDS for?
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.

How do SuDS work?
As surface water flows through SuDS, flow velocity is controlled and pollutants are removed. The SuDS may include the following:
• 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 or wetlands.
• infiltration systems, such as infiltration trenches and soakaways mimic natural recharge, allowing water to soak into the ground.

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