Surface Water Drainage

All major development will be required to use sustainable drainage systems (SuDS) for the management of surface water run-off. Where site specific Flood Risk Assessments are required in association with development proposals, they should be used to design appropriate systems and determine how SuDS can be used on particular sites.

What are sustainable drainage systems?

The main purpose of sustainable drainage systems is to mimic the natural drainage of a site before development.  This is achieved by capturing rainfall and allowing as much of it as possible to evaporate or soak into the ground close to where it fell. The rest is directed through a SuDS management process, which improves water quality, to the nearest watercourse to be released at the same rate and volumes as before development.

There are many different sustainable drainage system features available to suit the constraints of a site. These include green roofs, more natural features such as ponds, wetlands and shallow ditches called swales. Hard engineered elements are often used in high density, commercial and industrial developments.  These include permeable paving, canals, treatment channels, attenuation storage and soakaways.

Benefits of sustainable drainage systems

There are multiple benefits to using SuDS within development:

  • Pollutants, including metals and hydrocarbons from roads and car parks, that have entered rainwater are reduced on the way to a watercourse. Water entering a local watercourse is therefore cleaner as a result and this does not go on to harm wildlife habitats.
  • SuDS generally replace traditional underground piped systems that use gully grates or storm water drains at street level. This means any problems with the system are quicker and easier to identify than with a conventional system, and are likely to be cheaper and more straightforward to rectify.
  • SuDS will become increasingly important to control surface water if rainfall increases because of climate change. They can also provide other benefits in developments such as passive cooling, which will again help mitigate any increase in temperatures due to climate change.

Why do we need sustainable drainage?

Urbanisation reduces the amount of rainfall that can soak away into the ground and means that it has to be managed to prevent flooding. Traditionally this surface water has been combined with the foul sewerage system. More recent developments have used separate surface water sewers that discharge direct to local watercourses. Whilst this has advantages to combined sewers, there are environmental risks if misconnections occur between the two systems.

As towns have spread and density of development has increased, so too has the volume of surface water that these piped systems must cope with. Looking forward, the pressure on urban drainage systems will increase due to further development to meet the needs of our growing population and as a result of a changing climate.

Legislation

The Flood and Water Management Act 2010 is the key legislation relating to SuDS in England and Wales. From the 6 April 2015 Local Planning Authorities (District Councils) have the duty to ensure that fit for purpose SuDS are delivered on new developments, unless they are deemed inappropriate. The Lead Local Flood Authority, Oxfordshire County Council, has taken on the role of statutory consultee.

For further information please contact the Oxfordshire Drainage Team.