Understanding sustainable drainage systems
The impact of development
Urbanisation poses a significant flood risk because of the reduction in the amount of rainfall which can soak away into the ground. An increase in impermeable area on site will result in greater rainfall runoff, essentially increasing the speed and volume of flooding. A sustainable drainage system (SuDS) is designed to replicate, as closely as possible, the natural drainage from the Site (pre-development) to mitigate the flood risk on and off Site.
Why SuDS drainage is now important for land acquisition and development
- The feasibility of sustainable site drainage systems, SuDS, is a key constraint on the viability of the site development process.
- “It is essential that the consideration of sustainable drainage takes place at the land acquisition due diligence stage” (LASOO, 2015, Practice Guidance, Local Authority SUDS Officer Organisation).
- From April 6th 2015 SuDS are regulated by Local Planning Authorities and will be required under planning law for major developments in all cases unless demonstrated to be inappropriate.
- The Lead Local Flood Authority will require information as a statutory consultee on major planning applications with surface water drainage implications.
- The National Planning Policy Framework requires that new developments in areas at risk of flooding should give priority to the use of SuDS and demonstrate that the proposed development does not increase flood risk downstream to third parties.
SuDS design and runoff destination
SuDS mimic the natural ‘greenfield’ drainage by capturing rainfall and allowing as much as possible to soak into the ground close to where it fell through infiltration systems, or if infiltration is not feasible, slowing down its entry into the watercourse through retention systems. New developments will no longer be able to automatically connect to the sewer network.
There are many different SuDS features to suit the constraints and design of a site, including green roofs, rain gardens, ponds, wetlands, infiltration basins and swales (shallow ditches). In high density, commercial and industrial developments, permeable paving, attenuation storage and soakaways may be an option.
SuDS can significantly improve the quality of water leaving the Site and can enhance the amenity and biodiversity that a site has to offer.
Dr Paul Ellis, GeoSmart Innovation Director, is on the project steering group for Susdrain an exciting new community that provides a range of resources for those involved in delivering sustainable drainage systems (SuDS).
Contact us to find out more about our SuDS Reports and SuDS Infiltration Maps. These help developers assess the drainage potential and the associated design risks for a site at the master planning stage.