SUDS Hierarchy

SUDS Hierarchy1

The SUDS Hierarchy generally sets out the preferred method of selecting which Sustainable Drainage System should be used.

'Soft SUDS' such as ponds and swales are the preferred drainage systems as they mimic natural drainage and provide a number of benefits. 'Soft SUDS' are excellent drainage systems as they can be used to attenuate surface water flows, reduce the flow rate of surface water runoff, improve the quality of surface water runoff by removing hydrocarbons and pathogens and also promote and enhance biodiversity within a developed environment.

Smaller developments may not have the physical room for pond and swales therefore other drainage systems may be required. There is always preference to infiltration systems as they recharge natural ground water supplies, reduce the impact of excess flows to watercourses and surface water sewers and help to remove contaminants found in surface water. Care should be taken with infiltration systems when used in or near aquifer protection zones or close to buildings or structural foundations. It is recommended that the Environment Agency is consulted prior to constructing infiltration systems and soakaways.

There is also merit for using storage tanks such as oversized pipes and culverts where space is at a premium or simply not available. This can be complemented by discharging to natural drainage system such as a reed bed or small pond to give provide a final stage of treatment to the surface water runoff.

Where possible surface water should also be dealt with at source. It is not best practice to convey surface water to a large attenuation structure at the end of the development. SUDS such as rainwater harvesting systems, waterbutts, permeable surfaces can be used to deal with surface water runoff as it lands on a building, car park or road immediately. This helps to reduce flood risk and improves the quality of surface water runoff on exiting the system.

Rainwater harvesting and waterbutts encourage rainwater recycling which reduces the use of potable water supplies and retains rainwater on the property.