Sustainable Drainage Systems Design Guidance

The attenuation device(s) required to achieve the proposed surface water restriction from the site should be clearly identified. Above ground storage devices that provide water quality, ecological and amenity benefits should be proposed where practicable. 

This is in line with SuDS being required in accordance with the National Planning Policy Framework (NPPF) and Non-Technical Standards. Robust justification should be provided if below ground attenuation devices such as crates, tanks or oversized pipes are proposed.

Attenuation device(s) should always be located in accessible areas such as areas of open space, under access roads or shared parking areas. Thought should be given to how maintenance and/or potential replacement of the attenuation devices would be carried out over the lifetime of the development.

Devices serving developments in more than one ownership should not be located in areas such as private rear gardens where access and ownership arrangements are unclear.

Flows within long term storage areas should be infiltrated to the ground or discharged at low flow rate of maximum 2 l/s/ha. Runoff generated from rainfall events up to the 1 in 100 year rainfall event will not be allowed to leave the site in an uncontrolled way. 

Temporary flooding of designated areas to shallow depths and velocities may be acceptable, this will be considered on a site by site basis.