Significant differences in limiting discharge rates exist within Local Lead Flood Authority (LLFA) and Local Authority (LA) for SuDS design.
DEFRA and the EA have produced guidance within Report SC30219 “Rainfall runoff management for developments”. It is from here that the 5 l/s minimum outfall rate "rule" was established. This was widely adopted by many institutions including BRE and HR Wallingford together with many local authorities.
However this is no longer considered appropriate for some LA and LLFA. For example, one local authority will formally condition a planning application with flow rates from the site to be no lower than 5l/s whereas an adjacent LA will only consider flows at, or near to, greenfield rates and 1l/s is widely cited. In one LLFA a SuDS proforma points to the use of on online tool produced by HR Wallingford, which uses the 5 l/s "rule" as the basis for attenuation volume calculations; yet the same LLFA will not accept the 5l/s rule when bespoke calculations are produced.
Innervision can design SuDS schemes to meet various limiting discharge rates. Please get in touch with us for a free initial consultation.