The criterion requires no run-off to leave the developed site into the local watercourse(s) for a storm event that results in rainfall depths up to 5mm. It is not acceptable to collect the rainfall within an attenuation tank and allow the runoff to be released from the site at a restricted rate. This simply slows the rate at which it is released to the watercourse(s).
Compliance should be based upon zero runoff from the first 5mm rainfall for 80% of events during summer and 50% in winter. This is in line with the recommendations within Chapter 24.8 of The SuDs Manual (C753), CIRIA 2015.
The 5mm rainfall event is considered one of the most common rainfall events and, therefore, a system should be designed to prevent this run-off leaving the site thus protecting a receiving watercourse from pollution.
Where ground conditions prohibit the use of SuDs techniques to manage the rainfall onsite or it is not be possible for the first 5mm of rainfall to be prevented from leaving a site completely. An appropriately qualified professional must explain comprehensively why this criterion cannot be fully met and design a system to meet the intent of this criterion as far as possible. Where this can be justified and all other relevant criteria have been achieved, the credit can still be awarded.
16.08.2024 Clarification added on calculation requirements and text from KBCN1059 now included.
