Where using flow matching long term storage should be included.

The introduction of new areas of hardstanding as part of development is likely to increase runoff volumes from a site. When limiting all discharge back to the 1 in 1-year greenfield rate the benefit of the reduction is discharge rates for larger events will offset the negative impact that this increase in volume may have. However, if flow matching is used to manage discharge rates from a site then such benefits will not be gained, and it will be necessary to provide additional mitigation for the impact of increased volumes of water leaving the site. This can be achieved through the use of online long-term storage (LTS). Where ground conditions allow LTS should be delivered in a method similar to the hybrid approach mentioned earlier in this document. Infiltration should be used to manage discharge for the initial LTS volume and piped discharge should only be considered when this volume of water has been accounted for. On sites that do not allow this approach LTS volumes should be limited back to no greater than 2l/s/ha.