Brownfield sites should where possible revert the drainage back to its natural state.
|What the LLFA expect to see
At all stages of the planning process, when developing a brownfield site and once it has been proven that discharging to the 1 in 1-year greenfield rate (or matched rates) or as close as feasibly possible is not possible. The current brownfield rates accompanied by their calculations should be provided to ensure that a minimum 50% betterment is provided.
The LLFA expects surface water drainage schemes on brownfield development sites to follow the same principles as if the site is greenfield. If it is unfeasible to restrict runoff rates back to the 1 in 1-year greenfield runoff rate or the alternative flow matching approach, the LLFA will accept discharge rates to be restricted to as close as feasibly possible to the 1 in 1-year greenfield runoff rate or flow matching rates with limiting to a 50% betterment of the brownfield 1 in 1-year runoff rate or flow matching rates as a last resort. If this approach is to be chosen, then it should be clearly evidenced that restricting to the greenfield rate is unfeasible and that the new rate proposed is as close as feasibly possible to the 1 in 1-year greenfield runoff rate or flow matching rates. This supporting evidence should include a range of rates and the corresponding storage volumes associated with those rates. It should be noted that a 50% betterment is a last resort option and is only acceptable when lower rates are proven to be unviable.
Page updated: 17/02/2020