How should you calculate the “greenfield runoff rate” for the site drainage design?
The Greenfield runoff rate which is to be used for assessing the requirements for limiting discharge flow rates and attenuation storage for a site should be calculated for the whole development area (paved and pervious surfaces - houses, gardens, roads, and other open space) that is within the area served by the drainage network whatever size of the site and type of drainage system. Significant green areas such as recreation parks, general public open space etc., which are not served by the drainage system and do not play a part in the runoff management for the site, and which can be assumed to have a runoff response which is similar to that prior to the development taking place, can be excluded from the greenfield analysis.
The exclusion of large green areas is important when using the UKSuDS.com storage assessment tool as it is only valid for use where the paved proportion of the site is larger than the pervious proportion (PIMP is greater than 50%). Ignoring for a moment the issue of the over-extrapolation and validity of the approach, calculating a greenfield discharge rate for the whole site including this type of public open space, will result in a higher Qbar value than if it is excluded. If this is subsequently applied to the (smaller) area served by the drainage system, then the storage volume estimation will be under-sized.
Where green spaces cannot be excluded because they have been landscaped to alter their runoff characteristics or to assist in managing the runoff from extreme events and this results in a PIMP value which is less than 50%, then the use of the UKSuDS tool is not valid and other more appropriate runoff modelling and management techniques should be used for assessing the storage requirements. If it is applied it should be recognised that the estimated storage volume is under-predicted. The reason why the tool should not be used is that no runoff is assumed to take place from pervious areas in the development and this is partially compensated by using 100% runoff from paved areas (in line with sewers for Adoption).
The tool has been known to be used by applying only the paved area as being the site area. Applied in this way it is implying that all the vegetated areas of the site (gardens etc.) continue to discharge as they did prior to development. It will also result in small values of Qbar. This is a very conservative position, but is an approach that could be taken if it was thought to be appropriate for a particular situation.