What is “the greenfield runoff rate”?
Greenfield runoff is usually calculated as the peak rate of runoff for a specific return period due to rainfall falling on a given area of vegetated land. There are methods which predict only the peak rate of flow while others produce a runoff hydrograph.
Although greenfield runoff assessment is often a requirement for development sites, the approved formulae have all been derived from river catchment runoff data. The recent publication of ReFH2 and the plot scale method has been produced to try and make the predictions more relevant and accurate for site developments.
The extrapolation of these formulae to site scales is justified on the basis of providing consistent approaches that can be universally applied, rather than giving an accurate assessment of the actual runoff rate from a site. No method would claim to provide an accurate assessment of site scale runoff.
There are a number of factors which are not taken into account which can result in under-prediction of runoff rates from a site. These include:
- The vegetated land characteristics (whether it is treed, grassland, farmed or otherwise landscaped) is not used;
- The gradient of the site is not used;
- Rainfall intensity is not used.
However it must be stressed that although the formulae claim to give a greenfield flow rate, and that it might be much less than it might be, this does not invalidate the runoff control requirements to protect the catchment downstream.