Greenfield Runoff Rate
QBARThe rate of surface water runoff from a site in its undeveloped (greenfield) condition. New development is typically required to limit post-development discharge to this rate.
The greenfield runoff rate is the estimated rate at which surface water drains from a site in its natural, undeveloped state. In England, new development is almost universally required to limit its post-development surface water discharge to no more than the greenfield rate for the equivalent storm event, ensuring that the development does not increase flood risk downstream.
Greenfield runoff rates are calculated using one of two main methods:
- IH124 / FEH Statistical method: Used for smaller sites, this derives the mean annual flood (QBAR) from soil type, annual average rainfall, and site area. QBAR is then factored for specific return periods.
- FEH Rainfall-Runoff method: A more detailed approach using Flood Estimation Handbook data, suitable for larger or more complex catchments.
For most LLFA areas, the target discharge rate is the greenfield QBAR for the 1-year return period, with attenuation provided up to and including the 1 in 100 year event plus climate change. Some authorities set a minimum discharge rate (often 2 l/s or 5 l/s) to avoid impractically small orifice sizes at flow-control devices.
On brownfield (previously developed) sites, some LLFAs allow a pragmatic approach where the target is a significant reduction from existing runoff rates rather than strict greenfield equivalence, particularly where the existing drainage discharges to a combined sewer.
Aegaea calculates greenfield runoff rates for all surface water drainage designs, using the methodology accepted by the receiving LLFA or water authority.