Greenfield runoff rate estimation

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 publication of ReFH2 and the ReFH 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 a consistent approach 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 the value of the figure given, this does not invalidate the principle of controlling runoff to protect against flooding of the catchment downstream.

There are many methods that can be used for calculating greenfield runoff, however there are only two methods provided in the uksuds greenfield runoff rate estimation tool. These are:

• The equation derived in IH 124, Flood estimation for small catchments (Marshall and Bayliss, 1994);
• The FEH statistical equation by Kjeldsen et al. (2008).

In addition a default value of 3 l/s/ha can be used in the surface water storage volume design tool.

IH124 and FEH Statistical are both widely used though other options are available such as the ReFH2 tool by Wallingford HydroSolutions and UK Centre for Ecology and Hydrology (2015). 

IH124

Qbar = 0.00108 x (AREA)^0.89 x SAAR^1.17 x SPR^2.17

where:

• Qbar is the mean annual flood flow from a rural catchment (m³/s)
• AREA is the area of the site (km²)
• SAAR is the standard Average Annual Rainfall for the period 1941 to 1970 (mm).
• SPR is Standard Percentage Runoff coefficient for the SOIL category.

Information for SAAR and SOIL are available from paper maps which were issued by the Flood Studies Report (NERC, 1975; https://www.ceh.ac.uk/data/software-models/flood-estimation-handbook) and also subsequently in the Wallingford Procedure (Kellagher, 1981; https://eprints.hrwallingford.com/37/). 

The value for SAAR was reassessed in the Flood Estimation Handbook 1999 (Institute of Hydrology, 1999) as parameter AAR for the period 1961 – 90. This has subsequently been revised several times with the most current data given in the FEH22 rainfall model (available from the FEH Web Service at https://fehweb.ceh.ac.uk/). The differences between any of these values for SAAR (from FSR or FEH data) is likely to be small for most locations across the country. The advantage of using the digitally based information is the increased level of accuracy for a specific site. However this must be seen in the context of the derivation of the formula which uses this parameter value which was produced prior to the digital age. 

Similarly the value of SPR for the 5 SOIL types in the Winter Rainfall Acceptance Potential (WRAP) map can be replaced by the SPRHOST values for the greater differentiation of the 29 soil types based on IH126 Hydrology of Soil Types (Boorman et al, 1995) or from FEH (UKCEH FEH web service). Unlike the parameter SAAR, the two parameters of SPR and SPRHOST are said to not be exactly the same measure of the hydrological characteristics of soil. However the difference is considered to be sufficiently small to justify the substitution of the SPRHOST parameter value based on the advantage of being much higher resolution soil information. 

Appendix 5 of the Environment Agency document “Preliminary rainfall runoff management for developments” (SC030219 rev. E, 2013) gives an analysis comparing the two equations based on the substitution of the SPR parameter. As a result of this analysis, it is recommended that a lower bound limit to the value of 0.1 is used if used with the IH124 method.

The IH124 equation predicts a greenfield peak flow rate estimate for the mean annual flood called Qbar (a return period of approximately 1:2.3 years). This value is then factored by a growth rate parameter to give a flow rate for other return periods.

FEH statistical equation

The FEH statistical equation predicts the median annual flood called Qmed (a return period of 1:2 years).

Qmed = 8.3062 x (AREA)^0.851 x 0.1536^{(1000 / SAAR)} x FARL^3.4451 x 0.0460^{(BFIHOST x BFIHOST)}

where:

• Qmed is the median annual flow rate; the 1:2 year event (m³/s).
• AREA is the area of the catchment (km²).
• AAR is the standard average annual rainfall for the period 1961 to 1990 (mm).
• FARL is a reservoir attenuation function and is set at 1.0 and therefore has effectively been ignored. This means that areas which have water bodies which attenuate the runoff will over-predict the greenfield runoff rate.
• BFIHOST is the base flow index derived using the HOST classification.

In contrast to the FSR method of IH124, the ReFH based methods are all based on digital and higher resolution information. These formulae are generally regarded as being more accurate than the IH124 though this can only be assessed at the river catchment scale and not site scale. 

It is interesting to note that analysis for the FEH statistical equation peak flow formula found the use of BFIHOST (Base Flow Index) to produce a better correlation than the use of SPRHOST. As flood flows are not intuitively linked to river base flows this provides a warning to treat all these formulae with a degree of caution. 

To obtain the flow rate for any specific return period, a growth factor based on curves given in FSSR 14 (NERC, 1983) can be applied to both the FEH and the IH124 equations. In practice a hydrologist would carry out a detailed pooling analysis based on selecting similar catchments to obtain a likely growth curve for predicting values for higher return periods. However this requires a suitably skilled person to carry out such work. 

3 l/s/ha

A research study was carried out by HR Wallingford, “Storage requirements for rainfall runoff from greenfield development sites” (SR580; HR Wallingford, 2002), which looked at various throttle rates for limiting the discharge from the sites in the context of the peak flood flow of receiving rivers. In summary a throttle rate of 3 l/s/ha demonstrated that sufficient retention of surface water runoff was achieved to protect the river against flooding being exacerbated. However it recommended the use of IH124 for estimating greenfield runoff rates due to its ease of use to take account of the different soil type and runoff rates. 

However since that time, multiple alternative methods have been produced and recommended for estimating greenfield runoff rates in spite of the fact that these are all based on river flow data with catchments measure in tens of square kilometres. It is believed that the default value equivalent to Qbar or Qmed of 3 l/s/ha is adequate for general use for the purpose of estimating drainage runoff control, but that the alternative greenfield equations provided by the tools are suitable simple alternatives to use if the authorities or drainage design team prefer to base the estimate on a nationally derived greenfield equations. 

It should be noted that when the greenfield equations are applied with SOIL types 1 or 2 (or equivalent soil types for the FEH statistical equation) that Qbar and Qmed values are less than 2 l/s/ha. This value is considered to be a cut-off to avoid excessive storage volumes being calculated which are likely to based on storms with durations greater than 24 hours. 

ReFH2 plot scale equation

Information from ReFH2 is no longer used in the current surface water storage volume design tool, however if desired the value could be calculated within the ReFH2 software and then included within the surface water storage volume design tool as a “user specified” discharge flow rate. For information, some information is given here. 

The ReFH2 software has a module specifically developed for plot-scale application of the method. This method uses specific plot-scale equations for Time to Peak (Tp) and Baseflow Lag (BL) which are different to the catchment scale tool. The areal reduction factor for the rainfall is set to 1.0 and the winter design storm profile is recommended.

Tp is the unit hydrograph time to peak in hours and for plot scale applications is calculated by:

• Tp = a PROPWET^b AREA^c (1 + urbext₂₀₀₀)^d SAAR^e

BL is the baseflow recession constant or lag in hours and for plot scale applications is calculated by:

• BL = a PROPWET^b AREA^c (1 + urbext₂₀₀₀)^d BFIHOST^e

Tp and BL are evaluated by the software for a catchment area of 0.5 km². For greenfield sites, the urbext parameter will be set to zero. These values are then used to compute the design storm duration which is then applied to the site. The plot-scale models use AREA as an alternative to the catchment method descriptor DPLBAR (mean drainage path length), and also SAAR as an alternative to the catchment descriptor DPSBAR (mean drainage path slope). This is because DPLBAR and DPSBAR cannot be estimated at the plot-scale by the FEH catchment descriptor software. Because the formulae are applied at 50ha to define the design storm duration, the length of the event is often significantly longer than event durations calculated by other methods at site scale. This results in a lower intensity storm and therefore a lower predicted runoff peak flow rate than might give the actual peak runoff rate for a site. The recommendation for using a winter profile event, when typical events causing maximum peak runoff are summer events, is unclear.

Many studies have been carried out by CEH since the work on the IH124 equation and the much later FEH Statistical equation (Kjeldsen et al., 2008). The latest is the Environment Agency Report SC090031/R2 issued in March 2024. This report is another attempt at updating the runoff estimation for small sites to replace the original work of IH124, but the outcomes are inconclusive because there is virtually no data on greenfield runoff measurements which are of a suitable scale for development sites. Other plot scale research includes the ReFH2 method which developed a catchment scale model with a modification for use for plot scale analysis. The plot scale work was calibrated against a limited data set of sites with relatively moderate sized rainfall events. There is relatively little information provided on this work in the “The Revitalised Flood Hydrograph Model ReFH2: Technical guidance” (Wallingford HydroSolutions and UKCEH, 2015).

There has been continuous pressure to prevent the continued use of IH124 with subsequent equations giving more accurate answers. However the FEH statistical equation has stood up well against the further research. In practice it must be recognised that applying river derived formulae to development sites is far beyond their validity of use (and this is effectively confirmed by the latest research study) so both equations continue to be used, due to their ease of use, for the purposes of drainage design for assessing storage volumes for attenuation of surface water discharges. 

The greenfield runoff rate, which is usually used for assessing the requirements for the limiting discharge flow rate (and subsequently the attenuation storage volume), for a site should be calculated for the whole development area as submitted for planning (including all houses, gardens, roads/paved surfaces, and all open space) that is within the area served by the drainage network and that could generate runoff to the proposed drainage system, even if this is only likely to occur during very extreme rainfall events (this may be the case for grassed/vegetated areas). 

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. Open green spaces are useful areas for managing extreme events and therefore they would normally be included in the drainage analysis. A decision made to use only part of the site for assessing the limiting discharge rate will result in a smaller runoff rate and may therefore result in a greater storage volume being needed.

The exclusion of large green areas was important when using the previous (pre-2025) uksuds surface water storage volume estimation tool as it’s validity for use was limited to the paved proportion of the site being greater than 50%. This is no longer the case with the current storage tool.

The total site area is the whole area within the Planning “red line” boundary being considered for planning and development. The trend for some users or planners stipulating that only the paved and roof area should be used was originally driven by the incorrect practice of assuming no runoff taking place from grassed / vegetated surfaces. As the surface water storage volume estimation tool applies runoff from all grassed / vegetated surfaces, it is recommended that the whole site area is normally used for estimating the greenfield runoff rate which is subsequently used for the limiting discharge rate.