Greenfield runoff rate: comparing the accepted methods
Two accepted methods applied to the same field can differ by a factor of two on a small site. Here is what drives the difference, how the 50 hectare scaling works, and how to present it.
The greenfield runoff rate sets the discharge your site is allowed, and two accepted methods applied to the same field will give you different answers. On a small site the gap between an IH 124 estimate and a ReFH 2 estimate can be a factor of two, which is the difference between a scheme that needs a modest attenuation tank and one that needs a basin nobody left room for.
So establish the greenfield runoff rate method with the lead local flood authority before the layout is fixed. Doing the calculation twice costs less than redesigning the site.
Why the method chosen changes the answer
Both methods estimate a flood flow from catchment characteristics rather than from measurement, because almost no small site has a gauge on it. They use different descriptors, different equations and different underlying datasets, and they were calibrated on different catchment populations.
IH 124, from Institute of Hydrology Report 124, estimates the mean annual flood from area, rainfall and a soil index. ReFH 2, the revitalised flood hydrograph method in its second version, uses the Flood Estimation Handbook catchment descriptors, including a baseflow index derived from hydrology of soil types, and produces a hydrograph rather than a single peak.
Neither is wrong. They answer the question with different assumptions, and on catchments smaller than the ones either method was built for, the assumptions are doing most of the work.
Catchment descriptors and their sensitivity
The soil term dominates. In the IH 124 equation the soil index carries an exponent above two, so a shift in the assumed soil class moves the result more than anything else. ReFH 2 shows the same behaviour through the baseflow index: a permeable catchment produces a low greenfield rate, and a clay catchment a high one.
The standard IH 124 expression for the mean annual flood on a rural catchment is:
QBAR = 0.00108 x AREA^0.89 x SAAR^1.17 x SOIL^2.17
with area in square kilometres, standard average annual rainfall in millimetres, and the soil index dimensionless.
Illustrative example, and illustrative only. Take a notional 50 hectare catchment with a standard average annual rainfall of 700 millimetres and a soil index of 0.45. The calculation gives a mean annual flood of about 0.22 cubic metres per second, which is roughly 4.4 litres per second per hectare. Change the soil index to 0.3 and the rate falls to well under half that. Nothing about the site has changed except an assumption drawn from a mapped soil class.
That is the sensitivity to be transparent about. A drainage strategy quoting a rate to two decimal places, from descriptors it does not list, is asking to be queried.
Small catchment limitations
IH 124 was derived for catchments of roughly half a square kilometre and above. ReFH 2 has a comparable lower bound. Most development sites are far smaller.
Established practice is to calculate for a notional catchment at the smallest area the method supports, commonly 50 hectares, and then scale the result to the site area to give a rate per hectare. That is what the guidance in the SuDS Manual recommends, and it is why greenfield rates are quoted in litres per second per hectare rather than as an absolute flow.
Two consequences follow. First, the derived rate describes a 50 hectare catchment's behaviour applied to your two hectare site, which is an approximation rather than a measurement. Second, the resulting allowable discharge on a small site can be absurdly low. A rate of 4 litres per second per hectare on a 0.3 hectare plot gives 1.2 litres per second, which is a flow no practical orifice will pass reliably without blocking.
That is why most lead local flood authorities apply a minimum discharge rate, often 2 litres per second and sometimes 5, and why some publish a default greenfield rate per hectare to use where calculation is unreliable. Ask what the local position is before calculating.
Where each method is accepted
| IH 124 | ReFH 2 | |
|---|---|---|
| Inputs | Area, rainfall, soil index | Full FEH catchment descriptors |
| Output | Mean annual flood, scaled to return periods | Hydrograph and peak flows by return period |
| Lower area bound | Around 0.5 square kilometres | Comparable bound, below which use is extrapolation |
| Usual acceptance | Widely accepted for small sites, often as the default | Increasingly expected on larger and more sensitive schemes |
| Licence | Freely available equation | Software licence required |
In practice the split falls along scheme size. A single dwelling or a small infill site will be assessed against the authority's published rate or an IH 124 figure, and nobody will ask for more. A major application, or a site discharging to a watercourse with a capacity problem, will attract a request for the FEH descriptors and a ReFH 2 output.
Some authorities state a preference in their local SuDS guidance or their local flood risk management strategy. Read it. Where the authority names a method, use that method and present the alternative as a sensitivity rather than as your headline figure.
Site area against catchment area
This is the error that most often produces a wrong number, and it is not a hydrology error.
Greenfield rates are derived per hectare of catchment and applied per hectare of site. The site area used should be the area whose runoff is being controlled, meaning the developed area draining to the point of discharge, rather than the red line boundary of the application. Those are different on most sites.
Three specific traps:
- Including land outside the site that drains through it. Off-site flows passing through the site are not part of your allowable discharge and should be conveyed separately.
- Using the full red line area where part of the site is retained as open land discharging as it always has.
- Splitting a site across two outfalls and applying the whole-site rate to each.
State the contributing area, show it on a drawing with the drainage catchments marked, and reconcile it with the impermeable area schedule. That single drawing removes most of the correspondence.
Reconciling a difference between methods
When two methods disagree, do not average them. Explain them.
Set out both results with the inputs used. Identify which descriptor drives the difference, which is usually the soil or baseflow term. Say which method you consider more reliable for this catchment and why, with reference to the area bound and to any local guidance. Then propose the rate you are designing to, and say whether it includes a betterment reduction.
Betterment is a separate question that gets tangled up in this one. On a previously developed site the Defra non-statutory technical standards expect the rate to be brought as close to greenfield as reasonably practicable, and many authorities ask for a stated percentage reduction on the existing rate. That is a policy decision about the target, not a hydrological dispute about the greenfield figure, and keeping the two separate in the document makes both easier to agree.
Where storage volume rather than rate is the constraint, the interaction with source control matters, including whether rainwater harvesting counts as attenuation in the authority's view. It frequently does not.
Stating the method and inputs in the strategy
A Drainage Strategy should let a reviewer reproduce the rate without asking a single question. That means recording:
- the method and its version, with the software and release where relevant
- every catchment descriptor used, with its source and the date of the download
- the notional catchment area used for scaling, and the scaling step shown
- the return periods calculated, normally the 1 in 1, 1 in 30 and 1 in 100 year events
- any urban adjustment applied, and whether it has been applied to a rural rate by mistake
- the minimum discharge rate imposed by the authority, and the final design rate
- the contributing site area, cross-referenced to the drainage layout drawing
On small schemes this fits on one page, and it is the page that gets a drainage consultation response returned without conditions. The same discipline scales down to drainage evidence for a single new dwelling, where the calculation is trivial and the transparency still matters.
Where the site also sits in a flood zone, keep the discharge rate consistent with the modelling in the Flood Risk Assessment. Two documents quoting different rates for the same outfall is the most common internal inconsistency in a drainage submission, and it invites the authority to reject both.
This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Which greenfield runoff method should I use?
Ask the lead local flood authority first, because several publish a preference or a default rate per hectare. IH 124 is widely accepted for small sites and ReFH 2 is increasingly expected on major schemes or where the receiving watercourse has a capacity problem. Where the authority names a method, use it.
Why are greenfield rates quoted per hectare?
Because both methods have a lower catchment area bound of around half a square kilometre, well above the size of most development sites. Practice is to calculate for a notional catchment, commonly 50 hectares, and scale the result to a rate per hectare which is then applied to the contributing site area.
Which input has the biggest effect on the result?
The soil term. In the IH 124 equation the soil index carries an exponent above two, and ReFH 2 shows the same behaviour through its baseflow index. Shifting the assumed soil class can halve or double the rate without anything about the site changing, which is why the descriptors used must be listed.
What if the calculated discharge rate is too small to build?
That is common on small plots, where a few litres per second cannot pass through an orifice reliably without blocking. Most lead local flood authorities apply a minimum discharge rate, often 2 litres per second and sometimes 5. Establish the local minimum before designing the outfall control.
Which site area should the rate be applied to?
The area whose runoff is being controlled, meaning the developed area draining to the point of discharge, rather than the red line application boundary. Exclude off-site flows passing through the site and retained open land discharging as before, and show the drainage catchments on a drawing.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.