Calculating greenfield runoff rates for a drainage strategy
The greenfield rate is the benchmark every discharge is judged against. Most holding objections come from the inputs, not the method.
The greenfield runoff rate is the number everything else in a drainage strategy is measured against. National standards work by comparing what leaves the developed site with what would have left it undeveloped, so the discharge rate, the attenuation volume, the size of the basin and the cost of the whole scheme all follow from a figure calculated in the first hour of the design.
That is why reviewers spend more time on this page than on any other, and why most holding objections on drainage are about inputs rather than about method. Nobody disputes the equation. They dispute the area you used.
Why greenfield is the benchmark
The non-statutory technical standards for sustainable drainage set the framework. For a greenfield site, the peak runoff rate to any drain, sewer or surface water body for the one year and the hundred year events should not exceed the greenfield rate for the same event. For a previously developed site, the rate should be as close as reasonably practicable to the greenfield rate for the equivalent undeveloped site.
Two consequences follow.
Greenfield rate has to be calculated even on a brownfield site, because it is the target the reduction is measured towards.
And the rate is needed for more than one return period. The one year event drives the small orifice and the everyday behaviour of the system. The hundred year event, with a climate change allowance, drives the storage volume.
The regression approach and where it still fits
The long-standing approach estimates the mean annual flood from three inputs: the catchment area, the standard average annual rainfall for the location, and a soil index taken from published hydrology of soil types mapping.
It is simple, it is implemented in every drainage package, and for small development sites it remains the method most lead local flood authorities expect to see. It was derived from a study of rural catchments and it carries a lower limit on catchment size, which is the source of most of the trouble.
Below that limit the convention is to calculate the rate at the minimum catchment area the method supports and then scale it linearly by the actual site area. That convention is widely applied and widely accepted. State that you have used it, because a reviewer who sees a rate for a 0.3 hectare site with no explanation will assume the method was applied outside its range.
Catchment descriptor methods
The statistical method built on catchment descriptors takes a wider set of inputs: area, rainfall, a base flow index derived from soil, a measure of catchment wetness, a lake and reservoir attenuation factor, drainage path length and an urban extent term. Descriptors are obtained for a defined catchment outlet rather than for a site boundary.
It is the better method for a real catchment with a defined outfall, and it is what a reviewer will expect where the development sits within a larger drainage area or where the outfall receives flow from upstream.
Its weakness at small scale is the same as everything else in this field. Descriptors derived for a catchment of a few hectares are extrapolated well beyond the data the method was fitted to, and small changes in the urban extent term move the answer a long way.
Where the two methods disagree materially, say so and explain which you have adopted and why. A reviewer would far rather see a stated judgement than a single number presented as fact.
Choosing between them, and the sensitivities that matter
Four inputs do most of the damage.
Area. Use the contributing catchment, not the red line boundary and not the developed footprint alone. A site with a field draining onto it has a larger contributing area than the site itself, and one that discharges through two outfalls has to be split.
Rainfall. Standard average annual rainfall varies sharply over short distances in upland areas. Take it from the recognised source for the exact location, not for the nearest town.
Soil. The index steps between classes rather than varying smoothly, so a site on the boundary between two mapped classes gives two answers. Test the sensitivity and report it.
Urbanisation. On a site with an existing developed area, decide explicitly whether you are calculating the rate for the site as if wholly greenfield or as it is, and be consistent.
Then apply the local rules. Many authorities publish a minimum discharge rate per outfall, commonly a few litres per second, because a hydrobrake or orifice sized to a smaller flow blocks. Where the calculated rate is below the minimum, the permitted rate is the minimum and the storage is sized on that.
Presenting the calculation so a reviewer can repeat it
This is the part that gets the objection lifted. The reviewer is checking whether they can reproduce your number, and if they cannot, they ask for more information rather than accepting it.
Include:
- The method used, by name, with the version of the software or tool and the date the calculation was run.
- Every input, with its value, its units and its source, including a screenshot or reference for the rainfall and soil inputs.
- The contributing area, with a plan showing how it was drawn and where the outfalls are.
- The output rates for each return period used, with the growth relationship applied.
- Any conventions applied, such as scaling from a minimum catchment area, together with the reason.
- The permitted rate finally adopted, and whether it is the calculated rate or the authority's minimum.
Put all of it in the Drainage Strategy with the calculation output as an appendix rather than as an embedded image at the wrong scale. Cross-refer from the Flood Risk Assessment rather than repeating the figures, because two documents with slightly different rates in them is a certain objection.
Where the rate meets the rest of the design
The greenfield figure is only useful once it is connected to the storage calculation, and that connection is where two documents commonly diverge.
Storage volume is driven by the difference between what falls on the developed site and what is allowed to leave it, for a range of storm durations. The critical duration is rarely the one people assume: a small site with a low permitted discharge is often critical at a long duration rather than at the short intense storm, and the model has to be run across the full range to find it.
Then add the climate change allowance to the rainfall, not to the discharge rate, and state the allowance band used and the reason for choosing it. Half-drain time and the treatment of exceedance are checked as well, and a system that takes days to empty will attract a comment even where the peak rate is right.
Set the results out as a table of storm duration against required volume, with the critical case identified, so the reviewer can see the whole picture rather than a single headline volume.
The errors that trigger a holding objection
Using the red line area when part of the site drains elsewhere. Using a rate calculated for the whole site at an outfall serving half of it. Quoting a rate to three decimal places from a method with an accuracy of nothing like that. Applying a climate change allowance to the greenfield rate rather than to the developed runoff. Changing the layout after the calculation and not rerunning it. And presenting a spreadsheet output with no inputs visible.
None of those is a modelling failure. They are documentation failures, and they cost weeks.
Where the drainage evidence is being prepared by someone who is not a drainage engineer, professional review before submission is the sensible step, and for specialist technical reports qualified professional input may be required. To see which documents a site is likely to need, check a site by postcode or look through the flood and drainage report types.
This is general guidance rather than legal advice. Check the position with your lead local flood 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
What is QBAR?
The mean annual flood: the average of the annual maximum flows for a catchment, which corresponds roughly to a flood with a return period of a little over two years. It is the anchor value in greenfield runoff work because the estimation methods produce it directly, and rates for the one year, thirty year and hundred year events are derived from it using published growth relationships.
Which method should I use?
For a small development site, a regression approach using catchment area, standard average annual rainfall and a soil index is normally accepted and is what most drainage software implements. For a larger catchment with reliable descriptors, the catchment descriptor method is preferred. Whichever you use, state the method, the version, the inputs and the source of each input.
Why does my calculated rate come out below the minimum the council will accept?
Because a small site produces a tiny greenfield rate, and the orifice needed to restrict flow to it would be a few millimetres across and would block. Lead local flood authorities therefore set a minimum discharge per outfall, commonly a small number of litres per second. Check the local standard before sizing storage, because it changes the volume required.
Do the same rates apply to a brownfield site?
No. The national standards distinguish greenfield sites, where discharge should not exceed the greenfield rate, from previously developed sites, where discharge should be reduced as close as reasonably practicable to the greenfield rate. Where an existing positive connection is being replaced, evidence the existing rate rather than assuming it.
What soil information do I need?
A soil index derived from published hydrology of soil types mapping, or an equivalent descriptor. Take it from the recognised dataset rather than from a site investigation borehole log, because the methods were calibrated against the mapped classification. Where site testing shows conditions very different from the mapped class, say so and explain how it was handled.
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.