Greenfield run-off rates and how they are calculated
The greenfield rate sets the discharge a development is allowed, and therefore the attenuation volume the layout has to accommodate. One number, a great deal of consequence.
A greenfield runoff rate calculation estimates how fast surface water would leave your site if nothing had ever been built on it. Lead local flood authorities use that figure to set the discharge rate a new development is allowed, which in turn sets how much attenuation storage the scheme has to find space for. Get the rate wrong at the start and either the drainage design is unbuildable or the layout loses a plot to a basin that was never needed.
It is one number. It decides more of the site layout than most people expect.
What the rate is describing
The greenfield rate is the peak flow leaving an undeveloped catchment during a storm of a given return period, expressed in litres per second, often per hectare.
The reference figure is QBAR, the mean annual flood, which is the average of the annual maximum flows. Rates for longer return periods, the 1 in 30 year and 1 in 100 year events, are derived from it using growth factors. Those three rates are what a drainage strategy is normally tested against.
The point of the standard is simple enough. A roof and a driveway shed water far faster than a field, so a development that discharges at its own unrestricted post-development rate pushes a surge downstream. Restricting discharge to the greenfield rate means the site releases water at roughly the speed it always did, and stores the difference.
Estimation methods used in practice
Two approaches dominate for small and medium sites.
The IH124 method, from Institute of Hydrology Report 124, estimates QBAR from catchment area, standard average annual rainfall and a soil index. It was developed for small rural catchments and remains the common choice for sites of a few hectares, though it was not derived for very small areas, which is why a minimum catchment area is conventionally applied when using it.
The Flood Estimation Handbook statistical method uses catchment descriptors obtained from the national dataset and is generally preferred where the site sits within a gauged or well-described catchment. For larger schemes, or where the IH124 result looks implausible, this is the cross-check.
Whichever is used, state the method, the input values, the source of each input and the date obtained. An assessor who cannot reproduce your figure will substitute their own, and theirs will be conservative.
Catchment characteristics that drive the figure
Three inputs move the answer more than anything else.
Soil type, through the soil index or the hydrology of soil types classification, which describes how readily water infiltrates rather than running off. Heavy clay produces a much higher greenfield rate than free-draining sand.
Standard average annual rainfall for the location, which is a long-term average rather than a design storm.
Site area, which on small sites is the awkward one. The methods were calibrated on catchments of square kilometres, not on a 0.3 hectare plot, and the resulting rate for a small site can fall below anything a control device will reliably pass.
That is why a practical minimum discharge rate is usually applied. A flow control smaller than about 5 litres per second is prone to blockage, and many lead local flood authorities accept that figure as a floor. Check the local standard rather than assuming, because some authorities set a different minimum and some require a specific device type.
Betterment expected on brownfield sites
On previously developed land the baseline is different. The national non-statutory technical standards for sustainable drainage expect the peak runoff rate from a previously developed site to be as close as reasonably practicable to the greenfield rate, and in any event not to exceed the existing rate for the same event.
In practice most lead local flood authorities push for a meaningful reduction against the existing rate rather than parity, and several set their own required percentage in local standards or a supplementary planning document. The figure varies by authority, so find the published local standard before designing to a number someone remembered from another scheme.
Evidencing the existing rate matters too. A claim that an entire site is currently impermeable needs a survey showing what is actually there, because an officer looking at aerial imagery and seeing grass will ask.
Attenuation volume and storage design
Once the discharge rate is fixed, the storage volume follows from the difference between inflow and permitted outflow across the critical storm duration.
| Design element | What sets it |
|---|---|
| Discharge rate | Greenfield rate or the agreed brownfield betterment, subject to a practical minimum |
| Attenuation volume | Difference between runoff and permitted discharge over the critical duration |
| Climate change allowance | The current Environment Agency allowance for the location and lifetime |
| Urban creep allowance | Assumed future paving of gardens and open areas |
| Exceedance route | Where water goes when the design event is exceeded |
| Treatment stages | Water quality management appropriate to the land use |
Two allowances are easy to forget and expensive to add later. Climate change allowances for peak rainfall intensity are published by the Environment Agency and revised periodically, so take the current figure for the relevant river basin district and the development lifetime rather than copying one from an earlier report. Urban creep is an addition to impermeable area over time, and most authorities expect it on residential schemes.
Critical duration is a calculation, not an assumption. Long duration storms often govern for large attenuation volumes even where short intense storms govern for pipe sizing, and a strategy that tests only one duration is incomplete.
Lead local flood authority expectations
The lead local flood authority is a statutory consultee on surface water drainage for major development, and its published local standards usually sit above the national ones.
Common additions include a required percentage reduction on brownfield sites, a stated minimum discharge rate, restrictions on discharging to a public sewer where any alternative exists, requirements for infiltration testing to a specified standard, treatment stage expectations by land use, and long-term maintenance and adoption arrangements.
Ask for the local standards document at the start. It is usually published, it is usually specific, and designing against the national standards alone will produce a strategy the authority does not accept.
Presenting the calculation clearly
Show the working in a form that can be checked in ten minutes.
State the method used and why. Give every input with its source and the date it was obtained. Give the QBAR result and the rates for each return period assessed. State the proposed discharge rate and the flow control device type. Give the attenuation volume with the critical duration that produced it. State the climate change and urban creep allowances applied.
Then state the residual position: what happens in an event beyond the design standard, and where exceedance flows go. That last paragraph is the one flood risk officers read first, and drainage strategies that omit it get a holding objection.
The Drainage Strategy is where this calculation belongs, and it should be consistent with any Flood Risk Assessment prepared for the same site, because the two documents share the site levels and the outfall. Where both are needed, produce them together rather than sequentially.
Drainage design at this level is engineering work, and a professional reviewer should check the hydraulic calculations before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is QBAR?
QBAR is the mean annual flood, the average of the annual maximum flows leaving a catchment. It is the reference greenfield rate from which rates for longer return periods such as the 1 in 30 and 1 in 100 year events are derived using growth factors. Drainage strategies are normally tested against all three.
Which estimation method should be used?
The IH124 method is the common choice for small and medium sites, using catchment area, standard average annual rainfall and a soil index. The Flood Estimation Handbook statistical method, using catchment descriptors, is generally preferred for larger sites or as a cross-check where the IH124 result looks implausible.
Why is there a minimum discharge rate?
Because greenfield rates calculated for very small sites can fall below what a flow control device will reliably pass without blocking. A control smaller than around 5 litres per second is prone to blockage, and many lead local flood authorities accept that as a practical floor. Check the local standard, which can differ.
What is expected on a brownfield site?
The national non-statutory technical standards expect the peak rate to be as close as reasonably practicable to the greenfield rate and in no event to exceed the existing rate. Most lead local flood authorities press for a meaningful reduction against the existing rate, and several publish a required percentage in their local standards.
Do climate change allowances apply to the discharge rate?
They apply to the rainfall used to size storage rather than to the permitted discharge rate itself. Use the current Environment Agency allowance for peak rainfall intensity in the relevant river basin district over the development lifetime, and add an urban creep allowance on residential schemes, since both increase the volume required.
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.