Setting a discharge rate with no greenfield record
A brownfield site has no greenfield record and often no documented existing rate. Here is how to derive both, and how to get a limiting rate agreed before design freeze.
Setting a greenfield runoff rate for a brownfield site is the question that holds up more drainage strategies than any other, because the site has no greenfield record and the existing rate is often undocumented. The answer is to calculate a greenfield equivalent rate from catchment characteristics, establish the existing rate from evidence rather than assumption, and then agree the limiting discharge with the lead local flood authority before the levels are frozen.
Do it in that order. Designing the attenuation first and then asking for a rate to suit it never works, because the lead local flood authority sets the rate and the volume follows from it.
Why a previously developed site has no greenfield record
Greenfield run-off rates are derived from catchment characteristics, not from measurement at the site. There is no gauge on a half hectare yard in a town, and there never was. What the standard methods give you is an estimate of what an undeveloped catchment of that size, in that rainfall regime, on those soils, would have discharged.
On a brownfield site two further problems appear. The soils under the hardstanding have been compacted, disturbed or replaced, so the soil index that drives the calculation no longer describes what is there. And the existing drainage often discharges to a combined sewer with no record of a consented rate, which means the existing rate has to be reconstructed rather than looked up.
Both problems are soluble. Neither is solved by picking a round number.
Estimating the greenfield equivalent rate
The standard route is the IH124 method, which estimates the mean annual flood from the catchment area, the standard average annual rainfall for the location and a soil index. From that, growth curve factors give the rates for the 1 in 1, 1 in 30 and 1 in 100 year events. For larger or more complex catchments the revitalised flood hydrograph and statistical flood estimation methods are used instead.
Three practical points govern whether your figure will be accepted.
The method was developed for catchments larger than most development sites, so applying it to a site of a fraction of a hectare involves scaling that the guidance itself recognises as approximate. Say so in the strategy rather than presenting the output as precise.
The soil index should come from the published soils data for the location, using the undisturbed classification rather than an assumption drawn from a site investigation borehole log. Where the ground investigation shows made ground to depth, note the discrepancy and explain which figure you used and why.
The area used must be the same area throughout: the same red line, the same hectares, in the greenfield calculation, the existing rate calculation and the attenuation model. Mismatched areas between appendices are the single most common reason a lead local flood authority sends a strategy back.
What the technical standards actually require
The Defra non-statutory technical standards for sustainable drainage systems draw a clear line between two situations. For a greenfield site, the peak run-off rate must not exceed the greenfield rate for the relevant events. For a previously developed site, the peak run-off must be as close as reasonably practicable to the greenfield rate and must never exceed the existing rate.
That wording carries the whole argument. It does not say match the greenfield rate on a brownfield site. It says get as close as reasonably practicable, which makes the discussion one of feasibility, evidenced by what the site can physically accommodate.
Local policy often goes further. Many lead local flood authorities require a stated minimum betterment against the existing rate, with a higher figure inside a critical drainage area. In London the drainage hierarchy in the London Plan pushes schemes towards greenfield rates. Read the local plan policy, the authority's own SuDS guidance and any surface water management plan for the area before settling the figure.
Establishing the existing rate with evidence
You can only claim an existing discharge rate if you can show there is a lawful existing discharge. That means four kinds of evidence, and the strategy should present whichever exist.
- Asset records from the sewerage undertaker showing the connection, its size and its invert.
- A CCTV or manhole survey confirming the connection is present, live and of the recorded diameter.
- A measured or photogrammetric assessment of the existing impermeable area, with a plan showing what was included.
- Any historic consent, trade effluent agreement or planning condition recording a permitted rate.
Where none of that exists, expect the authority to treat the site as though it were greenfield for the purpose of setting the limit. That is a defensible position for them to take, and arguing against it without records rarely succeeds.
Calculate the existing rate with a recognised method from the measured impermeable area and the existing network, and state the method used. A rate derived by dividing a rule of thumb by the site area is not evidence.
Minimum practical rates and blockage risk
There is a floor below which flow controls stop working reliably. Small orifices block, and a control that blocks causes flooding upstream. In practice a discharge of around 5 litres per second is treated as the practical minimum for a conventional control, with vortex type devices and properly designed screens sometimes taken lower.
On small sites this creates a genuine conflict. A 0.2 hectare plot with a calculated greenfield rate well below that floor cannot be drained to the calculated figure through a single control. The accepted resolutions are to accept the practical minimum with the attenuation volume sized for the calculated rate, to use infiltration for part of the site so that only a residual flow needs a control, or to combine long term storage with a low flow control designed with maintenance access.
Say which resolution you have adopted and why. A Drainage Strategy that quietly designs to 5 litres per second without acknowledging the calculated rate reads as though the calculation was not done.
Agreeing the rate before design freeze
The sequence that works on a brownfield site looks like this.
Establish the greenfield equivalent rate and the evidenced existing rate. Take both to the lead local flood authority, with the impermeable area plan and the asset records, and ask for written agreement to a limiting rate and to the climate change and urban creep allowances to be applied. Then size the attenuation and fix the levels.
The allowances matter as much as the rate. Peak rainfall intensity allowances for climate change are published by the Environment Agency and vary by management catchment and epoch, so check the current figures rather than carrying forward an allowance from a previous scheme. Add an allowance for urban creep on residential development. Both change the volume more than a small change in the discharge rate does.
Where the discharge is to a public sewer, the undertaker's agreement runs separately from the planning process and has its own timescale. On sites where the connection needs a formal agreement, start it early, because it can outlast the determination period.
Recording the assumption in the strategy
The section that decides whether a drainage strategy is approved without a round of questions is the assumptions schedule. It should state, on one page: the site area and impermeable area used, the rainfall and soil data sources with dates, the greenfield rates for each return period, the existing rate with its evidence, the limiting rate agreed and the date of that agreement, the climate change and urban creep allowances, the attenuation volume, the design event and the exceedance route.
Officers do not need to be persuaded by prose. They need to be able to check the numbers against their own model in twenty minutes. A strategy that lets them do that gets approved.
Where the site is small, the proportionate approach is set out in the note on drainage evidence for a single new dwelling, and the question of whether harvesting counts towards storage is covered in the note on rainwater harvesting and attenuation credit. Where groundwater is the constraint rather than surface water, see the note on groundwater flooding evidence. The drainage strategy sits alongside the Flood Risk Assessment and the two should share every number.
This is general guidance, not 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
Do the technical standards require greenfield rates on brownfield sites?
No. The Defra non-statutory technical standards require peak run-off from a previously developed site to be as close as reasonably practicable to the greenfield rate and never to exceed the existing rate. That makes it a question of what the site can physically accommodate. Local policy often adds a minimum betterment figure, higher inside a critical drainage area.
How is a greenfield rate calculated for a small site?
Usually by the IH124 method, using catchment area, standard average annual rainfall and a soil index, with growth curve factors giving the rates for each return period. The method was developed for larger catchments, so applying it to a fraction of a hectare involves approximate scaling. State that in the strategy rather than presenting the output as precise.
What evidence proves an existing discharge rate?
Sewerage undertaker asset records showing the connection and its diameter, a CCTV or manhole survey confirming it is live, a measured plan of the existing impermeable area, and any historic consent or condition recording a rate. Without that evidence the authority will usually treat the site as greenfield for the purpose of setting the limit.
Is there a minimum discharge rate?
In practice around 5 litres per second is treated as the floor for a conventional flow control, because smaller orifices block and a blocked control floods upstream. Vortex devices with properly designed screens are sometimes accepted lower. Where the calculated rate falls below the floor, say which resolution you adopted and size the storage for the calculated figure.
When should the rate be agreed?
Before levels are fixed. Take the greenfield equivalent rate, the evidenced existing rate and the impermeable area plan to the lead local flood authority, and ask for written agreement to the limiting rate and the climate change and urban creep allowances. Attenuation volume follows from those figures, and retrofitting storage after level design is expensive.
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.