Greenfield runoff rate and why it sets the discharge limit

The greenfield runoff rate is the rate at which rainwater would leave a site if nothing had ever been built on it. It matters because the national technical standards for sustainable drainage use it as the ceiling: for a greenfield site, the peak rate discharged to a sewer, highway drain or watercourse should not exceed the greenfield rate for the same rainfall event. Every attenuation tank, every flow control and a good deal of the drainage budget follows from that single number, which is why it is worth understanding before a consultant hands you one.

What the rate is meant to represent

Undeveloped land absorbs, stores and slowly releases rainfall. Roofs and tarmac do not. The greenfield rate is the benchmark that describes the natural condition, and the design task is to make a developed site behave as though it were still in that state at the point of discharge.

The rate is usually expressed in litres per second, sometimes per hectare. It is estimated rather than measured, because nobody gauges a two-hectare field, and the estimate rests on catchment characteristics: area, soil type and its permeability classification, average annual rainfall, and a hydrological method.

The mean annual flood flow, often written as QBAR, is the common starting point. Longer return period rates are then derived from it using growth factors. Two consultants using different methods on the same field will not produce the same figure, which is why the method has to be stated rather than assumed.

The return periods that get assessed

Three events do most of the work in a drainage submission, and the standards address each.

The 1 in 1 year event governs the everyday condition and stops frequent small discharges scouring a watercourse. The 1 in 30 year event is the one most drainage authorities use for no-flooding-of-the-network. The 1 in 100 year event, with a climate change allowance added to the rainfall, is the design case for no flooding of property.

Climate change allowances are published by the Environment Agency by river basin district and time period, and they are revised. Use the current figures, name the allowance you applied and say which epoch it covers, because a drainage engineer reviewing the submission will check that first.

How the rate is estimated on a small site

The recognised hydrological methods were developed for catchments much larger than a housing plot, and applying them to half a hectare involves extrapolation. That is not a reason to avoid them, but it is a reason to state the assumptions.

A calculation submitted for review should show the catchment area used and whether it is the whole site or only the developed part, the soil classification and its source, the average annual rainfall figure, the method and the version, and the resulting rates for each return period.

Two arguments recur. The first is whether the rate should be calculated on the whole site or only on the area being developed, which on a large plot with a small building makes a substantial difference. The second is the minimum practical discharge. Flow control devices have a lower limit below which blockage becomes likely, commonly quoted around 2 litres per second, and where the calculated greenfield rate falls below that limit the authority has to accept a higher figure or a device it is content to maintain. Agree it rather than assume it.

Previously developed land and betterment

On a brownfield site there is no natural condition to return to, so the standards work differently. The discharge should be as close as reasonably practicable to the greenfield rate, and in any event must not exceed the existing rate for the same event.

In practice most lead local flood authorities look for betterment, a reduction against the existing positively drained rate. The percentage sought varies between authorities and is set out in local guidance rather than nationally, so check the local standard before designing the storage.

Establishing the existing rate needs evidence: a measured impermeable area from a survey or aerial imagery, the existing connection size and its recorded invert, and any historic drainage plans. An assumed existing rate based on the whole site being impermeable is the claim most often challenged.

Attenuation volume is the consequence

Lower the discharge rate and the storage volume rises, not proportionately but steeply. That relationship is the reason the rate is negotiated rather than simply accepted.

The volume has to go somewhere. On a tight urban site it becomes a tank under the car park or an oversized pipe, both of which carry cost and maintenance. On a larger site it can be a basin or a swale, which is usually less costly and delivers biodiversity and amenity alongside. Above-ground storage also gets counted in the biodiversity metric, which underground tanks do not.

So fix the rate early. Changing it after the layout is set means finding storage in a scheme that has no room for it, and that is how drainage ends up under the only area of public open space.

Agreeing the rate with the drainage authority

The lead local flood authority is the statutory consultee on surface water drainage for major development, and its drainage team will review the calculation. The sewerage undertaker has its own interest where the discharge is to a public sewer, and the Environment Agency where it is to a main river.

Get the rate agreed in principle at pre-application stage, in writing. A short technical note with the calculation and the proposed rate, sent before the layout is fixed, costs very little and settles the biggest variable in the Drainage Strategy.

Note that the sustainable drainage approval regime under Schedule 3 to the Flood and Water Management Act 2010 has not been brought into force, so approval currently runs through the planning system and the consultee arrangements rather than through a separate approving body. Check the position at the time you apply, because that change has been under discussion for some years and would add an approving body with its own timetable.

Evidence to submit

The drainage submission should let a reviewer reproduce the number without asking a question.

• The calculation output with all inputs shown, including method and version.
• A plan showing the contributing catchment area used.
• The soil classification source and the average annual rainfall figure.
• Existing impermeable area evidence where the site is previously developed.
• The proposed discharge rate for each return period, and the flow control device selected.
• The climate change allowance applied, with the epoch and the river basin district.
• Storage volume calculations following from the rate, with the critical duration identified.

Where the site is also in a flood zone, the discharge rate sits alongside the wider assessment in a Flood Risk Assessment, and the two documents must use the same climate change allowance. And if they do not, that inconsistency is the first thing a drainage officer will notice.

Browse all 66 PlanScope report types and start a draft to see which drainage and flood documents your site is likely to need.

This is general guidance, not 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.