Greenfield run-off rate and how it is estimated

The greenfield run off rate is the rate at which surface water would leave a site if it had never been built on, and on most schemes it is the ceiling the drainage design has to work back from. Defra's non-statutory technical standards for sustainable drainage systems set it as the limit for peak discharge from greenfield development, for both the 1 in 1 year and the 1 in 100 year events. Everything downstream of that figure moves when it moves: attenuation volume, orifice size, basin depth, even whether the scheme fits on the site.

So the estimate is worth getting right early, before the layout is fixed.

What the rate measures

It is a flow, given in litres per second, not a volume. The starting point is QBAR, the mean annual flood flow for the catchment, which in UK hydrology sits close to the 1 in 2.33 year event. Growth factors then scale QBAR up or down to the return periods the standards care about, usually quoted as Q1, Q30 and Q100.

Two things are commonly confused here. The rate applies to the site, so a 2 hectare red line boundary with 1.2 hectares of built development still starts from the whole contributing area unless you can show part of it drains somewhere else. And the rate is a peak, not an average, so it is compared against the modelled peak outflow from the proposed system rather than against an annual total.

The estimation methods in common use

Three approaches turn up in practice, and a lead local flood authority will usually accept any of them if the inputs are stated.

The IH124 regression was calibrated on gauged catchments of around half a square kilometre and upwards. Most development sites are far smaller, so the convention is to run the calculation at 50 hectares and scale the answer to the site area rather than feeding a tiny figure into a regression that was never fitted to it. If a hydrologist has not done that, the output can be wrong by a wide margin, and a reviewer who knows the method will spot it.

The three inputs that move the answer

Site area is the easy one, and the one most often argued over. Take it from the survey, state whether it is the red line or the contributing catchment, and say what you excluded.

The soil index matters more than anything else in the calculation. The regression weights it heavily, raised to a power above two, so moving from a freely draining soil class to a slowly permeable one changes the answer more than doubling the site would. Take the class from the published soil data for the location rather than from an infiltration test, because the two describe different things. Infiltration testing to BRE Digest 365 tells you whether soakaways will work. The soil index tells you what the rural catchment would have shed.

Standard average annual rainfall is taken from the 1961 to 1990 baseline series for the grid reference. It is a fixed historical input, so climate change allowances are applied to the proposed rainfall events, not to the greenfield baseline. Mixing those two up produces a limit that is too generous and a design that is too small.

Minimum practicable discharge rates

A flow control sized to pass two or three litres per second needs an orifice small enough to block on a leaf. That physical limit is why lead local flood authorities apply a floor to the discharge rate, commonly quoted somewhere between 2 and 5 litres per second, and why some set it per hectare instead. The figure is local policy rather than national standard, so check the authority's published surface water guidance before you design to it.

Where the calculated greenfield rate falls below that floor, the honest approach is to design to the floor, say so in the strategy, and show the attenuation volume both ways. Some authorities will then ask for long term storage or a longer half drain time as the trade-off. Illustrative example: a 1.8 hectare site on a slowly permeable clay soil can produce a 1 in 1 year greenfield rate of only a few litres per second, below any practical orifice size, so the design settles at the authority's stated minimum and the volume is sized accordingly.

Previously developed sites and betterment

The standards treat brownfield differently. Peak run-off has to be as close as reasonably practicable to the greenfield rate, and it must never be worse than the existing rate of discharge. That second limb is the one that needs evidence, because an existing connection you cannot prove is an existing connection the undertaker will discount.

Useful evidence includes the sewerage undertaker's asset records, a CCTV survey of the outfall, historic drainage layouts and a measured impermeable area take-off from the pre-development survey. Many authorities ask for a stated percentage reduction against that baseline, and some publish the figure in their local standards. Where the site has no positive connection at all, the greenfield rate is the only sensible starting point, whatever the site looked like before.

For a scheme that also sits in Flood Zone 2 or 3, the discharge limit sits inside the wider case made in a Flood Risk Assessment, and the two documents need to use the same numbers. Reviewers do check.

How the authority sets the limit

The lead local flood authority is the statutory consultee for surface water on major development, and its comments usually arrive as a design standard rather than an objection. Many publish a local SuDS guide setting the discharge limit, the minimum rate, the urban creep uplift and the rainfall events they want modelled.

Separately, the sewerage undertaker controls connection to a public sewer under the Water Industry Act 1991, and it can set its own conditions on rate and location. Those two limits are not always the same. Where they differ, design to the tighter one and explain the conflict in the Drainage Strategy rather than leaving the case officer to reconcile them.

Recording the method so it can be audited

A drainage strategy that states a rate without showing its working invites a holding objection. Set out the method by name, the software or tool used, each input with its source and date, and a short table of Q1, Q30 and Q100 alongside the proposed peak outflows for the same events. Add the urban creep allowance you applied and where the figure came from.

Then say plainly what is still unknown. If the soil index has been read from a map rather than confirmed on site, that belongs in the report, not in a reviewer's first question.

PlanScope drafts a Drainage Strategy from your site data and guided answers, and flags site-specific figures such as the greenfield rate as still to be confirmed rather than filling them in. Hydrological calculations need a suitably qualified author, and for specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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