A worked greenfield runoff rate calculation

The greenfield runoff rate is the flow that would leave your site if it had never been built on, and it is the number your drainage design has to hit. Discharge from a new development is normally limited to that rate, so everything else in the strategy, the size of the tank, the orifice on the outfall, the depth of the basin, follows from a single figure in litres per second. Work it out early. It decides whether a scheme has room for the drainage it needs.

The method below is the one most consultants use for small sites, with an illustrative worked example on a half hectare plot.

Why the greenfield rate sets the discharge limit

National policy expects development not to increase flood risk elsewhere, and the Defra non-statutory technical standards for sustainable drainage translate that into a rate: for greenfield sites the peak runoff leaving the development should not exceed the greenfield rate for the same event, and on previously developed land the discharge should be reduced as far as reasonably practicable.

Lead local flood authorities apply those standards through consultation responses and conditions. They are the statutory consultee for surface water drainage on major applications, and many authorities comment on minor schemes as well.

The county council or unitary authority holds that role, which on a two-tier area means the drainage comments come from a different body than the one determining the application. Allow for the extra week or two that adds.

The rate is not a formality. A 5 litre per second limit on a site with 2,500 m2 of roof and hardstanding means real storage volume somewhere, and there is no point discovering that after the layout is fixed.

What the calculation needs

For a site too small for a gauged flow record, the standard estimate comes from the Institute of Hydrology Report 124 method, which predicts the mean annual flood, usually written as QBAR, from three inputs.

1. Catchment area in square kilometres. The method was derived for catchments of 0.5 km2 and above, so for a small site the convention is to calculate at 50 hectares and scale the answer to the site area.
2. Standard average annual rainfall, in millimetres, for the site location.
3. A soil index derived from the hydrology of soil types mapping, running from 0.15 on freely draining soils to 0.50 on the least permeable.

The equation is QBAR = 0.00108 x AREA^0.89 x SAAR^1.17 x SOIL^2.17, giving a result in cubic metres per second. Rainfall and soil values come from published datasets rather than judgement, and you should state the source of each in the report.

A worked example on a half hectare plot

Illustrative example. A 0.5 hectare greenfield plot for four houses, with an assumed standard average annual rainfall of 700 mm and an assumed soil index of 0.45. Both inputs are assumptions for the purpose of showing the arithmetic, not values for any real location.

Calculating at the 50 hectare minimum, so AREA is 0.5 km2:

• 0.5^0.89 = 0.540
• 700^1.17 = 2,130
• 0.45^2.17 = 0.177
• QBAR = 0.00108 x 0.540 x 2,130 x 0.177 = 0.220 m3/s, or 220 litres per second

Divide by 50 hectares and the rate is 4.4 litres per second per hectare. For the 0.5 hectare site, that is 2.2 litres per second.

Which immediately raises the practical problem. A flow control device passing 2.2 litres per second needs a very small opening, and small openings block. Designers commonly adopt a minimum of around 5 litres per second for that reason, using a vortex control rather than a plain orifice, and lead local flood authorities usually accept the argument where it is made openly. Some will not. Ask before you design.

Turning the rate into attenuation volume

Storage volume is the difference between what falls on the impermeable area during the design event and what the outfall can release in the same period.

Illustrative continuation of the same example. Assume 2,500 m2 of impermeable area and an assumed rainfall depth of 60 mm over the critical six hour duration, including a climate change uplift.

• Runoff generated: 2,500 m2 x 0.060 m = 150 m3
• Released at 5 litres per second over six hours: 5 x 3,600 x 6 / 1,000 = 108 m3
• Storage required: 150 minus 108 = 42 m3

That arithmetic is a sanity check, not a design. Real sizing runs a range of storm durations to find the critical one, applies the current Environment Agency peak rainfall allowance for the relevant river basin district, epoch and percentile, and accounts for half drain-down times and for exceedance routing above the design event. Take the allowances from the published tables on gov.uk at the time you design, because they are revised.

Betterment on previously developed sites

Brownfield sites rarely discharge at greenfield rates without demolishing more than the scheme requires, so the standards ask for as close to greenfield as is reasonably practicable, with a reduction against the existing rate as the fallback.

Evidence the existing rate properly. Measure the impermeable area from a survey rather than from an aerial photograph, identify where each area currently drains, and be honest about roofs that already discharge to soakaway. Authorities have seen the trick of counting a whole site as impermeable to inflate the baseline.

Many authorities look for a stated percentage reduction. Others accept greenfield equivalence only. The requirement sits in the local plan or the authority's drainage guidance, so read it before you set the target.

One more point on brownfield sites. Where the existing drainage discharges to a combined sewer, the water undertaker may want the surface water taken out of that system entirely as a condition of any new connection, which changes the design rather than the rate. Ask the undertaker for a pre-development enquiry response early, because the answer can move an outfall to the far side of a site.

What the lead local flood authority checks

Reviewers look at a short list, and failing any item usually brings a holding objection.

• Whether the runoff calculation states its inputs and their sources
• Whether the discharge hierarchy was followed, testing infiltration first, then watercourse, then sewer
• Whether infiltration was tested to a recognised method and the results reported, including failures
• Whether the storage volume covers the 1 in 100 year event plus the current climate change allowance
• Whether exceedance flows above that event are shown routed away from buildings
• Who will maintain the system, and how it will be funded over its design life

Put the calculation in the Drainage Strategy rather than burying it in an appendix, and cross-refer it from the Flood Risk Assessment where one is required. If the two documents state different discharge rates, and they often do, the objection writes itself.

And keep the working visible. A reviewer who can follow the arithmetic will argue about the inputs. A reviewer who cannot will ask for the whole thing again.

The commonest avoidable failure is a mismatch between the drawn layout and the calculated volume. If the tank on the plan holds 30 m3 and the calculation asks for 42 m3, nobody reads any further.

This is general guidance, not legal advice. Check the position with your lead local flood authority or a qualified drainage engineer before relying on it, and note that for specialist technical reports, qualified professional input may be required.

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