An illustrative greenfield runoff calculation for a small site

A greenfield runoff rate calculation for a small site almost always lands below the rate any drainage engineer would let you build to. Work through the arithmetic for a third of a hectare and you get something near 1.5 litres per second, which is a hole a few millimetres across. That is why the answer on small sites is usually a discharge rate set by the lead local flood authority's minimum, with attenuation storage sized to hold the difference. The calculation still has to be presented, because the council needs to see that the rate you have used is justified rather than assumed.

What follows is one worked example, labelled illustrative throughout. The inputs are stated so you can see which numbers are site-specific and which are policy.

What greenfield runoff rate actually means

Greenfield runoff rate is the flow that would leave the site if it were undeveloped, and it is the benchmark against which a developed site's discharge is judged. Defra's non-statutory technical standards for sustainable drainage systems set the tests: for a greenfield site, the peak runoff rate for the 1 in 1 year event and the 1 in 100 year event should not exceed the rate for the undeveloped site for the same event. For previously developed land, the standard is betterment, with the discharge reduced as close to greenfield as is reasonably practicable.

The quantity most often quoted is QBAR, the mean annual flood flow. It sits roughly at the 1 in 2.3 year return period, and rates for other return periods are derived from it using growth factors. A Drainage Strategy will normally present QBAR, the 1 in 1 year rate, the 1 in 30 year rate and the 1 in 100 year rate, then state the proposed limiting discharge.

The inputs the calculation needs

Four things, and only four, drive the standard estimate.

1. Contributing area, in square kilometres, taken from the red line or the drained area, whichever the LLFA's standards specify.
2. SAAR, the standard average annual rainfall for 1961 to 1990 at the site, in millimetres.
3. SOIL index, derived from the HOST soil classification, running from 0.15 for the most permeable soils to 0.50 for the least.
4. The method itself. The IH124 regression is the long-standing approach for small rural catchments, and the reFH2 and FEH statistical methods are the current alternatives set out in CIRIA C753, The SuDS Manual.

The trap is scale. IH124 was derived from catchments of 0.5 km² and above, so applying it directly to a 0.35 hectare plot is outside the method's range. Standard practice is to calculate the rate for a 50 hectare catchment with the site's own SAAR and SOIL values, then scale the result pro rata by area. Say in the report that you have done this, and why.

An illustrative worked calculation

Take a site of 0.35 hectares, so 0.0035 km². Assume SAAR of 700 mm and a SOIL index of 0.45, both of which would in practice be read off the published soil and rainfall datasets for the actual grid reference. These figures are chosen to make the arithmetic visible and are not values for any real location.

The IH124 form is QBAR rural equals 0.00108 multiplied by area to the power 0.89, multiplied by SAAR to the power 1.17, multiplied by SOIL to the power 2.17, with area in square kilometres and the result in cubic metres per second.

Applied to a 0.5 km² catchment with those inputs, the result is 0.220 m³/s, which is 219.7 litres per second. Divide by 50 hectares and the rate is 4.39 litres per second per hectare. Multiply by the site's 0.35 hectares and the greenfield QBAR for the plot is 1.54 litres per second.

That number is the honest answer and it is also unbuildable. A flow control passing 1.5 l/s under a low head needs an orifice of a few millimetres, which blocks with silt, leaves and the contents of a builder's wheelbarrow.

Turning the rate into a storage volume

Storage volume is the gap between what arrives and what leaves during the critical event. The proper method routes a range of storm durations through the proposed system and reports the worst case. The arithmetic below shows the principle for a single duration.

Assume 1,800 m² of impermeable area on the 0.35 hectare plot, a runoff coefficient of 0.95, and a 1 in 100 year six-hour rainfall depth of 60 mm. That depth is an assumption made to demonstrate the sum, and a real assessment must use the depth-duration-frequency values for the site's own grid reference.

Add a 40 per cent uplift for climate change and the design depth becomes 84 mm. Runoff volume is 0.084 m multiplied by 1,800 m² multiplied by 0.95, giving 143.6 m³. If the outflow is held at 2 litres per second throughout the six hours, the volume leaving is 2 multiplied by 21,600 seconds divided by 1,000, so 43.2 m³. Storage required is 143.6 minus 43.2, which is 100.4 m³.

Raise the permitted discharge to 5 litres per second and the outflow over six hours becomes 108 m³, leaving 35.6 m³ of storage. The sensitivity of the storage volume to the discharge rate is the single most useful thing this exercise shows, and it is why the discharge rate is worth negotiating early.

Design events and the climate change uplift

Three return periods do the work. The 1 in 1 year event controls the frequency of discharge to the receiving system, the 1 in 30 year event is the point at which no surface flooding should occur within the site, and the 1 in 100 year event plus a climate change allowance is the volume the system must contain without flooding buildings or leaving the site uncontrolled.

The allowance itself changes. The Environment Agency publishes peak rainfall intensity allowances by management catchment and epoch, and they have been revised more than once. Do not carry a percentage forward from an old report. State the allowance you used, the epoch, the catchment and the date you checked the published table, and say in the report where the figure came from. The same discipline applies to peak river flow allowances if the site is also within Flood Zone 2 or 3.

Urban creep is a separate addition. Residential schemes are commonly asked to allow for future paving over of gardens, at a percentage set by the LLFA's local standards.

Where the lead local flood authority sets a floor

Most LLFAs publish local standards that override the arithmetic at the bottom end. The usual device is a minimum discharge rate per outfall, commonly 2 litres per second, sometimes 5, on the practical ground that smaller orifices and vortex controls silt up and are not maintainable. Some authorities instead set a floor of 2 litres per second per hectare, others accept the calculated greenfield rate but require a proprietary flow control with a maintainable minimum aperture.

Find the document before you design. It is usually a SuDS design guide or local flood risk management standards published by the county council or unitary authority, and it will also tell you the drainage hierarchy order, the discharge priority, the required infiltration test method and whether above-ground storage is expected. Where the proposed outfall is a public sewer rather than a watercourse, the water undertaker's own capacity position matters too, which is the territory of a sewer capacity enquiry.

Schedule 3 of the Flood and Water Management Act 2010, which would create statutory SuDS approving bodies in England, has not been brought into force. Check the current position before describing any approval route beyond the planning application and the LLFA's consultee role.

Presenting the calculation so it can be checked

A drainage consultee has minutes, not hours. Give them a single table with the site area, drained area, SAAR, SOIL index, method used, QBAR, the 1 in 1 and 1 in 100 year rates, the proposed limiting discharge and the storage volume. Then give the working, the software output and the reference for every input.

Three things get calculations sent back. Using the red line area when the LLFA asks for the positively drained area. Quoting a climate change allowance without naming the epoch and catchment. And presenting a single storm duration when the critical duration has not been tested. Above-ground storage also earns goodwill, because it is visible, maintainable and can double as planting, which links the drainage case to the amenity case made through the urban greening factor.

Write the maintenance regime while the numbers are fresh. Who inspects the flow control, how often, and how the silt gets out.

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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. 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.