Greenfield runoff rate: what it means and how it is set

The greenfield runoff rate is the rate at which rainfall would leave a site as surface water if the site were undeveloped, expressed in litres per second and usually quoted per hectare. It is the benchmark a lead local flood authority uses when deciding how much surface water a development may discharge, and on most sites it is the number that sizes the attenuation.

It is an estimate, not a measurement. Nobody gauges the flow off a field before it is built on, so the rate is derived from catchment characteristics using a published method, and the method chosen changes the answer.

What the rate represents

The rate describes peak flow, not volume. For a given storm return period, it is the highest instantaneous discharge the undeveloped catchment would produce.

Three values are usually quoted together. QBAR is the mean annual flood flow, which is the baseline figure. The 1 in 1 year, 1 in 30 year and 1 in 100 year rates are then derived from QBAR using growth factors. The 1 in 100 year greenfield rate is often the limit applied to the 1 in 100 year plus climate change event from the developed site, which is why a scheme has to store a great deal of water to meet it.

The national technical standards for sustainable drainage expect the peak runoff rate from a greenfield site to be no higher than the greenfield rate for the same event. On a previously developed site the expectation is that the rate should be as close to greenfield as reasonably practicable, and in any case no worse than the existing rate.

Methods used to estimate it

Three approaches appear in submitted calculations, and an authority will ask which one was used.

The Institute of Hydrology Report 124 method is the standard approach for small catchments. It derives QBAR from the catchment area, the standard average annual rainfall for the location, and a soil index describing how readily the soil sheds water. For sites below about 50 ha it is the usual choice, with a minimum area applied because the method becomes unreliable on very small catchments.

The Flood Estimation Handbook statistical method is used on larger catchments and where a gauged record exists nearby. It gives a different answer from IH124 on the same site, sometimes materially.

Soil based approaches using published soil and hydrology classifications supply the runoff parameter that both methods need. The data source matters: a national soil map read at 1:250,000 is coarse, and where the result is marginal an authority may accept site specific infiltration data instead.

State the method, the inputs and the data source in the calculation sheet. An attenuation volume presented without the greenfield calculation behind it is the commonest reason a drainage submission goes back.

Why previously developed sites are treated differently

A site already covered in buildings and tarmac discharges at a rate far above greenfield, so applying the greenfield limit to a redevelopment would often make the scheme undeliverable.

The policy answer is betterment rather than compliance. Calculate the existing rate from the current impermeable area, calculate the greenfield rate as though the site were undeveloped, and then show how close the proposal gets to greenfield. Where greenfield cannot be achieved, explain why with reference to the physical constraints: available space for storage, depth to the outfall, ground conditions unsuitable for infiltration, basement or contamination constraints.

Authorities differ in what they will accept, and some set a fixed expectation such as a stated percentage reduction on the existing rate. The London Plan takes a stronger position, aiming at greenfield runoff on all development. Read the authority's own guidance and any surface water management plan before fixing the strategy, and treat consultee standing advice from planning consultees as the starting point rather than the whole answer.

Minimum discharge rates and blockage risk

There is a practical floor to how small a discharge can be controlled.

A flow control device set to a very low rate needs a small orifice, and a small orifice blocks. Most lead local flood authorities will not accept a controlled discharge below around 5 litres per second for that reason, and some will accept lower where a vortex or hydrobrake style device with a larger throat is specified and a maintenance regime is secured.

That floor has a consequence on small sites. On a plot of a quarter of a hectare the calculated greenfield rate may be well below the minimum the authority will permit, so the control is set at the minimum and the attenuation is sized accordingly. Say so in the calculation rather than presenting an unachievable figure.

An illustrative calculation for a one hectare site

The sequence below is illustrative and the arithmetic should be run with the actual site inputs.

1. Establish the catchment area in hectares, taken from the red line or from the drained area where those differ.
2. Obtain the standard average annual rainfall for the location from the published dataset.
3. Obtain the soil runoff parameter for the site from the national soil and hydrology classification.
4. Calculate QBAR using the IH124 relationship, which combines the three inputs above.
5. Apply the growth factors for the region to derive the 1 in 1, 1 in 30 and 1 in 100 year rates.
6. Compare the 1 in 100 year greenfield rate with the achievable controlled discharge, and apply the minimum rate where the calculated figure falls below it.
7. Size the attenuation for the critical duration event at the 1 in 100 year plus climate change allowance, discharging at the agreed rate.
8. Add an allowance for urban creep on residential schemes, since paving of gardens over time increases the impermeable area.

For a lowland English hectare the 1 in 100 year greenfield rate commonly comes out at a single figure number of litres per second, which is why attenuation volumes on even modest sites run to hundreds of cubic metres. The climate change allowance is revised periodically, so take the current percentage from the published allowances rather than from an earlier report.

Evidence the authority asks for

A drainage submission that gets approved first time contains the calculation and its workings, not just the conclusion. A Drainage Strategy should include:

• The greenfield rate calculation with the method named, each input stated and its source identified.
• The existing rate calculation for a previously developed site, with the impermeable area measured and shown on a plan.
• Infiltration test results to the relevant standard where infiltration is proposed, including failed tests.
• Attenuation volume calculations for the critical duration, with the climate change allowance and the urban creep allowance stated separately.
• The flow control device type and its set rate, with the minimum rate position explained.
• Exceedance flow routing for events beyond the design standard, shown on a drawing.
• A maintenance plan with the responsible party named for the lifetime of the development.

Where the site is in Flood Zone 2 or 3, or is one hectare or more, a Flood Risk Assessment carries this material alongside the flood risk case. Drainage conditions are also among those most often varied later, and section 73 variation of planning conditions explains that route where a scheme changes.

Drainage design needs a suitably qualified author, usually a civil or water engineer, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which reports your site is likely to need.