Greenfield run-off rate: the drainage design benchmark

The greenfield run-off rate is the rate at which rainfall would leave your site if the site had never been built on. It is the benchmark every surface water drainage design is measured against, and on most schemes it is the number that decides how much attenuation storage you have to fit under the garden.

For a self-builder the practical version is blunter. Work out the greenfield rate early, because it sets the size of the tank, and the tank has to go somewhere.

What the rate means and why it is used

Undeveloped land absorbs, holds and slowly releases rainfall. Roofs, driveways and roads do none of that, so a developed site sheds water faster and in larger volumes. Left unmanaged, that difference is what floods the road at the bottom of the hill.

Drainage policy responds by capping discharge. The national non-statutory technical standards for sustainable drainage set the expectation that the peak run-off rate from a developed site should not exceed the greenfield rate for the same event, both for the 1 in 1 year storm and for the 1 in 100 year storm. Everything the design does, permeable paving, swales, basins, cellular storage, exists to hold water back so the outflow stays under that cap.

The rate is expressed in litres per second, usually litres per second per hectare for comparison between sites.

QBAR and the return periods that matter

The starting figure is QBAR, the mean annual flood flow: the flow expected on average once a year from a catchment of the given characteristics. It is estimated using published hydrological methods rather than measured, because nobody gauges a half-hectare paddock.

Three inputs drive the estimate: the catchment area, the standard average annual rainfall for the location, and the soil or hydrology of soil types classification, which describes how readily the ground sheds water. A clay site in the wet north-west produces a very different QBAR from a chalk site in Cambridgeshire of the same size.

From QBAR the design derives rates for the return periods the standards ask about, typically the 1 in 1 year, 1 in 30 year and 1 in 100 year events. Those three appear in almost every drainage strategy, because the 1 in 1 controls the frequent nuisance flows, the 1 in 30 tests that the network does not surcharge, and the 1 in 100 plus a climate change allowance tests that flooding is contained on site.

Small sites have a practical floor. Below roughly 5 litres per second, flow control devices become prone to blockage, and many lead local flood authorities set a minimum discharge rate for that reason. If your calculated greenfield rate is 1.2 litres per second, ask the authority what minimum it will accept before designing storage for a rate no orifice can reliably deliver.

Brownfield sites and the betterment expectation

Previously developed land already sheds water quickly, so the existing rate is not the greenfield rate. The standards ask for run-off to be reduced as close as reasonably practicable to the greenfield rate for the same event.

In practice authorities look for a stated reduction against the existing positively drained rate. Where the site drains to a public sewer, the water company may have its own expectation, and a reduction against the existing connection is often the pragmatic outcome on a tight urban plot.

Two evidential points make that argument work. Establish the existing impermeable area from survey rather than assumption, because an overstated baseline invites a challenge. And establish that the existing drainage is real and connected, since a yard that has always soaked away does not give you a rate to reduce.

How the rate drives attenuation volume

The arithmetic is simple in principle. Storage volume is the difference between what falls on the site during the design storm and what the site is allowed to release during the same period.

Squeeze the release rate and the storage grows. That is why the discharge rate is the single number with the most influence on the drainage budget, and why arguing it with the flood authority early is worth more than any refinement of the pipe network afterwards.

The design storm is the 1 in 100 year event with the appropriate climate change allowance applied to rainfall intensity, and the critical duration has to be found by testing a range rather than assumed. Short intense storms and long moderate ones produce different storage requirements, and the design has to work for whichever is worse.

Where the drainage evidence sits depends on the scheme. The distinction between what goes in a Flood Risk Assessment and what goes in a Drainage Strategy is worth getting right before either document is drafted.

Who checks the figures

For major development the lead local flood authority is a statutory consultee on surface water drainage, and its comments carry real weight with the case officer. For minor schemes the council may consult it informally or rely on its own drainage officer.

What they check, in roughly this order: the greenfield rate calculation and its inputs, the proposed discharge rate and the flow control device, the storage volume and the storm durations tested, the outfall and whether it is consented, exceedance routes for storms larger than the design event, and the maintenance arrangements over the lifetime of the development.

Outfall consent is the item most often missing. Discharging to a watercourse needs the consent of the riparian owner and, where there is a structure in an ordinary watercourse, land drainage consent from the flood authority. Discharging to a public sewer needs the water company's agreement, and a surface water connection is not automatic.

An illustrative calculation for a small site

The figures below are illustrative and assumed for the purpose of showing the method. They are not a design for any real site.

Take a plot of 0.4 ha, of which 0.15 ha will be roof, drive and path after development. Suppose the greenfield rate for the 1 in 100 year event has been calculated at 4 litres per second per hectare, giving 1.6 litres per second for the whole plot, and the flood authority applies a 5 litres per second minimum so the allowable discharge is 5 litres per second.

For a 6 hour design storm producing 60 mm of rainfall on the impermeable area, the volume arriving is 0.15 ha, or 1,500 m², multiplied by 0.06 m, which is 90 m³. Over those 6 hours the site may release 5 litres per second, which is 5 × 3,600 × 6 divided by 1,000, or 108 m³. On that duration nothing needs storing.

Now test a 15 minute storm producing 20 mm. The volume arriving is 30 m³ and the permitted release over 15 minutes is about 4.5 m³, so roughly 25 m³ has to be held. Test the range of durations and the largest of those residuals is the storage you build.

That is the whole logic, and it is why the flow control rate matters more than anything else in the design. The Drainage Strategy sets out the calculations and the maintenance plan for the council, while a Flood Risk Assessment deals with flooding to and from the site. Enter your postcode on PlanScope to see which of the two 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.