Greenfield runoff rate is the amount of rainwater that leaves a site as surface flow when it is undeveloped — and for most planning applications it is the number your drainage strategy has to match. Hit it, and your sustainable drainage design will usually satisfy the lead local flood authority. Miss it, and you risk a holding objection that stalls the whole application.
This guide explains what greenfield runoff rate means, how it is calculated, why the Non-Statutory Technical Standards for SuDS pin so much on it, and how to size a drainage strategy that stands up at planning.
What "greenfield runoff rate" actually means
When rain falls on a field, most of it soaks into the ground or is intercepted by vegetation; only a fraction runs off the surface into watercourses. When you cover that field with roofs, tarmac and paving, far more of the same rainfall runs off, and it runs off faster. Left unmanaged, that extra volume and speed is what overwhelms the drains and floods properties downstream.
The greenfield runoff rate — usually expressed in litres per second (l/s), often per hectare — is the benchmark of the natural, pre-developed condition. National policy asks new development to discharge surface water at a rate no greater than this, so the site behaves, from the drainage network's point of view, as if it had never been built on.
Why it matters for planning
The Non-Statutory Technical Standards for SuDS set the expectation that peak runoff rates and volumes from a developed site should, as far as reasonably practicable, not exceed the greenfield equivalent. For brownfield sites the target is a betterment on the existing (often much higher) rate — commonly a reduction of at least 20–50% depending on the authority.
Lead local flood authorities are statutory consultees on major applications, and increasingly comment on minor ones too. A flood risk assessment and drainage strategy that does not demonstrate greenfield (or better) discharge is one of the most common reasons for a surface-water drainage objection.
How the rate is calculated
Two established approaches dominate UK practice:
- IH124 and the FEH statistical method — industry-standard hydrological techniques that estimate runoff from catchment characteristics such as area, soil type (SOIL/HOST class) and rainfall.
- The rational method — sometimes used for very small sites, though it tends to be less favoured by reviewers for planning-grade work.
The key inputs are site area, the SOIL or hydrology-of-soil-types classification, and the standard-average-annual-rainfall (SAAR) for the location. From these the assessor derives the runoff for the critical return periods — typically the 1-in-1, 1-in-30 and 1-in-100 year events. A widely used convention is that where the calculated greenfield rate for a small site falls very low, a minimum practical discharge (often around 5 l/s per outfall, to avoid blockage) is applied.
From rate to drainage strategy
Once you know the permitted discharge rate, the drainage strategy has to store the difference between what falls and what is allowed to leave. That is where SuDS features — soakaways, permeable paving and attenuation — do their work. The design sequence is:
- Establish the greenfield (or brownfield betterment) discharge rate.
- Calculate the attenuation volume needed to hold back everything above that rate, up to the 1-in-100-year event plus a climate change allowance.
- Choose and size SuDS components following the drainage hierarchy — infiltration first, then discharge to watercourse, then to sewer as a last resort.
- Control the outfall with a flow-restricting device (a hydrobrake or orifice) set to the permitted rate.
The 2026 update to the flood map for planning, which added banded surface-water depth and climate change extents, gives assessors better data for sizing this storage — but the greenfield rate remains the target the storage is designed around.
Climate change and the future of SuDS standards
Attenuation volumes must include the current climate change allowance for peak rainfall intensity, which can add 40% or more to design storms depending on region and epoch. Looking ahead, the anticipated commencement of Schedule 3 of the Flood and Water Management Act 2010 would make SuDS approval a formal, standards-based process — reinforcing, not replacing, the greenfield-runoff principle.
How Fortress Associates can help
Our flood risk assessment service produces a site-specific FRA and SuDS drainage strategy for planning applications in England using live Environment Agency data, with the report ready in minutes and free of charge. It calculates greenfield (or brownfield betterment) runoff rates, sizes the attenuation your site needs, and sets out a drainage strategy to CIRIA C753 and the Non-Statutory Technical Standards. Explore our planning services or contact us to get started.
Sources & further reading
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