Greenfield runoff rates and the methods used to derive them

The greenfield runoff rate calculation sets the ceiling on what your site is allowed to discharge. Every attenuation volume, every flow control device and most of the drainage cost on a development follows from that one number, and it is agreed with the lead local flood authority rather than chosen by the designer.

The principle is straightforward. Surface water leaving a developed site should not leave faster than it would have done had the site remained undeveloped fields. Establishing that undeveloped rate on a site that has not been fields for a century, or that covers two hectares when the published methods were derived from catchments of several square kilometres, is where the difficulty starts.

Why greenfield rate is the benchmark

The non-statutory technical standards for sustainable drainage systems set the expectation. For greenfield sites, the peak runoff rate to any receiving watercourse must not exceed the greenfield rate for the same rainfall event, assessed for the 1 in 1 year and the 1 in 100 year events. For previously developed sites, the rate must be as close as reasonably practicable to the greenfield rate and must not exceed the rate for the existing developed site.

Those two sentences carry most of the design. The 1 in 1 year event governs everyday performance and the behaviour of the flow control. The 1 in 100 year event, with a climate change allowance applied, governs storage volume.

Local policy sits on top. Many lead local flood authorities require a stated reduction below the existing rate on brownfield land, and some apply a minimum standard of betterment regardless. Check the authority's own published standards before assuming the national position is the whole test.

The estimation methods

Three approaches appear in practice.

IH124 remains the workhorse. It estimates the mean annual flood, usually written QBAR, from the catchment area, the standard average annual rainfall and a soil index derived from national soil mapping. Rates for other return periods follow by applying regional growth factors to QBAR.

The method was developed from gauged catchments substantially larger than a housing site. Applying it to a 1.5 hectare plot is extrapolation, and every party to the discussion knows it.

Catchment descriptors and where they come from

Three inputs dominate.

Catchment area, which for a development site is the area draining to the discharge point, not the red line boundary. Get this right first, because it is the input most often taken carelessly and it scales the whole answer.

Standard average annual rainfall, taken from national datasets for the site's location. This varies enormously across England and is the reason an identical site produces different rates in Cumbria and in Essex.

The soil index, derived from national soil and hydrology mapping, which reflects how readily the soil sheds water. Where the mapping disagrees with what the site investigation found, say so and explain which you have used and why. Infiltration testing on the site is relevant evidence here.

Record the source and date of every descriptor in the appendices. A drainage consultee who cannot reproduce your inputs will not accept your output.

Small site limitations

This is the practical heart of the subject. The published methods were not derived from sites of the size most developments occupy, and applying them directly to a very small area produces rates so low that no flow control can reliably deliver them.

The convention that has emerged is to calculate the rate for a nominal minimum area, commonly 50 hectares, and then scale the result to the actual site area. It is a pragmatic fix rather than a scientific one, and different authorities apply it differently. Ask the lead local flood authority which convention it expects before you model anything.

Be explicit in the report about which convention you used. A drainage strategy that quietly applies a scaling rule without naming it invites a challenge, and the challenge usually arrives late.

Minimum discharge rates and blockage

A calculated greenfield rate of 0.8 litres per second on a small site is arithmetically fine and physically impractical. Delivering it needs an orifice small enough to block on a leaf.

Authorities therefore usually accept a minimum controlled discharge, commonly around 5 litres per second per outfall, on the basis that anything smaller cannot be maintained reliably. Vortex flow controls can hold lower rates with a larger aperture and are widely used where the calculated rate is very low, at the cost of a more complex device and a maintenance obligation.

Whichever route, the flow control needs a maintenance regime attached to it and somebody to carry it out. Long-term responsibility for these assets is a planning question in its own right, and our note on SuDS adoption and maintenance sets out who ends up holding it.

Betterment on previously developed land

On brownfield sites the existing rate is usually far above greenfield, sometimes by an order of magnitude, because the site is largely impermeable and drains straight to a sewer.

The national standard asks for a rate as close as reasonably practicable to greenfield. In practice authorities negotiate a percentage reduction from the existing rate, and the number depends on the site constraints, the receiving system's capacity and how much space is available for storage.

Evidence the existing rate properly. That means the impermeable area from a measured survey rather than an estimate, the existing drainage arrangement including any positive connection to a sewer, and the capacity of that connection. An assumed existing rate based on the whole site being impermeable will be challenged, and rightly.

Agreeing the rate with the consultee

Do this early, in writing, and before the layout is fixed.

1. Read the lead local flood authority's published standards and any local guidance on methods and minimum rates.
2. Establish the discharge point and the receiving system, and confirm the water company or riparian owner position on a connection.
3. Prepare the calculation with inputs and sources shown, and the convention used for small sites named.
4. Submit it at pre-application rather than with the planning application, and ask for confirmation of the rate in writing.
5. Feed the confirmed rate into the storage design, then check that the storage fits the layout before the layout is agreed.

The rate feeds directly into the Drainage Strategy and, where the site is in a flood zone or exceeds one hectare, into the Flood Risk Assessment as well. Both documents should quote the same figures and reference the same calculation. The wider expectations of the drainage consultee are set out in our guide to what the lead local flood authority checks.

One honest uncertainty: Schedule 3 of the Flood and Water Management Act 2010, which would introduce a separate SuDS approval regime in England, has been subject to repeated statements about implementation. Until it takes effect the planning route remains the mechanism, and the current position is worth checking rather than assumed, as covered in our note on where Schedule 3 SuDS approval stands.

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