Greenfield run-off rate section of a Drainage Strategy

The greenfield run-off rate is the number that decides how much attenuation storage a site needs. Set the allowable discharge low and the storage volume rises, which on a tight plot decides whether a scheme fits. Set it without evidence and the lead local flood authority sends the strategy back.

For a self-builder or a small developer this section is short, and it is the part of the Drainage Strategy that gets read first.

This is general guidance rather than legal advice, and local standards vary, so check what your lead local flood authority publishes before designing to any figure here.

Where the run-off rate sits in the strategy

The strategy has a fixed logic. Establish the existing situation. Calculate the allowable discharge. Choose the point of discharge using the hierarchy. Size the attenuation for the design events. Show the exceedance routes. Set out maintenance for the lifetime of the development.

The allowable discharge sits at step two and constrains everything after it. Which is why the section belongs near the front, with the inputs stated so a reviewer can reproduce the arithmetic without asking.

Where the site also sits in Flood Zone 2 or 3, the drainage work runs alongside the Flood Risk Assessment rather than inside it, and the two documents have to use the same climate change assumptions.

Estimating QBAR and what you need to do it

QBAR is the mean annual maximum flood: broadly, the flow you would expect once a year on average from an undeveloped catchment of that size.

For small sites it is estimated from three inputs. The contributing area in hectares. The standard average annual rainfall for the location. And a soil index describing how readily the ground sheds water rather than absorbing it. Rates for other return periods, typically one year, thirty year and hundred year, are derived from QBAR using published growth factors.

Two things reviewers check immediately. Whether the area used is the developable area or the whole site, since they can differ substantially on a plot with retained woodland. And whether the soil index came from a published dataset or from an assumption. Name the source for each input, with the date you retrieved it.

Very small sites produce very small numbers. A quarter hectare plot can generate a calculated greenfield rate low enough that no practical flow control device could deliver it, which is why minimum rates exist.

An illustrative calculation for a small plot

The following is illustrative and shows the shape of the calculation rather than real values for any site.

Take a plot of 0.3 hectares, currently grassland, being developed for four houses with an impermeable area of about 0.14 hectares once roofs, drives and paths are counted. The greenfield calculation for the site produces a QBAR figure in the low single digits of litres per second, and the hundred year greenfield rate a little above it.

The authority applies a minimum practical discharge rate, which is higher than the calculated hundred year greenfield figure, so the control is set at that minimum and the strategy says so explicitly. The attenuation volume is then sized to hold the difference between the hundred year plus climate change inflow and that permitted outflow, across a range of storm durations, with the critical duration reported.

The lesson is procedural. State whether the design is limited by the calculated rate or by the minimum practical rate, because a reviewer who cannot tell will assume the higher figure was chosen for convenience.

Betterment on previously developed land

Brownfield sites start with impermeable area already draining somewhere, often to a combined sewer with no restriction at all.

The national non-statutory technical standards expect run-off from previously developed sites to be reduced to as close to the greenfield rate as reasonably practicable, and in any event not to exceed the existing rate. Many lead local flood authorities go further and publish a required percentage betterment.

Evidence the existing rate properly. Measure the impermeable area from a survey or recent aerial imagery rather than assuming the whole site drains. Identify the existing connection and its condition, and where the discharge currently goes. Sites where the existing positive drainage cannot be found at all are usually treated as greenfield, which is a considerably harder starting point.

Attenuation volume and the events you test

Storage is sized for the hundred year event with the applicable climate change allowance for peak rainfall intensity, taken from the current published tables for the management catchment.

Test a range of storm durations. The critical duration for a small site with limited storage is often short and intense, while for a larger attenuation basin it can be several hours. Report the critical duration and the volume it produces, rather than a single figure with no context.

Half drain time matters too, because a system that has not emptied before the next storm has no capacity. Many authorities expect the storage to half empty within a stated period, and the flow control device has to deliver that at low heads as well as at peak.

Then show what happens when the design event is exceeded. Exceedance routes drawn on a plan, directing overland flow away from buildings and towards open space, are expected on anything above a single dwelling.

The discharge hierarchy and infiltration evidence

The hierarchy is settled and reviewers apply it in order.

1. Infiltration to ground, where the ground and groundwater conditions allow.
2. Discharge to a watercourse, with the consent of the riparian owner or drainage board.
3. Discharge to a surface water sewer, with the undertaker's agreement.
4. Discharge to a combined sewer, only where nothing above is possible.

Each step down requires evidence that the step above was tested and rejected. For infiltration that means soakage testing to a recognised method, at the depth and location the system will occupy, with the groundwater level recorded and an unsaturated zone maintained beneath the base of the feature. A single trial pit in the corner of a site, dug in summer, is not evidence.

Infiltration is also unsuitable in some settings regardless of the test result: shallow groundwater, contaminated ground where mobilising pollutants is a risk, source protection zones, unstable ground and steep slopes. Say why infiltration was rejected rather than leaving it unexplained.

What the lead local flood authority comments on

The authority is a statutory consultee on major development and comments on many smaller schemes by arrangement. Its responses cluster around a short list: inputs not stated, minimum discharge rates ignored, climate change allowance out of date, no exceedance plan, no maintenance regime, and no evidence for infiltration.

Maintenance is the one applicants treat as an afterthought. The authority wants to know who owns each component, who cleans it, how often, and how that is secured for the lifetime of the development, whether by a management company, an adoption agreement or a private arrangement recorded in the deeds.

If flood risk tests also apply to the site, what the sequential and exception tests must prove sets out how the drainage evidence fits into the wider case.

Scope the drainage work before the layout is fixed, because attenuation needs space. You can enter your postcode on PlanScope to see which reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.