QBAR & Return-Period Rates • IH124 / FEH • UKSuDS

Greenfield Runoff Rate Calculations The Discharge Rate Your Drainage Must Hit

Need to restrict your discharge to greenfield rates? We calculate QBAR and the 1, 30 and 100-year runoff rates using IH124 and FEH methods — the foundation every LLFA-compliant drainage strategy is built on.

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QBAR & Q1/Q30/Q100
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Greenfield Runoff Rate Calculations

The greenfield runoff rate is the rate at which rainfall would leave your site in its undeveloped state — and it is the rate to which your post-development surface water discharge usually has to be restricted. Get it wrong and the whole drainage strategy is wrong: too generous and the LLFA objects, too tight and you over-provide expensive storage.

We calculate the mean annual flood (QBAR) and the 1 in 1, 1 in 30 and 1 in 100-year greenfield rates using the recognised IH124 and FEH statistical methods through UKSuDS, and set the limiting discharge rate for your drainage design. For brownfield redevelopment we establish the existing positive discharge and the betterment the LLFA will expect. It’s a quick, standalone piece of work — or the starting point of a full strategy.

What’s Included

  • ✓ QBAR (mean annual flood) — calculated for your site’s area, region and characteristics
  • ✓ Return-period rates — the 1, 30 and 100-year greenfield runoff rates
  • ✓ Method selection — IH124 or FEH statistical method, chosen and justified for your catchment
  • ✓ Limiting discharge rate — the rate your drainage design must be restricted to, ready for the LLFA
  • ✓ Brownfield betterment — existing runoff established and the required reduction set out for redevelopment sites
  • ✓ Clear calculation output — UKSuDS output and a short note suitable for submission or to feed your strategy
Greenfield site from the air Natural watercourse on greenfield land River flowing through countryside

When Do You Need Greenfield Runoff Rates?

Want to learn more first? Read our greenfield runoff guide, or get a quote.

Restricting Your Discharge

LLFAs require post-development discharge restricted to greenfield rates (or existing rates with betterment). The calculation sets that target.

Sizing Attenuation Storage

The runoff rate directly drives how much attenuation storage you need — get it right and you avoid over-building expensive crates or tanks.

Part of a Drainage Strategy

Every SuDS drainage strategy starts here; we can do it standalone or as the first step of the full job.

Brownfield Redevelopment

Establishing existing positive discharge and the reduction the LLFA expects on previously developed land.

Discharging a Condition

Conditions that fix discharge to greenfield rates need the calculation as evidence — see condition discharge.

Feasibility & Appraisal

Early-stage runoff figures to test whether a site can drain viably before you commit to a layout.

How Greenfield Run-off Rates Are Calculated

The benchmark every drainage strategy is designed to meet.

The greenfield run-off rate is the rate at which rainfall would leave your site if it were undeveloped grassland. Lead Local Flood Authorities require new development to discharge surface water no faster than this rate — so the figure sets the size of your attenuation storage and the setting of your flow control. Getting it right, and using the method the LLFA expects, avoids an under-sized (refused) or over-sized (expensive) drainage scheme.

QBar and the return periods

QBar is the mean annual flood flow for the site. From it we derive the greenfield rates for the 1 in 1, 1 in 30 and 1 in 100 year events — the figures used to set the discharge limits for each storm.

IH124 and FEH methods

For small sites the IH124 method is standard; larger or more complex sites use the FEH statistical approach. We apply the method appropriate to the site area and the LLFA's guidance, and calculate on the impermeable (contributing) area.

What it feeds into

The greenfield rate, together with the developed run-off, determines the attenuation volume required and the flow control setting — so it is the first calculation in any surface water drainage strategy.

Frequently Asked Questions

What is the greenfield runoff rate?
It’s the rate surface water would run off your site in its natural, undeveloped state, for a given storm return period. Post-development discharge is normally restricted to this rate so development doesn’t increase downstream flood risk.
IH124 or FEH — which method?
Both are accepted. IH124 is the long-standing method; the FEH statistical approach is often preferred for larger or well-gauged catchments. We select and justify the appropriate method for your site and the LLFA’s expectations.
What about brownfield sites?
Previously developed sites discharge from existing hard surfaces. The LLFA usually expects a reduction (betterment) towards greenfield. We establish the existing rate and the achievable reduction.
Is there a minimum discharge rate?
In practice very low rates risk blockage, so a sensible minimum (often around 1–5 l/s depending on the LLFA) is used. We set a rate that is both compliant and buildable.
How quickly can you provide it?
A standalone greenfield runoff calculation is typically delivered within a couple of working days. We’ll confirm a fixed fee within 24 hours.
What is QBar?
QBar is the median (mean annual) flood flow — the run-off rate a site would generate in a typical year in its undeveloped state. It is the base figure from which the 1, 30 and 100-year greenfield rates are derived and is usually quoted in litres per second per hectare.
Do I need a greenfield run-off calculation for a small site?
Yes, in most cases. Even a single new dwelling that increases impermeable area is expected to limit discharge to greenfield (or, on previously developed land, to a percentage betterment of the existing rate). The LLFA will ask for the calculation with the drainage strategy.
Why do LLFAs insist on the greenfield rate?
Because developing land replaces grass with roofs and paving, sending water into drains far faster than before. Limiting discharge to the greenfield rate stops new development increasing flood risk downstream — a core requirement of the NPPF and the National SuDS Standards.

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