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.
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
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.
Frequently Asked Questions
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