SuDS Attenuation Calculator — Storage Volume & Crate Count

SuDS (Sustainable Drainage Systems) attenuation stores excess stormwater runoff and releases it slowly at greenfield runoff rates. This prevents flooding downstream by mimicking natural drainage. In England, SuDS are required for most new developments through the planning system (the National Planning Policy Framework and the non-statutory technical standards for SuDS). Schedule 3 of the Flood and Water Management Act 2010 — which would make SuDS mandatory via a separate SuDS approval body — was announced but had not been commenced in England as of 2026. Options include soakaways, permeable paving and swales.

Storage volume = Peak runoff VOLUME − Greenfield discharge VOLUME, where each volume is its flow rate multiplied by the storm duration. The peak runoff volume comes from the impermeable area multiplied by rainfall depth (intensity × duration) for the design storm; the greenfield discharge volume is the permitted greenfield rate × site area × duration. This calculator also credits any infiltration to pervious ground during the storm. Greenfield runoff rate is typically 2-5 l/s/ha for clay soils in the UK, and a safety factor of around 10% is commonly added. This matches the CIRIA C753 SuDS Manual simplified volumetric method.

Local authority SuDS policies typically require attenuation designed for the 1 in 100 year storm event plus a climate-change allowance. Since 10 May 2022 the Environment Agency replaced the old flat 40% national uplift with management-catchment peak-rainfall allowances that vary regionally (roughly +10% to +40%, given as a central 50th-percentile and an upper 95th-percentile figure for the 2050s and 2070s epochs) — so confirm the allowance for your catchment from the EA service rather than assuming 40%. Some authorities accept 1 in 30 year storage with safe exceedance routing for the larger 1 in 100 year plus climate-change event.

Attenuation crates (geocellular storage) are modular plastic units buried underground wrapped in geotextile membrane. They typically have a 95% void ratio, meaning 95% of the crate volume stores water. Common sizes are 1000mm x 500mm x 400mm. They are lightweight, stackable and can support traffic loading with appropriate cover depth.

Greenfield runoff rate is the rate at which rainwater naturally drains from undeveloped land. In England it is typically calculated using the IH124 or FEH statistical methods (Environment Agency report SC030219). For clay soils it is typically 2-5 l/s/ha. Most Lead Local Flood Authorities apply a minimum discharge of around 2 l/s/ha for low SOIL-class sites — and a practical minimum of about 5 l/s where small orifice outlets would otherwise block — so a very low calculated greenfield rate does not always translate into a smaller permitted discharge. Always confirm the controlling rate with your LLFA.