What is SuDS attenuation?

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. Under Schedule 3 of the Flood and Water Management Act, most new UK developments must incorporate SuDS.

How is the storage volume calculated?

Storage volume = (Peak runoff - Greenfield discharge) x Storm duration. The peak runoff comes from the impermeable area multiplied by rainfall intensity for the design storm event. Greenfield runoff rate is typically 2-5 l/s/ha for clay soils in the UK. A safety factor of 10% is commonly added.

What storm return period should I design for?

Building Regulations and local authority SuDS policies typically require attenuation designed for a 1 in 100 year storm event plus a 40% allowance for climate change. Some local authorities may accept 1 in 30 year with exceedance routing for the 1 in 100 year + climate change event.

What are attenuation crates?

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.

What is the greenfield runoff rate?

Greenfield runoff rate is the rate at which rainwater naturally drains from undeveloped land. In England, this is typically calculated using IoH124 or FEH methods. For clay soils, it's typically 2-5 l/s/ha. The local lead flood authority (LLFA) may specify the required greenfield rate for your site.

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SuDS Attenuation Calculator — Storage Volume & Crate Count

Calculate attenuation storage volume for sustainable drainage systems using greenfield runoff rates and storm return periods.

Total Site Area

Total site area in square metres

Impermeable Area

Percentage of roofs, driveways, paving

Soil Type

Infiltration rate: 0.001 m/hr

Storm Return Period

Rainfall intensity: 91 mm/hr

Storm Duration

Typical critical duration 60 minutes

Greenfield Runoff Rate

From LLFA or IoH124 calculation

How We Calculate This

This calculator determines the attenuation storage volume needed to limit site discharge to greenfield runoff rates for a given storm event.

Core formula

Storage Volume = (Peak Runoff Volume - Greenfield Discharge Volume)
Where Peak Runoff = Impermeable Area x Rainfall Intensity x Duration, and Greenfield Discharge = Greenfield Rate x Duration.

Crate sizing

Crate count = Storage Volume / (Crate Volume x Void Ratio). Standard geocellular crates have a 95% void ratio. Excavation volume accounts for the crate volume plus surrounding gravel and membrane.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.