Stormwater Attenuation — StormWater Solutions

Stormwater Attenuation is the process of temporarily storing surface water run-off during a storm event and releasing it at a controlled, reduced rate. The purpose is to limit the peak discharge to a downstream sewer or watercourse — typically the greenfield run-off rate — and so prevent flooding.

Why attenuation is required

Hard surfaces shed water far faster than greenfield land. A typical commercial site can produce ten or more times the peak run-off of the undeveloped land it replaces. Without attenuation, this peak overwhelms downstream infrastructure, contributing to flooding.

UK planning policy — supported by the Non-Statutory Technical Standards for SuDS and the SuDS Manual (CIRIA C753) — requires new developments to limit run-off rates to the greenfield equivalent. On brownfield sites, betterment of typically 30–50% versus pre-development is normally required.

Attenuation methods in the UK

Attenuation can be delivered through above-ground or below-ground storage:

• Attenuation ponds and basins — above-ground depressions that fill during storms, deliver multiple SuDS pillars including amenity and biodiversity.
• Underground tanks — corrugated steel pipe, geocellular crates or pre-cast concrete chambers below paved areas or open ground.
• Swales and detention channels — linear features combining conveyance and short-term storage.
• Oversized pipes — large-diameter pipework used as in-line storage where space is limited.
• Permeable pavement sub-base — the granular sub-base under permeable pavement contributes to attenuation volume.

Sizing the attenuation volume

Attenuation volume is calculated for the 1-in-100-year critical-duration storm with a climate change allowance — typically +40% in England under current Environment Agency guidance. The required volume equals the inflow volume minus the volume discharged at the permitted rate over the storm duration.

Sizing is normally performed in hydraulic modelling software (MicroDrainage, InfoDrainage, Causeway Flow) that runs the full set of design storm durations to find the critical event.

Related ViaCon solutions

ViaCon supplies attenuation tanks and the full ViaCon Storm Solutions range, including flood control systems and the broader stormwater solutions portfolio. Related glossary entries: attenuation tank, SuDS and flood risk management.

Frequently asked questions about stormwater attenuation

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