What Is an Attenuation Tank?
An attenuation tank is an underground structure designed to temporarily store excess stormwater runoff during periods of heavy rainfall. By holding water and releasing it at a controlled rate, attenuation tanks prevent downstream flooding, reduce peak discharge, and help developments meet local drainage requirements.
How Does an Attenuation Tank Work?
During a storm event, rainwater enters the attenuation tank faster than it can be discharged into the existing drainage network or watercourse. The tank stores the surplus volume and uses a flow control device — typically a vortex flow control or orifice plate — to restrict the outflow to a pre-agreed rate, often matching the greenfield runoff rate for the site.
Once the storm passes, the tank gradually empties, ready for the next rainfall event. This process is known as stormwater attenuation, and it is a key requirement in most modern planning applications across the UK.
Where Are Attenuation Tanks Used?
Attenuation tanks are installed beneath car parks, roads, playing fields, and landscaped areas on residential, commercial, and infrastructure developments. They are particularly common where surface-level SuDS features such as swales or ponds are not feasible due to space constraints.
Typical applications include new housing developments required to limit discharge rates, commercial sites with large impermeable areas, road and highway drainage schemes, and retrofitted urban drainage where existing capacity is insufficient.
Materials and Construction
Modern attenuation tanks are typically constructed from modular geocellular crates, corrugated steel pipes, or reinforced concrete chambers. Modular plastic systems offer lightweight handling and flexible configurations, while corrugated steel pipe solutions provide high structural capacity and long design life — particularly suited for deeper installations or sites with heavy traffic loading.
Attenuation Tank vs Soakaway
While both manage stormwater, they serve different purposes. An attenuation tank stores water temporarily and releases it at a controlled rate into the drainage system. A soakaway allows water to infiltrate directly into the surrounding soil. The choice depends on ground conditions, permeability testing, and local authority requirements.