An attenuation tank is an underground structure that temporarily stores surface water run-off during heavy rainfall and releases it at a controlled rate. By holding back the peak flow, the tank stops a development from increasing flood risk downstream and allows the site to meet the discharge rate set by the planning authority, sewerage undertaker or lead local flood authority.
How an attenuation tank works
During a storm, run-off from roofs, roads and car parks enters the tank faster than the outlet allows it to leave. The surplus is stored, and a flow control device — typically a vortex flow control or an orifice plate — restricts the outflow to the permitted rate, which on greenfield sites is normally the greenfield run-off rate. When the storm has passed, the tank drains down and is ready for the next event. The process is called stormwater attenuation, and it is the storage element of most SuDS schemes where there is no space for a pond.
Attenuation tank design and sizing
The required storage volume is the difference between the inflow generated by the design storm and the volume the permitted outflow can discharge during the same storm. The main inputs are the contributing impermeable area, the allowable discharge rate, the design return period (commonly the 1 in 100-year event), and allowances for climate change and urban creep. The calculation is run in hydraulic modelling software across a range of storm durations to find the critical one. Structural design then accounts for cover depth, traffic loading, groundwater level and flotation, and the layout must provide access for inspection and silt removal.
Types of attenuation tank
| Type | Strengths | Typical use |
|---|---|---|
| Geocellular plastic crates | High void ratio, light, flexible layout | Shallow installations under landscaping and light-traffic areas |
| Corrugated steel pipe tanks | High structural capacity, deep or shallow cover, man-entry access | Large volumes, roads, heavily trafficked car parks, infrastructure |
| Concrete pipes and box sections | Rigid, durable | Highway drainage, sites with concrete supply chain |
| Oversized pipes | Simple, combines conveyance and storage | Linear sites such as roads and rail |
The choice is driven by volume, available depth, loading, groundwater and the maintenance regime the adopting body requires. See attenuation crates for a closer look at geocellular systems.
Commercial and residential attenuation tanks
On a single house or small residential plot, attenuation is usually a small crate or tank feeding a restricted outlet, or a soakaway where ground conditions allow. Residential estates and commercial developments — retail parks, logistics sites, industrial yards — have large impermeable areas and strict discharge limits, so storage volumes run to hundreds or thousands of cubic metres. At that scale the tank sits under car parks or service yards, must carry HGV loading, and needs inspection access that an adopting body will accept, which is why prefabricated steel commercial attenuation tanks are widely specified.
Attenuation tank vs soakaway
Both manage surface water, but in different ways. An attenuation tank stores water temporarily and releases it at a controlled rate to a sewer or watercourse. A soakaway lets water infiltrate into the surrounding ground. Infiltration sits higher in the drainage hierarchy, so a soakaway is preferred where permeability testing and groundwater levels allow; where they do not, an attenuation tank is the alternative. To preserve the design storage over the tank’s life, fit an upstream silt trap that intercepts sediment before it enters the system.
Related ViaCon solutions
ViaCon manufactures prefabricated corrugated steel attenuation tanks for infrastructure and commercial developments, delivered to site ready for fast, low-disruption installation. See also ViaCon Storm Solutions and our stormwater solutions range. Related glossary entries: balancing pond, vortex separator and attenuation crates.
Frequently asked questions about attenuation tank
What is an attenuation tank?
An attenuation tank is an underground storage structure that holds surface water run-off during heavy rain and releases it through a flow control at a restricted rate, so that a development does not increase flood risk downstream.
How big does an attenuation tank need to be?
The volume is the difference between the run-off generated by the design storm and what the permitted outflow can discharge during that storm. It depends on the impermeable area, the allowable discharge rate, the design return period, and the climate change and urban creep allowances. A drainage engineer calculates it with hydraulic modelling software against the local authority’s requirements.
What is the difference between an attenuation tank and a soakaway?
An attenuation tank stores water and releases it at a controlled rate to a sewer or watercourse. A soakaway lets water infiltrate into the surrounding ground. Soakaways need permeable soil and enough depth to groundwater; where infiltration testing fails, an attenuation tank is used instead.
Can an attenuation tank be installed under a road?
Yes. Tanks under roads and heavily trafficked car parks must be designed for the vehicle loads and cover depth. Corrugated steel tanks are often chosen in these locations because of their structural capacity at both shallow and deep cover.
Do attenuation tanks need maintenance?
Yes. Silt and debris reduce storage volume and can block the flow control. Tanks should have inspection access, an upstream silt trap and a maintenance plan covering regular inspection of inlets, the flow control and the tank interior, with silt removed when it accumulates.