What is a Attenuation Tank

An attenuation tank is a key component in modern drainage and flood management systems, designed to temporarily store excess surface water runoff during periods of intense or prolonged rainfall. These tanks help mitigate the risk of sewer overload and localised flooding by capturing and holding large volumes of water, and then gradually releasing it into the drainage network at a controlled rate.

Used extensively in sustainable drainage systems (SuDS), attenuation tanks provide a passive and efficient means of managing stormwater in urban environments, where increasing impermeable surfaces such as roads, pavements, and rooftops limit the ability of rainwater to naturally infiltrate into the ground. Their application is particularly relevant in new developments, redevelopment schemes, and flood-sensitive zones where traditional drainage systems are insufficient to cope with peak flows.

The Function of Attenuation Tanks

Attenuation tanks are essentially underground storage chambers that intercept rainwater collected from roofs, car parks, and other hard surfaces. When rainfall intensity exceeds the capacity of the downstream drainage system, the attenuation tank temporarily retains the excess volume. The stored water is then discharged at a controlled rate—typically through a flow control device such as a hydrobrake or orifice plate—into the receiving sewer, watercourse, or treatment system once the capacity becomes available.

This staged release reduces the risk of surcharging in sewers, minimises flood hazards on-site and downstream, and promotes compliance with planning and environmental regulations aimed at sustainable water management.

The typical operation of an attenuation tank follows these phases:

  • Influx: During a rainfall event, surface water flows into the tank via gullies, channels, or downpipes. 
  • Storage: The tank retains the incoming water, preventing it from entering the main drainage system all at once. 
  • Release: A flow control chamber regulates the discharge of water at a pre-determined maximum flow rate. 
  • Emptying: The tank empties gradually, resetting its storage capacity for the next storm event. 

Depending on the scale of the system and site-specific requirements, attenuation tanks can be sized to accommodate anything from a minor rainfall event to a once-in-a-century storm.

Design and Construction

Attenuation tanks come in various forms and materials, allowing them to be tailored to the needs of a specific project. Their design depends on several key factors, including the anticipated volume of runoff, the rate of discharge required, site layout, soil conditions, and the available installation depth.

Common types of attenuation tanks include:

  1. Modular Cellular Systems
     These are lightweight, interlocking plastic crates or cells enclosed in a geotextile or impermeable membrane. They can be stacked or laid out in different configurations to match the site footprint and depth constraints. These systems are quick to install and ideal for confined or irregular spaces. 
  2. Concrete or Precast Chambers
     For sites with high load requirements (such as highways or industrial estates), precast concrete tanks offer durability and structural integrity. These are often installed as a series of chambers with integrated access points for maintenance. 
  3. Large-diameter Pipes or Culverts
     In some cases, oversized drainage pipes made of plastic or concrete are used to provide inline attenuation storage. These systems are suitable for linear installations along roads or site perimeters. 
  4. Stormwater Geocellular Units
     Similar to modular systems, these offer high storage volumes with a minimal footprint and are commonly used under car parks or landscaped areas. 

Materials must be resistant to corrosion, silt accumulation, and potential root ingress. The tank must also be adequately vented and designed to allow for inspection and cleaning.

Integration with Flow Control Systems

A vital feature of any attenuation tank is the flow control mechanism that governs the release of stored water. Without this, the tank would simply act as a temporary holding vessel with little impact on flood prevention. Flow control devices are installed at the outlet of the tank and sized to match the allowable discharge rate, which is often stipulated by local water authorities or planning regulations.

Common types of flow control systems include:

  • Hydrobrakes: Vortex flow control devices that restrict discharge without moving parts, using fluid dynamics to maintain a constant flow rate. 
  • Orifice plates: Simple, fixed-size openings that allow water to exit the tank at a set maximum rate. 
  • Penstocks: Manually or automatically controlled gates that can be used to isolate the tank for maintenance or emergency conditions. 
  • Smart flow control systems: In advanced installations, these use telemetry and sensors to dynamically manage outflows based on weather forecasts and real-time drainage conditions. 

The flow control system is typically housed in a dedicated chamber adjacent to or within the attenuation structure itself. It must be accessible for routine inspection and capable of operating reliably under variable hydraulic conditions.

Applications and Planning Requirements

Attenuation tanks are increasingly a requirement in planning permissions, especially in areas subject to strict runoff control policies. Developers are often required to demonstrate that new buildings or infrastructure will not increase the volume or rate of water discharged into existing drainage networks.

Typical applications include:

  • Residential developments: Managing runoff from driveways, roofs, and communal spaces. 
  • Commercial and industrial premises: Handling large roof areas and extensive paved surfaces. 
  • Retail parks and supermarkets: Where significant volumes of surface water are generated by car parks. 
  • Road schemes and transport infrastructure: Preventing highway flooding and protecting nearby watercourses. 
  • Retrofit SuDS: Adding resilience to existing drainage networks in urban redevelopment projects. 

In most cases, attenuation systems are designed in conjunction with other SuDS components, such as permeable paving, swales, rain gardens, or green roofs. This holistic approach helps mimic natural hydrological processes, improves water quality, and supports biodiversity.

Maintenance and Operational Considerations

While attenuation tanks are passive systems, they still require periodic maintenance to ensure long-term performance. Debris, silt, and organic material can accumulate over time, reducing storage capacity and compromising flow control devices.

Typical maintenance tasks include:

  • Inspecting and cleaning inlet and outlet pipes. 
  • Removing sediment from the base of the tank or filter membranes. 
  • Checking the functionality of flow control units. 
  • Inspecting access chambers for blockages or damage. 

The frequency of maintenance will depend on the local environment, upstream catchment characteristics, and tank design. Systems installed under highways or in industrial settings may require more frequent attention due to the risk of contamination by oils, litter, or heavy particulates.

Proper maintenance must be planned at the design stage, with access chambers, inspection ports, and cleaning routes integrated into the system layout. Failure to maintain an attenuation tank can lead to reduced performance, flooding, and breach of planning or environmental regulations.

Environmental and Regulatory Context

Attenuation tanks are central to meeting sustainable development and water management goals. In the UK, their use aligns with the requirements of the Flood and Water Management Act 2010, Building Regulations (Part H), and guidance provided by organisations such as the Environment Agency and CIRIA (Construction Industry Research and Information Association).

Key regulatory objectives that attenuation tanks help meet include:

  • Limiting peak discharge to greenfield runoff rates. 
  • Reducing surface water pollution by allowing sediments to settle before discharge. 
  • Supporting climate resilience in urban drainage design. 
  • Protecting watercourses from erosion and ecological disruption. 

Moreover, many local authorities now require SuDS strategies to be embedded in planning applications, and attenuation tanks are often a cornerstone of such schemes. Their inclusion is seen not only as good practice but also as a necessary step in ensuring that developments are future-proofed against the increasing frequency of extreme weather events.

Conclusion

Attenuation tanks are an essential solution in the management of stormwater and flood risk in both new and existing developments. By capturing and controlling the release of excess runoff, they help reduce pressure on drainage networks, prevent property damage, and support sustainable land use planning.

For engineers, planners, and contractors, understanding how to specify, install, and maintain attenuation systems is key to delivering resilient infrastructure that meets modern environmental and regulatory standards. When integrated effectively with other SuDS elements, attenuation tanks provide a powerful tool for balancing development needs with responsible water management—protecting both built environments and the natural ecosystems they inhabit.