Attenuation Tanks, Attenuation Crates & Underground Stormwater Tanks, Turnkey Solutions

Attenuation Tank Guide: Design, Installation and Benefits for Sustainable Drainage Systems (SuDS)

What is an Attenuation Tank?

An attenuation tank is an underground stormwater storage system designed to temporarily store excess surface water runoff during periods of heavy rainfall. The stored water is then released at a controlled rate into a drainage network, watercourse, sewer system or infiltration area.

Attenuation tanks are a key component of Sustainable Urban Drainage Systems (SuDS) and play a critical role in reducing flood risk, managing stormwater runoff and helping developments comply with planning and environmental requirements.

Modern attenuation tanks are commonly constructed using geocellular storage crates, providing a lightweight, high-capacity solution for underground stormwater storage.

Why are Attenuation Tanks Required?

As urban development increases, natural ground surfaces are replaced with impermeable materials such as roads, car parks, roofs and paved areas. These surfaces prevent rainwater from infiltrating naturally into the ground, resulting in increased runoff volumes and higher peak flows.

Without adequate stormwater management, this can lead to:

• Localised flooding

• Sewer overloading

• Watercourse erosion

• Pollution of natural water bodies

• Increased flood risk downstream

Attenuation tanks help mitigate these issues by storing stormwater during peak rainfall events and releasing it gradually through a flow control device.

Many Local Authorities and Lead Local Flood Authorities (LLFAs) now require attenuation measures as part of planning approvals for residential, commercial and infrastructure developments.

How Does an Attenuation Tank Work?

An attenuation tank operates by collecting runoff from drainage systems connected to roofs, roads, car parks and hardstanding areas.

The process typically follows these stages:

Collection

Surface water enters the underground drainage system and is conveyed into the attenuation tank.

Storage

The attenuation tank temporarily stores excess runoff generated during rainfall events.

Flow Control

A vortex flow control or orifice plate restricts the discharge rate leaving the tank.

Controlled Discharge

Water is released gradually at a predetermined rate, often based on greenfield runoff rates or Local Authority requirements.

Event Recovery

Once the storm has passed, the tank empties and becomes available for the next rainfall event.

Types of Attenuation Tanks

Geocellular Attenuation Tanks

The most common solution is a geocellular attenuation tank constructed from modular plastic crates.

Advantages include:

• High void ratios typically exceeding 95%

• Lightweight construction

• Fast installation

• Flexible sizing

• Cost-effective transport

• Suitable for deep installations

Geocellular storage systems can be configured to suit almost any project footprint and storage requirement.

Concrete Attenuation Tanks

Concrete tanks offer excellent structural strength but are generally heavier and more expensive to install.

They are often used where:

• Exceptional loading conditions exist

• Deep burial depths are required

• Long-term structural durability is prioritised

Large Diameter Pipe Systems

Oversized pipes can sometimes provide attenuation storage where space constraints prevent the use of conventional crate systems.

What are Attenuation Crates?

Attenuation crates are modular geocellular units that form the structural framework of a geocellular attenuation tank.

The crates are assembled to create a large underground storage void capable of retaining significant stormwater volumes.

Key benefits include:

• High storage efficiency

• Easy transportation

• Rapid installation

• Flexible layouts

• Reduced excavation requirements

Modern attenuation crate systems can support substantial traffic loads when correctly designed and installed.

Attenuation Tank Design

Proper attenuation tank design is essential to ensure compliance with planning requirements and achieve long-term performance.

Design considerations include:

Storage Volume

The tank must be sized to accommodate the required design storm event.

This is typically determined using hydraulic modelling software such as:

• MicroDrainage

• Causeway Flow

• InfoDrainage

Inflow Rates

The design must account for peak runoff entering the system.

Discharge Rate

The allowable discharge rate is often dictated by:

• Lead Local Flood Authority requirements

• Water Authority requirements

• Greenfield runoff calculations

• Site-specific planning conditions

Ground Conditions

Ground investigations help determine:

• Infiltration rates

• Groundwater levels

• Soil conditions

• Excavation requirements

Structural Loading

The attenuation tank must withstand:

• Vehicle loading

• Burial depth pressures

• Construction traffic

• Ground loads

Attenuation Tanks and SuDS

Attenuation tanks are a fundamental component of Sustainable Urban Drainage Systems.

SuDS aim to replicate natural drainage processes while providing multiple environmental benefits.

Attenuation tanks contribute to:

• Surface water management

• Flood prevention

• Runoff reduction

• Water quality improvement

• Climate resilience

They are commonly used alongside:

• Permeable paving

• Swales

• Rain gardens

• Bioretention systems

• Filter drains

• Ponds

• Wetlands

Flow Control Devices

A flow control device is essential to the operation of any attenuation tank system.

The flow control regulates discharge rates and prevents excessive outflow during storm events.

Common options include:

Vortex Flow Controls

Vortex flow controls use hydraulic principles to restrict discharge without moving parts.

Benefits include:

• Reliable operation

• Low maintenance

• Self-cleansing characteristics

• Excellent hydraulic performance

Orifice Plates

Orifice plates provide a simple and economical solution for smaller applications.

Flow Control Chambers

Flow control devices are typically housed within dedicated flow control chambers or manholes for maintenance access.

Installation of Attenuation Tanks

Correct installation is critical to the long-term performance of an attenuation tank.

Typical installation stages include:

Excavation

The excavation is prepared to the required dimensions and depth.

Formation Preparation

A suitable formation level is established and compacted.

Geotextile Installation

A geotextile membrane may be installed where infiltration is required.

Geomembrane Installation

For attenuation applications, an impermeable geomembrane is often used to prevent water loss.

Crate Assembly

Geocellular crates are assembled and positioned.

Pipe Connections

Inlet, outlet and inspection connections are installed.

Wrapping and Sealing

The tank is wrapped and sealed to create a watertight storage system.

Backfilling

Specified backfill material is placed and compacted in accordance with manufacturer recommendations.

Maintenance Requirements

Attenuation tanks generally require minimal maintenance, but routine inspections remain important.

Maintenance activities may include:

• Inspecting flow control devices

• Checking catchpits and silt traps

• Removing accumulated debris

• CCTV surveys

• Monitoring discharge performance

Regular maintenance helps maximise service life and ensure compliance with drainage requirements.

Benefits of Attenuation Tanks

Attenuation tanks provide numerous benefits for developers, contractors and asset owners.

Flood Risk Reduction

Storage of stormwater runoff helps reduce downstream flooding.

Planning Compliance

Supports compliance with SuDS and Local Authority requirements.

Space Efficiency

Underground installation preserves valuable surface land.

Flexible Design

Suitable for residential, commercial, industrial and infrastructure projects.

Long Service Life

Modern systems offer excellent durability and reliability.

Environmental Benefits

Supports sustainable water management strategies and climate adaptation measures.

Typical Applications

Attenuation tanks are used across a wide range of sectors including:

• Housing developments

• Commercial developments

• Retail parks

• Industrial estates

• Schools and universities

• Hospitals

• Sports facilities

• Highway projects

• Infrastructure developments

• Logistics facilities

Choosing the Right Attenuation Tank Supplier

Selecting the correct attenuation tank supplier is crucial to project success.

Look for a supplier that offers:

• Technical design support

• Hydraulic expertise

• Flow control integration

• High-performance geocellular systems

• Installation guidance

• Nationwide delivery

• Competitive pricing

An experienced supplier can help optimise tank sizing, reduce installation costs and ensure compliance with planning and drainage requirements.

Conclusion

Attenuation tanks are a proven and effective solution for managing stormwater runoff and reducing flood risk. Whether constructed using geocellular attenuation crates, concrete structures or alternative storage systems, attenuation tanks form an essential component of modern Sustainable Urban Drainage Systems (SuDS).

By combining properly designed attenuation storage with flow control devices and robust drainage infrastructure, developers can achieve compliant, sustainable and cost-effective surface water management solutions for projects of all sizes.

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