How do Attenuation Tanks work?

FLI Precast
FLI Precast

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Flooding causes devastating effects on a global level each year.

It damages properties, businesses and affects many people’s livelihoods and well-being.

Not only does it cause huge damage from a monetary perspective, but it can also damage vegetation as pollutants can be carried by flood water.

As more and more developments across Ireland and the United Kingdom are built on green fields, it is essential that flood prevention measures are put in place and invariably form part of the planning process.

A greenfield site is somewhere that has not been built on previously. It is often agricultural land but can also be undeveloped land in a city or rural area.

With this newly built infrastructure, rainwater is then unable to drain into the ground as the land will now be covered by houses, buildings, roads/pavements, or car parks.

Local Authorities now have strict guidelines for developers wishing to build or construct new projects on existing non-developed land.

Stormwater can no longer be collected and discharged openly back into the existing drainage network. The flow of stormwater must therefore be managed and controlled.

Therefore, the flow of stormwater must be managed and controlled.

All large-scale developments and projects must incorporate a mechanism to contain stormwater on site and release it slowly back into the drainage network at an agreed rate.

In many cases, the best way to achieve flood prevention is to detain the excess runoff in subsurface stormwater attenuation tanks.

At FLI Precast Solutions, we specialise in stormwater management. Our precast stormwater Attenuation Tanks allow projects across Ireland and the UK to manage stormwater and implement flood prevention measures robustly and affordably.

We have proudly partnered with and worked alongside water authorities, highways, and local councils in both the public and private sectors for over 30 years.

What is an Attenuation Tank ?

Attenuation tanks are large container/detention tanks that sit underground and store excess rainwater. This removes the risk of flooding in a residential area in a controlled manner.

These Attenuation Tanks act as a buffer and store any excess rainwater.

This dramatically reduces the risk of residential flooding as excess stormwater such as heavy rain or melted snow is safely collected and stored in the attenuation tank below ground level.

This method of flood prevention is particularly popular among large residential developments. Although the use of such tanks is not restricted solely to the residential sector.

Why is using an Attenuation Tank important?

The use of Attenuation Tanks will dramatically reduce the risk of flooding. Precast Attenuation Tanks are particularly effective.

They are durable and come with a design life of 100 years or more.

They also provide versatility as they can be used for large residential or commercial developments. These precast Attenuation Tanks have also been used as a flood prevention method in international airports where surface water runoff can be considerable.

Flooding costs the UK alone billions each year. There are millions of properties at risk. Therefore, the importance of prevention methods such as Attenuation Tanks cannot be understated.

Government studies such as the Flood and Coastal Defence project estimates that urban flooding costs the UK upwards of £320 million per year.

The study also estimates that properties across the UK will see a significant increase in the risk of flooding.

The number of properties at risk of localised flooding is set to increase by 10% each year until 2080.

Whether it is Attenuation Tanks or other methods, flood prevention in urban areas will only grow in importance.

Benefits of using Attenuation Tank

As well as being an extremely effective way of flood prevention. Precast stormwater Attenuation Tanks provide multiple other benefits.

  • 100-year asset life.
  • Flexible design to suit any shape and size.
  • Low ongoing maintenance requirements post installation.
  • Loadable to suit a wide range of environments and highly trafficked areas.
  • Capable of a high load capacity. Suitable for use under infrastructure such as car parks, railways, airports or commercial outlets.
  • Low Installation costs due to components being made built to order off-site (unique to FLI Precast Solutions)

The above illustrates some of the many additional benefits associated with using an Attenuation Tank for stormwater alleviation.

If you would like to discuss a project or design, please feel free to contact our team directly at [email protected]

How does an Attenuation tank work?

The purpose of an Attenuation Tank is to capture rainwater or stormwater from the surface and store it temporarily.

It is then slowly released in a controlled manner to restore the equilibrium with the environment that took place prior to any development.

During spells of heavy rainfall or melted snow or sleet, the Attenuation Tank holds the water within its system and minimises the risk of localised flooding.

The water is then released in a slow manner using a mechanism called a hydro brake at a previously agreed discharge rate with the adopting water authority.

The above infographic allows you to visually see the process of how an Attenuation Tank works. The process is as follows:

  1. Rainwater, snow, hail, or sleet falls from the clouds
  2. Surface water is safely collected and stored in the underground attenuation tank, particularly during a storm event where volumes can be considerable.
  3. The water is then slowly released back into the soil in a controlled manner using a mechanism called a hydro brake.

Process of ordering an Attenuation Tank

If you are looking to install an Attenuation Tank, we can provide you with a seamless solution.

Our years of experience mean we can combine a high-quality design with a streamlined installation process.

We can discuss a project and assess your needs with our simple 4 step process:

  1. Request a quotation via email at [email protected] or via phone at +44 1279 423303
  2. One of our design experts will assess your specifications and answer any questions you may have. From here a formal quotation will be sent to you directly.
  3. All being well, a detailed design if prepared and approved.
  4. Our modular precast Attenuation Tank arrives at your site as per the agreed date where we can also install depending on the agreement.

From a site perspective, the process of installing a precast stormwater Attenuation Tank is as follows.

  1. The client carries out the preparation and excavation of earthworks. This will get the project to the ‘formation’ level.
  2. FLI Precast mobilises to the site to install leveling strips that allow precast units to be stood upon. Precast units are then placed upon the stips and an in situ base is poured.
  3. The final step is for FLI Precast to hand over the completed structure to the client for backfilling and the installation of turrets. This allows for access and cleaning going forward.

What is the difference between a soakaway and an Attenuation Tank?

A soakaway and an Attenuation Tank have some very important differences.

A soakaway is designed to ‘soak’ water back into the soil. Attenuation Tanks hold the excess water and slowly discharge it back into the surface water system. Doing so in a controlled manner protects both the infrastructure and the environment.

The water stored in the Attenuation Tank is released via a flow-control chamber and is discharged slowly back into a local water source, often a river or reservoir.

This controlled process helps minimise the risk of localised flooding.

Looking to discuss a project? 💡 At FLI Precast Solutions, we also have experience installing soakaways. Do not hesitate to contact our team if this is an area you would like to discuss.

How to size an Attenuation tank?

Below we analyse the most important factors when sizing an Attenuation Tank.

1. Geographical Location

Your project location will have a significant impact on rainfall levels throughout the year.

The below table highlights the average rainfall in different locations across Europe. Measurements are in both inches and millimetres per month in 2021.


Average Annual Rainfall


Average Annual Rainfall


Days of Rainfall

















The above table illustrates how much rainfall levels can vary by location. These cities are not huge distances apart, but have varying levels of rainfall.

This is important as the higher the average rainfall levels in a location, the larger the Attenuation Tank that will be required.

2. Storm Regularity

How regular, or irregularly an area is affected by storms is an important factor. If there are frequent storms with heavy rainfall, additional measures should be in place.

In 2021 alone, Italy, Austria and Germany suffered significant storms and huge amounts of rainfall. This caused widespread flooding and damage.

Estimating the potential number of storms within a location will help determine what size tank is necessary.

3) Climate Change

In recent times, rainfall levels have become more unpredictable.

The Met Office has stated rainfall levels are increasing globally and are also becoming more intense.

Therefore, products such as Attenuation Tanks are playing an increasingly important role with increases in climate change and storm events being factored into the storage volumes needed.

4) Allowable release levels

If an Attenuation Tank is connected to a local water source, there will be restrictions on the level of allowable outflow.

The level of outflow must be calculated and released at a correct rate in order to avoid any potential flood risks.

Are there different types of Attenuation tanks?

Yes. There are different types of Attenuation Tanks which use different materials.

Precast concrete Attenuation Tanks are the most common type across the UK and Ireland.

However, there are some geocellular systems available. These use the modular/honeycomb nature of geocellular systems.

However, performance can be difficult to monitor if choosing this geocellular method. They can also be difficult to maintain.

It is for this among other reasons that a precast concrete Attenuation Tank is a more robust and long-term solution.

Attenuation Tanks are versatile and come in all shapes and sizes. At FLI Precast Solutions, we specialise in bespoke designs. Our versatility allows us the ability to design Attenuation Tanks for a wide range of projects.

Attenuation tank FLI Precast Solutions

If you have a specific design you would like to discuss, please do not hesitate to contact our team. 

Case Study - Attenuation Tank Installation

Bulkhaul Ltd

Bulkhaul is a business dedicated to the global transportation of bulk liquids, powders, and gases. FLI Precast’s engineers worked closely with Lynas on the hydraulic drainage design, but what set this project apart was the need for a loading capability, the like of which other manufacturer’s products just couldn’t provide. With containers potentially being stacked three high, the point loads above the tank were far in excess of anything you would typically find on other sites, but FLI Precast rose to the challenge.
Project Highlight
This project was completed on time and on budget.

This project shows the versatility of the FLI Precast Attenuation Tank offering. It highlights our tank in a non-residential setting and illustrates our design and engineering capabilities to cater for bespoke projects.

The importance of stormwater management

Stormwater management is of huge significance. Particularly as we continue to build large amounts of infrastructure and residential properties.

As we settle and build on land previously untouched, these actions have consequences that interfere with the natural water cycle.

Urbanisation increases surface imperviousness. This means there is a faster runoff of rainwater which equals a higher risk of flooding.

Therefore, precautionary measures are necessary. This allows for the management of stormwater and minimises the risk of flooding.

The UK government alone has implemented a £2.6 billion, six-year programme dedicated to improving flood defences.

Ireland has had a flood prevention programme in place since 2016. The cost to date is an estimated $400.

Attenuation Tanks and other resources allow us to control and store excess stormwater. From here it can be released slowly and in a controlled manner.

Where are Attenuation Tanks used?

Attenuation Tanks are versatile. The bespoke nature of our designs in particular provides versatility and flexibility.

We use our tanks across a wide range of sectors and are not limited to residential settings. Other sectors include:

  • Residential
  • Commercial
  • Aviation
  • Transportation
  • Communications

History of Attenuation Tanks

FLI Precast Solutions have been a leading supplier of stormwater Attenuation Tanks for almost 30 years.

This experience combined with our in-house design teams has allowed us to become the market leader for this method of flood prevention.

Attenuation tanks or “Carlow Tank” as they are also known, are the industry standard for stormwater attenuation. FLI Precast has played and continues to play a vital role in the stormwater attenuation sector.

From creating the design of the original “Carlow Tank” to being a leading provider of attenuation tanks almost 20 years later.

With an emphasis on climate change and unpredictable weather patterns, there has never been a greater need to protect our homes, businesses, and environment. 

Severe storms and flooding can have long-term effects on buildings and infrastructure. It can also have a huge financial impact. Therefore, it is important that flood alleviation methods are put in place.

About FLI Precast

At present FLI Precast produce around 1,000 tonnes per week and we have completed several thousand projects over the last 35 years, sending precast concrete as far as Australia. At present, we proudly employ over 100 staff both on and off-site.

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