How does a stormwater attenuation tank work?

Posted by Callum Vallance-Poole, on June 20, 2022.

Attenuation tanks and systems come in all shapes and sizes and can be used in a variety of environments. They play a vital role in the everyday protection of our environment by storing and releasing excess rainwater that is unable to naturally filter into the ground. In this article we explain how an attenuation tank works and go into more detail about why they are needed.

What is a stormwater attenuation tank?

Attenuation tanks are often confused with soakaway crates, and while there are some similarities between them, the way they operate is largely very different. A soakaway crate is designed to send water into the soil, while an attenuation tank stores and redirects it to a watercourse (river, lake or reservoir).

An attenuation tank is used to capture surface water run-off that occurs during prolonged periods of rain, reducing the risk of localised flooding. The tank features a flow control chamber that manages how quickly the water is released which ensures the water is discharged at a safe rate that won’t damage the environment.

Why are attenuation tanks needed?

The risk of localised flooding is increased in areas that have a lot of hard surfaces where rainfall is unable to naturally infiltrate into the ground. Developers are legally required to incorporate solutions for this into their plans and attenuation tanks provide a long-term, sustainable method of storing and discharging rainwater without damaging the environment.  

Large scale attenuation systems are typically used in commercial areas such as car parks and residential housing schemes, and there are also smaller scale attenuation systems available that are suitable for domestic settings. In general, they offer a long-term, cost-efficient solution for dealing with rainwater run-off and help to prevent the local sewer network from becoming overwhelmed.

The Environmental Agency predict there will be an increased risk of flooding in the UK over the next few decades. This is part of the reason why legislation has been introduced to ensure excess surface water is managed correctly, because if there is more rainfall, there is a higher chance that sewer networks will struggle to cope.

How does an attenuation tank work?

An attenuation tank works in a very simple way to store and release rainwater run-off. However, there is more to the design than just being a large storage tank. The key feature of an attenuation tank is the flow control valve, which ensures the amount of water being released is done so at a rate the reservoir or river can cope with.

When there is heavy rainfall, the excess water run-off flows down into the tank and is held there until it is the right time to release. A vortex is created by the energy inside the tank when there is a fast flow of water, which is then used to discharge the water at a safe rate for the receiving watercourse.

The vortex also plays a key role in lower maintenance requirements for the tank. It is in-effect a self-cleaning mechanism, which reduces the need for constant inspection and cleaning, which would otherwise add a significant amount to the cost. 

What are the benefits of an attenuation tank?

There are many benefits that come with the installation of an attenuation tank, including:

Water storage

Primarily, this is the main task of an attenuation tank. Storing water that cannot naturally filter into the ground so it can be safely discharged back into the environment.

Public safety

By storing rainwater, people, homes and businesses are given more protection against the risk of flooding. This is more likely in built up areas with harder, flatter surfaces, although specific lakes, ponds and swales are also created with attenuation tanks used to store heavy rainfall.

Placemaking

When development companies are planning new residential or commercial schemes, by law they must consider integrating sustainable drainage methods. Often this includes a combination of attenuation tanks and the building of new green spaces for natural water absorption.

Modular design

Modern attenuation crates offer more flexibility when it comes to installation. Underground storage requires a lot of strategic planning to accommodate the many infrastructures that are already in place (piping/cables etc.) and attenuation crates that allow modular construction make it easier for the systems to adapt and work vertically or horizontally, without compromising drainage performance.

Money saving

The cost of dealing with flood repairs can often extends into thousands of pounds and by helping to minimise localised flooding, attenuation tanks offer a cost-effective way of ensuring flood damage costs are minimised.

What to consider before installing an attenuation tank

Before installing an attenuation crate on your home property there are a few things to consider:

Contact the local planning department

Check with the local authority whether any land is protected, or if you are close by to a watercourse. Only proceed once you have confirmation, as it can save you a lot of time and money.

Soil percolation test

You need to determine the absorption rate of your soil by carrying out a soil percolation test. This is fairly easy to do and will tell you if an attenuation crate is the right option. Soil that is too soft will absorb the water and not release it, so the soakaway won’t have much use.

Confirm installation area

Some local authorities are very specific on where surface water can be collected and treated, so it’s a good idea to get as much information as possible before installation begins. Once this is sorted, you should install the crate at least 5 metres from your home – bearing in mind that the further the distance, the longer the pipes will need to be.

Check the installation area

Check the installation area for any pipes or cables that may be underground. You might find water mains, electricity cables or gas mains there and you want to ensure these are not damaged by installing an attenuation crate.

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!