Attenuation tanks may not be visible to the public, but beneath the ground they play a vital role in keeping towns and cities safe from the risk of flooding. Here we explain how an attenuation tank works and why they are so important.
What is an attenuation tank?
An attenuation tank is an important flood prevention system that is used to store excess rainfall before slowly releasing the water back into the watercourse (such as a river or reservoir) when it is safe to do so.
According to the Environment Agency, there were 91 flood warnings and 237 flood alerts issued by the body in 2020. More than 50mm (2 inches) of rain fell in less than 36 hours in many areas of the UK and without the many attenuation tanks that are currently installed, these figures would have been much higher.
A National Assessment of Flood Risk paper released by the agency stated that around 5.2 million properties in England (which is about 1 in 6) face the risk of flooding. 2.4 million properties and 5 million people that live and work in them are at risk of flooding from rivers or the sea, with one million at risk of surface water flooding.
However, we should bear in mind that this research was released in 2009 and given the ongoing effects of climate change to the weather and land, the figures are now likely much higher than the threat posed by flooding 13 years ago.
Why use an attenuation tank?
On average, there are 133 days of rain or snow in the UK every year, which equates to around 33 inches. The most affected places tend to be Northwest England, mid-Wales and Southwest England, although there are heavy periods of rainfall right across the country that can lead to serious localised flooding.
So, it’s not just a perception that the UK is a rainy country – it’s a fact backed up by statistics. This is something that local authorities have to bear in mind when it comes to planning and developing new infrastructure. Legislation has increased significantly in the past 10-20 years which means councils are now legally required to include systems that prevent flooding in their plans.
While the end goal is to design systems that better manage the flow at source – instead of downstream – this is where attenuation tanks and crates come in, providing a reliable backup to deal with excess rainfall to lower the risk of flooding.
The use of an attenuation tank offers several important benefits aside from storing water. It helps to keep people and buildings safer in built up areas that tend to feature a lot more concrete and harder surfaces. Their installation also enables development companies to incorporate more green spaces that naturally absorb water (tanks are kept below ground) helping to improve quality of life for communities. They are also a cheaper alternative compared to paying for repair and maintenance work in the aftermath of a serious flood, which can seriously impact a local authority’s budget and ability to invest in planned projects.
Can I use an attenuation tank in my home?
While large, purpose-built attenuation tanks are used in public spaces such as car parks and residential housing areas, there are also smaller domestic models that can help protect your home and the local environment against flooding.
If you live in an area that has a high level of rainfall, or if you reside nearby to a river or lake that is at risk of flooding during storm events, it is worthwhile considering a domestic attenuation tank. They come in the form of individual crates that can hold hundreds of litres, allowing for modular construction to suit the space you have on your land.
The installation process is relatively straightforward and in most cases you won’t need planning permission (although you should always check this with the local council). They are a good idea for installation beneath driveways where surface water can become trapped, or in back gardens that may become waterlogged if exposed to too much rain for long periods.
How does an attenuation tank work?
An attenuation tank follows a very simple process to receive, store and release excess rainwater:
Receiving water
When there is heavy rainfall that the local systems will struggle to cope with, an attenuation tank is ready to take in the water to help reduce the likelihood of flooding. The key element is a flow control chamber to regulate the amount of water being pumped into the tank. Depending on the size of the tank, it may feature two or three different sections, ensuring polluted water is released separately.
Water storage
Once the tank has received the water it is then stored inside the system until it is safe to steadily release it. The length of time it remains here depends on the amount of rainfall at ground level and how the local network is coping to drain it away. In most cases this around 24-48 hours after the storm event has ceased.
Water release
The purpose of an attenuation tank is to prevent flooding downstream, so when it is safe to release the water from the system it is returned to the watercourse at a regular rate. The rate is based on the capacity of the local network and calculated before installation, so the water can be released without overburdening the network and the treatment plant.
How much do attenuation tanks cost?
There is no fixed price for attenuation tanks as the requirements of every project are unique. Whether it’s a single attenuation crate or a large scale attenuation tank, there are several factors to consider that will determine the final cost. Once you have an idea of the size of tank you need you will then have a better understanding of the product cost. However, once groundworks begin it may throw up some unexpected obstacles or issues that need to be overcome, so this also needs to be factored in the overall price.
