Why is an attenuation tank needed?

Stormwater attenuation tanks are systems that many people aren’t aware of, despite the important role they play in our everyday lives. This is because they are installed underground and quietly work away dealing with excess water run-off that local sewer systems are unable to cope with. They can be installed in public and domestic locations and help to reduce the threat of localised flooding. Here we explain what an attenuation is, what it does and why it is needed.

An attenuation tank provides a long-term solution to reduce the threat of flooding caused by water run off after long periods of rain.

The tanks feature a flow control chamber that manages the amount of water being pumped into a local sewer system or watercourse (i.e., a river or reservoir). To ensure the system is not overpowered with too much water in one go, instead, water is released at a slower, sustainable rate the infrastructure is able to cope with. Some tanks can last for decades and require very little maintenance, safeguarding the immediate environment and nearby properties.

Stormwater attenuation tanks are included as a part of a Sustainable Urban Drainage (SuDS) setup, which means they must also meet the same legal requirements. This is laid out in Part H of the Building Regulations and has been in effect since 2002.

SuDS plays a key role in reducing flood risks that are caused by excessive levels of rainfall. Impermeable land cover is increased through development of new buildings, while also reducing the amount of rainfall that can be intercepted by vegetation and infiltrated into the ground. This creates a greater surface water runoff area which drainage systems struggle to cope with.

The Flood and Water Management Act 2010 requires developers to consider using sustainable drainage for all new developments. Local planning departments vet planning applications to ensure that rainwater from the proposed new site will not have an adverse effect on the local environment.

This is because the construction of domestic, commercial and industrial development creates more hard surfaces such as driveways, roads, car parks etc. Surface water run-off is increased as a by-product, meaning more water is diverted into sewage systems rather than naturally filtering into the ground.

The idea is to find more efficient ways of preventing flooding by designing systems that can deal with excess water flow at the source, rather than letting issues develop downstream that are more difficult to manage. Stormwater drains are designed to only deal with a certain level of water run-off and when too much water enters the sewer system at a rate it is unable to manage, the risk of flooding dramatically increases.

Government statistics state that 1 in 6 homes in England are at risk of flooding. And with climate change due to change weather patterns over the coming years, the UK will become increasingly vulnerable to extreme weather and an increased flooding risk.

Without a flow control device, attenuation tanks would simply release stored water at the same rate it enters the tank, defeating the entire purpose of the system. This is where a flow control valve becomes important, as it manages the water release rate, so it is discharged safely without causing harm to the surrounding environment.

Some flow control devices (such as orifice plates) work using a very simple principle: the smaller the pipe hole, the lower the rate at a given pressure (which is differential pressure either side of the hole/orifice). When the filter is closed no water can pass through, and it can be gradually opened to allow more water to enter until the maximum flow is reached.

Vortex flow regulator valves are becoming more commonplace and typically feature an intake, outlet and volute, and compared to orifice plates and throttle pipes, they offer better hydraulic performance. The inlet on this type of device is larger than a standard orifice – in some instances it can be twice while still generating the same outflow or allow discharge.

At low flow rates, this type of flow control valve will allow water to pass through the inlet and onto the outlet without restriction. At higher rates, stormwater will enter the tank to create a vortex, which is formed by its energy, and the control mechanism is able to manage the flow at a specified discharge rate.

The vortex also helps to reduce the amount of maintenance required for the tank, as it effectively cleans itself. This adds a further reason to invest in an attenuation tank, as it can largely be left to operate without any assistance.

If you only have a stormwater attenuation tank installed on your land, the water is not suitable for reuse and the system can only discharge back into the environment.

However, there are models that combine stormwater attenuation tanks with rainwater harvesting systems, which enable you to reuse water run-off that has been stored. This gives you the added benefit of protection against flooding while also offering the chance to lower your water bills if you live on a metered property.

While the water is not potable (i.e., drinkable) the water is perfectly suitable for watering plants, flowers and lawns, as well as washing down the driveway and cleaning the car and other similar vehicles. If you want to reduce the risk of flooding and harvest rainwater, this type of system offers the best solution, as it saves money and the hassle of installing two separate tanks, while also using up less space.

Investing in a combined rainwater harvesting and stormwater system can also add value to the price of your property, as it is an additional feature that will also prove to be a cost-saving add on for the new owners.