‘Attenuation’ means to make something less or weaker. In this case, it means lessening the flow of water run-off and managing the amount that is entering a local sewer system or watercourse, such as a river or reservoir.
An attenuation tank provides a long-term solution to the threat of flooding by storing excess water and then managing the rate at which it is released back into the environment.
An attenuation tank comprises a watertight area to store the excess water as well as a flow control system that slows the rate at which the water is then released.
An attenuation tank reduces the risk of flooding by storing excess water from long or heavy periods of rain before releasing it slowly into a watercourse or the sewer system through flow control mechanisms. This doesn’t just prevent flooding locally but, as the water is contained and delayed at source, it doesn’t transfer and increase the problem downstream.
Attenuation tanks work by temporarily storing excess water, like stormwater, before routing it to the sewer system or a watercourse as normal. However, the volume of water allowed to flow back into the system is controlled through a flow control chamber.
During periods of heavy rainfall, the excess water is fed into the attenuation tank where it is held until the time is right to release it back into the main system, preventing flooding both locally and downstream. The tanks can be concrete structures or crate systems that store the water while also allowing for loading on top of them.
Attenuation tanks can be categorised as being ‘online’ or ‘offline.’ In an online system, the water inlets and outlets are in different locations so that the total volume of water passes through. The structure fills when the flow rate exceeds the desired discharge rate at the outlet. Offline systems connect the inlet and outlet to the same inspection chamber, with the inlet being placed above the normal flow level so that the structure only fills up during periods of heavy rainfall.
Without flow control measures, attenuation tanks would simply release any stored water at the same rate it enters. A flow control valve manages the rate at which water is discharged, making sure it doesn’t overwhelm sewer systems or damage the surrounding environment.
Simpler flow control devices, such as orifice plates, have an outlet hole that can be completely closed or opened gradually to allow more water to pass through. The smaller the opening, the lower the flow rate as a result of differential pressure either side of the hole.
Vortex flow regulators include a volute alongside the inlet and outlet, which creates a vortex in the water at higher flow rates. The vortex means that the water cleans the tank as it passes through, reducing maintenance requirements, while also helping to control the flow.
Whichever flow control measures are used, the key is to release the water at a slow and sustainable rate that the infrastructure or environment can cope with.
As already discussed, the purpose of attenuation tanks is to prevent flooding by dealing with excess water flow at the source, storing the water temporarily before releasing it at a slower rate back into the sewer system or waterways.
This reduces the risk of localised flooding where storm drains become overwhelmed with the volume of water entering the sewer system as well as preventing flooding issues from occurring further downstream.
This issue has become increasingly pressing as urban developments including driveways, roads and car parks mean more hard surfaces that cannot soak up water. This creates more surface water run-off into the sewer system rather than being filtered naturally through the ground, overwhelming drainage systems.
Climate change has also increased the risk of heavy rainfall and flooding for parts of the world, including England, where statistics say 1 in 6 homes are at risk of flooding. Such statistics have led to building regulations including sustainable urban drainage systems (SuDS), of which stormwater attenuation tanks are a part.
SuDS seek to help offset the flood risks created by climate change, increased impermeable land cover, such as buildings and roads, as well the associated reduction in vegetation that can intercept and slow water flow rates.
In the UK, the Flood and Water Management Act 2010 requires developers to consider sustainable drainage management systems in new developments, while local planning departments check applications to make sure there is provision to stop water run-off from having an adverse effect on the surrounding environment.
Soakaways offer an alternative method of dealing with excess water, but these work differently to attenuation tanks. The main difference is that, were attenuation tanks store the water before releasing it back into the system, soakaways allow the water to soak back into the soil around it.
Soakaway crates are wrapped in a permeable membrane that allows the water to slowly seep through and back into the ground. The slow rate of the water permeating the membrane prevents the ground from becoming waterlogged and flooding. The design of a soakaway system typically requires the crate structure to be half emptied in under 24 hours
While some attenuation tanks are precast structures made of materials such as concrete, a great many of them are created from crates that are stacked into a structure before being covered with a protective geotextile and an impermeable geomembrane to create a water-tight structure.
The crates themselves create the underground void that can fill with water, but they need to be built to be structurally sound and able to withstand the loading from whatever is placed on top of them on the surface. However, it is the geomembrane liner that is doing the real work in ensuring a water-tight seal and this is where welding is important.
Here is a step-by-step guide to installing a crate-based attenuation tank:
- Step One: Firstly a pit needs to be dug to house the attenuation tank. This pit is typically lined with gravel before being covered with a layer of geotextile that acts as a protective layer for the geomembrane
- Step Two: The geomembrane is laid on top of the geotextile layer. If the tank is large, sections of this geomembrane layer may need to be welded together
- Step Three: The crates are then fitted together on top of the geomembrane base. The crates may be clipped together of placed on a base plate, depending upon their design
- Step Four: Once the crates are all in place end plates can be used to make sure the structure is rigid, after which the sides of the geomembrane are pulled up and sealed along with any pipe connections
- Step Five: A top layer of geomembrane is then placed over the structure and welded to the geomembrane side panels to create a watertight seal
- Step Six: A final protective layer of geotextile is placed over the top and down the sides of the tank to protect the geomembrane layer from damage
Attenuation tanks can last for decades with minimal maintenance, although this is reliant on the quality and integrity of the geomembrane welds. The successful performance of the lining system is related to the goemembrane seam weld, which can be integrity tested during its lifetime. Welding provides the most durable method of joining the geomembrane as taped seam joints can break down, leading to leaks and the failure of the system.
A storm event can lead to the collection of a large quantity of water, however this water is not suitable for reuse and so needs to be discharged back into the environment. However, there are systems available that combine stormwater attenuation with separate rainwater harvesting systems that allow for the re-use of rainwater that has been stored.
Rainwater is not potable, so cannot be used as drinking water, but is fine for watering gardens or cleaning with. Combining an attenuation tank with rainwater harvesting saves the cost of installing two completely separate systems, uses less space, and saves costs on metered water use.
Attenuation tanks provide a solution to excess water run-off, helping to protect property and the environment from stormwater. An attenuation tank stores excess water and then allows it to be released through the usual sewer systems or into watercourses once water levels have died down. This not only prevents localised flooding, but also prevents issues from being pushed downstream to other areas.
As more hard surfaces such as roads and buildings are built, there is an increased need to deal with water run-off that would otherwise have been soaked up by the ground.
However, attenuation tanks are only as good as their construction, which, for crate systems, means the integrity of the welds that join the water-tight geomembrane layer. For this reason it is important to make sure you use the correct processes and expertise to ensure the integrity of the welds.