What is an underground water attenuation system?

To comply it is often necessary to attenuate rainwater either at roof level or below ground. This article covers water attenuation below ground. Please refer to our article below for information on rainwater attenuation at roof level.

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What is a water attenuation system and how does it work?

A water attenuation system forms part of a Sustainable Drainage System (SuDS). It directs excess rainwater from the drainage system on buildings and built infrastructure such as roads into a water storage facility called an attenuation tank. It is used to reduce the risk of flooding in urban environments during heavy rainfall.

The attenuation tank features a flow-control chamber which controls whether the water is stored or slowly released back into the sewers or watercourses. The alternatives are traditional drainage systems, which flow directly into sewers and watercourses, and soakaway crate systems which allow the stored water to gradually soak into the surrounding soil.

It can be linked to either a siphonic drainage system, which eliminates air from the pipework for faster, more efficient flow or a gravity drainage system, where both air and water enters the pipework.

Find out more about siphonic drainage systems

What are the potential issues with a water attenuation system?

One of the key issues is that all the elements of the drainage system must be able to cope with the same flow of excess water. However, while the attenuation tank and guttering are typically designed to cope with 100-year storms, the pipe system is often only designed for 30-year weather events.

This isn’t an issue in normal rainfall, but during summer storms the capacity of the pipe system may not be able to cope with the sudden increase in water.

When a water attenuation system is used as part of a siphonic drainage system, this can result in flooding at high level within the building. When flooding occurs, it is often assumed that the issue lies with the guttering, when in fact it is the below-level pipe capacity that is the problem.

To prevent flooding, a vented manhole cover should be installed where the above-ground system meets the gravity underground drainage. This allows air within the system to vent, so it reaches maximum siphonic capacity quickly and effectively. If there is a lack of capacity within the underground system, water can flow out at ground level.

Similar issues can also occur on gravity systems, although these have less severe consequences. To prevent this, gullies should be used to discharge external downpipes and back inlet gullies with open grates should be fitted to the pipes servicing internal downpipes. These gullies prevent water from backing up within the downpipes by allowing it to escape outside of the building.