What are attenuation crates?

When there is a storm event, towns and cities will experience a large amount of rainfall in a short space of time, which can pose problems for the mains sewage network, leading to localised flooding. Attenuation crates offer a solution to this problem, but what are they and how do they work?

Attenuation crates (also known as stormwater crates) are systems used to store excess groundwater run-off so it can be safely discharged back into the environment to reduce the risk of floods. They plan an important role in protecting people, property, and local land from the build-up of rainwater during heavy downfall periods and can be used in domestic, public and commercial environments.

When developers are planning new projects, they are legally obliged to include sustainable methods of drainage that will lessen the strain that can be placed onto the local sewage network due to construction. This is particularly true in city and urban areas, as concrete and other harder surfaces do not absorb water as naturally as soil, which can quickly lead to rising water levels and flooding.

Many of the existing sewers we use today were built in the mid-to-late 19th century and while they can cope with most rainfall events, they can become overloaded and struggle to find the capacity to manage a high amount of rainfall in a short period of time. This increases the chances of localised flooding occurring, putting people, property, and the environment at risk.

There is also growing concern about the increase in extreme weather events that could see a sharp rise in rainfall in the UK. When drainage systems are placed under sustained pressure, it can lead to split or burst pipes or cracks forming in pipes that will develop later. With more strain being put onto sewage networks, systems like attenuation crates offer a viable solution that can lessen the load to keep people safe.

It is worth noting that attenuation crates offer a more practical solution compared to some traditional methods. Rubble was once considered the best way to deal with excess groundwater run-off, which relied on holes that featured drainage pipe ends and rubble. This is a time consuming and costly procedure, as large amounts of rubble are needed, as well as heavy machinery to dig the hole and lift the rubble into place.

In comparison, attenuation crates are lightweight and do not require the same amount of effort to construct a sustainable drainage system. Most crates are delivered flat-packed, so transporting them to various locations is very easy. Crate designs are also very simple, with most crates consisting of a few pieces of plastic that can be clipped together to become operational.

Traditional rubble methods also do not offer long-term solutions to flood risks. Sediment is washed into this type of soakaway which eventually builds up amongst the rubble. Over time, this ‘plugs’ the gaps that would have allowed water to pass through, essentially blocking the system and creating a build-up of standing water. This is not an issue that is faced by attenuation crates. They are designed to cope with a rise in sediment from surface run-off, with several routes the water can take so it avoids being blocked.

Even though attenuation crates are made from plastic, they are incredibly sturdy and durable, enabling them to withstand large weight pressures (when configured correctly) without becoming compressed. However, rubble tends to become compressed over time if heavy loads are placed above them, which reduces the space available for filtration and affects their performance levels.

Attenuation crates work using a very simple process, very much in a similar way to attenuation tanks. One of the key benefits of this type of system is the modular construction, which allows multiple crates to be built to suit the immediate environment. This enables horizontal and vertical systems to be built, which offers far more flexibility than some other systems.

Once installed, they can be connected to pipework and wrapped inside a geotextile. This attenuation system then receives excess groundwater run-off during periods of heavy rain, using pumps to manage the incoming stream of water. Depending on the capacity of the local sewage network, it is usually stored inside the crate system for around 1-2 days, before being redirected to the local sewer or watercourse at an acceptable flow rate.

If you are installing attenuation crates, you must ensure they are at least 5 metres away from your property and a minimum of 2.5 metres away from a boundary. You can find more details about this in Building Regulations Part H3.

Projects that involve hard landscaping may be required to provide SUDS ((Sustainable Urban Drainage System) to manage surface water run-off. This means the water is collected where it falls and wherever possible it is then reused.

Care must be taken to ensure that the water table (water situated below ground) does not touch the bottom of the pit at any point during the year (as table levels continually change). Before installation, a percolation test must be carried out by a professional to verify the suitability of the soil.

When the soil type is not classified as heavy, it is usually safe to assume that one crate is safe for 10 sq. metres of roof covering or driveway etc. Five crates will equate to 1 cubic metre, which will be suitable for 50 sq. metres of surface area. These calculations are based on crate dimensions of 1000mm (L) x 500mm (W) x 400mm (H).

It is recommended that the bedrock should always be at least more than one metre from the base of the drainage pipe, which is something a professional installation company should be aware of. And because pipework depth cannot be any lower than 700mm, this means that the attenuation crate should be installed at least 2 metres beneath the surface.