What are the different types of stormwater attenuation crates and benefits of each?

If you are new to stormwater attenuation crates and are not familiar with the different types of systems that are available, it may be difficult to know where to start. There are 5 main variations to choose from, each one offering different features benefits depending on your drainage need and installation environment. Here we explain more about the different types of attenuation crates and the main benefits of each one.

Single piece stormwater attenuation systems contain everything within one crate, including a flow control compartment, so there is no need for a separate chamber. This helps to simplify the installation process as there are fewer components to manage, which is helped further by being prefabricated and fully assembled, so they are ready to go straight into the ground.

Stackable stormwater attenuation crates offer a modular design that give you more flexibility over how the system is installed and set-up in the ground. One of the major benefits of using stackable crates is that the installation can be completed much faster and without the need to use specialist heavy equipment. These types of crates are more suitable for areas that cannot accommodate a long row of crates attached to each other. Instead, they are stacked on top of each other to retain a large storage holding and facilitate the release of rainwater into the surrounding environment.  

Light duty stormwater attenuation crates are best suited for pedestrian loads through to domestic vehicular loads. In most cases, light duty crates are installed in gardens and landscaping areas and have a load limit of around 20 tonnes. The depth of cover, tank depth, top surface cover, ground type (granular or cohesive) and usual loading traffic will all have to be accounted for to ensure this type of crate is suitable for the land.

Heavy duty stormwater attenuation crates are designed to accept large weight loads at ground level, often exceeding a 60-70 tonne load rating. The exact load a capacity is decided by the cover depth, soil type, surface finish and other similar factors, which can be determined by the installation company. A heavy-duty stormwater attenuation crate is typically installed in areas that experience large amounts of traffic on a regular basis, or for car parking areas that accommodate several domestic vehicles.  

Good maintenance of a stormwater attenuation crate ensures it remains in working order so you can enjoy consistent and efficient performance levels. An inspectable stormwater attenuation crate makes the task of maintenance much easier to manage as it is specifically designed with inspection in mind. They include inlets and outlets that enable you to assess the condition of the current as and when needed. Where it once used to be the case that attenuation crates were neglected after installation, that has changed because of increased calls for better sustainability, so inspection crates are far more commonplace today.

Stormwater attenuation crates are used below ground to manage surface water levels after heavy rainfall. Once installed, they are combined with a geotextile and waterproof pipework to create the attenuation system. The water captured during a storm even is retained in the tank and redirected to a local body of water or storm drain at a manageable rate to avoid flooding. A soakaway system can also be created using the attenuation crate. This is done by placing the create inside a permeable geotextile, so the water can then be filtered out into the surrounding environment.

Stormwater attenuation crates are used in a variety of different environments including roads, car parks, sports stadiums, shopping centres, pavements and more. They can be used on any site that can be improved with water infiltration.

Once you have a clearer idea of the different types of stormwater attenuation crates, you might then start to wonder how many you will need for your land. The answer will depend on the size of storage you need to install. Stormwater attenuation crates offer good flexibility as they can be stacked and most come with a void ratio of at least 90%, which helps to reduce any wastage of underground space. Other factors to take into consideration include the expected flow rate and the type of soil in the ground. In some cases you may even need at least 4-5 cates per cubic metre, although this will be dictated by the model and type of crate you install. 

Many first-time buyers of stormwater attenuation crates are often unsure about how deep the system should be. Experts recommend that the bedrock should not be within one metre of the drainage piping, in order to maintain proper filtration levels. This means installation should be at least 2 metres below the ground because pipework depth limits stand at 700mm.

If you are installing attenuation creates you should be aware of the following regulations for soakaways:

• They need to be a minimum of 2.5 metres away from the boundary
• Crates must be installed a minimum of 5 metres from the wall of any nearby building
• At no point in the year should the water table reach the bottom of the pit
• Installation points should not be located close to another soakaway or other areas where the water could be contaminated
• Percolation tests should be carried out before installation
• The size/volume should be based on a drainage calculation (which can be done once you know the surface are of the wate source and a percolation test has been carried out)

There is also a simple calculation you can carry out to get a rough idea of the size of soakaway you will need. For non-heavy/clay soil, it is usually safe to assume that one crate (1,000mm L x 500mm W x 400mm H) is enough for 10 square metres of roof covering or driveway etc. 5 crates equal 1 cubic metre and 1 cubic metre can be used for 50 square metres of surface area. So, once you have measured the total surface area, you should have a rough idea of the size of the soakaway.