Attenuation crates are modular plastic geocellular units assembled into an underground void that stores surface water run-off and releases it downstream at a controlled rate. Together with prefabricated steel tanks they form the two dominant families of below-ground attenuation storage on UK developments, and the choice between them is driven by depth of cover, traffic loading, ground conditions and whole-life inspection requirements.
How geocellular attenuation storage works
Run-off is routed from the surface water network into a crate structure wrapped in an impermeable geomembrane, where it is held during the storm peak and released through a flow control device at, or below, the greenfield run-off rate agreed with the lead local flood authority. Where the underlying soil is permeable and groundwater levels allow, the same structure can be wrapped in a geotextile instead so the stored volume infiltrates — the arrangement is then a soakaway rather than an attenuation tank. The two functions are frequently combined, with an infiltration base and an overflow to a piped outfall for events that exceed the infiltration capacity.
Void ratio, loading classes and cover depth
Crate systems are specified on three parameters. Void ratio is typically 90–96 per cent, so a crate field stores roughly nine to ten times the volume of the same footprint filled with granular material. Short-term compressive strength is declared for vertical and lateral loading, and the design must apply the long-term (creep-reduced) value over a 50- or 120-year design life. Cover depth then follows from the loading case: shallow landscaped areas, car parks and adoptable highways each demand a different crate class, and heavily trafficked areas often require a reinforced concrete slab above the crate field. CIRIA C737 sets out the structural and geotechnical design framework for modular geocellular drainage systems in the UK.
Inspection, maintenance and adoption
The main operational weakness of crate systems is access. Silt that enters the void cannot be removed without an inspection and jetting channel designed into the structure from the outset, and many crate types offer only limited internal access for CCTV survey. Pre-treatment is therefore not optional: a silt trap, catchpit or hydrodynamic separator upstream of the inlet protects the storage volume for the life of the asset. Water companies apply their own criteria for adoption of below-ground storage, and a system that cannot be inspected along its full length is unlikely to be adopted.
Crates versus a steel attenuation tank
Crates suit shallow, wide footprints, irregular plan shapes and sites where units must be carried by hand into constrained access. Corrugated steel attenuation tanks suit deeper installations, higher fill heights and heavy trafficking, offer full man-entry inspection along the barrel, and are installed as a single continuous structure rather than thousands of jointed cells. Where a site combines deep cover with an adoption requirement, the steel option normally has the stronger case; where the storage sits under landscaping at 600 mm cover, crates usually win on cost.
Related ViaCon solutions
ViaCon supplies corrugated steel attenuation storage as an alternative to geocellular crates. See our prefabricated steel attenuation tanks and the wider stormwater solutions range. Related glossary entries: attenuation tank, stormwater attenuation and soakaway.
Frequently asked questions about attenuation crates
What are attenuation crates?
Attenuation crates are modular plastic geocellular units assembled into an underground void that stores surface water run-off during a storm and releases it downstream at a controlled rate. They are wrapped in an impermeable geomembrane for attenuation, or a permeable geotextile where the volume is intended to infiltrate.
What is the void ratio of an attenuation crate?
Most proprietary crate systems declare a void ratio between 90 and 96 per cent, so the storage volume is close to the gross excavated volume of the crate field. Granular fill by comparison offers around 30 per cent.
How deep can attenuation crates be buried?
Cover depth depends on the declared long-term compressive strength of the crate and the traffic loading above it. Landscaped areas commonly allow shallow cover, while adoptable highway loading requires a higher crate class and often a reinforced concrete distribution slab. Deep installations under heavy fill are usually better served by a corrugated steel tank.
Do attenuation crates need a silt trap?
Yes. Silt that enters a crate field is very difficult to remove, so pre-treatment upstream of the inlet — a silt trap, catchpit or hydrodynamic separator — is essential to protect the storage volume and to satisfy adoption requirements.