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There are numerous ways to create a below ground stormwater attenuation system - in some situations, a traditional soakaway may still be the answer. However, the drive towards sustainable drainage (SuDS) schemes, has brought more sophisticated alternatives to the fore.
A typical example is the modular geocellular drainage system, often called "crates". Crates can provide many benefits and it is easy to understand why they have become popular. However, crates are not always the optimum solution, whatever the client expectation, and in many instances the ADS StormTech system gives significantly superior performance. ADS StormTech is the world's preferred proprietary brand of below ground SuDS attenuation system for a reason.
Footprint v storage volume
Whilst this is regarded as crates biggest strength, things are not always that simple. Whilst crates might be typically expected to provide a 95% void ratio and deliver high storage volumes for a specific footprint, StormTech's range of chamber sizes enables you to optimise the solution for the conditions of the site. The large chamber sizes enable you to get greater storage volumes and fast construction in areas with a tight footprint, whilst the smaller units are ideal for sites with shallow inverts. The ADS Design Tool allows the user to quickly produce an optimised design for your site that can fully integrate with CAD drawing files and specification documents.
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StormTech chambers come in a range of sizes for maximum design flexibility and performance
Hydraulic flow
Most crate systems take inflow into the entire tank storage space. With StormTech, however, the inflow is managed, and "First Flush" runoff is directed into the patented Isolator Row, an in-built water quality treatment device. Pollutants washed off a catchment surface during the initial stage of a rainfall event are directed exclusively into a discrete part of the StormTech system - where they can be separated from the cleaner runoff entering the system at higher flow rates - and retained safely for periodic removal. This means that only the Isolator Row will require cleaning, the other chambers are protected and remain clean throughout the life of the system.
Water Quality
There is no integral water quality treatment capability in a crate system. Instead, crates rely on upstream sediment removal systems and treatment trains. This could be something as basic - and potentially inadequate - as a catch pit, or more appropriately, a proprietary gravity separation sediment tank or hydrodynamic separator. Hydrocarbon removal will require a bypass or full-retention oil/water separator, depending on the intended use of the development and the level of risk of hydrocarbon pollution.
Not only can this lack of integral treatment in crates require significant additional capital investment in the treatment train (each manufactured treatment device can cost over £10,000, and multiple units may be required on a project), there is a potential additional operational cost in terms of inspection and maintenance.
The StormTech Isolator Row as an in-built water quality treatment device has 2 treatment stages: gravity separation and filtration. It is independently tested and validated to remove over 80% Total Suspended Solids (TSS). Often, this means that no additional sediment removal provision is required upstream.
An additional 2 treatment stages can take place within a StormTech system downstream of Isolator Row within the embedment stone: adsorption and biodegradation. This can lead to even greater removal efficiencies for TSS, hydrocarbons, metals and other surface water pollutants.
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StormTech's patented Isolator Row removes >80%TSS, hydrocarbons, metals and other surface water pollutants
Design standards
As a minimum, crate systems should fully comply with BS EN17150/BS EN17151 series and projects designed in accordance with CIRIA Report C737, Structural and Geotechnical Design of Modular Geocellular Drainage Systems. However, it should be noted that the side and top panels carry 100% of the weight without the ability to shed loads into soils or aggregate. Large beds of adjacent crates that fail can spread to the entire system. StormTech chambers exceed AASHTO LRFD design specifications and safety factors for HS-20 (i.e., HGV) live loads and deep burial earth loads. StormTech chambers also fully comply with ASTM F2418/ASTM F2922 product and ASTM F2787 design requirements. In contrast to crates, the arch design of StormTech shifts the load to the surrounding embedment stone whilst providing stability against buckling. BBA Certificate 07/4480 independently validates StormTech under strict assessment rules.
Cover depth
Many StormTech chambers can be installed with as little as a 450mm minimum cover depth under a road carrying HGV loads. This not only means reduced excavation and muck-away costs, but that flows can reach outfalls under gravity without the need for pumping. The minimum cover depth for crates under a road is typically 800mm. If a shallower cover depth is required, stronger, more expensive crates are needed. An alternative is to place a concrete slab over the crates, but this can make it impossible to access the system from above.
Inspection and maintenance
Even when crate systems can be accessed, the complex internal structure means cleaning is still difficult. Without expensive upstream silt removal, there is a risk of a build-up of material within the crates themselves. This will compromise the flow path and reduce the storage volume - which may eventually cause the whole system to fail. If the silt particles carry pollutants, they may flush downstream through the crates and result in water quality breaches, pollution, and fines. In contrast, Isolator Row is the only part of a StormTech system that requires cleaning. This is carried out using standard sewer jetting equipment and a vacuum tanker, with cleaning intervals typically between 3-7 years, depending on the nature of the site.
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Isolator Row requires cleaning every 3 - 7 years with standard sewer cleaning equipment
Ease of transport and installation
A 1000m3 crate system made up of flat-pack components may have over 10,000 separate parts for assembly and installation. The same 1000m3 tank made up of 400 litre pre-assembled crates will require 2,500 individual units. A 1000 m3 StormTech system would only require 200qty MC3500 Chambers, resulting in simpler and quicker installation. Additionally, StormTech chambers stack, meaning they're efficient to transport and take up minimal storage space on site.
Embodied Carbon
A crate typically has 45-60kg of injection moulded polypropylene per m3 of storage. A recent study indicated that a 300m3 tank would have an embodied carbon content of around 67 tCO2e over a 50-year design life. A StormTech system typically has 13-15kg of injection moulded polypropylene per m3 of storage. A 300m3 tank would have an embodied carbon content of around 25 tCO2e over a 50-year design life - a saving of over 60%.
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