Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | SEPTEMBER 2015 | 39 In the know Digging deeper: stormwater management W hen it comes to stormwater management tanks – or Sustainable urban Drainage Systems (SuDS) – the goal has always been to replicate natural drainage as closely as possible. In this sense, there is nothing new about SuDS. However, with demand on the rise and external factors like climate change playing a highly influential role, today's stormwater systems have had to evolve significantly to meet changing requirements. As a result, we've seen the introduction of pioneering new developments in system design and maintenance – which have made modern stormwater solutions more bespoke, reliable and efficient than ever. Recent changes to the Flood & Water Management Act have been a particular milestone for the stormwater industry. These stipulate that a project's SuDS design has to be approved as part of the planning application before work starts. This welcome development has cemented the importance of making effective drainage part of a project's plans from the outset, rather than an a‰erthought. At a practical level, it means today's specifiers need to make the right decisions about the system's hydraulic and structural design in advance. However, to ensure best long-term results, they Storming ahead Recent years have seen a marked development in the way stormwater systems are designed and maintained to ensure sustainable, long-term performance Kevin Reed naTional SPEcificaTion ManagER gRaf UK Stackable systems have made for easier transport and storage also need to plan ahead for on-site activity, including installation, and ongoing maintenance. designing for success Effective hydraulic design is all about ensuring SuDS are set up to handle the right stormwater conditions – both today and throughout the lifetime of the tank. Traditionally this has meant designing them with a 30-year storm in mind (the kind of weather event that is statistically likely to happen just once in a 30-year period). However, over the last five years, as the threat of climate change has heightened, we have seen a shi‰ in standard practice – and most systems are now designed for a 100- year storm, with a 20% contingency buffer on top of this. This change in approach has been reflected in a tightening of outgoing flowrate regulations for both greenfield and brownfield sites, to ensure that existing stormwater sewers and water courses are not overloaded for individual applications, even in extreme conditions. For attenuation systems, the rate stored water is released to the public sewer is usually set by the water company, Environment Agency or local authority. For infiltration applications the size of the soakaway will depend on the natural infiltration rate of the soil (determined by carrying out on-site testing). With so many factors at play, this can seem like a minefield at times, and as a result there are now so‰ware programmes (for example, Graf UK's Storm Flow programme) that provide accurate sizing support for specifiers. From a structural point of view, modular stormwater tanks have become the norm over the past 12 to 15 years – offering greater volume capacity and space efficiency than more traditional drainage solutions, such as rubble pits. Opting for below-ground systems also enables developers to free up on-site space that can be put to other uses. With the right system, it's possible to use this space as required, whether it's a play area, roadway or even a heavy- duty lorry park (with a load up to 60 tonnes). To this end, the structure and material of modular tanks is integral to performance, ensuring the system remains as lightweight

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