Sustainable Urban Drainage Systems might look straight forward, but contractors and designers are under increasing pressure not only to protect homes against flooding, but to improve water quality as well. B&E hears from Dr Jason Shingleton, Marketing and Development Directors of Polypipe Water Management Solutions about how the industry is coping with change.

ON AN undeveloped site, typically 15% to 20% of the rainwater becomes direct surface runoff, which then drains away slowly into the nearest watercourse without any need for control. However, in developed catchments direct run-off can be up to 80% of the rainfall volume, impacting dramatically on the existing drainage network.

With peak flow rates often increasing by a factor of ten, streams and rivers must cope with much larger and sudden run-off flows from a developed site. Integrated Sustainable Urban Drainage Systems (Suds) can control the excess stormwater at source, reducing the impact of downstream flooding and facilitating the carriage of water run-off at a controlled rate.

Quality control

As the varying environmental legislations have developed to encourage the use of Suds the debate has shifted to encompass the notion of not only controlling stormwater at source, but also improving water quality. One argument is to prevent water contamination in the first instance to ensure that no treatment is needed. However, in certain circumstances this is neither practical nor realistic. Water run-off from areas such as car parks, roofs and industrial areas is often contaminated with a wide range of pollutants, including oil, silt, organic debris, pesticides and detergents. These can all have a harmful effect on the environment downstream and with regards to dirt and other organic debris specifically, such contaminants can actually hinder the performance of any drainage solution by silting up the application.

The National Suds Working Group states that rainwater harvesting is the preferred option for stormwater management, and should be the first investigated. Not only does it control water at source but it also enables the user to benefit from up to 50% saving on water bills. Providing non-potable water to the user for their toilet flushing, washing machine and garden use provides a significant reduction in mains water consumption, which is an essential ingredient for the new Code for Sustainable Homes released by the Government in December 2006.

Within the code minimum levels of potable water consumption are set in order to meet the criteria for the varying levels of star rating. With an average household using 150 litres of water for each person a day, in order to reach the highest ratings it is fair to assume a rainwater harvesting system would need to be included within a mix of water efficient products. The rainwater harvesting tank helps to meet the minimum requirements for water consumption and can be integrated to meet the criteria for surface water run off.

If rainwater harvesting is not possible, or desired, the Building Regulations Approved Document H3 states that a suitable soakaway application should be sought. However, the use of soakaway is not always possible as ground conditions can often affect its use, such as in clay soils. The Environment Agency (EA) requires soakaways be used for roof water only before it suffers contamination from other sources.

Trouble with rubble?

The major concern with a rubble-filled soakaway is that there is a potential for contaminants from the rubble to seep back into the watercourse and for the soakaway to silt up and lose its effectiveness. Modular cells, should be installed with a silt trap to prevent silt migration, cleaning the water before allowing it to enter back into an existing watercourse.

In cases where a soakaway application cannot be used, the next logical step is to examine the use of attenuation.  This can be achieved by wrapping a Suds cell structure in a geotextile to create an impermeable tank to store stormwater ready for discharge at a controlled rate, via a flow control device. Where a cell structure is not suitable, for example in areas which may be subjected to differential movement such as airport runways or where adoption by the water authority is required, twin wall pipes with welded end plates can be used instead.

For any of the above systems to be fully effective they must have the most appropriate maintenance in place. Silt traps and petrol interceptors can be used, to filter the water before it enters the attenuation tank or cell structure. This is even more important when taking into consideration the Water Framework Directive which goes further than the Building Regulations and planning policy statements by stating that water run-off must be cleaned at source, not just controlled. Using silt traps, a fully maintainable system can be easily specified.

Going underground

The use of “hard Suds”, such as modular plastic Suds, as opposed to “soft Suds” such as lagoons, has increased in recent years, as architects and developers make the most of the space available to them. This is especially important with the introduction of the PPS3, which dictates a minimum number of dwellings per hectare, resulting in an efficient and less intrusive form of land use.

While soft drainage methods such as swales and balancing ponds have their place, modular Suds have exceptional drainage without impeding on the development footprint. Cells can be installed on a residential development underneath driveways and other trafficked areas to provide drainage without disturbing the aesthetic appeal of the site and taking up valuable land space that could otherwise be developed.

The Water Framework Directive will impact greatly on local government planners in exercising the need for water to be cleaned, as well as controlled, at source. It is expected to be introduced as early as 2010, and will provide a new challenge for installers and Building Control.

Suds do not have to simply be about the above in isolation, more importantly it can be about a holistic combination of various applications, be it rainwater harvesting used for attenuation and recycling, or rainwater harvesting used with soakaway to meet recycling, soakaway and attenuation requirements. There is a multitude of options, which can be tailored to meet even the most strict site constraints, whether a full end-to-end or a standalone application has been specified.

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