CPD: Drainage and stormwater management

A Wavin AquaCell Prime geocellular stormwater management unit has been installed at Hilton Group’s £60m hotel development in Bournemouth
As local authorities sanction more homes to be built on floodplains and climate change makes rainfall patterns increasingly unpredictable, stormwater management is now a top priority, says Tim Wootton.

In the current economic and environmental climate, stormwater management and flood control is uppermost in many people’s minds – but this has not always been the case. Continuing urban development, a changing climate and intensive rainfall has led to rainwater becoming an issue that just will not dry up without investment and planning.

Following a year that included the wettest, most prolonged winter on record and some of the most devastating floods in UK history, it is no surprise that surface water run-off has become a crucial consideration for every construction project.

Surface water has to be controlled in ways that avoid using an already overloaded sewer system. Many existing drainage systems can cause problems such as flooding, pollution or damage to the environment, and so are not sustainable in the long term.

As a result, housebuilders and developers across the UK are adopting best practice sustainable drainage systems (SuDS) early in the planning process to mitigate flood risk.

A sustainable solution

The more we urbanise our landscape, the more we reduce the amount of available land where rainfall is able to soak away into the ground. The traditional approach of combining the surface water with the foul sewage system is costly, but separate surface water sewers that discharge directly into local water courses pose a potential environmental risk if misconnections occur between the two systems.

Both of these outdated systems are already demonstrating key failings, so any increase in intensity of rainfall may cause them to be overwhelmed, which, in turn, would cause increased widespread flooding. As new developments are built – some on floodplains – the need for proper flood risk management is set to become more important than ever.

Sustainable drainage mimics natural processes by allowing rainfall to soak into the ground where possible, or by delaying discharges. Reducing both the volume and rate of surface water run-off to sewers and water courses helps to improve water quality, as well as the ecology and amenity value of water courses.

It is important, however, to remember that there is no single drainage solution for any one site.

Developing a SuDS strategy

As local authorities across the UK wave through developments on floodplains to satisfy demand for new housing, they have recognised the need to work with developers to manage the risk of surface flooding using SuDS.

Specifying drainage systems at the earliest stages of site selection and design will allow easier integration into developments, while at the same time influencing other aspects of the site and reducing impermeable areas, where possible.

Effective early engagement with stakeholders should also help deliver SuDS in multifunctional spaces such as car parks, footpaths and verges, gardens, landscaped areas, driveways, courtyards or communal spaces. This should improve cost effectiveness and integration within the development, resulting in better places to live.

The numerous and varied characteristics of a site make certain sustainable drainage functions more appropriate than others. The variety of SuDS components and design options available allows designers and planners to consider local land use, future scenarios and the needs of local people in drainage design.

Options that can be considered range from natural above-ground SuDS solutions such as swales, detention ponds, basins and permeable surfaces, to engineered solutions such as concrete culverts, plastic pipes, attenuation tanks and soakaways.

Benefits of modularisation

Faced with rising costs and stricter deadlines, modularisation is growing in popularity as contractors seek the next generation of efficient and economical products and systems.

Because of the numerous benefits both on and off site, modular rainfall management has become an increasingly popular choice at every stage of the supply chain, from the architect and specifier to the contractor and client.

The increased focus on SuDS compliance has led to a sharp increase in the use of geocellular stormwater management units. A high-strength, factory-engineered unit can be constructed to accommodate calculated run-off volumes, and provide greater assurance to developers, local authorities adopting the drainage systems, homeowners and insurers alike.

Wavin AquaCell is a modular technique for managing excessive rainfall and recharging local groundwater. It is available in four options, each suited for different applications in terms of the vertical loading and surface traffic above. The units can also be used in combination with each other (see case study, below).

The modular nature of AquaCell gives it flexibility of scale, ranging from just a few units acting as a soakaway, to thousands of units to create a vast underground reservoir. As part of a complete solution, it can work with traditional SuDS techniques to provide effective long-term flood protection.

In terms of assembly, geocellular AquaCell units can simply be clipped together to form underground structures for infiltration or the temporary storage of stormwater. For additional strength and stability, they can be designed and laid in a “brick bonded” format in the required volume before being wrapped in either geotextile or membrane options.

The fact that the tanks are buried allows the land above to be used as site amenity or to increase biodiversity. This can be safer and more valuable to some stakeholders and the wider public than the alternative of large ponds or basins.

A modular system is scaleable so it can be used for small-unit schemes, as well as large-scale developments

The ideal SuDS scenario is about managing the amenity value, and creating a happy medium between landscaped and engineered solutions. In the case of engineered solutions, the shape of the tank can be irregular to maximise land use. Housebuilders calculate the amount of storage required, depending on site conditions, and whether they are designing and building to protect against a one-in-30-year or a one-in-100-year rainfall event.

Specifying the right unit

There are five factors to take into account when designing a geocellular structure for trafficked or landscaped installations. These are depth of cover, soil type, surface finishing, presence of groundwater and the type of traffic/loading. The combination of these will determine which unit needs to be specified.

To guarantee the structural integrity of an engineered drainage system, any underground structure must be strong enough to support the loads to which it will be subjected without any unacceptable deflection.

The correct choice of geocellular unit must have appropriate proven top (vertical) and side (lateral) load-bearing capacity and deflection characteristics to suit site conditions. For example, the AquaCell Eco has a vertical loading capacity of 17.5 tonnes/sq m and lateral loading capacity of 4 tonnes/sq m.

These loading capacities define the maximum depth parameters for which each system is suitable. For example, the AquaCell Core is suitable for installations to a maximum depth of 4m, and can cope with vertical loading of 56 tonnes/sq m and/or heavy traffic from HGVs.

In certain situations – for example, a deep-running drainage network, other buried services running above the tank location, banked or sloping ground and an upper layer of clay that prevents infiltration – installations may have to have greater cover depths. In these cases a unit with a higher loading capacity will be required.

It is important to remember, however, that minimum depth of cover will vary according to whether or not the installation will be subject to trafficking by cars or HGVs.

For example, a landscaped or non-trafficked location with a 0.3m cover depth will typically require a minimum vertical strength of 17.5 tonnes/sq m. Alternatively, a car park with occasional light delivery traffic and 0.71m-0.75m cover depth will require minimum vertical strength of 40 tonnes/sq m.

Legal compliance

Before undertaking any installation it is important to consult best practice guidelines and to ensure compliance with legal requirements.

CIRIA (Construction Industry Research and Information Association) report C680, Structural Design of Modular Geocellular Drainage Tanks, provides guidance on how to specify attenuation and soakaway systems using plastic modular storage.

Wavin is the leading supplier of plastic pipe systems and solutions in Europe. The company provides essentials: plastic pipe systems and solutions for tap water, surface heating and cooling, soil and waste, rain and stormwater, distribution of drinking water, and gas and telecom applications. Wavin’s headquarters are in Zwolle, the Netherlands, and it has a presence in 24 European countries. The company employs around 6,000 people and reported revenue of €1.3bn for 2011. Outside Europe, it has a global network of agents, licencees and distributors. Since mid-2012, Wavin has been part of the Mexichem Group – a leader in plastic pipe systems, and in the chemical and petrochemical industry in Latin America.

Wavin UK

Established in 1962, Wavin Limited is the UK’s leading supplier of water management, plumbing, heating and drainage systems for the building, construction and utilities markets. Highly respected within the construction industry, Wavin has a reputation for innovation and leadership, and has an extensive product portfolio under the OSMA, Hep2O and Hepworth brands, which are available through plumbing and builders merchants throughout the UK.

The Flood and Water Management Act was enacted in 2010 to address the enormous physical and economic damage caused by the floods of 2007.

The legislation continues to guide housebuilders in preventing or dealing with surface water. It lays down guidance on the best way to alleviate the flood problems of previous years.

By standardising the design philosophy, it ensures sites are designed in the best way possible, and is augmented by information and guidance from organisations such as CIRIA.

The UK government and other interested bodies are promoting a more holistic approach to the design of sustainable drainage systems with a focus on solutions that mirror nature in terms of discharging water into the ground at the point of contact.

Developers that seek to maximise returns through land use tend to favour a mixture of conventional SuDS with engineered solutions to most efficiently address their site drainage needs but at minimum cost of land.

As concern continues to grow over climate change and urban development, geocellular stormwater management units are increasingly popular at sites across the UK.

From individual houses to large-scale housing and commercial schemes, geocellular units such as AquaCell can work with the more traditional SuDS solutions to provide a more effective flood protection system in the short and long term.

Tim Wootton is technical manager at Wavin

Home and dry in the Firth of Forth

Built on the south shore of the Firth of Forth, a new 450-home development in South Queensferry, Edinburgh, required an underground drainage system that would address surface water run-off and alleviate flood risk. A combination of more than 6,000 AquaCell geocellular stormwater management units from Wavin was specified.

Designed by EMA Architects, the joint venture development between Cala Homes and Barratt Homes will feature three-, four- and five-bedroom detached homes, townhouses, and two- and three-bedroom flats on the 35-acre site.

Following recommendations from civil engineer Halcrow Group, groundworks contractor Mulholland Contracts specified a series of attenuation tanks for three areas of the development to reduce the risk of flooding.

At the largest tank in the centre of the development, 3,610 AquaCell stormwater management units have been installed beneath the main bus turning point. Manufactured from specially reformulated, recycled material for trafficked and deep applications, 2,166 AquaCell Prime units were specified to create the first three layers of 19m x 19m x 2m cellular unit soakaway.

To complete the tank, Wavin supplied 722 AquaCell Core and 722 AquaCell Plus for the bottom two layers. Suitable for areas subject to regular and heavy traffic loading, AquaCell Core provided the required strength, while a layer of AquaCell Plus was specified for its inherent suitability for installations where extra depth is required. Installed at a depth of 4.48m, the tank was then backfilled with aggregate to 100mm above the units.

With the 450-home development set for completion in 2017, Wavin’s AquaCell range of systems will ensure that the new residents will benefit from a proven stormwater solution that delivers effective flood protection in both the short and long term.

This article has been created by Construction Manager in partnership with Wavin UK

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