Realistic rainwater storage

Ian Crickmore of Polypipe Terrain explains how adopting rainwater harvesting systems can provide sustainable drainage solutions in space-constrained urban areas.

Obsessing over rain is something of a national pastime, but capturing rainwater could help meet sites’ increased drainage demands, particularly in dense urban areas.

Average rainfall in the UK stands at 33.7 inches per year, and potable water can be retained through roof top attenuation systems. Using these solutions, rainwater can be harvested and utilised in non-potable applications, providing a valuable resource.

In urban areas however, limited ground space prevents rainwater from draining away naturally, placing immense strain on surface water drainage and creating flooding risks, which architects should try to mitigate in their design.

Addressing space constraints

Attenuation can occur on many levels, from ground level to podium decks and rooftops. Storm water is typically stored at ground level, but in dense urban areas this is

often not feasible, as it would require the use of premium priced land, carrying out expensive excavation or even removing underground facilities.

A more cost-effective solution is to place the water management and source control systems above ground level, on roofs and podium decks. This way architects can overcome the space constraints while providing an efficient solution to cope with a site’s drainage demands.

Adding green roofs or ‘blue’ roofs to a development can also provide numerous benefits, whether to capture and control the flow of water into drainage systems, or to store and utilise rainwater as a resource for the building.

Sustainable alternatives

Green roofs, incorporating building drainage and irrigation, can be planted over a waterproof membrane to store and attenuate rainfall before dispelling it to the drainage system at a controlled rate. Whether partially or fully covered in vegetation, green roofs can reduce air pollution and improve air quality, while also adding to the visual appeal of a building.

Storing rainwater at roof and podium levels saves potable water, and allows it to be harvested and reused within the building. When required, captured water can also be treated and reused for the irrigation of green roofs.

Besides preventing flooding caused by excess rainfall in urban areas, green roofs also boost bio-diversity by providing a habitat for wildlife, birds and insects.

An alternative to green roofs is the blue roof. It retains water and dispels it into sewer systems at a rate agreed between the property developer and the relevant water companies.

Using the same waterproof membrane as the green roof, the blue roof is explicitly designed to store and control water at rooftop and podium level. Using flow control outlets, the blue roof retains as much or as little water as is needed to avoid overwhelming drainage and sewer systems.

A solution for any level

At our company, we use an attenuation system to produce effective blue and green roof solutions. This engineered geocelluar system is suitable for use at any level and uses interlocking modular cells to create an exceptionally strong structural raft with a high resistance to bending. The system’s strength makes it ideal for creating horizontal structural rafts.

It can also be combined in layers using interlocking shear connectors and the proprietary jointing system, which increases in depth by 85 mm or 150 mm increments, making it useful in infiltration systems. This not only allows for flexibility when balancing the soil permeability and infiltration area of the units, but also in the level of residual water.

When used within car parks, or in any other heavily trafficked areas in an urban setting where rainwater could be polluted with oils and hydrocarbons, a SuDS (sustainable urban drainage systems) geotextile is used to treat storm water before it is dispersed back to the watercourse. Non-woven and needle punched, these filters contain specific treatment properties that remove oils from the water and provide hydrocarbon pollution treatment in SuDS applications.

With growing population densities placing increased pressures on surface water drainage systems, it is now more imperative than ever that architects design sustainable drainage solutions to capture and manage rainwater via systems that can be seamlessly incorporated into a city’s existing infrastructure.

In addition, specifying blue and green roofs can provide drainage in dense urban areas while also delivering cost and environmental benefits.

Ian Crickmore is technical director at Polypipe Terrain