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Sustainable drainage

With the emphasis currently on dealing with the aftermath of flooding, it’s easy to forget that we should be looking at the causes and solutions. Flooding is nothing new, it’s happened throughout history, but things have been made worse because of the way we’ve interfered with natural landscapes. We’ve constructed vast areas of impermeable surfaces that cause the water quickly to run off into watercourses, resulting in flash flooding.

Wherever possible the flow of water away from hard surfaces needs to be slowed. The simplest way of doing this is the humble water butt that collects water from roofs so it can be used when needed to water the garden. More sophisticated rainwater harvesting system allow rainwater from roofs to be stored and used not just in the garden but for flushing toilets, washing cars and even in washing machines.

Similarly, green or living roofs planted with sedum or indigenous wildflowers, herbs and meadow grasses not only help provide biodiversity but temporarily store water during a storm so its run-off is attenuated.

The rainwater run-off from paving, roads or driveways can be considerable. One way of dealing with this is through the use of permeable surfaces. Traditional materials such as shingle are ideal. Alternatively, specially designed permeable paviours can be used. In addition, sustainable drainage systems (SuDS) mange the water. These are designed to replicate as closely as possible the natural drainage from the site before development, avoiding the need for pipe and sewer arrangements or culverts which can become blocked and cause flooding.

Some housebuilders say that constructing SuDS is expensive but others believe that money can be saved by looking at the landscaping first as it cuts the need for major civil engineering. SuDS elements are generally relatively small scale and allow the water to soak into the ground and saturate the soil and vegetation, both managing run-off and contributing to the enhancement of the local environment.

SuDS take many forms, including shallow dips in the ground planted with vegetation that are know as swales; ponds; and storm-water storage systems consisting of buried, hollow plastic crates where water is temporarily held before soaking away naturally. As well as reducing the risk of flooding, such measures help recharge natural groundwater reserves, reduce pollution concentrations and provide habitats for wildlife.

Image: An ACO storm tank being installed to provide stormwater infiltration and attenuation Credit: ©ACO