How rooftop gardens can help combat flooding

In recent years, there seems to have been a rise in the extreme weather all over the world from terrible flooding in Bangladesh and Pakistan, the record cold snap in North America, to one of the wettest winters on record in the UK.

Extreme events are very difficult to tackle, and in some cases there is little we can do, other than increase our preparedness and our recovery response. However there is one thing we can do in response to smaller scale, more common events such as flooding from intensive rain showers. As winter closes in on, it’s worth looking at some of the ways we can better manage excess water.

People have tackled floods for centuries, but modern urban development has thrown up a set of new challenges. The more we develop the landscape, the more rainwater stays on the surface rather than sinking into the soil. That means water gets onto the roads and into drains more quickly, bypassing some aspects of the natural hydrological cycle.

Rooftop plants (including green-roofs and roofgardens) along with rainwater collectors and rain gardens can help slow things down and spread the impact of heavy rain out over a longer period. The idea is to replace some of the trees, grass, hollows and wetlands that have been lost to concrete, and so mimic a more natural flow of water.

One approach that aims to manage rainwater more naturally is known as Sustainable Urban Drainage Systems (SUDS). SUDS are particularly useful in helping to manage small but frequent floods from rainfall, just as the un-urbanised landscape would.

The system has three main aims: to catch and slow down the flow of water; to improve the quality of water by capturing and treating the pollutants it contains; and to benefit the local community by providing a green space that people can enjoy and where wildlife can flourish.

Reducing runoff

Managing water where it lands – at its source – is one of the most effective ways of reducing runoff. We can do this by creating green roofs and raingardens, for example, as well as by designing other mechanisms that slow down the flow of rainwater from our roads, roofs and driveways. These features are often linked to ponds, wetlands and temporary storage areas (called basins) which can hold and treat more water from a larger area.

Individual roof gardens, water collectors or raingardens may seem small-scale, but if many of these features are installed in an urban area and they also link up to a larger pond or wetland they can have a huge impact on slowing the flow, cleaning water of some of the pollutants it carries and also providing habitat, attractive settings and helping to recharge our streams and groundwater supplies in a more natural way.

Sustainable urban drainage is already being used to great effect in Portland, Oregon, where 3,500 trees have saved the city US$63m in pipe replacement, and in Malmö, Sweden, where 6km of water channels and ten retention pools helped an area that previously experienced chronic flooding. In Dunfermline, Scotland, sustainable drainage is being added to a greenfield pre-development.

Too much rain to drain

Sometimes it rains so heavily that no sensible drainage system could handle the flow. Where serious and sustained flooding is caused by unprecedented rain – as happened in the UK last winter – it has to be said that these small-scale systems are less effective. Sustainable drainage measures would probably have helped initially, but no form of water management could have accommodated rainfall on that scale.

Record-breaking rainfalls combined with high tides and high water tables presented us with a perfect combination of conditions that any traditional engineering would have been hard pressed to tackle. In these cases, we need to think about how to be more resilient to floods – how we can be more prepared, reduce the impact of the flood, and recover more quickly.

Researchers in the UK have been working on this for some time now, and in some cases such approaches have become law – in Scotland, for instance, all new developments since 2006 must have sustainable drainage.

SUDS can deliver sustainable solutions to our urban water management problems. They can give us healthier urban catchments, more livable neighbourhoods, and cleaner rivers and streams. And who doesn’t love a rooftop garden?

Published in collaboration with The Conversation

Author: Rebecca Wade is a Senior Lecturer in Environmental Science for Urban Water Technology Centre at the University of Abertay Dundee.

Image: A rooftop garden on a building in Durban. REUTERS/Rogan Ward.