Last month the Chinese city of Zhengzhou was inundated by record breaking rainfall. An entire average year’s rainfall dropped in just three days, and the consequences were catastrophic. Unable to absorb that quantity of water, streets flooded, rivers overflowed and houses collapsed. The subway line filled with water. 500 people had to be rescued from a flooded road tunnel, and 14 people drowned in their cars.
As greenhouse gases warm the atmosphere, these sorts of events become more likely. Warmer air holds more moisture, so rain events can become more extreme. There are examples closer to home, including the flash floods that closed nine London Underground stations this summer.
Dragan Savic, a professor of hydroinformatics, explains why this happens: “As the city has grown, we’ve concreted or asphalted over most of the land in the city, so there’s less capacity to soak up the rain that comes down.”
As this graphic from Yes Magazine shows, rainwater has to go somewhere. In nature half of it goes into the ground, and most of the rest evaporates. It’s different in an urban context. Rain falling on concrete and asphalt is not absorbed. A fraction of the rainwater goes into the ground, and the majority of it runs off the surface. This has to channeled safely through gutters and drains – but these were built before climate change. They don’t have the capacity to handle newly supercharged rainstorms, and so urban flooding becomes more likely.
As always, reducing emissions and avoiding the worst of climate change is the first priority. There are also adaptations that cities are going to need to consider. How do we make cities more permeable, so that water is absorbed?
There are many different ways to improve permeability, including rain plazas or absorbent paving. Grass verges to pavements have a number of advantages, including soaking up rainfall. Even more efficient are rain gardens, designed to flood and function as miniature roadside wetlands. Here’s one in Nottingham that has been retrofitted into a grass verge.
China has been a leader in this field, pioneering the idea of ‘sponge cities’ (which I’ve written about before). The city of Nanning, for example, has spent $1.6 billion to fit water absorbing pavements, green roofs and walls, and a large wetland park that can safely flood. After piloting a variety of techniques in three locations, Nanning is one of 30 cities in China that have received funding to become sponge cities in this way.
While there are some expensive retrofits and green infrastructure projects that can all help, there is a simple, cheap and obvious solution at hand: depaving. As it sounds, it involves digging up superfluous areas of paving and asphalt, and laying down more natural ground cover.
There are reasons why land gets paved over in the first place, and many areas are going to need to stay paved. But often paving is for perceived convenience, an approach that then backfires in heavy rainfall. It’s a false economy – saving on mowing a lawn for example, only to have to pay for drain maintenance or flood damage later.
I don’t have to go far to find suitable locations for depaving. Looking around my own neighbourhood and street, there are plenty of places where you cannot see natural ground. It is seamlessly paved in all directions, wall to wall, from the houses to the road and up the other side. We live at the top of a hill and aren’t going to get flooded ourselves, but that just means that the water that falls on us is going to end up as somebody else’s problem. So even here, we ought to be considering just how much paving we really need.
Depave is a little US charity that carried out depaving projects, (and from whom I have borrowed the header image) and they’re a good place to look for examples and guides for how to do it well. Their projects are modest and localised, but what they do is going to be a recurring theme in cities around the world in the coming years. Too many places have been too hasty with the concrete. All those unnaturally hard surfaces are creating a real problem, and it’s only going to get worse. As events in China have demonstrated, it’s no exaggeration to say that depaving may be a life-saving climate adaptation.