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Conservation Science for a Healthy Planet

How to Build a City That Doesn’t Flood? Turn it Into a Sponge

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From Iowa to Vermont and from San Francisco to Chicago, urban infrastructure is getting a reboot.

….Creating better stormwater management systems requires using green infrastructure elements in urban planning and restoring some of the rain-retention capacity that cities have lost to urbanization. These elements can be roughly broken into two categories: the man-made engineered replacements of the natural water pathways and the restorations of the original water routes that existed before a city was developed.

….Traditional road construction, made with asphalt, gravel and sand, is a very compacted structure that leaves little space between the particulates, and thus no room for the rainwater to seep through. In the construction industry that gap measure is described by the term “air void,” which is typically set at four percent for the traditional pavement mix, says Richard Willis, Director of Pavement Engineering and Innovation at National Asphalt Pavement Association.

One way to make cities spongier is to use permeable pavements, such as porous asphalt made with a lot of large stones rather than fine aggregates such as sand, and with added cellulose fibers to hold the porous asphalt together. This creates more pores, and increases the air void up to 15 or 20 percent, allowing more rainwater to seep through.

….Another way to make cities hold water is by building rain gardens and bioswales. A rain garden is a depression in the soil seeded with native plants that helps soak up rainwater. With that setup, house spouts can empty into a rain garden instead of a sewer, decreasing sewage overflows in heavy downpours. A bioswale is a rain garden on a larger, more engineered scale. It is constructed by creating deeper and larger depressions where water can temporarily accumulate and drain out slowly.

….Green infrastructure for sponge cities can also include non-engineered solutions—such as restoring urban forests and increasing their ability to absorb stormwater runoff. In Seattle, urban planners got rid of invasive species such as English Ivy and Himalayan blackberries and restored native evergreens that do a better job of stormwater retention.

….For countries in the developing world, which are on the frontlines of climate change, the problem is more urgent and monetary resources are a problem. In these countries, solutions that follow the Seattle model are increasingly being embraced, says Sarah Colenbrander, Senior Researcher at the London-based International Institute for Environment and Development. From Kampala, Uganda to Bangalore, India urban wetlands and woodlands are being restored in many cities. The biggest stumbling block, according to Postel, is scalability: can one-off examples work on a larger country-wide scale? That can only happen with a significant boost from policy implementation and top-down legislation, she says.
Studies found that local building codes often create needless impervious cover while giving developers little or no incentive to conserve the natural areas that are so important for the natural water flow. The world needs to rethink its cultural expectations of what a prosperous and successful city looks like, Colenbrander says: “Is it a city like Sydney or Los Angeles where everyone has a white picket fence and a nice garden? Or is it a city more like Hong Kong or even central London where people live much more densely and have a communal green space together so you have less of an ecological footprint?”….

A diagram of a water retention system. Credit: Pittsburgh Water and Sewer Authority

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