Urban floods make the news with alarming regularity. Just in the past few months, Hurricane Harvey submerged Houston, and the seasonal monsoon crippled cities in South Asia. Dramatic floods from increasingly severe storms come with a steep cost, both human and financial, and the problem will only get worse with climate change. One of the biggest culprits for the deadly toll these floods wreak? Urbanization.
As cities develop, miles of impervious pavement are laid over forest or wetlands, displacing the natural flood management systems like creeks, underground streams, or bogs. In a completely uninhabited landscape, rainfall integrates into the natural water cycle by four different ways: it either soaks all the way to the ground and becomes groundwater; runs down valleys into bodies of water and finds its way to the sea; is taken up by plants; or just evaporates. In urban or suburban sprawls with paved roads, highways, and parking lots, water has nowhere to go, so every heavy rain can turn into a flood.
The number of cities around the world is growing quickly. In her book, Replenish: The Virtuous Cycle of Water and Prosperity, Sandra Postel, the director of the Global Water Policy Project, reports that over the past 35 years, the number of cities in China alone has climbed from 193 to 653. As urban and suburban areas expand, the stormwater runoff problems will grow as well.
But now there’s a movement around the world to build smarter and “spongier” cities that can absorb rainwater instead of letting it flow through miles of pavement and cause damaging floods. 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.
Man-Made Solutions: Rain Gardens, Bioswales, and Porous Pavements
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.