- There’s a global movement 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.
….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.