How a city can be more like a forest

One defining characteristic of a city is that it is full of hard surfaces. Streets, sidewalks, buildings and bridges shed water when it rains. All that water has to go somewhere, and it usually gets there fast. So for a city to function more like a forest or a field, step one is to slow down the water.

The issue is becoming more urgent. According to the Natural Resources Defense Council (NRDC), a leading New York–based policy, science and advocacy organization, over the past 50 years the number of days with heavy precipitation events has increased more than 50 percent. If this trend continues, as climate models suggest it will, flash floods will pose an ever greater risk both to people and to the infrastructure built over decades — even centuries, in some places — to handle stormwater. On the flip side, droughts elsewhere will put drinking water supplies under greater stress and increase conflicts among agricultural, industrial and residential water users.

The systems that deliver water literally are crumbling beneath our feet. In 2009, the American Society of Civil Engineers — the same group that gave the nation’s levees a D — gave the same lousy grade to U.S. wastewater facilities. For example, as of 2010, the budget for Washington, D.C.’s water department, funded entirely from residents’ water bills, was so low and the water mains and other infrastructure in such disrepair that replacing the entire aging system would take 300 years.

One estimate says that at least an annual investment of $180 billion is necessary for urban water infrastructure to deal with rapid population growth. If even 10 percent of this investment could be made in green infrastructure, that would represent a figure greater than the combined annual budgets of the major conservation NGOs, greatly enhancing the goals of conservation while providing necessary water to people.

The role of water is so important to green infrastructure that some experts speak of blue-green or turquoise infrastructure. The reason is clear: in natural conditions, rocks, soil, plants and trees keep water where it falls, or slow water down on its way into wetlands, streams and rivers. As a result, only 10 percent of rain becomes runoff, half gets absorbed and the rest goes back into the air as water vapor. The páramo in Ecuador is a great example; a giant sponge that soaks up water where it falls. When the ground cannot absorb rain — whether because cattle have pounded it solid or because people have built office towers, apartment complexes, roads and parking lots on it — nearly all the rain washes away, carrying pollution with it, and with increasing frequency overrunning stormwater systems.

City/tree image by Parinya via Shutterstock

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