When a hard rain falls on a city, it can turn quickly into an ecological disaster. That’s because in 775 mostly older cities where sanitary and storm water sewers are combined underground, the rushing stormwater often overwhelms treatment plants, forcing a stew of oil, brake dust, heavy metals, pet and human feces into nearby waterways. The resulting pollution can shut down beaches, enter the food chain and contaminate drinking supplies.
The traditional and costly remedy for the problem of “combined sewer overflow” has been to build underground tunnels or sinks to capture the excess.
Basically, these cities are becoming better sponges.
“We appeal for guidance to the ecosystems that are being taken away,” explains North Carolina State University professor Bill Hunt, a stormwater expert. “We build a constructed stormwater wetland that’s modeled on a naturally occurring wetland. It’s not a perfect cognate,” he says. “But it’s close.”
That’s a big change from the past. “The old approach was to get stormwater into the pipe and into the ground as fast as possible,” explains Jennifer Drake, who teaches at the University of Toronto’s Department of Civil Engineering: “But in nature, it’s a much slower process and the path is quite long, the water meanders so it doesn’t end up overwhelming a downstream river or flooding the banks. So we want to create porous places within a city to act like a sponge.”
These places include vegetated swales and bio-retention cells, fancy names for low tracts of land that collect and filter water, green streets and green roofs, rain gardens and rain barrels, bump-outs, infiltration galleries and pervious pavement. They are all designed to capture water and hold it until it until the water has either naturally percolated through the soil and been absorbed by plant roots or released into the atmosphere through the plants’ leaves, a process known as evapotranspiration.
The beauty of an intensive investment in green infrastructure is that the impact goes well beyond curbing stormwater flow. Green roofs, road gullies, curbside bio-retention plots and swales planted with trees and flowers and designed to hold water while it seeps into the ground, not only clean the water, but provide carbon sequestration, which helps slow global warming and combat urban heat islands, where temperatures can be as much as 5 degrees higher than the surrounding countryside.
Ref: by T.R. Goldman