יום שישי, 21 באפריל 2017

Why Smart Cities Are Turning Themselves Into Sponges

Nearly 800 cities have problems with overflowing sewers. They’re using green techniques to keep the water from ever reaching the drain.

By T.R. Goldman

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. In large cities, such as St. Louis, such projects can total close to $5 billion. But engineers are discovering greener and often cheaper ways to prevent water flowing into storm drains by recreating (and in some cases improving upon) the earth’s natural filtering system.

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.

“You’re creating urban habitats,” says Drake, “so you’re also making pollinating spaces for bees, and public spaces that are beneficial for mental health.”

Here are five cities using innovative green technology to manage stormwater runoff before it ever hits the drain:

Chicago — A 55-gallon rain barrel might not solve the world’s stormwater runoff problems, since a 1,000-square-foot roof delivers more than 10 times that much water from a 1-inch rain. But rain barrels are still a vital tool, which is why the city spent $6 million over two years to give away 123,000 rain barrels to its customers. “It’s the gateway drug into storm water management because it gets people thinking about water and thinking about managing water,” says Hunt at North Carolina State University.

A police officer drives through floodwaters from 2016 Hurricane Matthew in North Carolina. | AP Images

The key, notes David St. Pierre, executive director of Chicago’s Metropolitan Water Reclamation District, is to encourage people to “disconnect” by removing their downspout from that direct connection to a main sewer line. “If you can take these direct stormwater connections out of system, that’s your highest return on investment,” he says. And while “it might be a rain barrel today, tomorrow it could be a 500-gallon cistern included underneath every new home, so that people could reuse their stormwater runoff in a very significant way.”

Fayetteville, N.C. — This eastern North Carolina city, home of Fort Bragg, took a dilapidated, two-block slice of downtown already slated for revitalization and turned it into the state’s first fully developed green street. A “road diet” eliminated three superfluous lanes, allowing a wider sidewalk and the creation of 13 sunken “landscape islands,” where water enters through a break in the curbing. The islands are filled with 3 feet of “expensive dirt mixed with sand and organic topsoil,” explains city engineer Giselle Rodriguez, who shepherded the project last year. Interlocking concrete pavers were put down in some parking areas, which also allow water to drain into the ground.

The goal of the $1.3 million project: to permit 85 percent of rainwater to drain into the ground instead of nearby Blounts Creek, where it would normally run. And it seems to work. Rodriguez says the green street was “able to tolerate Hurricane Matthew’s 15 inches of rain without a problem.”

Philadelphia — The nation’s fifth-largest city announced plans to spend $2.5 billion from 2011 to 2036 to turn 10,000 acres into green spaces. That includes one-third of the impervious areas in the older, urban core, which has a combined sewer and stormwater system. By doing so, the city hopes eventually to cut stormwater pollution in local rivers and waterways by 85 percent.

But that’s not all. As part of its stormwater management program, the city helps subsidize residents who want to replace their water-shedding driveways and concrete patios, with porous pavers. Philadelphia is also one of the few municipalities with a separate stormwater fee based on the percentage of a property’s impervious surface. New developments the size of three townhouse lots or bigger are required to put systems in place that will guarantee the capture of the first inch of any stormwater runoff.

Oak Creek, Wis. — You’ve heard of highly efficient homes with net-zero energy use. Why not highly efficient developments with net-zero stormwater use? The town of Oak Creek, a few miles south of Milwaukee on Lake Michigan, did just that with its 2016 Drexel Town Square development. The 85-acre site, a former Delphi Automotive plant, already had a large wetlands area, although it was dominated by cattails and red canary grass, an invasive species. So Sue Winnen, an environmental engineer for the town, decided to turn it into a “fully functioning wetland, with flowers and pollinators and birds, something that would soak up the water and hold it, while slowly releasing it into the ground.”

An architectural sketch of the Drexel Town Square development in Oak Creek.

Old parking lots were replaced with pervious pavers set on top of stone. After a rain, says Winnen, you can feel the water under the pavers as you walk on them. And a sloped parking lot sends that runoff into three bioswales on the opposite corner from the wetlands. The result: Despite a very wet spring, says Winnen, the amount of water flowing out of the entire development’s sole drainage pipe is the equivalent of a few garden hoses of water.

Toronto — North America’s first green roof bylaw took effect in Toronto in 2010 for commercial and residential buildings that had more than 20,000 square feet of floor space. Depending on the size of the building, anywhere from 20 to 60 percent of the available roof space must become a green roof, outfitted with a waterproof membrane, a growing medium, and plants. By the end of 2016, some 400 green roofs had either been built or were in the permitting process, covering the equivalent of about 60 NFL football fields.

“There’s not a sea of green roofs yet, just a patch here and a patch there,” says Jane Welsh, the team lead for environmental policy in the Toronto planning department, looking out over Canada’s largest city from her office on the 22nd floor. “But we get excited, of course. And there’s a very strong perceptual flavor that you’re starting to feel that Toronto is a green city.”

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