Green AlleysReduce Stormwater Runoff

August 11, 2010

Chicago has about 1,900 miles of public alleys—more than any other city in the world. The Chicago Department of Transportation's Green Alley Program is an environmentally minded approach to upgrading alleys that flood or need structural repair. Six alleys have been converted and more are in the works, says Janet Attarian, project director of the city's streetscape and urban design program.

The DOT is using permeable pavement in the form of permeable asphalt, concrete, and pavers to allow water to seep through the porous surface into the subsoil. Initial construction costs are higher than for conventional paved alleys, but green alleys can capture about 80% of rainwater, reducing strain on the sewer system. The pavement can withstand snow plowing and salting; icing is reduced because melting snow seeps into the surface.

High-albedo pavement is also being used. Its light coloration reflects sunlight away from the ground, reducing urban heat island effect. Recycled construction materials, such as concrete aggregate and slag (a by-product of steel production) are also being used in the green alley concrete mixes.

Green alleys are also being fitted out with dark sky-complaint light fixtures. The fixtures direct light downward—where it's most needed—reducing light pollution and glare. The fixtures also have metal halide lamps, which produce a white light, instead of the yellow light from existing sodium vapor lamps.

So far, the experimental green alleys have been performing well, and reaction from neighbors has been “overwhelmingly positive,” according to Chicago DOT spokesman Brian Steele.

By the end of the year, the city will have completed nearly 40 green alleys, including 10 traditional alleys with high-albedo concrete, seven alleys with open-bottom catch basins, and 21 alleys with permeable asphalt.