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In the alley-center infiltration approach, permeable material (asphalt, concrete, or pavers) is placed in the center of the alley and high-albedo concrete is placed along the outer edges. This way, water soaks through the center of the alley, but truck tires roll on top of the stronger pavement.
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    Each of Chicago's four pervious-pavement designs can be tweaked to fit just about any alley in the city. This is an example of full-alley infiltration using permeable pavement. Source: Chicago DOT

Here's a little-known fact about Chicago: The Midwest city has more miles of alleys—1900 in all—than any other city in the world. That's the equivalent surface area of five mid-sized airports.

That's also 3500 acres of impermeable surface in a city that's committed to going green.

But going green isn't the sole reason for the city's $900,000 alley-renovation pilot program, funded by general funds.

“One of residents' biggest complaints is flooding,” says Janet Attarian, project director of the city's streetscape and urban design program. Heavy rainfalls, or rain events that last for days, flood alleys. The water spills off into—and overwhelms—the city's combined sewer/stormwater system, causing basements to flood and the system to overflow into the Chicago River.

“Basements become short-term retaining ponds until the system clears enough for them to be emptied out,” says Attarian. “Plus, we're discharging contaminated water into the river because our system can't handle it.”

According to Attarian, the city's in-place solutions weren't good enough. For example, repaving a puddle-prone alley with asphalt worsens the ponding, causing excess water to run off into back yards instead of into the street where it's properly drained into the combined sewer system. And connecting sewer mains from the alleys to the city's sewer system is cost-prohibitive.

Other options were needed.

The answer: Reconstruct alleys with permeable surfaces that allow water to pass through and infiltrate the soil below. Made of either porous asphalt and/or concrete or paving stones shaped to leave gaps at the corners, these pavements would reduce flooding, recharge groundwater, and save taxpayer dollars that would otherwise be spent treating stormwater or tearing up streets to install stormwater collection systems.

The Chicago DOT chose five alleys with maintenance problems and ideal soil conditions to test four anti-flood paving models, each incorporating high-albedo concrete and either recycled concrete or permeable pavement (concrete, asphalt, or pavers) as a base material. Surfaces are pitched and graded to direct excess storm-water toward the center of the alley, where it drains into the existing street sewer system. Underneath is 1 foot of either gravel or crushed stone that traps water until it soaks into the ground. Naturally occurring iodes attack pollutants such as oils and antifreeze, helping to break them down before the water reaches aquifiers. Optional inlet structures or stormwater infiltration trenches are also installed to store and dispense water while the soil absorbs it. To see a diagram of Chicago's pervious pavement design, click here.