Since 2006, the Chicago Department of Transportation has been upgrading the city’s alleys with state-of-the-art green pavement materials and designs to better manage stormwater and prevent flooding. The agency also is testing use of reflective surfaces to reduce the urban “heat island” effect, and is increasing use of recycled materials for rehabilitation of alleys. Chicago’s Green Alley program was launched to help address rainwater collecting in alleys and flooding surrounding areas.  Additionally, the program helped meet goals to mitigate and adapt to the effects of climate change established in Chicago’s Climate Action Plan. Each of the city’s departments was charged with determining how climate change will affect its programs and taking action to help mitigate and adapt to the expected impacts, including increases in temperature and more frequent and severe flooding.

How the Program Works

Chicago’s urban landscape includes more than 1,900 miles of public alleys accounting for more than 3,500 acres of impervious surface, one of the largest alley networks of any city in the world, Leopold said. Rehabilitation of the city’s alleys using green techniques offered a good starting point to help relieve environmental stresses on the city’s transportation and sewer infrastructure. Most of the aging alleys throughout the city are not connected to the city’s storm sewer system and are prone to flooding. When flooding problems occur, instead of tearing up the alley and diverting water to the sewer system, officials now install permeable surfaces that slow down the flow of water and allow natural infiltration and recharge to the groundwater below.

The Green Alley program began with five pilot projects, and soon expanded for use on a regular basis. Rehabilitation using green infrastructure practices is taking place as the need arises to upgrade existing alleys. As of the end of 2009, the city will have installed more than 100 green alley designs throughout the city.

The Green Alley Handbook

To help get the word out on its sustainable infrastructure practices, the city published the Green Alley Handbook, which describes best management practices used in the program and examples from pilot projects. The handbook describes the following types of Green Alley techniques:

• improved drainage through proper pitching and grading of the alley;
• use of pavement materials such as permeable pavers, permeable concrete, and permeable asphalt;
• installation of “high albedo” pavement which is light in color and reflects sunlight away from the surface rather than absorbing and radiating heat.
• use of recycled construction materials, including recycled concrete aggregate used in concrete mix and as a base beneath surface paving, use of slag from industrial processes as a component of concrete mix, and use of ground tire rubber in porous asphalt and reclaimed asphalt pavement in non-porous asphalt;
• use of energy efficient, “dark sky compliant” lighting that directs light downward and reduces light pollution.

The handbook describes four applications that used different combinations of these techniques based on site conditions. These included use of green pavement materials with conventional drainage, use of full alley infiltration using permeable pavement, use of center alley infiltration using permeable pavement, and use of green pavement materials with a subsoil filtration system. It also recommends a variety of best management practices that adjacent property owners can use, including recycling, composting of yard waste and scraps, planting shade trees and native plants, use of permeable pavements and green roofs, installation of energy efficient and dark-sky lighting, and creation of naturalized detention and vegetated swales to encourage stormwater infiltration.

Lessons Learned

The agency has had some “lessons learned,” including the need for increased maintenance for the permeable surfaces. The pervious pavements need to be cleaned on a regular basis to maintain permeability, and cleaning must begin before the pavement becomes deeply clogged with debris. City officials have found they can get the job done by running their traditional street sweepers twice a year – in the fall and the spring – as part of a regular maintenance routine for the green alleys. Chicago DOT is continuing to monitor the performance of green alleys to determine whether maintenance practices are sufficient and to measure infiltration rates, pavement strength and durability, and reflective characteristics of the materials.

For more information, link to the Green Alley Handbook or contact David Leopold, Project Manager, Streetscape and Sustainable Design Program, Chicago DOT, at [email protected]. Access information on Chicago’s Climate Change Action Plan.