Pollutants that the water picks up like pesticides, fertilizers, oil and soap are harmful in any quantity. Others types including construction sediment, bare soil, agricultural land, pet waste or grass clippings and leaves can be harmful to creeks, rivers and lakes in sufficient quantities. Various other human activities, seen frequently in the city, like watering, car washing and malfunctioning septic tanks can also put water onto the land surface.
In cities like Chicago, none of the water that falls on hard surfaces like roofs, driveways, alleyways, parking lots or roads can seep into the ground. These impermeable surfaces create large amounts of runoff that pick up pollutants. The runoff flows from gutters and storm drains to streams. Runoff not only pollutes' but erodes streambanks. The mix of pollution and eroded dirt muddies the water and causes problems downstream.
Chicago's Mayor Daley implemented a Green Alley project after his Green Roof projects took initiative. From the City of Chicago Website, Chicago's Green Alley program incorporate a variety of characteristics:
- Permeable pavements (asphalt, concrete or pavers) that allow stormwater to filter through the pavement and drain into the ground, instead of collecting on hard surfaces or draining into the sewer system. The pavement can be used on the full width of an alley, or simply in a center trench.
- Open bottom catch basins: installed in alleys to capture water and funnel it into the ground
- High-albedo pavement, a lighter-colored surface that reflects sunlight instead of absorbing it, helping reduce the urban heat island effect
- Recycled materials, such as concrete aggregate, slag and recycled tire rubber
Other green alley techniques include using proper grading and pitch to facilitate drainage, and using dark sky-compliant light fixtures to reduce light pollution and provide uniform illumination.
Green Alleys are part of CDOT's "green infrastructure" -- which includes recycled construction materials, permeable pavements and other efforts.
The program began as a pilot in 2006, and through 2010, more than 100 Green Alleys have been installed.
The handbook below provides an overview of CDOT's Green Alley program.
Chicago has as much as 1900 miles of alley ways- enough concrete to build five major airports. The continuity of this program is crucial because of Chicago's relation and utilization of the Chicago River and Lake Michigan. These bodies of water are not only essential to Chicago's prosperity, but also, very vulnerable to pollution from our stormwater runoff.
Stormwater Management in the City of Chicago:
The City of Chicago owes its very existence to its location at the confluence of the Chicago River and Lake Michigan. Lake Michigan provides the City with an abundant water supply while the Chicago River serves as a highway to move goods and services critical to the City’s growth. Chicago has built a historic legacy in protecting these valuable water resources. To protect its water supply, engineers in the 1900s constructed the Chicago Sanitary and Ship Canal to reverse the Chicago River’s natural flow from eastward to westward, steering human and industrial waste away from Lake Michigan. In 1972, Chicago pioneered the use of deep tunnels to capture, convey, and store combined sewage during storms for later treatment.
Today, Chicago is taking a new comprehensive approach toward further improving the quality of its surface waters. Rather than through large scale engineering projects, the approach centers on simple storm water Best Management Practices (BMPs) at the source level to reduce the negative impacts of storm water runoff. Through various model projects, the City aims to demonstrate the efficacy of various BMP approaches, promote public acceptance and usage, and encourage modification of local ordinances to allow wide-spread usage of BMPs.