On July 2, Chicago was hit with a severe storm that dumped as much as nine inches in the western suburbs and left thousands of homes and businesses to deal with flooding. Built on a swamp, the city’s topography is low-lying and flat with a history of flooding. The impervious surfaces throughout the city prevent rain from being absorbed into the earth which exacerbates flooding issues, a problem experienced by many urban centers.

In 1975, the city began construction on the Tunnel and Reservoir Plan, better known as “The Deep Tunnel,” as a solution to the pollution and flooding issues wrought by its combined sewer overflow. The Deep Tunnel would collect and store rain and sewage until the water could be treated, thereby protecting Lake Michigan, its chief source of drinking water, and other inland waterways, while also preventing sewage backups into homes and businesses.

The Deep Tunnel consists of three massive reservoirs, two of which are complete, providing a current storage capacity of 3.5 billion gallons. However, that capacity can be overwhelmed by strong storms which prompts lock shutdowns and triggers releases into the river and backups into streets, homes and businesses. Such was the case with this storm when district officials made the decision to relieve pressure on the system and opened locks near Navy Pear allowing 1.1 billion gallons of waste to flow into Lake Michigan.[1]

Upon completion of the third reservoir, anticipated in 2029, the combined sewage capacity of The Deep Tunnel will increase to 17.5 billion; however, the changing climate is producing more frequent and powerful storms and infrastructure – no matter the size and scope – may not sufficiently mitigate the overflow. A 2010 study conducted by Wuebbles and associates determined that storms capable of triggering sewage releases into Lake Michigan (precipitation of more than 2.5 inches in a day) would increase by 50% by 2039, a point illustrated by the nearly 40 billion gallons in releases that have occurred since 2008.[2]

Many cities are moving away from large-scale infrastructure projects and instead incorporating smaller-scale, green or nature-based strategies to address pollution and flooding, such as rainwater harvesting, porous pavement, trees and rain gardens. Although the success of The Deep Tunnel is evident in the overall improved water quality in the area, science predicts that the frequency and strength of storms will increase and will, consequently, continue to overwhelm infrastructure and endanger water quality. Chicago, like many cities, will have to consider other options to navigate the future in preparation for the storm that swamps The Deep Tunnel.

[1] Hawthorne, Michael, and Adriana Pérez. “Chicago’s Flood-Control Project Can’t Contend with Climate Change.” Governing, 21 July 2023, www.governing.com/infrastructure/chicagos-flood-control-project-cant-contend-with-climate-change.

[2] Hawthorne, Michael, and Adriana Pérez. “Chicago’s Flood-Control Project Can’t Contend with Climate Change.” Governing, 21 July 2023, www.governing.com/infrastructure/chicagos-flood-control-project-cant-contend-with-climate-change.