Engineers marvel at Chicago's 'Deep Tunnel' flood prevention system, built over 50 years

Chicago's "Deep Tunnel" project, an immense underground system of tunnels and reservoirs, stands as a testament to engineering ambition, having taken over half a century to fully realize. This colossal infrastructure is designed to combat the city's persistent problem of massive flooding.

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the system currently has a capacity of more than 11 billion gallons for wastewater and stormwater.

The Deep Tunnel network channels stormwater through more than 110 miles of tunnels, directing it into three main reservoirs: Thornton, McCook (currently under construction), and Majewski. The system's primary goal is to protect the region's rivers and the vast Lake Michigan from overwhelming amounts of water. Official reports indicate the system currently holds over 11 billion gallons of stormwater and wastewater, with capacity expected to reach 17.5 billion gallons upon completion of the McCook reservoir's second phase.

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once the second stage of the McCook reservoir is completed, the total capacity will rise to 17.5 billion gallons.

Much of Chicago operates on a combined sewer system, where wastewater from homes and businesses flows through the same pipes as rainwater. Historically, these systems were designed to discharge untreated sewage directly into rivers, especially during heavy rainfall when treatment plants and collectors become overloaded. The Deep Tunnel aims to mitigate this by capturing excess water before it reaches the rivers or overwhelms treatment facilities.

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these combined systems were built before wastewater treatment technology existed, so they were designed to discharge the used water directly into the river.

However, recent heavy rainfall has pushed the Thornton and McCook reservoirs to near-maximum capacity, reaching 94% and 96-100% full, respectively. This near-overflow situation, particularly after a summer of intense storms, puts several Chicago suburbs, including Blue Island, Calumet City, and South Holland, at risk of flooding. Some experts suggest that while the Deep Tunnel is a significant achievement, further measures like developing more green spaces and alternative water storage solutions are necessary to fully prevent flooding.

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when it rains heavily, the collectors and treatment plants can become overloaded; then, the wastewater follows the original course of the sewers and reaches the river. This pollutes the water and increases the risk of flooding.