'Chicago to unveil giant new stormwater storage units to help combat basement, street flooding,' NBC 5 Chicago

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The City of Chicago Department of Water Management and the Metropolitan Water District of Greater Chicago Tuesday are set to unveil two giant underground stormwater storage units meant to help reduce flooding in parts of the city.

The $12 million investment, the MWRD says, will provide 1.7 million gallons of stormwater storage, "helping protect more 2,900 homes, businesses and other properties across Chicago's West Side," including the 28th, 29th and 37th wards.

Officials say the project will help handle excess stormwater on the west side during heavy rainfall, helping to reduce street and basement flooding.

The announcement comes as Cook County's stormwater reservoirs reached record capacity following torrential rain over the Fourth of July weekend. According to the latest estimates from the MWRD, the McCook Reservoir in suburban Bedford Park is at full capacity with more than 3.6 billion gallons of water in it.

According to officials, it's the sixth time in 2026 the reservoir has hit capacity, compared to the five times it had been full in the previous five years combined.

In addition, the Thornton Composite Reservoir, located near South Holland, is at 94% capacity, with more than 7.4 billion gallons of water being stored. According to estimates, it's the most water that has ever been in that reservoir since it was opened in 2015.

A press conference about the project, with MWRD President Kari K. Steele and Chicago Mayor Brandon Johnson, is set to take place at 9:30 a.m. Tuesday.

What is the "Deep Tunnel" system?

In all, more than 13 billion gallons, or nearly 20,000 Olympic-sized swimming pools' worth, of stormwater is being stored in the area's Tunnel and Reservoir Plan (TARP) system, according to MWRD.

The TARP system is also known as the "Deep Tunnel" system, capturing and storing stormwater and sewage that would otherwise overflow into area rivers and lakes during rainstorms. The system then transfers that water to treatment facilities, where it is processed and then released back into area rivers and streams.

That water makes its way through approximately 110 miles of tunnels, which are 8-to-33 feet in diameter and located hundreds of feet underground through the Chicago area. Those tunnels carry water into three separate reservoirs, including the Thornton and McCook reservoirs, along with the Majewski Reservoir near suburban Elk Grove Village.

If the system becomes too full of water, untreated sewage can make its way into area rivers and can overflow from sewers, leading to neighborhood and basement flooding. Officials may also be forced to release untreated water into Lake Michigan to alleviate pressure on the TARP system, which would lead to beach closures and potential health hazards.

Officials say the system is still able to contain the water that's fallen from recent storms, and the NBC 5 Storm Team has good news, as significant rainfall isn't appearing in the forecast for the foreseeable future, allowing the collected water to be processed and drained out of the reservoir system.

In coming years, MWRD says it is expanding capacity at the McCook Reservoir, which will in turn allow the entire system to contain up to 17.5 billion gallons of water in the event of heavy rainfall.

https://www.nbcchicago.com/news/local/chicago-to-unveil-giant-new-stormwater-storage-units-to-help-combat-basement-street-flooding/3958239/

https://www.nbcchicago.com/video/news/local/recent-storms-push-chicago-areas-flood-management-strategy-to-its-limits/3958588/