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Metropolitan Water Reclamation District of Greater Chicago

Q: Besides TARP, what else is MWRD doing to adapt to these intense rains that overwhelm the region? I know of at least two stormwater detention projects, one in Westchester if I recall and another in Winnetka. What else?

A: The Tunnel and Reservoir Plan has been the MWRD’s long-term control plan for the combined sewer areas of Cook County commencing in 1972. Bearing in mind that the Illinois State Water Survey publishes rainfall data periodically, in 1960, six inches of rain in 24 hours was considered a 100-year storm event. This rainfall data was updated in 1989, and 7.5 inches of rain in 24 hours was the updated 100-year event. The most current rainfall data, updated in 2019, indicates that a 100-year rain event is 8.58 inches in 24 hours. This updated rainfall data represents over a 40 percent increase in rainfall volume for which detention facilities should be sized. This is quite a change since the time that TARP was originally conceived. In other words, rainfall volume and intensity has increased dramatically since 1960. Read more about TARP 

From the late 1960s through the 1980s the MWRD, in collaboration with local municipalities, the Natural Resource Conservation Service, and/or the US Army Corps of Engineers, constructed roughly 3.1 Billion Gallons (BG) of storage across 33 flood control reservoirs that provide regional stormwater flood damage benefits. These flood control reservoirs serve the separate sewer areas of Cook County. The facilities are shown in the attached schematic.

In 2004, the MWRD was given authority for regional stormwater management in Cook County. Projects that are considered regional must have large, multijurisdictional impact and take many years to plan, design and construct. Examples of these projects are the completed Heritage Park Flood Control Facility in Wheeling, the Buffalo Creek Reservoir Expansion near Buffalo Grove and the nearly complete Addison Creek Reservoir in Bellwood.
Additionally, the District regulates stormwater runoff from new development outside of the City of Chicago through our Watershed Management Ordinance (WMO) that includes provisions for stormwater detention. Since the adoption of the WMO in 2014, over 633 million gallons of detention volume has been constructed on private property to comply with the WMO.

In order to be more responsive to flood mitigation needs in the communities we serve, the MWRD sought and was granted authorization in 2014, to acquire flood-prone properties and to plan, implement and finance local stormwater management projects.

Since then, the MWRD has completed or has in progress 229 projects [See attached graph] that provide flood mitigation to more than 17,000 structures. The total cost of all of these projects is $758 million and the MWRD’s portion is $506 million, demonstrating how the MWRD has leveraged public funds and delivered projects that we recognize are needed in this dynamic environment where we experience intense precipitation. The successful implementation of such a large number of projects relies upon our ability to partner with local communities and other governmental organizations through our Stormwater Partnership Program, Flood-Prone Property Acquisition Program and Green Infrastructure Partnership Program to address flooding through a mix of green and gray solutions and property buyouts.

We have created some project and program videos:
Stormwater Partnership program 
Flood Prone Property program 
Addison Creek Reservoir 
Buffalo Creek Reservoir 
Robbins Stormwater Park and Midlothian Creek Restoration projects are currently underway.
Green Alleys in Harwood Heights

Q: The McCook Reservoir was only 17% full as of 2 a.m. on July 2 but CSOs began occurring by 8:19 a.m. At 9:44 a.m. the North Branch Pumping Station began releasing sewage and storm water, even though the rainfall in that part of the region was lighter than in the Berwyn/Cicero area. Why did CSOs begin happening even though the reservoir was largely empty? Is this another case of local sewers unable to convey storm runoff to the interceptors and TARP quickly enough to prevent CSOs? What can be done about this?

A: According to the National Weather Service over July 2-3, “rainfall totals in and in the immediate vicinity of Chicago ranged from roughly 3 to 7 inches, though a few localized areas received over 8 inches of rainfall. The worst of the flooding occurred on the west and southwest sides of Chicago and in the near west and southwest suburbs”. There is no system able to handle that much water in just a few hours. To your assertion that CSOs occurred prior to TARP filling, yes, you are correct. During extreme events where the TARP system is rapidly filling, we activate the North Branch Pumping Station and Racine Avenue Pumping Station so that the large pumps can control the sewer elevations and prevent a rapid surge in the sewer system once TARP fills. These locations are different than most of our outfalls that are activated via gravity once TARP fills or conveyance limitations occur.

In some positive news, Calumet TARP Tunnel and Thornton Composite Reservoir that serve the south service area has captured more than 99.9% of CSOs since it was completed at the end of 2015, and there has not been an overflow in the last three years.

Q: Former Commissioner Shore once told me that people are just going to have to get used to having water/sewage backups in their basements. Is that still the case? What else can be done?

A: Over the long-term, we will add more capacity (6.5 billion gallons) with the addition of Stage 2 of McCook Reservoir, but more work is needed across Cook County.

These storms demonstrate the need to invest in both gray and green infrastructure to increase capacity for more storms. Combined sewers generally are capable of handling at most a 5-year storm event and as storms have increased in frequency and intensity, even the valuable gray infrastructure technologies cannot always keep up with all of the runoff, leading to flooded basements and surface flooding. That is why we have partnered with communities to deliver local stormwater and green infrastructure projects aimed at reducing the rate of runoff to the sewers.

Climate change threatens to alter precipitation, water resources and the reliable systems that the MWRD has installed to protect area water quality, public health and safety. We are developing regional climate action plans in collaboration with area partner agencies, while also improving our operations to reduce greenhouse gas emissions. In collaborating with other regional agencies, we’re working to address climate hazards that impact our area, and to make investments through partnerships with communities to minimize the impacts of climate change on people, places, and our infrastructure assets, and to keep our region and world safe for future generations.

While the intensity of the July 2-3, 2023, rain event was overwhelming to our system, we believe there are also measures homeowners can take to help protect their properties. There remain many homes and buildings that direct their roof load to the combined sewer. We encourage residents and property owners to consider the information provided in MWRD’s Green Neighbor Guide  and disconnect the building’s roof load and direct it to a rain barrel, rain garden, bioswale, or yard. In addition, homeowners can protect their basements from backup by installing overhead sewers or a backflow preventer. Finally, while we are talking, plant a tree! Trees provide many environmental benefits and also can intercept and absorb stormwater, reducing the load to the sewers. The MWRD gives away oak tree saplings every year through our Restore the Canopy Program. Since 2016 we have distributed 100,000 trees. Additionally, we issue Overflow Action Alerts through social media and emails to encourage the public to reduce their use of water during rain events: wait to run the dishwasher, shower and wash clothes.


Established in 1889, the Metropolitan Water Reclamation District of Greater Chicago (MWRD) is an award-winning, special purpose government agency responsible for wastewater treatment and stormwater management in Cook County, Illinois.


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