Thorn Creek Flood Control Diversion to Thornton Quarry

I already described the diversion of Salt Creek flood waters into the Elmhurst Quarry. While studying TARP, I learned that a Thornton Transitional Reservoir was finished in 2003 using the West Lobe of the Thornton Quarry to store flood waters from Thorn Creek. I also learned that the Thornton and McCook Reservoirs are called "composite" reservoirs. The Thornton Composite Reservoir uses the North Lobe of the Thornton Quarry. [UofI] When I learned the West Lobe was used to store Thorn Creek flood waters, I scanned the left bank of the creek looking for a diversion structure.

The rest of this information is from an IU study except for the satellite images.

IU

It turns out. I did not look far enough north along the creek.

A 100-year flood is 7400 cfs. The design goal is to divert 80% of that flow or 6200 cfs. This will lower the creek height a little over 6' during a heavy storm.

Below is a satellite image of the diversion structure. The 12" pipes on 3' centers along the creek edge are to screen out objects such as tree limbs. Then, at the intake bay, is a 6" grate. "This keeps pedestrians or animals from falling into the structure." The diversion structure has three 12' x 12' sluice gates.

3D Satellite

IU, diversion structure during construction

The diversion structure funnels water to a 22' diameter drop shaft that is 300' deep. This will accommodate a flow of 6700 cfs. At the bottom of the shaft is a deaeration chamber (200' x 32' x 60') that is connected to a 22' diversion tunnel bored to the West Lobe. I found this hole in the West Lobe. But according to the map, this would be the end of the diversion tunnel. An 8' dewatering tunnel connects the diversion tunnel to the Calumet TARP tunnel. The water will be run through the Calumet Water Reclamation Plant. This surprised me since this is storm water instead of sewage. But it is probably cheaper to use the TARP pumping station than build a second one for just storm water. "After dewatering, sediment and other debris that were settled in the reservoir will be disposed to off-site. Therefore, the Thornton Transitional Reservoir project has a double role: flood protection and water quality improvement for an area of approximately 300 square miles, which includes parts of the City of Chicago and its southern suburbs."

Oct 17, 2022: MWRD posted ten photos with the comment: "The cement pour began at 4 a.m. and concluded by 11 p.m. that night. It was one of the many grueling days of building a section of Deep Tunnel to link Thorn Creek over bank floodwaters to the MWRD’s Thornton Composite Reservoir. Today we visited this critical infrastructure that will protect nine communities from flooding and tie into our Tunnel and Reservoir Plan. Here about 250 feet below ground, crews have been working hard since March to complete the tunnel by the end of the year.  They blasted through a rock plug nearly 30 feet deep and then went to work forming and meticulously lining a 22-foot diameter tunnel by combining about 4,000 cubic yards of that cement with 8,000 pounds of rebar. Since 2003, the MWRD has leased the Thornton Transitional Reservoir in the West Lobe of the Thornton Quarry, and in that time it has captured more than 50 billion gallons of flood waters. But now it is time to turn it back over to mining operations and begin diverting water to the Thornton Composite Reservoir. Thanks to the dedication of our engineers and contractors this important tunnel is on schedule to be ready by the end of the year!"

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