Monday, January 15, 2018

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.

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."

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