A tunnel that now carries rainwater and sewage to the Thornton Reservoir in Sept. 1, 2015, before it became operational later that year. (Jose M. Osorio / Chicago Tribune)
Despite a series of downpours that brought several inches of rain to some parts of the south and southwest suburbs in recent days, the Thornton Reservoir — the Southland’s massive holding basin for the aftermath of storms — was only at a bit more than 27% of capacity Tuesday.
Some communities reported very minor flooding of some homes, but neither Cook County nor the state transportation department reported any road closings due to flooding.
According to the U.S. Geological Survey, over a seven-day period through late Monday afternoon, one monitoring station in Chicago Heights recorded nearly 6.4 inches of rain, the same total as a station on the Little Calumet River in South Holland.
A station in Harvey recorded 6.25 inches, and gauges in Crete and Frankfort each showed rainfall totals of 6 inches, according to USGS.
An Orland Park gauge recorded nearly 5.5 inches over the seven-day span and a station in Matteson had tallied a bit more than 5 inches. Monee had received 4.8 inches and Palos Park 4.3 inches for the weeklong period, according to the USGS.
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The National Weather Service said that from Thursday morning through Sunday morning, Lansing had received 5.7 inches of rain while Park Forest had been drenched with nearly 6 inches, almost 3 inches of which fell in a 24-hour period ending Sunday morning.
Much of that water finds its way to the Thornton Reservoir, a mined section of the Thornton Quarry on the north side of Interstate 294.
Sometimes referred to as the Grand Canyon of the south suburbs, it is the world’s largest stormwater reservoir with a capacity of just under 8 billion gallons.
A key element of the Metropolitan Water Reclamation District’s Tunnel and Reservoir Plan, often termed Deep Tunnel, the Thornton Reservoir first began taking combined storm and sanitary sewer water in late fall 2015.
The reservoir was a bit more than 27% of capacity, or 2.1 billion gallons, on Tuesday, according to the district. In May of last year, it reached nearly 50% capacity.
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Costing $429 million, the reservoir is designed to ease flooding on Chicago’s South Side and in 14 suburbs, including Calumet Park, Dolton, Markham, Riverdale and South Holland.
But along with diverting stormwater, the mammoth basin captures sewage from aged lines running under streets throughout Chicago’s South Side and many south suburbs, which disgorge their contents into streets, waterways and, in some cases, Lake Michigan during particularly heavy storms.
The combined stormwater and sewage is treated at the MWRD’s Calumet reclamation plant on the South Side.
The unsettled weather also spawned an EF0 tornado — the lowest rating on a scale that considers 28 different types of damage to structures and trees — that touched down briefly Saturday afternoon in Crete.
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With peak winds of 70 mph, it touched down just after 2 p.m. near Old Monee Road and Sangamon Street and traveled a bit more than 4 miles northeast and dissipated near Ronald Road just west of Illinois Route 394, according to the National Weather Service.
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The tornado’s width was 100 yards, according to the service.
“We were very lucky,” Crete Mayor Mike Einhorn said.
He said power was out in a small portion of the southwest area of the community, and a couple of trees in the tornado’s path were toppled.
“I just saw the path on the map produced by the NWS and it looks like it went right over my house,” Einhorn said.
mnolan@tribpub.com
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