Rain clears in Chicago but flooding is still a risk

Heavy rains in Chicago this week triggered a flash flood warning on Tuesday and put some city tunnels and reservoirs at capacity.

The skies will be clear this morning, but a few showers could come this afternoon, according to the National Weather Service Chicago. The heavy rain and thunderstorms for the past few days will cause flooding to continue along rivers. Before the weather dries out this weekend, there will be another round of rain and storms on Thursday.

This isn’t the most extreme weather event the Metropolitan Water Reclamation District (MWRD) has experienced, said principal engineer Brian Levy. The systems are working as expected and have handled a total of 3.7 inches of rain since Friday.

All that rainfall put two of the city’s tunnels for water storage at capacity, Levy said. Chicago has a combined sewer system which means both rain and sewage head to the same place. Tunnels and reservoirs store the water before it can get pumped to a treatment plant.

The main stream tunnel is holding 1.2 billion gallons and the Desplaines tunnel has about 400 million gallons. Both are 100 percent full.

Those two tunnels flow into the McCook Reservoir which is just about full with 3.4 billion gallons of sewage and rain water. The city’s last tunnel and reservoir in Calumet still has room. The tunnel is 77 percent full with 485 million gallons.

So what happens if the tunnels are reservoirs can’t store anymore water? The water will begin to flow into area waterways like the Chicago River or North Branch Canal.

After a storm subsides, the water is pumped into a treatment plant where it can get cleaned and discharged back into the area’s waterways.

Residents can help take pressure off the system by waiting to do laundry or putting off a shower until the weather dries up. “Every little bit counts,” said Levy.