Water Flow Underground
Most people don't realize the extent of hydraulic engineering that moves water around Chicago. Chicago has one of the largest underground water systems in the world, and the Metropolitan Water Reclamation District itself is the largest single user of energy in the State of Illinois to move water through the system.
Why is this?
Chicago's water history has been one of trying to control water coming in and out of the city. The City pipes nearly 2B gallons of water out of Lake Michigan for its daily use. The used wastewater is combined that with any rain that falls across the City through a combined sewer system, meaning that wastewater and stormwater are combined into one system. Pipe infrastructure is in place to collect and divert that combined water to the treatment plants, then to discharge to local waterways.
For any rainfalls over 2/3" the system gets overwhelmed by the volume of water, and it backs-up and overflows. This results in urban flooding, consisting of street flooding and basement flooding, as well as combined sewer overflows (CSO) into the rivers and sewage releases into Lake Michigan. There is simply too much stormwater draining into the system. Designers are investigating green infrastructure solutions to keep rainwater out of the system, and instead to use that natural rainwater to improve the development of gardens, parks, green streets, and urban farms and community gardens throughout the city.
Chicago's urban water statistics include the following:
+Chicago withdrawals 1.75 Billion gallons for potable water use from Lake Michigan every day. (Regulated by great lakes interstate and binational agreement.) This water is taken in through cribs 2-4 miles off shore, tunneled to the purification plants and distributed across the City by the Department of Water Management.
+ Chicago rainfall ranges from 37-40" annually, and rain quantities can vary from a fraction of an inch (the majority of rains) to several inches (the April 2013 rains were the highest in recent years). Climate change research indicates that rain will fall within increased quantity (inches) and intensity (over a shorter time).
+Wastewater (used from the Lake Michigan source), from household, commercial, and industrial uses, is combined with rainwater (collected primarily through downspouts, pavements and street inlets). This combined water moves into a network of 15’ interceptor pipes, leading to the treatment plants. Chicago's Stickney Treatment Plant is the 2nd largest in the world, processing approximately 1B gallons of water per day.
+When a larger rainfall occurs, and the combined water system is at capacity, the Deep Tunnel and Reservoir System, serves as a holding mechanism for water waiting to be treated. The Tunnels are 35’ in diameter (shown in the dark grey) are located 150-250’ below ground. Most tunnels line the major waterways as overflow mechanisms. Some tunnels also lead to the Reservoirs.
+When the Tunnels are at capacity, the combined water is tunneled to regional quarries currently being converting to Reservoirs for holding larger quantities of untreated water.
Statistics and quantities of Chicago's urban water system are shown on the map.
As of 2014, Chicago averaged a combined-sewer overflow weekly, and discharged untreated sewage to Lake Michigan twice annually. Street and basement flooding (the majority in people's homes) continue to occur on a regular basis.