Residents of the Chicago region have begun mopping up from the largest one-day rainfall recorded since records have been kept and you probably have lots of questions about it.
I'm a member of the nine-member elected board of commissioners of the Metropolitan Water Reclamation District of Greater Chicago ("District" or "MWRD"), the agency charged with wastewater treatment and stormwater management for Cook County. By Saturday morning, rains that had begun on Friday and continued through the night had begun to flood basements, fill detention ponds and spill into streets and yards.
Folks, this was one huge storm. By midday on Sunday, the rain gauges on the north side of the District's basin had recorded 7.26 inches of rain, with more still falling. Hanover Park recorded 9.45 inches at that time. A heavy one-day rain storm for us is 3 to 4 inches. The Chicago region gets 35-36 inches of rain a year on average. So we got 20 percent of our annual total in about just 30 hours!
Why was there so much flooding in streets and homes this weekend? Simply put, the water had nowhere else to go. This is no comfort to the thousands of people who have suffered substantial damage from this weekend's storm. You've seen the pictures.
Yet, consider this: approximately 42 percent of the land in Cook County is covered by some sort of impervious surface - roads, roofs, parking lots, driveways, brick patios. The rich and varied landscape of prairies and oak woods that characterized our region prior to the 1850s has been replaced by a concrete skin. Natural communities such as prairies and wetlands that would have absorbed or held much of the rain have been paved over. Rain hitting impervious surfaces like pavement and driveways and patios runs off and is mostly funneled into storm sewers or retention basins.
Under normal conditions - even up to 3 or 4 inches of rain - the systems of sewer and stormwater pipes is sufficient to handle the rainstorm's runoff. The MWRD's Deep Tunnel - 109 miles of tunnel carved out of bedrock limestone up to 300 feet underground - can hold 2.5 billions gallons of stormwater overflow at one time. The Tunnel was full from this weekend's rainstorm by 7:30 a.m. Saturday morning. Two of the reservoirs that are also part of the MWRD's flood control and stormwater overflow system are still being built but the reservoir near O'Hare filled with 350 million gallons of water (mgd) and the Thornton transitional reservoir in the southwestern part of Cook County held 2.25 billion gallons from this weekend's storm.
Still with so much rain, the MWRD was compelled to employ its last resort - opening the system of three gates and locks at Wilmette Harbor, at the mouth of the Chicago River near Navy Pier, and at the O'Brien lock on the south side of Chicago, to discharge stormwater overflow into Lake Michigan. The gates at Wilmette Harbor were opened at 6:18 a.m. Saturday, to draw down water levels in the swollen North Shore Channel. Roughly a billion gallons an hour flows into the lake at Wilmette under these emergency conditions.
And, for the first time in six years, the MWRD opened the lock near Navy Pier, and the O'Brien gate near 130th St. and Torrence Avenue in Chicago to lower water levels in the Chicago River and area waterways. Combined, the three gates released an estimated 4 billion gallons of water an hour into the lake. Though this water is primarily stormwater, it does contain some highly-diluted sewage. By midday Sunday, MWRD spokesman Jill Horist estimated the MWRD had discharged 99 billion gallons of stormwater overflow to the lake.
If we can release excess stormwater to the lake and capture billions of gallons in Deep Tunnel and detention basins, why do we still get flooded basements and rivers over their banks? Because, this weekend, we had more water than our pipes could hold.
In Chicago and 50 of the older suburbs in Cook County, we have what are called combined sewers. This means the same pipes that carry sewage and wastewater from your home also carry stormwater from the street drains. These pipes lead to larger interceptor pipes that carry combined stormwater and sewage to the wastewater treatment plants operated by the MWRD.
At the treatment plants, the MWRD removes most of the pollutants and separates the solid wastes from the liquid wastes and then discharges the treated wastewater - called effluent - into the Chicago area waterways. In fact, about 70 percent of the "flow" in the Chicago waterways is effluent discharged from the District's treatment plants.
Yet when we have big storms, so much water rushes into the combined sewer pipes that it backs up into people's basements or overflows into the streets (and, regrettably, into people's yards) because the pipes can only hold so much water. The city of Chicago and some suburbs have installed restrictors in many storm drains to slow the flow of stormwater into the pipes and to use the streets as temporary holding areas for stormwater overflow.
These restrictors help prevent backups from sewer pipes, but some residents don't like the ponding in the streets and unfortunately a few residents actually disable the restrictors.
Over the course of 26 hours since the storm began on Friday, September 12, the MWRD released approximately 62 billion gallons of water from the Chicago Area Waterways into Lake Michigan. At the far end of the system, 50 miles downstream from Wilmette, the Lockport Controlling Works (near Joliet) has also discharged an additional 13 billion gallons of stormwater to the Des Plaines River downstream (eventually heading towards the Mississippi River) during the same time frame for a total relief to the waterways of about 75 billion gallons. Yet, the MWRD estimates that more than 90 billion gallons of precipitation have fallen thus far this weekend across the MWRD service region of Cook County.
So, where did all that extra water go - the 10-15 billion gallons of water that the MWRD did not discharge to the lake or send downstream? You guessed it: your basement, my basement, yards and streets and fields all over the region.
What can we do about this, you ask? In my opinion, we can't build pipes or reservoirs big enough to hold such huge amounts of stormwater runoff - and municipal leaders agree that the costs would be astronomical and the federal support for such large-scale infrastructure projects is dwindling. Yet we can try to peel back some of the concrete skin we have laid over the landscape to capture rainwater where it falls and allow it to recharge our underground aquifers.
A suite of techniques known as "green infrastructure" is designed to capture and hold rainwater, slowing the flow into the sewers and allowing more water to evaporate or infiltrate into the ground. These techniques include green roofs, rain barrels, rain gardens, bioswales (a fancy word for a vegetated area that captures rain from a parking lot or street) and permeable pavement.
While these are most effective in smaller rain events - up to an inch of rain, for example - by slowing the flow into our sewers they can effectively turn a 4-inch rainstorm into a 2-inch rainstorm by reducing the amount of stormwater the treatment system must handle. Even these techniques would not have prevented all the flooding during Saturday's storm, however. But they would have taken some water out of the torrential flow into the stormwater system. Each year as more people and municipalities install green infrastructure, the more favorable impact the green infrastructure will have on the region.
This weekend's storm just happened to be a monster storm, the storm of the century. In the end, we may just have to live with the idea - and the reality - that our basements will flood now and then. I'm off to mop up.