February 23, 2018

A Hole in the Ground
Makes History

Weather watchers, data geeks, taxpayers, maybe even those among us who think that government can do no good—you’ll want to pay attention. I combed through data from the National Weather Service’s Chicago Area precipitation gauges, and the numbers are telling.

Since 1985, every single winter rainstorm in the Chicago region—that’s December 21 through March 20—with 1.67” or more in a single day has caused a reversal of the Chicago waterways into the lake—until February 20. Look closely at this graph:

Comparison of Winter Rain Events in Chicago Area

What was the difference this week? How did the Chicago region manage 2.1” of rain falling on snow-covered frozen ground in a single day without needing to discharge millions of gallons of stormwater overflow to the lake? The first stage of the McCook Reservoir went online in late December and filled to its 3.5 billion gallon capacity on Tuesday. The other sections of the Tunnel and Reservoir Plan operated as expected and filled as well. The engineers and operators at the Metropolitan Water Reclamation District lowered the water levels in the Chicago area waterways by releasing as much water as they could through gates at the Lockport powerhouse—giving the system more room to hold rain. All told, the tunnel and reservoirs held 9.2 billion gallons of water—that’s stormwater mixed with some sewage and many contaminants that otherwise would have poured onto our streets, backed up into our basements, and flushed pollution into our waterways.

Reversals wreak ecological havoc and threaten public health, so preventing them is critically important. Just consider the data from past winter reversals: 

  • Between December 26–27, 2008, Chicago was hit with about 1.83’’, causing a reversal at Wilmette and sending 460.8 million gallons of untreated stormwater into Lake Michigan.
  • Between March 7–8, 2009, Chicago received about 2.54’’, also causing a reversal at Wilmette and discharging 143.1 million gallons of untreated stormwater into Lake Michigan.

I don’t mean in any way to diminish the flooding that did occur this week or the property damage and trauma that many people experienced throughout Cook County. Winter rainstorms can be especially problematic because frozen ground acts like cement—no water can infiltrate. Everything runs off: snowmelt adds to rain and piles of snow in the streets can prevent storm drains from functioning as they should.

Before the storms began on February 19, weather watchers knew that the projected rain combined with snowmelt and frozen ground would likely result in flooding, perhaps even rivaling the catastrophic flooding we experienced in 2008. Yet, Cook County was better prepared thanks to the completion of the Thornton Reservoir in 2015, the first lobe of McCook Reservoir coming online in late 2017, and many local stormwater management projects. Much remains to be done—and MWRD allocated $65.6 million for stormwater projects in its 2018 budget—but it’s heartening indeed to see that the system performed well and know that circumstances could have been much, much worse but for the determination, dedication, and vision of the MWRD engineers and trustees half a century ago when the Tunnel and Reservoir Plan was conceived and begun.

A big thanks, too, to my whip-smart staff aide, Tim Oravec, who assisted with research for this newsletter and made the graph. 

McCook Reservoir fills with 3.5 billion gallons on February 20

McCook Reservoir fills with 3.5 billion gallons on February 20