Historic flooding in Milwaukee forces combined sewer overflow

MILWAUKEE - Saturday's storms flooded basements, cars, and streets.

It also forced the Milwaukee Metropolitan Sewerage District (MMSD) to send a combination of storm water and sewage into Lake Michigan and nearby rivers.

The FOX6 Weather Experts are calling this a "1,000-year storm."

To be clear, that's not to say it will happen once every 1,000 years. It means the chance of it happening every year is 1-in-1,000.

So here, those narrow odds align and a system built to store water couldn't keep up.

FREE DOWNLOAD: Get breaking news alerts in the FOX LOCAL Mobile app for iOS or Android

What is the Deep Tunnel?

What we know:

Backed-up basements, raging rivers, and cars floating, even underwater, on state highways, the damage from the weekend storm is widespread.

The Milwaukee Metropolitan Sewerage District says it could have been even worse without 28 miles of storage buried underground known as the "Deep Tunnel."

"Think of the Deep Tunnel as a big bath tub, and it's designed to hold water and wastewater until the treatment plants have the time and capacity to clean the water," said Bill Graffin, MMSD Public Information Manager.

MMSD can store more than a billion gallons of water a day thanks to its water treatment facilities, like Jones Island, and the Deep Tunnel, which is well underground. Looking back on Saturday and Sunday, that capacity may sound like a lot. But for this storm, it still wasn't enough.

Combined sewage overflow

What we know:

So, just before ten o'clock Saturday night, MMSD began what's known as a "combined sewage overflow."

"The biggest thing we try to prevent during a storm is basement back-ups. The only way you can do that is to have a sewer overflow. The overflow is basically a relief valve out in the system," added Graffin.

That overflow is from a pipe that combines storm water and whatever's flushed from homes and businesses.

Without the space to treat it, it's sent to the closest exit points, into rivers and lakes.

For this storm, it's in the Milwaukee River, Menomonee River, and Lake Michigan between McKinley and South Shore Beaches.

Bill Graffin Public says while that's not the preferred outcome, the district believes it's the safest.

SIGN UP TODAY: Get daily headlines, breaking news emails from FOX6 News

"It's a bigger health threat to have it in someone's basement, especially if it's not cleaned up properly," said Graffin.

And he says this could be the biggest overflow MMSD has ever reported.

Awaiting the numbers

What's next:

The district is still calculating just how many gallons were released, as they don't have sensors in their infrastructure to automatically know.

The law requires that they report that data to the Wisconsin DNR within five days.

It means all of that data is due tomorrow (Friday).