Through this note, I hope to answer questions and address incorrect information I have seen on social media about the deep tunnel system, the August 9-10 storm, and the timing of opening tunnel gates.  

First, I am the person who has the responsibility of closing the gates to the deep tunnels.  I am an engineer and I would NEVER purposefully cause basement backups in any of the 29 municipalities that drain to the Milwaukee Metropolitan Sewerage District’s (MMSD) sewer system. NEVER.

The municipal and MMSD sewer infrastructure is integrated throughout the region.  There are 3,000 miles of privately owned laterals that connect to 3,000 miles of municipal-owned sewers. Those sewers connect to 300 miles of MMSD’s larger and deeper metropolitan intercepting sewers. 

When I ask for the tunnel gates to be closed, we close all of them in two phases. I first close gates to try to stop overfilling the tunnel, and secondly, close tunnel gates when the tunnel is full. When the gates to the tunnel are closed, water is overflowed to the rivers. This sequence of tunnel gate closures is designed to help minimize the basement backups that might occur.

Additionally, the deep tunnel is not meant to control flooding.  The tunnel was built to reduce sewage overflows to the rivers and Lake Michigan. Since the original tunnel came online, we have significantly reduced the number of overflows to the rivers, from 50-60 per year to an average of two per year. So far in 2025, we have had one tunnel overflow in August. Yes, this was the largest overflow since the tunnel became operational, but it was also the largest storm, estimated at a 1000-year storm, since the tunnel became operational.There are no gates in our rivers or streams that control surface flooding.

On the evening of August 9th, the storm hit unexpectedly and inundated this region. The tunnel system started to fill quickly. By 9:55 pm we could see that the tunnel would overfill if we did not take some action, so we closed the first set of tunnel gates from the combined sewer system.  This action stopped water from entering the tunnel from the area around downtown Milwaukee and part of Shorewood.  When the tunnel gates close, the water flows by gravity to overflow points along the waterways.

During a normal storm, this would have been enough to stop the tunnel from overfilling. Unfortunately, after midnight on August 10th, a second round of heavy rainfall occurred.  The tunnel continued to fill with water from all of the municipalities. We closed the second set of tunnel gates at 3:05 am.

At this point, the tunnel system was completely full; it would not matter if the gates were open or closed, no more water could get into the tunnel. During a storm, you can easily monitor this information by visiting MMSD’s website and the Live Rain and Facility Information page, which refreshes the data every 5 minutes; it is real-time.

The Milwaukee region got hit with a record storm.  The tunnel filled up.  The rivers filled up. The regional and local sewers filled up. Streets were flooded, and unfortunately, basements flooded. No infrastructure in the world would be able to manage this amount of rain. The devastation we experienced is unreal, and I feel for all who had damage to their homes. I hope this helps, but if folks would like to talk further, please send me an email at kshafer@mmsd.com.