Understanding the Deep Tunnel Gate Operations During the Record Rainfall of August 9-10, 2025

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.

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.