MMSD Completes Computer Modeling of April 2026 Storm Impact on North East Portion of Bay View

(Milwaukee, WI) – Following the historic April 2026 storms that dropped 7-plus inches of rain on the Milwaukee Metropolitan Sewerage District (MMSD) service area, computer modeling indicates that slightly more than 400 homes in the Bay View were at elevated risk of backups during this historic storm. This is primarily due to the volume and intensity of rain that fell, but other factors such as elevation and groundwater levels can play a role.

The modeling also assessed whether debris found in the overflow pipe (CSO195) that services the area further contributed to the backups. According to the modeling completed, the backups would have occurred regardless of whether the sewer pipe was blocked by debris or not, but may have contributed to the severity of the backups. Moreover, an additional 200 homes in a defined area of Bay View were put into an elevated risk category due to the debris, but not throughout the entire neighborhood or elsewhere in the region (see map).

In July, MMSD invested more than $500,000 to gain access to the sewer and removed the debris in the underground overflow pipe. The infrastructure sits underwater below Lake Michigan water levels. CSO 195 will be modified to install a flap gate (or similar structure) to prevent future debris buildup in the pipe. Inspections will be performed every 6 months, depending on ice conditions, beginning in March 2027. MMSD also is inspecting additional underground pipes for debris.

“We recognize how difficult and disruptive basement backups are for residents, and we understand why those impacted want clear answers about what happened,” said Kevin Shafer, MMSD Executive Director. “While our modeling shows that the impact of the debris may have been limited to a specific area of Bay View during the historic storms our region experienced in April, it’s abundantly clear that we should have moved faster to identify and fix the issue. We must do better in the future, and we will. The modeling gives us a clearer understanding of the role the debris may have played, and we are changing how we inspect and manage CSO195 and other outfalls going forward.”

Overflow pipes are relief valves that allow excess water out of the sanitary sewer system, so it does not end up backing up in the pipes into the basements of houses and businesses.

The presence of debris in CSO195 may have affected the risk and severity of basement backups during the April storm. Basement backup risk can also be affected by the amount and intensity of rainfall, Lake Michigan water levels, groundwater conditions, basement depths, and how water moves through the sewer system.

In this case, the debris in the overflow pipe could not have caused basement backups outside of the sewershed that serves a defined area in Bay View. Similar to a watershed, the sewers in a sewershed only serve a specific geographic area that drains wastewater or stormwater into a shared endpoint.

From April 14 through April 18 of this year, severe storms dropped more than 7 inches of rain over portions of MMSD’s service area. Streets flooded in many areas of the 29 communities that MMSD serves where hundreds of basement backups were reported.

MMSD received 26 wet basement reports in the sewershed pictured on the map (see above) in Bay View after the storm. An important note: We know that not every homeowner that experiences a backup ends up reporting it. MMSD is working with government and community partners to ensure residents know how to report backups, which will better inform long-term investments in regional infrastructure.

MMSD's 10-year investment plan calls for $2.1 billion in improvements to regional water reclamation facilities and sewers to reduce the risk of overflows and basement backups.

#######