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MMSD explains how deep tunnel operations and extreme storms led to overflows and local impacts
Summary
Milwaukee Metropolitan Sewerage District staff explained how extreme August and April storms filled the deep tunnel, prompting gate closures and combined sewer overflows (CSOs). MMSD said overflows are driven by system capacity and that monitoring gaps complicate advance warnings for some tunnels.
Milwaukee Metropolitan Sewerage District (MMSD) staff told Brown Deer residents that recent extreme storms overwhelmed regional systems and forced overflows to rivers and Lake Michigan, even as the agency works to limit such discharges.
Slattery Marlin, who identified himself as MMSD’s municipal program manager, said the district operates the Jones Island and South Shore treatment facilities and manages the deep tunnel and diversion structures that store wet‑weather flows. "The deep tunnel treats 98.4% of the total volume of flow," Marlin said, explaining that the tunnel and its gates are designed to reduce basement backups but cannot capture every extreme event.
MMSD staff described how the tunnel and diversion structures work: during normal or moderate wet weather flows the system sends water into the deep tunnel for later treatment; once the tunnel reaches capacity, operators close a gate that effectively prevents more water entering the tunnel and allows excess to flow to rivers through combined sewer overflow (CSO) points. Marlin said the gates are closed only when the structure is full, and that closing the gate reduces the tunnel’s ability to accept further inflow, which is why large, rapid storms can trigger outfalls to waterways.
The agency put recent events in perspective: staff described the August storm as approximately a 1,000‑year event and the April storm as a 100‑year (1% annual chance) event. In both instances, Marlin said, the district saw CSOs. Residents at the meeting pressed MMSD on how tunnel fills relate to basement backups in Brown Deer; agency staff said impact depends on where neighborhoods tie into interceptors and on localized conditions, not only the regional tunnel status.
Several residents also raised monitoring concerns. One attendee noted that North Shore tunnel status has shown "no data for years," meaning local residents cannot see realtime fill levels for that facility; MMSD acknowledged monitoring gaps and said an emergency project is under way to restore communications where needed. The district did not give a firm completion date during the briefing but said it would provide follow‑up information to the village.
MMSD and village staff encouraged residents to join neighborhood alerts that send water‑drop or high‑flow notices by text during extreme events and to use MMSD resources such as pipe‑check guidance on preventing household contributions to inflow and infiltration (I&I).
