Combined sewers and basement flooding in metro Detroit: which communities share one pipe

In much of the region's older housing, sewage and stormwater run through the same pipe under the street. That single design decision, made a century ago, is the hidden plumbing behind many of metro Detroit's worst basement backups. Here is which communities verifiably have it, straight from the permits and public plans.

If you own a pre-1960 home with a basement in Wayne, Oakland, or Macomb county, the age of your house tells you a lot about your flood exposure, but the sewer under your street tells you the rest of the story. This page is for owner-occupants of those older homes, especially anyone with a finished basement, who wants to know one specific fact: is my community on a combined sewer? We went to the primary documents, the NPDES permits, county notification plans, and city engineering plans, and classified only the communities where a source actually says so. Sewer status is as of July 2026, per the sources listed on each row.

What a combined sewer is

The U.S. EPA's definition is the plainest: "In a combined sewer system, both wastewater and stormwater flow through the same pipes," and during wet weather "the combined flow of wastewater and stormwater can overwhelm the system" (EPA, Combined Sewer Overflow Basics). Michigan's environmental agency, EGLE, describes it the same way: "snowmelt, rainwater runoff, and wastewater (sewage) from inside houses and buildings are all combined in one pipe. When the pipes become full, excess combined sewage can be released at points throughout the system" (EGLE combined sewer overflow explainer).

Newer subdivisions were built differently, with one pipe for sanitary flow and a separate one for storm runoff. As the Great Lakes Water Authority puts it, "Newer sewer systems are constructed with separate storm and sanitary sewers, so they do not have the same capacity challenges" (GLWA CSO FAQ). That is why sewer design tracks housing age so closely in this region, and why the oldest communities carry the most combined pipe.

Why one pipe raises basement-backup risk

The mechanism is capacity. A combined pipe sized for sewage must also swallow every downpour. GLWA's own FAQ states it directly: "A combined sewer overflow may occur during a significant rain event because the collection system rapidly reaches capacity, which increases the risk of flooding and basement backups" (GLWA CSO FAQ). When a full combined sewer has nowhere to send water, one of the places it goes is backward, up floor drains and into the lowest connected point, which in a pre-1960 home is the basement.

Local governments say the same thing when they spend money on it. Macomb County describes its Chapaton basin upgrades as projects to "reduce combined sewer overflows and reduce the risk of basement flooding," needed "to prevent station failure that would result in sewage backing up into basements and neighborhoods" (Macomb County Public Works, 2023). None of this means a home on a combined sewer will flood, or that a home on separated sewers cannot. It means the structural odds are worse where the pipe is shared, especially in older housing. If a backup has already happened to you, see our guide to the 45-day claim deadline and who to file with.

Which communities verifiably have combined sewers

Two honest classes, nothing softer. Combined (citywide or nearly) means an operator, state, or federal document says the community's local system is combined citywide or that a combined district covers all or nearly all of it. Combined in part (district or portion) means a government document places a defined portion, an acreage or a named district, on combined sewers, or the community holds or appears on a combined-sewer-overflow (CSO) discharge permit; the rest of the community may be separated. One important distinction: nearly every community's sewage eventually reaches a regional interceptor that carries combined flow. That alone does not make the local system combined, and the table below is about local collection systems and defined combined districts only.

Wayne County (13 communities)

Oakland County (15 communities)

Fourteen of these sit, all or in part, in the George W. Kuhn (GWK) drainage district, formerly Twelve Towns: a combined-sewer district of about 24,500 acres whose storm-and-sanitary flow is treated at the GWK retention basin in Madison Heights. Oakland County's Water Resources Commissioner describes the facility as "capturing and treating combined stormwater and sanitary flow" for 14 municipalities. For several of these communities only part of the city lies in the combined district, so we label all of them "in part" rather than overclaim.

Macomb County (4 communities)

Why do the Grosse Pointes score so high?

Readers of our 2026 findings regularly ask why the Grosse Pointes, some of the metro's most valuable housing, sit near the top of the Basement Risk Index. The score itself is driven by housing age: the Pointes are overwhelmingly pre-1960 housing, built before sump pumps, backwater valves, and modern drainage were standard. The sewer record is consistent with that picture rather than contradicting it. Grosse Pointe Woods is the strongest case, its own 2023 plan states the city "owns and operates a combined sewer system that serves the entire city." Grosse Pointe Farms is combined with a separation program just beginning (310 acres in the first phases, 2026–2028). Grosse Pointe Park is combined in part, with separation work ongoing. The City of Grosse Pointe and Grosse Pointe Shores are not yet verified either way, and we say so rather than guess. Old housing and shared pipes arrived together, and in the Pointes both are still largely in place.

What this means for the Basement Risk Index

To be plain about our own methodology: sewer status is explanatory context on this page, not a scored input to the BRI. The index is built from U.S. Census housing-vintage measures, with the City of Detroit's score also incorporating its documented 311 water-in-basement record; suburban scores are modeled from housing data and labeled as modeled, as our methodology spells out. Combined sewers are part of the reason housing age predicts basement trouble so well here, which is also visible in our analysis of who bears Detroit's basement-flood burden, where the hardest-hit neighborhoods sit on the city's oldest housing and combined pipes. This table is the receipts for that context, community by community.

During storm season, the live Basement Backup Watch pairs this context with active flood alerts, the 48-hour rain outlook, and river levels for all three counties.

Sources

Every classification above traces to a named government or system-operator document, checked July 2026. The live discharge record for any Michigan CSO event is EGLE's public overflow list, linked below.

• U.S. EPA, Combined Sewer Overflow Basics and Combined Sewer Overflows in the Great Lakes Basin: Michigan (the state's 40 CSO permits).
• Michigan EGLE, What You Need to Know about Combined Sewer Overflows and Retention Treatment Basins; MiWaters public overflow list (live CSO/SSO discharge events).
• Great Lakes Water Authority, CSO FAQ and 2022–2026 Capital Improvement Plan, Appendix D.
• Wayne County, CSO Public Notification Plan (2018).
• Oakland County Water Resources Commissioner, retention treatment basins, George W. Kuhn basin, and CSO Public Notification Plan (2026); City of Southfield, RainSmart flyer (the GWK 14-community list).
• Macomb County, Public Works release on Chapaton and Martin projects (2023); Southeast Macomb Sanitary District, facilities and Milk River drain notification plan.
• City documents: Detroit DWSD sewer system upgrades; EGLE Highland Park environmental assessment; FEMA Hamtramck case study; Grosse Pointe Woods CWSRF project plan; Grosse Pointe Farms sewer separation project; Grosse Pointe Park infrastructure reports; City of Dearborn construction updates.