Why Metro Detroit Basements Flood: Combined Sewers, Storms and the 2014 and 2021 Events

What is a combined sewer, and why does it back up into basements?

A combined sewer is a single pipe network that carries both stormwater and household wastewater to a treatment plant. In heavy rain the volume can exceed what the pipe and plant can take, so the system overflows at designed outfalls, and the same pressure can push sewage back up the lowest drains in homes, usually basement floor drains.

Older cities built one set of pipes to take everything: roof runoff, street drainage, toilets and kitchens. In dry weather that works. A combined sewer's flow is small and goes to treatment. In a storm, runoff from roofs, driveways and streets floods into the same pipes, and the volume can be many times the dry-weather flow. When the pipe fills, engineers built in relief points called combined sewer overflows, which discharge the excess, diluted but untreated, to a river or lake instead of letting it back up everywhere. The U.S. EPA describes combined sewer overflows as a recognized source of pollution for exactly this reason.

The relief points do not always relieve enough. If the water in the pipe rises higher than the lowest drain in a house, it flows backward through the house connection. A basement floor drain, a basement toilet or a laundry sink are lower than almost everything else in the building, so they are where the water shows up first. That water is Category 3: it is sewage-contaminated, and it is a health hazard rather than a cleaning job. Our guide to sewer backup in a Dearborn basement covers what to do and what to throw out.

There is a second pathway that has nothing to do with the pipe type. Separated systems, with one pipe for sewage and a different one for rain, can also back up if rainwater gets into the sanitary line through cracks, connections or downspout connections, or if a storm sewer cannot take the flow and floods the street. Whether your street has a combined, separated or partly separated system is a fact about your street, not about metro Detroit in general, which is why the rest of this guide is careful about where it makes each claim.

Which metro Detroit communities have combined sewers?

Reports describe a largely combined system in Detroit, an undersized combined sewer in Hamtramck and a combined system in Ferndale. For Dearborn the sourced fact concerns its stormwater system, not a combined sewer. For other suburbs we have no sourced statement, so ask your city's Public Works office.

This is the point where regional explainers tend to overreach, so it is worth being plain about what is and is not established. The table below separates what published reports say for each place and avoids filling the gaps. A gap in the table means we did not find a source, not that the answer is no.

Detroit is the clearest case. Detroit's sewers are largely combined, so heavy rain fills basements with sewage in vulnerable neighborhoods, and that is why the Detroit Water and Sewerage Department (DWSD) runs a basement backup protection program, covered in our guide to that program. Four of the areas this site covers are Detroit neighborhoods: Warrendale, Southwest Detroit, Brightmoor and Far West Detroit. DWSD itself says, in connection with Brightmoor, that its combined system was sized for past rain, not today's heavier rain and snowmelt.

Great Lakes Water Authority (GLWA) and DWSD both appear in this picture, and the split is easy to garble. Broadly, GLWA treats and moves wastewater regionally, while DWSD runs Detroit's local system. For anything about your own street, the answer comes from your city's office, not from either authority by default.

What happened in the 11 August 2014 storm?

The 11 August 2014 storm overwhelmed metro Detroit's combined sewers, causing combined-sewer overflows and basement backups across Wayne, Oakland and Macomb counties. Detroit Metropolitan Airport at Romulus recorded 4.57 inches that day, breaking the date's earlier record of 2.06 inches set in 1964.

August 2014 is the regional reference point for basement flooding in the area, and it is safe to state it as a regional event because the reporting did. Combined sewer systems across three counties overflowed, basements backed up in all three, and the 4.57 inches recorded at the airport more than doubled the previous record for the date. Local officials described the flooding as overwhelming at the time, and the Christian Science Monitor article cited below carried that description.

The 2014 storm does not tell you whether your own street was on a combined line, and we are not going to suggest it does. It tells you how a system designed for earlier assumptions about rain behaves when a day delivers more than a single day's worth of normal summer rain. In metro Detroit that is the pattern worth understanding: it is not a gradual failure but a threshold, and when the threshold is crossed, the lowest openings in the system give way first.

For the climate background, Detroit Metropolitan Airport's 1991 to 2020 NOAA normals show about 33 inches of precipitation and about 44 inches of snow in a normal year, with June, August and July the wettest months. That is a normal year, and the storms that flood basements are the exceptions that land on top of it.

What happened in Dearborn in June 2021?

On 25 to 26 June 2021, over 7.5 inches of rain fell in Dearborn in about six hours, against a stormwater system built for roughly three inches in 24 hours. The city says east Dearborn is lower than west Dearborn and was developed before World War II, and that its northeast and southeast ends took the worst damage.

The City of Dearborn's flood information is the source for the numbers, and it is specific about the design standard. The stormwater system was built for about three inches in 24 hours. The storm delivered more than double that total, and delivered most of it in about a quarter of the time. As arithmetic and not as an engineering finding, the design figure averages about an eighth of an inch an hour, while the storm averaged more than an inch an hour. Pipes are not designed to flat averages, so treat that as an illustration of the scale of the mismatch.

The city also explains why the east side suffered most. East Dearborn sits at a lower ground elevation than west Dearborn, and it was developed before World War II, so its sewers were built to the standards of that time. Later infill removed several surface streams that once led to the Rouge River, leaving fewer places for water to go. West Dearborn, higher and less prone to flooding, was still damaged. The city statement is about the stormwater system and about ground elevation; it is not a statement that Dearborn's sewers are combined, and this guide does not make that claim.

If you live in east Dearborn and want to know what that means for your basement, the practical step is to ask Public Works what serves your address and what is planned. For the cleanup side, see our service pages on basement flooding cleanup and storm damage restoration.

How do the Rouge, Ecorse Creek and soil affect flooding?

Both the Rouge and Ecorse Creek watersheds are heavily urbanized, so rain runs off fast. The lower Rouge was straightened and a 4.2-mile section concrete-lined to move stormwater out quickly, and the Ecorse watershed has clay soil 20 to 65 feet deep that limits absorption. Clay is sourced for the Ecorse watershed only.

Surface water and sewers are linked. The Rouge bends southeast through Dearborn, and Henry Ford dammed it between 1910 and 1920 for the Fairlane Mansion. The lower stretch was later straightened, with a 4.2-mile section lined in concrete to carry stormwater out fast. Wayne County describes Hines Park, which follows the Middle Rouge through Garden City, Westland, Livonia and Redford toward Dearborn, as flood control that keeps the floodplain as open space, and says the Middle Rouge floods often because its basin is heavily urbanized. We have no basis to say more about Rouge soils, and we do not.

The Ecorse Creek watershed is better documented. It covers about 43 square miles, is 85 percent developed, has a low gradient and heavy silting, and sits on clay soil 20 to 65 feet deep that limits how much water the ground absorbs. Two branches join in Lincoln Park and the river meets the Detroit River between Ecorse and Wyandotte. Dearborn Heights is described as a low point in the Ecorse Creek and Rouge systems, taking water from upstream communities, and FEMA spent $3.18 million in 2024 buying 18 flood-prone homes along the creek.

In a past flood about 1,000 basements were reported flooded in Livonia, and the mayor said saturated ground and the volume of rain, not the sewer system, were to blame. We cannot give the year, since none was found. Another report put water four to five feet deep on Fenton at Midland in Redford in a past flood, also undated. These show that river and surface flooding and sewer backups are different problems that can arrive together, and they are treated differently by insurers: floodwater is a separate policy. See flood damage cleanup for the creek-and-river side.

What can a homeowner do about it?

Ask your city's Public Works office what serves your address, add a backwater valve and a working sump pump through a licensed plumber, and ask your insurer about a water-backup endorsement. Detroit addresses have a separate DWSD program. Federal flood insurance typically takes 30 days to take effect, so it cannot be bought during a storm.

Nothing a homeowner does will change a city's sewer system, but several things change what happens inside the house. A backwater valve in the house line stops sewage returning through floor drains, a working sump pump with a battery backup deals with groundwater, and keeping the lowest basement level clear of valuables limits the loss when something does get in. Valves and pumps are plumber's work, and the city building department can tell you whether a permit applies.

On the money side, the Michigan Department of Insurance and Financial Services (DIFS) says that water and sewer back-up is not automatically covered under a homeowners or renters policy, so discuss a back-up option with an agent before a flood. Federal flood insurance is a separate policy and typically takes 30 days to go into effect. When you report damage, DIFS advises not to simply call it a flood, because storm rainwater intrusion, sewer back-up or a failing sump pump may be covered under the policy. Our sewer backup insurance guide explains the endorsement.

If you do get water, treat it as sewage until a technician says otherwise, stay out of it, and call. Dearborn Water Damage Restoration answers the phone 24/7 and a crew is dispatched as fast as one is available; no arrival time can be promised after a storm that floods many homes at once.

• Ask Public Works or your city's DPW what serves your address, and whether any program exists locally.
• Ask a licensed plumber about a backwater valve and a sump pump with a battery backup.
• Ask your agent whether your policy has a water-backup endorsement and what its limit is.
• Photograph the lowest level now, so that a later claim has a baseline.
• Keep valuables, documents and electrics off the basement floor.