Overview of Stormwater in Detroit

When it rains in Detroit, stormwater falls on rooftops, streets, sidewalks and parking lots and then this storm water flows into our sewer system. The sewer system in Detroit is a combined sewer system. That means it carries both stormwater and sanitary sewage. During wet weather, too much stormwater can overload the combined system. This might cause basement backups, street flooding, and polluted wastewater to flow into the Rouge River, the Detroit River, and eventually, Lake Erie.

We all share the benefits of healthy water and the consequences of overloading Detroit’s combined sewer system. That’s why we also share the responsibility for controlling the amount of stormwater that flows into the system.

Learning about Detroit Stormwater and Solutions

Use these links to learn more about stormwater, the combined sewer system, and what you can do to be a part of Detroit’s stormwater solution.

How does untreated stormwater and sanitary sewage get into our waterways, like the Detroit River and Lake Erie?
What is DWSD doing about this problem?
What is Green Stormwater Infrastructure and how can it help Detroit?

How Does Untreated Stormwater and Sanitary Sewage Get Into the Rouge River, the Detroit River and Lake Erie?

In southeast Michigan, approximately three million residents and thousands of businesses send wastewater down their drains each day to a network of sewer pipes that lead to the Detroit Wastewater Treatment Plant (WWTP). The sewer pipes don’t only carry wastewater from toilets, showers, and drains—they also carry storm water when there’s wet weather. When wastewater from homes and businesses mix with stormwater, the Rouge and Detroit Rivers are in danger of becoming more polluted.

Stormwater in the Combined Sewer System. Combined sewers transport wastewater from homes and businesses with stormwater during wet weather in a single pipe. During storms, combined sewer systems collect the stormwater that runs off our streets and houses along with untreated sewage. That means the volume of flow in that single pipe can be 10 to 100 times greater than the volume of flow that passes through the pipe on a dry day. In southeast Michigan, 30 percent of the sewer systems (26 of 77 communities) that send flows to the Detroit WWTP are combined sewers. Combined sewers were constructed when development was much lower, meaning the pipes had to carry less flow.

During wet weather, there is a lot of pressure on the collection and treatment system. If flows exceed the capacity of the system, an overflow of untreated sewage and stormwater enters the Rouge and Detroit Rivers. These overflows are called combined sewer overflows, or CSOs. CSOs are a last resort to prevent sewer backups and basement flooding.

Stormwater in the Sanitary Sewer System. Many of the conventional sanitary sewers in the remaining 70 percent of the service area take on storm water flow from footing drains that are connected to the sanitary sewer rather than to a sump in the basement. Like combined sewers, sanitary sewers with stormwater flows can become overloaded and cause sanitary sewer overflows, or SSOs, to the Rouge and Detroit rivers.

Stormwater in the Municipal Separate Storm Sewer System (MS4). There are small portions of Detroit, as well as portions of other suburban communities, that have pipes and other conveyances (e.g., ditches) intended to transport only stormwater, referred to as a municipal separate storm sewer system, or an MS4. Unlike wastewater in the combined sewer system and sanitary sewer system, stormwater that travels through the MS4 is never treated and goes directly into rivers and lakes. Approximately four percent of the system is an MS4. When there is too much storm water in the MS4, streets and homes can experience flooding.

How is DWSD Managing Stormwater?

DWSD is responsible for managing facilities and programs to prevent untreated CSOs, as well as reducing the quantity and improving the quality of stormwater in Detroit.

Managing stormwater in the combined sewer system. DWSD staff manage and operate the collection system to prevent combined sewer overflows, or CSOs, from occurring. Facilities like retention treatment basins, or RTBs, temporarily store and treat combined sewage. From January through April 2014, these RTBs prevented more than 7.5 billion gallons of untreated combined sewage from overflowing into our waterways.

To prevent more overflows without expensive investments in more collection facilities, DWSD is working to reduce the amount of storm water that enters the combined sewer system through investments in  Green Stormwater Infrastructure approaches. This is part of DWSD’s Alternative Rouge River CSO Control Program, a 25-year phased plan that focuses on Green Infrastructure solutions and right-sized CSO control facilities.

Managing stormwater in the municipal separate storm sewer system (MS4). For stormwater entering the separate storm sewer system, DWSD has a National Pollutant Discharge Elimination System (NPDES) MS4 permit. This permit requires DWSD to develop and implement a stormwater management program (SWMP). The SWMP includes six types of management activities intended to improve the quality of stormwater before it enters the MS4.

Outfall on Rouge River


CSO basin



Why is it Important to Manage Stormwater from Development Projects?

The recent review of Detroit's city municipal code revealed that Detroit does not currently require development projects to manage stormwater post-construction.  Developed lands – such as roofs, parking lots and sidewalks – that does not allow rain water to infiltrate into the ground, contributes significantly to discharges into the sewer system and requiring development projects to manage this storm water is necessary to comply with its regulatory requirements under the National Pollutant Discharge Elimination System (NPDES) permits issued by the Michigan Department of Environmental Quality.  An inter-departmental City technical advisory committee has been developing a new a post-construction stormwater ordinance for more than year to address this code deficiency.


How Can You Get involved?

The City of Detroit has solicited input from a variety of stakeholders during this stormwater post-construction ordinance development process. The input process is still underway. Individuals and organizations interested in getting involved can participate in the upcoming outreach and input events regarding the new ordinance, as well as other municipal code updates being considered by the City to support green stormwater infrastructure.


Rainfall to Results: The future of stormwater

Based on input from leading stormwater professionals, the Water Environment Federation (WEF) Stormwater Institute drafted a report, Rainfall to results: The future of stormwater that details the challenges, opportunities, and pathways to improving the nation's stormwater systems to make them more efficient, effective and sustainable. Read full report.