A Southeast Michigan Flood Study

In August 2021, less than three months after some of the most extensive flooding in southeast Michigan, Sue Coffey became Interim CEO of the Great Lakes Water Authority. One of her top priorities was addressing the unprecedented rain events impacting the region. An engineer by trade, Coffey decided to send a letter to an entity that she knew could help find a solution: the U.S. Army Corps of Engineers (USACE). 

By 2022, an agreement was drawn up between GLWA and USACE to begin the Flood Risk Management study. Authorized and funded by Congress through the Water Resources Development Act, the general investigative study was launched to find ways to combat intense rain events and reduce the risk of flooding in southeast Michigan. 

Measuring the Rain

For individuals to understand flooding risk, they must understand how to discuss large rain events. There are three factors that scientists and engineers use to characterize a rain event. 

“There’s intensity, or how fast rain is falling for, say, an hour.” said Todd King, Chief Resiliency Officer at GLWA. “There’s duration, how long the event is, and there’s the frequency, or how often that event occurs.” 

Our system’s capacity for rain events is determined by a federal tool created by the National Oceanic and Atmospheric Administration (NOAA) National Weather Service, the Atlas 14 Precipitation Frequency Data Server, which provides an estimate of how much precipitation is typical for a given part of the country. Atlas 14 is used to calculate the amount of rainwater a sewer system must be built to handle during large storm events. Capacity varies depending on whether the sewers are combined (sanitary and storm) or separated design.  

“The city of Detroit and the older surrounding suburbs are all what we call combined. That means there’s only one pipe. Sanitary sewage and stormwater runoff go into the same pipe,” says King. 

Much of southeast Michigan’s wastewater infrastructure is designed with a combined system, meaning sewer and storm runoff all collect in the same pipe. Newer suburbs are designed with one pipe dedicated to sanitary and another pipe dedicated to storm. 

The Watershed

 
“The study was based on what we call the watershed,” Todd said. “Why is that important? Because when the water falls onto the ground, it stays within the watershed. The watershed defines the boundary.” 

Any rainwater that falls within that watershed boundary makes its way to the Clinton River, Detroit River or portions of Lake St. Clair, defining the study area. From there, the team will determine focus areas within the study boundary to develop alternative solutions that will provide the greatest benefit to the region. 

The Flood Study is a 6-step USACE process involving scientists, engineers and economists that work within the region or who work with agencies that have a large footprint on the system. The initial goal was to develop a Chief’s Report, a document that identifies solutions for flood mitigation, and outlines avenues for the region to receive needed federal funding to support this work over such a large region. The scope of the project originally covered 8.5 years at a cost of $11 million.  Recent redirection from the federal government administration currently in office has approved a more streamlined and focused effort over a four-year period with a $5 million budget. GLWA, as the non-federal sponsor, splits the effort and costs with USACE on a 50:50 basis.   

Building a Resilient System

Moving forward, King shares, the focus has centered on developing projects that align with the smaller scope of the study and building collaboration with Member Partners and planning partners such as SEMCOG and the Michigan Department of Transportation. 

“We’re going to first establish a technically sound approach to identify where these flooding problem areas are and then look for solutions and use the study to define projects that will help us address flooding on a more regional basis.” 

Moving forward, USACE and GLWA will continue to invite the public to participate in informational sessions & community engagement events to remain up to date and involved in the study. 

The Southeast Michigan Flood Risk Management study helps us dedicate resources to enhance our understanding of our region. With the help of USACE, GLWA gains a stronger sense of how best to create a resilient system, accommodate for excessive rain events, and improve the quality of life for residents impacted by flooding in our region today, and for generations to come.