SEPA

January 2025

Communities with Combined Sewers
Adapting to a Changing Climate:

Detroit, Michigan

Background

The Detroit Water arid Sewerage Department (DWSD) maintains
the city of Detroit's collection system which includes 3,000 miles
of storm sewers, sanitary sewers, and combined sewers and
serves just under 680,000 people. DWSD sends wastewater and
wet weather flows to the Great Lakes Water Authority (GLWA) for
treatment.1

Challenges

Key Information

•	Location: Detroit, Ml

•	Population served: 680,000

•	Permit Number: MI0022802

•	Key hazards: increasing storm
intensity, increased precipitation,
warming temperatures

Detroit's collection system is facing challenges related to aging
infrastructure, failing private lateral service lines, flooding,
basement backups, and combined sewer overflows (CSOs). In
the last decade, Detroit has experienced several historic flooding
events. In 2021, President Biden declared a national disaster when
Detroit experienced a major storm that resulted in more than 6 inches of
rain in a 12-hour period and led to flooding on streets and freeways, as
well as in residential and commercial basements. To plan for and adapt
to these challenges, DWSD and GLWA have focused on peak flow
control, reducing impervious areas in the sewershed, resilient
redevelopment and infrastructure upgrades, and partnerships to improve
resiliency to changing climate conditions.

Climate Impacts

Detroit's Climate Action Plan notes that total annual precipitation in
southeast Michigan has already increased by 11 percent when
comparing the 1981-2010 average to the 1961-1990 average. Moving
forward, Detroit expects these trends to continue, with more frequent and
more severe storms as well as more rain than snow due to warming
temperatures. These changes are likely to lead to increased flooding,
more basement backups, and CSOs.

Solutions

Climate Action Plan

Detroit's Climate Action Plan is organized around major themes with
goals, near-term and long-term action steps, indicators for success, and

*4*





JC15 I HEAVY RAINS CAUSE BIG FLOODS IN DETROIT

"l ¦ 111 II	III I I I I I HI I

Historic flooding events in Detroit. Flooded
Detroit neighborhood (top) and news report
of highway closures and street flooding
(bottom)

1 GLWA operates regional water and wastewater systems for communities in eight southeast Michigan counties, including the city of Detroit,
while DWSD operates the city's local water and wastewater systems. See DWSD and GLWA's roles and responsibilities for more details.


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Communities with Combined Sewers Adapting to a Changing Climate: Detroit, Michigan

appropriate methods for
assessment. One theme is parks,
public spaces, and water
infrastructure. This theme identifies
steps that address stormwater runoff
while also providing recreational,
habitat, and aesthetic benefits.

These steps include increasing tree
canopy citywide; developing a green
infrastructure plan to manage
stormwater, filter pollutants, and
improve public health; daylighting
streams that have historically been
paved over and polluted; and
expanding, protecting, and
maintaining natural corridors that
provide habitat.

Risk-based Asset
Management

In order to prioritize projects,

DWSD's asset management plan includes a risk model based on the consequence and probability of failure of
each asset. This model considers basement backups, environmentally sensitive areas, hard to maintain areas,
population density, employment rates, proximity to public transportation, asset condition, and hydraulics. The risk
assessment also checks whether each neighborhood can meet a level of service of conveying a 10-year design
storm. In collaboration with other city departments, each of these factors is assigned a weight, and weighted
scores are used to prioritize projects each year. The model is updated annually as more information is available.
DWSD has found that this model is approximately 80% accurate
based on field observations.

Map presenting neighborhood risk scores from DWSD's Capital Improvement Program
risk model

Stormwater Management

In 2018, in an effort to reduce flooding and CSOs, the city of
Detroit established stormwater management requirements
through a stormwater ordinance. The stormwater ordinance
requires management of 10-year and 100-year peak flow
volumes for new developments. As of 2023, 78 projects have
been completed with a total of 2.5 million gallons of retention
volume, which has reduced peak flows in the collection system
by 40 million gallons annually.

Additionally, DWSD has been working to mitigate flooding by
restoring capacity in the sewer system through sewer pipe
cleaning, catch basin inspection and cleaning, and the
construction of two new detention basins.

Green Infrastructure	Completed DWSD green infrastructure project that

DWSD has also prioritized implementing green infrastructure. In
2013, the Michigan Department of Environment, Great Lakes,

and Energy (EGLE) issued a National Pollutant Discharge Elimination System (NPDES) permit that required
DWSD to develop and implement a Green Stormwater Infrastructure Plan for 17 CSO outfalls. To date, DWSD
has implemented green infrastructure in municipal parks, replaced impervious surfaces with green space on
thousands of acres of vacant lots, planted thousands of trees throughout the city, and overseen a downspout

January 2025 vvEPA


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Communities with Combined Sewers Adapting to a Changing Climate: Detroit, Michigan

disconnection program. In total, these installations currently provide 2.2 million gallons of peak flow reduction per
year and reduce the annual volume of runoff discharged to the collection system by 53 million gallons.

Basement Backups

In 2021, DWSD conducted a study of basement backups to identify the most likely backup locations during
storms. Private lateral sewer line repairs can cost homeowners up to twenty thousand dollars, so Detroit provides
funding to customers to assist with lateral upgrades or replacements through their Basement Backup Protection
Program. Additionally, the program provides inspections, downspout disconnections, and installation of backwater
valves for homes in flood-prone neighborhoods to help mitigate backups.

Infrastructure Updates and Rehabilitation

GLWA has made several infrastructure upgrades and is planning more upgrades to improve resiliency against
aging infrastructure and flooding, reduce backups, and increase power reliability. GLWA updated several pump
stations to improve power reliability and redundancy and plans to update others to reduce the risk of flooding and
backups. GLWA is also planning to upgrade the Detroit River Interceptor and rehabilitate other portions of the
sewer system to optimize existing capacity, as well as extend the lifespan of existing infrastructure.

Enhancing Resiliency

To implement wet weather improvements including flood mitigation, DWSD and GLWA have obtained funding
from a number of grant programs including FEMA's Building Resilient Infrastructure and Communities and the
Clean Water State Revolving Fund. They have also partnered with entities like the Michigan Department of
Transportation (MDOT) on several large-scale tunnel, sewer, drainage area disconnection, and freeway removal
projects to improve stormwater management and reduce flooding.

Additional Information

For more information on the Detroit Water and Sewerage Department, contact Samuel Smalley, Chief
Operating Officer, at Samuel.Smallev@detroitmi.gov. For more information on the Great Lakes Water
Authority, contact Todd King, System Resiliency Officer, at todd.king@glwater.org. Additional
information about DWSD and GLWA's permit, CSOs, and climate adaptation can be found in the
following resources:

•	DWSD's Stormwater Management Plan

•	Great Lakes Water Authority Capital Improvement Plan

•	Detroiters Working for Environmental Justice

EPA Publication 830-F-25-008

January 2025 £EPA


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