United States
Environmental Protection Agency
Office of Water (4203)
Washington, DC 20460
EPA-841-B-00-005C
October 2000
Street Storage for
Combined Sewer Surcharge
Control
Skokie and Wilmette, Illinois
Introduction
This case study describes the use of street storage
and catch basin modifications to reduce the rate of
runoff entering combined sewer systems (CSSs).
These modifications help alleviate residential
basement flooding that results from CSS
surcharging during rainfall events. Because
building relief sewers would be both expensive
and disruptive, the communities of Skokie and
Wilmette, Illinois, were willing to try alternative
approaches. The communities decided to modify
street cross sections and storm drain inlets so the
street surfaces could store and convey runoff
during peak storm events and reduce hydraulic
loading to the combined sewer. This process
required extensive coordination with regional,
state, and local officials and residents to ensure
that safety and community acceptance concerns
were satisfied.
Project Area
The street surface storage projects were conducted
in the towns of Skokie and Wilmette, Illinois,
suburbs of Chicago. The entire 8.6 square miles
of Skokie and a 2-square-mile section of Wilmette
Key Concepts:
> Street Storage
> Flow Regulation
> Street & Inlet Modifications
> Source Control
LOW-IMPACT
DEVELOPMENT
CENTER
Figure 1. Runoff temporarily stored on a street
surface.
Project Benefits:
> Elimination of Surcharge
> Community Acceptance
> Cost Savings
are urban areas served by a CSS. Skokie has
approximately 23,000 households (65 percent of
which occupy single-family homes) and Wilmette
has approximately 9,000.
Project Description
To alleviate system surcharging, a strategy that
combined the following elements was used:
- Street storage
- Downspout disconnection
- Flow regulators
- Subsurface storage
- New storm and combined sewer systems
- Improvements to existing storm and
combined sewer systems
The goal of the project was to take full advantage
of the street and inlet system for stormwater
control as an alternative to installing expensive
underground facilities such as complex
configurations of storage vaults with flow
regulators or additional pipes for increased storm
drainage capacity.
The alternative street storage approach was based
primarily on installing a system of street berms 7
to 9 inches high at the curb line that detain water
on the street surface. Figure lisa photograph
that shows this technique. In addition, installing
flow regulation devices at catch basin outlets
reduced the rate of storm water flow to the CSS so
both the inlet structure and the street can be used
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Berm Overflow
Street Storage
of Stormwater
Longitudinal
Street Profile
Controlled
Discharge to
Combined ^
Sewer System
Catch Basin
Flow Regulator
Figure 2. Schematic diagram of the street storage system (Note: not to scale and great vertical exaggeration).
for storage. Figure 2 illustrates this concept.
Subsurface storage facilities were installed in the
street right-of-way and in other public areas as
part of the storm drainage system at critical points
in the system and in high-traffic areas, parking
areas, and pedestrian walkways where ponding
was not acceptable. Overall, street storage
accounts for over half of the total stormwater
storage capacity. The other half is accounted for
in subsurface and off-street storage.
Project Summary and Benefits
Many benefits were realized from this project.
First, researchers estimate a cost advantage from
using street storage over conventional sewer
separation systems to alleviate CSS surcharges.
Figure 3 shows the estimated costs for the Skokie
system to be approximately 38 percent of the
costs for conventional approaches to sewer
separation. A breakdown of costs associated with
the street storage approach reveals that berm/flow
regulator installation is a small fraction of the
overall project cost, as shown in Figure 4. The
street storage system could aid in traffic control
because the berms function much like speed
humps as traffic calming structures. Also, the
storage system reduces the frequency and volume
of combined sewer overflows, resulting in fewer
storm water-related pollutant events in receiving
waters. Since the system's installation in 1983,
250
Sewer Separation
Source Control
Figure 3. Cost comparison between the traditional
sewer separation approach and a source control
approach using street surface storage.
80
70
60
| 50
i 40
10
DBerms/Flow Regulators
D Sewers
D Supplemental Storage
Figure 4. Costs associated with the CSS surcharge relief
project.
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consisting of 2,900 flow regulators, the Skokie
Public Works Department has not reported any
problems with icing of ponded areas during winter
weather because the water typically remains on
the street surface for less than 30 minutes.
Several lessons were learned as a result of this
project. First, researchers emphasized the
importance of using a comprehensive approach to
stormwater management that explores funding
and cooperation from different sources, including
streetscaping and revitalization programs.
Second, early and frequent stakeholder
involvement and strong outreach and education
programs that clearly identify the benefits of street
storage were necessary to gain support for the
project from citizens. Finally, a comprehensive
inspection and maintenance program with training
for public works staff was essential to ensure that
street storage systems functioned as designed.
References
USEPA. 2000. Street Storage System for Control
of Combined Sewer Surcharge. EP A/600/R-
00/065. U.S. Environmental Protection Agency,
Washington, DC.
Walesh, S., andR. Carr. 1999. Street Surface
Storage for Control of Combined Sewer
Surcharge. In WRPMD '99: Preparing for the
21st Century, ed. M.E. Wilson. Proceedings of
the 26th Annual Water Resources Planning and
Management Conference, Tempe, Arizona, June
6-9, 1999.
Contact Information
Stuart G. Walesh, PE
Consultant
3006 Towne Commons Drive
Valparaiso, IN 46385-2979
Phone: (219)464-1704
Fax: (219)464-2978
stuwalesh@aol.com
Robert Can-
Senior Water Resources Engineer
Earth Tech
1020 North Broadway, Suite 400
Milwaukee, WI 53202
Phone: (414)225-5135
Fax: (414)225-5111
bob_carr@earthtech. com
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