In cooperation with the 

U.S. ARMY CORPS OF ENGINEERS, CHICAGO DISTRICT

Monitoring and Analysis of 

Combined Sewer Overflows, 

Riverside and Evanston, Illinois, 1997-99

By Andrew M. Waite, Nancy J. Hornewer, and Gary P. Johnson

Water-Resources Investigations Report 01-4121

U.S. Department of the Interior

U.S. Geological Survey

Urbana, Illinois

2002
ii 

U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this report

is for descriptive purposes only and does not imply

endorsement by the U.S. Government.

For additional information, write to: Copies of this report can be

purchased from:

District Chief U.S. Geological Survey

221 North Broadway Avenue Branch of Information Services

Suite 101 Box 25286

Urbana, Illinois 61801 Denver, CO 80225-0286

Internet address: http://il.water.usgs.gov Telephone: 1-888-ASK-USGS
iii

CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Physical setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Combined sewer overflow monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Operation and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Analysis of combined sewer overflow data at Riverside, Illinois. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Description of study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Storm volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Analysis of combined sewer overflow data at Evanston, Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

Description of study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

Storm volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

APPENDIX

CSO Database CD-ROM

(in pocket)

FIGURES

Figures 1. Map showing location of deep-tunnel system and Riverside and Evanston, Ill. . . . . . . . . . . . . . . . . . . 2

2-3. Diagrams showing:

2. Schematic of a combined sewer system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Schematic of a combined sewer overflow flow monitoring and water-quality sampling 

installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4-9. Photographs showing:

4. American Sigma 920 Long-Term Flow Monitor data logger and probe used to

measure flow in combined sewers during this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5. Sigma probe installation in 60-inch “short pipe” at Riverside, Ill. . . . . . . . . . . . . . . . . . . . . . . . 8

6. Sigma 910 meter with probe mounted on I-beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7. Sigma probe mounted on C-channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

8. Starflow Ultrasonic Doppler Meter mounted on a 3-inch wide stainless-steel band 

with an expanding clamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9. Rain-O-Matic rain gage mounted on Village of Riverside Fire Department building, 

Riverside, Ill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
iv 

FIGURES—Continued

Figure 10-12. Diagrams showing:

10. Schematic (plan) of sewer system used in measuring combined sewer overflows

at Riverside, Ill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

11. Schematic (profile) of sewer system near Gage Street, Riverside, Ill. . . . . . . . . . . . . . . . . . . . 13

12. Schematic (profile) of sewer system near Olmstead Avenue, Riverside, Ill.. . . . . . . . . . . . . . . 14

13-14. Photographs showing:

13. The deep tunnel that transports combined sewer overflow water to the 

West-Southwest Water Reclamation Plant (Stickney) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

14. Interceptor sewer and diversion weir in the “Gage Street pipe,” Riverside, Ill. . . . . . . . . . . . . 16

15-23. Graphs showing:

15. Discharge at the “Gage Street pipe” and the “long pipe,” Riverside, Ill., April 22, 1999 . . . . . 17

16. Stage and velocity at the “long pipe,” Riverside, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . . 19

17. Stage and velocity at the “brick pipe,” Riverside, Ill., June 23-27, 1999 . . . . . . . . . . . . . . . . . 20

18. Stage and velocity at the “brick pipe,” Riverside, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . . 22

19. Discharge at the “Gage Street pipe,” Riverside, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . . . 23

20. Stage and velocity at the “short pipe,” Riverside, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . . 25

21. Stages at the “outfall pipe” and the Des Plaines River, Riverside, Ill., April 21-26, 1999 . . . . 26

22. Stage and velocity at the “outfall pipe,” Riverside, Ill., April 21-26, 1999. . . . . . . . . . . . . . . . 27

23. Stage at the “long pipe” and discharge at the “outfall pipe” during the storm 

of July 19, 1998, Riverside, Ill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

24-26. Diagrams showing:

24. Schematic (plan) view of sewer system used in measuring combined sewer overflows 

at Evanston, Ill.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

25. Schematic (profile) of sewer system at Evanston, Ill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

26. Schematic (profile) of sewer system at Evanston, Ill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

27-30. Graphs showing:

27. Stage and velocity at the “siphon pipe,” Evanston, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . 35

28. Stage and velocity at the “new pipe,” Evanston, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . . . 37

29. Stage and velocity at the “outfall pipe,” Evanston, Ill., April 21-26, 1999 . . . . . . . . . . . . . . . . 39

30. Stage and velocity at the “ledge,” Evanston, Ill., April 21-26, 1999. . . . . . . . . . . . . . . . . . . . . 40

TABLES

Table 1. Flow mass balance for chamber 2 in the combined sewer system at Riverside, Ill., 

April 22-26, 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2. Flow mass balance for chamber 3 in the combined sewer system at Evanston, Ill., 

April 21-26, 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
v

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 

1929 (NGVD of 1929)—a geodetic datum derived from a general adjustment of the firstorder level nets of both the United States and Canada, formerly called Sea Level Datum of 

1929.

Altitude, as used in this report, refers to distance above or below sea level.

Concentrations of chemical constituents in water are given either in milligrams per liter 

(mg/L) or micrograms per liter (μg/L).

Multiply By Obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 0.004047 square kilometer

square foot (ft2) 0.09290 square meter

square mile (mi2

) 2.590 square kilometer

Volume

cubic foot (ft3) 0.02832 cubic meter

Flow rate

foot per second (ft/s) 0.3048 meter per second

cubic feet per second (ft3/s) 0.02832 cubic meters per second

Mass

pound, avoirdupois (lb) 0.4536 kilogram
ABSTRACT 1

ABSTRACT

The U.S. Geological Survey, in cooperation with

the U.S. Army Corps of Engineers, collected and analyzed flow data in combined sewer systems in Riverside

and Evanston, northeastern Illinois, from March 1997 to

December 1999. Continuous 2- and 5-minute stage and

velocity data were collected during surcharged and nonsurcharged conditions at 12 locations. Mass balances

were calculated to determine the volume of water flowing through the tide-gate openings to the Des Plaines

River and the North Shore Channel and to determine the

volume of water flowing past the sluice gate to the deep

tunnel.

The sewer systems consist of circular pipes ranging

in diameter from 0.83 feet to 10.0 feet, elliptical siphon

pipes, ledges, and tide and sluice gates. Pipes were constructed of either brick and mortar or concrete, and

ranged from having smooth surfaces to rough, pitted and

crumbling surfaces. One pipe was noticeably affected by

water infiltration from saturated ground.

During data analysis, many assumptions were

necessary because of the complexity of the flow data

and sewer-system configurations. These assumptions

included estimating the volume of water entering an

interceptor sewer at the “Gage Street pipe” at Riverside,

the effect of infiltration on the “brick pipe” at Riverside,

and the minimum velocity required for the meter to

make an accurate velocity determination. Other factors

affecting the analysis of flow data included possible

non-instrumented sources of inflow, and backwater conditions in some pipes, which could have caused error in

the data analysis. Variations of these assumptions potentially could cause appreciable changes to the final massbalance calculations.

Mass-balance analysis at Riverside indicated a total

inflow volume into chamber 3 of approximately

721,000 cubic feet (ft3) during April 22-26, 1999. Outflow volume to the Des Plaines River at Riverside

through the tide gate was approximately 132,000 ft3;

outflow volume to the deep tunnel through the sluice

gate was approximately 267,000 ft3. The mass-balance

analysis at Evanston indicated a total inflow volume into

chamber 3 of approximately 5,970,000 ft3 during April

21-26, 1999. The outflow volume to the North Shore

Channel through the tide gates at Evanston was approximately 2,920,000 ft3; outflow volume to the deep tunnel

through the sluice gates was approximately

3,050,000 ft3.

INTRODUCTION

The U.S. Army Corps of Engineers, Chicago District (Corps), is designing a flood-control reservoir as

part of the Chicago Underflow Plan/Tunnel and Reservoir Plan (CUP/TARP) that will provide storage for

combined sewage from the Des Plaines and Mainstream

tunnel systems (fig. 1). The proposed McCook reservoir

is intended to reduce flood damages and minimize the

release of untreated combined sewage into local area

waterways (U.S. Army Corps of Engineers, 1998). The

Des Plaines and Mainstream Tunnel systems (referred to

as the deep tunnel) are designed to transport combined

sewage to the flood-control reservoir when flows exceed

the capacity of the West-Southwest Water Reclamation

Plant (referred to as Stickney).

In order to design a cost-effective aeration system

for the reservoir, it is necessary to determine the quality

and quantity of water expected to enter the reservoir

during large rain storms. For this reason, the biochemical oxygen demand (BOD) and other water-quality characteristics of the influent combined sewage for rain

storms from 1 to 2 in. and greater are required. The

Metropolitan Water Reclamation District of Greater

Chicago (MWRDGC) collected water-quality samples

at the Riverside and Evanston combined sewer overflow

(CSO) locations, the Calumet pumping station, and at

the Racine pumping station (fig. 1). To determine the

BOD load during these storms, flow data, in addition to

water-quality data, are required. The U.S. Geological

Survey (USGS), in cooperation with the Corps, monitored and analyzed combined-sewer flow data at Riverside and Evanston, Ill. (fig. 1). The data-collection

locations at the Riverside and Evanston combined sewers represent only two of many potential overflow points

in the Des Plaines and Mainstream Tunnel systems. The

qualitative and quantitative data collected at these two

locations will be used to estimate, on a larger scale, the

Monitoring and Analysis of Combined Sewer Overflows,

Riverside and Evanston, Illinois, 1997-99

By Andrew M. Waite, Nancy J. Hornewer, and Gary P. Johnson
2 INTRODUCTION

Figure 1. Location of deep-tunnel system and Riverside and Evanston, Ill.

(modified from http://www.mwrdgc.dst.il.us/plants/tarpmap.htm).
COMBINED SEWER OVERFLOW MONITORING 3

quality and quantity of water that might be expected in

the reservoir, and will aid in the design of a reservoir

aeration system.

Purpose and Scope

This purpose of this report is to describe the flow

data collection and analysis completed as part of the

monitoring of combined sewer overflows in Riverside

and Evanston, Ill., from 1997 to 1999. This report also

describes the reasons for choosing the flow-monitoring

locations, the type of equipment used, and the procedures followed in collecting and analyzing the flow data.

Flow and rainfall data collected by the USGS and waterquality data collected by the MWRDGC are presented

in a CD-ROM included at the back of this report. Water

level (stage) and velocity data were collected in 2- and

5-minute intervals.

Physical Setting

The service area for the Des Plaines and Mainstream systems of the TARP consists of 252 mi2 of the

Chicago metropolitan area (U.S. Army Corps of Engineers, 1998). Dry-weather flow, including both domestic

and industrial wastewater, stormwater, and inflow and

infiltration are transported in these tunnels. The communities of the service area contain approximately 3 million people and are serviced by more than 4,400 mi of

combined sewers (U.S. Army Corps of Engineers,

1998).

The Des Plaines tunnel system covers approximately 32 mi2, which includes part or all of

20 communities. This service area contains a population

of approximately 263,000 and mainly consists of residential property. The Mainstream tunnel system covers

approximately 220 mi2, which includes part or all of

21 communities. This service area contains a population

of approximately 2.7 million, and mainly consists of

residential property with some commercial and industrial areas (U.S. Army Corps of Engineers, 1998).

Acknowledgments

Various individuals and agencies deserve acknowledgment for assistance in this project. The staff of

MWRDGC assisted with field operations and provided

access to the confined sewer space. Mr. Tony Bednarz,

Village of Riverside Fire Department, and Mr. Michael

Whalen, City of Evanston Fire Department, offered their

department staffs as stand-by rescue teams for USGS

personnel as well as performing confined-space rescue

training on site. In addition, Mr. William Renaker, Fox

Lake Fire Department, and the Illinois Fire Service

Institute, provided confined-space rescue training for

USGS personnel.

COMBINED SEWER OVERFLOW

MONITORING

Methodology

Combined-sewer flows were determined by collecting water level (stage) and velocity data in 2- and

5-minute intervals. Flow generally was calculated using

the basic equation

Q = AV, (1)

where Q is discharge, in cubic feet per second (ft3/s);

A is the wetted area in the pipe, in square feet

(ft2); and

V is the water velocity, in feet per second (ft/s).

The hydraulic characteristics of the combined sewers were complex because of the pipe configurations,

pipe junctions, and chambers. Furthermore, the operation of the sluice gates leading to the deep tunnel caused

reverse flows through many of the pipes. Because of the

complexity of each sewer system, alternate methods of

determining and estimating velocities, and, consequently, flows, were required. For example, velocity

estimates were required at times when water was flowing, but sometimes velocities were too slow for the flow

meter to measure; also, velocity-magnitude estimates

were required when sewer-system turbulence induced

unreasonably high velocities.

Generally, a combined sewer system is a gravityflow sewer that is designed to carry residential, commercial, industrial wastewater, and stormwater runoff

(fig. 2). The actual wastewater flow, or dry-weather flow,

usually is only a fraction of the stormwater design flow.

Dry-weather flow is diverted from the combined sewer

to a wastewater-treatment plant through interceptor sewers, which are designed to transport only up to what the

wastewater-treatment plant can safely handle (Metcalf

& Eddy, Inc., 1991).

Diversion structures are used in combined sewers to

control the flow between the combined sewer and the

interceptor sewer (Metcalf & Eddy, Inc., 1991). In the

case of the monitoring locations discussed in this report,

the flow-diversion structures consisted of diversion

weirs (fig. 2).

During periods of stormwater runoff, the combined

sewer often transports more water than the interceptor

sewer can handle. In this case, the water in the combined

sewer rises and overflows the weirs and empties into a
4 COMBINED SEWER OVERFLOW MONITORING

Figure 2. Schematic of a combined sewer system (modified from Metcalf & Eddy, Inc., 1991).
COMBINED SEWER OVERFLOW MONITORING 5

receiving waterway. In order to keep the receiving

waterway from backing up into the sewer system, tide

gates can be utilized (fig. 2) (Metcalf & Eddy, Inc.,

1991). Tide gates generally allow water to flow out of

the sewer system when the stage inside the system is

higher than the water in the receiving stream.

The MWRDGC’s water-quality data collection at

Riverside and Evanston began on May 1, 1995. In addition to collecting samples at Riverside and Evanston, the

MWRDGC also collected samples at the Calumet

pumping station and the Racine pumping station (fig. 1).

Water-quality samples were collected with automatic

samplers during rain storms when 1 to 2 in. or more of

rain fell. Both the sampler intake lines and the program

parameters were arranged in such a way that samples

automatically would be collected when water began to

flow through the tide gates to the receiving streams.

During these storms, stages in the sewer systems were

expected to rise high enough so that discharges would

result to the receiving waterways. Samples were processed and analyzed for BOD, and sometimes for suspended solids and ammonium-N. To determine

constituent loadings, or the quantity of constituent per

unit volume of water (for example, a BOD load), water

volumes also were needed. Originally, water-quality

sampling and computer flow modeling together were to

be used to indicate the constituent loading. However,

because of the hydraulic complexity of the systems at

Riverside and Evanston, the Corps deemed it necessary

to collect flow data in the combined sewers in addition

to the water-quality data. Flow data collection began as

early as March 1997 at some locations. In order for the

water-quality data to be used to compute constituent

loads, flow and water-quality data needed to be collected

concurrently at the same locations. These data would be

used to compute constituent loads and to calibrate a

computer model that could be used to simulate flows

and loads for periods prior to and subsequent to flow

data collection. The model also may be used to estimate

the total quantity and quality of water from all of the

possible combined sewer overflow points that may be

stored in the proposed McCook reservoir.

Equipment

A typical CSO monitoring installation for this study

consisted of an automatic water-quality sampler for collecting samples during periods of high flow and a flowmonitoring device with a water depth and velocity sensor (fig. 3).

Portable long-term flow meters were installed to

collect flow data in the combined sewers. Most flow

meters installed at both Riverside and Evanston were

manufactured by American Sigma, Inc. and are referred

to as Sigma meters (fig. 4).

The Sigma meters consist of a data logger in a

watertight cylindrical enclosure. This enclosure houses

the 6-volt batteries, and all the electronics and circuitry.

The data-logger interface allows the Sigma meter to

communicate with a modem or portable computer for

the purpose of remotely monitoring flow and/or downloading data. A probe that contains a submersible pressure transducer (stage sensor) and an ultrasonic Doppler

velocity meter (velocity sensor) is connected to the

Sigma meter. The probe is 0.8 in. high, 1.5 in. wide, and

5 in. long. Minimal flow disturbance results from the

probe’s low profile. The Sigma meter models that were

used in this study included the American Sigma 910, to

which one probe was connected, and an American

Sigma 920 and 930, to which two and three probes were

connected, respectively. The multiple-probe Sigma

meters were used at locations where more than one

stage and velocity measurement were required.

The stage sensor is a diaphragm that deflects under

the pressure of the water column above it. Ten transducers capable of measuring stage ranging from 0.018 to

11.5 ft of water, with an accuracy of +/- 0.023 ft (American Sigma, Inc., 1996b). These 10 transducers were

located at the “long pipe,” “brick pipe,” “short pipe,”

and the “Gage Street pipe” at Riverside, and the “ledge,”

“new pipe,” “outfall pipe,” and three probes in the “Lake

Street pipe” at Evanston. Pipe and probe locations are

described in detail later in the report. Two transducers

measured stage ranging from 0.018 to 34.6 ft of water,

with an accuracy of +/- 0.07 ft and were located at the

“short pipe” and the “floor” at Evanston.

The velocity sensor operates according to the Doppler-shift principle. The probe transmits an ultrasonic

acoustic signal (1 MHz) into the water column and waits

for a return signal bounced off reflectors or particulate

matter. The return signal is received back at the probe

with a different, or shifted, frequency. The magnitude of

the frequency shift is proportional to the magnitude of

the velocity, and the direction of the frequency shift is

proportional to the direction of the velocity (Michael

Simpson, U.S. Geological Survey, written commun.,

1999). The velocity sensor measures the average velocity, as opposed to discrete velocities, in the water column. The Sigma meter can measure velocities ranging

from –5.0 ft/s to 20.0 ft/s with an accuracy of +/- 2 percent (American Sigma, Inc., 1996b). Negative velocities
6 COMBINED SEWER OVERFLOW MONITORING

Figure 3. Schematic of a combined sewer overflow flow monitoring and water-quality sampling installation

(modified from Metcalf & Eddy, Inc., 1991).
COMBINED SEWER OVERFLOW MONITORING 7

result if reverse flow occurs. The minimum velocity

required for “good” velocity measurement is

approximately from 0.20 to 0.30 ft/s (Joe Prell,

American Sigma, Inc., oral commun., 1999). If the

water is clear and has few reflectors, or if the water is

pooled, allowing the particulate matter to settle out, or if

the velocity is below the detection limit of the meter

(from 0.20 to 0.30 ft/s), a zero velocity may be recorded.

The Sigma meters were programmed to record

stage and velocity every 5 minutes, regardless of the

depth of water over the probe. The Sigma meters were

capable of recording battery voltages; however, in an

effort to maximize the amount of available data-logger

memory, battery voltages were not recorded. The

5-minute recording interval was chosen because stages

in the sewers can quickly change from dry-pipe to fullpipe conditions in a matter of minutes. It is likely that

with a longer data-recording interval (for example,

15 minutes), the data resolution to account for abrupt

changes in stage and velocity would not have been adequate to determine flow rates.

Selection for the placement of the probe is critical

for proper operation of the velocity sensor. To obtain the

most accurate velocity measurement possible, various

items need to be considered. For example, turbulence

induced by obstructions, vertical drops, pipe bends, and

elbows can cause poor velocity measurements. Ideally,

if the probe is mounted in a pipe near an outfall, or is

near a vertical drop, it should be located at least 10 times

the maximum expected water level away from the disturbance (American Sigma, Inc., 1996b). If the probe is

located in a pipe with surcharged or backwater conditions such that the pipe would become full, then the

probe should be mounted at least 10 pipe diameters

upstream from the end of the pipe or any flow disturbance.

In general, a series of stainless-steel straps and ringstyle bands were used to mount the probes to the pipes

(fig. 5). The probe was mounted near the end of the strap

with the cable fastened to the strap with duct tape to

avoid accumulating debris. The strap, which only

spanned a portion of the pipe circumference, then was

bolted to the wall of the pipe, with the probe placed at

the bottom of the pipe. In certain cases, the strap and

probe assembly was rotated and mounted slightly higher

on the wall of the pipe to keep the probe from fouling by

accumulating debris.

Ring-style stainless-steel bands were used in pipes

of small diameter, generally 28 in. or less. In the case of

the stainless-steel band, the probe would be mounted on

the band and the cable again would be duct taped to the

band. The assembly would be inserted into the pipe and

expanded with an expanding clamp or scissor jack,

causing the band to snugly adhere to the pipe.

In addition to the techniques described above, other

techniques sometimes were required for probe installation. For example, at the “Lake Street pipe” the probes

were mounted on the flange of an I-beam and lowered

Figure 4. American Sigma 920 Long-Term Flow Monitor data logger and

probe used to measure flow in combined sewers during this study

(modified from http://www.americansigma.com/920.htm).
8 COMBINED SEWER OVERFLOW MONITORING

into the water (fig. 6). This mounting was done because

the pipe surface was pitted, broken, and crumbling.

Also, the water at this location was too deep for a stainless-steel band installation.

Each I-beam was approximately 14 in. tall, 10 in.

wide, and 31 in. long, and weighed about 175 pounds.

The weight of the I-beams generally prevented their

movement during high-flow conditions. Because more

than one probe was located in the same pipe, each probe

was mounted on a separate I-beam, and the I-beams

were offset. Offsetting the I-beams ensured that no interference resulted with the velocity signal during velocity

measurements.

After May 5, 1998, the water routing downstream

of the probes at the “Lake Street pipe” was altered,

which caused the stages in this pipe to drop approximately 1.70 ft. In this condition, the I-beams could no

longer be used to mount and secure the probes. The Ibeam flange, and, consequently, the probe, often was out

of the water. An alternative to this installation was

mounting and securing the probes with C-channels

(fig. 7). Again the probes were mounted on small stainless-steel plates and were then mounted on the web of

three different C-channels. Each C-channel was approximately 4 in. tall, 12 in. wide, and 48 in. long, and

weighed about 180 pounds. The weight of the C-channels generally prevented their movement during highflow conditions. The C-channels also were anchored to

the bottom of the pipe. This helped prevent their movement during high-flow conditions.

In addition to the 8 Sigma meters with 12 probes,

one Unidata Starflow Ultrasonic Doppler Instrument

(referred to as the Starflow meter) was used for measuring flow. The Starflow meter (fig. 8) consists of a probe

with a stage sensor, velocity sensor, and internal thermistor. Interface cables connected to the probe are

accessible through an environmental enclosure. The

probe dimensions are 1.0 in. high, 2.5 in. wide and

9.5 in. long.

The Starflow meter operates in the same manner as

the Sigma meter. The stage sensor is a diaphragm that

deflects under the pressure of the water column above it.

The transducer can measure stage ranging from 0.0 to

16.4 ft of water, with an accuracy of +/- 0.01 ft (Unidata,

1995). The velocity sensor in the probe operates

according to the Doppler-shift principle. The velocity

sensor measures the average velocity, as opposed to discrete velocities, in the water column. The Starflow meter

can measure velocities ranging from -14.7 ft/s to

14.7 ft/s, with an accuracy of +/- 2 percent, although the

Figure 5. Sigma probe installation in 60-inch “short pipe” at Riverside, Ill.
COMBINED SEWER OVERFLOW MONITORING 9

Figure 6. Sigma 910 meter with probe mounted on I-beam.

Figure 7. Sigma probe mounted on C-channel.
10 COMBINED SEWER OVERFLOW MONITORING

minimum velocity required for the Starflow meter to

make a “good” velocity measurement is approximately

0.07 ft/s (Unidata, 1995). Negative velocities result if

reverse flow occurs.

For the Starflow meter to collect representative

velocity data, an adequate amount of reflectors or particulate matter must be in the water. If the water is very

clear or has few reflectors, or if the water is pooled,

allowing the reflectors to settle out, an accurate velocity

measurement cannot be made. In these cases, the Starflow meter reports the last “good” velocity measurement

as the present velocity.

The Starflow meter was programmed to operate

using an event-based logging sequence. The Starflow

meter was inactive and did not log any data until water

was sensed over the probe. Once the probe detected

water, data logging began; when water was no longer

detected over the probe, logging was discontinued. The

Starflow meter was programmed to record stage, velocity, water temperature and battery voltage at 2-minute

intervals. Filling the logger memory was not a concern

because the logger only was collecting data when water

was detected over the probe.

In addition to the Sigma and Starflow flow meters, a

Rain-O-Matic rain gage (fig. 9) was installed on the roof

of the Village of Riverside Fire Department building.

The fire department building is approximately 0.3 mi

northwest of Olmstead Street, where the “long pipe,”

“brick pipe” and “short pipe” probes were located. The

rainfall data collected with this rain gage will be useful

for relating the quantity of rain that fell in the watershed

to the amount of water that flowed through the sewer

system.

The rain gage contains a 6.25-in. funnel, which

drains into a spoon that tips when full. Each 0.01 in. of

precipitation causes the spoon to tip. Because the rain

gage does not have the capability to log and store data,

a HOBO contact-closure event data logger also was

installed with the rain gage.

Figure 8. Starflow Ultrasonic Doppler Meter mounted

on a 3-inch wide stainless-steel band with an

expanding clamp (modified from

http://unidata.com.au/car/6705.htm).

Figure 9. Rain-O-Matic rain gage mounted on

Village of Riverside Fire Department building,

Riverside, Ill.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 11

Operation and Maintenance

Operation and maintenance of the Sigma and Starflow meters required site visits on a monthly basis.

Because of the composition of combined sewage, the

probes were prone to fouling from debris accumulation

and grease/oil film development. If the leading edge of a

probe became sufficiently fouled, the velocity signal

strength would be adversely affected and a zero velocity

would be recorded. For this reason, the probes needed to

be cleaned once per month. Additionally, the data logger

in the Sigma meter was used in “wrap” memory mode.

In this mode, when the memory becomes full, the newest readings overwrite the oldest readings (American

Sigma, Inc., 1996a). Because of the large volume of data

collected, the Sigma 910 memory became full after

approximately 32 days of continuous operation. Therefore, data needed to be retrieved monthly. Finally,

because of the frequent measurement and logging interval of 5 minutes, the Sigma meters required monthly

battery replacement.

Because access to the pipes was required for operation and maintenance of the meters, and because each of

these pipes was considered a permit-required confined

space, permits were filled out before every entry into the

sewer. All confined space protocols were followed

according to American Industrial Hygiene Association

(1995). In addition, personal protective clothing was

worn, including multiple layers of gloves, Tyvec suits,

and chest waders.

ANALYSIS OF COMBINED SEWER

OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Description of Study Area

The Village of Riverside, Ill., has a population of

approximately 8,800 (http://www.pe.net/~rksnow/ilcountyriverside.htm#statistics) and is located

along the Des Plaines River, approximately 11 mi west

of Chicago, Ill. (fig. 1). The combined sewer system

where data were collected (fig. 1) services a total area of

305 acres (0.48 mi2) (Darren Olson, Christopher B.

Burke Engineering, Ltd., written commun., 2000), and

is predominantly residential.

Two Sigma meters with four probes were installed

in the Riverside sewer system at the “Gage Street pipe,”

“long pipe,” “brick pipe,” and the “short pipe” (fig. 10).

The Starflow meter was used to monitor the “outfall

pipe.” Elevations of each of these pipes and probe locations are relative to the USGS datum at the gaging station on the Des Plaines River at Riverside, (figs. 11 and

12) and were determined using a laser level. The meters

were set such that a stage of 0.00 ft indicated dry-pipe

conditions.

The “Gage Street pipe” is a 48.5-in. diameter brick

pipe with a drainage area of approximately 210 acres

(0.33 mi2) (Darren Olson, Christopher B. Burke Engineering, Ltd., written commun., 2000). Dry-weather

flow is diverted by a diversion weir (fig. 10) to an interceptor sewer and is conveyed to Stickney. During periods of stormwater flow, the stage in the “Gage Street

pipe” can exceed the height of the diversion weir just

downstream of the interceptor sewer. In this case, water

overflows the weir and enters chamber 1 (fig. 10) near

the entrance to the “long pipe.” Overflow water can take

two possible paths from this chamber. The first path is

through the “long pipe.” The bottom of the “long pipe”

in chamber 1 is nearly the same elevation as the chamber floor; therefore, when water enters chamber 1 it

immediately begins to flow through this pipe towards

chamber 2 (fig. 10). If the stage in chamber 1 rises sufficiently because of backwater conditions (described

later), it also may flow through a rectangular tide gate

(3.75 ft by 4.25 ft) in chamber 1 (second path) to the

Des Plaines River. This tide gate has a switch-closure

alarm that is activated when the gate opens. The alarm is

relayed to Stickney and signals the sluice-gate operator

to open the sluice gate (described later).

Between chamber 1 and 2, a small, unmonitored,

10-in. diameter connecting sewer (fig. 10) may contribute water to the “long pipe” during periods of stormwater flow. This combined flow is monitored in the

54-in. diameter smooth concrete “long pipe” (fig. 10),

upstream of chamber 2.

A 27.5-in. diameter “brick pipe” combined sewer

(fig. 10), with a drainage area of approximately 83 acres

(0.13 mi2) (Darren Olson, Christopher B. Burke Engineering, Ltd., written commun., 2000) also can contribute stormwater flow into chamber 2. Dry-weather flow

is diverted by a diversion weir to an interceptor sewer, as

in the “Gage Street pipe.” When stormwater overflows

the diversion weir, it flows through the “brick pipe” into

chamber 2. The probe in the “brick pipe” was mounted

approximately half the distance between the diversion

weir and chamber 2 (fig. 10).

From chamber 2, there are two possible flow paths

and the first is through the “short pipe” (60-in. diameter,

mildly pitted concrete pipe) (fig. 10) and into chamber

3. An 18-in. diameter sewer pipe with a drainage area of

approximately 12 acres (0.02 mi2) (Darren Olson,

Christopher B. Burke Engineering, Ltd., written commun., 2000) enters near the top of chamber 3 and was
12 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Figure 10. Schematic (plan) of sewer system used in measuring combined sewer overflows at Riverside, Ill.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 13 Figure 11. Schematic (profile) of sewer system near Gage Street, Riverside, Ill.
14 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Figure 12. Schematic (profile) of sewer system near Olmstead Avenue, Riverside, Ill.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 15

not instrumented. Therefore, the flow contribution into

chamber 3 is unknown. The second path is through the

2.5-ft square tide gate, through the “outfall pipe,” and

into the Des Plaines River. Water that enters chamber 3

is diverted by a single 8-ft by 8-ft sluice gate that is fully

closed during dry-weather periods. When the sluice gate

is closed, a backwater condition can result, allowing the

stage in chamber 3 to increase rapidly and cause reverse

flow through the “short pipe.” In the event of reverse

flows, the stage in chamber 2 can increase to the point

that water starts to flow in reverse directions through

both the “long pipe” and “brick pipe.” If the stage continues to increase, water also may discharge to the Des

Plaines River through a 2.5-ft by 2.5-ft tide gate near

Olmstead Ave. in chamber 2. This tide gate has a

switch-closure alarm that is activated when the gate

opens. The alarm is relayed to Stickney and signals the

operator to open the sluice gate. If the backwater condition persists, water can back up through the “long pipe”

into chamber 1 and discharge through the tide gate in

chamber 1 to the Des Plaines River, as described previously.

If the tide gate in chamber 2 opens, water empties

into the “outfall pipe” (fig. 10). This 27.5-in. diameter

brick pipe discharges directly to the Des Plaines River.

This pipe is instrumented with the Starflow meter, which

begins recording data once it senses water over the

probe. When the tide gates in both chamber 2 and/or

chamber 1 open, personnel at Stickney usually open the

sluice gate leading to the deep tunnel. Once the sluice

gate is opened, the water in the sewer system flows

through the open sluice gate and drops approximately

235 ft through a vertical shaft to the deep tunnel

(fig. 13). Opening the sluice gate causes the stage in the

sewer system to decrease and causes the water to stop

discharging to the Des Plaines River.

The deep tunnel (up to 33 ft in diameter) conveys

the CSO to Stickney to be treated and released. In addition to conveying water to Stickney, the deep tunnel also

can store excess stormwater if Stickney is utilized to full

capacity. Storage in the deep tunnel will continue until

the treatment facility is capable of treating and releasing

the stormwater. If Stickney is over capacity and all or

most of the deep tunnel is used for storage, the sluice

gates may be closed to keep any additional stormwater

from entering the drop shaft and deep tunnel, which

reduces the chance of hydraulic surging (Patrick Connolly, Metropolitan Water Reclamation District of

Greater Chicago, written commun., 2000). During the

periods when the sluice gates are closed, the stormwater

collects in the sewer system upstream of the sluice gate

until a sufficient level is reached to again cause the tide

gate in chamber 2 (and possibly in chamber 1) to open

and discharge to the Des Plaines River.

Storm Volumes

A detailed description is given below of the

assumptions and methods used to analyze the flow data

during the storm of April 22-26, 1999, at Riverside

beginning at the “Gage Street pipe” in downstream

order toward the sluice gate. The measured flow and calculated volumes are affected greatly by limitations of

the flow meters and the numerous associated assumptions and unknown factors. Generally, the same assumptions and methods were used for the quantification of

other storm volumes. Differences in assumptions made

in the analysis of the April 1999 storm and other storms

are given at the end of this section.

Figure 13. The deep tunnel that transports combined

sewer overflow water to the West-Southwest Water

Reclamation Plant (Stickney)(modified from

http://www.mwrdgc.dst.il.us/plants/tarp.htm).
16 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

All of the dry-weather flow at the “Gage Street

pipe” is diverted into an interceptor sewer by means of a

diversion weir downstream of the probe (fig. 14).

Because no flow-monitoring equipment was installed in

the interceptor sewer in the “Gage Street pipe,” it was

necessary to estimate the quantity of water diverted by

the weir in order to compute the volume of water that

overflowed the weir during rain storms.

To estimate the flow into the interceptor sewer from

the “Gage Street pipe,” three small stage rises were analyzed. The magnitude of these rises was such that the

depth of the water approached, but did not exceed, the

height of the weir (0.85 ft). This result was confirmed in

that no water was detected at the “long pipe” probe during these periods. This condition indicates that all the

flow in the “Gage Street pipe” was diverted into the

interceptor sewer. It was determined that a maximum

average discharge of 3.75 ft3/s was flowing through the

“Gage Street pipe” during these periods, and, consequently, through the interceptor sewer. Therefore, it was

assumed that any flow rate in the “Gage Street pipe” that

exceeded 3.75 ft3/s overflowed the weir. This flow rate

could be an over- or underestimate, depending on the

flow conditions downstream in the interceptor sewer. If

backwater conditions were present, caused by the submerged interceptor sewer outlet, it is possible that the

sewer would accept less than 3.75 ft3/s of stormwater.

This condition indicates that during a storm more water

could overflow the weir than estimated. However, if the

interceptor sewer allowed for free-orifice flow, meaning

that the interceptor outlet was a free outfall and not submerged, it is possible that more than 3.75 ft3/s was flowing through the interceptor. This condition indicates that

during a storm less water than estimated could overflow

the weir.

When water overflows the weir, it should be

detected at the “long pipe” probe in the form of a stage

and velocity increase within 5 to 10 minutes. This

method of estimating the overflow provided reliable

results, as water was detected at the “long pipe” at

nearly the same time that the flow in the “Gage Street

pipe” exceeded 3.75 ft3/s (fig. 15). A calculated volume

of 357,009 ft3 overflowed the “Gage Street” weir during

the April 1999 storm.

When the “long pipe” probe detected increased

amounts of water on April 23, the stage and velocity

increased rapidly. This water then combined with the

water from the “brick pipe” in chamber 2 and flowed

through the “short pipe” into chamber 3. After chamber

3 filled, reverse flow, measured as a negative velocity,

occurred through the “short pipe” and then through the

“brick pipe” and “long pipe.” The first negative velocity

at the “long pipe” occurred at 12:50 am (00:50, 24-hour

time) on April 23 at a stage of approximately 3.65 ft.

This negative velocity appears reasonable, because

chamber 3 likely filled and caused water to back up

throughout the sewer system. At 01:35 and 01:40 on

April 23, the stage indicated that the 54-in. “long pipe”

was surcharged, meaning that the static pressure of the

water above the probe was greater than the pipe diameter. However, the stage did not exceed 9.58 ft, at which

point backwater from the “long pipe” would have

caused the stage in chamber 1 to reach the bottom of the

Figure 14. Interceptor sewer and diversion weir in the “Gage Street

pipe,” Riverside, Ill.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 17 Figure 15. Discharge at the “Gage Street pipe” and the “long pipe,” Riverside, Ill., April 22, 1999.
18 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

tide gate in chamber 1, potentially allowing for

discharge to the Des Plaines River. During this

surcharged period, two large consecutive negative

velocities in the “long pipe” were recorded (-7.07 and

-5.28 ft/s) resulting in large negative flow rates (-112

and -84 ft3/s, respectively). Because more water was

being added to the sewer system during this period, it

was likely that turbulence resulted throughout the entire

sewer system, potentially causing exaggerated negative

velocity magnitudes. These negative velocities indicate

that the entire pipe cross section was traveling in a

reverse direction from 5 to 7 ft/s. This result is unlikely.

It is more likely that the water was turbulent in the

vicinity of the probe and that negative velocities resulted

but at a lower magnitude than recorded. However,

because it was impossible to verify flows during highflow conditions, the recorded velocity data were

accepted and used in the flow analysis. The stages and

the velocities in the “long pipe” recorded during the

April 1999 storm are shown in figure 16.

Beginning at 10:55 on April 23, velocities in the

“long pipe” were “noisy,” indicating that the data had

“random and persistent disturbances that reduced the

clarity or quality of [the] signal” (Riverside Publishing

Company, 1988) and fluctuated between 0.00 and

approximately –0.50 ft/s (fig. 16). These negative velocities probably are erroneous, because they were not confirmed by velocities in the “short pipe” or the “brick

pipe” (0 ft/s) measured during this period. In general,

because these velocities in the “long pipe” are noisy,

they normally would be considered zero because no

flow likely was detected. However, when the stage is

changing, it is obvious that water is being added to or

removed from the sewer system. If the velocity is noisy,

or more realistically zero during a period of rising or

falling stage, it is apparent that water is moving in the

sewer system. However, no velocity was detected by the

Sigma meter. This result probably is because the actual

velocity is lower than the minimum meter detection

limit (from 0.20 to 0.30 ft/s). During periods when the

velocity was lower than the detection limit and the stage

was rising, velocities were changed to zero. This change

was done because during any stage increase, it was

impossible to determine whether the velocities should

be positive or negative, because of more water entering

the system, or because of backwater or turbulent conditions. Alternately, during periods when the velocity was

lower than the detection limit, and if the stage was

decreasing, water was discharging from the system,

most likely through the sluice gate. In general, the

velocities were changed to one-half the average minimum velocity detection limit of the Sigma meter

(0.125 ft/s). This change was done because it was

assumed that water was flowing through the sluice gate,

but slowly enough to be undetected by the Sigma meter.

One-half the average minimum detectable velocity commonly is used when computing flow that is below the

detection limit for these types of meters (Jennifer Miller,

U.S. Army Corps of Engineers, oral commun., 2000).

From 10:55 to 11:35 on April 23, velocities in the

“long pipe” were noisy and changed to zero because the

stage was still rising (fig. 16). From 11:40 on April 23 to

03:15 on April 24, the stage was decreasing and these

velocities were changed to 0.125 ft/s. It is likely that

water was released slowly through the sluice gate to the

deep tunnel. From 03:20 on April 24 to 08:55 on April

26, the noisy and zero velocities were changed to zero

because the stage primarily remained constant, indicating that the sluice gate was closed, and only very minor

seepage may have resulted.

Stage decreased rapidly at the “long pipe” from

09:00 to 09:05 on April 26. The corresponding measured positive velocities indicated that the sluice gate

was opened, allowing all the water to flow in a positive

direction out through the sluice gate and to the deep tunnel. A calculated volume of 647,714 ft3 flowed through

the “long pipe” during the April 1999 storm period

(table 1).

Water in the “brick pipe” empties into chamber 2.

Although the “brick pipe” stage plot for the April 1999

storm is similar to the “long pipe” stage plot, the velocity plots are different. On March 17, 1999, the “brick

pipe” probe stage sensor was found to be faulty. The

sensor was replaced and moved approximately 4 ft

downstream of the original location on April 14, 1999.

At the new location, a small opening near the top of the

pipe allowed infiltration into the pipe, ranging from

small drips during dry weather, to more substantial

trickling when the surrounding ground was saturated.

In addition, throughout the length of the “brick pipe,”

approximately 0.10 ft of pooled water was present

because of its variable slope. The infiltration from the

top of the “brick pipe” dripped into the pooled water in

the vicinity of the probe, and caused the velocity sensor

to record this irregular dripping water as noisy positive

and negative velocities. This problem was detected and

the probe was moved another 2 ft downstream in the

pipe on June 25, 1999. No further problems resulted.

The velocity data before and after the probe was relocated on June 25 are shown in figure 17.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 19 Figure 16. Stage and velocity at the “long pipe,” Riverside, Ill., April 21-26, 1999.
20 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Figure 17. Stage and velocity at the “brick pipe,” Riverside, Ill., June 23-27, 1999.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 21

During the period when the velocities in the “brick

pipe” were affected by infiltration, two large storms

occurred in April 1999 and June 1999. A plot of the

stage and velocity data for the “brick pipe” during the

April 1999 storm is shown in figure 18.

During the large stage rise on April 23, the velocity

in the “brick pipe” was positive, as expected when the

water flows toward the sluice gate. From 10:55 to 11:35

on April 23, the zero velocities were not changed,

because it was unknown whether the stage rise was

caused by water entering the system in a positive or negative (reverse flow) direction.

As the flow through the “brick pipe” increased, the

water combined with water from the “long pipe” in

chamber 2 and flowed through the “short pipe” into

chamber 3. Because the sluice gate was closed, the

water filled chamber 3 and flowed back through the

“short pipe,” and then through the “long pipe” and

“brick pipe.” As the “brick pipe” surcharged, the velocities fluctuated between small positive and negative values, indicating turbulent conditions in the sewer system.

During the stage rise, the velocity data indicated a

period of both zero, and small positive velocities

(0.30 ft/s), similar to velocities from the “long pipe.”

However, near the beginning of the stage recession on

April 23, the velocities consistently were negative,

which is an unrealistic result. Also, during the period

when the stage was constant, throughout the latter half

of the storm, the velocities fluctuated between positive

and negative values, and then became mostly negative

throughout the end of the storm. These velocities most

likely are erroneous. As mentioned earlier, errors probably were caused by infiltration near the probe. If these

data were correct, reverse flow resulted in the “brick

pipe” during most of the storm, which also is unlikely.

Many of the negative velocities in the “brick pipe”

measured during the storm probably are not correct.

First, the measured velocities were not all negative, indicating considerable turbulence in the sewer system.

Negative velocities were not confirmed by velocities

measured in other pipes. Second, because the diversion

weir upstream of the “brick pipe” probe was 0.85 ft

high, it is unlikely that the water in the “brick pipe” (at

an average depth of 0.95 ft) was flowing in a reverse

direction at an average velocity of 1.50 ft/s. Additionally, the rainfall data collected from the rain gage on the

Village of Riverside Fire Department building indicate

that most of the rainfall ended at about 02:00 on April

23. During the period from 03:30 April 24 to 08:50 on

April 26, when the stage primarily was constant and the

velocity primarily was negative, no water was contributed by overflow at the “Gage Street” weir (fig. 19).

Therefore, the only water that possibly could be

added to the system, allowing for reverse flow in the

“brick pipe,”was seepage through the tide gate in chamber 2 because of the high stage of the Des Plaines River,

and the two non-instrumented sources of inflow, including the 10-in. diameter inflow pipe upstream of the

“long pipe” probe and the 18-in. diameter pipe in chamber 3. It is unlikely that any of these sources of inflow

contributed much water because the 12-acre (0.02 mi2

)

drainage area for the 18-in. diameter pipe only was

approximately 4 percent of the total drainage area of

305 acres (0.48 mi2) for the entire system. Also, no substantial negative velocity was measured with the Starflow meter in the “outfall pipe.” Finally, velocity plots of

data in the “brick pipe” indicate that prior to April 1999,

velocities were noisy and similar to those in the “long

pipe,”whereas during most of April and June 1999, the

negative velocities appeared largely overexaggerated. It

only was during the period when the probe was affected

by infiltration from April 1999 to June 1999 that velocities appeared noisy during large stage rises.

From 11:40 on April 23 to 03:25 on April 24, the

stage was decreasing. The velocities in the “brick pipe”

were changed to 0.125 ft/s, as it was assumed that water

slowly was released through the sluice gate to the deep

tunnel. From 03:30 on April 24 to 08:50 on April 26, the

velocities were changed to zero, because the stage basically remained constant. This constant stage indicates

that the sluice gate was closed, and only very minor

seepage may have been occurring.

As the stage in the “brick pipe” rapidly decreased at

the end of the storm because the sluice gate opened, two

unlikely negative velocities were recorded. If the probe

is in still water, an internal gain adjustment is boosted

automatically in an effort to allow the probe to detect

very small movements in the water (Joe Prell, American

Sigma, Inc., oral commun., 1999). The Sigma meter

might record the initial turbulence as the correct velocity

because the gain is boosted. This condition resulted during this stage recession. When the sluice gate was

opened, some turbulence probably was induced in the

sewer system, which caused the Sigma meter to record a

negative velocity rather than a positive velocity. This

negative velocity also supports the assumption that the

water in the “brick pipe” was pooled, and not flowing in

a reverse direction (negative flow) throughout the storm,

as described earlier. As the velocities during the stage

recession were very likely to have been positive as the

water flowed toward the sluice gate, they were changed

to positive values for the purpose of data analysis.

A calculated volume of 72,726 ft3 flowed through the

“brick pipe” during the April 1999 storm period

(table 1).
22 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Figure 18. Stage and velocity at the “brick pipe,” Riverside, Ill., April 21-26, 1999.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 23 Figure 19. Discharge at the “Gage Street pipe,” Riverside, Ill., April 21-26, 1999.
24 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Water in chamber 2 flows through the “short pipe”

to chamber 3. A plot of the stage and velocity data for

the “short pipe” during the April 1999 storm is shown in

figure 20.

This 60-in. diameter pipe was surcharged for the

majority of the April 1999 storm. This stage plot closely

resembles plots of the “long pipe” and “brick pipe.” The

velocities near the beginning of the storm are positive

during the stage rise, as expected. Once chamber 3

filled, water backed up into the “short pipe” and the rest

of the sewer system; however, no negative flows were

detected in the “short pipe,” unlike in the other pipes.

Data indicate that at 00:40 on April 23, the stage in the

“short pipe” rose more than 6.0 ft in a 5-minute period,

and more than 8 ft in a 10-minute period. During this

rapid stage rise, it is possible that negative velocities

occurred between recorded velocity values. Five minutes after the pipe became surcharged, the water stopped

flowing, as indicated by the zero velocities. From 00:50

to 11:40 on April 23, no changes were made to the

velocities. Velocities primarily were zero, because the

stage was rising. During this time, it was impossible to

determine whether the velocities are positive, because

more water was added to the system, or negative,

because of backwater or turbulence. From 11:45 on

April 23 to 03:35 on April 24, the stage was decreasing

and the velocity was changed to 0.125 ft/s. The water

probably was released slowly through the sluice gate to

the deep tunnel. From 03:40 on April 24 to 08:55 on

April 26, recorded velocities of 0.00 ft/s remained

unchanged, because the stage basically remained constant. These results indicate that the sluice gate was

closed, and only very minor seepage may have been

occurring.

At the end of the storm, from 08:55 to 09:10 on

April 26, the stage decreased rapidly and positive velocities were recorded. Positive velocities indicate that the

sluice gate was opened, allowing all the water in the

sewer system to flow in a positive direction through the

gate and to the deep tunnel. A calculated volume of

267,106 ft3 flowed through the “short pipe” during the

April 1999 storm period (table 1).

When the stage in chamber 2 rises sufficiently,

water can flow out of the sewer system through a tide

gate to the “outfall pipe” to the Des Plaines River, and

should be detected by the Starflow meter. When the Des

Plaines River stage exceeds approximately 4.70 ft above

the datum at the USGS gaging station on the Des

Plaines River at Riverside, river water can flow from the

river into the “outfall pipe” and over the Starflow meter

(fig. 21).

During the April 1999 storm, the stage in the “outfall pipe” rose smoothly and steadily, whereas the velocity remained nearly zero (fig. 22) because the rise in

stage recorded by the Starflow meter actually was a

reflection of the rise in stage of the Des Plaines River.

The first time that an appreciable positive velocity was

detected by the Starflow meter was at 23:04 on April 22,

while large negative velocities were detected at 23:48.

These velocities resulted because of the turbulence of

the river water inside the pipe, rather than water discharging through the tide gate, because the stage inside

the sewer was not high enough to cause a gate opening.

The difference in elevation between the “long pipe”

probe and the bottom of the tide gate in chamber 2 is

3.15 ft; therefore, the “long pipe” stage must exceed

3.15 ft for the tide gate in chamber 2 to potentially open.

Identical velocities were recorded by the Starflow

meter for extended time periods because good velocity

readings could not be made consistently due to the lack

of reflectors in the pooled water. During these periods,

the Starflow meter reported the last “good” velocity

measurement as the present velocity. For the purpose of

data analysis, it was assumed that if any repeated velocities were present, they were not valid and not used in the

data analysis. A calculated volume of 131,686 ft3 flowed

through the “outfall pipe” during the April 1999 storm

period (table 1).

To verify the analytical methods used in data analysis, a flow mass balance was calculated for chamber 2.

The two inflows into this chamber are the “long pipe”

and the “brick pipe,” whereas the two outflows are the

“short pipe” and the “outfall pipe.” The volume of water

calculated in the “Gage Street pipe” was not used in the

mass balance; instead, this volume was used to verify

the volume calculated in the “long pipe.” Also, the quantity of water measured through the “short pipe” is the

same as what flowed through the sluice gate to the deep

tunnel. Mass-balance results for chamber 2 are given in

Table 1. Flow mass balance for chamber 2 in the

combined sewer system at Riverside, Ill., April 22-26,

1999

Flow into chamber 2 Volume

(cubic feet)

Long pipe: 647,714

Brick pipe (drainage area of 83 acres): 72,726

Total: 720,440

Flow out of chamber 2

Short pipe (to the sluice gage): 267,106

Outfall pipe (to the Des Plaines River): 131,686

Total: 321,648

Difference (Total in - Total out): 321,648
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 25 Figure 20. Stage and velocity at the “short pipe,” Riverside, Ill., April 21-26, 1999.
26 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS

Figure 21. Stages at the “outfall pipe” and the Des Plaines River, Riverside, Ill., April 21-26, 1999.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT RIVERSIDE, ILLINOIS 27 Figure 22. Stage and velocity at the “outfall pipe,” Riverside, Ill., April 21-26, 1999.
28 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

table 1. The total calculated inflow to chamber 2 from

the “long pipe” and the “brick pipe” was 720,440 ft3

(table 1). The total calculated outflow in the “short pipe”

(to the sluice gate) and the “outfall pipe” (to the Des

Plaines River) was 398,792 ft3.

Generally, the same assumptions and methods were

used in the analysis of the June and December 1999

storms. However, some variations were required and are

described below. During the June 1999 storm, the Starflow meter was not operating properly and no flow data

were recorded. To determine the flow through the “outfall pipe,” a storm on July 19, 1998, was investigated as

the Starflow meter was operating properly. It was important that during the July 19, 1998, storm the Des Plaines

River stage measured at the USGS gage on the Des

Plaines River at Riverside was low enough that no water

was backed up into the “outfall pipe” and no river water

was present on the downstream side of the tide gate.

This condition was similar to the June 1999 storm.

A relation between the “long pipe” stage and the “outfall pipe” discharge was established (fig. 23); then the

discharges for the July 19, 1998, storm were related to

the “long pipe” stage during the June 1999 storm to

determine the total outflow to the Des Plaines River.

Finally, during the December 1999 storm, the volume of water that reportedly overflowed the “Gage

Street” weir exceeded the volume of water detected at

the “long pipe” probe. Because the “long pipe” probe

detects water from the “Gage Street” overflow, along

with the water from the 10-in. diameter inflow pipe

upstream of the “long pipe” probe, the water volume

through the “long pipe” should be equal to or greater

than the volume that overflows the “Gage Street” weir.

During this storm, four large negative velocities were

measured in the “long pipe” during the rise on December 4, 1999, accounting for a considerable volume of

water supposedly flowing in the reverse direction

(nearly 94,000 ft3). Although negative velocities during

this period are realistic, the magnitude of these velocities appears unreasonably large. It was assumed that

there was sufficient turbulence near the probe to cause

the large negative velocities. For the purpose of data

analysis, these velocities were reduced to one-tenth their

original magnitude so that the quantity of water detected

at the “long pipe” probe was greater than the quantity

that overflowed the weir at “Gage Street.”

ANALYSIS OF COMBINED SEWER

OVERFLOWDATAATEVANSTON,ILLINOIS

Description of Study Area

The city of Evanston is a suburb of Chicago, Ill.,

has a population of about 73,200

(http://www.pe.net/~rksnow/ilcountyevanston.htm#statistics), and primarily most of the city is located between

the North Shore Channel and Lake Michigan (fig. 1).

The combined sewer system where data were collected

as part of this study services an area of 2,313 acres

(3.61 mi2) that is predominantly residential and commercial. Six flow meters with nine probes were installed

in the Evanston sewer system at the following USGS

named pipes: the “Lake Street pipe” (three probes),

“siphon pipe,” “new pipe,” “short pipe,” chamber 3

“floor,” chamber 3 “ledge,” and “outfall pipe” (fig. 24).

Elevations of each of these pipes and probe locations are

relative to an arbitrary datum (figs. 25 and 26) and were

determined using a laser level. The meters were set such

that a stage of 0.00 ft indicated dry-pipe conditions.

The “Lake Street pipe” is a 120-in. generally circular pipe with a flat bottom. The pipe is rough concrete

and has a drainage area of 1,738 acres (2.72 mi2) (Darren Olson, Christopher B. Burke Engineering, Ltd.,

written commun., 2000). Dry-weather flow is diverted

by a diversion weir (fig. 2) to an interceptor sewer

(“siphon pipe”) and is conveyed to the North-Side Water

Reclamation Plant (fig. 1). The elliptical “siphon pipe”

is located just upstream of the “Lake Street pipe” overflow weir and allows for a maximum water depth of 3.87

ft before surcharging. During stormwater flow periods,

the stage in the “Lake Street pipe” can exceed the height

of the weir just downstream of the “siphon pipe” and

overflow into chamber 3 (fig. 24). There are three possible paths for the overflow water in this chamber. The

first path includes two 8-ft by 8-ft sluice gates that divert

the water from chamber 3 and convey the water to the

deep tunnel. During dry-weather flow periods, one

sluice gate is open approximately 5 percent (approximately 5 in.) and the other gate is closed (Pat Connolly,

Metropolitan Water Reclamation District of Greater

Chicago, written commun., 2000). Although one sluice

gate is partially open, water can collect in chamber 3

more rapidly than it can flow through the partially open

gate. The second path is taken when the stage in chamber 3 rises sufficiently, causing reverse flow through the

“short pipe” and “new pipe.” The third path is taken

when the system becomes surcharged and water flows

through tide gate and into the North Shore Channel.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS 29 Figure 23. Stage at the “long pipe” and discharge at the “outfall pipe” during the storm of July 19, 1998, Riverside, Ill.
30 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

Figure 24. Schematic (plan) view of sewer system used in measuring combined sewer overflows at Evanston, Ill.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS 31

Figure 25. Schematic (profile) of sewer system at Evanston, Ill.
32 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

Figure 26. Schematic (profile) of sewer system at Evanston, Ill.
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS 33

The “new pipe” is a 112-in., smooth, concrete,

storm-relief sewer (fig. 24) with a drainage area of

575.4 acres (0.90 mi2) (Darren Olson, Christopher B.

Burke Engineering, Ltd., written commun., 2000). The

“new pipe” conveys stormwater and provides storage for

water that backs up from chamber 3. Water flowing past

the “new pipe” probe in a positive direction enters

chamber 1 and flows through a pipe into chamber 2. The

water then flows from chamber 2 through the “short

pipe” and enters chamber 3. A 48-in. diameter concrete

pipe enters into chamber 1, but does not contribute water

to the sewer system (Nancy Hornewer, U.S. Geological

Survey, oral commun., Jan. 2000). This pipe can provide

storage during periods of storm-water flow.

 The “short pipe” is a 108-in., smooth, concrete

sewer (fig. 24) that conveys stormwater from the “new

pipe,” through two baffle walls, which help diminish

velocities, over a 2.63-ft vertical drop, and finally to

chamber 3. The “short pipe” also provides storage for

water that backs up from chamber 3.

When overflow from the “Lake Street pipe” and

water from the “short pipe” enters chamber 3, it is soon

detected by the “floor” probe (fig. 24), which is located

1.33 ft above the chamber floor and near the 2.63-ft vertical drop. Because of extensive turbulence in chamber

3, velocities could not be accurately measured here and,

thus, the floor probe only was used to measure stage.

If the stage in chamber 3 exceeds the 2.63-ft drop at

the baffle walls, reverse flow can occur through the

“short” and “new pipe.” As the stage rises, water can

enter the 48-in. diameter pipe that will provide storage

until the water can be released. If severe backwater conditions are present in the “short pipe,” it can cause

chamber 2 (containing a tide gate) (fig. 24) to fill with

water.

The “outfall pipe” is a 108-in., smooth, concrete

overflow sewer (fig. 24) located downstream of the tide

gate in chamber 2. If the stage in chamber 2 is high

enough so that the tide gate opens, the water flows

through the gate and into the “outfall pipe,” and finally

to the North Shore Channel. This tide gate has a switchclosure alarm that is activated when the gate opens. The

alarm is relayed to Stickney and signals the sluice-gate

operator to open the sluice gate(s).

If the stage is high enough in chamber 3, a tide gate

that is located near the top of chamber 3 (fig. 24) can

open. A probe is located on the ledge (referred to as the

“ledge” probe) just upstream of the tide gate. This tide

gate also has a switch-closure alarm that is activated

when the gate opens, signaling the operator to open the

sluice gate(s). If the stage is high enough so that the tide

gate opens in chamber 3, the water flows through the

gate opening and into an unmonitored outfall, and

finally to the North Shore Channel.

Once the sluice gates are opened, the water in the

chambers flows through the open sluice gates (fig. 24)

and drops approximately 179 ft through a vertical shaft

to the deep tunnel (fig. 13). Opening the sluice gates

stops water from discharging to the North Shore Channel.

Storm Volumes

A detailed description is given below of the

assumptions and methods used in data analysis during

the storm of April 21-26, 1999, at Evanston, beginning

at the “Lake Street pipe” in downstream order toward

the sluice gates (fig. 24). The measured flow and calculated volumes are greatly affected by limitations of the

flow meters, and the numerous assumptions made in

analysis and unknown factors not considered. As with

the analysis at Riverside, generally the same assumptions and methods were used for quantification of other

1999 storms.

Three Sigma probes were installed in the “Lake

Street pipe” because they were prone to fouling from

debris accumulation and grease/oil film development.

The probes were mounted on C-channels and located in

line, resulting in the most fouling at the most upstream

probe. Therefore, the velocities measured at this probe

often were recorded as zero. However, the two downstream probes weren’t as prone to fouling. Of the two

downstream probes, one was a standard stage/velocity

probe, and the other was a Keppler stage/velocity probe.

The Keppler probe functions best in clear water because

the velocity sensor in a Keppler probe has “larger crystals and, therefore, a greater beam/return signal sensitivity to reflectors in the water” (Joe Prell, American

Sigma, Inc., oral commun., 1999). Because of the Keppler probe’s greater sensitivity to particles in the water,

the recorded velocities can be higher than the actual values if the probe is used in water containing high quantities of particulate matter, including combined sewage.

The manufacturer stated that when the velocity between

the standard and the Keppler probe differed substantially, the standard probe likely would indicate a more

representative average velocity (Joe Prell, American

Sigma, Inc., oral commun., 1999).

For the purposes of data analysis in the “Lake Street

pipe,” the data from the two downstream probes were

used. Because the Keppler probe sometimes reported

higher velocities than those reported by the standard

probe, data from the standard probe were used if the

instantaneous velocities measured by the two probes dif-
34 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

fered by more than 0.50 ft/s. If the instantaneous

velocities differed by less than 0.50 ft/s, the two

velocities were averaged.

Two additional inflow pipes, 21.5- and 12-in. diameter, respectively, connect to the “Lake Street pipe” on

opposite sides (fig. 24) downstream of the Sigma

probes, and were, therefore, not measured. On October

5, 1999, during dry-weather conditions, discharge measurements were made in an attempt to quantify the flow

entering the “Lake Street pipe” through these pipes.

Although the instantaneous flow rates were very low

(less than 0.10 ft3/s), it was assumed that flows could be

substantially higher during stormwater flow periods.

Therefore, 0.50 ft3/s was added to each instantaneous

measurement made in the “Lake Street pipe” during the

1999 storms. During the April 1999 storm, this

0.50 ft3/s represented a total volume of 186,300 ft3.

A calculated volume of 12,372,224 ft3 flowed through

the “Lake Street pipe” during the April 1999 storm

period (table 2).

All the dry-weather flow from the “Lake Street

pipe” is diverted by a diversion weir (fig. 2), through the

“siphon pipe” interceptor sewer and conveyed to the

North-Side Water Reclamation Plant. The time-of-travel

for water from the “Lake Street pipe” to the “siphon

pipe” probe was examined using the timing of four stage

peaks at both locations. However, no direct correlation

between the stage peaks at the “Lake Street pipe” and

the “siphon pipe” were found. For this reason, no time

correction was made for time-of-travel between the two

pipes.

Throughout the April 1999 storm, the data collected

at the “siphon pipe” generally indicated large stage rises

along with consistent positive velocities (fig. 27).

Between 23:05 and 23:15 on April 22, however, the

stage dropped more than 7.0 ft in 10 minutes, whereas

the velocity increased from approximately 1.0 ft/s to

over 3.0 ft/s. This large change in stage and velocity

probably is attributable to a sudden release of water to

the treatment plant. Between 00:45 and 00:55 on

April 23, a period of zero velocities was recorded. Zero

velocity could have been recorded if the leading edge of

the probe was fouled, not allowing for the velocity signal to be emitted, although this condition is unlikely.

Zero velocities also could have resulted if the water

actually stopped moving because the treatment plant

was at, or over, capacity. From 03:10 to 03:55 on April

23, a second period of zero and negative velocities again

was recorded. Again, zero velocities could have been

caused by fouling of the upstream side of the probe;

however, because negative velocities also were

recorded, it is more likely that the treatment plant

reached capacity and could not handle any additional

inflow. Thus, backwater conditions and eventually turbulence or reversed flow resulted. These negative velocities were measured during a very large stage rise (more

than 8.0 ft), which also indicates the treatment plant

may have reached capacity and could not accept additional inflow. During this period of zero and negative

velocities at the “siphon pipe,” velocity at the “Lake

Street pipe” dropped from nearly 3.0 ft/s to less than

1.0 ft/s while the stage increased, indicating that the

“siphon pipe” was not accepting more water from the

“Lake Street pipe.” Thus, the water overflowed the weir

and entered chamber 3, which quickly filled. This second period of both zero and negative velocities in the

“siphon pipe,” from 03:10 to 03:55 on April 23, verifies

the results during the first period from 00:45 to 00:55 on

April 23. Because of this condition, the “siphon pipe”

data were accepted for the analysis. A calculated volume of 7,277,579 ft3 flowed through the “siphon pipe”

during the April 1999 storm period (table 2).

The volume of water overflowing the diversion weir

at the “Lake Street pipe” was calculated by subtracting

the total volume through the “siphon pipe” from the

“Lake Street pipe.” It was assumed that any instantaneous volume of water in the “Lake Street pipe” that

exceeded that of the “siphon pipe” overflowed the weir

and entered chamber 3. A calculated volume of

5,094,645 ft3 overflowed the weir and entered

chamber 3.

Chamber 3 receives “Lake Street pipe” overflow as

well as water from the “new pipe” conveyed by the

“short pipe.” Because of limitations with the location of

the “short pipe” probe, explained below, the data from

the “new pipe” were used in the analysis. The “short

pipe” probe was located in a 108-in. diameter pipe

(approximately 25 ft long). Downstream of the “short

pipe” were velocity-diminishing baffle walls and a

Table 2. Flow mass balance for chamber 3 in the

combined sewer system at Evanston, Ill., April 21-26,

1999

Flow into chamber 2 Volume

(cubic feet)

Lake Street Pipe (drainage area of 1,738 acres): 12,372,224

Siphon Pipe (water withdrawn from Lake Street Pipe): 7,277,579

New Pipe (drainage area of 575 acres): 850,309

Total: 5,944,954

Flow out of chamber 2

Outfall Pipe (to the North Shore Channel): 1,651,312

Ledge (to the North Shore Channel): 1,267,903

Total: 2,919,215

Volume to sluice gages and deep tunnel

(Total in - Total out):

3,025,739
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS 35Figure 27. Stage and velocity at the “siphon pipe,” Evanston, Ill., April 21-26, 1999.
36 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

2.63 ft vertical drop leading into chamber 3. The probe

was mounted near the middle of the “short pipe”

approximately 15 ft from the 2.63-ft drop (fig. 26).

Because this pipe becomes surcharged, it would have

been ideal to locate the probe at least 10 pipe diameters

upstream from the vertical drop, but this was not possible because the length of the “short pipe” was less than

the required 10 pipe diameters. Flow was affected by the

vertical drop (which could cause drawdown), but also by

the velocity-diminishing baffle walls (which would

cause turbulence). Although the data from the “short

pipe” probe were not used for flow or volume calculation, the stage data were used for comparison to determine stages inside and outside the sewer system,

indicating when the tide gates in chambers 2 and 3 may

have opened.

In contrast, the “new pipe” probe was located in a

112-in., smooth, concrete, storm-relief pipe that receives

stormwater flows as well as provides storage for backwater from chamber 3. Just downstream of the “new

pipe” probe is a small bend at the manhole access

(fig. 24), which possibly could have induced some turbulence; however, because the bend was small, the flow

was unlikely to be substantially affected. In addition,

chamber 1 was downstream of the “new pipe” probe,

and could have caused some disturbance; however,

because this chamber is much farther away than the

sources of turbulence at the “short pipe,” “new pipe”

data were deemed more reliable. There were no apparent flow disturbances or sources of turbulence upstream

of the probe.

Three distinct, large stage rises are indicated in the

“new pipe” (fig. 28) during April 21-26, 1999. The

water-level rises were due to stormwater from upstream

sources and backwater from chamber 3. As the stage

increased near the beginning of each rise, a positive

velocity was measured. Positive velocities indicate that

storm water was conveyed to the “new pipe” from

upstream sources. Shortly after each rise began, the

velocities became negative. Negative velocities indicate

that water filled chamber 3 to the point that water backflowed through the “short pipe” and the “new pipe.”

During the peak of each rise, the stage became relatively

steady and the velocities became zero. This resulted

because water that entered chamber 3 from “Lake Street

pipe” overflow was released through the tide gates in

chambers 2 and 3, before an additional stage increase

could be realized at the “new pipe.” During the latter

part of the stage recession, generally on April 25-26,

velocities fluctuated between zero and small positive

values (an average of about 0.4-0.5 ft/s). Zero velocities

were recorded probably because the water velocity was

below the detection limit of the meter. Because the stage

was changing, it is evident that water was moving

through the sewer system. Therefore, when a zero velocity was recorded, it was changed to 0.125 ft/s.

During normal dry-weather operation, one 8-ft by

8-ft sluice gate in chamber 3 is open approximately

5 percent (approximately 5 in). When a tide-gate alarm

is received at Stickney, one or both of the sluice gates

are opened between 10 and 20 percent (approximately

10 to 20 in); however, exact gate openings are unknown.

When the stage began to decrease in the “new pipe” during each of these three rises in April, not only was water

flowing through the tide gates in chamber 2 and 3, but

the sluice gate was partially open. During the stage

recessions, the water was flowing through the partially

opened sluice gate to the deep tunnel, although positive

velocities were not detected at the “new pipe” probe;

instead, zero velocities were recorded. During the recession of April 21-22, the stage in the “new pipe” dropped

approximately 13 ft before a positive velocity again was

detected (1.81 ft/s). During this period, velocity was not

detected by the Sigma meter because the velocity in the

“new pipe” was lower than the Sigma meter’s detection

limit of 0.20 to 0.30 ft/s. During the periods when the

velocity was lower than the detection limit, the velocities were not changed if the stage was rising. In these

cases, it was impossible to determine whether the velocities should have been positive because more water was

being added to the sewer system; or negative, which

would have resulted because of backwater or turbulence.

Alternately, during these same periods when the velocity was below the detection limit, if the stage was

decreasing, it is evident that water was being removed

from the system through the tide gate in chambers 2 and

3, and also through sluice-gate openings. During these

periods, the velocities were changed to 0.125 ft/s, as

described in the “Storm Volumes” section for Riverside.

A calculated volume of 850,309 ft3 flowed through the

“new pipe” during the April 1999 storm period (table 2).

Although the “floor” probe was installed to verify

stage peaks detected at the “ledge” probe and elsewhere,

the “short pipe” stage was used for this verification

instead because values at the “short pipe” more closely

verified values recorded at the “ledge probe.” When the

stage in chamber 3 rises higher than the 2.63-ft vertical

drop, flow can reverse and cause chamber 2 to fill. The

tide gate in chamber 2 then can open and water can flow

through the gate opening to the “outfall pipe,” which is

located downstream of this tide gate, and finally into the

North Shore Channel (fig. 1). Alternately, when the

North Shore Channel stage rises, water can flow from

the river into the “outfall pipe,” but the tide gate theore-
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS 37Figure 28. Stage and velocity at the “new pipe,” Evanston, Ill., April 21-26, 1999.
38 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

tically prevents flow into the sewer system. A plot of the

“outfall” stage and velocity data for the April 1999

storm is presented in figure 29.

The stage in the “outfall pipe” rose smoothly

because the North Shore Channel was high enough to

back up into the pipe. The rise in stage is a reflection of

the rise in stage of the North Shore Channel, which is

the reason for the nearly zero velocity measured early

on April 21. During three distinct periods (from 21:55

on April 21 to 01:55 on April 22; from 13:20 to 18:00

on April 22; and from 19:25 to 23:05 on April 22) positive velocities were detected in the “outfall pipe”

because of the tide-gate openings. In addition to these

periods of consistent positive velocities, smaller magnitude, inconsistent positive velocities were recorded but

were not used for data analysis because they were

assumed to be affected by turbulence from the North

Shore Channel. The stage on the upstream side of the

tide gate from the CSO was compared to the stage on

the downstream side of the tide gate from the North

Shore Channel. During the periods of inconsistent positive velocities, because stage was higher at the North

Shore Channel than in the sewer system, it was decided

not to include these velocities in the data analysis, based

on the assumption that the gate was not open. Finally,

flow might have occurred from the North Shore Channel

around the edges of the tide gate in chamber 2 and into

the sewer system during periods when the North Shore

Channel stage was high when compared to the stage in

the sewer system. This condition resulted at the tide gate

in chamber 3 (described later), and because both tide

gates are at approximately the same elevation, the

assumption that reverse flow could have resulted

through the tide gate in chamber 2 is valid. A calculated

volume of approximately 1,651,312 ft3 flowed through

the “outfall pipe” during the April 1999 storm period

(table 2).

If the stage rises in chamber 3, water can exceed the

height of the “ledge” at the tide gate in chamber 3 and

be detected by the “ledge” probe. A plot of the “ledge”

probe stage and velocity data is presented in figure 30.

During the first appreciable stage and velocity rise

from 21:55 on April 21 to 00:50 on April 22, velocities

mostly were positive, indicating flow out through the

tide gate. However, at 22:20, 22:25, and 22:55 on April

21, negative velocities were detected, indicating turbulence near the probe probably caused by the tide gate

closing momentarily. The second appreciable stage rise

indicated the same trend with consecutive positive

velocities and one negative velocity at 13:55 on

April 22, indicating turbulence near the probe. For the

purpose of data analysis, the negative velocities during

these two periods were changed to zero, because water

probably was not flowing back through the gate. After

the second period of flow through the gate, the water

inside the sewer decreased and remained high enough

that the water still was over the “ledge” and on the gate,

but not high enough to open the gate. During this period,

velocity values were both positive and negative, indicating turbulence on and near the “ledge.” This portion of

the data was not included in the analysis, because water

probably did not flow through the gate opening during

this period. The third appreciable stage rise was accompanied by a large velocity rise, indicating that water

once again flowed through the gate. However, following

this rise, at 23:10 on April 22, the stage in the sewer

decreased below the level of the tide gate. However,

because the North Shore Channel stage was high

enough to submerge part of the tide gate, water from the

North Shore Channel began to seep around the edges of

the gate causing reverse flow into the sewer system. This

condition can be seen from 23:10 on April 22 to 03:40

on April 23; from 00:30 to 10:00 on April 24; from

15:00 to 16:55 on April 24.

The flow calculated from the “ledge” probe data

was affected by turbulence because of the probe location. According to the manufacturer’s specifications, the

probe should be located at least 10 times the maximum

expected depth away from any flow obstruction; however, the location of the “ledge” probe did not permit

this installation. Additionally, turbulence at the “ledge”

probe was induced when water overflowed the “Lake

Street” weir, directly toward the “ledge” probe. A calculated volume of 1,267,903 ft3 flowed over the “ledge”

during the April 1999 storm period (table 2).

Because no probe directly measured the water flowing to the sluice gates, as in the “short pipe” at Riverside, the volume of water through the sluice gates at

Evanston was calculated based on a mass balance in

chamber 3. The inflows into chamber 3 include the

water overflowing the “Lake Street” weir, the “new

pipe,” conveyed by the “short pipe,” and seepage around

the tide gates. The outflows are the “outfall pipe,” the

“ledge” gate and the sluice gates. Mass-balance results

for chamber 3 are given in table 2. The total inflow to

chamber 3 from the “Lake Street pipe” overflow and the

“new pipe” was 5,944,954 ft3, whereas the total outflow

through the tide gates to the North Shore Channel was

2,919,215 ft3. The remainder of the volume was

3,025,739 ft3 and flowed through the sluice gates to the

deep tunnel (table 2).

Similar assumptions and analytical methods generally were used during the June 1999 and December

1999 storms as used with the April 1999 storm. How-
ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS 39Figure 29. Stage and velocity at the “outfall pipe,” Evanston, Ill., April 21-26, 1999.
40 ANALYSIS OF COMBINED SEWER OVERFLOW DATA AT EVANSTON, ILLINOIS

Figure 30. Stage and velocity at the “ledge,” Evanston, Ill., April 21-26, 1999.
SUMMARY 41

ever, one minor variation in flow analysis at the “new

pipe” was required. On August 10, 1999, the probe in

the “new pipe” was moved about 3 ft upstream and up

along the side of the pipe to keep it from being buried by

debris. The adjusted probe elevation was 1.09 ft above

the bottom of the pipe. Therefore, during the start and

end of the December 1999 storm, water was not

detected by the probe until it reached a depth of 1.09 ft.

The amount of water that flowed past the probe at the

beginning of the storm was estimated by examining six

other storms and taking the average volume of water

flowing past the probe before the stage reached 1.09 ft.

It was estimated that the total volume of water undetected by the probe during the rising stage was approximately 0.60 percent (approximately 1,802 ft3) of the

entire storm volume (317,291 ft3).

SUMMARY

The U.S. Geological Survey, in cooperation with

the U.S. Army Corps of Engineers, collected and analyzed flow data in combined sewer systems in Riverside

and Evanston, Ill., during 1997-99. Water-quality data

were collected by the Metropolitan Water Reclamation

District of Greater Chicago (MWRDGC) at Evanston,

Riverside, the Calumet pumping station, and the Racine

pumping station from May 1, 1995, to December 9,

1999. This study was done in an effort to provide data

and analysis that can be used in the design of a floodcontrol reservoir as part of the Chicago Underflow

Plan/Tunnel and Reservoir Plan.

The sewer systems consist of circular pipes ranging

in diameter from 0.83 ft to 10.0 ft; elliptical siphon

pipes; ledges; and tide and sluice gates. Stage and velocity data were collected at 12 locations at Riverside and

Evanston, Ill., from March 1997 to December 1999

using American Sigma ultrasonic flow meters and a

Starflow ultrasonic Doppler instrument. The instrument

probes were mounted on stainless-steel straps, stainlesssteel bands, I-beams, and C-channels. Operation and

maintenance of the meters required monthly site visits.

Some probes were prone to fouling from debris accumulation and grease/oil film development. Flow mass balances were performed for the sewer systems at

Riverside and Evanston to determine the quantity of

water flowing through the sluice-gate openings to the

deep tunnel and to determine the quantity of water flowing through tide-gate openings to receiving waterways.

Because of the complexity of the flow data and

data-collection locations at both the Riverside and Evanston locations, various assumptions were required. In six

pipes, during periods when the stage was decreasing, if

the velocity of the water was below the minimum detection limit of the Sigma meters (from 0.20 to 0.30 ft/s),

then one-half of the average minimum detection velocity (0.125 ft/s) was applied in the analysis. In addition,

unquantified volumes of water may have seeped around

the edges of the tide gates and into the sewer system

from the receiving waterways, as indicated at one site in

Evanston. It is unknown what effect this volume would

have on the final volume calculation results. The diversion from the “Gage Street pipe” into the interceptor

sewer at Riverside was not measured and, therefore, was

estimated. One probe at Riverside was affected by infiltration, which caused measurement of erroneous velocity data during much of the April 1999 and June 1999

storms. Velocity corrections were made throughout

these periods.

The mass balance at Riverside for the storm during

April 22-26, 1999, indicated a total inflow into

chamber 2 of 720,440 ft3, whereas the outflow to the

Des Plaines River was 131,686 ft3 and the outflow

through the sluice gate to the deep tunnel was

267,106 ft3. The mass balance at Evanston for the storm

during April 21-26, 1999, indicated a total inflow into

chamber 3 of 5,944,954 ft3, whereas the outflow to the

North Shore Channel was 2,919,215 ft3 and the outflow

through the sluice gates to the deep tunnel was calculated to be 3,025,739 ft3.

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American Industrial Hygiene Association, 1995, Confined

space entry–An AIHA protocol guide: Fairfax, Va.,

American Industrial Hygiene Association, 53 p.

American Sigma, Inc., 1996a, American Sigma InSight

Data Analysis software user’s guide: Medina, N.Y.,

American Sigma, Inc., 99 p.

_____1996b, American Sigma long term flow monitor–

Operating and maintenance manual: Medina, N.Y.,

American Sigma, Inc., 66 p.

Metcalf & Eddy, Inc., 1991, Wastewater engineering

treatment, disposal, and reuse (3d ed.): McGraw-Hill,

Inc., 1,334 p.

Riverside Publishing Company, 1988, Webster’s II new

Riverside University dictionary: Boston, p. 798.

Unidata, 1995, Starflow ultrasonic doppler instrument–

user’s manual: Lynn MacLaren Publishing, [102] p.

U.S. Army Corps of Engineers, 1998, Special re-evaluation

report and final environmental impact statement:

Chicago, Ill., U.S. Army Corps of Engineers

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