U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5205 

Prepared in cooperation with the Chicago District of the U.S. Army Corps of Engineers

Comparison of Index Velocity Measurements Made With a 

Horizontal Acoustic Doppler Current Profiler and a ThreePath Acoustic Velocity Meter for Computation of Discharge 

in the Chicago Sanitary and Ship Canal near Lemont, Illinois 

EXPLANATION

Elevation, in feet above North American

 Vertical Datum of 1988

High: 568.14

Low: 548.98

N

Sloughed bank

Buried

cables

Cross-channel AVM

cables

Up-looking

ADCP

(Installed near midchannel within

the acoustic velocity meter (AVM) 

measurement volume)

Horizontal acoustic Doppler current 

 profiler (H-ADCP)

Gage

house

Measurement cross section

162 feet

20o

44.5

o

Flow
Cover: Schematic showing the instrument configurations superimposed on the channel bathymetry at the U.S. Geological Survey 

gaging station on the Chicago Sanitary and Ship Canal near Lemont, Illinois. 
Comparison of Index Velocity Measurements 

Made With a Horizontal Acoustic Doppler 

Current Profiler and a Three-Path Acoustic 

Velocity Meter for Computation of Discharge 

in the Chicago Sanitary and Ship Canal near 

Lemont, Illinois 

By P. Ryan Jackson, Kevin K. Johnson, and James J. Duncker 

Prepared in cooperation with the Chicago District of the U.S. Army Corps of Engineers 

Scientific Investigations Report 2011–5205

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012 

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Jackson, P.R., Johnson, K.K., and Duncker, J.J., 2012, Comparison of index velocity measurements made with a 

horizontal acoustic Doppler current profiler and a three-path acoustic velocity meter for computation of discharge 

in the Chicago Sanitary and Ship Canal near Lemont, Illinois: U.S. Geological Survey Scientific Investigations 

Report 2011–5205, 42 p. 
iii

Contents

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

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

Methods...........................................................................................................................................................2

Data Collection......................................................................................................................................2

Continuous Deployments............................................................................................................2

Synoptic and Short-Term Deployments ...................................................................................6

Data Processing....................................................................................................................................7

Index Velocity Computations .....................................................................................................7

Calibration Measurements.........................................................................................................7

Acoustic Velocity Meter (AVM)........................................................................................8

Horizontal-ADCP (H-ADCP)...............................................................................................9

Stage-Area Rating .......................................................................................................................9

Discharge Computation ..............................................................................................................9

Results ...........................................................................................................................................................10

Flow Characterization ........................................................................................................................10

Velocity Magnitude and Direction ..........................................................................................10

Distribution of Observed Velocities ...............................................................................10

Primary and Secondary Flows and Flow Angle...........................................................10

Temporal Variability..........................................................................................................17

Frequency Analysis .................................................................................................17

Flow Variability Owing to Barge Passage............................................................19

Flow Variability Owing to Lockages......................................................................21

Reverse Flows ...................................................................................................................23

Stratification and Salinity Variation ...............................................................................24

Velocity Profiles .........................................................................................................................25

Vertical................................................................................................................................25

Transverse..........................................................................................................................28

Comparison of Instrument Performance ........................................................................................30

Instrument Reliability ................................................................................................................31

Velocity Statistics ......................................................................................................................32

Rating Curves..............................................................................................................................34

Computed Discharge.................................................................................................................37

Conclusions...................................................................................................................................................40

References Cited..........................................................................................................................................42
iv

Figures

1. Map of the Chicago Area Waterway System showing the Chicago Sanitary and 

Ship Canal, Calumet Sag Channel, North Branch Chicago River, Little Calumet River, 

Grand Calumet River, and the other waterways in and near Chicago, Illinois...................3

2. Schematic showing the instrument configurations superimposed on the channel 

bathymetry at the U.S. Geological Survey gaging station on the Chicago Sanitary 

and Ship Canal near Lemont, Illinois .........................................................................................4

3. Schematic of the channel cross section showing the three paths of the acoustic 

velocity meter at the U.S. Geological Survey gaging station on the Chicago Sanitary 

and Ship Canal near Lemont.......................................................................................................5

4. Schematic of the horizontal acoustic Doppler current profiler deployed at the 

U.S. Geological Survey gaging station on the Chicago Sanitary and Ship Canal 

near Lemont...................................................................................................................................6

5. Drawing showing a moving-boat discharge measurement with a boat-mounted 

acoustic Doppler current profiler and differential global positioning system and 

the resulting velocity magnitude data in the cross section...................................................7

6. Schematic of the up-looking acoustic Doppler current profiler deployed near 

midchannel at the U.S. Geological Survey gaging station on the Chicago Sanitary 

and Ship Canal near Lemont, Illinois .........................................................................................8

7. Graph showing frequency-distribution diagrams of velocity magnitude 

observations for each of the three paths of the acoustic velocity meter in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois, December 2005–

January 2010................................................................................................................................11

8. Graph showing frequency-distribution diagrams of velocity magnitude 

observations for each of the nine cells of the horizontal acoustic Doppler current 

profiler in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–January 2010..................................................................................................11

9. Graph showing primary and secondary flows as a function of Vmean for the 

Chicago Sanitary and Ship Canal near Lemont, Illinois .......................................................12

10A. Graph showing percent difference from the measured discharge as a function 

of the flow angle in the Chicago Sanitary and Ship Canal near Lemont, Illinois .............14

10B. Graph showing residuals as a function of flow angle in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois .......................................................................................15

11A. Graph showing percent difference from the measured discharge as a function 

of the secondary to primary velocity ratio in the Chicago Sanitary and Ship Canal 

near Lemont, Illinois ...................................................................................................................16

11B. Graph showing residuals (rated discharge minus measured discharge) as a 

function of the secondary to primary velocity ratio in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois .......................................................................................17

12. Graph showing power spectra for acoustic velocity meter path and horizontal 

acoustic Doppler current profiler cell velocity data, November 2006–January 2010, 

in the Chicago Sanitary and Ship Canal near Lemont, Illinois ............................................18

13. Graph showing seiche periods for the Chicago Area Waterways System 

compared to the observed dominant periods in the velocity data in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois .......................................................19
v

14. Graphs showing 15-minute average vertical velocity profiles measured near 

the center of each of the nine cells of the horizontal acoustic Doppler current 

profiler by using a boat-mounted acoustic Doppler current profiler in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois, May 20, 2008 ..............................27

15. Graph showing 26-hour average vertical velocity profile measured by using 

an up-looking, bottom-mounted acoustic Doppler current profiler in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois, May 20–21, 2008 ........................28

16. Graph showing 4-year average vertical velocity profiles measured by the 

three paths of the acoustic velocity meter in the Chicago Sanitary and 

Ship Canal near Lemont, Illinois, December 2005 to February 2010...................................29

17. Graph showing transverse normalized velocity profiles in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois, as measured by the horizontal acoustic 

Doppler current profiler and boat-mounted acoustic Doppler current profiler 

for a range of flow velocities ....................................................................................................30

18. Graph showing index velocity rating curve for the acoustic velocity meter 

deployed in the Chicago Sanitary and Ship Canal near Lemont, Illinois...........................34

20. Graph showing comparison of rated discharges for the acoustic velocity meter 

and horizontal acoustic Doppler current profiler for 33 discharge measurements 

in the Chicago Sanitary and Ship Canal near Lemont, Illinois ............................................35

19. Graph showing index velocity rating curve for the horizontal acoustic Doppler 

current profiler deployed in the Chicago Sanitary and Ship Canal near 

Lemont, Illinois ............................................................................................................................35

21A. Graph showing errors in rated discharge relative to the measured discharge 

for the acoustic velocity meter and horizontal acoustic Doppler current profiler 

deployed in the Chicago Sanitary and Ship Canal near Lemont, Illinois...........................36

21B. Graph showing residuals in rated discharge relative to the measured discharge 

for the acoustic velocity meter and horizontal acoustic Doppler current profiler 

deployed in the Chicago Sanitary and Ship Canal near Lemont, Illinois...........................37

22. Scatterplot of the computed discharge for the acoustic velocity meter and 

horizontal acoustic Doppler current profiler deployed in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois, November 10, 2006, to December 31, 2010 ...........38

23. Quantile-quantile plots of mean daily, monthly, and annual computed discharges 

for the acoustic velocity meter and horizontal acoustic Doppler current profiler 

deployed in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 10, 2006, to December 31, 2010 .............................................................................39

24. Boxplots of the difference in computed discharge for the acoustic velocity meter 

and horizontal acoustic Doppler current profiler deployed in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois, November 10, 2006, to December 31, 2010 ...........40
vi

Tables

1. Primary and secondary velocities observed during discharge measurements 

in the Chicago Sanitary and Ship Canal near Lemont, Illinois ............................................13

2. Dominant frequencies and periods for the acoustic velocity meter and horizontal 

acoustic Doppler current profiler velocity data for October 2007–January 2010 

and November 2006–January 2010 in the Chicago Sanitary and Ship Canal 

near Lemont, Illinois ...................................................................................................................18

3. Barge/tow passages in the Chicago Sanitary and Ship Canal near Lemont, 

Illinois, May 20, 2008 (14:00) to May 21, 2008 (15:00).............................................................20

4. Lockages recorded at Lockport Lock on the Chicago Sanitary and Ship Canal, 

near Lemont, Illinois, May 20, 2008 (14:00), to May 21, 2008 (15:00)....................................22

5. Velocity pulse characteristics recorded in the Chicago Sanitary and 

Ship Canal near Lemont, Illinois, associated with lockages at Lockport Lock, 

May 20, 2008 (14:00), to May 21, 2008 (15:00)..........................................................................22

6. Flow-reversal observations in the Chicago Sanitary and Ship Canal near 

Lemont, Illinois, November 2006–January 2010.....................................................................23

7. Vertical velocity profile properties for a range of flow conditions in the 

Chicago Sanitary and Ship Canal near Lemont and Romeoville, Illinois...........................26

8. Comparison of missing data by individual path/cell for the two index 

velocity meters in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–December 2010..............................................................................................31

9. Comparison of missing data by number of paths/cells for the two index 

velocity meters in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–December 2010..............................................................................................32

10. Comparison of velocity statistics by individual path/cell for the two index 

velocity meters in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–January 2010..................................................................................................33
vii

Conversion Factors 

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

meter (m) 3.281 foot (ft)

mile (mi) 1.609 kilometer (km)

Flow rate

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

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

mile per hour (mi/h) 1.609 kilometer per hour (km/h) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Instrument and data frequencies are give in hertz (Hz).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C). 

Salinity is given in parts per thousand (ppt). 

Comparison of Index Velocity Measurements Made With 

a Horizontal Acoustic Doppler Current Profiler and a 

Three-Path Acoustic Velocity Meter for Computation of 

Discharge in the Chicago Sanitary and Ship Canal near 

Lemont, Illinois 

By P. Ryan Jackson, Kevin K. Johnson, and James J. Duncker 

Abstract

The State of Illinois’ annual withdrawal from Lake 

Michigan is limited by a U.S. Supreme Court decree, and the 

U.S. Geological Survey (USGS) is responsible for monitoring 

flows in the Chicago Sanitary and Ship Canal (CSSC) near 

Lemont, Illinois as a part of the Lake Michigan Diversion 

Accounting overseen by the U.S. Army Corps of Engineers, 

Chicago District. Every 5 years, a technical review committee 

consisting of practicing engineers and academics is convened 

to review the U.S. Geological Survey’s streamgage practices 

in the CSSC near Lemont, Illinois. The sixth technical review 

committee raised a number of questions concerning the flows 

and streamgage practices in the CSSC near Lemont and 

this report provides answers to many of those questions. In 

addition, it is the purpose of this report to examine the index 

velocity meters in use at Lemont and determine whether the 

acoustic velocity meter (AVM), which is now the primary 

index velocity meter, can be replaced by the horizontal acoustic Doppler current profiler (H-ADCP), which is currently the 

backup meter. 

Application of the AVM and H-ADCP to index velocity measurements in the CSSC near Lemont, Illinois, has 

produced good ratings to date. The site is well suited to index 

velocity measurements in spite of the large range of velocities and highly unsteady flows at the site. Flow variability 

arises from a range of sources: operation of the waterway 

through control structures, lockage-generated disturbances, 

commercial and recreational traffic, industrial withdrawals 

and discharges, natural inflows, seiches, and storm events. 

The influences of these factors on the index velocity measurements at Lemont is examined in detail in this report. Results of 

detailed data comparisons and flow analyses show that use of 

bank-mounted instrumentation such as the AVM and H-ADCP 

appears to be the best option for index velocity measurement 

in the CSSC near Lemont. Comparison of the rating curves 

for the AVM and H-ADCP demonstrates that the H-ADCP is a 

suitable replacement for the AVM as the primary index velocity meter in the CSSC near Lemont. 

A key component to Lake Michigan Diversion Accounting is the USGS gaging station on the CSSC near Lemont, 

Illinois. The importance of this gaging station in monitoring 

withdrawals from Lake Michigan has made it one of the most 

highly scrutinized gaging stations in the country. Any changes 

in streamgaging practices at this gaging station requires 

detailed analysis to ensure the change will not adversely 

affect the ability of the USGS to accurately monitor flows. 

This report provides a detailed analysis of the flow structure 

and index velocity measurements in the CSSC near Lemont, 

Illinois, to ensure that decisions regarding the future of this 

streamgage are made with the best possible understanding of 

the site and the characteristics of the flow. 

Introduction 

The construction of the Chicago Sanitary and Ship Canal 

(CSSC) in the late 1800s and the subsequent reversal of the 

Chicago River in 1900 allowed Chicago to overcome its issues 

with wastewater and flourish as a city. With the construction 

and subsequent flow reversal came scrutiny of the withdrawals from Lake Michigan and a U.S. Supreme Court decree 

limiting Illinois’ diversion of Great Lakes water. This decree 

established the need for diversion accounting and continuous 

measurement of the discharge of water out of Lake Michigan 

through the CSSC. To ensure that the best engineering practices and technology are being used in diversion accounting, 

a technical review committee consisting of practicing engineers and academics is convened every 5 years to review the 

U.S. Geological Survey’s streamgage practices in the CSSC 

near Lemont, Illinois (Ill.). With locks and control works 

throughout the Chicago Area Waterway System (CAWS) and 

significant influences from industry, the CSSC is a highly 
2 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

unnatural waterway with continuous perturbations that propagate from the lakefront to the confluence with the Des Plaines 

River at Lockport Lock and Dam (fig. 1). Continuous and 

accurate measurement of discharge in the CSSC requires innovative technology, measurement redundancy, and extensive 

data analysis. This report describes the comparison of index 

velocity measurements for two types of acoustic velocity 

meters installed at the U.S. Geological Survey (USGS) gaging 

station on the CSSC near Lemont, Ill.. 

The intense scrutiny of the Lake Michigan diversion 

accounting requires a high degree of accuracy and reliability 

in the USGS discharge measurements and the methods to 

compute continuous discharge in the CSSC. Continuity in 

measurements and methods is very important as technology 

improves and new instrumentation is deployed. It is critical that, whenever possible, an overlap in instrumentation be 

provided and proper comparison between instrumentation 

be made to ensure continuity and consistency of the record. 

The current primary gage in the CSSC near Lemont, Ill., is an 

acoustic velocity meter (AVM). This time-of-travel meter measures a bank-to-bank average velocity at a specified depth in 

the measurement reach (index velocity), which is related to a 

mean cross-sectional velocity by an index velocity rating. The 

use of AVMs has declined in recent years, and this technology 

is being replaced by the horizontal acoustic Doppler current 

profiler (H-ADCP), also called an acoustic Doppler velocity 

meter (ADVM). The H-ADCP measures velocity in a series 

of “cells” at a specified distance in front of the bank-mounted 

unit. Technical limitations prevent the unit from measuring 

near either bank (the unmeasured near-bank distance is a function of the unit configuration as well as channel geometry and 

water depth). Currently, an H-ADCP is the backup gage in the 

CSSC near Lemont and provides measures of discharge during 

periods of missing record from the primary gage. The lack of 

support and replacement parts for the AVM has made it necessary to consider using the H-ADCP as the primary gage with 

AVM as the backup gage. This report presents a comparison 

of the index velocity measurements and index ratings for the 

two instruments during a period of overlapping deployment. 

In addition, the flow at the site is characterized. The report 

discusses whether the H-ADCP can reliably function as the 

primary discharge gage while maintaining the continuity and 

accuracy of the record. 

The unsteadiness and unique flow structure in the 

CSSC make comparison of instrumentation critical prior to 

any instrumentation change. The inherent differences in the 

measurement techniques employed by each instrument may 

potentially lead to biases in reported discharges. As a part of 

this comparison, the flow characteristics and unsteadiness in 

the CSSC near Lemont, Ill., are discussed herein, as are the 

potential effects of these influences on the reported discharge. 

Methods

Data Collection

This section describes the index velocity instrumentation 

used in both the continuous monitoring gages installed on the 

CSSC near Lemont, Ill., and the synoptic measurements used 

to calibrate the instruments and investigate the flow structure 

and unsteadiness. 

Continuous Deployments 

The USGS has operated a continuous monitoring gaging 

station on the CSSC near Lemont, Ill., since 2004. Before this, 

the station was located near Romeoville, Ill., and was moved 

in 2004 because of interference from the electronic fish barrier 

operated by the U.S. Army Corp of Engineers (Kevin K. Johnson, U.S. Geological Survey, written commun., 2011). The 

installation includes a three-path Accusonic ORE 7510 GS 

acoustic velocity meter (AVM), a Teledyne RD Instruments 

Channel Master acoustic Doppler velocity meter (H-ADCP) 

with upward-looking acoustic stage sensor, a ParaScientific 

pressure sensor (PS-2), a staff gage, two Campbell Scientific 

electronic dataloggers (CR10X), a high-data-rate satellite 

transmitter (DCP), and a Sierra Wireless Raven XT Modem. 

The gaging station is on the right bank in a relatively straight 

reach with vertical walls, a channel top width of about 162 ft, 

and mean depth of about 25 ft (fig. 2). The station is in an area 

of the CSSC where the dolomite bedrock did not extend to 

the land surface, so set blocks were placed atop the bedrock 

to form the top part of the canal walls. Along the right bank 

within the measurement section of the AVM, the set block 

retaining wall has collapsed, and a significant amount of earth 

and rock has sloughed into the canal. The year of the collapse 

is unknown, but it occurred before gage installation. Heavy 

barge traffic required that the AVM transducers and H-ADCP 

be mounted in notches cut into the canal walls to protect the 

instrumentation. 

The AVM transmits sound 229.1 ft between transducers 

on opposite banks at an oblique angle (44.5 degrees) to the 

channel orientation, and the difference in the traveltime of the 

sound pulses traveling with and against the flow is directly 

related to the mean velocity of the water in the cross section. 

Because the transducers are integrated into the wall of the 

canal, the full width of the canal is sampled. To provide a better estimate of depth-averaged flow than can be obtained with 

a single transducer, three transducer sets are mounted at elevations of 570.2, 565.0, and 559.6 ft (NAVD 1988) for paths 1, 

2, and 3, respectively (fig. 3). The AVM requires cables to be 

laid along the riverbed between the transducers at opposite 

banks, making the instrument susceptible to damage from boat 

traffic and debris moving along the bed and creating a liability 

in data continuity. The cables have been severed once since the 

installation of the system in 2004, and commercial divers were 

required for repair. 
Methods 3

10 MILES

10 KILOMETERS

5

0 5

0

COOK COUNTY

WILL COUNTY

LAKE COUNTY

COOK COUNTY

42°15'

41°45'

41°30'

42°

88°15' 88° 87°45' 87°30'

Des

R i v e r

P l a i n e s

Chicago

Sanitary and Ship Canal

Calumet Sag

Channel

Salt

Creek

Little Calumet River

Calumet

R

Cr

Willow

Des P l a i n e s

River

N o r t h

Branch

Chicago

River

South Branch

Chicago River

North Shore Channel

ILLINOIS

INDIANA

LAKE

MICHIGAN

Grand

Calumet R

McHENRY CO

Lockport Lock and Dam

O'Brien Lock

and Dam

Chicago Lock

Wilmette

Control

Structure

1

2

Illinois

Map

area

ILLINOIS

River

Base from U.S. Geological Survey 1:100,000 Digital Data

Albers Equal-Area Conic Projection

Standard Parallels 33 ̊ and 45 ̊, central meridian −89 ̊

COOK COUNTY

DU PAGE COUNTY

1

EXPLANATION

U.S. Geological Survey (USGS) 

 streamflow-gaging station 

 at Lemont, Illinois (05536890)

USGS streamflow-gaging station 

 at Romeoville, Illinois (05536995)

2

Figure 1. Map of the Chicago Area Waterway System (CAWS) showing the Chicago Sanitary and Ship Canal 

(CSSC), Calumet Sag Channel, North Branch Chicago River, Little Calumet River, Grand Calumet River, and the other 

waterways in and near Chicago, Illinois. 
EXPLANATION Elevation, in feet above North American Vertical Datum of 1988 High: 568.14 Low: 548.98 N Sloughed bank Buriedcables Cross-channel AVM cables Up-looking ADCP (Installed near midchannel within the acoustic velocity meter (AVM) measurement volume) Horizontal acoustic Doppler c

Gage house Measurement cross section 162 feet 20o 44.5o Flow Figure 2. Schematic showing the instrument configurations superimposed on the channel bathymetry at the U.S. Geological Survey gaging st

Sanitary and Ship Canal near Lemont, Illinois.
Methods 5

551.5

0 18 36 54 72 90 108 126 144 162

576.7

Elevation above North American Vertical Datum of 1988 (NAVD 88), in feet

Distance, in feet

576.7, NAVD 88 (mean)

Channel cross section and bathymetry simplified for schematic

555.0

560.0

565.0

570.0

575.0

PATH 1 (570.2, NAVD 88)

PATH 2 (565.0, NAVD 88)

PATH 3 (559.6, NAVD 88)

Figure 3. Schematic of the channel cross section showing the three paths of the acoustic velocity meter (AVM) at the 

U.S. Geological Survey gaging station on the Chicago Sanitary and Ship Canal near Lemont, Illinois (looking upstream)..

The Channel Master H-ADCP requires only one set of 

bank-mounted transducers and does not require cross-channel 

cables. The H-ADCP utilizes the acoustic principle of the 

Doppler shift to measure water velocity. The H-ADCP is 

oriented perpendicular to the flow, and sound is transmitted 

out into the channel from two transducers along 20-degree 

divergent beams and is reflected and scattered by particles in 

the water (fig. 4). The moving particles reflect some sound 

back to the unit, which records the frequency of the sound and 

the time since the emitted pulse. The frequency of the returning sound is different from that of the emitted pulse because 

of the speed of the particles (Doppler shift). By using multiple 

beams and range gating based on timing of return pulses, the 

instrument computes a two-dimensional velocity vector for 

a series of segments (called cells) across the channel at the 

elevation of the instrument. Because the instrument is both a 

transmitter and receiver, a segment of data close to the face 

of the instrument (called the blanking region) is lost owing to 

acoustic ringing. On the opposite bank, the divergent beams 

impinge on the canal walls at an angle of 20 degrees, causing 

a loss of data (6 percent of the channel width or about 10 ft) 

owing to the phenomenon of side-lobe interference. Therefore, 

unlike the AVM, the H-ADCP does not measure the full width 

of the flow and measures along only one path in the vertical (565.1 ft; NAVD 1988). Theoretically, these differences 

could lead to potential biases in the computed discharge under 

certain flow conditions. The instrument is configured such that 

the measured section is broken into 9 cells with equal widths 

of 15 ft (fig. 4). The first cell starts 15 ft from the right bank 

(looking downstream) to accommodate the blanking region 

and the last cell ends 12 ft from the left bank to allow for theside-lobe interference region. 
6 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

551.5

0 18

15 30 45 60 75 90 105 120 135 150

36 54 72 90 108 126 144 162

555.0

560.0

565.0

570.0

575.0

576.7 

(mean)

Elevation above North American Vertical Datum of 1988 (NAVD 88), in feet

Distance, in feet

Elevation 565.1, NAVD 88

o 1.5

o

20

cell 1 cell 2 cell 3 cell 4 cell 5 cell 6 cell 7 cell 8 cell 9

Channel cross section and bathymetry simplified for schematic

Figure 4. Schematic of the horizontal acoustic Doppler current profiler (H-ADCP) deployed at the U.S. Geological Survey gaging 

station on the Chicago Sanitary and Ship Canal near Lemont, Illinois (looking upstream). 

Synoptic and Short-Term Deployments 

The AVM and H-ADCP are both calibrated by relating 

the index velocities measured by the AVM and H-ADCP to 

a mean cross sectional velocity (discharge/rated area) determined from a discharge measurement, which is made with 

an acoustic Doppler current profiler (ADCP) deployed from 

a moving boat that traverses the canal from bank to bank 

(hereafter called a boat-mounted ADCP or BM-ADCP). Typically, a 600- or 1,200-kHz Teledyne RDI Rio Grande ADCP is 

used and is often integrated with a Trimble AG132 differential 

global positioning system (GPS) for georeferencing of the 

velocity data. A manned boat on a tag line is typically used 

for precise control, though remote-controlled boats have been 

used at this site on occasion. Using the same principles as 

the H-ADCP, the four-beam BM-ADCP can compute threedimensional velocity profiles over a zone of the water column 

below the instrument (fig. 5). This is done while the boat is 

moved across the canal, allowing the flow field across the 

channel and channel bathymetry to be measured. The instrument uses either bottom-tracking or GPS to track its location 

as the boat traverses the channel. Using open-channel flow 

theory to account for unmeasured areas near the banks, bed, 

and surface, one can compute the discharge by integrating 

across the section (Mueller and Wagner, 2009). In addition 

to a reliable measurement of the total discharge, the movingboat ADCP measurements yield valuable high-resolution, 

three-dimensional velocity data that can be used to examine 

the flow structure at or near the gaging station. A typical 

discharge measurement takes less than 1 hour to complete, and 

these measurements are made routinely throughout the year 

(approximately every 8 weeks) and during periods of high 

flow. 
Methods 7

Global Positioning 

System (GPS) signal

Figure 5. Drawing showing a moving-boat discharge 

measurement with a boat-mounted acoustic Doppler current 

profiler (BM-ADCP) and differential global positioning system 

(GPS) and the resulting velocity magnitude data in the cross 

section. 

To examine the vertical velocity distribution at the site, a 

bottom-mounted, up-looking ADCP (Teledyne RDI 600-kHz 

Rio Grande) was deployed approximately midchannel in the 

path of the AVM in May 2008 for several days and redeployed 

from March 2009 to June 2010. This stationary instrument 

continuously measures three-dimensional velocity profiles 

in the water column above the unit (fig. 6). This unit was 

tethered to shore with a safety line and communication and 

power cable, enabling retrieval of the instrument, data logging, 

and a continuous source of power. In addition, a string of six 

temperature and conductivity probes was deployed within 

the notch on the right bank to determine whether any stratified conditions exist at this site. Jackson and others (2008) 

discovered density currents driven by salinity differences from 

road-salt runoff at upstream sites in the CAWS near Lake 

Michigan. Data from the up-looking ADCP was logged to a 

tablet personal computer while the DCP, phone modem, and 

data loggers recorded and transmitted data from the thermistors and conductivity sensors. 

Data Processing 

Index Velocity Computations 

The basis of index velocity ratings lies in the ability to 

relate, through an empirical relation, the velocity at a point 

or section within a channel, Vindex, to the mean velocity of the 

rated cross section defined as Vmean = Qmeasured/Arated, where Arated

is the rated area of the cross section defined by the stage-area 

rating curve and Qmeasured is the measured discharge. The continued success of this rating curve is dependent on the repeatability of the flow structure and, ultimately, the repeatability of 

the index velocity in the cross section for a given discharge. If 

the flow structure differs substantially within the measurement 

volume for the same discharge, the rating will fail to produce 

a meaningful relation. Index velocity ratings have found use 

in tidal systems, highly controlled systems with locks and 

dams, and in surface water systems with significant backwater 

effects. Unlike a more commonly used stage-discharge rating, 

index velocity ratings can account for backwater effects and 

can overcome hysteresis, which can occur in stage-discharge 

ratings (Morlock and others, 2002). However, hysteresis in 

index velocity ratings is possible in tidally affected areas 

(Ruhl and Simpson, 2005). 

Calibration Measurements 

The index velocity ratings for the instruments in the 

CSSC near Lemont were developed through calibration 

measurements made concurrently with the continuous monitoring. For a range of flows, the discharge was measured at 

or near the index velocity meters by using a BM-ADCP and 

following standard USGS measurement procedures outlined in 

Mueller and Wagner (2009). The discharge reported for each 

measurement was then divided by the rated area determined 

from the stage-area rating (using a time-weighted mean stage 

for the measurement period) to compute Vmean. A regression 

was performed between Vmean and Vindex to determine the index 

velocity rating. This rating is continually checked by making 

routine and event-driven discharge measurements at the site 

and checking the measurement against the rating.
8 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

551.5

0 18 36 54 72 90 108 126 144 162

555.0

560.0

565.0

570.0

575.0

Distance, in feet

Up-looking acoustic Doppler current

profiler (ADCP)

576.7 

(mean)

Channel cross section and bathymetry simplified for schematic Elevation above North American Vertical Datum of 1988 (NAVD 88), in feet

where 

Vi is the 10-minute average velocity of the i

th

acoustic path, and 

np is 3 for valid data on all three paths. 

V (1)

n

AVM V

p

i i

np

= ∑ =

1

1

If all three paths report a valid velocity for the 10-minute 

sampling period, the AVM index velocity VAVM is computed 

as the arithmetic mean of the velocities for each of the three 

paths:

Acoustic Velocity Meter (AVM)

At the USGS gaging station on the CSSC near Lemont, 

Ill., the AVM reports the mean water velocity along each of the 

three acoustic paths at 2-minute and 10-minute intervals. The 

actual sampling frequency is faster than this (about 1 Hz), but 

the data are averaged over 2-minute and 10-minute intervals 

for each path to reduce the noise and yield an accurate mean 

velocity for each path. Though both values are logged, the 

10-minute average velocity is used in the computation of the 

rated discharge to reduce variability in measurements. 

Figure 6. Schematic of the up-looking acoustic Doppler current profiler deployed near midchannel at the U.S. Geological 

Survey gaging station on the Chicago Sanitary and Ship Canal near Lemont, Illinois. 
Methods 9

If any individual path returns a missing velocity (no measurement made) or invalid velocity (erroneous as determined 

internally by the AVM) during the 10-minute sampling period, 

then the reported mean for that path is not valid and is reported 

as missing data. If one or two of the paths report missing data 

for the 10-minute average, path coefficients are multiplied by 

the remaining good path velocities prior to arithmetic averaging to determine VAVM. Path coefficients are used to correct 

VAVM for the effect of missing or invalid data. Path coefficients 

were determined by using the 10-minute path velocities from 

December 19, 2005, to January 3, 2008. Approximately 

93,000 values (86 percent of record) were used, which comprised all valid and non-zero path velocities for each of the 

three paths. The AVM path coefficients Ciwere determined by 

dividing the three path arithmetic mean VAVM by the 10-minute 

average velocity along each path:

V

V

C AVM

i

= i (2)

where 

i = 1, 2, 3. 

The approximately 93,000 path coefficients for each path were 

averaged to define a single path coefficient for each path of 

the AVM. The computed path coefficients for AVM paths 1, 

2, and 3 are 0.9300, 1.0134, and 1.0836, respectively. In the 

event that all three path velocities are invalid for a 10-minute 

averaging interval, VAVM for that time period is left blank and 

reported as missing data. 

Horizontal-ADCP (H-ADCP)

At the USGS gaging station on the CSSC near Lemont, 

Ill., the H-ADCP reports the mean water velocity in each of 

the nine cells across the channel at 1-minute intervals. The 

actual sampling frequency is faster than this (about 1 Hz), but 

the data are averaged over 1-minute intervals for each cell to 

reduce the noise. 

The H-ADCP index velocity VH-ADCP is computed as the 

arithmetic average of the nine cross-channel cell velocities,

V

n H ADCP V

c

b i

n

i

c

− = = ∑1

1 (3)

where 

Vbi is the 1-minute average cell velocity in the 

i

th cell, and 

nc equals 9 for valid data in all cells. 

If data from any cell are invalid (as determined by internal 

screening in the instrument), the average is computed from 

the remaining cells with no coefficients applied to account for 

the missing cells. In the event that all nine cell velocities are 

invalid, VH-ADCP for that time period is left blank and reported 

as missing data. 

Stage-Area Rating 

Index velocity ratings require a stage-area rating curve 

to transform a measured stage or gage height (water-surface 

elevation relative to a gage datum) to cross-sectional area prior 

to computation of discharge. Stage-area ratings are built from 

a cross-section survey and must be continually checked to 

ensure that the cross section is not changing. The stage-area 

rating for the CSSC near Lemont has been stable since installation of the gage in 2004. The rating is occasionally checked 

by using data collected during discharge measurements at 

the same cross section as defined in the stage-area rating (see 

fig. 2). The stage-area rating for the CSSC at Lemont, Ill., is 

defined as 

A G CSSCL C = + SSCL 139. . 83 670 39 (4)

where 

ACSSCL is the rated area for the CSSC near 

Lemont, and 

GCSSCL is the gage height at CSSC near Lemont. 

A report containing more detailed information on the development of the stage-area rating curve for the CSSC near Lemont 

is currently (at the time of this publication) under review 

(Kevin K. Johnson, U.S. Geological Survey, written commun., 

2011). 

Discharge Computation 

Once Vindex is determined (that is, VAVM or VH-ADCP), the 

index velocity rating for the instrument is used to determine 

the mean cross sectional velocity Vmean. The observed gage 

height is used in conjunction with the stage-area rating for the 

site to determine the rated area of the cross section. The rated 

discharge Qrated is then computed as the product of the mean 

cross-sectional rated velocity Vmean and the rated area Arated. 
10 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Results 

Flow Characterization 

This section is dedicated to analysis of the observed flows 

in the Chicago Sanitary and Ship Canal near Lemont, Ill. The 

analysis evaluates in detail the sampling configuration of the 

instruments and potential effects of the flow structure on the 

measured index velocities for each instrument. 

Velocity Magnitude and Direction

Distribution of Observed Velocities

Velocity data from the AVM and H-ADCP were analyzed 

to determine the range of flows observed in the CSSC near 

Lemont, Ill. The data used in the analysis consisted of 10-minute data from the AVM for November 2006–January 2010 

and 10-minute data from the H-ADCP for the same period. 

Individual velocities observed for each path of the AVM and 

each cell of the H-ADCP were analyzed independently. 

For the selected period for analysis, the CSSC near 

Lemont, Ill., exhibits a skewed distribution of flow velocities, with the greatest occurrence of velocities in the 0.5- to 

0.75-ft/s range. To a lesser extent, velocities in the range 3.5 to 

4.0 ft/s were also relatively common (figs. 7 and 8). Both the 

AVM and the H-ADCP registered this skewed distribution in 

all paths and cells (except cell 9 of the H-ADCP). Both instruments registered negative velocities and velocities as high as 

5 ft/s; however, the range of observed velocities was slightly 

wider for the H-ADCP, likely because of the smaller measurement volume used in this system (15-ft cell width for the 

H-ADCP compared to a 162-ft channel width for the AVM). 

This skewed distribution is likely caused by the operation of 

the powerhouse and lock at Lockport, Ill. The primary peak 

at 0.5 to 0.75 ft/s arises from “normal” power generation at 

the powerhouse, when about 2,000 ft3/s is discharged through 

the turbines. However, in advance of storms, the Metropolitan 

Water Reclamation District of Greater Chicago (MWRDGC) 

will draw down the canal to provide storage by increasing 

flow through the powerhouse and opening sluice gates at the 

powerhouse and at the controlling works upstream of Lockport. These rapid drawdown events in the canal and the flood 

events that follow appear to generate a range of flows with 

peak velocities around 4 ft/s. A third, hardly perceptible peak 

occurs in the range 2 to 3 ft/s, and this may be associated with 

flows resulting from lockages and/or operational changes in 

power generation and flow structures. 

The highest velocities occur nearest the surface and 

within H-ADCP cells 5, 6, and 7. With the exception of cell 9, 

which exhibits a low bias over the entire range of observed 

flows (discussed in detail later in this report), the high flows 

appear to be biased to the left bank, as do the negative flows. 

Detailed discussions of the transverse velocity distribution and 

flow reversals are also presented later in this report. 

Primary and Secondary Flows and Flow Angle

The index velocity methods applied in the CSSC near 

Lemont utilize the streamwise component of the velocity 

(parallel to the course of the river) in the computation of the 

index velocity. For these techniques to be effective, flow at the 

chosen sites must be relatively unidirectional over the range 

of flows at the sites. Uncertainty in the index velocity rating 

can be introduced if the flow direction (or direction of primary 

velocity) within the measurement volume of the index velocity 

meter changes under similar discharge conditions. Therefore, 

from the 61 discharge measurements made before April 2010, 

a subset of 29 was selected to complete a detailed analysis of 

the flow direction near Lemont. The selected measurements 

included GPS data and valid compass calibrations (used to 

georeference velocity data), requirements for a proper analysis. These measurements were collected and processed by use 

of USGS standard methods (Mueller and Wagner, 2009). 

The 29 discharge measurements each consisted of at 

least 4 transects (but often 8 or more) at a fixed measurement 

section just upstream of the notch in the right bank (fig. 2). 

A transect is defined as a single traverse of the instrument 

from one bank of the river to the alternate bank. By using the 

Matlab-based program Velocity Mapping Toolbox (VMT; 

Daniel R. Parsons, University of Leeds, written commun., 

2011), the BM-ADCP data for each measurement were 

mapped to the fixed measurement section and interpolated to 

a regular grid with horizontal grid node spacing of 0.1 m and 

vertical grid node spacing equal to the bin size (variable), and 

flow components were averaged across all transects at each 

grid node. Flow components computed include streamwise 

and transverse velocity (as set by the channel orientation near 

Lemont), primary and secondary velocities defined by both 

the zero net secondary discharge (ZSD) and Rozovskii (ROZ) 

definitions (Lane and others, 2000), and vertical velocity. 

Primary velocity is the component of velocity in the direction 

that maximizes downstream discharge and minimizes crossstream discharge for either the whole cross section (ZSD definition) or for each individual velocity profile (ROZ definition). 

Secondary velocity is the component of velocity perpendicular 

to the primary velocity in the transverse (or lateral) direction. 

This analysis yielded an average velocity cross section for 

each measurement. By using the same procedure, the acoustic 

backscatter recorded by the ADCP during the measurement 

was averaged to produce an average backscatter cross section 

for each measurement. Acoustic backscatter is the intensity 

of the signal returned to the instrument from particles in the 

water column and is generally proportional to the concentration and size of particles in the water column (and may 

be used as a surrogate for suspended sediment with proper 

calibration). Primary and secondary velocities are used in this 

analysis to determine the magnitude of flow perpendicular to 

the direction of discharge. Although not currently used for rating development, these components of flow may help explain 

some uncertainty in the index velocity rating at Lemont. 
Results 11

-1 0 1 2 3 4 5 6 101

102

103

104

105

Velocity, in feet per second

Number of observations

Path 1 (n = 105,881)

Path 2 (n = 133,217)

Path 3 (n = 133,329)

EXPLANATION

Figure 7. Frequency-distribution diagrams of velocity magnitude observations for each of the three paths of the acoustic 

velocity meter (AVM) in the Chicago Sanitary and Ship Canal near Lemont, Illinois, December 2005–January 2010. 

-1 0 1 2 3 4 5 6 100

101

102

103

104

105

Velocity, in feet per second

Number of observations

EXPLANATION

Cell 1 (n = 166,965)

Cell 2 ( = 167,123) n

Cell 3 ( = 167,123) n

Cell 4 ( = 167,123) n

Cell 5 ( = 167,123) n

Cell 6 ( = 167,123) n

Cell 7 ( = 167,123) n

Cell 8 ( = 167,123) n

Cell 9 ( = 167,123) n

Figure 8. Frequency-distribution diagrams of velocity magnitude observations for each of the nine cells of the 

horizontal acoustic Doppler current profiler (H-ADCP) in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–January 2010. 
12 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

As the discharge and mean velocity in the CSSC near 

Lemont decreases, the primary flow direction shows greater 

deviation from the streamwise direction (fig. 9; table 1). 

Primary flow directions determined by using the zero net 

secondary discharge method (Lane and others, 2000) were 

found to deviate from the streamwise direction by as much as 

4.3 degrees at very low flows. Flow angles of this magnitude 

will lead to less than a 1-percent negative bias in the index 

velocity (errors are proportional to 1−cos(θ), where θ is the 

absolute value of the flow angle) as the index velocity meters 

generally only use the streamwise velocity in the computation of the index velocity. However, it is important to note 

that the flow angle determined here is an average flow angle 

determined by rotating the section such that the net secondary discharge is zero. Therefore, individual flow angles at any 

point in the cross section may be substantially different from 

that reported here. An index velocity meter samples only a 

small fraction of the cross section, and if the flow angles vary 

substantially, the variation will result in greater errors in the 

measurement of an index velocity. 

The magnitude of the secondary velocity increases relative to the primary velocity as the mean velocity decreases in 

the CSSC near Lemont (fig. 9). On the basis of the 29 discharge measurements analyzed, the greatest ratios of secondary to primary velocity are observed at low flows. For Vmean

< 0.6 ft/s, secondary velocities increase in magnitude and can 

exceed 10 percent and be as much as 82 percent of the magnitude of the primary velocity. For Vmean > 0.6 ft/s, the secondary 

velocities are less than 10 percent of the primary velocity, and 

the ratio further decreases with increasing Vmean. Secondary 

flows can arise from channel curvature, channel confluences, 

bathymetry, environmental effects such as wind, and stratified 

flows. The CSSC near Lemont is subject to all these factors.

Primary flow direction, in degrees

Secondary velocity/primary velocity

Measured discharge/rated area, in feet per second

236

238

240

242

244

246

248

250

252

254

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Secondary/primary velocity ratio (zero net secondary discharge (ZSD) definition)

Secondary/primary velocity ratio (Rozovskii (ROZ) definition)

Primary flow direction

Channel orientation

EXPLANATION

Figure 9. Primary and secondary flows as a function of Vmean for the Chicago Sanitary and Ship Canal near Lemont, Illinois. 
Results 13

Table 1. Primary and secondary velocities observed during discharge measurements in the Chicago Sanitary and Ship Canal near 

Lemont, Illinois. 

[Vmean = Q/Ar, where Q is the measured discharge and Ar is the rated area; ZSD, zero net secondary discharge; ROZ, Rozovskii definitions]

Measurement 

number

Vmean, in cubic feet 

per second

Secondary/primary 

velocity ratio 

(ZSD)

Secondary/primary 

velocity ratio 

(ROZ)

Primary flow 

direction, in degrees 

from true north

Flow angle1, 

in degrees

17 0.35 0.237 0.214 242.6 2.9

23 .99 .058 .053 246.8 −1.3

34 .42 .045 .034 247.3 −1.8

35 .41 .038 .024 246.3 -.8

36 .65 .094 .089 248.6 −3.1

37 .88 .098 .092 248.7 −3.2

38 3.65 .023 .019 246.6 −1.1

39 .57 .099 .091 245 .5

40 1.02 .057 .05 247.2 −1.7

41 1.10 .061 .057 244.8 .7

42 .55 .094 .089 245 .5

43 .63 .1 .098 244.2 1.3

44 .53 .125 .117 244.8 .7

45 .19 .498 .458 243.5 2

46 .06 .823 .578 249.8 − 4.3

47 .15 .627 .561 242.1 3.4

48 .69 .04 .032 245.2 .3

49 2.20 .029 .025 245.5 0

50 2.18 .04 .034 245.2 .3

51 1.07 .034 .029 244.8 .7

52 .92 .051 .044 245.6 −.1

53 .82 .048 .04 245.1 .4

54 .73 .043 .039 245.5 0

55 .79 .045 .04 245.8 −.3

56 .80 .049 .043 245.6 −.1

57 .46 .066 .059 247 −1.5

58 .68 .047 .04 245.3 .2

60 .35 .062 .058 246.6 −1.1

61 .34 .058 .054 246.6 −1.1

1

From streamwise flow direction (245.5 degrees from true north near Lemont, Ill.).
14 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

To assess the effect of flow angle and secondary flows 

on index velocity measurements near Lemont, the percent 

difference and residuals of the rated discharge from the measured discharge were plotted against the flow angle (figs. 10A

and 10B) and the ratio of secondary and primary velocities 

(figs. 11A and 11B). The percent difference data show a weak 

trend in which errors in the rated discharge increase for larger 

flow angles and larger secondary to primary velocity ratios. 

Residual plots have considerable scatter and are generally 

inconclusive. This relatively simple analysis may help explain 

why certain measurements do not fall in line with the rating 

(for example, measurements 45–47). Though the H-ADCP 

and AVM show the same general trend, the AVM appears to 

be more susceptible to errors in flows with relatively large 

flow angles and secondary flows. Individual differences in the 

rated discharge from the measured discharge for the AVM and 

H-ADCP can exceed 40 percent at lower discharges (less than 

about 500 ft3/s). These errors greatly exceed those computed 

by using simple geometry and the mean flow angle, indicating 

that the limited sample volume of the instruments may result 

in unbalanced and disproportionately large flow angles compared to the overall cross section. However, secondary flows 

often occur in places where flows are complex, and complex 

flows are inherently more difficult to accurately measure. The 

relationship between rated discharge errors and secondary 

currents (and flow angle) may also be a product of an overall 

increase in discharge-measurement uncertainty in nonuniform 

flows. 

0.01

0.10

1.00

10.00

100.00

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Percent difference from measured discharge (absolute value) 

Flow angle, in degrees (absolute value)

y = 2.2877e0.422x

2 R = 0.1873

y = 0.8109e0.9641x

2 R = 0.407

A

EXPLANATION

Horizontal acoustic Doppler current profiler (H-ADCP)

Acoustic velocity meter (AVM)

AVM

H-ADCP

Figure 10A. Percent difference from the measured discharge as a function of the flow angle in the Chicago Sanitary and 

Ship Canal near Lemont, Illinois. 
Results 15

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Flow angle, in degrees (absolute value)

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

Residual (Q - Q ), in cubic feet per second Rated Measured

B

EXPLANATION

Horizontal acoustic Doppler current profiler (H-ADCP)

Acoustic velocity meter (AVM)

Figure 10B. Residuals (rated discharge minus measured discharge) as a function of flow angle in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois. 
16 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

0.01

0.10

1.00

10.00

100.00

0.01 0.1 1

Percent difference from measured discharge (absolute value) 

y = 74.966x

2 R = 0.3938

1.2261

y = 26.774x

2 R = 0.3226

0.7307

Secondary/primary velocity ratio

A

EXPLANATION

Horizontal acoustic Doppler current profiler (H-ADCP) (Rozovskii (ROZ) definition)

Acoustic velocity meter (AVM) (ROZ definition)

AVM (ZSD definition)

H-ADCP (zero net secondary discharge (ZSD) definition)

H-ADCP (ROZ definition)

AVM (ROZ definition)

Figure 11A. Percent difference from the measured discharge as a function of the secondary to primary velocity ratio in 

the Chicago Sanitary and Ship Canal near Lemont, Illinois. 
Results 17

0.01 0.1 1

Secondary/primary velocity ratio

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

Residual (Q - Q ), in cubic feet per second Rated Measured

B

EXPLANATION

Horizontal acoustic Doppler current profiler (H-ADCP) (Rozovskii (ROZ) definition)

Acoustic velocity meter (AVM) (ROZ definition)

AVM (ZSD definition)

H-ADCP (zero net secondary discharge (ZSD) definition)

Figure 11B. Residuals (rated discharge minus measured discharge) as a function of the secondary to primary velocity 

ratio in the Chicago Sanitary and Ship Canal near Lemont, Illinois. 

Temporal Variability 

Frequency Analysis 

The flow in the CSSC near Lemont, Ill., is highly variable 

in response to turbine and sluice gate changes and lockages 

at Lockport, Chicago Lock and Chicago River Controlling 

Works, and O’Brien Lock and Dam. In addition, temporal 

variations in the flow are also caused by variability in barge 

traffic, sewage-treatment-plant discharge, powerplant coolingwater discharge, industrial intake and discharge, wind, seiches, 

and tributary inflows. Although some of these processes 

produce nonperiodic variations in the flow near Lemont, many 

likely contain some form of periodicity, whether it is a diurnal 

variation in effluent discharge, routine control-gate changes, 

or seiche periods set by the flow depth and channel geometry. 

Pulsing of the flow near Lemont is common and is identifiable 

in the velocity records of the AVM, H-ADCP, and up-looking 

ADCP. 

Velocity records (2-minute data for the AVM and 1-minute data for the H-ADCP) were examined for the periods 

October 2007–January 2010 (AVM) and November 2006–

January 2010 (H-ADCP) to determine whether any dominant 

frequencies in the data exist. Power spectra were independently generated for each cell of the H-ADCP and each path of 

the AVM (fig. 12). Dominant frequencies are evident as local 

peaks in the power spectrum; these were identified, and the 

corresponding frequencies were documented. 
18 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

10-7 10-6 10-5 10-4 10-3 10-2

Power spectrum, in decibels

Frequency, in hertz

-60

-50

-40

-30

-20

-10

0

10

1

2

9 8

7

5 6

3 4

EXPLANATION

Acoustic velocity 

 meter (AVM) path 1

AVM path 2

AVM path 3

Horizontal acoustic Doppler 

 current profiler (H-ADCP) cell 1

H-ADCP cell 2

H-ADCP cell 3

H-ADCP cell 4

H-ADCP cell 5

H-ADCP cell 6

H-ADCP cell 7

H-ADCP cell 8

H-ADCP cell 9

Figure 12. Power spectra for acoustic velocity meter (AVM) path and horizontal acoustic Doppler current profiler (H-ADCP) cell 

velocity data, November 2006–January 2010, in the Chicago Sanitary and Ship Canal near Lemont, Illinois. (Numbered arrows identify 

dominant-frequency peaks in the record.) 

Although analyzed independently, all paths of the AVM 

and all cells of the H-ADCP showed numerous dominant 

frequencies, with corresponding periods in the range of 16.1 to 

348.9 minutes (table 2). A total of eight dominant frequencies 

(peaks 2–9) were identified in all records, and an additional 

weak peak (peak 1) was identified in some of the H-ADCP 

cell data and the AVM path 1 data. The frequencies and 

periods associated with the peaks identified in figure 12 are 

listed in table 2. In spite of the higher sampling frequency of 

the H-ADCP, the high-frequency data (greater than approximately 10-3 Hz) appear to be overwhelmed by the noise of the 

instrument. The plateau of the H-ADCP data at high frequency 

is indicative of the effect of instrument noise (Anderson and 

Lohrmann, 1995) and masks oscillations in the data at periods 

less than about 15 minutes. The AVM data show a similar 

response at high frequencies, though the noise floor for the 

AVM is slightly lower than for the H-ADCP. 

The CAWS is a closed system with locks and control 

structures regulating flow on the inlets and outlets of the 

system. The primary influences on this system with respect 

to flow unsteadiness are control changes and lockages at 

Lockport. Lockport Lock and Dam is a large structure with 

a 39.5-ft head difference across the structure, and changes in 

flow at Lockport are felt throughout the CAWS. By using shallow-water wave theory, the periods for various seiche modes 

were computed for the CAWS on the basis of an approximate 

mean depth and reach length (Dean and Dalrymple, 1984). 

Because the system has multiple branches, this analysis 

was performed on each of the three primary arms: Lockport 

to Wilmette, Lockport to Chicago Lock, and Lockport to 

O’Brien Lock (see fig. 1). These three reaches have corresponding approximate mean depths of 20.3 ft, 24.9 ft, and 

17.4 ft, respectively (computed by using a distance-weighted 

mean and bathymetry from the USGS and U.S. Army Corps of 

Engineers). Reach lengths are approximately 50.0 mi, 36.3 mi, 

and 35.1 mi, respectively. The first 15 seiche modes were 

computed for each branch of the system. Although an infinite 

number of modes are theoretically possible, the lower modes 

are likely present in nature as higher frequency modes are 

damped out by friction. 

Table 2. Dominant frequencies and periods for the acoustic 

velocity meter (AVM) and horizontal acoustic Doppler current 

profiler (H-ADCP) velocity data for October 2007–January 2010 

(AVM) and November 2006–January 2010 (H-ADCP) in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois. 

Peak number1 Frequency (hertz) Period (minutes)

1 4.777E−05 348.9

2 9.590E−05 173.8

3 1.896E−04 87.9

4 2.408E−04 69.2

5 3.041E−04 54.8

6 4.505E−04 37

7 5.020E−04 33.2

8 8.726E−04 19.1

9 1.035E−03 16.1

1

Peaks are identified in the spectra presented in figure 12.
Results 19

At least half of the dominant frequencies identified in 

the CSSC near Lemont are consistent with seiche modes, 

and seiches in the Lockport to Wilmette reach appear to 

make the largest contribution to flow variability near Lemont 

(fig. 13). Peaks 1, 2, and 4 in the power spectrum (fig. 12) 

can be explained by the modes 1, 2, and 5 seiches for the 

Lockport to Wilmette reach. Peak 3 compares well with the 

Lockport to Wilmette mode 4 and Lockport to O’Brien mode 

3 seiches. Peak 6, a well pronounced peak in both the AVM 

and H-ADCP data, may be owing to the superposition of the 

Lockport to Chicago Lock mode 6 seiche and the Lockport 

to O’Brien mode 7 seiche. Finally, the period associated with 

peak 9 is equal to the Lockport to Chicago lock mode 14 

seiche period. In addition to these seiche modes that are well 

correlated with the observed periods present in the data, several other modes show reasonably good correlation and may 

be able to explain the spectrum peaks within the uncertainty of 

the seiche mode periods. 

Flow Variability Owing to Barge Passage

The effect of barge/tow passages through the CSSC near 

Lemont was investigated by using the up-looking ADCP data 

from May 2008 and June 2009. Because the up-looking ADCP 

was placed midchannel, barges passing through the measurement volume can be identified in the data as the flow is 

disturbed and the beams impinge on the bottom of the vessel, 

causing a sudden change in the acoustic return. Therefore, the 

up-looking ADCP data were used to identify the start and end 

times of a barge passage by marking the start and end times 

of these disturbances. In addition, because the instrument has 

four diverging beams and a compass, the orientations of the 

instrument’s beams are known; and by analyzing the intensity 

of the acoustic returns of each beam at the beginning of each 

barge disturbance, the direction of travel (that is, upstream or 

downstream) can be identified. Finally, the ferrous metals in 

the vessels create a disturbance in the local magnetic field and 

cause erroneous readings from the compass on the up-looking 

ADCP during the time of passage when the barge is directly 

over the ADCP. Therefore, the compass readings are a good 

indicator of barge/tow passages. 

348.9

173.8

87.9

69.2

54.8

37

33.2

19.1

16.1

10.0

100.0

1,000.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Seiche period, in minutes

Seiche mode

344.2

172.1

86.0

68.8

37.5

16.1

87.1

37.3

EXPLANATION

Lockport to Wilmette

Lockport to Chicago Lock

Lockport to O'Brien

Peak 1

Peak 2

Peak 3

Peak 4

Peak 5

Peak 6

Peak 7

Peak 8

Peak 9

Theoretical seiche periods

Observed periods

Figure 13. Seiche periods for the Chicago Area Waterways System (CAWS) compared to the observed dominant periods in the 

velocity data in the Chicago Sanitary and Ship Canal near Lemont, Illinois. 
20 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

A total of 23 barges/tows traversed the measurement 

site in a period of 25 hours on May 20–21, 2008 (table 3). An 

additional 29 barge/tow passages were identified in up-looking 

ADCP data during June 18–24, 2009, and were used for 

comparison to the 2008 data. The 2008 up-looking ADCP data 

were collected by using high-resolution water mode 11, which 

is subject to data loss in turbulence and high-velocity flows. 

Therefore, barge/tow passages resulted in significant data loss 

in which all or nearly all velocity data from the up-looking 

ADCP were lost for an average of more than 4 minutes during 

each passage. However, the 2009 up-looking ADCP data were 

collected by using water mode 12, a more robust configuration that is more tolerant of turbulent flows. Very little data 

loss occurred during barge/tow passages with the up-looking 

ADCP in water mode 12. 

Heading changes associated with compass errors owing 

to local magnetic field disturbance can be high during barge/

tow passages. The heading changed over the range of 5 to 

30 degrees with a median value of 10 degrees for the 23 barge/

tow passages in the 2008 up-looking ADCP data (table 3). 

In nearly all cases, the errors caused a heading deviation that 

alternated about the true heading (that is, a positive bias followed by a negative bias or vice versa). Because the instrument is in a stable position on the bed (but not fixed to the 

bed), the heading was stable in the absence of barges/tows. 

Table 3. Barge/tow passages in the Chicago Sanitary and Ship Canal near Lemont, Illinois, May 20, 2008 (14:00) to May 21, 2008 (15:00). 

[CST, Central Standard Time; DS, downstream; US, upstream; --, median not applicable]

Barge number Date Time, CST Disturbance time1

, 

in minutes Direction of travel Compass change, in degrees

1 5/20/08 15:36:15 7.28 DS 30

2 5/20/08 17:19:35 4.62 DS 18

3 5/20/08 19:09:23 5.38 DS 11

4 5/20/08 20:00:39 3.65 DS 12

5 5/20/08 20:06:40 7.47 US 15

6 5/20/08 20:37:56 6.85 US 21

7 5/20/08 23:07:42 2.08 US 10

8 5/21/08 00:01:21 6.03 DS 30

9 5/21/08 00:14:43 2.37 DS 10

10 5/21/08 00:34:23 5.87 US 20

11 5/21/08 01:10:17 2.68 DS 10

12 5/21/08 02:06:28 2.40 US 8

13 5/21/08 05:49:08 4.62 DS 8

14 5/21/08 06:23:26 6.02 US 10

15 5/21/08 07:47:15 1.75 DS 12

16 5/21/08 08:31:11 4.28 DS 10

17 5/21/08 09:20:52 7.30 US 10

18 5/21/08 10:06:35 4.43 DS 9

19 5/21/08 10:18:11 2.53 US 30

20 5/21/08 10:52:19 2.52 DS 8

21 5/21/08 13:21:56 2.37 DS 5

22 5/21/08 14:13:03 3.32 US 8

23 5/21/08 14:49:33 4.12 DS 9

Median -- -- 4.28 -- 10.0

1

Disturbance time denotes the time the flow was disturbed leading to significant loss of data from the 600-kHz Rio Grande acoustic Doppler current profiler 

with water mode 11. 
Results 21

The 2009 up-looking ADCP data had erroneous compass readings owing to a malfunctioning heading, pitch, and roll sensor, 

so it could not be used for comparison. The compass errors 

caused by barges may be problematic if the up-looking ADCP 

is ever used as an index velocity meter at the site. Index velocity computations require one to use the compass to determine 

instrument orientation (or use an instrument with no compass 

and a known, fixed orientation) so that the streamwise velocity 

can be determined. Nevertheless, these disturbances are short 

and well defined and may be useful as a flag or marker for data 

filtering. Compass deviations may also be a useful tool for 

counting the frequency of barge passages should that ever be 

required. 

The barge/tow passages observed in May 2008 caused 

regular data loss in path 1 of the AVM but no data loss in 

paths 2 and 3. AVM data were analyzed for the periods of 

barge/tow passages identified from the up-looking ADCP data, 

and the analysis revealed that path 1 of the AVM regularly lost 

14 to 28 minutes of data after a barge/tow passage, resulting in 

between 20 to 30 minutes of invalid data when averaged over 

10-minute intervals. AVM paths 2 and 3 showed no data loss 

for the same barge/tow passages, indicating the disturbance is 

likely owing to blocking of AVM path 1 by fully loaded barges 

(9 ft draft) and air entrainment by the tow propellers and is 

confined to the near-surface layer. In general, for a median 

flow velocity of 0.75 ft/s, a 0.1- to 0.3-ft/s (13- to 40-percent) 

decrease in the flow velocity over all paths was associated 

with downstream-bound barges/tows, whereas a 0.2- to 0.5-ft/s 

(27- to 67-percent) increase in flow velocity accompanied 

upstream-bound barges/tows. Impacts of the barges/tows 

likely vary substantially with the size, loading, and speed of 

the barges. 

The H-ADCP showed little to no data loss from 29 barge/

tow passages in June 2009. Data from the June 2009 up-looking ADCP deployment were used in place of the May 2008 

data because of a contamination of the H-ADCP data during 

the May 20–21, 2008, study (data were invalid because of 

human error in programming the instrument). Of the 29 barge/

tow passages during the analyzed period, only 7 passages 

caused any loss of data in the H-ADCP, and the data loss was 

minimal. In general, only one cell (but up to three cells) of 

the H-ADCP were lost in response to the barge/tow passages. 

Data were lost in these cells for only 2 to 3 minutes after each 

passage. Fifteen of the passages have an associated change in 

measured velocity (taken as the mean of the H-ADCP cells), 

with changes ranging from approximately 0.2 to 0.5 ft/s (12 to 

31 percent) for a mean velocity of 1.62 ft/s. Although the flow 

rate for this period was higher than for the May 20–21, 2008, 

data, the flow disturbances are similar in magnitude. The sustained flow velocity reached 3.7 ft/s for a 28-hour period during June 19 and 20, 2009, and although only one barge passed 

during this period, a velocity increase of 0.1 ft/s (2.6 percent) 

was observed during the passage. Therefore, although barge/

tow passages may not create a large loss of H-ADCP data, 

they introduce uncertainty in the computed discharge at the 

site by modifying the velocity field for all flow rates observed 

at the site, resulting in errors or bias in the index velocity 

measurements. However, temporal averaging of the computed 

discharge at the daily, monthly, and annual time scales can 

substantially lessen any influence on the computed discharge 

from navigation. 

Flow Variability Owing to Lockages

A significant source of variability in the flow in the 

CSSC near Lemont, Ill., is lockages at Lockport Lock, 11.5 mi 

downstream. Lockport Lock and Dam has an average 39.5-

ft head difference across the structure and requires a large 

volume of water (approximately 2.6 million cubic feet) to fill 

the lock (U.S. Army Corps of Engineers,1986). Each time the 

lock is filled, the flow in the canal increases and a flood wave 

propagates upstream. Based on shallow-water wave theory, the 

rate of propagation of this wave upstream, or wave celerity, is 

approximately 23 ±3 ft/s after accounting for frictional effects 

(assuming a 20-percent decrease in wave celerity owing to 

friction; Geyer and Chant, 2006). Therefore, flood waves generated by the lock should affect the gage near Lemont approximately 40 to 51 minutes after the start of the filling of the lock 

chamber (for a mean channel depth of 25 ft). 

To examine this source of variability, the data from the 

up-looking ADCP and the AVM were utilized for the period 

of May 20 and 21, 2008. During this 25-hour period, a total 

of eight vessels locked through at Lockport, with two of the 

vessels requiring two lockages to transit the lock because of 

their large size (table 4). Table 4 also lists the status of the 

lock chamber during filling because the rate at which it is 

filled is increased significantly when the chamber is empty 

(Pat Wharry, Lockport Lock Master, oral commun., 2010). By 

use of the up-looking ADCP and AVM data, velocity “pulses” 

associated with lockages were identified in the records and are 

listed in table 5, along with their general characteristics. 

Increased velocity pulses near Lemont owing to lockages at Lockport Lock have a duration of about 33 minutes 

and an average midchannel velocity of about 1.1 ft/s for the 

time period analyzed (table 5). The time- and depth-averaged 

midchannel flow velocity based on the data from the up-looking ADCP for this period (May 20–21, 2008) was 0.55 ft/s; 

increases in velocity above the average velocity ranged from 

0.6 to 0.7 ft/s (109 percent to 127 percent) for pulses generated by filling an occupied lock chamber to 0.7 to 0.8 ft/s 

(127 percent to 145 percent) for filling an empty lock chamber 

at Lockport Lock. The three paths of the AVM show a similar 

response to the up-looking ADCP, with about a 0.1-ft/s smaller 

peak pulse velocity (likely owing to spatial and temporal 

averaging of the data). The 33-minute average pulse duration 

is consistent with the time it takes to fill the lock under normal 

operating conditions (about 30 minutes for an occupied lock 

chamber) and is close to the 37-minute period observed in the 

AVM and H-ADCP data (table 2, peak 6). 
22 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Table 4. Lockages recorded at Lockport Lock on the Chicago Sanitary and Ship Canal, near Lemont, Illinois, May 20, 2008 (14:00), to 

May 21, 2008 (15:00). 

[Data provided by the U.S. Army Corp of Engineers Rock Island District; CST, Central Standard Time; --, median not applicable]

Lockage 

number

Arrival time, 

CST

Lockage 

start time, 

CST

Lockage 

end time, 

CST

Lockage 

duration, 

in minutes

Travel 

direction Cuts1

Chamber 

status 

during fill

Chamber 

filled during 

lockage?

1 5/20/08 14:50 5/20/08 14:50 5/20/08 16:20 90 Upstream 1 Occupied Yes

2 5/20/08 14:39 5/20/08 16:20 5/20/08 17:29 69 Downstream 1 Occupied No

3 5/20/08 18:53 5/20/08 18:53 5/20/08 19:52 59 Downstream 1 Empty Yes

4 5/20/08 20:39 5/20/08 20:39 5/21/08 00:28 229 Upstream 2 Occupied Yes (×2)

5 5/20/08 21:20 5/21/08 00:47 5/21/08 01:46 59 Upstream 1 Occupied Yes

6 5/21/08 04:06 5/21/08 04:06 5/21/08 07:11 185 Upstream 2 Occupied Yes (×2)

7 5/21/08 10:06 5/21/08 10:06 5/21/08 10:39 33 Downstream 1 Occupied No

8 5/21/08 12:03 5/21/08 12:03 5/21/08 12:30 27 Downstream 1 Empty Yes

9 5/21/08 13:44 5/21/08 13:44 5/21/08 14:15 31 Upstream 1 Occupied Yes

10 5/21/08 14:25 5/21/08 14:25 5/21/08 17:31 186 Upstream 2 Occupied Yes

Median -- -- -- 64 -- -- -- --

1

 “Cuts” denotes the number of lockages required to lock through a raft of barges (that is, two cuts means the load was split into two to pass through 

the lock).

Table 5. Velocity pulse characteristics recorded in the Chicago Sanitary and Ship Canal near Lemont, Illinois, associated with 

lockages at Lockport Lock, May 20, 2008 (14:00), to May 21, 2008 (15:00).

[CST, Central Standard Time; ft/s, foot per second; NA, not applicable]

Pulse 

number

Pulse start, 

CST

Pulse end, 

CST

Pulse 

duration, 

in minutes

Maximum 

pulse 

velocity, 

in ft/s

Associated 

lockage 

number

Pulse 

traveltime, 

in minutes

Lock 

loading1

included?

Adjusted 

pulse 

traveltime,2

in minutes

Comments

1 5/20/08 16:13 5/20/08 16:44 31.4 1.09 1 83.4 Yes 65.4 NA

2 5/20/08 19:37 5/20/08 20:00 23.0 1.30 3 44.8 No 44.8 NA

3 5/20/08 22:01 5/20/08 22:48 47.2 1.18 4 82.8 Yes 64.8 NA

4 5/20/08 23:21 5/20/08 23:55 33.9 .86 4 NA NA NA NA

5 5/21/08 02:01 5/21/08 02:31 30.2 .85 5 74.3 Yes 56.3 NA

6 5/21/08 05:17 5/21/08 05:51 33.9 .92 6 71.3 Yes 53.3 NA

7 5/21/08 06:37 5/21/08 07:42 65.3 1.52 6 NA NA NA NA

8 5/21/08 12:25 5/21/08 12:58 32.7 1.52 8 22.5 No 22.5 Lock prefilled?

Median NA NA 33.3 1.1 NA NA NA NA NA

1

Specifies whether the traveltime estimate includes the time to load the lock and close the gates. 

2

Adjusted traveltime removes 18 minutes from the traveltime estimate to account for lock loading (Pat Wharry, U.S. Army Corps of Engineers, 

oral commun., 2010).
Results 23

The pulse traveltime from Lockport to Lemont is consistent with shallow-water wave theory. The traveltime to 

reach Lemont for a lock pulse generated by filling an unoccupied lock chamber at Lockport is approximately 45 minutes 

(table 5, pulse 2), well within the range predicted by shallowwater wave theory. Lockage number 7 was not used in this 

analysis because it appears that the lock was prefilled prior to 

the arrival time of the vessel (the start-of-lockage time appears 

to be stamped after the fill began, as supported by the very 

short pulse traveltime). For the case of a pulse generated by 

the filling of an occupied lock chamber, the start-of-lockage 

time is assigned when a vessel is permitted to enter the lock, 

and the filling of the lock starts after the vessel is secure in 

the lock and the gates are closed. The typical time required 

for this process is 15–20 minutes (Pat Wharry, Lockport Lock 

Master, oral commun., 2010). Therefore, the pulse traveltimes 

for these lockages were adjusted by the 18 minutes to remove 

the time prior to the start of lock filling. The adjusted pulse 

traveltimes for a pulse generated by filling an occupied lock 

range from 53 to 65 minutes, with a mean of 60 minutes, and 

are slightly longer than that predicted by shallow-water wave 

theory. These differences likely are owing to a combination of 

uncertainty about the exact time the lock began to fill and possibly a deviation from shallow-water wave theory.

Reverse Flows

The 1-minute H-ADCP data and 2-minute AVM data 

for the period November 2006–January 2010 were averaged over 10-minute periods, and reverse flows during each 

10-minute observation were identified in each record. Table 6 

shows the results of this comparison, including the percentage of observations with reverse flow in all cells/paths and in 

each individual cell/path, the median and maximum negative 

velocities observed during reversals, and the maximum period 

of sustained reverse flow.

Flow reversals occur in the CSSC near Lemont, Ill., but 

they only occur a very small percentage of the time. Data indicate that full channel flow reversals occur less than 0.1 percent 

of the time near Lemont (table 6). However, partial flow 

reversals (reversal flow in part of the cross section) occur more 

frequently. With the exception of the left bank of the canal, 

partial flow reversals generally occur between 0.13 percent 

and 0.48 percent of the time. Flow reversals on the left bank 

occurred 1.4 percent of the time during the observation period.

Table 6. Flow-reversal observations in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–January 2010.

[Based on 10-minute averaged velocity data; ft/s, foot per second; AVM, acoustic velocity meter; H-ADCP, horizontal 

acoustic Doppler current profiler]

Instrument Path/cell

Percent of 

observations with 

reverse flow

Median velocity 

during reversals, 

in ft/s

Maximum 

negative velocity 

during reversals, 

in ft/s

Maximum period 

of flow reversal, 

in minutes

AVM All 0.07 −0.04 −0.24 80

1 .13 −.04 −.24 160

2 .23 −.04 −.22 90

3 .32 −.03 −.25 160

H-ADCP All .08 −.04 −.56 70

1 .46 −.05 −.57 120

2 .41 −.05 −.51 150

3 .39 −.05 −.50 150

4 .41 −.05 −.45 170

5 .44 −.05 −.67 180

6 .42 −.04 −.52 180

7 .48 −.04 −.84 110

8 .66 −.02 −.64 190

9 1.4 −.01 −.33 200
24 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Flow reversals in the CSSC near Lemont, Ill., occur 

more regularly near the streambed and near the left bank. 

Table 6 shows that the AVM records indicate almost 3 times 

the number of flow reversals in path 3 (nearest the bed) than in 

path 1 (nearest the surface). It is important to note that partial 

flow reversals near the bed may be missed in index velocity 

measurements with the H-ADCP because of the lack of nearbed velocity measurements. However, given their relatively 

low occurrence rate and low magnitudes, it is unlikely that 

missed near-bed partial flow reversals will lead to large errors 

in computed discharge. Table 6 also shows that the H-ADCP 

records indicate more than 3 times the flow reversals in cell 9 

(nearest the left bank) than in cell 1 (nearest the right bank). 

Both cells 8 and 9 indicate that flow reversals are more common on the left bank of the channel. 

Flow reversals near Lemont are generally small in 

magnitude. Median values of flow velocity range from −0.01 

to −0.05 ft/s; however, individual 10-minute averages range 

from −0.22 to −0.84 ft/s. It should be noted that the maximum 

reverse flow velocity observed by the H-ADCP is more than 

twice that observed by the AVM for path 2. This difference 

may be owing to the higher noise levels and smaller sampling 

volume in the H-ADCP measurements compared to the AVM 

measurements. 

The maximum values of reverse flow generally occurred 

in periods of sustained reverse flow, thus allowing periods of 

upstream transport. Reverse-flow periods ranged from 90 to 

200 minutes in individual cells/paths, whereas full-cross-section reversals occurred for periods of 70 to 80 minutes. Such 

reversals could have implications for transport of contaminants and biological constituents upstream. 

Stratification and Salinity Variation

Stratification of the water column can lead to densitydriven flows in the CAWS (Jackson and others, 2008). Such 

stratified flows in the CAWS arise primarily from differences 

in the temperature and salinity (owing to road-salt runoff) over 

the depth. In a branched system such as the CAWS, variable 

flow depths and numerous inflows can cause significant differences in density between branches, thus leading to the formation of density currents (Jackson and others, 2008). Index 

velocity measurements can be susceptible to errors in stratified 

flows and flows with variable salinity. Changes in salinity 

cause a change in the speed of sound, a fundamental property 

used by all acoustic meters. Stratified conditions can significantly change the velocity distribution in the water column 

by generating complex or bidirectional flows, leading to large 

errors in index velocity measurements (Jackson and others, 

2008). In addition, ray bending can occur as sound emitted 

by the acoustic instrument passes through a strong stratification, altering its direction of travel and leading to contamination or loss of data. Salinity variations are common in tidal 

locations and are accounted for by using a mean salinity for 

speed-of-sound computations (Levesque and Oberg, in press). 

In general, ADCPs must account for salinity to accurately 

measure water velocities. The AVM is a traveltime meter and 

uses the difference in traveltime for a sound pulse moving 

with the current and against the current. In general, an AVM is 

not susceptible to errors caused by variable salinity provided 

the speed of sound does not change significantly between the 

two pulses. 

Data from a thermistor and conductivity string deployed 

by the USGS in the CSSC near Lemont from March 2009 to 

May 2010 reveal that the canal does stratify in this reach. The 

primary contributor to the stratification is temperature, the 

largest temperature stratification occurring between September 

and April during sustained periods of low flow. During these 

months, the temperature difference between the streambed and 

the water surface can be greater than 5°C and is likely a result 

of wastewater inflows and discharges of cooling water from 

power generation plants in the system upstream. The Calumet 

Sag Channel upstream of Lemont may be also causing periods 

of stratified flow at Lemont (upstream-propagating density 

currents were documented by the authors just upstream of 

the confluence of the Calumet Sag Channel and the CSSC in 

December 2010). During the rest of the year, the temperature 

difference in the water column is generally less than 0.2°C. 

By comparison, the flow shows little overall stratification with 

respect to salinity, with typical changes over the water column 

of approximately 0.025 ppt for most of the year. However, the 

overall salinity does increase dramatically during the winter 

months in this system, with salinities approaching 2 ppt (and 

specific conductance values greater than 3,500 μS/cm) following winter storm events, a consequence of road-salt runoff 

(Jackson and others, 2008). In general, high-flow events break 

the stratification, and the entire water column has a uniform 

temperature and salinity for a period following high flows.

To estimate the potential error caused by not accounting 

for salinity variations in index velocity measurements in the 

CSSC near Lemont, BM-ADCP discharge measurements collected during a period of elevated salinity were recomputed to 

account for a salinity of 1 ppt and 2 ppt rather than zero. For 

the two measurements analyzed, this resulted in an increase 

in discharge of 0.23 and 0.47 percent for the 1 ppt and 2 ppt 

increase in salinity, respectively. In addition, the average 

velocities for each measurement with and without the salinity 

correction were compared and found to have a mean increase 

of 0.1 to 0.5 percent when accounting for a salinity of 1 ppt. 
Results 25

Therefore, owing to unaccounted-for changes in salinity in the 

CSSC, the H-ADCP can be expected to be biased low by less 

than about 0.5 percent during periods of high salinity, which 

typically occur in the winter following storm events (Jackson and others, 2008). The discharge measured by the AVM 

should not be affected by the salinity change, owing to the 

principles of operation of the AVM. A comparison of 480 daily 

mean discharges between March 15, 2009, and December 31, 

2010, computed by using the AVM (not affected by salinity 

variation) and H-ADCP (affected by salinity variation) found 

that the difference in computed discharge (QH-ADCP – QAVM) 

was weakly correlated (r = 0.31) with specific conductance 

measured just below the depth of the H-ADCP (approximately 560.8 ft NAVD 88). This weak, positive correlation is 

opposite in sign to that predicted by correction of discharge 

measurements for salinity increases; therefore, it is likely that 

this weak correlation is owing to seasonal changes in canal 

operation (discretionary diversion of Lake Michigan water 

ceases during the winter) rather than variations in specific 

conductance. 

Velocity Profiles 

Vertical 

Vertical velocity profiles in the CSSC near Lemont, Ill., 

were analyzed for three different temporal scales: 15-minute 

average profiles from a BM-ADCP, a 26-hour average profile 

obtained from an up-looking ADCP, and long-term average 

profiles from approximately 4 years of data from the AVM. 

Investigating the vertical flow profile at this range of scales 

allows one to look past effects from turbulence and complex 

flow structure and effects from the highly variable flows seen 

in the CSSC near Lemont, Ill. 

For this analysis, near-instantaneous flow profiles were 

obtained near the center point of each cell of the H-ADCP by 

using a BM-ADCP on May 20, 2008. This experiment yielded 

15-minute, temporally averaged flow profiles at nine points 

across the channel in the CSSC near Lemont. In addition, data 

from the up-looking ADCP deployed for a 26-hour period 

during May 20–21, 2008, was averaged and used to define a 

short-term, temporally averaged vertical velocity profile. To 

define a long-term, temporally averaged flow profile, velocity 

data in all three paths of the AVM near Lemont were averaged 

for the period December 2005 to February 2010. In addition 

to averaging all the observed values for each path, the velocity 

data were grouped into three categories based on the path 2 

velocity (< 1.0 ft/s, 1.0 to 3.0 ft/s, and > 3.0 ft/s) and averaged 

within each velocity range (grouping approximately follows 

the primary ranges of velocity observed at the site as, shown in 

figs. 7 and 8). In all cases, the vertical flow profiles were compared to the logarithmic and generalized power law models for 

velocity distribution in open channels (Chen, 1991). Both the 

one-sixth power law and generalized, variable exponent power 

law were compared to the observed data (table 7). The onesixth power law is equivalent to Manning’s equation for mean 

velocity in a uniform flow (Chen, 1991). 

Near-instantaneous vertical velocity profiles can vary 

significantly across the channel near Lemont. Figure 14 shows 

the 15-minute average velocity profiles obtained with a BMADCP on a tag line. In spite of averaging nearly 1,500 individual velocity profiles at each location, profiles at many sections 

show evidence of turbulence and complex flows and fit poorly 

with either the log law or the power law. These poor fits lead 

to great variability in the shear velocity, u*, and roughness 

length, z0 (table 7). The flow approaches a more logarithmic 

profile near the center of the channel (cells 4 and 5) and the 

left bank (cells 8 and 9). Much of the flow variability can be 

attributed to unsteadiness and barge traffic. During these measurements, a flood wave passed through the CSSC at Lemont, 

causing the discharge to nearly triple (1,225 to 3,490 ft3/s) and 

then drop again to 1,500 ft3/s. Smaller fluctuations of 500 to 

1,000 ft3/s were superimposed on this flood wave. The AVM 

path velocities showed a nonuniform response to this flood 

wave. Path 3 of the AVM (nearest the bed) saw a fourfold 

increase in velocity during the flood wave, whereas path 1 saw 

a twofold increase in velocity. In addition, two barges passed 

the site during these measurements, and data collection was 

suspended for a period before and after barge passage. However, the effect of the downstream-bound barges can be seen 

on the flow profiles in cells 1, 2, 6, and 7 (a barge passed the 

site after measurements in cell 1 and cell 6). Deceleration also 

is evident at the top of each of the measured profiles. Although 

winds from the northwest at 10–15 mi/h were observed in the 

area during the measurements, field notes recorded a observation of “light upstream wind” during the measurements. Flow 

disturbance owing to the ADCP and manned boat can cause 

flow deceleration near the surface. Mueller and others (2007) 

showed that flow disturbance from just the ADCP can extend 

beyond the blanking distance of the instrument and contaminate velocity data closest to the unit. The deceleration seen in 

the measured profiles near the surface is believed to be primarily owing to a combination of wind and disturbance from the 

manned boat.
26 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Table 7. Vertical velocity profile properties for a range of flow conditions in the Chicago Sanitary and Ship Canal near Lemont and 

Romeoville, Illinois.

[ft/s, foor per second; u, streamwise velocity, in ft/s; u*, shear velocity; κ, Von Karman constant; z, height above the bed, in feet (ft); z0, roughness length; 

R2, coefficient of determination; a, power law coefficient; n, power law exponent; CP, Chen product CP, = κ*a*n*e; e, base of natural logarithms; “Position” is 

the location of the measured profile in the channel; BM-ADCP, boat mounted acoustic Doppler current profiler; AVM, acoustic velocity meter; 

--, not applicable; <, less than; >, greater than]

Instrument Position

Mean 

velocity, 

in ft/s

Log law, u/u* = 

1/κ ln(z/z0) 

Power law, 

u/u* = a(z/z0)

n

Power law, 

u/u* = a(z/z0)

1/6

u*, 

in ft/s

z0, 

in ft R2 a n R2 CP a n R2 CP

BM-ADCP1 Cell 1 0.42 0.099 1.828 0.876 1.32 0.634 0.857 0.933 3.25 0.167 0.426 0.603

Cell 2 .45 .060 .507 .939 2.44 .358 .934 .973 4.49 .167 .691 .833

Cell 3 .36 .029 .069 .875 4.29 .207 .859 .990 5.29 .167 .830 .982

Cell 4 .80 .062 .053 .973 4.51 .197 .966 .991 5.32 .167 .945 .989

Cell 5 .69 .099 .623 .963 2.18 .397 .992 .963 4.36 .167 .687 .809

Cell 6 .57 .100 1.071 .919 1.62 .519 .982 .935 3.89 .167 .569 .723

Cell 7 .43 .027 .015 .914 5.85 .153 .885 .995 5.32 .167 .879 .989

Cell 8 .47 .036 .050 .952 4.71 .189 .934 .994 5.34 .167 .922 .991

Cell 9 .38 .042 .265 .983 3.12 .283 .967 .984 4.89 .167 .818 .908

Up-looking 

ADCP

Center .54 .047 .097 .991 3.96 .225 .997 .992 5.22 .167 .935 .970

AVM -- .75 .053 .040 .994 5.09 .176 .997 .998 5.37 .167 .994 .998

< 1.0 .047 .058 .995 4.76 .188 .998 .998 5.35 .167 .985 .993

1 to 3 .064 .004 .994 7.29 .123 .996 .999 5.09 .167 .873 .945

> 3.0 .199 .006 .994 6.80 .132 .991 .999 5.21 .167 .923 .968

Romeoville 

ADCP2

Center .95 .066 .030 -- 4.88 .182 -- -- 5.35 .167 -- --

2.23 .151 .025 -- 5.17 .173 -- -- 5.35 .167 -- --

1

Velocity and discharge were highly unsteady during measurements. 

2

From González and others (1996).
Results 27

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

0

5

10

15

20

25

-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5

0 0.2 0.4 0.6 0.8 1

0

5

10

15

20

25

Height above bottom, in feet

-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

0 0.1 0.2 0.3 0.4 0.5

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Velocity, in feet per second

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

Cell 1 Cell 2

Cell 4 Cell 5 Cell 6

Cell 8 Cell 9

Cell 3

Cell 7

Observed

Logarithmic

Power law (exponent = 1/6)

Power law (exponent = n)

EXPLANATION

Figure 14. Fifteen-minute average vertical velocity profiles measured near the center of each of the nine cells of the horizontal 

acoustic Doppler current profiler (H-ADCP) by using a boat-mounted acoustic Doppler current profiler (BM-ADCP) in the Chicago 

Sanitary and Ship Canal near Lemont, Illinois, May 20, 2008. 

The short-term average flow profile measured by the 

up-looking ADCP in the CSSC near Lemont, Ill., during the 

26-hour deployment in 2008 fits a logarithmic profile with 

slight deviation near the boundaries (fig. 15). The fitting 

parameters, including the shear velocity and roughness length, 

are given in table 7. The roughness length, z0, is considerably 

larger than that measured by a BM-ADCP in 1994 and 1995 in 

the CSSC at Romeoville, Ill., by Gonzales and others (1996). 

It is also larger than values obtained from fitting 15-minute 

averaged data in cells 4 and 8 (two of the best log law fits). 

The increase in roughness length is likely owing to a improper 

fit of the near-bed data by the log law (fitting only the lower 10 

percent of the data results in u* = 0.032 ft/s and z0 = 0.017 ft). 

Like the May 20, 2008, BM-ADCP data, this 26-hour uplooking ADCP dataset includes highly unsteady flow. During 

the measurement period, seven flood waves passed through the 

CSSC at Lemont with at least 2,000 ft3

/s increase in flow rate. 

One flood pulse consisted of a 4,500 ft3

/s increase (250 ft3/s to 

4,710 ft3/s) in just over 3 hours. Although discharge averaged 

approximately 2,000 ft3/s during the measurement period, the 

range of discharges observed ranged from −50 to 4,710 ft3/s. 

Although much of the variability in shear velocity and roughness length is likely owing to unsteadiness, a combination of 

the increased bed roughness created by sloughed banks near 

Lemont and the drag created by the up-looking ADCP and 

mount sitting on the bottom could also lead to an increased 

roughness length. An ADCP without the up-looking mount 

is capable of producing a flow disturbance out to 1.6 ft from 

the instrument under the range of flows observed during this 

deployment (Mueller and others, 2007). Unlike the profiles 

measured at Romeoville, Ill., which fit the one-sixth power 

law, the one-sixth power law fails to fit the profile from the 

up-looking ADCP near Lemont, Ill. The profile near Lemont is 

closer to a one-fifth (or even one-fourth) power law based on 

26 hours of velocity data. 
28 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

5

10

15

20

25

Mean height above bottom, in feet

Mean velocity, in feet per second

Observed

Logarithmic

Power law (exponent = 1/6)

Power law (exponent = n)

EXPLANATION

Figure 15. Twenty-six-hour average vertical velocity profile measured by using an up-looking, bottom-mounted acoustic 

Doppler current profiler in the Chicago Sanitary and Ship Canal near Lemont, Illinois, May 20–21, 2008. 

Although instantaneous and short-term averaged vertical 

velocity profiles may deviate from a logarithmic profile (as 

indicated by boat-mounted and up-looking ADCP data), the 

long-term time-averaged data indicate the vertical velocity 

profile in the CSSC near Lemont, Ill., is a logarithmic distribution under a range of flow conditions (fig. 16). In addition, figure 16 shows the averaged data fit with a one-sixth power law 

primarily for low velocities (< 1 ft/s). Higher flow velocities 

exhibit a deviation for the one-sixth power law yet continue to 

fit a logarithmic distribution. On average, power law exponents approach a one-eighth power law at velocities greater 

than 1.0 ft/s, though only three points are fitted in the profile 

and there are far fewer observations at high flows. 

Transverse 

Although the multipath AVM near Lemont can describe 

the vertical velocity distribution to some extent, it cannot yield 

any information about the transverse velocity distribution. 

However, the multi-cell configuration of the H-ADCP can 

describe the transverse velocity profile near middepth in the 

CSSC near Lemont, Ill., for a wide range of flow conditions. 

Data from the H-ADCP for the period November 2006–

January 2010 were used to define the average transverse 

velocity profile for a range of flows. The data were grouped 

according to index velocity range (that is, nine-cell average) 

and temporally averaged within each respective cell for the 

entire record. In addition, ADCP data collected from a boatmounted ADCP during discharge measurements with GPS 

data were extracted, mapped to the fixed measurement cross 

section, depth averaged, and spatially averaged within each 

of the nine cells defined by the configuration of the H-ADCP. 

To ensure accurate positioning of the data within the cross 

section, those measurements collected at nearby cross sections 

and those without GPS data were omitted from this analysis. 
Results 29

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

0

5

10

15

20

25

30

Height above bottom, in feet

Mean velocity, in feet per second (ft/s)

AVM path 1

AVM path 2

AVM path 3

Vpath 2 < 1.0 ft/s

1.0 ft/s >= Vpath 2 <= 3.0 ft/s Vpath 2 > 3.0 ft/s

Free surface

EXPLANATION

Logarithmic

Power law (exponent = 1/6)

Power law (exponent = n)

Observed

All data

Less than

Greater than

<

>

AVM Acoustic velocity meter

Figure 16. Four-year average vertical velocity profiles measured by the three paths of the acoustic velocity meter (AVM) in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois, December 2005 to February 2010. Data are grouped according to path 2 

velocity range, and line types are as defined in figure 15. 

Of the 61 discharge measurements collected near Lemont, Ill., 

before April 2010, 29 measurements met the above criteria 

and were used in this analysis. For this analysis, the velocity 

data have been normalized by the maximum velocity in the 

cross section, allowing a collapse of the wide range of flow 

magnitudes near Lemont. 

In general, the H-ADCP data show that the transverse 

flow distribution near Lemont is skewed to the right bank 

(looking downstream) for flows less than 3.0 ft/s and skewed 

to the left bank for flows greater than 3.0 ft/s (fig. 17). As the 

flow velocity increases in the cross section near Lemont, the 

velocity on the left bank increases by as much as 60 percent, 

whereas the flow velocity on the right bank decreases by about 

3 percent. This skew to the left bank at high velocities is likely 

related to the channel curvature near Lemont, which would 

generate higher flows to the outside of the bend. In general, 

the maximum velocities occur in cells 3 and 4 at low flows 

and cells 5 and 6 at high flows. 

The sharp change in the velocity in cell 9 immediately 

adjacent to the left bank deserves further discussion. Data 

from the H-ADCP show a 40-percent decrease in flow velocity 

between cell 8 and cell 9 and a deviation from the standard 

beta distribution fit for transverse velocity profiles (Seo and 

Baek, 2004). There are several possible reasons for this discontinuity. First, impingement of the acoustic beam on the far 

wall, the bed, or contamination by side-lobe interference could 

be creating a low bias in cell 9. However, the far edge of cell 9 

is 12 ft from the left bank and is outside of the 10-ft side-lobe 

zone (6 percent of channel width). Cell 9 is also 9.0 ft from the 

bottom at the far bank, so beam impingement on the bottom 

is unlikely. Also, there is no evidence of abrupt signal attenuation in cell 9 during instrument beam checks. Although past 

beam check data do not support side-lobe interference from 

the bed or the water surface, it is possible that the interference 

is subtle and may not be recognized in the data. More rigorous 

analysis of the beam-check data will be required in the future 

at this site. 
30 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

160 140 120 100 80 60 40 20 0

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Distance from right bank, in feet

Normalized velocity

Cell 9 Cell 8 Cell 7 Cell 6 Cell 5 Cell 4 Cell 3 Cell 2 Cell 1

Horizontal acoustic Doppler 

 current profiler (H-ADCP) (all)

H-ADCP (less than 1 foot

 per second (< 1 ft/s))

H-ADCP (1 to 3 ft/s)

H-ADCP (> 3 ft/s)

Boat mounted acoustic Doppler 

 current profiler (BM-ADCP) (all)

BM-ADCP (< 1 ft/s)

BM-ADCP (1 to 3 ft/s)

BM-ADCP (> 3 ft/s)

8-cell beta distribution fit

9-cell beta distribution fit

EXPLANATION

Figure 17. Transverse normalized velocity profiles in the Chicago Sanitary and Ship Canal near Lemont, Illinois, as measured by the 

horizontal acoustic Doppler current profiler (H-ADCP) and boat-mounted acoustic Doppler current profiler (BM-ADCP) for a range of 

flow velocities. 

Another possible explanation for lower velocities in cell 9 

is the proximity to the canal wall. Cell 9 is 3 ft closer to the 

boundary (12 ft) than cell 1, which is 15 ft from the right bank. 

The no-slip condition at the bank requires a sharp decrease in 

the velocity near the banks, and cell 9 may be showing an artifact of this flow deceleration. However, the 29 measurements 

made with a boat-mounted ADCP at the site do not show (on 

average) this flow deceleration in cell 9. A third explanation 

for the discontinuity would be the influence of the Calumet Sag Channel confluence upstream. Low flows from the 

Calumet Sag Channel, which joins the left bank of the CSSC 

0.7 mi upstream of Lemont, could be hugging the left bank, 

generating a shear layer and causing a sharp decrease in flow 

velocity. Although time-averaged boat-mounted discharge 

measurements do not reflect this discontinuity, several individual measurements have shown a sharp 45- to 65-percent 

decrease in velocity near the left bank. The abrupt decrease in 

velocity is associated with an increase in backscatter, possibly 

indicating the presence of a second water mass made up of 

Calumet Sag Channel water hugging the left bank (aerial photos show the Calumet Sag Channel water is often more turbid 

than CSSC water, owing to shallower depth and input from 

numerous tributaries). Finally, located between the diverging beams of the H-ADCP is a mound on the bed near the left 

bank that is entirely within the width of cell 9 (see fig. 2). It 

is possible that this mound, which is 3.8 ft off the otherwise 

uniform bottom, could be causing a flow disturbance within 

the measurement volume of the H-ADCP cell 9 and causing 

the low bias. Nevertheless, the likely cause for this discontinuity in cell 9 is likely a combination of the above factors that 

manifest themselves under conditions that are not accurately 

captured in discharge measurements.

Comparison of Instrument Performance 

This section presents comparisons of instrument performance with regard to index velocity measurements. This 

analysis includes a comparison of instrument reliability with 

a discussion of missing record, a comparison of velocity 

statistics including variance in the path and cell velocities, and 

a comparison of the most recent ratings curves developed for 

these instruments. 
Results 31

Instrument Reliability

The USGS streamgage on the CSSC near Lemont, Ill., 

must be accurate and reliable. Periods of missing data occur 

at all streamgages, but the importance of this gage for diversion accounting dictates that missing data must be minimized 

and measurement redundancy must be provided for. “Missing 

data” is defined as data that are lost because of an instrument 

malfunction or service outage (for example a power failure) 

or are marked invalid. “Invalid data” is defined as velocity 

data that are collected while the instrument is operational, but 

the data have been marked as invalid during internal instrument checks (for example, turbulence from a barge may cause 

correlations to fall below a predefined threshold set by the 

manufacturer). Data are considered valid if the instrument is 

functioning properly and velocity data have passed all internal 

instrument checks. 

Since the installation of the gage near Lemont, the AVM 

has been the primary index velocity meter and the H-ADCP 

has been the backup. The remainder of this section examines 

the data records from these two instruments and compares 

their reliability for the period November 2006–December 2010. Velocity data (10-minute data) were used in this 

analysis. Missing data occur when either an invalid value 

(defined by each instrument for specific configuration settings) 

or no value is reported for a path/cell of the instrument during 

a sampling interval. Therefore, the percentage of missing data 

for each instrument includes both invalid velocity data and 

lost data from when the instrument was out of service. This 

is particularly important to note for the AVM, which was out 

of service from January 7, 2008, to May 20, 2008, because 

of severed cross-channel cables that had to be replaced by 

divers (requiring a service contract that delayed the repair); 

from September 7, 2009, to December 15, 2009, because of an 

equipment failure and unavailability of replacement parts; and 

from June 23, 2010, to July 9, 2010, because of a power-supply issue with the AVM. The only significant period of missing 

record for the H-ADCP was from July 12, 2007, to July 25, 

2007, during an equipment malfunction. 

The AVM has a substantially higher percentage of missing record compared to the H-ADCP over the same period 

from November 2006–December 2010 (table 8). During this 

period, the AVM returned no valid data on any path 16.7 

percent of the time compared to 0.89 percent invalid data for 

the H-ADCP. The primary reason for this large difference is 

the time the AVM was out of service because of an instrument 

failure from severed cross-channel cables. If one considers 

only the time during which the instrument was operational, the 

percentage of missing data drops considerably. Table 8 lists the 

percentage of missing data by AVM path and H-ADCP cell. 

Overall, during the time the instruments were operational, the 

AVM had invalid data on path 1 approximately 19.8 percent of 

the time, whereas it had invalid on paths 2 and 3 less than 0.03 

percent of the time. This finding indicates that path 1 is highly 

susceptible to erroneous data, likely produced by barge/tow 

traffic, but that these disturbances do not invalidate velocities 

on paths 2 and 3. During a 26-hour period in May 2008, there 

were 23 barge/tow passages through the measurement reach, 

resulting in approximately 30 percent invalid data on path 1 

(an average of 20 minutes lost per vessel). The AVM path 2 

percentage of invalid data (during instrument operation) of 

0.01 percent is very close to the average percentage of invalid 

data (during instrument operation) for all cells of the H-ADCP 

of 0.03 percent (found by omitting dates of instrument failure 

in July 2007). This finding indicates that the AVM path 2 and 

the H-ADCP located at the same elevation in the water column 

are equally reliable on average. With regard to individual path/

cell reliability, the most reliable path of the AVM is path 2, and 

the least reliable is path 1. Cell 6 proved to be the most reliable for the H-ADCP, and cell 1 was the least reliable. 

Table 8. Comparison of missing data by individual path/cell for 

the two index velocity meters in the Chicago Sanitary and Ship 

Canal near Lemont, Illinois, November 2006–December 2010. 

[AVM, acoustic velocity meter; H-ADCP, horizontal acoustic Doppler current 

profiler]

Instrument Path/cell

Percent 

missing 

(total)

Percent 

invalid 

(with 1 or 

more valid 

path/cell)

Percent lost 

(all paths/

cells 

missing)

AVM 1 36.5 19.8 16.7

2 16.8 .01 16.7

3 16.8 .02 16.7

H-ADCP 1 .99 .10 .89

2 .97 .08 .89

3 .97 .08 .89

4 .97 .08 .89

5 .96 .07 .89

6 .93 .04 .89

7 .95 .05 .89

8 .98 .09 .89

9 .95 .06 .89
32 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Analysis of the instrument records by number of valid 

paths for the AVM and cell for the H-ADCP reveals that the 

AVM reports valid data on all paths only 63.5 percent of 

the time compared to valid data for all cells of the H-ADCP 

98.7 percent of the time (table 9). The primary reasons for 

this difference is the large period of missing data for the AVM 

during service outages and the contamination of path 1 by 

vessel traffic. The comparison also shows that loss of data 

on two paths of the AVM is rare (occurs only 0.03 percent of 

the time). Loss of one cell on the H-ADCP is most common 

(occurs 0.32 percent of the time), and the likelihood of missing 

more than one cell decreases with increasing number of cells 

lost (with the exception of all nine cells invalid). This result 

correlates well with the 6-day period in June 2009, where 

impacts of barge/tow passages on the H-ADCP were typically 

limited to contamination in only one cell. For the H-ADCP, it 

is relatively rare to have partially valid data. Generally, either 

all the cells are valid or invalid for a sampling period. 

AVM and 1-minute data for the H-ADCP) for each for the 

instruments were used to compute 10-minute averages. Basic 

statistics were then computed for each instrument record, and 

the results are given in table 10. 

The minimum and maximum velocities observed for each 

instrument compare well, but there are subtle differences. The 

minimum velocities (all negative or upstream) recorded by 

the AVM are generally consistent across all three paths, with 

a slight increase in negative flow near the bed. However, the 

H-ADCP shows significant variability in minimum (upstream) 

velocities across the cross section. The highest upstream flows 

occur in cells 5, 7, and 8, indicating flow reversals are more 

pronounced toward the left bank (however, cell 9 shows the 

lowest negative flow). Averaging across all nine cells results in 

a mean minimum velocity of −0.21 ft/s which compares very 

well with the minimum velocity observed on path 2 of the 

AVM (−0.22 ft/s). The maximum observed velocities do not 

compare as well between instruments. The H-ADCP nine-cell 

average maximum velocity is almost 4 percent higher than the 

maximum velocity observed on path 2 of the AVM. The higher 

H-ADCP maximum velocity is likely owing to the fact that 

data near the banks are not measured with the H-ADCP (lower 

flows near the banks would lower the mean velocity). Finally, 

the highest velocities occur near the surface (as expected) and 

closer to the left bank at high flows, as previously discussed in 

the velocity profiles section.

The median observed velocities for each instrument 

show a skew in the flow to the right bank and a 10.1 percent 

higher median velocity for path 2 of the AVM compared to 

the H-ADCP. The bias of the flow to the right bank on average was discussed in the velocity profiles section and may be 

owing to influence from the confluence with the Calumet Sag 

Channel upstream and other factors. The 10.1-percent difference in median flow velocity for the two instruments at the 

same elevation in the water column is counterintuitive; one 

would expect that the H-ADCP would have a greater median 

velocity than AVM path 2 because the nine-cell average does 

not include data from unmeasured zones near the banks, where 

the velocity is lower. One possible explanation for the greater 

median flow velocity registered by the AVM is the reduced 

cross-sectional area in the AVM cross section compared to the 

H-ADCP cross section owing to a sloughed bank. Figure 2 

shows the sloughed bank just downstream of the upstream 

notch in the canal wall on the right bank. By using bathymetry collected by the USGS and University of Illinois in 2010, 

cross sections were extracted at the upstream notch and halfway between the upstream and downstream notches. There is 

a 13.7 percent decrease in cross-sectional area within the AVM 

measurement volume between the notches. This reduction in 

flow area is capable of producing an increase in velocity magnitude in the AVM measurement volume that is proportional 

to the observed difference in the median velocities for the two 

instruments. 

Table 9. Comparison of missing data by number of 

paths/cells for the two index velocity meters in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 2006–December 2010.

[AVM, acoustic velocity meter; H-ADCP, horizontal acoustic Doppler 

current profiler]

Instrument

Number of 

missing 

paths/cells

Percent of 

observations 

missing 

AVM 0 63.5

1 19.7

2 .03

3 16.7

H-ADCP 0 98.66

1 .32

2 .08

3 .03

4 .01

5 .005

6 .002

7 .001

8 .0

9 .89

Velocity Statistics

The statistics of the observed velocity records for each 

path of the AVM and each cell of the H-ADCP can provide 

insight into instrument performance. For November 2006–

January 2010, the velocity data (2-minute data for the 
Results 33

Table 10. Comparison of velocity statistics by individual path/cell for the two index velocity meters 

in the Chicago Sanitary and Ship Canal near Lemont, Illinois, November 2006–January 2010.

[ft/s, foot per second; AVM, acoustic velocity meter; H-ADCP, horizontal acoustic Doppler current profiler]

Instrument Path/cell

Minimum 

observed 

velocity, 

in ft/s

Maximum 

observed 

velocity, 

in ft/s

Median 

observed 

velocity, 

in ft/s

Velocity 

variance1

, 

in (ft/s)2

AVM 1 −0.24 5.06 0.80 0.0413

2 −.22 4.81 .76 .0450

3 −.25 4.47 .70 .0457

H-ADCP 1 −.57 4.56 .72 .1964

2 −.51 4.85 .76 .1972

3 −.50 5.10 .78 .2036

4 −.45 5.19 .78 .2100

5 −.67 5.42 .76 .2103

6 −.52 5.59 .72 .2024

7 −.84 5.54 .68 .1940

8 −.64 5.17 .62 .1848

9 −.33 4.16 .39 .1523

9-cell 

average

−.21 5.00 .69 .1251

1

Mean value of variance computed over 10-minute averaging intervals.

the meter. Theoretically, random errors should decrease proportional to ns

−0.5 , where ns

 is the number of samples. If each 

particle in the measurement volume is considered a sample 

of the velocity, then the random error should decrease with 

distance from the meter because of the larger number of particles in the sampling volume. However, this explanation does 

not account for the distribution of velocity variance across the 

channel and the sharp decrease in variance in cell 9. Therefore, the decrease in variance in cell 9 (and to a lesser extent 

cell 8) may be owing to less turbulence in the flow. This latter 

explanation supports the hypothesis that cell 9 resides primarily in Calumet Sag Channel water, which may hug the left 

bank under typical flow conditions and which possess a lower 

turbulence intensity (and higher backscatter) than the main 

flow in the CSSC. This hypothesis could be tested through a 

targeted study with a BM-ADCP. Another possible explanation for this lower variance in cell 9 is side-lobe interference. 

If the side lobe of the acoustic signal is impinging on the bed, 

then the variance may be lower because of the contamination 

of the velocity data in cell 9 with data from a stationary object 

(the bed). However, it is important to note that no side-lobe 

interference has been confirmed for the H-ADCP at the time of 

this publication. 

The mean variance in the 10-minute average velocity 

records is nearly 3 times higher for the nine-cell average of the 

H-ADCP than for path 2 of the AVM. This result implies that 

the H-ADCP has significantly more noise in the data than the 

AVM. Although the AVM has a larger sampling volume (about 

45 percent, owing to the 44.5-degree angle of the paths across 

the channel) and longer unit value averaging (2 minutes) compared to the H-ADCP (1 minute), this increased spatial and 

temporal averaging is likely not enough to account for this difference in variance. Further comparison is difficult because the 

instruments’ operation involves very different measurement 

techniques. The AVM uses a bulk flow measurement technique 

that is inherently less variable than the H-ADCP technique, in 

which bulk flows are reconstructed from velocities of individual particles in the water. Finally, the velocity variance is 

similar across all paths of the AVM and relatively uniform for 

all cells of the H-ADCP with the exception of cell 9 near the 

left bank (and to a lesser extent, cell 8). Cell 9 of the H-ADCP

showed a 24 percent lower variance than the mean variance 

of the other eight cells, indicating that the velocity in cell 9 

is less variable than that in the other cells. Instrument noise 

should be uniform across all the cells; however, in spite of the 

fact that the cell sizes are all equal, the 1.5-degree beam width 

results in an increase in sampling volume with distance from 
34 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Rating Curves 

The index velocity rating curves for the AVM and 

H-ADCP are linear in the velocity-velocity plane and do not 

appear to have a stage dependence. This relation is expected 

given the limited range of stage (approximately 573.7 to 

580.6 ft above NAVD 88). Figures 18 and 19 show the rating 

curves for the AVM and H-ADCP, respectively. The index 

velocity ratings for the AVM and H-ADCP as determined by 

linear regression are 

V V mean = − AVM 0. . 8914 0 0161 (5)

V V mean = + H A− DCP 0. . 8866 0 0239 (6)

where 

VAVM and VH-ADCP are the index velocities for the AVM and 

 H-ADCP, respectively. 

The coefficients of determination for the linear regressions 

are 0.9968 and 0.9955 for the AVM and H-ADCP ratings, 

respectively, and the standard error of the regression for the 

AVM is 0.045 compared to 0.051 for the H-ADCP. These 

regression statistics indicate both rating curves fit the observed 

data well, with a slightly better fit for the AVM rating. A total 

of 45 moving-boat ADCP discharge measurements collected 

between January 2005 and March 2010 were used to develop 

the AVM rating, whereas 42 measurements collected between 

June 2005 and March 2010 were used to define the H-ADCP 

rating. 

For the 33 measurements that are common to both the 

AVM and H-ADCP ratings, the rated discharges computed 

with both ratings are very close to equal. Figure 20 shows 

the rated discharges plotted against one another with a line 

of equality shown for comparison. Over the full range of the 

measurements, the instruments compute very similar rated 

discharges with a slight deviation at very low discharges. 

At low flows, the H-ADCP computes a slightly higher rated 

discharge than the AVM. On the basis of the data presented 

above, this difference is likely owing to the complications in 

measurements resulting from secondary flows that occur at the 

site primarily under low-flow conditions. 

Figure 18. Index velocity rating curve for the acoustic velocity meter (AVM) deployed in the Chicago Sanitary 

and Ship Canal near Lemont, Illinois. 

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

Measured velocity (Q/A ), in feet per second Rated

Acoustic velocity meter velocity, in feet per second

EXPLANATION

Rating-development measurements

Rating-verification measurements

Linear regression (rating-development measurements)

y = 0.8914x - 0.0161 2 R = 0.9968
Results 35

Figure 19. Index 

velocity rating curve 

for the horizontal 

acoustic Doppler 

current profiler 

(H-ADCP) deployed 

in the Chicago 

Sanitary and Ship 

Canal near Lemont, 

Illinois. 

Figure 20. Comparison of rated discharges for the acoustic velocity meter (AVM) and horizontal acoustic Doppler 

current profiler (H-ADCP) for 33 discharge measurements in the Chicago Sanitary and Ship Canal near Lemont, Illinois. 

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

Measured velocity (Q/A ), in feet per second Rated

Horizontal acoustic Doppler current profiler velocity, in feet per second 

EXPLANATION

Rating-development measurements

Rating-verification measurements

Linear regression (rating-development measurements)

y = 0.8866x + 0.0239 2 R = 0.9955

EXPLANATION

Rating-development measurements

Rating-verification measurements

X = Y

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Horizontal acoustic Doppler current profiler rated discharge, in cubic feet per second

Acoustic velocity meter rated discharge, in cubic feet per second
36 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

Differences from the measured discharge are greatest at 

low flows and least at high flows for both instruments (fig. 21). 

At low flows (< 700 ft3/s), the rated discharge for the H-ADCP 

is biased high and the rated discharge for the AVM is biased 

low. Low flows are inherently difficult to measure, owing to 

flow variability and instrument noise. On the basis of all the 

measurements used in the rating curve development, the AVM 

is biased high relative the measured discharge by 0.52 percent 

(median percent difference) and the H-ADCP is biased high by 

1.43 percent (median percent difference). Overall, the AVM is 

the more accurate gage, with an overall median error of about 

one-third that of the H-ADCP based on 45 measurements for 

the AVM and 42 for the H-ADCP. In addition, the variance of 

the errors for the H-ADCP is 41 percent higher than the error 

variance for the AVM, indicating a greater variability in the 

rated H-ADCP discharge with respect to the true discharge. In 

general, for flows in the range 1,000 to 5,000 ft3/s (the range 

most typical for the CSSC), most computed (instantaneous) 

discharges are within 10 percent of measured discharge. The 

percent errors in rated discharge for both instruments decrease 

with increasing discharge, and both instruments are within 2 

percent of the measured discharge for flows greater than 8,000 

ft3/s. It is important to note that these percent differences are 

for short-duration discharge measurements, which are subject to instrument noise and flow variability. These errors are 

distributed about zero with median values that are less than 1.5 

percent, and temporal averaging of the computed discharge at 

daily, monthly, and yearly timescales will reduce the error in 

the computed discharge. The next section compares computed 

discharge for both instruments at these timescales. 

For comparison purposes, the uncertainty of mean annual 

discharge computed by means of the AVM and H-ADCP have 

been assessed by using the methodology of Duncker and others (2006). Assuming no uncertainty in BM-ADCP measurements, the estimated uncertainties in mean annual discharge 

are 0.91 percent and 1.6 percent for the AVM and H-ADCP, 

respectively. If one assumes a 1-percent uncertainty in both the 

rating-curve slope and intercept (an attempt at including the 

unknown uncertainty of BM-ADCP measurements), the estimated uncertainties in annual mean discharge are 3.3 percent 

and 4.4 percent for the AVM and H-ADCP, respectively. 

Percent difference from observed discharge

−20.00

Observed discharge, in cubic feet per second

EXPLANATION

Acoustic velocity meter (AVM) (development measurement)

Horizontal acoustic Doppler current profiler (H-ADCP) (development measurement)

AVM (development measurement)

H-ADCP (development measurement)

A

−40.00

−60.00

80.00

60.00

40.00

0.00

20.00

100 1,000 10,000

Figure 21A. Errors in rated discharge relative to the measured discharge for the acoustic velocity meter (AVM) and 

horizontal acoustic Doppler current profiler (H-ADCP) deployed in the Chicago Sanitary and Ship Canal near Lemont, 

Illinois. 
Results 37

Observed discharge, in cubic feet per second

B EXPLANATION

Acoustic velocity meter (AVM) (development measurement)

Horizontal acoustic Doppler current profiler (H-ADCP) 

 (development measurement)

AVM (development measurement)

H-ADCP (development measurement)

100 1,000 10,000

−200

Difference from observed discharge, in cubic feet per second

−400

−600

800

600

400

0

200

Figure 21B. Residuals in rated discharge relative to the measured discharge for the acoustic velocity meter (AVM) and 

horizontal acoustic Doppler current profiler (H-ADCP) deployed in the Chicago Sanitary and Ship Canal near Lemont, 

Illinois. 

Computed Discharge

This section is dedicated to the comparison of computed 

discharge from the AVM and H-ADCP index velocity 

measurements in the CSSC near Lemont, Ill. Data used in 

this analysis include daily mean discharges for November 10, 

2006, to December 31, 2010. From the computed discharge 

records for each instrument, days with estimated values or 

flagged data in either record have been removed from both 

records. This process ensures that only days in which both 

instruments were operational are being compared. In addition 

to comparing daily mean discharges, the following analysis 

also compares the monthly and annual mean discharges 

computed by using the index velocity method for the two 

instruments. It is important to understand that, for this 

analysis, annual mean values are for calendar years and not 

water years (October 1 to September 30) and that the annual 

mean for 2006 is a partial mean value (November 10, 2006, 

to December 31, 2006). 

For the range of flows observed in the CSSC near 

Lemont, Ill., the AVM and H-ADCP mean daily, monthly, 

and annual computed discharges compare very well. Figure 22 is a scatterplot comparing the computed discharge data 

for both instruments at the three different time scales. For 

reference, a line of equal discharge also is plotted. The daily 

mean discharge data cloud lies along the line of equality with 

some scatter about the line. Above about 7,000 ft3/s, the data 

plot primarily to the right of the line of equality, indicating 

that flows above 7,000 ft3/s have a slightly lower computed 

discharge for the H-ADCP compared to the AVM. At flows 

above 14,000 ft3/s, the data conform more closely to the line 

of equality. Both the monthly and annual mean discharges 

compare well between the AVM and H-ADCP and show considerably less variability both in range of observed discharge 

and deviation from the line of equality. The remainder of this 

section is dedicated to a more quantitative comparison of 

the discharge records by comparing the discharge distributions from each instrument and the difference between daily, 

monthly, and annual mean discharge for each instrument. 
38 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

Horizontal acoustic Doppler current profiler computed discharge, in cubic feet per second

Acoustic velocity meter computed discharge, in cubic feet per second

EXPLANATION

Daily mean

Monthly mean

Annual mean

X = Y

Figure 22. Scatterplot of the computed discharge (daily, monthly, and annual mean) for the acoustic velocity meter (AVM) and 

horizontal acoustic Doppler current profiler (H-ADCP) deployed in the Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 10, 2006, to December 31, 2010. 

At a significance level of 5 percent, the distributions of 

daily, monthly, and annual mean discharges for the AVM and 

H-ADCP are not significantly different. At each time scale, 

the data distributions were compared by use of a two-sample 

Kolmogorov-Smirnov test and, in each case, the null hypothesis that the two samples come from the same continuous 

distribution was accepted at the 5-percent significant level. To 

further illustrate this equality in distributions, quantile-quantile 

plots are shown in figure 23 for each time scale. Data with 

equal distributions will fall along the Yquantile = Xquantile line. The 

data from the AVM and H-ADCP show no significant deviation from this line. The solid part of the line in each figure represents the region between the 25th and 75th percentiles. The 

distributions for both the daily and monthly mean discharges 

are highly skewed, whereas the distribution for annual mean 

discharge is symmetric (though only five data points are available for analysis).

Although differences between computed discharge 

for the AVM and H-ADCP are significantly different (in a 

statistical sense) for the daily mean values, differences at the 

monthly and annual time scales are not significantly different. Differences in computed discharge were computed for 

each pair of daily, monthly, and yearly mean values for the 

two instruments, plotted in boxplots (fig. 24), and compared 

by using a sign test. The sign test analyzes the differences and 

determines whether dataset 1 is generally larger (or smaller, or 

different) than dataset 2. At the 5-percent significance level, 

the test indicates that the computed daily mean discharges 

for the H-ADCP are lower than the AVM discharges (median 

difference of −59 ft3

/s). However, monthly and annual mean 

discharges computed for the H-ADCP are not statistically 

different from the AVM computed discharges when evaluated 

with the same sign test at the 5-percent significance level. 
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 

0

2,0004,0006,0008,000

10,00012,00014,00016,00018,000

A. Daily mean

1,000 2,000 3,000 4,000 5,000 6,000 

1,000

2,0003,0004,0005,0006,0007,000

Horizontal acoustic Doppler current profiler computed discharge quantiles, in cubic feet per second B. Monthly mean

2,600 2,700 2,800 2,900 3,000 3,100 3,200 3,300 3,400 

2,600

2,8003,0003,2003,4003,600

Acoustic velocity meter computed discharge quantiles, in cubic feet per second

C. Annual mean

Figure 23. Quantile-quantile plots of mean daily, monthly, and annual computed discharges for the acoustic velocity meter (AVM) and horizontcurrent profiler (H-ADCP) deployed in the Chicago Sanitary and Ship Canal near Lemont, Illinois, November 10, 2006, to December 31, 2010. 
40 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

-300

-200

-100

0

100

200

300

Daily mean

Difference in computed discharge, in cubic feet per second

Median = -59; 

n = 1,211

Monthly mean

Median = -35; 

n = 60

Lower quartile line, 25th percentile

10th percentile; 10 percent of values

 are less than the value indicated 

 at the endpoint of the line 

Outlier

90th percentile; 10 percent of values

 are greater than the value indicated

 at the endpoint of the line

Upper quartile line, 75th percentile

Median line, 50th percentile

Annual mean

Median = -34; 

n = 5

EXPLANATION

Figure 24. Boxplots of the difference in computed 

discharge (daily, monthly, and annual mean) for the 

acoustic velocity meter (AVM) and horizontal acoustic 

Doppler current profiler (H-ADCP) deployed in the 

Chicago Sanitary and Ship Canal near Lemont, Illinois, 

November 10, 2006, to December 31, 2010. 

Therefore, in spite of having a median differences of −35 ft3/s 

and −34 ft3/s for the monthly and annual mean discharges, 

respectively, these differences could be attributed to chance 

and are not statistically significant according to the sign test 

at the 5-percent significance level. This finding indicates that 

temporal averaging of the data on a monthly and annual basis 

reduces the variability between the two datasets, thus making 

any differences statistically insignificant. Therefore, for the 

purposes of Lake Michigan diversion accounting in which the 

diversion is assessed on the basis of an annual mean discharge, 

the use of the H-ADCP-computed discharge in place of the 

AVM computed discharge is acceptable. 

Conclusions 

The State of Illinois’ annual withdrawal from Lake 

Michigan is limited by a U.S. Supreme Court decree, and the 

U.S. Geological Survey is responsible for monitoring flows 

in the Chicago Sanitary and Ship Canal (CSSC) near Lemont, Ill., as a part of the Lake Michigan Diversion Accounting overseen by the U.S. Army Corps of Engineers, Chicago 

District. Every 5 years, a technical review committee consisting of practicing engineers and academics is convened to 

review the U.S. Geological Survey’s streamgaging practices 

in the CSSC near Lemont, Ill. The sixth technical review 

committee raised a number of questions concerning the flows 

and streamgaging practices in the CSSC near Lemont, and 

this report provides answers to many of those questions. In 

addition, this report examines the index velocity meters in use 

at Lemont to determine whether the acoustic velocity meter 

(AVM), which is now the primary index velocity meter, can 

be replaced by the horizontal acoustic Doppler current profiler 

(H-ADCP), which is currently the backup meter. Although 

the AVM has performed well as the primary meter at Lemont, 

the unavailability of replacement parts and difficulty in making timely repairs have made it necessary to find a suitable 

replacement meter. 

The flows in the CSSC near Lemont, Ill., cover a wide 

range of velocities and are highly unsteady. The primary 

source of unsteadiness appears to be the control of the Chicago Area Waterway System (CAWS) through three primary 

lock and dam structures. Control changes and lockages at the 

downstream end of the flow at Lockport, Ill., cause changes in 

the stage and flow elsewhere on the system, and the disturbances propagate through the system in the form of flood 

waves and seiches, both of which can be identified in the discharge records near Lemont. Withdrawals and discharges from 

powerplants and industry on the canal and wastewater inflows 

from three large water-reclamation plants further add variability to the flow. Storm-driven inflows from several tributaries 

on the Calumet Sag Channel and North Branch Chicago River 

and inflows from local runoff and combined-sewer overflows 

are yet other sources of flow variability, primarily during high 

flows. Localized and short-term flow variation near Lemont 
Conclusions 41

is caused by commercial water traffic, which can be heavy in 

the Lemont area. A true understanding of the flow variability 

in the CSSC near Lemont requires one to unravel the complex 

time series that makes up the gage records at Lemont and trace 

each individual fluctuation in flow back to its source. Although 

this task is basically impossible, tools such as spectral analysis 

applied in this report can identify sources of flow variability 

that have a periodic component. Whereas large-scale seiches 

driven by control changes and lockages at Lockport appear to 

play a large role in the flow variability near Lemont, there is 

still much to be understood about flows in the CAWS. Understanding the role of each source of flow variability will require 

detailed hydrodynamic modeling in which each source of flow 

variability is represented accurately.

The success of an index-velocity-based discharge 

measurement depends on the flow characteristics at the site. 

If the velocity distribution in the measurement cross section 

is unique for every discharge (does not change for the same 

discharge), then one can theoretically develop an index velocity rating to compute discharge. In the CSSC near Lemont, 

the success of the index velocity method relies on the ability 

of the flow in the channel to conform to a set velocity distribution at every discharge and overcome local, transient flow 

disturbances. An examination of the vertical and transverse 

velocity profiles for a range of flows and temporal averaging 

periods revealed that although instantaneous flow profiles may 

be highly variable, temporally averaged profiles are generally 

consistent with open-channel flow theory. Therefore, temporal 

averaging is necessary near Lemont to average out the shortterm variations in the flow owing to turbulence and the sources 

of flow variability discussed above. The length of the averaging interval is a tradeoff between the loss of resolution in the 

data caused by averaging true fluctuations in the discharge 

(which can occur over the course of a few minutes) and loss of 

accuracy in the index velocity rating caused by turbulence and 

other sources of flow variability causing local disruptions to 

the velocity distribution. Although an analysis of the averaging interval was not completed, the authors plan to complete 

such an analysis in the near future. Such an analysis would 

reveal whether the error in the rated discharge is dependent 

on sampling time and may help identify an averaging interval that minimizes this error. Finally, secondary flows were 

most prominent at low discharges and appeared to be partially 

responsible for large differences between the rated and measured discharge. Secondary flows have the ability to disrupt 

index velocity measurements because they introduce a flow 

angle, causing an erroneous index velocity to be computed. 

If the secondary flows were consistent at the site (that is, not 

varying in magnitude for a given discharge), they could be 

accounted for in the index velocity rating. Unfortunately, the 

secondary flows observed near Lemont appear to vary considerably at low flows and thus appear to be a source of error. 

Use of bank-mounted instrumentation such as the AVM 

and H-ADCP appears to be the best option for index velocity 

measurement in the CSSC near Lemont. At this site, mounting 

instruments to the bed is very difficult, and up-looking ADCPs 

are placed on the channel bed and mounted on frames that do 

not have a fixed orientation. Therefore, up-looking ADCPs 

at this site rely on their compass and pitch and roll sensors to 

determine the correct orientation of the velocities measured 

above the unit. Commercial traffic caused significant disruptions to the compass readings on the deployed up-looking 

ADCPs, and a failure of the compass and pitch and roll sensors on an extended deployment corrupted a large dataset. 

Reliance on these sensors to determine an accurate and stable 

index velocity makes an up-looking ADCP less reliable than 

a bank-mounted H-ADCP (unless one installs the up-looking 

ADCP in a fixed, known orientation and uses the instrument 

coordinate system). The additional vulnerability of the instrument on the streambed to damage from commercial traffic and 

debris makes it less attractive than the H-ADCP for an AVM 

replacement. 

Application of the AVM and H-ADCP to index mean 

channel velocity in the CSSC near Lemont, Ill., has produced 

good ratings to date. The site is well suited to index velocity 

measurements, and although the flows at the site display substantial primary and secondary flow variability, it is unlikely 

that one could find an alternative site between Romeoville 

and the Calumet Sag Channel confluence with the CSSC 

where these effects would not be present or would be lesser 

in magnitude. Location of the gage in this reach is essential 

to accurate accounting of the Lake Michigan Diversion. This 

reach is highly industrial and is a hub for commercial traffic 

on the CAWS. The current site of the gage is upstream of the 

main hub of commercial activity. Several aspects of the site 

add complexity to the analysis, including the sloughed right 

bank in the measurement reach and the proximity to the Calumet Sag Channel. The sloughed bank appears to be causing a 

flow acceleration through the measurement reach and may be 

responsible for influencing secondary circulation under certain 

flow conditions. The proximity to the confluence of the CSSC 

and Calumet Sag Channel may be responsible for flow deceleration near the left bank as observed in the H-ADCP data and 

moving-boat discharge measurements. Although both these 

factors have the ability to create changes in the flow structure 

and disrupt the index velocity rating, their overall effect on the 

ratings are small.

Comparison of the rating curves for the AVM and 

H-ADCP has revealed that the H-ADCP is a suitable replacement for the AVM as the primary index velocity meter near 

Lemont. Because of the smaller sampling volume and inherent uncertainty associated with Doppler measurements, the 

H-ADCP has slightly higher uncertainty than the AVM with 

regard to discharge computations. However, on the basis of 

velocity data collected during the past 4 years, the H-ADCP 

is more reliable than the AVM and is much less susceptible to 

disruptions from commercial traffic. Through a formal analysis of the effect of sampling time and averaging interval on 

measurement uncertainty near Lemont, it may be possible to 

further reduce the uncertainty in the H-ADCP. An added benefit of using the H-ADCP as the primary index velocity meter 

is the increased resolution of flow structure near Lemont. The 
42 Comparison of Index Velocity Measurements for Computation of Discharge, Chicago Sanitary and Ship Canal, Lemont, Ill.

multicell design and two-dimensional velocity measurement 

capability of the instrument provides more information about 

the flow, making the H-ADCP an essential tool to understanding the influence of the Calumet Sag Channel on flows near 

Lemont.

The USGS gaging station on the CSSC near Lemont, Ill., 

is a key component to Lake Michigan Diversion Accounting . 

The importance of this station in monitoring withdrawals from 

Lake Michigan has made it one of the most highly scrutinized 

gaging stations in the country. Any changes in streamgaging 

practices at this gage requires detailed analysis to ensure the 

change will not adversely affect the ability of the USGS to 

accurately monitor flows. This report has provided a detailed 

analysis of the flow structure and index velocity measurements in the CSSC near Lemont, Ill., to ensure that decisions 

regarding the future of this gaging station are made with the 

best possible understanding of the site and the characteristics 

of the flow. 

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