Discharge Measurements Using a

Broad-Band Acoustic Doppler 

Current Profiler

By Michael R. Simpson

___________________________________

United States Geological Survey

OPEN-FILE REPORT 01-1

SACRAMENTO, CALIFORNIA

2001

8016-20
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not 

constitute endorsement by the U.S. Geological Survey.

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

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

District Chief

U.S. Geological Survey

Placer Hall, Suite 2012

6000 J Street

Sacramento, CA 95819-6129
Contents III

CONTENTS

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

Purpose and Scope....................................................................................................................................................... 1

A Short History of Acoustic Doppler Current Profiler Discharge Measurement ........................................................ 1

Chapter 1: Theory of Operation ............................................................................................................................................ 3

Basic Acoustic Velocity Measurement Principles ....................................................................................................... 3

The Physics of Sound ....................................................................................................................................... 3

The Doppler Principle Applied to Moving Objects........................................................................................... 3

Measuring Doppler Shifts Using Acoustic Backscatter .................................................................................... 4

Measuring Doppler Shifts from a Moving Platform.......................................................................................... 5

Radial Motion .......................................................................................................................................... 5

Acoustic Doppler Current Profiler Beam Geometry ......................................................................................... 6

Calculating Three-Dimensional Velocity Components ........................................................................... 6

Beam Scenarios ....................................................................................................................................... 6

The Fourth Beam and Error Calculations................................................................................................ 7

Acoustic Doppler Current Profiler Water-Velocity Profile Measurements.................................................................. 8

Acoustic Doppler Current Profiler-Measured Profiles Compared with Conventional Current Meter 

Measurements...................................................................................................................................... 8

Time Gating: Measuring Doppler Shifts from Different Depths....................................................................... 8

Bottom Tracking................................................................................................................................................ 9

Acoustic Doppler Current Profiler Limitations for Velocity-Profile Measurements ......................................... 9

Range Limitations.................................................................................................................................... 9

Side-Lobe Interference ............................................................................................................................ 11

Effects of Different Beam Angles............................................................................................................ 12

Blanking Distance.................................................................................................................................... 13

Instrument Development: Solving the Problem of Velocity-Measurement Uncertainty ............................................. 13

Random and Bias Error ..................................................................................................................................... 13

Random Error .......................................................................................................................................... 13

Bias Error................................................................................................................................................. 13

Pitch and Roll .................................................................................................................................................... 14

Beam-Angle Error ............................................................................................................................................. 15

Narrow-Band and Broad-Band Doppler Shift Measurements........................................................................... 15 

Narrow-Band Doppler Shift Measurements ............................................................................................ 16

Broad-Band Doppler Shift Measurements .............................................................................................. 17

Differences Between Phase-Shift Measurements and Lag-Spacing Measurements (Time Dilation) ............... 21

Bottom-Tracking Limitations ............................................................................................................................ 21

The Broad-Band Acoustic Doppler Current Profiler: Overcoming the Self-Noise Problem....................................... 22

Error Sources Unique to Broad-Band Acoustic Doppler Current Profilers....................................................... 22

Random Uncertainty Caused by Self Noise ................................................................................................................ 23

Summary...................................................................................................................................................................... 26

Chapter 2: Acoustic Doppler Current Profiler Discharge-Measurement Principles.............................................................. 27

Parts of an Acoustic Doppler Current Profiler Discharge Measurement..................................................................... 27

Velocity Cross-Product Measurement Using an Acoustic Doppler Current Profiler................................................... 27

General Equation .............................................................................................................................................. 27

The General Equation, as Applied to Acoustic Doppler Current Profiler Moving-Boat Measurements........... 27

Properties of the Acoustic Doppler Current Profiler Measured Cross Product ................................................. 28

Integrating the Cross Product Over the Water Depth .............................................................................. 28

Estimating Cross Products in the Unmeasured Portions of the Profile ................................................... 29

Blanking Distance.................................................................................................................................... 29
IV Contents

Side-Lobe Interference............................................................................................................................ 29

Evolution of the One-Sixth Power-Curve Estimation Technique ........................................................... 30

Integration, By Time, Over the Width of the Cross Section ................................................................... 31

Estimating Discharge Near the Channel Banks .......................................................................................................... 31

Determination of Total River Discharge—Putting it All Together.............................................................................. 32

Discharge-Measurement Software .............................................................................................................................. 32

Summary ..................................................................................................................................................................... 33

Chapter 3: R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes.......................... 35

Measurement Modes—Why?...................................................................................................................................... 35

Water Modes ............................................................................................................................................................... 35

Water Mode Zero (WM0) ................................................................................................................................. 35

Water Mode 1 (WM1)....................................................................................................................................... 35

Water Mode 4 (WM4)....................................................................................................................................... 37

Water Mode 5 (WM5)....................................................................................................................................... 37

Ambiguity Velocity Revisited ................................................................................................................. 37

Water Mode 7 (WM7)....................................................................................................................................... 37

Water Mode 8 (WM8)....................................................................................................................................... 38

Range/Speed “Windows” for Water Modes 1, 5, and 8 .................................................................................... 38

Bottom-Track Modes .................................................................................................................................................. 38

The Bottom Reflection ...................................................................................................................................... 38

Bottom-Track Mode 4 (BM4)........................................................................................................................... 40

Bottom-Track Mode 5 (BM5)........................................................................................................................... 40

Summary ..................................................................................................................................................................... 40

Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment............................................................. 41

Where Do We Start?.................................................................................................................................................... 41

Acoustic Doppler Current Profiler Equipment............................................................................................................ 41

Acoustic Doppler Current Profiler Pressure Case and Transducer Assembly................................................... 41

Power Supply and Communications Interface.................................................................................................. 42

Acoustic Doppler Current Profiler Discharge-Measurement Software............................................................. 45

Documentation .................................................................................................................................................. 45

Ancillary Equipment ................................................................................................................................................... 45

Measurement Platform or Vessel Requirements ............................................................................................... 45

Laptop Computer .............................................................................................................................................. 48

Acoustic Doppler Current Profiler Mounts....................................................................................................... 51

Range Finder or Method for Estimating Distance to Shore.............................................................................. 52

Trolling Motors/Plates....................................................................................................................................... 55

Miscellaneous Measurement Equipment .......................................................................................................... 55

Installation of the Broad-Band Acoustic Doppler Current Profiler............................................................................. 56

Mounting the Acoustic Doppler Current Profiler on the Vessel........................................................................ 56

Deck-Unit and Power-Supply Connections ...................................................................................................... 57

Acoustic Doppler Current Profiler Cables and Connectors .............................................................................. 58

Broad-Band Acoustic Doppler Current Profiler...................................................................................... 58

Workhorse Rio Grande...................................................................................................................................... 59

Summary ..................................................................................................................................................................... 60

Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration ................................................ 63

Discharge-Measurement Software—“Transect”......................................................................................................... 63

Transect Configuration................................................................................................................................................ 63

Creation of a Preliminary Configuration File Using Transect Modules ........................................................... 63

Communications Setup ........................................................................................................................... 63

Calibration Setup..................................................................................................................................... 64

Planning Setup ........................................................................................................................................ 66

The Configuration File, in Detail ...................................................................................................................... 68

The Communications Section ................................................................................................................. 69

Ensemble Out Section............................................................................................................................. 69

Acoustic Doppler Current Profiler Hardware Section ............................................................................ 70

Direct Commands Section ...................................................................................................................... 70
Contents V

Water-Track Commands................................................................................................................ 72

WSnnn............................................................................................................................. 72

WNnnn ............................................................................................................................ 72

WPnnnn........................................................................................................................... 72

WFnnnn........................................................................................................................... 73

WDnnn nnn nnn .............................................................................................................. 73

Bottom-Track Commands............................................................................................................. 73

BAnnn ............................................................................................................................. 73

BCnnn ............................................................................................................................. 73

BPnnn.............................................................................................................................. 73

BXnnnn ........................................................................................................................... 73

&Rnn............................................................................................................................... 73

General Commands....................................................................................................................... 74

ESnn ................................................................................................................................ 74

EXnnnnn ......................................................................................................................... 74

Recording Section ......................................................................................................................... 74

Calibration Section.................................................................................................................................. 75

Processing Section .................................................................................................................................. 77

Graphics Section ..................................................................................................................................... 77

History Section........................................................................................................................................ 77

Finishing the Preliminary Configuration File: Required Commands ............................................................... 77

Transect Release Enhancements (2.80 and Later) ............................................................................................ 80

Summary ..................................................................................................................................................................... 84

Chapter 6: Data Acquisition.................................................................................................................................................. 85

Operation of Transect Software .................................................................................................................................. 85

Loading the Configuration File ......................................................................................................................... 85

Starting the Transect Acquire Module .................................................................................................... 85

Transect-Data Displays ........................................................................................................................... 86

Transect-Data Recording......................................................................................................................... 89

Summary ..................................................................................................................................................................... 89

Chapter 7: Discharge-Measurement Procedure .................................................................................................................... 91

Cross-Section Reconnaissance.................................................................................................................................... 91

Premeasurement Checkout.......................................................................................................................................... 91

Boat-Maneuvering Techniques ................................................................................................................................... 92

Starting the Cross-Section Traverse ............................................................................................................................ 92

During the Cross-Section Traverse (Transect Tips and Tricks) .................................................................................. 93

Ending the Cross-Section Traverse ............................................................................................................................. 94

Alternate Techniques Used During Low-Flow Conditions......................................................................................... 94

What Constitutes a “Good” Discharge Measurement? ............................................................................................... 95

Archival of Acoustic Doppler Current Profiler Discharge-Measurement Data .......................................................... 95

Summary ..................................................................................................................................................................... 98

Chapter 8: Discharge-Measurement Review and Assessment .............................................................................................. 101

Configuration-File Review.......................................................................................................................................... 101

River Conditions ............................................................................................................................................... 101

Acoustic Doppler Current Profiler Hardware ......................................................................................... 101

Acoustic Doppler Current Profiler Direct Commands............................................................................ 101

Calibration Section.................................................................................................................................. 102

Transect Software Playback ........................................................................................................................................ 102

Missing Ensembles ..................................................................................................................................................... 102

Error Caused By Sediment Movement Near the Bottom............................................................................................ 103

Large Magnitudes of the Unmeasured Layers ............................................................................................................ 104

Low-Velocity Measurements ...................................................................................................................................... 105

Power-Curve-Fit Applicability.................................................................................................................................... 105

Edge Values ................................................................................................................................................................. 106

Shiptrack ..................................................................................................................................................................... 107
VI Contents

Summary ..................................................................................................................................................................... 108

Chapter 9: Discharge-Measurement Error ............................................................................................................................ 111

A Review of Major Acoustic Doppler Current Profiler Velocity-Measurement Limitations and Uncertainties......... 111

Limitations ........................................................................................................................................................ 111

Range Limitations ................................................................................................................................... 111

Side-Lobe Interference............................................................................................................................ 111

Unmeasured Velocity Due to Blanking Distance and Transducer Draft................................................. 111

Random and Systematic Uncertainty................................................................................................................ 112

Random Uncertainty Due to Self Noise and Lag Distance..................................................................... 112

Systematic Uncertainty Due to Velocity Ambiguity ............................................................................... 113

Systematic Uncertainty in Speed of Sound Due to Temperature............................................................ 113

Systematic Uncertainty in Speed of Sound Due to Salinity.................................................................... 114

Systematic Uncertainty Due to Incorrect Beam Geometry..................................................................... 114

Minimizing Uncertainty in Velocity Profiles .......................................................................................... 114

Errors Affecting the Accuracy of Discharge Measurements....................................................................................... 115

Simplified Random-Error Model ...................................................................................................................... 115

Bias Error .......................................................................................................................................................... 118

Instrument-Caused Bias Error................................................................................................................. 118

Beam-Angle Errors ................................................................................................................................. 119

Depth-Measurement Errors..................................................................................................................... 119

Speed-of-Sound Errors Due to Temperature and Salinity Gradients ...................................................... 119

Bias Error Due to Incorrect Estimation of Unmeasured Velocities

Near the Water Surface and Channel Bottom ..................................................................................... 120

Operator-Caused Bias Error.................................................................................................................... 120

Incorrect Transducer Draft ............................................................................................................ 121

Improper Broad-Band Acoustic Doppler Current Profiler Mounting ........................................... 121

Incorrect Edge-Distance Estimates ............................................................................................... 121

Incorrect Estimated Edge Shapes.................................................................................................. 121

Bottom Movement ........................................................................................................................ 122

Configuration-File Error ............................................................................................................... 122

Poor Choice of Cross Sections...................................................................................................... 122

Common Sense Rules ............................................................................................................................. 122

Summary ..................................................................................................................................................................... 122

References Cited ................................................................................................................................................................... 123
Contents VII

FIGURES

Chapter 1. Theory of Operation

1.1.–1.40. Graphic illustrations showing:

1.1. Christian Johann Doppler.............................................................................................................................. 3

1.2. Stationary wave observer .............................................................................................................................. 4

1.3. Moving wave observer .................................................................................................................................. 4

1.4. Magnified view of backscatterers.................................................................................................................. 4

1.5. An acoustic pulse being backscattered.......................................................................................................... 5

1.6. Reflected pulse showing two Doppler shifts................................................................................................. 5

1.7. Effect of radial motion on Doppler shift ....................................................................................................... 5

1.8. Velocity components ..................................................................................................................................... 6

1.9. Downward-looking, convex-head acoustic Doppler current profiler ............................................................ 6

1.10. Boat-mounted acoustic Doppler current profiler with the “Janus” configuration....................................... 6

1.11. Northwest-moving water-velocity vector and the resulting Doppler shifts from a 

hypothetical, three-beam sonar ....................................................................................................................... 7

1.12. Northeast-moving water-velocity vector and the resulting Doppler shifts for a

hypothetical, three-beam sonar ....................................................................................................................... 7

1.13. Acoustic Doppler current profiler measuring a homogeneous velocity field.............................................. 8

1.14. Nonhomogeneous velocity field bounded by the acoustic Doppler current profiler 

beams .............................................................................................................................................................. 8

1.15. Analogy of a conventional current-meter string to an acoustic Doppler current 

profiler (ADCP) profile.................................................................................................................................... 8

1.16. Acoustic Doppler current profiler (ADCP) time gating .............................................................................. 9

1.17. Short and long bottom-track pulse .............................................................................................................. 9

1.18. Spectrograph of received Doppler signal .................................................................................................... 10

1.19. Phase change due to size and speed differences of scatterers..................................................................... 10

1.20. Echo returned from a cloud of particles...................................................................................................... 10

1.21. Acoustic Doppler current profiler transducer beam pattern ........................................................................ 11

1.22. Reflected signal strength indicators (RSSI) for a four-beam acoustic Doppler current profiler................. 12

1.23. Polar plot of 10-ping broad-band acoustic Doppler current profiler velocity averages .............................. 14

1.24. Polar plot of about 200-ping broad-band acoustic Doppler current profiler velocity averages .................. 15

1.25. Pitch and roll axes for a boat-mounted acoustic Doppler current profiler .................................................. 16

1.26. Bin positions during an acoustic Doppler current profiler roll occurrence................................................. 16

1.27. Freeway strobe-light system used to measure vehicle speed ...................................................................... 17

1.28. Narrow-band acoustic Doppler current profiler (ADCP) shift measurement ............................................. 17

1.29. Acoustic pulse pair approaching a stationary particle................................................................................. 18

1.30. Acoustic pulse pair reflected from a stationary particle.............................................................................. 18

1.31. Acoustic pulse pair approaching a moving particle .................................................................................... 18

1.32. Acoustic pulse pair reflected from a moving particle.................................................................................. 19

1.33. Acoustic pulse pair with a small reflected pressure wave ........................................................................... 19

1.34a. Race track analogy during the first strobe flash ........................................................................................ 20

1.34b. Race track analogy during the second strobe flash ................................................................................... 20

1.35. Explanation of ambiguity velocity.............................................................................................................. 21

1.36. Description of time dilation compared with phase-angle difference .......................................................... 22

1.37. Effect of bottom-track pulse width on bias caused by bottom movement .................................................. 23

1.38. Narrow pulse pairs compared with wide pulse pairs................................................................................... 24
VIII Contents

FIGURES—Continued

1.39. Phase-coded pulse pair................................................................................................................................ 24

1.40. Effects of code element lag on correlation.................................................................................................. 25

Chapter 2. Acoustic Doppler Current Profiler Discharge-Measurement Principles

2.1–2.8. Graphic illustrations showing:

2.1. Cross-product vectors during a cross-section traverse.................................................................................. 28

2.2. Properties of the water-velocity/boat-velocity cross product........................................................................ 28

2.3. Acoustic Doppler current profiler-beam pattern showing side-lobe features................................................ 29

2.4. Hypothetical shape of a parasitic, side-lobe pattern...................................................................................... 29

2.5. Example velocity profile showing measured and missing values.............................................................. 30

2.6. Example velocity profile of one-sixth power-curve fit and typical values................................................. 30

2.7. Unmeasured areas in a typical acoustic Doppler current profiler

discharge-measurement cross section.............................................................................................................. 31

2.8. Edge-value estimation scheme described by equations 2.8, 2.9, and 2.10.................................................... 32

Chapter 3. R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes

3.1–3.3. Graphic illustrations showing:

3.1. Choosing the proper measurement mode is difficult..................................................................................... 35

3.2. Depth/range/speed-operational “windows” for water modes 1, 5, and 8...................................................... 36

3.3. Acoustic Doppler current profiler-backscattered intensity with depth showing 

the bottom reflection........................................................................................................................................ 39

Chapter 4. Acoustic Doppler Current Profiler Hardware and Ancillary Equipment

4.1. Graphic illustration, “Which equipment do we need?” ................................................................................................. 41

4.2–4.24. Photograph showing:

4.2. A 1,200-kilohertz broad-band acoustic Doppler current profiler with attached pipe brackets ..................... 42

4.3. An R.D. Instruments, Inc., 600-kilohertz Workhorse “Rio Grande” acoustic Doppler

current profiler ................................................................................................................................................. 43

4.4. Front and rear view of an R.D. Instruments, Inc., acoustic Doppler current profiler

deck unit .......................................................................................................................................................... 44

4.5. Connector cable for attachment of R.D. Instruments, Inc., broad-band acoustic

Doppler current profiler to accompanying deck unit....................................................................................... 45

4.6. Tethered acoustic Doppler current profiler discharge measurement platform .............................................. 46

4.7. Radio-controlled, 12-foot, broad-band acoustic Doppler current profiler platform...................................... 46

4.8. Two views of an acoustic Doppler current profiler side-swing mount on a 30-meter 

(95-foot) vessel................................................................................................................................................ 47

4.9. Side-swing mount on a 6-meter (20-foot) Boston Whaler for an acoustic 

Doppler current profiler................................................................................................................................... 48

4.10. Side-swing mount on a 4.5-meter (15-foot) Boston Whaler for an acoustic

Doppler current profiler................................................................................................................................... 49

4.11. Acoustic Doppler current profiler mount for an inflatable dinghy.............................................................. 50

4.12. Laptop computer screen in diffuse sunlight ................................................................................................ 50

4.13. Laptop computer with missing plastic doors and port covers..................................................................... 51

4.14. Inexpensive acoustic Doppler current profiler mount ................................................................................. 51

4.15. Aluminum acoustic Doppler current profiler mount ................................................................................... 52

4.16. Sea-chest mount for a narrow-band acoustic Doppler current profiler ....................................................... 53

4.17. Mount used by the U.S. Geological Survey Tampa, Florida, Subdistrict ................................................... 54

4.18. Hydraulic mount used by the U.S. Geological Survey Indiana District ..................................................... 55

4.19. “Toggle” mount used by the U.S. Geological Survey Kentucky District ................................................... 56

4.20. Swing mount variation used by the U.S. Geological Survey Illinois District............................................. 57

4.21. Detachable swing mount used by the U.S. Geological Survey Idaho District............................................ 58

4.22. Optical range finders used to estimate near-shore distances....................................................................... 58

4.23. Typical battery-operated trolling motor used when making acoustic Doppler

current profiler discharge measurements......................................................................................................... 59

4.24. Two views of a steering adaptor that connects the trolling motor to the main engine................................ 60

f

f
Contents IX

FIGURES—Continued

4.25. Graphic illustration showing interconnections of the broad-band acoustic Doppler current

profiler (BB-ADCP) deck unit with other components of the acoustic Doppler current profiler 

discharge-measurement system.................................................................................................................................... 61

Chapter 5. Broad-Band Acoustic Doppler Discharge-Measurement System Configuration

5.1–5.9. Graphic illustrations showing:

5.1. BB-TALK terminal emulator help screen ..................................................................................................... 64

5.2. Transect main menu showing menu choices................................................................................................. 65

5.3. Communication acoustic Doppler current profiler (ADCP) submenu.......................................................... 65

5.4. Transect main menu showing calibration menu choices............................................................................... 66

5.5. Calibration offsets submenu screen .............................................................................................................. 66

5.6. Calibration scaling submenu screen.............................................................................................................. 67

5.7. Transect main menu showing planning menu choices.................................................................................. 67

5.8. Planning setup submenu screen .................................................................................................................... 68

5.9. Planning acoustic Doppler current profiler submenu screen......................................................................... 69

5.10–5.11. Graphics showing:

5.10. Communication, ensemble out, and acoustic Doppler current profiler (ADCP) hardware

sections of the configuration file...................................................................................................................... 70

5.11. Direct command and recording sections of the configuration file .............................................................. 71

5.12. Graphic illustration showing planning acoustic Doppler current profiler (ADCP) menu in expert 

mode (Alt-M)............................................................................................................................................................... 72

5.13–5.20. Graphics showing:

5.13. Calibration section of the configuration file................................................................................................ 76

5.14. Processing, graphics, and history sections of the configuration file ........................................................... 78

5.15. Hardware and direct command sections of the configuration file............................................................... 79

5.16. Recording section of the configuration file ................................................................................................. 80

5.17. Calibration section of the configuration file................................................................................................ 81

5.18. Graphics and history sections of the configuration file............................................................................... 82

5.19. Edge-slope coefficient in the Transect 2.80+ configuration file.................................................................. 83

5.20. Transect 2.80+ directives in the processing section of the configuration file ............................................. 84

Chapter 6. Data Acquisition

6.1–6.7. Graphic illustrations showing:

6.1. Transect software main menu........................................................................................................................ 85

6.2. Transect software Acquire introductory screen............................................................................................. 86

6.3. Transect software Acquire profile menu screen ............................................................................................ 87

6.4. Transect software Acquire velocity contour plot screen ............................................................................... 87

6.5. Transect software Acquire intensity contour plot.......................................................................................... 88

6.6. Transect software Acquire shiptrack plot...................................................................................................... 88

6.7. Transect software Acquire tabular display screen......................................................................................... 89

Chapter 7. Discharge-Measurement Procedure

7.1. Graphic illustration showing improper discharge-measurement technique................................................................... 91

7.2–7.8. Photographs showing:

7.2. Discharge-measurement cross section on Dutch Slough near Oakely, California........................................ 91

7.3. Broad-band acoustic Doppler current profiler mount being deployed vertically.......................................... 92

7.4. Discharge-measurement log sheet/note......................................................................................................... 92

7.5. Acoustic Doppler current profiler vessel at the beginning of a discharge

measurement ................................................................................................................................................... 93

7.6. Acoustic Doppler current profiler vessel near center channel....................................................................... 93

7.7. Acoustic Doppler current profiler vessel at the end of a cross-section traverse............................................ 94

7.8. Winching system used to move an acoustic Doppler current profiler vessel

at slow speeds ................................................................................................................................................. 94

7.9–7.10. Graphics showing:

7.9. Example of an acoustic Doppler current profiler (ADCP) discharge-measurement note (front).................. 96

7.10. Example of an acoustic Doppler current profiler discharge-measurement 

note (back) ...................................................................................................................................................... 97
X Contents

FIGURES—Continued

7.11. Graph showing discharge-measurement data from a tidally affected 

gaging station on a tributary of the San Joaquin River ................................................................................ 98

7.12. Discharge-measurement log sheet................................................................................................................ 99

Chapter 8. Discharge-Measurement Review and Assessment

8.1–8.11. Graphic illustrations showing:

8.1. Transect containing missing ensembles caused by a loss of bottom tracking............................................... 102

8.2. Transect software shiptrack plot of an anchored vessel ................................................................................ 104

8.3. Transect software edge-value screen showing unmeasured layers ............................................................... 104

8.4. Transect shiptrack plot of acoustic Doppler current profiler-measured velocities 

on a slow-moving river.................................................................................................................................... 105

8.5. Transect software discharge profile plot........................................................................................................ 106

8.6. Transect software edge-value screen............................................................................................................. 106

8.7. Transect software shiptrack plot.................................................................................................................... 107

8.8. Transect software shiptrack plot with operator’s initials............................................................................... 108

8.9. Transect software profile plot of “initials”.................................................................................................... 108

8.10. Transect shiptrack screen ............................................................................................................................ 109

8.11. Transect subsectioning menu screen ........................................................................................................... 109

Chapter 9. Discharge-Measurement Error 

9.1–9.2. Graphic illustrations showing:

9.1. Screen shot of Transect software showing tabular output............................................................................. 112

9.2. Screen shot of BB-SETUP software showing a typical setup for a 1,200-kilohertz

broad-band acoustic Doppler current profiler ................................................................................................. 113

9.3–9.4. Graphs showing:

9.3. Depth error due to speed of sound that is uncorrected for temperature........................................................ 114

9.4. Depth error due to speed of sound that is uncorrected for salinity ............................................................... 114

9.5–9.9. Graphic illustrations showing:

9.5. Discharge error using a boat speed of about 0.3 meter per second (1 foot per second)

and about 0.9 meter per second (3 feet per second) ........................................................................................ 116

9.6. Discharge error using a boat speed of about 0.9 meter per second (3 feet per second)

in 9 meters (30 feet) of depth ......................................................................................................................... 116

9.7. QERROR setup screen.................................................................................................................................. 117

9.8. Discharge error with a mean water velocity of about 0.3 meter per second

(1.0 foot per second) ....................................................................................................................................... 118

9.9. Discharge error in deep water [9 meters (30 feet)] with a mean river velocity of

about 0.15 meter per second (0.5 foot per second).......................................................................................... 119

9.10–9.11. Graphs showing:

9.10. Discharge error in shallow water [3 meters (10 feet)] with a mean river velocity of 

about 0.15 meter per second (0.5 foot per second).......................................................................................... 120

9.11. Exaggerated instance of depth error due to limitations of the acoustic beams ........................................... 121

TABLES

Chapter 1. Theory of Operation

1.1. Comparison of narrow-band and broad-band single-ping standard deviation................................................... 26

Chapter 3. R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes

3.1. Mode 1 single-ping standard deviation using three different values for the

WV command .............................................................................................................................................. 36

3.2. Setup and performance values for water mode 5 operation (WZ05)................................................................. 38

3.3. Setup and performance values for water mode 8 operation (WZ05)................................................................. 39

3.4. Minimum depth ranges for bottom-track modes 4 and 5 .................................................................................. 40

Chapter 5. Broad-Band Acoustic Doppler Discharge-Measurement System Configuration

5.1 Optimum bin size (WSnnn) for acoustic Doppler current profiler (ADCP) dischargemeasurement applications ................................................................................................................................. 72
Contents XI

TABLES—Continued

Chapter 9. Discharge-Measurement Error 

9.1 Approximate maximum depth range for 300-, 600-, and 1,200-kilohertz acoustic 

Doppler current profiler (ADCP) systems ......................................................................................................... 111

9.2 Approximate depth-averaged single-ping precision for 1,200-, 600-, and 300-kilohertz

broad-band acoustic Doppler current profiler-measured water velocities using mode 1

operation, 20° beams, and WV190 .................................................................................................................... 115

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS, AND ACRONYMS 

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

°F = (1.8 × °C) + 32

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both

the United States and Canada, formerly called Sea Level Datum of 1929.

Abbreviations and Acronyms

dB decibel CFG configuration (file)

Hz hertz CPU central processing unit

hp horsepower CV coefficient variation

K, kB kilobyte DC direct current

kHz kilohertz DOI U.S. Department of the Interior

Mb megabyte EGA extended graphics array

ppt parts per thousand GPS Global Positioning System

rpm revolutions per minute IBM International Business Machines

s second LCD liquid crystal display

V volt LED luminescent electronic display

W watt MS DOS Microsoft Disk Operating System

AC alternating current PC personal computer

ADCP acoustic Doppler current profiler QA quality assurance

ASCII American standard code for information interchange RDI R.D. Instruments, Inc.

BB-ADCP broad-band acoustic Doppler current profiler RSSI reflected signal strength indicator

BNC bayonet style coaxial USGS U.S. Geological Survey

CD-ROM compact disk, read-only memory

Multiply By To obtain

centimeter (cm) 0.3937 inch (in.)

centimeter per second (cm/s) .3281 foot per second (ft/s)

cubic meter per second (m3

/s) 35.3147 cubic foot per second (ft3/s)

cubic meter per second per second (m3/s/s) 35.3147 cubic foot per second per second (ft3/s/s)

decibel per meter (dB/m) .3048 decibel per foot (dB/ft)

decimeter (dm) 3.937 inch (in.)

kilometer (km) .6214 mile (mi)

meter (m) 3.281 foot (ft)

meter per second (m/s) 3.281 foot per second (ft/s)

millimeter (mm) .03937 inch (in.)

millimeter per second (mm/s) .03937 inch per second (in./s)

square meter per second per second (m2

/s/s) 10.7639 square foot per second per second (ft2/s/s)
Introduction 1

Discharge Measurements Using a

Broad-Band Acoustic Doppler

Current Profiler

By Michael R. Simpson

INTRODUCTION 

The measurement of unsteady or tidally affected 

flow has been a problem faced by hydrologists for many 

years. Dynamic discharge conditions impose an 

unreasonably short time constraint on conventional 

current-meter discharge-measurement methods, which 

typically last a minimum of 1 hour. Tidally affected 

discharge can change more than 100 percent during a 

10-minute period. Over the years, the U.S. Geological 

Survey (USGS) has developed moving-boat dischargemeasurement techniques that are much faster but less 

accurate than conventional methods. For a bibliography 

of conventional moving-boat publications, see Simpson 

and Oltmann (1993, page 17).

The advent of the acoustic Doppler current 

profiler (ADCP) made possible the development of a 

discharge-measurement system capable of more 

accurately measuring unsteady or tidally affected flow. 

In most cases, an ADCP discharge-measurement 

system is dramatically faster than conventional 

discharge-measurement systems and has comparable or 

better accuracy. In many cases, an ADCP dischargemeasurement system is the only choice for use at a 

particular measurement site.

ADCP systems are not yet “turnkey”; they are 

still under development, and for proper operation, 

require a significant amount of operator training. Not 

only must the operator have a rudimentary knowledge 

of acoustic physics, but also a working knowledge of 

ADCP operation, the manufacturers' dischargemeasurement software, and boating techniques and 

safety.

Purpose and Scope

The purpose of this report is to describe ADCP 

operating techniques, fundamental ADCP theory, 

ADCP discharge-measurement theory, and vessel 

mounts and operating techniques required for ADCP 

discharge measurements. It is not intended to replace 

the “hands-on,” USGS-approved training required for 

all ADCP discharge-measurement system operators. 

This report only describes the Microsoft Disk 

Operating System (MS DOS) version of the dischargemeasurement software, Transect, and will have to be 

revised to include future versions that run under 

Microsoft Windows. The report also only describes the 

configuration and use of the R.D. Instruments, Inc. 

(RDI), broad-band ADCP (BB-ADCP). Subsequent 

modifications to the BB-ADCP will be included in 

updates to this report if these modifications 

significantly change the configuration of the ADCP or 

if discharge-measurement techniques must be changed. 

This report also will be updated if another 

manufacturer’s ADCP is used to measure discharge by 

a significant number of USGS users.

A Short History of Acoustic Doppler Current 

Profiler Discharge Measurement

In 1982, an ADCP was used to measure the 

discharge of the Mississippi River at Baton Rouge, 

Louisiana (Christensen and Herrick, 1982). The test 

results were encouraging, differing less than 5 percent 

from simultaneous measurements made with conventional moving-boat methods. However, the computer 

software and hardware were incapable of processing 

the velocity data provided by the profiler on a real-time 

basis; the discharge measurements had to be computed 

after the fact. Although this technology looked very 

promising at the time of these tests, computer and 

Doppler signal-processing technology had not 

progressed to the level needed to collect and compute 

reliable river and estuarine discharge measurements.

In 1985, the USGS purchased a narrow-band 

ADCP to be used for the development of a dischargemeasurement system (an explanation of the differences 

between narrow-band and BB-ADCPs is given in 
2 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

chap. 1). This discharge-measurement system was 

successful (Simpson and Oltmann, 1993) but, because 

of minimum depth limitations, the narrow-band ADCP 

was usable only in rivers and estuaries with depths 

greater than 3.4 meters (m) [11.5 feet (ft)].

Because of depth and measurement precision 

limitations of the narrow-band ADCP, the ADCP 

manufacturer began exploring a slightly different area 

of acoustic technology, which has been termed “broadband.” In 1991, a prototype BB-ADCP was developed 

and tested. The BB-ADCP short-term random error 

(standard deviation of measured water velocities) was 

reported to be an order of magnitude lower than that of 

the narrow-band system and, because of this, could be 

optimized for shallow-water operation.

The USGS was interested in this technology 

because of its application to shallow water and 

purchased one of the first production models of the 

BB-ADCP from RDI in February 1992 to be evaluated 

for use in the measurement of river discharge. This 

evaluation is ongoing in the California District and has 

been joined by several other districts of the USGS. 

Morlock (1996) evaluated the BB-ADCP for discharge 

measurement at selected locations throughout the 

United States. This evaluation compared discharges 

measured utilizing a BB-ADCP with discharges 

measured at 12 stream-gaging sites having stable stagedischarge relations. The results of the evaluation 

showed that the BB-ADCP system can be used to 

accurately measure discharge at sites similar to those 

measured by Morlock (1996).

The manufacturer has since developed smaller 

versions of the BB-ADCP. Two such instruments are 

the “Workhorse” with a bottom-tracking option 

(chap. 1) and the “Rio Grande,” which is a Workhorse 

with built-in bottom-tracking capability. Because the 

manufacturer has four types of ADCPs capable of 

measuring river discharge (narrow-band, broad-band, 

Workhorse, and Rio Grande), they will be referred to as 

ADCP discharge-measurement systems in this report, 

and the instrument will be referred to as an ADCP, 

unless details specific to a particular model are 

discussed.

The ADCP discharge-measurement system may 

be the only feasible, accurate method for measuring 

discharge in tidally affected rivers and estuaries, as well 

as in rivers or canals with unsteady flow (Simpson and 

Oltmann, 1993). The ADCP also has proven useful as a 

substitute for conventional discharge-measurement 

techniques in many upland rivers with depths too deep 

for wading measurements. 

On the other hand, the setup and operation of the 

ADCP for discharge measurements is complicated, 

compared with conventional methods. Configuration of 

the ADCP for discharge-measurement use requires a 

working knowledge of conventional dischargemeasurement principles, as well as a basic knowledge 

of acoustics and Doppler shift measurement techniques. 

With the help of this report, attendance at a USGSapproved ADCP discharge-measurement course, and 

the manufacturer’s documentation, it is hoped that an 

interested individual can gain the skills needed to 

accurately measure river discharge using an ADCP.
Chapter 1: Theory of Operation 3

CHAPTER 1: THEORY OF OPERATION

Basic Acoustic Velocity Measurement Principles

Before the operational aspects of an ADCP 

measurement can be understood, some of the basic 

physical properties of sound and sound propagation 

through different mediums must be examined. This 

chapter introduces basic acoustic Doppler velocity 

measurement principles and some of the problems 

associated with such measurements. Much of the 

information in this chapter was taken from R.D. 

Instruments, Inc. (1989, 1996).

The Physics of Sound

What commonly is perceived as sound is a 

vibration of our eardrums caused by the arrival of 

pressure waves. The eardrum transfers the pressurewave information to parts of the inner ear where the 

mechanical energy of the pressure waves is converted 

to an electrical signal. The brain interprets this 

electrical signal as sound.

Sound waves can occur in most media (water, air, 

and solids) and are similar to water waves; sound waves 

have crests and troughs that correspond to bands of 

high and low air pressure, and water waves have crests 

and troughs that correspond to high and low water-level 

elevations. Pitch (frequency) of sound waves increases 

as the wavelength (the distance between the wave 

peaks) becomes shorter. This frequency, or pitch, 

usually is expressed in hertz (Hz). One hertz equals one 

wave (cycle) per second. The human ear can hear 

frequencies from about 40 Hz to about 24 kilohertz 

(kHz) (24,000 Hz). These frequencies are dubbed the 

“sonic” frequencies. Sound frequencies below about 

40 Hz are called “subsonic,” and sound frequencies 

above 25,000 Hz are called “ultrasonic.”

The Doppler Principle Applied to Moving Objects

The ADCP uses sound to measure water velocity. 

The sound transmitted by the ADCP is in the ultrasonic 

range (well above the range of the human ear). The 

lowest frequency used by commercial ADCPs is around 

30 kHz, and the common range used by the USGS for 

riverine measurements is between 300–3,000 kHz.

The ADCP measures water velocity using a 

principle of physics discovered by Christian Johann 

Doppler (1842) (fig. 1.1). Doppler’s principle relates 

the change in frequency of a source to the relative 

velocities of the source and the observer. Doppler 

(1842) stated his principle in the article, “Concerning 

the Coloured Light of Double Stars and Some Other 

Constellations in the Heavens,” while working in 

Prague, Czechoslovakia. The Doppler principle can be 

described best using the water-wave analogy 

(figs. 1.2, 1.3).

Figure 1.2 shows a stationary observer watching 

a series of waves that are passing at a rate of one wave 

per second. This rate is analogous to a transmit 

frequency of 1 Hz. In figure 1.3, the wave observer is 

boating toward the wave source at a rate of four wavelengths per second. Because the waves are passing at a 

rate of one wave per second, the observer notices the 

passage of five waves during each second of his boating 

trip. He senses that the rate of the passing waves is 5 Hz, 

though the wave source is still emitting waves at 1 Hz. 

This phenomenon is known as the Doppler effect.

Many people have experienced the Doppler 

effect while on a busy street. The sound of a car horn 

seems to drop in frequency as the car passes and recedes 

from the observer. The apparent lowering of frequency 

is called the Doppler shift (fig. 1.3). The car is a moving 

sound-wave source; therefore, when the car is 

approaching an observer, the frequency of the sound 

waves striking the observer’s ear drums is proportional 

to the speed of the car (in wavelengths per second) plus 

the frequency of the car horn in hertz. When the car is 

receding from the observer, the frequency of the sound 

waves striking the observer’s ear drums is proportional 

Figure 1.1. Christian Johann Doppler.
4 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Wave rate: 

 one wave per second

Boat speed:

 zero waves per second

Wave rate:

 one wave per second

Boat speed:

 four waves per second

Transducer

Acoustic

pulse

Magnified view 

of backscatterers

to the frequency of the car horn (in hertz) minus the 

speed of the car (in wavelengths per second). If the 

exact source frequency is known and the observed 

frequency can be calculated, equation 1.1 can be used 

to calculate the Doppler shift due to the relative 

velocities of the source and observer (R.D. Instruments, 

Inc., 1989):

(1.1)

where

 = the Doppler shift frequency, in hertz;

 = the transmitted frequency of the sound 

from a stationary source, in hertz;

FD FS

V

C

---  

  =

FD

FS

 = relative velocity between the sound source 

and the sound wave receiver (the speed at 

which the observer is walking toward the 

sound source), in meters per second; and

 = the speed of sound, in meters per second.

Note that:

•If the observer walks faster ( increases), the 

Doppler shift ( ) increases

•If the observer walks away from the sound ( is 

negative), the Doppler shift ( ) is negative

•If the frequency of the sound ( ) increases, the 

Doppler shift ( ) increases

•If the speed of sound ( ) increases, the Doppler 

shift ( ) decreases

Measuring Doppler Shifts Using Acoustic Backscatter

An ADCP applies the Doppler principle by 

bouncing an ultrasonic sound pulse off small particles 

of sediment and other material (collectively referred to 

as backscatterers) that are present, to some extent, even 

in optically clear water. A magnified view of the water 

column and the backscatterers “illuminated” by the 

sound pulse are shown in figure 1.4. There are, of 

course, exceptions to every rule, and in tow tanks and 

some natural rivers, backscatterer density can be too 

low for the proper operation of an ADCP.

The ADCP transmits an acoustic “ping,” or pulse 

into the water column and then listens for the return 

echo from the acoustic backscatterers in the water 

column. Upon receiving the return echo, the ADCP’s 

onboard signal-processing unit calculates the Doppler 

shift using a form of autocorrelation (the signal is 

compared with itself later). A schematic diagram of a 

transmitted acoustic pulse (ping) and the resulting 

reflected acoustic energy are shown in figure 1.5. Note 

V

C

V

FD

V

FD

FS

FD

C

FD

Figure 1.2. Stationary wave observer.

Figure 1.3. Moving wave observer.

Figure 1.4. Magnified view of backscatterers.
Chapter 1: Theory of Operation 5

Transmitting

Receiving

Transmitted acoustic ping

Reflected acoustic energy

Scatterers

Transducer

Transducer

Moving

sound pulse

Moving

backscatterers

First Doppler shift (as perceived by scatterers)

Second Doppler shift (as perceived by transducer)

Transducer

Wave rate:

 one wave per second 

Boat speed: 

 four waves per second

One wave

per second

Five waves

per second

Two and three-fourths and three-fourths

 waves per second

B

A

C

that very little reflected acoustic energy is reflected 

back (backscattered) towards the transducer in 

figure 1.5; most of the acoustic energy is absorbed or 

reflected in other directions.

Measuring Doppler Shifts From a Moving Platform

When the scatterers are moving away from the 

ADCP, the sound (if it could be perceived by the 

scatterers) shifts to a lower frequency. This shift is 

proportional to the relative velocity between the ADCP 

and the scatterers (fig. 1.5). Part of this Doppler-shifted 

sound is backscattered towards the ADCP, as if the 

scatterers were the sound source (fig. 1.6). The sound is 

shifted one time (as perceived by the backscatterer) and 

a second time (as perceived by the ADCP transducer) 

(R.D. Instruments, Inc., 1989).

Because there are two Doppler shifts, 

equation 1.1 becomes equation 1.2:

(1.2)

If the sound source and receiver move, relative to 

the Earth, but stay at a fixed distance from one another, 

there is no Doppler shift. The Doppler shift exists only 

when sound sources and receivers move in relation to 

FD 2 FD

V

C

---  

  =

each other. The Doppler shift between the source and 

the Earth exactly cancels the opposite shift between the 

Earth and the receiver (R.D. Instruments, Inc., 1989).

Radial Motion

Only radial motion, which is a change in distance 

between the source and receiver, will cause a Doppler 

shift. Figure 1.7 shows 1-second boat tracks in three 

different directions, relative to the wave source. Boat 

vector A is parallel with the wave direction; therefore, 

vector A encounters the full component of Doppler 

shift. Boat vector B is moving at an angle to the wave 

direction and encounters only a part of the Doppler shift 

component, whereas boat vector C (normal to the wave 

source) encounters no Doppler shift.

Figure 1.5. An acoustic pulse being backscattered.

Figure 1.6. Reflected pulse showing two Doppler shifts.

Figure 1.7. Effect of radial motion on Doppler shift.
6 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Component of

velocity seen

by ADCP

Water velocity

Magnitude and

direction

ADCP transducer



Forward

Port Starboard

Aft

Mathematically, this means the Doppler shift 

results from the velocity component in the direction of 

the line between the source and receiver (R.D. 

Instruments, Inc., 1989) as shown in equation 1.3:

(1.3)

where 

= the angle between the relative-velocity vector 

and the line between the ADCP and 

scatterers (fig. 1.8).

Acoustic Doppler Current Profiler Beam Geometry

Calculating Three-Dimensional Velocity Components

In a vessel-mounted system, the transducers are 

mounted near the water surface and aimed downward. 

Figure 1.9 shows a typical ADCP. Note that there are 

four independently working acoustic beams with each 

beam angled 20–30o from the vertical axis of the 

transducer assembly. This configuration of beams is the 

so-called “Janus” configuration, named after the Greek 

God, Janus, who could simultaneously look forward 

and backward.

Beam Scenarios

To visualize the three-dimensional capabilities of 

the “Janus” configuration, refer to figure 1.10 while 

reading the following scenarios:

•If the starboard (90o left of forward) beam has a 

positive Doppler shift, the port (90o right of 

FD 2 FS

V

C

---  

  = cos( ) θ

θ

forward) beam has a negative Doppler shift, and 

the forward and aft beams have no Doppler shift, 

then the water is flowing from starboard to port 

(or the water is still and the boat is sliding 

starboard)

•If the forward beam has a negative Doppler shift, 

the aft beam has a positive Doppler shift, and the 

Figure 1.8. Velocity components. ADCP, acoustic Doppler current 

profiler.

Figure 1.9. Downward-looking, convex-head acoustic Doppler 

current profiler.

Figure 1.10. Boat-mounted acoustic Doppler current profiler with 

the “Janus” configuration.
Chapter 1: Theory of Operation 7

Beam 1

Beam 2

Beam 3

0

+

_

Small positive

Doppler shift

0

+

_

0

+

_

Large positive

Doppler shift

Large negative

Doppler shift

N

Beam 1

Beam 2

Beam 3

0

+

_

Small negative

Doppler shift

0

+

_

0

+

_

Large positive

Doppler shift

Large negative

Doppler shift

N

starboard and port beams have no shift, then the 

water is flowing under the boat from aft to 

forward (or the water is still and the boat is 

backing)

•If the forward and port beams have a positive 

Doppler shift of magnitude 1 and the aft and 

starboard beams have a negative Doppler shift 

of magnitude 1, then water is passing under the 

boat from a point halfway between forward and 

port with a magnitude of the square root of 2, or 

1.414 (or the water is still and the boat is 

crabbing toward the forward port quarter at a 

magnitude of 1.414)

•If all four beams have a positive Doppler shift, 

the water is flowing upward toward the hull of 

the boat (or the water is still and all personnel on 

the boat should don their life jackets)

The computation of velocity in three dimensions 

(x, y, z) requires at least three acoustic beams (figs. 1.11 

and 1.12). Figure 1.11 shows a northwest-moving 

water-velocity vector and the resulting Doppler shifts 

from each of three acoustic beams (a hypothetical, 

three-beam sonar). Because the water-velocity vector is 

almost exactly at right angles to beam 2, the resulting 

Doppler shift on beam 2 is small. The water-velocity 

vector is approaching and is almost aligned with 

beam 3; therefore, beam 3 has a large, positive Doppler 

shift. The water-velocity vector is receding from, and is 

almost aligned with, beam 1; therefore, beam 1 has a 

large negative Doppler shift.

Figure 1.12 shows a northeast-moving watervelocity vector. Note that because the water-velocity 

vector almost is at right angles to beam 3, the resultant 

Doppler shift is small. The water-velocity vector is 

approaching beam 1 at an angle; therefore, beam 1 has 

a large positive Doppler shift. Beam 2 has a large 

negative Doppler shift because the water-velocity 

vector is receding from, but not perfectly parallel with, 

beam 2. This configuration also could measure vertical 

velocities. If all three beams have positive Doppler 

shifts, then a vertical component of water velocity 

moving toward the transducers is present. By using 

simple trigonometry, velocity components in three 

orthogonal coordinates can be calculated from Doppler 

shifts measured with three sonar beams.

The Fourth Beam and Error Calculations

Some ADCP manufacturers use a four-beam 

configuration and the fourth redundant beam is used to 

compute an error velocity. This error velocity can be 

used to test the assumption that flow volume of water 

bounded by the four measurement beams is homogeneous. Velocity homogeneity means that the water 

velocities do not change significantly in magnitude or 

direction within the confines of the acoustic beam 

footprint. Figure 1.13 shows a homogeneous velocity 

field bounded by the four beams of an ADCP.

If a velocity field existed as in figure 1.13, the 

ADCP would provide a nearly zero error velocity. Error 

velocity is defined as the difference between a velocity 

measured by one set of three beams versus a velocity 

measured by the other set of three beams during the 

same time frame. A difference between these two 

measurements could be caused by a bad or corrupt 

beam velocity or by nonhomogeneity (fig. 1.14). In 

practice, a small error velocity almost always exists 

because complete homogeneity of the velocity field 

rarely occurs during field measurements.

Figure 1.11. Northwest-moving water-velocity vector and the 

resulting Doppler shifts from a hypothetical, three-beam sonar.

Figure 1.12. Northeast-moving water-velocity vector and the 

resulting Doppler shifts for a hypothetical, three-beam sonar.
8 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Depth Cell 

ADCP Conventional

current meter

Acoustic Doppler Current Profiler Water-Velocity 

Profile Measurements

Acoustic Doppler Current Profiler-Measured Profiles 

Compared with Conventional Current Meter Measurements

The ADCP is best known for its capability to 

measure profiles of water velocity. A velocity profile 

can be compared roughly to using a number of pointvelocity meters that are suspended in the vertical axis of 

a water-velocity field (fig. 1.15). Theoretically, the 

velocity measured by each conventional current meter 

is analogous to the velocities measured at the center of 

ADCP depth cells (fig. 1.15). However, the analogy 

between a string of current meters and an ADCP profile 

is not perfect. Current meters measure water velocity at 

individual points in the vertical profile, whereas 

velocities that are measured by the ADCP and assigned 

to individual depth cells are really the center-weighted 

mean of velocities that are measured throughout the 

sample window (fig. 1.15).

Time Gating: Measuring Doppler Shifts from Different 

Depths

The ADCP profiling capability is accomplished 

by time gating (and sampling) the received echo at 

increasingly longer time intervals as the acoustic-beam 

wave fronts vertically traverse the water column 

(fig. 1.16).

The analogy of a Navy sonar operator can be used 

to understand time gating. The sonar operator presses a 

button on the sonar console causing a “ping” to be 

transmitted by the ship’s sonar transducer. As soon as 

the ping is transmitted, the sonar operator activates a 

stopwatch and begins listening to the returned echo. 

The sound from the ping travels through the water very 

Figure 1.13. Acoustic Doppler current profiler measuring a 

homogeneous velocity field.

Figure 1.14. Nonhomogeneous velocity field bounded by the 

acoustic Doppler current profiler beams.

Figure 1.15. Analogy of a conventional current-meter string to an 

acoustic Doppler current profiler (ADCP) profile. 
Chapter 1: Theory of Operation 9

cell 1

cell 2

cell 3

cell 4

echo echo echo echo

Transmit

pulse 

start end Gate 1 Gate 2 Gate 3 Gate 4

TIME

Blank

Bin 1

Bin 2

Bin 3

Bin 4

RANGE FROM ADCP

Entire beam does not

illuminate the bottom

at one time 

Entire beam does

illuminate the bottom

at one time 

Short pulse Long pulse

fast, but at a finite speed that can be calculated. The 

operator hears a continuous, low-intensity echo that is 

caused by the sound reflecting off of particles in the 

water as the ping speeds toward the ocean floor. The 

operator hears an abrupt increase in echo amplitude and 

frequency and immediately presses the stop button on 

the stopwatch. The echo anomaly was caused by a 

submerged submarine. The operator then calculates the 

distance to the submarine using the elapsed time from 

the stopwatch and a speed-of-sound equation 

(Urick, 1975). The speed of the submarine, relative to 

the Navy ship, also can be calculated and is 

proportional to the Doppler shift of the returned echo.

If the sonar operator was replaced by a computercontrolled receiver and time circuit, the received echo 

could be recorded and “sliced up” into small pieces 

with each piece corresponding to a received time. Each 

slice (depth cell) would be from an increasingly deeper 

part of the water column. The speed of the particles in 

each depth cell could be measured by calculating the 

Doppler shift of the echo in each depth cell. For this 

purpose, most ADCPs contain a computer-controlled 

receiver and timer circuit for each acoustic beam, as 

well as sophisticated signal-processing hardware to 

calculate Doppler shifts.

The ADCP transmits a ping along each acoustic 

beam and then time gates the reception of the returned 

echo on each beam into depth cells. Speed and direction 

are then calculated (using a center-weighted mean of 

the velocities measured in the depth cell) and assigned 

to the center of each depth cell (bin) over the measured 

vertical.

Bottom Tracking

To measure absolute water velocities (water 

velocities relative to the Earth), the ADCP must sense 

and measure the velocity of the ADCP, relative to the 

river bottom (bottom tracking). If the velocity of the 

water is known, relative to the ADCP, and the velocity 

of the ADCP is known, relative to the river bottom, then 

the water velocity, relative to the bottom, can be 

calculated. The bottom-track pulse is somewhat longer 

than the water-track pulse to properly ensonify the 

bottom (fig. 1.17). All ADCP instruments that are 

designed to measure discharge have the ability to 

calculate vessel velocity using a bottom-track pulse. 

The bottom-track ping also is used to measure the 

profiled depth range from each beam. These depthrange measurements are averaged to obtain a depth for 

the measured velocity profile.

Acoustic Doppler Current Profiler Limitations for VelocityProfile Measurements

Range Limitations

Reception and calculation of Doppler shift from 

a returned echo requires sophisticated electronic 

circuitry as well as high-speed digital signal-processing 

algorithms. The ADCP contains circuitry and microprocessors capable of resolving very small changes in 

echo Doppler shift that are needed for accurate watervelocity and bottom-velocity measurement. However, 

there are some problems associated with echo reception 

and water-velocity measurement using ADCPs.

In an ADCP, the backscattered echo amplitude 

falls off as a function of range, frequency, and pulse 

width, as well as the attenuating properties of the water 

mass. In ADCP systems, the uncertainty (random error) 

of the returned velocity measurement is strongly 

affected by changes in the backscattered echo 

amplitude. Figure 1.18 shows a spectrograph 

illustrating the relationship among transmit frequency, 

echo amplitude, and spectral width. For accurate waterFigure 1.16. Acoustic Doppler current profiler (ADCP) time gating. 

Adapted from R.D. Instruments, Inc., (1989).

Figure 1.17. Short and long bottom-track pulse. Adapted from 

R.D. Instruments, Inc., (1989).
10 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Doppler Shift

Transmit

frequency Doppler frequency

 average

Echo

amplitude

DB

Spectral

width

Frequency

Transmitted acoutic pulse (ping)

Reflected acoustic pulse (ping)

Pulses have a different

phase at time t 

Transmitted

Receiving

Transducer

Transducer

Scatterers

Scatterers

velocity measurement, the returned signal should have 

a very small spectral width (as shown by the dotted line 

labelled “Doppler frequency average” in figure 1.18), 

but, in reality, there are several factors that “spread” or 

“smear” the returned frequencies over a larger part of 

the frequency spectrum, as bounded by the solid line in 

figure 1.18.

The first factor is the finite duration of the 

acoustic transmit pulse. If the pulse is t seconds long, 

the reflected signal has a frequency spread of 1/t Hz, 

about the center frequency (Urick, 1975). This effect is 

the dominant source of spectral spreading of the 

received Doppler shifted echo. This spectral spreading 

causes increased random error in the determination of 

Doppler shift and, therefore, increased random error in 

the measurement of water velocity. The smaller the 

pulse width, the greater the amount of random error.

Figure 1.19 shows a signal being backscattered 

from scatterers of different sizes that are moving at 

different speeds, which results in the arrival of signals 

at differing phases. Because the autocorrelation 

technique needs accurate phase information to 

calculate frequency shift, this spreading effect causes 

random error in the determination of Doppler shift.

In natural waters, the reflected signal is affected 

significantly by scatterer velocities in the cloud of 

particulate matter that is “illuminated” by the ultrasonic 

pulse (fig. 1.20). The result of this cloud-scattering 

effect is to increase the spectral width of the return 

signal. 

Although the dominant source of spectral 

spreading is the transmit pulse length, the measured 

spectral width can be an indicator of velocity 

uncertainty. As the signal-to-noise ratio decreases near 

the end of the profile, the spectral width increases. This 

increase translates directly into velocity uncertainty. 

Thus, changes in spectral width are related to velocity 

uncertainty, especially in the last one-third of the 

profiling range (R.D. Instruments, Inc., 1989).

The echo amplitude is the measure of the energy 

in the echo and varies with a dynamic range of many 

Figure 1.18. Spectrograph of received Doppler signal. DB, decibels.

Figure 1.19. Phase change due to size and speed differences of 

scatterers.

Transmit pulse 

Cloud scattered echo

Transducer

Exaggerated redition of particle cloud

Scatterer

-

Figure 1.20. Echo returned from a cloud of particles.
Chapter 1: Theory of Operation 11

Water surface Transducer draft

Blanking distance

Transducer 

Distance that corresponds to

electronics and transducer

recovery time (after ping)

Side lobe

Side lobe

Main beam

Profiled

area

Area of side lobe

interference

Distance along the

acoustic wave-front where

side lobe interference

causes loss of reception of

the main-beam acoustic signal.

Side lobe interference is caused by the 

reflection (from the bottom) of the

downward pointing side lobe. 

orders of magnitude; therefore, it is converted to 

decibels. The echo amplitude is a function of several 

things:

•Transmit pulse power

•Transmit pulse length

•Reflective quality of scatterers

•Quantity of scatterers

•Absorption coefficient of the water

When the echo amplitude is high, the signal-tonoise ratio of the returned echo is high. However, when 

the echo amplitude drops below a certain level, the 

signal-to-noise ratio drops, increasing the spectral 

width, which, in turn, increases the uncertainty of the 

velocity measurement. This effect increases with range 

because the echo signal-to-noise ratio decreases with 

range. The signal-to-noise ratio also can decrease if 

there are too few particles in the water column. 

Ironically if there are too many particles (as with a very 

high sediment concentration), signal-to-noise ratio, 

from the far bin, echoes can be reduced because of 

absorption, beam spreading, and attenuation.

Side-Lobe Interference

Most transducers that are developed using 

present technology have parasitic side lobes that are 

emitted 30–40o off the main beam acoustic beam. Sidelobe interference is caused when the parasitic side lobe 

of an acoustic beam strikes the bottom before the main 

beam finishes traversing the total depth (fig. 1.21).

When the side lobe strikes the bottom, it usually 

swamps the receivers with an increased amplitude 

signal that smears the velocity information that is being 

gathered from the main-beam echo. On a 1,200-kHz 

BB-ADCP system, the loss of vertical profiling range 

because of this effect is approximately 15 percent with 

30o-beam angles and 6 percent with 20o-beam angles. 

Figure 1.22 shows a screen shot of a typical 

Doppler profile showing the backscattered amplitude 

or, using an RDI-coined term, the “reflected signal 

Figure 1.21. Acoustic Doppler current profiler transducer beam pattern.
12 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Bottom Ping - RSSI 

RSSI - Reflected Signal Strength Indicator

strength indicator” (RSSI). Notice that the RSSI falls 

off logarithmically with depth until the side lobes strike 

the bottom. A sharp increase in the RSSI occurs when 

the side lobes hit the bottom because the bottom is such 

a strong reflector. Another reflection is seen below the 

first bottom reflection. This is called a multiple, and it 

occurs when the signal is reflected off the bottom, 

travels to the surface, and is reflected off the surface. 

Note that the multiple is twice the depth as the 

“original” bottom echo.

Effects of Different Beam Angles

Although using smaller beam angles increases 

the percentage of the profile that is measured, the 

precision of water-velocity measurements also is 

reduced because of decreased coupling with the 

horizontal. The formula for this calculation is shown in 

equation 1.4 (R.D. Instruments, Inc., 1989):

Rm = D A cos( ) (1.4)

where

 = maximum measurable range, in meters;

 = distance from the ADCP to the channel 

bottom, in meters; and

 = angle of the beam relative to the vertical, in 

degrees.

The standard deviation of single-ping watervelocity measurements decreases as a function of 

beam-pointing angle (angle between beam and the 

vertical). If standard-deviation values are known for a 

given beam angle, they can be predicted for other beam 

angles using the ratio of the sines of the angles (eq. 1.5).

(1.5)

where 

 = predicted single ping standard deviation of 

measured horizontal water velocity, in 

centimeters per second;

Rm

D

A

σu

σ

a

sin( ) A

sin( ) a = ------------------------

σu

Figure 1.22. Reflected signal strength indicators (RSSI) for a four-beam acoustic Doppler current profiler. 
Chapter 1: Theory of Operation 13

= single-ping standard deviation of measured 

horizontal water velocity, in centimeters 

per second, using beam angle a;

 = beam angle a, in degrees; and

 = beam angle of predicted measurement, in 

degrees.

Note that approaches infinity as the beam 

angle approaches zero (fig. 1.9).

Blanking Distance

After transmitting acoustic pulses, the 

transducers and associated electronics must rest a short 

time before receiving the reflected acoustic signals. A 

good analogy of this effect is a large gong. The 

vibrations from a gong take a long time to die out 

(sometimes several minutes). A transducer ceramic is 

similar to a miniature gong in that the pulse (ping) 

vibrations at the 1,200-kHz resonant frequency must be 

allowed to die out before the transducer is used as a 

listening device. These tiny vibrations last about 

170 microseconds. During that time, the acoustic pulse 

has traveled about 0.3 m (0.98 ft) if sound velocity is 

assumed to be 1,500 meters per second (m/s) 

[4,921 feet per second (ft/s)], and velocity measurements cannot be made within that distance (fig. 1.21).

The actual distance to the first measured bin 

depends on several factors:

•Blanking distance

•Speed of sound

•Operating mode

•Bin size

•Transmit frequency

•Transducer beam angles

Instrument Development: Solving the Problem of 

Velocity-Measurement Uncertainty

Aside from the instrument errors discussed in the 

previous section, most early ADCP measurement 

systems suffered from “sloppiness,” or in more 

technical terms, “velocity-measurement uncertainty.” 

Single-ping velocity uncertainty for an RDI narrowband 1,200-kHz ADCP purchased in 1986 (using 

default settings), for example, was 13 cm/s (0.43 ft/s). 

Luckily this “sloppiness” is, for the most part, random 

and can be reduced by data averaging. “Narrow-band” 

is defined later in this chapter. The following section 

discusses the reasons for velocity-measurement 

uncertainty and some techniques used to reduce it.

σ

a

A

a

σu

Random and Bias Error

When using an ADCP, two types of errors 

contribute to velocity uncertainty; random error and 

bias. Bias errors are sometimes called systematic errors. 

Random error can be reduced by data averaging; bias 

error cannot. A thorough understanding of these two 

types of errors is a crucial prerequisite to the assessment 

of ADCP velocity and discharge measurement 

accuracy.

Random Error

ADCP sources of random error are as follows:

•Pulse length—The shorter the pulse length for a 

given frequency in a narrow-band ADCP, the 

greater the random error

•Transmit frequency—the lower the frequency at 

a given pulse length (or lag distance), the greater 

the random error

•Signal-to-noise ratio—the lower the signal-tonoise ratio, the greater the random error

•Beam angle—As the beam angle approaches 

vertical, random error approaches infinity

A random velocity-vector error is composed of a 

random magnitude and a random direction. Figure 1.23 

shows 1,920 pings (individual velocity measurements) 

taken on a lake near Sacramento, California, using a 

BB-ADCP. Velocities were taken in still water from 

depth cell number 10 (bin 10) and averaged into 

10 ping groups that gave a total of 192 velocity 

averages. The north and east components were 

averaged for 10 pings each and the resultant averages 

were converted to polar coordinates, then plotted. 

Notice that the error pattern is similar to the pattern of 

a shotgun blast, with the errors evenly distributed 

around zero. A directional bias would show as a 

nonuniform pattern of data points distributed along one 

of the directional axes.

Random error is reduced by the square root of the 

number of samples. When data averaging reduces 

random error magnitudes below the value of bias errors, 

further averaging becomes superfluous. Figure 1.24 

shows the same set of data as in figure 1.23 averaged for 

about 200 pings; the same set of data yields nine 

averages.

Bias Error

A velocity-vector bias has a fixed magnitude and 

direction that either is constant or proportional to the 

measured velocity. Bias error is nonrandom and, 

therefore, cannot be reduced by data averaging. 

Fortunately, in most cases, bias error in ADCPmeasured velocities and discharge measurements is 
14 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

foot per second

270 degrees

90 degrees

180 degrees 360 degrees

1.2 0.8 0.4 0 0.4 0.8 1.2

small. Examining figure 1.24 reveals that there may be 

a small bias error or an actual water velocity in the lake 

at a direction of 250o. Random and bias error are 

discussed in more detail in this report in the sections on 

velocity and discharge-measurement errors.

Pitch and Roll

Pitch is defined as rotation along the fore/aft axis 

of the ADCP, whereas roll is defined as rotation in the 

direction of the starboard/port axis of the ADCP. Most 

ADCP systems contain instruments that detect the 

magnitude of pitch and roll, as well as methods to 

correct ADCP-measured velocities for the effects of 

pitch and roll. Figure 1.25 shows the pitch and roll axes 

as they apply to a boat-mounted ADCP.

Corrections for pitch and roll of an ADCP must 

address velocity corrections and depth corrections. The 

velocity corrections are needed because the geometry 

of the beam angles change, with regard to the flow field 

(eq. 1.3, fig. 1.8), during instances of pitch and roll.

Changes in bin depths also are evident during 

pitch and roll occurrences (fig. 1.26). Bottom depths 

and bin depths must be “remapped” during an ADCP 

pitch and roll occurrence. For small angles of pitch and 

roll, these corrections are not significant unless velocity 

profiles in all three orthogonal coordinates are desired. 

Values of horizontal water velocity are a function of the 

cosine of the pitch and roll, which is insignificant for 

angles less than 5o. However, if accurate vertical 

velocities are desired, even small amounts of pitch and 

roll can significantly affect accuracies. ADCPs 

commonly are designed with pendulum-type pitch and 

roll sensors, which are affected by acceleration. 

However, if an ADCP is expected to be used primarily 

aboard a vessel in areas having large waves, then fastresponding gyroscope systems should be used to 

compensate for pitch and roll.

Figure 1.23. Polar plot of 10-ping broad-band acoustic Doppler current profiler velocity averages.
Chapter 1: Theory of Operation 15

foot per second

270 degrees

90 degrees

180 degrees 360 degrees

1.2 0.8 0.4 0 0.4 0.8 1.2

Beam-Angle Error

Errors in the beam angles could have been a 

significant source of bias error with early ADCP 

systems before the manufacturer instituted qualityassurance procedures to minimize this type of error. 

Beam-angle errors are best detected on a fixed distance 

course. The manufacturer has developed a computer 

program that accurately calculates ADCP beam-angle 

errors, based on data that are collected on the fixed 

distance, lake, or bay course. Beam-angle errors also 

will show as biases during intercomparison tests with 

conventional discharge measurements or other 

discharge-measurement devices.

Beam-angle errors can be eliminated in recently 

developed (after 1993) ADCP firmware by introducing 

corrections into the ADCP system flash memory. This 

procedure should be done only by the manufacturer. 

Suspect systems should be sent to the manufacturer for 

beam-angle testing and recalibration.

Narrow-Band and Broad-Band Doppler Shift Measurements

“Narrow band” is not a very descriptive term and 

is used here only because the term is used in the 

industry to describe a certain type of ADCP instrument. 

The term is used to describe a pulse-to-pulse incoherent 

ADCP. This means that in a narrow-band ADCP, only 

one pulse is transmitted into the water, per beam per 

measurement (ping), and the resolution of Doppler shift 

must take place during the duration of the received 

pulse. In the case of RDI-manufactured narrow-band 

ADCPs, this is accomplished using an autocorrelation 

technique.

The broad-band (BB) ADCP was developed by 

RDI in an attempt to solve some of the measurement 

uncertainty problems seen with the narrow-band ADCP. 

In a BB-ADCP, the Doppler shift is resolved by 

transmitting two pulses of the same shape that are in 

phase with each other (pulse-to-pulse coherent). The 

following section describes, in detail, the operation of 

narrow-band ADCPs and BB-ADCPs.

Figure 1.24. Polar plot of about 200-ping broad-band acoustic Doppler current profiler velocity averages.
16 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Pitch

axis

+

-

Roll

axis

+

-

USGS

Depth cell Depth cell

mapping

Pitch or roll angle

Narrow-Band Doppler Shift Measurements

Doppler shift can be described as the perceived 

frequency shift of a transmitted (and then reflected) 

signal caused by the movement of the reflector. Doppler 

effect also can be described as the magnitude of the 

phase difference between two coherent (but independent) samples of a reflected signal. The following 

analogy provides an explanation of narrow-band 

Doppler shift velocity measurements.

Joel Gast (R.D. Instruments, Inc., oral 

commun., 1992) has likened a narrow-band Doppler 

shift measurement to the measurement of automobile 

speed on a freeway (at night) using a strobe light and a 

high-speed camera. Consider a freeway at night with 

traffic moving at a steady rate of speed. A camera has 

been placed near the freeway and posts have been 

installed at set distances within the camera's field of 

view. A strobe light is actuated and, while the freeway 

is illuminated by the single-strobe pulse, the camera 

takes two high-speed photographs. When the 

investigator examines the photographic negatives he 

finds that by lining up (synchronizing) the images of the 

cars on the two photographic negatives, the distance 

traveled by the cars can be determined by measuring the 

apparent shift in position of the reference posts 

(fig. 1.27). The auto speeds also can be calculated by 

multiplying intrapost distance by the lag time between 

the two photos. If the strobe flashes become acoustic 

pulses, the cars become reflective particles in the water 

column, and the negatives become the received 

Figure 1.25. Pitch and roll axes for a boat-mounted acoustic Doppler 

current profiler.

Figure 1.26. Bin positions during an acoustic Doppler current profiler roll occurrence.
Chapter 1: Theory of Operation 17

SHIFT

PHOTOGRAPH

NUMBER ONE

PHOTOGRAPH

NUMBER TWO

COMPOSITE PHOTOGRAPH

T

L = Lag

DEPTH

Pulse Length

Pulse Length = TL + n * TL

RDI sets n = 4/30

Narrow-band ADCP 

T

L = f (Bin Size) 

Ambiguity is resolved

reflected signals, this scenario becomes roughly 

analogous to the workings of a narrow-band ADCP 

system.

The drawback to such a system is that the strobe 

pulse dissipates very quickly and the two photos must 

be taken while the same cars are still illuminated by the 

strobe. This means that time lags are very short and the 

distance traveled by the cars (reflectors) is very short; 

therefore, the car speeds cannot be measured precisely. 

Because of these limitations, velocity measurements 

made using the narrow-band technology are “noisy” 

(have a relatively high random error). Figure 1.28 

shows a diagram of a narrow-band Doppler shift 

measurement. The signal is sampled twice during the 

reception of the reflected signal. The lag-time between 

each measurements is shown as TL. Using an autocorrelation technique, the Doppler shift is then calculated. In the narrow-band ADCP, the pulse length 

depends on the lag (TL) which is a function of bin size. 

A filter scheme that looks at the whole returned signal 

is used to resolve ambiguity.

Broad-Band Doppler Shift Measurements

Using the freeway analogy, if the investigator 

decides to install another camera a distance of ten or 

more car lengths (parallel to the freeway) from the first 

camera, he could actuate a strobe, take a picture with 

the first camera, wait a short time, actuate another 

strobe, then take a picture with the second camera. If the 

strobes are timed correctly, the cars will travel from the 

field of view of the first camera into the field of view of 

the second camera during the time between photos. The 

investigator synchronizes the positions of the cars on 

the two negatives and finds that there is a much longer 

lag time (time between each strobe versus the time 

between two photos taken during the same strobe) and 

that the cars traveled a longer distance. The investigator 

then can calculate the speed of the cars with much 

greater precision than with the single-strobe system. 

The distance between the cameras and the time between 

each strobe must be chosen carefully. If the investigator 

waits too long between strobes, random movement 

between the cars (passing, slowing down, speeding up, 

and so forth) will render the two negatives 

“unmatchable” (uncorrelated). Transmitting a pair of 

pulses (strobes) into the water allows for much longer 

lag times (therefore, more precision) than the narrowband system. The investigator finds, however, that there 

are some “costs” associated with this technique.

One of the most significant costs is self noise. 

The description of self noise again uses the freeway 

analogy. Suppose that, because of limitations in 

photographic technology, the freeway cameras have no 

shutters. Because the investigator must leave the 

camera shutters open, both cameras will “see” the 

traffic illuminated by the two strobes. However, only 

50 percent of the “scenery” will be usable to both 

cameras for correlation purposes. For example, the film 

in camera one is exposed once during the first strobe. 

The cars then travel out of the field of view of camera 

one and into the field of view of camera two. However, 

the film in camera two already has been exposed by the 

flash of the first strobe and, thus, any cars photographed 

have left the field of vision. When the second strobe 

flashes, the film in both cameras is again exposed 

(double exposed) and the cars that were first 

photographed by camera one are now photographed by 

camera two. Because the film has been double exposed, 

only 50 percent of the scenery in each exposure 

contains cars that are common to both cameras.

Figure 1.27. Freeway strobe-light system used to measure vehicle 

speed.

Figure 1.28. Narrow-band acoustic Doppler current profiler (ADCP) 

shift measurement. RDI, R.D. Instruments, Inc.
18 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Transducer

Pulse 2

Pulse 1

Stationary

particle

Lag A

Transducer

Pulse 1

Pulse 2

Lag B

Stationary

Particle

As in the film of the freeway cameras, the 

reflected wave front from the first BB-ADCP pulse-pair 

is again “exposed” by the incident wave front of the 

second pulse and, therefore, is subject to the same 

“double exposure.” The increased noise due to this 

50-percent correlation is reduced by data averaging 

(very narrow pulses can be used, and, therefore, large 

amounts of data can be collected and averaged). 

Without a technique called phase coding (discussed 

later in this section) and a high signal-processing rate, 

BB-ADCP velocity measurements would be less 

precise (because of self noise) than measurements 

made by the narrow-band ADCP system.

The BB-ADCP cannot only measure the phase 

angle differences between pulse pairs, but can measure 

the change in lag spacing between transmitted and 

received pulse pairs (time dilation). This pulse-pair 

measurement concept can be visualized using a series 

of illustrations depicting a stationary particle, a moving 

particle, and the effects of these particles on lag times 

between reflected pulse pairs in a liquid medium. 

Figure 1.29 shows a transmitted pulse pair from a 

stationary source approaching a stationary particle.

Figure 1.30 shows the same pulse pair reflected 

from a stationary particle. In the case of a stationary 

particle, lag A (fig. 1.29) is equal to lag B (fig. 1.30). 

Now assume that the particle is moving away from the 

transducer. Figure 1.31 shows the aforementioned 

transmitted pulse pair approaching a moving particle. 

Figure 1.32 shows the pulse pair after their reflection 

from the moving particle.

Note that, in the case of a moving particle, the 

reflected pulse lag distance (lag B) has increased 

because the particle’s movement delays the reflection 

of the second pulse, relative to the first. Although this 

can be thought of as a time-domain phenomena, it is 

really a description of Doppler effect. The transmitted 

pulse repetition frequencies appear to vary in 

accordance with changes in the speed and direction of 

the transducer and (or) reflector.

Lag distance between the reflected pulses 

increases as the transducer and particle move apart. The 

opposite occurs if the particle and transducer move 

together. The difference between transmitted and 

received lag distance is proportional to the speed of the 

particle (relative to the transducer) or the transducer 

(relative to the particle). The difference in lag distance 

is exaggerated in these examples to aid comprehension. 

Figure 1.29. Acoustic pulse pair approaching a stationary particle. In reality, the ratio of the sound speed in water 

Figure 1.30. Acoustic pulse pair reflected from a stationary 

particle.

Transducer

Pulse 2

Pulse 1

Lag A

Moving

particle

Figure 1.31. Acoustic pulse pair approaching a moving particle.
Chapter 1: Theory of Operation 19

Transducer

Pulse 1

Pulse 2

Lag B

Moving

particle

Delayed

reflection

Same

scatterers

Pulse

b 

Pulse

a 

Pulse

b 

Pulse

a 

 

Time t Time t+1

Transducer

A small, reflected

pressure wave moving

towards the transducer

[1,500 m/s (4,921 ft/s)] to the particle speed 

[0–2 m/s (0–6.6 ft/s)] results in very small lag 

differences. If a particle is moving slowly, the lag 

differences will be small and hard to measure. For that 

reason, discharge measurements of flows with low 

water velocities (0.05 m/s or less) are imprecise using 

the BB-ADCP discharge-measurement system unless 

special methods are employed. The accurate measurement of these lag differences is discussed in the next 

section. 

Self noise can be visualized as shown in 

figure 1.33. When pulse “a” illuminates an object, a 

small pressure wave is reflected back toward the 

transducer. This pressure wave contains pure information about the speed of the object that caused the 

reflection. The passage of pulse “b” through the 

reflected pressure wave again illuminates scatterers in 

the vicinity of the pressure wave and contaminates the 

pure speed information of the pressure wave with 

unwanted noise. This contamination causes a dramatic 

increase in the single-ping random error of the velocity 

determination.

Another cost for using the increased lag spacing 

available with the BB-ADCP system is velocity 

ambiguity. The freeway analogy is not appropriate to 

explain the velocity ambiguity phenomenon, therefore, 

a circular racetrack analogy is used (figs. 1.34a, b). 

Suppose the investigator decides to mount cameras and 

strobe systems in a helicopter that hovers above a 

circular racetrack at night. Both cameras are mounted 

so that the racetrack is within their field of view. The 

circumference of the racetrack has been estimated and 

some reference poles (visible around the edge of the 

track) have been identified. The investigator tests the 

strobe and finds that just the outlines of the cars can be 

seen. The camera shutter is opened, and the first of two 

strobe flashes is actuated. After a short time, the other 

flash is actuated. The investigator rotates the developed 

negatives to synchronize the car outlines and estimates 

their speed by multiplying the distance between the 

poles by the time between strobe flashes.

This speed-measurement system works well until 

the car speeds increase substantially or the investigator 

decides to increase the time between strobe flashes (lag 

times) to improve measurement precision. After the first 

strobe flash, the cars complete one lap (past the spot 

where they were first photographed) before the second 

strobe flash. When the investigator attempts to 

synchronize the negatives, he becomes confused 

because he cannot determine how many (if any) laps the 

cars have completed or whether the cars have gone 

forward or backward.

The “ambiguity velocity” is the velocity the cars 

must achieve before this confusing circumstance 

happens. If the strobe flashes are temporally close, the 

ambiguity velocity is high (higher than the cars 

Figure 1.32. Acoustic pulse pair reflected from a moving particle.

Figure 1.33. Acoustic pulse pair with a small reflected pressure 

wave.
20 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

normally travel) but measurement precision is lower 

because the cars have traveled a shorter distance 

between strobe flashes. If the investigator lengthens the 

time between strobe flashes (lag) to improve the 

measurement precision, the ambiguity velocity 

becomes lower and, therefore, more troublesome.

The primary method of measurement used by 

BB-ADCP systems is the measurement of phase-angle 

differences between the pulse pairs. This measurement 

is subject to ambiguity errors because the yardstick 

used to make these measurements actually is one-half 

of one cycle at the transmitted frequency. Figure 1.35 

shows the error factor when the speed of the measured 

velocity exceeds the ambiguity velocity. The colored 

circle represents one cycle of transmitted energy with a 

possible phase measurement capability of 0–360o. In 

this example, we will let 1 millimeter per second 

(mm/s) equal 1o of phase change. We have no trouble 

measuring plus and minus 10 mm/s or even plus and 

minus 170 mm/s using our one-half cycle yardstick, but 

when the measured velocity is 190 mm/s, our yardstick 

reads a velocity of −170 mm/s. This is an error of 

360 mm/s. Notice that this ambiguity velocity is 

180 mm/s and is equivalent to 180o on our circular 

yardstick. The measurement error (when the ambiguity 

velocity is exceeded) is always equal to two times the 

ambiguity velocity.

The BB-ADCP (and even the narrow-band 

ADCP) may report an erroneous velocity caused by 

ambiguity when scatterer velocities are high enough 

that their movement between lags exceeds one-half of 

the transmitted frequency wavelength. Because a 

1,200-kHz BB-ADCP uses the change in phase of a 

1,200-kHz sinusoid for a Doppler shift “ruler,” it is 

impossible to identify which cycle of the reference 

signal to use when calculating the phase shift between 

the two returned reflections when the reflected signal 

phase shift is greater than one-half of one wave length. 

To solve this problem, the manufacturer has included a 

signal processing technique that corrects for ambiguity 

errors. This technique takes additional time, however, 

and somewhat slows the water ping rate. Shortening the 

lag distance increases the ambiguity velocity, causing it 

to be more noticeable and less bothersome, especially if 

water and bottom velocities are significantly lower than 

the ambiguity velocity. However, decreasing the lag 

distance also increases noise (standard deviation of 

measured velocities).

The ambiguity velocity of a 1,200-kHz 

BB-ADCP can be estimated by equation 1.6 

(L.Gordon, R.D. Instruments, Inc., written commun., 

1992):

(1.6)

where

 = the ambiguity velocity, in meters per 

second; and

 = the lag specified by the BB-ADCP “&L” 

command.

The maximum allowed ambiguity velocity is 

5.2 m/s (17 ft/s) (L = 9) for a 1,200-kHz system. 

Contamination of the measured data shows up as 

velocity spikes at the ambiguity interval (twice the 

ambiguity velocity). This means that if ambiguity errors 

are suspected, the (unaveraged) data should be 

examined for velocity spikes of the opposite sign that, 

when compared with the last good measured velocity, 

have a difference of about twice the ambiguity velocity.

Ua

47

L = -----

Ua

L

Figure 1.34a. Race track analogy during the first strobe flash.

Figure 1.34b. Race track analogy during the second strobe flash.
Chapter 1: Theory of Operation 21

Am biguity Velocity

-V +V

0

180

360

-V +V

0

180

360

-V +V

0

180

360

-V +V

0

180

360

-V +V

0

180

360

-V +V

0

180

360

Example: 180 m m /s = 180 degree phase change

10 d = 10 m m /s

-10 d = -10 m m /s 

170 d = 170 m m /s

-170 d = -170 m m /s 

190 d = -170 m m /s

Expected = 190, measured = -170, result = 360 mm/s error 

An ambiguity error will be 2 multiplied by ambiguity velocity

Differences Between Phase-Shift Measurements and 

Lag-Spacing Measurements (Time Dilation)

An unambiguous measurement can be obtained 

from the returned Doppler information by looking at 

the change in lag spacing (figs. 1.31, 1.32) but this 

method of measuring time dilation is much less precise 

than the measurement of phase-angle difference. 

Figure 1.36 shows the differences between phase angle 

and time dilation. Note that even though the ambiguity 

velocity has been exceeded in the last example, the 

time-dilation measurement still provides a “ball-park” 

measurement of velocity. The time-dilation method 

usually is used to resolve ambiguities.

Bottom-Tracking Limitations

To measure discharge, the ADCP must sense and 

measure the velocity of the ADCP, relative to the river 

bottom (bottom tracking). If the velocity of the water is 

known, relative to the ADCP, and the velocity of the 

ADCP is known, relative to the river bottom, then the 

water velocity, relative to the bottom, can be calculated. 

The bottom-track pulse must be somewhat longer than 

the water-track pulse to properly illuminate the bottom 

(fig. 1.17). In many cases, a group of water-velocity 

pings is averaged along with one or more bottom-track 

pings to form an averaged ensemble. To compute 

discharge, the ADCP must provide the horizontal water 

and boat velocity components, depth, and time between 

ensembles. The ADCP discharge-measurement 

software discussed in chapter 2 uses these data to 

calculate a vector cross product at each bin and then 

uses an extrapolation scheme to estimate the cross 

products in the unmeasured areas near the top and 

bottom of the profile (eq. 2.2, chap. 2).

The discharge-measurement software then 

integrates these cross products over the profile depth to 

obtain a mean, depth-weighted cross product for each 

ensemble. The depth-weighted cross products are 

summed during the cross-section traverse to produce 

the discharge measurement (eq. 2.2, chap. 2). Bottom 

tracking is required by the discharge-measurement 

software to compute discharge. If the river bed is 

moving, or if the bottom-track velocities are affected by 

material moving near the bottom, then the cross product 

will be biased. This problem sometimes can be 

Figure 1.35. Explanation of ambiguity velocity. mm/s, millimeter per second; V, velocity; d, degree.
22 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

alleviated by shortening the bottom-track pulse length 

(as shown in fig. 1.37) using the “&R” direct command 

as discussed in chapter 5. If shortening the pulse width 

does not work and if the river bed is moving at a speed 

of more than a few centimeters per minute, this bottomtrack error cannot be eliminated and other means must 

be used to determine boat velocity (chap. 8).

The Broad-Band Acoustic Doppler Current 

Profiler: Overcoming the Self-Noise Problem

Error Sources Unique to Broad-Band Acoustic Doppler 

Current Profilers

In the previous section we discussed general 

ADCP errors. To enable a more complete 

understanding of the broad-band technology, we will 

touch upon those error sources again showing their 

effects on BB-ADCP systems. Although many sources 

of error for narrow-band ADCP systems have been 

discussed by Hansen (1986), Theriault (1986), 

Chereskin and others (1989), and Simpson and 

Oltmann (1993), few investigators have identified and 

itemized the error sources that can degrade accuracy 

and precision of the more recent BB-ADCP technology. 

Some of the known sources of error that affect the 

accuracy of velocity measurements (and, therefore, 

discharge measurements) and errors due to the physical 

limitations of the system are listed in this section. The 

signal processing technology required to accomplish 

water-velocity measurements with the BB-ADCP is 

extremely complex; the manufacturer and users will 

undoubtedly find new error sources during operational 

use of the BB-ADCP system. Major error sources in the 

narrow-band ADCP systems were identified over a 

period of 5 years (1986–91), and it is possible that the 

same length of time will be required to fully understand 

the sources of error in the BB-ADCP system, as well.

Errors that affect the performance of BB-ADCPs 

for velocity measurements (and, therefore, discharge 

measurements) can be either random or bias. As 

discussed earlier in this chapter, random errors can be 

reduced by data averaging; bias errors cannot. For 

purposes of this report, random errors are assumed to be 

Gaussian (normally distributed about the mean) and are 

expressed as standard deviations (in percent) of the 

measured mean quantity. Bias errors have sign and 

Figure 1.36. Description of time dilation compared with phase-angle difference.

0

40

400

B A

B A

Phase Angle and Time Dilation
Chapter 1: Theory of Operation 23

Long Pulse

Small area illuminated Large area illuminated

Short pulse

near bottom

sediment

suspended

sediment

Bottom

Long backscattered pulse width

Short backscattered

pulse width

magnitude and are expressed as a percent of the “true” 

mean quantity, where “true” is defined as unbiased.

This report will not attempt a complete 

discussion of BB-ADCP bias error sources related to 

the physics of the acoustic signal (other than beamangle errors and depth-measurement errors) because 

many of these sources are not yet documented and are 

beyond the scope of this report. Bias errors for the 

narrow-band ADCP system (with some application to 

the BB-ADCP system) are discussed in Simpson and 

Oltmann (1993). The most overwhelming source of 

velocity-measurement error in the BB-ADCP is 

random uncertainty due to self noise.

Random Uncertainty Caused by Self Noise

Whenever a pair of pulses is used to measure 

water velocity (using lag times associated with the 

BB-ADCP), only a 50-percent correlation can be 

obtained from the scenery illuminated by the pulse pair 

when attempts are made to synchronize (autocorrelate) 

the reflected signals (as discussed earlier in this 

chapter). Figure 1.33 shows an acoustic beam with a 

pair of long acoustic pulses being transmitted into the 

water (pulse “a” and pulse “b”). Directly after the 

transmission of the pulse pair, the receiver begins 

“listening” to the pressure waves that have been 

reflected from scatterers in the water column. At time t, 

the reflected signals from scatterers in the water mass, 

illuminated by pulse a, begin their return path toward 

the transducers in the form of a small pressure wave. 

The small pressure wave advances toward the 

transducer for a short time before it is illuminated by 

pulse b traveling in the opposite direction. The passage 

of pulse b causes instantaneous reflected signals to be 

superimposed on the original reflected signals in the 

small pressure wave, creating a “double exposure.” This 

is the 50-percent decorrelation effect caused by self 

noise (discussed above). The reflected signals (actually 

a continuum of reflected signals) travel back to the 

transducer. These signals are received and shunted to a 

delay-line register (or scratch-pad memory) for a short 

time while the BB-ADCP signal processor applies an 

autocorrelation technique to the received signals in an 

Figure 1.37. Effect of bottom-track pulse width on bias caused by bottom movement.
24 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Transducer

100

narrow

pulses

Pulse b

Pulse a

Pulse b

Pulse a

Transducer

Pulse 2 of a phase- coded group

Pulse 1 of a phase-coded group

attempt to synchronize (match) reflections obtained 

from the same water mass (data separated by the time 

between t and t + 1, as shown in fig. 1.33). If the lag is 

matched correctly, the autocorrelation function of the 

reflected signals will reach a peak. Because the two 

pulses were transmitted coherently (with the same 

phase), amplitude and phase information can be 

calculated from the function output. By using the phase 

information, the speed of the reflective particles can be 

determined, however, measurement precision is limited 

because of self noise.

Suppose very narrow (short) pulses are 

transmitted at a and b (fig. 1.38). These pulses are so 

narrow that 100 of them can be placed into the space 

occupied by the original long pulses. This modification 

will increase measurement precision by the square root 

of the number of samples (in the case of 100 samples, 

by a factor of 10). With this increased precision (even 

with the 50-percent level of self noise), the BB-ADCP 

capabilities surpass those of the narrow-band ADCP by 

almost one order of magnitude. This increased 

precision is gained at great cost because of the limited 

amount of energy the narrow pulses can deliver into the 

water. This energy loss caused by the narrow pulses is 

so great that it renders the system nearly unusable. To 

overcome this energy loss, the manufacturer developed 

a design innovation that incorporates most of the 

advantages of wide and narrow pulses. A wide pulse is 

transmitted (therefore, delivering more energy into the 

water than a narrow pulse), but is logically split into 

many small segments called code elements, each 

having a phase shift of either 0o or 180o (fig. 1.39).

The coding order of these phase shifts is pseudo 

random (behaves numerically like a random sequence). 

This technique has previously been applied to radar 

signals and some spread-spectrum communications 

signals (Minkoff, 1992), but the BB-ADCP 

manufacturer probably is the first to use this technique 

for water-velocity-measurement sonar.

The consequence of transmitting this phasecoded pulse-pair series into the water is that even 

though the pulses are long, the signal processor still 

must wait the full lag period (a to b) before achieving an 

autocorrelation peak of significant amplitude 

(fig. 1.40). In other words, because of the phase coding, 

it is difficult for the autocorrelation algorithm to realize 

a peak at the wrong interval.

The objective of the manufacturer is to achieve 

decorrelation of adjacent pulse pairs and, therefore, a 

Figure 1.38. Narrow pulse pairs compared with wide pulse pairs. Figure 1.39. Phase-coded pulse pair.
Chapter 1: Theory of Operation 25

Contents of a delay line

 register with matching

 code element spacing

Contents of a delay line

 register with a delay difference

of one code element

Autocorrelation function

output

Autocorrelation function

output

Y

X

Y

X

greater effective N (number of samples used for data 

averaging). Effective N as opposed to actual N is 

discussed in chapter 9. Obviously, there are times when 

accidental correlation occurs because currently there 

are only two phase choices, but, overall, a much greater 

precision is achieved using phase coding (because of 

improved signal-to-noise ratio) than by simply using a 

narrow pulse pair. The principle reason the 

manufacturers named the ADCP “broad-band” was that 

its bandwidth was increased to accommodate the signal 

processing of narrow-pulse pair (coded or not). The 

amount of random uncertainty in velocity measurements due to self noise has not been formally presented 

by the BB-ADCP manufacturer, but presumably is 

contained in the performance data for overall system 

random uncertainty (noise) values predicted by the 

manufacturer's error model and presented later in this 

chapter. When a phase coding method is used to reduce 

random uncertainty due to self noise, its effectiveness is 

greatly dependent on the order and number of the 

pseudo-random code elements used to construct the 

measurement pulse pair.

Figure 1.40. Effects of code element lag on correlation.
26 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

The single-ping random error of a narrow-band 

ADCP is significantly higher than a BB-ADCP for a 

given operating frequency and bin size. However, the 

signal processing requirements of the BB-ADCP 

system are much greater than those of the narrow-band 

ADCP, slowing the ping rate markedly. The ping rate of 

a narrow-band ADCP system can be as high as 10 pings 

per second (or higher, depending on depth and 

transducer frequency), whereas the maximum ping rate 

of a BB-ADCP system is less than three pings per 

second (using present technology). Table 1.1 is an 

example of the net result of these effects (for 30o beamangle systems).

The narrow-band ADCP has a 5-second average 

standard deviation that is comparable to the BB-ADCP 

if the BB-ADCP bin size is one-fourth that of the 

narrow-band ADCP. This means that the BB-ADCP has 

a higher resolution and a smaller bin size, and can be 

used in shallower water than the narrow-band ADCP 

system for a given operating frequency.

Future systems using the narrow-band ADCP 

technology should not be ruled out. If an adaptive 

scheme were used to increase the narrow-band ADCP 

ping rate and reduce the narrow-band ADCP bin size in 

shallow water, discharge measurements using either 

system would have comparable accuracies. Because of 

larger, more energetic pulses, the narrow-band ADCP 

system also has a slightly greater range for a given 

frequency than the BB-ADCP system, as well as a more 

robust bottom-tracking ability.

Summary

Narrow-band acoustic Doppler current profilers 

(ADCPs) and broad-band acoustic Doppler current 

profilers (BB-ADCPs) use the Doppler principle to 

measure profiles of water velocity. To measure 

discharge, they also must measure velocity of the 

ADCP, relative to the river or estuary bottom.

Narrow-band ADCPs use a measurement method 

called pulse-to-pulse incoherent velocity measurement, 

which means that the ADCP transmits single, 

independent acoustic pulses from each beam and 

resolves the Doppler frequency shift during the duration 

of a single pulse. The frequency determination method 

usually is an autocorrelation technique.

BB-ADCPs use two or more coherent 

(synchronized) acoustic pulses in a scheme called the 

pulse-to-pulse coherent method. The frequency 

determination method usually is an autocorrelation 

technique that measures the phase angle difference and 

the time difference (spacing) between the transmitted 

and received pulse pairs to determine Doppler shift.

The single-ping random error of a narrow-band 

ADCP is significantly higher than a BB-ADCP for a 

given operating frequency and bin size, however, the 

signal processing requirements of the BB-ADCP 

system are much greater than those of the narrow-band 

ADCP, slowing the ping-rate markedly. 

Increasing the lag distance between the pulse 

pairs in a BB-ADCP system lowers the single-ping 

standard deviation (to a point). Longer lag times 

increase the chances of ambiguity errors and should be 

used with caution, especially when averaging data.

Table 1.1. Comparison of narrow-band and broad-band single-ping standard deviation

[m, meter; ft, foot; cm/s, centimeter per second; ft/s, foot per second] 

Property Narrow-band Broad-band

Single-ping standard deviation................................................................ 13.5 cm/s (0.44 ft/s) 6.4 cm/s (0.21 ft/s)

Minimum bin size ................................................................................... 1. m (3.3 ft) .25 m (.82 ft)

Pings per second ..................................................................................... 8 2

Standard deviation of a 5-second average............................................... 2.1 cm/s (.07 ft/s) 1.81 cm/s (.06 ft/s)
Chapter 2: Acoustic Doppler Current Profiler Discharge-Measurement Principles 27

CHAPTER 2: ACOUSTIC DOPPLER CURRENT 

PROFILER DISCHARGE-MEASUREMENT 

PRINCIPLES

In chapter 1, narrow-band ADCP and BB-ADCP 

velocity measurements were discussed in detail. In this 

chapter, we will discuss the methods used to calculate 

discharge from data collected using an ADCP. A basic 

knowledge of conventional river dischargemeasurement techniques is necessary to understand 

how an ADCP measures discharge. Conventional 

ADCP discharge-measurement techniques are covered 

in Buchanan and Somers (1969).

Parts of an Acoustic Doppler Current Profiler 

Discharge Measurement

Just how is an ADCP used to measure discharge? 

The ADCP could be used as a conventional current 

meter. If an ADCP were mounted to a boat, the operator 

could position the boat at 30 or more stations (verticals) 

in the cross section. Velocities and depths could then be 

taken at each vertical, and the discharge calculated 

using the area/velocity method ( ). Such a 

method would be only a slight improvement over the 

conventional boat-tagline discharge-measurement 

techniques.

The unique ability of the ADCP to continuously 

collect water-velocity profile data and ADCP-velocity 

(boat-velocity) data, relative to the bottom, lends itself 

to the use of a more sophisticated method of dischargemeasurement integration. A velocity vector cross 

product at each depth bin in a vertical profile is 

calculated using ADCP-collected data. This cross 

product is first integrated over the water depth 

measured by the ADCP and then integrated, by time, 

over the width of the cross section. The following 

equations illustrate the integration method. The reader 

should try to envision them as descriptions of the 

discharge-measurement algorithm and mechanics.

Velocity Cross-Product Measurement Using an 

Acoustic Doppler Current Profiler

General Equation

The general equation (eq. 2.1) for determining 

river discharge through an arbitrary surface, , is

(2.1)

Q AV =

s

Qt V f ⋅ n ds s

∫ =

where

 = total river discharge, in cubic meters per 

second;

 = mean water-velocity vector, in meters per 

second;

 = a unit vector normal to at a general point; 

and

 = differential area; in meters.

The General Equation, as Applied to Acoustic Doppler 

Current Profiler Moving-Boat Measurements

The above is just a form of the familiar equation 

 integrated over a cross section. For movingboat discharge applications, the area is defined by the 

vertical surface beneath the path along which the vessel 

travels. The dot product of will equal zero when 

the vessel is moving directly upstream or downstream, 

and will equal when the vessel is moving normal to 

; both vectors are in the horizontal plane.

Because the ADCP provides vessel-velocity and 

water-velocity data in the vessel’s coordinate system, it 

is convenient to recast equation 2.1 in the following 

form (eq. 2.2) (Christensen and Herrick, 1982):

(2.2)

where

 = total cross-section traverse time, in seconds;

 = total depth, in meters;

 = mean vessel-velocity vector, in meters per 

second;

 = a unit vector in the vertical direction;

 = vertical differential depth, in meters; and

 = differential time, in seconds.

The derivation of this equation by Christensen 

and Herrick (1982) is summarized in Simpson and 

Oltmann (1993). The equation originally was 

formulated by Kent Dienes (R.D. Instruments, Inc., oral 

commun., 1986).

The cross-product algorithm is well suited to 

ADCP discharge-measurement systems. Translated 

into nonmath terms, the above can be described as the 

cross product of the boat velocity and the water velocity 

first integrated over the cross-section depth and then 

integrated, by time, over the cross-section width 

(fig. 2.1).

Qt

V f

n ds

ds

Q AV =

s

V f ⋅ n

V f

V f

Qt ( ) ( ) V f × Vb ⋅ k d t z d

O

d

∫

O

T

= ∫

T

d

Vb

k

dz

dt
28 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Flow

+

-

f > 0 f = 0

f = 0 f = 0

Boat traversing moving water Boat traversing still water

Boat idle in still water Boat idle in moving water

The cross product part of equation 2.2, 

, can be converted to rectangular 

coordinates to facilitate plugging in boat- and vesselvelocity vectors (eq. 2.3).

, (2.3)

where

= cross component of the mean water-velocity 

vector, in meters per second;

 = fore/aft component of the mean watervelocity vector, in meters per second;

 = cross component of the mean vesselvelocity vector, in meters per second; and 

 = fore/aft component of the mean vesselvelocity vector, in meters per second.

This is simply called the velocity cross product, 

which can be represented as shown in equation 2.4:

, (2.4)

where

 = the cross product of the water-velocity and 

boat-velocity vectors.

Properties of the Acoustic Doppler Current Profiler 

Measured Cross Product

Figure 2.2 shows the properties of the cross 

product. Note that the boat must be traversing a velocity 

field before the cross product becomes greater than 

( ) Vf × Vb ⋅ k

( ) Vf × Vb ⋅ k = a1b2 – a2b1

a1

a2

b1

b2

f a1b2 a2b1 = –

f

zero. In practice, several ADCP pings often are 

averaged to help reduce random error. This group of 

averaged water- and bottom-track velocity 

measurements is called an ensemble. The cross product 

is calculated from the averaged ensemble velocities and 

is expressed in units of square meters per second per 

second.

Integrating the Cross Product Over the Water Depth

When the cross product is integrated over depth, 

the resulting value is in cubic meters per second per 

second, and by substituting in the values for the 

beginning and ending times of each ensemble, a 

discharge value (cubic meters per second) is determined 

for each measured ensemble. The discharges for each 

ensemble then are summed to obtain total channel 

discharge (fig. 2.1). The mechanics of this operation 

require casting equation 2.2 into a form usable by the 

ADCP measurement software (eq. 2.5):

(2.5)

where

 = measured channel discharge (doesn’t 

include the unmeasured near-shore 

discharge), in cubic meters per second;

 = number of measured discharge subsections;

 = index of a subsection;

 = depth of a subsection, in meters;

 = value of cross product at depth ;

Qm fz dz

0

d

∫

i = 1

Ns

= ∑ ti

Qm

Ns

i

d

fz z

Figure 2.1. Cross-product vectors during a cross-section traverse. Figure 2.2. Properties of the water-velocity/boat-velocity cross 

product.
Chapter 2: Acoustic Doppler Current Profiler Discharge-Measurement Principles 29

Side lobe Main beam Side lobe

Draft

Blanking

distance

Area

of

measured

discharge

Side lobe

interference

Streambed

Maximum

slant

range

 = integrated vertical depth of subsection , in 

meters; and

 = elapsed traveltime between the ends of 

subsections and , in seconds.

Estimating Cross Products in the Unmeasured Portions of the 

Profile

Problems arise when trying to implement the 

above summation for several reasons. To understand 

those reasons, the limitations of an ADCP watervelocity measurement are reexamined in the following 

sections.

Blanking Distance

Blanking distance was discussed in chapter 1, 

but is examined again here. Figure 2.3 is a modified 

version of figure 1.21 showing the unmeasured parts of 

a vertical-velocity profile. As discussed in chapter 1, 

the small ceramic element in the transducer is like a 

miniature gong. The large voltage spike of the transmit 

pulse bangs it like a hammer, and the vibrations, as well 

as the residual signal, must die off before the transducer 

can be used to receive incoming signals. This means 

that incoming signals are not used if they are received 

during a short period after the signal is transmitted. 

This time period is equivalent to a distance traveled by 

the signal known as the blanking distance. The 

blanking distance plus the transducer draft (depth of the 

transducer face below the water surface) compose a 

part of the near-surface profile that is not measured by 

the ADCP.

dz i

ti

i i – 1

Side-Lobe Interference

Most transducers emit unwanted (parasitic) side 

lobes of acoustic energy at 30–40o angles off the main 

beam. Figure 2.3 shows a slice of a typical transducer 

beam pattern. The side lobe probably is a single, hollow 

cone with its apex at the transducer (fig. 2.4). This is 

only the author’s mental image of the three-dimensional 

side-lobe structure, but the image probably is a 

simplistic view of the true pattern. This threedimensional structure should be kept in mind when 

profiling close to obstacles. Most acoustic beams have 

parasitic side lobes (including external depth sounders 

that may be mounted on the vessel). As discussed in 

detail in chapter 1, up to 15 percent (depending on the 

beam angle) of the water column near the bottom 

cannot be measured because of interference from these 

side lobes.

The combination of the effects of transducer 

draft, blanking distance, and side-lobe interference 

yields a profile that is incomplete. To properly compute 

discharge for each subsection, the cross-product values 

near the water surface and near the bottom must be 

estimated.

As shown in figure 2.5, values are not provided 

at or near the water surface and below a point equal to 

85–94 percent of the total depth. The percent of 

unmeasured profile area depends on the beam angle. 

The unknown values are labeled at the water 

surface and at the channel bottom. The simplest 

method of estimating these values would be to let 

at the surface equal and to let at the bottom equal 

the last measured value ( ) and approximate the 

f

f f1

fn

f f1

f2 fn

fn – 1

Figure 2.3. Acoustic Doppler current profiler-beam pattern showing 

side-lobe features. Figure 2.4. Hypothetical shape of a parasitic, side-lobe pattern.
30 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

1 1.5 2 2.5 3 3.5 4

f VALUE, IN SQUARE FEET PER SECOND, PER SECOND

DISTANCE FROM WATER SURFACE (

Z), IN FEET

WATER SURFACE

f 2

f 3

f 4

f 5

f n

Z6 Z5 Z4 Z3 Z2 Z1

0.5

f VALUES

Measured

Missing

f 1

0

2

4

6

8

10

12

1 1.5 2 2.5 3 3.5 4

f VALUE, IN SQUARE FEET PER SECOND, PER SECOND

12

10

8

6

4

2

0

DISTANCE FROM WATER SURFACE, IN FEET

WATER SURFACE

f 2

f 3

f 4

f 5

f 6

0.5

f VALUES

Estimated

Measured

1/6 power

 curve

f 1

integral in equation 2.5 using a trapezoidal calculation 

(eq. 2.6):

, (2.6)

where

 = depth-weighted mean value in 

measurement subsection , in square 

meters per second per second;

 = index of depths and respective values;

 = number of measured and estimated values; 

and

 = depth from water surface of respective 

measured and estimated values.

Although equation 2.6 appears intimidating, it is 

nearly the same method as used on standard USGS 

discharge-measurement notes to sum discharge. In this 

case, however, it is used from surface to bottom rather 

than across the width of the river.

Evolution of the One-Sixth Power-Curve Estimation Technique

The above method might work, but, in most 

cases, water at the surface is moving faster than water 

deeper in the profile, and the water near the bottom of 

the profile slows to zero velocity as it nears the channel 

bed (assuming a stationary bed). There are exceptions 

gi

fj f + j + 1

2 -----------------

 

 

 

i = 1

n – 1

= ∑ zj + 1 z – j ( )

gi f

i

j f

n f

zj

f

to this rule, especially in an estuary, but for general use, 

the top-most and bottom-most values in an ADCPmeasured profile must be estimated to calculate an 

accurate ADCP discharge measurement.

The author attempted to calculate discharge using 

several different methods for profile estimation 

(Logarithmic and general power law), but found that 

because of “noisiness” of the ADCP-profile data, the 

resulting least-squares-derived estimates were 

unrealistic, especially near the upper part of the profile. 

A method using a one-sixth power law (Chen, 1989) 

eventually was chosen because of its robust noise 

rejection capability during most streamflow conditions. 

A full discussion of the one-sixth power law and its 

derivation can be found in Simpson and 

Oltmann (1993). The power law estimation scheme is 

an approximation only and emulates a Manning-like 

vertical distribution of horizontal water velocities. 

Different power coefficients can be used (one-half to 

one-tenth) to adjust the shape of the curve fit to emulate 

profiles measured in an estuarine environment or in 

areas that have bedforms that produce nonstandard 

hydrologic conditions and provide alternate estimation 

schemes under those circumstances. Figure 2.6 shows a 

one-sixth power curve drawn through the same set of 

 values that were illustrated in figure 2.5.

In cases where bidirectional flow exists (water is 

moving one direction at the surface and is moving the 

opposite direction near the channel bottom), the powercurve estimation scheme will not work. In these cases, 

the unmeasured areas must be estimated using other 

methods.

f

Figure 2.5. Example velocity profile showing measured and missing 

f values.

Figure 2.6. Example velocity profile of one-sixth power-curve fit 

and typical values. f
Chapter 2: Acoustic Doppler Current Profiler Discharge-Measurement Principles 31

Area of measured discharge

Unmeasured area due to side-lobe interference

Unmeasured area due to blanking distance and transducer draft

Unmeasured

near-shore

discharge

Unmeasured

near-shore

discharge

In most cases, points at the top and the bottom of 

the profile can be estimated using the one-sixth powercurve estimation scheme. The estimated points then are 

used with the actual measured points, to calculate a 

depth-weighted mean cross product for each ensemble. 

Discharge then can be calculated for each ensemble 

pair because the time between each ensemble is known.

Integration, By Time, Over the Width of the Cross Section

The summation in equation 2.5 is accomplished 

by equation 2.7

(2.7)

where

 = midsection discharge between measurement 

subsection and subsection , in cubic 

meters per second;

 = depth-weighted mean value in 

measurement subsection , in square 

meters per second per second; and

 = vessel traveltime between measurement 

subsection and , in seconds.

Equation 2.5 is used to calculate the main 

channel discharge by summing all Q values (eq. 2.7) 

collected during the cross-section traverse. Unfortunately, before the total channel discharge can be 

calculated, two other areas need estimation schemes 

(fig. 2.7).

Estimating Discharge Near the Channel Banks

The power-curve fitting scheme estimates values 

in the areas at the top and bottom of the profile 

(blanking/draft distance and side-lobe interference 

qi git = i

qi

i l – 1

gi f

i

ti

i i – 1

area), but, because of these and other ADCP depth 

limitations, shallow areas near the edges of the 

riverbank cannot be measured. The near-shore areas are 

estimated using a ratio interpolation method presented 

by Fulford and Sauer (1986), which can be used to 

estimate a velocity at an unmeasured location between 

the riverbank and the first or last measured velocity in a 

cross section. The equation for the estimate is 

equation 2.8

(2.8)

where

= a location midway between the riverbank and 

first or last ADCP-measured subsection;

 = estimated mean velocity at location , in 

meters per second;

 = measured mean velocity at the first or last 

ADCP-measured subsection, in meters per 

second;

 = depth at subsection , in meters; and

 = depth at the first or last ADCP-measured 

subsection, in meters.

Fulford and Sauer (1986) defined and as 

depth and velocity at the center of the first or last 

measured subsection and not the near-shore edge of the 

subsection, as presented in equation 2.8. However, 

because the ADCP subsections purposely are kept very 

narrow at the start and finish of each measurement, the 

differences between the two applications are not 

significant (Simpson and Oltmann, 1993). Figure 2.8 

illustrates the estimation method used for near-shore 

discharge. With this method, discharge can be estimated 

by assuming a triangular discharge area between 

subsection and the riverbank, which reduces 

equation 2.6 to equation 2.9

(2.9)

Remembering that , discharge in the 

estimated area can be calculated by equation 2.10.

(2.10)

where 

 = estimated edge discharge, in cubic meters 

per second; and

= distance to the riverbank from the first or last 

ADCP-measured subsection, in meters.

Ve

de

-------- Vm

dm

= ---------

e

Ve e

Vm

de e

dm

dm Vm

m

Ve = 0.707Vm

Q AV =

Qe

0.707VmLdm

2 = ------------------------------

Qe

Figure 2.7. Unmeasured areas in a typical acoustic Doppler current L

profiler discharge-measurement cross section.
32 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

e

L

dm

de v v e m

ve

vm

de

dm

e

L

Estimated mean velocity at

 location e 

Measured mean velocity at

 first or last ADCP-measured 

 subsection

Depth at subsection e 

Depth at first or last ADCP-

 measured subsection 

Distance to the riverbank from the

 first or last ADCP-measured

 subsection

 

 

A location midway between the

 riverbank and first or last ADCP- 

 measured subsection

EXPLANATION

The discharge-measurement software calculates 

depth ( ) from the average of the depths measured on 

all four beams. The distance ( ) to the riverbank from 

the first or last discharge measurement subsection is 

provided by the system operator using estimation 

dm

L

techniques described in the chapter on dischargemeasurement techniques.

The triangular ratio-interpolation method works 

well in parabolic-shaped natural channels, however, it 

does not work well in rectangular concrete channels or 

natural channels with nonstandard slopes near the 

banks. In these cases, a bank-slope coefficient can be 

used to properly depict the channel-bank geometry (see 

chapter 5.)

Determination of Total River Discharge—Putting 

it All Together

Using all the methods and equations described in 

this chapter, total river discharge ( ) can be calculated 

by equation 2.11

(2.11)

where

 = total channel discharge [the sum of all 

values collected during the discharge 

measurement traverse (eq. 2.7)], in cubic 

meters per second;

 = near-shore discharge estimate on the left 

side of the channel, in cubic meters per 

second; and

 = near-shore discharge estimate on the right 

side of the channel, in cubic meters per 

second.

Discharge-Measurement Software

Based on the above principles, the USGS, as well 

as the manufacturers of ADCPs, have written computer 

programs designed to collate ADCP data collected 

during a cross-section traverse and compute discharge 

in real time. Because of the difficulty in maintaining 

and updating such a program, the USGS has elected to 

use manufacturers’ proprietary programs for this 

capability, provided the proper algorithms are used and 

quality assurance (QA) criteria are met. The current QA 

plan (Lipscomb, 1995) requires that the whole system 

(ADCP and discharge-measurement software) meet 

certain standards.

The USGS has an archtype computer program 

written in Pascal that can be used as a basis for a 

vendor-created software package. Because the archtype 

computer program has not been translated into machine 

code that will run on IBM personal computers and 

compatibles, most ADCP system operators utilize 

“Transect,” a suite of proprietary software modules 

developed by RDI for use with their ADCPs. The 

Qt

Qt Qm Qei Qer = + +

Qm qi

Qei

Qer

Figure 2.8. Edge-value estimation scheme described by equations 

2.8, 2.9, and 2.10. ADCP, acoustic Doppler current profiler.
Chapter 2: Acoustic Doppler Current Profiler Discharge-Measurement Principles 33

Transect software includes the estimation methods used 

by the USGS in the archtype program and has proven to 

be a useful, accurate discharge-measurement tool. 

Other manufacturers currently are developing similar 

software.

Summary

Unlike conventional discharge-measurement 

methods, acoustic Doppler current profiler (ADCP) 

discharge-measurement software does not calculate 

discharge directly from area and velocity data. The 

ADCP discharge-measurement software calculates a 

water/boat velocity vector cross product in each bin 

before integrating the cross products over the subsection depth. The resulting subsection discharges then 

are summed over the width of the cross section.

Discharge cannot be measured near the water 

surface because of the draft required by the transducer 

(depth below water surface) and transducer blanking 

distance. Discharge cannot be measured near the 

channel bed primarily due to side-lobe interference. 

Cross products in these unmeasured portions of the 

channel cross-section usually are estimated using a onesixth power-curve estimation scheme (unless the profile 

shape dictates other methods). Discharges in the 

unmeasured portions of the cross-section near the edges 

of the riverbank are estimated using a ratiointerpolation method.

The U.S. Geological Survey (USGS) has 

developed a discharge-measurement archtype program; 

however, most ADCP system operators are using a 

proprietary suite of software modules called “Transect” 

developed by R.D. Instruments, Inc., for use with their 

ADCPs. Transect includes the estimation methods used 

by USGS in the archtype program and has proven to be 

a usable, accurate discharge-measurement tool. Other 

manufacturers also are developing dischargemeasurement software.
34 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

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Chapter 3: R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes 35

Water Modes?

Deep - Shallow

Fast - Slow

Bottom Modes?

Deep - Shallow

CHAPTER 3: R.D. INSTRUMENTS, INC., BROADBAND ACOUSTIC DOPPLER CURRENT PROFILER 

MEASUREMENT MODES

Measurement Modes—Why?

One of the most compelling features of ADCP 

systems is their versatility. Versatility comes with a 

cost, however, and the cost is added complexity. Most 

ADCPs have several water-measurement modes 

(fig. 3.1) and bottom-measurement modes. These 

modes are chosen based on environmental conditions 

(fast/slow or shallow/deep water and current shear). 

A simple matrix of mode choices for different 

measurement conditions would be desirable for this 

chapter, but, unfortunately, it’s not that easy! Several 

environmental factors may play a part in the choice of 

measurement modes.

In an ADCP manufactured by RDI, these modes 

are set using direct commands placed in a configuration 

file (chap. 5). The BB-ADCP commands W (water 

mode) and B (bottom mode) can be set to different 

values depending upon the flow and bottom conditions 

expected at the discharge measurement site. Mistakes 

in setting these values may cause unrecoverable errors 

in the measurement of discharge. This chapter, 

therefore, is devoted to the explanation of the different 

operational modes available for ADCPs manufactured 

by RDI. It is quite likely that when this report is 

published, operational modes for ADCPs manufactured 

by RDI will be available that are not covered in this 

chapter. The manufacturer releases technical notes and 

bulletins when such changes are developed, which are 

available on the manufacturer’s web site 

(www.rdinstruments.com).

Parameters that dictate the operation of waterand bottom-track measurement modes are set by direct 

commands sent to the profiler from the Transect 

configuration file (chap. 5). These direct commands 

take the form of W$nnn, or B$nnn, where W = water 

mode and B = bottom mode. The “$” signifies the 

water- or bottom-mode parameter that receives the 

numeric value, nnn. These direct commands are 

discussed in detail in this chapter.

Water Modes

The water-measurement modes juggle different 

lag distances, pulse lengths, code element 

combinations, and adaptive schemes to measure water 

velocity under many hydrologic conditions. Some of 

these modes should be used with caution because errors 

can result if they are misapplied. Direct commands for 

these modes take the form of WMn, where n is 0, 1, 4, 

5, 7, or 8, depending on the mode used.

Water Mode Zero (WM0)

Water mode zero (direct command WM0) is 

referred to as the “expert” mode because it allows the 

user to set virtually all aspects of the watermeasurement ping structure. Water mode zero will not 

be addressed further in this report because it is almost 

never used under routine measurement conditions. 

When this mode is used, it is used in connection with 

instructions from the manufacturer and usually is used 

for special circumstances where other modes are 

inadequate.

Water Mode 1 (WM1)

Water mode 1 (direct command WM1) is called 

the “dynamic” mode. This is a general purpose mode 

that should be used (in preference to mode 4) for most 

routine measurement conditions—with one caveat; the 

ambiguity velocity must be set correctly for stream 

conditions. Operators can modify the ambiguity 

velocity (and, thereby, the lag spacing of the mode 1 

ping structure) using the WV command. Proper 

adjustment of the WV value allows mode 1 to be used 

in shallow and deep water and in current shear where 

other modes fail. Ambiguities in the measurement of 

velocity are not automatically resolved in water mode 

and the maximum expected velocity of the boat 

(relative to the water) must be estimated before setting 

the WV command. If the value is set too low, an 

ambiguity error could be introduced into the velocity 

measurement (see chap. 1).

The default WV value (lag spacing) for mode 1 

allows a high ambiguity velocity [480 centimeters per 

Figure 3.1. Choosing the proper measurement mode is difficult.
36 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

MAXIMUM RELATIVE WATER SPEED (M/S)

Depth-range

MODE 1 OPERATION

Mode 5/8 window for WZ05

Mode 8 window for WZ05

Mode 5 window for WZ10

Mode 8 window for WZ10

0 0.5 1 2 10

M5/8 M8

M5/8 M5/8 M8

45 m (600 kHz, 50-cm cell)

12 m (1200 kHz, 25-cm cell)

7.0 m (600 kHz)

3.5 m (1200 kHz)

3.5 m (600 kHz)

1.75 m (1200 kHz)

second (cm/s), 16 ft/s]. The single-ping standard 

deviation of a velocity measurement using this default 

setting is about 19 cm/s (0.62 ft/s). This magnitude of 

uncertainty usually produces an unacceptable precision 

for the resulting discharge measurement, unless the 

measurement vessel is slowed to less than one-third of 

the speed of the absolute water velocity (water velocity 

relative to the Earth) during the cross-section traverse. 

Fortunately, the mode 1 default lag spacing can be 

changed by altering the WV command. Note: Most of 

the ambiguity-velocity directives expect radial (alongbeam) ambiguity velocity, not horizontal ambiguity 

velocity. Radial ambiguity velocity can be determined 

using the following formula (eq. 3.1) if the desired 

horizontal ambiguity velocity is estimated

(3.1)

where

= radial ambiguity velocity, in centimeters 

per second;

 = horizontal ambiguity velocity, in 

centimeters per second; and

 = transducer beam angle, in degrees.

The horizontal ambiguity velocity can be 

estimated by adding the highest expected ADCP speed 

(relative to the water) and the highest expected absolute 

water velocity. This value is and is plugged 

into equation 3.1 to arrive at a reasonable value for the 

WV command. It is multiplied by 1.5 in the equation 

for safety reasons. Usually, it is better to have a bit of 

extra noise (standard deviation of velocities) in the 

measurement than to inadvertently average in an 

ambiguity error. Table 3.1 shows the single-ping 

standard deviation for three different ambiguityvelocity values.

Mode 1 can be used in slightly more shallow 

water than mode 4 [1 m (3.3 ft)], but modes 5 or 8 are 

preferred for medium-to-slow velocities [below 

10 cm/s (0.33 ft/s)] because of the increased resolution 

available with these modes. Mode 1 sometimes can be 

used in shallow water where modes 5 and 8 do not work 

because of high velocities and shear. Figure 3.2 shows 

the depth and velocity range windows wherein each 

mode is designed to function properly. Notice that 

mode 1 encompasses a much larger range of depths and 

velocities than do the other modes.

To increase the range of mode 1 in water depths 

near the edge of the ADCP measurement capability 

(depends on frequency), the operator can toggle the 

WB command from 0 to 1. According to the 

manufacturer, setting the WB command to 1 enables 

greater ADCP range by narrowing the received 

Vradial 1.5 Vhorizontal = ( )sinθ

Vradial

Vhorizontal

θ

Vhorizontal

bandwidth (which improves the signal-to-noise ratio). 

Unfortunately this also increases the velocity 

measurement standard deviation by a factor of 2. See 

chapter 9 (table 9.1) for the maximum recommended 

ranges for ADCPs of different frequencies. The WB 

command only can be used for mode 1 operation. 

Setting WB to anything other than zero in other water 

modes will cause unpredictable results.

For routine discharge measurements in swiftflowing waters with depths greater than 2 m (6.5 ft), 

mode 1 is the mode of choice. The operator must set the 

WV command for the proper ambiguity velocity. Water 

Table 3.1. Mode 1 single-ping standard deviation using three 

different values for the WV command

[ADCP, acoustic Doppler current profiler; kHz, kilohertz; cm/s, centimeter 

per second; ft/s, foot per second] 

ADCP

frequency

(kHz)

Ambiguity velocity

(WV command)

Single-ping

standard deviation

300 480 cm/s (15.78 ft/s) 19.35 cm/s (0.63 ft/s)

300 190 cm/s (6.23 ft/s) 14.07 cm/s (.46 ft/s)

300 60 cm/s (1.99 ft/s) 6.87 cm/s (.23 ft/s)

600 480 cm/s (15.78 ft/s) 19.35 cm/s (.63 ft/s)

600 190 cm/s (6.23 ft/s) 14.08 cm/s (.46 ft/s)

600 60 cm/s (1.99 ft/s) 6.87 cm/s (.23 ft/s)

1200 480 cm/s (15.78 ft/s) 19.37 cm/s (.64 ft/s)

1200 190 cm/s (6.23 ft/s) 14.08 cm/s (.46 ft/s)

1200 60 cm/s (1.99 ft/s) 6.87 cm/s (.23 ft/s)

Figure 3.2. Depth/range/speed-operational “windows” for water 

modes 1, 5, and 8. m, meter; kHz, kilohertz; cm, centimeter; 

M/S, meter per second.
Chapter 3: R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes 37

modes 2 and 3 are obsolete and are no longer used for 

riverine measurements.

Water Mode 4 (WM4)

Until recently, water mode 4 (default for the BBADCP) was used as the general production mode for 

water-discharge measurement. This mode is no longer 

recommended by RDI for discharge measurement 

because of hard-to-detect errors that can be introduced 

with automatic mode switching. Mode 4 uses a single 

set of pulses with a long lag that is dependent on depth 

cell length. The lag is either one-half the depth cell 

length or has a horizontal ambiguity velocity of 

160 cm/s (5 ft/s), whichever is greater. The value of 

160 cm/s (5 ft/s) is used for most depth cell sizes. Using 

the minimum depth cell size for a given operational 

frequency [25 centimeters (cm), 10 inches (in.)] for a 

1,200-kHz BB-ADCP, for example, will decrease the 

lag, raise the ambiguity velocity slightly, and increase 

the measured water-velocity standard deviation. The 

ambiguity is resolved on this single set of pings by a 

proprietary signal-processing algorithm developed by 

the manufacturer (R.D. Instruments, Inc., 1996).

If necessary, mode 4 can be used in high- and 

low-velocity conditions. When depths become too 

shallow [2 meters (m) and below], or if the signal return 

is too weak, the ADCP will shift automatically to 

mode 1 operation. The operator is not notified when 

automatic mode switching occurs and it is 

recommended that an appropriate WV command be 

inserted into every configuration file that uses mode 4 

operation. The WV command value can be estimated 

using equation 3.1 (see Water Mode 1 discussion).

Again, mode 4 is no longer recommended for 

discharge measurement use. It is only available on BBADCP systems and should be used only if directed by 

the manufacturer, or if mode 1 will not work.

Water Mode 5 (WM5)

Water mode 5 (“low and slow” mode) uses a true 

version of pulse-to-pulse coherent signal processing. 

The two transmitted pulses are completely independent 

of each other, but are synchronized in phase. To 

eliminate self noise in mode 5, the second pulse is not 

sent until the first pulse dies out. This procedure creates 

a long lag with low standard deviation of the measured 

water velocity.

Ambiguity Velocity Revisited

One problem with extra long lags is a low 

ambiguity velocity. Using the racetrack analogy from 

chapter 1, the first strobe flash (analogous to an 

acoustic pulse) shows the car with the racing stripe 

leading the pack of racers (fig. 1.34a). The second 

strobe flash reveals that the pack of racers apparently 

has passed the racing-striped car and won the race 

(fig. 1.34b). Or does it? If the racing-striped car was 

actually moving three or four times faster than the other 

racers, it not only won the race but has almost “lapped” 

the rest of the racers. In this case, the racing-striped car 

has exceeded the velocity at which the timekeeper 

could determine its speed without ambiguity. In the 

case of mode 5 operation, this ambiguity velocity is 

relatively low [50 cm/s (1.7 ft/s) ADCP speed, relative 

to the water].

A long lag time increases the possibility of 

decorrelation between pulses. Using the above 

racetrack analogy, the photographer could overlay the 

negative of the first strobe flash on the negative of the 

second strobe flash and then rotate the negatives until 

the “pack” of cars are aligned. When the pack of cars 

are aligned, the checkered flag on the first negative will 

be a certain distance from the checkered flag on the 

second negative. By using this distance and the time 

between strobes, the speed of the pack of cars can be 

computed. This method of speed computation works 

unless the time between strobes becomes so long that 

the cars in the pack change position, lanes, or speeds. If 

this happens, the cars will not “match up” when the 

negatives are rotated. The two negatives are then said to 

be uncorrelated. This effect can occur using mode 5 if 

there is shear and turbulence in the water column.

Because of the above two effects, mode 5 is not 

usable in water with substantial velocity, shear, or 

turbulence. However, in slow-moving water with little 

current shear, mode 5 can be used to obtain highly 

precise discharge measurements. Because of the high 

precision of mode 5, smaller bin lengths can be used, 

which enables mode 5 to be used in shallow water. 

Direct command changes must be made for mode 5 

operation. The WZ command sets the starting length of 

the mode 5 and mode 8 lags and it is recommended that 

a command of WZ05 be used as a starting point for 

mode 5 and mode 8 operation. Table 3.2 describes 

water mode 5 performance for the specified setup.

Water Mode 7 (WM7)

Water mode 7 (extended range mode) is used to 

obtain water profiling at ranges 10–15 percent greater 

than profiling ranges available in other modes. Mode 7 

converts the BB-ADCP system to a system similar to a 

narrow-band ADCP, thereby increasing the signal-tonoise ratio and range. This mode is practical only for 

75-, 150-, and 300-kHz BB-ADCPs and is intended for 

open-ocean use where extended depth range is needed. 

Mode 7 water-velocity measurements have 
38 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

approximately 2.5 times the standard deviation of 

velocities measured with a mode 4 ping using the same 

depth cell size.

Water Mode 8 (WM8)

Water mode 8 is for shallow water and will 

sometimes work in conditions where mode 5 will not. 

This mode uses a pulse-to-pulse coherent configuration 

much like mode 5 to calculate velocity, but employs 

sophisticated signal processing techniques to reduce 

lag times. A mode 8 ping has a single-ping standard 

deviation 10 times greater than a mode 5 ping of the 

same bin size, but can be used in shallow water in much 

the same manner as mode 5. Setups for mode 8 are the 

same as the setups for mode 5. Table 3.3 gives the 

conservative values provided by the manufacturer for 

mode 8 performance (R.D. Instruments, Inc., 1995). 

Under certain circumstances, the operator may see up 

to a 30 percent improvement over the maximum depth 

range and maximum velocities listed in table 3.3; 

however, it is best not to expect optimum performance 

under field conditions.

Range/Speed “Windows” for Water Modes 1, 5, and 8

Figure 3.2 shows a graphic rendition of the 

depth-range/speed operational “windows” for water 

modes 1, 5, and 8. Notice that mode 1 is, by far, the 

most robust operational mode, with the largest 

operational range of depth and relative water speed. For 

slow velocities with shallow depths, mode 5 is the 

mode-of-choice, especially with 600-kHz ADCPs. 

Mode 8 sometimes will work in shallow water where 

relative water velocities are too high for mode 5 

operation. The higher standard deviation of the mode 8 

measurements usually requires some data averaging, or 

a slow cross-section traverse (at speeds below that of 

the water speed, relative to the Earth).

Bottom-Track Modes

Bottom-track modes are implemented 

independently of the water-velocity measurement 

modes and sometimes are set differently. There are two 

bottom-track modes currently used by the BB-ADCP: 

modes 4 and 5. Bottom-track modes 1–3 are obsolete. 

Other bottom-track modes may be available by the time 

this report is published.

Bottom tracking is done by proprietary firmware 

schemes built into the ADCP by the manufacturer; 

however, all such schemes must rely on the assumption 

that the bottom reflection obeys basic laws of physics. 

These principles of bottom tracking should be known 

by the ADCP operator to properly evaluate bottom 

tracking under variable field conditions.

The Bottom Reflection

When an acoustic signal strikes the river bottom, 

the reflected signal normally is much stronger (by 

orders of magnitude) than the reflected signal from 

scatterers in the water mass. It is no surprise, therefore, 

that the standard deviation of the bottom-track velocity 

measurement is about 10 times less than the water-mass 

Table 3.2. Setup and performance values for water mode 5 operation (WZ05)

[kHz, kilohertz; cm, centimeter; cm/s, centimeter per second; m, meter; ft/s, foot per second; ft, foot. WF, WS, keyboard commands] 

Acoustic Doppler

current profiler

operating

frequency

 (kHz)

Blanking

distance

WF,

(cm)

Depth cell

size, WS

(cm)

Single-ping

standard

deviation

(cm/s)

(ft/s)

Range to first

depth cell

(m)

(ft)

Minimum

profiling

range

(m)

(ft)

Maximum

profiling

range

(m)

(ft)

600 25 (WF25) 10 (WS10) 0.5 cm/s

(0.16 ft/s)

0.35 m

(1.15 ft)

0. 9 m

(2.95 ft)

7.0 m

(23.0 ft)

600 25 (WF25) 25 (WS25) .3 cm/s

(.010 ft/s)

.50 m

(1.64 ft)

1.6 m

(5.25 ft)

7.0 m

(23.0 ft)

600 25 (WF25) 50 (WS50) .2 cm/s

(.007 ft/s)

.75 m

(2.46 ft)

2.2 m

(7.18 ft)

7.0 m

(23.0 ft)

1200 25 (WF25) 5 (WS05) .6 cm/s

(.020 ft/s)

.30 m

(.98 ft)

.8 m

(2.63 ft)

3.5 m

(11.5 ft)

1200 25 (WF25) 10 (WS10) .4 cm/s

(.013 ft/s)

.35 m

(1.15 ft)

.9 m

(2.95 ft)

3.5 m

(11.5 ft)

1200 25 (WF25) 25 (WS25) .3 cm/s

(.010 ft/s)

.50 m

(1.64 ft)

1.6 m

(5.25 ft)

3.5 m

(11.5 ft)
Chapter 3: R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes 39

0

1

2

3

4

5

6

7

8

9

10

DEPTH , IN METERS

140 145 150 155 160 165 170

BACKSCATTERED AMPLITUDE IN COUNTS

Threshold

velocity measurement. Figure 3.3 shows a 1,200-kHz 

ADCP backscattered intensity (measured in counts) 

that decreases with depth until the acoustic beam 

strikes the river bottom at a depth of approximately 

7 m (23 ft). Data averaged from a series of water-track 

pulses were used to construct figure 3.3, but a single 

bottom-track pulse backscattered-intensity plot would 

look much the same. 

Modern bottom-track schemes are self adaptive 

to stream conditions and use factors such as correlation 

magnitude, correlation side peak location, and (or) 

spectral width to determine the presence of the bottom 

echo. Still, bottom-track failures can occur when one or 

more of the parameter identification criteria are not met 

by the output signal or, for some reason, are met at the 

wrong time.

In the case of heavy sediment load, which causes 

high absorption and scattering of the acoustic signal, 

the weakened bottom reflection (from deep water) 

cannot activate a detection threshold. In some cases, 

when this happens, the ADCP firmware is programmed 

to try other (more robust) bottom-tracking modes 

before flagging the data as “bad.”

Another problem occurs during periods of high 

flow when heavy sediment loads are moving on or near 

the channel bed; the bottom-track velocities become 

biased. The physical process that causes this 

phenomenon is theorized only at this time. Attempts 

have been made to overcome this bias problem by using 

lower frequency systems and special bottom-track 

schemes. A separate method of measuring vessel 

velocity that eliminates the need for bottom tracking 

can be used; the input from a differential global positioning systems (GPS) receiver and depth sounder is 

used to replace the data from the ADCP bottom-track 

pulse. Because this method requires that the operator 

have an indepth understanding of GPS systems, as well 

Table 3.3. Setup and performance values for water mode 8 operation (WZ05)

[kHz, kilohertz; cm, centimeter; cm/s, centimeter per second; m, meter; ft/s, foot per second; ft, foot. WF, WS, keyboard commands] 

Frequency

(kHz)

Blanking

distance

WF

(cm)

Depth cell

size, WS

(cm)

Single-ping

standard

deviation

 (cm/s)

(ft/s)

Range to first

depth cell

(m)

(ft)

Minimum

profiling

range

(m)

(ft)

Maximum

profiling

range

(m)

(ft)

600 25 (WF25) 10 (WS10) 8.2 cm/s 

(.27 ft/s)

0.35 m

(1.15 ft) 

0.6 m

(1.97 ft)

7.0 m

(23.0 ft)

600 25 (WF25) 25 (WS25) 5.2 cm/s

(.17 ft/s)

.50 m

(1.64 ft)

.9 m

(2.95 ft)

7.0 m

(23.0 ft)

600 25 (WF25) 50 (WS50) 3.7 cm/s

(.12 ft/s)

.75 m

(2.46 ft)

1.4 m

(4.59 ft)

7.0 m

(23.0 ft)

1200 25 (WF25) 5 (WS05) 11.0 cm/s

(.36 ft/s)

.30 m

(.98 ft)

.5 m

(1.64 ft)

3.5 m

(11.5 ft)

1200 25 (WF25) 10 (WS10) 7.8 cm/s

(.26 ft/s)

.35 m

(1.15 ft)

.6 m 

(1.97 ft)

3.5 m

(11.5 ft)

1200 25 (WF25) 25 (WS25) 5.0 cm/s

(.164 ft/s)

.50 m

(1.64 ft)

.9 m

(2.95 ft)

3.5 m

(11.5 ft)

Figure 3.3. Acoustic Doppler current profiler-backscattered 

intensity with depth showing the bottom reflection.
40 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

as significant investment in additional GPS and depth 

sounding equipment, it will not be discussed in this 

report. For more information on using this method for 

discharge measurements during conditions of bottomsediment movement, contact the ADCP manufacturer.

Bottom-Track Mode 4 (BM4)

Bottom-track mode 4 is a general-purpose 

bottom-track mode. This mode can unambiguously 

determine the speed of the ADCP, relative to the 

channel bottom, under most conditions. The detailed 

detection scheme used is proprietary and copyrighted 

by RDI, but mode 4 uses the correlation side-peak 

position to resolve velocity ambiguities and it lengthens 

the lag at predetermined depths to improve variance. 

Bottom-track mode 4 can be used with water-track 

modes 1, 4, 5, 7, and 8; however, because of processing 

limitations, the standard deviation of bottom-track 

velocity data (using mode 4) increases as depth 

decreases.

Bottom-Track Mode 5 (BM5)

Bottom-track mode 5 is the default mode for 

most discharge-measurement and velocity-profiling 

use. This mode uses a pulse-to-pulse coherent 

technique that has a lower variance in shallow water 

than bottom-track mode 4 by a factor of up to four. In 

very shallow water at slow speeds, mode 5 variance is 

lower than mode 4 by a factor of 100. Although mode 5 

has a very precise measurement capability, it has a 

slightly slower ping rate than mode 4. Bottom-track 

mode 5 will automatically switch to bottom-track 

mode 4 when water is too deep (or too fast) for mode 5 

operation. Because of this adaptive capability, mode 5 

is the mode of choice for small rivers and estuaries 

(table 3.4). Table 3.4 lists the minimum bottomtracking depths for bottom-track modes 4 and 5.

At least one other ADCP manufacturer has 

developed the software and system firmware (bottomtracking algorithms) for the measurement of river 

discharge; however, at the time this report was written, 

no detailed information was available for analysis and 

publication.

Summary

Water-track modes and bottom-track modes are 

independent of each other and must be carefully 

chosen, depending upon the stream conditions. In 

general, a good starting point for measurements at an 

unfamiliar site is water mode 1 (WM1) and bottomtrack mode 5 (BM5). In cases of slow-moving, shallow 

flow, water mode 5 should be tried first, followed by 

water mode 8, followed by modified versions of mode 1 

(use bottom-track mode 5 in all cases). In cases of high 

velocities, high shear, abrupt boat movements, or 

dynamic depth conditions, default and modified 

versions of mode 1 can be tried with a slow crosssection traverse to reduce random error. In such cases, 

bottom-track mode 4 probably should be used.

The theory of ADCP discharge measurement has 

been discussed in chapters 1–3. Chapters 4–9 introduce 

“practical” techniques and equipment.

Table 3.4. Minimum depth ranges for bottom-track modes 4 and 5

[kHz, kilohertz; m, meter; ft, foot] 

Frequency

(kHz)

Depth

(m)

(ft)

1200 0.8 m (2.6 ft)

600 .8 m (2.6 ft)

300 1.5 m (5 ft)
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 41

wd40 

CHAPTER 4: ACOUSTIC DOPPLER CURRENT 

PROFILER HARDWARE AND ANCILLARY 

EQUIPMENT

In this chapter we discuss the equipment needed 

to measure river discharge with an ADCP (fig. 4.1). 

Although a generic discussion of ADCPs is provided in 

this report, the following three sections cover the 

ADCP manufactured by RDI. The reason for this is 

twofold:

•Until recently, RDI was the only manufacturer 

of ADCP systems that could be utilized for 

discharge measurements (bottom-track 

algorithms). Another vendor (Sontek) has 

developed a bottom-tracking ADCP that can be 

used for discharge measurements, but the 

system is not yet widely used by the USGS

•As discussed in chapter 2, RDI has written 

discharge-measurement software that is 

integrated with the ADCP to calculate discharge 

and to store data (Transect software). 

Knowledge of the details of this software is 

required for the collection of accurate discharge 

measurements.

When another ADCP manufacturer produces a 

capable system that is used by a significant number of 

USGS offices, a description of that system (and its 

associated software) will be included in a follow up to 

this report.

Where Do We Start?

The following equipment is needed to measure 

river discharge with an ADCP:

ADCP system

•Pressure case and transducer assembly

•Power supply and communications interface

•Discharge-measurement software

•Manufacturer’s documentation

Ancillary equipment

•Measurement platform or vessel

•ADCP mounting assembly

•Laptop computer

•Range finder or method for estimating distance 

to shore

Of course, the measurement equipment is 

worthless unless used by a competent, well-trained 

ADCP operator. If a boat is used, the skipper must be 

experienced in boat-maneuvering techniques required 

for ADCP discharge measurement and must have 

attended a U.S. Department of Interior (DOI) boat 

safety course.

Acoustic Doppler Current Profiler Equipment

Acoustic Doppler Current Profiler Pressure Case and 

Transducer Assembly

The direct-reading BB-ADCP is configured 

much like the narrow-band ADCP (Simpson and 

Oltmann, 1993); however, the transducer and 

electronics are contained in the same cylindrical 

enclosure (pressure case). The diameter of the 

enclosure is approximately 150–220 millimeters (mm), 

depending on the configuration. A convex or concave 

transducer assembly on one end of the cylindrical 

canister employs an orthogonal (Janus) beam aiming Figure 4.1. “Which equipment do we need?”
42 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

pattern with the three or four transducer beams angled 

20o (or optionally 30o) outward from the center axis of 

the assembly. The transducer assembly diameter 

depends on the frequency of operation and configuration. For most work, the concave transducer head 

has proven to be more durable because it is protected 

somewhat from floating debris and direct contact with 

the river bottom. Figure 4.2 shows a BB-ADCP with 

a 1,200-kHz concave transducer assembly with pipe 

brackets attached.

RDI has introduced another version of the 

BB-ADCP into its product line called the “Workhorse.” 

The Workhorse line of ADCPs can be purchased with 

or without bottom-tracking capability. A river version 

of the Workhorse (the Rio Grande, fig. 4.3) is available 

with bottom-tracking and operational modes suited to 

river-discharge measurements.

Power Supply and Communications Interface

For BB-ADCP direct-reading units, the 

manufacturer supplies a deck unit with the BB-ADCP 

that can convert several power sources to voltages 

required by the BB-ADCP. The deck unit can accept 

120-volt (V) alternating current (AC), 20- to 60-volt 

Figure 4.2. A 1,200-kilohertz broad-band acoustic Doppler current profiler with attached pipe brackets.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 43

direct current (DC), or 12-volt DC (fig. 4.4). The deck 

unit has a power switch on the front panel as well as two 

luminescent electronic display (LED) indicators. One 

LED indicator lights up when power is applied to the 

deck unit, and the other LED indicator lights up when 

data are being transmitted to and from the ADCP. 

A 13-mm (0.5-inch) diameter cable connects the BBADCP to the deck unit assembly. At the ADCP end, the 

cable is potted to an underwater connector (fig. 4.5). 

The shell connector at the deck unit end of the cable has 

Figure 4.3. An R.D. Instruments, Inc., 600-kilohertz Workhorse “Rio Grande” acoustic Doppler current profiler.
44 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

24 pins and uses a bayonet-style connector. The 

overhang on the back of the deck unit can hinder the 

ability to easily disengage this connector and many 

users do not fully engage the bayonet mechanism. 

Connectors for RS-232 serial communications, power 

connections, and fuse holders also are on the back panel 

of the deck unit. The fuses protect the ADCP and deck 

unit in the event of short circuits and incorrect (reverse) 

battery polarity.

The manufacturer also markets an ADCP product 

known as the “Workhorse Rio Grande” (fig. 4.3) that 

does not require a separate deck unit. The Workhorse 

is powered directly from a DC source, or optionally 

from a 120-volt AC to a 12-volt DC converter. The 

Workhorse communications cable and the power 

supply cable are attached to the pressure case by way 

of a submersible connector.

Figure 4.4. Front and rear view of an R.D. Instruments, Inc., acoustic Doppler current profiler deck unit.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 45

Acoustic Doppler Current Profiler Discharge-Measurement 

Software

To measure river discharge, the ADCP system is 

controlled by discharge-measurement software 

(chaps. 2, 3). The configuration of this software is 

discussed in detail in chapter 5.

Documentation

The manufacturer provides several pieces of 

documentation that are essential for the proper 

operation and setup of the BB-ADCP including 

“The Direct Reading and Self-Contained BB-ADCP 

Technical Manual” (R.D. Instruments, Inc., 1995). For 

those using the Workhorse Rio Grande, RDI provides 

“Workhorse Rio Grande Technical Manual P/N 957-

6101-00,” November 1999. Also needed is the Transect 

Program users manual for the broad-band acoustic 

Doppler current profilers (R.D. Instruments, Inc., 

1994). The technical manual is shipped with each 

ADCP; however, the Transect computer software and 

manual must be purchased separately. The Transect 

software and users manual are shipped as computer 

diskettes. Both manuals are updated periodically and 

hard copies of these updates can be obtained from the 

manufacturer for a fee. The manufacturer also maintains an internet web site that provides downloadable 

copies of the latest ADCP documentation updates, 

technical bulletins, and complete manuals 

(www.rdinstruments.com). The operator also should 

become familiar with the “Quality assurance plan for 

discharge measurements using broad-band acoustic 

Doppler current profilers” (Lipscomb, 1995).

Ancillary Equipment

Measurement Platform or Vessel Requirements

Discharge measurement platform or vessel 

requirements will vary, depending on the size and flow 

rates of the rivers and streams. For example, if 

measurements are in large rivers or estuaries, the 

BB-ADCP may be mounted on a 9- to 15-m (30- to 

50-foot) vessel with an enclosed cabin. If, on the other 

hand, discharge measurements are on small, slowmoving rivers, the vessel of choice may be a 5- to 6-m 

(16- to 20-foot) skiff. For very small streams or rivers 

with minimal wave action, the BB-ADCP may be 

mounted on an inflatable boat, such as a Zodiac.

For improved safety while taking measurements 

that would be hazardous using manned platforms or to 

avoid traffic hazards, the USGS Indiana District has 

developed a tethered platform (fig. 4.6). For flood work 

and bridge scour measurements, the USGS Office of 

Surface Water has developed a prototype radiocontrolled platform (fig. 4.7).

Figure 4.5. Connector cable for attachment of R.D. Instruments, 

Inc., broad-band acoustic Doppler current profiler to 

accompanying deck unit.
46 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

The proper boat choice will depend on the 

topography and hydrology of the area of interest; 

however, it is best to have several alternative vessels for 

discharge measurement use. In the USGS California 

District, three boats are used for ADCP discharge 

measurements. Figure 4.8 shows a 30-m (95-foot) 

vessel with a side-swing mount. Using this 

configuration, measurements in the estuary can be 

obtained under all but the worst of conditions.

Figure 4.9 shows a similar mount on a 6-m 

(20-foot) Boston Whaler. This vessel can be used in 

estuaries, rivers, and in river tributaries. A large 

150-horsepower (hp) engine is used to get from place to 

place quickly when making discharge measurements in 

an estuary or river delta. A smaller engine is used when 

making discharge measurements in small rivers and 

Figure 4.6. Tethered acoustic Doppler current profiler discharge measurement platform.

Figure 4.7. Radio-controlled, 12-foot, broad-band acoustic Doppler 

current profiler platform.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 47

slow-moving water. This vessel can be rigged with 

canvas for inclement weather.

Figure 4.10 shows a side-swing mount on a 

4.5-m (15-foot) Boston whaler. This vessel is easily 

trailerable and is used when measuring small rivers. 

The main engine has a four-cycle, 45-horsepower 

engine that can be idled at low speeds for discharge 

measurement. This configuration is used mainly in fair 

weather; however, canvass also can be rigged to make 

the vessel usable in inclement weather.

When making discharge measurements in 

narrow rivers, a trolling plate can be used on the main 

engine or an electric trolling motor can be used to slow 

the vessel. For accurate measurements in very slowvelocity water, this vessel can be pulled on a tagline.

In Sweden, BB-ADCP operators have used a 

small, inflatable dinghy when making discharge 

measurements (fig. 4.11). The advantage of this type of 

vessel is that launch ramps are not needed. The dinghy 

can be inflated on the riverbank and the equipment set 

up for use in less than 30 minutes.

It is imperative that not only the correct boat be 

used for any given set of river and weather conditions, 

but also that boat operators be properly trained. Correct 

Figure 4.8. Two views of an acoustic Doppler current profiler side-swing mount on a 30-meter (95-foot) vessel.
48 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

operation of the boat is vital to obtaining high-quality 

discharge and velocity measurements. The DOI 

requires that boat operators attend a DOI motor boat 

operators training course before operating a USGS boat 

or vessel. All passengers on USGS vessels should wear 

life jackets (type V or better) and observe all U.S. Coast 

Guard and USGS water safety regulations.

Laptop Computer

The computer selection for running the 

BB-ADCP software is very important. Because of 

the amount of data processing required, the computer 

must have an i286 central processing unit (CPU) or 

equivalent (an i386, i486, or Pentium CPU is desirable), 

and also must be IBM compatible. The computer at 

least must be capable of displaying extended graphics 

array (EGA) compatible graphics. Because of the 

amount of data storage required, a hard drive 

(nonvolatile ram drive) should be used with at least 

20 megabytes (Mb) of available storage space.

The computer screen display should be visible in 

direct and diffuse sunlight (fig. 4.12). Those computers 

that do not rely on backlighting for display illumination 

Figure 4.9. Side-swing mount on a 6-meter (20-foot) Boston Whaler for an acoustic Doppler current profiler.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 49

seem to have the most visible display in direct sunlight. 

Standard monochrome liquid crystal display (LCD) 

and LEDs have the best visibility. Active matrix color 

displays have fair-to-poor visibility, and dual scan color 

LCD displays have poor-to-no visibility in direct 

sunlight. The computer purchase should be based on 

the computer’s ability to run the Transect software and 

on the visibility of the screen display in direct sunlight.

For routine collection of streamflow data, a 

rugged laptop computer is desirable. Several 

manufacturers now produce laptops with antiglare 

screen coatings, shock-mounted hard drives, and waterresistant keyboards and access panels. Standard laptops 

have minimal protection from the elements; rain and 

dust protection are not provided and many have flimsy 

plastic doors covering the port connections. These 

plastic doors fall off or are easily broken (fig. 4.13).

Do not rely on the internal laptop battery to 

provide power for an all-day measurement session 

because most laptop batteries will not last beyond about 

3 hours and many will not last 1 hour before requiring 

recharge. Most of these rechargeable batteries tend to 

Figure 4.10. Side-swing mount on a 4.5-meter (15-foot) Boston Whaler for an acoustic Doppler current profiler.
50 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

gradually lose capacity with age. The measurement 

system operator should attempt to power the computer 

from an external battery with a large capacity, such as 

an external 12-volt automobile or deep-cycle marine 

battery. Many of the newer laptops do not directly 

accept 12-volt power and require special adapters for 

power conversion. These adapters should be purchased 

with the computer, if possible, because they may be 

unavailable with ensuing computer model changes. 

Most adapters have a cigarette lighter plug at the 

12-volt end of the adaptor. The cigarette lighter plug 

can be replaced with two alligator clips that can be 

attached directly to battery terminals. Another 

alternative for laptop power is to purchase a 200-watt 

(W) inverter. There are several small efficient inverters 

on the market today that do not draw excessive power 

under idle conditions. The inverter is connected to the 

12-volt batteries and the computer AC adaptor is 

plugged into the inverter.

The data-processing computer is connected to the 

BB-ADCP through a serial connection on the back of 

the deck unit. Normally, this serial connection is 

RS-232c; however, for long cables (longer than 200 ft), 

RS-422 protocol should be used. RS-422 protocol 

requires the use of a separate converter box as well as 

changes in the internal BB-ADCP switch settings 

(R.D. Instruments, Inc., 1995). The RS-232c serial 

connection is provided by way of a standard IBM 

personal computer (PC) 9-pin female to 25-pin male 

adaptor cable, which is available in most computer 

shops. A null-modem adaptor is not needed.

The computer must be protected from direct 

sunlight and heat during data collection. The LCD 

screen turns dark and unusable if it remains in direct 

sunlight too long. The computer also can be damaged 

by the heat of direct exposure to the sun. One solution 

is to place the computer inside an empty cooler that is 

turned on its side. The cooler shades the computer and 

Figure 4.11. Acoustic Doppler current profiler mount for an 

inflatable dinghy. 

Figure 4.12. Laptop computer screen in diffuse sunlight.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 51

LCD from direct sunlight and also protects the 

computer from splashing water.

Acoustic Doppler Current Profiler Mounts

For discharge measurement or profiling, the 

BB-ADCP is positioned with the transducer assembly 

facing downward into the water column with beam 

three oriented forward. The BB-ADCP can be mounted 

in many different ways, but two methods have been 

used for most deployments; the side-swing mount and 

the “sea chest” mount.

Figures 4.14 and 4.15 show two types of sideswing mounts that have been used successfully. An 

inexpensive mount made from 51-mm (2-inch) 

aluminum pipe and 51-mm by 305-mm (2-inch by 

12-inch) lumber is shown in fig. 4.14, and a more 

expensive mount fabricated from 6061-T aluminum is 

shown in fig. 4.15. These types of mounts have been 

used on boats that range in length from 4–30 m 

(14–95 ft) and on streams 3.5 m (10 ft) wide and on 

rivers as wide as 1.6 km (1 mi). These mounts have the 

disadvantage of being far from the boat keel and, 

therefore, are subject to altitude changes caused by 

pitch and roll. However, for most applications, these 

mounts produce acceptable results.

Figure 4.16 shows a “sea-chest” mount that has 

been used successfully for narrow-band ADCPs and 

can be modified for use with BB-ADCPs. The “seachest” mount has the advantage of being close to the 

Figure 4.13. Laptop computer with missing plastic doors and port 

covers.

Figure 4.14. Inexpensive acoustic Doppler current profiler mount. 
52 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

boat keel and, therefore, less subject to altitude changes 

due to pitch and roll. The position also places the 

BB-ADCP away from edges of the boat wake where 

entrained air is present. Entrained air can cause loss of 

bottom track.

ADCP operators at Tampa, Florida, have 

developed a variant of the swing mount (fig. 4.17) that 

allows the BB-ADCP to be easily swung aboard the 

measurement vessel. A hydraulic mount was designed 

and installed on a “john boat” in the USGS Indiana 

District (fig. 4.18). A unique variant of the transom 

mount was designed by Harry Hitchcock of the USGS 

Kentucky District (fig. 4.19). An inboard variation on 

the transom swing mount is being used by the USGS 

Illinois District (fig. 4.20). An easily detachable swing 

mount was designed and used by the USGS Idaho 

District (fig. 4.21). 

Range Finder or Method for Estimating Distance to Shore

Edge discharges are estimated in the Transect 

software using a technique similar to that used when 

making conventional discharge measurements. The 

unmeasured area between the boat and the river edge is 

estimated using the last measured mean velocity, the 

last measured depth, and the distance from the boat to 

shore. The algorithm assumes a triangular-shaped area 

Figure 4.15. Aluminum acoustic Doppler current profiler mount.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 53

for this estimate. If the river channel is rectangular, 

these edge estimates can be doubled and an adjustment 

made for the roughness of the edges.

Estimating the distance to shore can be done with 

the naked eye; however, such distance estimates are 

almost always short of the true distance (underestimated). The reason for this is unclear, but is 

possibly due to lack of visual clues between the boat 

and shore. The most reliable way of ensuring accurate 

edge estimates is to set buoys out from the shore at 

known distances (measured with a tape or distance 

meter). The Transect software is then started and 

stopped at these buoys. This method is not always 

possible when large numbers of discharge 

measurements are needed at different locations within a 

short time period.

Figure 4.16. Sea-chest mount for a narrow-band acoustic Doppler current profiler.
54 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

There are several types of distance-measurement 

devices on the market that have been used to increase 

the accuracy of the edge estimates without the need to 

set buoys or onshore devices. The devices can be placed 

into three categories:

•Optical

•Sonic

•Infrared laser

Good results have been obtained with 

inexpensive, optical range finders (fig. 4.22) that use 

parallax and a focusing device to estimate distance. The 

operator identifies a rock or object at the stream edge 

and then rotates a knob to converge two images of the 

object. The distance is then read from a scale on the 

device. This method requires a little practice but, with 

properly calibrated range finders, acceptable accuracy 

can be obtained up to about 180 m (600 ft).

Sonic devices that have been tried, require a 

vertical wall for a signal return. Riverbank edges 

generally do not have topographies that enhance 

acoustic reflections. To be usable, these devices need 

special sonic targets (corner reflectors) placed on the 

riverbanks. These devices can be useful if the operator 

Figure 4.17. Mount used by the U.S. Geological Survey Tampa, Florida, Subdistrict.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 55

is able to deploy the sonic targets at the cross-section 

edges.

Laser devices are cumbersome and more delicate 

than the other range finders, but can be more accurate 

over longer distances. They can be used without targets 

(corner reflectors) up to about 75 m (250 ft) and up to 

hundreds of meters (thousands of feet) with targets. The 

major drawback to these devices are their cost and 

durability. They require precision optics, which are 

delicate and easily damaged.

Trolling Motors/Plates

Discharge measurements typically require the 

use of slow boat speeds, especially in low watervelocity conditions. The main boat engine usually is too 

large for use when making discharge measurements. A 

battery-operated trolling motor (fig. 4.23) with variable 

speeds or a small gasoline-driven marine engine may be 

used to maneuver the boat at the slow speeds required 

for making a discharge measurement.

If an electric trolling motor is used, extra deepcycle marine batteries should be included in the 

discharge-measurement equipment inventory. When 

using a trolling motor, a full day of discharge 

measurements may require two or more fully charged, 

deep-cycle marine batteries. If a gasoline-powered 

trolling motor is used, the operator may wish to 

purchase a four-cycle marine engine. A four-cycle 

engine is quieter than a two-cycle engine, does not 

smoke as much, idles for longer times at low speeds 

without spark plug fouling, and does not require an 

oil/gas mixture for operation.

Inexpensive steering adapters (fig. 4.24) can link 

smaller trolling motors to the main engine steering, and 

remote throttles and gearshifts also can be console 

mounted. Electric trolling motor steering even can be 

done with a foot pedal.

Miscellaneous Measurement Equipment

Miscellaneous discharge-measurement 

equipment may include the following:

•Buoys with attached anchors for marking crosssection edges

•Tag-line systems for measuring very slow 

velocities

Figure 4.18. Hydraulic mount used by the U.S. Geological Survey 

Indiana District.
56 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

•External depth sounders and navigation 

equipment

•Rain/sun canopy and computer-protection gear

Installation of the Broad-Band Acoustic Doppler 

Current Profiler

Mounting the Acoustic Doppler Current Profiler on the 

Vessel

The ADCP is mounted on the measurement 

vessel using the mounting systems described 

previously. In most cases, beam three is oriented toward 

the bow of the vessel, however, if the vessel will be 

measuring discharge next to bridge piers or vertical 

walls, the ADCP should be oriented such that the beams 

are at 45o angles to the axis of the vessel. This 

orientation allows the closest approach to the vertical 

wall.

Care must be taken to eliminate magnetic fields 

or ferrous materials from the vicinity of the pressure 

case. Not only the ADCP mounts, but all nuts, bolts, and 

other fasteners must be made from nonferrous 

materials.

Figure 4.19. “Toggle” mount used by the U.S. Geological Survey Kentucky District.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 57

Deck-Unit and Power-Supply Connections

The other components of the BB-ADCP system 

also must be mounted or placed in safe (splash-free) 

areas on the measurement vessel. There are some 

precautions to be observed when installing the system 

components. The deck unit (if used) should be 

positioned out of direct sunlight (excessive heat) and 

away from exposure to moisture (the housing for the 

deck unit electronics is not waterproof). The deck unit 

should be opened and examined to ensure that all 

components (especially the DC-to-DC convertor block) 

are firmly attached to the board socket by some means 

other than electrical connector pins. The manufacturer 

has corrected this problem, but several of the early deck 

units had no mechanical attachments on the DC-to-DC 

converter block. The block would become dislodged 

during transportation and the deck unit would not 

function when connected to 12-volt DC power.

The connections on the back of the deck unit are 

shown in figure 4.25. Using the deck unit, the 

BB-ADCP may be powered with 110- to 120-volt AC 

power, 12-volt DC power, or 20- to 60-volt DC power. 

The AC power is supplied to the deck unit using a 

standard IBM personal computer-style pigtail 

Figure 4.20. Swing mount variation used by the U.S. Geological Survey Illinois District.
58 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

connection. The 12-volt DC power is supplied using a 

user-designed cable connected to the labeled terminal 

block on the back of the deck unit. Polarity is important 

when connecting to this terminal block using this cable 

connector. Early versions of the deck unit did not have 

input diode protection and could be damaged by 

improper DC polarity. The input polarity should be 

double checked before applying power. Most 

experienced BB-ADCP users begin a daily measuring 

session with two fully charged marine deep-cycle 

batteries. One fully charged 12-volt battery should 

power the BB-ADCP and computer for the whole day. 

The second battery is for backup.

A range of 20–60 V DC can be supplied to the 

deck unit through the bayonet style coaxial (BNC) 

connector (fig. 4.25). This voltage also must have 

proper polarity, as it is delivered directly to the 

BB-ADCP. The voltage must be supplied using a 

properly insulated (center positive) female BNC 

connector. IMPORTANT!!! These voltage magnitudes 

can be lethal, especially around water. Make sure all 

connections are insulated properly.

The voltage entering the 20- to 60-volt connector 

does not go through the DC-to-DC convertor before 

being sent to the BB-ADCP; therefore, it is not “stepped 

up” to the nominal BB-ADCP transmit voltage of 50. 

Voltages less than 30 may not supply the BB-ADCP 

with enough transmit power to enable profiling in the 

deeper depth ranges, especially if the water has few 

scatterers. Voltages greater than 60 may damage BBADCP power regulation components.

Acoustic Doppler Current Profiler Cables and Connectors

Broad-Band Acoustic Doppler Current Profiler

A multi-pin shell connector on the BB-ADCP 

cable connects to a 24-pin bayonet socket on the back 

of the deck unit. This connector assembly is not very 

Figure 4.21. Detachable swing mount used by the U.S. Geological 

Survey Idaho District.

Figure 4.22. Optical range finders used to estimate near-shore 

distances.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 59

sturdy and should be protected from undue twisting and 

mechanical pressure. The overhang at the back of the 

deck unit prevents easy disengagement of the bayonet 

mechanism, and many operators only partially engage 

the mechanism to facilitate easy removal. A right-angle 

shell connector can be ordered (when specifying the 

cable length) that may provide greater protection than 

the standard shell connector.

The molded underwater connector on the 

BB-ADCP end of the cable has a plastic alignment key 

that can become worn. Slight misalignment caused by 

wear of the alignment key can cause connector pins to 

bend when mating the connectors and tightening the 

locking ring. The O rings on both connector ends 

should be lubricated regularly with silicone grease and 

the connector rocked slightly to enable proper 

connector mating. If the alignment key or keyway is 

worn excessively, the connectors should be replaced.

It is possible that a long, coiled BB-ADCP cable 

can cause improper BB-ADCP operation. Noise or 

interference can be introduced (induced) into the coiled 

cable. Excess BB-ADCP cable should not be coiled, but 

flaked (using a non-overlapping S-shaped pattern) 

along the deck. It is best to order a short cable for use 

when the deck unit is mounted close to the BB-ADCP.

Workhorse Rio Grande

There is no deck unit for the Workhorse Rio 

Grande ADCP. Power is supplied to the electronics in 

the pressure case through a combination power and 

communications cable. The communications cable is 

terminated with a standard DB-9 RS-232 connector that 

mates with most laptop communication ports. The 

power cable is terminated with alligator clips that can 

be attached to a 12-volt battery. A users guide (R.D. 

Instruments, Inc., 1999) for connecting and configuring 

the Rio Grande is provided with the instrument or can 

be obtained from the RDI web site 

(www.rdinstruments.com).

Undoubtedly, other equipment will be needed 

that is unique to each operator or measurement site. 

Most operators build a “kit” containing ADCP 

measurement equipment, manuals, and field forms to be 

loaded before each deployment, thereby reducing the 

chances of forgetting to pack a vital piece of 

measurement gear.

Figure 4.23. Typical battery-operated trolling motor used when 

making acoustic Doppler current profiler discharge measurements.
60 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Summary

The acoustic Doppler current profiler (ADCP) 

discharge-measurement system consists of the 

following items:

An ADCP system with bottom-tracking 

capability

•Pressure case and transducer assembly

•Power supply and communications interface

•Discharge-measurement software

•Manufacturers’ documentation

Ancillary equipment

•Safe measurement platform or vessel

•ADCP mounting assemblies

•Laptop computer

•Range finder or method for estimating distance 

to shore

•A knowledgeable ADCP operator and welltrained vessel operator.

Figure 4.24. Two views of a steering adaptor that connects the trolling motor to the main engine.
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment 61

BB-ADCP 1/0 CABLE

TO J17

DECK UNIT (REAR VIEW)

FROM CHANNEL 1

(J20 RS-232 or J19 RS-422)

TO COMPUTER EXTERNAL RS-232 (J28)

TO COMPUTER

FROM (J22)

TO GYRO

TO

ENSEMBLE-OUT

DEVICE

FROM 

NAVIGATION

DEVICE 

TERMINAL/COMPUTER

INPUT POWER (CHOOSE ONE INPUT VOLTAGE)

DC 20-60 VOLTS

DC 12 VOLTS

AC 98-264 VOLTS

+ _

INPUT

98-264 VAC INPUT

12 VDC

INPUT

20-60 VDC

FUSE 5A FUSE 10A FUSE 5A

CHANNEL 1 CHANNEL 2 CHANNEL 3

RS 422 (J19)

RS 232 (J20) RS 232 (J24)

RS 484 (J21) RS 422 (J23)

GYRO

SERIAL RS 232 (J28) SYNCHRO/STEPPER (J22)

ADCP (J17)

CAUTION

ELECTROSTATIC SENSITIVE DEVICE

OBSERVE PRECAUTIONS

Figure 4.25. Interconnections of the broad-band acoustic Doppler current profiler (BB-ADCP) deck unit with other components of the 

acoustic Doppler current profiler discharge-measurement system. Adapted from R.D. Instruments, Inc., (1995). DC, direct current; 

AC, alternating current; V, volt.
62 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

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Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 63

CHAPTER 5: BROAD-BAND ACOUSTIC DOPPLER 

DISCHARGE-MEASUREMENT SYSTEM 

CONFIGURATION

Discharge-Measurement Software—”Transect”

Although the BB-ADCP can be used as a 

velocity-profiling instrument, this report primarily 

addresses its usefulness as part of a dischargemeasurement system. The purpose of this chapter is to 

discuss proprietary software used to configure the 

BB-ADCP or Workhorse Rio Grande and to acquire 

data from these ADCPs to measure discharge. This 

software is called “Transect.” Most of the information 

covered in this chapter also can be found in the Transect 

Users Manual for the BB-ADCP (R.D. Instruments, 

Inc., 1995). This chapter will not dwell on the small 

details of configuring the BB-ADCP or Rio Grande, 

but will attempt to touch on the important issues and 

provide tips to the neophyte ADCP operator.

Transect is composed of a series of stand-alone 

software modules that incorporate the dischargemeasurement algorithms discussed in chapter 2 and 

also include the following features:

•A graphical user interface

•Graphical output of ADCP velocity and 

discharge-measurement data

•Tabular output of ADCP velocity and dischargemeasurement data

•Command and control interface to the ADCP

•Raw and processed data storage and playback

Transect also includes algorithms for estimating 

unmeasured parts of the water column and cross 

section. The Transect software has the ability to accept 

and record (along with the velocity data) external 

navigation and depth inputs.

Transect Configuration

The configuration file is at the heart of the 

Transect software. The configuration file can be 

thought of as the interface between the ADCP and the 

Transect software modules. Commands in the 

configuration file tell the Transect software how to 

communicate with the ADCP, how to configure the 

ADCP, and how to modify the ADCP data-collection 

parameters for proper discharge measurement. It is 

possible to use the communication, calibration, and 

planning modules of the Transect software to build a 

trial configuration file on the basis of rough stream 

parameters. Although this method is used below to 

illustrate some of the modules in the Transect software, 

it is recommended that neophyte operators obtain 

preliminary configuration files from ADCP training 

classes or the USGS Office of Surface Water ADCP 

users’ group Web site (http://il.water.usgs.gov/adcp/). 

Experienced users usually bypass the Transect 

communication, calibration, and planning modules and 

directly modify preexisting configuration files using a 

text editor to fit the desired stream conditions.

To familiarize the reader with the Transect 

communication, calibration, and planning modules, we 

will first discuss a method for creating a preliminary 

configuration file, and then discuss each section of the 

configuration file, in detail.

Creation of a Preliminary Configuration File Using Transect 

Modules

Communications Setup

The Transect software provides a communication 

test and setup whereby the user can establish communication with the BB-ADCP processor. The user also 

can use any terminal-emulation software or the software supplied by the manufacturer called “BBTALK” 

to establish communication with the BB-ADCP. 

BBTALK is a simple terminal emulator with filecapture capability that can be used to initially check the 

BB-ADCP communication parameters if the Transect 

software cannot establish communication.

Instructions for using the emulator are in a disk 

file called 'BBTALK.DOC' that is shipped with the 

Transect software utilities. If the operator cannot 

establish communication with the BB-ADCP, then the 

BB-ADCP technical manual should be consulted and 

the communications and power connections should be 

double checked. Different baud rates and bit combinations can be tried on a trial-and-error basis in the event 

that the BB-ADCP was setup on a nondefault baudrate/bit combination. This will be practical only for 

semistandard parity bit and stop bit combinations (for 

example, no parity bit, eight data bits, one stop bit). It is 

possible to set the BB-ADCP to a nonstandard baud rate 

with a combination of stop bits and parity that is 

difficult (if not impossible) to determine by trial and 

error. If all standard methods produce “garbage” output 

to the BBTALK screen, the ADCP should be 

disassembled and the master reset button on the CPU 

board should be pushed (R.D. Instruments, Inc., 

1995–1999). This will reset the BB-ADCP to factory 

default communication values. This should, however, 

be done only after the failure of all other attempts to 

establish communication with the ADCP.

By using the BB-TALK terminal emulator 

(fig. 5.1), the operator can invoke the BB-ADCP 

diagnostics menu with the PT command. PT0 displays 
64 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

the test menu. PT200 performs an entire suite of 

diagnostic tests. The BB-ADCP technical manual 

provides a detailed description of the diagnostic tests. 

Parts of the PT200 diagnostic test often will indicate 

failure if the ADCP transducer assembly is not 

immersed in water. In fact, certain parts of the PT200 

diagnostic test may fail (with a healthy ADCP) if the 

transducers are immersed in a shallow bucket of water 

that is placed on a hard surface.

For best results, the diagnostic tests should be 

performed at the river cross section with the ADCP 

immersed in water. If the tests must be done in the 

office or lab, the transducer surfaces should be cleaned 

with soap and water (to remove grease), and then 

immersed in a plastic pail of water sitting on a foam 

pad. The manufacturer should be called if the 

BB-ADCP fails any of the PT200 suite of diagnostic 

tests using this scenario. Discharge measurements 

should not be done if the diagnostic tests indicate a 

failed subsystem. A PT200 test should be done before 

every discharge-measurement series, and the test 

output should be captured to a file and archived.

The communications menu selection of the 

Transect software main menu (fig. 5.2) allows the 

operator to establish communication with the 

BB-ADCP, provides the proper communication 

parameters for the construction of the configuration 

file, and provides access to direct (terminal emulation) 

communication with the BB-ADCP for checkout, 

configuration, and debugging of configuration 

problems. The communications parameters established 

in a session using the communication submenu are 

added to the communication section of the 

configuration file.

The communications ADCP submenu (fig. 5.3) 

provides for the setup and checkout of initial BB-ADCP 

communication parameters. Once these parameters 

have been determined, the trial configuration file can be 

saved with these parameters included. The communication menu also provides for the setup of communications between navigational devices, external readouts, 

and the external sensors (if any are required). The setup 

of these devices is covered in detail in the Transect 

software documentation (R.D. Instruments, Inc., 1999) 

and is not covered in this report.

Calibration Setup

After establishing communication with the 

BB-ADCP (and, thereby, setting up communication 

parameters in the configuration), the operator then 

should go to the calibration submenu for the next level 

of configuration file construction. In the calibration 

menu within the main menu (fig. 5.4), two submenus 

(offsets and scaling) will help create a “trial” 

configuration file when saved.

The calibration offsets submenu (fig. 5.5) 

provides the operator with the opportunity to set the 

BB-ADCP time and to enter compass alignment offsets 

(if the compass is separate from the BB-ADCP) or to 

enter compass magnetic corrections.

Figure 5.1. BB-TALK terminal emulator help screen.
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 65

The next menu entered should be the calibration 

scaling submenu (fig. 5.6). The calibration scaling 

submenu provides the operator with the option to 

change several important velocity-profile scaling and 

estimation parameters. If salinity at the measurement 

site is known, or can be accurately estimated, it can be 

entered in this menu. Speed of sound can be entered 

manually if it is known, but, in almost all cases, should 

be computed for each ensemble. The discharge 

extrapolation scheme should be left at “power” for the 

top estimation method, and “power” for the bottom 

estimation method, unless examination of the profiles 

indicates that these values should be changed or 

bidirectional flow exists in the measurement cross 

section. The power-curve exponent also should be left 

at 0.16670, unless otherwise indicated (chap. 8). Pitch 

and roll compensation should be left at the default 

setting of YES, which allows the Transect software to 

automatically compensate for vessel pitch and roll 

during the discharge measurement. The absorption 

Figure 5.2. Transect main menu showing menu choices. BB-ADCP, broad-band acoustic Doppler current profiler.

Figure 5.3. Communication acoustic Doppler current profiler (ADCP) submenu. 
66 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

coefficient and echo intensity scale also should be left 

at default values, unless otherwise indicated. The value 

of these parameters has no effect on discharge- or 

velocity-measurement results. The configuration file 

should be saved with the appropriate name upon exiting 

the calibration menu.

Planning Setup

The next main menu is the planning menu 

(fig. 5.7). The planning menu, has two submenus (set 

and ADCP) that, when used properly, will complete the 

initial creation of the configuration file.

Figure 5.4. Transect main menu showing calibration menu choices. BB-ADCP, broad-band acoustic Doppler current profiler.

Figure 5.5. Calibration offsets submenu screen. ADCP, acoustic Doppler current profiler
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 67

The planning setup submenu (fig. 5.8) prompts 

the operator for the deployment name, primary drive, 

secondary drive, recorded data types, measurement 

processing, and engineering units used for tabular and 

graphics output. If an operating frequency for the 

ADCP has not been specified, the setup menu will 

prompt the operator to provide an entry.

The deployment name should be a unique, fourletter identifier for the measurement site. For example, 

the identifier FRE5 could be used for Sacramento River 

at Freeport, California, session 5 (fig. 5.8). Future 

releases of Transect may support the use of a longer 

identifier. The deployment name has two functions; it 

becomes the name of a MS DOS root-level directory 

Figure 5.6. Calibration scaling submenu screen. ADCP, acoustic Doppler current profiler. 

Figure 5.7. Transect main menu showing planning menu choices. BB-ADCP, broad-band acoustic Doppler current profiler.
68 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

where all ADCP data collected with this configuration 

will be stored and it also forms the first four characters 

of each filename of ADCP data.

The primary and secondary drives in the setup 

submenu are usually the same and are used to construct 

the data storage path name. For example, data from the 

above example would be stored in the FRE5 directory 

on the C drive (C:\FRE5). The secondary drive is used 

in cases where the Transect software runs out of storage 

space on the primary drive. When hard disk space is 

limited, the secondary drive can be changed to indicate 

another hard disk or a floppy drive.

Data recorded with Transect may be stored in 

three different types of files: raw, processed (averaged), 

and navigation. Raw data alwaysshould be recorded for 

USGS discharge measurements. These data provide the 

operator with the ability to “play back” any discharge 

measurements that were recorded from unprocessed 

BB-ADCP data-output files. Processed data can be 

recorded if additional information needs to be stored 

during the discharge-measurement series. Raw data 

files contain unmodified data recorded directly from the 

ADCP. Processed data files typically contain averaged 

ADCP data, but also may contain data from an external 

navigation device, configuration-file data, and 

estimates of edge discharges.

Navigation data should not be recorded unless 

the BB-ADCP is connected to external navigation 

devices such as GPS or Loran C systems. The recording 

and processing of navigation data requires a recent 

version of the Transect software (4.00 or later) to 

execute properly. This report will not cover the 

operation of external navigation interfaces.

The processing selection (fig. 5.8) controls data 

averaging done by the Transect software (not in the 

BB-ADCP) and is used for processed data output. The 

raw ensemble data can be averaged with time (traverse 

time) or space (cross-section distance). This setting will 

affect the number of ensembles that are displayed 

during a cross-section traverse but will not affect the 

recording of raw ADCP data. The averaging of raw data 

is affected by the number of measurement pings 

averaged by the ADCP firmware (discussed in chap. 6).

The graphics and tabular displays can be 

annotated in either standard or metric units. This setting 

does not affect the raw data.

The planning ADCP submenu (fig. 5.9) allows 

the operator to input ship speed and the length of 

transect, and then provides the operator with a display 

of estimated transect time, first and last depth-cell 

positions, time between ensembles, and standard 

deviation of individual ensemble velocity averages. The 

operator also is informed of the amount of available 

disk space and disk space required for the transect.

If the operator has saved the configuration file at 

each of the above described menu steps, a configuration 

file has been created, much like the one in figure 5.10.

The Configuration File, in Detail

As explained in the previous section, the ADCP 

configuration file is used by the Transect software to 

communicate with, configure, and control an ADCP. 

Figure 5.8. Planning setup submenu screen. ADCP, acoustic Doppler current profiler.
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 69

The configuration file also controls the type of data 

collected and where the data are stored on the host 

computer. Configuration files are standard text files that 

can be opened with a generic text editor.

The first line of a configuration file verifies that 

this is an official RDI configuration file and must be 

present for proper loading. If the line becomes altered 

or corrupted, the operator will get a message that reads 

“the file is not a Transect configuration file.” If the 

above described error occurs, the line can be corrected 

(with the exact text shown in fig. 5.10) using a text 

editor.

The configuration file consists of the following 

sections:

•The communications section

•The calibration section

•The recording section

•The ADCP hardware section

•The direct command section

•The processing section

•The graphics section

•The history section

Each section of the configuration file addresses 

different aspects of ADCP command and control, as 

well as Transect software data storage and 

visualization. We will discuss each section of the 

configuration file in detail in the following paragraphs. 

Sections within the configuration file are enclosed in 

braces.

The Communications Section

The first section is the communication section, 

which sets up the Transect software to “talk” with the 

ADCP. The section contains directives in the form 

Device 5sp (Switch sp Port sp Baud-rate sp 

Paritybits sp Databits sp Stopbits) [port baud 

parity databits stopbits]

where 

sp = space.

Items in parentheses should be edited to match 

the device configuration. Items in brackets are 

reminders or comments and should not be changed. For 

example, if an ADCP were setup to communicate at 

38,400 bauds per second with no parity, eight stop bits, 

and one data bit through the COM1 port of a computer, 

the first directive line would read: ADCP (ON COM1 

38400 N 8 1) [Port Baud Parity Databits Stopbits].

There are directives for all allowed devices that 

might communicate with the Transect software through 

the various computer ports. The ADCP directive line is 

mandatory; however, the other directive switches 

should be turned off unless needed (fig. 5.10). These 

directives also can be set from the communication 

menu in the Transect software.

Ensemble Out Section

The ensemble out section (fig. 5.10) contains a 

series of toggles that enable or disable output of the 

various ensemble data. If the ENSOUT directive in the 

communication section is ON, then information from 

Figure 5.9. Planning acoustic Doppler current profiler submenu screen. ADCP, acoustic Doppler current profiler.
70 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

BEGIN RDI CONFIGURATION FILE — This line must be first in all RDI configuration files

COMMUNICATIONS — Begin communication section

 

ADCP — A Must ON COM1 9600 N 8 1 — Must be set to ADCP rate

ENSOUT 

NAV OFF COM3 9600 N 8 1

EXTERNAL OFF COM5 9600 N 8 1

UseSoftwareBreak (NO) — This is set to yes when using a radio modem.

ADCP HARDWARE

Firmware (5.57)

Angle (20)

Frequency (1200)

System (SHIP)

Mode (1)

Orientation (DOWN)

Pattern (CONVEX)

Switch Port Baud Parity Databits Stopbits 

Optional...see

explanation in text.

This section (ADCP hardware) will fill with pertinent information

when you start the acquire program (see text). If you have set these

values incorrectly, you will receive a warning message after the ADCP

responds.

— This is for

 GPS or Loran

{

}

}

}

OFF COM2 9600 N 8 1

OFF COM4 N 8 1 REFOUT 9600

each ensemble is transmitted from the selected 

computer port. This section normally is not used for 

ADCP discharge measurements. Details for obtaining 

real-time ensemble data are available in the Transect 

software manual (R.D. Instruments, Inc., 1995). The 

ensemble out section also can be set up from the 

communications menu in the Transect software.

Acoustic Doppler Current Profiler Hardware Section

The ADCP hardware section (fig. 5.10) is a list of 

parameters that describe the ADCP hardware 

configuration. These parameters are not actually 

transmitted to the ADCP but are checked against actual 

ADCP hardware parameters upon ADCP “wake up.” 

Missing parameters are filled in to match ADCP 

hardware. If the Transect software discovers a 

discrepancy between parameters in the configuration 

file ADCP hardware section and configuration data 

transmitted by the ADCP, a warning is displayed. 

Additional information regarding this check is given 

later in this chapter.

Direct Commands Section

Direct commands are commands interpreted by 

the ADCP, much like a computer operating system 

interprets commands issued by a user. Direct 

commands can affect the outcome of a discharge 

measurement, therefore, it is imperative that the ADCP 

operator fully understand the purpose and outcome of 

the basic direct command set. The direct commands 

section is the most important part of the configuration 

file (fig. 5.11). Some direct commands are mandatory 

for proper discharge measurement. Erroneous 

commands can cause improper operation of the ADCP. 

The most important of the direct commands are the 

water- and bottom-mode commands (WM, WV, and 

BM). These commands were discussed in chapter 3, 

and are not discussed here.

Figure 5.10. Communication, ensemble out, and acoustic Doppler current profiler (ADCP) hardware sections of the configuration file. 

RDI, R.D. Instruments, Inc.; GPS, global positioning system.
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 71

DIRECT COMMANDS — Direct command section: These commands are very important 

 and are covered in detail later in chapter 5 of this report.

WS25 — Bin size

BX300 — Bottom-track depth

WNXXX — Number of water-measurement bins

WF50 — Blanking distance

WV170 — Mode 1 ambiguity velocity

WZ5 — Mode 5 lag 

WD111100000 — Output data types

WM1 — Water mode 1 — Number of bottom pings

WP5 — Number of water pings

BP4 — Number of bottom pings

BM5 — Bottom-track mode

TP000006 — Time between pings

EX10111 — Coordinate transformation flags

ES0 — Salinity

{

}

}

{

RECORDING — Beginning of the ADCP recording setup section

Deployment (FRE5) — Unique four-letter deployment name

Drive 1 (C) — Primary data storage drive

Drive 2 (C) — Secondary (or emergency) data drive

ADCP (YES) — Set YES to record ADCP data (a MUST)

Average (NO) — Averaged data are not usually recorded

Navigation (NO) — Set to No external GPS data are recorded

StartRecording (NO) — Set to yes to turn recording on by default

 when Transect is first started. 

Some direct commands may be controlled from 

menus within the Transect software. Direct commands 

are discussed in detail in the technical manuals for the 

BB-ADCP, Rio Grande, and narrow-band ADCP 

(R.D. Instruments, Inc., 1995–1999) and many of 

them are not referenced in this report. However, an 

understanding of the most important commands is 

required for proper creation of configuration files.

Direct commands, like all other BB-ADCP 

parameters, can be edited into the configuration files 

without using the Transect software menus. However, 

the expert mode (alt-M) in the ADCP planning submenu provides an easy way to enter direct commands 

into the configuration file. This mode allows the 

operator to specify the direct commands that will be 

sent to the BB-ADCP during Transect initialization 

(fig. 5.12).

Figure 5.11. Direct command and recording sections of the configuration file. ADCP, acoustic Doppler current profiler; GPS, Global 

Positioning System.
72 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

The direct command numeric arguments are set 

by the manufacturer and are in metric units. To avoid 

confusion, standard-unit conversions will not be 

presented in the following discussion. All direct 

commands listed in the technical manual are important; 

however, some are set by default and almost never 

change, some are not changed except under rare 

conditions, and some are changed dynamically by the 

instrument. Some direct commands must be changed 

by the operator for optimum BB-ADCP operation in 

changing environments and are discussed below.

When discussing the direct commands, the 

syntax for each command will be shown as CCnnnn, 

where CC is a two-letter command; and nnnn is an 

integer parameter. For example, the water ping 

command will be referred to as WPnnnn, where nnnn is 

the number of water pings averaged for a data 

ensemble.

Water-Track Commands

WSnnn

WSnnn is the command used to set the length, in 

centimeters, of the depth cell used for water-velocity 

measurement (0–999), and controls the vertical 

resolution of the measured-velocity profile. In most 

cases, this value of WS can be determined using 

table 5.1

These settings provide the best resolution in the 

event that velocity profile information must be 

extracted from the discharge measurement. By default, 

these values are placed in the direct command section 

when a new configuration file is created with the ADCP 

frequency specified. In some cases, these values may 

have to be changed (in very deep or very shallow water, 

for example). Consult the BB-ADCP technical manual 

before doing so. The WS parameter also may be set in 

the planning menu of the Transect software.

WNnnn

WNnnn is the command used to set the number 

of water-velocity measurements (depth cells) in the 

vertical profile, and can have a numeric range of 1–128. 

This value should be set so that WS multiplied by WN 

exceeds the maximum expected depth in the cross 

section by a safe margin. CAUTION! Failure to 

properly set this parameter can cause loss of dischargemeasurement accuracy, loss of vertical resolution, and 

incomplete velocity profiles. This parameter also can be 

set using the planning menu in the Transect software.

WPnnnn

WPnnnn is the command that sets the number of 

water-velocity measurements (pings) that are averaged 

together to form a data ensemble (range from 0 to 

Figure 5.12. Planning acoustic Doppler current profiler (ADCP) menu in expert mode (Alt-M). 

Table 5.1. Optimum bin size (WSnnn) for acoustic Doppler current 

profiler (ADCP) discharge-measurement applications

[kHz, kilohertz] 

ADCP

water mode

WSnnn for

1,200-kHz ADCP

WSnnn for

600-kHz ADCP

WSnnn for

300-kHz ADCP

Mode 1 25 50 100

Mode 5 5 10 20

Mode 8 5 10 20
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 73

6384). As a general rule it is recommended that this 

value be set to 1 because if an ambiguity error occurs, 

it will not be averaged with other pings and therefore 

can be readily identified. If data are averaged by using 

a WP command greater than 1 and an ambiguity error 

occurs in one of the ensembles, it will be averaged 

along with the good pings and will cause a bias error 

that may not be recognized when the data are played 

back. In situations where many ambiguity errors occur, 

the discharge measurement will be biased. This bias 

can occur using water modes 5 and 8 and sometimes is 

seen using mode 1 during dynamic conditions.

In some cases during low-flow measurements, 

when using mode 1, a modest number of pings can be 

averaged to help reduce the standard deviation of the 

discharge measurement. Averaging pings in the ADCP 

by using a non-zero WP command can increase the 

total number of pings collected during a discharge 

measurement because the ADCP firmware processes 

data faster than the Transect software. However, using 

a non-zero WP command is dangerous (because of the 

possibility of disguised velocity ambiguities) and 

should be used with caution.

WFnnnn

WFnnnn is a command used to set transducer 

blanking distance (range 0–9999 cm); the blankingdistance command defines the regions near the 

transducer faces where no velocity data should be 

collected. Depth cells close to the transducer faces may 

be corrupted because transmitted energy has not 

completely dissipated from the electronics or 

transducer ceramics. If this command is set too low, it 

may cause corruption of the first measured velocity bin. 

This corruption can be very hard to detect. For 

1,200- and 600-kHz BB-ADCP units, the recommended minimum setting for this value is 50 for mode 

1 and 30 for modes 5 and 8 operation. For Rio Grande 

units, the recommended setting for this value is 25 for 

all modes. Minimum recommended values for this 

parameter also depend upon operating frequency.

WDnnn nnn nnn

WDnnn nnn nnn is the water-data output from 

the BB-ADCP. The toggles for these parameters are set 

to 1 or 0 (1 = enabled, 0 = disabled) for the following 

data: velocity, correlation, intensity, percent good, 

status, and P0, P1, P2, and P3 (Px = external parameter 

data 0–3). For example, WD111100000 outputs 

velocity, correlation, intensity, and percent good.

Bottom-Track Commands

BAnnn

BAnnn is the bottom-track evaluation amplitude 

minimum (1–255 counts). This command defaults to 

30, but can be changed to 25 when bottom movement is 

suspected. Values lower than 25 should not be used 

because of the possibility of velocity-measurement 

contamination by sediment moving above the bottom. 

When bottom-sediment movement is present, using 

lower frequency ADCPs (300- or 600-kHz) may 

sometimes help eliminate bottom-movement bias 

(chap. 3) (example BA25).

BCnnn

BCnnn is the bottom-track correlation magnitude 

minimum (1–255 counts). The BB-ADCP flags any 

data having a correlation magnitude less than this value 

as bad data—perfect correlation (solid target) is 

255 counts and the default value is 220 counts. This 

value can be set slightly lower when bottom tracking is 

difficult to acquire (example BC210).

BPnnn

BPnnn is the number of bottom pings that are 

averaged for each data ensemble (1–999 pings). This 

value should be set to the same value as that of the WP 

command, or one less than the WP command if more 

than one water ping is averaged for each data ensemble 

(example BP01).

BXnnnn

BXnnnn is the maximum bottom-track search 

depth, in decimeters [80–9999 decimeters (dm)]. This 

value should be set to match the deepest expected depth 

plus a safety factor. CAUTION! Error in this parameter 

in water deeper than 8 m can cause loss of bottom track 

and invalid discharge measurements because of missing 

ensembles. This parameter is set in decimeters, rather 

than centimeters, and, therefore, is more prone to 

operator setup errors (example BX100).

&Rnn

The &Rnn command, while not a standard 

bottom-track command, must be set properly in the 

BB-ADCP because the default setting is not optimized 

for river discharge measurement. The value nn is the 

length of the bottom-track pulse, in percent of total 

depth. In BB-ADCP firmware, the default for this is 

30 (30 percent). The manufacturer suggests that a value 

of 20 (20 percent) is more applicable to riverine 

environments. The recommended setting for this 

command is &R20, and it should be included in the 

direct commands section of the configuration file so that 

the default value is overwritten.
74 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

General Commands

ESnn

ESnn is the estimated salinity, in parts per 

thousand (0–40). It is important to set this parameter 

properly because the BB-ADCP default setting is for 

open ocean (ES35), which can cause dischargemeasurement errors in freshwater if undetected. An 

ES0 command is necessary in the configuration file if 

measuring freshwater. Salinity in brackish or oceanic 

waters should be measured with a salinimeter. Unless 

the salinity is known, this command always should be 

set for zero salinity (ES0) because the value can be 

changed after the fact in the Transect calibration section 

(if it is discovered that brackish water was present, for 

example). If the value is set to the wrong salinity (other 

than 0) in the direct command section, it is much harder 

to correct the resulting discharge values.

EXnnnnn

EXnnnnn is the set of coordinate transformation 

flags, where each n in the command is set to a 1 or a 0, 

according to the following encoding scheme:

•EX00nnn = no transformation (beam 

coordinates)

•EX01nnn = instrument coordinates

•EX10nnn= ship coordinates

•EX11nnn = Earth coordinates

•EXnn1nn = uses tilts (pitch and roll) in 

transformation

•EXnnn1n = allows three-beam solutions if one 

beam is below WC threshold

•EXnnnn1 = allows bin mapping

This command allows the BB-ADCP to 

transform beam coordinates into other coordinate 

systems. The Transect software can do the above 

transformations if radial-beam velocities (beam 

coordinates) are recorded (the default mode). For 

robust operation, the BB-ADCP probably should be set 

for ship coordinate transformation because the data are 

transformed in the Transect software rather than 

adjustments made to the raw recorded data.

If velocity profiles are required, it is preferable to 

collect raw data in beam coordinates because of the 

difficulty in changing coordinate systems after 

conversion to Earth coordinates. (It is difficult to “back 

out” raw-beam coordinates if there is no ping-by-ping 

record of the corrections for pitch, roll, and heading.)

When recording in ship or beam coordinates 

while averaging ensemble data in the profiler, pitch, 

roll, and sharp boat movements and heading changes 

should be kept to a minimum. Corrections for these 

events are calculated by the Transect software at the end 

of the ensemble (not for each ping). For example, if 

averaging is set to five water pings and five bottomtrack pings per ensemble and beam coordinate 

transformation is selected, coordinate transformation 

and pitch and roll correction will not occur until the 

averaged ensemble is transmitted to the computer from 

the ADCP. In the above case, the transformation and 

correction occurs at the end of a 4- to 5-second interval. 

If the boat has pitched or rolled significantly during the 

5-second period, the correction may be applied 

incorrectly or not applied at all. For velocity 

coordinates, the Transect software calculation is not 

affected by the coordinate system being used, however 

the software does require exact synchronization 

between the bottom-track vector and the water-track 

vector. If both coordinates are Earth transformed at the 

end of a 5-second interval and there are abrupt heading 

changes during the interval, the synchronization of 

these vectors may not be achieved because of 

time/phase lag in the onboard flux-gate compass. For 

these reasons, despite the lower ADCP ping-rate 

performance, the WP and BP commands probably 

should be set to average 1 ping for each data ensemble 

(WP01 and BP01) in windy conditions or conditions 

with a significant amount of pitch and roll.

Another reason for setting WP and BP to 1 is so 

ambiguity errors (chap. 3) will be more apparent. If 

data are averaged along with an ambiguity error, the 

error could be masked because of the averaging. This 

can be especially devastating in mode 5 because 

ambiguity errors are more likely to occur.

Pitch and roll corrections should be applied, 

unless the tilt sensors are defective. If tilt sensors are 

transmitting invalid data, they can be disabled to collect 

discharge data.

In most cases, three-beam solutions always 

should be enabled so that velocities can be calculated in 

the event that data are lost from one beam, however, 

there is some controversy as to the proper setting for 

three-beam solutions. Please check the USGS ADCP 

users web site (http://il.water.usgs.gov/adcp/) for latebreaking instructions.

Bin-depth mapping should be enabled so that the 

measurement bins are kept at the same depth for each 

beam during a pitch and roll event.

For example, an EX00111 command does no 

coordinate conversion (beam coordinates, uses pitch 

and roll in the transformation, allows three-beam 

solutions for missing beams, and allows bin-depth 

mapping for pitch and roll).

Recording Section

The recording section of the configuration file 

(fig. 5.11) contains information that is used by the 
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 75

Transect software to set up the data recording paths, as 

well as specifications for drive priority, averaged data 

recording, and navigational data recording. Required 

sections are as follows:

•Deployment (cccc), where cccc is a fourcharacter file name to be used as a prefix for the 

data file. For example, if the operator picks 

deployment (TUFA), resulting file names will 

be TUFA001r.000, TUFA002r.000, and so forth. 

The deployment name and configuration file 

name should be the same to keep the correct 

configuration file synchronized with the correct 

data set (version 2.72 and earlier). Version 2.80 

(and later) of the Transect software stores a 

configuration file with each transect, thereby 

eliminating the synchronization problem.

•Drive 1 (d) where d is the path name for the 

primary data-storage drive. For example, if 

drive C:\ is the primary data drive, then the 

proper directive is drive 1 (C).

•Drive 2 (d) where d is the path name for the 

secondary data-storage drive. For example, if 

drive C is nearly full and is the primary datastorage drive, the Transect software will 

automatically switch to the secondary storage 

drive upon filling C drive with data. If a 

formatted floppy disk is placed in drive A for 

such an emergency, then the proper directive 

would be drive 2 (A).

•ADCP (YES/NO) specifies that the Transect 

software record (or not record) ADCP raw 

(unaveraged) data. This always should be set to 

YES.

•Average (YES/NO) specifies that the Transect 

software record (or not record) averaged data. 

This directive usually is set to NO, unless the 

operator wishes to record averaged data. 

Because raw data can be averaged on playback, 

this directive usually is not needed, even if 

averaged data are desired.

•Navigation (YES/NO) specifies that the 

Transect software record (or not record) 

external-navigation data. This is set to YES if 

external-navigation data from Loran C or GPS 

systems are to be recorded with each ensemble. 

Any external RS-232 data can be recorded in 

this manner and synchronized with the ADCP 

data.

Calibration Section

The calibration section of the configuration file 

(fig. 5.13) contains important information that supplies 

the Transect software with data that are vital to proper 

coordinate transformation and discharge calculation. 

Note that these data can be changed upon playback and 

are not part of the raw data file.

•ADCP depth, in meters, is the draft (depth below 

water surface) of the ADCP transducer faces. 

This parameter is used by the Transect software 

to calculate the depths at the bin centers and is 

vital to the proper calculation of the curve fit 

estimation data as well as subsection depth.

•Heading/magnetic offsets are values that are 

primarily used to correct the internal flux gate 

compass of an ADCP to true north. One 

parameter adjusts for magnetic anomalies (such 

as metal objects), whereas the other adjusts for 

magnetic declination obtained from 

navigational charts or maps.

•Transducer misalignment is a value that is 

nonzero only when the compass is separate from 

the transducer assembly. This value is used to 

correct the azimuth of the transducer assembly 

to the azimuth of the compass (in degrees), and 

is primarily used with gyroscopic-based heading 

systems.

•The intensity and absorption values are scale 

factors used to correct the raw, backscattered 

intensity counts from the ADCP into decibel 

values corrected for receiver gain and range. The 

intensity scale factor is used to convert raw 

counts to decibels and should be left at the 

default value unless other values are indicated. 

From the calibration menu, the absorption value 

can be set to a default value, set to a known 

value, or calculated by the Transect software as 

a function of frequency, water temperature, and 

salinity. All three choices can be made from the 

Transect calibration software by tabbing to the 

appropriate selection in the sound absorption 

coefficient part of the scaling submenu.

•Salinity should be set to the measured or 

estimated salinity at the measurement site.

•Speed-of-sound correction and pitch-and-roll 

compensation normally should be set to YES, 

unless the temperature or tilt sensors are 

providing erroneous data.

•Tilt misalignment, pitch offset, and roll offset 

normally are set to zero unless the tilt sensors are 

mounted external to the ADCP case.

•Top discharge estimate can be set to POWER or 

CONSTANT. If it is set to POWER, then the 

Transect software uses the power-curve 

coefficient to estimate velocities in the 

unmeasured area near the water surface. If it is 

set to CONSTANT, then the Transect software 
76 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

CALIBRATION — ADCP calibration section (discussed in detail in chapter 5 of this report ).

ADCP depth (0.16 m) — ADCP draft (distance below water surface).

DBTDraft (0.00 m) — Depth sounder draft (if an external depth sounder is used).

Heading / Magnetic offset (0.00 10.00 deg) — For setting compass alignment to 

Transducer misalignment (0.00 deg) — For aligning compass to ADCP 

Intensity scale (0.43 dB/cts)

Absorption (0.278 dB/m)

Salinity (0.0 ppt) — Set to correct salinity value here (or zero if in doubt).

Speed of sound correction (YES)

Pitch & roll compensation (YES)

Tilt Misalignment (0.00 deg)

Pitch_Offset (0.000 deg) 

Roll_Offset (0.000 deg)

Top discharge estimate (POWER)

Bottom discharge estimate (POWER)

Power curve exponent (0.1667) — Normally set to 0.1667 unless the velocity profiles appear

 to be nonstandard.

 Normally left at default values.

 

Normally set to POWER.

 Normally set to 0.00 unless the tilt sensors are mounted

 external to the ADCP case.

 Normally set to YES.

(if compass is true north.

separate).

{

}

uses the same cross product as is used for the 

uppermost measured bin to estimate nearsurface, unmeasured discharge. Chapter 8 

contains a discussion on how to examine the 

measured profiles to determine the proper 

estimation technique.

•Bottom discharge estimate can be set to 

POWER or CONSTANT. If it is set to POWER, 

then the Transect software uses the power-curve 

coefficient to estimate velocities in the 

unmeasured area near the channel bed. If it is set 

to CONSTANT, then the Transect software uses 

the same cross product as is used for the bottommost measured bin to estimate near-bottom, 

unmeasured discharge. Chapter 8 contains a 

discussion on how to examine the measured 

profiles to determine the proper estimation 

technique.

•Power-curve exponent usually is set to 0.1667 

for a “Manning-like” water-velocity profile. 

Under most conditions, this value should be set 

to the one-sixth power setting (0.1667), even in 

the estuary. A good test of this value is to pick a 

transect and average a series of ensembles in the 

deeper, faster part of the cross section. The 

averaged profile then should be then viewed 

from the discharge profile plot in the Transect 

software playback menu. If the plotted power 

curve fits the data points, the one-sixth exponent 

value can be used. If the plotted power curve 

does not fit the data, other exponent values can 

be tried until a good fit is obtained. If the 

velocity profile is distinctly nonstandard, or 

crosses zero (bidirectional) then a power-curve 

fit should not be used (use CONSTANT for top 

and bottom velocity estimates). This technique 

will be discussed in more detail in chapter 8.

Figure 5.13. Calibration section of the configuration file. ADCP, acoustic Doppler current profiler.
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 77

Processing Section

The processing section of the configuration file 

(fig. 5.14) contains directives to the Transect software 

that control ensemble averaging, profiled depth, and 

internal depth-sounder data, as well as directives that 

control the output of an external velocity monitor 

(refout):

•Average every () directive can be set to a time 

value or a spatial value. For example, if the 

value is set to “average every [5.00 seconds 

(s)],” then the Transect software will display an 

averaged output to the console every 5 s. This 

parameter also can be set to a metric distance 

value. “Average every (5.00 m)” will send an 

averaged output to the console every 5 m (16 ft). 

During data acquisition (acquire) it usually is 

best to set this value to zero for faster updates to 

the computer screen. The setting of this value 

will not affect the raw-data file.

•Depth-sounder value must be set [(YES) or 

(NO).] If the ADCP has an internal fifth-beam 

depth sounder, this value should be set to YES; 

otherwise, it should be set to NO. (This 

normally should be set to NO, unless you have a 

special system.)

•BTM layer percent () is a value that overrides 

the normal maximum bottom profile range 

percentage (85 percent with 30o transducer 

angle and 94 percent with 20o transducer 

angles). This directive need not be used unless 

the operator wishes to change the default 

profiling range for the ADCP. This change must 

be made for old firmware versions for mode 5 

operation and for other special circumstances. 

The accuracy of the Transect software output 

data can be seriously degraded if this value is set 

incorrectly. Inclusion of this command is not 

recommended unless you are trying to use water 

mode 5 with an older BB-ADCP with phase 2 

firmware.

The remainder of the values in the processing 

section are not discussed in this report, and are used 

only if data are being sent to an external reference via 

an RS-232 port. Information on these settings is in the 

BB-ADCP or Rio Grande technical manual 

(R.D. Instruments, Inc., 1995–1999).

Graphics Section

The graphics section of the configuration file 

controls the Transect software display scaling factors. 

This section is best changed from within the Transect 

software in either the Acquire or playback software 

using the F6 special function key. These settings are 

discussed in chapters 6 and 8.

History Section

The history section of the configuration file is 

modified by the Transect software. This section gives 

the version number of the Transect software used with 

the configuration file and should not be changed by the 

operator.

Finishing the Preliminary Configuration File: Required 

Commands

Before the preliminary configuration file that we 

created in the first part of this chapter can be used for 

data collection, some additional commands must be 

added, either by using an editor or by using the expert 

mode in the Transect software calibration menu. The 

sections must be checked, changed, or modified 

(figs. 5.15–5.18).

Entries in the ADCP hardware section that are 

added by the Transect software during the first run of 

the Acquire module are shown in figure 5.15. These 

entries also can be added using a text editor such as 

Microsoft Disk Operating System (MS DOS) Edit. The 

Transect software will check the values against values 

returned from the ADCP and will deliver a warning to 

the operator if they do not match.

Entries in the direct command sections that must 

be checked and entered manually, if necessary, also are 

shown in figure 5.15. The command syntax and purpose 

are discussed in the Direct Commands Section earlier in 

this chapter. The critical entries to check are as follows:

•WPnnn is the water pings per ensemble. This 

value normally should be set to 1 (example 

WP001).

•BPnnn is bottom pings per ensemble. This value 

normally should be set to the same value as the 

WP value (example BP001).

•ESnn is estimated salinity, in parts per thousand. 

This value should be set to zero, unless the 

operator is sure of the actual salinity value 

(example ES0). The Transect software can 

calculate speed-of-sound using salinity values 

after-the-fact.

•EXnnnnn is for coordinate transformation. If 

beam coordinates are selected, then EX00111 

should be used.

•BXnnnn is bottom-track maximum search 

depth, in decimeters. This value should be set 

slightly deeper than the expected maximum 

depth at the measurement site. Loss of bottom 

track can result if this value is too shallow, and 
78 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

GRAPHICS — Graphics parameter section.

Units (English)

Velocity Reference (BOTTOM)

East_Velocity (-1.0 1.0 ft/s)

North_Velocity (-1.0 1.0 ft/s)

Vert_Velocity (-0.5 0.5 ft/s)

Error_Velocity (-0.5 0.5 ft/s)

Depth (1 35 bin)

Intensity (50 200 counts)

Discharge (-1 1 ft3/s)

East_Track (-211 211 ft)

North_Track (-190 231 ft)

Ship track (1 bin 0.5 ft/s)

Proj_Velocity (-1.0 1.0 ft/s)

Proj_Angle (250.0 deg from N)

Bad_Below_Bottom (YES)

Line1 ('Standard' Config file for 1200 kHz)

Line2 (Mode 1 -- 25cm bins -- 5 WP 4 BP -- Ship)

These data should be left alone 

unless the ADCP output is being 

sent to an external monitor or 

device other than the computer 

running the Transect software.

 Information stored in these lines

 is displayed on Transect's output

 data plots.

 This section should not be modified.

 Transect stores the version number here

 that was used to collect discharge data.

{

}

}

{

}

PROCESSING — Beginning of the ADCP processing parameter section.

Average every (0.00 s) — Normally set to 0.00 unless you are data averaging

Use Depth Sounder (NO) — Set to NO unless the ADCP has a depth-sounder beam.

MaxFileSize (1200)

External_formats (N N N N N) [HDT HDG RDID RDIE]

External_decode (N N N N) [heading pitch roll temp]

Start_Shore_distance (-1) [cm]

End_Shore_distance (-1) [cm]

Edge_distance_prompt (NO)

Use GPS For Btm (0)

HISTORY

SOFTWARE (BB-TRANSECT)

Version (4.05)

END RDI CONFIGURATION FILE — This is a required end-of-file statement.

These values are set in the Transect program and should 

not be changed directly by editing the configuration file.

Figure 5.14. Processing, graphics, and history sections of the configuration file. ADCP, acoustic Doppler current profiler; GPS, Global 

Positioning System; RDI, R.D. Instruments, Inc.
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 79

ADCP HARDWARE

Firmware (5.57)

Angle (20)

Frequency (1200)

System (SHIP)

Mode (1)

Orientation (DOWN)

Pattern (CONVEX)

This section now has information that should

correspond to the current ADCP hardware 

on your system.

{

}

}

}

}

{

{

{

DIRECT COMMANDS

BX200 — The maximum bottom-track depth is 200 decimeters (20 meters).

WS50 — Water bin size is 50 centimeters.

WN060 — Set for 60 depth bins.

WF50 — Blanking distance is 50 centimeters.

WV170 — Mode 1 ambiguity velocity is set for 170 centimeters per second.

WD111100000 — Data collected are velocity, correlation, intensity, and percent good.

WM1 — Water-measurement mode 1.

WP1 — One water ping averaged per ensemble.

BP1 — One bottom ping averaged per ensemble.

BM5 —Bottom-measurement mode 5.

EX10111 — Coordinates are ship, with pitch and roll correction and bin mapping enabled.

ES0 — Salinity is set to zero.

&R20 — Bottom-track bin size is set to 20 percent of the measured depth.

ADCP (ON COM1 9600 N 8 1) [Port Baud Parity Databits Stopbits]

ENSOUT(OFF COM2 9600 N 8 1) [Port Baud Parity Databits Stopbits] 

NAV (OFF COM3 9600 N 8 1) [Port Baud Parity Databits Stopbits]

REFOUT(OFF COM4 9600 N 8 1) [Port Baud Parity Databits Stopbits]

EXTERNAL (OFF COM5 9600 N 8 1) [Port Baud Parity Databits Stopbits] 

UseSoftwareBreak (NO)

BEGIN RDI CONFIGURATION FILE

COMMUNICATIONS

ENSEMBLE OUT

ENS CHOICE (NNNNNNNN)[Vel Corr Int %Gd Status Leader BTrack Nav]

ENS OPTIONS (Bottom 1 8 18) [Ref First Last Start End]

ENS TYPE (RAW) [RAW (default) or AVERAGED data transmitted]

Figure 5.15. Hardware and direct command sections of the configuration file. RDI, R.D. Instruments, Inc.; ADCP, acoustic Doppler current 

profiler.
80 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Deployment (FRE5) — Make sure to update this deployment name when

 using the CFG file in another area.

Drive 1 (C)

Drive 2 (C)

ADCP (YES)

Average (NO)

Navigation (NO)

StartRecording (NO)

{

}

RECORDING

excessive ensemble times can result if it is too 

deep. For example, a BX0250 command sets the 

bottom-track maximum search depth at 25 m.

•WMnn, BMnn, and WVnnnn are water and 

bottom mode commands. These commands 

should be added per instructions in chapter 3.

The recording and calibration sections of the 

completed configuration file are shown in figures 5.16 

and 5.17, respectively. If the configuration file is 

renamed for use at another measurement site, the fourcharacter deployment name must be changed to one 

matching the new site. If the configuration file is reused 

at another site, the proper transducer draft must be 

entered. Failure to do so can cause significant errors 

when measuring shallow/wide rivers. The two-line 

comment in the graphics section (fig. 5.18) also should 

be updated when reusing the configuration file.

Transect Release Enhancements (2.80 and Later)

For purposes of brevity, Transect software 

releases 2.80 and later hereafter will be referred to as 

Transect 2.80+. The following enhancements are 

included in Transect software release 2.80+:

•A copy of the configuration file is saved with 

each discharge data file with the letter C 

imbedded in the file name. Example—If the raw 

data file name is JUNK001R.000, then the 

configuration file is JUNK001C.000.

•The user is prompted for starting and ending 

edge distances. These values are stored in the 

configuration file and in the processed data file, 

if one has been generated.

•The unique configuration file can be loaded 

automatically and used during the Transect 

software playback.

•The Transect module “Acquire” starts with 

recording turned off. This feature eliminates two 

keystrokes at the start of each measurement. 

Percent good is redefined by the user in 

processed data files.

•The Transect software will extrapolate discharge 

in missing bins.

•Default data file size is changed from 

300 kilobytes (K) to 1,200 K.

•Other enhancements are included as detailed in 

the Transect version 2.80 software 

documentation.

Transect 2.80+ operation requires some 

additional commands within the configuration file and 

at the DOS prompt (or in the Autoexec.bat file).

Earlier versions of the Transect software allowed 

only for a triangular-shaped edge slope. Transect 2.80+ 

allows the operator to select an edge slope that can vary 

from square to no slope at all (fig. 5.19). Entering a -1 

directs the Transect software to use a triangular-shaped 

edge slope (default). Entering a -2 directs the Transect 

software to use a square-edged slope, and entering a 

value between 1 and 0 directs the software to use that 

value as a slope coefficient where 0.91 is nearly square, 

0.35 is almost triangular, and 0.05 is nearly flat 

(fig. 5.19).

In the processing section of the configuration file, 

there are three new items used by Transect 2.80+. If 

Edge_distance_prompt is set to YES, then the software 

will prompt the operator for edge-distance estimates 

during data acquisition (acquire). The values that are 

Figure 5.16. Recording section of the configuration file. ADCP, acoustic Doppler current profiler; CFG, configuration (file).
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 81

ADCP depth (0.16 m)

DBTDraft (0.00 m)

Heading / Magnetic offset (0.00 0.00 deg)

Transducer misalignment (0.00 deg)

Intensity scale (0.43 dB/cts)

Absorption (0.278 dB/m)

Salinity (0.0 ppt)

Speed of sound correction (YES)

Pitch & roll compensation (YES)

Tilt Misalignment (0.00 deg)

Pitch_Offset (0.000 deg)

Roll_Offset (0.000 deg)

Top discharge estimate (POWER)

Bottom discharge estimate (POWER)

Power curve exponent (0.1667)

Edge_slope coefficient (-1.00000) [-1=Trianglar(0.3535):-2=Square(0.91):User]

OneCycleK (0.0000000)

OneCycleOffset (0.0000000)

TwoCycleK (0.0000000)

TwoCycleOffset (0.0000000)

DBTOffset (0.00 m)

DBTScaleFactor (NO)

GPSLead (0.00 s)

— This value must be updated when copying the

 CFG file for use in a new deployment.

{

CALIBRATION

}

entered by the operator are saved in the configuration 

file in the Start_Shore_distance and the 

End_Shore_distance variables (fig. 5.20).

Several DOS environmental variables also must 

be set before running Transect 2.80+. DOS 

environmental variables can be compared to memory 

“mail boxes.” The system operator puts a directive into 

a mail box, and when the Transect starts, it checks the 

mail box, reads the mail, and takes appropriate action. 

In the case of Transect 2.80+, four environmental 

variables (mail boxes) are used to send directives to the 

Transect software. The first environmental variable is 

typed at the DOS prompt, or in the Autoexec.bat file; 

(set AUTOSAVECFG = Y). This environmental 

variable tells the Transect software to save a 

configuration file with each save of a raw- (or 

processed-) data file. The file names have the following 

form. If the deployment name in the RECORDING 

section of the configuration file is, ARKN, for example, 

then the files saved are

•ARKN001R.000 for the raw-data file;

•ARKN001P.000 for the processed-data file (if 

requested);

•ARKN001C.000 for the configuration file (this 

is the new one!);

•ARKN001N.000 for the navigation file (if 

requested).

Figure 5.17. Calibration section of the configuration file. ADCP, acoustic Doppler current profiler; GPS, Global Positioning System.
82 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Proj_Angle (250.0 deg from N)

Bad_Below_Bottom (YES)

Line 1 (Sacramento River at Freeport)

Line 2 (Mode 1 -- 25cm bins -- 5 WP 4 BP -- Ship)

Remember to update this twoline message when redeploying

the ADCP in a new area

(Graphics section in part)



}

}

{

SOFTWARE (BB-TRANSECT) 

Version (4.05)

END RDI CONFIGURATION FILE

HISTORY

If AUTOSAVECFG = N or is missing, the 

Transect software will not save a configuration file 

when it saves a data file.

The second environmental variable is 

AUTOLOADCFG. Transect 2.80+ will autoload 

the matching configuration file when a data file is 

loaded for playback if the environmental variable 

AUTOLOADCFG is set to Y; (set 

AUTOLOADCFG = Y). These first two directives 

greatly enhance the post-processing of the Transect 

software data files. Although still necessary, the 

operator is not dependent only upon the log sheets for 

the recording of edge distance estimates. The operator 

also will be assured that changes to the transducer draft 

and other configuration directives will not be 

inadvertently applied to the wrong data files during 

playback.

The third environmental variable is 

STARTRECORDOFF, which can effectively eliminate 

one minor annoyance that is present in earlier versions 

of the Transect software; (set 

STARTRECORDOFF = Y).

This variable tells the Transect software to start 

the Acquire software with the data recording toggled 

OFF. Many operators prefer to start the cross-section 

traverse with the recording turned off because 

discharge should not be collected with zero good bins 

or only one good discharge bin. Recording can begin 

when the operator observes that velocities are being 

properly measured (chap. 7). Older versions of the 

Transect software started with recording turned on and 

the operator was required to toggle recording off before 

starting ADCP data collection.

The fourth and final environmental variable 

relates to the collection of processed-data files. Older 

versions of the Transect software used an unusual 

definition of percent good to describe the data in each 

averaged bin (number of good four-beam and threebeam Earth transformations). In Transect 2.80, the 

operator can define the meaning of percent good in a 

processed-data file by setting the following 

environmental variables; (set USEPERCVEL = Y).

If this variable is set to Y, percent good will equal 

the number of good velocity values in each bin, 

averaged together by the Transect software; (set 

USEPERCQ=Y).

If this variable is set to Y, percent good will equal 

the number of good discharges in each bin averaged 

together by the Transect software. Note: the “redefined” 

percent good will be saved to the processed-data files, 

but nothing is saved to indicate which of the above two 

definitions were used.

To ensure that the above environmental variables 

are set each time the Transect software is run, the 

operator should edit them into the Autoexec.bat file in 

the boot drive root directory.

The additions to the Transect configuration file, 

discussed above, can be entered with a text editor or 

added from the Transect software calibration menu. In 

the Transect software calibration menu, the 

Figure 5.18. Graphics and history sections of the configuration file. ADCP, acoustic Doppler current profiler; RDI, R.D. Instruments, Inc.
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration 83

ADCP depth (0.16 m)

DBTDraft (0.00 m)

Heading / Magnetic offset (0.00 0.00 deg)

Transducer misalignment (0.00 deg)

Intensity scale (0.43 dB/cts)

Absorption (0.278 dB/m)

Salinity (0.0 ppt)

Speed of sound correction (YES)

Pitch & roll compensation (YES)

Tilt Misalignment (0.00 deg)

Pitch_Offset (0.000 deg)

Roll_Offset (0.000 deg)

Top discharge estimate (POWER)

Bottom discharge estimate (POWER)

Power curve exponent (0.1667)

Edge_slope coefficient (-1.00000) [-1=Trianglar(0.3535): 2 = 

 Square(0.91):User]

 Enter "-1" for a Trianglar edge slope (0.3535) (default)

 Enter "-2" for a Square edge slope (0.91)

 Enter a coefficient between 0 and 1 (see diagram)

Edge-slope coefficients

1.00

0.91

0.35

0.00

CALIBRATION

}

{

Figure 5.19. Edge-slope coefficient in the Transect 2.80+ configuration file. ADCP, acoustic Doppler current profiler.
84 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Average every (0.00 s)

Use Depth Sounder (NO)

MaxFileSize (1200) — This value defaults to 1200 kilobytes. When the file size reaches 

 1200 kilobytes, it is saved and the file extension is incremented

by 001 and a new file is started.

External_formats (N N N N N) [ HDT HDG RDID RDIE ]

External_decode (N N N N) [ heading pitch roll temp ]

Start_Shore_distance (-1) [ cm ]

End_Shore_distance (-1) [ cm ]

These values will be added by the operator during

 transect if ... 

Edge_distance_prompt (YES) — ... THIS value is set to YES (prompts the operator for edge 

 values).

PROCESSING

{

}

configuration file must be loaded before the operator 

moves to the scaling screen. In the DISCHARGE 

EXTRAPOLATION box, the “Prompt for Edge Shore 

Distances” should be set to YES with the spacebar. The 

“Edge Shore Coefficient” for the channel should be 

selected. If the “Edge Shore Coefficient” is unknown, 

TRANGLE should be selected.

Summary

The Transect software requires a companion 

configuration file for each measurement session. The 

configuration file contains instrument and recording 

setup data that enable the Transect software to properly 

measure the cross-section discharge and to store the 

resulting data. This configuration file can be created 

from within the Transect software or can be modified 

from a preexisting configuration file using a text editor. 

The configuration file should be checked before use 

with an American standard code for information 

interchange (ASCII) text editor to verify that the correct 

direct commands and recording parameters have been 

included and that unwanted commands are not present.

Figure 5.20. Transect 2.80+ directives in the processing section of the configuration file.
Chapter 6: Data Acquisition 85

CHAPTER 6: DATA ACQUISITION

Operation of Transect Software

As described in chapter 5, Transect is a series of 

stand-alone software modules that are run from an 

executable menu program named Transect.exe. 

Transect.exe is an MS DOS-based computer program 

that can be run from an MS DOS prompt or from a 

batch file. There are several command line switches that 

can be used when running Transect, but we will discuss 

only the “/m” switch. The /m switch runs the Transect 

software in monochrome mode. It can be used when 

running Transect on a laptop with a monochrome liquid 

crystal display or on a laptop with a color screen if 

increased contrast is desired. For example, if the 

operator wishes to start Transect in monochrome mode, 

he would simply invoke Transect by typing 

TRANSECT /M at the command prompt. The default 

(color) mode is invoked by simply typing TRANSECT 

at the command prompt. Unlike unix, MS DOS 

commands can by typed in upper or lower case.

In chapter 5 we discussed the communication, 

calibration, and planning modules of the Transect 

software. In this chapter we will discuss the operation 

of the Acquire module.

Loading the Configuration File

During Transect initialization, the Transect 

software must obtain a valid configuration file. The 

proper configuration file can be loaded manually into 

the Transect software from any of the menus by pressing F3 and supplying a configuration file name (for 

example, by typing SACU.CFG). The Transect 

software stores the name of the last configuration file 

used in a small file called TRANSECT.PTR. 

TRANSECT.PTR may be edited and modified prior to 

entering the Transect software. For example, the 

TRANSECT.PTR file contains the following 

command: C:\SACU.CFG. When the Transect software 

starts, it will look for a file named SACU.CFG for its 

initial configuration. Any valid file path name can be 

entered into TRANSECT.PTR. For example, when 

TRANSECT.PTR is edited and C:\SACU.CFG is 

replaced with C:\BBTRAN26\SACU.CFG, the 

Transect program will find and load SACU.CFG from 

the C:\BBTRAN26 directory.

Starting the Transect Acquire Module

Entry into the Acquire menu (fig. 6.1) causes the 

Transect software to load a configuration file using the 

path name in the TRANSECT.PTR file. If a valid file 

name is not present in TRANSECT.PTR or if 

Figure 6.1. Transect software main menu. BB-ADCP, broad-band acoustic Doppler current profiler.
86 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

TRANSECT.PTR is not present, Transect will prompt 

the operator for a configuration file name. After loading 

the configuration file, Transect sends a “break” signal 

to the ADCP and waits a short time for a response. This 

is termed “waking up” the ADCP. If the ADCP does not 

wake up, usually it is because the operator has failed to 

turn on the power from the deck unit or that a communications or power cable has not been connected. If all 

attempts to communicate with the BB-ADCP from the 

Acquire menu meet with failure, the operator can do 

one of two things: exit the Transect software and try to 

establish communication with the ADCP using 

BBTALK or another terminal-emulator program, or 

enter the Transect software communication ADCP 

submenu and select auto connect. The Transect software will cycle through all combinations of baud rates 

and configuration combinations until it successfully 

establishes communications or it makes so many failed 

passes that the operator finally looses patience and calls 

for help. The BB-ADCP technical manual contains 

trouble-shooting procedures in cases of BB-ADCP 

failure or apparent faulty operation.

Once the ADCP is communicating properly, the 

introductory screen for the Acquire menu will appear 

on the monitor. This screen (fig. 6.2) contains helpful 

information on the setup and configuration of the 

ADCP:

•ADCP communication parameters;

•ADCP firmware parameters—firmware-release 

version, beam angle, operating frequency, 

coordinate system, operating mode, orientation 

system, and head configuration (pattern);

•Recording selections;

•ADCP operating variables: depth cell length, 

number of depth cells, pings per ensemble, time 

between pings, and blanking distance;

•Configuration file processing parameters.

The BB-ADCP operator should inspect the data 

for accuracy before continuing with the Transect 

software session.

Transect-Data Displays

The F10 or escape key then can be pressed to 

invoke the Transect software profile menu.

The Acquire menu provides many different ways 

to view ADCP data as they are being collected. 

Typically, most users will view incoming data using the 

Tabular display. Some of the available data displays are 

described below:

•Profiles of water velocity, echo intensity, and 

discharge cross product may be displayed 

(fig. 6.3).

Figure 6.2. Transect software Acquire introductory screen. ADCP, acoustic Doppler current profiler.
Chapter 6: Data Acquisition 87

•Color or monochrome plots of north velocity, 

east velocity, vertical velocity, error velocity, 

and velocity along a user set azimuth may be 

displayed (fig. 6.4).

•Backscattered intensities (fig. 6.5) also may be 

contour plotted. This can be useful for providing 

the operator with a relative indication of 

Figure 6.3. Transect software Acquire profile menu screen.

Figure 6.4. Transect software Acquire velocity contour plot screen. 
88 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

suspended-sediment concentration in the 

measured cross section.

•The movement of the boat can be displayed in 

the so-called shiptrack plot (fig. 6.6). The 

operator also can display velocity vector “stick” 

plots at specified depths.

•Measured velocities, intensities, and 

correlations may be displayed in tabular form 

Figure 6.5. Transect software Acquire intensity contour plot. ADCP, acoustic Doppler current profiler.

Figure 6.6. Transect software Acquire shiptrack plot. ADCP, acoustic Doppler current profiler.
Chapter 6: Data Acquisition 89

(fig. 6.7). The tabular screen is the most useful 

of the many displays available because of the 

information it imparts. The operator can tell 

quickly if error velocities are high, or if beam 

measurements or bottom tracking are bad, and 

scale factors need not be preset.

Transect-Data Recording

Data are recorded under the file name given in 

the RECORDING section (figs. 5.11 or 5.16) of the 

configuration file. Recording of the data to a data file is 

controlled by toggling the F5 key. When the Transect 

software is instructed to start recording (by toggling the 

F5 key), the discharge-measurement registers (but not 

the ensemble counter) are set to zero. The F4 key, on the 

other hand, wakes up the ADCP, starts the ADCP 

pinging, and resets the discharge-measurement 

registers and ensemble counter. Chapter 7 presents a 

detailed description of the use of the F4 and F5 keys.

Summary

Transect software data acquisition is invoked 

using the Acquire menu. The Transect software always 

loads the last used configuration file. There are two 

ways to load a new configuration file. The first is to edit 

the TRANSECT.PTR file and insert the path name of 

the configuration file before starting the Transect 

software. The second is to use the F3 key when starting 

the Transect software. The software then will prompt 

the operator for a configuration file name.

Actuating the F4 function key starts the data 

collection by the acoustic Doppler current profiler 

(ADCP). Recording can be toggled on and off with the 

F5 key.

Data displays during data acquisition can be 

selected by the operator. The tabular view is one of the 

more useful displays available during data collection. 

Some operators prefer to collect data while displaying 

profile plots of velocity magnitude and direction.

Figure 6.7. Transect software Acquire tabular display screen. ADCP, acoustic Doppler current profiler.
90 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

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Chapter 7: Discharge-Measurement Procedure 91

CHAPTER 7: DISCHARGE-MEASUREMENT 

PROCEDURE

 In this chapter we will discuss procedures for 

making discharge measurements (fig. 1) utilizing the 

ADCP and the Transect software. The following topics 

will be discussed in detail:

•Cross-section reconnaissance

•Premeasurement checkout

•Boat-maneuvering techniques

•Transect software tips and tricks during the 

discharge measurement

•Alternate discharge-measurement techniques 

for low-flow conditions

•What constitutes a “good” discharge 

measurement

•Recent Transect software enhancements

Cross-Section Reconnaissance

Figure 7.2 shows a 91-m- (300-ft-) wide stream 

with tidally affected flows having a roughly trapezoidal 

cross section with average depths of 4.5–6 m 

(15–20 ft). This cross section on Dutch Slough near 

Oakley, California, is typical of rivers in the northern 

California Sacramento/San Joaquin River Delta. Proper 

cross-section reconnaissance is vital to the precision 

and accuracy of ADCP discharge measurements. The 

measurement vessel depth sounder or the ADCP depthrange measurements are useful tools in determining the 

usability of the cross section for ADCP discharge 

measurements.

In general, the ADCP operator should look for a 

cross section with a roughly parabolic, trapezoidal, or 

rectangular shape, having an average depth of at least 

1.5 m (5 ft). The measurement sometimes can be made 

at locations having less depth, if water modes 5 or 8 are 

employed (chap. 3).

Cross sections with asymmetrical bottom 

topographies should be avoided, if possible. For 

example, the operator should avoid a measurement 

cross section that is very shallow on one side and deep 

on the other.

Average water velocity also is an important factor 

in choosing a measurement cross section. Cross 

sections exhibiting slow [less than 10 cm/s (0.30 ft/s)] 

average velocities should be avoided. Although 

measurements can be made under these conditions, 

special techniques must be employed (discussed later in 

this chapter).

Premeasurement Checkout

After the boat is launched and the ADCP 

equipment is set up, care should be taken to obtain an 

accurate transducer depth measurement.

If a side-swing mount is used, the weight of the 

operator(s) can cause an unwanted pitch of the ADCP 

vessel (fig. 7.3). This pitch angle may cause an 

erroneous reading of the ADCP depth. For example, 

when the operator moves to the side of the vessel to 

measure transducer depth, the vessel pitches to that side 

because of the operator’s shifted weight. The operator 

notes the depth of the ADCP below water surface, and 

then returns to his normal position in the vessel causing 

the vessels pitch to return to the original value. This 

Figure 7.1. Improper discharge-measurement technique.

Figure 7.2. Discharge-measurement cross section on Dutch Slough 

near Oakely, California.
92 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

recorded depth measurement will be incorrect. Because 

the transducer depth measurement is used in the 

Transect software to compute discharge, the error in 

ADCP depth sometimes can cause a significant bias of 

the ADCP discharge measurement. This is true 

especially for wide, shallow rivers.

Before the discharge measurement begins, the 

operator should note any conditions relevant to the 

discharge measurement on the discharge-measurement 

log sheet (fig. 7.4). Wind, bidirectional flow, eddies, 

standing waves, passing boats, and sediment conditions 

are just some of the things that should be noted for later 

analysis of the discharge measurements.

Before a discharge measurement begins, the 

operator should complete the following tasks:

•Meet all DOI boat-safety requirements and 

make sure all required life jackets, throwable 

devices, fire extinguishers, and horns are in 

good working condition aboard the ADCP 

vessel.

•Determine the “unpitched” transducer depth and 

enter it on the log sheet and in the configuration 

file.

•Perform a PT200 diagnostic test using 

BB-TALK or other terminal emulator software. 

Save the test results to an ASCII file placed in 

the deployment directory.

•Synchronize the computer, ADCP, and operator 

watch times.

•Perform a short reconnaissance of the cross 

section to determine shallow areas and the shape 

of the cross section so that unmeasured areas 

near the bank can be characterized. If the cross 

section is unsuitable for any reason (too shallow 

in places, for example), select another 

measurement cross section.

•If buoys are used to aid the estimation of edge 

distances, they should be deployed and the 

distance to shore from each buoy should be 

measured and noted on the dischargemeasurement log sheet.

•If range finders are used to determine edge 

distances, they should be checked for proper 

calibration.

•Record weather, hydrological, and other 

physical phenomena pertinent to the discharge 

measurement on the discharge-measurement 

note.

•Make sure that the right configuration file has 

been loaded properly into the Transect software.

•Make sure that the power supply has been turned 

on and the ADCP has been “awoken.”

Boat-Maneuvering Techniques

Boat-maneuvering techniques for discharge 

measurements when using the ADCP and the Transect 

software do not require the precision once needed for 

conventional moving-boat discharge measurements. 

However, there are some basic maneuvers that improve 

accuracy and allow smooth transitions between 

measurements.

Starting the Cross-Section Traverse

For a typical measurement, the operator must 

maneuver the boat close to, and parallel with, the 

riverbank (fig. 7.5). The boat should be maneuvered in 

as close as possible to the bank without grounding 

(bottoming out) the boat motor propeller or the 

Figure 7.3. Broad-band acoustic Doppler current profiler mount 

being deployed vertically.

Figure 7.4. Discharge-measurement log sheet/note.
Chapter 7: Discharge-Measurement Procedure 93

BB-ADCP transducer assembly. Performing this 

maneuver takes practice.

While the boat is somewhat stationary, the 

operator should start the Transect software and set the 

Acquire display to tabular velocity mode (initial 

setting). The tabular mode setting is optional as the 

Transect software will collect data in any display mode. 

However, the tabular mode enables the operator to 

determine if there is an adequate number of depth cells 

(having good discharge) before starting the 

measurement traverse. This capability provides the 

greatest advantage over the other display modes, 

especially when starting and ending the discharge 

measurement.

At this point, the operator is beginning the 

discharge measurement and must accomplish several 

tasks quickly:

•The distance to shore must be estimated by 

some means and recorded.

•The operator must turn the ADCP data 

recording off by pressing the F5 key and must 

start the ADCP pinging by pressing the F4 key.

•While looking at the tabular display, the 

operator must verify that the ADCP is collecting 

at least two good bins of velocity data.

•When the operator is satisfied that accurate data 

are being collected and the boat is in the correct 

position to start the discharge measurement, he 

must press the F5 key to begin recording and 

wait until two good ensembles have been 

collected. During this period (about 5 s), the 

boat should be barely moving toward center 

channel.

During the Cross-Section Traverse (Transect Tips 

and Tricks)

When the Transect software begins collecting 

data, the operator should verify that there is an “ADCP 

PINGING” message near the upper right of the monitor 

display and that a transect recording file is opened (file 

name visible at the lower right of the Transect software 

screen). At this time the boat should have just begun 

traversing the river cross section. The operator must 

quickly scan the initial ensemble display to determine if 

everything is operating correctly. Signs of improper 

operation are flagged “bad” in all columns and rows or 

“bad” in all rows of an individual column. The display 

columns correspond to north velocity, east velocity, 

vertical velocity, error velocity, and percent good. If the 

incoming data appear correct, the operator should 

continue the transect with the same course and a 

slightly increased speed (approximately that of the 

water or slightly less). As the boat enters faster flow, 

engine revolutions per minute (rpm) and boat heading 

may have to be adjusted slightly to enable a smooth 

traverse (fig. 7.6).

Uniform boat speed during a transect is more 

important than steering a straight course. The course 

may be allowed to change slightly and slowly, if 

necessary, during the traverse. However, rapid course 

and boat heading changes can introduce errors into the 

measurement. The key element here is to DO 

EVERYTHING SLOWLY, including course changes, 

the speed of the vessel itself, and even the speed of 

persons moving around onboard the measurement 

vessel. Sharp accelerations of the measurement vessel 

in any direction should be minimized or eliminated.

Figure 7.5. Acoustic Doppler current profiler vessel at the 

beginning of a discharge measurement.

Figure 7.6. Acoustic Doppler current profiler vessel near center 

channel.
94 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Ending the Cross-Section Traverse

As the vessel approaches the opposite edge of the 

measuring section, the boat should be slowed by slowly 

changing the heading to a more upstream direction and 

slowing the boat motor. The boat then can creep (crab) 

toward the bank. When the operator decides that the 

approach cannot be continued further, an edge value is 

determined, and the F5 key is pressed to end the 

transect.

At the end of a cross-section traverse (fig. 7.7), 

the boat heading is changed just enough so that the boat 

stops its bankward movement and begins to slowly 

creep in the direction of center channel. At this point, 

the operator may press the F5 key again to begin 

another transect and obtain a starting distance value. 

Slow “crabbing” at the start and finish of each cross 

section works better than nosing the boat into the bank 

and then backing away. The ADCP should not be 

allowed to pass over the boat propeller vortex during 

the discharge measurement. Entrained air in the vortex 

will cause failure of the BB-ADCP bottom track and 

result in lost ensembles.

The operator should practice the above described 

technique a few times before an actual transect session 

is begun so that you (and the boat operator) can become 

accustomed to the flow conditions at this location. The 

more practice you have in making these measurements, 

the more uniform will be the measurement results. 

When maneuvering near the riverbank, you must 

remember that making a large heading adjustment 

away from the bank (swinging the bow away from the 

bank) will bring the stern (and, therefore, the engine 

prop and shaft) into contact with the bank or bottom. 

This maneuver can produce highly undesirable results. 

The boat will crab slowly away from the bank with only 

minor adjustments needed in the boat steering.

Alternate Techniques Used During Low-Flow 

Conditions

The above described technique works only when 

there is sufficient stream velocity to allow the boat to 

crab. At very low stream velocities [less than 10 cm/s 

(0.33 ft/s)], the boat must be turned VERY SLOWLY to 

enable a direct crossing of the stream. In some cases, 

the best approach is to raise the engine and pull the boat 

slowly (at the stream velocity or less) across the stream 

with ropes or a tag-line (fig. 7.8).

The winching system shown in figure 7.8 was 

used to measure leakage through control structures in 

the USGS Illinois District (Oberg and Schmidt, 1994). 

Because the velocities measured were very slow 

Figure 7.7. Acoustic Doppler current profiler vessel at the end of a 

cross-section traverse.

Figure 7.8. Winching system used to move an acoustic Doppler 

current profiler vessel at slow speeds.
Chapter 7: Discharge-Measurement Procedure 95

[around 10 cm/s (0.3 ft/s)], the ADCP was winched 

slowly across the control structure opening to reduce 

the random error in the discharge measurement 

(chap. 9).

To gain reasonably accurate measurements in 

very slow moving water, special setup commands must 

be used to increase the measurement pulse lag times 

(chap. 5). The use of these long lag times causes the 

ambiguity velocity to be very low. Therefore, the boat 

must be moved across the stream ever so slowly (as if 

rows of dominoes were balanced on the gunnels). 

What Constitutes a “Good” Discharge 

Measurement?

The ADCP operator should consult Lipscomb 

(1995) for guidelines on discharge-measurement 

requirements and documentation procedures. However, 

because these instruments are relatively new and 

sometimes used for special purposes, common sense 

also will dictate the procedures in some cases. General 

procedures conforming to the quality-assurance plan 

are presented here, but the ADCP operator is cautioned 

to observe conditions and to document as much 

information as possible during the transect session. 

With the advent of electronic instruments used for data 

recording, valuable information can sometimes “fall 

through the cracks” and may not be missed until the 

ADCP operator reviews the data back at the office.

Because a BB-ADCP transect may be made 

much faster than a conventional discharge measurement, multiple transects usually are averaged to 

increase discharge-measurement accuracy. During 

fairly uniform conditions, a group of four or more 

transects usually are averaged for a final dischargemeasurement value. If one transect differs significantly 

(±5 percent) from the rest, and there is no rationale to 

discard it, at least four additional transects should be 

obtained and the average recomputed. The average of 

these transects can be reported as an individual 

discharge measurement and logged on a USGS ADCP 

discharge-measurement note (fig. 7.9). The back of the 

USGS measurement note has space for 14 transects, 

their edge-value estimates, and other pertinent data 

(fig. 7.10).

In figure 7.10, the values labeled standard error 

(range) and standard error (percent) near the bottom of 

the note are critical for measurement results in steadyflow conditions. For fairly uniform flow conditions, at 

least four ADCP discharge measurements should be 

made to determine mean and standard-deviation values. 

From these data, the standard error, in percent of mean 

discharge, should be calculated. The standard error, in 

percent of mean discharge, also is known as the 

coefficient of variation (CV). The CV is calculated by 

dividing the standard deviation (in discharge units) of 

the four transect discharges by the mean discharge. If 

the CV is larger than 5 percent, additional measurements should be made. A large CV can be an indication 

that the ADCP ping rate is too slow, or that the boat 

should be slowed during the cross-section traverse to 

collect more pings. Outliers or single, bad, transects can 

cause a large CV, however, they should not be simply 

thrown out without an attempt to discover the reason for 

their imprecision. When in doubt, it is always advisable 

to collect additional transects.

For measurements of net flow in tidally affected 

estuaries and rivers, many discharge measurements 

may have to be made (more than can be logged on the 

example ADCP discharge-measurement form). 

Figure 7.11 shows a series of transects made at a tidally 

affected gaging station on a tributary of the San Joaquin 

River.

In these cases, a discharge-measurement log 

sheet (fig. 7.12) may be used with pertinent data logged 

on a discharge-measurement note (fig. 7.9) that is 

attached to the log sheet. This log sheet is used by the 

USGS California District. Standard-deviation 

calculations for tidally effected discharge measurements are meaningless unless the “true mean” 

discharge can be calculated. Because of the difficulty in 

calculating the coefficient of variation for tidally 

effected discharge (or any dynamically changing 

discharge), its inclusion on the discharge-measurement 

note example (fig. 7.9) is of questionable significance.

Where tides or discharge conditions are changing 

rapidly, the operator may not wish to average transects 

and, in these cases, a single transect constitutes a 

discharge measurement. In most cases, many 

measurements will be required to have as much 

coverage as possible of the dynamic flow time series. 

Single measurements should be logged on a BB-ADCP 

discharge-measurement note (fig. 7.9) and if many are 

made, they should be logged on a dischargemeasurement log sheet (fig. 7.12) with an attached 

USGS discharge-measurement note. Again, standarddeviation calculations or coefficient of variation 

calculations, in these cases, are of questionable value.

Archival of Acoustic Doppler Current Profiler 

Discharge-Measurement Data

Transect software data files should be placed on 

archival media in the following form:
96 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Figure 7.9. Example of an acoustic Doppler current profiler (ADCP) discharge-measurement note (front).
Chapter 7: Discharge-Measurement Procedure 97

•Site deployment directory name NNNN (where 

NNNN is the four-letter site designator)

•Set of transect raw data files

{NNNN001R.00

 |

 |

 \/

 NNNN0XXR.00

}

where XX is the last transect number in the 

series,

•Transect session *.CFG file NNNN.CFG

•Transect session documentation file 

NNNN.DOC

The operator must create an ASCII file, using a 

text editor, to be archived with each set of Transect data. 

This electronic NNNN.DOC file is needed to preserve 

Transect edge-value estimates and other site-specific 

information in the event that hard copies of log sheets 

are lost or misplaced. This DOC file must be drafted as 

soon as possible after the completion of the Transect 

session and stored in the Transect session directory. At 

a minimum, the file should contain the following 

information:

•Site location

•Date, start time, and end time

•Names of measurement staff

•Description of measurement vessel and 

equipment

•Transducer depth (depth from water surface to 

the center of the transducer assembly faces)

•BB-ADCP serial number and firmware-revision 

number

•An electronic copy of the measurement log, 

which is helpful for reconstituting ADCP 

discharge measurements. The log copy should 

include: start time; Transect file name; startedge estimate; end-edge estimate; discharge; 

and remarks about site conditions, including 

indication of eddies or reverse flow at bank 

edges; and remarks about passing boat traffic 

and wind conditions (pitch and roll)

•Any pertinent information about instrument 

setup or changes to the setup during the Transect 

session

These data should be archived to safe media as 

soon as possible following a Transect session. Safe 

Figure 7.10. Example of an acoustic Doppler current profiler discharge-measurement note (back).
98 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

-2,000

0

2,000

4,000

6,000

8,000

DISCHARGE, IN CUBIC FEET PER SECOND

2:52 3:07 3:21 3:36 3:50 4:04 4:19 4:33 

TIME, IN HOURS

DISCHARGE MEASUREMENTS AT DUTCH SLOUGH

media may consist of a compact disk, read-onlymemory (CD-ROM), optical media (30-year), or a disk 

drive on a computer system that is backed up regularly. 

The computer hard disk of the laptop used for data 

collection is not considered “safe media.”

Summary

A smooth bank-to-bank transect technique is 

essential for accurate discharge measurements. The 

operator always should remember that slow, smooth 

boat movements are desirable.

For steady-state flows, at least four transects 

should be made and the results averaged to calculate a 

“discharge measurement.” If one measurement is more 

than 5 percent from the mean, four additional transects 

should be collected and averaged. A CV should be 

calculated from the discharge-measurement series by 

dividing the standard deviation, in units of discharge, by 

the mean of the series. If the CV is greater than 

5 percent, the ping rate should be increased or the boat 

slowed during the transect to collect more pings per 

transect. Additional transects should be made until a 

series of four is obtained having a CV of less than 

5 percent.

For discharge measurements made in tidally 

affected rivers and estuaries, the CV value and standard 

deviations cannot be computed easily and, therefore, 

are not used. However, comparison and common sense 

sometimes can detect inaccurate transect discharges. In 

these cases, a single transect can be used as a discharge 

measurement, although many measurements usually 

are made during a discharge-measurement session to 

define the dynamic flow conditions.

Suitable log sheets and discharge-measurement 

notes must accompany the discharge-measurement 

series. The operator should store a copy of the 

configuration file that is used for the series in the 

Transect software deployment directory with a 

documentation file containing edge values and other 

pertinent information (described above). A documentation file should be stored with the Transect software 

data files describing stream conditions and pertinent 

information not collected by the Transect software.

All Transect data files and accompanying 

configuration and documentation files should be 

archived to “safe” media as soon as possible after the 

measurements are made. Although paper-copy records 

can be kept for records computation, acoustic Doppler 

current profiler discharge-measurement archives should 

be such that the discharge measurements could be 

wholly recreated from data stored on electronic media. 

Figure 7.11. Discharge-measurement data from a tidally affected 

gaging station on a tributary of the San Joaquin River.
Chapter 7: Discharge-Measurement Procedure 99

Figure 7.12. Discharge-measurement log sheet. ADCP, acoustic Doppler current profiler.
100 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

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Chapter 8: Discharge-Measurement Review and Assessment 101

CHAPTER 8: DISCHARGE-MEASUREMENT 

REVIEW AND ASSESSMENT

Individual transects should be checked for 

discharge-measurement errors and inconsistencies as 

soon as possible after collecting the dischargemeasurement series. In most cases, this examination is 

done at the office or in a motel suite, but it can be done 

at the measurement site. The data are examined by 

using the Transect software to reconstruct the 

measurement from the stored raw data and 

configuration files. This is termed “playing back” the 

discharge measurement. During playback, the edge 

discharges also can be estimated, and the power-curve 

(or other) estimation scheme used near the top and 

bottom of the profile can be examined for correctness 

and changed, if necessary.

Proper review and assessment of ADCP 

discharge measurements is almost as important as the 

techniques and instrument setup used to collect the 

original data. Improper instrument setup and faulty 

measurement techniques can be revealed during post 

processing and, in some cases, corrections can be made 

to improve discharge-measurement accuracy.

Review of ADCP discharge measurements 

should include the following steps.

•Configuration file review with focus on the 

applicability of the following sections to river 

conditions:

ADCP hardware setup

ADCP direct commands

ADCP calibration constants

•Transect software playback with focus on the 

following subjects:

Missing data ensembles

Possible bottom-sediment movement

Magnitudes of discharge in the 

unmeasured (estimated) parts of the 

cross-section near the top and bottom 

of the profile

Technique used for low-velocity 

[< 10 cm/s (0.33 ft/s)] measurements

Power-curve-fit applicability

Reasonableness of edge values

Shiptrack examination

These assessment procedures are discussed in the 

following paragraphs.

Configuration-File Review

The first thing that should be checked is the 

applicability of the configuration file to the data file and 

to river conditions. The ADCP discharge-measurement 

notes should indicate the location of the configuration 

file used for the discharge-measurement series or 

contain a listing of the file.

River Conditions

The configuration file should be examined in a 

text editor to determine if the proper setup and 

configuration commands were used for the stream or 

river being measured. All sections should be checked, 

but the most important are the ADCP hardware, the 

direct command, and the calibration sections:

Acoustic Doppler Current Profiler Hardware

•First examine the ADCP Hardware section of 

the configuration file.

(example)

ADCP HARDWARE

{

Firmware (5.45)

Angle (20)

Frequency (1200)

System (BEAM)

Mode (4)

Orientation (DOWN)

Pattern (CONCAVE)

}

Are these entries compatible with the ADCP used 

to make the discharge measurements? Is the proper 

mode being used for the river or stream in question?

Acoustic Doppler Current Profiler Direct Commands

•Next examine the Direct Commands section of 

the configuration file.

(example)

DIRECT COMMANDS

{

WS25

WF50

BX200

WN060

WD111100000

WP00001

BP001

WM1

BM5

ES0

WE0450

}

Is the blanking distance (WF) adequate for the 

transducer frequency? Is the bin size (WS) proper for 

the transducer frequency? Are enough depth bins 

specified (WN) to cover the range of depth measured? 

A few playback profiles may have to be examined 
102 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

before this question is answered. Is the bottom track 

maximum depth (BX) greater than the maximum 

stream depth? If BX is less than the maximum depth, 

there will be missing ensembles. Are the bottom and 

water modes (BM,WM) specified correct for the 

measurement application? Is an ES0 (salinity of zero) 

command present?

Calibration Section

•Finally, examine the calibration section.

(example)

CALIBRATION

{

ADCP depth (.40 m)

Heading/Magnetic offset (0.00 

 0.00o)

Transducer misalignment (0.00o)

Intensity scale [0.43 decibels (

dB) per count)]

Absorption (0.440 dB/m)

Salinity [0.0 parts per thousand

 (ppt)]

Speed of sound correction (YES)

Pitch and roll compensation (YES)

Tilt Misalignment (0.00o)

Pitch_Offset (0.000o)

Roll_Offset (0.000o)

Top discharge estimate (POWER)

Bottom discharge estimate 

(POWER)

Power curve exponent (0.16670)

Does the transducer draft correspond to the draft 

entered on the ADCP discharge-measurement notes? 

Have the proper offsets (if any) been entered? Is the 

proper estimation scheme being used for the bottom and 

the surface, and is the proper power-curve exponent 

being used?

Transect Software Playback

When the configuration file has been verified, the 

Transect software data files should be loaded and 

replayed using the Transect software playback menu. 

When the playback section is first entered, the software 

looks for the last loaded configuration file (in 

TRANSECT.PTR) for information about file locations. 

The operator either can load a new configuration file or 

load an individual data file by pressing the F3 key. The 

operator can load a list of files from a deployment by 

pressing the F8 key. During playback, the data should 

be examined with a critical eye for the anomalies 

discussed in the sections below. Common sense also 

should be used when reviewing Transect software data 

files.

Missing Ensembles

A velocity contour plot of the ADCP discharge 

measurement should first be examined for missing 

ensembles. Missing ensembles show as vertical black 

lines in the contour plot, as shown in figure 8.1. 

Figure 8.1 shows a transect containing missing 

Figure 8.1. Transect containing missing ensembles caused by a loss of bottom tracking. 
Chapter 8: Discharge-Measurement Review and Assessment 103

ensembles caused by a loss of bottom tracking. Note 

that, in this case, the missing ensembles would contain 

a significant amount of flow if they were present. The 

discharge calculated from the transect described above 

will be underestimated unless the missing data are 

estimated.

The following procedure can be used (using one 

of Transect’s hidden commands) to correct transects 

containing bad bottom-track velocities:

•In the playback section of Transect, load the file 

to be used in the normal way.

•Put playback into single-step mode by toggling 

the F4 key, and rewind the file using the ALT W 

command.

•Type F6 to open the ‘scale menu’ and enter 

7 m/s (20 ft/s) for the water-speed (current 

sticks) value. Type F10 to return to the main 

menu.

•Hit the space bar to single step through the data. 

Write down the ensemble number for any 

ensemble that has anomalously long current 

sticks, or high or bad speed indicated in the 

bottom-track portion (near the right side of the 

screen). After stepping through the entire file, 

type ALT W to rewind the file. Let’s assume 

ensemble 40 was bad.

•Set caps lock, scroll lock, and num lock on.

•Single step through the file to the ensemble 

preceding the bad ensemble (39). Type 

CTRL-END (simultaneously). A small window 

will open in the upper left corner of the screen 

with the message ENTER ACCESS CODE:. 

Type 3200 then ENTER (on a laptop computer, 

the zero key may be remapped to another 

location on the keyboard as a result of pressing 

num lock). The bottom-track velocity now is 

locked into its present value. Single step to the 

next ensemble (40). The bottom-track velocity 

should be the same as the preceding ensemble 

(39). If the next ensemble (40) is good, open the 

access code window again and type 3200 to 

release the lock. If the next ensemble (41) is 

bad, continue stepping through the file until the 

ensemble preceding the last bad ensemble is 

reached and then release the lock. The locking 

procedure described above is then repeated until 

all bad ensembles have been corrected and the 

end of the file is reached. The total discharge 

will then include estimates for missing 

ensembles, based on extrapolated bottom-track 

velocities.

•Edge estimates should be done in the usual way.

The method described above should be used with 

caution. It only should be used to “fill in” one or two 

missing ensembles, at most. When the method is used, 

it should be so noted on the discharge-measurement log 

sheet and the discharge measurement should be rated 

accordingly. If most of the measurements collected 

during a measurement session contain missing 

ensembles, the discharges should be remeasured at a 

more favorable cross section (usually with slower 

velocities) or should be remeasured when conditions 

become favorable.

Error Caused By Sediment Movement Near the 

Bottom

Errors caused by bottom movement generally 

show as negatively biased discharge measurements. 

These errors are introduced into the dischargemeasurement cross product as apparent upstream boat 

movement. Sometimes the quantity of sediment moving 

near the bottom is enough to completely attenuate the 

bottom echo, causing catastrophic loss of bottom track, 

which shows as missing ensembles in the playback 

data. Figure 8.1 shows a velocity contour plot with 

missing ensembles caused by loss of bottom track.

When the ADCP is affected by bottom-sediment 

movement but has not lost bottom track, one might 

expect the resulting shiptrack plot to be curved in the 

upstream direction, even though the vessel is moving 

normal to the flow. In reality, the boat usually is pushed 

downstream by the current and, therefore, the 

information from the shiptrack plot can be misleading.

If bottom movement is suspected, the vessel 

should be anchored near the highest flow area in the 

cross section and data collected for at least 10 minutes. 

Examination of the resulting shiptrack plot will reveal 

the magnitude of the bottom-track error as a upstreamgoing shiptrack. The length of the track, in meters, 

multiplied by the elapsed time can be used to calculate 

the speed of the apparent upstream boat movement.

Figure 8.2 shows a shiptrack plot taken with the 

boat at anchor. The plot shows an apparent upstream 

boat movement of 18.7 m (61.5 ft) and an elapsed time 

of 612 s. This indicates an apparent velocity 

measurement error of 0.03 m/s (0.10 ft/s). It would be 

tempting to use this value to correct discharge 

measurements, however, the bottom probably is moving 

at different speeds in the cross section (slower near the 

edges and faster near the center, for example). A 

correction based on a single bottom-movement 

measurement would be questionable. Again, ADCP 

measurements sometimes can be made during 

conditions of bottom-sediment movement by 

substituting differential GPS-positioning inputs for 
104 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

ADCP bottom-tracking data. See the manufacturers’ 

web sites for information on using GPS systems for 

bottom tracking (www.rdinstruments.com) and 

(www.sontek.com).

Large Magnitudes of the Unmeasured Layers

Figure 8.3 shows the ending edge estimate 

(ALT E) screen with the show unmeasured discharge 

(ALT U) switch invoked. The unmeasured discharge 

magnitudes near the top and bottom of the channel are 

summed with the measured discharge (circled area in 

fig. 8.3). If the magnitude of the discharge in these 

unmeasured areas is greater than 30 percent of the 

measured discharge, special attention must be paid to 

the applicability of the power-curve estimation scheme 

used. A deeper cross section should be located and 

used, if possible.

Figure 8.2. Transect software shiptrack plot of an anchored vessel.

Figure 8.3. Transect software edge-value screen showing unmeasured layers.
Chapter 8: Discharge-Measurement Review and Assessment 105

Low-Velocity Measurements

Most of the errors seen during Transect software 

playback are caused by too few ensembles being 

collected for a given river velocity. Even seasoned 

ADCP operators sometimes fall prey to this error. It 

cannot be over emphasized that SLOWER river 

traverses are BETTER when it comes to the measurement of discharge in rivers having slow mean 

velocities. Slow traverses allow more ensembles to be 

collected, thereby reducing the amount of random error 

in the discharge measurement. In general, the boat 

should traverse the river at about the same velocity as 

the water. In large, wide rivers, this often is not possible 

(or feasible), and in such cases, equation 9.1 in 

chapter 9 should be used to estimate the proper boat 

velocity. The large standard deviation of ADCPmeasured (slow) velocities can be seen in the shiptrack 

plot of figure 8.4.

The boat speed for the traverse (fig. 8.4) was 

close to that of the water. Even though there was a large 

single-ping standard deviation, the standard deviation 

of the resulting group of discharge measurements was 

small. If any single discharge measurement has a 

coefficient of variation larger than 5 percent, the ADCP 

and boat operator should be cautioned to decrease 

vessel speeds during discharge measurements at the site 

in question for similar or lower velocities.

Power-Curve-Fit Applicability

Figure 8.5 shows a discharge profile with 

measured data, cross-product curve fit, and the resulting 

discharge profile.

As can be seen from figure 8.5, the actual 

measured profile differs significantly from the one-sixth 

power-curve fit. In this profile, velocity near the surface 

is less than the velocity in the middle of the profile (as 

shown by the arrow). This may be caused by such things 

as wind shear or a flow reversal near the surface. If 

many or all of the measured profiles differ in this 

manner, then the TOP estimation scheme should be 

changed from POWER to CONSTANT in the 

configuration file, and the data replayed. The circled 

area indicates the profiled area where measurement bias 

can occur because of incorrect top layer discharge 

estimates. Generally, the bottom estimation scheme 

should be left at POWER. The exception to this rule is 

when significant numbers of profiles are bidirectional. 

A bidirectional profile is one wherein the velocities are 

stratified with the top-most velocities moving in a 

different direction than the bottom-most velocities. In 

cases of stratified flow, the estimation scheme should be 

set to CONSTANT at the top and CONSTANT at the 

bottom. This method will introduce some errors but the 

errors will pnot be as great as those caused by setting 

the estimation scheme to POWER.

Figure 8.4. Transect shiptrack plot of acoustic Doppler current profiler-measured velocities on a slow-moving river. 
106 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Edge Values

Figure 8.6 shows the Transect software playback 

edge-value estimate screen, which is accessible by 

pressing ALT-E after Transect playback. Note! This 

feature is available only during playback at this time. 

This menu allows the operator to insert start and end 

edge distance estimates. The software then calculates 

an estimated discharge for each edge and adds it to the 

total discharge. Transect software version 2.80 (and 

later versions) allows the operator to enter edge values 

while acquiring the discharge measurement. These edge 

values are saved in the configuration file and can be 

loaded during playback.

Figure 8.5. Transect software discharge profile plot. ADCP, acoustic Doppler current profiler.

Figure 8.6. Transect software edge-value screen. 
Chapter 8: Discharge-Measurement Review and Assessment 107

Note that in figure 8.6, the start edge-discharge 

estimate is negative, the main body of discharge is 

positive, and the end edge discharge is positive. This 

phenomena can be caused by two things:

•The start-edge discharge is moving in the 

opposite direction from the main body of 

discharge and the end-edge discharge [this can 

occur because of eddies or momentum effects 

(fig. 8.7)].

•Initial boat movement was toward the starting 

shore rather than away from it.

The sign (+ or −) assigned to the cross-product 

calculation is based on the boat course, in relation to the 

flow direction. If the water is flowing uniformly in one 

direction and the boat is traversing the flow uniformly 

without changing course, the sign of the cross product 

should remain constant. If the boat reverses course (or 

the flow changes direction during the traverse), the sign 

of the cross product will change. If this occurs at either 

or both edges, the edge velocities will have a sign that 

differs from that of the main flow body (fig. 8.6). In 

recent versions of the Transect software (version 4.00 

and later), corrections have been made to eliminate 

incorrect signs (+ or −) near the riverbank edges, 

however, the operator still should carefully examine the 

edge-value water velocities.

Shiptrack

The operator should take care to start the 

Transect software only after the boat begins the crosssection traverse and to stop the Transect software before 

making the course reversal at the end of the traverse. 

During the discharge measurement, the ADCP operator 

also should note any observed longitudinal-flow 

reversals on the discharge-measurement log sheet. 

Longitudinal-flow reversals can take place at high flow 

(due to eddies) and near slack tide (due to momentum 

effects). Longitudinal-flow reversals should be noted 

during the review of shiptrack plots. Then the reviewer 

should determine whether the reversal is actual or 

caused by initial boat movement toward shore.

The shiptrack plot shown in figure 8.7 also 

should be examined for discrepancies in the motion of 

the discharge-measurement vessel. In general, a good 

discharge measurement occurs when the measurement 

vessel makes a smooth traverse from bank to bank. An 

extreme example of irregularities in the shiptrack is 

illustrated in figure 8.8. Examination of the velocityprofile data in figure 8.9 does not reveal these 

irregularities. If the shiptrack plot of this measurement 

had not been examined, it may have been erroneously 

labeled as a good measurement.

The shiptrack plot should be examined for 

“hooks” (fig. 8.10) near the edges that indicate that the 

F5 key was either pressed too early in the transect 

(before the vessel had started its move toward the far 

bank) or too late (after the boat operator has reversed 

course at the end of the transect). Such “hooks” can be 

edited out of the data using the F8 subsectioning menu 

(fig. 8.11).

Figure 8.7. Transect software shiptrack plot.
108 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Summary

Transects should be replayed as soon as possible 

after being collected to check for possible errors in 

acoustic Doppler current profiler (ADCP) setup and 

discharge-measurement technique. Configuration files 

should be examined to see if proper ADCP setup 

parameters were used for the measurement. The powercurve estimation scheme should be checked for 

correctness and the edge-value estimates should be 

checked.

Discharge-measurement technique should be 

checked for correctness. In particular, the boat speed 

should be slow enough to reduce the dischargemeasurement standard deviation to a value under 

5 percent (of mean discharge). In tidally affected areas 

or large shallow, slow-moving rivers, equation 9.1 

Figure 8.8. Transect software shiptrack plot with operator’s initials.

Figure 8.9. Transect software profile plot of “initials”. 
Chapter 8: Discharge-Measurement Review and Assessment 109

(chap. 9) can be used to assess discharge-measurement 

standard deviation.

If bottom movement is suspected, the boat 

should be anchored in an area of the highest channel 

flow and transects should be collected for at least 

10 minutes. The resulting shiptrack plot should be 

examined for apparent upstream boat movement.

If a few missing ensembles are discovered during 

playback, they can be estimated using the technique 

described in this chapter. If there are many missing 

ensembles present in the transect data, the discharges 

should be remeasured at a different cross section or 

when conditions become more favorable.

Magnitudes of the discharges in the unmeasured 

layers should be examined and, if they comprise a 

significant portion of the total discharge, the operator 

should attempt to locate a deeper cross section for 

future measurements.

Figure 8.10. Transect shiptrack screen. ADCP, acoustic Doppler current profiler.

Figure 8.11. Transect subsectioning menu screen. 
110 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

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Chapter 9: Discharge-Measurement Error 111

CHAPTER 9: DISCHARGE-MEASUREMENT ERROR

In this chapter we will discuss the major sources 

of error in ADCP discharge measurements. We will first 

review velocity-measurement uncertainty (chap. 1) and 

then discuss random and systematic ADCP dischargemeasurement errors. A simple discharge-measurement 

error model is presented to aid the operator in 

premeasurement planning and error assessment.

A Review of Major Acoustic Doppler Current 

Profiler Velocity-Measurement Limitations and 

Uncertainties

A short review of ADCP velocity-measurement 

uncertainty and measurement limitations will help in 

understanding discharge-measurement errors. Detailed 

explanations and examples of these uncertainties are 

discussed in chapter 1.

Limitations

Range Limitations

The signal strength of an acoustic pulse 

decreases logarithmically with distance from the 

transducer face (range). As the signal strength and 

signal-to-noise ratio decreases, the spectral width of the 

returned signal increases. This increase in spectral 

width with range causes an increased standard 

deviation of the measured velocity with range. At some 

range the return echo is unusable. This limiting range is 

largely dependent on transducer frequency and, to a 

much lesser extent, on transmit pulse length and beam 

angle for any given ADCP. The usable range of an 

ADCP also is affected by the number of scatterers in the 

water column. Below is a conservative estimate of 

ADCP maximum range for several transducer 

frequencies (table 9.1).

Side-Lobe Interference

When a parasitic side lobe strikes the bottom, the 

returning echo drowns out the reception of the echo 

from the main beam. In an ADCP with 20o-beam 

angles, this loss of reception occurs at a point equal to 

about 94 percent of the total depth. This area of sidelobe interference near the bottom (equal to 6 percent of 

total depth) can be calculated using equation 1.4. Using 

this equation, the Transect program “throws out” 

discharge data in the area of side-lobe interference and 

then estimates the velocities near the bottom using 

power-curve fits or other estimation techniques. 

However, if the operator is collecting velocity profiles, 

the velocities may not be flagged “bad” by the Transect 

software. To determine the depth at which the side-lobe 

interference will affect the data, the operator must 

examine backscattered intensities. In a normal profile, 

the backscattered intensities start at around 140 counts 

(or higher) and “drop off” with depth. Velocity 

measurements become questionable at the point where 

intensities increase (or stop decreasing). Looking at 

beam 1 in the tabular plot in figure 9.1, we see that the 

intensities start at 145 and increase to 147 in the 

following bin because of ringing (mechanical and 

electronic resonance). This error in the top-most bin 

occurs if the blanking distance is set too short (see the 

next section on Blanking Distance). The intensities 

decrease until about 123 counts where they “flatten” 

and then begin to increase. The depth at which the 

counts flatten and do not decrease is the depth at which 

the velocities may become affected by side-lobe 

interference. The area between the last good bin center 

and the bottom must be estimated by some means to 

obtain mean velocity in the vertical profile.

Unmeasured Velocity Due to Blanking Distance

and Transducer Draft

After transmitting the acoustic signal, the ADCP 

transducers and electronics must “recuperate” briefly 

before they can receive the incoming echoes. During 

recuperation, the acoustic signal travels a short distance 

(blanking distance). Acoustic reflections cannot be 

received within the area between the transducer face 

and the blanking distance.

The transducer assembly must be submerged 

adequately so that it does not break the water surface 

during pitch and roll events. The depth of the transducer 

faces below water surface is called transducer draft.

To calculate the distance to the center of the first 

bin, it is necessary to use the BB-SETUP software 

module, available from RDI, as part of the Transect 

software package. From the screen capture shown in 

figure 9.2 it can be seen that if you are using a 

Table 9.1. Approximate maximum depth range for 300-, 600-, and 

1,200-kilohertz acoustic Doppler current profiler (ADCP) systems

[kHz, kilohertz; m, meter; ft, foot] 

ADCP frequency

(kHz)

Range

(m)

(ft)

300 20 m (390 ft)

600 48 m (157 ft)

1,200 15.4 m (50.5 ft)
112 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

1,200-kHz BB-ADCP with 0.25-m (0.82-foot) bins, a 

blanking distance of 0.5 m (1.64 ft) and an ambiguity 

velocity (WV) of 190 cm/s (6.2 ft/s), the center of bin 1 

will be at a range of 0.85 m (0.28 ft) from the face of the 

transducers. If the transducer draft is 0.25 m (0.82 ft), 

the center of the first bin is 0.85 + 0.25 = 1.10 m (3.6 ft) 

below water surface. Water velocity in the area between 

the water surface and the first bin center must be 

estimated to calculate mean water velocity in the 

vertical profile.

Random and Systematic Uncertainty

In chapter 1, we discussed ADCP velocitymeasurement uncertainty in detail. In the following 

paragraphs we will touch on each of the major sources 

of ADCP velocity-measurement uncertainty so that we 

can more readily grasp the causes of dischargemeasurement error.

ADCP velocity-measurement uncertainty can 

take two forms:

•Random uncertainty

•Systematic uncertainty (bias)

Random uncertainty can be reduced by data 

averaging, but systematic uncertainty cannot. If the 

magnitude of systematic uncertainty is known, it can 

sometimes be corrected using adjustment factors or 

coefficients.

Random Uncertainty Due to Self Noise and Lag Distance

The most significant source of random 

uncertainty in a BB-ADCP water-velocity 

measurement is caused by self noise (freeway analogy 

in chap. 1). The magnitude of this uncertainty is 

affected by ADCP transducer frequency, transmit pulse 

width, lag distance, and many other factors. This 

uncertainty directly affects ADCP water-velocity 

measurement precision. Techniques such as pseudo 

random phase encoding, data averaging, and increasing 

the lag distance are used to help reduce the effects of 

this uncertainty. The magnitude of this uncertainty can 

be predicted using the BB-SETUP program that is 

shipped with the Transect software files. Figure 9.2 

shows a screen shot of a typical 1,200-kHz broad-band 

ADCP setup scenario.

In mode 1 operation, the operator can control the 

lag distance by changing the value of the ambiguity 

velocity (WV command). Notice that for an ambiguity 

velocity of 190 cm/s the single-ping standard deviation 

(an indicator of velocity-measurement random 

uncertainty) is 14.08 cm/s (0.46 ft/s). If the operator 

lowers the ambiguity velocity to 90 cm/s (2.95 ft/s) for 

example, the single ping standard deviation drops to 

10.3 cm/s (0.34 ft/s). There are practical upper and 

lower limits to the ambiguity-velocity setting; setting 

the ambiguity velocity too high will produce 

unacceptable random uncertainty in the velocity 

measurement and setting the ambiguity velocity too low 

may introduce ambiguity uncertainty (chap. 1). In 

Figure 9.1. Screen shot of Transect software showing tabular output.
Chapter 9: Discharge-Measurement Error 113

mode 5 operation, the velocity-measurement 

uncertainty is greatly reduced (by about a factor of 10) 

because of the large lag spacing. However, because this 

large lag spacing lowers the ambiguity velocity, mode 5 

only can be used in conditions where the bottom and 

water velocities are low [less than 50 cm/s (1.64 ft/s)], 

relative to the ADCP.

Systematic Uncertainty Due to Velocity Ambiguity

If the operator is recording single-ping 

ensembles, velocity-ambiguity uncertainties show as 

“spikes” in the data that are the opposite sign of the 

“true” velocity. The difference between the “true” 

velocity and the erroneous velocity is twice the 

magnitude of the ambiguity velocity (WV command in 

mode 1). A detailed explanation of this uncertainty is in 

chapter 1. For mode 1 operation, the operator can raise 

the value of the ambiguity velocity to eliminate these 

uncertainties, but the single-ping velocitymeasurement standard deviation will increase, causing 

decreased measurement precision. The ambiguityvelocity setting is a trade off and usually can be set to a 

“safe” value using equation 3.1 in chapter 3. If the 

ambiguity-velocity values are set to a high value 

because of possible ambiguity uncertainties, the 

operator may have to slow the boat during the crosssection traverse to improve the discharge-measurement 

standard deviation (discussed later in this chapter).

For mode 5 operation, the ambiguity velocity 

(WZ command) is set automatically by the ADCP, 

depending on water depth and depth-cell length. If, 

during mode 5 operation, ambiguity uncertainties are 

encountered, the operator either must switch 

measurement modes or look for a cross section having 

lower velocities and (or) current shear.

Systematic Uncertainty in Speed of Sound Due to Temperature

Speed-of-sound calculations that are not 

corrected for temperature can cause velocitymeasurement errors and depth errors as great as 

7 percent. Fortunately the Transect software uses data 

from a thermistor in the transducer assembly to correct 

Figure 9.2. Screen shot of BB-SETUP software showing a typical setup for a 1,200-kilohertz broad-band acoustic Doppler current profiler.
114 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

2

0

0 10 30 20 40

-2

-4

-6

-8

TEMPERATURE, IN DEGREES CELSIUS

DEPTH ERROR, IN PERCENT

3

2.5

0 10 30 20 40

2

1.5

1

0.5

0

-0.5

SALINITY IN PARTS PER THOUSAND

DEPTH ERROR IN PERCENT

speed-of-sound calculations for temperature variations 

in the water near the transducer. Under normal stream 

conditions, this correction reduces speed-of-sound 

errors to insignificant levels. Sometimes, however, the 

water column becomes temperature stratified. Unlike 

horizontal water-velocity errors, depth-measurement 

errors can be introduced by temperature gradients in a 

stratified water column. Figure 9.3 shows depth errors, 

in percent, due to improper temperature compensation 

for speed of sound. Fortunately temperature gradients 

must be extremely high (10oC or more) to cause a 

significant error in a BB-ADCP depth measurement. 

Gradients of this magnitude sometimes can occur 

during the summer in slow-moving water.

Speed-of-sound errors also can be caused by a 

faulty thermistor in the transducer assembly. An error in 

temperature causes an erroneous calculation of water 

density, which results in a speed-of-sound uncertainty.

Systematic Uncertainty in Speed of Sound Due to Salinity

The speed-of-sound equation in the Transect 

program (R.D. Instruments, Inc., 1995) depends on a 

user-supplied salinity value to calculate speed-of-sound 

corrections (as discussed in the velocity-measurement 

error section in the first part of this chapter). This value 

is specified in the Transect software configuration file. 

If the operator has entered an incorrect value or has 

forgotten to enter the proper value, depths (as well as 

velocities) are calculated incorrectly. Depth errors as 

high as 3 percent can be caused by speed-of-sound 

calculations that are not corrected for salinity (fig. 9.4). 

Fortunately “ball park” salinity values usually will 

reduce this error to less than 1 percent.

Velocity errors due to salinity-caused speed-ofsound variations can be at least as serious, therefore, 

many boat operators carry a salinimeter (or 

conductivity meter) with them to the dischargemeasurement site. Salinity can be calculated and 

entered into the configuration file prior to the discharge 

measurement, however, a better approach is to enter 

zero salinity into the configuration file (ES0). The 

correct salinity can be plugged into the configuration 

file during playback. An incorrectly entered salinity 

using the ES direct command can be very hard to 

correct after the fact.

Systematic Uncertainty Due to Incorrect Beam Geometry

The largest source of systematic uncertainty in an 

ADCP is caused by uncorrected errors in the 

measurement of the beam-pointing angles. These 

uncertainties usually are in older BB-ADCP units that 

were sold before the manufacturer instituted stringent 

quality-control procedures. Late model BB-ADCPs and 

Workhorse Rio Grandes have transducer assemblies 

that meet more exacting beam-angle tolerance 

requirements. Each ADCP is tested on a distance course 

and corrections are saved in the ADCP firmware for any 

measured beam-angle discrepancies. Older ADCPs can 

be upgraded by the manufacturer and then tested in a 

lake to identify and correct beam-angle uncertainties.

Minimizing Uncertainty in Velocity Profiles

Many scientific studies require accurate watervelocity profile measurements and discharge 

measurements. Many of the velocity-measurement 

errors described above can be minimized or eliminated 

by using the following common sense rules:

•Use the proper water mode for the job. If the 

velocities are dynamically changing, or the 

vessel is affected by pitch and roll, use water 

mode 1. In shallow water that is placidly 

moving, mode 5 may be used. In shallow, fastmoving water, mode 8 may be used.

Figure 9.3. Depth error due to speed of sound that is uncorrected 

for temperature.

Figure 9.4. Depth error due to speed of sound that is uncorrected 

for salinity.
Chapter 9: Discharge-Measurement Error 115

•Collect enough data at the profile location to 

ensure that the measured water-velocity 

standard deviation is within usable limits (after 

the data are averaged).

•If the unmeasured bottom and top velocities are 

to be estimated, ensure that the measured 

portion of the profile (after averaging) is 

congruent with the estimation scheme used. For 

example, if the measured part of the profile is 

bidirectional, a one-sixth power-curve 

estimation scheme is improper.

Errors Affecting the Accuracy of Discharge 

Measurements

If heavy sediment loads are not present and a 

moving river bed problem does not exist, errors in 

ADCP discharge measurements usually are caused by 

improper river traverse rates or erratic movements of 

the discharge-measurement vessel. These random 

errors usually cause imprecision in the discharge 

measurement (scatter). Reducing the traverse rate and 

eliminating erratic vessel movements can reduce 

random error.

The next most common error types are depth 

errors caused by incorrect transducer draft, depth errors 

due to speed-of-sound variations, and cross-product 

errors due to improper application of the power-curve 

fitting scheme. These sources of error are biases that 

cannot be improved by slowing the traverse rate or 

averaging the data (see bias error section later in this 

chapter). Random errors in discharge measurements 

made with the BB-ADCP system roughly can be 

predicted if the following values are known:

•ADCP water-velocity measurement precision

•ADCP bottom-track velocity measurement 

precision

•The standard deviation of naturally occurring 

water-velocity pulsations (at the ADCPmeasurement time scale)

•Approximate average cross-sectional water 

depth

•Approximate average water speed

•BB-ADCP bin size

•BB-ADCP ping rate

Table 9.2 gives the approximate depth-averaged, 

water-velocity standard deviation for 1,200-, 600-, and 

300-kHz BB-ADCPs with 20o beam angles using 

mode 1 operation with an ambiguity velocity of 

190 cm/s (6.23 ft/s).

Mode 5 bottom-track standard deviation is much 

less than the water-velocity standard deviation for all 

ADCP frequencies [0.2–0.3 cm/s (0.006–0.009 ft/s)].

Simplified Random-Error Model

The precision of a discharge measurement may 

be computed using the following algorithm in 

equation 9.1:

(9.1)

where

 = standard deviation of discharge 

measurement, in percent;

 = estimated approximate mean stream 

velocity, in centimeters per second;

 = BB-ADCP water-velocity precision, in 

centimeters per second (table 9.2.);

 = BB-ADCP bottom-track precision, in 

centimeters per second (table 9.2.);

 = estimated standard deviation of natural 

pulsations, in percent of mean velocity; a 

value of usually 8–12 percent (at the time 

interval used for an ensemble by the 

BB-ADCP);

 = depth error due to round off and resolution 

limitations;

 = average number of bins in the vertical; and

 = total number of subsection measurements.

Equation 9.1 is derived using the following 

assumptions:

•A 15-percent bin-to-bin correlation

•A 0-percent subsection-to-subsection 

correlation

•A smooth rectangular channel

σq

100

Xw

Vm

------  

 

2

100

Xb

Vm

------  

 

2

σp

2 σz

2 + ++

0.75NbNs

= -----------------------------------------------------------------------------------------

σq

Vm

Xw

Xb

σp

σz

Nb

Ns

Table 9.2. Approximate depth-averaged single-ping precision for 

1,200-, 600-, and 300-kilohertz broad-band acoustic Doppler current 

profiler-measured water velocities using mode 1 operation, 

20° beams, and WV190

[kHz, kilohertz; m, meter; ft, foot; cm/s, centimeter per second; ft/s, foot 

per second] 

Frequency

(kHz)

Depth

cell size

Average standard deviation

for depth range

1,200 0.25 m

(0.8 ft)

 14.1 cm/s (0.46 ft/s)

600 .50 m

(1.64 ft)

14.1 cm/s (.46 ft/s)

300 14.1 cm/s (.46 ft/s)
116 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

RIVER VELOCITY, IN FEET PER SECOND

RIVER VELOCITY, IN FEET PER SECOND

DISCHARGE ERROR, IN PERCENT DISCHARGE ERROR, IN PERCENT

VESSEL SPEED 0.3 METER PER SECOND

(1 FOOT PER SECOND)

VESSEL SPEED 0.9 METER PER SECOND

(3 FEET PER SECOND)

25

20

15

10

5

0

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

14

12

10

8

6

4

2

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

DISCHARGE ERROR, IN PERCENT

RIVER VELOCITY, IN FEET PER SECOND

VESSEL SPEED O.9 METER PER SECOND

(3 FEET PER SECOND)

The 0.75 constant in equation 9.1 is a rough 

approximation of the Markov model output when there 

is a sample (bin-to-bin) correlation of 15 percent 

(Matalas and Langbein, 1962). Matalas and Langbein 

(1962) present an equation for the calculation of 

effective N for any correlation coefficient, and this 

equation should be used in error models that are 

developed for the estimation or prediction of 

BB-ADCP (or narrow-band ADCP system) dischargemeasurement error.

Depth error due to BB-ADCP round off and 

resolution limitations ( ) is estimated by the 

manufacturer to be 4 percent of the measured vertical 

depth range on an individual beam (Joel Gast, R.D. 

Instruments, Inc., oral commun., 1997). Because four 

beams are averaged for the depth measurement, the

error becomes percent for each ping averaged 

during the discharge measurement.

Figure 9.3 shows graphs of mode 1, 1,200-kHz, 

error model output for a 61-m- (200-foot-) wide river, 

with boat speeds of about 0.3 and 0.9 m/s (1.0 and 

3.0 ft/s). Figure 9.3 reveals that a boat operator 

traversing a 61-m- (200-foot-) wide, 4.6-m- (15-foot-) 

deep river could use a boat speed of about 0.3 m/s 

(1 ft/s) for the measurement of all mean river velocities 

above 0.15 m/s (0.5 ft/s). Figure 9.5 also shows that 

discharge-measurement error increases with the boat 

speed. This increase occurs because the BB-ADCP is 

collecting fewer pings during the cross-section 

traverse, with a resultant increase in random error 

(because fewer data are averaged). The boat operator 

must remember that the precision of a discharge 

measurement can change dramatically with changes in 

the total number of pings and the total number of bins 

sampled during the traverse. The total number of depth 

bins depends on the water depth. Figure 9.6 shows the 

same scenario as in figure 9.5, but with an average 

cross-section depth of 9 m (30 ft) rather than 4.3 m 

(15 ft). Note that the discharge-measurement error 

decreases to magnitudes similar to those shown in 

figure 9.5. This decrease occurs because there are twice 

as many bins averaged in the vertical profile.

Although equation 9.1 can be programmed into a 

spreadsheet and used to predict dischargemeasurement uncertainty, a small executable software 

application (QERROR) has been developed (for 

mode 1 use) that is more easily used in the field. 

Figures 9.7–9.9 show screen shots from the QERROR 

application. Figure 9.7 shows the input screen. The 

measurement units are in mixed systems on the input 

screen for two reasons:

σz

4

4

------ = 2

•USGS field office personnel are more adept at 

estimating widths and depths in the standardmeasurement system.

Figure 9.5. Discharge error using a boat speed of about 0.3 meter 

per second (1 foot per second) and about 0.9 meter per second 

(3 feet per second).

Figure 9.6. Discharge error using a boat speed of about 0.9 meter 

per second (3 feet per second) in 9 meters (30 feet) of depth.
Chapter 9: Discharge-Measurement Error 117

•ADCP input parameters always are requested in 

SIS units.

Figure 9.8 is the output screen from the 

QERROR application and it illustrates the importance 

of slow-vessel traverse rates when measuring rivers 

with slow mean velocities. Note that the error is 

reasonable when the boat is moving at speeds less than, 

or equal to, the water velocity. A good rule of thumb is 

to traverse the river at about the same speed as the mean 

water velocity. However, this rule becomes 

inapplicable when measuring wide, deep rivers, with 

slow water velocities. The operator would not be able to 

complete such a measurement within a reasonable time 

frame. In cases of wide, deep rivers, the operator can 

use equation 9.1 or the QERROR application to 

estimate discharge-measurement error based on boat 

speed, average water depth, and estimated mean water 

velocity.

Figure 9.9 shows the same measurement 

scenario as used for figure 9.8 with a mean river 

velocity of 0.15 m/s (0.5 ft/s). Notice that boat speeds 

as high as 0.6 m/s (2 ft/s) could be used for accurate 

measurement of the above described river, partly 

because the river (figs. 9.6 and 9.7) is 9 m (30 ft) deep. 

If the river depth were shallow [3 m (10 ft) or less], the 

boat would have to be slowed to the mean water 

velocity to maintain a reasonable precision (a CV of 

less than 5 percent) (fig. 9.10).

Examination of fig. 9.6 reveals that the length of 

time required for the river traverse increases 

exponentially as mean river velocities approach 0. The 

requirement for this extended averaging period begins 

to defeat the purpose of the BB-ADCP measurement 

system at very shallow depths and low velocities. Water 

mode 5 should be used, if possible, in these cases 

because it will reduce the above described dischargemeasurement errors by almost an order of magnitude, 

however, its use is limited to rivers having little shear or 

turbulence. For 600-kHz BB-ADCPs, water mode 8 can 

be useful in shallow water if the vessel is slowed to 

reduce the higher random error of mode 8 operation.

Figure 9.7. QERROR setup screen. ADCP, acoustic Doppler current profiler.
118 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Bias Error

Discharge-measurement random error can be 

reduced by data averaging (slowing the vessel speed for 

the measurement transect), as discussed above. Bias 

(systematic) errors cannot be reduced by data 

averaging.

Bias error can be separated into two classes: 

instrument error and operator error. Instrument error is 

due to physical, electrical, or acoustical limitations of 

the BB-ADCP instrument (or defects in the BB-ADCP 

hardware and firmware). Operator error is caused by 

improper BB-ADCP installation, setup, and, in some 

cases, application.

Instrument-Caused Bias Error

There are many sources of instrument bias error. 

Some are more significant than others. This report will 

not attempt a discussion of ADCP systematic error 

sources related to the physics of the acoustic signal 

(other than beam-pointing angles and depth 

measurements) because many of these sources are not 

yet documented and are beyond the scope of this report. 

The manufacturer is aware of many of these types of 

errors and reports that the two most important of these 

are water-velocity measurement errors, due to selective 

absorption (nonuniform signal absorption in the water 

mass over the transmitted signal spectrum), and 

bottom-track errors due to terrain effect (the leading 

edge of the acoustic beam is far20ther away from the 

transducer than is the trailing edge when it impinges the 

channel bottom). These and other errors are thought by 

the manufacturer to be small and insignificant for most 

applications (Joel Gast, R.D. Instruments, Inc., oral 

commun., 1992). The following paragraphs will discuss 

or revisit instrument errors that significantly can affect 

the accuracy of an ADCP discharge measurement. 

These errors are as follows:

•Beam-angle errors.

•Depth-measurement errors.

•Speed-of-sound errors due to temperature and 

salinity.

•Bias errors caused by improperly estimating the 

unmeasured portions of the cross section.

•Operator-caused bias errors.

Figure 9.8. Discharge error with a mean water velocity of about 0.3 meter per second (1.0 foot per second). ADCP, acoustic Doppler current

profiler.
Chapter 9: Discharge-Measurement Error 119

Beam-Angle Errors 

Errors in the beam-pointing angles (discussed in 

the velocity-error section) have an equivalent affect on 

the accuracy of discharge measurements. 

Depth-Measurement Errors 

The Transect software uses depth measured from 

the four acoustic beams to calculate mean depth for 

each discharge-measurement subsection. These depthmeasurement errors can come from two sources:

•Depth sampling errors due to limitations of the 

acoustic beams and bin sizes.

•Depth errors due to improper estimation of 

speed of sound.

Figure 9.11 shows an exaggerated instance of 

depth error due to limitations of the acoustic beams. If 

only a few depth measurements were taken, per crosssection traverse, this type of error would be significant. 

However, the averaged depth is calculated using all four 

beam depths for each ensemble and, if the ensembles 

are kept short enough, the mound (fig. 9.8) will be 

integrated into the total cross-section averaged depth. A 

typical BB-ADCP discharge measurement collects 

many more depth measurements during the crosssection traverse than are collected using conventional 

methods.

Near the bank edges, the BB-ADCP beams 

oriented toward shore will show shallow depths, 

whereas the beams oriented toward the channel will 

show greater depths. An average of all four beams will 

approximate the vertical depth from the center of the 

BB-ADCP transducer assembly to the bottom. In pitch 

and roll conditions, averaged depth measurements from 

all four acoustic beams will be more accurate than 

depths measured by a single, vertically placed, depth 

sounder because of the large beam “footprint” or 

pattern.

Speed-of-Sound Errors Due to Temperature and Salinity Gradients

Speed-of-sound errors (discussed in the ADCP 

Velocity-Measurement Limitations and Uncertainties 

section of this chapter) affect the accuracy of discharge 

measurements in the same way that they affect the 

accuracy of velocity measurements. These uncertainties 

Figure 9.9. Discharge error in deep water [9 meters (30 feet)] with a mean river velocity of about 0.15 meter per second (0.5 foot per second). 

ADCP, acoustic Doppler current profiler.
120 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

are subtle and hard to spot in the collected data. Close 

attention to detail is required by the ADCP operator to 

eliminate speed-of-sound uncertainties.

Bias Error Due to Incorrect Estimation of Unmeasured Velocities

Near the Water Surface and Channel Bottom

Errors of this type are called extrapolation errors. 

An example of a nonstandard velocity profile is shown 

in chapter 8 to illustrate this error. The extrapolation 

scheme used to estimate cross products near the water 

surface and channel bed assumes a “Manning-like” 

velocity profile. The unmeasured area near the bottom 

usually is not a problem because the velocity must go 

to zero at some point close to the bed. However, the 

unmeasured area near the water surface is problematic, 

particularly in wind-affected cross sections and in 

estuaries. Wind effects can cause nonstandard profiles 

that are significantly biased (near the water surface). In 

these cases, the POWER estimation scheme (used in 

the Transect program) can be changed to CONSTANT. 

In the estuaries and other backwater-affected 

sites, gravitational circulation sometimes can cause 

nonstandard profiles as well as bidirectional flow. 

Examination of the Transect program playback files can 

sometimes reveal nonstandard profiles and can provide 

the basis for a revised power-fit coefficient (chap. 8). In 

cases of bidirectional flow, the estimation scheme 

should be changed to CONSTANT on the bottom and 

CONSTANT at the surface because a power function 

extrapolation mathematically cannot cross zero. In 

bidirectional flows, the velocities may be positive 

(flowing downstream) in the upper portion of the water 

column and be negative (flowing upstream) in the lower 

portion of the water column.

Fortunately, extrapolation errors can be corrected 

during data playback. The raw recorded data are not 

affected by changing the extrapolation scheme, and the 

operator and office analyst can use trial-and-error 

techniques to reduce bias error that is due to incorrect 

estimation of unmeasured cross products.

Operator-Caused Bias Error

Operators can introduce a number of bias errors 

affecting a discharge measurement. Many of these 

Figure 9.10. Discharge error in shallow water [3 meters (10 feet)] with a mean river velocity of about 0.15 meter per second (0.5 foot per 

second). ADCP, acoustic Doppler current profiler.
Chapter 9: Discharge-Measurement Error 121

errors have been covered in previous chapters, but are 

summarized here.

Incorrect Transducer Draft

This error probably is the most common 

operator-caused bias error. Missetting the transducer 

draft can cause significant discharge-measurement 

errors, particularly in wide, shallow channels.

Improper Broad-Band Acoustic Doppler Current Profiler Mounting

If the BB-ADCP is mounted to the side of the 

boat keel and in the boat wake, entrained air can cause 

occlusion of one or more BB-ADCP beams. A 

BB-ADCP that is mounted too high near the boat hull 

can suffer from the “good side/bad side” effect. The 

“good side/bad side” effect manifests itself in a series 

of discharge measurements that seem to be 

directionally biased. For example, discharges measured 

from right to left bank seem to be significantly different 

from discharges measured from left to right bank. 

Victor Levesque (U.S. Geological Survey, oral 

commun., 1996) has quantified this error and has 

associated it with the venturi effect of the boat hull on 

flowing water: water passing under the boat hull is 

slowed on the streamward side of the boat, and 

accelerated on the downstream side of the boat. This 

effect causes bias in the uppermost bins. Averaging 

even numbers of transects can help reduce this error.

Incorrect Edge-Distance Estimates

Distances to the riverbank are almost always 

underestimated unless the operator is very close to the 

riverbank when the distance is estimated. This 

underestimation can cause a significant bias error, if 

undetected.

Incorrect Estimated Edge Shapes

In narrow channels, significant bias errors can 

result from incorrectly characterizing the near-bank 

geometry. Rectangular-shaped edge-discharge areas 

can contain significant amounts of unmeasured 

discharge if the triangular edge algorithm is used for 

edge-discharge estimation.

Figure 9.11. Exaggerated instance of depth error due to limitations of the acoustic beams.
122 Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

Bottom Movement

As a general rule of thumb, all discharge 

measurement series taken where velocities are greater 

than 1 ft/s or where bottom movement is suspected 

should be followed or preceded by a bottom movement 

check (chap. 8).

Configuration-File Error

Every BB-ADCP operator undoubtedly has 

collected transects with the wrong configuration file or 

has inserted incorrect direct commands in the 

configuration file (at least once). Attention to detail will 

help eliminate this error.

Poor Choice of Cross Sections

Because the BB-ADCP is such a versatile 

instrument, it is easy to measure discharge in a cross 

section that normally would be rejected for use with 

conventional methods. “Pan-handle”-shaped cross 

sections with deep channels near one side of the river 

are examples of poor cross sections. The beam “footprint” (with accompanying side lobes) becomes larger 

as depths increase. Discharge in a submerged canyon 

may be unmeasurable because the beams and side lobes 

impinge the canyon walls. Proper reconnaissance and 

experience will help in choosing a cross section that 

eliminates bias problems due to this effect.

Common Sense Rules

By following the common sense rules listed 

below, errors in discharge measurements can be 

reduced or eliminated:

•Be a smooth operator! The BB-ADCP 

discharge-measurement system will give more 

consistent results if rapid movements and course 

changes are kept to a minimum. Smooth boat 

motion is more important than a straight-line 

course

•Be observant! Are the edge flows moving in the 

same direction as the main body of flow? Did 

the wind come up? Did a motorboat pass the 

bow of the measurement vessel during the 

transect? This information may be needed 

during playback to properly evaluate the 

discharge measurement

•Is the discharge-measurement vessel moving 

slowly enough? The more pings collected during 

a discharge measurement, the more precise the 

measurement

•Examine discharge data on site, if possible. 

Problems with improper setup may not appear 

until the measurements are replayed

•Don’t trust an “eyeball” edge estimate. Most 

people tend to underestimate the distance to 

shore unless the vessel is very close to shore 

when the estimation is attempted

•Don't be afraid to ask for help! Experienced 

BB-ADCP operators carry a cellular phone, a 

long list of phone numbers, and are not afraid to 

ask, “What gives here?” Only novice operators 

are too proud to ask for help.

Summary

If high sediment loads are not present, errors in 

acoustic Doppler current profiler (ADCP) discharge 

measurements usually are caused by traversing the river 

too fast during the discharge measurement. As a rule, 

the vessel should traverse the river at the approximate 

speed of the mean water velocity to obtain consistently 

precise discharge measurements. When measuring 

wide, deep rivers or estuaries, the operator should use 

equation 9.1 to estimate correct vessel traverse rates, if 

possible. If the operator checks the standard deviation 

of the discharge measurements at the measurement site 

he or she can easily determine if the discharge 

measurements have too much scatter (5 percent or 

greater). If such is the case, the boat traverse rate can be 

slowed and the measurements can be redone. Most bias 

errors can be eliminated with proper attention to detail.
References Cited 123

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