U.S. GEOLOGICAL SURVEY
Open-File Report 01-01
Sacramento, California 2001
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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 discharge-measurement 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 discharge-measurement 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' discharge-measurement software, and boating techniques and safety.
Introduction
Purpose and Scope
A Short History of Acoustic Doppler Current Profiler Discharge Measurement
Chapter 1: Theory of Operation
Basic Acoustic Velocity Measurement Principles
The Physics of Sound
The Doppler Principle Applied to Moving Objects
Measuring Doppler Shifts Using Acoustic Backscatter
Measuring Doppler Shifts from a Moving Platform
Radial Motion
Acoustic Doppler Current Profiler Beam Geometry
Calculating Three-Dimensional Velocity Components
Beam Scenarios
The Fourth Beam and Error Calculations
Acoustic Doppler Current Profiler Water-Velocity Profile Measurements
Acoustic Doppler Current Profiler-Measured Profiles Compared with Conventional Current Meter Measurements
Time Gating: Measuring Doppler Shifts from Different Depths
Bottom Tracking
Acoustic Doppler Current Profiler Limitations for Velocity-Profile Measurements
Range Limitations
Side-Lobe Interference
Effects of Different Beam Angles
Blanking Distance
Instrument Development: Solving the Problem of Velocity-Measurement Uncertainty
Random and Bias Error
Random Error
Bias Error
Pitch and Roll
Beam-Angle Error
Narrow-Band and Broad-Band Doppler Shift Measurements
Narrow-Band Doppler Shift Measurements
Broad-Band Doppler Shift Measurements
Differences Between Phase-Shift Measurements and Lag-Spacing Measurements (Time Dilation)
Bottom-Tracking Limitations
The Broad-Band Acoustic Doppler Current Profiler: Overcoming the Self-Noise Problem
Error Sources Unique to Broad-Band Acoustic Doppler Current Profilers
Random Uncertainty Caused by Self Noise
Summary
Chapter 2: Acoustic Doppler Current Profiler Discharge-Measurement Principles
Parts of an Acoustic Doppler Current Profiler Discharge Measurement
Velocity Cross-Product Measurement Using an Acoustic Doppler Current Profiler
General Equation
The General Equation, as Applied to Acoustic Doppler Current Profiler Moving-Boat Measurements
Properties of the Acoustic Doppler Current Profiler Measured Cross Product
Integrating the Cross Product Over the Water Depth
Estimating Cross Products in the Unmeasured Portions of the Profile
Blanking Distance
Side-Lobe Interference
Evolution of the One-Sixth Power-Curve Estimation Technique
Integration, By Time, Over the Width of the Cross Section
Estimating Discharge Near the Channel Banks
Determination of Total River Discharge--Putting it All Together
Discharge-Measurement Software
Summary
Chapter 3: R.D. Instruments, Inc., Broad-Band Acoustic Doppler Current Profiler Measurement Modes
Measurement Modes--Why?
Water Modes
Water Mode Zero (WM0)
Water Mode 1 (WM1)
Water Mode 4 (WM4)
Water Mode 5 (WM5)
Ambiguity Velocity Revisited
Water Mode 7 (WM7)
Water Mode 8 (WM8)
Range/Speed "Windows" for Water Modes 1, 5, and 8
Bottom-Track Modes
The Bottom Reflection
Bottom-Track Mode 4 (BM4)
Bottom-Track Mode 5 (BM5)
Summary
Chapter 4: Acoustic Doppler Current Profiler Hardware and Ancillary Equipment
Where Do We Start?
Acoustic Doppler Current Profiler Equipment
Acoustic Doppler Current Profiler Pressure Case and Transducer Assembly
Power Supply and Communications Interface
Acoustic Doppler Current Profiler Discharge-Measurement Software
Documentation
Ancillary Equipment
Measurement Platform or Vessel Requirements
Laptop Computer
Acoustic Doppler Current Profiler Mounts
Range Finder or Method for Estimating Distance to Shore
Trolling Motors/Plates
Miscellaneous Measurement Equipment
Installation of the Broad-Band Acoustic Doppler Current Profiler
Mounting the Acoustic Doppler Current Profiler on the Vessel
Deck-Unit and Power-Supply Connections
Acoustic Doppler Current Profiler Cables and Connectors
Broad-Band Acoustic Doppler Current Profiler
Workhorse Rio Grande
Summary
Chapter 5: Broad-Band Acoustic Doppler Discharge-Measurement System Configuration
Discharge-Measurement Software--"Transect"
Transect Configuration
Creation of a Preliminary Configuration File Using Transect Modules
Communications Setup
Calibration Setup
Planning Setup
The Configuration File, in Detail
The Communications Section
Ensemble Out Section
Acoustic Doppler Current Profiler Hardware Section
Direct Commands Section
Water-Track Commands
WSnnn
WNnnn
WPnnnn
WFnnnn
WDnnn nnn nnn
Bottom-Track Commands
BAnnn
BCnnn
BPnnn
BXnnnn
&Rnn
General Commands
ESnn
EXnnnnn
Recording Section
Calibration Section
Processing Section
Graphics Section
History Section
Finishing the Preliminary Configuration File: Required Commands
Transect Release Enhancements (2.80 and Later)
Summary
Chapter 6: Data Acquisition
Operation of Transect Software
Loading the Configuration File
Starting the Transect Acquire Module
Transect-Data Displays
Transect-Data Recording
Summary
Chapter 7: Discharge-Measurement Procedure
Cross-Section Reconnaissance
Premeasurement Checkout
Boat-Maneuvering Techniques
Starting the Cross-Section Traverse
During the Cross-Section Traverse (Transect Tips and Tricks)
Ending the Cross-Section Traverse
Alternate Techniques Used During Low-Flow Conditions
What Constitutes a "Good" Discharge Measurement?
Archival of Acoustic Doppler Current Profiler Discharge-Measurement Data
Summary
Chapter 8: Discharge-Measurement Review and Assessment
Configuration-File Review
River Conditions
Acoustic Doppler Current Profiler Hardware
Acoustic Doppler Current Profiler Direct Commands
Calibration Section
Transect Software Playback
Missing Ensembles
Error Caused By Sediment Movement Near the Bottom
Large Magnitudes of the Unmeasured Layers
Low-Velocity Measurements
Power-Curve-Fit Applicability
Edge Values
Shiptrack
Summary
Chapter 9: Discharge-Measurement Error
A Review of Major Acoustic Doppler Current Profiler Velocity-Measurement Limitations and Uncertainties
Limitations
Range Limitations
Side-Lobe Interference
Unmeasured Velocity Due to Blanking Distance and Transducer Draft
Random and Systematic Uncertainty
Random Uncertainty Due to Self Noise and Lag Distance
Systematic Uncertainty Due to Velocity Ambiguity
Systematic Uncertainty in Speed of Sound Due to Temperature
Systematic Uncertainty in Speed of Sound Due to Salinity
Systematic Uncertainty Due to Incorrect Beam Geometry
Minimizing Uncertainty in Velocity Profiles
Errors Affecting the Accuracy of Discharge Measurements
Simplified Random-Error Model
Bias Error
Instrument-Caused Bias Error
Beam-Angle Errors
Depth-Measurement Errors
Speed-of-Sound Errors Due to Temperature and Salinity Gradients
Bias Error Due to Incorrect Estimation of Unmeasured Velocities
Near the Water Surface and Channel Bottom
Operator-Caused Bias Error
Incorrect Transducer Draft
Improper Broad-Band Acoustic Doppler Current Profiler Mounting
Incorrect Edge-Distance Estimates
Incorrect Estimated Edge Shapes
Bottom Movement
Configuration-File Error
Poor Choice of Cross Sections
Common Sense Rules
Summary
References Cited
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