USGS

Discharge Measurements Using a Broad-Band Acoustic Doppler Current Profiler

By Michael Simpson

 

U.S. GEOLOGICAL SURVEY

Open-File Report 01-01

Sacramento, California 2001




Full report (3.8 MB PDF)
Cover (12 K gif)
Contents, Introduction, and Chapters 1-3 (980 K PDF)
Chapters 4-6 (1.1 MB PDF)
Chapters 7-9 and References Cited (1.1 MB PDF)

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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 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.

Contents

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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