����������������������������������������������

�������������������������������������������������������������

������������������������������������

������������������������������������������

������������������������������������������������������������������������������

����������������������������������������

�������������������������������

����������������������
Cover. Sacramento River at Freeport Bridge, Freeport, California. (Photograph by Cathy Munday, U.S. Geological Survey)
Investigation of Hydroacoustic FlowMonitoring Alternatives at the Sacramento 

River at Freeport, California: Results of the 

2002–2004 Pilot Study

By Catherine A. Ruhl and James B. DeRose

In cooperation with the Sacramento Regional Wastewater Treatment Plant and 

the California Department of Water Resources

Scientific Investigation Report 2004-5172

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ruhl, C.A., DeRose, J.B., 2004, Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River 

at Freeport, California: Results of the 2002–2004 Pilot Study: U.S. Geological Survey Scientific Investigations Report 

2004-5172, 18 p.
iii

Contents

Executive Summary ....................................................................................... 1

Abstract ................................................................................................... 2

Introduction ................................................................................................ 2

Purpose and Scope ................................................................................. 2

Acknowledgments ................................................................................. 2

Principles of Operation of Acoustic Flow-Monitoring Systems .................................... 4

Acoustic Velocity Meter ........................................................................... 4

Acoustic Doppler Velocity Meter ............................................................... 4

Calculation of Discharge ................................................................................. 5

Equipment Configuration ................................................................................. 5

Acoustic Velocity Meter ........................................................................... 5

Short-Range Acoustic Doppler Velocity Meter ................................................ 5

Long-Range Acoustic Doppler Velocity Meter ................................................ 5

Downward-Looking Acoustic Doppler Current Profiler ....................................... 6

Proof-of-Concept Study ................................................................................. 6

Calibrations ................................................................................................ 7

Area Calibration .................................................................................... 7

Velocity Calibration ................................................................................. 8

Acoustic Velocity Meter ..................................................................... 8

Short-Range Acoustic Doppler Velocity Meter .......................................... 9

Long-Range Acoustic Doppler Velocity Meter .......................................... 9

Outfall Building Datalogger Programming and Error-Checking Algorithm ........................ 10

Data Transmission ....................................................................................... 12

Cellular Digital Packet Data ..................................................................... 12

Radio Signal .......................................................................................... 12

Results ...................................................................................................... 13

Conclusions ................................................................................................ 17

References ................................................................................................ 18
iv

Figures

Figure 1. Map showing study location, Sacramento River at Freeport, California ......................... 3

Figure 2. Schematics of standard acoustic stream-gaging stations ................................................ 4

Figure 3. Schematic showing discharge measurement from a boat using a profiling 

downward-looking acoustic Doppler velocity meter. ......................................................... 6

Figure 4. Graph showing relationship between index velocity measurements 

collected using a short-range acoustic Doppler velocity meter and 

an acoustic velocity meter ...................................................................................................... 7

Figure 5. Graph showing acoustic velocity meter (AVM) calibration relationship ........................ 8

Figure 6. Graph showing short-range acoustic Doppler velocity-meter 

calibration relationship ............................................................................................................ 9

Figure 7. Graph showing long-range acoustic Doppler velocity-meter 

calibration relationship .......................................................................................................... 10

Figure 8. Graph showing discharge record recorded at the Outfall Building showing 

results from the error-checking program ........................................................................... 11

Figure 9. Graph showing comparison of primary and secondary discharge records and 

selected discharge record recorded at the Outfall Building, based on an 

error-checking program ......................................................................................................... 12

Figure 10. Graph showing comparison of short-range acoustic Doppler velocity-meter 

record received at the U.S. Geological Survey office and selected discharge 

record recorded at the Outfall Building .............................................................................. 13

Figure 11. Graph showing tidal discharge data collected from the three hydroacoustic 

systems during the 2002–2004 pilot study ........................................................................... 14

Figure 12. Graph showing comparison of tidally averaged discharge data collected from 

the three hydroacoustic systems during the 2002–2004 pilot study ............................... 16

Tables

Table 1. Comparison of acoustic Doppler velocity meter configuration settings used during 

the 2002–2004 pilot study at the Sacramento River at Freeport, California ..................... 5

Table 2. Differences among calculated discharge records from three hydroacoustic 

stream-gaging systems during the 2002–2004 pilot study at the Sacramento River 

at Freeport, California ............................................................................................................. 15

Table 3. Record comparisons of hydroacoustic stream-gaging systems used during the 

2002–2004 pilot study at the Sacramento River at Freeport, California ......................... 15

Table 4. Statistical comparison of measured and calculated discharge values during the 

2002–2004 pilot study at the Sacramento River at Freeport, California ......................... 17
v

Conversion Factors, Abbreviations, and Acronyms

Multiply By To obtain

acre 0.4047 hectare (ha)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

foot (ft) 0.3048 meter (m)

Abbreviations

hrs hours

kHz kilohertz

sec seconds

Acronyms

ADVM acoustic Doppler velocity meter

AVM acoustic velocity meter

CADWR California Department of Water Resources

CDMA code division multiple access

CDPD cellular digital packet data

LR-ADVM long-range acoustic Doppler velocity meter

MPD median percent difference

RMS root-mean-squared

SR-ADVM short-range acoustic Doppler velocity meter

SRWTP Sacramento Regional Wastewater Treatment Plant

USGS U.S. Geological Survey
Investigation of Hydroacoustic Flow-Monitoring 

Alternatives at the Sacramento River at Freeport, 

California: Results of the 2002–2004 Pilot Study

by Catherine A. Ruhl and James B. DeRose

Executive Summary

The Sacramento River at Freeport (Freeport) is a tidally 

affected channel approximately 620 feet wide located at the 

northern boundary of the Sacramento—San Joaquin River 

Delta, California. In 1978, an acoustic velocity meter (AVM) 

was installed at Freeport to monitor the flow. The AVM was 

calibrated successfully and has been used continuously since 

that time. Although the calibration has been stable, an increasing number of maintenance problems prompted a search 

for alternatives to monitor discharge at this location. Two 

sideward-looking acoustic Doppler velocity meters (ADVMs) 

were tested in a pilot study from 2002–2004: a short-range 

SonTek SL-1500 (SR-ADVM) and a long-range SonTek 

SL-500 (LR-ADVM). The pilot study was conducted over 

a wide range of hydrologic conditions, and both sidewardlooking ADVMs performed well at this location and have been 

calibrated successfully. 

The SR-ADVM and the LR-ADVM have good calibrations and data-recovery rates. Based on data collected through 

February 2004, the SR-ADVM had an r2=0.997 and a datarecovery rate of 97 percent; the LR-ADVM had an r2

=0.996 

and a data-recovery rate of 91 percent. The discharge data 

from the SR-ADVM have been selected as the primary record 

because this instrument has a robust calibration and a higher 

data-recovery rate. The discharge data from the LR-ADVM 

are the secondary record and provide real-time data redundancy, thereby minimizing data loss. 

Two issues require further attention: (1) although the 

SR-ADVM data have been selected as the primary record, 

there appears to be some problems with the discharge records, 

as compared to the discharge records from the other systems; 

and (2) there are problems with the radio-transmission system 

between the ADVM systems on the Freeport Bridge and the 

datalogger in the Outfall Building. 

To confirm the stability of all the calibrations, additional 

discharge measurements should be collected, particularly in 

flow conditions below 30,000 cubic feet per second. These 

additional measurements should help resolve the discrepancies 

between the SR-ADVM discharge data and that from the other 

two systems. Based on the analysis of the new discharge measurements, there may be changes to the ADVM calibrations 

and in the determination of which ADVM system will provide 

the primary record.

Reliability of the radio-transmission signal between the 

ADVM systems on the Freeport Bridge and the datalogger 

in the Outfall Building continue to be problematic. A comparison of the 15-minute time series telemetered directly 

to the U.S. Geological Survey (USGS) offices by a cellular 

digital packet data system, and the 1-minute data transmitted by radio to the Outfall Building, show that there are 

data-transmission problems between the Freeport Bridge and 

the Outfall Building. The datalogger at the Outfall Building 

records the “selected discharge”. The selected discharge is 

determined through automated error-checking protocols to 

confirm that the incoming data meet quality-control criteria. 

In the event that the primary record (SR-ADVM) data fail to 

meet the criteria, the secondary record (LR-ADVM) data are 

selected automatically. In the event that both records fail to 

meet the quality-control criteria, then the last data value is 

retained. During January and February 2004, 56 percent of the 

selected discharge record was the primary record, 27 percent 

of the selected discharge record was the secondary record, and 

the remaining 17 percent was the retained value. However, 

the USGS record shows that the time series was complete and 

in no instance was the criteria met for rejecting the primary 

record, indicating that the radio transmission was not working 

between the Freeport Bridge and the Outfall Building.

To provide additional time to identify and test a robust 

data-transmission system between the ADVM systems on the 

Freeport Bridge and the datalogger in the Outfall Building, 

the AVM can continue to be operated. However, in the event 

that problems arise with the AVM, the ADVM systems are 

available to provide backup data. During this time, additional 

calibration data should be collected as part of the standard 

operation and maintenance protocol of the station. In the event 

of complete AVM failure prior to the resolution of the datatransmission problems, the ADVM system with the strongest 

calibration and best data-recovery rate should be selected as 

the primary replacement system and the other should be used 

as the secondary system. 
2 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study

Abstract

The Sacramento River at Freeport is a tidally affected 

channel approximately 620 feet wide located at the northern boundary of the Sacramento—San Joaquin River Delta, 

California. In 1978, an acoustic velocity meter was installed at 

Freeport to monitor the flow. The acoustic velocity meter was 

calibrated successfully and has been used continuously since 

that time. Although the calibration has been extremely stable, 

an increasing number of maintenance problems prompted a 

search for alternatives to monitor discharge at this location. 

Two sideward-looking acoustic Doppler velocity meters were 

tested in a pilot study from 2002–2004: a short-range system 

and a long-range system. The pilot study was conducted over 

a wide range of hydrologic conditions and both sidewardlooking acoustic Doppler velocity meters have performed well 

at this location and have been calibrated successfully. As of 

February 2004, the short-range system had a robust calibration 

and a higher data-recovery rate, therefore, it was selected as 

the primary replacement of the acoustic velocity meter, with 

the long-range system providing real-time data redundancy to 

minimize data loss. 

Introduction

The Sacramento—San Joaquin Delta (Delta) is a complex 

network of tidally influenced channels located in the heart 

of California’s Central Valley (fig. 1). More than 20 million 

people depend on the Delta for drinking water, 4.5 million 

acres of cropland are irrigated with Delta water, and several 

endangered fish species reside in or migrate through the Delta. 

Approximately 80 percent of the water entering the Delta 

is conveyed by way of the Sacramento River past Freeport. 

During the study period, daily averaged river flows on the 

Sacramento River at Freeport ranged from approximately 

9,000 cubic feet per second (ft3/s) to 75,000 ft3/s, and tidal 

flows ranged from 300 ft3/s to 77,000 ft3/s. 

The U.S. Geological Survey (USGS) has been using 

acoustic technology to monitor flow in tidally affected areas 

since the late 1970s. Use of acoustic flow meters has expanded 

since that time and the USGS and the California Department 

of Water Resources (CADWR) currently operate more than 

20 stations in the Delta. The use of acoustic technology in 

stream gaging across the United States has been growing, and 

an increasing number of stream-gaging stations are employing 

these approaches (Morlock and others, 2002). Standard “stagedischarge” stream-gaging techniques are not appropriate for 

tidally affected locations because the relationship between 

stage and discharge is variable over time (Laenen, 1985).

Purpose and Scope

The purpose of this report is to present general information regarding acoustic velocity meter (AVM) technology and 

to document the results of a pilot study from September 2002 

to February 2004 investigating flow-monitoring alternatives 

at the Sacramento River at Freeport. The objective of the pilot 

study, done in cooperation with the Sacramento Regional 

Wastewater Treatment Plant (SRWTP) and CADWR, was to 

find a viable alternative to the existing AVM flow-monitoring system at Freeport. The study entailed installation of two 

sideward-looking acoustic Doppler velocity meters (ADVMs) 

and the calibration and maintenance of these systems during 

2002–2004. It is unusual to have three independent systems 

to measure discharge at a single station. The data collected 

from all three systems during the pilot study were analyzed 

and compared to identify the best alternative for monitoring 

discharge at Freeport. In addition, data-transmission equipment and a program to conduct preliminary error checking 

were tested as a part of the study. The resulting discharge data 

are transferred or will be transferred electronically in nearreal time to three agencies: USGS, CADWR, and SRWTP. 

On-going operation and maintenance of the ADVM systems 

are scheduled to become part of the continuing monitoring 

program.

Acknowledgments

We sincerely thank the cooperators who funded this 

study: SRWTP and CADWR. In addition, there were many 

participants who made this study possible. The field staff, 

who have worked many long hours installing, calibrating, 

and troubleshooting the station: Jon Yokomizo, Jim George, 

Curt Battenfeld, Mike Simpson, Bill Brazelton, Laureen 

Fong-Frydendal, Rob Hilditch, and Greg Smith from the 

USGS, and Larry Bezanson from SRWTP. Additional thanks 

to Mike Simpson from the USGS for contributing his thorough 

knowledge of acoustics; he was involved with the installation 

and calibration of the original AVM system and his participation in this project has been invaluable both for understanding the history of the station and for his technical expertise. 

Cathy Munday at the USGS took the cover photograph of 

the Freeport Bridge. Finally, many thanks to the reviewers 

and production staff who have helped with this report: John 

Kemp, Mike Simpson, Robert Meyer, Carmen Lee, Mitchell 

Maidrand, Bill Hendrix, Stephen Ito, Art Hinohosa, Carol 

Sanchez, Phil Contreras, Jeremy Woods, Charlie Kaehler, and 

Kimberly Engelking
����

���������

�������

��������

����

���������

����������

�������������

������

������

������������������

�

������

������

����

�������������������

���

Radio 

Transmission

Path

Environmental

enclosure and

pressure bubbler

Environmental

enclosure

������

�����

����

�����

�����

SR-ADVM

��������

�������

��������

��������

����

��������

������

Existing AVM System

Installed 1978

Environmental

enclosure - with

wire weight and

manometer

LR-ADVM

N

Transducer

 A

Transducer

 B

����������

���������

�������������

�����

����

���

���

���

Rio Vista

Pittsburg

Antioch

Freeport

Sacramento

Lodi

Fairfield

Sacramento

River

R

R

Mokelumne

Dry Cr Cosumnes

� � ��������

� � �������������

�

��

����������

�

��� ���

���

��

���

��

��� ���

Hood

Stockton

San Joaquin

River

Sacramento

River

�

Figure 1. Study location, Sacramento River at Freeport, California. (A) station location map, (B) acoustic Doppler velocity meter mounting system, (C) photo of Freeport Bridge (photo by C. Munday). ADVM, acoustic Doppler velocity meter; LR-ADVM, long-range acoustic 

Doppler velocity meter; SR-ADVM, short-range acoustic Doppler velocity meter; AVM, acoustic velocity meter; ACC9, building name.

Introduction 3
�

Vp

Vp

Vi

45

�

�

�

�

�

Vp = ������������������������������� �

�������������������

� �

�������������������������������������������� ����������������������������������������

Vi = ����������������������

(Vi = Vp / Cos �)

�������������

�������������

Transducer

and mount

Central

processing

unit

����������

Sideward

looking 

ADVM Velocity measured

along acoustic beam

Water-surface

elevation gage

Water-surface

elevation gage

����������

������

����������

�����

�����������

�����������

Figure 2. Schematics of standard acoustic stream-gaging stations. (A) acoustic velocity meter installation and (B) acoustic Doppler 

velocity meter installation. ADVM, acoustic Doppler velocity meter.

4 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study

Principles of Operation of Acoustic 

Flow-Monitoring Systems

Acoustic Velocity Meter

AVMs work on a “time of travel” principle. The AVM 

system is comprised of two transducers that are mounted 

diagonally across a channel and connected to a central 

processing system by cables. An acoustic signal that has a 

component travelling in-line with the water velocity (from 

transducer A to B) will move faster than an acoustic signal that 

has a component travelling against the water velocity (from 

transducer B to A). The water velocity along the acoustic path 

(Vp) is proportional to the difference, in time, required for 

the acoustic signal to travel between the transducers in each 

direction (Accusonic Technologies, 2003). Index velocity (Vi) 

is determined by measuring the difference in the time required 

for an acoustic signal to travel between the two transducers 

and a knowledge of the transducer geometry (specifically the 

distance between the transducers and the angle of the acoustic 

path with respect to the principal flow direction) (fig. 2A).

Acoustic Doppler Velocity Meter

ADVMs utilize a pair of monostatic transducers–

transducers that both send and receive an acoustic pulse. An 

acoustic pulse of a known frequency is sent out into the water 

column along the acoustic beam. A fraction of that acoustic 

pulse then is reflected by small particles in the water back 

to the transducer at a different frequency. The water velocity 

within the acoustic beam (Vi) can be determined based on the 

change in the acoustic frequency and the geometric configuration of the transducers (SonTek Corporation, 2000) (fig. 2B).
Table 1. Comparison of acoustic Doppler velocity meter configuration settings used during the 2002–2004 pilot study at the 

Sacramento River at Freeport, California

[kHz, kilohertz; sec, seconds; ft, feet; m, meter, n/a, not applicable]

 Parameter

Short-range 

acoustic Doppler 

velocity meter

Long-range 

acoustic Doppler 

velocity meter

System configuration

System type Sideward looking Sideward looking

Serial number E600 C420

Sensor frequency 1,500 kHz 500 kHz

Number of beams 2 2

Vertical beam (stage) Yes No

Program configuration

Average interval 40 sec 40 sec

Sample interval 900 sec 900 sec

Cell begin 16.4 ft (5 m) 49.2 ft (15 m)

Cell end 65.6 ft (2 m) 147.6 ft (45 m)

Profiling mode n/a Yes

Number of cells n/a 5

Cell size n/a 16.4 ft (5 m) 

Blanking distance n/a 49.2 ft (15 m)

Equipment Configuration 5

Calculation of Discharge

River discharge is a function of the cross-sectional area 

and the cross-sectionally averaged velocity. However, because 

these parameters cannot be directly measured on a real-time 

basis, surrogate parameters that can be measured easily in the 

field are used and then calibrated to calculate the discharge. In 

tidally affected channels, stage and index velocity are measured directly in the field. Stage is calibrated to determine the 

cross-sectional area. Index velocity is calibrated to determine 

the cross-sectionally averaged velocity. These two values are 

used to calculate discharge as:

 (1)

where Q is discharge, 

 A is cross-sectional area, and

 is cross-sectionally averaged velocity.

Equipment Configuration

During the pilot study, a total of three independent equipment configurations were used to monitor the discharge at 

Freeport: an AVM, a short-range ADVM (SR-ADVM), and a 

long-range ADVM (LR-ADVM). Table 1 compares the configuration settings for the two ADVMs. In addition, a profiling 

ADVM is mounted in a downward looking position to a 

boat-and is used periodically to directly measure the discharge, 

and these data are used to calibrate the equipment (Simpson 

and Oltmann, 1993; Simpson and Bland, 1999; and Simpson, 

2001).

Acoustic Velocity Meter

The AVM system at Freeport is an Ocean Research 

equipment model 7410 Accusonic flowmeter. This AVM system measures index velocity and a Stevens manometer measures water level. The AVM is located approximately 630 feet 

(ft) downstream of the Freeport Bridge. The transducers are 

mounted to aluminum “H” beams driven into the channel bottom and must be serviced by divers. Average hourly data are 

recorded; however, the SRWTP receives high-frequency (every 

minute or less) real-time data from this station to manage their 

effluent stream.

Short-Range Acoustic Doppler Velocity Meter

The SR-ADVM system at Freeport is a SonTek SL-1500. 

This system has an integrated upward-looking stage beam. 

The SR-ADVM is installed on a sliding aluminum pipe mount 

that is bolted to the eastern Freeport Bridge fender (fig. 1) as 

near to the center of the channel as possible to best characterize the flow. This system is programmed to provide 15-minute 

time series; however, to provide higher-frequency data to the 

SRWTP, data are transferred to the Outfall Building as a rolling 5-minute average at 1-minute intervals. In addition, the 

datalogger at the Outfall Building is programmed to check 

for errors. The error-checking protocols are discussed in the 

Outfall Building Datalogger Programming and Error-Checking 

Algorithm section of this report.

Long-Range Acoustic Doppler Velocity Meter

The LR-ADVM system at the Freeport Bridge is a 

SonTek SL-500. Stage is measured with a pressure bubbler 

system. The LR-ADVM is installed with a similar mount as 

the SR-ADVM and is bolted to the western Freeport Bridge 

fender. The LR-ADVM was programmed to sample the 

velocities between 50 ft and 150 ft from the face of the instrument. Although this system is capable of measuring velocities 

approximately 400 ft away from the face of the instrument, 

the sample volume was reduced because of obstructions with 

the Freeport Bridge fender. This system is capable of reporting profile data (or lateral variability in the velocity structure); 

however, for the purposes of calibration and reporting discharge, a bulk average of the entire sample volume is used. 

This system is programmed to provide 15-minute time series; 

however, to provide higher-frequency data to the SRWTP, data 

are transferred to the Outfall Building as a rolling 5-minute 

average at 1-minute intervals. In addition, the datalogger at 

the Outfall Building is programmed to check for errors. The 
�����������������������������

velocity

vectors

�����

���� �� ����

�������������������������������

������

������

��������������

��������������������������

�������������������������������

Figure 3. Discharge measurement from a boat using a profiling downward-looking acoustic Doppler velocity meter.

6 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study

error-checking protocols are discussed in the Outfall Building 

Datalogger Programming and Error-Checking Algorithm section of this report.

Downward-Looking Acoustic Doppler Current 

Profiler 

Periodically discharge at Freeport was directly measured 

using a boat-mounted downward-looking ADVM profiler 

(fig. 3). Discharge measurements were collected over the full 

range of expected flows to fully characterize the index velocity relationship (Simpson, 2001). In addition, full-tidal cycle 

measurements were collected in September and October 2002 

to ensure proper characterization of flood-ebb asymmetry. 

Proof-of-Concept Study

In January 2002, the USGS installed the SR-ADVM 

on the Sacramento River at Freeport (fig. 1) as part of a 

month-long proof-of-concept study. The results of this preliminary study indicate that sideward-looking ADVM technology would be a feasible replacement for the existing AVM. 

Although no calibration measurements were collected during 

the proof-of-concept study, a direct comparison of the measured index velocities (fig. 4) indicates that the SR-ADVM 

successfully monitors velocities in a large, tidally influenced 

channel. The existing AVM has a consistent rating, and discharge measurements have been consistently within 5 percent 

of the calculated flows from the AVM (water year 2001 Sacramento River at Freeport station analysis). Because the AVM 

rating is stable, the index-velocity data collected by the AVM 

were compared directly to the index-velocity data collected 

by the SR-ADVM, and a strong correlation (r2 = 0.990) exists. 

Moreover, ebb-flood asymmetries were not observed on the 

Sacramento River at Freeport.
������ �� ���� ������ � �����

����������� ����������� � �����

������� ����������� ����������� � �����

���������� ����������� �� ������ �����

� � ������� � �� � ������ � � � ������

��� ����� �� ���������� � ������

��� ��� ��� ��� ��� ��� ���

���

���

���

���

���

���

���

���

���

���

���

����������� �������� ������� �������� �����

����� ��������� �� ���� ��� ������

���

����� �������

�

�����

�����

�������

�� �� ���� ��� ����

��

����������� ����

���� �����

Figure 4. Relationship between index velocity measurements collected using a short-range acoustic Doppler velocity meter and an 

acoustic velocity meter. RMS, root-mean-squared.

Calibrations 7

There were two periods of deviation between the AVM 

and the SR-ADVM relation: January 18 at 2245 through 

January 19 at 1315 (14.5 hrs) and February 9 from 0745 

through 0945 (2 hrs). Although the exact cause of these discrepancies is unknown, it was likely a transient obstruction of 

the acoustic beams such as floating debris in the channel.

In August 2002, the LR-ADVM was installed on the 

western Freeport Bridge fender. Stage and index-velocity data 

collection by both ADVM systems began in September 2002. 

Calibration measurements have been collected periodically 

since October 2002.

Calibrations

Area Calibration

The original area calibration was not adjusted during this 

pilot study. Periodic comparisons between the original calibration and subsequent cross-sections suggest that this section 

of the channel is stable and that no calibration is needed. The 

equation describing the relationship is:
����������������

�����������������������

� ��� ��� ��� ��� ��� ��� ���

�

���

���

���

���

���

���

���

���

���������������������������

�������������������������������

���������������������������������������

��������������������������������������

�����������������������

��������������������������������

����������������������������������

������������������������������������

�����������������������������

����������������������������

������������������������������

�

�

����������������������

��������������������

���

Figure 5. Acoustic velocity meter (AVM) calibration relationship. RMS, root-mean-squared.

8 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study

A = 2.269499 * H2 + 131.0353 * H – 25761.21 (2)

where A is the cross-sectional area, and

 H is the water-surface elevation.

Velocity Calibration

In all cases, least-squares regression analysis was used to 

develop the calibration relationships.

Acoustic Velocity Meter

This system was calibrated in 1978 using a “k-curve” 

relationship (Laenen, 1985) based on direct measurements of 

the river flow. No modifications to the calibration relationship have been made since that time. Directly measured check 

measurements are collected regularly and are consistently 

within 5 percent of the calculated discharge time-series (water 

year 2001 Sacramento River at Freeport station analysis). To 

ensure that the site has been stable over time, a recalibration 

effort using the index velocity method was undertaken in 2003 

using discharge measurements collected from March 2000 

to February 2003; a difference of approximately 1 percent 

between the existing and the new calibrations for tidal and 

daily discharge was found. This suggests that the Freeport 

station has been stable over time. The results of the recalibration effort, which included data collected over a wide range of 

hydrologic conditions, show a strong calibration (r2 = 0.998) 

(fig. 5).
����������� ����

����������� �����������

����� �����������

� ��� ��� ��� ��� ��� ��� ��� ��� ���

�

���

���

���

���

���

���

���

���

����� ��������� �� ���� ��� ������

���

�

�������

�� �� ���� ��� ����

��

������ �� ���� ������ � ���

����������� ����������� � �����

������� ����������� ����������� � �����

���������� ����������� �� ������ �����

� � ������ � � � ������

��� ����� �� ���������� � ������

Figure 6. Short-range acoustic Doppler velocity-meter calibration relationship. RMS, root-mean-squared.

Calibrations 9

Short-Range Acoustic Doppler Velocity Meter

The calibration of the SonTek SL-1500 SR-ADVM 

changed slightly since the beginning of the pilot study as 

additional calibration measurements were collected. The latest calibration was developed in June 2003 when additional 

high-flow measurements were incorporated into the calibration (fig. 6). Periodic check measurements have been collected since that time. This system has a strong calibration 

(r2 = 0.997) with data being collected in flow conditions ranging from 1,100 ft3

/s to 72,500 ft3/s. The check measurements 

suggest that the calibration has been stable in the lower and 

middle range and can be extrapolated for high-flow conditions. 

The calculated discharge values were within 5 percent of the 

measured low-to-moderate flows and within 2 percent of the 

measured high-flow data.

Long-Range Acoustic Doppler Velocity Meter

The calibration of the SonTek SL-500 LR-ADVM 

changed slightly since the beginning of the pilot study as 

additional calibration measurements were collected. The latest calibration was developed in June 2003 when additional 

high-flow measurements were incorporated into the calibration (fig. 7). Periodic check measurements have been collected since that time. This system has a strong calibration 

(r2 = 0.996) with data being collected in flow conditions ranging from 1,100 ft3

/s to 72,500 ft3/s. The check measurements 

suggest that the calibration has been stable in the lower and 

middle range and can be extrapolated for high-flow conditions. 

The calculated discharge values were within 7 percent of the 

measured low-to-moderate flows and within 2 percent of the 

measured high-flow data.
����������� ����

����������� �����������

����� �����������

������ �� ���� ������ � ���

����������� ����������� � �����

������� ����������� ����������� � �����

���������� ����������� �� ������ �����

� � ������ � � � ������

��� ����� �� ���������� � ������

� ��� ��� ��� ��� ��� ��� ��� ��� ���

����� ��������� �� ���� ��� ������

�

���

���

���

���

���

���

���

���

���

�

�������

�� �� ���� ��� ����

��

Figure 7. Long-range acoustic Doppler velocity-meter calibration relationship. RMS, root-mean-squared.

10 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study

Outfall Building Datalogger 

Programming and Error-Checking 

Algorithm

The Outfall Building datalogger is programmed to provide a rolling 5-minute average every minute. These 1-minute 

data at the Outfall Building are converted to analog data and 

transferred to the SRWTP through a 4–20 milliamp converter.

Error-checking and programmatic decision-making 

algorithms are used to determine which system is providing 

the best possible data. Because two ADVM flow-monitoring 

systems were installed as part of the pilot study, a datalogger 

was installed at the Outfall Building to perform automated 

error checking. Based on the strong data-recovery rate and the 

calibration relationship, the SR-ADVM has been selected as 

the system providing the “primary record”; the LR-ADVM 

provides the “secondary record”. The purpose of the third 

datalogger programmed with data-quality and error-checking 

algorithms is to minimize data loss and to fully utilize the 

redundancy of the independent stream-gaging instrumentation. 

Two approaches to check for errors are used: percent 

difference and absolute difference. Under most conditions the 

percent difference approach is used; however, in low-flow conditions (less than 1,000 ft3

/s), the absolute difference approach 

is used because high-percent differences may result from 

small denominator values, even though the absolute difference 

between subsequent measurements is small. If the discharge 

is greater than 1,000 ft3/s and the percent difference between 

subsequent measurements is greater than 15 percent, then 

the program automatically selects the discharge value from 

the secondary record (fig. 8, open circles). The program also 

calculates the magnitude of the difference between the current 

measurement and the previously recorded measurement. If the 

discharge is less than 1,000 ft3/s and the absolute difference 
�����

�����

�����

�����

������

������

������

������

������

������

�� � � �� �� ��

������� ���� �������� ����

���������� �� ����� ���� ��� ����

��

�������� ���������

��������� ���� ��������

������� ����

Figure 8. Discharge record recorded at the Outfall Building showing results from the error-checking program.

Outfall Building Datalogger Programming and Error-Checking Algorithm 11

between the two measurements is greater than 500 ft3/s, then 

the program will reject the new value and select the discharge 

value from the secondary record.

In the event that both the primary and secondary record 

values are rejected as “poor-quality data”, the selected 

discharge value is held constant until a good data point is 

received from one of the two ADVM systems. The program 

parameters of 1,000 ft3/s, 500 ft3/s, and 15 percent were 

selected from an analysis of the data. These parameters have 

proven to be conservative enough to not cause large erroneous 

shifts, while allowing for the use of the secondary record when 

the primary record is in question.

Analysis of the data reported to the datalogger at the 

Outfall Building shows that the error-checking protocols are 

working as expected and provide a continuous data record. 

Based on the error-checking algorithm, the program selected 

the secondary discharge a total of 232 times during a 3-week 

period (fig. 8, open circles). There were a total of 11 instances 

that a missing-data indicator was recorded in the final discharge record (fig. 8, solid circles). A closer look at a 2-day 

period shows that the error-checking algorithm is working as 

expected (fig. 9). When the primary record is not available, the 

program switches automatically to the secondary record and 

provides a complete record.
������������������������������������������������������

�

������

������

������

�����

�

������

������

������

�����

�����������������������������������

���������������� ����������������

������������������

��������

�������������������������

���������������������������������������������������������������������

����������������������������������������������������������������������

�

�

Figure 9. (A) Comparison of primary and secondary discharge records and (B) selected discharge record recorded at the Outfall Building, based on an error-checking program.

12 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study

Data Transmission

The data-collection system at Freeport transmits data in 

two ways: (1) cellular digital packet data (CDPD) transmission 

from the two ADVM systems mounted on the Freeport Bridge 

to the USGS; and (2) radio-signal transmission from the two 

ADVM systems mounted on Freeport Bridge to the datalogger 

at the Outfall Building. 

Cellular Digital Packet Data 

This telemetry system has been utilized elsewhere in the 

USGS Delta Flows Network and has proven to be reliable. 

No problems have been identified in this application and 

the USGS continues to receive reliable real-time data from 

the ADVMs mounted on the Freeport Bridge. Although the 

vendor is phasing out the CDPD system, the USGS is testing another digital data transmission system elsewhere in the 

Delta Flows Network: code division multiple access (CDMA) 

Digital Cellular. We do not anticipate significant problems 

launching the CDMA system.

Radio Signal

Radios have been used under different conditions elsewhere in the USGS Delta Flows Network without substantial 

problems; however, with the SRWTP’s requirement of data 

transfer to the Outfall Building every minute have proven 

radio data transmission to be unreliable. Numerous attempts at 

improving the data-recovery rate at the Outfall Building have 

been unsuccessful and currently it is unclear if interference is 

from the Freeport Bridge structure, is electromagnetic, or if 

other problems are degrading the performance of this system. 

The problems associated with this approach suggest the need 

for a different communication system for the long-term implementation of a flow-monitoring station on the Freeport Bridge. 

A comparison of the 15-minute time series received 

through the CDPD system at the USGS and the 1-minute data 

recorded as “selected discharge” at the SRWTP Outfall Building show that there are substantial data-transmission problems 

between the Freeport Bridge and the Outfall Building (fig. 10). 

During January and February 2004, 56 percent of the “selected 

discharge” record was from the primary system (SR-ADVM), 

27 percent of the selected discharge record was from the 
���������

�

������

������

������

������

������

������

������

������

����������������������������������������������

�����������������������������������������������

������������ �������������

��������������������������������������������������������������������

�����������������������������������

�����������������������������������������������������

��������������������������������������������������������

�����������������������������������

Figure 10. Comparison of short-range acoustic Doppler velocity-meter record received at the U.S. Geological Survey office and selected 

discharge record recorded at the Outfall Building.

Results 13

secondary system (LR-ADVM), and the remaining 17 percent 

was the retained value. However, the USGS record shows that 

the primary-record time-series is complete and in no instance 

is the criteria for rejecting the primary record (discharge 

below 1,000 ft3/s with a change of more than 500 ft3/s between 

subsequent discharge measurements or a change of more than 

15 percent between subsequent discharge measurements) met. 

This result indicates that the radio transmission between the 

Freeport Bridge and the Outfall Building was not working. 

The high-frequency variability in the 1-minute time-series 

record at the Outfall Building is due to the shorter averaging 

time; transitions between the primary and secondary records; 

and data-transmission problems.

Results

It is unusual to have three, independent flow-monitoring 

systems at a single location. Inevitably, even small differences 

among the results lead to questions about the overall performance of each of the instrumentation configurations. Each 

system has a strong calibration (r2 values ranging from 0.996 

to 0.998), and all three systems performed well during the 

17-month period of the pilot study (fig. 11). A comparison of 

the resulting discharge calculations shows that the difference 

between the discharge time-series datasets is less than 5 percent overall (table 2).
������������

���� ��� ��� ��� ��� ��� ��� ��� ��� ���� ���� ��� ���� ��� ��� ��� ��� ���

�

������

������

������

������

�����������������������������������

���������������������������������������������������������

�

������

������

������

������

��������������������������������������������������������

�

������

������

������

������

���������������������������������

���� ���� ����

Figure 11. Tidal discharge data collected from the three hydroacoustic systems during the 2002–2004 pilot study. (A) short-range acoustic Doppler velocity meter; (B) long-range acoustic Doppler velocity meter; and (C) acoustic velocity meter.

14 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study
Table 2. Differences among calculated discharge records from three hydroacoustic stream-gaging systems during the 

2002–2004 pilot study at the Sacramento River at Freeport, California

[ft3/s, cubic feet per second; AVM, acoustic velocity meter; SR-ADVM, short-range acoustic Doppler velocity meter; LR-ADVM, long-range 

acoustic Doppler velocity meter]

 Systems compared

Tidal flows Daily flows

Median percent

difference

Median difference

(ft3/s)

Median percent

difference

Median difference 

(ft3/s)

AVM and SR-ADVM 4.77 1120 4.41 780

AVM and LR-ADVM 3.52 740 2.70 460

SR-ADVM and LR-ADVM 3.98 830 4.17 840

Systems compared

Tidal flows, in percent Daily flows, in percent

Within 5 

percent

Within 10 

percent

Within 15 

percent

Within 5 

percent

Within 10 

percent

Within 15 

percent

AVM and SR-ADVM 62 87 96 61 99 >99

AVM and LR-ADVM 82 96 99 73 99 >99

SR-ADVM and LR-ADVM 96 >99 >99 61 97 >99

Table 3. Record comparisons of hydroacoustic stream-gaging systems used during the 2002–2004 pilot study at the 

Sacramento River at Freeport, California

[AVM, acoustic velocity meter; SR-ADVM, short-range acoustic Doppler velocity meter; LR-ADVM, long-range acoustic Doppler velocity 

meter; >, greater than]

Results 15

The median percent difference (MPD) was determined as 

the median of the absolute value of the difference between the 

two time-series records divided by the first time-series record. 

For example, for the comparison of the AVM and the SRADVM, the MPD was calculated as follows:

MPD = median [abs ( AVM

AVM SR ADVM

Q

Q � Q � 100* )]; (3)

where MPD is the median percent difference,

 Q is the discharge, 

and the subscript describes the equipment used to collect the 

data. The median difference was determined as the median of 

the absolute value of the difference between the two timeseries records.

Overall, each independent stream-gaging system compares well with the others (table 3). The largest discrepancies are between the daily flows measured by the SR-ADVM 

system and the other two systems, with 61 percent of the 

data being reported within 5 percent. The best correlation is 

between the tidal flows measured by the two ADVM systems, 

with 96 percent of the data being reported within 5 percent. 

The USGS has the following standards for categorizing 

discharge records: A record is classified as “excellent” when 

95 percent of the daily discharge data are within 5 percent of 

the real value; “good” when 95 percent of the data are within 

10 percent of the real value; “fair” when 95 percent of the data 

are within 15 percent of the real value; and “poor” if the data 

do not meet these criteria (Kennedy, 1983).

Based on the initial calibration results and data-recovery 

rates, the data collected by the SR-ADVM were selected as the 

primary record and the LR-ADVM data were classified as the 

secondary record. Comparisons between the daily-flow data 

from the two ADVM systems show that, in general, the data 

from the SR-ADVM and the LR-ADVM compared very well 

until March 2003 when a shift occurred and the SR-ADVM 

tended to report higher discharge values than the LR-ADVM 

and the AVM (fig. 12). After March 2003, the SR-ADVM 

reported daily discharge values approximately 800 ft3/s, or 

4.6 percent higher than the AVM and the LR-ADVM when 

flows were less than 30,000 ft3/s (fig. 12B, C). These higher 

discharge values reported by the SR-ADVM may have been 

caused by a slight change in the SR-ADVM alignment. The 

SR-ADVM was found to be rotated approximately 5 degrees 

from the original position and was repositioned in June 2004. 

In December 2003, the LR-ADVM reported higher discharge 

values than the SR-ADVM. It is unclear what caused this 

change in the relationship between these two instruments; it 

may be due to the high flows recorded at the end of the study 

period. A similar, though less dramatic, change in the relationship occurred during high flows in early 2003. 

Similar comparisons between the SR-ADVM and the 

AVM and the LR-ADVM and the AVM show greater differences between the instruments during periods of high flow 
������������

�

�

�

��� ������� �������

������������������

���������������������

������������������������

����������

������������������������

������

������

������

������

�

�����

�����

�����

�

������

������

������

���� ��� ��� ��� ��� ��� ��� ��� ��� ���� ���� ��� ���� ��� ��� ��� ��� ���

���� ���� ����

��

���

�

� �������

� ���

� ��� � �������

� ���

� ��� � �������

� �������

� �������

� ������� � ��� � ������� � ��� � ������� � �������

Figure 12. Comparison of tidally averaged discharge data collected from the three hydroacoustic systems during the 2002–2004 pilot 

study. (A) discharge records, (B) difference between discharge records, and (C) percent difference between discharge records. AVM, 

acoustic velocity meter; SR-ADVM, short-range acoustic Doppler velocity meter; LR-ADVM, long-range acoustic Doppler velocity meter; 

Q, discharge.

16 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study
Table 4. Statistical comparison of measured and calculated discharge values during the 2002–2004 pilot study at the Sacramento 

River at Freeport, California

[ft3/s, cubic feet per second; SR-ADVM, short-range acoustic Doppler 

velocity meter; LR-ADVM, long-range acoustic Doppler velocity meter; 

AVM, acoustic velocity meter]

Hydroacoustic 

system

Mean

difference 

(ft3/s)

Median

difference 

(ft3/s)

Standard 

deviation

(ft3/s)

SR-ADVM 95 100 890

LR-ADVM –559 –169 1,179

AVM 805 611 1,268

Conclusions 17

(fig. 12). Both of the ADVM instruments measured lower 

discharge during the periods of high flow suggesting that 

the AVM is overestimating discharges above 60,000 ft3/s by 

3,000–4,000 ft3/s (fig. 12B); comparison of the percent differences over the entire period of record shows that the percent 

difference is relatively consistent between periods of low flow 

(fall 2003) and high flow (spring 2003) (fig. 12C).

The best method to assess the accuracy of the various 

instrumentation systems is to compare the calculated discharge 

results from the three hydroacoustic instruments with the 

direct discharge measurements collected with a downwardlooking ADVM profiler (table 4). The SR-ADVM showed 

the least difference between the measured and calculated 

discharge values, as measured by the mean and median differences, and had the lowest variability, as measured by the 

standard deviation. The LR-ADVM tends to be slightly biased 

towards underestimating the discharge and the AVM tends to 

be biased towards overestimating the discharge. The standard 

deviations for the LR-ADVM and the AVM are higher than 

those for the SR-ADVM.

The higher discharge values that are apparent when 

comparing the time-series data of the SR-ADVM with that 

of the AVM and LR-ADVM (fig. 12) are not apparent when 

comparing the SR-ADVM results with measured data (fig. 6). 

The low-flow period in 2002, when most of the low-flow calibration measurements were collected, shows good agreement 

among all three systems. However, beginning in March 2003, 

the SR-ADVM consistently reported higher discharge results 

than the other two systems. The cause of this offset is unclear; 

however, the check measurements do not indicate that there 

are any substantial problems with the SR-ADVM calibration. 

All three stations have high data-recovery rates. During 

the period of study, the SR-ADVM had a 97-percent complete 

record, the LR-ADVM had a 91-percent complete record, and 

the AVM system had a 99-percent complete record. Most of 

the missing data associated with the ADVM systems were lost 

during the first 2 months of the pilot study when programming problems caused several periods of data to be lost. The 

programming problems have been addressed and data loss has 

declined significantly. Since November 2002, the SR-ADVM 

has a 99-percent complete record and the LR-ADVM has a 

95-percent complete record.

Conclusions

The acoustic velocity meter (AVM), the short-range 

acoustic Doppler velocity meter (SR-ADVM), and long-range 

acoustic Doppler velocity meter (LR-ADVM) configurations 

have been calibrated successfully, with strong relationships 

with the measured data. Although the AVM system provides 

high-quality data and has a strong calibration (r2=0.998) and a 

high data-recovery rate (99 percent during the 2002–2004 pilot 

study), the system is more than 25 years old and operation and 

maintenance costs are increasing substantially. It is becoming 

more difficult to obtain replacement parts and, in some cases, 

the parts are no longer manufactured. In addition, data recovery had been a problem prior to the pilot study, with several 

periods of missing data in fall 2000.

Both sideward-looking ADVM systems are viable alternatives to the existing AVM system. Moreover, maintenance 

of the ADVM equipment is easier because the equipment can 

be reached without diver assistance and replacement parts are 

readily obtainable. The SR-ADVM is the best replacement 

system based on its calibration (r2=0.997) and data-recovery 

rate (97 percent). However, check measurements collected 

in flow conditions below 30,000 ft3/s should be conducted 

to confirm the stability of the calibration and to resolve the 

apparent discrepancy between the SR-ADVM and the other 

two systems. In addition, there continue to be significant problems with data transmission between the ADVM systems on 

the Freeport Bridge and the datalogger in the Outfall Building.

To provide time to identify and test a robust datatransmission system, the AVM can continue to be operated 

as the primary system. However, in the event that problems 

arise with the AVM, the ADVM systems are available to 

provide backup data. During this time, additional calibration 

data should be collected as part of the standard operation and 

maintenance protocol of the station. In the event of complete 

AVM failure prior to the resolution of the data-transmission 

problems, the ADVM system with the strongest calibration 

and best data recovery rate should be selected as the primary 

replacement system. 
References

Accusonic Technologies, 2003, Multipath flowmeter systems 

theory and operating principle: East Falmouth, Mass., 4 p. 

Accessed June 4, 2003, at URL http://www.accusonic.

com/technology.html

Kennedy, E.J., 1983, Computation of continuous records of 

streamflow: U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. A13, 53 p.

Laenen, A, 1985, Acoustic velocity meter systems: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A17, 38 p. 

Morlock, S.E., Nguyen, H.T., and Ross, J.H., 2002, Feasibility 

of acoustic Doppler velocity meters for the production of 

discharge records from U.S. Geological Survey streamflowgaging stations: U.S. Geological Survey Water-Resources 

Investigations Report 01-4157, 56 p.

Simpson, M.R., 2001, Discharge measurements using a broadband acoustic Doppler current profiler: U.S. Geological 

Survey Open-File Report 01-1, 134 p.

Simpson, M.R. and Bland, Roger, 1999, Techniques for 

accurate estimation of net discharge in a tidal channel: IEEE 

Sixth Working Conference on Current Measurement, San 

Diego, California, 1999, Proceedings, p. 125-130.

Simpson, M.R., and Oltmann, R.N., 1993, Dischargemeasurement system using an acoustic Doppler current 

profiler with applications to large rivers and estuaries: 

U.S. Geological Survey Water-Supply Paper 2395, 32 p.

SonTek Corporation, 2000, SonTek ADVM-series instruments 

technical documentation: San Diego, California, 77 p.

18 Investigation of Hydroacoustic Flow-Monitoring Alternatives at the Sacramento River at Freeport: Results of Pilot Study