Application of Acoustic and Optic Methods for 

Estimating Suspended-Solids Concentrations in 

the St. Lucie River Estuary, Florida

By Eduardo Patino and Michael J. Byrne

Prepared in cooperation with the 

SOUTH FLORIDA WATER MANAGEMENT DISTRICT

Scientific Investigations Report 2004-5028

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:

Patino, Eduardo, and Byrne, M.J., 2004, Application of acoustic and optic methods for estimating suspendedsolids concentrations in the St. Lucie River Estuary, Florida: U.S. Geological Survey Scientific Investigations Report 

2004-5028, 23 p.
iii

Contents

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

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

Purpose and Scope ............................................................................................................................. 3

Description of Study Area .................................................................................................................. 3

Acknowledgments ............................................................................................................................... 3

Methods of Investigation and Data Analysis ........................................................................................... 3

Measurement of Acoustic Backscatter Strength .......................................................................... 3

Measurement of Turbidity .................................................................................................................. 4

Sampling and Determination of Suspended-Solids Concentrations ........................................... 4

Collection of Tidal Stage, Discharge, Salinity, and Temperature Data ....................................... 4

Analysis of Laboratory and Field Data ............................................................................................. 7

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods .................... 7

Suspended-Solids Concentration to Acoustic Backscatter Strength Relation ........................ 7

North Fork Site ............................................................................................................................ 7

Speedy Point Site ........................................................................................................................ 9

Suspended-Solids Concentration to Turbidity Relation .............................................................. 12

North Fork Site .......................................................................................................................... 12

Speedy Point Site ...................................................................................................................... 13

Qualification of Empirical Equations .............................................................................................. 13

Sensitivity Analysis ............................................................................................................................ 14

Time-Series Estimates of Mean Suspended-Solids Concentrations ........................................ 20

Summary and Conclusions ........................................................................................................................ 22

References Cited ........................................................................................................................................ 23

FIGURES

 1. Map showing location of monitoring sites in the St. Lucie River Estuary study area ...... 2

 2. Diagram showing instrument setup for the monitoring sites in the St. Lucie River 

Estuary ........................................................................................................................................... 4

 3-6. Photographs showing:

 3. Extent of biofouling of turbidity and salinity probes between field maintenance 

visits at a monitoring site in the St. Lucie River Estuary ............................................ 5

 4. Sampling equipment used for suspended-solids measurements ............................ 5

 5. Collection of an integrated water sample for suspended-solids concentration 

analysis ............................................................................................................................... 6

 6. A monitoring site (Steele Point) in the St. Lucie River Estuary ................................. 6

 7-10. Graphs showing:

 7. Measured to estimated suspended-solids concentration relation at the 

North Fork site using acoustic backscatter strength data ...................................... 10

 8. Measured to estimated suspended-solids concentration relation at the 

Speedy Point site using acoustic backscatter strength data ................................. 12

 9. Suspended-solids concentration to turbidity relation at the North Fork site ....... 13

 10. Suspended-solids concentration to turbidity relation at the Speedy Point site .. 14
iv

 11-14. Graphs showing distribution of parameter data and residuals used in the 

suspended-solids concentration to:

 11. Acoustic backscatter strength regression analysis at the North Fork site .......... 15

 12. Acoustic backscatter strength regression analysis at the Speedy Point site ..... 16

 13. Turbidity regression analysis at the North Fork site ................................................. 17

 14. Turbidity regression analysis at the Speedy Point site ............................................. 18

 15-20. Graphs showing:

 15. Acoustic backscatter strength to suspended-solids concentration relation 

at the North Fork and Speedy Point sites ................................................................... 19

 16. Salinity to suspended-solids concentration relation at the North Fork and 

Speedy Point sites .......................................................................................................... 19

 17. Temperature to suspended-solids concentration relation at the North Fork 

and Speedy Point sites ................................................................................................... 20

 18. Effect of temperature on acoustic backscatter strength ......................................... 20

 19. Local point to mean cross-sectional relation for suspended-solids 

concentration at the North Fork site ............................................................................ 21

 20. Local point to mean cross-sectional relation for suspended-solids 

concentration at the Speedy Point site ....................................................................... 21

 21. Time-series plot showing discharge and mean cross-sectional suspended-solids

concentrations at the North Fork site .................................................................................... 22

TABLES

 1. Point sample data from the North Fork site used for regression analyses ........................ 8

 2. Point sample data from the Speedy Point site used for regression analyses ................. 10

 3. Suspended-solids concentrations for the North Fork and Speedy Point sites used 

to develop local to mean cross-sectional relations ............................................................. 22
v

Conversion Factors, Acronyms, Abbreviations, 

and Datums

Multiply By To obtain

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2) 2.59 square kilometer

Acronyms

ABS Acoustic backscatter strength

ADCP Acoustic Doppler current profiler

ADVM Acoustic Doppler velocity meter

EDI Equal discharge increment

GOES Geostationary Operational Environmental Satellite

NWIS National Water Information System

PVC Polyvinyl chlorinated

SJRWMD St. Johns River Water Management District

SFWMD South Florida Water Management District

SWIM Surface-Water Improvement and Management

USGS U.S. Geological Survey

Water-Quality Abbreviations

mg/L milligrams per liter

NTU nephelometric turbidity units

ppt parts per thousand

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 1929); horizontal coordinate information is referenced to the North American Datum of 

1983 (NAD83).
vi
Abstract

Acoustic and optic methods were applied to estimate 

suspended-solids concentrations in the St. Lucie River Estuary, 

southeastern Florida. Acoustic Doppler velocity meters were 

installed at the North Fork, Speedy Point, and Steele Point 

sites within the estuary. These sites provide varying flow, 

salinity, water-quality, and channel cross-sectional characteristics. The monitoring site at Steele Point was not used in 

the analyses because repeated instrument relocations (due to 

bridge construction) prevented a sufficient number of samples 

from being collected at the various locations. Acoustic and 

optic instruments were installed to collect water velocity, 

acoustic backscatter strength (ABS), and turbidity data that 

were used to assess the feasibility of estimating suspendedsolids concentrations in the estuary. Other data collected at the 

monitoring sites include tidal stage, salinity, temperature, and 

periodic discharge measurements.

Regression analyses were used to determine the relations 

of suspended-solids concentration to ABS and suspendedsolids concentration to turbidity at the North Fork and Speedy 

Point sites. For samples used in regression analyses, measured 

suspended-solids concentrations at the North Fork and Speedy 

Point sites ranged from 3 to 37 milligrams per liter, and 

organic content ranged from 50 to 83 percent. Corresponding 

salinity for these samples ranged from 0.12 to 22.7 parts per 

thousand, and corresponding temperature ranged from 19.4 

to 31.8 °C. Relations determined using this technique are site 

specific and only describe suspended-solids concentrations 

at locations where data were collected. The suspended-solids 

concentration to ABS relation resulted in correlation coefficients of 0.78 and 0.63 at the North Fork and Speedy Point 

sites, respectively. The suspended-solids concentration to turbidity relation resulted in correlation coefficients of 0.73 and 

0.89 at the North Fork and Speedy Point sites, respectively. 

The adequacy of the empirical equations seems to be limited 

by the number and distribution of suspended-solids samples 

collected throughout the expected concentration range at the 

North Fork and Speedy Point sites. Additionally, the ABS 

relations for both sites seem to overestimate at the low end and 

underestimate at the high end of the concentration range. 

Based on the sensitivity analysis, temperature had a 

greater effect than salinity on estimated suspended-solids 

concentrations. Temperature also appeared to affect ABS data, 

perhaps by changing the absorptive and reflective characteristics of the suspended material. Salinity and temperature had no 

observed effects on the turbidity relation at the North Fork and 

Speedy Point sites.

Estimates of suspended-solids concentrations using ABS 

data were less “erratic” than estimates using turbidity data. 

Combining ABS and turbidity data into one equation did 

not improve the accuracy of results, and therefore, was not 

considered.

Introduction

The Indian River Lagoon extends about 156 mi along 

the central-east coast of Florida. The St. Lucie River Estuary 

is a major tributary at the southern end of the lagoon (fig. 1). 

Drainage modifications and increased land use in the St. Lucie 

watershed have substantially increased wet-season flows and 

reduced dry-season flows entering the estuary. Anthropogenic alterations have caused a high volume of stormwater 

discharges, producing large salinity fluctuations and sedimentation rates. The transport of these sediments as suspended 

load decreases light penetration with increased turbidity, and 

deposition of the sediments creates layers of fine-grained, 

nutrient-rich muck within the estuary. These processes have 

contributed to detrimental effects to the seagrass communities 

and overall health of the estuarine system. To restore and protect environmental resources of the Indian River Lagoon and 

the St. Lucie River Estuary, the South Florida Water Management District (SFWMD) and the St. Johns Water Management 

District (SJWMD) initiated the Indian River Lagoon SurfaceWater Improvement and Management (SWIM) Plan (Steward 

and others, 1993). 

Application of Acoustic and Optic Methods for 

Estimating Suspended-Solids Concentrations in the 

St. Lucie River Estuary, Florida

By Eduardo Patino and Michael J. Byrne

Introduction 1
Figure 1. Location of monitoring sites in the St. Lucie River Estuary study area.

2 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Improving techniques to calculate sediment loads in these 

environments on a continuous basis will increase the accuracy of records and enhance the overall understanding of the 

estuarine system. Specifically, managers need to accurately 

determine suspended-solids loads entering the estuary, suspended-solids transport and deposition characteristics within 

the estuary, and the relation of suspended-solids transport to 

tidal flows and freshwater inflows. In 1999, the U.S. Geological Survey (USGS), in cooperation with the SFWMD, began 

an investigation to test the use of data from acoustic and optic 

instruments as surrogates for the estimation of suspendedsolids concentrations. Turbidity sensors were installed at an 

existing flow and salinity monitoring network in the St. Lucie 

River Estuary system where acoustic instruments were already 

in place. Water samples were collected to determine suspended-solids concentrations at the North Fork, Speedy Point, 

and Steele Point monitoring sites (fig. 1). The existing monitoring network is the result of a cooperative effort between the 

USGS and SFWMD to provide hydrologic and water-quality 

data for the estuary.

Purpose and Scope

This report describes the feasibility of using acoustic and 

optic methods to improve the accuracy of estimating highly 

organic suspended-solids concentrations in the St. Lucie River 

Estuary. Acoustic backscatter strength (ABS), turbidity, salinity, and temperature data are presented and analyzed, as well 

as data from water samples collected for laboratory determination of suspended-solids concentrations. Regression analyses 

are performed to determine the suspended-solids concentration 

to ABS relation and the suspended-solids concentration to 

turbidity relation. Empirical and site-specific estimation models are presented for suspended-solids concentrations using 

time-series data from acoustic and optic instruments. Relations 

between point and mean cross-sectional suspended-solids concentrations also are addressed as a necessary step for calculating time series of mean suspended-solids concentrations.

Description of Study Area

The St. Lucie River Estuary supports a large variety of 

plants and animals including seagrass, marine fish, and invertebrates. The estuary is located in parts of Martin and St. Lucie 

Counties and has two major forks—the North Fork and the 

South Fork (South Florida Water Management District, 1999, 

p. 2). Five tributaries to the estuary provide drainage for a 

watershed that encompasses about 820 mi2. Ten Mile Creek 

is a major tributary to the North Fork; the C-23 and C-24 

canals drain agricultural and residential lands and discharge 

freshwater into the estuary through control structures S-48 and 

S-49; Old South Fork is a major tributary to the South Fork; 

and St. Lucie Canal (C-44) connects Lake Okeechobee to the 

South Fork (fig. 1). The St. Lucie Canal also provides drainage 

for agricultural lands and Lake Okeechobee through a series 

of gated spillways and a lock chamber (for boat traffic) at 

structure S-80.

The study was conducted at three monitoring sites 

representing different flow magnitudes, water-quality, and 

channel cross-sectional characteristics. The North Fork site is 

just north of Kellstadt Bridge along Port St. Lucie Boulevard 

in southern St. Lucie County; the Speedy Point site is beneath 

Roosevelt Bridge along U.S. Highway 1 in northern Martin 

County; and the Steele Point site is beneath Evans Crary 

Bridge along the coast in northern Martin County (fig. 1)

Acknowledgments

The authors are grateful to Gordon Hu from the SFWMD 

for his input throughout the study. Special thanks is extended 

to the SFWMD staff at the Martin/St. Lucie Service Center for 

use of their facilities.

Methods of Investigation and Data 

Analysis

Instrument installation and field data collection and 

analysis were performed similarly at the North Fork, Speedy 

Point, and Steele point monitoring sites. Data collected at the 

Steele Point site, however, could not be used in the analysis 

because multiple instrument relocations due to bridge construction resulted in inconsistent data. All field data except 

suspended-solids concentration were recorded at 15-minute 

intervals. Water samples for determination of suspended-solids 

concentration were collected on a monthly and event-driven 

basis. A typical instrument setup for water sampling at the 

monitoring sites in the St. Lucie River Estuary is shown in 

figure 2.

Measurement of Acoustic Backscatter Strength

Acoustic Doppler velocity meter (ADVM) instruments 

were used to measure an index of the mean water velocity at 

the monitoring sites and to collect data pertaining to the ABS 

received by the sensor. The instruments were placed at a fixed 

location vertically (about middepth) and measured a section 

about 19 ft in length, between 6 and 25 ft from the ADVM 

transducer faces. The ADVM produces a short pulse of sound 

at a known frequency, which is reflected (scattered) in all 

directions by particulate matter in the water. The acoustic 

return signal received by the ADVM has a frequency change 

(Doppler shift) proportional to the velocity of the scattering 

material (SonTek, 1997). The water velocity measured by the 

ADVM represents an index velocity that has a direct relation 

to the mean channel velocity.

ABS is a measure of the strength of acoustic return 

signals reflected from material in suspension within the body 

of water measured, and is reported in counts. The St. Lucie 

River Estuary is an environment that promotes extensive 

biological growth on all submerged structures, including 

the instruments at the monitoring sites. Biofouling of the 

Methods of Investigation and Data Analysis 3
transducer faces of the ADVM can substantially affect ABS 

measurements, and periodic cleaning of transducer faces 

is necessary to maintain accuracy. Corrections can be used 

to account for decreasing signal strength due to biological 

growth, but these connections usually decrease the accuracy 

of the estimates, as described later for the North Fork site.

Measurement of Turbidity

Turbidity was measured at two depths at the Speedy Point 

site to document any vertical stratification in the distribution 

of suspended solids, and at only one depth at the North Fork 

site because the stream at this site appears to be well mixed. 

Turbidity is recorded in nephelometric turbidity units (NTU), 

which is a measure of suspended-solids content (cloudiness) 

in water. Instruments used in this study measure turbidity 

by projecting a light beam into a volume of water and then 

measuring the light scattered off suspended particles present in 

the water (YSI Incorporated, 1998). Biofouling was observed 

on turbidity and salinity probes (fig. 3); therefore, frequent 

field visits for instrument maintenance and calibration were 

necessary.

Sampling and Determination of SuspendedSolids Concentrations

Water samples were collected to represent local and mean 

cross-sectional suspended-solids concentrations using point 

and depth-integrated (top to bottom) sampling techniques. 

Local space, in this report, represents an area that is at the 

altitude of the ADVM transducer and about 9 ft from the 

transducer toward the center of the stream. Local suspendedsolids concentrations were measured by collecting water 

samples using a horizontal Van Dorn bottle type sampler. 

Cross-sectional concentrations were measured by collecting 

depth-integrated samples using the equal discharge increment 

method (EDI) and the DH-59 sampler as described by the U.S. 

Interagency Committee on Water Resources, Subcommittee on 

Sedimentation (1965). The two samplers used in the study are 

shown in figure 4. All water samples were collected in accordance with USGS prescribed methods (Edwards and Glysson, 

1988). Collection of a water sample using the DH-59 sampler 

is depicted in figure 5.

Traditional laboratory techniques used in the determination of total suspended-solids concentrations were modified to 

address potential bias in the results as compared to those used 

for suspended-sediment concentration analyses (Gray and 

others, 2000). Analyses performed at the USGS Ocala Laboratory in Florida were modified to process: (1) entire water 

samples rather than aliquots from samples; and (2) all bottles 

from depth-integrated cross-sectional samples. The suspendedsolids concentrations were obtained by drying sample contents 

at 105 °C. The volatile component was determined by baking 

the contents at 500 °C and subtracting the inorganic residue 

from the suspended-solids concentration value. To avoid misinterpretation, laboratory results are presented as suspendedsolids concentrations in this report.

Collection of Tidal Stage, Discharge, Salinity, 

and Temperature Data

Tidal stage, salinity, and temperature data were collected in 15-minute intervals at the North Fork and Speedy 

Point sites, and miscellaneous discharge measurements 

were taken for velocity calibrations and rating development. 

All 15-minute data were recorded and transmitted every 4 

hours by way of Geostationary Operational Environmental 

Satellite (GOES) into the USGS National Water Information 

System (NWIS) database. A monitoring site (Steele Point) in 

the St. Lucie River Estuary is shown in figure 6.

Figure 2. Instrument setup 

for the monitoring sites in 

the St. Lucie River Estuary.

4 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Figure 3. Extent 

of biofouling of 

turbidity and 

salinity probes 

between field 

maintenance visits 

at a monitoring 

site in the St. Lucie 

River Estuary.

Figure 4. Sampling 

equipment used for 

suspended-solids 

measurements.

Methods of Investigation and Data Analysis 5
Figure 5. Collection 

of an integrated (top to 

bottom) water sample 

for suspendedsolids concentration 

analysis.

Figure 6. A monitoring site (Steele 

Point) in the St. Lucie 

River Estuary. ADVM 

is acoustic Doppler 

velocity meter.

6 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Tidal stage data were recorded to determine water depth 

and to calculate the stage-dependent cross-sectional area 

needed for discharge computations. These data were collected 

using an incremental shaft encoder attached to a pulley that 

suspends a stainless-steel tape, weight, and float inside an 

8-in-diameter polyvinyl chlorinated (PVC) pipe stilling well. 

Discharge measurements were performed using a boatmounted acoustic Doppler current profiler (ADCP). From 

these measurements, velocity ratings were developed to compute time-series records of discharge at the monitoring sites.

Salinity and temperature were recorded as water-quality 

characteristics to determine changes in water density. Because 

water density directly affects the speed and absorption of 

sound, density changes within the volume of water measured 

by the ADVM can affect the velocity and ABS values reported 

by the instrument.

Analysis of Laboratory and Field Data

As previously mentioned, suspended-solids concentration 

was determined at the USGS Ocala Laboratory, and temperature and salinity were determined in the field. Suspendedsolids concentrations for samples used in regression analyses 

(described later) ranged from 3 to 37 mg/ L, and organic 

content ranged from 50 to 83 percent. The suspended-solids 

concentrations and organic content varied depending on the 

monitoring site location and flow conditions at the time of 

sampling. Observed data suggest that no substantial temperature stratification occurs at the monitoring sites, and substantial salinity stratification occurs only during large freshwater 

releases from structures S-48, S-49, and S-80 along the C-23, 

C-24, and C-44 canals, respectively (fig. 1). For the samples 

just mentioned, corresponding salinity ranged from 0.12 to 

22.7 ppt depending on the magnitude and duration of freshwater releases from structures S-48, S-49, and S-80; water 

temperature ranged from 19.4 to 31.8 °C.

Estimation of Suspended-Solids 

Concentrations Using Acoustic and 

Optic Methods

The collection of water samples for the computation 

of suspended-solids transport is a labor intensive and costly 

process. Additionally, no direct correlation exists between 

suspended-solids concentration and discharge in tidal streams; 

the magnitude and chemical composition of freshwater inflow 

affect suspended-solids concentration because of flocculation 

of dissolved organics. For these reasons, it is advantageous 

to have a reliable method for estimating suspended-solids 

concentration using surrogate parameters. Measuring surrogate 

parameters such as ABS and turbidity, and developing relations between these and suspended-solids concentration, can 

improve the accuracy of suspended-solids transport estimates 

as well as increase temporal resolution of the data by allowing 

for the computation of time-series records.

The equations used in this study to test the feasibility of 

using ABS and turbidity data to estimate suspended-solids 

concentrations are as follows:

 Suspended solids = 10ABS [a + b log(salinity) 

+ c log(temperature)] + d, (1)

and

 Suspended solids = a ́ Turbidity + b ́, (2)

where a, b, c, d, a ́, and b ́ are regression coefficients, and 

ABS, salinity, temperature, and turbidity are field measured 

parameters. Equation 1 is exponential because acoustic data 

are reported in counts (a logarithmic, internal instrument unit), 

and regression analyses using ABS data must be performed 

with all raw linear data converted to a logarithmic scale.

The use of ABS and turbidity data collected at the 

monitoring sites in the St. Lucie River Estuary for estimating suspended-solids concentrations with equations 1 and 2 

is summarized herein. A description of obtained ABS and 

turbidity relations is followed by a discussion on the quality 

of empirical equations and a sensitivity analysis describing the 

response that each relation shows to certain field parameters. 

Combining ABS and turbidity data into one equation did not 

improve the accuracy of results, and therefore, is not used nor 

described herein. A sample of time-series estimates of suspended-solids concentrations also is presented. The computation of suspended-sediment transport (loads) is not presented 

because of pending verification of estimating equations used 

in the study and the large amount of missing or insufficient 

field data.

Suspended-Solids Concentration to Acoustic 

Backscatter Strength Relation

Multivariable regression techniques were used to determine the best possible relation of suspended-solids concentration to ABS at the North Fork and Speedy Point sites. Salinity 

and temperature were included in the empirical equations to 

account for the effects of density change.

North Fork Site

At the North Fork site, 38 of 49 collected water samples 

were used to determine the relation of suspended-solids 

concentration to ABS in the local space. All data collected 

at this site and used for the regression analysis are presented 

in table 1. Eleven samples were not used because of missing, bad, or erroneously collected ABS data. The measured 

suspended-solids concentrations for water samples used in this 

analysis ranged from 3 to 18 mg/L, and corresponding average 

salinity and temperature ranged from 0.2 to 12.9 ppt and 19.4 

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 7
Date Time

Suspended-solids 

concentration

(milligrams per liter)

Salinity

(parts per thousand)

Temperature

(degrees 

Celsius)

Acoustic backscatter

 strength

Turbidity

(NTU)

Total

Inorganic

Organic

Organic to

inorganic ratio

No. 1No. 2

Average

ABS

 (counts)

Adjusted ABS

(counts)

06-02-99 1600 18 7 11 1.57 9.8 13.0 11.4 29.7 155 195 BD

06-24-99 1230 13 6 7 1.17 .5 .5 .5 28.5 151 191 BD

06-24-99 1404 11 5 6 1.20 .5 .5 .5 28.7 148 188 BD

06-24-99 1526 8 4 4 1.00 .5 .5 .5 29.2 140 180 BD

06-24-99 0948 8 4 4 1.00 .5 .5 .5 28.2 BD BD BD

07-14-99 1640 10 3 7 2.33 .6 .6 .6 31.8 146 186 13

07-14-99 1750 10 3 7 2.33 .6 .6 .6 31.3 149 189 14

07-15-99 1105 13 5 8 1.60 .7 .6 .6 30.9 152 192 20

07-15-99 1206 15 5 10 2.00 .7 .6 .6 30.9 157 197 17

07-15-99 1325 11 4 7 1.75 .6 .6 .6 30.9 150 190 14

07-15-99 1423 6 2 4 2.00 .6 .6 .6 31.2 139 179 10

07-15-99 1515 7 3 4 1.33 .6 .6 .6 31.1 138 178 7.3

07-15-99 1610 6 2 4 2.00 .6 .6 .6 31.5 138 178 11

08-11-99 0843 7 2 5 2.50 4.1 4.3 4.2 30.8 126 166 BD

08-11-99 1010 6 2 4 2.00 6.1 6.4 6.2 30.9 130 170 11.3

08-11-99 1134 6 2 4 2.00 6.3 8.6 7.4 31.2 131 171 6.3

08-11-99 1300 6 2 4 2.00 7.1 8.7 7.9 31.3 133 173 5.6

09-17-99 1230 4 2 2 1.00 MD MD MD MD MD MD MD

10-14-99 0930 4 2 2 1.00 .3 .3 .3 26.6 147 147 BD

10-14-99 1103 3 2 1 .50 .3 .3 .3 26.5 142 142 BD

10-14-99 1238 6 3 3 1.00 .3 .3 .3 26.7 161 161 BD

10-14-99 1355 6 2 4 2.00 .2 .2 .2 26.9 157 157 BD

10-18-99 1353 13 4 9 2.25 .1 .1 .1 25.0 BD BD BD

10-18-99 1547 8 3 5 1.67 .1 .1 .1 25.2 BD BD BD

11-19-99 0843 5 2 3 1.50 1.1 1.2 1.1 21.6 148 148 5.6

11-19-99 1340 5 3 2 .67 .6 .6 .6 22.0 155 155 3.1

12-05-99 0830 9 3 6 2.00 4.9 5.2 5.0 19.4 151 151 BD

12-05-99 0903 11 4 7 1.75 5.0 5.2 5.1 19.5 153 153 BD

12-05-99 1118 9 3 6 2.00 5.2 6.2 5.7 19.7 147 147 BD

12-05-99 1212 10 4 6 1.50 5.2 5.5 5.4 19.8 149 149 BD

12-05-99 1322 9 3 6 2.00 3.7 4.3 4.0 20.0 156 156 BD

12-05-99 1351 8 3 5 1.67 3.1 3.4 3.2 19.8 156 156 BD

12-14-99 0906 4 1 3 3.00 1.1 1.2 1.1 21.8 133 133 BD

01-06-00 1207 7 2 5 2.50 7.3 7.4 7.4 20.7 150 150 BD

01-27-00 1624 3 2 1 .50 MD MD MD MD MD MD MD

102-11-00 1823 14 2 2 1.00 9.4 11.8 10.6 18.5 155 155 3.6

102-11-00 1827 14 2 2 1.00 9.4 11.7 10.5 18.5 154 154 3.6

102-11-00 1830 15 2 3 1.50 9.4 11.7 10.5 18.5 154 154 3.6

8 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida

Table 1. Point sample data from the North Fork site used for regression analyses. 

[Values shown in bold red were used to determine the suspended solids to acoustic backscatter relation and the suspended solids to turbidity relation. 

ABS, acoustic backscatter strength; NTU, nephelometric turbidity units; BD, bad data; MD, missing data]
to 31.8 °C, respectively. All ABS data collected from June 2 

to August 11, 1999, had to be adjusted by correcting recorded 

ABS values based on the difference between readings before 

and after cleaning the transducers. ABS readings increased 

by 40 counts after transducers were cleaned in August 1999. 

A constant correction was applied because there is no available information to determine a correction prorated between 

the time biofouling started and the instrument cleaning date. 

Because density affects the speed of sound in water, 

salinity and temperature were both included in the multivariable regression analysis used to determine the best possible 

relation of suspended-solids concentration to ABS. This 

empirical relation is expressed as:

 Suspended solids = 10ABS [0.02876 + 0.00032 log(salinity)

- 0.01106 log(temperature)] - 1.30367. (3)

The correlation coefficient is:

R2 = 0.78.

The relation obtained between measured and estimated 

suspended-solids concentrations using equation 3 is shown in 

figure 7. The one-to-one line shown figure 7 does not represent a regression line of suspended-solids concentrations as a 

function of ABS because estimated suspended-solids concentrations are a function of temperature and salinity as well 

as ABS. This plot shows actual measured suspended-solids 

concentrations relative to estimated concentrations in order to 

depict how well equation 3 fits measured concentrations at the 

North Fork site.

Speedy Point Site

At the Speedy Point site, 33 of 42 collected water samples were used to determine the relation of suspended-solids 

concentration to ABS in the local space. All data collected at 

this site and used in the regression analysis are presented in 

table 2. Nine samples were not used in the regression analysis 

because of missing or bad ABS data or because of questionable suspended-solids sample data. The measured suspendedsolids concentrations for samples used in this analysis ranged 

from 4 to 23 mg/L, with corresponding average salinity and 

temperatures ranging from 0.2 to 22.7 ppt and 20.7 to 31.8 °C, 

respectively. No corrections to ABS data were necessary 

at the Speedy Point site. The empirical equation developed 

for estimating suspended-solids concentrations at this site is 

expressed as:

 Suspended solids = 10ABS [0.03247 - 0.00204 log(salinity)

 - 0.01547 log(temperature) - 0.23927. (4)

The correlation coefficient is:

R2 = 0.63.

Date Time

Suspended-solids 

concentration

(milligrams per liter)

Salinity

(parts per thousand)

Temperature

(degrees 

Celsius)

Acoustic backscatter

 strength

Turbidity

(NTU)

Total

Inorganic

Organic

Organic to

inorganic ratio

No. 1No. 2

Average

ABS

 (counts)

Adjusted ABS

(counts)

102-11-00 1834 15 2 3 1.50 9.0 11.6 10.3 18.5 154 154 3.4

02-24-00 0905 5 2 3 1.50 4.7 9.3 7.0 21.2 144 144 BD

02-24-00 1407 5 2 3 1.50 6.0 11.0 8.5 21.9 156 156 BD

03-23-00 1754 7 2 5 2.50 10.0 10.2 10.1 24.3 142 142 7.9

03-24-00 1610 6 2 4 2.00 12.3 13.4 12.9 24.7 156 156 9.7

03-24-00 1629 6 2 4 2.00 11.9 12.5 12.2 24.6 153 153 9.9

03-24-00 1653 6 2 4 2.00 11.1 11.3 11.2 24.6 153 153 9.8

03-24-00 1711 7 2 5 2.50 10.6 11.1 10.9 24.6 153 153 8.8

03-24-00 1727 6 2 4 2.00 10.4 10.5 10.5 24.5 151 151 8.3

04-28-00 1449 12 7 5 .71 7.9 8.0 7.9 26.6 BD BD 13

05-16-00 1715 21 4 17 4.25 MD MD MD MD MD MD MD 

1Data from this sample were not used in the regression analysis because they were erroneously collected outside of local space at the North Fork site; data 

are shown solely for documentation.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 9

Table 1. Point sample data from the North Fork site used for regression analyses. (Continued)

[Values shown in bold red were used to determine the suspended solids to acoustic backscatter relation and the suspended solids to turbidity relation. ABS, 

acoustic backscatter strength; NTU, nephelometric turbidity units; BD, bad data; MD, missing data]
Figure 7. Measured to estimated suspended-solids concentration relation at the North Fork site 

using acoustic backscatter strength data.

Date Time

Suspended-solids 

concentration

(milligrams per liter)

Salinity

(parts per thousand)

Temperature 

(degrees Celsius)

Acoustic backscatter 

strength (counts)

Turbidity 

(nephelometric 

turbidity units)

Total

Inorganic

Organic

Organic to

 inorganic ratio

No. 1 No. 2

Average

No. 1 No. 2

Average

No. 1 No. 2

Average

06-03-99 0915 5 2 3 1.5 MD MD MD MD MD MD MD MD MD MD

06-24-99 0920 8 3 5 1.7 3.4 6.3 4.9 29.3 29.4 29.4 154 8.9 8.8 8.85

06-24-99 1118 10 4 6 1.5 2.5 3.1 2.8 29.4 29.4 29.4 145 8.1 7.2 7.65

06-24-99 1200 10 4 6 1.5 2.4 2.9 2.6 29.4 29.3 29.4 151 8.1 6.8 7.45

06-24-99 1332 10 4 6 1.5 2.1 2.3 2.2 29.6 29.5 29.6 142 9.9 7.5 8.7

06-24-99 1501 12 5 7 1.4 2.0 2.2 2.1 29.9 29.7 29.8 151 BD BD BD

07-14-99 1552 14 4 10 2.5 6.4 7.2 6.8 31.8 31.3 31.6 158 6.0 11.0 8.50

07-14-99 1711 14 6 8 1.3 6.0 6.6 6.3 31.9 31.6 31.8 153 8.5 13.0 10.75

07-15-99 1020 6 2 4 2.0 8.7 10.1 9.4 30.8 30.7 30.8 146 4.5 16.0 10.25

07-15-99 1134 6 2 4 2.0 9.4 9.5 9.4 30.9 30.8 30.9 151 5.1 15.0 10.05

07-15-99 1259 6 2 4 2.0 10.1 9.2 9.7 31.7 31.6 31.7 133 5.0 11.0 8.00

Table 2. Point sample data from the Speedy Point site used for regression analyses.

[Values in bold red were used to determine the suspended solids to acoustic backscatter relation and the suspended solids to turbidity relation. MD, missing data; 

BD, bad data]

10 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Table 2. Point sample data from the Speedy Point site used for regression analyses. (Continued)

[Values in bold red were used to determine the suspended solids to acoustic backscatter relation and the suspended solids to turbidity relation. MD, missing data; 

BD, bad data]

Date Time

Suspended-solids 

concentration

(milligrams per liter)

Salinity

(parts per thousand)

Temperature 

(degrees Celsius)

Acoustic backscatter 

strength (counts)

Turbidity 

(nephelometric 

turbidity units)

Total

Inorganic

Organic

Organic to

 inorganic ratio

No. 1 No. 2

Average

No. 1 No. 2

Average

No. 1 No. 2

Average

07-15-99 1351 5 2 3 1.5 10.0 9.4 9.7 31.8 31.6 31.7 152 4.9 9.9 7.40

07-15-99 1449 8 3 5 1.7 9.7 9.4 9.6 31.4 31.1 31.3 171 6.7 12.0 9.35

07-15-99 1543 7 2 5 2.5 8.8 8.8 8.8 31.6 31.3 31.5 149 5.9 11.0 8.45

08-11-99 0921 7 2 5 2.5 15.4 18.2 16.8 30.5 30.6 30.6 131 3.7 13.0 8.35

08-11-99 1041 6 2 4 2.0 17.6 19.8 18.7 30.5 30.5 30.5 146 8.0 14.8 11.40

08-11-99 1205 8 3 5 1.7 15.3 17.1 16.2 31.0 30.8 30.9 148 .8 11.0 5.90

08-11-99 1323 8 3 5 1.7 13.3 15.3 14.3 31.4 31.1 31.3 134 3.2 11.0 7.10

09-17-99 1155 8 2 6 3.0 MD MD MD MD MD MD MD MD MD MD

10-14-99 1125 18 6 12 3.5 .2 .2 .2 27.0 27.0 27.0 133 13.0 18.0 15.50

10-14-99 1305 19 6 13 2.2 .2 .2 .2 27.1 27.2 27.2 131 17.0 22.0 19.50

10-14-99 1504 16 5 11 1.7 .2 .2 .2 27.1 27.2 27.2 126 13.0 20.0 16.50

10-14-99 0956 18 14 14 .6 .2 .2 .2 27.1 27.1 27.1 1126 3.6 6.1 14.85

10-18-99 1238 16 5 11 2.2 .13 .13 .13 25.3 25.3 25.3 BD 15 16 15.5

10-18-99 1505 13 3 10 3.3 .12 .12 .12 26.0 25.9 26.0 BD 12 14 13

11-19-99 0925 21 6 15 2.5 3.2 3.7 3.4 21.4 21.5 21.5 122 33 24 28.5

11-19-99 1239 20 7 13 1.9 2.7 3.3 3.0 22.0 21.9 22.0 127 28 25 26.5

11-19-99 1514 23 8 15 1.9 2.9 3.0 2.9 22.2 22.2 22.2 134 23 31 27

12-14-99 1011 6 1 5 5.0 8.3 11.1 9.7 22.4 22.6 22.5 128 MD 8.5 MD

01-06-00 1519 10 3 7 2.3 11.0 13.0 12.0 21.4 21.1 21.3 126 7.8 5.4 6.6

01-06-00 1557 6 2 4 2.0 10.8 12.7 11.8 21.4 21.2 21.3 130 7.4 6.1 6.75

01-27-00 1313 6 2 4 2.0 MD MD MD MD MD MD MD MD MD MD

02-24-00 1246 18 5 13 2.6 22.5 22.3 22.4 20.7 20.7 20.7 153 16 22 19

02-24-00 1337 17 5 12 2.4 22.8 22.6 22.7 20.9 20.8 20.9 147 18 18 18

03-23-00 1617 9 3 6 2.0 19.0 19.7 19.4 24.3 24.4 24.4 131 6.8 13 9.9

03-24-00 1351 4 2 2 1.0 20.9 22.2 21.6 24.3 24.0 24.2 120 1.6 7.1 4.35

03-24-00 1457 5 2 3 1.5 20.4 22.2 21.3 24.2 23.9 24.1 122 2.2 8 5.1

03-24-00 1530 4 2 2 1.0 19.8 21.9 20.9 24.1 23.9 24.0 123 2.9 11 6.95

04-14-00 1118 6 2 4 2.0 18.3 19.9 19.1 23.2 23.3 23.3 121 6.7 6.4 6.55

04-28-00 1300 32 10 22 2.2 4.8 MD MD 25.9 25.8 25.9 BD 44 36 40

04-28-00 1323 37 11 26 2.4 4.9 MD MD 26.0 25.7 25.9 BD 45 36 40.5

05-04-00 1246 20 7 13 1.9 4.2 4.7 4.5 24.8 24.7 24.8 BD 26 17 21.5

1Questionable suspended-solids sample. The acoustic backscatter strength and turbidity data were not used in regression analyses and are shown here solely 

for documentation.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 11
Figure 8. Measured to estimated suspended-solids concentration relation at the Speedy Point site 

using acoustic backscatter strength data.

The relation obtained between measured and estimated 

suspended-solids concentrations using equation 4 is shown in 

figure 8. As in figure 7, the one-to-one line shown in figure 

8 does not represent a regression line of suspended-solids 

concentrations as a function of ABS because estimated suspended-solids concentrations are a function of temperature 

and salinity as well as ABS. This plot shows actual measured 

suspended-solids concentrations relative to estimated concentrations in order to depict how well equation 4 fits measured 

concentrations at Speedy Point.

Erratic patterns in the acoustic data resulted in larger 

variance as compared to data collected at the North Fork site. 

Excessive turbulence within the volume of water measured by 

the instrument probably affected the consistency of ABS data. 

Turbulence develops at this site in the higher water velocity 

zones near the large concrete footings where instrumentation 

was installed. Observed flow patterns at the Speedy Point site 

and results obtained from correlation analyses suggest better 

results probably would have been obtained if instruments had 

been located in an area with less turbulence. Results obtained 

at the North Fork site also suggest that the technique used can 

provide better results under favorable measuring conditions.

Suspended-Solids Concentration to Turbidity 

Relation

Simple linear regression techniques were used to determine the best possible relation of suspended-solids concentration to turbidity at the North Fork and Speedy Point sites. 

Salinity and temperature had no observed effects on the 

turbidity relations.

North Fork Site

At the North Fork site, 20 of 49 collected water samples 

were used to determine the relation of suspended-solids concentration to turbidity in the local space (fig. 9). All turbidity 

data collected at this site and used for the regression analysis 

are presented in table 1. Twenty-nine water samples were not 

used in the regression analysis because of bad, missing, or 

erroneously collected turbidity data. The measured suspendedsolids concentrations for water samples used in this analysis 

ranged from 5 to 15 mg/L, and corresponding average salinity and temperature ranged from 0.6 to 12.9 ppt and 21.6 to 

31.8 °C, respectively. The empirical equation developed for 

estimating suspended-solids concentrations using turbidity 

data at this site is expressed as:

12 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
 Suspended solids = 0.614 Turbidity + 1.49. (5)

The correlation coefficient is:

R2 = 0.73.

Speedy Point Site

At the Speedy Point site, 36 of 42 collected water 

samples were used to determine the relation of suspendedsolids concentration to turbidity in the local space (fig. 10). 

All turbidity data collected at this site and used for the regression analysis are presented in table 2. Six samples were not 

used in the regression analysis because of missing or bad turbidity data. The measured suspended-solids concentrations for 

samples used in this analysis ranged from 4 to 37 mg/L, and 

corresponding average salinity and temperatures ranged from 

0.12 to 22.7 ppt and 20.7 to 31.8 °C, respectively. Even though 

good correlation was obtained using turbidity data from each 

of two sensors at this site, average turbidity was used for this 

analysis. The empirical equation developed for estimating 

suspended-solids concentrations using turbidity data at this 

site is expressed as: 

 Suspended solids = 0.799 Turbidity + 1.76. (6)

The correlation coefficient is:

R2 = 0.89.

Figure 9. Suspended-solids concentration to turbidity relation at the North Fork site. R2 is the 

correlation coefficient.

Qualification of Empirical Equations 

Generally, the accuracy of empirical equations is directly 

related to how well relevant variables are represented. In 

this study, equations describing suspended-solids concentrations were calibrated for the North Fork and Speedy Point 

sites because of site-specific: (1) suspended sediment characteristics (composition and size distribution); and (2) water 

sampling locations relative to site instrumentation. Thus, 

each relation is only applicable to the site for which it was 

developed, as evidenced by comparing the ABS and turbidity 

relations obtained for each site.

The distribution of parameter data used to develop 

equations 3 and 4, and relations between residuals and these 

parameters are shown in figures 11 and 12. For the purpose 

of these analyses, residuals are calculated by subtracting 

measured from estimated suspended-solids concentrations 

using the corresponding empirical equation. The adequacy 

of these equations appears to be limited by the number and 

distribution of samples throughout the concentration range, 

and by changes in the physical characteristics (specifically, 

the organic to inorganic ratio) of the material in suspension, as 

shown by apparent trends in figures 11 and 12.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 13
Trends in the residuals to suspended-solids concentrations graphs indicate that equations 3 and 4 appear to overestimate at the lower range of suspended-solids concentrations 

and underestimate at the higher range. The accuracy of data 

calculated using ABS relations as described by equations 3 

and 4 may be improved by: (1) increasing the number of water 

samples to obtain a better distribution of measured suspendedsolids concentrations throughout the expected range; and 

(2) further studying the effects that changes in the physical 

composition of suspended-solids, including the organic ratio 

and varying particle-size distribution, may have on the established relations.

The distribution of parameter data used to develop 

equations 5 and 6, and relations between residuals and these 

parameters are shown in figures 13 and 14 (residuals are 

calculated as previously described). Again, the adequacy of 

these equations for describing suspended-solids concentrations 

may be limited by the number and distribution of samples 

throughout the expected concentration range. Graphs of the 

relations between residuals to suspended-solids concentrations 

in figures 13 and 14 show either weak or nonexistent trends 

for available data. However, limited measurements were taken 

throughout the observed concentration range, and increasing 

the number of water samples to obtain a more complete 

distribution of measurements may improve the accuracy of 

the relations.

Sensitivity Analysis

A sensitivity analysis was performed based on field data 

collected at the North Fork site on June 2, 1999, and at the 

Speedy Point site on October 14, 1999. This sensitivity analysis was designed to qualify the response of suspended-solids 

concentration relations to changes in primary and secondary variables. In this study, field parameters are classified as 

primary and secondary variables depending on their role in the 

suspended-solids concentration relations. Turbidity and ABS 

are considered to be the primary variables, whereas salinity 

and temperature are considered to be the secondary variables 

because their role is defined as possible “modifiers” of the 

suspended-solids concentration relations to ABS and turbidity.

Turbidity relations for the North Fork and Speedy Point 

sites were developed without use of secondary variables and 

are shown in figures 9 and 10, respectively. No further tests 

were necessary because suspended-solids concentration only 

appears to be related to turbidity; salinity and temperature had 

no observed effects on the turbidity relations. 

Figure 10. Suspended-solids concentration to turbidity relation at the Speedy Point site. R2 is the 

correlation coefficient.

14 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Figure 11. Distribution of parameter data and residuals used in the suspended-solids 

concentration to acoustic backscatter strength regression analysis at the North Fork 

site. Circles indicate measurements used in regression analysis.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 15
Figure 12. Distribution of parameter data and residuals used in the suspended-solids 

concentration to acoustic backscatter strength regression analysis at the Speedy 

Point site. Circles indicate measurements used in regression analysis.

ACOUSTIC BACKSCATTER

STRENGTH, IN COUNTS

SALINITY, IN PARTS PER THOUSAND

TEMPERATURE, IN DEGREES CELSIUS

ACOUSTIC BACKSCATTER

STRENGTH, IN COUNTS

0 5 10 15 20 25

SALINITY, IN PARTS PER THOUSAND

TEMPERATURE, IN DEGREES CELSIUS

0 5 10 15 20 25

MEASURED SUSPENDEDSOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

MEASURED SUSPENDEDSOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

0 1 2 3 4 5 6

NUMBER OF SAMPLES NUMBER OF SAMPLES NUMBER OF SAMPLES NUMBER OF SAMPLES NUMBER OF SAMPLES

RESIDUALS, IN MILLIGRAMS

PER LITER

RESIDUALS, IN MILLIGRAMS

PER LITER

RESIDUALS, IN MILLIGRAMS

PER LITER

RESIDUALS, IN MILLIGRAMS

PER LITER

RESIDUALS, IN MILLIGRAMS

PER LITER

14

12

10

8

6

4

2

0

0 5 10 15 20 25

10

8

6

4

2

0

12

10

8

6

4

2

0

ORGANIC TO INORGANIC RATIO

FOR SUSPENDED-SOLIDS SAMPLES

ORGANIC TO INORGANIC RATIO

FOR SUSPENDED-SOLIDS SAMPLES

0 5 10 15 20 25

TREND LINE

-10

-10

-10

-10

-10

-8

-8

-8

-8

-8

-6

-6

-6

-6

-6

-4

-4

-4

-4

-4

-2

-2

-2

-2

-2

0

0

0

0

0

2

2

2

2

2

4

4

4

4

4

6

6

6

6

6

110 120 130 140 150 160 170 180

20 25 30 35 20 22 24 26 28 30 32

0

2

4

6

8

10

12

14

16

18

0

2

4

6

8

10

12

14

16

18

1 2 3 4 5 6

120 130 140 150 160 170 180

16 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Figure 13. Distribution of parameter data and residuals used in the suspended-solids 

concentration to turbidity regression analysis at the North Fork site. Circles indicate 

measurements used in regression analysis.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 17
Figure 14. Distribution of parameter data and residuals used in the suspended-solids 

concentration to turbidity regression analysis at the Speedy Point site. Circles indicate 

measurements used in regression analysis.

18 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
The ABS relations were developed using ABS as the 

primary variable, and salinity and temperature as secondary 

variables. The sensitivity analysis for ABS relations used four 

tests to determine how the relation responds to each variable. 

The first test was performed by keeping all variables constant, 

except for ABS, and computing suspended-solids concentrations for several ABS values within the expected range for 

each site. As anticipated, suspended-solids concentrations 

increased in response to increasing ABS values at the North 

Fork and Speedy Point sites (fig. 15).

Salinity is one of two secondary variables used to account 

for possible density effects on ABS data because of its role in 

determining acoustic signal absorption as sound travels across 

a volume of water. The second test was performed by keeping all variables constant, except for salinity, and computing 

suspended-solids concentration for several salinity values 

within the expected ranges at the North Fork and Speedy Point 

sites (fig. 16). The effect of salinity was found to be minimal; 

however, it differed between both sites as reflected by the different signs for regression coefficients associated with salinity in equations 3 and 4. This difference may be the result of 

an insufficient number and distribution of suspended-solids 

measurements taken throughout the salinity range. Additional 

suspended-solids measurements throughout the salinity range 

could help determine if the salinity term is necessary in the 

suspended-solids equations.

Temperature is the other secondary variable used to 

account for density effects on ABS data. For the third test, all 

variables were kept constant, except for temperature. Suspended-solids concentrations were computed for several temperature values within the expected range at the North Fork 

and Speedy Point sites; the responses are shown in figure 17. 

Based on results obtained with data from both sites, the effect 

of temperature in equations 3 and 4 is much greater than that 

of salinity.

The North Fork site yielded the best available ABS data, 

and was used to develop an equation that describes suspendedsolids as a function of ABS and water density. Water density 

was estimated as a function of water temperature and salinity 

using an equation described by Millero and Poisson (1981), 

and then used to develop the following equation:

Figure 15. Acoustic backscatter strength to suspended-solids 

concentration relation at the North Fork and Speedy Point 

sites. All variables were kept constant while varying acoustic 

backscatter strength.

Figure 16. Salinity to suspended-solids concentration relation 

at the North Fork and Speedy Point sites. All variables were 

kept constant while varying salinity.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 19
 Suspended solids = 10ABS [-1.24762 + 0.4188 log(density)] 

- 0.54035. (7)

The correlation coefficient is:

R2 = 0.63.

The fourth test was used to provide insight 

about how temperature may affect ABS relations, 

in addition to the effects related to temperatureinduced changes in water density. North Fork 

data collected on June 2, 1999, and equations 

3 and 7 were used to solve for ABS; all variables were held constant, except for temperature 

(fig. 18). Temperature values encompassing 

expected ranges for the North Fork site were 

used to estimate corresponding ABS values. 

The results suggest that temperature may have 

effects on the ABS relations that are unrelated to 

water density, perhaps by inducing temperaturedependent changes in the absorptive and reflective characteristics of the suspended material 

within the sampled volume of water.

Time-Series Estimates of Mean 

Suspended-Solids Concentrations

The potential value of using surrogate data for the 

calculation of time-series records of mean cross-sectional 

suspended-solids concentrations lies in the ability to produce 

data on a continuous basis. This is especially true for periods 

when data would otherwise not be available, such as during 

and immediately following significant meteorological events 

such as tropical storms or hurricanes. Because mean crosssectional concentrations are needed, established local relations 

must be extrapolated horizontally to represent mean crosssectional concentrations. This is accomplished by collecting 

simultaneous samples within local and cross-sectional space, 

and performing correlation analyses to develop relations 

between the two. 

The location where monitoring instruments are placed in 

a given stream can greatly affect the ability to obtain accurate 

relations between local (near sensors) and mean concentrations. Vertical and horizontal stratification can affect how well 

data collected within a small section of a stream can represent 

the mean cross-sectional conditions. For example, a narrow 

stream channel, such as at the North Fork site (about 300 ft 

wide), is more likely to have good correlation between point 

and cross-sectional data than a wide stream channel, such as at 

the Speedy Point site (about 1,000 ft wide).

Because the intent of this report is to document the 

feasibility of using these techniques and not to produce actual 

records of suspended-sediment loads, the relations described 

herein are presented for reference only; more data will be 

necessary to improve and verify these relations. The local 

(point) to mean cross-sectional suspended-solids concentration 

relations for data available for the North Fork and Speedy 

Point sites are shown in figures 19 and 20, respectively. 

Figure 17. Temperature to suspended-solids concentration 

relation at the North Fork and Speedy Point sites. All variables 

were kept constant while varying temperature.

Figure 18. Effect of temperature on acoustic backscatter strength.

20 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida
Figure 19. Local point to mean cross-sectional relation for suspended-solids 

concentration at the North Fork site. R2 is the correlation coefficient.

Figure 20. Local point to mean cross-sectional relation for suspended-solids 

concentration at the Speedy Point site. R2 is the correlation coefficient.

The local to mean relation for the North Fork site is:

 Suspended solidsmean = 0.8381 Suspended solidslocal

+ 1.8484, (8)

and the local to mean relations for the Speedy Point site is:

 Suspended solidsmean = 1.1839 Suspended solidslocal

 + 8.5405. (9)

The correlation coefficients are R2 = 0.81 for the North Fork 

site, and R2 = 0.57 for the Speedy Point site.

Data used to develop these relations are presented in 

table 3. Local suspended-solids concentrations were measured 

by collecting water samples in the local space as previously 

defined. Cross-sectional concentrations were measured by 

collecting depth-integrated samples using the EDI method and 

a DH-59 sampler.

A sample of time-series records of estimated suspendedsolids concentrations was generated using ABS and turbidity 

data from the North Fork site and equation 8 to transform 

local estimates to mean cross-sectional concentrations. These 

estimates are presented with discharge and measured mean 

suspended-solids concentration in figure 21. The data from 

the North Fork site (fig. 21) suggest that time-series records of 

suspended-solids concentrations estimated using ABS data are 

less “noisy” than those estimated using turbidity.

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods 21
Summary and Conclusions

Acoustic and optic methods were applied in a study to 

estimate suspended-solids concentrations in the St. Lucie 

River Estuary, southeastern Florida. Instrument installation 

and field data collection and analysis were performed at the 

North Fork site in southern St. Lucie County and at the Speedy 

Point site in northern Martin County. These sites provide varying flow, salinity, water-quality, and channel cross-sectional 

characteristics. A third monitoring site, Steele Point, was 

not used in the analysis because bridge construction resulted 

in several instrument relocations that prevented a sufficient 

number of measurements from being taken at any of the 

locations used.

Acoustic Doppler velocity meter instruments were 

installed to measure an index of the mean water velocity at 

surface-water monitoring sites and to record information about 

the strength of the return signal—acoustic backscatter strength 

(ABS)—received by the sensor, a parameter affected primarily 

by the amount of material in suspension. Additionally, optic 

sensors were installed to measure the turbidity of the water 

at these sites. Water samples were collected using point and 

depth-integrated samplers to measure local and mean crosssectional suspended-solids concentrations, respectively.

Regression analyses were used to determine the relations 

of suspended-solids concentration to ABS and suspendedsolids concentration to turbidity at the North Fork and Speedy 

Point sites. Usable measurements of suspended-solids concentrations ranged from 3 to 18 mg/L at the North Fork site 

and from 4 to 37 mg/L at the Speedy Point site. The organic 

content of suspended solids measurements made at both 

sites ranged from 50 to 83 percent. The suspended-solids 

concentrations and organic content varied depending on the 

monitoring site location and flow conditions at the time of 

Figure 21. Discharge 

and mean cross-sectional 

suspended-solids concentrations at the North 

Fork site. ABS is acoustic 

backscatter strength.

Date Local point 

samples

Cross -sectional 

samples

North Fork Site

10-18-99 13 15

10-18-99 8 12

11-19-99 5 6

11-19-99 5 6

12-05-99 9 9

12-05-99 11 9

12-05-99 9 7

12-05-99 10 8

12-05-99 9 8

12-14-99 4 4

01-06-00 7 7

02-24-00 5 7

03-24-00 6 8

03-24-00 6 9

03-24-00 7 119

05-16-00 21 20

Speedy Point Site

11-19-99 21 24

11-19-99 20 21

11-19-99 23 27

01-06-00 10 17

02-24-00 18 39

03-24-00 4 14

03-24-00 5 16

04-28-00 32 51

05-04-00 20 49

1Biased sample due to bottom sediment resuspension during 

sampling; data were not used in the analysis and are shown here 

solely for documentation.

22 Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations...Florida

Table 3. Suspended-solids concentrations for the North 

Fork and Speedy Point sites used to develop local to 

mean cross-sectional relations.

[units are in milligrams per liter]
sampling. For samples used in the regression analyses, salinity 

ranged from 0.12 to 22.7 parts per thousand, and temperature 

ranged from 19.4 to 31.8 °C. 

Salinity and temperature data were collected as waterquality characteristics and to account for water-density effects 

on ABS data. These data were used as secondary variables 

in regression analyses to develop the estimation equations. 

Salinity and temperature data used for the ABS relation ranged 

from 0.2 to 12.9 ppt and 19.4 to 31.8 °C, respectively, at the 

North Fork site and ranged from 0.2 to 22.7 ppt and 20.7 to 

31.8 °C, respectively, at the Speedy Point site. 

All relations described herein are site specific and are 

only useful for estimating suspended-solids concentrations for 

locations where data were collected. Because of an insufficient 

number of equal discharge increment samples, all suspendedsolids concentration relations developed as part of this study 

are relations in the local space (near the sensors) and do not 

represent mean suspended-solids concentrations of the entire 

cross section. The suspended-solids concentration to ABS 

relations for the North Fork and Speedy Point sites resulted 

in correlation coefficients of 0.78 and 0.63, respectively. 

The suspended-solids concentration to turbidity relation at 

the North Fork and Speedy Point sites resulted in correlation 

coefficients of 0.73 and 0.89, respectively. 

The adequacy of these empirical equations appears to be 

limited by the number and distribution of suspended-solids 

samples taken throughout the expected concentration range 

at both sites. Additionally, the ABS relations for both sites 

seem to overestimate at the low end and underestimate at the 

high end of the concentration range. The physical composition (organic to inorganic ratio) of the suspended material in 

samples taken also seems to partly determine how well the 

ABS relations represent suspended-solids concentrations, as 

shown by the analysis of North Fork data. Combining ABS 

and turbidity data into one equation did not improve the accuracy of results, and therefore, this approach was not considered 

in the final analysis.

Based on the sensitivity analysis, temperature had a 

greater effect than salinity on estimated suspended-solids 

concentrations. In addition to affecting ABS data by changing 

water density, temperature also appeared to affect ABS data 

by changing the absorptive and reflective characteristics of the 

suspended material. Salinity and temperature had no observed 

effects on the turbidity relation at the North Fork and Speedy 

Point sites.

In a mathematical comparison, estimates of suspendedsolids concentrations using ABS data were “less” erratic than 

estimates using turbidity data. The overall results of the study 

suggest that this technique has the potential to adequately 

describe time-series estimates of suspended-solids concentrations in the St. Lucie River Estuary. However, the accuracy 

with which the empirical equations describe these estimates 

may be improved by: (1) increasing the number of water samples to obtain better distributions of measured suspended-solids concentrations and local to mean cross-sectional relations; 

and (2) further studying the effects that changes in the physical 

composition of suspended-solids may have on ABS relations. 

References Cited

Edwards, T.K., and Glysson, G.D., 1988, Field methods for 

measurement of fluvial sediment: U.S. Geological Survey 

Open-File Report 86-531, 118 p.

Gray, J.R., Glysson, G.D., Turcios, L.M., and Schwartz, G.E., 

2000, Comparability of suspended-sediment concentration 

and total suspended solids data: U.S. Geological Survey 

Water-Resources Investigations Report 00-4191, 20 p.

Millero, F.J., and Poisson, Alain, 1981, International one 

atmosphere equation of state for seawater: Deep-Sea 

Research, v. 28A, p. 625-629.

SonTek, 1997, SonTek Argonaut—Acoustic Doppler current 

meter: San Diego, Calif., Technical Documentation, 127 p.

South Florida Water Management District, 1999, Focus on the 

St. Lucie River: West Palm Beach, November, unnumbered.

Steward, Joel, Virnstein, Robert, Haunert, Dan, and Lund, 

Frank, 1993, Draft Surface Water Improvement and Management (SWIM) Plan for the Indian River Lagoon: West 

Palm Beach, South Florida Water Management District.

U.S. Interagency Committee on Water Resources, Subcommittee on Sedimentation, 1965, Instructions for sampling with 

depth-integrating sediment samples US D-49 and DH-59: 

Report O, 175 p.

YSI Incorporated, 1998, Environmental monitoring systems—

Operations manual: Yellow Springs, Ohio, YSI Publication, 

255 p.

References Cited 23