U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 9

Application of Acoustical Methods for 

Estimating Water Flow and Constituent 

Loads in Perdido Bay, Florida

Prepared in cooperation with the

Florida Department of Environmental Protection

7-4101

By J.W. GRUBBS and JOHN R. PITTMAN

Tallahassee, Florida

1997
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

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

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Federal Center

Denver, CO 80225

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

District Chief

U.S. Geological Survey

227 North Bronough Street

Suite 3015

Tallahassee, Florida 32301
Contents III

CONTENTS

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

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

Description of Study Area .................................................................................................................................................... 2

Methods of Study .................................................................................................................................................................. 4

Methods Used to Measure Discharge and Characterize Velocity Fields .................................................................... 4

Estimation of Continuous and Daily-Mean Discharge from Data Collected at the 

Gaging Station .............................................................................................................................................. 5

Collection of Stage Data and Development of Stage-Area Relation ................................................................ 5

Collection of Water-Velocity Data and Development of Index Velocity-Mean Velocity Relation .................. 5

Computation of Continuous and Filtered Daily-Mean Discharge .................................................................... 6

Water Quality and Constituent Loading ...................................................................................................................... 6

Results of Study .................................................................................................................................................................... 7

Stage Observations and Stage-Area Relation ............................................................................................................. 7

Discharge Measurements ............................................................................................................................................ 9

Acoustic Velocity Meter Velocity Measurements and Index Velocity-Cross Section Velocity Rating ...................... 11

Continuous Discharge Estimates and Net-Downstream Flows .................................................................................. 13

Water Quality and Constituent Loads ......................................................................................................................... 15

Spatial Surveys of Temperature, Specific Conductance, Dissolved Oxygen, and pH ..................................... 15

Analyses of Synoptic Water-Quality Samples .................................................................................................. 17

Estimates of Constituent Loads ........................................................................................................................ 20

Summary ............................................................................................................................................................................... 22

References ............................................................................................................................................................................ 23

FIGURES

1. Map showing location of study area .......................................................................................................................... 3

2. Graph showing stage and index velocity at Perdido Bay at U.S. Highway 98 near Pensacola, Florida, during 

September 7-24, 1995 ................................................................................................................................................ 8

3. Graph showing stage-area rating ............................................................................................................................... 9

4. Flow fields determined from discharge measurements made on April 27, 1995 ...................................................... 11

5. Flow fields determined from discharge measurements made on September 7-8, 1995 ............................................ 12

6. Relation between mean cross-section velocity and index (Acoustic Velocity Meter) velocity in Perdido Bay at 

U.S. Highway 98 near Pensacola, Florida ................................................................................................................. 13

7. Cross-sectional area and computed discharge in Perdido Bay at U.S. Highway 98 near Pensacola, Florida. ........... 14

8. Temperature observations from April and September surveys of Perdido Bay at U.S. Highway 98......................... 16

9. Specific conductance observations from April and September surveys of Perdido Bay at U.S. Highway 98........... 17

10. Dissolved oxygen oxygen observations from April and September surveys of Perdido Bay at U.S. Highway 98 ... 18

11. pH observations from April and September surveys of Perdido Bay at U.S. Highway 98 ........................................ 19
IV Contents

TABLES

1. Summary statistics of continuous stage, area, velocity, and discharge data, August 18-September 28, 1995 ........... 8

2. Discharge measurements made with the Acoustic Doppler Current Profiler at U.S. Highway 98 ............................. 10

3. Temperature, specific conductance, dissolved oxygen, and pH at 1400 hours, April 28, 1995, approximately 

1,000 feet from mouth of Elevenmile Creek, 10 feet from center of channel ............................................................ 18

4. Temperature, specific conductance, dissolved oxygen, and pH at 1500 hours, April 28, 1995, approximately 

1,000 feet from mouth of Elevenmile Creek, in center of channel ............................................................................. 19

5. Temperature, specific conductance, dissolved oxygen, and pH at 1530 hours, April 28, 1995, at mouth of 

Perdido River .............................................................................................................................................................. 19

6. Water quality data from synoptic water quality samples ............................................................................................ 21

7. Loads computed from the product of net-discharge and constituent concentrations .................................................. 21
Introduction 1

Application of Acoustical Methods for 

Estimating Water Flow and Constituent Loads 

in Perdido Bay, Florida

By J.W. Trey Grubbs and John R. Pittman

Abstract

Water flow and quality data were collected 

from December 1994 to September 1995 to 

evaluate variations in discharge, water quality, 

and chemical fluxes (loads) through Perdido Bay, 

Florida. Data were collected at a cross section 

parallel to the U.S. Highway 98 bridge. 

Discharges measured with an acoustic Doppler 

current profiler (ADCP) and computed from 

stage-area and velocity ratings varied roughly 

between cubic feet per second during a 

typical tidal cycle. Large reversals in flow 

direction occurred rapidly (less than 1 hour), and 

complete reversals (resulting in near peak netupstream or downstream discharges) occurred 

within a few hours of slack water. Observations of 

simultaneous upstream and downstream flow 

(bidirectional flow) were quite common in the 

ADCP measurements, with opposing directions of 

flow occurring predominantly in vertical layers. 

Continuous (every 15 minutes) discharge 

data were computed for the period from August 

18, 1995, to September 28, 1995, and filtered 

daily mean discharge values were computed for 

the period from August 19 to September 26, 1995. 

Data were not computed prior to August 18, 1995, 

either because of missing data or because the 

velocity rating was poorly defined (because of 

insufficient data) for the period prior to landfall of 

hurricane Erin (August 3, 1995). The results of 

the study indicate that acoustical techniques can 

yield useful estimates of continuous 

(instantaneous) discharge in Perdido Bay. Useful 

estimates of average daily net flow rates can also 

be obtained, but the accuracy of these estimates 

will be limited by small rating shifts that 

introduce bias into the instantaneous values that 

are used to compute the net flows.

 Instantaneous loads of total nitrogen ranged 

from -180 to 220 grams per second for the samples 

collected during the study, and instantaneous loads 

of total phosphorous ranged from -10 to 11 grams 

per second (negative loads indicate net upstream 

transport). The chloride concentrations from the 

water samples collected from Perdido Bay 

indicated a significant amount of mixing of 

saltwater and freshwater. Mixing effects could 

greatly reduce the accuracy of estimates of net 

loads of nutrients or other substances. The study 

results indicate that acoustical techniques can 

yield acceptable estimates of instantaneous loads 

in Perdido Bay. However, estimates of net loads 

should be interpreted with great caution and may 

have unacceptably large errors, especially when 

saltwater and freshwater concentrations differ 

greatly.

INTRODUCTION

Estuaries and tidally-affected streams are important natural resources valued for their recreational, 

economic, ecological, and aesthetic benefits. Pollutant 

discharges to estuaries and tidally affected streams are 

regulated to protect their water quality. Effective regulation of these pollutant discharges requires an under-

±10 000 ,
2 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

standing of tidal variations in flow and chemical flux 

rates (constituent loads). Until recently, a key limitation in improving our understanding of flow and loading variations in tidally-affected streams and estuaries 

was the difficulty and large expense associated with 

successfully measuring continually varying flow in 

these areas. New technology, employing acoustic techniques, is now available to accurately measure flow 

and determine average velocities in stream cross sections. This technology has also made it possible to 

estimate constituent loads in tidal areas with a much 

greater degree of accuracy.

In 1994, the U.S. Geological Survey (USGS) 

began a pilot study in cooperation with the Florida 

Department of Environmental Protection (FDEP) to 

evaluate the effectiveness of acoustical methods for 

measuring flows and estimating constituent loads in 

Perdido Bay. The specific objectives of the study were 

to determine daily mean discharges in Perdido Bay, 

vertical and horizontal velocity variations across flowmeasuring sections, and estimate constituent loads for 

selected flow conditions. The purpose of this report is 

to describe the methods employed in the study, and the 

results of analyses of water-flow, water-quality, and 

constituent-loading data. Descriptions of instantaneous and daily mean discharge values (estimated 

from data collected near the U.S. Highway 98 bridge 

across Perdido Bay) are presented for the period 

August 18 through September 28, 1995. Water-quality 

data and constituent-load estimates at this cross 

section are presented for selected periods during the 

spring and summer of 1995. Additional water-quality 

data collected near the mouths of the Perdido River 

and Elevenmile Creek during the spring of 1995 are 

also presented in the report. 

DESCRIPTION OF STUDY AREA

Perdido Bay covers an area of approximately 

28 square miles along the common border of southwestern Alabama and the western panhandle of Florida, near 

the city of Pensacola, Florida (fig. 1). Approximately 10 

square miles of the bay lie above the U.S. Highway 98 

bridge. The average depth of the bay is approximately 7 

feet, with deeper parts (10 feet and greater) of the bay 

occurring near the mouth of the Perdido River and 

downstream from the U.S. Highway 98 bridge (Livingston, 1992). Depths of approximately 15 feet were also 

observed near the center of the channel at the U.S. 

Highway 98 bridge during the study.

Perdido Bay forms the outlet for the Perdido 

River Basin, which drains an area of 913 square miles. 

An additional 130 square miles of land contribute runoff 

to the bay above the U.S. Highway 98 bridge (where 

most of the data were collected during the study). Several rivers and streams discharge into the bay, the most 

prominent being the Perdido River and its tributaries, 

the Blackwater and Styx Rivers and Dyas and Brushy 

Creeks. Elevenmile Creek and Bayou Marcus are two 

smaller streams that also discharge to the Perdido Bay 

upstream of U.S. Highway 98. The average annual runoff to Perdido Bay is approximately 2,000 cubic feet per 

second, based on a simple extrapolation of average runoff values obtained from gaging stations located on the 

streams mentioned above.

Average annual precipitation at Pensacola is 

approximately 63 inches, and ranges from less than 45 

to greater than 80 inches during more extreme years. 

Precipitation is commonly associated with the passage 

of cold fronts during the winter and intense, convective thunderstorms during the summer. Late winter, 

early spring, and summer are typically the wettest 

periods, while the late spring and fall months are 

typically the driest. Average annual potential evaporation in the area is approximately 47 inches per year 

(Kohler and others, 1959).

Flow within the bay is affected by tidal fluctuations which are associated with astronomic (sunmoon-earth interaction) and meteorologic forces. Data 

from tide gages in the bay indicate that the tides are 

typically diurnal (one high and one low tide per day), 

although semidiurnal tides (two high and two low 

tides per day) with small differences between lower 

low water and higher high water did occur on an 

approximately 2-week cycle during the study. 

Although tide tables have not been developed for 

Perdido Bay, the National Oceanic and Atmospheric 

Administration (NOAA) tide tables (NOAA, 1994) 

indicate that average tidal range in Perdido Bay is less 

than 0.5 foot. In the current study, however, the 

observed tidal range was approximately 0.8 foot. 

Strong winds, when aligned with the longitudinal 

orientation of the bay can also induce water-level 

changes of up to 0.5 foot (Livingston, 1992; Niedoroda, 1992). However, this orientation (northeast to 

southwest) roughly parallels the direction with the 

lowest frequency of strong winds (Livingston, 1992; 

Niedoroda, 1992).
Description of Study Area 3

Location

of

Study

Area

FLORIDA

ALABAMA GEORGIA

EXPLANATION

PERDIDO RIVER

BASIN BOUNDARY

Perdido River

River

Styx

Mobile

Bay

88°00′

GULF OF MEXICO

0 10 MILES

0 10 KILOMETERS

31°00′

29°30′

87°30′

ALABAMA

Creek

Dyas

FLORIDA

Perdido

Bay

Blackwater River

Brushy

Creek

Pensacola

Blackwater River

Creek

Elevenmile

Bayou Marcus Creek

Styx River

Perdido

Bay

0 2 4 MILES

0 2 4 KILOMETERS

ALABAMA

FLORIDA Perdido River

Figure 1. Location of study area.
4 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

Various forms of agriculture, including silviculture, are the dominant land use in the northern and western parts of the contributing area to Perdido Bay. 

Residential, commercial, and industrial land uses occur 

in the southeastern part of the area, primarily in and 

around Pensacola. Perdido Bay receives organic carbon, 

color, and nutrient loads from a large paper mill that discharges effluent to Elevenmile Creek. Effluent from 

sewage treatment plants is another source of nutrient 

and organic carbon loadings to the bay.

METHODS OF STUDY

To satisfy the study objectives, three basic datacollection activities were conducted during the study. 

These include (1) monitoring water-surface elevation 

(stage) and water velocity on a “continuous” basis 

(data collected every 15 minutes) at a gaging station 

established in the boat channel under the center span 

of the bridge, (2) periodic discharge measurements, 

and (3) periodic water-quality measurements of 

nutrients and other chemical constituents and physical 

properties. Discharge measurements were used to 

(1) develop equations relating discharge to velocity 

and water-level data collected at the gaging station, 

(2) characterize vertical and horizontal variations in 

water velocity, and (3) compute chemical loads from 

the water-quality data. The following sections of the 

report describe the methods used to make discharge 

measurements and characterize the spatial variability 

of velocity (velocity field characterization), estimate 

continuous and daily mean discharge from data 

collected at the gaging station, and characterize water 

quality and constituent loads.

Methods Used to Measure Discharge and 

Characterize Velocity Fields

A boat-mounted broad band acoustic Doppler 

current profiler (ADCP) was used to measure discharge and evaluate velocity fields at the U.S. Highway 98 cross section. The ADCP uses sound to 

measure water velocity. An ADCP uses the Doppler 

effect by bouncing an ultrasonic sound pulse off small 

particles of sediment and other zooplankton (scatterers) that are present in the water column (Simpson and 

Oltmann, 1992). The Doppler effect refers to the frequency change of the transmitted sonar signal caused 

by the relative motion between the ADCP and the 

scattering material in the water column (RD Instruments, 1992). Since this suspended material is moving 

at the same velocity as the water, the magnitude of the 

Doppler effect is directly proportional to the current 

velocity (RD Instruments, 1992, p. 1-3). By sampling 

segments of the returning sonar signal in successive 

time increments, the instrument measures velocities in 

discrete depth increments in the water column called 

depth cells (acoustic signals from shallow cells return 

to the ADCP before signals from deeper cells, thus 

earlier segments of the signal correspond to shallower 

data). The area of these depths cells is determined by 

the ADCP as it measures the elapsed time and velocity 

of the vessel relative to the stream bottom. The ADCP 

also measures depth to the bottom as the vessel moves.

There are limitations to the ADCP technology, 

however. The ADCP does not measure discharge near 

the surface, bottom, and at the sides of the transect 

where the water is too shallow or vessel placement is 

not possible. Velocities at the surface are not measured 

due to the submergence of the transducer head beneath 

the water surface (transducer draft) and due to a 

“blanking” distance below the transducer which corresponds to electronic and transducer recovery time after 

signal transmission. For the 1,200 kilohertz ADCP 

used in this study, the blanking distance is about 

0.98 foot (Mike Simpson, U.S. Geological Survey, 

written commun., 1997). The transducer draft and 

blanking distance are measured and used by the 

system software to estimate the surface discharge (by 

extrapolating a velocity value from the shallowest 

velocity data measured by the ADCP). 

The acoustic wave front from each transducer 

face consists of a main beam and side lobes. Near the 

bottom, where side lobe interference causes loss of 

reception of the main beam acoustic signal, no velocities or discharges can be determined. Again, discharge 

in this region near the bottom is estimated by the 

system software by extrapolating a velocity value 

from the deepest measured velocity data. For the 

1,200-kHz system used in this study, the bottom 

6 percent of the water column velocities are estimated 

(Simpson and Oltmann, 1992). Discharge in the near 

shore segments, where the depth of water is too 

shallow for the system to operate, is computed by 

estimating the near shore area (as one half of the depth 

at the last known vertical velocity sounding multiplied 

by the distance from that sounding to the shore) and 

the mean velocity of the near shore area (as mean 

velocity of the last non-missing vertical velocity 

sounding multiplied by 0.707).
Methods of Study 5

Estimation of Continuous and Daily-Mean 

Discharge from Data Collected at the 

Gaging Station

Continuous stage and water-velocity measurements were made at a gaging station established in the 

center of the channel beneath the U.S. Highway 98 

bridge. These stage and “index” velocity data were 

collected so that the total area and mean velocity of the 

cross section at U.S. Highway 98 could be estimated 

on a continuous basis. Note that the term, index velocity, refers to the velocity measured at the gage, and is 

generally different from the mean cross-sectional 

velocity because the velocity measured by the gaging 

station samples only a small part of the total cross section. The discharge through the cross section is equal 

to the product of the cross-sectional area and mean 

velocity of the cross section. Thus, continuous discharge can be estimated from the continuous stage and 

index velocity data if relations can be developed 

between stage and cross-sectional area, and between 

index velocity and mean cross-sectional velocity. The 

following sections describe procedures used to collect 

stage and index velocity data, and to develop stagearea and index velocity-mean velocity relations.

Collection of Stage Data and Development of 

Stage-Area Relation

Stage data were collected using a float-type gage 

and digital shaft encoder, which were installed in a gage 

house mounted above a stilling well. Stage data were collected every 15 minutes under routine monitoring operations and at shorter intervals (2 to 5 minutes) when 

discharge measurements were being made. The gage was 

installed on the eastern bridge fender near the middle of 

the U.S. Highway 98 cross section. A vertical staff gage 

(Rantz and others, 1982) was installed at the downstream 

end of this fender. This gage was used a reference for correcting stage data that were in error due to vertical movements of the gage house or loss of datum correction 

information (stored in the encoder) due to lightning 

strikes. The datums of the staff and float gages were 

determined using a level and rod, and were used to calculate stage above sea level (using the National Geodetic 

Vertical Datum of 1929). The stage data should have an 

error standard deviation of about 0.01 to 0.02 foot, except 

for periods between April 27 and July 19 and between 

July 19 and August 16, 1995, when the gage house 

dropped by 0.30 foot and 0.27 foot, respectively. Concurrent staff-gage and float-gage readings were made at the 

ends of these two time intervals, and a time-weighted 

correction was applied to data within these intervals.

Cross-sectional areas were estimated from depth 

sounding (depth and distance) data from the ADCP 

discharge measurements. Depth soundings were 

converted to bottom elevations by subtracting depth 

observations from the stage observed during a given 

ADCP measurement. This resulted in a bottom elevation and distance (from right or left bank) data set for 

each ADCP measurement. A stage-area data set was 

then created by processing each of the bottom elevation and distance data sets (from individual ADCP 

measurements) with a channel geometry analysis 

program (Regan and Schaffranek, 1985). The stagearea data sets from all of the ADCP measurements 

were then plotted together, and a piecewise-linear 

equation relating stage and area was developed from 

the data by graphically fitting a line through the data.

Collection of Water-Velocity Data and 

Development of Index Velocity-Mean Velocity 

Relation

The index velocity observations were made by 

measuring the mean velocity of a portion of the middle 

of the U.S. Highway 98 cross section (between the two 

bridge fenders on either side of the boat channel) using 

an acoustic velocity meter (AVM; Rantz and others, 

1982; Laenen, 1985) from December 13, 1994, to 

September 28, 1995. An AVM installation consists of 

one or more pairs of sonic transducers and a transceiver that transmits and receives acoustic signals to 

and from the transducers. At the U.S. Highway 98 

gage, one pair of transducers was deployed at an average depth of 3.5 feet. Transducers were separated by a 

distance of approximately 100 feet and the resulting 

acoustic path between the two transducers was oriented approximately 60 degrees from the direction of 

water flow. The AVM measures the difference 

between the upstream and downstream travel time of 

acoustic signals (sound moves faster downstream, and 

the difference in upstream and downstream travel 

times is inversely proportional to the water velocity), 

and computes an index velocity (water velocity passing through the acoustic path between the AVM transducers) using the following equation (Laenen, 1985):

v (1) i

b

2cos( ) Θ --------------------- 1

tus

----- 1

tds

= – -----
6 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

where is the computed index velocity, is the 

length of the acoustic path, is the angle between the 

acoustic path and water flow direction, and and 

 are the respective upstream and downstream 

travel times for the acoustic signal. Index-velocity data 

were collected every 15 minutes under routine monitoring operations and at shorter intervals (2 to 5 minutes) when discharge measurements were being made.

Measurements of the mean water velocity of the 

cross section ( ) were obtained by dividing measured 

discharge (from the periodic ADCP discharge measurements) by the cross-sectional area at the time of 

measurement (as determined from the stage-area 

rating and the mean stage at the time of the ADCP 

measurement). Linear equations relating index velocity to the mean cross-sectional velocity were then 

developed by (1) plotting the mean cross-sectional 

velocities versus the mean of the index velocities 

measured during a given ADCP measurement, and 

(2) drawing a best fit line through segments of the 

versus plot that exhibited a linear relation. This set 

of linear equations, together, define the relation 

between the mean cross-section velocity and index 

velocity (otherwise referred to as the velocity rating).

Computation of Continuous and Filtered DailyMean Discharge

Once stage-area and - ratings were developed, discharge estimates were computed using all of 

the stage and index velocity data. This was accomplished by substituting the continuous stage and index 

velocity values into the stage-area and - ratings, 

respectively, and multiplying the resulting crosssectional area and mean velocity estimates to determine cross-sectional discharge ( ). Application of 

this method resulted in a data set with estimates of 

instantaneous discharge at 15-minute intervals.

Daily mean discharge data were computed by 

applying a low-pass filter (Godin, 1972) to the instantaneous discharge data set. The discharge data are 

filtered for two reasons. First, daily-mean discharges 

computed from unfiltered instantaneous values may be 

misleading because the daily-mean values will have a 

cyclic behavior with an amplitude and period that 

differs from that of the original data. These differences 

are related to the differences between the period over 

which the data are averaged and the periods of cyclic 

components contained in the original data (Lew 

Delong, U.S. Geological Survey, written commun., 

June 1993). Filtering is also necessary to evaluate the 

nontidal component of flow occurring in a tidal reach. 

For example, Perdido Bay continuously receives large 

inflows from the Perdido River which results in a net 

downstream flow of water through the bay. Water also 

flows in and out of the bay because of tidal currents. 

Evaluating the magnitude of flow within the bay for 

periods as short as a few days requires filtering out the 

tidal flow components. In this study, daily mean 

filtered discharges were computed from filtered 

instantaneous discharge values that resulted from 

applying the Godin filter to the unfiltered instantaneous discharge estimates (which were calculated 

from stage and index velocity data).

Water Quality and Constituent Loading

Seven water-quality samples were collected in 

April and September 1995 for analysis of nutrients and 

other chemical constituents and physical properties. 

The April samples were collected shortly after a 

period of seasonal, high flows, while the September 

samples were collected during a period of average 

flow. The April and September samples were collected 

using the equal-discharge method and either a bottle 

sampler or a point sampler (during vertically-stratified 

conditions in April). The equal-discharge method was 

implemented by selecting three to five locations (verticals) in the cross section that were centered in subsections of the cross section that represented equal 

proportions of the total discharge (for example, if 

samples were collected at five verticals, each vertical 

would be located at the center of a subsection containing 20 percent of the total discharge through the cross 

section). The samples collected at each vertical sampling station were then composited in a churn splitter, 

which was used to thoroughly mix the samples to 

obtain a discharge-weighted sample for the cross 

section. A bottle sampler was used to collect the five 

samples collected during September 1995. This type 

of sampler was employed because there were no 

abrupt changes in temperature or salinity with depth 

(no thermocline or halocline, respectively), indicating 

well-mixed conditions during this period. The bottle 

sampler consisted of an open 1-liter polyethylene 

bottle which was held in a metal carrier that was 

suspended by nylon cord. The sampler was deployed 

by lowering and raising it at a constant rate between 

the water surface and the bay bottom (samples 

obtained in this manner were, therefore, depthvi b

Θ

tus

tds

V

vi

V

vi V

vi V

Q

ˆ
Results of Study 7

integrated samples). A point (Klemmer) sampler was 

used to collect the two samples on April 28, 1995. 

This type of sampler was used to collect samples at 

depths of 3.2 and 8 feet, which was above and below a 

thermocline and halocline that occurred during this 

time.

The samples that were collected as described 

above were analyzed for dissolved chloride, nutrients 

(nitrogen and phosphorous constituents), dissolved 

solids, suspended solids, color, turbidity, and specific 

conductance. These analyses were performed in accordance with methods described in Skougstad and others 

(1979). Five of these samples were collected during 

periods when discharge measurements were made 

using the ADCP. Loading rates were calculated from 

the nutrient, solids, and chloride analyses by multiplying the reported concentrations of these constituents 

by the measured discharge and the appropriate conversion factor:

(2)

where is the loading rate for a given constituent, in 

grams per second; is the concentration of a given 

constituent in milligrams per liter; and is the 

discharge, in cubic feet per second.

 Measurements of temperature, specific conductance, dissolved oxygen, and pH were also made at a 

variety of locations in the U.S. Highway 98 cross section to evaluate spatial variations in these properties. 

These measurements were made when water-quality 

samples were collected on April 27 and 28, and September 8. Spatial variations in these properties were 

evaluated by making measurements at 2- to 3-foot 

depth increments at 5 vertical sampling stations 

located at the center of subsections representing 20 

percent of the total discharge through the cross section. Measurements at 3-foot depth increments were 

made at a single vertical location near the gaging station on September 28. Similar measurements were 

made near the mouths of the Perdido River and Elevenmile Creek on April 28. In addition to these synoptic 

surveys, temperature data were collected on a “continuous” basis (once every 15 minutes) in the middle of 

the cross section (adjacent to the stage recorder) at 

three depths (approximately 2, 6, and 10 feet below 

the water surface) from July 30, through September 

28, 1995.

RESULTS OF STUDY

Stage Observations and Stage-Area 

Relation

Stage data were collected on a continuous basis 

(once every 15 minutes) from December 13, 1994 to 

September 28, 1995. Tidal effects were readily 

apparent in the continuous stage data, which exhibited 

a diurnal fluctuation pattern during all periods of a 

given lunar month, except during periods approximately 7 and 21 days after a spring tide, when a 

semidiurnal pattern was often apparent (fig. 2). This 

semidiurnal pattern is evident in figure 2 during 

September 9-11 and September 23-24. Summary 

statistics of the continuous stage data are shown in 

table 1 for the period from August 18 to September 28, 

1995. Data prior to this period are not included in 

table 1 because discharge was not computed prior to 

August 18 (as described later in the report in the 

discussion of the velocity rating). Note that the peak 

measured stage (3.68 feet) during the study occurred 

prior to this period and was due to storm surge that 

coincided with the passage of Hurricane Erin during 

August 3-4, 1995.

The stage-area rating that was developed from 

the ADCP depth soundings is shown in figure 3. These 

data are grouped by measurement trip in figure 3, 

which makes it possible to evaluate changes in the 

stage-area relation that may have occurred between 

trips. The data shown in figure 3 show two distinct 

“bands,” with one band of data corresponding to the 

ADCP measurements made in April and July and 

another corresponding to the measurements made 

September. The cross-sectional areas in September 

were approximately 3,600 to 4,400 square feet greater 

than those in April and July, possibly a result of scour 

in the U.S. Highway 98 section of Perdido Bay after 

Hurricane Erin. Differences were also evident between 

the early and late September measurements. The areas 

from the September 8-9 measurement period were generally 1,600 to 1,800 square feet greater in area than the 

September 26-28 data, although the significance of this 

difference is questionable because most of the September 8-9 stage-area data were within the range of variation of the September 26-28 data. Accordingly, the 

early and late September data were lumped together 

and an equation describing the stage-area relation was 

developed by graphically fitting a line through the data 

(fig. 3). This relation was applied to stage data 

L = 0.02832CQ

L

C

Q
8 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

SEPTEMBER 1995

3

2

1

0

-1

-2

-3

STAGE, IN FEET ABOVE SEA LEVEL

INDEX VELOCITY, IN FEET PER SECOND

STAGE

INDEX VELOCITY

Figure 2. Stage and index velocity at Perdido Bay at U.S. Highway 98 near Pensacola, Florida, during 

September 7-24, 1995.

Table 1. Summary statistics of continuous stage, area, velocity, and discharge data, August 18-September 28, 1995

[Negative values indicate velocities or discharge in the upstream direction. ft, feet; ft2, square feet; ft/s, feet per second; ft3/s, cubic feet per second]

Variable Mean Standard 

deviation Maximum Minimum 90th

percentile

75th

percentile Median 25th

percentile

10th

percentile

Number of 

observations

Stage, in ft 1.14 0.28 1.85 0.32 1.55 1.35 1.11 0.92 0.79 3,828

Area, in ft2 38,300 1,100 41,000 35,300 39,900 39,100 38,200 37,500 37,000 3,828

Index velocity, , in ft/s -0.06 0.25 1.49 -1.7 0.29 0.10 -0.08 -0.23 -0.38 3,814

Mean cross section 

velocity, , in ft/s

0.05 0.28 1.40 -0.62 0.42 0.26 0.12 -0.25 -0.29 3,814

Computed discharge, , 

in ft3/s

2,000 10,800 54,600 -24,200 16,000 10,100 4,500 -9,700 -11,200 3,814

vi

V

Q

ˆ
Results of Study 9

collected after the passage of Hurricane Erin. A 

similar procedure was used to fit a stage-area relation 

from the April and July ADCP data and applied to the 

prehuricane period. Summary statistics describing the 

cross-sectional areas that were estimated from the 

continuous stage data are shown for the period of 

August 18 to September 28, 1995, in table 1. As seen 

in the table, cross-sectional areas ranged from 35,300 

to 41,000 square feet.

Discharge Measurements

Discharge measurements were made with the 

ADCP during four periods, April 27-28, July 21, 

September 7-8, and September 27-28 (table 2). 

Collection of these data was necessary for developing 

a velocity rating, evaluating spatial and temporal flow 

variations, and computing loads for selected constituents. Changes in the magnitude, direction, and spatial 

patterns of flow are discussed in this section. In the 

following discussion, positive values of net flow are 

referred to as net-downstream flow (toward the Gulf 

of Mexico). Conversely, negative values of net flow 

are referred to as net-upstream flow or flow in a direction opposing the natural flow of freshwater in the 

Perdido River, which is toward the Gulf of Mexico.

Several patterns were evident in the measured 

cross-sectional velocity profiles made with the ADCP. 

First, bidirectional (simultaneous downstream and 

upstream) flow is quite common. No periods of static 

flow were seen for the whole cross section; rather net 

flow reversals occurred as upstream and downstream 

flow regions equaled each other in magnitude. 

-1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

STAGE, IN FEET ABOVE SEA LEVEL

55,000

50,000

45,000

40,000

35,000

30,000

25,000

AREA, IN SQUARE FEET

STAGE-AREA RATING

SEPTEMBER 26-28, 1995

SEPTEMBER 8-9, 1995

APRIL 27, 1995

JULY 21, 1995

Figure 3. Stage-area rating.
10 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

During high- or low-tide conditions, bidirectional flow 

still occurred even as the volume of water in the 

measured cross section was at a maximum or minimum, respectively. The actual reversal of net 

discharge through the measured cross section occurred 

after the tidal extremes. This is illustrated in figure 4 

where the upper cross section shows the velocity profile at low tide at 1904 hours on April 27, 1995, when 

the discharge was approximately 9,700 cubic feet per 

second downstream. During this period, a relatively 

high downstream flow region existed in the upper part 

of the water column and a relataively low upstream 

flow region surrounded by a near static flow region 

occurred deeper in the water column in the eastern half 

of the cross section. The lower cross section in figure 4 

shows the velocity profile during rising tide 4.5 hours 

later at 2330 hours. Discharge has reversed and is 

Table 2: Discharge measurements made with the acoustic 

Doppler current profiler at U.S. Highway 98

[Negative discharge values indicate net upstream flow]

Date Time

Discharge, 

in cubic feet

per second

04/27/95 1804 9,700

04/27/95 2007 3,400

04/27/95 2037 3,900

04/27/95 2322 -1,600

07/21/95 0246 -3,700

09/07/95 1532 -13,800

09/07/95 1555 -14,900

09/07/95 1613 15,200

09/07/95 1634 15,700

09/07/95 1656 17,400

09/07/95 1738 13,400

09/07/95 1805 11,300

09/08/95 0753 -11,000

09/08/95 0812 -12,300

09/08/95 0831 -10,900

09/08/95 0850 -12,000

09/08/95 0945 -11,600

09/08/95 1008 -13,400

09/08/95 1032 -12,800

09/08/95 1054 -13,200

09/08/95 1116 -11,600

09/08/95 1312 -11,200

09/08/95 1331 -8,900

09/08/95 1352 -5,500

09/08/95 1413 -2,300

09/26/95 1925 -9,000

09/26/95 1952 -10,900

09/26/95 2016 -9,900

09/26/95 2043 -9,100

09/27/95 0803 12,800

09/27/95 0828 12,800

09/27/95 0847 12,400

09/27/95 0908 9,800

09/27/95 0937 11,300

09/27/95 1026 9,000

09/27/95 1044 5,600

09/27/95 1104 3,200

09/27/95 1245 600

09/27/95 1315 -500

09/27/95 1453 -9,100

09/27/95 1511 -10,000

09/27/95 1545 -9,000

09/27/95 1611 -9,000

09/27/95 1627 -9,000

09/27/95 1644 -8,300

09/27/95 1702 -8,500

09/27/95 1720 -9,000

09/27/95 1738 -8,400

09/27/95 1922 -10,500

09/27/95 1941 -9,800

09/27/95 1922 -11,300

09/27/95 2052 -13,100

09/27/95 2134 -12,400

09/27/95 2151 -11,800

09/27/95 2216 -10,400

09/27/95 2234 -11,200

09/27/95 2322 -10,400

09/28/95 0804 12,800

09/28/95 0826 12,400

09/28/95 0857 12,500

09/28/95 0939 11,300

09/28/95 1010 10,000

09/28/95 1033 9,300

09/28/95 1051 9,500

09/28/95 1112 9,100

09/28/95 1139 7,700

09/28/95 1200 9,600

09/28/95 1219 8,400

09/28/95 1257 6,800

09/28/95 1323 4,600

09/28/95 1341 5,600

09/28/95 1404 3,600

Table 2: Discharge measurements made with the acoustic 

Doppler current profiler at U.S. Highway 98 --(Continued)

[Negative discharge values indicate net upstream flow]

Date Time

Discharge, 

in cubic feet

per second
Results of Study 11

approximately 1,600 cubic feet per second upstream. 

At that time, the lower depth upstream flow region had 

grown in both size and extent whereas the downstream 

flow region had decreased extensively.

 Second, the net flow reversals occur quickly. 

For example, the data collected on September 27 indicate a change from a net-downstream flow of 

620 cubic feet per second to a net-upstream flow of 

510 cubic feet per second occurring within 30 minutes. 

Changes from net flows of several thousand cubic feet 

per second probably occurred over slightly longer 

periods.

Finally, while the spatial patterns of flow (velocity or flow fields) can be highly variable and complex, 

there was a similar pattern seen in both the April and 

September ADCP measurements. Much of the flow in 

the section often concentrated in cells of relatively 

high flow. During ebbing conditions, downstream flow 

typically occurs in the shallower water depths of the 

cross section with mixed (upstream and downstream) 

flow occurring deeper in the section (fig. 5, top profile). During flooding conditions (fig. 5, bottom profile), upstream flow generally occurs at greater depths, 

middle channel, with mixed (bidirectional) or slow 

flow in shallower depths.

Acoustical Velocity Meter Velocity 

Measurements and Index Velocity-Cross 

Section Velocity Rating

Tidal effects were apparent in the continuous 

data, exhibiting a pattern that was similar to the pattern 

in stage data. Maximum downstream and upstream 

index velocities generally occurred midway through 

the ebbing and flooding tides, and stagnant values 

occurred near high and low tides (fig. 2). Summary 

statistics for the continuous data are shown in 

table 1. No longer-term (study-period time scale) 

trends were evident in the data.

The velocity rating ( versus ) and the data 

points that were used to fit the rating are shown in 

figure 6. This velocity rating was developed from the 

previously described stage-area ratings and the AVM 

measured index velocities and ADCP discharge data 

collected during September 1995. The rating is comprised of three linear segments corresponding to 

 foot per second, foot 

per second, and foot per second. Analysis 

of figure 6 indicates that most of the data points used 

to fit the lower, middle, and upper segments of the 

rating are within plus or minus 0.05, 0.17, and 0.04 

foot per second of the rating, respectively. The larger 

Figure 4. Flow fields determined from discharge measurements made on April 27, 1995.

vi

vi

V vi

vi < –0.14 0.14 vi – ≤ < –0.12

vi ≥ –0.12
12 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

scatter around the middle segment of the rating is due 

to the steep slope of this segment. This steep slope 

represents an abrupt shift in the velocity rating associated with conditions near slack (low) tide as the tidal 

current reverses from ebbing to flooding flow. 

Because of this abrupt transition, discharge values 

calculated during these conditions can have large 

errors. This is evident in the ADCP measurements 

made shortly after low tide on September 27. Differences between calculated and measured discharges for 

three measurements made during this period ranged 

from approximately -7,600 to 12,800 cubic feet per 

second. The summary statistics of the continuous 

data indicate that less than 2 percent of the data fell 

within the range of values that correspond to the middle, “high-error,” segment of the rating curve. The 

velocity rating shown in figure 6 was used to compute 

mean cross-sectional velocities ( ) from the continuous index velocity values collected by the AVM. Summary statistics of these computed values are shown 

in table 1.

 Data collected prior to August were not 

included in table 1 because the velocity rating was not 

applicable to all of the index-velocity measurements 

collected during the study. The cross-sectional 

velocity and index-velocity data collected during the 

periods when ADCP measurements were made in 

April and July did not fit the velocity rating described 

above (fig. 6). This indicates that a shift in the velocity 

rating may have occurred at some time prior to 

September 1995. One possible explanation for such a 

shift is that changes may have occurred during 

Hurricane Erin, which struck Pensacola on August 3, 

1995, and produced a storm surge of approximately 

2.5 feet in Perdido Bay. Unfortunately, the prehurricane ADCP data are insufficient to quantify possible 

shifts in the velocity rating or develop a prehurricane 

velocity rating (equipment problems prevented more 

ADCP data from being collected during the April and 

July trips). As described previously, differences 

between the pre- and posthurricane stage-area 

relations suggest that significant scouring of the U.S. 

Highway 98 cross section may have occurred due to 

the hurricane. Such changes could result in shifts in 

the velocity rating by changing the relative depth 

(depth divided by total depth) at which the AVM transducers are suspended. Because insufficient data were 

available to define a prehurricane velocity rating, discharge estimates were not computed from the 

continuous stage and index velocity data for the 

prehurricane period.

vi

vi

V

V

Figure 5. Flow fields determined from discharge measurements made on September 7-8, 1995.
Results of Study 13

Continuous Discharge Estimates and 

Net-Downstream Flows

Continuous discharge estimates were computed 

for the period from August 18 to September 28, 1995, 

using the continuous stage and index-velocity, and the 

stage-area and velocity ratings described previously. 

These data were then processed with a low-pass filter 

to eliminate the tidal components from the discharge 

data. These data were subsequently averaged to obtain 

daily mean discharge values that represent net downstream flows. Data were not computed prior to August 

18, 1995, either because of missing data or because the 

velocity rating was poorly defined for the period prior 

to landfall of hurricane Erin (August 4, 1995). This 

section presents the results of these computations and 

describes the temporal variability, errors, and limitations associated with the continuous and filtered daily 

mean discharge estimates.

 As previously mentioned in the methods 

section, continuous discharge estimates were computed by substituting the continuous stage and index 

velocity values into the stage-area and - ratings, 

respectively, and multiplying the resulting crosssectional area and mean velocity estimates to determine cross-sectional discharge ( ). The resulting 

data show a pattern of daily fluctuations that are similar to that seen in the index-velocity data (figs. 2 and 

7), although the peaks are flatter and the between-peak 

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

INDEX VELOCITY, IN FEET PER SECOND

0.5

0.4

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

-0.4

MEAN CROSS-SECTIONAL VELOCITY, IN FEET PER SECOND

VELOCITY RATING

04/27-28/95

09/08-09/95

09/26-28/95

07/21/95

Figure 6. Relation between mean cross-section velocity and index (Acoustic Velocity Meter) velocity in Perdido Bay at 

U.S. Highway 98 near Pensacola, Florida.

vi V

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ˆ Qˆ
14 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

transitions (between net-upstream and downstream 

flow) steeper for the hydrograph. This pattern 

arises because the computed values are more sensitive to variations in index velocity (and less sensitive 

to variations in stage) and because of the nonlinear 

shape of the velocity rating (the velocity rating is 

much steeper for slack water conditions), as shown in 

figure 3. Summary statistics describing the distribution 

of values computed from the continuous stage and 

index-velocity data are presented in table 1.

The accuracy of the continuous discharge 

estimates (computed from individual stage and indexvelocity observations at the U.S. Highway 98 gage) 

was assessed by evaluating the differences between 

the ADCP measured discharge values and corresponding discharge estimates computed from the stage and 

index-velocity data collected at the time of the ADCP 

discharge measurements. Most of these “residual 

differences” were within a range of plus or minus 

2,000 cubic feet per second. The upper and lower 

quartile values of the residuals were 1,100 and -780 

cubic feet per second, respectively, and the 90th and 

10th percentiles were -2,100 and 1,600 cubic feet per 

second, respectively. Additionally, no obvious bias 

was evident in plots of these residuals. These data suggest that the errors in the continuous discharge estimates for the posthurricane period were generally less 

than plus or minus 2,000 cubic feet per second, which 

is equivalent to approximately 20 percent of the typical flow rates during ebbing or flooding conditions. 

The errors associated with the continuous discharge 

estimates during slack water conditions can be much 

larger; however, the periods during which errors of 

this magnitude are likely to occur are limited because 

of the brevity of slack water periods.

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

SEPTEMBER 1995

50,000

40,000

30,000

20,000

10,000

0

-10,000

-20,000

-30,000

-40,000

-50,000

AREA, IN SQUARE FEET

DISCHARGE, IN CUBIC FEET PER SECOND

CROSS-SECTIONAL AREA

COMPUTED DISCHARGE

Figure 7. Cross-sectional area and computed discharge in Perdido Bay at U.S. Highway 98 near Pensacola, Florida.

Q

ˆ

Q

ˆ

Q

ˆ
Results of Study 15

Filtered daily mean discharge values ( ) were 

computed for the period from August 18 to September 

26, 1995. The filtered daily mean discharge values were 

compared to runoff from upstream gages as a check on 

the reasonableness of the discharge values computed at 

the U.S. Highway 98 gage. In this comparison, runoff to 

Perdido Bay upstream of U.S. Highway 98 ( ) was 

estimated by adding the daily mean streamflows from 

the Perdido River at Barrineau Park, Florida (station 

number 02376500) and the Styx River near Elsanor, 

Alabama (station number 02377570) and multiplying 

the result by 1.775, which is the ratio of the combined 

drainage areas of these two stations and the approximate drainage area to the Perdido Bay upstream from 

U.S. Highway 98. These estimated daily mean discharges to Perdido Bay upstream of U.S. Highway 98 

were then averaged over the period for which filtered 

daily mean discharge values were available from the 

U.S. Highway 98 gage (August 17, 1995, and later). 

This average discharge to Perdido Bay (exptrapolated 

from upstream gages) was approximately 1,600 cubic 

feet per second, which compares favorably to the average of the filtered daily mean discharges at the U.S. 

Highway 98 gage, 1,590 cubic feet per second.

Filtered mean daily flow values should be more 

accurate than the continuous data because the random 

error of such estimates is inversely proportional to the 

number of instantaneous discharge estimates used to 

calculate the average. The improvement in accuracy 

resulting from averaging is limited, however, because 

the continuous estimates are still subject to a variable 

(with time) bias that occurs due to subtle shifts in the 

rating curve. For example, a shift as small as 0.01 foot 

per second in the velocity rating would result in a bias 

of approximately 400 cubic feet per second, which is 

approximately 20 percent of the mean daily filtered 

value during the posthurricane period. This type of 

error is not reduced when daily average values are 

computed from instantaneous values, and detection 

and elimination of bias as small as this is probably not 

possible. Determination of the probable magnitude of 

these “random” shifts would require more ADCP 

measurement trips than were possible in this study. 

Such an exercise would yield valuable insight into the 

accuracy with which daily average flow rates could be 

estimated from AVM-equipped gages in tidal environments in which the magnitude of net flows is small in 

comparison to gross tidal inflows and outflows. The 

reader is referred to a report by Sloat and Gain (1995) 

for further discussion of the errors associated with 

instantaneous and average flow estimates obtained 

from continuous AVM data. The report by Sloat and 

Gain also describes the deployment of multiple AVM 

paths. Such an approach could yield more accurate 

discharge estimates in future AVM deployments in 

Perdido Bay because of the bidirectional nature of 

flow in the bay and the large width of the bay cross 

section.

The results described above indicate that the 

usefulness of the discharge estimates obtained in 

Perdido Bay using an AVM depends on the intended 

application of the data and is limited by the accuracy 

of the data. For example, the data are not very useful 

for estimating total inflow or outflow (for periods of a 

day or longer) because the errors associated with these 

estimates are much larger than the estimates themselves (the uncertainty of the sum of the individual 

instantaneous estimates is proportional to the number 

of estimates in the summation). However, the AVM 

data are useful for estimating instantaneous flow rates 

if errors on the order of 20 percent are acceptable. 

Data with this level of accuracy could be quite useful 

for calibrating or verifying a hydraulic model of 

Perdido Bay over periods of time that would be 

impractical for continuous measurement with an 

ADCP. Similarly, estimates of mean daily values may 

also be useful if errors on the order of 20 percent are 

acceptable.

Water Quality and Constituent Loads

Spatial Surveys of Temperature, Specific 

Conductance, Dissolved Oxygen, and pH

As described in the methods section, surveys of 

temperature, specific conductance, dissolved oxygen, 

and pH were conducted on April 27-28, and on September 8, 1995, (during water-quality sample collection) to evaluate the spatial variability of water quality 

during these periods. The results of these surveys are 

illustrated in the boxplots shown in figures 8-11. The 

plots were constructed by grouping measurements 

made at the same depth but at several different vertical 

sampling stations. The boxplots of the temperature and 

specific conductance data illustrate the abrupt thermoclines (fig. 8) and haloclines (fig. 9) present during 

the April surveys, as well as the absence of a strong 

halocline and a reversing (decreasing from surface to 

the middle depths, then increasing with depth towards 

the bottom) trend in the temperature profile during the 

Qf

Qus
16 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

September survey. Data collected from the surveys in 

both months indicate that horizontal variations in temperature and specific conductance were generally 

small when compared to variations with depth. Measurements of temperature and specific conductance 

were made on September 28, 1995, at a single vertical 

sampling station near the gage. The conductance profile on this date was similar to that of the September 8 

survey (conductance increased with depth but a strong 

halocline was not evident). The temperature measured 

on September 28 increased with depth, although the 

magnitude of the difference between near surface and 

near bottom measurements ranged from 0.3 to 1.1 °C 

from the early morning survey to the late morning survey, respectively. Measurements of temperature, specific conductance, dissolved oxygen, and pH were also 

made at selected vertical sampling stations near the 

mouths of the Perdido River and Elevenmile Creek 

(fig. 1) on April 28 and are presented in the tables 3-5.

Relations between water depth and dissolved 

oxygen and pH were also evident in the April 27-28, 

and September 8 cross-sectional surveys (figs. 10 and 

11). Dissolved oxygen concentrations generally 

increased with depth in the April surveys, although the 

differences in median concentrations were less significant because the horizontal variability of concentrations was generally greater (approaching or exceeding 

the range of concentrations from the water surface to 

bay bottom) at depths greater than or equal to 6 feet. 

Dissolved oxygen concentrations measured in the 

April surveys ranged from less than 6 to greater than 

10 milligrams per liter (fig. 10), which was equivalent 

to dissolved oxygen saturation levels of 70 to 

140 percent under the observed temperature and 

(5)

(5)

(5)

(5)

(5)

20

20 20

30

30 30

22

22 22

24

24 24

26

26 26

28

28 28

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 3

and more than 1.5 times the interquartile

range outside the quartile

Outlier data value more than 3 times the

interquartile range outside the quartile

(5) Number of observations

TEMPERATURE

(A) APRIL 27, 1995 SURVEY

(4)

(4)

(4)

(4)

(2)

DEPTH, IN FEET DEPTH, IN FEET

(B) APRIL 28, 1995 SURVEY

(4)

(5)

(4)

(3)

(5)

TEMPERATURE, IN DEGREES CELSIUS

(C) SEPTEMBER 8, 1995 SURVEY

Figure 8. Temperature observations from April and September surveys of Perdido Bay at U.S. Highway 98.
Results of Study 17

salinity conditions. Dissolved oxygen concentrations 

ranged from less than 2 to greater than 8 milligrams 

per liter during the September 8 survey (fig. 10). 

Differences between dissolved oxygen concentrations 

at the various depth intervals were also more distinct 

in the September 8 survey because the concentration 

ranges at individual depth intervals were much smaller 

than the overall concentration range (from water 

surface to the bay bottom).

Measured pH values ranged from 6.5 to 8.1 in 

the April surveys and generally increased with depth, 

although this pattern was less distinct in the April 28 

data because of greater horizontal variability in the 

shallower (less than 6 feet deep) data (fig. 11). A 

distinct pattern of decreasing pH with increasing depth 

was evident in the September survey data, with pH 

values ranging from 7.5 to 8.0 (fig. 11).

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 3

and more than 1.5 times the interquartile

range outside the quartile

Outlier data value more than 3 times the

interquartile range outside the quartile

(5) Number of observations

DEPTH, IN FEET DEPTH, IN FEET

(5)

(5)

(5)

(5)

(5)

40,000

40,000 40,000

0

0 0

10,000

10,000 10,000

20,000

20,000 20,000

30,000

30,000 30,000

SPECIFIC CONDUCTANCE

(A) APRIL 27, 1995 SURVEY

(4)

(4)

(4)

(4)

(2)

(B) APRIL 28, 1995 SURVEY

(4)

(5)

(4)

(3)

(5)

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

(C) SEPTEMBER 8, 1995 SURVEY

Figure 9. Specific conductance observations from April and September surveys of Perdido Bay at U.S. Highway 98.

Analyses of Synoptic Water-Quality Samples

Seven water samples were collected in April and 

September 1995 and the results of the analyses of these 

samples are shown in table 6. Two samples were 

collected concurrently on April 28: one above the 

halocline (at a depth of 1 meter), and one below the 

halocline (at a depth of 2.5 meters). The remaining 

samples, which were collected in September, were a 

composite of depth-integrated samples collected at 

several vertical sampling stations along the U.S. Highway 98 cross section. The April 28 samples were collected on an ebbing tide, approximately 6 hours after 

high tide. The September 8 sample was collected 

approximately 2 hours before a high tide. Samples were 

collected within 2 hours of low and high tides and on 

flooding and ebbing tides on September 27-28.
18 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

Table 3. Temperature, specific conductance, dissolved 

oxygen, and pH on April 28, 1995, approximately 1,000 feet 

from mouth of Elevenmile Creek, 10 feet from center of 

channel

[Latitude: 30°27’34” Longitude: 87°22’45”; bottom depth, 7.5 feet; time of 

observation approximately 1500 hours]

Depth, 

in feet

Temperature, 

in degrees 

Celsius

Dissolved 

oxygen, in 

milligrams 

per liter

Specific conductance, in microsiemens per 

centimeter

pH

1 23.7 3.6 590 7.1

3 21.3 3.0 630 7.1

5 20.4 3.0 690 7.1

7 20.4 3.7 700 7.1

The largest differences in the water-quality data 

occurred in the conductance, chloride, solids, and 

color values. Specific conductance values were lower 

for the April samples (ranging from approximately 

1,600 to 9,400 microsiemens per centimeter) than the 

September samples (which ranged from approximately 

27,000 to 29,000 microsiemens per centimeter). Chloride and solids concentrations showed a similar pattern 

with significantly higher concentrations occurring in 

the September samples. Color values exhibited the 

opposite pattern, with significantly higher values 

occurring in the April samples. The chloride and solids 

concentrations from the shallow and deep samples 

collected on April 28 are consistent with poor mixing 

of shallow, fresher water with deeper, more saline 

water at the time of sample collection.

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 3

and more than 1.5 times the interquartile

range outside the quartile

Outlier data value more than 3 times the

interquartile range outside the quartile

(5) Number of observations

DEPTH, IN FEET DEPTH, IN FEET

(5)

(5)

(5)

(5)

(5)

1

1 1

11

11 11

2

2 2

4

4 4

6

6 6

8

8 8

10

10 10

DISSOLVED OXYGEN

(A) APRIL 27, 1995 SURVEY

(4)

(4)

(4)

(4)

(2)

(B) APRIL 28, 1995 SURVEY

(4)

(5)

(4)

(3)

(5)

DISSOLVED OXYGEN, IN MILLGRAMS PER LITER

(C) SEPTEMBER 8, 1995 SURVEY

Figure 10. Dissolved oxygen oxygen observations from April and September surveys of Perdido Bay at U.S. Highway 98.
Results of Study 19

 

 

Table 4. Temperature, specific conductance, dissolved 

oxygen, and pH on April 28, 1995, approximately 1,000 feet 

from mouth of Elevenmile Creek, in center of channel

[Latitude: 30°27’34” Longitude: 87°22’45”; bottom depth, 10.3 feet; time 

of observation approximately 1400 hours]

Depth, 

in feet

Temperature, 

in degrees 

Celsius

Dissolved 

oxygen, in 

milligrams 

per liter

Specific conductance, in 

microsiemens 

per centimeter

pH

1 23.8 3.9 480 7.1

2 22.7 3.9 530 7.1

4 20.4 2.2 690 7.1

6 20.4 2.2 720 7.0

8 20.4 1.9 720 7.0

10 20.3 2.4 880 7.0

Figure 11. pH observations from April and September surveys of Perdido Bay at U.S. Highway 98.

0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

14

EXPLANATION

25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 3

and more than 1.5 times the interquartile

range outside the quartile

Outlier data value more than 3 times the

interquartile range outside the quartile

(5) Number of observations

DEPTH, IN FEET DEPTH, IN FEET

(5)

(5)

(5)

(5)

(5)

pH

(A) APRIL 27, 1995 SURVEY

(4)

(4)

(4)

(4)

(2)

(B) APRIL 28, 1995 SURVEY

(4)

(5)

(4)

(3)

(5)

6.4 6.4

6.4

8.2 8.2

8.2

6.5 6.5

6.5

7.0 7.0

7.0

7.5 7.5

7.5

8.0 8.0

8.0

pH UNITS pH UNITS

(C) SEPTEMBER 8, 1995 SURVEY

Table 5. Temperature, specific conductance, dissolved 

oxygen, and pH on April 28, 1995, at mouth of Perdido River

[Latitude: 30°27’02” Longitude: 87°23’45”; bottom depth, 16 feet; time of 

observation approximately 1530 hours; -----, missing value]

Depth, in 

feet

Temperature, in 

degrees 

Celsius

Dissolved 

oxygen, in 

milligrams 

per liter

Specific 

conductance, in 

microsiemens per 

centimeter

pH

1 22.0 6.5 110 5.6

6 21.7 6.3 120 5.7

7.5 19.6 6.9 300 5.4

8 ----- 6.6 2,500 5.4

9 20.3 4.8 3,700 6.7

12 21.4 6.7 19,400 6.6

15 22.0 7.0 20,900 6.5
20 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

The concentrations of the various nitrogen and 

phosphorous constituents that were analyzed showed 

little variability among the samples (table 6). Total 

nitrogen values ranged from approximately 0.2 to 

0.7 milligrams per liter, with the bulk (70 to 

100 percent) of the total nitrogen occurring as organic 

nitrogen. Detectable nitrate plus nitrite nitrogen 

concentrations were only present in the April 28 samples. The most significant difference (with respect to 

nitrogen constituents) between the shallow and deep 

samples from April 28 were the ammonia values; the 

ammonia concentration in the deeper sample was four 

times higher than the concentration in the shallower 

sample. Concentrations of phosphorous constituents 

were also low or below reporting limits in the samples 

collected (table 6).

Estimates of Constituent Loads

As described previously, loads were calculated 

for periods when synoptic water quality samples and 

ADCP discharge data were collected concurrently. The 

results of the constituent load calculations and discharge values at the time of sample collection are 

shown in table 7. The magnitude of measured 

discharges during sample collections ranged from 

approximately -9,100 to 12,000 cubic feet per second. 

The direction of net flow was downstream (positive 

flow) during the ebbing conditions on September 28 

(tables 3 and 4), as expected. The direction of net flow 

was upstream (negative flow) during the four measurements that were made during flooding tides or 

near the high and low tides (tables 3 and 4).

The chloride and solids loads were the largest 

fluxes among the chemical constituents that were 

evaluated because chloride and solids concentrations 

were two to four orders of magnitude larger than the 

nutrient concentrations. The estimates of chloride loads 

ranged from -3,000 to 3,300 kilograms per second, and 

total solids and suspended solids ranged from -5,600 to 

6,700 and -41 to 84 kilograms per second, respectively. 

Note that negative flows indicate that the net transport 

of a chemical constituent is upstream.

The computed loads of nitrogen and phosphorous constituents were much smaller than those of 

chloride and solids. Total nitrogen loads ranged from -

180 to 220 grams per second. The bulk of this load 

was represented by organic nitrogen, with roughly 

equal proportions of dissolved and undissolved 

organic nitrogen loads for all but the September 8 sample. Total organic plus ammonia nitrogen loads, less 

their dissolved ammonia component, ranged from 

approximately -170 to 220 grams per second. In contrast, none of the total nitrate plus nitrite or dissolved 

ammonia loads exceeded 6 grams per second. Total 

phosphorous loads ranged from -10 to 11 grams per 

second. The loading rates in table 4 suggest that at 

least half of the phosphorous load in the September 

27-28 samples was comprised of phosphorous in a 

form other than orthophosphate.

It should be noted that the loads described above 

are instantaneous loads and are therefore not necessarily representative of net downstream or upstream loads 

that occur over longer time intervals. For example, 

even if the concentration of a given substance is 

constant during a day, the load might vary over four 

orders of magnitude over the course of a tide cycle as 

the discharge changes from 10,000 cubic feet per 

second in the upstream direction during a flooding tide 

to 10,000 cubic feet per second in the downstream 

direction during an ebbing tide. One approach that is 

commonly used to estimate net loads is to estimate 

instantaneous loads continuously (for example, every 

15 minutes) and filter these loads to remove tidal 

effects (Stoker and others, 1996). This results in instantaneous net loads that are independent of the loads 

caused by the tidal movement of water. Continuous 

estimates of instantaneous loads are typically obtained 

by developing a relation between concurrent measurements of discharge and load, which is then applied to 

the unfiltered continuous discharge estimates. Average 

(net) loads over periods of a day or longer could be 

computed by averaging the filtered instantaneous 

loads. An alternative approach for estimating these 

average loads would be to multiply chemical concentrations by daily mean filtered discharge values. This 

approach might be suitable if concentrations are 

approximately constant during a given day.

Application of either of the above approaches 

for computing net loads over periods of a day or 

longer may yield estimates of questionable accuracy in 

a system like Perdido Bay, where gross tidal inflows 

and outflows are much larger than the net flows 

(inflows minus outflows). Under these conditions, 

chemical concentrations may be greatly affected by 

the mixing of saltwater and freshwater. For example, 

the chloride concentrations observed in the September 

samples ranged from 9,200 to 10,000 milligrams per 

liter, which is several orders of magnitude larger than 
Results of Study 21

Table 6. Water quality data from synoptic water quality samples[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter, total and suspended solids represent residue after evaporating at 105 °C]

Sample 

Date

Sample

time

Approximate 

tidal condition

Chloride, 

dissolved,

in mg/L

Color, in 

platinumcobalt units

Total

solids,

in mg/L

Suspended 

solids,

in mg/L

Specific 

conductance,

in μS/cm

Turbidity, in 

nephelomtric turbidity units Nitrogen, total, in mg/L

Ammonia + 

organic 

nitrogen, 

dissolved,

in mg/L

Ammonia 

+ organic 

nitrogen, 

total,

in mg/L

Organic 

nitrogen, 

dissolved,

in mg/L

Ammonia

nitrogen, 

dissolved,

in mg/L

Nitrate + 

nitrite 

nitrogen, 

dissolved, 

in mg/L

Nitrate + 

nitrite 

nitrogen

total,

in mg/L

04/28/95 1245-13151

1Sample collected above halocline at depth of 1 meter.

Ebbing 440 100 970 70 1,590 10 0.54 0.30 0.44 0.28 0.02 0.10 0.10 

04/28/95 1245-13152

2Sample collected below halocline at depth of 2.5 meters.

Ebbing 2,800 60 6,200 18 9,430 8.0 0.59 0.31 0.51 0.23 0.08 0.08 0.08 

09/08/95 1315-1400 1 hour before 

high

9,200 30 17,000 74 26,600 1.8 0.22 0.23 0.22 0.21 0.02 <0.02 <0.02 

09/27/95 1520-1540 2 hours after 

low

9,200 20 18,000 48 27,000 3.8 0.67 0.25 0.67 0.24 0.01 <0.02 <0.02 

09/27/95 0942-1012 Flooding 9,900 20 19,000 100 28,700 3.0 0.59 0.26 0.59 0.25 0.01 <0.02 <0.02 09/28/95 0022-0044 3 hours 

before high

10,000 20 19,000 140 28,700 4.0 0.61 0.24 0.61 0.22 0.02 <0.02 <0.02 

09/28/95 0927-0956 Ebbing 9,500 15 19,000 240 28,400 3.8 0.63 0.27 0.63 0.26 0.01 <0.02 <0.02 

Table 7. Loads computed from the product of net-discharge and constituent concentrations[Positive load values indicate net-seaward loads and negative load values indicate net-landward loads; kg/s, kilograms per second; g/s, grams per second]

Sample 

date

Sample

time

Discharge, in 

cubic feet 

per second

Chloride, 

dissolved,

in kg/s

Solids, 

total,

in kg/s

Suspended 

solids, in 

kg/s

Nitrogen, 

total,

in g/s

Ammonia + 

organic nitrogen, dissolved,

in g/s

Ammonia + 

organic nitrogen, total, 

in g/s

Organic

 nitrogen,

 dissolved,

in g/s

Ammonia 

nitrogen,

dissolved,

in g/s

Nitrate + 

nitrite 

nitrogen,

dissolved,

in g/s

Nitrate + 

nitrite 

nitrogen, 

total,

in g/s

Nitrit

nitroge

dissolv

in g/s

09/08/95 1215-1300 -11,400 -3,000 -5,500 -24 -71 -71 -71 -68 -6 <-6 <-6 -3 09/27/95 1420-1440 -9,100 -2,400 -4,600 -12 -170 -65 -170 -62 -3 <-5 <-5 <-3 09/27/95 1942-2012 -9,800 -2,700 -5,200 -28 -160 -72 -160 -69 -3 <-6 <-6 <-3 09/27/95 2322-2344 -10,400 -2,900 -5,600 -41 -180 -71 -180 -65 -6 <-6 <-6 -3 09/28/95 0827-0856 12,400 3,300 6,700 84 220 95 220 91 4 <7 <7 <4 
22 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida

chloride concentrations in upstream runoff. If we 

assume that chloride concentrations were fairly constant during the period August 18 though September 

28, 1995, (when continuous discharge data are available), and that mixing is insignificant, then equation 2 

could be used to estimate net loads of chloride during 

this period by multiplying the average of the filtered 

daily mean discharges in this period (1,600 cubic feet 

per second) and the average chloride concentration 

observed in the early and late September samples 

(9,600 milligrams per liter). This results in an estimated net load of approximately 440 kilograms per 

second. This number is probably 3 orders of magnitude larger than the actual net loading rate that 

occurred in late August and September because the 

terrestrial loads that were input to Perdido Bay during 

and shortly before this period were probably on the 

order of 100-1,000 milligrams per second. A similar 

calculation for net solids loads would also yield unreasonably large values. These large errors are the result 

of differences in the volume-weighted concentrations 

in the incoming and outgoing tidal flows. Errors of this 

magnitude occur with concentration differences as 

small as 1 percent, even if the more sophisticated 

approach of averaging instantaneous filtered loads is 

employed.

Estimates of net loads of nutrients are also 

affected by mixing, although these effects are less 

obvious because differences between saltwater and 

freshwater concentrations of nutrients are much 

smaller than corresponding differences in chloride or 

solids concentrations. For example, multiplying the 

average of the filtered daily mean discharges from late 

August through September (1,600 cubic feet per second) and the average total nitrogen concentration 

observed in the early and late September samples 

(0.54 milligrams per liter) results in a net load of 

approximately 24 grams per second, which is within 

the range of plausible net loads from terrestrial 

sources. However, small differences between nutrient 

concentrations in saltwater and freshwater can affect 

the significance of this estimate. For example, if the 

average daily tidal inflow rate were 10,000 cubic feet 

per second and tidal outflow rate (tidal inflow plus terrestrial runoff) were 11,400 cubic feet per second, then 

a concentration difference of only 0.05 milligram per 

liter in the inflowing (saltier) and outflowing (fresher) 

water could result in an error of 70 percent in the 

above estimate of net nitrogen load. This hypothetical 

error is certainly plausible for this example, which 

assumed that nitrogen concentrations were stable 

during the late August-September period. The magnitude of this error is also plausible (and probably 

conservative) even if more sophisticated approaches 

are employed, such as computing continuous loads 

from continuous, discharge-weighted samples. The 

errors associated with the more commonly used technique of estimating and filtering continuous loads 

should be considerably larger than this, because 

continuous nutrient concentrations are typically estimated from discharge-concentration relations (as 

opposed to using measured concentrations from 

continuous discharge-weighted samples). The above 

results indicate that acoustical techniques can yield 

acceptable estimates of instantaneous loads in Perdido 

Bay; however, estimates of net loads should be interpreted with great caution and may have unacceptably 

large errors, especially when saltwater and freshwater 

concentrations differ greatly.

SUMMARY

Water flow and quality data were collected from 

December 1994 to September 1995 to evaluate variations in discharge, water quality, and chemical fluxes 

(loads) through Perdido Bay, Florida. Data were 

collected at a cross section parallel to the U.S. Highway 98 bridge. Discharges measured with an acoustic 

Doppler current profiler (ADCP) and computed from 

stage-area and velocity ratings varied roughly between 

 cubic feet per second during a typical tidal 

cycle. Large reversals in flow direction occurred 

rapidly (less than 1 hour) and complete reversals, 

resulting in near peak net-upstream or downstream 

discharges, occurred within a few hours of slack water. 

Observations of simultaneous upstream and downstream flow (bidirectional flow) were quite common 

in the ADCP measurements, with opposing directions 

of flow occurring predominately in vertical layers. The 

observed flow fields were complex and changed over 

time, with flow often concentrated in cells of strong 

flow that strengthened and weakened through a tide 

cycle. Instantaneous discharge values were computed 

at 15-minute intervals and filtered daily values were 

computed for the period from August 18 to September 

28, 1995. Data were not computed prior to August 18, 

1995, either because of missing data or because the 

velocity rating was poorly defined (due to insufficient 

data) for the period prior to the passage of Hurricane 

Erin (August 3-4, 1995). The results of the study indi-

±10 000 ,
References 23

cate that acoustical techniques can yield useful estimates of continuous (instantaneous) discharge in 

Perdido Bay. Useful estimates of average daily net 

flow rates can also be obtained, but the accuracy of 

these estimates will be limited by small rating shifts 

that introduce bias into the instantaneous values that 

are used to compute the net flows.

Concentrations of total nitrogen ranged from 

0.22 to 0.67 milligram per liter, with organic nitrogen 

representing the bulk of total nitrogen. Concentrations 

of phosphorous constituents were low (less than 

0.04 milligram per liter) or below detection limits in 

the samples collected. Distinct thermoclines and haloclines were observed during sample collection on 

April 28, but were absent during sample collections in 

early and late September. Instantaneous loads of total 

nitrogen ranged from -180 to 220 grams per second for 

the sample collected during the study, and instantaneous loads of total phosphorous ranged from -10 to 

11 grams per second. The chloride concentrations 

from the water samples collected from Perdido Bay 

indicated a significant amount of mixing of saltwater 

and freshwater. Mixing effects could greatly reduce 

the accuracy of estimates of net loads of nutrients or 

other substances. The study results indicate that acoustical techniques can yield acceptable estimates of 

instantaneous loads in Perdido Bay; however, estimates of net loads should be interpreted with great 

caution as they may have unacceptably large errors, 

especially when saltwater and freshwater concentrations differ greatly.

REFERENCES

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Report on physical and chemical processes affecting 

the management of Perdido Bay: Results of the 

Perdido Bay interstate project Report to the Florida 

Department of Environmental Regulation, January 25, 

1991.

Godin, Gabriel, 1972, The analysis of tides: University of 

Toronto Press, 264 p.

Kohler, M.A., Nordenson, T.V., and Baker, D.R., 1959, 

Evaporation maps for the United States: U.S. 

Department of Commerce, Weather Bureau, Technical 

Paper Number 37.

Laenen, Antonius, 1985, Acoustic velocity meter systems: 

U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. A17, 38 p.

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Drainage System, part IV Physical Background 

conditions: Environmental Planning and Analysis 

Report, July 1992.

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coast of North and South America including 

Greenland: Riverdale, Maryland, 301 p.

Niedoroda, A.W., 1992, Perdido Bay System Nutrient 

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Engineering Report, June 1992.

Rantz, S.E., and others, 1982, Measurement and 

computation of streamflow: U.S. Geological Survey 

Water-Supply Paper 2175, v. 1 and 2, 631 p.

Rappaport, E.N., 1995, Preliminary report: Hurricane Erin, 

31 July - 6 August 1995: U.S. Department of 

Commerce, National Hurricane Center

Regan, R.S. and Schaffranek, R.W., 1985, A computer 

program for analyzing channel geometry: U.S. 

Geological Survey Water-Resources Investigations 

Report 85-4335, 49 p.

RD Instruments, 1992, Technical manual for acoustic Doppler 

current profilers: RD Instruments, San Diego, CA.

Simpson, M.R., and Oltmann, R.N., 1992, Dischargemeasurement system using an acoustic Doppler current 

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U.S. Geological Survey Open-File Report 91-487, 49 p.

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analysis of inorganic substances in water and fluvial 

sediments: U.S. Geological Survey Open File-Report 

78-679, 159 p.

Sloat, J.V. and Gain, W.S., 1995, Application of acoustic 

velocity meters for gaging discharge of three lowvelocity tidal streams in the St. Johns River Basin, 

northeast Florida: U.S. Geological Survey WaterResources Investigations Report 95-4230, 26 p.

Stoker, Y.E., Levesque, V.A., and Fritz, E.M., 1996, 

Discharge, water-quality characteristics, and nutrient 

loads from McKay Bay, Delaney Creek, and East Bay, 

Tampa, Florida, 1991-1993: U.S. Geological Survey 

Water-Resources Investigations Report 95-4167, 47 p.

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tidal-period variations from time-series data using lowpass filters: Journal of Physical Oceanography, v. 12, 

no. 1, p. 112-115.
24 Application of Acoustical Methods for Estimating Water Flow and Constituent Loads in Perdido Bay, Florida