Prepared in cooperation with the 

South Carolina Department of Health and Environmental Control

Tidal Flow Dynamics and Background Fluorescence 

of the Atlantic Intracoastal Waterway in the Vicinity 

of Sullivan’s Island and the Isle of Palms, 

South Carolina, 2011–12

11/23/2011 12/3/2011 12/13/2011 12/23/2011 1/2/2012

Date

1/12/2012 1/22/2012 2/1/2012

–3,500

–3,000

–2,500

–2,000

–1,500

–1,000

–500

0

500

1,000

1,500

35

37

39

41

43

45

47

49 Zero flow

51

53

55

Residual flows, in cubic feet per second

Specific conductance, millisiemens per centimeter

Open-File Report 2013–1077

U.S. Department of the Interior

U.S. Geological Survey
Cover. The Atlantic Intracoastal Waterway by Sullivan’s Island looking to the northeast. Photograph by Victor Levesque, U.S. Geological 

Survey. Inset shows low-pass filtered specific conductance for Charleston Harbor (orange line), Sullivan’s Island (white line), Isle of Palms 

(green line) and residual flows for Sullivan’s Island (blue line).
Tidal Flow Dynamics and Background 

Fluorescence of the Atlantic Intracoastal 

Waterway in the Vicinity of Sullivan’s 

Island and the Isle of Palms, South Carolina, 

2011–12

By Paul A. Conrads, Celeste A. Journey, Jimmy M. Clark, and Victor A. Levesque 

Prepared in cooperation with the 

South Carolina Department of Health and Environmental Control

Open-File Report 2013–1077

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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U.S. Government.

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Conrads, P.A., Journey, C.A., Clark, J.M., and Levesque, V.A., 2013, Tidal flow dynamics and background fluorescence 

of the Atlantic Intracoastal Waterway in the vicinity of Sullivan’s Island and the Isle of Palms, South Carolina, 

2011–12: U.S. Geological Survey Open-file Report 2013–1077, 20 p.
iii

Contents

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

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

Data Collection...............................................................................................................................................3

Continuous Velocity, Stage, Temperature, and Specific Conductance Data..............................5

Discrete Flow Measurements ............................................................................................................6

Computation of Continuous Tidal Flow Data .............................................................................................8

Development of Stage-Area Curves..................................................................................................8

Development of Index-Velocity Ratings............................................................................................8

Continuous Tidal Flow Record ............................................................................................................9

Assessment of Water-Quality and Background Fluorescence............................................................13

Basic Water Chemistry at Profile Stations .....................................................................................13

Background Fluorescence and Specific Conductance ...............................................................16

Summary........................................................................................................................................................20

References Cited..........................................................................................................................................20

Figures

1. Map showing location of shellfish harvesting areas, index velocity stations,

remote index velocity station, flow measurement transects, and water-quality

sampling stations near Sullivan’s Island and Isle of Palms, South Carolina ......................2

2. Photographs showing views of the Isle of Palms and Sullivan’s Island monitoring

stations on the Atlantic Intracoastal Waterway .....................................................................4

3. Graphs showing gage height and index velocity data for the Isle of Palms station

and the Sullivan’s Island station.................................................................................................6

4. Graphs showing temperature and specific conductance data for the Isle of Palms

station and the Sullivan’s Island station ...................................................................................7

5. Graphs showing stage-area ratings for the Isle of Palms station, the Sullivan’s

Island station, and the Breach Inlet station.............................................................................8

6. Graphs showing index-velocity ratings for the Isle of Palms station, the Sullivan’s

Island station, and the Breach Inlet station.............................................................................9

7. Graphs showing flow and residual flow for the Isle of Palms station, the Sullivan’s

Island station, and the Breach Inlet station...........................................................................10

8. Graph showing residual flow for the Isle of Palms, the Sullivan’s Island, 

and the Breach Inlet stations ...................................................................................................11

9. Graphs showing specific conductance and residual flow at the Isle of Palms 

and Sullivan’s Island stations ...................................................................................................12

10. Graph showing low-pass filtered specific conductance for Charleston Harbor,

Sullivan’s Island, and Isle of Palms and residual flows for Sullivan’s Island....................12

11. Maps showing fluorescence concentrations and specific conductance in the

Atlantic Intracoastal Waterway and nearby tidal creeks on January 24, 2012,

during high-tide conditions, Sullivan’s Island and Isle of Palms, South Carolina............18

12. Maps showing fluorescence concentrations and specific conductance in the

Atlantic Intracoastal Waterway and nearby tidal creeks on January 24, 2012,

during low-tide conditions, Sullivan’s Island and Isle of Palms, South Carolina .............19
iv

Tables

1. U.S. Geological Survey river monitoring stations near Isle of Palms and Sullivan’s

Island ..............................................................................................................................................3

2. Water-quality station locations in the Atlantic Intracoastal Waterway near

Sullivan’s Island and Isle of Palms, South Carolina, on January 24, 2012...........................4

3. Specifications for water-quality sensors on the YSI 6600 EDS-V2 sonde...........................5

4. Minimum and maximum flow measured during floodtide and ebbtide at the Isle of

Palms, Sullivan’s Island, and Breach Inlet stations ................................................................7

5. Minimum, maximum, and average computed flows and residual flow for the period 

December 4, 2011, to January 26, 2012 ...................................................................................10

6. Descriptive statistical summary of the water-quality measurements collected at

selected sites in the Atlantic Intracoastal Waterway near Sullivan’s Island and

Isle of Palms, South Carolina, on January 24, 2012...............................................................13

7. Water-quality measurements collected at selected points along the cross section

in the channel during high-tide conditions at selected sites in the Atlantic

Intracoastal Waterway near Sullivan’s Island and Isle of Palms, South Carolina,

on January 24, 2012 ....................................................................................................................14

8. Water-quality measurements collected in the center of the channel during

low-tide conditions at selected stations in the Atlantic Intracoastal Waterway

near Sullivan’s Island and Isle of Palms, South Carolina, on January 24, 2012................17
v

Conversion Factors, Datum, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Flow velocity

foot per second (ft/s) 0.3048 meter per second (m/s)

Flow rate

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

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

°F=(1.8×°C)+32

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Specific conductance is given in millisiemens per centimeter at 25 degrees Celsius (mS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L).

Abbreviations

ADCP acoustic Doppler current profiler 

ADVM acoustic Doppler velocity meter

BI Breach Inlet (station)

CH Customs House (station)

EDS Extended Deployment System

EST Eastern Standard Time

GIS Geographic Information System

IDW inverse distance weighted

IOP Isle of Palms (station)

kHz kilohertz

NPDES National Pollutant Discharge Elimination System 

R2 coefficient of determination

SI Sullivan’s Island (station)

SL side looker

USGS U.S. Geological Survey

Tidal Flow Dynamics and Background Fluorescence of 

the Atlantic Intracoastal Waterway in the Vicinity of 

Sullivan’s Island and the Isle of Palms, South Carolina, 

2011–12

By Paul A. Conrads, Celeste A. Journey, Jimmy M. Clark, and Victor A. Levesque 

Abstract

To effectively plan site-specific studies to understand the 

connection between wastewater effluent and shellfish beds, 

data are needed concerning flow dynamics and background 

fluorescence in the Atlantic Intracoastal Waterway near the 

effluent outfalls on Sullivan’s Island and the Isle of Palms. 

Tidal flows were computed by the U.S. Geological Survey 

for three stations and longitudinal water-quality profiles were 

collected at high and low tide. Flows for the three U.S. Geological Survey stations, the Atlantic Intracoastal Waterway by 

the Isle of Palms Marina, the Atlantic Intracoastal Waterway 

by the Ben M. Sawyer Memorial Bridge at Sullivan’s Island, 

and Breach Inlet, were computed for the 53-day period from 

December 4, 2011, to January 26, 2012. The largest flows 

occurred at Breach Inlet and ranged from -58,600 cubic feet 

per second (ft3/s) toward the Atlantic Intracoastal Waterway 

to 63,300 ft3/s toward the Atlantic Ocean. Of the two stations 

on the Atlantic Intracoastal Waterway, the Sullivan’s Island 

station had the larger flows and ranged from -6,360 ft3/s to 

the southwest (toward Charleston Harbor) to 8,930 ft3

/s to the 

northeast. Computed tidal flow at the Isle of Palms station 

ranged from -3,460 ft3

/s toward the southwest to 6,410 ft3/s 

toward the northeast. The synoptic water-quality study showed 

that the stations were well mixed vertically and horizontally. 

All fluorescence measurements (recorded as rhodamine 

concentration) were below the accuracy of the sensor and the 

background fluorescence would not likely interfere with a dyetracer study.

Introduction

Proposed changes in water-quality limits for fecal coliform may have potential effects on the open shellfish beds 

(those beds where harvesting is allowed) near the effluent outfalls of the Sullivan’s Island and the Isle of Palms wastewater 

treatment plants (fig. 1). A number of alternative study 

approaches can be used to better understand the potential connections between wastewater effluent and the shellfish bed, 

including dye-tracer studies, sediment sampling for wastewater indicators, and thermal plume tracking. To effectively plan 

such studies, data are needed concerning the flow dynamics 

and background fluorescence of the Atlantic Intracoastal 

Waterway near the effluent outfalls on Sullivan’s Island and 

the Isle of Palms in South Carolina. Tidal flow dynamics of 

the Atlantic Intracoastal Waterway, located on the landward 

side of the barrier islands along the South Carolina coast, are 

quite complex because of reversing tidal flows, interconnected 

tidal creeks, multiple connections between barrier islands 

and the Atlantic Ocean, wetting and drying of extensive tidal 

marshes, and semidiurnal tides with a 5- to 7-foot (ft) vertical 

range. Often, flows in these systems are characterized by large, 

bidirectional tidal excursions during the flood and ebb tides 

and by small residual (net) flows over the tidal cycle. 

Dye fluorometry is an established method for tracing the 

movement of water in many hydrologic settings. When used 

as a tracer, the concentration of a dye is directly proportional 

to its fluorescence. Background fluorescence can interfere or 

enhance the fluorescence of the dye, thereby causing overestimation of the movement and dispersion of the dye (Hartel 

and others, 2007). Elevated background fluorescence can be 

caused by optical brighteners commonly found in the effluent of wastewater treatment plants and septic tanks. Optical 

brighteners have the ability to fluoresce and interfere with the 

measurement of rhodamine dye used as a hydrologic tracer. 

Other sources of background fluorescence include naturally 

occurring humic substances that produce the “blackwater” of 

coastal waters, synthetic organic compounds that can reach 

the water during rainfall runoff, and various by products from 

pulp and paper production (Hartel and others, 2007). 

The U.S. Geological Survey (USGS), in cooperation with 

the South Carolina Department of Health and Environmental 

Control, initiated a data-collection effort in 2011 to measure 

the tidal flow dynamics and background fluorescence in the 
2 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

8

10

11

20

19

Morgan

Creek

Inlet Creek

Swinton Creek

Creek

Conch

Hamlin Creek

Breach

 Inlet

S e ven Reaches

The

Cove

Shem Creek

Charleston

Harbor

ATLANTIC OCEAN

Gray

Bay

Atlantic

Intracoastal

Waterway PALM BOULEVARD

ISLE OF PALMS CONNECTOR

BEN SAWYER BOULEVARD

CHARLESTON

COUNTY

2

3

4

6

7

9

12

13

14

15

16

17

18

21

1

5

Base from ESRI® Data and Maps, 2010

Universal Transverse Mercator projection, zone 17

Goat Island

Isle of Palms

Marina

0 0.5 1 MILES

0 0.5 1 KILOMETERS

Isle of

Palms

Mount

Pleasant

Sullivan's 

Island

Ben M. Sawyer 

Memorial 

Bridge

Sampling location not used

 and identifier

10

Sampling location and identifier

EXPLANATION

Swamp / wetland

Shell-fish harvesting boundary

Approved

Prohibited

Restricted

Flow measurement transect

Wastewater treatment plant

Remote index velocity station

 and identifier

BI

Index velocity station

 and identifier

SI

Specific conductance station

 and identifier

CH

15

79°52' 79°50' 79°48' 79°46'

32°48'

32°46'

BEN SAWYER BOULEVARD

Charleston

Mount

Pleasant

CHARLESTON

COUNTY

1 Charleston

Harbor

SI

BI

IOP CH

Study area

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

Figure 1. Location of shellfish harvesting areas, index velocity stations, remote index velocity station, flow measurement transects, 

and water-quality sampling stations near Sullivan’s Island and Isle of Palms, South Carolina. [BI, Breach Inlet station; CH, Charleston 

Harbor station; IOP, Isle of Palms station; SI, Sullivan’s Island station] 
Data Collection 3

Atlantic Intracoastal Waterway in the vicinity of Sullivan’s 

Island and Isle of Palms. An important part of the USGS 

mission is to provide scientific information for the effective 

water-resources management of the Nation. To assess the 

quantity and quality of the Nation’s surface water, the USGS 

collects hydrologic and water-quality data from rivers, lakes, 

and estuaries using standardized methods and maintains the 

data from these stations in a national database. This data collection effort will contribute to the understanding of complex 

flow and water-quality dynamics of near barrier islands along 

the South Atlantic coast.

The purpose of this report is to characterize tidal-flows 

and background fluorescence in the Atlantic Intracoastal 

Waterway in the vicinity of Sullivan’s Island and Isle of 

Palms. The methods used to collect continuous velocity, stage, 

temperature and flow are described, as well as those used for 

discrete flow measurements. This report documents the development of three index velocity sites for the computation of 

continuous, including the development of stage curves, indexvelocity ratings, and computation of continuous flow record. 

Water quality and background fluorescence are assessed using 

basic water chemistry data collected at profile stations. 

Data Collection

To compute continuous tidal flows, two index-velocity 

stations were installed on the Atlantic Intracoastal Waterway at 

the Isle of Palms Marina and at the Ben M. Sawyer Memorial 

Bridge (referred to as the IOP station and SI station, respectively for this report; table 1 and fig. 1). Each station was 

equipped with an acoustic Doppler velocity meter (ADVM), 

water-level sensor, and temperature and specific conductance 

sensors. The ADVM, temperature, and conductance sensors for the IOP station were attached to a mooring piling 

at the southwestern end of the floating docks (fig. 2A). The 

ADVM, temperature, and specific conductance sensors for the 

SI station were attached to the northeastern wooden fenders 

(fig. 2B). Stations were visited approximately every 3 weeks 

during the deployment to perform quality-assurance checks 

and to exchange batteries.

In addition to monitoring tidal velocity, gage height 

(also referred to as stage), and specific conductance, physical measurements of flow were made during approximately 

one-half of a tidal cycle at the index-velocity stations. These 

measurements were used to develop stage-area relations for 

the index-velocity stations and to develop index-velocity to 

mean-velocity relations for each measured cross section. 

Breach Inlet (fig. 1) flow was measured and used as 

a remote index-velocity station to estimate the tidal flow 

exchange in the inlet between the Isle of Palms and Sullivan’s 

Island. No monitoring instrumentation was installed at Breach 

Inlet (referred to as the BI station in this report, table 1), but 

physical measurements of flow were made for approximately 

one-quarter of a tidal cycle at the BI station. These measurements were used with the SI station stage data to develop a 

stage-area relation for the BI station, and the velocity data at 

the SI station were used to develop a relation between the SI 

index velocity and the mean velocity at the BI station. No time 

correction was applied to the SI station data used at the remote 

index-velocity site at the BI station.

To measure the background fluorescence of the Atlantic 

Intracoastal Waterway, a synoptic water-quality study was 

conducted on January 24, 2012. Prior to field data collection, 21 potential locations were identified for data collection 

and established in a Geographic Information System (GIS) 

point coverage. Of these 21 potential stations, 5 were omitted 

for logistical reasons (stations 8, 10, 11, 19, and 20) (fig. 1). 

Selected water-quality vertical profile locations included 

stations near the effluent-receiving creeks, tidal creeks, and 

within the main body of the Atlantic Intracoastal Waterway 

(fig. 1 and table 2). Depth profiles of background fluorescence, 

Table 1. U.S. Geological Survey river monitoring stations near Isle of Palms and Sullivan’s Island.

[USGS, U.S. Geological Survey; AIW, Atlantic Intracoastal Waterway; IOP, Isle of Palms; V, velocity; SC, specific conductance; GH, gage height; WT, water 

temperature; Q, flow; SI, Sullivan’s Island; BI, Breach Inlet; CH, Customs House; S.C., South Carolina]

Station name USGS 

station number

Acronym of 

station name

 used in this 

report

Physical

 properties

Period

 of record

Longitude 

(NAD 83)

Latitude

 (NAD 83)

AIW at Isle of Palms Marina 

at Isle of Palms, S.C.

324822079454000 IOP V, SC, GH,

WT, Q

December 2, 2011‒

January 28, 2012

-79°45'40" 32°48'22"

AIW at the Ben Sawyer Bridge

 at Sullivan’s Island, S.C.

324623079503000 SI V, SC, GH,

WT, Q

December 2, 2011‒

February 7, 2012

-79°50'30” 32°46'23"

Breach Inlet at S.C. Highway 

703 at Sullivan’s Island, S.C.1

324634079484400 BI Q December 2, 2011‒

January 28, 2012

-79°48'44” 32°46'34" 

Cooper River at Customs House

 at Charleston, S.C.

21720710 CH SC October 1, 1996‒

February 20, 2013

-79°55'26” 32°46'49"

1

Remote index-velocity site and used the gage height and velocity data from the Sullivan’s Island station.
4 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

A B

Figure 2. Views of the A, Isle of Palms, and B, Sullivan’s Island monitoring stations on the Atlantic Intracoastal Waterway. 

Photographs by Victor Levesque, U.S. Geological Survey.

Table 2. Water-quality station locations in the Atlantic Intracoastal Waterway near Sullivan’s Island and Isle of Palms, South Carolina, on 

January 24, 2012.

[AIW, Atlantic Intracoastal Waterway; L, low tide; H, high tide; WWTP, wastewater treatment plant; S.C., South Carolina]

Station 

number1

(fig. 1)

Station description

Location 

relative to 

AIW

Within 

shellfish 

harvesting 

areas

Crosssection 

profile

Tide

1 AIW in The Cove near Sullivan’s Island Main channel No No L

2 AIW at mouth of The Cove nr Sullivan’s Island Main channel No No H, L 

3 Unamed cove off AIW at the WWTP outfall in Sullivan’s Island Cove No No H, L

4 AIW at the east side of S.C. Highway 703 (Ben Sawyer) Bridge in Sullivan’s Island Main channel No No H, L

5 AIW between the mouths of Narrows and Conch Creeks nr Sullivan’s Island Main channel No No L

6 West fork of Conch Creek nr Sullivan’s island within shellfish harvesting area Tidal creek Yes No H

7 East Fork of Conch Creek near Sullivan’s Island within shellfish harvesting area Tidal creek Yes No H

9 AIW before the mouth of Inlet Creek near Sullivan’s Island Main channel No No H

12 AIW before the mouth of Swinton Creek near Isle of Palms Main channel No Yes H, L

13 AIW at the Isle of Palms connector bridge at Isle of Palms Main channel No No H, L

14 Hamlin Creek near its mouth near Goat Island at Isle of Palms in shellfish 

harvesting areas

Tidal creek Yes Yes H

15 AIW at Goat Island Main channel No No H

16 AIW at Isle of Palms Marina index-velocity station Main channel No No H, L

17 AIW at the Isle of Palms WWTP outfall east of Isle of Palms Connector Bridge Main channel No Yes H, L

18 Hamlin Creek north of AIW in Isle of Palms Tidal creek Yes No H

21 Hamlin Creek south of AIW in Isle of Palms Tidal creek Yes No H

1

Five stations were omitted for logistical purposes (stations 8, 10, 11, 19, and 20).
Data Collection 5

dissolved oxygen, pH, specific conductance, and water temperature were measured at 14 stations during high tide (8:30 

to 11:30 a.m. EST) and at 9 stations during low tide (13:00 to 

14:30 p.m. EST). High-tide vertical profiles were measured at 

stations 2–4, 6, 7, 9, 12–18, and 21. Low-tide vertical profiles 

were created at stations 1–5, 12, 13, 16, and 17. At stations 12, 

14, and 17 during high-tide conditions, cross-sectional profiles 

were measured at 25-percent, 50-percent, and 75-percent 

widths along the cross section to assess horizontal mixing. 

Where water depth made it feasible, measurements were 

made at 1-ft, 5-ft, 10-ft, and 15-ft depth intervals to assess 

vertical mixing. 

 Two YSI 6600-V2 Extended Deployment System (EDS) 

water-quality sondes were used in the data-collection effort. 

The primary sonde was equipped with dissolved oxygen, pH, 

specific conductance, water temperature, and rhodamine sensors. The rhodamine sensor was used to measure the background fluorescence; specifications for the water-quality sensors are listed in table 3. The secondary sonde was equipped 

with dissolved oxygen, pH, specific conductance, and water 

temperature probes and provided field verification of the 

primary sonde readings. Data from the secondary sonde are 

not presented in this report. On January 24, 2012, prior to field 

data collection, the sensors on both sondes were calibrated 

as described in the YSI User’s Manual and the U.S. Geological Survey (USGS) National Field Manual (Wilde, variously 

dated; YSI Incorporated, 2011). 

Continuous Velocity, Stage, Temperature, and 

Specific Conductance Data

The index-velocity stations were operated from December 2, 2011, to February 7, 2012. The two stations were instrumented with a SonTek Argonaut-SL (side-looker) ADVM 

equipped with a vertical acoustic beam for measuring water 

level (SonTek, 2009) and YSI 600R water-quality sampling 

sonde equipped with temperature and specific conductance 

sensors. At the IOP station, the ADVM, and temperature and 

conductance sensors were mounted approximately 2 ft above 

the channel bottom. At the SI station, the ADVM, temperature, and specific conductance sensors were mounted about 

6 ft above the channel bottom. The IOP station ADVM was 

configured to measure a horizontal distance range from 6.6 

to 26.2 ft (2 to 8 meters) from the instrument. The SI station 

ADVM was configured to measure a horizontal range from 

9.8 to 39.4 ft (3 to 12 meters) from the instrument. Both stations used a 240-second averaging interval at the start of each 

quarter hour (15-minute measurement interval). Because of 

the bidirectional flow paths at these stations, velocity and flow 

toward the northeast were defined as positive and velocity and 

flow toward the southwest were defined as negative. For the 

BI station, positive velocity and flow were toward the Atlantic 

Ocean, and negative velocity and flow were toward the Atlantic Intracoastal Waterway. 

The time-series gage-height and index-velocity data for 

the IOP and SI stations indicate a typical semidiurnal tidal 

cycle of 2 low and 2 high tides per lunar day (fig. 3). Because 

of power supply problems, data ends for the IOP station on 

January 28, 2012. Data collection was discontinued for the SI 

station on February 7, 2012. The period of record for the BI 

station is identical to that of the SI station. The gage-height 

data are referenced to a relative datum for each station. During 

the data-collection period, the gage height ranged from 0.55 

to 7.72 ft (7.17 ft total range) at the IOP station and ranged 

from 3.36 to 12.28 ft (8.92 ft total range) at the SI station. The 

velocity time-series data for the two stations are different in 

magnitude and primary direction. Maximum velocity ranged 

from -0.52 foot per second (ft/s) to the southwest (toward 

Charleston Harbor) and 0.91 ft/s to the northeast for the IOP 

station and ranged from -2.10 ft/s to the southwest to 1.91ft/s 

to the northeast for the SI station.

The time-series of temperature and specific conductance 

data are shown in figure 4; temperature data ranges were 

almost identical for the IOP and SI stations, ranging from 7.4 

to 17.4 °C at the IOP station and from 7.5 to 17.5 °C at the 

SI station. The specific conductance data for the two stations were similar in pattern, but differed in range. Specific 

Table 3. Specifications for water-quality sensors on the YSI 6600 EDS-V2 sonde.

[°C, degrees Celsius; mS/cm, millisiemens per centimeter; %, percent; ROX, rates of oxidation; mg/L milligrams per liter; 

μg/L, micrograms per liter; resolution, the smallest measurable increment; accuracy, the closeness of the measurement to the 

actual value]

Sensor Sensor 

type Range Resolution Accuracy

Temperature YSI 6560 -5 to 50 °C 0.01 °C +/- 0.15 °C

Specific conductance YSI 6560 0 to 100 mS/cm 0.001 to 0.1 mS/cm +/- 0.5% or 0.001 mS/cm

Optical dissolved oxygen YSI ROX 0 to 50 mg/L 0.01 mg/L +/- 1 % or 0.1 mg/L 

pH YSI 6561 0 to 14 units 0.01 units +/- 0.2 units

Rhodamine fluorescence YSI 6130 0 to 200 μg/L 0.1 μg/L +/- 5% or 1 μg/L
6 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

–1.00

–0.75

–0.50

–0.25

0

0.25

0.50

0.75

1.00

0

1

2

3

4

5

6

7

8

A. Isle of Palms station 

Stage

Index velocity

–2.5

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

2.5

3

4

5

6

7

8

9

10

11

12

13

12/1/2011 12/11/2011 12/21/2011 12/31/2011

Date

1/10/2012 1/20/2012 1/30/2012 2/9/2012

Velocity, in feet per second

Gage height, in feet

B. Sullivan’s Island station

Velocity Gage height

Figure 3. Gage height and index velocity data for A, the Isle of Palms station and B, the Sullivan’s Island station.

conductance ranged from 49.3 to 53.6 millisiemens per 

centimeter (mS/cm) at the IOP station and ranged from 40.6 to 

52.8 mS/cm at the SI station. The median specific conductance 

of 52.2 mS/cm for the IOP station was greater than the median 

value of 50.4 mS/cm for the SI station. 

Discrete Flow Measurements

Once during the 60-day deployment, discrete tidalcycle flow measurements were made over a 10- to 13-hour 

period in the Atlantic Intracoastal Waterway near the Isle of 

Palms Marina and at the Ben M. Sawyer Memorial Bridge. 

Additionally, discrete measurements were made over a 5- to 

6-hour period at Breach Inlet. Measurements were made 

with Teledyne RD Instruments 1,200 kilohertz (kHz) Rio 

Grande acoustic Doppler current profilers (Teledyne RD 

Instruments, 2008) on January 23 and 24, 2012. Acoustic 

flow measurement and processing procedures described 

by Mueller and Wagner (2009) were followed for the flow 

measurements.

The flow measured at the IOP station on January 23, 

2012, ranged from –2,170 to 2,400 cubic feet per second 

(ft3

/s), and flow at the SI station ranged from -4,630 to 

-107 ft3/s on January 23 and from -3,360 to 4,010 on January 24, 2012 (table 4). Although flow at the SI station did not 

reverse direction during the half tidal cycle on January 23, 

it was necessary to measure positive flows for a short period 

on January 24 in order to develop the index-to-mean velocity 

relation. The flow measured at the BI station on January 24 

ranged from -3,470 to 35,960 ft3/s. The range of flow measured at the BI station is not representative of a typical tidal 

cycle. Data collection at the BI station was limited by the 

logistics of one field crew measuring flow at both the SI and 

BI stations and the priority of measuring positive flow at the 

SI station on January 24. 
Data Collection 7

Date

40

42

44

46

48

50

52

54

6

8

10

12

14

16

18

20

12/1/2011 12/11/2011 12/21/2011 12/31/2011 1/10/2012 1/20/2012 1/30/2012 2/9/2012

Specific conductance, in millisiemens 

per centimeter 

Temperature, in degrees Celsius

B. Sullivan’s Island station

Temperature

Specific 

conductance

40

42

44

46

48

50

52

54

6

8

10

12

14

16

18

20

A. Isle of Palms station

Temperature

Specific 

conductance

Figure 4. Temperature and specific conductance data for A, the Isle of Palms station and B, the Sullivan’s Island station.

Table 4. Minimum and maximum flow measured during floodtide and ebbtide at the Isle of Palms, Sullivan’s Island, and Breach Inlet stations.

[USGS, U.S. Geological Survey; Min, minimum; ft3/s, cubic foot per second; Max, maximum; AIW, Atlantic Intracoastal Waterway; S.C., South Carolina; 

na, data not available]

Station name USGS station number

January 23, 2012 January 24, 2012

Min 

(ft3

/s)

Max 

(ft3/s)

Min 

(ft3/s)

Max 

(ft3/s)

AIW at Isle of Palms Marina at Isle of Palms, S.C.1 324822079454000 -2,170 2,400 na na

AIW at the Ben Sawyer Bridge at Sullivan’s Island, S.C.1 324623079503000 -4,630 -107 -3,360 4,010

Breach Inlet at S.C. Highway 703 at Sullivan’s Island, S.C.2 324634079484400 na na -3,470 35,960

1

Postive flow is toward the northeast and negative flow is toward the southwest.

2

Positive flow is toward the Atlantic Ocean and negative flow is toward the Atlantic Intracoastal Waterway.
8 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

Computation of Continuous Tidal Flow 

Data

The computation of continuous flow at the indexvelocity stations was accomplished by following procedures 

described in Levesque and Oberg (2012). The first step was 

to develop stage-area ratings to establish the relation between 

tidal stage (gage height) at the station and cross-sectional 

area. The second step was to develop index-to-mean velocity 

ratings to convert the index velocity to a mean velocity for 

the cross section. The first two steps were accomplished by 

using the data from the tidal-cycle flow measurements. Flow 

was then computed by multiplying the cross-sectional area 

(computed using the stage-area rating) by the mean velocity 

(computed using the index-to-mean velocity rating).

Development of Stage-Area Curves 

The cross-sectional areas measured during the tidalcycle measurements were related to the measured gage height 

at the time of the measurements for the two index-velocity 

stations (fig. 5). Stage-area ratings that relate stage to crosssectional area were developed using a selected acoustic 

Doppler current profiler (ADCP) transect and the USGS 

software AreaComp2-Version2 (http://hydroacoustics.usgs.

gov/indexvelocity/AreaComp.shtml). AreaComp2 uses the 

stage and the projected area at the time of measurement and 

computes the cross-sectional area for any given stage. The BI 

station used the gage height data from the SI station because 

of the proximity of their locations.

Development of Index-Velocity Ratings 

Side-looking ADVMs measure a horizontally oriented 

range-averaged velocity at a fixed elevation in the water column. The measured velocity typically is not the mean velocity 

for the cross section. In order to compute continuous flow at 

the index-velocity stations, the index velocity is calibrated to 

the measured-mean velocity of the cross section. Measuredmean velocities were computed from the tidal-cycle flow 

measurements made on January 23 and 24, 2012, by dividing 

the measured flow by the area determined from the stagearea rating. Simple-linear regression analysis was used to 

relate the index velocities to the measured mean velocities 

for the cross section for the three stations (fig. 6). Regression 

analysis for the IOP and SI stations indicated that the relation 

between the index velocity and the measured-mean velocity 

B. Sullivan’s Island station

A. Isle of Palms stage-area rating

C. Breach Inlet stage-area rating

y = 12.285x2 + 366.35x + 1,900.6

R2

 = 0.9997

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

y = 1.7602x2

 + 572.68x + 7,884.7

R2

 = 0.9999

8,000

9,000

10,000

11,000

12,000

13,000

14,000

15,000

16,000

17,000

18,000

0 2 4 6 8 10 12 14 16 18

Stage, in feet

y = 6.8329x2 + 344.18x + 709.72 

R2

 = 0.9999

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

Cross-sectional area, in square feet

Figure 5. Stage-area ratings for the A, Isle of Palms station, 

B, the Sullivan’s Island station, and C, the Breach Inlet station.
Computation of Continuous Tidal Flow Data 9

B. Sullivan’s Island station

A. Isle of Palms station

C. Breach Inlet station

–2.0

–1.5

–1.0

–0.5

0.0

0.5

1.0

1.5

2.0

–2.0 –1.5 –1.0 –0.5 0 0.5 1.0 1.5 2.0

Measured mean velocity, in feet per second

–1.0

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

–1.0 –0.5 0 0.5 1.0

Index velocity, in feet per second

–1.0

–0.5

0

0.5

1.0

1.5

2.0

2.5

3.0

–1.0 –0.5 0 0.5 1.0 1.5

Linear (positive flow 

measurements)

Negative flow 

measurements

Positive flow 

measurements

Linear (negative flow

measurements)

Negative flow 

measurements

Positive flow 

measurements

Linear (negative flow 

measurements)

Linear (positive flow 

measurement)

Flow measurements

Linear (flow 

measurements)

Figure 6. Index-velocity ratings for the A, Isle of Palms station, 

B, the Sullivan’s Island station, and C, the Breach Inlet station.

had changes in slope because of changes in flow direction. 

Residual plots of the simple-linear regression model were 

used to separate the data into two groups (positive and negative directions). Simple linear regression analysis was used on 

each data group to improve the accuracy of the index velocity 

rating at the IOP and SI stations. Because of limited flow data 

at the BI station, the data could not reliably be separated into 

positive and negative groups, so a simple-linear model was 

used for the BI index-to-mean velocity rating.

The velocity rating covers a range of mean velocity 

from –0.7 to 0.7 ft/s for the IOP station (fig. 6A) and –1.2 to 

0.7 ft/s for the SI station (fig. 6B), and -0.3 to 2.7 ft/s for BI 

station (fig.6C). The standard error of the IOP station velocity 

ratings were 0.21 ft/s in the positive direction and 0.16 ft/s 

in the negative direction. The coefficients of determination 

(R2) were 0.15 and 0.77 in the positive and negative direction, 

respectively. For the SI station velocity ratings, the standard 

error was 0.02 ft/s and 0.09 ft/s in the positive and negative 

direction, respectively, and the R2s were 0.995 and 0.94 in the 

positive and negative direction, respectively. The BI station 

velocity rating standard error was 0.47 ft/s and a R2 of 0.79. 

The lower R2 and higher standard error of the IOP station 

velocity rating, as compared to the SI station rating, is likely 

a result of complex flow patterns at the IOP ADVM location. 

The IOP station is located close to the confluence with Morgan Creek and the bend in the Atlantic Intracoastal Waterway 

to Seven Reaches (fig. 1). The flow patterns caused by the 

confluence of these flow paths contributed to the decreased 

accuracy of the IOP station velocity rating. The accuracy of 

the BI station velocity rating accuracy was adversely affected 

by limited number of available flow measurements and the 

use of data from of a remote ADVM that was not located in 

the Breach Inlet channel. Even with reduced accuracy, however, a reliable estimation of the continuous flow and residual 

flow direction is possible.

Continuous Tidal Flow Record 

The continuous (15-minute interval) flow data at the 

index-velocity stations are a product of the mean velocity and 

cross-sectional area. The mean velocity is computed using 

the 15-minute index-velocity data and the index velocity 

rating, and the cross-sectional area was computed using the 

15-minute stage data and the stage-area rating. The flows into 

the study reach of the Atlantic Intracoastal Waterway were 

different in magnitude and direction for each station (table 5 

and fig. 7). The flow magnitude at BI station was substantially greater than the IOP or SI station flows and ranged from 
10 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

Table 5. Minimum, maximum, and average computed flows and residual flow for the period 

December 4, 2011, to January 26, 2012.

[ft3/s, cubic foot per second]

Station

Maximum 

negative flow 

(ft3

/s)

Maximum 

positive flow

(ft3/s)

Maximum 

residual

negative flow

 (ft3

/s)

Maximum 

residual

positive flow

(ft3

/s)

Average 

residual 

flow

(ft3

/s)

Isle of Palms1 -3,460 6,410 -814 2,010 616

Sullivan’s Island1 -6,360 8,930 -3,020 1,040 -866

Breach Inlet2 -58,600 63,300 -24,600 11,300 -5,770

1

Positive flow is toward the northest and negative flow is toward the southwest.

2

Positive flow is toward the Atlantic Ocean and negative flow is toward the Atlantic Intracoastal Waterway.

Flow

EXPLANATION

Residual flow

A. Isle of Palms station

B. Sullivan’s Island station

C. Breach Inlet station

Flow, in cubic feet per second

–80,000

–60,000

–40,000

–20,000

0

20,000

40,000

60,000

80,000

–6,000

–4,000

–2,000

0

2,000

4,000

6,000

8,000

11/23/2011 12/3/2011 12/13/2011 12/23/2011 1/2/2012

Date

1/12/2012 1/22/2012 2/1/2012 2/11/2012

–8,000

–6,000

–4,000

–2,000

0

2,000

4,000

6,000

8,000

10,000

Figure 7. Flow and residual flow for the A, Isle of Palms station, B, the Sullivan’s Island station, and C, the 

Breach Inlet station.
Computation of Continuous Tidal Flow Data 11

-58,600 ft3/s to 63,300 ft3/s. The magnitude of flows at the IOP 

station ranged from -3,460 ft3/s to the southwest to 6,410 ft3/s 

to the northeast. At the SI station, the flows ranged from 

-6,360 to 8,930 ft3/s. 

The residual flow directions and magnitudes were determined by using a low-pass filter to remove the semi-diurnal 

tidal signals from the 15-minute instantaneous computed flow 

data (Roberts and Roberts, 1978; Mathworks, 1998). Residual 

flow hydrographs for the IOP, SI, and BI stations are shown in 

figures 7 and 8. The differences in the magnitude and direction 

of the residual flow at the three stations is evident in figure 8. 

The average residual flows for the study period are 616 ft3/s 

to the northeast at the IOP station, -866 ft3/s to the southwest 

at the SI station, and -5,770 ft3/s into the Atlantic Intracoastal 

Waterway for the BI station (table 5). 

The specific conductance at the IOP and SI stations 

are shown with the respective residual flow in figure 9. The 

specific conductance appears to be negatively correlated with 

the residual flow direction (generally northeast or southwest). 

Specific conductance generally decreases when residual 

flow to the northeast increases. To determine the hydraulic 

interaction between Charleston Harbor and the Atlantic 

Intracoastal Waterway, the specific conductance for the SI, 

IOP, and Cooper River at the Custom House (CH) stations 

were smoothed using a low-pass filter of nested 13- and 

25-hour moving window averages and plotted along with the 

residual flows at the SI station (fig. 10). The lower specificconductance water of Charleston Harbor and the decrease 

in specific conductance at the SI and IOP stations during 

periods of positive flow in the Atlantic Intracoastal Waterway 

may indicate the transport of lower specific-conductance 

water from Charleston Harbor to the northeast into the Atlantic Intracoastal Waterway during periods of positive flows to 

the northeast.

Isle of Palms

Sullivan's Island

Breach Inlet

11/23/2011 12/3/2011 12/13/2011 12/23/2011 1/2/2012

Date

1/12/2012 1/22/2012 2/1/2012 2/11/2012

–30,000

–25,000

–20,000

–15,000

–10,000

–5,000

0

5,000

10,000

15,000

20,000

Residual flow, in cubic feet per second

Figure 8. Residual flow for the Isle of Palms, the Sullivan’s Island, and the Breach Inlet stations.
12 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

A. Isle of Palms station

B. Sullivan’s Island station

11/23/2011 12/3/2011 12/13/2011 12/23/2011 1/2/2012

Date

1/12/2012 1/22/2012 2/1/2012 2/11/2012

38

40

42

44

46

48

50

52

54

–4,000

–3,000

–2,000

–1,000

0

1,000

2,000

3,000

4,000

Residual flow, in cubic feet per second

Specific conductance, millisiemens per centimeter

Residual flow

Specific conductance

38

40

42

44

46

48

50

52

54

–4,000

–3,000

–2,000

–1,000

0

1,000

2,000

3,000

4,000

Residual flow

Specific conductance

Figure 9. Specific conductance and residual flow at the A, Isle of Palms and B, Sullivan’s Island stations.

EXPLANATION

SI-Specific conductance

IOP-Specific conductance

CH-Specific conductance

SI-Flow

11/23/2011 12/3/2011 12/13/2011 12/23/2011 1/2/2012

Date

1/12/2012 1/22/2012 2/1/2012

–3,500

–3,000

–2,500

–2,000

–1,500

–1,000

–500

0

500

1,000

1,500

35

37

39

41

43

45

47

49 Zero flow

51

53

55

Residual flows, in cubic feet per second

Specific conductance, millisiemens per centimeter

Figure 10. Low-pass filtered specific conductance for Charleston Harbor (CH), Sullivan’s Island (SI), and Isle of 

Palms and residual flows for Sullivan’s Island (SI).
Assessment of Water-Quality and Background Fluorescence 13

Assessment of Water-Quality and 

Background Fluorescence 

Depth, water temperature, specific conductance, dissolved oxygen, pH, and fluorescence were summarized using 

statistical techniques. Water temperature, specific conductance, dissolved oxygen, and pH were comparable between 

the primary and secondary sonde, having a maximum variability at a site was 1.8 percent. Individually, the average 

variability between sondes of water temperature and specific 

conductance measurements was about 0.3 percent. The average variability of pH measurements between sondes was only 

0.1 percent. Dissolved oxygen measurements displayed the 

greatest variability between sondes and had an average variability of 1.5 percent.

Measurements of background fluorescence in rhodamine concentrations and specific conductance at the selected 

stations were used as input into the established GIS point 

coverage. The inverse distance weighted (IDW) interpolation 

technique using ArcGIS Spatial Analyst (ESRI, 2011) was 

applied to the data to interpolate values between the measured locations and produce a surface layer of the data that 

represented the study area. The IDW interpolation implements the assumption that things closer to one another are 

more alike than those farther apart (ESRI, 2011). Therefore, 

sampled points closest to areas of interpolation will exert 

more influence on the interpolation than sampled points 

farther way.

Basic Water Chemistry at Profile Stations

During high-tide and low-tide conditions, water-quality 

measurements were collected at selected locations in the study 

area (table 6-7, fig. 1). During high-tide conditions, water 

temperatures ranged from 12.2 to 14.4 °C and had a median of 

12.5 °C at all stations and all depths (table 6). Maximum water 

temperatures were measured during ebb tide at station 18, 

located on Hamlin Creek north of the Atlantic Intracoastal 

Waterway in the Isle of Palms (table 7). Specific conductance 

ranged from 49.2 to 53.1 millisiemens per centimeter (mS/cm) 

and had a median of 52.9 mS/cm. Dissolved oxygen concentrations ranged from 7.6 to 8.5 milligrams per liter (mg/L) 

and had a median of 8.3 mg/L. For the water temperature and 

barometric pressure conditions (764 millimeters of mercury) 

at the time of sampling, the dissolved oxygen concentrations 

measured at the stations in the Atlantic Intracoastal Waterway represented about 75-percent saturation during high-tide 

conditions. Measurements of pH were relatively stable among 

profile stations and depths, ranging from 7.8 to 8.0.

At the 3 transects (stations 12, 14, and 17) where 3 points 

were measured during high tide (table 7), horizontal (bankto-bank) variability along the profile transect was less than 

1.2 percent for water temperature, specific conductance, 

dissolved oxygen, and pH, indicating well-mixed conditions. 

In general, water temperature varied between 0 and 0.1 °C; 

specific conductance varied between 0 and 100 mS/cm (0 to 

0.2 percent of reading); dissolved oxygen varied between 0 

and 0.1 mg/L; and pH varied less than 0.1 standard units. 

Table 6. Descriptive statistical summary of the water-quality measurements collected at selected sites in the Atlantic Intracoastal 

Waterway near Sullivan’s Island and Isle of Palms, South Carolina, on January 24, 2012.

[ft, foot; °C, degrees Celsius; mS/cm, millisiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter]

Statistic Depth (ft) Water temperature 

(°C)

Specific 

conductance 

(mS/cm)

Dissolved oxygen 

(mg/L)

pH

(units)

Fluorescence, field 

(μg/L)

High-tide conditions

Minimum 1.0 12.2 49.2 7.6 7.8 0.0

1st Quartile 1.0 12.3 52.7 8.1 8.0 0.2

Mean 4.9 12.6 52.4 8.2 8.0 0.3

Median 5.0 12.5 52.9 8.3 8.0 0.3

3rd Quartile 5.0 12.8 53.0 8.4 8.0 0.4

Maximum 15.0 14.4 53.1 8.5 8.0 0.7

Low-tide conditions

Minimum 1.0 12.7 48.4 7.8 7.9 0.0

1st Quartile 1.0 13.5 50.8 7.8 7.9 0.1

Mean 4.2 13.7 51.8 8.1 7.9 0.2

Median 5.0 13.7 52.6 8.1 7.9 0.2

3rd Quartile 5.0 13.8 52.7 8.2 7.9 0.2

Maximum 10.0 15.0 52.9 8.5 8.0 0.5
14 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway Table 7. Water-quality measurements collected at selected points along the cross section in the channel during high-tide conditions at selected sites in the Atlantic Intracoastal Waterway near Sullivan’s Island and Isle of Palms, South Carolina, on January 24, 2012.—Continued [RB, right bank; US, upstream; NE, northeast; %, percent; ft, foot; °C, degrees Celsius; mS/cm, millisiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; AIW, Atlantic Intracoastal Waterway; WWTP, wastewater treatment plant; S.C., South Carolina; ND, not determined] Station number (fig. 1) Station description Cross section location from RB looking US/NE (%) Time Depth (ft) Water temperature (°C) Specific conductance (mS/cm) Dissolved oxygen (mg/L) pH (units) Fluorescence, field (μg/L)

2 AIW at mouth of The Cove near Sullivan’s Island 50 1035 1 12.8 49.2 8.5 8.0 0.2

1036 5 12.6 49.9 8.4 8.0 0.31037 10 12.5 50.6 8.3 8.0 0.4

3 Unamed cove off AIW at the WWTP outfall in Sullivan’s Island 50 1030 1 13.1 49.9 8.0 7.9 0.3

1031 3 13.1 50.1 7.9 7.9 0.5

4 AIW at the east side of S.C. Highway 703 (Ben Sawyer) Bridge in 

Sullivan’s Island

50 1023 1 12.7 49.3 8.4 8.0 0.3

1024 5 12.5 50.2 8.3 8.0 0.31025 10 12.6 51.1 8.2 8.0 0.01026 15 12.7 52.6 7.9 7.9 0.6

6 West fork of Conch Creek nr Sullivan’s Island within shellfish 

harvesting area

50 1052 1 13.1 52.9 8.0 8.0 0.3

1053 5 13.1 52.9 8.0 8.0 0.31054 10 13.1 52.9 8.0 8.0 0.1

7 East Fork of Conch Creek near Sullivan’s Island within shellfish 

harvesting area

50 1100 1 12.9 52.9 8.1 8.0 0.4

1101 5 12.9 52.9 8.0 8.0 0.11102 10 12.9 52.9 8.0 8.0 0.1

9 AIW before the mouth of Inlet Creek near Sullivan’s Island 50 1009 1 12.6 51.7 8.3 8.0 0.2

1010 5 12.6 51.8 8.2 8.0 0.2

1011 10 12.5 51.9 8.2 8.0 0.2

12 AIW before the mouth of Swinton Creek near Isle of Palms 50 0826 1 12.2 52.6 8.5 8.0 0.5

0827 5 12.2 52.7 8.5 8.0 0.30830 10 12.2 53.0 8.5 8.0 0.3

12 AIW before the mouth of Swinton Creek near Isle of Palms 25 0835 1 12.2 53.1 8.5 8.0 0.3

0837 5 12.2 53.1 8.5 8.0 0.2

12 AIW before the mouth of Swinton Creek near Isle of Palms 75 0840 1 12.3 53.1 8.4 8.0 0.7

0841 5 12.3 53.1 8.4 8.0 0.4

13 AIW at the Isle of Palms connector bridge at Isle of Palms 50 0845 1 12.2 52.9 8.4 8.0 0.2

0850 5 12.2 53.0 8.4 8.0 0.60851 10 12.2 53.0 8.4 8.0 0.3
Assessment of Water-Quality and Background Fluorescence 15

Table 7. Water-quality measurements collected at selected points along the cross section in the channel during high-tide conditions at selected sites in the Atlantic Intracoastal Waterway near Sullivan’s Island and Isle of Palms, South Carolina, on January 24, 2012.—Continued[RB, right bank; US, upstream; NE, northeast; %, percent; ft, foot; °C, degrees Celsius; mS/cm, millisiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; AIW, Atlantic Intracoastal Waterway; WWTP, wastewater treatment plant; S.C., South Carolina; ND, not determined]

Station 

number 

(fig. 1)

Station description

Cross section 

location from RB 

looking US/NE 

(%)

Time Depth (ft) Water temperature (°C) Specific conductance (mS/cm) Dissolved oxygen (mg/L) pH (units) Fluorescence, field (μg/L)

14 Hamlin Creek near its mouth near Goat Island at Isle of Palms in 

shellfish harvesting areas

25 0911 1 12.4 52.8 8.3 8.0 0.2

0912 5 12.4 52.9 8.3 8.0 0.2

14 Hamlin Creek near its mouth near Goat Island at Isle of Palms in 

shellfish harvesting areas

50 0914 1 12.3 52.9 8.4 8.0 0.4

0915 5 12.3 53.0 8.4 8.0 0.60916 10 12.3 53.0 8.4 8.0 0.5

14 Hamlin Creek near its mouth near Goat Island at Isle of Palms in 

shellfish harvesting areas

75 0917 1 12.4 53.0 8.3 8.0 0.1

0918 5 12.4 53.0 8.3 8.0 0.3

15 AIW at Goat Island 50 0925 1 12.8 52.9 8.1 8.0 0.2

0926 5 12.7 52.9 8.2 8.0 0.30927 10 12.5 53.0 8.2 8.0 0.5

16 AIW at Isle of Palms Marina index-velocity station 50 0935 1 13.1 52.8 8.0 7.9 0.2

0936 5 13.0 52.8 7.9 7.9 0.40937 10 13.0 52.8 7.9 7.9 0.10938 15 12.8 52.9 7.8 7.9 0.0

17 AIW at the Isle of Palms WWTP outfall east of Isle of Palms 

Connector Bridge

25 0859 1 12.4 53.0 8.4 8.0 0.3

0900 5 ND ND ND ND 0.3

17 AIW at the Isle of Palms WWTP outfall east of Isle of Palms 

Connector Bridge

50 0901 1 12.4 53.0 8.3 8.0 0.2

0902 5 12.4 53.0 8.3 8.0 0.30903 10 12.3 53.0 8.3 8.0 0.4

17 AIW at the Isle of Palms WWTP outfall east of Isle of Palms 

Connector Bridge

75 0905 1 12.3 52.9 8.4 8.0 0.6

0906 5 12.3 53.0 8.4 8.0 0.6

18 Hamlin Creek north of AIW in Isle of Palms 50 1119 1 14.4 52.7 7.6 7.8 0.4

1120 5 14.3 52.8 7.6 7.8 0.3

21 Hamlin Creek south of AIW in Isle of Palms 50 0953 1 12.4 53.0 8.4 8.0 0.3

0954 5 12.3 53.1 8.4 8.0 0.60956 10 12.3 53.1 8.4 8.0 0.4

21 Hamlin Creek south of AIW in Isle of Palms 75 1000 1 12.5 53.0 8.3 8.0 0.3

1002 10 12.4 53.0 8.3 8.0 0.2
16 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

During low-tide conditions, water temperatures collectively ranged from 12.7 to 15.0 °C and had a median value of 

13.7 °C for all stations and all depths (tables 6 and 8). Maximum water temperatures were measured at station 3, located 

in an unnamed cove off the Atlantic Intracoastal Waterway 

near the wastewater treatment plant outfall in Sullivan’s 

Island (table 8). Specific conductance ranged from 48.4 to 

52.9 mS/cm and had a median of 52.6 mS/cm. Dissolved 

oxygen concentrations ranged from 7.8 to 8.5 mg/L and had a 

median of 8.1 mg/L. For the water temperature and barometric 

pressure conditions (764 millimeters of mercury) at the time of 

sampling, the dissolved oxygen concentrations measured at the 

stations in the Atlantic Intracoastal Waterway also represented 

about 75-percent saturation during low-tide conditions. Measurements of pH were relatively stable among profile stations 

and depths, ranging from 7.9 to 8.0.

Stations were well mixed vertically, with water temperature, specific conductance, dissolved oxygen, and pH, varying minimally with depth (table 8). The percent variability in 

water temperature with depth ranged from -2.6 to 0.1 percent 

from the water surface to channel bottom during high-tide 

conditions and from -7.2 to -0.2 percent during low-tide conditions. A negative percentage indicated a decreasing trend from 

surface to bottom, whereas a positive percentage indicated 

increasing trend from surface to bottom. The variation in specific conductance with depth (from surface to bottom) ranged 

from 0 to 6.2 percent during high-tide conditions and from 0 

to 4.2 percent during low-tide conditions. Dissolved oxygen 

measurements varied from -6.1 to 0.9 percent from surface to 

bottom during high-tide conditions and from -5.2 to 0.1 percent during low-tide conditions. Measurements of pH were 

the least variable with depth, ranging from -0.8 to 0.4 percent 

during high-tide conditions and from -0.6 to 0.4 percent during 

low-tide conditions. 

Background Fluorescence and Specific 

Conductance

Background fluorescence (recorded as rhodamine) 

concentrations were all below 1 microgram per liter (μg/L), 

which is the accuracy level of the sensor (table 3), so measurements should be compared with caution. Additionally, 

fluorescence varied vertically and horizontally, whereby 

small differences in magnitude produced large differences in 

percentages. Therefore, differences in mixing for fluorescence 

were expressed in micrograms per liter of rhodamine, rather 

than in percent. In general, horizontal variability at the profile 

transects at stations 12, 14, and 17 and vertical (depth) variability in fluorescence ranged from 0.1 to 0.4 μg/L. Ranges of 

horizontal and vertical variability at individual stations were 

similar to ranges of spatial (among station) variability, increasing the uncertainty when comparing stations.

During high-tide conditions, an apparent pattern of 

slightly elevated fluorescence (near 0.5 and 0.6 μg/L) in specific regions of the Atlantic Intracoastal Waterway and nearby 

coves and tidal creeks was observed in the IDW distribution 

map (fig. 11). These regions included Hamlin Creek, the 

Atlantic Intracoastal Waterway near the mouth of Hamlin 

Creek (stations 14, 18, and 21), especially at the greater (5 ft 

and 10 ft) depths (fig. 11C,E. Fluorescence appeared to be 

higher at near-surface (1-ft) depths than greater depths (5 ft 

and 10-ft) during high tide in Conch Creek (fig. 11A,C,F) 

(stations 6 and 7) and highest at greater-depth (10 ft) during 

high tide in Hamlin Creek (fig. 11A,C,F) (stations 14 and 18). 

The IDW interpolation of specific conductance, also mapped 

for high-tide conditions, indicated small increases in specific 

conductance from the southwest (Sullivan’s Island) to northeast (Isle of Palms) in the Atlantic Intracoastal Waterway, 

and in Hamlin and Conch Creeks (fig. 11B,D,F). This spatial 

change in specific conductance was observed at all 3 depths in 

the water column.

A pattern of slightly elevated fluorescence was also 

observed during low-tide conditions in the region of Hamlin 

Creek and the Atlantic Intracoastal Waterway near the mouth 

of Hamlin Creek in the IDW distribution map (fig. 12A,C,E), 

but unlike the case during high-tide conditions, not in the other 

tidal creeks. Near the Ben M. Sawyer Memorial Bridge (State 

Highway 703) at Sullivan’s Island, slightly higher fluorescence was observed at 1- and 5-ft depths (fig. 12A,C). Spatial 

distribution of specific conductance during low-tide conditions 

had a similar pattern of increasing specific conductance from 

southwest (Sullivan’s Island) to northeast (Isle of Palms) in 

the Atlantic Intracoastal Waterway and (fig. 12B,D,F). Graphs 

showing fluorescence concentrations and specific conductance 

in the Atlantic Intracoastal Waterway and nearby tidal creeks 

on January 24, 2012, during low-tide conditions, Sullivan’s 

Island and Isle of Palms, South Carolina.

Although areas within the Atlantic Intracoastal Waterway 

and tidal creeks appeared to have slightly elevated fluorescence, all measured concentrations were below the accuracy of 

the sensor. Therefore, the measured background fluorescence 

would be unlikely to interfere with a dye-tracing study, if sufficient dye were to be released to raise levels well above the 

accuracy level of 1 μg/L. Additionally, changes in water temperature, pH, and dissolved oxygen concentrations have been 

identified to affect dye fluorescence in ambient waters, and 

results from this study indicated a relatively well-mixed system in terms of those factors (YSI Incorporated, 2001, 2011). 

Low measured concentrations produced a large amount of 

uncertainty about whether the apparent slight elevation of the 

fluorescence was related to optical brighteners in wastewater 

effluent, turbidity (not measured during the study) interference 

with the fluorescence probe sensor, high naturally occurring 

organic compounds interference, or simply poor sensor performance at concentrations below its accuracy level. 
Assessment of Water-Quality and Background Fluorescence 17

Table 8. Water-quality measurements collected in the center of the channel during low-tide conditions at selected stations in the Atlantic Intracoastal Waterway near Sullivan’s Island and Isle of Palms, South Carolina, on January 24, 2012.[ft, foot; °C, degrees Celsius; mS/cm, milliSiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; AIW, Atlantic Intracoastal Waterway; WWTP, wastewater treatment plant; S.C., South Carolina]

Station 

number 

(fig. 1)

Station description Time Depth (ft) Water temperature (°C) Specific conductance (mS/cm) Dissolved oxygen (mg/L) pH (units) Rhodamine, field (μg/L)

1 AIW in The Cove near Sullivan’s Island 1415 1 14.3 48.4 8.1 7.9 0.3

1416 5 13.7 50.0 8.2 7.9 0.21418 10 13.4 50.5 8.1 7.9 0.2

2 AIW at mouth of The Cove near Sullivan’s Island 1405 1 13.8 50.5 8.2 7.9 0.13 Unamed cove off AIW at the WWTP outfall in Sullivan’s Island 1400 1 15.0 50.8 8.3 7.9 0.24 AIW at the east side of S.C. Highway 703 (Ben Sawyer) Bridge in Sullivan’s Island 1350 1 13.7 50.3 8.5 8.0 0.1

1352 5 13.3 52.2 8.3 8.0 0.4

5 AIW between the mouths of Narrows and Conch Creeks near Sullivan’s Island 1340 1 13.8 51.3 8.4 8.0 0.0

1341 5 13.6 52.4 8.1 7.9 0.21342 10 13.4 52.5 7.9 7.9 0.0

12 AIW before the mouth of Swinton Creek near Isle of Palms 1335 1 13.8 52.7 7.8 7.9 0.2

1337 5 13.8 52.7 7.8 7.9 0.2

13 AIW at the Isle of Palms connector bridge at Isle of Palms 1327 1 13.9 52.7 7.8 7.9 0.2

1330 5 13.8 52.8 7.8 7.9 0.3

16 AIW at the Isle of Palms Marina index-velocity station 1310 1 13.6 52.7 8.2 7.9 0.1

1312 5 13.0 52.8 8.2 8.0 0.21313 10 12.7 52.9 8.2 8.0 0.1

17 AIW at the Isle of Palms WWTP outfall east of Isle of Palms Connector Bridge 1320 1 13.7 52.7 7.8 7.9 0.2

1321 5 13.6 52.8 7.8 7.9 0.51322 10 13.6 52.8 7.8 7.9 0.5
18 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

2 3

4

6 7

9

12

13

14

15 16

17

18

21

1

5

2

3

4

6 7 9

12

13 14 15

16

17

18

21

1

5

2 3

4

6 7 9

12

13

14 15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13

14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

CH

A

RLESTO

N

CH

A

RLESTO

N

CH

A

RLESTO

N

CH

A

RLESTO

N

CH

A

RLESTO

N

CH

A

RLESTO

N

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Isle of Palms Isle of PalmsIsle of Palms Isle of Palms

Isle of Palms Isle of Palms 701

17

703

701

17

703

701

17

703

701

17

703

701

17

703

701

17

703

EXPLANATION

Fluorescence, in  micrograms per  liter

0 to 0.10.11 to 0.20.21 to 0.30.31 to 0.40.41 to 0.5

0.51 to 0.6

Tidal marshes

EXPLANATION

Conductance, in  millisiemens per  centimeter

48.35 to 4949.01 to 5050.01 to 5151.01 to 5252.01 to 53.05

Tidal marshes

High tide

1-foot depth5-foot depth

10-foot depth

Base from ESRI® Data & Maps, 2010Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

0 1 2 MILES0 1 2 KILOMETERS

N

ACE

BDF

Figure 11. Fluorescence concentrations and specific conductance in the Atlantic Intracoastal Waterway and nearby tidal creeks on January 24, 2012, during high-tide conditions, Sullivan’s Island and Isle of Palms, South Carolina.
Assessment of Water-Quality and Background Fluorescence 19

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15 16

17

18

21

1

5

2 3

4

6 7 9

12

13 14

15

16

17

18

21

1

5

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Mount

Pleasant

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Sullivan's

Island

Isle of

Palms

Isle of

Palms

Isle of Palms Isle of Palms

Isle of

Palms

Isle of

Palms

CHARLESTON

CHARLESTON

CHARLESTON

CHARLESTON

CHARLESTON

CHARLESTON

701

17

703

701

17

703

17

701

703

701

17

703

701

17

703

701

17

703

EXPLANATION

Conductance, in  millisiemens per  centimeter

48.35 to 4949.01 to 5050.01 to 5151.01 to 5252.01 to 53.05

EXPLANATION

Fluorescence, in  micrograms per  liter

0 to 0.10.11 to 0.20.21 to 0.30.31 to 0.40.41 to 0.5

0.51 to 0.6

Tidal marshes Tidal marshes

Base from ESRI® Data & Maps, 2010Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

0 1 2 MILES0 1 2 KILOMETERS

N

Low tide

1-foot depth5-foot depth

10-foot depth

ACE

BDF

Figure 12. Fluorescence concentrations and specific conductance in the Atlantic Intracoastal Waterway and nearby tidal creeks on January 24, 

2012, during low-tide conditions, Sullivan’s Island and Isle of Palms, South Carolina.
20 Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway

Summary 

Proposed changes in the water-quality limits for fecal 

coliform may have potential effects on the shellfish beds near 

the effluent outfalls of the Sullivan’s Island and the Isle of 

Palms wastewater treatment plants. To effectively plan a site 

specific study to understand the connection between wastewater effluent and the shellfish beds, data are needed on the 

flow dynamics and background fluorescence of the Atlantic 

Intracoastal Waterway near the effluent outfalls on Sullivan’s 

Island and the Isle of Palms. To compute continuous tidal 

flows, two index-velocity stations were installed and operated on the Atlantic Intracoastal Waterway at the Isle of Palms 

Marina and at the Ben M. Sawyer Memorial Bridge near Sullivan’s Island. Tidal flows were estimated for Breach Inlet as a 

remote index-velocity station using the continuous 15-minute 

stage and velocity data from the Sullivan’s Island station. To 

measure the background fluorescence of the Atlantic Intracoastal Waterway, a synoptic water-quality study was conducted to collect longitudinal water-quality profile locations 

along the Atlantic Intracoastal Waterway and included stations 

near the effluent receiving creeks, tidal creeks, and coves.

The two index-velocity stations measured concurrent 

data for the 53-day period of December 4, 2011, to January 

26, 2012. Tidal flow and residual flow for the 53-day period 

on the Atlantic Intracoastal Waterway at the Isle of Palms 

stations ranged -3,460 cubic feet per second (ft3/s) toward the 

southwest (toward Charleston Harbor) to 6,410 ft3/s toward 

the northeast. For Sullivan’s Island, the flows ranged from 

-6,360 ft3/s to the southwest to 8,930 ft3/s toward the northeast. The largest flows occurred at Breach Inlet and ranged 

from -58,600 ft3/s toward the Atlantic Intracoastal Waterway 

and 63,300 ft3/s toward the Atlantic Ocean. The residual flow 

during the 53-day period was -5,770 ft3/s into the system 

toward the Atlantic Intracoastal Waterway at the Breach Inlet 

station. At the Isle of Palms station, the residual flow for the 

same period was 616 ft3/s toward the northeast where it was 

-866 ft3/s toward the southwest at the Sullivan’s Island station.

Synoptic longitudinal water-quality profiles at high and 

low tide were sampled on January 24, 2012. Profile data 

included depth, water temperature, specific conductance, 

dissolved oxygen, pH, and background fluorescence. Generally, the stations were well mixed vertically and horizontally 

with respect to water temperature, pH, and dissolved oxygen. 

Maps of background fluorescence and specific conductance 

were generated for high- and low-tide and for 1-, 5-, and 

10-foot depth using an inverse distance weighted interpolation technique. All the fluorescence measurements were below 

the accuracy of sensor. The background fluorescence would 

unlikely interfere with a dye-tracer study.

References Cited

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YSI Incorporated, 2001, Water tracing, in situ dye fluorometry, 

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YSI Incorporated, 2011, 6-Series multiparameter water quality 

sondes user manual: YSI Incorporated, accessed March 

15, 2012, at http://www.ysi.com/media/pdfs/069300-YSI-6-

Series-Manual-RevH.pdf.
Manuscript approved for publication on xxxxxxx

Editorial review by xxxxxxxx

Illustrations prepared by Kim Swidarski

Layout by xxxxxxx

For additional information regarding this publication, contact:

Director

USGS South Carolina Water Science Center

720 Gracern Road, Suite 129

Columbia, SC 29210

phone: 803–750–6100

e-mail: dc_sc@usgs.gov

Or visit the South Carolina Water Science Center Web site at:

http://sc.water.usgs.gov/

Document prepared by the Raleigh and West Trenton Publishing 

Service Centers
Conrads, P.A., and others —Tidal Flow Dynamics and Background Fluorescence of the Atlantic Intracoastal Waterway—Open-File Report 2013–1077