�������

������

������

�������

�������

������� �������

�������

�������

�������

�������

������

�

��

��� ����� ������������

� ���� ������������

�����������

����

�����

�������

�������� �������

� � �� ��

����������

��������

����� ������ �����

������ �����

�����

������ ������

�����

�����

���� ����

��

��

�������� �����

����

���� ����

���

����

������

������

���

��� ��� ����� �����

������

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3041

April 2005

South Carolina Coastal Erosion Study

Printed on recycled paper

Introduction

Much of the South Carolina coast is 

heavily developed, supporting local populations, infrastructure, and a large tourism 

industry. Local economies are greatly 

affected by damage and property loss due 

to coastal erosion, which occurs primarily during severe storm events. In 1989, 

Hurricane Hugo caused over $5 billion in 

damages to coastal residences and industry. Understanding the factors controlling 

coastal erosion, sediment transport, and 

shoreline change is essential to the successful management of coastal resources.

In 1999, the U.S. Geological Survey 

(USGS), in partnership with the South 

Carolina Sea Grant Consortium, began 

a study of processes affecting shoreline 

change along the northern coast of South 

Carolina, focusing on the Grand Strand 

Figure 1. Map showing the location of the Grand Strand study area off South 

Carolina and an image of sidescan-sonar data collected in 1999–2003. The 

image reveals the acoustic character of the sea floor as variations in the 

strength of the return signal are represented as gray-scale values; areas of 

low backscatter (low reflectance) are displayed as dark tones, and areas of 

high backscatter (high reflectance) are displayed as light tones. Surficial grab 

samples and video images show areas of low backscatter to be characterized 

by fine to medium sand, silt, and mud. Coarse sand, shell hash, gravel, and 

outcropping strata characterize areas of high backscatter. A–A’ marks the 

location of the seismic profile displayed in figure 3. The offshore sand body 

oriented oblique to the shoreline is a potential source of beach-nourishment 

material. The base map was compiled from bathymetric and topographic data 

from the National Geophysical Data Center (NGDC) U.S. coastal relief model. 

Vertical exaggeration (25x) and an illumination source were applied to the 

data, yielding shaded-relief images. The dark-blue line parallel to the shore is 

an artifact of data processing.

Geologic Mapping

South Carolina’s Grand Strand is centered on a 100-kilometer (km)-long arcuate stretch of shoreline within the apex 

of Long Bay (fig. 1). Unlike the sandy 

barrier beaches that characterize most of 

the U.S. Atlantic margin, the central part 

of the Grand Strand is a mainland beach, 

not backed by an estuary. Numerous 

borings in onshore coastal plain deposits 

have identified younger Holocene beach 

deposits (formed ~10,000 years ago to 

present) that are attached to older Pleistocene headlands (ice-age deposits formed 

1.8 million to 10,000 years ago) in this 

region (fig. 2). These relict headlands 

most likely formed at a slightly higher 

sea level and continue to influence the 

shape of the shoreline today as sea level 

rises toward its former highstand position. To the north and south of Myrtle 

Beach, several small barrier islands with 

relatively large tidal inlets are separated 

from the older Pleistocene mainland by 

narrow, marsh-filled estuaries.

region (fig. 1). Previous work along the 

U.S. Atlantic coast shows that the structure and composition of older geologic 

strata located seaward of the coast heavily influence the coastal behavior of areas 

with limited sediment supply, such as the 

Grand Strand. By defining this geologic 

framework and identifying the transport 

pathways and sinks of sediment, geoscientists are developing conceptual models 

of the present-day physical processes 

shaping the South Carolina coast.

The primary objectives of this research 

effort are (1) to provide a regional synthesis of the shallow geologic framework 

underlying the coastal upland, shoreface, 

and inner continental shelf and to define 

its role in coastal evolution and modern 

beach behavior; (2) to identify and model 

the physical processes affecting coastal 

ocean circulation and sediment transport 

and to define their roles in shaping the 

modern shoreline; and (3) to identify 

sediment sources and transport pathways 

and eventually to construct a regional 

sediment budget.
High-resolution geophysical systems 

(sidescan sonar, swath bathymetry, and 

seismic reflection) were used to map the 

surficial and subsurface, or framework, 

geology of the inner continental shelf 

in water depths ranging from 4 to 14 

meters (m) (fig. 1). These acoustic data 

define the distribution of surficial sediment (mud, sand, gravel, and rock), the 

geomorphology, or shape, of the seabed, 

and the stratigraphy and structure of units 

underlying the inner shelf. The surveys 

extend 10 km seaward of the coast and 

include sediment sampling, bottom video, 

and photography to calibrate or verify the 

geophysical data.

Sea-floor mapping reveals a generally sediment starved, low-relief inner 

shelf that dips gently seaward. Extensive 

areas lack significant sediment deposits 

and are characterized by high backscatter 

on the sidescan-sonar imagery (fig. 1). 

Seismic-reflection data show Cretaceous/

Tertiary strata (pre-ice-age deposits 

formed 145 to 1.8 million years ago) 

that are folded, dipping to the southeast, 

and locally incised by a network of filled 

paleochannels cut by the ancestral Pee 

Dee River during previous sea-level 

lowstands (fig. 3). The location and trend 

of the channels appear to be controlled 

by differential erosion of the underlying 

strata. The younger channel-fill deposits 

and older underlying units are truncated 

by a regional unconformity (gap in the 

geologic record due to erosion or nondeposition) that was eroded during multiple 

sea-level cycles.

A patchy, discontinuous veneer of 

low-backscatter, relatively well sorted, 

fine to medium sand overlies this flatlying, erosional surface (figs. 2 and 3). 

Small channels observed within this 

veneer likely represent remnants of 

tidal inlets in a barrier-island chain that 

formerly existed seaward of the modern 

coast. The thickness of sand deposits 

overlying the erosional surface is variable; large accumulations, locally more 

than 6 m thick, generally are found in 

tidal-delta complexes near modern inlets 

(figs. 2 and 4). One exception is a large 

shore-oblique sand body offshore of Myrtle Beach, which is not associated with a 

modern inlet (fig. 4). The sand deposit is 

10 km long, 2 km wide, and more than 3 

m thick; it represents a potential source of 

beach-nourishment material.

Figure 2. Geologic cross section B–B’ and map of the Grand Strand area, 

South Carolina, displaying onshore and offshore surficial geology and locations of coastal plain borings. Data from the borings were used to generate the 

geologic cross section. Surficial geologic maps indicate that older Pleistocene 

deposits compose the mainland beaches along the Grand Strand. These relict 

deposits were formed when sea level was higher than the present level and 

continue to influence the shape of the modern shoreline as sea level rises. 

Younger Holocene deposits are generally centered on modern inlet systems. 

Cross section modified from Putney and others (2004); onshore geology modified from McCartan and others (1984) and Owens (1989). The offshore surficial 

geology was defined through seismic profiles. Older Cretaceous folded and 

dipping strata rise close to the sea floor in the north, indicating an absence 

of younger units in this region. The Cretaceous strata deepen to the south, 

as evidenced by younger Tertiary and Pleistocene deposits in the middle and 

southern regions of the survey area. Younger Holocene sediments are primarily located near modern inlets. An exception is the sand body lying oblique to 

the coast offshore of Myrtle Beach (see fig. 4). Geologic ages modified from 

International Commission on Stratigraphy (2004). MSL, mean sea level.

�������

������

������� ������

�������

�������

������� �������

�������

�������

�������

�������

�������

������

� ��

�������� ����� ������ ����� ������

������

���������

������

��

���

����

����

��

���

����

����

�����������

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

� �

����������

�������

������ �� ������

��������

�����

������ ������

�����

�����

���������

������

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

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

�������� ������� ������� �������

������� ����� �������

� � �� ��

����������

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

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

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

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

�� �����������

�

��

������ �����

������ �����

�����
Figure 3. Interpreted (bottom) and uninterpreted (top) versions of a representative seismic profile collected offshore of Murrells Inlet, S.C. Older 

Cretaceous/Tertiary units are incised by Pleistocene (ice-age) paleochannels formed by the ancestral Pee Dee River during sea-level lowstands. 

The channels are truncated by an unconformity eroded during multiple 

sea-level cycles. A patchy, discontinuous veneer of younger Holocene 

sediments overlies the unconformity. Vertical scale is displayed as twoway traveltime in milliseconds and approximate depth in meters below 

mean sea level (MSL); depth is calculated by assuming 1,500 meters per 

second for the speed of sound in water. See figure 1 for location.

���

��

��

��

���

��

��

���

��

��

���

��

��

��

� ��� � � ��

����������������

������

�

�

�����

�

�

��

�

����

�� ��

���

����

�

�

�

��

�������� ������������ � �����

������������

� ��

������������ �������

�

���

�� ��

�

�

�

�

��

��������� �������

����������� ��������

���� ������� �� ������ ����� ����

�������������������

���� �� ��� ������� ����� ����

������������

�������� �� ����������������������

��������

������� ����� ��� �� ��������

�����������

������� ������

Figure 4. Map of the Grand Strand area, South Carolina, displaying 

the thickness of offshore Holocene sediment as defined through 

seismic profiles. Sediment thickness ranges from 0.5 to more than 6 

meters, and the thickest deposits are near tidal inlets. An exception 

is the sand body lying offshore of Myrtle Beach, which is a potential 

source of beach-nourishment material. Cretaceous/Tertiary strata and 

(or) Pleistocene deposits are at or near the sea floor in regions devoid of 

Holocene sediment or overlain by a thin veneer of sediment (displayed as 

areas of no color). Sediment thickness less than 0.5 m was not resolved by 

the seismic system.

��������

����� ������ �����

������ �����

�����

������ ������

�����

�����

���� ����

������� �������

������

������

�������

�������

������� �������

�������

�������

�������

�������

�������

������

� � �� ��

����������

���

� ������

���������
Figure 5. Deployment of a USGS tripod used for oceanographic studies. 

The tripod is configured with various instruments used to measure pressure, currents, salinity, temperature, suspended-sediment concentrations, 

and wave characteristics. Photograph by Dann Blackwood (USGS).

Study of Present-Day Physical Processes

Insights derived from the geologic framework studies have 

guided oceanographic experiments to better understand the 

transport of sediment within Long Bay (fig. 1). These studies 

aim (1) to measure and model the oceanographic circulation in 

the region, (2) to identify the processes maintaining the offshore 

sand body, (3) to quantify changes in wave energy propagation 

in the nearshore due to the presence of this sand body, and (4) to 

identify the consequences of removal of the sand body.

To accomplish these goals, oceanographic and sediment-transport observational systems (fig. 5) were deployed for a period of 6 

months (November 2003 to April 2004) at eight sites to measure 

pressure, currents, salinity, temperature, suspended-sediment 

concentrations, and wave characteristics. Initial results show that 

local wind events strongly correlate with peaks in wave energy. 

Winds also drive near-surface currents. However, bottom currents near the sea floor are more complex and are controlled by 

a combination of wind forcing, upwelling or downwelling flows, 

and stratification. Additionally, local waves provide energy that 

resuspends bottom sediments, which have increased concentrations in the water column during strong wind events. This combination of wind-driven surface currents and near-bottom circulation 

determines the direction and magnitude of sediment transport 

within Long Bay.

Sediment Budget

Defining the transport rates and volumes of sediment that enter 

and leave a segment of coast is critical to the effective management of coastal areas. Geomorphic evidence suggests that erosion of the inner shelf contributes sandy sediment to the coastal 

system. Integrating oceanographic studies and geologic mapping 

will yield a comprehensive view of the physical processes shaping the modern shoreline and, ultimately, will define the regional 

sediment budget for the Grand Strand. 

References Cited

International Commission on Stratigraphy, 2004, International stratigraphic chart: available online at http://

www.stratigraphy.org/chus.pdf

McCartan, Lucy, Lemon, E.M., Jr., and Weems, R.E., 1984, 

Geologic map of the area between Charleston and Orangeburg, 

South Carolina: U.S. Geological Survey Miscellaneous Investigations Map I–1472, 2 sheets, scale 1:250,000.

Owens, J.P., 1989, Geologic map of the Cape Fear region, Florence 1°x2° quadrangle and northern half of the Georgetown 

1°x2° quadrangle, North Carolina and South Carolina: U.S. 

Geological Survey Miscellaneous Investigations Map I–1948–

A, 2 sheets, scale 1:250,000.

Putney, T.R., Katuna, M.P., and Harris, M.S., 2004, Subsurface 

stratigraphy and geomorphology of the Grand Strand, Georgetown and Horry Counties, South Carolina: Southeastern Geology, v. 42, no. 4, p. 217–236.

By Jane F. Denny, Wayne E. Baldwin, William C. Schwab, 

John C. Warner, and M. Richard DeVoe

For more information, please contact:

Jane F. Denny

William C. Schwab

U.S. Geological Survey

384 Woods Hole Road

Woods Hole, MA 02543–1598

Telephone: 508–457–2311 (Denny)

 508–457–2211 (Schwab)

E-mail: jdenny@usgs.gov

 bschwab@usgs.gov

Web site: http://woodshole.er.usgs.gov/

M. Richard DeVoe

South Carolina Sea Grant Consortium

Charleston, SC 29401

E-mail: Rick.DeVoe@seagrant.org