PAST, PRESENT AND FUTURE INLETS
OF THE OUTER BANKS BARRIER
ISLANDS, NORTH CAROLINA
David J. Mallinson, Stephen J. Culver, Stanley R. Riggs, J.P. Walsh,
Dorothea Ames, and Curtis W. Smith
Department of Geological Sciences
Thomas Harriot College of Arts and Sciences and
Institute for Coastal Science and Policy
East Carolina University
This photograph looks west toward Hatteras Village, across Isabel Inlet that
formed during Hurricane Isabel (2003). The remnants of Highway 12 are seen in the foreground, and the former
location of the ocean shoreline is in the white surf beyond the inlet. Photograph is by S.R. Riggs.
PAST, PRESENT AND FUTURE INLETS
OF THE OUTER BANKS BARRIER
ISLANDS, NORTH CAROLINA
A White Paper by
David J. Mallinson, Stephen J. Culver, Stanley R. Riggs, J.P. Walsh
1
,
Dorothea Ames, and Curtis W. Smith
2
Members of the
NORTH CAROLINA COASTAL GEOLOGY
COOPERATIVE RESEARCH PROGRAM
Department of Geological Sciences
Thomas Harriot College of Arts and Sciences and
1
Institute for Coastal Science and Policy
East Carolina University, NC 27858
December, 2008
2
Now at Marshall Miller and Associates, 11277 Airpark Road, Suite 203, Ashland, VA 23005
Oregon Inlet
Hatteras Inlet
Ocracoke Inlet
Drum and Ophelia Inlet
Buxton Inlet
Isabel Inlet
The work summarized in this report is the product of the collaborative eorts of many researchers associated
with the North Carolina Coastal Geology Cooperative, a multi-year research program led by East Carolina
University, the United States Geological Survey, and the North Carolina Geological Survey, with contributions
from scientists at the University of Delaware, the University of Pennsylvania, and the Virginia Institute of Marine
Sciences. We would additionally like to acknowledge the assistance of Jim Watson, John Woods, Ron Crowson,
and numerous graduate students. This work was funded in part by the USGS Cooperative Agreement award
02ERAG0044, and a UNC General Administration Research Competitiveness award. Other support came
from the U.S. National Park Service, the U.S. Fish and Wildlife Service, Environmental Defense, NC Division
of Coastal Management, and the NC Division of State Parks and Recreation. Sources of aerial photographs
include: USACE-FRF, Duck, NC; USACE-Wilmington; CHNS, Manteo; NC State Database; NC DOT; U.S.
Geological Survey.
1
For centuries, humans have depended on inlets as a means
of navigating between the ocean and the protected coastal
waters behind barrier islands. Inlets along the North
Carolina Outer Banks barrier islands oered access to the
rst English settlers in the New World during the late
16th century, and continue to oer access for commercial
and recreational vessels. Although promising passage
to sheltered waters, the dynamic, shifting sands of the
inlet shoals have led to the grounding and destruction
of numerous vessels, contributing to the infamous label
“graveyard of the Atlantic” for the North Carolina coast.
With time, some inlets provided access to port towns
which became locations for trade, and which would
provide a local pilot to help navigate ships through the
shifting channels. Thus, inlets became an important
economic asset. Today, inlets are still vital to navigation,
trade, and commerce, especially commercial and
recreational shing.
In addition to their clear historical and economic
signicance, inlets provide a vital service to the
maintenance of estuaries and barrier islands, and play
a fundamental role in the evolution of transgressive
(landward migrating) barrier islands. In spite of their
name, inlets could more appropriately be termed “outlets”
as they provide an exit for fresh water owing down the
rivers. Within the estuaries, the fresh riverine water mixes
with salty ocean water to produce the mixed salinity or
brackish waters. The riverine ow volumes and ocean
storm dynamics determine the residence time of water
within the estuaries, which is important to biological
systems.
Inlets also provide a pathway for sand to be transferred
from the shorezone on the ocean-side of the island to
the estuarine side of the island. The sand is deposited
as vast ood-tide deltas which are colonized by marsh
plants upon inlet closure. The resulting back-barrier
shoals and marshes maintain island width, and provide
a shallow platform over which the island may migrate
landward. The occurrence of inlet channel sediments
and ood-tide delta sediments beneath the barrier islands
aects the variety of sediment available to the beach
system as the ocean shoreline recedes. As such, island,
beach, and shoreface morphologies are related to the
occurrence of paleo-inlet channels beneath the islands.
In turn, the island geomorphology is a key factor in
determining where future inlets are most likely to occur.
Thus, it is important to understand the dynamics of past
inlets and their relationship to the sediment budget and
island geomorphology. Inlets are vital to the short-term
maintenance of barrier island systems and their estuaries,
and long-term barrier island evolution in response to
ongoing sea-level rise.
A digital version of this document, along with reports on related research funded by a grant from the University of North
Carolina system, can be accessed at the North Carolina Coastal Hazards Decision Portal: http://www.coastal.geology.ecu.edu/
NCCOHAZ/.
2
Inlets are a fundamentally important part of our coastal
system by virtue of their roles in both human activities and
barrier island maintenance and evolution. But inlets are not
xed in space and time and this fact is at the root of several
inlet-related coastal management issues. This White Paper
is produced for coastal managers, agencies, business owners,
coastal residents, etc., to provide a general overview of the
workings of inlet systems as we struggle to live with their
dynamic nature during a time of sea-level rise and high storm
activity.
Inlets provide for the interchange of fresh and marine
waters within the estuarine system. The number and size
of inlets is naturally adjusted to (i.e., is in equilibrium
with) the volume of water discharged from the rivers and
the amount of water that enters and exits the estuaries
daily due to astronomical and wind tides (the “tidal
prism”). Typically, where tidal energy is high, such as in
southern NC, many inlets are required to accommodate
the exchange of seawater during a tidal cycle, resulting in
more inlets and shorter islands. Where the tidal range is
minimal, inlets act primarily as outlets for fresh water that
ows into the estuaries from the rivers. This situation
results in fewer inlets and longer barrier islands. Currently,
within the Albemarle-Pamlico Estuarine System (APES)
the river discharge is relatively low and the average length
of time that water stays in the sounds (residence time) is
approximately 11 months. Except in the vicinity of inlets,
astronomical tides within the APES are small (<1 ft.).
However wind tides associated with a variety of weather
systems increase the volume of water being exchanged
between the estuaries and ocean. Storm events frequently
result in the formation of ephemeral inlets to accommodate
this additional exchange. Due to the low volume of
freshwater discharge and small astronomical tidal
prism, few inlets occur along the Outer Banks north
of Cape Lookout, and currently include New Drum,
New Old Drum, and Ophelia inlets in Core Banks,
Ocracoke Inlet, Hatteras Inlet, and Oregon Inlet (Fig.
1).
An inlet consists of a variety of geomorphic
components (Fig. 2). The inlet channel that separates
the adjacent islands is the throat channel and consists
of a central main ebb channel and anking marginal
ood channels. The cross-sectional area of the throat
channel conforms to the volume of water that must
pass through it. When the water volume is decreased,
the channel will tend to shoal. If the volume
increases, the channel will deepen and/or widen.
Depending on wave and current patterns, sand
transport between islands may take a circuitous route
to bypass the intervening inlet. Sand is moved along
the beach and nearshore area parallel to the coastline
in response to waves as they encounter the shallow
coastal environments. Along the Outer Banks, sand
generally moves from north to south in the longshore
current owing to the cumulative high energy wave
action from the northeast. As sand encounters the
MODIS satellite image showing the location of active
inlets between the Virginia state line and Cape Lookout, NC.
Satellite image courtesy of Institute for Marine Remote Sensing,
College of Marine Science, University of South Florida.
3
Cartoon
showing the
generalized
morphology
and
terminology
of features
associated with
inlets.
inlet during ood tides, it is moved into the marginal ood
channels and is deposited on the ood-tide delta (FTD)
(Fig. 2). As the tide ebbs, sediment is transported seaward
through the main ebb channel to be deposited on the ebb-
tide delta (ETD). The ETD sand is reworked by waves
and currents into shoals, which migrate across the ETD
platform and merge with the beach down-current of the
inlet. In this manner, both the FTD and ETD temporarily
store and episodically release sand to the nearby beaches and
coastal system.
The location of the main ebb channel also plays a
fundamental role in erosion and deposition around an inlet.
The main ebb channel may shift its location during storms.
If the channel intersects the island on one side of the inlet,
rapid erosion of the island shoreline may ensue. At the
same time, sand that was originally deposited on the ETD
on the up-current side of the ebb channel may now be on
the down-current side, and may naturally nourish the beach
on the adjacent island.
The shape and size of the inlet, the FTD, and the ETD
depend upon the amount of sand moving along the coast,
the wave energy and the tidal range. Depending on the
angle of wave approach, and the volume of sand moving
along the shore, the up-current side of the inlet may accrete
sand, building a spit, while the down-current side erodes.
The net consequence of this process on the Outer Banks is
southward inlet migration.
An important point is that the natural transport and
deposition of sand within an inlet environment and
the adjacent beaches is in equilibrium with the natural
coastal dynamics. Any interruption of the natural sand
transport across an inlet, either by dredging and enlarging
the channels, mining sand from the ETD, or by installing
terminal jetties, will increase shoreline erosion on the down-
current side of the inlet.
Flood-tide delta and channel-ll sediments of the inlet
throat are commonly preserved beneath transgressive
barrier islands and are important components of island
evolution during transgression (Godfrey and Godfrey, 1976;
Herbert, 1978; Heron and others, 1984; Inman and Dolan,
1989; Riggs and Ames, 2003; Culver and others, 2006).
Following inlet closure, shallow FTD deposits may become
marshes that serve as a shallow water platform where storm-
driven island overwash processes deposit large lobes of
sand, thus building island elevation (Godfrey and Godfrey,
1976; Riggs and Ames, in press). FTD shoals may also be
reworked and incorporated into the back-barrier shorezone,
thereby increasing island width and elevation (Riggs and
Ames, 2003; Culver et al., 2006; Smith et al., 2008; Riggs
and Ames, in press) (Figs. 3 and 4).
4
An illustration of a sequential model of barrier island evolution in response to shoreline
recession and inlet formation. A) Active ood- and ebb-tide deltas (FTD and ETD, respectively)
form in association with an inlet. B) As the inlet closes, the ETD collapses, causing temporary
and localized shoreline accretion, while adjacent areas continue to erode; the FTD is abandoned,
and a platform marsh and marsh islands develop on FTD shoals, increasing the island width.
C) Continued shoreline erosion narrows the island more rapidly in areas underlain by ne FTD
sediments while slower erosion occurs where coarse sands associated with the inlet throat
channel occur. D) The narrow portion of the island breaches during a storm and cross-island ow
and down-cutting create a new inlet. Erosion accelerates in adjacent areas underlain by ne FTD
sediment, continuing the evolutionary succession.
Aerial
photographs of
various regions
of the Outer
Banks showing
different
successional
stages within
the spectrum of
inlet and barrier
island evolution
(CIR DOQQ
from NC State
Database; black
and white aerial
photographs
from NPS
Archives at
Cape Hatteras
National
Seashore
Manteo).
5
Although six inlets currently occur north of Cape Lookout,
numerous additional inlets have dissected the island chain
episodically during historical times (post 1585 A.D.) (Stick,
1958; Fisher, 1962). The rst map of northeastern North
Carolina was made in 1590 and illustrates the occurrence
of numerous inlets along the northern Outer Banks (Fig.
5). Studies have attempted to identify the location of
paleo-inlets based upon historical maps (Stick, 1958; Payne,
1985), island geomorphology (Fisher, 1962; Riggs and
Ames, in press; Ames and Riggs, in press) or stratigraphy,
as derived from cores and geophysical data (Susman, 1975;
Moslow and Heron, 1978; Herbert, 1978; Susman and
Heron, 1979; Heron et al., 1984; Smith, 2004; Mallinson
et al., 2005; Smith, 2006; Culver et al., 2006; Smith et
al., in press). Such studies are signicant in understanding
island geomorphology and the processes of island formation
and evolution.
Fisher (1962) attempted to recognize paleo-inlets along
the Outer Banks by analyzing aerial photographs for the
occurrence of geomorphologic features (i.e., the shape of
the land surface) related to paleo-inlets. He also provided
the positions and dates of known historic inlets (Fig. 6).
Recent studies using modern geophysical techniques (Smith,
2006) have demonstrated that the inlets identied in Fisher’s
1962 study represent only a fraction of the total number of
inlet channels underlying the Outer Banks (Fig. 6).
Modern advances in geophysical technology, specically
ground penetrating radar (GPR) (Fig. 7), have allowed for
rapid acquisition and interpretation of shallow subsurface
data to dene the geology beneath barrier islands (Fitzgerald
The White-deBry map was made in 1590 and shows the occurrence of numerous inlets along the NC coast.
6
et al., 1992; Bristow et al., 2000; Neal and Roberts, 2000;
Barnhardt et al., 2002; Jol et al., 1996, 2002; Havholm et
al., 2004; Culver et al., 2006). Ground penetrating radar
works by transmitting radar (radio) waves into the ground
and recording the energy as it is reected from boundaries
associated with changes in sediment type beneath the
surface. The data provide a prole of the geology such that
dierent layers of rock or sediment can be mapped and
interpreted (Fig. 7). Inlet channels that are lled with sand
have distinctive appearances in the GPR data (Figs. 7B and
C).
Map illustrating the approximate locations and dates of existence of documented historic inlets (red arrows)
and previously undocumented inlet channels (blue arrows) discovered using ground-penetrating radar data.
7
(A) A photograph showing the process of collecting ground penetrating radar data using an all-terrain
vehicle. The GPR antenna is the orange box being pulled across the ground surface. (B) GPR data illustrating a
migrating inlet channel (from Salvo). (C) GPR data illustrating a non-migrating inlet channel (Chickinacommock Inlet
north of Rodanthe). The location of the data are shown on Figure 6.
More than 100 km (60 miles) of GPR data were acquired
to dene the locations and characteristics of old inlet
channels (paleo-inlets) from Oregon Inlet to Ocracoke Inlet
(Smith, 2006) (Fig. 8). Based upon these data, sediment
cores were collected to provide sediment for determining the
age of inlet activity and dening the role of inlet formation
in barrier island evolution.
GPR data reveal that paleo-inlet channels constitute 60%
to 70% of Hatteras and Pea Islands between Oregon Inlet
and Cape Hatteras (Fig. 8). Two main types of paleo-inlet
channels (non-migrating and migrating) were classied based
on geometry and ll patterns. The paleo-inlet channels are
cut into older ood-tide delta deposits that correspond to
older inlet activity when barriers existed further seaward.
Flood-tide delta deposits are generally overlain by marsh
peat and storm overwash sediments. Channel-ll sediments
occur under the widest portions of the island, whereas
narrow portions of the island are underlain by the FTD
and overwash sediments. This relationship is attributed to
the successional stage of island evolution in response to
rising sea level (Fig. 3), and indicates that the narrow island
segments are now in need of new inlets and deposition of
new FTD’s to increase island width.
8
Location map and interpreted ground penetrating radar data along Highway 12 from Oregon Inlet to Buxton, showing the locations of inlet
channels (yellow-shaded), and associated deposits.
9
The current active inlets along the Outer Banks of
North Carolina include Oregon Inlet (opened in
1846), Hatteras Inlet (opened in 1846), Ocracoke Inlet
(opened prior to 1585), New Drum Inlet (opened by
the U.S. Army Corps of Engineers in 1971), New-Old
Drum Inlet (opened in 1999 by Hurricane Dennis) and
Ophelia Inlet (opened in 2005 by Hurricane Ophelia)
(Fig. 1). Two other inlets, New Inlet (closed in 1945)
and Drum Inlet (closed in 1971), were recently active
and closed naturally. In addition, two inlets were
recently active, but were closed by the USACE, including
Isabel Inlet (opened in 2003 by Hurricane Isabel) and
Buxton Inlet (opened in 1962 by the Ash Wednesday
nor’easter) (Fig. 6).
Oregon Inlet (Figs. 9 and 10) opened by a hurricane
in 1846 near the site of a previous inlet (Gun or Gunt
Inlet) which closed in 1798. Between 1846 and 1989,
the inlet migrated approximately 2 miles south of its
original location (Fig. 9B). In 1962-1963 the Oregon
Inlet Bridge was built (Fig. 10), but the inlet continued
to migrate causing the throat channel to migrate from
under the xed navigation span and Pea Island to be
almost severed from the bridge. Consequently, inlet
dredging was increased to preserve the navigation
channel, a rock jetty was emplaced on the south bank
in 1989-1991, and a rock revetment was emplaced
around the south base of the bridge to prevent further
migration. However, the constrained location of the
south bank, and the continued southward spit growth
on the north bank caused Oregon Inlet to narrow and
deepen. The narrower throat channel resulted in rapid
scour beneath the central bridge pilings. As a result, rocks
were emplaced beneath the free-hanging pilings.
Oregon Inlet is an extremely dynamic inlet which, under
natural conditions, would likely continue to migrate
southward. The high energy and dynamic character of the
inlet conict with the static human infrastructure (bridge and
road), often pitting management policies and local interests
against natural coastal dynamics. Continually shifting sand
shoals and channels have necessitated increased dredging to
maintain navigability for commercial and recreational vessels
from nearby ports.
Controversies persist with the Oregon Inlet Bridge. The
bridge has exceeded its life expectancy and needs to be
replaced. One option is to rebuild it at its current location,
which would require continued nancial expenditures to
nourish the beach on Pea Island, to continually replace
the constructed dune ridges and Highway 12 (which are
frequently destroyed by storms) and to emplace kilometers of
(A) Figure shows a 1998 aerial photograph of
Oregon Inlet (NC State Database). (B) The 1998 aerial
photograph of Oregon Inlet showing superimposed
shorelines from 1849, 1932, and 1962 (following the 1962 Ash
Wednesday storm), illustrating the large degree of shoreline
variation and inlet migration.
10
sand bags in an attempt to hold the
island in place. Another option being
considered is to build a 17-mile-
long causeway in the sound behind
Oregon Inlet to Rodanthe, by-
passing the rapidly eroding Pea Island
barrier segment, and allowing the
Pea Island Wildlife Refuge to revert
to a natural state without needing
continued beach nourishment (Riggs
et al., 2008). From a long-term
nancial, management, and scientic
perspective, this latter option is more
viable but it conicts strongly with
local and shorter-term interests.
Hatteras Inlet (Fig. 11) opened
during the same hurricane that
opened Oregon Inlet in September
of 1846. During the Civil War, this
inlet was used extensively since it was
more navigable than Ocracoke Inlet
(Stick, 1958). In fact, it was used
by the Federal eet that captured
two Confederate forts near Hatteras
Village in 1861, and again in 1862
when Roanoke Island was attacked.
Today, Hatteras Inlet is used only
by small craft as the inlet shoals and
channels are subject to continual
change. Transportation between
Hatteras Island and Ocracoke Island
is maintained via the state-run ferry
system.
Ocracoke Inlet (Fig. 12) occurs on
all of the 16th and 17th century maps of coastal NC. New
data indicate that Ocracoke Inlet is located within a former
river valley (Pamlico Creek) that drained the Pamlico Sound
basin during the Last Glacial Maximum approximately
20,000 years ago (Mallinson et al., in review) (Fig. 13). It is
likely that the occurrence of this river valley beneath the inlet
accounts for its stability and longevity.
An oblique aerial photograph of Oregon Inlet and the Oregon Inlet
Bridge. Courtesy of the U.S. Army Corps of Engineers, Field Research Facility in
Duck, NC.
Figure showing the 1998 aerial photograph of Hatteras Inlet (NC State
Database).
Ocracoke Inlet has oered a navigable route for private
and commercial vessels for centuries. Prior to the opening
of Oregon Inlet in 1846, ships traveling to ports on the
mainland (Bath, Edenton, Washington, New Bern, etc.) only
had the option of using Ocracoke Inlet or Hatteras Inlet. In
1715 Ocracoke Inlet was designated an ocial port of entry
for access to the mainland communities, and required ocial
11
Figure showing the 1998 aerial photograph of Ocracoke Inlet (NC
State Database).
A map showing the topography of southern Pamlico Sound and the
Ocracoke Inlet area as it appeared during the last glacial maximum approximately
20,000 years ago when this area was dry land (based upon seismic data; Mallinson et
al., in review). Ancient river channels (blue paleo-channels) were mapped beneath the
modern southern Pamlico Sound and the inner continental shelf. Note that Ocracoke
Inlet occurs where Pamlico Creek passes beneath the modern barrier island trend. The
modern day coastline is included for the purpose of spatial orientation.
12
harbor pilots (Stick, 1958; Riggs and Ames,
2007). As a result, Ocracoke Village (on the
southwestern end of Ocracoke Island), and
Portsmouth Village (on the northeastern end
of Core Banks) ourished. Ocracoke Village
is still a thriving community with a tourist-
based economy. Portsmouth Village is now
a historical site within the Cape Lookout
National Seashore.
Drum Inlet (Fig. 14) initially opened in about
1899, but then closed naturally by 1919. It
was then reopened during a major hurricane
in 1933. Early attempts (beginning in 1938)
at dredging by the U.S. Army Corps of
Engineers did little to maintain a navigable
channel for commercial sherman (Stick,
1958; Riggs and Ames, 2007). By 1971, the
inlet was nearly closed, and the U.S. Army
Corps of Engineers proceeded to open New
Drum Inlet several miles to the southwest. A
channel was dredged and blasted through the
island to create New Drum Inlet in 1971.
The purpose of New Drum Inlet was to
allow commercial shing vessels to transit
between the ocean and several small coastal
communities. However, due to rapid shoaling,
no commercial vessels have ever used the inlet
(Riggs and Ames, 2007). In 1999, Hurricane
Dennis reopened Drum Inlet, which is now
referred to as New-Old Drum Inlet. In 2005,
Hurricane Ophelia opened an inlet southwest
of New Drum Inlet. Currently Ophelia Inlet
is expanding, and has nearly merged with New
Drum Inlet.
Buxton Inlet (Fig. 15) opened in March
1962, during the Ash Wednesday storm which
was an intense nor’easter. A bridge built over the inlet was
destroyed in a second nor’easter in December of the same
year. Following the bridge destruction, the U.S. Army Corps
of Engineers decided to ll the inlet using dredged sand from
the shallow sound behind the island at a location referred
to as “The Haulover”. The dredged hole still exists in this
A gure showing two aerial photographs of Core Banks that
illustrate the location of New Drum Inlet in 1998, and Ophelia and New-
Old Drum Inlets in 2006 (NC State Database). New-Old Drum Inlet
opened in 1999 during Hurricane Dennis in the same location that Old
Drum inlet occurred (paleo-Old Drum Inlet in the 1998 photograph).
Ophelia Inlet opened in 2005 during Hurricane Ophelia.
area between Avon and Buxton, and is commonly referred
to as “Canadian Hole”. Because this inlet was quickly
closed, no signicant ood-tide delta developed, and the
island continues to narrow and is vulnerable to future inlet
opening. The ocean shoreline at this location has receded
approximately 2500 feet since 1852 with a net loss of 76%
of island width by 1998 (Riggs and Ames, 2008).
13
(A) Buxton Inlet as it appeared following construction of the Highway 12 bridge shortly
after opening in 1962 (photograph courtesy of National Park Service Archives at Cape Hatteras
National Park, Manteo). (B) A photograph of Buxton Inlet following destruction of the bridge during
a nor’easter in December, 1962 (USACE, 1963). (C) An oblique aerial photograph of Buxton Inlet in
1963 showing the dredging and lling operation; note the pipeline feeding sand to the beach from
dredging operations in the sound (USACE, 1963). (D) An oblique aerial photograph showing the
Buxton Inlet location following inlet closure by the USACE (USACE, 1963).
(A) An aerial
photograph of Isabel Inlet
indicating the location of
the ground penetrating
radar survey following
lling of the channels,
shown in B) (NC State
Database). Note the
location of three channels
that developed, which are
seen within the GPR data.
For clarity, channel anks
are dened with the dashed
black line. Also, note the
occurrence of peat in the
subsurface, and exposed on
the shoreface.
14
Isabel Inlet formed in the Outer Banks during Hurricane
Isabel in September 2003 (Figs. 16, 17 and 18). GPR data
were collected here to illustrate the characteristics of a known
modern inlet. The inlet was lled within 40 days by the U.S.
Army Corps of Engineers using sand dredged from Hatteras
Inlet navigation channel to the southwest. Historical records
indicate that the Isabel Inlet region, along the narrow barrier
between Hatteras and Frisco, has experienced inlet activity in
the past. Isabel Inlet is classied as a non-migrating inlet that
has opened twice in 70 years (in 1933 and 2003). Following
the most recent opening, the pilings associated with a bridge
built during the 1933 opening were re-exposed (Fig. 18).
Figure showing the digital elevation model of Isabel Inlet that was made by Geodynamics, Ltd. (modied
from Freeman et al., 2004) showing the beginnings of ood-tide delta (Flood Shoal) and ebb-tide delta (Ebb Shoal)
formation, and channels scoured to 6 meters (20 feet) below sea level. The inlet was lled by the USACE before a
signicant ood-tide delta could form.
A photograph
looking
northeast
across the
newly formed
Isabel Inlet.
Notice the
pilings in
the water
(right side of
photograph)
which are
the remains
of a bridge
built in 1933.
Photograph
courtesy of
Gary Owens.
15
Due to increased global temperature, sea level may rise
between 1.5 feet to 2.6 feet above modern mean sea level by
the year 2100 (Church and White, 2006; Overpeck et al.,
2006; IPCC, 2007; Pfeer et al., 2008; Riggs et al., 2008).
In addition, there is growing evidence that warmer global
temperatures are already increasing the destructive potential
of hurricanes (Emanuel, 2005; Mann and Emanuel, 2006)
and may increase the frequency of hurricanes that reach
category 4 or 5 (Webster et al., 2005; Elsner et al., 2008).
Because barrier islands respond relatively rapidly to changes
in sea level and storm activity, global climate change has the
potential to signicantly alter both the morphology and
the evolution of barrier islands in the future, an important
component of which is inlet activity. With accelerating
climate change and sea-level rise, it has become increasingly
important to forecast future barrier island conditions, in
order to make long-term coastal management plans and
policies. Road maintenance plans, development policies,
hazard mitigation, and emergency response plans depend
upon an understanding of local erosion rates, and the
potential for the creation of new inlets. Some barrier island
segments are clearly in danger of developing inlets in the near
future. These are the narrow and low barrier island segments
that experience the highest erosion rates, such as the island
segment immediately north of Rodanthe, or the Buxton
and Isabel Inlet areas. In areas with low sand volume (sand-
starved segments; Riggs et al., 2008), where the underlying
geologic units are not resistant to erosion, major storm surge
and cross-island ow can cut a channel substantially below
sea level that results in post-storm tidal ow. The result is an
inlet.
Several investigations have conducted coastal hazard
assessments of the Outer Banks and other coastal areas,
including the potential for future inlet formation. These
assessments address a range in spatial scales and have dierent
purposes; some are more regional and qualitative in focus
while others are detailed and quantitative. For example,
Pilkey et al. (1998) produced maps of coastal vulnerability
to hurricanes and winter storm damage, including inlet
hazard areas, based upon the occurrence of past inlets, island
width and elevation, forest cover, dune height and width,
erosion or accretion rates, and various human impacts.
Riggs et al. (in press) produced maps of inlet vulnerability
based on knowledge of
the geologic framework,
geomorphology and erosion
rates (Fig. 19). In a more
quantitative assessment
and covering a much larger
area, Thieler and Hammar-
Klose (1999) produced a
Coastal Vulnerability Index
(CVI) for the U.S. coastline.
The CVI is a quantitative
measure of the collective
risk to coastal hazards and is
derived from six parameters
including geomorphology,
coastal slope, relative
sea-level change, shoreline
erosion rates, mean tidal
range and mean wave
height. This index, however,
lumps the potential eects
of many factors into one
measure and, therefore, its
relevance to specic hazards
A map from Riggs et al. (in press) showing the potential for future inlet
formation based upon barrier island geomorphology.
16
over a given timescale is dicult to
determine.
Walsh et al. (unpublished data) used
LiDAR elevation data to quantify the
barrier island cross-sectional island
volume, and these data were then
employed as a proxy for the risk of
forming a new inlet. The data are in
an ArcGIS shapele, enabling them
to be overlain with other datasets
and used to determine the number
and value of homes and specic
infrastructure at risk, etc. The data
are provided on the internet at the
North Carolina COastal HAZards
(NC COHAZ) Decision Portal, a
recently created web site aimed at
communicating hazard information
(http://coastal.geology.ecu.edu/
NCCOHAZ/maps/inlet_potential.
html).
The work by Walsh et al.
(unpublished) is a good example of
the type of data and tools needed
for coastal decision making and,
more specically, is a rst step
towards quantifying the risk of
future inlet opening. However, much
improvement is still needed. Not
only must the method of predicting
hazards be improved through the
coupling of geophysical data with
geospatial models but also public
education and the tools for hazard
communication, data collection and
integration need to be strengthened.
It would be useful to have coastal
hazard prediction tools which identify risk areas in advance
(48 hours) of approaching storms. Also, a system which
instantly incorporates ood observations with elevation and
infrastructure data could aid in emergency response eorts.
East Carolina University geologists and geographers and
the RENaissance Computing Institute at ECU ( http://
www.ecu.edu/renci/) are working with other researchers
and managers across the state to develop such useful hazard-
specic tools.
The inlet-opening potential maps of Walsh et al. identify
several sites of concern (Fig. 20). The former inlet locations
of Buxton Inlet (opened during the Ash Wednesday storm of
1962), New Inlet (re-opened for several years by a hurricane
in 1933) and Isabel Inlet are characterized as “Very High
Inlet Potential”. Beyond these areas, several other potential
inlet locations are highlighted, including portions of eastern
Ocracoke Island, the island segment between Avon and
Inlet-opening potential along the Outer Banks (http://coastal.
geology.ecu.edu/NCCOHAZ/maps/inlet_potential.html). Opening potential is
based on measurements of sub-aerial island volume. Categories are dened by
quartiles of the total population measured. Note the key and that the locations
of inlets in last century are areas mapped as “Very High Potential” (red).
17
Salvo, the Rodanthe area and nally the northern end of Pea
Island. Inlets in these areas would all aect trac ow along
Highway 12, the main transportation route along the Outer
Banks.
Beyond simple inlet formation, it is now understood that
areas with very high inlet potential also have the potential to
erode catastrophically to the point of barrier island collapse;
that is, the erosion below sea level of long segments of the
barriers (Culver et al., 2007). The collapse of portions of
Dauphin Island, Alabama and the Chandeleur Islands, LA
following the impacts of hurricanes Ivan and Katrina are
modern examples of what could happen to segments of the
Outer Banks (http://coastal.er.usgs.gov/hurricanes/katrina/
lidar/dauphin-island.html) (Fig.21). As sea level rises, a
barrier island will respond either by migrating landward
across the underlying substrate or by disintegrating if there
is not sucient sand volume to maintain relief above sea
level (Sallenger, 2000). As storms occur more frequently,
or with more intensity, the process of inlet creation and
barrier disintegration or collapse may proceed more quickly.
Based on geologic evidence, Riggs and Ames (2003) suggest
that large portions of the Outer Banks of North Carolina
could disappear within the next several decades if sea level
continues rising at the current rate or if one or more major
hurricanes were to directly impact the Outer Banks (Fig.
22). Similar collapse occurred approximately 1,000 years
ago during the warm climatic interval known as the Medieval
Warm Period (Culver et al., 2007). Given the importance of
barrier islands as coastal landforms, changes in barrier island
morphology, especially the possible increase in inlet activity
and disintegration or collapse of barriers altogether, would
have serious socio-economic implications. Understanding
and predicting the response of coastal systems and landforms
to sea-level rise and climate change is critical for eective
coastal planning and to develop management eorts that
can adapt to rising sea level and increased storm activity, as
evidenced by the recent Hurricane Katrina disaster.
Photographs showing portions of the Chandeleur Islands, LA before and after Hurricane Katrina. The top
photographs are from July 17, 2001, before the hurricane. The bottom two photographs are from August 31, 2005,
two days following Hurricane Katrina. The yellow arrows point to the same location in each photographic pair (http://
coastal.er.usgs.gov/hurricanes/katrina/photo-comparisons/chandeleur.html).
18
Conceptual model shows the potential evolution of the North Carolina coastal system
in response to a 2 foot sea-level rise and increased tropical storm intensity, both of which are
possible by 2100. The future mainland shoreline and wetland environments (marsh, pocosin swamp
forest) are superimposed upon the modern shoreline and elevation conguration. Greater shoreline
recession, ecosystem migration and marsh development in northern Pamlico Sound is likely to occur
where the tidal range will be enhanced. Segmentation of the barrier islands in numerous vulnerable
locations may occur in response to a 2 foot/century rate of rise and increased hurricane activity
causing enhanced tidal interchange.
19
As a result of the highly dynamic nature of inlets and their
adjacent shorelines, measures are being taken to responsibly
manage development around inlets. One signicant measure
which is undergoing review by the North Carolina Division
of Coastal Management is the redenition of Inlet Hazard
Areas, which include the ocean beaches adjacent to inlets
where the rate of shoreline change is more rapid and variable
than on other ocean beaches. Within these newly dened Inlet
Hazard Areas, shoreline setback regulations are being revised
to account for the high variability in shoreline erosion and
accretion.
Additionally, there are some controversial measures that are
being explored by local communities to “stabilize” the inlets
and adjacent beaches. These proposals include sand mining
of the ETD to nourish beaches and installing terminal
jetties on one or both sides of the inlets to stabilize the
inlet. However, these measures would interrupt the natural
sand transport mechanism and alter the sediment budget,
destabilizing the inlet and diminishing the quantity of sand
available to the backside of the island for back-barrier island
maintenance. Ultimately, these endeavors lead to increased
erosion and narrowing of the barrier island (Fig. 23).
Another controversial management issue involves the dredging
of the ebb channel to maintain a xed navigation channel. If
the dredge spoil sediment has the appropriate characteristics,
it is sometimes used to nourish beaches adjacent to the
inlet. However, frequently the most cost eective method of
dredge spoil disposal is to deposit it oshore, where it may be
lost from the beaches. Furthermore, the dredged navigation
channel interrupts the natural sand bypassing process, and
may result in the deposition of sand farther oshore and at
greater depths than under natural conditions, resulting in a
decrease of sediment available for the beaches (Pilkey et al.
1998). The net eect of removing this sand is an increase in
shoreline recession rates.
Inlets adjust naturally to changing hydrodynamic conditions
imparted by climate change, including storms and sea-level
rise. Inlet adjustment is a natural process that only becomes
a “hazard” or “natural disaster” when human structures and
infrastructure are in the way. Responsible management of the
inlet resources means designing policies and infrastructure that
are adaptive to the changing conditions. For example, instead
of building bridges across the inlet throat, which naturally
migrates rapidly due to high current activity, they could be
built across the FTD and shallow water platform (i.e., the
Hatteras Flats) behind the islands where sediments are more
stable. Instead of closing newly formed inlets, they should
be allowed to remain open at least long enough to build a
substantial FTD for the long-term maintenance
and stability of the barrier island. Access across
inlets could be accommodated by high speed ferries
such as those described by Riggs et al. (2008). A
sustainable coastal infrastructure necessitates the
ability to be exible as opposed to static; to be able
to change and adapt to the natural dynamics of the
coast. It is these natural dynamics and the constant
change that provide the fundamental beauty of the
Outer Banks to which so many are attracted.
Aerial photograph of Ocean City
Inlet, which formed in 1933 (Google Earth;
NASA, 2005). The 1933 shoreline is shown
in red, the photograph is from 2005. Jetties
were built in 1933 and 1934 in an attempt to
stabilize the inlet for navigation. The result was
a disruption in the sediment transport processes
and a large increase in erosion rates (up to
40 feet/year), reduction in island elevation,
and loss of critical beach and dune habitats
on Assateague Island down-drift of the inlet.
The effects extend for approximately 9 miles
southward from the inlet.
20
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Village and came within minutes and/or inches of opening additional inlets between Avon and Buxton and on
the northeast end of Ocracoke Island.
