NEW TOPSAIL INLET, NC Merritt Ross Willingham McL

Transcription

EFFECTS OF INLET MIGRATION ON BARRIER ISLAND PLANFORM AND
OCEANFRONT CHANGE: NEW TOPSAIL INLET, N.C.
Merritt Ross Willingham McLean
A Thesis Submitted to the
University of North Carolina Wilmington in Partial Fulfillment
of the Requirements for the Degree of
Master of Science
Department of Geography and Geology
University of North Carolina Wilmington
2009
Approved by
Advisory Committee
Paul A. Thayer
Michael S. Smith
William J. Cleary
Chair
Accepted by
DN: cn=Robert D. Roer, o=UNCW,
ou=Dean of the Graduate School &
Research, [email protected], c=US
Date: 2010.10.27 12:34:57 -04'00'
Dean, Graduate School
This thesis has been prepared in a style and format
consistent with
The Journal of Coastal Research
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TABLE OF CONTENTS
ABSTRACT ................................................................................................................................... vi
LIST OF TABLES ........................................................................................................................ vii
LIST OF FIGURES ..................................................................................................................... viii
INTRODUCTION .......................................................................................................................... 1
Background ................................................................................................................................. 2
Tidal Inlets .............................................................................................................................. 2
Study Area .................................................................................................................................. 3
Regional Geology ....................................................................................................................... 7
Regional Tidal Inlets ................................................................................................................... 7
Sediment Bypassing .................................................................................................................... 8
Inlet Hazard Zones .................................................................................................................... 10
OBJECTIVES ............................................................................................................................... 13
Inlet Migration .......................................................................................................................... 13
Shoreline Change ...................................................................................................................... 13
Ebb Delta: ................................................................................................................................. 14
Bypassing Events ...................................................................................................................... 14
Management and Inlet Hazard zones ........................................................................................ 14
METHODS ................................................................................................................................... 15
Rate of Change Database .......................................................................................................... 15
Channel Orientation and Barrier Morphology Database .......................................................... 18
Ebb-Tidal Deltas ....................................................................................................................... 19
Area and Volume .................................................................................................................. 19
RESULTS ..................................................................................................................................... 22
Total Inlet Migration ................................................................................................................. 22
Long-term Inlet Characteristics ................................................................................................ 23
Short-term Inlet Changes .......................................................................................................... 31
Inlet Bypassing Cycles .............................................................................................................. 31
Inlet Migration Patterns ............................................................................................................ 33
Oceanfront Change ................................................................................................................... 37
Net Change 1938-2006 ............................................................................................................. 40
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Topsail Island ........................................................................................................................ 40
Hutaff Island ......................................................................................................................... 41
Net Change Intervals................................................................................................................. 41
Net Change 1938-1949 ............................................................................................................. 41
Hutaff Island ......................................................................................................................... 42
Net Change 1949-1962 ............................................................................................................. 44
Topsail Island ........................................................................................................................ 44
Hutaff Island ......................................................................................................................... 46
Net Change 1962-1974 ............................................................................................................. 46
Topsail Island ........................................................................................................................ 46
Hutaff Island ......................................................................................................................... 48
Net Change 1974-1999 ............................................................................................................. 48
Topsail Island ........................................................................................................................ 49
Hutaff Island ......................................................................................................................... 49
Net Change 1999-2006 ............................................................................................................. 51
Topsail Island ........................................................................................................................ 51
Hutaff Island ......................................................................................................................... 53
Cumulative Shoreline Change .................................................................................................. 53
Topsail Island ............................................................................................................................ 54
Northern Shoreline Reach (T-1 through T-16) ..................................................................... 54
Southern Shoreline Reach ..................................................................................................... 54
1974-1999 ............................................................................................................................. 58
1999-2006 ............................................................................................................................. 63
Hutaff Island ............................................................................................................................. 63
1938-1962 ............................................................................................................................. 63
1962-2006 ............................................................................................................................. 66
DISCUSSION ............................................................................................................................... 73
Effects of Inlet Migration: ........................................................................................................ 74
Direction of Migration: ............................................................................................................. 74
Rate of Inlet Migration: ............................................................................................................ 78
Long-term Rates of Migration: ................................................................................................. 78
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Short-term Rates of Migration: ................................................................................................. 79
Tidal Delta Changes:................................................................................................................. 86
Flood Tidal Delta ...................................................................................................................... 86
Ebb Tidal Delta ......................................................................................................................... 90
Oceanfront Shoreline Change ................................................................................................. 100
Shoreline Change Intervals ..................................................................................................... 101
1938-1949 ........................................................................................................................... 101
1949-1962 ........................................................................................................................... 103
1962-1974 ........................................................................................................................... 107
1974-1999 ........................................................................................................................... 108
1974-1982 ........................................................................................................................... 109
1982-1990 ........................................................................................................................... 110
1990-1999 ........................................................................................................................... 112
1999-2006 ........................................................................................................................... 117
Future Changes and Implications............................................................................................ 119
CONCLUSIONS......................................................................................................................... 121
LITERATURE CITED ............................................................................................................... 124
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ABSTRACT
New Topsail Inlet, located 40 km northeast of Wilmington, NC, separates Topsail Island, a
developed barrier from Hutaff Island, an undeveloped barrier. Since the inlet opened in the late 1720’s the
inlet has migrated ~11 km to the southwest. A GIS based analysis of 61 sets of historic aerial
photographs (1938-2006) provided data on migration rates, the morphologic changes, the periodicity of
ebb delta breaching events and oceanfront changes associated with migration.
Since 1938, New Topsail Inlet has migrated southwest at an average rate of 26 m/yr. Migration
rates have varied from 95 m/yr (1945-49), to 11m/yr (1956-62). Four ebb delta-breaching events occurred
between 1978 and 2007. During the largest event (1978-1986) the outer segment of the ebb channel was
repositioned by 68º. Prior to channel reorientation, the inlet was migrating at 62 m/yr. Immediately prior
to the breaching event, the ebb channel briefly reversed its direction of movement, migrating at 23m/yr to
the northeast. Subsequent to shoal breaching, migration accelerated to 73 m/yr to the southwest due to the
large volume of material by-passed to the updrift Topsail Beach shoulder.
As a consequence of the southwesterly migration and ebb channel deflection, the southern portion
of Topsail Beach has been characterized by complex temporal and spatial oceanfront changes. Increased
rates of accretion (34 m/yr) and erosion (-15 m/yr) were common. This is due in part to the development
and movement of a large shoreline protrusion and in part to the truncation of the trailing shoreline as the
inlet moves south. The shoreline bump, which develops when the inlet is oriented to the north, accretes
rapidly as bypassed shoals weld to the updrift beach. However, when the channel switches, the large
bump soon becomes an erosion hotspot. Both the shoreline bump and the erosion hotspots migrate with
the inlet, leaving behind a beach with typical erosion rates.
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LIST OF TABLES
Table 1. Summary of Inlet Bypassing Episodes ........................................................................... 34
Table 2. Summary of Yearly Migration Rates and Channel Orientation ..................................... 36
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LIST OF FIGURES
Figure 1. Location map of the study area. ...................................................................................... 4
Figure 2. Map illustrating the migration pathway of New Topsail Inlet ........................................ 5
Figure 3. Map showing the current Inlet Hazard Areas. ............................................................... 12
Figure 4. Map depicting the location of the transects ................................................................... 17
Figure 5. Bathymetric maps of New Topsail Inlet ........................................................................ 21
Figure 6. Map showing historic island outlines of Topsail and Hutaff Islands. ........................... 24
Figure 7. Line plot illustrating possible inverse relationship between ebb-tidal delta volume and
channel orientation. ....................................................................................................................... 27
Figure 8. Composite images show the bypassing episode from 1982 – 1987 .............................. 32
Figure 9. Composite image illustrating the 4 bypassing episodes during the short-term study ... 35
Figure 10. Map illustrating the study area and the portion of Topsail Island that is zoned as an
Inlet Hazard Area .......................................................................................................................... 38
Figure 11. Line plot depicting cumulative channel migration from 1938 to 2006. ...................... 39
Figure 12. Map illustrating historic shorelines and inlet positions during 1938 and 1949. .......... 43
Figure 13. Map depicting historic shorelines and inlet positions during 1949 and 1962 ............. 45
Figure 14. Map of the study area depicting historic shorelines and inlet positions from 1962 and
1974............................................................................................................................................... 47
Figure 15. Map of the study area, inlet positions and net oceanfront changes between 1974 and
1999............................................................................................................................................... 50
Figure 16. Image of the study area in 2006 .................................................................................. 52
Figure 17. Bar graph showing the total cumulative changes for the transects located in the
northern shoreline reach. ............................................................................................................... 55
Figure 18a. Two line plots illustrate the movement of the shoreline between 1938 and 1974 .... 56
viii
Figure 18b. Two Line plots illustrate the shoreline movement of the southern transects ............ 57
Figure 19. Bar graph displaying the cumulative shoreline change experienced by transects in the
southern shoreline reach from1974-1999 ..................................................................................... 59
Figure 20. Line plot illustrating shoreline movement over the southern shoreline reach during
the period between 1974 and 1982 ............................................................................................... 60
Figure 21.Two line plots display the movement of the shoreline across transects in the southern
shoreline reach from 1982-1990 ................................................................................................... 61
Figure 22. Two line plots illustrating changes in the shoreline during the mini period from 19901999............................................................................................................................................... 62
Figure 23. Bar graph showing the cumulative change data for transects 18-27 for the time period
between 1999 and 2006. ............................................................................................................... 64
Figure 24. Two line plots show the various positions of the oceanfront shoreline during the
period 1999-2006. ......................................................................................................................... 65
Figure 25. Bar graph displaying cumulative shoreline loss across all transects within the Hutaff
shoreline reach from 1938-1962 ................................................................................................... 67
Figure 26. Bar graphs showing cumulative shoreline change data on Hutaff Island for all other
time periods between 1962 and 2006............................................................................................ 68
Figure 27. Line plot illustrating shoreline position change on Hutaff Island between 1938 and
1982 for transects 30-31................................................................................................................ 69
Figure 28. Line plot showing the movement of the shoreline across transects 32-34 on Hutaff
Island for the time period 1982-1990............................................................................................ 70
Figure 29. Line plot displaying shoreline position for transects 34 and 35 on Hutaff between late
1990 and 1999 ............................................................................................................................... 71
Figure 30. Line plot shows shoreline change for transects 35 and 36 for the time interval 19992006............................................................................................................................................... 72
Figure 31. Images serve as photographic documentation of re-curved dune ridges on the southern
portion of Topsail Island ............................................................................................................... 76
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Figure 32. Composite image illustrating changes to the inlet’s morphology incurred over the
course of the study ........................................................................................................................ 80
Figure 33. Composite image showing the movement of the flood ramp (illustrated by the orange
triangles) from 1962 to 1974. During this period, ........................................................................ 81
Figure 34. Composite image showing the completed migration .................................................. 82
Figure 35. Composite image showing the final position of the flood ramp ................................. 83
Figure 36. Bar and line graph that illustrates the connection between channel migration and the
orientation of the ebb channel ....................................................................................................... 85
Figure 37. Line plots illustrate the inverse relationship between inlet minimum width and
migration rate. ............................................................................................................................... 87
Figure 38. Line plots for the inlet’s minimum width and the acreage of the ebb delta ................ 93
Figure 39. A) Line plots illustrate the inverse relationship between the overall slowing migration
of the inlet and the increase in ebb delta siz ................................................................................. 94
Figure 40. Map of the study area illustrating the ideal ebb delta and channel configuration for
accelerated shoreline advancement on Hutaff Island.................................................................... 96
Figure 41. Map of the study area showing positions of the ebb delta and channel orientation prior
to the large protrusion build out on Hutaff Island......................................................................... 98
Figure 42. Map of the study area showing the ebb delta configuration subsequent to the
configuration favored for shoreline advancement on Lea/ Hutaff. ............................................... 99
Figure 43. Composite image illustrates changes to the Hutaff Island planform incurred due to
inlet migration ............................................................................................................................. 116
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INTRODUCTION
During the past several decades, all coastal communities in North Carolina have become
important tourist destinations that have experienced rapid growth and increased land values. The
single most important variable affecting development is the presence of a wide oceanfront beach.
Most of these developed shorelines are situated within chronic erosion zones and the greatest
hazards are associated with contemporary inlets. All developed barrier island oceanfronts
experiencing problems related to inlet-induced erosion, have erosion rates10-15 times the
average annual erosion of the mid-barrier oceanfront segments. During the past two decades,
80% of the insurance claims for erosion threatened buildings were related to erosion along inlet
shorelines. Currently many additional structures are threatened. As a result, inlets have drawn the
attention of coastal managers as communities continue to develop and attempt to mitigate land
loss within these erosion hot spots.
The designation of inlets as areas of environmental concern necessitates special
consideration. Coastal regulators have recognized detailed site-specific studies are needed to
effectively manage areas influenced by inlets. Currently the N.C. Division of Coastal
Management is attempting to redefine the areas falling within inlet hazard zones. However, the
lack of a sufficient database and an under-developed appreciation of the processes that determine
the short and long-term changes of the associated oceanfront shorelines have limited re-zoning
procedures. The intent of this paper is to present the effects New Topsail Inlet exhibits on its
adjacent barrier islands, and how the inlets movement plays a role in the shoreline change on
both islands.
Background
Tidal Inlets
Tidal inlet systems are breaks in the shoreline, which provide a connection between the
ocean and the back barrier bays, marshes, lagoons and tidal creek systems. In addition to flushing
out these areas, tidal inlets play a major role in the sediment budget, retaining large volumes of
sand impounded from the littoral system. Inlets also control the erosion and accretion patterns
over extensive shoreline stretches. Various factors such as, throat size, ebb shoal geometry,
bypassing events, and migration habit contribute to the length of an inlets influence, however the
width of influence is typically many times the current dimension of the specific inlet (SEABERGH
and KRAUS, 2003; FENSTER and DOLAN, 1996; BRUUN, 1990,1996).
The main features associated with tidal inlets are, tidal deltas and recurved spit-inlet-fill
sequences related to inlet migration (HAYES, 1980). HAYES (1980) proposed the following terms
for tidal deltas: ebb-tidal delta (seaward shoal) and flood tidal delta (landward shoal). Each of
these deltas is composed of a series of morphologic features. Common features of a typical ebb
tidal delta include a main ebb channel, flanked on either side by channel-margin linear bars, a
large broad sheet of sand called the swash platform, large swash bars formed by breaking and
shoaling waves and a large, steep, seaward slope called the terminal lobe. Depending on the
location on the ebb channel, the inlet system may have one or more marginal flood channels. The
morphology of the ebb tidal delta is a function of local tidal regime and wave energy.
Less than one percent of North Carolina’s shoreline is currently occupied by inlets;
however, inlets have had a significant influence in the shaping of the current shoreline. 13 of the
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inlets found within North Carolina’s shoreline are located in Onslow Bay. During the past 200
years these inlets have influenced 65% of the barrier shorelines within the bay and up to 100%
of some shoreline stretches. The 13 inlets found within Onslow Bay are a mixed group of stable
and migrating inlet systems (CLEARY, 1996). Eight of these inlets border developed shorelines,
six of which have been somewhat modified. Generally, stable inlet systems occur in the
regressive shorelines of the northern bay while migrating systems are found within the
transgressive barrier segment.
Study Area
Topsail Beach is the southernmost town of three communities located along Topsail
Island in Pender County N.C. The town is located updrift (NE) of New Topsail Inlet, which is
located 70 km north of Cape Fear and 40 km northeast of Wilmington (Figure 1). The inlet
separates developed Topsail Island from undeveloped Hutaff Island. During the early 1990’s, the
inlet was approximately 400 m wide and had a cross-sectional area of 675 m2 (CLEARY, 1994),
currently the inlet is 700m wide and has an unknown cross sectional area.
New Topsail Inlet has historically influenced the morphology and sedimentology of this
coastal segment through a long history of migration (Figure 2). After opening just south of Sloop
Point in the late 1720’s, the inlet has migrated nearly 11km in a southwesterly direction
(CLEARY, 1994). Recent migration has occurred at rates of nearly 30 m/yr, migrating
approximately 1.6 km between 1938 and 2006. Previous inlet migration is evidenced
3
Figure 1. Location map of the study area. Red boxes on image inlays indicate position of blown
up study area. Base image from PENDER COUNTY GIS DEPT (2006).
4
Figure 2. Map illustrating the migration pathway of New Topsail Inlet, several historic inlet
positions are shown. Inlet migration evidenced by long back barrier channel (Banks Channel)
and the presence of several strings of marsh islands. Base image from PENDER COUNTY GIS
DEPT (2006), 1849-1873 shoreline from NCDCM (2006).
5
within the back barrier by a long (10km) channel (Banks Channel) paralleling the landward side
of the island (Figure 2). Accompanying this feature are a series of narrow marsh islands, built
from previous flood tidal deltas as storm waves and flood currents reworked the sediments
(CLEARY et al., 1996). The hydrography of the inlet was modified in the 1930s with the dredging
of the Atlantic Intracoastal Waterway (AIWW) and the associated channel that connects the
estuary and the inlet (CLEARY et al.,2003).
New Topsail Inlet lies in a mixed energy setting with a mean tidal range of 0.91 m and an
average wave height of 0.73 m (CLEARY, 1994). The island is oriented northeast to southwest,
exposing the inlet to waves propagating from these directions. According to a US Army Corps of
Engineers study (1989) the dominant direction of wave approach is from the south-southwest,
accounting for over half the annual wave energy. Based on these data it was assumed that the
dominant direction of sediment transport was northward. It was estimated by the US Army Corps
of Engineers that 55 % of the 500,000 m3/yr gross rate of sediment transport moves in a
northerly direction across the inlet (CLEARY, 1994.; JARRETT, 1976).
Due to the migration of New Topsail Inlet and the periodic ebb-tidal breaching events, its
ebb-tidal delta rarely conforms to the previously discussed standardized models. It is common
for the outer portion of the ebb channel to be oriented at varying angles to adjacent barrier
islands. The deflection and reorientation of the ebb channel plays a significant role in shoreline
change along adjacent barriers. Due to the breakwater effect of the ebb-tidal delta, slight changes
in its morphology can affect adjacent oceanfront shoreline changes.
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Regional Geology
The barrier islands of Onslow Bay are situated in the southeastern part of the North
Carolina coastal zone, causing the majority of shoreline features to be controlled by the preHolocene stratigraphic framework of the shore-face (RIGGS, CLEARY and SNYDER, 1996).
Tertiary and Cretaceous stratigraphic units characterize the shore-face stretch between
Cape Lookout and Cape Fear. These units, along with interlaying Quaternary sediments form a
platform that many of the barriers are built upon. The underlying stratigraphy controls the
morphology of the shore-face while influencing sediment composition and fluxes as well as
modern beach dynamics (RIGGS, CLEARY and SNYDER, 1996). The perched barriers are
composed of thin assorted layers in which surficial beach sands top older eroding stratigraphic
units such as unconsolidated sands and muds, compacted muds, limestones, and sandstones. The
composition of individual barriers as well as the underlying geology also affects the position of
regional inlets and their ability to migrate.
Regional Tidal Inlets
Historically, the area south of Topsail Island has been influenced by several inlets. Maps
and charts from the 1880’s show three inlets occupying areas of what is now Hutaff Island. The
inlets responsible for modifying the shoreface of this area where: Old Inlet, Old Topsail Inlet,
and Sidbury Inlet. An 1880 T-sheet survey map placed Old Inlet 1 km northeast of Old Topsail
Inlet’s location at that time. Old Inlet has not been recorded on any maps or viewed in any aerials
7
since the 1880’s; its previous location is now an area that is being modified by New Topsail
Inlet’s southwestern shoulder.
Old Topsail Inlet, the most prominent of the three historic inlets, previously separated
Hutaff Island into two smaller islands, Lea Island to the northeast and Coke Island to the
southwest. Similar to many inlets in the region, Old Topsail inlet was migratory, moving 1.3 km
to the southwest between 1938 and 1998 (MCGINNIS, 2004). The inlet continually reduced in
size during this time, eventually closing between September 1997 and June 1998.
Historically, Sidbury Inlet has shown a transient nature. The small inlet was located 2.1
km northeast of Rich Inlet and has been recorded four times. The inlet is shown on T- sheets
from 1857 and 1880, and has also been recorded more recently opening from1909 - 1925 and
from 1959 – 1962 (GAMMILL, 1990).
A study by MCGINNIS (2004) showed that between 1938 and 2002, variations in inlet
position at Rich and Sidbury Inlets, and the migration of Old Topsail Inlet suggested that 35 % of
Hutaff Island, in 2002, would have been underlain by inlet fill. In addition to this data,
identification of previous inlet features, such as marsh islands, coupled with recorded locations
on T-sheets suggest that the entire Hutaff Island shoreline, in 2002, was underlain by inlet fill.
Sediment Bypassing
Inlet sediment bypassing is the process by which sediment moves from one side of the
inlet to the other. Generally, the direction of movement is from the updrift to the down drift side
of the inlet throat (FITZGERALD et al., 2001). BRUUN and GERRITSEN (1959) who originally
8
described this process identified two primary modes of transport: 1) bar bypassing- sand
bypasses inlets by wave action, moving around the inlet, along the terminal lobe; and 2) tidal
bypassing- sediment is transported into the channels during a flood current and subsequently
moves seaward through the channel system during ebb flow conditions.
Since BRUUN and GERRITSEN’S (1959) pioneering work, a number of additional models
of natural bypassing have been proposed by FITZGERALD et al., (2001) that pertain to the study
area include: ebb delta breaching, outer channel shifting and spit platform breaching.
Ebb-delta breaching occurs at inlets with stable throat positions (FITZGERALD et al., 2001;
FITZGERALD and PENDLETON, 2002). At these inlets, longshore transport produces sediment
accumulation on the updrift margin, promoting the deflection of the main channel. In time,
breaching of the ebb shoals leads to a realignment of the ebb channel and subsequent bypassing
of large quantities of sediment to the down drift shoulder (KOMAR and INMAN, 1970; GAUDIANO
and KANA, 2001.).
The second model that pertains to the conditions at New Topsail Inlet involves the
shifting of the outer ebb channel that initiates a bypassing episode similar to ebb tidal delta
breaching but is limited to the extreme seaward position of the ebb channel. The main portion of
the ebb channel remains in a fixed position while the outer channel is deflected in a down drift
direction. As in the ebb tidal-delta breaching model, the channel will realign along a new path
with time and the cycle begins anew.
The third model applicable to the study area involves spit platform breaching. This type
of bypassing commonly occurs at migrating inlets where the updrift spit is fronted by a large
intertidal platform (FITZGERALD et al., 2001). Large quantities of sand are bypassed when a new
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channel breaches the protruding platform leading to down drift bypassing of the segmented
shoal.
An extensive analysis of aerial photographs indicates sediment bypassing at New Topsail
Inlet is complex and differs from the aforementioned models. In addition to wave related
transport of sediment around the periphery of the shoals, ebb delta breaching and outer channel
deflection are the principal mechanisms of bar bypassing. Spit platform breaching is rare and
only occurs during large storm events. Examination of several photographic sets indicates ebb
delta breaching and channel deflection events can lead to either updrift or down drift bypassing
as the inlet migrates along its pathway. Changes along the adjacent oceanfront shorelines
sometimes occur quite rapidly whereas others are slow and lag relative to the movement of the
inlet. Morphologic planform changes occur at varying distances from the main inlet channel and
are directly connected to the alignment of the ebb channel.
Inlet Hazard Zones
According to the N.C. Division of Coastal Management Handbook (2003), inlet hazard
zones cover ranges from 75 m for stable inlets, to 1.3 km for migrating inlets. Studies of nearby
New River Inlet and Rich Inlet (Cleary, 2002., CLEARY et al., 2003, and CLEARY et al., 2004)
indicate that the shoreline change patterns related to the respective inlets can extend 2-3.5 km
along the adjacent shorelines. A study of the mixed energy barrier islands of Virginia (FENSTER
and DOLAN 1996) suggested that the zone of inlet influence could extend for as much as 6.8 km
along the bordering barrier shorelines. The data from the above-mentioned investigations clearly
point to the fact each inlet has unique site-specific characteristics that are important in the
10
determination of the inlet hazard zone. Important parameters include the size of the inlet, its
migration habit, the ebb channel’s alignment history, the associated shape changes of the ebbtidal delta and the local wave climate (FITZGERALD et al., 1983). Figure 3 shows the current inlet
hazard zone for New Topsail Inlet, which is approximately 825m long (NCDCM, 2006), and
encompasses a small portion of the inlet’s zone of influence.
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Figure 3. Map showing the current Inlet Hazard Areas. Image illustrates the extent of the study
area and current inlet hazard zones. Base image from PENDER COUNTY GIS DEPT(2006), inlet
hazard area shapefile from NCDCM (2006).
12
OBJECTIVES
The primary objective of this study is to examine the long-term effects of inlet migration
on adjacent barrier island morphology. This issue has been addressed through the development
of a robust data set consisting of spatial and temporal shoreline changes as well as changes in
inlet components responsible for oceanfront change. The main objective has been supported with
a suite of sub-objectives that aid in the understanding of the migrating inlet system and
subsequent shoreline change. These sub-objectives have been broken into five sections: inlet
migration, shoreline change, ebb delta, bypassing events, and management and inlet hazard
zones.
Inlet Migration:
Determine length of total inlet migration.
Determine migration rates since the advent of aerial photography (1938).
Find out if current migration rates are consistent with previous time periods.
Reveal how varying migration rates affect the shoreline.
Shoreline Change:
Determine long-term shoreline change rates.
Determine short-term changes in shoreline position with respect to inlet behavior.
Determine change in shoreline morphology with respect to inlet behavior.
13
Ebb Delta:
Determine how the position, size and shape of the ebb delta affect erosion and accretion
patterns along the shoreline.
How do ebb-tidal breaching events affect the shorelines morphology?
Determine periods for the various cycles of ebb-tidal delta symmetry changes.
Bypassing Events:
Determine how frequently large-scale bar bypassing events occur.
Determine the change in shoreline morphology due to shoal welding events.
What is the related time frame for the attendant shoreline change to occur?
Management and Inlet Hazard zones:
Determine the length of inlet influence on the updrift and down drift shorelines.
Find out how the zone of inlet influence changes with channel deflection, re-orientation
of the ebb channel, welding of large swash bars and ebb delta symmetry.
Is the current delineation of the inlet hazard zone appropriate for Topsail Beach?
14
METHODS
The investigation relied primarily on data derived from a large photogrammetric
database. Vertical aerial photographs were used to track changes in inlet position, inlet shoulder
positions, inlet width, ebb channel orientation, and ebb delta position. The photographs were also
used to observe bypassing cycles and determine migration history, migration rate, and shoreline
change rates. Two databases were developed through the rectification and digitization of the
aerial photos, one to monitor shoreline change, and the other to monitor inlet processes.
Rate of Change Database
Seventeen sets of historical aerial photographs from 1938-2006 were used to compile the
GIS database. Photos were obtained from the USACE Wilmington District, NCDCM, the
UNCW coastal geology laboratory and Coastal Planning and Engineering. The images were
scanned between 300 and 600 dpi (yielding approximately 3ft./pixel) using an Epson Perfection
1650 scanner and exported as uncompressed TIFF files. Simple rectification of the photographic
sets was completed using ERDAS ARC GIS 9.1 software, geo-referencing to a base layer of
2002 digital orthophotos. The photos were rectified in the North Carolina State Plane 1983
coordinate system using between 8 and 15 ground control points per photograph. The variation
in number of ground control points was due to the lack of identifiable features in photos prior to
the island development. The target root-mean-square (RMSE) value remained below 3 yielding
less than 10 ft. of error.
15
After rectification, the photos were imported into ArcView 3.1 for analysis. Once in
ArcView, shorelines were traced and digitized, using the high-water line (HWL) as the primary
shoreline indicator (DOLAN et al., 1980; Moore, 2000; PAJAK and LEATHERMAN, 2002). This
feature is often used to digitize shoreline positions due to its ease of recognition on historical
aerial photographs (CROWELL et al., 1991). The use of the HWL as an indicator has several
inherent sources of error including photographic quality, digitizing techniques and operator error.
CROWELL et al. (1991) determined worst-case error estimates for rectified aerial photos at ~ 25
ft, including distortion of photos, error in the delineation of the HWL from good quality photos,
digitizer error, and digitizer-operator error in the calculations (ANDERS and BYRNES, 1991).
Shoreline change rates were determined by positional comparison of historical high water
lines. Measurements were taken with the use of an offshore baseline, which paralleled the coast
and had 41 shore normal transects spaced 152.4 m (500 ft) apart attached to it (Figure 4) The
SCARPS! (Simple Change Analysis of Retreating and Prograding Systems) extension of
ArcView, created by C.W. JACKSON (2004), was used to collect shoreline change data along
each transect. SCARPS! generates several different shoreline change rates commonly used by
the state and local government, including, end point rate (EPR), linear regression rate (LRR), and
average of end point rates (AER). For a summary of various shoreline change methods please
refer to DOLAN et al., 1991.
16
Figure 4. Map depicting the location of the transects used in the SCARPS! shoreline change data
analysis.
17
For this study it was determined that EPR was the easiest and most effective way of
measuring both long-term and short-term shoreline change. As the primary disadvantage of
using EPR is the loss of data between the endpoint shorelines, cumulative shoreline change rates
were also calculated. This improved the ability to recognize important shoreline change patterns
and trends as well as beach nourishment projects on Topsail Beach.
In addition to shoreline change data, the photo sets were subjected to several other
measurements including minimum width, channel migration rate and inlet shoulder migration
rate. Inlet minimum width was measured using the measurement tool in ArcView where as the
other two measurements were completed though the construction of another baseline, which
crossed the inlet perpendicular to the inlet channel. The base line was constructed as an extension
of the adjacent islands centerlines. Ebb channel migration, or inlet migration was completed
using digitized ebb channels, starting with the earliest channel (1938) as the zero point.
Migration was noted as the distance along the baseline, between the zero point and the channel
being measured. Distances between individual ebb channels were also recorded to determine
variations in migration rate. The same process was completed for the inlet shoulders.
Channel Orientation and Barrier Morphology Database
In addition to the 16 sets of photographs used for shoreline change analysis, 45 more sets
of photographs were used in the observation of inlet processes. These photos were examined at
¼ year, ½ yr and 1 yr intervals over a twenty-two year period (1982 -2003). The photos were
selected based on availability, quality and amount of change occurring since the prior image set.
18
All of the air photos used to compile the database were subjected to the same processing steps
described for the shoreline change analysis photo sets.
These images were primarily used to calculate short-term shoreline change rates and
observe the temporal scale of the inlet’s cyclic bypassing and bar welding events. This was
completed through the digitization of the ebb channels and input into the GIS for analysis.
Channel orientations were calculated using SCARPS! with output as azimuth values.
Ebb-Tidal Deltas
Ebb-tidal deltas were hand digitized using the breaker line as a guide for the seaward
portion of the ArcGIS polygon. The interior portion of the ebb-tidal deltas were digitized along
the High Water Line (HWL) and then brought to a baseline inside the inlet, which acted as the
landward cutoff of the delta. While this is an unorthodox definition of the ebb tidal delta, it
provided constancy in the computation of the ebb delta footprint.
Area and Volume
The area of the polygon was then calculated using the ArcGIS extension X-tools. The
extension output yields the area in a number of measurements including square yards, square
meters and acres. X-tools also calculates the perimeter of the polygon based on the measurement
selected.
19
Estimated volumes were calculated based on a conversion factor developed by Coastal
Planning and Engineering (Pers. Com. JARRETT, 2008). The conversion factor is simply:
The Approximate Ebb Delta Volume was calculated by CPE using fathometer data
collected in 2007. The data was entered into MATLAB and the difference between two sets of
bathymetry was calculated (Figure 5). Based on surface estimates in MATLAB, the ebb shoal
volume was approximately 5,454,031 cubic yards (Pers. Com. DAY, 2008). Therefore:
The approximate volumes of historic ebb-tidal deltas were calculated by multiplying the area of
the ebb-tidal delta by 4.27 yards. In order to maintain consistency, cubic yards were converted to
cubic meters.
20
Figure 5. Bathymetric maps of New Topsail Inlet. The purple line marks the area used for
calculating ebb delta volume. The white lines indicate the location of a CPE sand resource area.
(A) Shows bathymetry of New Topsail Inlet with current ebb-tidal delta, (B) Show bathymetry
of the same area with the ebb- tidal delta removed.
21
RESULTS
The results of this study are presented in two separate parts, a long-term study from 19382006 and a short-term study from 1982-2006. The long-term study consists of 17 photo sets,
which exhibit long-term traits and patterns. The image sets in this study are spaced sporadically
and depend on availability. The long-term study focuses on both inlet change and shoreline
change; however, it was primarily used to calculate shoreline change rates. The short-term study
consists of 44 photo sets and mainly focused on short-term inlet changes. The photo sets cover
changes in time periods ranging from 4 months to 1 year, sufficient for in depth analysis of inlet
change patterns.
Total Inlet Migration
From the opening of New Topsail Inlet during the late 1720’s until the completion of the
study period in 2003 the inlet migrated a total distance of 11.5 km. The average migration rate
over the inlet’s lifespan was 41.8 m/yr. It is likely this migration occurred in several stages of
varying migration rates; however, this data covering the time period from the 1720’s to 1849
have not been analyzed in any detail.
The earliest data incorporated in this section of the study is a shoreline which was
mapped from 1849-1873 (NCDCM, 2007). From 1849-1873 until the advent of aerial
photography (1938) New Topsail Inlet migrated 2.27km to the southwest yielding an average
migration rate between 25.5 m/yr and 34.9 m/yr (Figure 2). From 1938 through 2006 (the
detailed study period) the inlet migrated 1.66 km at an average rate of 25.5 m/yr. These data
22
produce a total migration distance of 3.93 km over a period of 130 to 153 years and yield a
migration rate between 25.5 m/yr to 30.2 m/yr (Figure 6).
The migration distance and migration rate during the study period (1938-2003) were
calculated using 17 inlet positions. The migration rate during each of these eras varied
considerably ranging from a low of 1.1 m/yr. to a high of 73.7 m/yr.
Long-term Inlet Characteristics
During the base year of 1938, New Topsail Inlet had a minimum width of 398 m. The
main ebb channel had a shore normal orientation of 148.9º and was located in the southern
portion of the inlet, directly adjacent to Hutaff Island. The resulting ebb-tidal delta occupied
191.4 acres and contained approximately 3,029,110 m3. The ebb shoal was positioned almost
directly in front of the inlet corridor.
In the 6.58 years between the 1938 and 1945 photograph sets, the main ebb channel of
New Topsail Inlet migrated 111.6 m to the northeast. The northward trend was also followed by
the inlet’s northern shoulder eroding 114 m; however, the southern shoulder of the inlet also
eroded 140 m, producing a significantly wider inlet corridor with a minimum width of 548.1 m.
Although still in a shore normal orientation, the main ebb channel has reoriented itself to an
azimuth of 129.8º, with the outer portions of the channel in a more central location within the
inlet corridor. Because of channel orientation, the ebb tidal delta had an elongated shape, with
the majority of the sediment north of the inlet’s centerline. Although the minimum width
increased by 150 m, the area occupied by the ebb-tidal delta only increased by 1.5 acres.
23
Figure 6. Map showing historic island outlines of Topsail and Hutaff Islands. Note how the
morphology of the islands has changed through time. Base image from PENDER COUNTY GIS
DEPT (2006).
24
From 1945 to 1949, New Topsail Inlet reversed direction and migrated 346 m to the
south at an average rate of 73 m/yr, the highest rate of movement during the study period. The
southern inlet shoulder eroded 116 m while the northern shoulder accreted 78 m, primarily the
result of a bypassing cycle taking place during this time. The main ebb channel has moved
directly adjacent to Hutaff Island and is in a near shore parallel position with the southern island.
As a result of the bypassing episode and the alignment of the main ebb channel, the inlet’s
minimum width increased to 564 m and the ebb-tidal delta stretched into a flat, elongated shoal
protecting 1065 m of the Hutaff shoreline. The elongated shape of the ebb-tidal delta also caused
its area and volume to increase dramatically. The ebb shoal covered 225.58 acres and had an
approximate volume of 3,568,790 m3.
From 1949 to 1956 (6.3 years) the main inlet channel migrated 19 m to the southwest at
an average rate of 3 m/yr. The slow rate of migration may have been the result of a large
bypassing event initiated by the jump in channel position during 1949. Although the main ebb
channel has barely moved, the inlet shoulders both changed dramatically. The northern inlet
shoulder accreted at 40.3 m/yr and developed a large spit protruding into the interior portion of
the inlet throat. While the northern shoulder accreted, the southern shoulder eroded a rate of 32
m/yr for a total of 207 m of net erosion. Although both inlet shoulders have responded in a
similar fashion, the inlet’s minimum width has decreased from 564 m to 324 m, a result of the
formation of the spit. The exterior portion of the main ebb channel has reoriented itself to a more
central location and exited the shoal at 144º. Because of channel realignment, decreased inlet
minimum width and the previous bypassing episode, the ebb-tidal delta decreased in both area
and volume, covering 151.5 acres and containing 2,397,900 m3 of sediment.
25
Over the next 3.5 years, the main ebb channel migrated 49 m to the southwest along the
inlet baseline; however the exterior portion of the channel deflected to a position further south
than in 1956, exiting the main ebb shoal at a more northern orientation of 137º. Although there
was 124 m of accretion on the northern shoulder, this was primarily a result of the erosion of the
spit and the repositioning of the sediment contained within. The erosion of the spit also led to a
150 m increase in the inlet’s minimum width. The new channel orientation and increase in inlet
width may have caused an increase in the size of the ebb delta shoal (Figure 7). The shoal
covered an area of 220.28 acres and had an approximate volume of 3,485,035 m3.
Between 1959 and 1962 there was very little movement of the interior portion of the
main inlet channel, however, buildup of the ebb-tidal delta caused the outer portion of the
channel to shift northward and exit the ebb shoal at 145º. Even though the main ebb channel only
migrated 2.9 m (a measurement within the margin of error), the southern inlet shoulder eroded
155 m due to the formation of a marginal flood channel. The channel likely served as an exit
point for the main ebb channel, and was the beginning of a large bypassing event. The
orientation of the main ebb channel and previous welding of ebb shoal swash bars caused a
reduction in the size of the ebb-tidal delta, which occupied 209.7 acres and had an approximate
volume of 3,318,790 m3.
Unfortunately, due to the lack of available aerial photographs, the next observation period
was quite large and little interpretation can be done for the 11.6 years. Between 1962 and 1974
26
Figure 7. Line plot illustrating possible inverse relationship between ebb-tidal delta volume and
channel orientation. Correlation between the two first starts in 1962 after the inlet recovered
from several large storms destroying the ebb shoal and the outer portion of the inlet channel was
deflected to the north.
27
the main ebb channel migrated 569 m to the southwest at an average rate of 49 m/yr. Both the
northern and southern inlet shoulders responded accordingly. The northern shoulder accreted 391
m and the southern shoulder eroded 267 m. The inlet’s minimum width reduced 22 m to 367 m.
The main ebb channel was oriented in a near shore perpendicular position, exiting the ebb shoal
at 174º. The ebb delta volume has decreased to 3,162,230 m3.
Between 1974 and 1978, the main inlet channel migrated 111 m at a rate of 31.4 m/yr.
The inlet channel was pressed against Hutaff Island and maintained a shore perpendicular
orientation. The channel exited the ebb shoal at 180º. The northern inlet shoulder accreted at 20
m/yr, building 73 m of beach, where as the southern inlet shoulder eroded nearly 100 m. The
position of the channel near Hutaff Island caused the formation of a large protrusion on the
northern end of the island (discussed further in shoreline change section of results). A bypassing
event occurred during the period between photographs and formed a small spit on the southern
end of Topsail Island. The orientation of the channel has further reduced the volume of the ebb
shoal to 3,126,525 m3.
From 1978 to 1982, the main inlet channel migrated 200 m with the inlet’s northern
shoulder accreting 228 m at a rate of 58 m/yr. Meanwhile, the south shoulder eroded 168 m at a
rate of 43.2 m/yr. The inlet’s minimum width increased by 10 m to 388 m. The main inlet
channel remained pressed against Hutaff Island, with the exterior portion of the channel moving
to a more northerly orientation of 173.1º. The position of the main ebb channel caused continual
growth of the shoreline bump on Hutaff Island, directly south of the inlet corridor. The reorientation of the inlet channel has caused the ebb tidal delta to increase in volume by 506,090
m3 and to occupy an area of 229.6 acres.
28
During the 6.6 years between the 1982 and 1989 photographic sets, the inlet migrated 204
m at a rate of 30 m/yr. The northern inlet shoulder trailed behind at a rate of 16 m/yr while the
southern shoulder eroded at an accelerated rate of 33 m/yr. The varying rates of migration as
well as other factors such as increased water flow, lead to an increase in minimum width by 217
m to a new width of 605 m. The main ebb channel remained pressed against Hutaff Island,
however, the outer portion of the channel continued its movement to the north and was oriented
at 168.2º. As shown in Figure 7, the ebb tidal delta increased in area and had a volume of
approximately 3,801,936 m3.
From 1989 to 1993, New Topsail Inlet migrated 132 m at a rate of 26 m/yr. Both inlet
shoulders migrated at slightly faster rates, the northern shoulder accreted at 30 m/yr and the
southern inlet shoulder eroded at 36 m/yr. The inlet’s minimum width has decreased to 489 m
due to the new shape of the inlet complex. The most dramatic change within the inlet complex
was the change in path and location of the main inlet channel. The channel has started flowing in
a “C” shape and exited at a very northern orientation of 126.7º. Because of the new channel
position, a large channel margin linear bar has formed on the southern side of the main inlet
channel. The orientation of the channel has also affected the inlet complex in two ways. 1) The
ebb tidal delta increased in size, occupying 291.4 acres and contained approximately 4,610,650
m3 of sediment. 2) The orientation and position of the ebb channel orientation caused the buildup
of a large shoreline bump on the southern end of Topsail Island.
Between 1993 and 1999, the main inlet channel migrated 128 m at a rate of 24 m/yr, both
the northern and southern shoulders migrated at similar rates of 20 m/yr and 23 m/yr
respectively. The inlet’s minimum width increased by 154 m due to the repositioning of sand
along the southern end of Topsail Island; the minimum width during 1999 was 643 m. It is likely
29
a bypassing event took place during the time between photographic sets. This was evidenced by
a small spit building up on the southern end of Topsail Island and by the relocation and
reorientation of the main ebb channel. The exterior portion of the channel moved 465 m to the
south and exited at an orientation of 148.6º. As a possible result of the new orientation, the ebb
tidal delta volume decreased to 4,552,300 m3.
From 1999 to 2003, the main inlet channel migrated a total of 16 m; however, the inlet
shoulders behaved very differently. The northern inlet shoulder accreted 174 m as a result of
welding swash bars and spit development. The southern inlet shoulder eroded 99 m in its
continual march to the southwest. The inlet’s minimum width has decreased slightly to 614 m.
The main inlet channel has maintained its now characteristic “C” shape although there was a
slight linear bar breach during 2002. The exterior portion of the channel exits at the breach site
and has an orientation of 131º. It is important to note this orientation caused the channel to exit
the inlet corridor pressed against the southern end of Topsail Island and resulted in the buildup of
a large shoreline protuberance. The shoreline bump is likely due to the accretion of sand that
would otherwise be trapped in the ebb tidal delta. The ebb tidal delta increased in size and
occupied 338.5 acres while containing 5,355,935 m3 of sediment.
From 2003 to 2006, the main inlet channel migrated 87 m at a rate of 21 m/yr. The main
inlet channel has reoriented itself to a more southerly position of 147.5º. Despite this
reorientation, the inlet complex has maintained its “C” shape. The new position of the inlet
channel has caused a smoothing of the shoreline on Topsail Island, causing the shoreline bump to
be reworked. The channel’s new position has also allowed a large spit or accreting swash bar to
form off the southern tip of Topsail Island. The inlet’s minimum width has increased to 701 m,
possibly due to the widening planform of Topsail Island. The ebb tidal delta’s volume reduced to
30
4,172,880 m3, possibly due to large welding events and the repositioning of the main inlet
channel.
Short-term Inlet Changes
This section of the study was used to identify trends within the inlet complex that may
have been identified during the long-term study, but could not be expanded upon due to the time
gap in the data sets. The following presents results on sand bypassing, migration trends, erosion
and accretion rates of the inlet shoulders (tied into bypassing events), and how channel
orientation, ebb delta volume and migration rate are all interrelated.
Inlet Bypassing Cycles
During the period covered by the short-term portion of the study (April 1982 – October
2003), New Topsail Inlet was subject to 4 sand bypassing\ channel re-orientation periods. Of the
4 bypassing events, only one completed a full cycle. The completed cycle likely began in 1978
(out of the scope of the intensive study period), and reached completion in early 1987. The main
inlet channel started at a position of 180.6°, and slowly re-oriented itself to a terminal position of
106.1°, experiencing 74° of change through the cycle. Figure 8 shows the channel orientations
throughout 1978-1987. The change in orientation was accompanied by change in ebb delta
position and size, as well as the full development of the channel linear bars.
31
Figure 8. Composite images show the bypassing episode from 1982 – 1987. Blue shapes
highlight changing orientation of shoals; the black dotted line illustrates the outer extent of the
ebb tidal delta. The highlighted shorelines and channels correspond to the dates of the pictures.
32
During the cycle, the main inlet channel migrated 153 m to the southwest. However, the
migration did not occur in a constant rate or direction. The channel migrated to the south from
1982-1984, then switched, migrating north during 1985-1986, and finally switched back to a
southwesterly direction in 1987. The yearly migration rates ranged from -24 m/yr to 72 m/yr.
The inlet’s minimum width and ebb delta volume both increased during the episode; the inlet’s
minimum width ranged from 391 m in 1982 to 560 m right before the channel jumped to its new
position in 1987, and the ebb delta increased in volume from 3,593,400 m3 in 1982 to 4,052,140
m3 in 1987.
The three other bypassing episodes ranged in length of time and degrees of change, with
two of the cycles ending with pre-mature breaching events. The third episode began at the end of
1994 and has not yet reached completion. A summary of results from all the bypassing episodes
can be seen in Table 1, whereas channel locations of the 4 episodes are displayed in Figure 9.
Inlet Migration Patterns
In addition to the migration rates calculated during the long-term study, migration rates
were recorded in detail over the span of the short-term study. Although data may have been
collected on a sub-yearly basis, this data was compiled to represent rate of migration and
therefore distance traveled per year. The data collected during the short-term study are similar to
those collected during the long-term study; however, they present a more accurate representation
of how the inlet complex migrates and how its components are tied together. A summary of the
migration data for the main inlet channel, northern inlet shoulder, and southern inlet shoulder is
presented in Table 2.
33
Table 1. Summary of Inlet Bypassing Episodes
Period
° Of Change
° Of Jump
Migration
Rate (m/yr)
Migration
Distance (m)
Change in
Ebb Delta Volume
1978 to 01/13/1987
74°
62°
31.8
153
+ 535,000 m3
01/13/1987 to 1/11/1990
28°
24°
25.9
77.7
- 267,500 m3
Pre-mature breaching
episode.
1/11/1990 to 11/3/1994
26.8°
40.6°
23.3
91
+ 841,000 m3
Pre-mature breaching
episode.
11/3/1994 to 10/13/2006
32.7°
Unknown
17.1
152.2
+ 535,000 m3
34
Notes
Channel moved through
complete cycle.
Channel in re-orientation
process.
Figure 9. Composite image illustrating the 4 bypassing episodes during the short-term study. A)
Shows complete bypassing cycle from 04/7/2982 to 01/13/1987. B) Shows small and incomplete
cycle from 01/13/1987 to 1/14/1991. C) Illustrates channel position during incomplete cycle
from 1/11/1990 to 11/13/1994. D) Channel orientations during current bypassing episode, the
cycle began near 11/13/1994.
35
Table 2. Summary of Yearly Migration Rates and Channel Orientation
Year
Inlet Channel
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
68.76
68.78
31.4
-17.0
-24.2
72.1
7.5
8.0
-8.8
57.5
33.8
17.4
-2.7
95.4
-62.1
101.9
34.0
32.6
-10.9
-13.9
3.9
-18.8
Migration Rate (m/yr)
N. Shoulder
84.7
84.7
-91.8
11.7
-46.0
-44.0
26.1
97.8
140.4
1.2
59.0
-80.2
13.3
-25.6
145.1
41.8
94.5
185.5
84.0
72.0
41.6
29.8
S. Shoulder
Azimuth
61.0
61.0
24.0
4.8
29.0
-9.6
42.8
38.8
76.5
28.0
85.1
10.2
55.6
10.4
-41.1
74.9
29.7
33.7
158.1
-28.1
50.7
68.9
174.02
160.32
143.4
110.5
105.1
157.4
148.4
139.4
158.6
151.2
140.6
129.7
133.4
168.2
161.3
161.0
155.0
142.9
131.6
132.8
141.8
141.2
36
The relationship between migration of the inlet channel and how the inlet shoulders
respond will be explained in the discussion, as will the relationship between channel orientation
and migration rate.
Oceanfront Change
The next phase in determining New Topsail Inlet’s influence on its adjacent barrier
islands was to determine long and short-term oceanfront change rates and patterns. The primary
focus of the study was to gather an extensive database of shoreline change rates within the
current Inlet Hazard Areas (IHA), which extends from Transects 18 to 28 on Topsail Island
(Figure 10). However, change rates were also calculated for the shorelines, which continued past
the IHA. As the dataset outside the IHA is not as complete, various temporal periods were used
to group the data. These data were split into periods, which coincide with changes in inlet
migration rate.
Figure 11 illustrates cumulative inlet migration. The variations in slope indicate periods
of faster and slower migration and interval time periods were based on these changes, yielding
EPR’s for 1938-1949, 1949-1962, 1962-1974, 1974-1999 and 1999-2006. In addition to the
interval time periods, EPR was calculated for the whole study area from 1938-2006.
Cumulative shoreline change data was also calculated for the study area. Due to the migration of
the inlet and the lack of aerial photographs, cumulative change could not be calculated for the
entire study area. As previously stated, the IHA portion of the study area has the most
comprehensive data set, but other sections of the shoreline were also selected to illustrate the
presence of “erosion hotspots” and bump progradation. These results are primarily presented in
the DISCUSSION.
37
Figure 10. Map illustrating the study area and the portion of Topsail Island that is zoned as an
Inlet Hazard Area. The transects within the IHA were the primary focus for shoreline change
analysis. Please note green lines that indicate the portion of the study area which has been in
continuous existence since 1938. IHA shapefile from NCDCM (2006).
38
Figure 11. Line plot depicting cumulative channel migration from 1938 to 2006. Red lines
illustrate changes in migration rate and intervals for shoreline change analysis.
39
Net Change 1938-2006
Of the 6,097 m (20,000 ft) of shoreline analyzed, only 2,439 m (8000 ft) or 40% of the
shoreline has been in continual existence since 1938 (Figure 10). The other 60% of the shoreline
either has been subject to inlet influence (inlet shoulder or re-worked material) or has been the
location of Old Topsail or New Topsail Inlets.
Topsail Island
Transects 1-28 covered the Topsail Island portion of the study area, however, only
transects 1 -17 had continuous shoreline change data during 1938-2006. Net erosion occurred
along the majority of the oceanfront, from transects 1-15. This shoreline reach experienced an
average loss of -38.4 m at a rate of -0.57 m/yr. The greatest amount of erosion occurred along
transect 9, which lost 65.2 m of beachfront at a rate of -0.96 m/yr. The transect with the least
amount of erosion was transect 15, which lost -2.95 m of beachfront at a rate of -0.04 m/yr. The
extremely slow rate of erosion was likely due to the transect’s proximity to the 1938 inlet
position and the pattern of island widening occurring at the southern end of Topsail Island.
The two southern most transects 16 and 17 in the shoreline reach both experienced net
accretion from 1938 -2006. The accretion experienced at transect 17 was largely due to its
position on the 1938 inlet shoulder and cannot be considered true oceanfront change. However,
transect 16, which represents true oceanfront change gained 30.6 m of beach front at a rate of
0.45 m/yr. The accretion at transect 16 is likely due to its proximity to the 1938 inlet position and
the previously mentioned pattern of island widening.
40
Hutaff Island
On Hutaff Island only 2 transects were in continuous existence from 1938 -2006.
Transects 35 and 36 both experienced an average net erosion of -147.5 m at a rate of -2.2 m/yr.
All other transects covering the Hutaff Island shoreline (23-41) are either now re-worked
material forming the southern tip of Topsail Island or were in a position that overlaid New
Topsail or Old Topsail Inlet.
The five transects which overlap both the 1938 Hutaff Island shoreline and the 2006
Topsail Island shoreline can be used to exhibit the idea of shoreline straightening. Transects (2327) have prograded an average of 230 m. However, because the transects overlap two different
islands, this data is erroneous and cannot be used for more than speculation.
As a side note, it is important to remember that these data represent only the changes
which occurred between 1938 and 2006 and do not incorporate any of the changes that took
place between the two dates. Those changes will be discussed in another section of the
RESULTS and in the DISSCUSSION.
Net Change Intervals
As previously mentioned, oceanfront change was also calculated for five interval periods
corresponding to inlet migration rates. These data were calculated to see if any relationship
between inlet migration rate and oceanfront change rates exist.
Of the 6,097m (20,000 ft) of shoreline analyzed, 4,420 or 72.5% was in continual
existence between 1938 and 1949. The other 27.5% was either the location of an inlet or part of
an inlet shoulder (Figure 11).
41
Topsail Island
During the period from 1938 to 1949, transects 1-16 recorded shoreline change data on
Topsail Island. Transects 1-10 all had net erosion occur along them, losing an average of -23.8 m
at a rate of -2.11 m/yr. The greatest amount of erosion took place at T-3 where -36.8m of
beachfront was lost at a rate of -3.26 m/yr. The slowest rate of change occurred at T-10, which
eroded at -0.71 m/yr for a net loss of -7.98 m.
Transects 11-17 (See Figure 12) all experienced net accretion during the time period. An
average of 38.9 m was gained across the shoreline reach at a rate of 3.4 m/yr. The smallest
amount of accretion occurred at T-11, the most northern portion of the shoreline reach. T-11
gained 5.2 m of beachfront at a rate of 0.46 m/yr. Transect 17, located in the southernmost part
of the shoreline reach and therefore closest to the 1949 inlet corridor, accreted at a rate of 11.6
m/yr and gained 131.6 m of beachfront.
Hutaff Island
From 1938 to 1949, transects 23-36 recorded shoreline change along Hutaff Island. The
2,134 m shoreline reach experienced net erosion along all transects. The average loss along the
shoreline was -64.4 m, which eroded at a rate of -5.72 m/yr. These rates however, are not an
accurate reflection of how the island changed. Both the northern and southernmost transects (T23 and T-36) had very high oceanfront losses of -165.7 m and -143.3 m respectively. T-23
eroded at a rate of 14.6 m/yr where as T-36 eroded at a slightly lesser rate of -12.7 m/yr. The
smallest rate of change took place near the middle of the island along T-29, which eroded at a
rate of -0.38 m/yr. The morphological changes, which occurred to Hutaff Island are displayed in
Figure 12. The erosion that took place effectively changed the islands shape from a concave
drumstick shape into a linear, straighter shoreline.
42
Figure 12. Map illustrating historic shorelines and inlet positions during 1938 and 1949. T-17
was did not record data as it was part of the inlet shoulder in 1938.
43
During this time interval, shoreline change data was collected from 4,115.8 m of
oceanfront. Transects 1-17 collected data on Topsail Island and Transects 27-36 collected data
on Hutaff Island (Figure 13). 67.5% of the study area was part of a continuous shoreline during
this time period. The remaining transects were subject to inlet influence or inlet position.
Topsail Island
From 1949-1962 T1-17 recorded shoreline changes along Topsail Island, however, the
pattern, which was recorded, was different during this time period. Instead of having two distinct
shoreline reaches, one which suffered from net erosion and the other experiencing net accretion,
there are three shoreline reaches; two zones of accretion, separated by a zone of erosion.
Transects 1-4 and 14-17 all experience net accretion, where as transects 5-13 experienced net
erosion.
The accretion zones consisted of a large shoreline bump surrounding the Topsail Beach
pier, and an area in close proximity to New Topsail Inlet. The average rate of change over both
accretion zones was 1.37 m/yr, gaining 16.9 m of beachfront over the 12.3-year period. The
transects with the highest accretion rates were T-3 and T-16; both transects prograded 28.8 m at
a rate of 2.3 m/yr. The least amount of shoreline change occurred at T-17, which gained only
4.1 m of beach at a rate of 0.33 m/yr.
The erosion zone, which was located 405 m south of the pier had an average zone wide
loss of -14.1 m, which occurred at a rate of -1.1 m/yr. The highest rate of erosion took place at T11, this section of shoreline lost -30.7 m of material at a rate of 2.38 m/yr. The area least
44
Figure 13. Map depicting historic shorelines and inlet positions during 1949 and 1962. Note that
not only has New Topsail Inlet migrated, but Old Topsail Inlet has also migrated, extending the
length of Hutaff Island past T-39.
45
effected by erosion was just north of the southern accretion zone, at T-13, where -0.39 m of
beachfront was eroded during the 12.3 year period.
Hutaff Island
During the period from 1949-1962, net erosion occurred at all transects but one along the
Hutaff Island shoreline. Transects 27-35 all recorded net erosion where as T-36 recorded net
accretion. The beachfront gained at this location was a result of Old Topsail Inlet’s migration
south and the subsequent island building that occurred. The rest of Hutaff Island eroded an
average of -38.1 m at a rate of -3.08 m/yr. The highest recorded loss took place at T-32 where 65.5 m of beach was eroded at a rate of -5.3 m/yr.
During the period from 1962-1974, 30 out of 41 transects recorded shoreline changes
over 70% of the study area. Transects 1-19 recorded changes on Topsail Island and transects 2939 recorded shoreline changes on Hutaff Island (Figure 14).
Topsail Island
Nearly all transects on the Topsail Island shoreline reach recorded net erosion; T-2, T-5
and T-19 recorded net accretion. T-2 and T-5 had minimal gains of 2.3 m and 2.7 m respectively.
T-19 had a much higher rate of accretion and gained 44.3 m of beachfront at a rate of 3.8 m/yr.
This large accretion rate was likely due the proximity of the inlet complex and its influence.
46
Figure 14. Map of the study area depicting historic shorelines and inlet positions from 1962 and
1974.
47
Topsail Island lost an average of 9.29 m of beachfront material at a rate of 0.80 m/yr. The
shoreline reach, which excluded T-19, had an average net change of -12.2 m and eroded at a rate
of 1.05 m/yr. The greatest loss occurred at T-16, the previous site of a large accretion zone
during the 40’s and 50’s. The beach at T-16 eroded 30.1 m at a rate of 2.6 m/yr. Transects 14 and
15 were also heavily eroded and lost 22.8 m and 25.9 m of beach respectively. The lowest rate of
erosion took place at T-7, which lost 0.21 m/yr and had a total loss of -2.41m.
Hutaff Island
The transects along the Hutaff Island reach (T-29 – T-39) recorded both net erosion and
net accretion. Similar to previous years the flanks of the island were more dramatically
affected by shoreline change processes than the center of the island. Overall, Hutaff Island
accreted an average of 40.0 m at a rate of 3.4 m/yr. The transects closest to New Topsail Inlet
changed the most, building out 111.9 m and 113.6 m at T-29 and T-30. These transects had
accretion rates of 9.6 m/yr and 9.7 m/yr respectively. The net erosion that occurred on the
southern portion of the island was a result of Old Topsail Inlet’s migration to the south. T-38 lost
the most material eroding 20.7 m at a rate of 1.78m/yr.
Unfortunately, the extent of aerial photographs during 1999 did not cover the entire study
area and limited the amount of shoreline change data collected. As a result, transects 1-8 did not
record any changes between 1974 and 1999 or between 1999 and 2006. However, data was
recorded along the rest of the Topsail and Hutaff Island shoreline reaches, accounting for 50% of
48
the study area; the other 50% was either previous or current inlet position or unavailable (Figure
15).
Topsail Island
In similar fashion to previous time periods, the Topsail Island shoreline can be divided
into two shoreline reaches based on net change. Transects 8-17, the northern shoreline reach,
experienced net erosion, losing 7.7 m of material at a rate of 0.32 m/yr over all transects.
Minimal accretion took place at T 8-10 where the largest gain was 1.8 m. The highest erosion
rate occurred at T-15, which lost 21.6 m of beachfront at a rate of 0.9 m/yr.
The southern shoreline reach (transects 18-22) gained an average of 52 m, pushing
seaward at 2.14 m/yr, however the shoreline reach did not accrete in a uniform fashion. Instead,
accretion rates ranged from a minimum of 0.39 m/yr at T-18 to a maximum of 5.48 m/yr at T-22.
Once again, accelerated rates were likely due to the proximity of the inlet complex during 1978.
Hutaff Island
Two transects , T-34 and T-35, recorded net accretion from 1962 to 1974; both transects
gained 24.2 m of beachfront at a rate of 1.0 m/yr. All other transects on Hutaff Island recorded
net erosion. As a whole, Hutaff Island eroded and average of 36.6 m at a rate of 1.5 m/yr.
Transect 40, the farthest transect from New Topsail Inlet to record oceanfront change, lost 82.9
m of beach at a rate of 3.41 m/yr.
49
Figure 15. Map of the study area, inlet positions and net oceanfront changes between 1974 and
1999. Note that due to the length of the time period nearly 600 m of shoreline on Topsail Island
was not analyzed.
50
The 1999-2006 data period is the best record of changes, which occurred on the southern
end of Topsail Island. 3,506.1 m of shoreline or 57.5% of the study area was used to calculate
shoreline change data between 1999 and 2006. Of that, 2,743.9 m of the shoreline was part of the
Topsail Island reach (not including transects 1-8). Essentially this resulted in 762.1 m of new
shoreline being analyzed that did not exist in 1974 (Figure 16).
Topsail Island
Once again, the Topsail Island shoreline can be divided into two sections based on net
change. The northern section was comprised of transects 8- 19 and was subject to net erosion.
This portion of the Topsail shoreline lost an average of 12.6 m of beachfront at a rate of 1.5 m/yr.
T-11 suffered the highest loss of 19.8 m at a rate of 2.42 m/yr.
The southern shoreline reach, comprised of transects 20-27 accreted at the accelerated
rate of 11.5 m/yr, gaining an average of 94.6 m across all transects. However, the same pattern of
increasing accretion rates tied to the proximity of the inlet complex was present in the data set.
The smallest beachfront gain occurred at T-20, which gained 10.2 m at a rate of 1.26 m/yr. The
largest seaward progression took place at T-27, which gained 224.5 m of material at an
astounding rate of 27.4 m/yr. The rates of change between T-20 and T-27 increased steadily as
T-27 was approached.
51
Figure 16. Image of the study area in 2006. Shorelines from 1999 and 2006 show inlet position
and oceanfront change during the time period. Topsail Island shoreline south of T-20 had not
been included in previous time periods.
52
Hutaff Island
Transects 35-40 recorded shoreline change data along Hutaff Island; all transects but T35 recorded net accretion. The average rate of accretion along all transects was 0.93 m/yr for a
total progression of 7.6 m. The average rate of change for the transects that recorded net
accretion was 2.6 m/yr, gaining 12.7 m of beachfront material. The erosion that occurred at T-35
(6.4 m) was likely a factor of the transect’s proximity to the inlet complex.
Cumulative Shoreline Change
In addition to net change, cumulative shoreline change data was also collected for the
study area. This data gave insight into the processes occurring between the net change interval
dates. As the shoreline is constantly changing, cumulative change data is essential in tracking
day to day changes related to storm events, beach nourishment projects and the everyday
movement of the shoreline.
This dataset combines shoreline change rates from the long-term study (17 shorelines)
and the more temporally intensive short-term study (45 shorelines). When compiling this data
several problems presented themselves. First, the migration of New Topsail Inlet makes it
impossible to attain cumulative shoreline data from 1938 to 2006 as over 1.6 km of shoreline
analyzed did not exist in 1938. Second, by combining the long-term and short-term studies, the
number of transects with change data for every date interval was greatly reduced. Transects that
had data during the long-term study may not have data available during the short-term study
simply because of the limited nature of the short-term photographic database. A large number of
photograph sets used during the short-term study only had 1 or 2 photos as the sets were taken to
track inlet changes.
53
Topsail Island
As a result of the problems described above, data from 1938-1974 will be used to discuss
the northern portion of the island (T-1 to T-16). The more recent data will focus on the current
inlet hazard zones, which cover the southern reach of Topsail Island (T-17 to T-27).
Northern Shoreline Reach (T-1 through T-16)
Data for this section of Topsail Island was compiled from the following dates: 1938,
1945, 1949, 1956, 1959, 1962, and 1974. All transects within this shoreline reach experienced
periods of both erosion and accretion, however at most locations, erosion outweighed accretion.
Transects 1-13 recorded a cumulative landward movement of the shoreline where as transects
14-16 recorded a cumulative seaward movement of the shoreline. Figure 17 shows the net
cumulative change experienced by each transect in the northern shoreline reach. In addition to
Figure 17, Figure 18A and 18B illustrate how shoreline advance and retreat took place during
1938 to 1974. The first and last points on the data plots represent the net change of the shoreline
between 1938 and 1974 for the representative transect. As can be seen in the graphs, there are
several positions on the beach that were subject to large movements in the shoreline, Figure 18B
is a particularly good example. Reasons behind the shorelines movement for this position as well
as others will be explained in the DISCUSSION.
Southern Shoreline Reach
As previously discussed, the southern portion of the Topsail Island shoreline presented
several problems that made it difficult to present the data in a consecutive and understandable
54
Figure 17. Bar graph showing the total cumulative changes for the transects located in the
northern shoreline reach. These changes illustrate the balance of erosion or accretion that has
occurred at each transect location.
55
Figure 18a. Two line plots illustrate the movement of the shoreline between 1938 and 1974.
Positive values denote regressive shoreline and negative values denote a transgressive shoreline.
56
Figure 18b. Two Line plots illustrate the shoreline movement of the southern transects located in
the northern shoreline reach. Notice how transects located next to each other react to events in a
similar fashion.
57
matter. As most of the problems have been spatial, the southern shoreline reach was divided into
several spatial limits based on different time periods. The goal was to use as much data as
possible while keeping it organized in manageable chunks and similar time intervals as the net
change data.
1974-1999
In order to include as much data as possible, the period from 1974 to 1999 was broken
into several smaller intervals. The mini intervals were: 1) 1974-1982, this time period had data
from three shorelines and covered transects 14-22, 2) 1982-1990, which had data available from
16 shorelines and covered transects 17-24, and 3) 1990-1999, which included data from 22
shorelines and spanned from T-18 to T-25. Figure 19 illustrates the total cumulative shoreline
changes that occurred at each transect during the mini time intervals. Notice that trends seen in
the net change data, such as an increase in accretion as the inlet complex is approached and an
apex point, which indicates a shift between accretion and erosion, are prevalent in this data set.
Unfortunately, the limited shoreline data from 1982 did not allow data to be collected outside the
inlet’s zone of influence, limiting what can be determined about how the inlet effected shoreline
change during the early 1980’s. The data displayed in Figure 19 does not represent how the
shoreline behaved during the period, just the total amount of erosion or accretion that occurred.
Figures 20-22 illustrate the habits of shoreline position by transect. The line plots in Figures 2022 show that although cumulative shoreline change was either dominated by erosion or
accretion, both processes occur along each transect within the shoreline reach.
58
Figure 19. Bar graph displaying the cumulative shoreline change experienced by transects in the
southern shoreline reach from1974-1999. The cumulative change for transects with more than
one column can be determined by adding the respective changes.
59
Figure 20. Line plot illustrating shoreline movement over the southern shoreline reach during
the period between 1974 and 1982. Accretion of transects 14-18 and erosion of T-19 was likely
related to the New Topsail Inlet’s migration to the southwest.
60
Figure 21.Two line plots display the movement of the shoreline across transects in the southern
shoreline reach from 1982-1990. The more dramatic movements recorded by T-21 through T-24
are related to the proximity of New Topsail Inlet and the repositioning of the main ebb channel.
61
Figure 22. Two line plots illustrating changes in the shoreline during the mini period from 19901999. Again changes in the southern part of the shoreline reach were more dramatic than changes
occurring along the more northern transects.
62
1999-2006
The most current period, incorporated 11 shorelines and spanned an area from T-18 to T27 (the area of Topsail Island covered by the current inlet hazard zones). Figure 23 shows the
total cumulative change, which occurred within the shoreline reach. As can been seen in Figure
19, this shoreline reach was dominated by accretion during the time period. However, Figure 24,
which illustrates the movement of the shorelines within this reach, shows that large erosional
events also take place along this section of the beach.
Hutaff Island
Several of the problems that occurred in presenting the cumulative data for the Topsail
shoreline were also present in the data for Hutaff Island. The two main problems within this data
set were the lack of historic aerial photographs, and the substantial amount of erosion that
occurred on the northern end of the island. Unlike the situation on Topsail Island, where the
island out grew the 1938 shoreline, the opposite has happened here; erosion has claimed the
shoreline on all but two transects. In order to deal with New Topsail Inlet’s migration, the
cumulative shoreline change data for Hutaff Island has been split into several time intervals.
1938-1962
The shoreline reach from 1938 to 1962 stretched from T-25 to T-35 and contained the
largest shoreline reach on Hutaff Island for computing cumulative change. This time interval
dataset is the most comprehensive portion of cumulative shoreline change data as all other time
periods were limited to two or three transect spans. All transects within the shoreline reach were
characterized by cumulative erosion, however, transects located at the islands extremities were
63
Figure 23. Bar graph showing the cumulative change data for transects 18-27 for the time period
between 1999 and 2006. This shoreline reach is located within the current inlet hazard zone for
New Topsail Inlet and is extremely prone to oceanfront change.
64
Figure 24. Two line plots show the various positions of the oceanfront shoreline during the
period 1999-2006. The figure shows that although the area is predominately accretionary,
erosion does occur.
65
subject to higher levels of erosion (Figure 25). The proximity of both New Topsail and Old
Topsail Inlets played a dramatic role in how the shoreline moved at these locations.
1962-2006
Cumulative data for several time intervals within this time period were plotted and can be
seen in Figure 26. Again, large erosion and accretion events were usually influenced, if not
controlled by the proximity of New Topsail Inlet. Typically, large accretion events took place
when the main channel of New Topsail Inlet exited the inlet throat directly adjacent to Hutaff
Island. The position of the inlet channel and its protective shoals allowed for the buildup of a
large shoreline bump (illustrated in Figure 26) during 1962-1982. As the inlet’s ebb channel
shifted northward and the outer portion of the channel was repositioned, the shoals were
reoriented and erosion of the shoreline bump took place. In addition to channel shifting, the
continual southern migration of the inlet complex caused continual erosion and reshaping of
Hutaff’s inlet shoulder, causing cumulative shoreline loss for much of the period between 1982
and 2006.
Positional changes in the shoreline recorded during the course of the study period are
displayed through figures 27-30. As a way of dealing with the continual loss of shoreline and to
limit the effect of the inlet shoulder, these graphs were created using transects that were in
existence for the entire time period being analyzed and were not in areas being reshaped as part
of the inlet complex.
66
Figure 25. Bar graph displaying cumulative shoreline loss across all transects within the Hutaff
shoreline reach from 1938-1962. The higher level of erosion present for transects 25-26 and 3435 is due to the proximity of New Topsail Inlet to the north and Old Topsail Inlet to the south.
67
Figure 26. Bar graphs showing cumulative shoreline change data on Hutaff Island for all other
time periods between 1962 and 2006. Limited data displayed for certain time intervals is due to
lack of shoreline information.
68
Figure 27. Line plot illustrating shoreline position change on Hutaff Island between 1938 and
1982 for transects 30-31. There is clear bump development which took place on the northern end
of the island from 1962 to1980. This was a direct result which occurred because of location of
the main ebb channel and its exit position adjacent to Hutaff Island.
69
Figure 28. Line plot showing the movement of the shoreline across transects 32-34 on Hutaff
Island for the time period 1982-1990. The previous shoreline bump which was built from 1962
to 1980 was eroded during the early 80’s as a result of outer ebb channel repositioning. The
small bump developed during 1988- late 1989 was also due to channel repositioning.
70
Figure 29. Line plot displaying shoreline position for transects 34 and 35 on Hutaff between late
1990 and 1999. The zigzag position of the shoreline during the late 1990’s was caused by New
Topsail Inlet’s migration to the south. The position of transects 34 and 35 during these years was
starting to be influenced by the erosional aspects of the inlet complex.
71
Figure 30. Line plot shows shoreline change for transects 35 and 36 for the time interval 19992006. Accretion and erosion during this time period was a result of the change in shape of the
southern inlet shoulder and subsequent oceanfront realignment.
72
DISCUSSION
Increased coastal development up and down the eastern seaboard has lead to the
construction of permanent structures in areas sensitive to inlet influence. The dynamic nature of
these systems and the resulting oceanfront realignment related to them has lead to the necessity
for the development of a large scale inlet management plan. As many of the eastern seaboards
inlets remain unmodified, they largely act as individualistic systems. Generally, these inlets have
certain similarities and components, but numerous factors influence how the overall inlet
complex works as a whole.
The detailed study and analysis of specific inlet systems, such as New Topsail Inlet, will
not only further the scientific understanding of how adjacent barrier islands are affected by
inlets, but will also allow site specific management plans to be developed. With the use of these
management plans, state and local government agencies will be in a more able position to
regulate development within sensitive areas, and prevent development that mirrors the mistakes
in the past.
This study is currently the most in-depth and complete data set assembled for any inlet
system in North Carolina. Several aspects of the study have made this statement to be true,
including the long and short-term components as well as the extensive aerial photograph
database, which allowed quarterly review of inlet changes on certain occasions. Hopefully the
understanding gained through this study can be used in future monitoring of New Topsail Inlet
and the information applied to other migrating systems with similar traits.
73
Effects of Inlet Migration:
The first thing that comes to mind when discussing the New Topsail Inlet system is long
periods of migration. And although the migration of the inlet complex has been continuous, it
has also been complex. Unfortunately, a detailed history of migration rates pre-1849 was not
possible. However, some understanding of the past can be gained by the current conditions
taking place within the inlet system. The complexities in the migration of the inlet complex stem
from two factors, 1) the direction of migration and 2) the rates at which the system is migrating.
Direction of Migration:
In order to fully understand how the New Topsail Inlet complex migrates, short-term and
long-term changes must be taken into account. Over the 68 years which this study analyzed in
detail, New Topsail Inlet migrated a total of 1.66 km to the southwest, reversing its direction
only once, and indicating that New Topsail Inlet migrates unilaterally. The direction of migration
is not only supported through photographic evidence, but also by the presence of Banks Channel,
a long, island parallel channel that separates the southern portion of Topsail Island from the
mainland (Figure 2). Flanking the marsh side of Banks Channel are a series of marsh islands,
created by the stabilization and vegetation of previous, reworked flood tidal delta shoals
(CLEARY et al., 1979).
Another indicator of New Topsail Inlet’s migration is the presence of re-curved dune
ridges along the southern portion of Topsail Island. As the inlet migrated, it produced a series of
curved beach ridges separated by low lying marshy areas, indicating lateral sedimentation
processes (HAYES, 1980). Unfortunately, the development of Topsail Island has made the re-
74
curves difficult to see in recent photographs, however Figure 31 illustrates several re-curves in
photos from 1938 and 2008.
A feasibility study conducted by the USACE (1989) estimated the gross rate of sediment
transport across New Topsail Inlet to be approximately 500,000 m3/yr, 55% or 275,000 m3
moving across the inlet throat in a northerly direction. More recently, the USACE used their
shoreline model GENESIS to predict the sediment transport potential for Topsail Island
(USACE, 2006). The data in this study concluded that the predicted average gross transport
along Topsail Beach amounted to 433,500 m3/yr with an average transport of 217,291 m3 to the
north.
What is curious and difficult to understand is that despite the predicted gross transport
rate to the north, New Topsail Inlet has historically continued its migration to the southwest. In
fact, all previous inlets found in the area, including Old Topsail Inlet and Sidbury Inlet, migrated
to the south as well. Although inlets typically migrate in the direction of net littoral drift, there
are rare instances, in unique settings and conditions, where inlets migrate in the opposite
direction of net drift (AUBREY AND SPEER, 1984). It seems a combination of 2 factors proposed
by AUBREY AND SPEER (1984) have contributed to New Topsail Inlet’s continual up-drift
migration, 1) attachment of ebb tidal delta bars to the down-drift barrier spit (Topsail Island) and,
2) disrupted ebb tide discharge around the channel/barrier bend.
The formation, migration and welding of wave driven ebb delta swash bars as well as the
breaching and bypassing of elongated ebb delta channel lobes within New Topsail Inlet has
resulted in the episodic accretion of sand on Topsail Island. During the short study period from
1982-2003, no less than 4 bypassing episodes were observed, the largest of which took 9 years to
75
Figure 31. Images serve as photographic documentation of re-curved dune ridges on the southern
portion of Topsail Island. These ridges are one of the main components supporting long-term
unilateral migration
76
reach completion and added 180 linear meters of sand to the northern inlet shoulder. Although
the shoal did indeed weld itself to the southern tip of Topsail Island, it would be difficult to
assume that all 180 m of property were amassed simply by this bypassing episode as the shoal
took 3 years to move across the inlet throat. However, for historic perspective it is interesting to
note the bypassing episodes as a potential drive mechanism for inlet migration. Therefore,
assuming that the inlet opened in 1720, approximately 31 bypassing episodes of a similar nature
could have occurred, resulting in 5,700 linear meters of sand transported through the inlet.
In addition to the bypassing episodes, it is thought that another driving factor of New
Topsail Inlet’s up drift migration is a sedimentation pattern similar to what is present in
meandering rivers. In this scenario, channel curvature found within the inlet throat provides a
mechanism for migration by modifying bed shear gradients, causing the outer bank of the
channel to erode and the inner bank to accrete (AUBREY and SPEER, 1984). In this type of
depositional environment, the growth of the inner bank stems from the re-worked flood tidal
delta. In regards to morphology, New Topsail Inlet has been similar to a meandering river for
quite some time, and has both a steep outer bank (eroding shoulder of Hutaff Island) and an
accreting point bar on the inner shoulder (Topsail Island). Historically, New Topsail Inlet has
exhibited a similar morphologic pattern since the advent of aerial photography. Prior to that,
maps and charts of the area have continually shown Topsail Island with a long parallel channel
running its length, where previously deposited sediments could have been easily gathered for
deposition on the interior shoulder as the water flowed through the bend in the inlet throat.
77
Rate of Inlet Migration:
Due to the extensive photographic documentation of New Topsail Inlet, both long-term
and short-term inlet migration rates have been calculated. Obviously, the long-term rates
illustrate more established trends in the inlet’s speed and direction of migration. These changes
can then be paired with shoreline change data in order to determine how inlet morphology
changes cause changes to adjacent island planforms. On the other hand, the short-term data is
more useful in determining how the inlet responds to quick changes, such as bypassing events. It
also illustrates the inlet’s highly variable migration rates and frequent reversals in the direction of
migration.
Long-term Rates of Migration:
The migration of New Topsail Inlet is complex and it is difficult to speak about different
components singularly, however the variation in migration rates likely reflects changes in ebb
channel orientation, longshore transport rates, inlet width and changes in the tidal prism.
Between 1938 and 2006, New Topsail Inlet’s rate of migration ranged from 73.75 m/yr to 1.12
m/yr in a southerly direction and 29.75 m/yr to 2.41 m/yr in a northerly direction. The average
migration rates for the 16 intervals used within the long study illustrate the extreme variability in
migration rates. Despite the complexity of the migration mechanism it is evident that the ebb
channel migration rates generally decreased from 1962 to 2003 (Figure 11). Data taken from
more recent aerial photographs suggest that the migration rate of the inlet increased from 2003 to
2006. Long-term migration rates were grouped into similar rates of change and averaged in order
to understand how the speed of the inlet affected the morphology of the adjacent islands during
the study.
78
The historic aerial photographs collected for the study illustrate the changing
configuration of the inlet, ebb channel position, and adjacent oceanfront shorelines during
representative years between 1938 and 2006. Figure 7, a composite cartoon of the changing
adjacent island morphology, clearly illustrates the changes that have occurred along both Topsail
Beach and Hutaff Island since 1938. However, in order to comprehend the changes occurring to
the inlet complex, Figures 32-35 were used to illustrate the morphologic changes that occurred
within New Topsail Inlet and likely affected both its migration pattern and rate. Examination of
Figures 32 and 33 show that between 1938 and the late 1950s the morphology of the Hutaff
Island inlet margin was substantially different than in the early 1960s. By 1962, both barriers had
achieved a position of relative equilibrium that suffered minimal changes until 1995 when a
portion of the Hutaff Island inlet margin was eroded substantially. The erosion lead to the
exposure of a relict tidal delta (likely deposited by one of the previous three inlets that occupied
this coastal stretch), characterized by sand filled creek mouths and tidal marsh, and may have
accounted for decreased channel migration rates.
Short-term Rates of Migration:
As a result of the in-depth photographic documentation available, it was possible to
reveal how New Topsail Inlet migrates and changes on a sub-yearly basis. These small-scale
(time frame) changes have lead to an understanding of the relationship between channel
orientation, migration rate, and channel width. It has also illustrated how the inlet shoulders
respond to migration and bypassing events.
79
Figure 32. Composite image illustrating changes to the inlet’s morphology incurred over the
course of the study. The orange triangles indicate the position of the flood ramp and its migration
to its current position behind the Topsail Island spit.
80
Figure 33. Composite image showing the movement of the flood ramp (illustrated by the orange
triangles) from 1962 to 1974. During this period, increased curvature of the main inlet channel
began to limit space available in the back barrier.
81
Figure 34. Composite image showing the completed migration of the flood ramp from behind the
inlet throat, to its new position behind the Topsail Island Spit. It continues to change its
orientation so that it faces the incoming flood tide.
82
Figure 35. Composite image showing the final position of the flood ramp within Banks Channel.
At this point it has successfully oriented itself to face the incoming flood tide.
83
Both channel orientation and inlet width seem to have an effect on inlet migration. The
orientation of the channel and its migration rate has a direct relationship where the migration rate
slows down as the configuration of the channel moves to a lower azimuth (shore parallel
orientation). Figure 36 illustrates how the migration rate of the channel continually slows down
until a by-passing event occurs and the migration rate jumps back up. During this process, the
inlet continuously deposits sand in the ebb tidal delta, which, due to the nature of the inlet is
situated off the southern inlet shoulder as an extension of Hutaff Island. Over time, the dominant
northerly longshore transport deflects the outer portion/ ebb channel of the inlet to the north,
increasing the length of the main inlet channel, shifting the ebb tidal delta to a more central
location, and slowing down the rate of migration.
As time passes and the ebb channel continues its northward march, the orientation of the
channel causes the development of a large shoreline bump on Topsail Island. The protrusion is
built out through a combination of swash bar migration and welding as well as sediment
deposition on the leeward side of the ebb tidal delta. These built up portions of the shoreline do
not remain permanently, but soon become erosion “hotspots” after the inlet has shifted to the
south and no longer supplies sediment. During the buildup process, the channel reaches a near
shore parallel orientation and the inlet slows its migration rate.
Once the channel becomes hydraulically inefficient, or some outside factor takes place
(such as a large storm event), the ebb delta is breached, bypassing a large quantity of sand and
causing an acceleration in migration rate (Figure 36 and Figure 8). The change in channel
curvature not only increases the migration rate of the channel but also increases the accretion rate
of the northern inlet shoulder, likely due to the recently by-passed ebb-delta.
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Figure 36. Bar and line graph that illustrates the connection between channel migration and the
orientation of the ebb channel. Migration rates of the inlet slow as the ebb channels deflection
nears Topsail Island. After the cycle of deflection is completed and the ebb channel jumps to its
new orientation, the inlet increases its rate of migration.
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The inlet’s minimum width seems to have an inverse relationship with the inlet’s
migration rate. Although the correlation between these two components of the inlet complex only
develops for a short time prior to being interrupted by a bypassing episode, Figure 37 illustrates a
clearly developed inverse relationship from 1988 and 1994. Simple physics would tell us that
this relationship exists due to the need to get the same quantity of water from the back barrier
area through the inlet throat in the same amount of time available as when the inlet has a wider
channel. In order to accomplish this, the rate of flow must be increased and thus the amount of
friction would increase also. As such, one might think this would result in increased erosion of
the southern inlet shoulder and an increased migration rate. Although this scenario has been
documented at New Inlet (HASBROUCK, 2008), the direct relationship between inlet width and
migration rate is not as noticeably present at New Topsail Inlet. Perhaps the complexity of the
migration cycle prevents an easy link being established on a short-term basis.
Tidal Delta Changes:
In addition to location changes of New Topsail Inlet, morphologic changes to both the
flood and ebb tidal deltas have also occurred because of inlet migration.
Flood Tidal Delta
Although the flood tidal delta is not as prominent a feature at the ebb tidal delta within
the New Topsail Inlet complex, the changes in its location and morphology have been quite
dramatic. Initially in 1938, the flood ramp was situated nearly directly behind the inlet throat, in
a seaward facing position as illustrated through Figure 32, picture A. However progression of the
inlet to the south coupled with changes in the position of the main inlet channel changed the
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Figure 37. Line plots illustrate the inverse relationship between inlet minimum width and
migration rate. Although the relationship is frequently interrupted by jumps in the channel
position, small areas of continuity (from 1988-1994) distinguish its existence.
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interior morphology of the inlet complex, forcing the flood ramp to shift positions. Inspection of
Figures D2 A, B, C, D and D3 illustrate positional and morphological changes of the flood ramp
within the inlet complex. In Figure 32 A, B,C and D, the flood ramp remains in a central
position, close to the inlet throat. However changes in the morphology of the Hutaff shoulder
starting in 1956 (Figure 32, D) gradually force the flood delta further into the back barrier. The
continued elongation of the Hutaff shoulder through 1959 and 1962 (Figure 33, A and B) kept
the flood delta in a back barrier position, effectively lengthening the main inlet channel.
Unfortunately, the largest data gap occurs between 1962 and 1974 during which a
significant change to the position of the flood ramp occurs, moving from directly behind the inlet
throat to a position sheltered by the southern portion of Topsail Island and perched on the
previously formed marsh islands. Because of its new position, the orientation of the flood ramp
has deflected towards the south, directly facing the incoming flood tide. Speculation into the
change to the interior morphology might suggest that as the inlet progressed southward, it moved
into an area previously occupied by its predecessor, Old Topsail Inlet. Previously deposited flood
deltas (marsh islands) then caused a dramatic change in the shape of northern Hutaff Island and
increased resistance to channel migration. In fact, in the 12.38 years prior to 1962 (February
1956 – March 1962) the southern inlet shoulder eroded 98.1 m at a rate of 7.92 m/yr where as in
the 11.66 years subsequent to 1962 (March 1962- December 1974) the southern inlet shoulder
eroded at a severely decreased rate of 1.96 m/yr for a total of 22.9 m. The resultant shape change
(northern most tip pointing towards Topsail Island, Figure 33, C and D) to northern Hutaff Island
and the decreased erosional pattern of the inlet shoulder removed the space previously occupied
by the flood ramp (Figure 33, A and B), and forced it into its new position behind Topsail Island
(Figure 33, C and D).
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From 1982 onward, the continued migration of the inlet and the resultant changes to both
the back barrier and the main inlet channel caused the flood delta to be pushed father behind
Topsail Island (Figures 34 and 35). The increase in channel curvature, which first started in 1962
(Figure 33, B) but was exemplified in the early 1980’s and 1990’s (Figure 34) may have
contributed to the location of the flood ramp by limiting back barrier space between the now
exposed mash islands behind Hutaff Island and the southern portion of Topsail Island. As a
result, the flood ramp was positioned within Banks Channel at the start of the elongated main
inlet channel. It is quite evident that’s the increased curvature of the main inlet channel caused
the complete realignment of the flood ramp which sits perpendicular to Topsail Island and the
inlet throat (Figure 34 and 35). It is quite possible that the repositioning of the flood delta within
Banks Channel has contributed to the decrease in the inlet migration rate experienced in 1962.
Additionally, the movement of the flood ramp may be a contributing factor to the
increase in island width experienced by the southern portion or Topsail Island. The interior
position of the flood delta has led to the buildup and development of large channel margin bars
within and behind the inlet throat. The resultant position of the channel margin bars has lead to
bar welding on the interior shoreline of the island and contributed to the trend of island widening
seen since the beginning of the study. Maximum island width for the southern end of Topsail
Island has ranged from a minimum of 363m in 1938 to a maximum of 661m in 2003. Obviously,
the interior welding of channel margin bars was not the sole component which caused the
increase in island width, both sand bypassing episodes and swash bar welding as well as channel
position also contributed to the increase seen since 1938. However, the increase in channel
curvature and movement of the flood tidal delta contributed to changes in island planform.
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Ebb Tidal Delta
Between 1938 and 2006, the ebb tidal delta occupied areas from a minimum of 151 acres
in 1956 and a maximum of 336 acres in 2003 and had volumes ranging between 2.4 million m3
and 5.3 million m3. The time averaged surface area of the ebb delta was 256 acres with a volume
of 3.9 million m3. Figure 7 illustrates that up until 1978 large fluctuations or increases in the
area of the ebb delta were minimal, with the exception of 1956 when the surface area dropped
from 225 acres to 151 acres and the volume dropped from 3.5 million m3 to 2.4 million m3. The
large fluctuation was likely due to the combined effect of the landfall of Hurricane Hazel in
October 1954, Hurricane Connie on 8/11/1955 and Hurricane Diane on 8/17/1955, and the
subsequent readjustment of the inlet complex.
It is however, interesting to note that no other recorded storm events ever triggered such a
dramatic response from the inlet. In fact, it seems that other storm events, including, the great
Atlantic Hurricane (9/44), Bertha (7/96), Fran (9/96), Bonnie (8/98), Floyd (9/99) or any of the
numerous nor’easters had much impact on the inlet complex at all. The combined effect of
Hurricane Bertha, a category 2 storm, and Hurricane Fran, a category 3 storm, caused a decrease
in the surface area from 319 acres to 268 acres and a loss of 764,554 m3 between 6/27/1996 and
2/16/1997. Hurricane Bonnie, a category 3 storm, caused a decrease in surface area of 31 acres
between 8/21/1998 and 3/21/1999. Hurricane Floyd a category 2 storm had so little impact that
the ebb delta actually increased in size from 287 acres to 292 acres between the time period
3/2/99 and 4/5/2000. This is especially interesting in that storm events are known triggering
mechanisms for ebb delta breaching episodes, however in this location, they are not. It is
possible that the impact to the inlet complex viewed in 1956 was a result of the close succession
of three storms with the initial damage occurring during Hazel (category 3) and then continued
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by Connie (category 2) and Diane (category 1) which followed 10 months later and were less
than a week apart.
Since 1978, changes in the size of the ebb delta have continued to increase, with small
fluctuations occurring in recent years as a result of the aforementioned storm interference and
bypassing episodes. Prior to 1978, the time averaged surface area of the ebb delta was 198 acres.
Since 1978, the time averaged surface area has been 280 acres, indicating a significant increase
in size because of the changing inlet dynamics. Inlet minimum width, migration rate, and
channel orientation are the three primary factors that have likely influenced the increase in size
of the ebb tidal delta. As the functions taking place at New Topsail Inlet are complex, it is
important to note that although the three components are discussed singularly below, minimum
width, migration rate and channel orientation are all interlinked and influence the size of the ebb
shoal as a complex system.
A direct correlation and two inverse relationships describe how inlet minimum width,
migration rate, and channel orientation have affected the changes observed to the ebb shoal. The
direct relationship exists between the inlet’s minimum width and the ebb delta, where as
migration rate and channel orientation have inverse relationships. As the inlet continued its
progression to the south, back barrier areas and tidal prism expanded. The increased tidal flow
leads to gradual inlet widening and an increase in the inlet’s minimum width over time. Figure
38 indicates that because of the increased tidal flow, the inlet moved more sand from the back
barrier to the ebb shoal or had greater sand trapping abilities for sand entering the inlet complex
via longshore transport.
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The general decrease in migration rate that has occurred since 1962 has lead to the
general increase in size of the ebb tidal delta, while at the same time, the ebb tidal delta has
responded to changes in migration rate in an inversely correlated fashion. Figure 39A indicates
that typically larger delta sizes are associated with lower rates of migration. This is especially
true after the inlet started taking on its modern morphology in 1962. This correlation is of no
great surprise as the slower the inlet moves along its migration pathway, the longer it will have to
build a larger ebb shoal. There are however a few exceptions, such as the pairing between the
smallest ebb delta size of 151 acres and the slowest period of migration which occurred from
1949 – 1962 (Figure 39B). The strange relationship between the very slow migration rate and the
small ebb delta was likely due to the previously discussed storm activity during this period and
the resultant destruction of the ebb shoal.
The second inverse relationship that exists relates the increase in ebb delta size to main
inlet channel orientation. Figure 7 clearly illustrates the relationship between ebb delta size and
channel orientation where the ebb delta is larger when the main inlet channel occupies a lower
azimuthal position and exits the inlet complex in more shore parallel position. The relationship
can then be linked to the migration rate as the lower azimuthal positions occur during the slowest
periods of migration (Figure 36). The geometry of the ebb shoal post 1962 was primarily
asymmetrical, with the main inlet channel separating the northern and southern portions of the
delta. The northern portion of the delta is typically much smaller than its southern counterpart,
and portions of it may weld to Topsail Island, forming small shoreline progradations. The
shoreline bumps typically build out as swash bars weld to the island and via direct deposition of
sand carried out of the inlet throat. The southern portion of the ebb delta is typically and
extended shoal which may or may not extend from the northern portion of Hutaff Island, out
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Figure 38. Line plots for the inlet’s minimum width and the acreage of the ebb delta show the
direct relationship between the increase of the channel minimum width and the size of the ebb
delta.
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Figure 39. A) Line plots illustrate the inverse relationship between the overall slowing migration
of the inlet and the increase in ebb delta size. B) line plots display migration rates to show the
corresponding link between inlet migration rate and ebb delta size.
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across the center line of the inlet throat (not the main inlet channel). The asymmetrical geometry
of the ebb shoal acts as a breakwater for Topsail Island and amplifies the progradation taking
place on the southern tip of the island.
Changes in the inlet morphology since 1962 have primarily allowed the above to be the
dominant configuration of the ebb delta in recent times. Between 1962 and 2006, only one
interval from 1974-1982 had a different ebb shoal configuration for an extended period of time.
Between 1974 and 1982, the main inlet channel had an extremely limited amount of curvature,
and exited the inlet throat pressed against Hutaff Island, causing the northern portion of the
shoreline to build out (Figure 33, C and D, Figure 34, A). In fact, transects on the northern end of
Hutaff Island recorded a net gain across the board, from T-31 to T-37. The highest build out was
recorded at T-32 which prograded a total of 192.2 m at a rate of 25.9 m/yr. T-37, which was
located 1.1 km away from the main inlet channel in 1982, recorded the smallest amount of
accretion, but still prograded 36.8 m at a rate of 4.9 m/yr. The ebb delta orientations which
facilitated the large progradation, along with historic shorelines and channel locations are
presented in Figure 40. Figures 41 and 42 illustrate previous and subsequent ebb shoal
configurations that have been more typically present post 1962 and generally assist accretion on
the southern tip of Topsail Island. The extremely short but rapid accretionary period experienced
on Hutaff Island is a perfect example of how inlets can dramatically affect shoreline change and
why building codes in inlet hazard areas need to be maintained.
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Figure 40. Map of the study area illustrating the ideal ebb delta and channel configuration for
accelerated shoreline advancement on Hutaff Island.
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Figure 41. Map of the study area showing positions of the ebb delta and channel orientation prior
to the large protrusion build out on Hutaff Island and illustrates a more typical delta
configuration.
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Figure 42. Map of the study area showing the ebb delta configuration subsequent to the
configuration favored for shoreline advancement on Lea/ Hutaff. These deltas illustrate a more
typical orientation of the ebb channel and favor build out on the Topsail Island shoreline.
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Oceanfront Shoreline Change
Throughout the course of the study, changes to both Topsail and Hutaff Islands have
primarily been related to inlet migration and changes to the inlet’s morphology. Changes in the
inlet’s migration rate have not only affected the erosion experienced by the northern shoulder of
Hutaff Island, but have also influenced erosion rates on Topsail Island, where chronic erosion is
the norm. Planform changes on both islands have been directly related to the inlet’s southward
march, ultimately adjusting the geometry of both the leading and trailing barriers. Figure 6
illustrates the dramatic changes in barrier planform experienced by Hutaff Island and also
illustrates how the lengthening of the trailing barrier (Topsail Island) and updrift shoreline
truncation has caused erosion to become commonplace along Topsail Beach.
Paired with the migration of the inlet complex, the changing morphology of the inlet,
specifically the deflection and realignment of the ebb channel, caused dramatic shoreline change
fluctuations to adjacent barriers. Of the varying alignments experienced by the ebb channel, the
time limited shore normal alignment lead to a nearly symmetrical ebb shoal, which resulted in
accretion occurring on both Hutaff and Topsail Islands. On the other hand, the far more common
alignment with the ebb channel deflected to the north favors accretion on Topsail Island and has
lead to the buildup of several large shoreline protrusions over the course of the study. Not only
does the skewed ebb channel lead to accretion of the northern shoulder, but the associated
asymmetrical ebb shoal leaves Hutaff exposed to oncoming wave energy out of the southeast.
Furthermore, because of the skewed morphology of the inlet, the southern marginal flood
channel develops off the northern tip of Hutaff and has lead to rapid erosion of the inlet shoulder
and adjacent oceanfront shoreline.
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Changes that occurred during the study period were broken into sections based on both
inlet migration rates and major changes seen in inlet morphology. Both net change and
cumulative net change were calculated for shoreline changes. Net change intervals for Topsail
Island have shown that for the history of the study, transects outside the zone of direct inlet
influence, which ranges from ~1200 m to ~1600 m, are subject to chronic erosion. Net change
rates within these shoreline reaches varied from a maximum of -2.1 m/yr from 1938-1949, to a
minimum of -0.3 m/yr from 1974-1999. Unfortunately net change intervals do not illustrate a
direct relationship to migration rates of the same interval and are generally of no use when
measuring shoreline change along the Topsail reach as they do not take into account localized
progradation or beach nourishment activities. Accretion and erosion was recorded during all net
change intervals on Topsail Island.
Shoreline Change Intervals
1938-1949
It is difficult to speculate as to what caused shoreline change during the early throws of
the study. Inspection of the photographic database indicates that the 1938 Topsail shoreline had
just been subject to a small period of accretion, which resulted in a shoreline bump near T-16.
Post Topsail build out, the main inlet channel shifted south and caused the buildup of the Hutaff
shoreline in a fashion similar to the period from 1974-1982. Between 1938 and 1949 erosion was
commonplace along the northern reach of the Topsail shoreline, which spanned T-1 through T10 and was located ~1130 m north of the inlet throat. The average erosion rate along the northern
reach was 2.1 m/yr during 1938-1949. However, erosion rates varied significantly between 1938-
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1945, which had an average rate of -3.97 m/yr, and 1945-1949, which had an average rate of 0.51 m/yr. The increased rates of the 38-45 period and likely linked to the Great Atlantic
Hurricane, which made landfall in the area on 8/1/1944.
Adjustment of the main inlet channel within the inlet corridor between 1938 and 1945 led
to a change in symmetry of the ebb tidal delta and may have caused a small bar welding event to
occur on Topsail Island, which caused the transgression of the shoreline an average of 10.7 m
between T13 and T-16 at a rate of 3.28 m/yr. It is also at this point in time that the trend of island
widening, recorded through the rest of the study, begin. The development of the small bulbous
shape at the southern tip of Topsail Island caused the shoreline to prograde 15.1 at T-15 and 17.4
m at T-16. Further movement of the inlet channel to the south between 1945 and 1949 lead to a
major restructuring of the ebb shoal which facilitated continued progradation of the Topsail
shoreline; transects 11-16 experienced as much as 22 m of shoreline change. The expansion of
the ebb tidal delta extended its wave sheltering effect as far north as T-11 and allowed T-11
through T-13 to infill shoreline areas previously left behind during the shoreline bump build out
of 1938-1945. Development of a large marginal flood channel prevented further accretion from
occurring on transects 14 or 15.
Change in the channel position between 1939 and 1945 lead to the complete truncation of
the northern tip of Hutaff Island. Areas previously protected by the ebb shoals wave sheltering
breakwater effect, were left exposed to oncoming wave energy and were further scoured away by
the development of a large marginal flood channel. Shoreline change on Hutaff ranged from a
minimum of -6.6 m at a rate of ~ -1 m/yr near the center of the island, to a maximum of -114.5 m
at a rate of -17.4 m/yr along both the northern and southern flanks of the island. A shift in the
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main inlet channel or the ebb channel of Old Topsail Inlet was the likely culprit for the extreme
erosion, which occurred on the southern portion of the island.
Further migration to the south by New Topsail Inlet’s main channel between 1945 and
1949 resulted in additional shoreline loss along Lea/ Hutaff Island. Shoreline change was likely
accelerated by the location of the inlet channel as it exited the inlet corridor pressed against the
southern inlet shoulder in a direction of 165º. Shoreline loss during the period ranged from 15.4
m to 56.8 m. Although extensive shoreline loss was recorded, the extent of the ebb shoal did
allow some accretion to occur at T-28 through T-30. The overall realignment of the main inlet
channel between 1938 and 1949 had a decimating effect on Hutaff Island geometry and
essentially changed the island from a small, concave, drumstick shape to a longer, linear
planform model.
1949-1962
Coupled with inlet related changes, several events, which affected shoreline change, took
place during this period. Specifically, a sequence of three documented hurricanes made landfall
within a ten month period between 10/1954 and 8/1955, and the Jolly Roger fishing pier was first
constructed in 1954 (MCALLISTER, 2006).
Shoreline change along the Topsail reach was characterized by three shoreline change
zones, two zones of accretion separated by a zone of erosion. The first accretion zone was
located along transects 1-4 and encompassed the newly constructed pier; the second zone of
accretion was located within direct influence of New Topsail Inlet and occupied transects 14-17.
The shoreline reach between T-5 and T-13 was predominately erosional although some accretion
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did occur along T-5 and T-6 because of pier construction. The shoreline build out observed at T5 and T-6 between 1949 and 1959 was short lived and later reversed between 1959 and 1962,
negating nearly all shoreline progradation. Shoreline changes between 1949 and 1962 amounted
to -1.6 m at T-5 and less than -0.5 m at T-6. Oceanfront changes at T-13, located at the southern
end of the erosional reach, amounted to -0.7 m as a result of inlet induced progradation from
1948-1956 and the resultant truncation as the inlet migrated to the southwest and change in ebb
cannel orientation. Ultimately, the change recorded at these two locations was the smallest
difference recorded along the Topsail Island reach.
The average total loss of the reach between T-5 and T-13 was -14.1 m at a rate of -1.1
m/yr. Although there was a high frequency storm period from 1954-1955, it was not visible in
the record between datasets as erosion rates from 1949-1956 did not differ greatly from those
recorded between 1956-1959 or 1959-1962. Interestingly, Figure 18, plots B and C, illustrate that
two transects within the reach eroded significantly more than surrounding transects. T-11
suffered a total of -30.7 m of shoreline change and the oceanfront at T-12 was eroded 22.6 m
where as the cumulative loss just outside the small reach was 19.9 m and 0.7 m respectively.
This accelerated loss was likely due to the truncation of the trailing shoreline after inlet induced
buildup occurred during 1945-1949 and is in effect a mini erosion “hotspot”.
What is interesting about the northern accretion zone is that its development was entirely
dependent on the construction of the Jolly Roger fishing pier. Transects 1- 4, which previously
experienced erosion rates as high as 4.9 m/yr and an overall average loss of 4.0 m/yr, had
accretion rates as high as 3.0 m/yr at T-3 (directly south of the pier) and 2.3 m/yr at T-2 (directly
north of the pier). It is obvious that the construction of the pier played a part in the accretion
experienced along this shoreline reach, however if the accretion was due to the pilings
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interrupting longshore sediment transport or if the accretion was a result of material placed on
the beach as part of construction remains to be seen. Another interesting bit of information
surrounding the pier is that it was completely destroyed by Hurricane Hazel in 1954, and many
of the pilings snapped off at or near sand level (MCALLISTER, 2006). It would have been
interesting to see if the remaining portion of the pilings the acted at as a groin during Hurricane
Hazel and the subsequent two storms, interrupting longshore transport and facilitating positive
shoreline change during these events. By 1962, the accretion zone at T-1 through T-4 had nearly
disappeared, and T-3 and T-4 were the only transects to record positive shoreline change from
1959 -1962.
Inlet related changes occurred along the southern portion of the shoreline reach between
T-13 and T-17 from 1949-1956, and between T-13 and T-19 from 1956 -1962. Changes in the
orientation of the ebb channel from 1949-1956 caused a decrease in the size of the ebb delta
shoal and skewed the position of the shoal in a northerly direction. The new orientation of the
ebb channel and the skewed ebb shoal lengthened the period of accretion occurring from T-13 to
T-17 and lead to a 56 m advancement of the shoreline at T-16 and a 42 m advancement at T-15
(Figure 18 plot D). The continual build up of the shoreline reach from T-14 to T-16 will lead to
the chronic erosion of this area in the future and the development of the first problematic erosion
“hotspot”.
Continued migration of the inlet complex during 1956-1962, coupled with a shoal
bypassing episode during late 1959, extended Topsail Islands measurable shoreline by two
transects and caused the northern inlet shoulder to migrate ~390 m to the south. The last period
of accretion for the shoreline reach between T-15 and T-16 occurred from 1956-1959 when
minimal growth was recorded by both transects. T-14, which was located to the north of the
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inlet’s protective zone, eroded 15.3 m from 1956-1959. By 1962, the shoreline reach
encompassed by transects 5-16 experienced net erosion and the greatest changes were recorded
at T-15 and T-16. As illustrated by Figure 18.2 D, the migration south and subsequent shoreline
truncation caused the beach at T-15 and T-16 to erode an average of 30.4 m at a rate of -11.7
m/yr. New shoreline growth moved southward with the migration of the inlet complex, and
caused an average shoreline advancement of 78.3 m at T-18 and T-19 which accreted at a rate of
12.8 m/yr from 1956-1962.
Near reach wide erosion was experienced along the shoreline reach of Hutaff Island with
the exception of T-36 from 1949-1962. The accretion at T-36 amounted to a 48.9 m shoreline
advancement that was a direct result of Old Topsail Inlet’s migration and channel realignment.
Transects 27-35 eroded an average of 48.3 m and the greatest loss was experienced at T-25
which was subject to the development of a large marginal flood channel from 1959-1962.
The period from 1938-1962 was the largest length of time that cumulative data could be
collected along an extensive shoreline reach on Hutaff Island. The encompassing tidal inlets and
their subjective migration pathways severely limited the amount of contiguous shoreline
available for data collection. Rapid migration and photographic data limitations from this period
forth limited all other datasets to a small number of transects. As illustrated through Figure 25,
Hutaff Island experienced severe erosion across all transects from 1938 -1962, with losses
nearing 200 m at the flanks of the island. As the Hutaff Island shoreline tract was limited to ~
1500 m it is difficult to assess the cause of erosion near the center of the island, where as it is
quite clear that adjustments to inlet morphology were responsible for the accelerated erosion at
the islands flanks.
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1962-1974
Unfortunately, as previously mentioned, a large gap in the photographic database exists
between 1962 and 1974. Not only did this limit observation of inlet change during the period, but
it also limits the amount of shoreline change that can be calculated. Because of the data gap, ~
500 m of new oceanfront shoreline could not be analyzed. That being said, the length of the
shoreline reach and the lack of significant storm activity during the time period yield typical
shoreline change rates.
The shoreline change zone on Topsail Island stretched from T-1 to T-18 and generally
experienced net erosion. The average loss over the shoreline reach was 12.2 m at a rate of -1.05
m/yr. Averaged rates included accelerated rates recorded from the erosion hotspot located from
T-14 to T-16 where erosion rates ranged from 1.96 m/yr to 2.59 m/yr and losses totaled 22.8 m
to 30.2 m. From 1959-1974 transects within the erosion hotspot lost an average of 49.3 m of
beachfront at a rate of -3.47 m/yr. T-16 which suffered the highest loss, suffered 64.8 m of
shoreline change at a rate of -4.5 m/yr. Although erosion rates in this area slowed from 19741978 and some accretion occurred on T-14 and T-16 it was not due to natural changes. Instead,
the decreased rates of erosion and small periods of accretion were likely linked to the
construction of a small groin field on the southern end of Topsail Island or the undocumented
placement of beach fill.
Accelerated rates of erosion continued to plague the small transect reach through 1986
when frequent nourishment projects were used to negate the ongoing erosion. However, by this
time, cumulative losses experienced from 1959 -1986 resulted in an average shoreline retreat of
95.4 m. The continual erosion of the shoreline reach from T-14 to T-16 continued on through the
rest of the 80’s and 90’s and is still a problem for the Sea Vista motel, which was built during the
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construction boom of the late 60’s. During the time of construction, the beach in front of the
motel was more than 100 m wide.
To the south of the 1962-1974 erosional reach, build out of the shoreline caused 44.36m
of accretion to occur at a rate of 3.8 m/yr at T-19. The accretion at the area was likely a result of
swash bar welding. By 1974 the Topsail Island planform had extended 387 m past the 1962 inlet
shoulder and spurred new construction on the southern end of the island. Three finger canals,
which exit into Banks Channel, were also excavated into the backside of the extended spit.
Shoreline change data for Lea/ Hutaff Island was recorded over a shoreline reach
encompassed by T-29 to T-39. Nearly all transects recorded net accretion during the time period.
The overall average shoreline change recorded was 39.9 m at a rate of 3.4 m/yr. These average
changes included the net erosion, which occurred from T-36 to T-38. Shoreline advancement
totals for T-29 to T-35 ranged from 13.9 m at T-35 to 113.6 m at T-29. Increased rates of
accretion were likely spurred by the realignment of the ebb channel at174º and the resultant
positional change of the ebb shoal. Migration of New Topsail Inlet into the previously occupied
inlet pathway of Old Topsail Inlet resulted in a dramatic change to the geometry of Hutaff Island
(Figure 6). The progression of the inlet to the south has also lead to the pirating of back barrier
tidal creeks which were previously part of the Old Topsail Inlet tidal prism.
1974-1999
In a similar fashion to the RESULTS, the time covered by this interval has been broken
down into smaller intervals so shore line changes can be discussed in more detail. The mini
intervals are 1974-1982, 1982-1990, and 1990-1999. As stated in the RESULTS section, the
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changes during the times discussed primarily deal with the southern portion of Topsail Island and
the current IHA.
1974-1982
During this time interval, shoreline change data for the Topsail reach was collected along
transects 14-22. The migration of the inlet channel 311 m to the south and the southern
orientation of the ebb channel, which ranged from 173º to 180º, lead to the extension and
truncation of the southern portion of Topsail Island. As previously discussed, extensive shoreline
erosion has been the norm for the northern transects located on Topsail Beach, and although this
is the southern portion of the Topsail shoreline, cumulative net erosion of 17.1 m was recorded
over transects 14 – 19. The transitional apex between erosion and accretion was located between
T-19 and T-20, approximately 1200 m north of the inlet centerline and ~1400 m north of the
main inlet channel. Accretion values ranged from 14 m at T-20 to 105.5 m at T-22 and were
likely a direct result of inlet proximity and infilling of previous island curvature near the inlet
shoulder.
The overall trend of accretion displayed in Figure 20 was likely due to the construction of
a small groin field between T-20 and T-15 or undocumented nourishment activities related to
maintenance dredging of Old Topsail Creek or Banks Channel. The subsequent trend of erosion
from 1978-1982 may have been related to the removal of the groin field, but is more likely due
to the natural truncation of the island, stemming from both inlet migration and channel
alignment.
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The southern alignment of the inlet channel during this period led to extensive build up of
the shoreline on the Hutaff Island. In a similar fashion to what occurs along the Topsail Island
oceanfront, the alignment of the inlet caused a large protrusion to form on the northern tip of the
island. The shoreline bump initially spanned transects 29-31 in 1974, but migrated south as the
inlet continued along its route and the ebb channel changed orientation. The bump was
subsequently positioned between T-30 to T-32 in 1978 and T-31 to T-36 in 1982, when
expansion of the bump lead to extensive progradation occurring from 1974 to 1982. The average
shoreline transgression during this period was 108.5 m, but ranged from a maximum build up of
192.2 m at T-32 and a minimum build up of 36.8 m at T-37. The period from 1974 to 1982 was
the last extensive building period (Figure 26) on Hutaff Island as the inlet’s morphology soon
changed completely.
1982-1990
Cumulative changes from 1982-1990 were recorded along the Topsail Island shoreline by
transects 17- 24. All transects along the shoreline reach recorded cumulative accretion during the
time period. Cumulative net accretion ranged from 16.3 m at T-17 to 200.9 m at T-24; gradual
increases in the amount of seaward movement were recorded as the inlet corridor was
approached. Although cumulative changes along the shoreline reach resulted in net accretion
across the board, shoreline change plots in Figure 21 indicate that erosion is still commonplace
Topsail Island. The up down behavior and sharp increases in shoreline position displayed by the
plots is most likely a recorded of small nourishment projects or placement of dredged material on
the beach. Information from the USACE indicates that between 1982 and 1988 four small-scale
nourishment projects took place on Topsail Beach. Maintenance dredging of Old Topsail Creek,
110
Banks Channel and the AIWW yielded the total disposal of ~ 272,182 m3 of beach fill material
along the islands oceanfront shoreline while two small scale nourishment projects from 10/85 12/85 and from 1/88-9/88 involved the placement of ~ 45,875 -115,450 m3 along Topsail Beach.
The placement of the above beach fill coupled with several natural events that built out the
shoreline, lead to the reversal of previously discussed trend of shoreline retreat.
In addition to the documented nourishment activity a large bypassing episode and
complete channel realignment occurred during the time interval. Breif periods of shoreline build
up caused by swash bar welding and the northward orientation of the ebb channel were recorded
along T-20 and T-21 during 1985 and 1986. Despite the welding events in the record,
quantifying shoreline change as a result of inlet influence during this period is nearly impossible
as the nourishment projects placed along the shoreline reach are subject to inlet influence and do
not sustain extended lifetimes. However, inspection of the photographic database makes it clear
that that much of the shoreline advancement during the late 80’s and early 1990 was a result of
the bypassed ebb shoal welding to the southern portion of Topsail Island.
Interestingly, in addition to the accelerated accretion, which took place along the
shoreline reach, two storms worthy of mention passed through the study area. The first was a
large-scale nor’easter, which took place over the 28th and 29th of March, 1984, and was
associated with several tornadoes in eastern North Carolina. The second was a category 2
hurricane (Diana) which made landfall south of Cape Fear on 9/12-9/13 1984. The nor’easter
caused accelerated erosion rates over the shoreline reach encompassed by T-17 to T-24 and
caused time averaged erosion rates ranging from 5.75 m/month to 21.9 m/ month. The average
loss over the shoreline reach was 12.2 m/month (Figure 21b), but storm induced damage was the
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likely culprit for all recorded erosion. On the other hand, accretion was recorded on 7 out of 9
transects during the time in which Hurricane Diana made landfall (04/1984-10/1984).
The Hutaff data set for 1982-1990 was limited to a small shoreline reach of ~ 600 m and
4 transects (T-31 through T-34), a result of limited photographic coverage. Although the
shoreline reach was limited, data clearly showed that erosion was the norm for the leading barrier
islands oceanfront. Inspection of the photographic database clearly indicates that the erosion
suffered by the Hutaff shoreline was related to inlet migration and the changing orientation of the
ebb channel. As a result of the previous build out during 1974-1982, transects nearest to the inlet
corridor (T-31 and T-32) experienced the largest change in oceanfront position and receded and
average of 156.6 m. Time averaged change rates recorded over the shoreline reach ranged from
less than -1.0 m/month to -13.1 m/month. As indicated by Figure 28, periods of accretion did
occur during the interval; however, they were greatly outweighed by the ongoing erosion.
Shoreline loss on Hutaff during this period was primarily related to inlet migration to the
south, channel deflection to the north and the corresponding changes to the morphology of the
ebb tidal delta. The limited to non-existent wave sheltering effect of the ebb shoal, paired with
the expansion of the marginal flood channel along the Hutaff oceanfront was generally
responsible for shoreline retreat on the northern portion of the island. The periods of accretion
illustrated in Figure 28 (transects 31 and 32, and 33-34 to a lesser extent) were caused by small
bar welding events and the expansion of the ebb shoal off the northern tip of the island.
1990-1999
The continued progression of the inlet throat on its southward trek during this period was
paired with both and increase in island width and island length as well as the resultant island
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truncation along Topsail Island. Coupled with inlet migration, changes to the oceanfront
shoreline resulted from ebb channel deflection and its eventual realignment, a small bypassing
episode which began in 1994, placement of beach fill from maintenance dredging operations, the
land fall of three large hurricanes, and the resultant emergency beach nourishment projects
subsequent to storm damage. The cumulative net effects of resultant change were recorded along
a shoreline reach ~ 1200 m long and through T-18 to T-25. Shorter periods of shoreline change
were recorded over longer reaches but were dependent on photographic documentation.
Over the Topsail shoreline reach, transects 18-24 recorded a net loss of beachfront which
ranged from a maximum of 31.8 m at T-21 to a minimum of 7.6 m at T-19. The average loss for
the shoreline reach was 19.4 m. Transect 25, the closest transect to the inlet complex was the
only transect which recorded cumulative net accretion.
Accretion, which was recorded during the early portion of the time interval, was likely
due to the tail end of a large welding event, which occurred near the end of 1989, and the
possible placement of dredge materials along the Topsail oceanfront. Subsequent erosion during
1990-1991 was a likely result of changes to the position of the main inlet channel and its average
heading of 157º. The ebb channel remained at this heading until late 1991 when it started its
cyclic deflection to the north. Reach wide net accretion, which soon followed, was likely caused
by the continued deflection of the ebb channel from 11/1993-01/1995 and the accompanied
deposition and welding of swash bars. Transects 16-24 recorded a mixture of both accretion and
erosion during this short time period. The average shoreline advancement recorded from T-16 to
T-24 was 37.2 m and ranged from a maximum of 56.7 m at T-24 to a minimum of 24 m at T-21.
Time averaged change rates ranged from 12.1 m/month to less than 1 m/month.
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Reach wide shoreline erosion occurred during two periods during the interval, the first
was from 06/1996-09/1996 and the second was from 08/1998-03/1999. Both periods were
subsequent to hurricane landfall, hurricanes Bertha and Fran during 06/96-09/96 and Hurricane
Bonnie during 08/19/-03/99. A third period of near reach wide erosion occurred from 10/199105/1992, and was likely due to an undocumented winter storm event. The average shoreline
retreat during 06/96-09/96 was 33.8 m and had losses ranging from 78.5 m at T-25 to 12.5 m at
T-21. The time averaged losses during this period ranged from 26.1 m/month to 4.1 m/month.
Storm damage losses for Hurricane Bonnie were not as extensive as the average reach wide loss
was 20 m and the maximum amount of shoreline retreat was 33.1 m. In unassociated events, an
average retreat of 16.6 m was recorded by the shoreline reach encompassed by T-17 to T-24
from 10/1991-05/1995, at rates, which ranged from -24 m at T-20 to -4.1 m at T-24. Meanwhile
the build out of the shoreline occurred at T-25 and T-26 because of swash bar welding and the
infill of a previously eroded inlet shoulder.
Data from the USACE indicates that the total documented volume of beach fill placed on
Topsail Beach from 1990-1999 was ~298,940 m3. Inspection of the both the shoreline change
data and the photographic database indicate that there were at least two beach fill projects; the
first, subsequent to shoreline loss recorded from 10/1991-05/1992, and the second as an
emergency fill project post Hurricane Fran. Fill from the first project (10/1991-05/1992) was
placed between T-12 and T-17. Unfortunately, data gaps in the shoreline database prohibited the
amount of buildup, which occurred from being recorded, but cursory measurements indicate that
~ 45 m of change occurred between the HWL in 1989 and the HWL at the end of the project. As
a portion of the project took place within the current IHA, much of it was quickly eroded and
inlet influences soon dominated changes in the area. In a situation similar to that described
114
above, much of the fill operation which took place post Hurricane Fran was not documented,
however the average progradation over all transects (T-18 to T-25) was 26.5 m during the six
month period in which the project took place. It is quite likely that the net loss recorded during
1990-1999 would have been much higher had these operations not taken place.
The shoreline change record for Hutaff Island for 1990-1999 is very limited as T-34 and
T-35 were the only transects in continual existence (with shoreline data) for the period.
Inspection of Figure 29 illustrates that the first shoreline setback corresponds to the start of ebb
channel deflection in 10/1991, the subsequent expansion of the marginal flood channel and the
shifting of the ebb shoal to the north. The first significant period of shoreline accretion took
place during 6/1996 and was a direct result of ebb channel realignment. The channel shift from
136º to 173º, which occurred between 11/1994, and 01/1995 was associated with several bar
welding events. The continued periods of accretion and erosion exhibited by the plots in Figure
29, from 1996 to late 1998, were the result of continual changes to the morphology of the ebb
delta. When the ebb channel began its second deflection to the north in 08/1998, erosion again
became the norm for the shoreline reach.
In addition to the shoreline changes caused by the inlet channel, migration of the inlet
throat caused large changes to the planform of Hutaff Island. As the inlet progressed south,
continued erosion on the southern inlet shoulder caused a breach between Hutaff and previously
deposited, back barrier mash islands (Figure 43). The breach eventually lead to the
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Figure 43. Composite image illustrates changes to the Hutaff Island planform incurred due to
inlet migration and the subsequent breach between Hutaff and previously deposited marsh
islands in the back barrier.
116
closure of Old Topsail Inlet in 10/1998 (CLEARY and MARDEN, 1999) as its main feeder channel
became part of the New Topsail Inlet tidal prism. The increase in tidal prism had a direct effect
on the shape of the inlet throat and increased its minimum width from 483 m in 11/1993 to 700
m in 11/1994.
1999-2006
The most recent period of recorded oceanfront change was similar to previous time
periods and was characterized by inlet migration, channel deflection, island truncation and beach
nourishment operations. Cumulative oceanfront change observations were recorded over a ~
1200 m shoreline reach and by transects 18-27. Long-term shoreline change data was also
available for the period and allowed positional changes to be observed over a longer reach
between T-6 and T-26. Unfortunately, the gap between 03/2003 and 01/2005 was too long to
record short-term changes which occurred to a nourishment project that was underway in March,
2003.
Near reach wide accretion was the net cumulative change recorded by the shoreline
encompassed by T-17 to T-27. Shoreline advancement ranged from 10.1 m at T-19 to 244.8 m at
T-27. The amount of accretion sustained by each transect increased with proximity to the inlet
(Figure 23). Transect 18, which was located on the cusp of inlet influence, was the only transect
where net erosion was recorded.
A similar trend was displayed by the long-term shoreline change data, where T-19
through T-26 also recorded net accretion. The long-term accretion values ranged from 10.3 m at
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T-19 to 163 m at T-26. Shoreline retreat was recorded by all other transects within the extended
shoreline reach (T-6 to T-18) despite the fact that as much as 63 linear meters of beach fill was
placed along Topsail Beach in 2003. Accelerated erosion values as high as 28 m/yr at T-14
indicate that the project was short lived and likely influenced by the proximity of New Topsail
Inlet.
Limited movement of the ebb channel during this period, which had a directional average
of 138º (range of 147º-127º), coupled with a total channel migration of -30.2 m caused a
dramatic change to the planform of Topsail Island. Changes facilitated by ebb channel
orientation included the buildup of a large shoreline protrusion near T-24/T-25, and caused an
increase of 88 m to the islands width from 1999-2003. Accretion on the southern tip of Topsail
Island was likely influenced by both channel position and the wave sheltering effect of the ebb
shoal. The accelerated buildup rates may have been further influenced by the placement and
immediate erosion of the large beach fill project directly north of T-18. Further advances of the
shoreline position from 2003 to 2006 at T-26 and T-27 were the result of infill between the inlet
shoulder and the large bump created from 2002-2003.
Changes along Hutaff Island during the period were recorded along a very limited
shoreline reach. Cumulative changes were only available for T-35 and T-36 as the shoreline in
front of T-34 eroded completely between 10/2003 and 01/ 2005. Although erosion was the
dominant shoreline change factor from 1999-2006 small period of accretion also occurred. The
shoreline advancement, illustrated by Figure 30, during 2001 was a result of ebb swash bar
welding, as was the buildup during 2002. As there were no ebb-channel realignment events
during this period, erosion along Hutaff Island was likely caused by the deflection of the ebb
channel to the north and the loss of the breakwater effect of the ebb shoal. In a fashion similar to
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the previous periods, development and expansion of the marginal flood channel was the likely
culprit for the recorded erosion along the shoreline reach and the planform changes to the Hutaff
inlet shoulder.
Future Changes and Implications
The long history of inlet migration and the increase in migration rate from -29.7 m/ yr
during 2002-2003 to 21.4 m/yr during 2003-2006 suggest that New Topsail Inlet will not stop its
migration to the southwest anytime soon. It is likely however, that migration rates will remain
similar to those of the recent past as it continues to move through areas previously occupied by
Old Topsail Inlet. The continued migration of the inlet will likely lead to an increase in its tidal
prism and therefore an increase in the average minimum width; a trend, which has been observed
throughout its history. In fact, the inlet’s minimum width increased by ~ 75 m during the last
three years of the study. As discussed, the increase in width will lead to an increase in both the
inlet’s sand trapping capabilities and the size of the ebb shoal, ultimately leading to a larger area
subject to inlet influence.
The deflection and realignment of the ebb channel will continue to affect both the leading
and trailing barriers. Although previous bypassing events ranged from 3 to ~7 years it is difficult
to predict when the next one might take place. What is clear though is that as the inlet continues
to migrate, shoreline truncation of the trailing barrier (Topsail Island) will occur.
The consequence of inlet migration will remain the same as it has in the past and will
ultimately result in shoreline retreat. Although erosion has occurred along the length of the
Topsail Shoreline reach, areas of maximum erosion will continue to occur in the same locations
as periods of maximum accretion. The issue here is that previously wide stretches of beach front
119
that assured home builders that their houses would be safe will soon erode as shoreline
regression continues. Currently, homes built to the southeast of the southernmost finger canal are
of concern. These houses will likely face fates similar to the Sea Vista Motel (transect 15), who’s
continued presence on the beach has been entirely based on periodic beach nourishment projects.
In fact, several of the homes on the eastern side of the road are already dangerously close to the
2006 HWL. The continued migration and truncation of the island will soon eroded the last of the
recurved dune ridges protecting these structures and they will likely be exposed to the open
oceanfront in five to ten years.
Erosion mitigation along Topsail Beach will continue to remain a problem for the town as
shoreline retreat continues to threaten structures along the oceanfront. Previous mitigation efforts
undertaken by the town have proved to be short lived and ineffective as displayed by the
complete erosion of the last beach nourishment project in less than two years. The possible
reduction in funding from the Federal Government for both navigational and maintenance
dredging operations as well as beach nourishment projects could create a severe problem for
Topsail Beach as these procedures would become the responsibility of the state and local
government.
The extent of the current IHA clearly does not cover all areas of inlet influence. The
continued and accelerated erosion of fill placed on the beach is a clear indication of inlet
influence outside the realm of the IHA. Setback limitations and size restrictions to new
construction on the island must be applied farther north of the inlet in order to for costly mistakes
to be reduced. This is of particular importance for future projects if the Federal Government does
indeed reduce or halt funding for aforementioned operations. The extent of the newly proposed
IHA for Topsail Beach is a much more realistic representation of the area subject to inlet
120
influence. These extended restrictions would reduce costs to both local governments and
taxpayers as new construction locations, similar to the houses on the southern end of the island,
would be subject to tighter regulations.
CONCLUSIONS
The continual and long-term migration of New Topsail Inlet between 1938 and 2006
caused an overall shift of 1750 m in the position of the inlet. Migration rates during this period
ranged from a minimum of 1.1 m/yr from 1959 -1962 to a maximum of 73.7 m/yr during 19451949. Although uncommon, two periods of northern migration did occur, the first from 19381945, the second from 2001-2003. Migration rates during these two periods were -16.9 m/yr and
-13.1 m/yr respectively. Migration of the inlet was accompanied by several long lasting changes
to the inlet morphology. Changes included variations in the inlet minimum width, which was
initially 398.8 m in 1938 but continued to increase with the migration of the inlet and the
increase in tidal prism. By 2006, the inlet minimum width was 701.8 m, the widest it had been
during the study.
The changes in channel position and orientation of the ebb channel lead to numerous
changes in the size, shape, and position of the ebb tidal delta. Ebb channel orientation ranged
from a near shore normal position of 148º to a maximum of 180º and a minimum of 104º.
Changes to both the direction of the ebb channel and the position of the main inlet channel
caused variations to the size and location of the ebb tidal delta. Ebb delta size ranged from a
minimum of ~ 2,394,790 m3 during 1956 to a maximum of ~ 5,348,990 m3 during 2003. The
cyclic deflection of the ebb channel often caused the ebb shoal to be skewed to the north and
exhibit asymmetrical morphology. Morphology that resembled the classic inlet occurred
121
infrequently, and was only seen when the ebb channel maintained a shore normal position and a
symmetrical ebb tidal delta was formed.
Changes to the interior inlet morphology began in 1956 when the inlet main inlet channel
started to exhibit increased curvature. The shift in channel orientation with in the back barrier
was likely a result of the inlet migrating into a space previously occupied by Old Topsail Inlet
and its associated flood shoal. The change to the interior morphology resulted in a lack of space
for New Topsail Inlet’s flood shoal and caused it s eventual migration and reorientation with in
Banks Channel, behind the Topsail Island spit. The displacement of the flood shoal started
sometime between 1962 and 1974 and ended when the flood ramp had completely reoriented
itself in line with Banks Channel and the incoming flood tide during 1982.
Several bypassing events were associated with the migration of the main inlet channel
and the cyclic deflection of the ebb channel. During the short-term intensive study, from 1982 to
2003, four bypassing events of various lengths took place. The events ranged from ~ 3 years to
greater than 7 years in length and reached various degrees of completion. Only one event moved
through the complete cycle; ebb channel orientation ranged from 180º near the start of the cycle
to 106º before the ebb shoal was breached. Bypassed sediment amounts ranged from a minimum
of 267,590 m3 to a maximum of 841,010 m3 and channel jumps between realignments ranged
from 62º to 24º.
Both the Topsail Island and the Hutaff Island shoreline reaches were primarily
characterized by net erosion and shoreline retreat. Net accretion did occur along small shoreline
tracts within the current zone of inlet influence. Migration of the inlet complex and the deflection
of the ebb shoal played a large role in shoreline position. Migration of the inlet resulted in the
122
erosion of the leading barrier and the truncation of the trailing shoreline. Areas that previously
experienced accelerated accretion soon became erosion “hotspots” as the inlet migrated and the
geometry of the island shifted. Ebb channel deflection and the wave sheltering effect of the
skewed ebb shoal were often the cause of accelerated accretion and the buildup of large shoreline
protrusions. Typical rates of erosion along the Topsail Island shoreline reach were ~ 1 m/yr, but
also experienced accelerated rates that ranged from 2.4 m/yr to 5.6 m/yr. Rates of erosion for the
Hutaff Island reach were typically much higher and compared with storm damage erosion on
Topsail Island; erosion rates as high as 13.1 m/month were recorded on the Hutaff shoreline
reach.
Erosion rates were typically higher along the Hutaff Island shoreline due to the exposed
nature of the beachfront. The typically skewed geometry of the ebb shoal severely limited the
wave sheltering effect of the delta. Also, the geometry of the ebb would typically lead to the
development and expansion of a marginal flood channel just off the tip of the island. The
position of the marginal flood channel accelerated erosion along the shoreline tract by scouring
away the beachfront and portions of the southern inlet shoulder.
123
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Engineers Wilmington District, 2,076 p.
127

NEW TOPSAIL INLET, NC Merritt Ross Willingham McL

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