Estuarine Shoreline Inventory-1977

1rl77
ESTUARINE SHORELINE INVENTORY
FOR
PENDER, NEW HANOVER, AND BRUNSWICK
COUNTIES, NORTH CAROLINA
Thomas Scott Hartness and Daniel R. Pearson
DCM COPY
DCM COPY

STATE OF NORTH CAROLINA
DEPARTMENT OF NATURAL AND ECONOMIC RESOURCES
COASTAL RESOURCES COMMISSION
ESTUARINE SHORELINE INVENTORY
FOR
PENDER, NEW HANOVER, AND BRUNSWICK
COUNTIES, NORTH CAROLINA
by
Thomas Scott Hortness and Daniel R. Pearson
Department of Geology
EAST CAROLINA UNIVERSITY
Greenville, North Carolina
March 1977
This report was funded by the National Oceanic Atmospheric Administration ( grant no. 04— 6-
158— 44095) through the North Carolina Coastal Resources Commission.

CONTENTS
ABSTRACT..........................................................ii
ACKNOWLEDGMENTS..................................................iii
INTRODUCTION.......................................................1
PREVIOUS INVESTIGATIONS............................................I
GEOLOGIC SETTING...................................................3
ESTUARINE SHORELINE CLASSIFICATION.................................4
Variables Affecting Erosion...................................6
Shoreline Types..............................................11
COUNTY SHORELINE INVENTORY........................................17
Pender County................................................20
New Hanover County...........................................25
Brunswick County.............................................30
CONCLUSIONS AND RECOMMENDATIONS...................................45
REFERENCES........................................................47

ABSTRACT
Shoreline erosion within navigable sections of the estuaries of
Pender, New Hanover, and Brunswick counties has been studied. Through
the use of a shoreline classification scheme, patterns of shoreline
erosion versus shoreline characteristics are delineated. Variables
affecting shoreline erosion rates are: bank height and composition;
type and frequency of waterway traffic; fetch and orientation of the
shoreline; and proximity of shoreline to a tidal channel. Field
observations and laboratory analysis of shoreline surveys indicate
that the major cause of shoreline erosion within the Atlantic Intra-
coastal Waterway and adjacent waters is due to boat wake activity.
Erosion along the Cape Fear River shoreline is minimal, with greatest
erosiona function of seasonal storm activity. A series of estuarine
shoreline maps indicating bank type and erosional activity has been
compiled for use in determining areas -of erosional activity.
Conclusions and recommendations indicated by this investigation
are:
1) Shoreline erosion along the AIWW is primarily a function of
man's activities.
2) Certain critical areas of eroding shoreline could be controlled
through utilization of proper shoreline protection techniques or
establishment of "no -wake" zones.
3) 'Proper land use planning coupled with effective zoning ordi-
nances could reduce the possibility of property damage due to seasonal
storm activity.
4) Landowners along coastal shorelines should be made aware of all
the consequences of owning estuarine shoreline property.
ii

ACKNOWLEDGEMENTS
It is a pleasure to thank the Coastal Resources Commission for
contacting us to conduct this study. Mike Black of the CRC was re-
sponsible for coordinating the efforts of numerous state agencies to
provide cars, boats, etc. and giving constant support to our efforts.
Access to aerial photographic coverage was graciously provided by the
USDA Soil Conservation Service offices in Pender, New Hanover, and
Brunswick counties. Special thanks go to three East Carolina University
faculty members: Dr. Vince Bellis and Dr. Stanley Riggs who made
numerous helpful suggestions; and to Dr. Mike O'Connor who provided
useful ideas as well as editorial commentary.

INTRODUCTION
Trends for the past few years indicate that coastal shoreline
development in North Carolina is progressing at an ever-increasing
rate. The need for a better understanding of shoreline erosion pro-
cesses has prompted numerous investigations through various institu-
tions and agencies(USDA Soil Conservation Service, 1975; Bellis,
O'Connor, and Riggs, 1975; U.S. Army Corps of Engineers, 1971). This
report evaluates estuarine shoreline erosion along the Atlantic Intra-
coastal Waterway(AIWW) and adjacent waters, and segments of the Cape
Fear River in Pender, New Hanover, and Brunswick counties, North
Carolina(figure 1). Increasing development along the AIWW necessi-
tates evaluation of the shoreline erosion situation for the tri-county
area. Although the Cape Fear River below Wilmington has not under-
gone the rapid shoreline development experienced along other large
coastal rivers, the potential for development is high, so this area
is included in this report.
PREVIOUS INVESTIGATIONS
The United States Department of Agriculture Soil Conservation
Service conducted a preliminary study of estuarine shoreline erosion
for New Hanover and Brunswick counties under the direction of L. D.
Hunning(Hunning, 1975). Comparative analyses of aerial photographs
for a 25-30 year period were made with net erosion being calculated
by the distance of shoreline migration between photographs. Results

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Figure I.
PENDER
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Location Map
miles
O 5 10 15

indicated shoreline erosion to be minimal, so a more detailed study
of these two counties was not undertaken.
A study by the U. S. Army Corps of Engineers(1971) summerized
the general characteristics of ocean and estuarine shoreline, and
located specific areas of erosional activity. However, the report
did not attempt to define the mechanisms or variables involved in
estuarine shoreline erosion.
GEOLOGIC SETTING
The Pender-New Hanover -Brunswick estuarine shoreline is primarily
composed of marshland peat or Pliocene and Pleistocene sands and gravels.
A prominent topographic feature known as the Suffolk Scarp parallels
the shoreline throughout much of the study area. This escarpment
forms many of the high banks that occur along the waterway. Seaward
of this scarp lies the Pamlico Terrace --a gently undulating surface
with elevations averaging less than ten feet Both the Suffolk Scarp
and Pamlico Terrace are believed to have been created during one of
the last high stands in sea level, either 40,000 or 100,000 years ago.
The Suffolk Scarp is considered by geologists to be a remnant shoreline,
and the Pamlico Terrace an associated off -shore ocean floor.
For the past 18,000 years, worldwide eustatic sea level has been
rising. Recent estimates set the current rate of rise to be between
0.5-1.5 feet per century. This "flooding" effect of coastal drainage
systems by a transgressing sea is responsible for the formation of the
estuarine river systems of eastern N. C., such as the Cape Fear and
Neuse River systems. Most shoreline erosion is a result of this rising
sea level.
3

Recent studies by Balaz(1974) and Hicks(1972) suggest that eastern
N. C. is currently experiencing vertical crustal movements in the earth's
crust. Two major tectonic elements, the Cape Fear Arch and the Pamlico
Basin(fig. 2), influence the rate of inundation for coastal counties.
The Pamlico Basin is subsiding at a rate of 5 mm/year, whereas the Cape
Fear Arch is rising at an average rate of 5-6.5 mm/year(Balaz, 1974).
This uplift in the Pender-New Hanover -Brunswick county area counteracts
the effect of eustatic sea level rise and slows the overall rate of
shoreline erosion. This tectonic effect, coupled with a general absence
of large, open -water bodies along southeastern N. C., explains the
rather slow natural erosion of estuarine shoreline.
ESTUARINE SHORELINE CLASSIFICATION
For the purpose of evaluating the various factors involved in
estuarine shoreline erosion, a shoreline classification scheme was
devised(modified after Bellis, O'Connor, and Riggs, 1975). Bank height
and composition, general floral types, land useage, shoreline protection
features, and severity of erosion are included in the classification.
The shoreline was surveyed by boat, with continuous typing of the
shoreline recorded on base maps. Aerial photographs, navigational
charts, and topographic maps greatly facilitated the field classification.
In order to obtain additional information concerning erosion and general
environmental processes, numerous interviews were conducted with land-
owners residing along the shoreline. Through the use of this classifi-
cation scheme, patterns of erosion versus shoreline features were
discerned.
M

V I R G IN I A
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9A MILES
'9 AN 0 20 40 60 80 100
BASEMENT ROCK SLOPE DIRECTION -ARCH
XBASEMENT ROCK SLOPE DIRECTION- BASIN
Figure 2.
5

Three major shoreline types are included in the classification
(table 1). The severity of erosion of identical shoreline types may
differ greatly due to local variables which influence the extent and
character of erosion.
Table 1. Estuarine Shoreline Classification for Pender, New Hanover,
and Brunswick Counties, North Carolina(modified after Bellis and others,
1975) .
SHORELINE TYPE
I. Marsh Grass
%EROSION RATE(ft/yr)
1
II. Sand Banks
A. Bluff( + 20 feet) >2
B. High Bank(5-20 feet) ?2
C. Low Bank(1-5 feet) 2
III. Sand Banks with Marsh Fringe 1
A. Bluff(+ 20 feet)
B. High Bank(5-20 feet)
C. Low Bank(1-5, feet)
*Rates are approximations and vary according to local conditions.
Variables affecting erosion
Major variables which must be taken into account when evaluating
the erosion potential and erosion rate for any given area are discussed
below.
1. Type and frequency of waterway traffic --Without exception, each
AIWW shoreline landowner interviewed attributes the shoreline erosion
problem to boat wakes produced by fast-moving, deepdraft, pleasure craft
and barges. During the spring and fall the AIWW becomes the major north -
south passageway for pleasure boats travelling the east coast. Yachts can,
0

at higher speeds, throw wakes up to four feet high, creating a tremen-
dous amount of wave energy that is ultimately expended upon the shoreline.
Thus, shoreline erosion along the waterway is almost totally attributable
to boat wakes.
Barge traffic utilizes the AIWW yearround, but due to the general
barge design and slow speeds virtually no wake is produced. Barge
movement does, however, create a water "withdrawl-surge" action as it
passes along the waterway. Figures 3 through 6 illustrate the types of
water motion produced by the passage of a barge. Although this surge
does contribute to shoreline erosion, the overall effect is minimal
when compared to the high energy waves produced by yachts.
� k+
Figure 3. Narrow stretch of AIWW with typical marsh shoreline
soon to be affected by wake action of passing barge.
7

Figure 4. As barge passes, the water is drawn away from the
shoreline. The amount of water level change is related to
the width of the channel.
Figure 5. Water surges back against the shoreline causing
erosion of the marsh.
C

Figure 6. Normal water level returns, but secondary water
oscillations continue for some time, keeping fine sediments suspended.
Since erosion along the waterway is caused by large boat traffic,
the erosion rate will be a function of the type and frequency of traffic
that travels the waterway. Local residents agreed that the erosion
rate has increased over the past few years due to an increase in vessel
traffic, size, and speed.
2. Tide level --The 3-4 foot tidal range in the study area(U. S.
Coast and Geodetic Survey, 1971) greatly affects the intensity of
erosion for a given shoreline type. At high tide, boat wakes will pass
over the low marshshore and break on the marsh grass, thus not causing
any erosion of the marsh peat. Bank shorelines can be severely eroded
at high tide if boat wakes break against the bank. At low tide, very
little erosion occurs as long as extensive mud flats or sand shoals lie
seaward of a particular bank or marsh.
9

3. Width of waterway --The intensity of shoreline erosion activity
along the AIWW is also a function of distance from the main waterway
channel. Generally, the closer the shoreline is to the channel, the
greater the erosion potential. Where the waterway is wide, mud flats or
shoals occur on one or both sides of the channel. These are shallow
water areas at high tide, thus aiding in dissipation of wave energy.
At low tide these flats and shoals separate the high tide shoreline
from the water, thus serving as a buffer zone to oncoming wakes(fig. 7).
As previously stated, barges create withdrawl-surge wave motions along
their paths. Narrower stretches of the waterway are particularly
susceptible to this effect primarily due to the short distance between
the shoreline and moving vessel.
Figure 7. Sand shoals exposed at low tide act as a buffer
zone, dispersing wake energy, protecting the bank from erosion.
E

t
f 4. Fetch and orientation of shoreline --Wind patterns for this area
4
annually shift from a northeast direction in the fall and winter months
to the southwest during the spring and summer(U. S. Naval Weather Service,
1970). Erosion due to wind -induced wave activity is a function of the
geographical orientation and fetch of the shoreline. The fetch along
most of the estuarine coastline is too small for generation of higher
energy waves that contribute to shoreline erosion. There are reaches of
the Cape Fear River with fetches greater than one mile. However, most
of the river is quite shallow with numerous shoals and dredge spoil
islands that together inhibit creation of large waves that would erode
the shoreline.
5. Proximity to a tidal channel --Tidal channels intersect the
waterway at numerous points. Due to the scouring effect of changing
tides, hanks along the mouths of large tidal channels may be sites for
erosion(fig. 8). In many instances, erosion has cut I:ack to expose
dredge spoil to wave action, thereby increasing sediment influx into
the waterway.
Shoreline types
The salt marsh shoreline will be discussed as a single unit,
although most of the AIWW shoreline is lined by a marsh border that
may fringe a low, high, or bluff -type bank.
1. Salt marsh peat --Salt marsh shoreline fringes most of the AIWW
and low-lying drainages of the Cape Fear River. Most of the seaward
side of the AIWW is comprised of broad expanses of salt marsh and
dredge spoil islands(fig. 9). Significant erosion of the marsh peat

*L;.n�l W11
Figure 8. Erosion along tidal channel exposing peat beds.
occurs during low tide. At high tide, boat wakes tend to ride over
the marsh peat and into the grass marsh, where the baffling effect of
the grass dissipates the wave energ},. As the tide level begins to drop,
waves tend to break upon, or directly in front of the peat. Undercutting
of the peat will proceed until the undercut peat will sag and eventually
break off in large blocks and fall into the water(fig. 10).
Salt marsh serves as an important energy -absorbing buffer zone in
areas where a higher bank borders the landward edge of the marsh. As
long as this fringe exists, the low, high, or bluff bank is protected
from wave activity. Without this fringing marsh, the bank would erode
at a rapid rate, forcing the landowner to utilize some form of shore-
line protective measure.
2. Low, high, and bluff bank --In areas where no marsh fringe exists,
the banks will erode quite readily since they are usually composed of
12

Figure 9. Seaward view of extensive salt marsh(taken from
atop spoil pile).
unconsolidated sands(fig. 11). Typically, the seaward edge of the
bank is composed of sand or mud flats which are exposed &-ring low
tide. These flats cause boat wakes to break far away from the bank at
low tide, thus inhibiting erosion of the bank. At high tide, waves
will travel over these shoals and break directly against the bank.
Undercutting of the bank causes trees and shrubs to fall into the
waterway, where they become navigational hazards. Sediment derived
from the bank will be moved over the flat and into the waterway channel.
The volume of sediment produced is a function of the erosion rate and
bank height.
Periodicallv the AIWW channel is dredged with spoil dumped on
bordering marshlands. Figure 12 illustrates how, in some instances,
the overall effect can be visualized as the transfer of sediment from one
side of the waterway to the other side.
13

Figure 10. Erosion of marsh grass shoreline. The peat is
undercut, breaking off in blocks.
2
Figure 11. Slumping trees resulting from the erosion of a
high bank.
14

Schematic Sequence of Erosion -Deposition -Dredging in the AIWW
12a.
12b.
Figure 12a. Natural system - pre -waterway
Figure 12b. Initial dredging of waterway
- 4 -
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12c.
?' troslon Erosion
Deposition
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Figure 12c. System trying to reach equilibrium by erosion of the bank and deposition in the channel.
Figure 12d. Redredging channel and placing spoil on adjacent shore.

COUNTY SHORELINE INVENTORY
The following series of maps are segments of the tri-county shore-
line indicating bank type and areas of active shoreline erosion. A
series of photographs is included to aid the user in understanding the
nature of the shoreline. Table 1 gives approximate erosion rates for
the various shoreline types.
17

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Figure 14. Eroding low bank without marsh fringe.
Figure 15. Failing bulkhead replaced by rip -rap.
22

Figure 16. Examples of poor attempts to protect shoreline
property are numerous.
Figure 17. Although somewhat successful, the rip -rap protection
system is far from being aesthetically pleasing.
23

Figure 18. Position of waning tree gives evidence of shoreline
recession.
Figure 19. Recession of protective marsh grass.
24

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Figure 20. Properly maintained bulkheading proves to },e effective
for controlling erosion.
Figure 21. Such bulkheading as this will prove to be effective
after eventual loss of protective marsh fringe.
26

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Figure 22. This modern shoreline is encroaching upon a relict
forest.
Figure 23. The marsh fringe serves as a buffer zone between the
water and the mainland.
29

30

RAN
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Figure 24. Narrow sections of the waterway with high bank shore-
lines are particularly vulnerable to erosion.
Figure 25. Escarpments along the Cape Fear River are created by
high energy wave erosion during severe storms.
31

Figure 26. The exposed tree trunk is indication that the shore-
line has receded.
32

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CW
R�ver

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Figure 27. Low bank developments such as this may be damaged
due to flooding caused by seasonal storms.
44
E-
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Figure 28. Rip -rap composed of materials such as concrete blocks,
bricks, etc. are somewhat effective although aesthetically unattrac-
tive.
34

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35

Figure 29. Bulkheads such
as this one are expensive to
build and require periodic
maintainance.
Figure 30. Failure of this bulkhead resulted in the loss of
valuable property over a ten-year period.
36

Figure 31. Erosion of this marsh results from boat wake action
at low tide.
Figure 32. Undercutting has resulted in the felling of trees
and shrubs along this low bank.
37

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=1 I—
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38

a�RL� f3eSaT !,
Figure 33. Stairways such as this one frequently fall victim to
shoreline erosion. Note that due to the accelerated erosion rate,
the bank recedes before destruction of the stairs.
Figure 34. Note the fence
posts exposed due to slumping
of this bluff.
39

Figure 35. Shoreline protection measures such as this are not very
effective at controlling erosion.
Figure 36. Stabilization of this high bank has been accomplished
through the use of concrete -filled bags. In such critical erosion
areas, the measure will prove to be only a temporary solution.
40

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42

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Lithe River Inlet
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43

Figure 37. The owner of this restaurant will soon have to utilize
some type of shoreline protection measure in order to protect his
property(note the power pole).
Figure 38. Failure of bulkhead due to wake activity on AIWW.
This structure is so located that it receives the full impact of
the boat wake.
44

CONCLUSIONS AND RECOMMENDATIONS
The results of this study indicate that rates of erosion may
vary considerably even between adjacent segments of the shoreline.
Although the erosion inventory presented in this report serves as
a guide to estuarine shoreline erosion, it is not a replacement for
on -site evaluation of any area. Man-made or natural changes of the
shoreline may alter the erosion process along any section of the
study area.
The following conclusions have resulted through careful analysis
of the estuarine shoreline erosion problem for the tri-county area:
1. Most of the AIWW utilizes man-made passageways, and the erosion
problem along the waterway is induced primarily by the activities of
man. An; alteration of the shoreline that must be made in order to cope
with erosion should be viewed as solutions to a man-made problem.
Thus, any aesthetic arguments over environmental changes due to shoreline
modification must consider that most of the AIWW is not part of the natural
system and has to be approached as such.
2. Portions of eroding bank shoreline can and should be controlled
through the use of shoreline protection techniques. Landowners would
benefit because property loss would be controlled, and the filling -in
of the waterway channel due to sediment derived from eroding shoreline
would be reduced.
3. For certain areas, no economically feasible shoreline modification
could reduce the erosion problem. Erosion of marshland and stretches of
narrow waterway channels can be controlled through the establishment of
"no -wake" zones, thus removing the mechanism(wakes) for erosion. This
45

would be particularly beneficial at high tide, when erosion potential
of bank shoreline is generally the greatest.
4. Zoning ordinances that would establish land use codes along the
estuarine shoreline could restrict landowners from building at areas of
severe erosion or areas of high erosion potential. Low bank areas along
the AIWW and the Cape Fear River are particularly vulnerable to flooding
caused by seasonal storms, such as hurricanes. Large storms have raised
water levels as high as ten feet above mean sea level in the tri-county
area, causing property damage due to flooding and increased shoreline
erosion. Low-lying areas should be considered as undesirable locations
for future real estate developments. Building code regulations could
create set -back lines for developments, thus removing permanent structures
from immediate danger by shoreline erosion.
5. Finally, a "do-nothing" approach to the shoreline erosion situa-
tion should be considered. The purpose of the AIWW is to allow safe,
efficient inland passage for commercial and private water traffic.
Since construction of the AIWW predates most real estate developments
along its shoreline, then erosion of the shoreline could be considered
as a problem only for those people who purchased shoreline property
without full consideration of the consequences of owning waterfront lots.
W.

REFERENCES
Balazs, E. I., 1974, Vertical crustal movements on the Middle Atlantic
Coastal Plain as indicated by precise leveling: Nat. Geod. Surv.,
U. S. Dept. of Commerce, Rockville, Md., 19 p.
Bellis, Vincent, O'Connor, M. P., and Riggs, S. R., 1975, Estuarine
shoreline erosion in the Albemarle -Pamlico region of North Carolina:
Univ. of N. C. Sea Grant Pub. UNC SG-75-2.9, Raleigh, N. C., 67 p.
Hicks, S. D., 1972, Vertical crustal movements from sea level measure-
ments along the east coast of the United States: Jour. of Geophys.
Res., v. 77, p. 5930-5934.
Hunning, L. D., 1975, Inland shoreline erosion study: Unpub. report to
USDA Soil Conserv. Service.
U. S. Army Corps of Engineers, 1971, National shoreline study- state of
N. C. regional inventory report: Wilmington, N. C., 77 p.
U. S. Coast and Geodetic Survey, 1971, East coast of North and South
America- Tide tables, p. 230-231.
U. S. D. A. Soil Conservation Service, 1975, Shoreline erosion inventory;
North Carolina, 30 p.
U. S. Naval Weather Service Command, 1970, Summary of synoptic meteoro-
logical observations- North American coastal marine areas, v. 3,
p. 159-316.
47