01 - Avon - AppD-Littoral Process

APPENDIX D

LITTORAL PROCESSES
Avon Village Beach Nourishment Project
Dare County, North Carolina

Prepared for:

Dare County Board of Commissioners
Bob Woodard, Chairman
954 Marshall C Collins Drive, Manteo NC 27954

Prepared by:

PO Box 8056, Columbia SC 29202 –8056

[2525–JULY 2021]

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TABLE OF CONTENTS
TABLE OF CONTENTS ..................................................................................................... i
1.0 INTRODUCTION ..................................................................................................... 1
2.0 BEACH CONDITION SURVEYS AND EROSION ANALYSIS ............................................... 2
2.1 Data Collection Methods ....................................................................................................... 3
2.1.1 Survey Stationing History ........................................................................................... 3
2.1.2 Vertical Datum ............................................................................................................. 6
2.1.3 Data Collection Methodology ..................................................................................... 6
2.2 Beach Profiles ........................................................................................................................ 9
2.3 Profile Volume Analysis Methodology ................................................................................. 14
2.3.1 Profile Volume Approach .......................................................................................... 14
2.3.2 Reference Contours and Calculation Boundaries .................................................... 17
2.4 Historical Erosion Rate ......................................................................................................... 19
2.4.1 Methodology of Aerial Photograph Analyses ........................................................... 19
2.4.2 Historical Shorelines ................................................................................................. 22
2.4.3 Recent Variations in Shoreline Change Rates .......................................................... 27
2.4.4 Rhythmic Shoreline Changes .................................................................................... 34
2.5 Volume Analysis of July 2020 Survey ................................................................................... 37
2.5.1 July 2020 Field Work ................................................................................................. 37
2.5.2. Unit Volume Analyses .............................................................................................. 41
3.0 COASTAL PROCESSES ........................................................................................... 48
3.1 Wave Climate ........................................................................................................................ 49
3.1.1 Real-Time Wave Buoy – Station 41025 ..................................................................... 49
3.1.2 Wave Information Studies – Station 63230 .............................................................. 52
3.2 Wave Modeling ..................................................................................................................... 57
3.2.1 Model Capabilities ..................................................................................................... 57
3.2.2 Model Assumptions ................................................................................................... 58
3.3 Shoreline Evolution Modeling .............................................................................................. 59
3.4 Model Setup .......................................................................................................................... 61
3.4.1 STWAVE Model Grid ................................................................................................... 62
3.4.2 GenCade Model Grid ................................................................................................. 62
3.4.3 Model Grid Size .......................................................................................................... 64
3.4.4 Model Bathymetry ..................................................................................................... 64
3.4.5 Wave Climate Analysis .............................................................................................. 69
3.4.6 Model Parameters ..................................................................................................... 69
3.5 STWAVE Model Results ......................................................................................................... 69
3.6 GenCade Model Calibration ................................................................................................. 78
3.7 GenCade Model Results ....................................................................................................... 79
3.8 Conclusions .......................................................................................................................... 82
REFERENCES .............................................................................................................. 83
Attachment 1-A) Baseline and Control
Attachment 1-B) Station Coordinates for July 2020 Survey
Attachment 2) Beach and Inshore Profiles
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1.0 INTRODUCTION
This appendix supplements information in the main text of this document and provides additional
data and analyses of erosion, wave climate, numerical modeling, and littoral processes in the
proposed Avon Village nourishment project area. It covers the follow ing topics:
 Field data collection for beach condition surveys
 Beach and inshore profiles
 Volume analysis for defining beach condition
 Historical erosion rates analysis
 Wave climate (NDBC wave buoy and WIS hindcast data)
 Wave transformation modeling to evaluate wave field for before and after
dredging conditions
 Shoreline evolution modeling to evaluate longshore sediment transport
with and without the nourishment project
 Profile adjustment after nourishment and project longevity

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2.0 BEACH CONDITION SURVEYS AND EROSION ANALYSIS
2.1 Data Collection Methods
2.1.1 Survey Stationing History
Coastal Science & Engineering (CSE) established a project baseline encompassing the length of
Hatteras Island from Oregon Inlet to Cape Point using existing monuments. Stationing is in
standard engineering units beginning near the Oregon Inlet jetty (station 0+00) and ending in the
Cape Hatteras National Seashore (CHNS) (station 1983+77).* Intermediate control points mark
the turning points and azimuths along the baseline.
[*Stationing in engineering nomenclature is shorthand for distances along a line. In this case,
station numbers increase from north to south , so station 420+00 (for example) is 42,000 f eet (ft)
or ~8 miles south of the starting point near Oregon Inlet. Station 1792+50 (for example) is 179,250
ft or ~34 miles from the starting point.]
Attachment 1-A lists the control points and applicable stationing along the baseline. The total
length of the baseline is ~198,377 linear feet (lf), and stations provide a convenient measure of
distances along the shore. Table 2.1 lists some reference stations and localities along the baseline.

TABLE 2.1. Baseline (BL) and stationing along Hatteras Island for the current project at reference localities.
See Attachment 1-A for a list of control monuments (turning points) along the baseline.
Station Monument # Locality Note
0+00 ― Oregon Inlet jetty North end of BL
~347+00 ― Pea Island 2011 breach inlet ―
~635+00 ― Mirlo Beach ―
686+44 ― Rodanthe BL turning point
~712+00 ― Rodanthe Pier ―
~1573+00 ― Village of Avon ―
~1590+00 ― Avon Pier ―
~1880+00 ― Village of Buxton North end of development
1928+11 CHL1 Old Hatteras Lighthouse site ―
1983+77 BYRD Cape Point area South end of BL

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The same baseline was utilized in the present Avon study to set up evenly spaced beach profile
lines at 500 ft spacing along the survey area. The Village of Avon community along the oceanfront
begins near Station 1500+00 and ends near Station 1680+00, encompassing a total of 18,000 lf (~3.4
miles) of shoreline, and the proposed project area covers approximately 73 percent of the Avon
shoreline from 15500+00 to 1682+00 (2.5 miles). The study area for a feasibility study included
some upcoast and downcoast shoreline of CAHA, extending from stations 1470+00 to 1700+00,
totaling 23,000 lf (~4.36 miles) (CSE 2020). The proposed project area is within this study area.
Appendix 1-B lists the control points and applicable stationing along the baseline for the Avon
study, and the survey lines are illustrated in Figure 2.1.
Five reaches are delineated based on the geographic locations and the volu me analyses as shown
in Figure 2.1. Reach 1 is CAHA land managed by the National Park Service (NPS) immediately
upcoast of Avon Village. Reaches 2 and 3 cover 9,500 ft of the Village north of Avon Pier and Reach
4 is the priority study area extending ~8,000 ft south of the pier to Askins Creek North Drive . Reach
5 encompasses the 3,000 ft of shoreline from Askins Creek North Drive into CAHA. The proposed
project area is from stations 1550+00 to 1682+00, encompassing the entire Reaches 3 and 4 with
some extensions on both the northern and southern ends of these two reaches.

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FIGURE 2.1. Map of the Avon study area in the feasibility study showing the “Hatteras Island baseline” (CSE
2020) and stationing in engineering form. Bathymetric survey lines of the study are shown in the figure.
Five reaches are delineated ba sed on the beach survey analyses. The offshore sand search area is marked
by the blue box, and the blue dots represent the core locations. Red dots along the beach show the
locations of sand sampling. Offshore coring and sand sampling on the beach are discussed in the
Environmental Assessment – Appendix A – Geotechnical Data Analysis.

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2.1.2 Vertical Datum
The vertical datum for CSE’s profile data collecti on was NAVD 88 (North American Vertical Datum
of 1988) which is ~0.4 ft above present mean tide level (MTL) in Dare County (NOAA-NOS). Figure
2.2 illustrates the vari ous relationships among key reference datums for the closest tidal station at
Cape Hatteras (NC) fishing pier, which is ~10 miles southwest of the Avon project area. At the pier,
mean ocean tide range is 3 .0 ft with an average spring tide of 3.5 ft (NOAA Tides and Currents,
station 865 4400). Mean high water (MHW) is 1.05 ft above NAVD; mean tide level is 0.4 5 ft below
NAVD; and mean low water (MLW) is 1.94 ft below NAVD. The horizontal datum used in CSE’s data
collection is NAD’83 (North American Datum of 1983, Zone : NC 3200).
2.1.3 Data Collection Methodology
Hydrographic data collection methodology followed
procedures outlined in the USACE Hydro graphic Surveying
Manual (EM 1110–2–1003; January 2002, updated April
2004). Data were collected in the horizontal datum of North
American Datum of 1983 (NAD’83) and were measured in US
survey feet using State Plane Coordinates in the zone N C–
3200 for Hatteras Island. The vertical datum was the North
American Vertical Datum of 1988 (NAVD 88) measured in feet.
CSE has been upgrading survey equipment and software
over the years to apply the most up-to-date industry
standards to the Avon project area. Two RTK-GNSS
(Trimble® Model R10 GNSS) units were used for the data
collection. The R10 GNSS and TSC3 Data Collector with
built-in modem allow CSE to access real -time networks
(RTN) nationwide where available. This feature eliminates
the need for an on-site “base station .”
The offshore work was performed using an Applanix™ POS
MV SurfMaster positioning system, a state -of-the-art
navigation system used to facilitate vessel tracking and
duplication of survey lines. It incorporates the GPS Azimuth
Measurement System (GAMS) and solves for position in six
degrees of freedom (boat position and orientation, heading, heave, roll, and velocity over the
bottom). This system is linked onboard the survey vessel in “real time” to an Odom™ Echotrac
CV100 and SMSW200 –4a transducer for depth measurements. The s ounder has a depth range of
0.8 ft to 1,000 ft with a 4° beam width and is designed to operate in very shallow depths such as
FIGURE 2.2. Key reference datums at Cape
Hatteras (NC) fishing pier (~9 miles southwest
of the project area).
[Source: NOAA-Tides and Currents Station ID
8654400]
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shoals of inlets or the inner surf zone. The unit has a resolution of 0.1 ft and an accuracy of 0.01
meter (0.03 ft) plus or minus 0.1% of depth.
CSE’s navigation and surveying system for overwater work enable s the recording of direct
measurements of the bottom without the need for tide corrections. Offshore profiles were
collected at 50 Hz but were filtered in the office to elimin ate spikes and provide a 10 –16-point
floating average. Smoothed offshore data were edited to a manageable size and merged with
subaerial data, which extend from the foredune to low-tide wading depth.
CSE maintains up-to-date software licensing and support for processing overwater and overland
data, including HYPACK™ 2021 and Trimble® Business Center (TBC). HYPACK™ is the industry
standard for preparing raw survey data for plotting, export to CAD, and other final products. It
allows CSE’s field team to qu ickly detect missing data, anomalies, and spikes while in the field to
ensure the tracklines match closely from survey to survey. The TBC software similarly processes
raw survey data and facilitates QA/QC and data export to CAD and GIS software.
Inshore surveys were obtained at higher tide stages to fill in the gap of the land -based data
collected around lower tide stages. The survey profiles extended from low -tide depth into the
nearshore area and the outer surf zone (~3,500 –5,000 ft from the baseline). Figure 2.3 shows
representative field data collection photos.

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FIGURE 2.3. Field data collection methods involved subaerial survey using RTK-GPS at low tide and hydrographic
surveys by boat at high tide.

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Data collected in x-y-z format were used directly to develop a digital terrain model (DTM), which
provides a three-dimensional picture of the beach, the longshore bar, and the offshore z one.
Figure 2.4 illustrates DTMs of the Avon project area from the most recent survey (July 2020) by
color-coded, smooth-contour maps using the indicated elevation/depth intervals for each color.
Light colors indicate the dune-beach zone and longshore bar; deep blue represents water
depths >30 ft. The bathymetry DTM s show relatively smooth, continuous morphology of a
longshore bar (yellow-green color band) (inside the 20-ft depth contour) along the northern half of
Avon. The longshore bar diminishes to the south and an inshore bar develops in shallower water.
2.2 Beach Profiles
Although sediment transport and morphology changes in the nearshore are three -dimensional, it
is customary in beach analysis to separately consider the cross -shore and planform (ie –
alongshore) evolution. Survey data (coll ected in x-y-z format) were converted to x-z (distance-
elevation) pairs to compare beach conditions among profile lines. No recoverable historical
profiles into deep water were found prior to 20 20; therefore, we believe the data collected by CSE
in July 2020 was the first set of comprehensive beach profiles for this area extending from the
frontal dunes to deep water.
Representative profiles from the July 2020 survey are shown in Figure 2.5. [See Figures 2.1 and the
aerial photo insert at the right -hand corner of Figures 2.5a-c for general locations.] Attachment 2
contains the set of 47 long profiles obtained by CSE in July 2020. Profiles north of Avon Pier in July
2020 exhibited a well-defined longshore b ar with a crest at −10 to −14 ft NAVD positioned ~1,000 to
1,200 ft offshore along the shoreface. This longshore bar broadens and flattens to the south from
Avon Pier with a deeper crest (than the northern section) at about −20 ft. The offshore bar dwind les
at the south end of Avon. Instead, a small inshore bar at −6 ft to −10 ft emerges south of Station
1660+00 in July (see Fig 2.5c).

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FIGURE 2.4. Color-coded topography and bathymetry Digital Terrain Models (DTM) from the July 2020 beach
condition survey for the Avon feasibility study (CSE 2020). Note: the proposed nourishment project area is
mainly along Reaches 3 and 4.

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FIGURE 2.5a. Representative profiles for the Avon study area. [UPPER] at Station 1485+00 in the middle
of Reach 1 (CAHA) showing a frontal dune at +18 ft NAVD and a second dune ~200 ft landward of the frontal
dune at about +20 ft NAVD, indicating a healthy beach condition and high level of storm resiliency. [LOWER]
at Station 1530+00 in Reach 2 (north Avon) showing a foredune extending to +24 ft NAVD and broad dry-
sand beach situated ~500−800 ft from the toe of the dune. A small inshore bar with crest at −10 ft NAVD is
positioned ~1,000 ft from the baseline.

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FIGURE 2.5b. Representative profiles for the Avon study area. [UPPER] at Station 1570+00 in Reach 3 (mid
Avon) showing a foredune nearly at +24 ft NAVD and approximately 100 ft of dry-sand beach from the toe
of the dune. The longshore bar with the crest around −14 ft NAVD in the northern reaches migrated
landward by a few hundred feet going south. [LOWER] at Station 1615+00 in Reach 4 (half mile south of Avon
Pier) showing lack of dune, escarpment at the dune face, narrow dry-sand beach, and steeper beach face.
The longshore bar becomes flatter with a deeper crest.

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FIGURE 2.5c. Representative profiles for the Avon study area. [UPPER] at Station 1660+00 in Reach 4
(~1,000 ft north of Askins Creek North Drive) showing a foredune at +24 ft NAVD and approximately
30−50 ft of dry-sand beach from the toe of the dune. An inshore bar with a broad crest around −12 ft
NAVD occurred at this locality in July. [LOWER] at Station 1685+00 in Reach 5 (near the last house at the
south boundary of Avon) showing a foredune at +24 ft NAVD and approximately 200 ft of dry-sand beach
from the toe of the dune. An inshore bar occurred near the low-tide terrace around −6 ft NAVD.

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2.3 Profile Volume Analysis Methodology
2.3.1 Profile Volume Approach
Volume variations along the Avon project area were estimated using standard methods (average -
end-area method), common cross-shore boundaries, and contour datums. Two primary lenses (ie
– volumes between particular reference contours) were use d to analyze beach/inshore profiles
using CSE's Beach Profile Analysis System (BPAS) software which facilitates statistical analysis,
volume change calculations, and graphing. Profile volumes are a convenient way to determine the
condition of the beach and compare one area with another. As Figure 2.6 illustrates, the active
littoral zone encompasses a broad area between the dunes and some limiting offshore depth.
Each profile incorporates complex topography , which changes continuously as the beach adjusts
to varying wave energy, sediment supply, and tide range. Storms modify the profile by shifting
sand from the dry beach and foredune to the outer surf zone. After storms, fair -weather waves
tend to move sand back to the visible beach and reshape protect ive longshore bars. If this cycle
of offshore/onshore sediment transport remains balanced over time, the beach will be stable with
no net loss of volumes in the profile. However, if more sand moves offshore or down coast over
time than returns to the vis ible beach, there will be a net loss and a specific volume erosion rate.

FIGURE 2.6. Representative profile of the littoral zone illustrating the principal features between the dune and offshore.
The profile varies with changes in wave energy, the passage of storms, and differences in sediment quality. The Avon
erosion analysis takes into account the cycle of beach profile changes and focuses on the sand volumes in the entire
littoral zone. [Based on Komar 1998]

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FIGURE 2.7. Variation in equilibrium barrier island and foreshore profiles for Louisiana and
North Carolina. Coarser sandy sediments [typically 0.4 millimeters (mm) grain size] (left) lead to
steeper profiles and less volume in the base of North Carolina barrier islands relative to Louisiana
which is founded on fine-grained sediments (typically ~0.1 mm grain size) (right). Ocean is to the
right on the diagram. Note: 1 meter ≈ 3.28 ft. [From CSE 2011]

While it is possible to approximate the equilibrium shape of beach profiles by simple analyt ical
equations (Dean 1991, 2002), each site has a unique set of coastal processes (wa ves, tides, and
nearshore currents) as well as complex admixtures of sediment. The morphology and slopes
across the surf zone vary significantly as sediments of differing sizes are sorted by waves. The
coarsest material tends to concentrate in shallow wa ter at the inshore breaker line. Finer sands
tend to accumulate on the longshore bar (if present) and foredune.
As Komar (1998) and many others have shown, coarser sediments tend to produce steeper
foreshore slopes (see Fig 2.6) than fine sand, assuming wave energy is similar. The implication is
that less coarse sand is required to establish a profile in equilibrium with the local waves and tides
compared with a beach consisting of very fine sand. This is illustrated in Figure 2.7. The example
in the graphic compares a typical cross -section through a Louisiana barrier island with the North
Carolina coast. Much of the Louisiana coast consists of very fine sand and experiences relatively
low wave energy. Barrier islands are low -relief with very broad platforms extending miles offshore.
North Carolina Outer Banks barriers are composed of much coarser sands which tend to
equilibrate at steeper slopes despite higher wind and wave energy. As a result, a cross -section
through a North Carolina barrier isla nd will have higher relief compared to Louisiana.

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Researchers have found that basic differences among beach profiles can be distinguished by
simple measures of profile volumes (eg – Verhagen 1992, Kana 1993). Profile volumes convert a
two-dimensional measure of the beach to a “unit volume” measure , as illustrated in Figure 2.8. By
using common datums and similar starting points (say, near the dune crest), it is possible to
calculate the volume of sand contained in a unit -length of beach.
Profile volumes integrate all the
small-scale perturbations across
the beach and provide a simple
objective measure of beach
condition (Kana 1993 , Kana et al
2015). They provide quantitative
estimates of sand deficits or
surpluses when compared
against a target or desirable
beach condition.
The examples of profile volumes
in Figure 2.8 show a “normal”
beach with a representative unit
volume of 100 cy/ft measured to
low-tide wading depth.
A normal healthy profile is gen -
erally considered to consist of a
stable foredune and a dry beach
that is wide enough to undergo
normal seasonal and storm
changes without adverse impact
to the dune or backshore
development.

FIGURE 2.8. The concept of unit-width profile volumes for a series of beach
profiles showing an eroded beach with a deficit, a normal beach, and a beach
with a volume surplus. Profile volumes integrate small-scale perturbations
in profile shape and provide a simple objective measure of beach condition.
Indicated quantities are realistic for many East Coast beaches within the
elevation limits shown. [After Kana 1990]
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The other profiles in the graphic illustrate values for an eroding beach (in this case, backed by a
seawall) and a beach with a sand surplus. For this simple example, the unit volume of the eroded
profile is 50 cy/ft, or ~50 percent of the normal beach. The third profile illustrates a beach with a
surplus of sand along the dry beach and wet -sand beach relative to a normal healthy profile. Such
areas often reflect accreting conditions where shallow bars are welding to the beach near inlets.
The calculation limits can be arbitrary as long as they are consistently applied. Ideally, they should
encompass the entire active zone of profile change for the time period(s) of interest.
It should be readily apparent that at least 50 cy/ft must be added to the eroded profile in Figure
2.8 to achieve a normal, healthy profile. In actuality, much more sand is required to account for
the area between low-tide wading depth and the offshore limit of significant sand movement (see
Fig 2.6). Analyses such as these are necessarily site -specific, but they are practical measures of
sand deficits and erosional losses over time.
2.3.2 Reference Contours and Calculation Boundaries
The normal limit of significant change in bottom elevation (ie – “depth of closure*” – DOC) for the
Avon project area was determined to be −24 ft NAVD (USACE/NPS 2015). This depth is based on
estimates of DOC at decadal scales at Duck (Birkemeier 19 85), Bogue Banks (Olsen 2006), Nags
Head (Kaczkowski & Kana 2012), Buxton (USACE/NPS 2015). Therefore, the seaward calculation
limit of unit volumes is referenced from −24 ft NAVD.
*Depth of Closure is where successive profiles (ie – cross-sections of the beach zone) tend to
converge (or close) over a given period of time, suggesting that major changes in bottom elevation
are not occurring beyond that point. It is the depth that FEMA uses when calculating incident-
related sand volume loss if a declared disaster occurs to an engineered beach. During rare storms ,
the observed closure depth can be in deeper water, meaning sand exchange can b e further
offshore, as CSE’s monitoring data confirm.
**NAVD (North American Vertical Datum of 1988) is a fixed reference elevation between mean high
and mean low water. Presently, the NAVD datum is ~0.78 ft above mean tide level in the Avon
project area.
The landward calculation limit of unit volumes is set at the dune crest (CSE 2020). This calculation
provides a measure of how much extra sand is contained in the profile sea ward of the dune crest
relative to the quantity in the active beach zone.
The utility of this approach is that the relative condition o f the beach from locality to locality can
be objectively compared. Areas that contain a stable dune and wide beach seaward of dune crest
can serve as a reference healthy condition for areas of high erosion and large sand deficits.

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FIGURE 2.9. The beach zone used for calculating sand quantities along the Avon project area. The zone of interest
extends from the dune crest to a specified offshore depth (FEMA reference at −24 feet NAVD), in effect, a large sand box
over which waves shape the beach and shift sand around.

Volume changes within the calculation limits (ie — “sand box”) for the Avon project area are
estimated using standard methods (average -end-area method) and common cross -shore
boundaries and contour datums. Three lenses (ie – volumes between particular reference
contours) were used in the feasibility study to evaluate levels of dune protection, dry beach and
construction berm adjustments, wet beach condition, inshore surf zone, and the outer surf zone
(CSE 2020). These three lenses are described as follows.
 Lens 1 (D unes ) — Volume Above +6 ft NAVD — The nourishment construction berm
varied between +6 ft and +7 ft NAVD (CSE 2019; USACE/NPS 2015). For the purpose
of feasibility study and planning, this elevation is set at +6 ft NAVD. The volume
above the berm elevation is a measure of the sand quantities shifted toward the
dunes and upper beach. Therefore, this is a measure of storm and flood protection
levels associated with the project or gains in dune volume due to post-project buildup
above the contour.
 Lens 2 (Beach to Low Tide Terrace ) — This lens encompasses the active beach to low -
tide wading de pth (from +7 to −6 ft NAVD). It includes the primary recreational portion
of the beach and the surf zone , where most wave -breaking occurs.
 Lens 3 (Underwater to FEMA Reference Depth ) — This lens represent s the outer
breaker zone and extends to the FEMA re ference depth (from −6 ft to −24 ft NAVD). It is
the area beyond which there is relatively little change in bottom.

Lens 2
Lens 3
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2.4 Historical Erosion Rate
2.4.1 Methodology of Aerial Photograph Analyses
CSE used the project baseline to reference sh oreline positions depicted on various historical aerial
photographs. Following standard practice, shoreline position from photographs generally
references a morphologic feature. Two commonly identified points are the seaward vegetation
line, which usually corresponds to a point near the foredune crest along Hatteras Island, and the
“dry-sand wet-sand” contact line on the active beach which approximates mean high water. We
also reviewed shoreline change rates determined by others using similar map/photo s ources and
methodology.
Before presenting results of the “linear” shoreline change analyses completed by the state of North
Carolina (ie – official erosion rates) (NCDENR 2012) and CSE’s present analyses, it is useful to
consider how such shoreline delinea tions may not represent consistent positions. Figures 2.10
and 2.11 illustrate ways that the beach condition at a particular time or erosion stage can vary.
Figure 2.10 shows a sequence of idealized profiles as they evolve over a storm period. The initi al
profile includes a vegetated dune, dry -sand beach, and sloping beach face. Mean high water (MHW)
falls along the sloping beach some distance seaward of the dry beach level. This reflects the fact
that breaking waves produce runup above the high tide l evel pushing sand to a higher level. If a
storm impacts the profile, the sloping beach cuts back leaving a low scarp (2 nd panel). Given
sufficient time during a storm, the scarp may cut back to the base of the dune. Further erosion
then undermines the foredune leaving a narrow remnant (4 th panel). Mean high water shifts
landward in proportion to the intensity of the storm as illustrated in Figure 2.10. If a large dune is
present, it will take a major storm to erode it away. After the dune crest is los t, storm waves can
overtop the remnant feature and drive sand inland producing a “washover” and depositing sand
inland. Delineation of MHW shoreline, therefore, depends on the stage of the beach and whether
the imagery was obtained before or soon after a storm. Note that in these examples, the seaward
vegetation line along the dune does not move as much as MHW until the dune is washed away
(bottom panel).

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FIGURE 2.10. A sequence of beach conditions associated with erosion events
illustrating the greater variation in “shoreline” position using mean high water
(red arrows) versus the seaward vegetation line (green arrows). [Source: CSE©]

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Figure 2.11 shows how some profiles grow seaward over time. The initial profile is the same a s the
previous figure but with a sustained dry -sand beach, wave runup leaves a debris line (“wrack”)
which serves as a seedbed for pioneering plants. The plants begin to trap sand and an incipient
dune starts to form along the wrack lines. If the beach r emains stable or grows seaward, the
incipient dune will grow and become a new foredune (lower panel). Thus, interpreting the seaward
vegetation line is difficult for the “Time 1” condition in the middle panel, but relatively easy once
vegetation is well established (top and bottom panels).
Because shoreline position interpretation from aerial photos or historical charts can be
problematic, short time intervals between aerial photos will tend to yield more variable rates of
change than long time intervals. The official erosion rates for Avon utilize a ~60 –65-year period to
eliminate some of the errors associated with the stage of the beach at the time of the available
photography (see next section).

FIGURE 2.11. Beach profile evolution under stable or accreting conditions
showing emergence and growth of a new foredune and the effect on
shoreline position using the seaward vegetation line (green arrows).
[Source: CSE©]

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[2525–Appendix D] Avon Village, Dare County, North Carolina 22
2.4.2 Historical Shorelines
NC Department of Natu ral Resources (NCDNR – formerly NC Department of Environment and
Natural Resources – NCDENR) periodically publishes official, long -term, average annual
oceanfront erosion rates (“setback factors”) for North Carolina. An early analysis was prepared by
Tafun et al (1979) who applied the “end -point method,” which was retained by NCDENR in
subsequent updates. The end-point method computes average annual rates at each shoreline
transect using the earliest and most -current shoreline position. The earliest shor elines considered
in prior analyses are typically based on NOAA’s National Ocean Service (NOS) “T -sheets” from the
1920s to 1930s. More recent shorelines are interpreted from controlled aerial photography using
the “wet/dry” line at the edge of the surf z one. This line approximates local MHW at the time of the
photography (Overton & Fisher 2003). NCDENR (2012) details the various blocking and smoothing
algorithms applied in developing official rates along Dare County. That study utilized imagery from
1946/49 and July 2009 (endpoints), which predated a series of significant storm events over the
past decade (eg – Hurricane Sandy 2012; Matthew 2016). Figure 2.12 shows the results of the
NCDENR (2012) analysis and is included here to illustrate the variabil ity along Hatteras Island near
Avon.
A striking aspect of the NCDENR (2012) shoreline change rates along Hatteras Island is their large
variation alongshore. Long barrier islands with few active inlets often exhibit more uniform
shoreline change rates (Hayes 1994). This is certainly the case north of Oregon Inlet along most of
Bodie Island or along Bogue Banks (NCDENR 2012). By comparison, some short segments of
Hatteras Island have zones of moderate accretion (>5 ft/yr) in close proximity to zones of hi gh
erosion (>10 ft/yr). The Avon area demonstrates both conditions. Avon extends from NCDENR
stations ~7315 to ~7425 (See Figure 2.12). The south half of the project area experienced erosion
at 2–5 ft per year between 1949 and 2009 (NCDENR 2012, Fig 2.13–2.15). By comparison, the north
half of Avon accreted at 1–4 ft/yr during the same period. This result is consistent with findings of
Everts et al (1983), who determined rates for 1949 –1980 at five transects along Avon (by 1.0 -minute
latitude intervals, or every ~6,000 ft) (Table 2.2).

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[2525–Appendix D] Avon Village, Dare County, North Carolina 23

Latitude Location 1849–1915 1849–1980 1915–1949 1915–1980 1949–1980
35° 22΄ North Avon 4.0 0.2 –8.5 –3.9 1.2
35° 21΄ ~ Pier 0.8 –0.5 NO –1.9 NO
35° 20΄ –0.1 –0.3 –1.3 –0.6 0.1

35° 19΄ 0.3 –1.0 –3.2 –2.0 –1.1
35° 18΄ South Avon –0.4 –2.0 –5.6 –3.7 –1.4
Mean Ocean Shoreline Changes from Oregon Inlet to Cape Hatteras
1852–1917 1917–1949 1949–1980 1852–1980
0.4 –2.9 –1.3 –1.1
TABLE 2.2. Historical accretion (+) and erosion (–) rates for Avon at ~1 nautical mile (ie – one minute of latitude)
transects using high tide lines as determined by US Coast and Geodetic and present-day NOAA surveys. Results
in meters per year; 1 meter = ~3.28 ft. [Source Everts et al 1983]

FIGURE 2.12 Long-term shoreline change rates for Hatteras Island around Avon derived from historical aerial
photography (1946/49 and 2009) showing smoothed and blocked data by transect as prepared by NCDENR and
their consultants. Shoreline segments in red exhibit long-term erosion, whereas, green indicates accreting areas.
Avon, situated between transect IDs ~7320 and 7420, shows trends of erosion along the southern half of the
community and accretion north of Avon Pier. [Source: NCDENR 2012, Figs 2.13–2.15]
Coastal Science & Engineering Littoral Processes
[2525–Appendix D] Avon Village, Dare County, North Carolina 24
The Everts et al (1983) “30 -yr” shoreline change rate for Avon (1949–1980) ranged from +1.2 to –1.4
meters per year (~+3.9 to –4.6 ft/yr). NCDENR (2012) “60-year” rates, using 1949–2009 imagery,
ranged from +5.5 to –4.5 ft/yr. NCDENR (2012) published “setback” factors for Avon based on these
data (Fig 2.13). Stable and accreting areas are assigned a 2 ft/yr setback factor irrespective of the
specific accretion rate. Figure 2.13 shows the highest erosion along South Avon at 4.5 ft/yr (Note :
Avon Pier is situated near Dune Way where the rates go from 2 to 3 ft/y r at the left-center). The
figure also indicates prior maximum erosion at 3 ft/yr for the 2003 rates. This yields the official
erosion rates as determined by NCDNR.
NCDNR-Division of Coastal Management (NCDCM) now maintains a web -link to linear erosion rate
data for Dare County (NCDENR.maps.arcgis.com). CSE downloaded the most recent NCDNR rates
in June 2020 and noted there have been slight increases in erosion rates since the 2012 maps were
published (Fig 2.14). For example, the maximum setback factor has increased from 4.5 ft/yr to 6.0
ft/yr along south-central Avon as of 2020. Accretion remains the predominant trend north of the
pier and along CAHA at the south end of the Village. Data on the NCDCM website show continued
accretion along North Avon at rates up to 4.6 ft/yr near Wahoo Circle. The highest official erosion
rate (2020) is 6.6 ft/yr in the Ocean View Drive neighborhood near Pampas Drive around 4,000 ft
south of Avon Pier.
The present prorated average official erosion rate (or “setback factor”) along South Avon is ~4.7
ft/yr. In next section we will discuss how these linear change rates can be converted to equivalent
volumetric rates in the Avon setting. Using an adopted ratio of dry beach loss to volumetric loss
(1.15) yields an average annual loss rate of roughly 5.4 cy/ft/yr along South Avon (applicable to
~10,750 lf).

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[2525–Appendix D] Avon Village, Dare County, North Carolina 25

FIGURE 2.13. Official (2012) erosion rates (“setback factors”) for the Avon area as determined by NCDENR (now NCDNR) Division of Coastal Management using 1949 – 2009 aerial imagery.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 26

FIGURE 2.14. NCDCM official erosion rates (“setback factors”) in feet per year for Avon. Note ~35% increase
since the 2012 condition and ~100% increase since the 2003 condition on Figure 2.5. [Source: NCDNR-DCM 2020]

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[2525–Appendix D] Avon Village, Dare County, North Carolina 27
2.4.3 Recent Variations in Shoreline Change Rates
As the previous section of this report describes, official erosion rates (setback factors) have
increased along Avon between 2003 (NCDENR 2012) and 2020. North Avon remains healthy and
accretional, but South Avon exhibits roughly 100% higher erosion rates. Official state rates
appropriately consider a relatively long period of time (40 –70 years) in the analysis to avoid short-
term bias associated with storm events and seasonal variations in the high water line.
CSE completed an independent analysis of shoreline change, using a combination of published
lines and aerial photo interpretations. A total of 12 d ates of surveys and imagery were utilized
spanning the years 1980 to February 2020. Everts et al (1983) performed a rigorous analysis of
earlier shorelines and presented average rates for several periods between ~1850 and 1980 (See
Table 2.2).
CSE downloaded NCDCM shapefiles and digitized selected recent images to generate a set of
historical shoreline maps for the project area (Figure s 2.15a and 2.15b). Each panel includes the
“1852” and “1946” shorelines for general comparison. CSE used the project b aseline and
measured offset distances between the baseline and each shoreline at 500 ft intervals.
Results for individual stations were averaged by reach, using five segments of shoreline exhibiting
similar trends of accretion or erosion. A cursory view of the shoreline maps (Figure s 2.15a-b) shows
that the 1852 and 1946 shorelines fall well seaward of recent shorelines along South Avon, but
track close to recent shorelines along the north end of the Village. To help distinguish recent
trends, CSE plotted the average shoreline position over time along each reach. The first graphs
(Fig 2.16 upper and lower) plot the data from the earliest two surveys to the present (from 1852
and 1946, respectively). These graphs confirm the “century” trends of shorelin e advance north of
Avon and “75-yr” advance since the 1940s along North Avon. All other reaches exhibit shoreline
retreat since the 1850s or 1940s. The other distinct trend is accelerated recession along South
Avon, starting around 2000.
Figures 2.17a and 2.17b plot the data for “Forty-Year,” “Twenty-Year,” “Ten-Year,” and “Five-Year”
periods along with trend lines to isolate change rates by reach for each period. The graphs also
include an overall average for Reaches 3 –5 which represent the principal pr oject area extending
from Station 1555+00 (~3,500 ft) north of Avon Pier to the approximate southern Village limit.
Reaches 1 and 2 at the north end of the study area generally show accretion for all periods. Earlier
data have been omitted in each graph to isolate the trends for the period.
The “Forty-Year” and “Twenty -Year” trends show an increase in average erosion rates from ~7.5
ft/yr to 10 ft/yr (Fig 2.17a) along Reaches 3–5. Reach 4, encompassing ~8,000 lf between stations
1590+00 and 1670+00 increased from ~10.7 to 14.4 ft/yr. These rates are higher than official erosion
rates (described in Section 2.4.2) because they encompass shorter and more recent time periods.
Coastal Science & Engineering Littoral Processes
[2525–Appendix D] Avon Village, Dare County, North Carolina 28
As the graphs depict, shoreline retreat was at a slower rate in the 1980s and 199 0s compared with
the 2000s and 2010s.
Figures 2.17a-b shows the shoreline movement trends during the past decade. The Ten -Year
trends are similar to the Twenty -Year trends in magnitude. However, the Five -Year trend (Figure
2.17a-b, lower) shows a decreas e in shoreline retreat to –6.4 ft/yr (Reaches 3–5 combined) and –
3.7 ft/yr (Reach 4), albeit based on only three survey dates.
The worst erosion along Avon in recent years has impacted a ~4,000 lf section from ~1,000 to 5,000
ft south of Avon pier. Figure 2.18 shows the change in shoreline position for representative stations
in this area since 1997. Stations closest to the pier (Fig 2.18 upper) show recession rates averaging
over 17 ft/yr (1997 –2000). At 3,000–4,000 ft south of the pier this “Twenty -Year” trend is ~16 ft/yr
recession. These results are in distinct contrast to changes between 1946 and 1998 (Fig 2.18 lower).
The limited data available shows a recession in this same area at under 3 ft/yr for the earlier period.

FIGURE 2.15a. Historical shorelines for the Avon area used in the present
analysis (various sources) as given in the text.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 29

FIGURE 2.15b. Historical shorelines for the Avon area used in the present analysis (various sources).

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[2525–Appendix D] Avon Village, Dare County, North Carolina 30

FIGURE 2.16. Changes in average shoreline position by reach along Avon for 1852 to 2020 (upper)
and 1946 to 2020 (lower). Note the general acceleration of erosion for most reaches since 2000.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 31

FIGURE 2.17a. “Forty-Year” and “Twenty-Year” erosion trends for Avon averaged by reach. The value
before the “x” in each equation is the erosion (or accretion) factor in feet per year. Rates in the black boxes
are averages for Reaches 3, 4, and 5, the main reaches of interest.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 32

FIGURE 2.17b. “Ten-Year” and “Five-Year” erosion trends for Avon averaged by reach. The value before
the “x” in each equation is the erosion (or accretion) factor in feet per year. Rates in the black boxes are
averages for Reaches 3, 4, and 5, the main reaches of interest.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 33

FIGURE 2.18. Trend in erosion for two periods at stations within the most critically eroded section
of beach. Note much higher rate of retreat for 1997–2020 compared with 1946 to 1998 (lower).
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[2525–Appendix D] Avon Village, Dare County, North Carolina 34
2.4.4 Rhythmic Shoreline Changes
The previous section analyzed typical shoreline positions by r each using averages of numerous
points along the shoreline. A characteristic of Hatteras Island beach is systematic variation in
visible beach width i n the longshore direction. This variation was apparent in February 2020 at the
time of a UAV flight for the orthorectified aerial photos as shown in Figure 2.19. The figure shows
conditions on February 18th along the critically eroded section of Avon sou th of the pier. Three or
four sections of the beach were considerably wider than adjacent areas that day, particularly
around stations 1620+00, 1650+00, and 1675+00.
Rhythmic beach features alongshore occur at various scales and are generally associated with
inshore bathymetric variations and the particular stage of the beach cycle (Wright and Short 1983).
Where an offshore bar is higher or closer to shore, the dry -sand beach in its lee will tend to be wider.
Conversely, where the bar is lower or missi ng, more wave energy can reach the shore and scallop
out the visible beach. Similar features were described by Fenster and Dolan (1993) for the Avon
area in a University of Virginia study. Inman and Dolan (1989) referred to large -scale variations in
beach width as “erosion-accretion waves” and noted there is a tendency for these features to
migrate alongshore in the direction of predominant (or “net”) sediment transport. CSE has
documented the scale and magnitude of such features at Pine Island (Currituc k County) and Nags
Head (CSE 2015, 2018).
At Avon, some historical shorelines evaluated by CSE show an amplitude (beach width) fluctuation
of over 150 ft and wave length (spacing between wide or crest sections) of ~2,500 –3,000 ft (Fig 2.20).
The February 2020 aerial image (Fig 2.19) showed rhythmic shoreline topography at these scales.
Figures 2.15a-b shows an aerial photo obtained in September 2018. In that case, the typical
amplitude of the features is around 50 ft and the spacing between crests is of the order of 1,000 ft.
These differences reflect the seasonality of the beach cycle.* The summer is usually a time of
beach buildup under lower wave conditions. This tends to smooth out or fill in erosional arcs and
thereby reduce differences in beach width. So an aerial photo in September is likely to exhibit less
variation in beach width (unless a hurricane has impacted before the survey!). Fall and winter
storms shift sand offshore and produce more irregular topography in the surf zone. This can l ead
to large-scale variations in visible beach width illustrated by the 2020 image.
[*Beach cycle – Systematic changes in the width and slopes of the visible beach and development
of underwater bars based on changes in wave energy. Generally, this refers to the process of
onshore-offshore sand transport between fair weather and storm conditions. If the beach cycle is
in balance, there are no net losses of sand in the littoral zone.]

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[2525–Appendix D] Avon Village, Dare County, North Carolina 35

FIGURE 2.19. Rectified aerial photo
of south Avon (note pier at the top of
the image) via UAV showing large-
scale variations in beach width.
These longshore sand waves or
rhythmic topography are common
during certain beach cycles,
particularly in winter after the visible
beach has eroded. They tend to
propagate to the south along Avon
in the direction of predominant sand
transport. [Source: CSE©]
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[2525–Appendix D] Avon Village, Dare County, North Carolina 36

Beach width variations described above become important if the seasonal magnitude exceeds the
average beach width. For example, if the “trough” of a longshore sand wave is too deep, it will cut
into the toe of the foredune and potentially impact developed property. Yet, some short time later,
the same section of beach may widen by hundreds of feet as the crest of the sand wave propagates
into the area. The implications of this for the present study is that successful beach restoration
has to account for the cyclic and localized beach width variatio ns as well as the long-term
shoreline erosion rate. The official setback factors published by the state (see Fig 2.13) are long-
term averages and generally well below 10 ft/yr for any point along Avon, and may alternate
between erosion and accretion for p articular localities during the same period. One of the
principal objectives of the present study is to determine how much sand is needed along Avon
beach to provide sufficient width for alongshore sand waves to propagate through without adverse
impacts to property and community infrastructure.
Section 2.5 describes how we can reduce some of the short -term variability in beach condition by
volumetric analysis. Rather than considering just one contour along the beach such as MHW,
volume analysis evaluates the entire active profile in the beach zone and computes reference
quantities that integrate all the small -scale variations in the beach and surf zone (Kana 1993; Kana
et al 2015).
FIGURE 2.20. Variation in high water shoreline position by station for September 2018 and
February 2020 illustrating rhythmic longshore variations in beach width (see text for
discussion).

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[2525–Appendix D] Avon Village, Dare County, North Carolina 37
2.5 Volume Analysis of July 2020 Survey
2.5.1 July 2020 Field Work
Field data collection began in July 2020 after delays associated with weather and island closures
due to the COVID-19 pandemic. As briefly discussed in Section 2.2, CSE completed the first
comprehensive survey of Avon beach using a project baseline and surv ey profiles from the dune
line out to ~35 ft water depths at 500 ft spacing between profiles from stations 1470+00 to 1700+00.
Table 2.3 summarizes the field data collection performed by CSE. Bathymetric survey lines are
shown in Figure 2.1 in the previous section.

Beach Condition Survey Completion Date
Orthoimagery of the Priority Area from Avon Pier to Askins Creek North Drive 18 February 2020
Beach Condition Survey — 500 ft spacing from stations 1470+00 to 1700+00,
a total of 47 profiles encompassing 23,000 linear feet (~4.35 miles) including 2,500
linear feet the Cape Hatteras National Seashore adjacent to the Avon Village.
6–8 July 2020

Sediment samples along 9 transects — 1470+00, 1500+00, 1530+00, 1560+00,
1590+00, 1620+00, 1650+00, 1670+00 , 1700+00 at four locations, ie – foredune, dry
beach, wet beach, low tide mark.
7 July 2020
Oblique aerial imagery of the survey area at low tide. 15 July 2020
Orthoimagery of the dune and dry -sand beach along bird closure areas
(stations 1470+00 to 1485+00) 15 July 2020

Ground photos were taken at representative monitoring stations. This offers a simple visual
assessment of dry beach width, dune condition, vegetative growth, escarpments, and general
condition of the beach through time. Photos wer e also taken of any areas or features of par ticular
importance or interest observed during the survey and site visits (Figure 2.21a-d).
CSE utilized the P4P system to take oblique aerial photos during the July 2020 survey and multiple
beach inspections through time. These photos provide unique perspectives of the littoral zone up
to an above-ground level of 400 ft (FAA ceiling for UAS). Selected aerial photos marked with
notations of reaches are shown in Figure 2.22a-c.
TABLE 2.3. CSE’s field work completed for the Avon Village shoreline assessment and beach nourishment study.
Coastal Science & Engineering Littoral Processes
[2525–Appendix D] Avon Village, Dare County, North Carolina 38
FIGURE 2.21b. Ground photo of the Avon study area (July 2020) at Station 1600+00 looking south. This section
of beach experienced accelerated erosion over the past few years, and is within the proposed project area.

FIGURE 2.21a. Ground photo of the Avon study area (July 2020) at Station 1600+00 looking north. Avon Pier
is shown near the top of the photo. This section of beach experienced accelerated erosion over the past few
years, and is within the proposed project area.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 39
FIGURE 2.21c. Ground photo of the Avon study area (July 2020) at Station 1555+00 looking south. Avon Pier is
shown near the top of the photo. This section of beach (Reach 3) is relatively healthy compared to Reach 4, and
can be used as a feeder area for Reach 4 in the proposed nourishment project.
FIGURE 2.22a. Aerial photo taken on 15 July 2020 showing the highly eroded section of Avon (Reach 4).

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[2525–Appendix D] Avon Village, Dare County, North Carolina 40
FIGURE 2.22b. Aerial photo taken on 15 July 2020 showing the middle and south sections of Avon.
FIGURE 2.22c. Aerial photo taken on 15 July 2020 showing the middle and north sections of Avon and
CAHA. The northern boundary of the proposed nourishment project is located near the northern end of
Reach 3.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 41
2.5.2. Unit Volume Analyses
Although sediment transport and morphology changes in the nearshore are three -dimensional,
it is customary in beach analysis to separately consider the cross -shore and planform (ie –
alongshore) evolution. Survey data (collected in x -y-z format) were converted to x -z (distance to
baseline—elevation) pairs for purposes of comparing beach conditions among cross -shore
profile lines.
A long-time rule -of-thumb used by the Corps of Engineers since the first nourishment project at
Coney Island (NY) in 1923 assumes a loss of one (1) square foot (ft²) of dry beach area is roughly
equivalent to a loss of 1 cubic yard (cy) of sand (CERC 1984). This ratio has also been assumed for
some analyses for NCDOT (Prof. M. Overton, NC State University, pers comm, October 2013). It can
be shown that this ratio varies according to the dimensions of the active zone of profile change,
but remains constant between fixed contours regardless of foreshore slope (Bruun 1962, Hands
1981, Dean 2002, Kana et al 2015). Normally, the vertical dimension considered extends from the
dry-beach elevation to the depth of closure (DOC). For example, if the average height of the dry
beach is +7 ft NAVD and the local DOC is −20 ft, there will be 27 cubic feet (cf) contained in o ne (1)
ft² of “beach” area (Fig 2.23).

Conveniently, 27 cf equals 1 cy, so the volume (cy) to area (square feet) ratio equals 1. This ratio
is >1 for beaches that exhibit a deeper DOC and <1 for beaches with a shallow DOC. Figure 2.24
illustrates how the volumetric erosion rate varies with the linear erosion rate, as well as the local
DOC.

FIGURE 2.23. Volume equivalents on a beach . Example assumes the active beach zone extends from the dry -sand
beach elevation at +7 ft to an offshore depth of 20 ft. Therefore, 1 ft² of dry beach area represents ~27 cf of profile
volume. This ratio remains constant by simple geometry for a parall elogram with equal end -surface area. This
concept can also be used to convert linear shoreline change to equivalent unit volume change. For example, 1 ft of
dry beach recession, in this example, is equivalent to 27 cf (per foot of shoreline length) sand loss, ie – 27 cf = 1 cy.
The ratio varies with the vertical dimensions of the littoral zone (Bruun 1962, Hands 1981) (After Kana et al, 2015).
Coastal Science & Engineering Littoral Processes
[2525–Appendix D] Avon Village, Dare County, North Carolina 42

At Hunting Island (SC), for example, the inshore area is a relatively constant −12 ft NAVD (de facto
DOC), and the dry beach equilibrates around +7 ft. This means 1 ft² of area loss on the visible beach
equates to about 0.7 cy of volume loss [ie (7+12)/27 ≈ 0.7]. Similarly, if DOC off Avon is assumed to
be −24 ft NAVD and the average dry -beach elevation is +7 ft NAVD, 1 ft² of beach area loss will
equate to about 1.15 cy of volume loss [ie (7+24)/27 ≈ 1.15]. CSE used this latter ratio to convert
linear erosion rates to volumetric rates in the project areas.*
[*Overton and Fisher (2005) assumed a similar dry -beach elevation, but a deeper limit of sand
exchange (−30 ft MSL), which equates to a ratio of 1.37 (ie – 1 ft of beach widening requires 1.37
cy/ft of nourishment). The authors report that the ratio “. . . would need to be refined during the
engineering design phase of the beach nourishment project.” (pg 7). Their report was prepared
prior to major nourishment projects in the Northern Outer Banks an d the availability of post-
nourishment profiles into deep water. CSE has adopted –24 ft NAVD for DOC in the nearby Buxton
area based on surveys between 2013 and 2020. While the DOC assumed by Overton and Fisher
(2003) is deeper and therefore more conserv ative, CSE believes –24 ft NAVD and a ratio of 1.15 is
realistic along Avon for project planning.]
FIGURE 2.24. Example of the relationship between unit volume erosion rate and linear erosion rate for
two beaches. The solid line shows a variable linear erosion rate alongshore, much like the trend and
magnitudes for Avon. The upper dashed curve shows an estimated equivalent unit volume erosion rate
for high-energy sites with a relatively deep limit of normal sand movement (DOC). The lower dashed
curve shows the equivalent volume for a lower-energy site where the DOC is shallower. The ratio that
CSE assumed for Avon is 1.15, based on a DOC ≈ 24 ft NAVD.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 43
Unit volumes for the Avon study area were calculated to determine the quantity of sand in one
linear foot of beach at each lens and each survey station. Th ese unit volumes were then used to
calculate the average unit volume along each reach. The unit volumes of the three lenses at each
survey station are listed together with the profile plots in Attachment 2. Unit volumes are the key
elements to determine the sand deficit and formulate shoreline management plans.
Figures 2.25, 2.26, and 2.27 show unit volumes station by station for each lens and cumulative
lenses using the July 2020 survey. The five reaches delineated for the study area are marked in the
figures, and the horizontal lines (in red) show the average unit volume of each reach. The average
unit volumes of each reach are summarized in Table 2.4.

FIGURE 2.25. Unit volumes by station in the foredune areas (Lens 1) along the Avon study area using the July
2020 survey.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 44

FIGURE 2.26. Unit volumes by station in the foredune and beach areas (Lenses 1 and 2) along the Avon
Village study area using the July 2020 survey.
FIGURE 2.27. Unit volumes by station in the foredune, beach, and underwater areas (Lenses 1, 2, and 3,
ie – entire profile between the foredune and –24 ft NAVD) along the Avon study area using the July 2020
survey.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 45

2.5.2.1 Dune – Lens 1 (From Dune Crest to +6 ft NAVD)
The first reference lens uses the dune crest as a starting point. The dune crest is a convenient point
of comparison because it tends to mark the seaward edge of stable vegetation and defines the
morphology and shoreline azimuth along beaches. The dune crest position varies les s than the
daily high watermark (surf swash line) or any contour along the intertidal beach zone. Variations
in dune position or seaward vegetation line can obviously occur in areas where the dunes have
been manipulated by scraping or breached during stor ms. To account for such variations, CSE
checked the indicated dune position on surveyed profiles against the overall shoreline morphology
and adjusted the reference calculation starting point as necessary to minimize volume variations
associated with majo r offsets in dune position among adjacent stations.
The +6 ft NAVD contour was chosen because most of the nourishment construction berm s along
the northern Outer Banks are designed at +6 ft with some areas up to +7 ft NAVD. The volume from
the dune crest to +6 ft elevation is a measure of storm and flood protection levels. It is also the
zone that will accumulate aeolian transport and grow dunes after nourishment.
The July 2020 survey results show that the 8,500 linear feet of shoreline along Reaches 1 an d 2 (~1.6
miles along CAHA and North Avon Village) are relatively healthy with dune volumes of 56 cubic
yards per foot (cy/ft) of shoreline and 66 cy/ft (respective ly). The 8,000 ft of shoreline along Reach
4 (south of the Avon Pier to Askins Creek North Drive) has the lowest dune volume averaging 13
cy/ft, which is 20 cy/ft less than Reach 3 (3,500 ft north of the Avon Pier), 27 cy/ft less than Reach 5
(3,000 ft south of Askins Creek North Drive), and 43–53 cy/ft less than the most northern sections
of Avon along Reaches 1 and 2.

TABLE 2.4. Summary of average unit volumes by reach in the three reference lenses along the Avon study area using
the July 2020 survey.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 46
2.5.2.2 Beach – Lens 2 (From +6 ft NAVD to –6 ft NAVD)
The second lens uses –6 ft NAVD contour as a reference elevation. It includes the dry -sand beach
and the wet-sand beach extending to low -tide wading depth. This is not on ly the primary
recreational portion of the beach but also the inner surf zone where a significant proportion of
wave-breaking and energy dissipation occurs.
The July 2020 survey results show that unit volumes in this lens exhibit similar trends as Lens 1.
Reach 4 has the lowest unit volume of 60 cy/ft, which is 33 cy/ft less than the adjacent north area
(Reach 3) and 39 cy/ft less than the adjacent south area (Reach 5). If compared to the most
northern section of Avon (Reach 2) and CAHA (Reach 1), the unit volume of Reach 4 in Lens 2 is only
~30% of the unit volumes in those two northern reaches. In simple terms, low unit volumes in this
part of the littoral zone correspond to narrower or steep beaches .
2.5.2.3 Underwater – Lens 3 (From –6 ft NAVD to –24 ft NAVD)
The third lens is determined to be between –6 ft and –24 ft NAVD contours. This represents the
outer surf zone extending seaward from low -tide wading depth to the closure depth set forth for
the FEMA post-storm restoration criteria.
The overall trend of July 2020 survey results of Lens 3 is consistent with those of Lenses 1 and 2.
Reach 4 has the lowest unit volume of 676 cy/ft, which is 66 cy/ft less than the adjacent south area
(Reach 5) and 74 cy/ft less than the adjacent north area (Reach 3). If compared to the most northern
section of Avon and CAHA, the underwater volume of Reach 4 (Lens 3) only contains about 70% of
the Reach 1 and Reach 2 volumes. This equates to ~250–300 cy/ft less sand in the profile.
2.5.2.4 Entire Profile – Lenses 1, 2, and 3 (From Dune Crest to –24 ft NAVD)
The three reference lenses, combined, represent the full active littoral profile between the
foredune and deep water during normal annual conditions along Avon beach. The present results
indicate that Reach 4 (south of the Avon Pier) has the lowest unit volume of 748 cy/ft, which is 128
cy/ft less than Reach 3, 133 cy/ft less than Reach 5, 440 cy/ft less than Reach 1, and 471 cy/ft less
than Reach 2.
These profile volume differences along Avon provide a basis for the proposed nourishment volume
determinations. We used the results to determine the profile deficit with respect to an ideal or
target beach condition.
2.5.2.5 Formulation of the Proposed Nourishment Project
To improve longevity and protect a res tored dune, a considerable amount of additional volume
would be needed along the recreational beach into shallow water. Nourishment longevity
increases geometrically with project length, so we have incorporated additional sections of beach
to preserve sand in the critically eroding area much longer . Applying the volumetric data discussed
in the previous sections, the County decided a “Five-Year” Plan involving 1,000,000 cy over 13,200
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[2525–Appendix D] Avon Village, Dare County, North Carolina 47
lf of shoreline as shown in Figure 2.28 . This would encompass Reaches 3 and 4 and extend several
hundred feet to the north and to the south. The proposed project area is from Station 1550+00 to
Station 1682+00 which starts ~4,000 ft north of Avon Pier and extends to the south end of Avon.
The Five-Year Plan assumes an aver age deficit volume of 53 cy/ft and advance nourishment of ~6.8
cy/ft/yr. These adopted erosion rates are equivalent an estimate erosion rate of 140,000 cy/yr
along the 13,200-ft project area.

FIGURE 2.28. Proposed beach nourishment at Avon showing the project limits and offshore borrow area.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 48
3.0 COASTAL PROCESSES
Avon is subject to coastal processe s (winds, waves, tides, and currents) typical of the northern
North Carolina coast. The Outer Banks in this area is exposed to ocean -swell waves originating
from the southeast and storm waves associated with nor ’easters. The highest waves are generally
associated with tropical storms and may occur in phase with hurricane su rges. Hurricane waves
can approach from all onshore directions as the storms track through the area. The spring tide
range is ~3.5 ft (NOAA-NOS 1983), and tides are semi -diurnal. Previous studies and geomorphic
evidence suggest that net longshore transport (ie – sand movement in the littoral zone) is
predominantly southerly (Inman & Dolan 1989). A thorough coastal processes study was
conducted for an adjacent site at Buxton, ~2 miles downcoast of the proposed project area
(USACE/NPS 2015 – Appendix A – Littoral Processes). This section details littoral processes
affecting the proposed Avon project area and addresses specific questions regarding the potential
impact of the proposed project on these processes.
The use of an offshore borrow area can influence waves, thereby modifying local sand transport
rates. Depending on the geometry of the borrow area, the excavation may effectively reduce wave
heights in part of the affected area and cause wave heights to increase elsewhere. To quantify the
changes in waves due to the borrow area and potential impact on sedimen t transport, wave height
over the potential borrow site was analyzed to compare pre -dredge conditions with anticipated
post-dredge conditions. Sediment transport was examined to determine how local increases in
wave energy density due to the presence of the borrow area might affect the regional sand -
transport potential.
The placement of nourishment sand on the beach may pot entially impact sediment trans port
along other strategic locations. Closure depth (the approximate limit of measurable bottom
change over particular time scales) was examined in the Buxto n area (~2 miles south of the Avon
project area) for the 2017–2018 project because it is an important consideration in locating the
borrow site (USACE/NPS 2015). It is beneficial for borrow sites to be located offshore of the depth
of closure location so that they will be independent of the littoral system at decadal time scales
for planning. Borrow site locations shoreward of the closure depth position may simply shift
sediment within the littoral zone and have minimal impact on the net sand volume change.
The steady–state spectral wave model (STWAVE) was used in this study to evaluate the changes of
wave patterns before and after dredging of the proposed borrow area. The generalized model for
simulating shoreline change (GenCade) was used to evaluate the impact on sediment transport
caused by the placement of nourishme nt sand. The GenCade model is a next -generation
combination of previous long -term planform evolution of beach models GENESIS (GENEralized
Model for Simulating Shoreline) and Cascade. GenCade is a regional model for calculating coastal
sediment transport, morphology change, and sand bypassing at inlets and engineered structures.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 49
Both models are developed and approved by the USACE and have been widely used by coastal
engineers and community planners in predicting the behavior of shorelines and sediment
transport. Information on each model is available in USACE (2001), Hanson and Kraus (1989),
Larson et al (2006), and Frey et al (2012).
3.1 Wave Climate
Offshore wave information is typically obtained from a wave gauge or a global/regional scale wave
hindcast or forecast. Nearshore wave information is required for littoral processes analysis and
for the design of almost all coastal engineering projects. Waves drive sediment transport and
nearshore currents, induce wave setup and runup, excite harbor osc illations, and impact coastal
structures. The longshore and cross-shore gradients in wave height and direction can be as
important as the magnitude of these para meters for some coastal design criteria .
Two types of wave stations are available offshore of the study area. One is a real-time wave buoy
at Diamond Shoals located ~19 miles offshore of Avon with 18 years of wave records from 2003 to
2020, and the other is a hindcast wave station located ~11 miles offshore with 40 years of records
from 1980 to 20 19.
3.1.1 Real-Time Wave Buoy – Station 41025
Station 41025 at Diamond Shoals (NC), owned and maintained by the National Data Buoy Center
(NDBC), appears to be the closest real-time wave buoy to the Avon project site. The station is
located at 35.010 N 75.454 W, ~15 miles southeast of Cape Hatteras (Fig 3.1). The water depth at
the station is ~160 ft, and the watch circle radius is 122 yards . This station has recorded wind and
wave data since 2003; however, there were no wave direction records until 20 12. Desiring 8760
hourly records for a typical year, there are four years (ie – 2011, 2012, 2013, and 2019) when actual
records are less than 65 percent of expected records. In addition, for 2011, March, April, and June
have nearly full records; for 2012 and 2013, no month has nearly full records; and for 2019, July
and August have nearly full records. Detailed description of the station and the collected data are
located at NDBC’s website (https://www.ndbc.noaa.gov/adcp_data.php?station=41025 ).
The wave height, dominant period, and direction analyses based on available data are listed in
Table 3.1. It shows that June, July, and August have the lowest wave heights compared to other
months.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 50
FIGURE 3.1. Station 41025 at Diamond Shoals (NC), owned and maintained by the National Data Buoy Center
(NDBC), appears to be the closest real-time wave buoy to the Avon project site. The station is located at 35.010° N
75.454° W, ~15 miles directly offshore of Cape Hatteras (NC) and ~19 miles from Avon in water depth of ~160 ft. The
hindcast wave station from the USACE’s Wave Information Studies (WIS) Station 63230 is located at 35.35° N and
75.33° W, ~11 miles southeast of Avon in water depth of ~60 ft.
TABLE 3.1. Monthly average wave climate from 2003 through 2020 at NDBC wave buoy
station 41025 at Diamond Shoals (NC). [Source: NDBC] Wave direction uses meteorological
convention. A direction of 0° corresponds to a wave arriving from True North. Similarly, a
direction of 90° corresponds to a wave from due east. Wave direction records are available
only for the period after 2012 at this station. Four years (2011, 2012, 2013, and 2019) have less
than 65% of expected records.

18-Year Record (2003–2020) at Diamond Shoals
Wave Height (ft) Dominant Wave
Period (s) Wave Direction (°)
January 5.91 7.96 119
February 6.00 7.93 127
March 6.27 8.59 121
April 5.74 8.04 113
May 4.82 7.60 130
June 3.94 7.27 145
July 3.94 7.05 155
August 3.58 7.75 135
September 5.54 9.19 104
October 5.15 8.45 100
November 5.64 8.28 102
December 5.68 8.03 116
Average 5.18 8.01 122
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FIGURE 3.2. The monthly average wave climate from 2003–2020 at NDBC Wave Buoy Station 41025 at Diamond
Shoals (NC) near Avon compared with the wave climate at the USACE Field Research Facility at Duck (NC). The criteria
for safe dredging apply to hopper-dredge and suction-cutterhead dredge operations are generally in waves less than
5 feet per guidance by dredging contractors and CSE’s project experience (USACE 2010). The graph shows that
average monthly wave height exceeds 5 feet from September to April in the proposed project area. Calmest
conditions occur in June and July when average wave heights are less than 4 feet.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 52
3.1.2 Wave Information Studies – Station 63230
The Wave Information Studies (WIS) is a project sponsored by the US Army Corps of Engineers
(USACE) that generates consistent, hourly, long -term (20+ years) wave climatology along all US
coastlines, including the Great Lakes and US island territories. Unlike a forecast, a wave hindcast
predicts past wave conditions using a computer model and observ ed wind fields. By using value-
added wind fields, which combine ground and satellite wind observations, hindcasted wave
information is generally of higher accuracy than forecast wave conditions and is o ften
representative of observed wave conditions. Hindcast data available from each site include hourly
wind speed, wind direction, bulk wave parameters (significant wave height, period, and direction),
as well as discrete directional wave spectra at 1 - to 3-hour intervals. WIS wave direction uses
meteorological convention: a direction of 0° corresponds to a wave arriving from true n orth.
Similarly, a direction of 90° corresponds to a wave from due east.
The closest WIS station to the project site is station 63230. It is located ~10.5 miles southeast of
Avon at 35.25° N and –75.33° W in water depth of ~60 ft (see Fig 3.1 for location). This station has
hindcast data for 40 years between 1980 and 2019. Figure 3.3 is a polar histogram of the frequency
of occurrence of wave heights and directions based on the 40-year record. The shoreline azimuth
is 10° due north, marked by a solid black line in Figure 3.3. Table 3.2 lists the percentage of
occurrence of wave height and period by direction . Most waves (79.8 percent) are from the
northeast to the south (45°–180°), but the northerly waves are generally larger than those from
other directions. Waves coming from the 45° band from the east-northeast, east, east-southeast
to the southeast (ie – 67.5° to 135° band) occur 45.9 percent of the time, and waves comin g from
east-northeast and southeast have the highest occurrence of 12.4 percent and 12.1 percent
(respectively) compared to the other directions.
The series of graphics in Figure 3.4 shows the monthly polar histogram s of wave directions and
wave heights. In late spring and summer months between May and August, waves are mainly from
the southeast with most wave heights smaller than 1 m (~3 ft), and the rest of the year waves are
mainly from northeast to east with most wave heights between 1 and 2 m (~3 and ~6 ft).

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TABLE 3.2. Percentage of occurrence of wave directions in 16 bands with 22.5° increment, associated wave
heights (in feet), and wave periods (seconds). Note: The shoreline azimuth of the Avon project area is ~10°
from true north. Wave direction uses meteorological convention. A direction of 0° corresponds to a wave
arriving from true north. Similarly, a direction of 90° corresponds to a wave from due east.
Direction
from °True
40-Year Record (1980–2019) at WIS 63230
Percentage of Occurrence
(%) Mean Wave Height (ft) Mean Wave Period (s)
0 ± 11.25 2.4 5.05 5.4
22.5 ± 11.25 6.7 5.25 5.9
45 ± 11.25 10.9 5.41 6.4
67.5 ± 11.25 12.4 5.02 6.9
90 ± 11.25 10.7 4.10 7.0
112.5 ± 11.25 10.7 3.58 6.9
135 ± 11.25 12.1 3.64 6.7
157 ± 11.25 11.7 4.04 6.0
180 ± 11.25 11.2 4.36 5.6
202.5 ± 11.25 6.8 4.69 5.4
225 ± 11.25 1.7 4.63 5.2
247.5 ± 11.25 0.5 4.63 5.2
270 ± 11.25 0.4 4.69 5.2
292.5 ± 11.25 0.4 4.82 5.2
315 ± 11.25 0.5 4.89 5.1
337.5 ± 11.25 0.9 5.09 5.2
All Directions 100 4.62 5.8

FIGURE 3.3.

Wave rose of WIS station 63230
showing the occurrence frequency
of wave direction and wave height
based on the 40-year record
between 1980 and 2019. Avon
shoreline azimuth is ~10° from the
north as marked by the black solid
line in the figure.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 54

FIGURE 3.4a. January to April — Monthly wave roses of WIS station 63230 showing the frequency of occurrence of wave direction and
wave height each month based on the 40-year record between 1980 and 2019. Avon shoreline azimuth is ~10° from the north as marked
by the black solid line in the figures.
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[2525–Appendix D] Avon Village, Dare County, North Carolina 55
FIGURE 3.4b. May to August — Monthly wave roses of WIS station 63230 showing the frequency of occurrence of wave direction and
wave height each month based on the 40-year record between 1980 and 2019.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 56
FIGURE 3.4c. September to December — Monthly wave roses of WIS station 63230 showing the frequency of occurrence of wave
direction and wave height each month based on the 40-year record between 1980 and 2019.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 57
3.2 Wave Modeling
3.2.1 Model Capabilities
The purpose of applying nearshore wave transformation models is to quantitatively describe the
change in wave parameters (wave height, period, direction, and spectral shape) between the
offshore and the nearshore (typically depths of 40 meters (120 ft) or less. In relatively deep water,
the wave field is relatively homogeneous on the scale of kilometers . However, in the nearshore,
where waves are strongly influenced by variations in bathymetry, water level, and current s, wave
parameters may vary significantly on a scale of tens of feet.
STWAVE is an easy -to-apply, flexible, robust, half -plane model for nearshore wind/wave growth
and propagation (USACE 2001). STWAVE simulates depth-induced wave refraction and shoaling,
current-induced refraction and shoaling, depth - and steepness-induced wave breaking, and
diffraction. It also replicates parametric wave growth (due to wind input and wave-wave
interaction) and white capping that redistribute and dissipate energy in a growing wave field.
A wave spectrum is a statistical representation of a wave field. Conceptually, a spectrum is a linear
superposition of monochromatic waves that describes wave energy distribution as a function of
frequency (one-dimensional spectrum) or frequency and direction (two -dimensional spectrum).
The peak period of the spectrum is the reciprocal of the frequency of the peak of the spectrum.
The wave height (significant or zero -moment) is equal to four times the square root of the area
under the spectrum. STWAVE is based on the assumption that the relative phases of the spectral
components are random, and thus phase information is not tracked (ie – it is a phase-averaged
model).
In practical applications, wave -phase information throughout a model domain is rarely known
accurately enough to initiate a phase -resolving model. Typically, wave -phase information is only
required to resolve wave-height variations near coastal structures for detailed, near -field
reflection and diffraction patterns. Thus, for these situations, a phase -resolving model should be
applied. For the proposed Avon nourishment plan (ie – comparison of pre- and post-dredging wave
patterns and determination of relative impacts of the proposed project ), STWAVE has proven
sufficient (Ekphisutsuntorn et al 2010, Kuang 2010, Kaczkowski & Kana 2012 , USACE/NPS 2015).
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[2525–Appendix D] Avon Village, Dare County, North Carolina 58
3.2.2 Model Assumptions
The typical assumptions made in the STWAVE model are:
a) Mild bottom slope and negligible wave reflection. STWAVE is a half-plane model, meaning
that wave energy can propagate only from the offshore toward the nearshore (±87.5°
from the x-axis of the grid, which is typically the approximate shore-normal direction).
Waves reflected from the shoreline or steep bottom features travel in directions outside
this half plane and are thus neglected. Forward-scattered waves (eg – waves reflected
off a structure but traveling in the +x direction) are also ignored.
b) Spatially homogeneous offshore wave conditions. The variation in the wave spectrum
along the offshore boundary of a modeling domain is rarely known, and for domains on
the order of tens of kilometers, it is expected to be small. Thus, the input spectrum in
STWAVE is constant along the offshore boundary.
c) Steady-state waves, currents, and winds. STWAVE is formulated as a steady-state model.
A steady-state formulation reduces computation time and is appropriate for wave
conditions that vary more slowly than the time it takes for waves to transit the
computational grid. For wave generation, the steady-state assumption means that the
winds have remained steady sufficiently long for the waves to attain fetch-limited or fully
developed conditions (the duration of the winds does not limit waves).
d) Linear refraction and shoaling. STWAVE incorporates only linear wave refraction and
shoaling, thus does not represent wave asymmetry. Model accuracy is therefore reduced
(wave heights are underestimated) at large Ursell numbers.
e) Depth-uniform current. The wave-current interaction in the model is based on a current
that is constant through the water column. If strong vertical gradients in the current
occur, their modification of refraction and shoaling is not represented in the model. For
most applications, three-dimensional current fields are not available.
f) Bottom friction is neglected. The significance of bottom friction on wave dissipation has
been a topic of debate in wave-modeling literature. Bottom friction has often been
applied as a tuning coefficient to bring model results into alignment with measurements.
Although bottom friction is easy to apply in a wave model, determining the proper
friction coefficients is difficult. Also, propagation distances in a nearshore model are
relatively short (tens of kilometers), so that the cumulative bottom friction dissipation is
negligible. For these reasons, bottom friction is neglected in STWAVE.
g) Linear radiation stress. Radiation stress is calculated based on linear wave theory.
The governing equations and other aspects of the model can be found in the USACE’s (2001)
technical report.
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3.3 Shoreline Evolution Modeling
G enCade was used in this study to evaluate longshore sediment transport during va rious
stage s of the design life following the proposed beach renourishment project . Results we re
used to evaluate the impact of the proposed renourishment and borrow -area dredging on
longshore transport at the beach.
GenCade combines the engineering power of GENESIS and the regional processes capability of
the Cascade model. It calculates shoreline change, wave -induced longshore sand transport, and
morphology change at inlets on a local to regional scale and can be applied as a planning or
engineering too l. Both STWAVE and GenCade are operated within the Surface -water Modeling
System (SMS) interface, bringing the functionality of a georeferenced environment together with
accessibility to other USACE numerical models. It provides a rapid assessment of mul tiple
engineering alternatives in a robust , self-contained operating platform. As such, it serves as an
economically viable application for shoreline change analysis.
Predicting long -term shoreline change plays a vital role in planning and manag ing coastal zones
and regional sediment management. Shoreline change is driven not only by natural processes
such as wave- and current-induced sediment transport but by engineering activities such as
beach nourishment and the placement of coastal structures. GenCade calculates shoreline
change, wave-induced longshore sand transport, and morphology change along open coasts
and at inlets on a local to regional scale.
The key module used in GenCade for this study is GENESIS. GENESIS is designed to simulate
long-term shoreline changes at coastal engineering sites resulting from spatial and temporal
differences in longshore sediment transport (Hanson & Kraus 1989). The longshore extent of the
modeled reach can range from <1 mile to 50 miles, and simulation periods can range from 1
month to 10 years. The shoreline evolution portion of the numerical modeling system is based
on one-line theory, which assumes that the beach profile shape remains unchanged. This allows
shoreline change to be described uniquely in terms of translating a single point on the profile.
The 0 ft NAVD contour was used as the shoreline for this study .
The structure of GENESIS was originally developed by Hanson (1987) in a joint research effort
between the University of Lund (Sweden) and the Coast al Engineering Research Center (CERC),
US Army Engineer Waterways Experiment Station (WES). It has been tested, revised , and
upgraded since it was developed and is widely used by coastal engineering and planning
communities for predicting the behavior of shorelines and long shore transport. Project sites
include stretches of coast in the United States such as Alaska, California, Louisiana, New Jersey,
New York, Texas, Florida , and the Carolinas. Additionally, there are applications along the
coastlines outside of the United States in countries such as Sweden, Japan, Thailand , and China
Coastal Science & Engineering Littoral Processes
[2525–Appendix D] Avon Village, Dare County, North Carolina 60
(Horikawa & Hattori 1987, Hanson et al 1989, Beumel & Beachler 1994, Bodge et al 1996, Ebersole
et al 1996, ERDC 2005, Ravens & Sitanggang 2007, ACRE 2008, Juh 2008, Ekphis utsuntorn et al
2010, Kuang 2010 , Kaczkowski & Kana 2012 , USACE/NPS 2015).
The GENESIS model (Hanson & Kraus 1989) is the primary numerical model of beach
nourishment planform evolution and was introduced by Dr. Robert Dean of the University of
Florida in his textbook , Beach Nourishment, Theory and Practice (2002). It has been described as
“a must for nourishment designers and a starting point for coastal scientist s interested in
nourishment performance” (reviewed by Marcel Stive, Chair of Coastal Engineer ing, Delft
University of Technology). Several project examples using this model are analyzed in th is book.
As concluded by Dean (2002) and also addressed in numerous articles in the coastal engineering
literature, several key factors should be take n into consideration to have a successful
application of the model . They are listed below.
 Representative wave data or reliable hindcasts are available.
 Historical shoreline position and the longshore distribution of volume changes for
substantial periods are available.
 Proper calibration and verification of the model.
 Appropriate model setup, including domain coverage, grid size , and actual bathymetry.
 An external wave transformation model capable of transforming the wave data from
offshore to the reference poi nt as required by the GENESIS model.
When the GENESIS model was used for the proposed Avon project, the above-listed key factors
were satisfied , except there was no historical shoreline position for model calibration or
verification. Historic annual erosi on rates determined from the previous sections of this report
were used to calibrate the se diment-transport model. Consequently, the model results were not
used to evaluate shoreline evolution, but rather the relative impact of the proposed project on
longshore sediment transport .
The internal wave -transformation model within GENESIS was used in this study to
mathematically simulate wave propagation from the reference point to the breaking point and
to the beach. Th is internal model determined the break ing wave characteristics used to
calculate the actual longshore sediment transport.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 61
I n the following sections, the details of model setup, application s for beach nourishment design
templates , and the impact of the proposed dredging and nourish ment will be discussed. The
conclusions of the engineering study will be provided after the discus sion. A brief outline of
each section is listed below:
 Section 3.4 STWAVE and GenCade model setup, including wave climate analysis,
model domain setup, bathymetry a pplication, and determination of model
parameters
 Section 3.5 STWAVE model results of pre- and post- dredging scenarios
 Section 3.6 GenCade model calibration and evaluation on sediment transport rates
 Section 3.7 GenCade model results to evaluate the impact of proposed
nourishment and borrow area dredging on longshore transport at the beach
 Section 3.8 Conclusions
3.4 Model Setup
The task of model setup includes determining the computational domain, building up the model
grid, designating model parame ters, and generating input data files. Input data of a typical
STWAVE model and a GenCade model include the wave field at the offshore boundary (wave height,
period, and direction), bathymetry over the model domain, initial shoreline position, measured
shoreline position and sediment transport rates for calibration purposes (if applicable), and
coastal engineering activities (coastal structure positions or beach fill characteristics if applicable).
The STWAVE model output includes the wave field over the computational domain, and the
GenCade model output includes the shoreline position and longshore transport rates at user -
specified time steps.
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3.4.1 STWAVE Model Grid
As discussed in previous sections, the proposed project area starts from station 1550+00 and
extends southward to station 1682+00, covering 2.5 miles (13,200 ft) from north to south. A
STWAVE grid extends about 2 miles beyond the north boundary and ~1.6 miles beyond the south
boundary of the project site. Extensions of the model domain be yond the project area ensure that
possible edge effects from the model boundary do not influence results in the area of interest.
Model sensitivity testing in the previous study has determined that such extents ensure proper
model function without edge ef fects (USACE/NPS 2015).
The STWAVE model grid was also extended seaward from the shoreline to a distance of about 3 .8
miles. The seaward boundary is parallel to the general shor eline trend with an azimuth of 10° from
due north (Fig 3.5). This seaward boundary is defined as the y-axis of the STWAVE model, and the
axis perpendicular to the y -axis pointing in the shoreline direction is denoted as the x -axis. The
two axes are shown as thick black lines in Figure 3.5. The south and onshore boundaries are
marked with red lines in the same graphic.
The grid encompasses both the project area and the identified borrow area. STWAVE operated
within the Surface -water Modeling System (SMS) interface requires the model to be set up in
metric units. For this study, the grid origin is at 934500 meters (m) East and 186200 m North in
North American Datum 1983 State Plane (NAD’83) North Carolina Zone 3200, and the grid
dimensions are 6,600 by 10,000 m in the x and y directions (respectively). The cell size is 100 m in
both directions.
3.4.2 GenCade Model Grid
The GenCade model boundary is parallel to the y -axis of the STWAVE grid and is denoted by a green
line with an arrow pointing from north to south in Figure 3.5. The grid origin is at 3045900 ft East
and 616700 ft North (NAD’83), and the grid length is 35,000 ft.
Ideally, the GenCade model boundary should not only cover the project area , but should also
extend some distance beyond the north and south ends of the project to eliminate any possible
boundary effects and to evaluate the shoreline performance of adjacent beaches. CSE surveyed
~1 mile north and south of the project limits in July 2020, and the data provide sufficient coverage
for the model application in this study.
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FIGURE 3.5. STWAVE and GenCade model boundaries and grid coverage.

y
E 934500 m
N 186200 m
x
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[2525–Appendix D] Avon Village, Dare County, North Carolina 64
3.4.3 Model Grid Size
Generally speaking, if the grid cell size is smaller, the shoreline simulation model results are more
detailed. However, reducing the grid size increases the STWAVE compu tation time. Model
sensitivity tests in the previous study with different spatial resolutions ranging from 50 ft to 500 ft
have been conducted (USACE/NPS 2015), and the optimum grid size determined for this study is
100 m for STWAVE and 100 ft for GenCade models. [Note: STWAVE requires all numbers in metric ,
and GenCade requires in feet.]
3.4.4 Model Bathymetry
The setup of the STWAVE and G enCade models requires applying offshore and nearshore data to
develop the bathymetry and topography in the model domain. Relative elevations on different
vertical datums published by NOAA’s National Ocean Service Tides and Currents at an adjacent
site at Cape Hatteras Fishing Pier are illustrated in Figure 2.2. A detailed description of CSE’s
bathymetric data collection methods and data analys es is presented in Section 2. The scatter data
used in this study and shown in Figure 3.6 include:
(1) CSE’s beach condition survey from the foredunes to deep water beyond –30 ft
NAVD at 500-ft intervals in July 2020.
(2) CSE’s bathymetric survey in an offshore sand search area in July 2020.
(3) NOAA Digital Elevation Model (DEM) dated 2012 with a uniform horizontal
resolution of 100 feet in the areas outside of CSE’s survey coverage.
The shoreline used in this study is defined as the 0 -m contour line relative to the NAVD 88 datum
extracted from the July 2020 beach condition survey.
Figure 3.5 shows the locations of the proposed excavations, with Area 1 to 10 ft and Area 2 to 6 ft
below the existing grade in the offshore borrow area. Grid origin is consistent across all models.
Figure 3.7 shows the bathymetry ac ross the grid area after interpolation across the model domain
before dredging. The after-dredging scatter data with greater depths in the proposed excavation
areas are visible in Figure 3.8, with Figure 3.9 showing the grid bathymetry after interpolation
across the model domain. For all, water depth along the seaward boundary is relatively uniform
around 17 m.
This is a conservative scenario for impact analysis, as it explores over 3.4 million cubic yards of
sand excavation versus the proposed project's 1 million.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 65
FIGURE 3.6. Combined bathymetric data collected by CSE in July 2020 and the NOAA Digital Elevation Model (DEM)
with a horizontal resolution of 100 feet (dated 2012). See Figure 3.5 for model origins, “x” and “y” axes, and coverage.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 66
FIGURE 3.7. Model coverage and interpolated bathymetry using the scatter data in Figure 3.6 for the before
dredging condition. See Figure 3.5 for model origins, “x” and “y” axes, and coverage.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 67
FIGURE 3.8. Combined bathymetric data collected by CSE in July 2020 and the NOAA Digital Elevation Model
(DEM) with a horizontal resolution of 100 feet (dated 2012). The elevations in the offshore borrow area reflect the
after-dredging condition after the 10-ft excavation before the existing grade. See Figure 3.5 for model origins,
“x” and “y” axes, and coverage.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 68
FIGURE 3.9. Model coverage and interpolated bathymetry using the scatter data in Figure 3.8 for the after
dredging condition. See Figure 3.5 for model origins, “x” and “y” axes, and coverage.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 69
3.4.5 Wave Climate Analysis
Obtaining satisfactory wave data is a necessary and crucial task in preparing and executing wave
and shoreline-evolution models. There are no site -specific, long-term wave records for the Avon
project area, but there are at least two wave data sources in the vicinity of the site as discussed in
the previous sections (ie – NDBC wave buoy 41025 and the WIS station 63230; see Figure 3.1 for
their locations). Although the NDBC wave buoy has real-time measurements, this station was not
used because it is located ~19 miles from Avon to the southeast.
The WIS station 63230 is located ~11 miles southeast of Avon and has 40 years of hindcast data
between 1980 and 2019. This station was chosen because of the long -term wave records, and the
net transport generated under the wave climate of this station agreed with historical observations
(USACE/NPS 2015).
3.4.6 Model Parameters
The parameters used in the GENESIS model include sand and beach data and longshore sand
transport calibration coefficients. The sand and beach data are determined from the analysis in
the geotechnical study (CSE 2021) and are listed below:
 Effective grain size = 0.289 mm
 Average berm height = 7 ft NAVD
 Closure depth = –24 ft NAVD
Volumetric erosion studies at the project area show tha t long-term average annual erosion rates
are estimated to be 140,000 cy/yr as discussed in Section 2.5.2 . The transport parameters K 1 and
K2 required in the model were adjusted within the recommended range to obtain the best fit of
simulated volumetric transport rate with historical data.
3.5 STWAVE Model Results
The borrow area for this project has an average depth of ~40 ft NAVD and a total area of ~250 acres.
It is located considerably outside the depth limits of significant sediment motion of the active surf
zone. Sediment removal from the borrow sites will result in offshore depressions , possibly 6-10 ft
below the present bottom. To determine if the tot al removal of the sediment from the borrow sites
would impact the concentration of longshore wave energy and littoral sediment transport
potential, STWAVE was used to simulate wave transformation over the borrow sites by comparing
conditions before and after dredging.
The STWAVE model results for the before-dredging and after-dredging scenarios are shown in
Figures 3.10 and 3.11 for comparison .
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[2525–Appendix D] Avon Village, Dare County, North Carolina 70

FIGURE 3.10. STWAVE simulated wave heights and directions for before-dredging scenario. The vertical lines represent
the cross-sections in y direction that are analyzed in Figure 3.12. Lines top to bottom illustrate y-axis distance from origin
of 100 m, 1000 m, 1500 m, 1700 m, 1900 m, 2100 m, 2300 m, 2500 m, 2700 m, 2900 m, 3100 m, 3300 m, 3500 m, 3700 m,
4000 m, 4500 m, 5000 m, 6000 m, 8000 m, and 9900 m. The proposed borrow area is located approximately 1700 to 3600
m from the origin, and the center of the borrow area is located around 2600 m from the origin. The arrows represent the
wave directions.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 71
FIGURE 3.11. STWAVE simulated wave heights and directions for after-dredging scenario.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 72
To evaluate wave pattern changes before and after dredging, 20 horizontal cross-sections were
chosen over the computational domain. These cross-sections effectively cover the study area ,
including the designated borrow area. Wave heights across horizontal cross -sections are plotted
in a series of graphics in Figure 3.12. Some key coordinates, dimensions, and relative distance to
the model origins are listed below.
 STWAVE Model Origin: x0 = 934500 m, y0 = 186200 m
 STWAVE Grid Length in “x” Direction equals: 6,600 m
 STWAVE Grid Length in “y” Direction equals: 10,000 m
 Project Northernmost Limit to STWAVE Origin: y = ~3,300 m
 Project Southernmost Limit to STWAVE Origin: y = ~7,300 m
 Borrow Area to Model Origin: 1,700 to 3,600 m in the “y” direction and 1,300–3,200 m in the
“x” direction
In Figure 3.1 2, the left side of the x -axis represents onshore, and the right side of the x-axis
represents offshore. The first four and the last six plots in this graphic (ie – y-distance to origin =
100 to 1,700 m and 4,000 to 9,900 m) show almost no difference in wave height before and after
dredging. The intermediate ten plots (ie – y-distance to origin = 1,900 to 3,600 m) show minor
differences in wave height between these two scenario s. This is where the proposed borrow area
is located. Since the project area “y” distance to origin is between 3,300 and 7,300 m, the results
indicate that borrow-area dredging has no or negligible impact on the wave field in the offshore
area. Wave height between the two scenarios became more noticeable where the borrow area is
located (ie – y-distance to origin = 1,700 to 3,600 m and x-distance to origin = 1,300 to 3,200 m ), but
the greatest difference was less than 10 percent of the height. In addition, the wave height
difference diminishes toward the shore.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 73
FIGURE 3.12. STWAVE simulated wave height comparisons at different horizontal cross-sections parallel to
the “x” axis (ie – with constant distance to the origin in the “y” direction) as illustrated in Figure 3.10. “0” is the
offshore model boundary and shoreline is at the left side of the graph where waves break and diminish.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 74
FIGURE 3.12(continued). STWAVE simulated wave height comparisons at different horizontal cross-
sections parallel to the “x” axis (ie – with constant distance to the origin in the “y” direction) as illustrated in
Figure 3.10. “0” is the offshore model boundary and shoreline is at the left side of the graph where waves
break and diminish.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 75
FIGURE 3.12(continued). STWAVE simulated wave height comparisons at different horizontal cross-
sections parallel to the “x” axis (ie – with constant distance to the origin in the “y” direction) as illustrated in
Figure 3.10. “0” is the offshore model boundary and shoreline is at the left side of the graph where waves
break and diminish.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 76
FIGURE 3.12(continued). STWAVE simulated wave height comparisons at different horizontal cross-
sections parallel to the “x” axis (ie – with constant distance to the origin in the “y” direction) as illustrated in
Figure 3.10. “0” is the offshore model boundary and shoreline is at the left side of the graph where waves
break and diminish.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 77
FIGURE 3.12(continued). STWAVE simulated wave height comparisons at different horizontal cross-
sections parallel to the “x” axis (ie – with constant distance to the origin in the “y” direction) as illustrated in
Figure 3.10. “0” is the offshore model boundary and shoreline is at the left side of the graph where waves
break and diminish.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 78
In conclusion, the STWAVE model results indicate that borrow -area dredging will not impact
the wave patterns along the project beach; the impact will be concentrated in the dredged
area and its immediately vicinity. The most significant increase will be ~10 percent of the
local wave height (expected to occur in the center of the borrow area ).
The pre-project depth in the borrow area is 10–35 ft deeper than the estimated DOC (ie -24 ft NAVD),
and the results show that the proposed maximum excavation of 6ft to 10 ft will not significantly
alter wave patterns at the shore and will only locally modify waves within the immediate borrow
area. The results also show that the location of the borrow area will not significantly alter sand
transport processes and rates over the excavation area, and will not impede or modify
normal onshore sand transport.
3.6 GenCade Model Calibration
Proper application of GenCade requires calibration by adjusting the various model parameters
until it can reasonably reproduce historical shoreline change or longshore sediment transport
rates over a given time interval. The key module of GenCade is GENESIS which was calibrated and
verified in the study for an adjacent project site in Buxton . In that study, the simulated rates for
total volumetric erosion and net longshore sediment transport were compared with historical
erosion rates (USAC/NPS 2015). Because the proposed Avon project area does not have more than
two sets of complete survey data for the calibration procedure, the net longshore sediment
transport rates and total volumetric erosion rates simulated were used to compare with the
estimated historical erosion rates of 140,000 cy/yr (as discussed in Section 2.5 of this report).
Figure 3.13 shows calculated average annual net longshore sediment transport rates along the
modeled shoreline over a specific 3-year period between 2017 and 2019. Moving from left to right
along the horizontal axis represents the shoreline from north to south, and p ositive transport rates
denote net sand movement to the south. Some key reference distances are listed below:
Project Northernmost Limit (sta 1550+00) to GenCade Origin: ~13,200 ft
Project Southernmost Limit (sta 1862+00) to GenCade Origin: ~26,400 ft
Figure 3.13 shows that the sediment transport rate varie s along the shoreline and is southerly,
which is consistent with the historical trend of sediment movement and spit accretion in this area.
Transport rates increase markedly along certain sections south of the Avon Pier . This increasing
transport rate explains the higher erosion observed in that area. The simulated average annual
net transport rate for the project area was ~140,400 cy. This average annual rate is consistent with
the adopted historical net annual erosion rate of 140,000 cy/yr.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 79
FIGURE 3.13. Average annual longshore net sediment transport rates over 3-year period. Positive rates
denote net sand movement to the south.

In conclusion, the net longshore sediment transport rate predicted by the GenCade model agrees
closely with the estimated volumetric loss rates of ~140,000 cy/yr along the proposed Avon project
area. Due to the lack of historical shoreline measurements for this area, the model cannot be
further calibrated for shoreline evolution. Because the model will be primarily used to evaluate
the impact of the proposed nourishment and offshore borrow area dredging on sediment transport
rates, shoreline position is not expected to hinder the comparative results .
3.7 GenCade Model Results
The proposed project calls for pumping a maximum of 1 million cubic yards of beach quality sand
from the designated offshore borrow area onto 13,200 linear feet of ocean beach. The designed
berm height is 7 ft NAVD, and the average fill density is ~76 cy/ft. Fill densities will vary from north
to south according to the historical erosion rates. The 4,000-ft section north of the Avon Pier is
scheduled to receive an average of 43 cy/ft, and the 9,200 -ft section south of the Avon Pier is
scheduled to receive an average of 90 cy/ft during nourishment. GenCade was used to determine
if nourishment on the beach and the removal of the sediment from the proposed borrow sites
would impact longshore sediment transport potential . These results were compared for pre- and
post-project conditions .
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[2525–Appendix D] Avon Village, Dare County, North Carolina 80
FIGURE 3.14. Comparison of annual net longshore sediment transport rates before and after the proposed
renourishment project.
Net sediment transport rate before and after the project are plotted in Figure 3.14. The average
annual net transport rate before the project for the shoreline segment between stations 1550+00
and 1682+00 (or a distance to G enCade origin between ~13,200 ft and ~26,300 ft) was ~140,400
cy/yr; the rate was ~140,000 cy/yr after the project. These rates are a few hundred yards different
before and after the project. It indicates that nourishment and borrow area dredging will cause
negligible changes in the net longshore sediment transport rate. The rates will ch ange locally
where the beach fill is conducted, but there will be no changes approximately 0.5 mile north or
south of the project area.
The series of images in Figure 3.15 illustrates the shoreline position over a three -year period after
completion of the proposed Avon nourishment. Red lines represent the initial shoreline before
nourishment, and green lines represent the shoreline positions at specific times after the project,
as indicated on the top of the images. At the end of the three -year period, the shoreline is expected
to be seaward of the initial shoreline in most of the sections, signifying a substantial amount of
nourishment sand placed along that section of oceanfront would remain along those areas.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 81
FIGURE 3.15. Shoreline position at 0 ft NAVD contour before nourishment (red) and its evolution (green) over a
three-year period after the proposed 1 million cubic yard project. Red lines represent the initial shoreline, and green
lines represent the shoreline at specific intervals after nourishment. In the final image three years post-
nourishment, the shoreline is seaward of the initial shoreline for most of its length, signifying a substantial amount
of nourishment sand remains.

T = 0 Days T = 98 Days T = 365 Days T = 730 Days T = 1095 Days
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[2525–Appendix D] Avon Village, Dare County, North Carolina 82
3.8 Conclusions
STWAVE and GenCade have been widely applied in coastal engineering and planning projects to
predict wave field and longshore transport behavior . They were used in this study to simulate
wave patterns and longshore sediment transport rate s before and after the proposed Avon
nourishment project. Results were used to evaluate the impact of borrow area dredging and beach
fill on wave height and longshore transport rates a long the study area at the Cape Hatteras
National Seashore in front of the Avon Village.
The STWAVE mode l results indicate that borrow area dredging will not cause any measur able wave
pattern changes in the project area, and the impact will be concentrated within the dredged area
and its immediately adjacent ocean bottom. The most significant wave height increase will be no
greater than 10 percent of the local wave height and is expected to occur in the borrow area. The
pre-project depth in the borrow area is 10–30 ft deeper than the estimated DOC in this setting, and
therefore well beyond any expected zone of normal exchange of sediment with the beach. T he
STWAVE model results show that the proposed excavation s up to 6 ft to 10 ft will have only a minor
local impact on waves in the immediate borrow area and negligible impact on waves at the beach .
The results also show that sand transport will not be significantly modified over the borrow area
and that normal onshore sand transport will continue uninterrupted.
The GenCade model was calibrated using long-term historical erosion rates of 140,000 cy/yr. The
calibrated model results yielded ~140,400 cy/yr annual net sediment transport rate s, which are in
close agreement with the estimated rates. The calibration results show that the model can capture
the overall sediment transport pattern and can be used to evaluate the relative changes of
sediment transport rates before and after nourishment and offshore borrow -area dredging.
The model simulation for potential after -project longshore transport along two s horeline
segments resulted in only minor changes comp ared to the before -project condition (of the order
of hundreds of cubic yards). The model results indicate that nourishment and borrow area
dredging will cause negligible changes in the longshore sediment transport rate. The rate
will change locally where beach fill is conducted, but there will be no changes ~0.5 mile north
or south of the fill area.

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[2525–Appendix D] Avon Village, Dare County, North Carolina 83
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