01 - Buxton - AppD-Littoral Processes

APPENDIX D
LITTORAL PROCESSES
Beach Renourishment to Protect NC Highway 12
at Buxton, Dare County, North Carolina
Prepared for:
Dare County Board of Commissioners
Bob Woodard, Chairman
954 Marshall C Collins Drive, Manteo NC 27954
Prepared by:
High Volue Services COS
5ustoinoble Solutions
PO Box 8056, Columbia SC 29202-8056
[2403 M-J U LY 20211

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TABLE OF CONTENTS
TABLEOF CONTENTS.................................................................................................................. iii
1.0 INTRODUCTION.................................................................................................................... 1
2.0 BEACH CONDITION SURVEYS AND EROSION ANALYSIS.............................................................. 3
2.1
Data Collection Methods........................................................................................................
3
2.1.1 Survey Stationing History..............................................................................................
4
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 Priorto the Initial 2017-2018 Nourishment ........................................
19
2.4.1 Historical Erosion Rates Prior to 2017...........................................................................
19
2.4.2 Historical Shorelines...................................................................................................
21
2.4.3 Equivalent Volumetric Erosion Rates............................................................................
27
2.5
Volume Analysis During and After the 2017-2018 Nourishment ...............................................
30
2.5.1 Overview of the 2017-2018 Nourishment Construction .................................................
30
2.5.2 Year 1 (2018) Post -Project Volume Analysis..................................................................
43
2.5.3 Year 2 (2019) Post -Project Volume Analysis..................................................................
46
2.5.4 Year 3 (2020) Post -Project Volume Analysis..................................................................
50
2.5.5 Updated Erosion Rate after the Initial 2017-2018 Nourishment (2018-2020)..................
56
3.0 COASTAL
PROCESSES..........................................................................................................
59
3.1
Wave Climate.......................................................................................................................
60
3.1.1 Real -Time Wave Buoy -Station 41025..........................................................................
60
3.1.2 Wave Information Studies - Station 63230...................................................................
63
3.2
Wave Modeling.....................................................................................................................
68
3.2.1 Model Capabilities.......................................................................................................
68
3.2.2 Model Assumptions.....................................................................................................
69
3.3
Shoreline Evolution Modeling...............................................................................................
70
3.4
Model Setup.........................................................................................................................
72
3.4.1 STWAVE Model Grid.....................................................................................................
73
3.4.2 GenCade Model Grid....................................................................................................
73
3.4.3 Model Grid Size...........................................................................................................
75
3.4.4 Model Bathymetry.......................................................................................................
75
3.4.5 Wave Climate Analysis.................................................................................................80
3.4.6 Model Parameters.......................................................................................................
80
3.5
STWAVE Model Results..........................................................................................................
80
3.6
GenCade Model Calibration...................................................................................................
89
3.7
GenCade Model Results........................................................................................................
92
3.8
Conclusions..........................................................................................................................93
REFERENCES.............................................................................................................................
95
Attachment
1A) Baseline and Control
Attachment1B)
Key Station Coordinates for Surveys
Attachment
1C) Station Coordinates for Surveys
Attachment
2) Beach and Inshore Profiles
Attachment
3) Compaction Results for Year 1 (2018) and Year 2 (2019) after 2017-2018 Project Completion
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI iii Buxton, Dare County, North Carolina

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Coastal Science & Engineering
[2403M-Appendix D]
Littoral Processes
Buxton, Dare County, North Carolina

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
Buxton renourishment project area. It covers the following topics:
• Field data collection for beach condition surveys
• Beach and inshore profiles
• Volume analysis for defining beach condition
• Overview of the 2017-2018 nourishment project
• Historical erosion rates and updated erosion rates since the 2017-2018
nourishment project
• Volume losses due to hurricanes Florence (September 2018) and Dorian
(September 2019)
• 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 renourishment project
• Profile adjustment after renourishment and project longevity
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 1 Buxton, Dare County, North Carolina

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Coastal Science & Engineering
[2403M-Appendix D]
Littoral Processes
Buxton, Dare County, North Carolina

2.0 BEACH CONDITION SURVEYS AND EROSION ANALYSIS
2.1 Data Collection Methods
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 feet (ft)
or -8 miles south of the starting point near Oregon Inlet. Station 1792+50 (forexample) 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, and stations provide a convenient measure of
distances along the shore. The Buxton community along the oceanfront begins near station
1880+00. The purpose of establishing one baseline for Hatteras Island (east coast) is to facilitate
future island -wide erosion analyses.
During the planning phase of the initial nourishment project in 2013, the study area included some
upcoast and downcoast areas and extended north and south from Buxton between stations
1720+00 (near Haulover Beach access in CAHA) and 1980+00 (Cape Point area of CAHA) (USACE-
USDOI-NPS 2015). Stations in this area were used to mark profile locations and compare variations
in the beach condition. 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
0+00
Monument #
—
Locality
Oregon Inlet jetty
Note
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
—
-1880+00
—
Village of Buxton
North end of development
1928+11
CHLI
Old Hatteras Lighthouse site
—
1983+77
BYRD
Cape Point area
South end of BL
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 3 Buxton, Dare County, North Carolina

2.1.1 Survey Stationing History
CSE's original data collection plan for a feasibility study of the initial nourishment project called
for profiles encompassing the littoral zone at a spacing of 1,000-2,000 feet (ft). After CSE's initial
deployment to the field in August 2013, the team received information about the USACE/NCDOT
emergency nourishment plan for the S-curve at Rodanthe (USACE 2013). The Corps established a
baseline specific to that project which used stations beginning -1.6 miles north of Mirlo Beach.
USACE station 0+00 corresponds to CSE station 548+94.
USACE officials (R Keistler, USACE-Wilmington, pers. comm., August 2013) provided information
on their control points and profile lines for the emergency project. To develop consistent profile
lines for future reference and comparison, CSE modified its 2013 data collection plan to match
USACE profile locations to the extent practicable. This means that CSE's initial profiles fall on odd -
numbered stations because of the offset between CSE's baseline and the USACE baseline. For
example, USACE profile line 80+00just north of Mirlo Beach is equivalent to CSE profile line 628+94.
At Buxton, initial profiles were run on odd -numbered stations in anticipation of potential future
federal work in the area. Thus, "Phase 1" Buxton profiles in 2013 are positioned slightly south
(-63 ft) of even -numbered stations (ie - 1850+63, 1860+63, etc).
Following a decision by Dare County to proceed with detailed planning for the initial Buxton
project in 2014, profile spacing was set at 500 ft to provide detail during the Phase 2 study of the
initial nourishment project. These lines were run from even -numbered stations along the Buxton
project area to simplify nomenclature and references (ie - 1850+00, 1855+00, etc). After these
adjustments, the Phase 2 beach survey data does not perfectly overlay the Phase 1 data.
Figure 2.1 shows the comparisons of CSE's baseline, profile stationing, and profile azimuths for the
Buxton area during the Phase 1 and Phase 2 studies for the initial nourishment project (USACE-
USDOMPS 2015). Azimuth information via starting and ending coordinates for each line is in
Attachment 1-B. The 500-ft spacing station numbers and profile azimuths adjusted in the Phase 2
study have been used in CSE's beach conditional surveys since 2014 (ie - red lines in Figure 2.1).
A total of 20 stations around Buxton were profiled between the existing structures/foredunes and
deep water in August 2013, and 52 stations were surveyed in October 2014. Profile spacing was
initially 1,000 ft along the center of the reach and -2,000 ft at the ends of the reach. Profile lines
are shore -perpendicular to the baseline in most cases.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 4 Buxton, Dare County, North Carolina

1700+Op
1720+00
a 1740+00
c
L
o t 1760+00 c
y ,
� m
O U
U
0 1780+00 O
ZU
N
E 1800+00
`D m w
a m
c 1820+00 Q
1840+00
1860+00
WXtQ 1880+00
1900+00
1920+00
Old Cape Hatteras Light Site .
Cape Hatteras Light. 1g40+00
1960*00
1980+QD
August 2013
i
October 2014
1
0 3,000
Scale
Cape Hatteras Datum:
SPCS NAD'83 (Feet) NC Zone3200
FIGURE 2.1. CSE's baseline, profile stationing, and profile azimuths established during the Phase
1 study in 2013 for the initial Buxton project, and further adjusted during the Phase 2 study in 2014
(red lines). Phase 1 profiles (August 2013) fell on odd -numbered stations for reasons given in the
text. Phase 2 profiles at 500-ft spacing (October 2014) are positioned at even -numbered stationing
and have been used since 2014.
Coastal Science & Engineering Littoral Processes
[2403M—Appendix DI 5 Buxton, Dare County, North Carolina

2.1.2 Vertical Datum
The vertical datum for CSE's profile data collection 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 various
relationships among key reference datums for the
closest tidal station at Cape Hatteras (NC) fishing pier,
which is -7 miles southwest of the Buxton 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 8654400). Mean high water (M HW) is 1.05 ft above
NAVD; mean tide level is 0.45 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 Hydrographic
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 NC-3200 for
Hatteras Island. The vertical datum was the North
American Vertical Datum of 1988 (NAVD'88) measured in
feet.
2.0
1.5
MHHW (1.40 ft)
1.0
MHw(1.05 ft}
$ D.5
00
0
0.0
NAVD88 (0 ft)
z
0
7 -0.5
MTL(-0.45f1j
NGV❑ 29 (-0.56ftj
a
W
w -1.5
-2 0
MLW (-1.94 ft)
M LL.N (- 2.0 6 ft)
-2.5
-3.0
FIGURE 2.2. Key reference datums at Cape
Hatteras (NC) fishing pier (-7 miles southwest
of the project area).
[Source: NOAA-Tides and Currents Station ID
8654400]
CSE has been upgrading survey equipment and software over the years to apply the most up-to-
date industry standards to the Buxton 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 ApplanixT" 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 overthe bottom). This
system is linked onboard the survey vessel in "real time" to an OdomT" Echotrac CV100 and
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 6 Buxton, Dare County, North Carolina

SMSW200-4a transducer for depth measurements. The sounder 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 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 enables 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 eliminate 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 HYPACKT" 2021 and Trimble° Business Center (TBC). HYPACKT" 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 quickly 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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 7 Buxton, Dare County, North Carolina

FIGURE 2.3. Field data collection methods involved subaerial survey using RTK-GPS at low tide and hydrographic
surveys by boat at high tide.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] R Buxton, Dare County, North Carolina

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 zone.
Figure 2.4 illustrates DTMs of the Buxton project area from the most recent survey (August 2020)
by color -coded, smooth -contour maps usingthe indicated elevation/depth intervals for each color.
Red and orange are the dune -beach zone, yellow marks the longshore bar, and blue represents
water depths >30 ft. The bathymetry DTMs show relatively smooth, continuous morphology of a
longshore bar (yellow -green color band) inside the 20-ft depth contour along Buxton positioned
about 1,200 ft offshore with a crest elevation of (-)-12 ft to -14 ft. A deep trough of (-)-30 ft is
located between two offshore bars with a bottom elevation of (-)-30 ft along most of the project
area, particularly in front of the Village of Buxton.
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 (collected 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 2013; therefore, we believe the data collected by CSE
in August 2013 was the first set of comprehensive beach profiles for this area extending from the
frontal dunes to deep water.
Table 2.2 lists beach profile surveys that CSE conducted between 2013 and 2020, along with the
primary purposes of these data collection efforts. Profiles of these surveys at representative
locations are shown in Figure 2.5. [See Figure 2.4 for general locations]. Attachment 2 contains a
set of profiles obtained by CSE from May 2017 to August 2020. Profiles prior to 2017 can be found
in the previous reports (CSE 2013; USACE-USDOI-NPS 2015 -Appendix A- Littoral Processes). Most
of the Buxton area profiles exhibited alongshore bar with a crest at -13-15 ft NAVD. The bar crest
is broader, deeper, and farther offshore at the southern end of Buxton along the National Seashore
(south of the groin field at the old Cape Hatteras Lighthouse location).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 9 Buxton, Dare County, North Carolina

TABLE 2.2. Beach profiles survey that CSE conducted between 2013 and 2020 along with the primary purposes of
these surveys. CSE is scheduled to perform a "Year 1" condition survey in August 2021 after completion of the
nresent nrniect_
Survey Date
Primary Purpose
August 2013*
Initial planning for the 2017-2018 project
Detailed planning and preliminary design for permitting for the
October 2014
2017-2018 project
October 2015
Facilitating the final design of the 2017-2018 project for bidding
August 2016
Updating erosion rates
May 2017
Determining pre -construction condition of the 2017-2018 project
Pre -hurricane season and Year 1 post -project condition after the
June 2018
2017-2018 project
October 2018
Post -Hurricane Florence
Pre -hurricane season and Year 2 post -project condition after the
June 2019
2017-2018 project
November2019
Post -Hurricane Dorian
Pre -hurricane season and Year 3 post -project condition after the
August 2020
2017-2018 project
*August 2013 profile data were collected at 1,000-2,000 ft spacing. All the other data sets
were collected at 500-ft even spacing.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 10 Buxton, Dare County, North Carolina

1740+00 )
12
1760+00
Pamlico Sound
1780+00
1800+00
0-
10
1880+00
Buxton
12 -
1soo+oo
1szo+oo._
Cape
• Hatteras
Light
Cape Hatteras
National Seashore
Cape Hatteras
National Seashore
STA 1770+50
U)
E
J
E
W
w
0
a
0 Atlantic Ocean
a
STA 1925+50
F--t NA', D
20 ft
loft
Oft
_10IT
-30 ft
-40 A
-50 ft
Proposed
Borrow Area
0 2,000
Scale
FIGURE 2.4. Color -coded topography and bathymetry DTM interpolated from the most recent survey in August 2020
for the Buxton project area. Note variations in water depth (and profile geometry) between the outer bar and the beach.
The "salient" in the shoreline between stations 1890+00 and 1980+00 is sand retained by groins at the old Cape Hatteras
Lighthouse site. The alignment of the offshore bar is straighter than the shoreline north and south of the salient.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 11 Buxton, Dare County, North Carolina

30
20
j 10
Q
z 0
c
o -10
-20
w
-30
-40
30
20
�j 10
Q
z 0
c
o -10
d -20
W
-30
-40
National Park Station 1790+00
- May 2017
-Jun 2019
Oct 2018
_
+7 FT NAVD
Jun 2019
------------------------------------------------
Nov 2019
-6 FT NAVD
-Aug 2020
r--- - -------------------------------------------
i
19 FT NAVD
---------------------------------
---- zaFTNAVD --------
0
500
1000
1500 2000
Distance from Baseline (ft)
Date
Vol to +7
Vol to -6
Vol to -19 Vol to -24
May 2017
17.9
113.3
482.8 826.1
Jun 2018
21.1
159A
580.2 950.5
Oct 2018
22.5
147.0
556.9 915.5
Jun 2019
26.6
156.6
568.2 922.9
Nov 2019
24.0
153.3
547.1 881.9
Aug 2020
27.6
148.8
536.2 907.0
Phntoa 1 January 21161MC
National Park Station 1860+00
2500 3000 3500
- May 2017 =
Jun 2018 -
Oct 2018 =
+7 FT NAV Jun 2019
F----------------------------------------------- Nov 2019
FT NAVD -Aug 2020 -
r--- - ----------------------------------
I -
-19 Fr NAVD
-
L------------------------ 24FTNAVD -------
0
500
1000 1500 2000
Distance from Baseline (ft)
Date
Vol to +7
Vol to -6
Vol to -19 Val to -24
May 2017
24.9
115.5
421.1 730,3
Jun 2018
27.6
165.4
605.2 943.0
Oct 2018
28.3
184.4
580.1 920.6
Jun 2019
28.3
176.8
559.7 908.3
Nov 2019
33.0
121.8
532.6 906.8
Aug 2020
33.0
152.8
580.2 905.2
Phato: 11 January 20161MC
2500 3000 3500
FIGURE 2.5a. Representative profiles for the Buxton project area at stations 1790+00 and 1860+00 in CHNS.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 12 Buxton, Dare County, North Carolina

30
20
j 10
Z 0
v
o -10
7
SD -20
W
-30
-40
30
20
j 10
Q
z 0
o -10
aT -20
W
-30
-40
Buxton Station 1890+00
- May 2017
i -Jun 2018 Oct 2018
+7 FT NAVD Jun 2019
- ------------------------------------------------- Nov2019
s FT NAVD -Aug 2020 -
r-- ---------------------------------------------------
-19 FT NAVD
r---------- - ------ -
L----- - ----- -- ------------------24FTNAVD .-------- -
0
500
1000
1500 2000
Distance from Baseline (ft)
Date
Vol to +7
Vol to -6
Vol to -19 Val to -24
May 2017
11.9
76.1
360.5 630.2
Jun 2018
14A
96.5
466A 765.3
Oct 2018
17.3
110.9
455.2 757.3
Jun 2019
18.0
122.0
435.2 709.5
Nov 2019
11.2
71.4
381.2 674.3
Aug 2020
2.3
44.1
353.8 651.5
Phola: 11 January 20161MC
Buxton Station 1905+00
2500 3000 35(
May 2017
Jun 2019 -
Oct 2018 =
+7 FT NAVD Jun 2019 -
Nov 2019 -
6 FT NAVD -Aug 2020 -
r------------ -------------------
-19 FT NAVD
---- -- -------------------------
L---- - - - - -- ----------- - -----.24FTNAVD -------
0
50o
1000 1500 2000
Distance from Baseline (ft)
Date
Vol to +7
Vol to -6
Val to -19 Val to -24
May 2017
3.6
45.9
308.7 572.7
Jun 2018
6.4
95.0
417.4 7237
1�
Oct 2018
9.5
101.1
377.7 679.2
Jun 2019
7.3
81.6
351.0 667.9
Nov 2019
0.0
25.6
307.3 630.0
Aug 2020
0.0
34A
313.8 578.1
Photo: 11 January2616 PC
250C 3000 3500
FIGURE 2.5b. Representative profiles for the Buxton project area at stations 1890 and 1905 in the Village of Buxton.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 13 Buxton, Dare County, North Carolina

2.3 Profile Volume Analysis Methodology
2.3.1 Profile Volume Approach
Volume variations along the Buxton project area were estimated using standard methods
(average -end -area method) and common cross -shore boundaries and contour datums. Two
primary lenses (ie - volumes between particular reference contours) were used 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 protective longshore bars.
If this cycle of offshore/onshore sediment transport remains balanced overtime, 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 visible beach, there will be a net loss and a specific volume
erosion rate.
Foreshore
Backshore Dune Berm Beach Face Low Tide Terrace
"Foredune" "Dry Beach" "Wet Beach" Runnel I Ridge
�I Inner Surf Zone L
Normal Breaker
7.nne
Swu..h Z.-
Fairweather Profile
Storm Profile
Trough I Longshore Bar
Su f Zane
Storm Breaker
Zane
G'
—High Nuter
—Low Water
I
I
Li.k Of .Norma! —11
P-00 Change I
"Closure"
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 Buxton
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]
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 14 Buxton, Dare County, North Carolina

While it is possible to approximate the equilibrium shape of beach profiles by simple analytical
equations (Dean 1991, 2002), each site has a unique set of coastal processes (waves, 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 water 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 island will have higher relief compared to Louisiana.
10 10
— NC Barrier Island L
5 — LA Barrier Island 5 a)
o
a
0 — 0 2 a�
O J
c
> N
N C/�
-5 -5 w
ca
m
-10 -10 ZE
300 200 100 0 100 200 300 400 500 600
Distance (meters)
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 lessvolume 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 z 3.28 ft. [From CSE 2011]
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 15 Buxton, Dare County, North Carolina

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.
SURVEY
MONUMENT
loft
5ft
Oft
o
-5 ft
FG570f7 LW
0 100 200 300 400
DISTANCE - FEET
SURVEY '
MONUMENT ,.'pVN�s •� �PG�
oe
yP�
Qy
loft �
sft
Oft
-5 ft
DISTANCE - FEET
SURVEY
MONUMENT
.G
loft
sft
0ft
-5 ft
ERODED BEACH
NORMALBEACH
500 600
BEACH WITH SAND SURPLUS
0 100 200 300 400 S00
DISTANCE - FEET
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]
Coastal Science & Engineering
[2403M-Appendix D]
Littoral Processes
16 Buxton, Dare County, North Carolina

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 overtime.
2.3.2 Reference Contours and Calculation Boundaries
The normal limit of significant change in bottom elevation (ie - "depth of closure*" - DOC) for the
Buxton project area was determined to be -24 ft NAVD (NPS/USACE 2015). This depth is based on
estimates of DOC at decadal scales at Duck (Birkemeier 1985), Bogue Banks (Olsen 2006), and Nags
Head (Kaczkowski & Kana 2012). 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 be 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 Buxton project area.
The landward calculation limit of unit volumes is set at the seawardmost structure in the
immediate area (edge of Highway NC 12 or seaward face of a building (USACE-USDOI-NPS 2015).
This calculation provides a measure of how much extra sand is contained in the profile seaward of
structures relative to the quantity in the active beach zone.
While the selection of volume calculation limits is arbitrary, the utility of this approach is that the
relative condition of the beach from locality to locality can be objectively compared. Areas that
contain a stable dune and wide beach seaward of structures can serve as a reference healthy
condition for areas of high erosion and large sand deficits.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 17 Buxton, Dare County, North Carolina

Volume changes within the calculation limits (ie — "sand box") for the Buxton 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) are used in the planning and design phases to evaluate levels of dune protection, dry
beach and construction berm adjustments, wet beach condition, inshore surf zone, and the outer
surf zone (USACE-USDOI-NPS 2015). These three lenses are described as follows.
• Lens 1 (Dunes) — Volume Above +7 ft NAVD — The 2017-2018 nourishment
construction berm was designed at +7 ft NAVD, and the proposed renourishment
construction berm is designed at the same elevation. The volume above the
+7 ft NAVD 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 depth (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 represents the outer
breaker zone and extends to the FEMA reference depth (from -6 ft to -24 ft NAVD). It is
the area beyond which there is relatively little change in bottom.
+30ft
+20ftloft
z Lens 2
Oft Low Tide Terrace (-6 ft) MHW +1.05 ft
MLW -1.94 ft
i0ft FEMA Reference Depth (-24 ft NAVD) Lens 3
zo ft
-30 It
FIGURE 2.9. The beach zone used for calculating sand quantities along the Buxton project area. The zone of interest
extends from the foredune 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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 18 Buxton, Dare County, North Carolina

2.4 Historical Erosion Rate Prior to the Initial 2017-2018 Nourishment
2.4.1 Historical Erosion Rates Prior to 2017
While CSE was planning and designing the initial nourishment, no recoverable historical profiles
encompassing the beach, surf zone, and inshore area to a depth of at least -24 ft NAVD were
located. Lacking such data, a standard method of estimating volumetric erosion rates is by
extrapolation from linear rates (CERC 1984).
A long-time rule -of -thumb used by the Corps of Engineers since the first nourishment project at
Coney Island (NY) assumes a loss of 1 square foot (ft2) of beach area is roughly equivalent to a loss
of 1 cubic yard of sand (CERC 1984). This ratio has also been assumed forsome analyses for NCDOT
(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). Typically, 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 1 ft2 of "beach" area (Fig 2.10).
Conveniently, 27 cf equals 1 cy, so the volume (cy) to area (sf) ratio equals 1. This ratio is >1 for
beaches that exhibit a deeper DOC and <1 for beaches with a shallow DOC. Figure 2.11 illustrates
how the volumetric erosion rate varies with the linear erosion rate and the local DOC.
z
+10 ft 1 ft,
+7 ft
Oft
l I- Mean Tide
101c 1 27 Cubic Feet --*-' .
1 cy/ft
-20 ft
Distance - Variable
FIGURE 2.10. 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 ft' of dry beach area represents -27 cf of profile
volume. This ratio remains constant by simple geometry for a parallelogram 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. 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
[2403M-Appendix D] 19 Buxton, Dare County, North Carolina

16
16
14
14
,
12
12
Example:
8 ft/yr = 10 cyift/yr
�%
n
10
10
`
�``� �
m
ED
8
`
8
m
� �
\
o
6
ti
6
c
4
\ ��
4
��
!
w
2
2
!
y
0
0
Station Alongshore
Volume Equivalent 1
Volume Equivalent 2 Linear Erosion Rate
--1.25 x Linear Rate
--.75 x Linear Rate
DOC --26-27 ft
DOC --12-14 ft
FIGURE 2.11. 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 Buxton (Source: NCDENR 2012). The upper dashed curve shows an estimated equivalent
u nit 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 Buxton is 1.15, based on a DOC z 24 ft NAVD.
For example, at Hunting Island (SC), 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 Buxton 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 z 1.15]. CSE used this latter ratio to convert
linear erosion rates to volumetric rates in the project area.*
('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 the Nags Head project and the availability of post -nourishment profiles into deep water. CSE (2014)
reported nearly 100 percent retention of nourishment sand after two years between the foredunes and -19 ft NAVD at Nags
Head. Post Irene and Sandy data at Nags Head also show evidence of accretion between -19 ft and -24 ft which
significantly exceeds the losses to -19 ft. This implies some onshore transport likely occurred during Irene and Sandy in the
zone beyond the -19-ft contour. The ratio assumed by Overton and Fisher (2005) is more conservative, but for the time
scales under consideration in the present study, CSE believes a ratio of 1.15 is realistic for planning.]
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 20 Buxton, Dare County, North Carolina

2.4.2 Historical Shorelines
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 is retained by NCDENR in recent updates. The end -point
method computes average annual rates at each shoreline transect using the earliest and most -
current shoreline position. The earliest shorelines considered in prior analyses are typically based
on NOAA's National Ocean Service (NOS) "T-sheets" from the 1920s to the 1930s. More recent
shorelines are interpreted from controlled aerial photography using the "wet/dry" line at the edge
of the surf zone, which approximates local MHW at the time of the photography (Overton & Fisher
2003). The most recent update of official shorelines (NCDENR 2012) utilized 1946/49 and July 2009
imagery (end points) from the US Department of Agriculture (USDA). NCDENR (2012) details the
various blocking and smoothing algorithms applied in developing the official rates along Dare
County. Figure 2.12 shows the results of the NCDENR (2012) analysis.
A striking aspect of the NCDENR 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. This is certainly the case north of Oregon Inlet along most of Bodie Island
or Bogue Banks (NCDENR 2012). By comparison, some short segments of Hatteras Island have
zones of moderate accretion (>5 feet per year-ft/yr) near zones of high erosion (>10 ft/yr). The
Buxton area has been highly erosional, but the rate diminishes in the vicinity of the Cape Hatteras
Lighthouse groins. The longest "stable" segment of Hatteras Island is between Salvo and Avon,
where measured change rates are within a narrow range of 1-5 ft/yr (see Fig 2.12).
CSE obtained the "shape files" of shorelines developed by NCDENR or their consultants at NC State
University and East Carolina University, and plotted them against the baseline for the present
study. CSE also digitized the June 2012 wet/dry line and added it to base maps of Buxton. An
earlier shoreline from the mid-1800s is also depicted. Distances from the CSE baseline to the
shoreline were measured at CSE profile transects and then used to check shoreline change rates
for several intermediate periods. The shorelines are illustrated in Figure 2.13, and the data are
tabulated by CSE stations in Table 2.3. Figure 2.14 plots selected results for the Buxton project
area. The official NCDENR long-term erosion rates (Figs 2.12 & 2.15) show focused high erosion
approaching 10 ft/yr in the area north of Buxton Village along the National Seashore. The rates at
CSE project profiles (see Fig 2.14) are consistent with NCDENR official rates, but show some
interesting variations according to the time period used.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 21 Buxton, Dare County, North Carolina

Cape Hatteras to Avon (raw, smoothed. & blockad data)
Cal. Ha--
negative (gre sn)= accrete on, n 0sfun (red) = eros Ion
12
A41'
10
�
9
8
7
g
.Q
■
htJ
3
4
1
.2
7, - 7,100 7,126
7,1aa 7,1160
7,18e 7�0 12i0 7wM, 7,a50 7x80 7A 7,320
7W 7,960 7,380 7,400 7.420
]96fi and ZO(19 S/iaYefrnes
iranseo ID
Avon to Rodanth■ (raw, smoothed, & blocked data)
„,ti,� , . , �.
negative;green}= accretion, pOsmve {rerp =eroslon
13
1 2
11
40
10
�
9
8.
p
g
7
4
$
6
44
i
V
W 3
2
1
0
S
a�
4
7,460 F,500
7,550 7,600 7.650 F,700 7,750
7,808
7A50 7,No 7Aso
M*and,0099naY m'r
iranuct ID
Rodanthe to Oregon inlet (raw, smoothed, & blocked data)
Raasmrle (er4( negatNe (gre en}= accrev on, pnsave "u) = erosion
22
Pee island (Nabonel Seeshorel
.at
18
16
1e -
12
-
y 8 r -
0
iFP
7,966 am 8A2o BA70 8.060 moo 8.100 8,120 8,140 8.160 8,160 8�00 8X0 8,240 8A0 8,280 8,3m 6,320 6,300
iransect D
1ed8 294® axe 2" Z_ffi
FIGURE 2.12. Long-term shoreline change rates for Hatteras Island derived from historical aerial photography (1946/49 and 2009)
showing smoothed and blocked data by transect as prepared by NCDENR/NCDCM and their consultants. [Source: NCDENR 2012,
Figs 34-36]
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 22 Buxton, Dare County, North Carolina

Historical Shorelines
1849 -1873
1925 -1946
1933 -1952
1970-1988
1998
• Waves
2004
2009
2012
I �
i
0 2,000
—
Scale (Feet)
r
aso+aa I
Salvos
900+00
U � i
- G
N �
4 � 920+00 I m
I r
I U
r O
i
f
I
I U
r
sao+oa I
1800+00
I
f
----- ------
r
I
I
G
�
!
U
,azo+oa
i
o
I
I
V
aI
I
------- ---
�
�
I
u,
�saa+ao
I -
------- m
0
U
I
I
0.
I�
I
I
I
,
--
18fi0+00
,
,
I
I
- --
• Buxton
I
I
I
,900+00
I
�-
1P i
Cape Hatteras Light
ssowo I Q - - I -
19so+00
I i -
i
I
I
I i l
980+00 -
i 1R+00 I
Rodanthe Area 1983+77 �� _ Buxton Area
PaneI03 i Panel04
FIGURE 2.13. Historical (approximate MHW) shorelines for Salvo and Buxton from GIS shape files prepared by NCDENR
and consultants from INC State University and East Carolina University. The 2012 shoreline (wet/dry line) was digitized
by CSE from 2012 imagery. The shoreline "salient' between stations 1920+00 and 1940+00 is associated with groins
placed in the 1970s to protect Cape Hatteras Lighthouse (which was eventually moved -1,600 ft to the southwest).
[Courtesy: NCDENR 2012]
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 23 Buxton, Dare County, North Carolina

TABLE 2.3. Historical shorelines (distance from baseline in feet).
CSE Station
1925-46
1970-88
1998
20D4
2009
1744+87
517
300
328
257
301
1777+68
604
294
214
251
242
1790+63
735
332
277
250
189
1800+63
719
377
252
238
134
1810+63
794
369
225
242
100
1821+84
803
414
271
215
138
1830+63
809
371
266
218
104
1840+63
719
384
196
119
95
1850+63
716
358
197
173
101
1860+63
717
398
217
198
101
1870+63
680
365
233
196
163
1880+63
650
300
210
187
166
1890+63
627
368
259
227
170
1900+63
581
439
277
274
180
1901+75
576
443
295
266
198
1910+63
560
396
353
201
229
1920+63
516
456
393
282
259
1929+11
490
358
324
241
219
1940+63
629
243
43
152
-37
1950+63
723
122
-89
80
-13
1960+63
827
93
6
64
112
1970+63
893
272
61
159
250
1983+77
1,012
630
265
361
351
Coastal Science & Engineering
[2403M-Appendix D]
Littoral Processes
24 Buxton, Dare County, North Carolina

4.0
Buxton Area - Linear Shoreline Change Rates - (ft/yr)
2.0
-
0.0
,
. . . . . . .
OEM
Note: Median Dates Assumed:
MOMMOMMAMMOMME
-2.0 -
1915146� 1935
Bch
0"
1970/88 1979
-4.0 -
-6.0 -
-8.0 -
-10.0 -Buxton
Groins
Village
-12.0 -
-14.0 -
1970/88 to 2012 —1925146 to 2012
-16.0 -
-18.0
I y5
�'NI
19�ox63 19�oN63 196ox�,3 1g�3k�'l
10 1�9ok63 1g1oX63 1$3ox63 1$�oX63 1��oX63 1��ox63 1go1X
FIGURE 2.14. Linear shoreline change rates (ft/yr) at CSE stations derived from historical shorelines 1920s to 2009,
digitized by the State of North Carolina (source: NCDENR 2012). The 2012 shoreline was digitized by CSE from 2012
imagery. [Source: ESRI ArcGIS World Imagery]
Coastal Science &Engineering Littoral Processes
[2403M-Appendix D] 25 Buxton, Dare County, North Carolina

► 780+00 ID:7870
p 2,000 _ Waves
Scale (Feet) 800+00 r
840+90 --
rl
2
a
-
I
�
0 860+00
t
ro
CO
a�
U
0
U
O
880+00
E
_
ro
F-��
EL Salvo
ro
i
i
920+00
ID: 7782
{ I
Jr lJ
940+00 2
980+0o ID:7760
2
9sa+ao
Rodanthe Area
Panel 03 I,,. 771A
i 4760�
4
1800+
b
5
13
9820+i
o
a�1840+
_ 1880+00
• Buxton
1920+00
7s4o+oo
Cape Hatteras Light
+sso+oQ
+sao+ �
198 0 0
D: 7258
5
D: 7257
7.5
ID, 7248
9
ID 7240
Cm
O
10
c
ro
ID, 7208
9.5
ID, 7193
8
ID 7189
6.5
ID 7177
5
ID 7161
7.5
ID', 7152
11
ID7135
10
ID: 71213 Buxton Area
g Panel 04
FIGURE 2.15. NCDENR (2012) official shoreline change rates and setback factors (ft/yr) for Salvo and the Buxton project
area as published by the state. As Figure 2.16 showed, some segments along Salvo with the minimum "setback factor"
of 2 ft/yr are accretional (eg - between transects ID 7738 and 7870). [Source: NCDENR 2012]
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 26 Buxton, Dare County, North Carolina

The erosion rates in the Buxton project area show long-term maxima around station 1840+00
(-4,000 ft north of the Village of Buxton/National Seashore boundary). Maximum rates were
around 9 ft/yr, excluding the area south of the old Hatteras Lighthouse groins. The section of the
National Seashore north of Buxton breached in the early 1970s and has washed over Highway NC
12 several times, including as recently as Hurricane Sandy in October 2012 (NCDENR 2012,
unpublished data).
As Figures 2.12 and 2.14 showed, erosion rates at the groins have been lower than adjacent areas,
indicating their stabilizing impact on that portion of the shoreline. Downcoast of the groins,
shoreline erosion rates were much greater between 1925/46 and 1970/88 (generally preceding
groin construction) than the period since 1970/88. Some of the reduction in the erosion rate over
the past few decades may be related to an -1.3 million cubic yard nourishment project placed
along Buxton in -1973 (S Rogers, North Carolina Sea Grant, pers. comm., August 2013).
2.4.3 Equivalent Volumetric Erosion Rates
CSE used official long-term erosion rates published by NCDENR (2012) and determined the rate at
each surveyed profile line (Table 2.4). An equivalent volumetric change rate was calculated using
the factor 1.15 as discussed in Section 2.4.1. Table 2.4 lists the estimated average annual volume
erosion rates at CSE stations within the project area. The equivalent rates at Buxton range up to
12.6 cubic yards per foot per year (cy/ft/yr) (station 1821+84).
Net annual erosion losses between stations were estimated using the average -end -area method,
which averages the unit rates for adjacent stations and applies the average over the shoreline
distance between stations (see Table 2.4). These subtotals were summed for designated reaches
for purposes of estimating net yearly losses in critically eroding areas.
CSE reviewed the locations of profile volume minima (see Section 2.3) and NCDENR linear erosion
rates, and delineated two priority reaches along the project area for possible beach restoration.
At Buxton, the critically eroding reaches are considered to be the-11,500-ft shoreline segment
between stations 1805+56 and 1920+63, or an expanded length incorporating the-13,750-ft
segment between stations 1790+63 and 1928+11 (USACE-USDOI-NPS 2015).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 27 Buxton, Dare County, North Carolina

TABLE 2.4. Official long-term (linear) erosion rates in the Buxton area and the estimated equivalent unit volume erosion rate
(cy/ft) using the conversion factor: 1 linear ft =1.15 cy/ft. The net volume rate between profile stations is computed by the
average -end -area method. [DCM - North Carolina Department of Environment & Natural Resources, Division of Coastal
Management] (after CSE 2013 and USACE-USDOI-NPS 2015)
CSE
Station
1744+87
USACE
Station
DCM
Transect ID
7277
Distance To
Next (ft)
DCIM Rate
(ft/yr)
Equiv Vol Rate
(cy/ft/yr)
Net Vol Rate
To Next (cy/yr)
1759+63
7268
1777+68
N/A
7257
7.5
9.0
-
1790+63
8.0
9.6
-
1792+44
7248
819
9.0
10.8
8,845
1800+63
493
9.0
10.8
5,620
1805+56
7240
507
10.0
12.0
6,084
1810+63
1,121
10.0
12.0
13,788
1821+84
N/A
879
10.5
12.6
10,812
1830+63
1,000
10.0
12.0
12,000
1840+63
1,000
10.0
12.0
11,700
1850+63
7208
1,000
9.5
11.4
11,400
1860+63
667
9.5
11.4
7,204
1867+30
7198
333
8.5
10.2
3,297
1870+63
N/A
1,000
8.0
9.6
9,300
1880+63
143
7.5
9.0
1,244
1882+06
7189
857
7.0
8.4
6,942
1890+63
1,000
6.5
7.8
7,500
1900+63
112
6.0
7.2
739
1901+75
7177
888
5.0
6.0
5,328
1910+63
N/A
1,000
5.0
6.0
5,400
1920+63
748
4.0
4.8
1,795
1928+11
7161
At Terminal Groin
1940+63
1950+63
1960+63
1970+63
N/A
1983+77
(1792+44 to 1928+11)
13,567
9.5
128,998
Totals
(1805+56 to 1920+63)
11,507
10.2
114,533
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 28 Buxton, Dare County, North Carolina

The northernmost boundary is -8,750 ft north of Buxton, and the southernmost boundary is
situated at the old Hatteras Lighthouse site. Average annual volumetric erosion rates for these
two reaches are estimated to be-114,500 cy/yr and-129,000 cy/yr (respectively) (see Table 2.4).
The average annual unitvolume erosion rates are 10.2 cy/ft/yr for the most critically eroding reach
and 9.5 cy/ft/yr for the expanded reach. In the Buxton area, the highest erosion rate is along CH NS
near station 1821+84, -5,800 ft north of the Village of Buxton, whereas the greatest profile deficit
is in the Village of Buxton.
For final design and planning purposes, CSE is evaluating a slightly longer shoreline to improve
project longevity and provide extended taper lengths for nourishment. The initial nourishment
plan called for a maximum project length of 15,500 linear feet in the Buxton area, and the
average annual erosion rate was adopted to be 114,500 cy/yr (ie - 9,542 cy per month).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 29 Buxton, Dare County, North Carolina

2.5 Volume Analysis During and After the 2017-2018 Nourishment
2.5.1 Overview of the 2017-2018 Nourishment Construction
The proposed renourishment project is a maintenance activity following the completion of the
2017-2018 Buxton nourishment project. Therefore, some project information is included in this
report to provide a background of Dare County's sediment management framework in this area.
2.5.1.1 Project Sponsor
The beach restoration project was sponsored by Dare County, North Carolina, and Dare County
served as project owner and administrator.
Construction funding for the federal Dare County hurricane protection and beach erosion control
project has not yet been appropriated. Therefore, Dare County used local funding for the
2017-2018 project without imposing additional costs to the state of North Carolina or the US
Government to address the urgent problem of erosion in the project area.
2.5.1.2 Project Setting
Dare County encompasses -89 miles of ocean shoreline from the Town of Duck to Hatteras Inlet.
The northern 30 miles (on Bodie Island) includes the towns of Duck, Southern Shores, Kitty Hawk,
Kill Devil Hills, and Nags Head (from north to south). There is a 5-mile undeveloped portion of Cape
Hatteras National Seashore (Seashore) at the southern end of Bodie Island. The southern
-53 miles on Hatteras Island encompass Cape Hatteras National Seashore and the communities of
Rodanthe, Waves, Salvo, Avon, Buxton, Frisco, and Hatteras (Fig 2.16). Approximately 16 miles are
developed, and 38 miles are undeveloped along the oceanfront. In total, 50 percent of Dare
County's ocean shoreline is developed, and 50 percent is undeveloped and held in permanenttrust
by the Cape Hatteras National Seashore.
The Buxton project area encompasses -3-miles (15,500 linear feet) of Hatteras Island in the Cape
Hatteras National Seashore, including -2.2 miles (11,000 linearfeet) of natural oceanfront and -0.8
mile (4,500 linear feet) in front of the developed Village of Buxton. Figure 2.17 shows the project
limits, total nourishment volume, and the proposed offshore borrow area.
2.5.1.3 Background, Purpose, and Need
The location of the 2017-2018 project area is on Hatteras Island in the Village of Buxton in the Outer
Banks of North Carolina. One of the largest preserved parcels of the Outer Banks, the Seashore offers
beachcombing, birding, fishing, camping, wind -surfing, and kite -boarding to beachgoers and road
trippers each year. The area is known for its abundant recreational, natural, and cultural resources,
including such historic locations as the Cape Hatteras Lighthouse, the Chicamacomico Life Saving
Station, the Wright Brothers National Memorial, and the Fort Raleigh National Historic Site.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 30 Buxton, Dare County, North Carolina

Alharmarle Sound
n Hags Head
n
m
N
m ¢ Q� Orag�aa lalef
� m
tulttPy Paint
Pea Island
National
Wildlife Refuge
+ Rodanthe
Is
12
Cape HaNeras
National Seashore
a Avon
Pamlico Sound
Buxton -am- PROJECT SITE
N 35 o 16' 36"
Hatteras W 75 ° 31' 00"
J�
i
kafferas 1a1at
Ocracoke Island
Cape Lookout w A t r r) t i c O c e a n
National Seashore Ocracoke lalef
FIGURE 2.16. Location of Buxton and Cape Hatteras National Seashore, Dare County, North Carolina.
Coastal Science & Engineering Littoral Processes
[2403M—Appendix D] 31 Buxton, Dare County, North Carolina

N: 590760
I N 591 M
E' 3032750
Mile 58 6
t E' W51780
J
4 3.000
r7
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4
Mile 59.
n
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a
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U
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Q
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m �
iA fl
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+
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Cape Halleras light r
� r •
1�
wr
• r�
• �r
Q Revised Borrow Area
8,400 ft x 1,700 ft (-300 ac}
r--� No Work Area (-28 ac)
• Sediment Borings
N:554760
1_
1 N:554760
E:3012M
E WS1750
Cape Nattaras
Datum:
SKS NAo'M (Feet) NG7_ane 3200
FIGURE 2.17. Map illustrating the fill schedule and project limits of the 2017-2018 Buxton nourishment
project. The sand source is an offshore borrow area situated about 9,000 feet from the former site of the
Cape Hatteras Lighthouse. No Work Area is marked on the map to avoid possible culture resources.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 32 Buxton, Dare County, North Carolina

NC 12 is the only highway linking all the Seashore islands and villages along the ocean. Before NC
12 was built, islanders drove on the beach to access homes and businesses. Seven villages —
Hatteras, Frisco, Buxton, Avon, Salvo, Waves, and Rodanthe (from south to north) —occupy
Hatteras Island, which includes a year-round population of - 4,300 people (2010 Census). Buxton
is the largest of the villages, known for world -class surf fishing and the Cape Hatteras Lighthouse.
The Hatteras Lighthouse is a registered National Historic Landmark and a National Historic Civil
Engineering Landmark since its relocation inland in 1999 (Booher & Ezell 2001).
Portions of the island, such as Waves, Salvo, and south Buxton, enjoy wide and stable beaches,
which have been accumulating sand. Other areas are narrow and have sustained extensive erosion
along the oceanfront, notably around Rodanthe and East Buxton (NCDENR 2012).
Shoreline change rates along the Outer Banks oceanfront are variously reported to average 2.6 feet
per year (Everts et al. 1983) to 4.5 feet per year (NCDENR 2012). At several localities, including
south Nags Head, Pea Island, Mirlo Beach, and the Buxton Canadian Hole just north of the village,
erosion rates have exceeded 10 feet per year over the past 50 years (NCDENR 2012). Coincidentally,
these sites have experienced chronic dune breaching, overwash onto NC 12, or the formation of
temporary breach inlets.
Hatteras Island plays a vital economic role in the state and local economy. During peak tourist
season, the Island receives up to 50,000 visitors daily, which has stimulated notable growth in
rental properties and businesses in recentyears. Astudyforthe Outer Banks Visitors Bureau (Lane
2013) found that Hatteras Island's tourism expenditures totaled $204 million in 2011, with a state
tax contribution of $10.3 million and $9.4 million in local taxes. Also, in 2011, island real estate
generated >$9 million annually in Dare County property taxes and $2.1 million in occupancy tax
collections. However, that same year, it was estimated that $2 million was lost in annual
occupancy rates dueto a two -month (Septemberand October) closure of NC 12fordune rebuilding
and road repairs.
In response to erosion, NC 12 has been realigned at some localities, including the proposed project
area north of Buxton Village. The highwaywas washed out by a breach inlet in 1962 and was severely
overwashed during other storms between 1962 and 1973 (Fisher 1967, Everts et al. 1983). After the
1960s storms, a realignment of NC 12 shifted the road as close as practicable to Pamlico Sound (NPS
1980). Beach nourishment between 1966 and 1973 reportedly helped mitigate breach events for over
20 years (Lighthouse View Motel, J. Hooper, former Dare County commissioner, pers. comm., April
2014). However, continued loss of sand along the Buxton Canadian Hole has resulted in more
frequent road repairs by NCDOT due to foredune breaches. The most recent major repairs were in
response to Hurricane Irene (August 2011) and Hurricane Sandy (October 2012).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 33 Buxton, Dare County, North Carolina

Maintenance of NC 12 has been an issue for decades and remains the subject of intensive study by
NCDOT. NCDOT developed a feasibility study for NC 12 in the Buxton area to consider alternatives
for interim (5-year) protection and longer -term (50-year) protection (NCDOT 2015). Any
improvements or modifications to NC 12 are subject to existing Easements and agreements
between the National Park Service and NCDOT. While NCDOT studies were underway, Dare County
reviewed the options for protecting NC 12, infrastructure, and maintenance of the beach under
coastal zone management (CZM) rules and regulations. Dare County determined that a wider
beach was needed in the Buxton area to restore the sand deficit, protect the foredune and
infrastructure, and maintain access via NC 12 in the project area with minimum disruption to the
economy and the environment.
Erosion around 2015 had undermined 51 houses and motel units alongthe Eastern shore of Buxton
Village, leading to emergency measures. These include sand -bagging to protect structures along
-1,500 linear feet of oceanfront at the south end of the proposed project area. Sand bags
eliminated a recreational beach and related habitats along -10% of the beach in the project area.
The County commissioned a feasibility study to evaluate erosion and beach restoration
alternatives (CSE 2013). Detailed surveys into deep water documented that erosion over several
decades has left a significant sand deficit in the Buxton area relative to adjacent sections of the
beach. Dune -breach events have generally occurred in the areas of Hatteras Island where dunes
are low, the beach is narrowest, and there is less sand seaward of buildings and infrastructure
compared to a normal stable beach. The breaches at Pea Island and Mirlo Beach during Hurricane
Irene (2011) are examples. The County decided to place 2.6 million cubic yards of beach -quality
sand along a 3-mile length of Hatteras Island in the Cape Hatteras National Seashore (Seashore),
including -0.8 mile in front of the Village of Buxton, and the project was completed between June
2017 and February 2018. The 2017-2018 project served the following purposes according to the
County:
1) Provided a wider beach and buffer storm waves along a critically eroding section of
Hatteras Island.
2) Reduced the frequency of storm damages to North Carolina Highway 12 (NC 12) and
existing community infrastructure.
3) Replaced erosion losses and augment the regional supply of beach sand by using a
non -littoral borrow source of compatible sediments from an offshore borrow area.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 34 Buxton, Dare County, North Carolina

2.5.1.4 Permitting
Dare County obtained permits under the National Environmental Policy Act (NEPA) and the state
Coastal Area ManagementAct (CAMA) permitting process, including preparation of comprehensive
Environmental Assessment and its seven appendices (USACE-USDOI-NPS 2015). The
environmental documentation was necessitated by the need to accomplish the work during the
summer months. The principal environmental issues at the National Seashore were impacts to
sensitive beach -nesting birds and sea -turtle species duringthe breedingseason through noise and
physical disruption, increased sand compaction and hardness, and changes in moisture content
and beach slope. As part of the Southeast Region, the project area is subject to the same turtle -
protection measures and "take statements" as Florida, where most nests are located (NMFS 1997).
Virginia Beach (VA), 100 miles north of Buxton, is situated along the same bight but is under the
Northeast (USA) environmental protection rules, where turtle nesting is of less concern.
Sea turtles, while present in low numbers in some Northeast Region waters, are not subject to the
Southeast Region "take statements," which severely limit the use of hopper dredges. Buxton
Village (east shore) had no sea turtle nest in the years before the 2017 project due to the narrow
dry beach and frequent overwash to the toe of the dune. The average nest density in the OBX
(north of Oregon Inlet) is <0.2 nests per mile per year (source: NCWRC database). By comparison,
South Carolina beaches average upward of 15 nests per mile peryear (NMFS 1998).
As mitigation for summer dredging, the project was required to provide turtle monitoring on the
beach each night during construction and endangered species monitoring onboard the dredges.
Open -net turtle trawling ahead of the dredge was also required if hopper dredging was used for
purposes of turtle safety.
Dare County, serving as the Owner of the project, procured the following state and federal permits
and certification:
1. North Carolina Department of Environment Quality 401 Water Quality
Certification (DWR #15-1087) - Issued on 23 November 2015.
2. North Carolina Department of Environment Quality Major CAMA Permit
(#136-15) - Issued on 15 December 2015.
3. National Park Service Special Use Permit (GOV16-5700-014) -Issued on
11 March 2016 and revised on 27 November 2017 for an extension.
4. U.S. Army Corps of Engineers (USACE), Department of the Army Permit
(#SAW 2015-01612) - Issued on 24 March 2016.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 35 Buxton, Dare County, North Carolina

2.5.1.5 Project Bids
CSE prepared a final design, plans, specifications, and bid documents, and made them available
to contractors on 15 March 2016 following review by County officials. Bids were received and
opened at 2 pm on 7 April 2016. Because work involved excavations offshore, ocean -certified
dredges were required under US Coast Guard regulations. Therefore, only five US dredging firms
are qualified to perform this work. Four (out of five) firms, having ocean -certified equipment, bid
on the project: Dutra Group (Dutra), Great Lakes Dredge & Dock Company (GLDD), Manson
Construction, and Weeks Marine. The County's reserved construction budget (limit) was
$22,963,175.
Bids were requested for mobilization and pumping of a base quantity of 2.2 million cubic yards
over the length of the project and an alternate (supplementary) quantity of up to 400,000 cubic
yards. Bidders were requested to submit two bids, Bid A for 2016 construction and Bid B for 2017
construction. Details of the bids received from the four responding bidders are listed in Table
2.5. Two out of the four bidders (ie - Dutra and Weeks) submitted bids for Bid A, but both bids to
complete the maximum permitted work were nearly 50 percent more than the County's budget
(Dutra proposed $33,980,000, and Weeks Marine proposed $34,150,00). All four bidders
submitted bids for Bid B, and Weeks Marine offered the lowest bid, equating to $22,150,000
(including mobilization/demobilization and pumping) for 2.6 million cubic yards (maximum
volume allowed under the permits). The net price equated to (-)$8.52 per cubic yard (in -place
volume based on measurements on the beach). The total construction price for Bid B was -$12
million lower than Bid A, and $813,175 below the County's budget. The remaining funds could
be used for post -project monitoring, sand fencing, legal, administration, contingencies, and
future renourishment study.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 36 Buxton, Dare County, North Carolina

TABLE 2.5. Bid tabulation for beach restoration at Buxton, Dare County, North Carolina.
BID TABULATION
Beach Restoration at Buxton, Dare County, North Carolina
2 PM April 7, 2016
Bidder 1
Bidder 2
Bidder 3
Bidder 4
Name
Dutra Group
2350 Kerner Blvd.
San Rafael, CA
94901
Great Lakes
Dredge & Dock Co.
2122 York Roak
Oakbrook, Illinois
60523
Manson Construction
5985 Richard Street
Jacksonville, Florida
32216
Weeks Marine Inc.
304 Gaille Drive
Innwoods Business Park
Covingon, LA 70433
Addendum Acknowledged
1 1/
1 1I
1 1I
1 l
Bid Items
Base Bid Al - Mob/Demob
4,600,000
No Bid
No Bid
4,850,000
Base Bid A2 - Dredging and
Placement of 2.2 MCY
24,860,000
No Bid
No Bid
25,300,000
Alt Bid A3 - unit price of 400,000 CY
11.30
No Bid
No Bid
10.00
Total Price = Al+A2+A3*400,000
33,980,000
No Bid
No Bid
34,150,000
Base Bid B1 - Mob/Demob
4,400,000
8,190,000
9,750,000
4,350,000
Base Bid B2 - Dredging and
Placement of 2.2 MCY
21,560,000
19,910,000
33,000,000
15,400,000
Alt Bid B3 - unit price of 400,000 CY
9.80
9.05
15.00
6.00
Total Price = Bi+B2+B3*400,000
29,880,000
31,720,000
48,750,000
22,150,000
Document List
Bid Form C-410
Noncollusion Affidavit C-420
1I
Bid Bond C-430
1I
List of Subcontractors C-440
Equipment List C-450
Hopper 1 Cutterhead C
Hopper 2 Cutterhead 0
Hopper 1 Cutterhead 0
Hopper 2 Cutterhead 1
Iran Divestment Act Certification
Business License Number
61566
67410
63823
Other Supporting Documents
Owner Representatives Opening Bids: Dustin Peele David Clawson Wally Overman Dorothy Hester Gary Gross
Engineer Representatives Opening Bids: Tim Kana (PG 1752) Haiciina Kaczkowski (PE 37281)
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 37 Buxton, Dare County, North Carolina

2.5.1.6 Construction
Construction was completed between 21 June 2017 and 27 February 2018, and approximately
2,600,000 cubic yards of beach quality sand was placed along the 15,500-foot project area
according to the design (Figure 2.17). It is the largest beach nourishment project ever completed
near Cape Hatteras, and the primary purpose of the project was to protect NC Highway 12.
Summer dredging was permitted forth is particular project because of inclement wave conditions
in the winter months (Figure 2.18). The Contractor (Weeks Marine) elected to use a cutterhead
dredge (CRMcCaskill) to start the nourishment on 21 June 2017. Production of construction lagged
due primarily to rough sea conditions that frequently curtailed operations and led to mechanical
breakdowns. As of 22 August 2017, -1.1 million cy of sand (-42 percent of the total contract volume)
was placed on the beach by the cutterhead dredge.
FIGURE 2.18. Graph showing the monthly average wave climate from 2003-2020 at NDBC Wave Buoy Station 41025 at
Diamond Shoals (NC) near Buxton (Source: NDBC). The criteria for safe dredging apply to hopper -dredge operations
using ocean -certified equipment. Suction-cutterhead dredges generally cannot operate safely in waves greater than 3
feet (USACE 2010). The graph shows that average monthly wave height exceeds 5 feet from September to April in the
Buxton project area. Calmest conditions occur in June and July when average wave heights are -3.7 feet. The bars at
the bottom of the graph show approximate range of dates when certain protected species may be present in or near the
project area.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 38 Buxton, Dare County, North Carolina

In September2017, four named hurricanes (Irma, Jose, Katia, and Maria) impacted the project area,
and wave heights were significantly higher than the required safe operating conditions for a
cutterhead dredge. Construction had to cease for over 50 days until 11 Oct when the Contractor
started to use a hopper dredge (RN Weeks). Despite the intermittent work schedule, construction
moved forward. As of 22 December 2017, nourishment in front of the narrow isthmus of NC
Highway 12 was completed, and the highly vulnerable section of the road was protected.
Weeks Marine's newly constructed hopper dredge, Magdalen, passed sea trials and obtained the
operation certificate just in time in January 2018, when the project was -80 percent completed.
With twice the capacity of the RN Weeks, the Magdalen finished the last -0.5 million cy of work and
delivered her last load at 1:30 pm on 27 February 2018, almost five months later than the
Contractor's original estimate. The three dredges that the Contractor used are listed in Figure 2.19.
The Contractor's production rates relative to the offshore wave heights are shown in Figure 2.20
and Figure 2.21.
Construction Delays and Contract Time Extension
The contract time of the agreement between Dare County and the Contractor, Weeks Marine,
required construction to be completed by December 15, 2017. In light of the Contractor's multiple
delays, Weeks Marine submitted a request for a contract time extension on 6 December 2017. In
the request, Weeks Marine analyzed the wave data at the NOAA Station 41025 at Diamond Shoals
(NC) and determined there have been a total of 54 days from 15 June to 31October 2017 when the
wave heights exceeded the average records of the previous six years (2011-2016) at this station.
Therefore, Weeks Marine requested that 54 calendar days be added to the contract time and that
the project completion date be moved from 15 December 2017 to 7 February 2018.
CSE analyzed the wave data provided by StormGeo (headquartered in Bergen, Norway) for the
Buxton borrow area over the same period of time (ie - the 138 days from 15 June to 31 October
2017). If a 5-foot (ft) wave is considered the threshold of safe operation condition for a cutterhead
dredge (USACE-USDOI-NPS 2015), the dredge must shut down when the peak wave heights exceed
that threshold. [Note: Peak wave heights are defined as occasional individual wave heights over a
1-hour period.]
As summarized in Table 2.6, StormGeo wave data show there were a total of 60 more days than the
historical records at Diamond Shoals when peakwave heights exceeded the operational threshold.
These higher -than -average wave conditions curtailed dredging operations for extended periods
and contributed to mechanical breakdowns. High waves and shoaling also prevented routine
maintenance dredging at Oregon Inlet and precluded inlet access at various times, further adding
to delays for Weeks Marine's support vessels.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 39 Buxton, Dare County, North Carolina

The project team assumed the project would be completed by 7 February (as proposed by the
Contractor in their time extension request) and the weather over the final two months would
follow this higher -than -average pattern, with additional delays. Therefore, the total weather days
could be as high as 80 days.
TABLE 2.6. CSE wave analyses using data provided by StormGeo."
*The following quotes are from an email from StormGeo explaining the methodology used in their wave
forecast for the Buxton project (c/o McLean Barnett, MarineWatch Manager of StormGeo, 2 August 2017).
"Regarding the buoy data for the forecast, we use several tools to generate each forecast. Our forecasters
use many global, regional and ensemble models to generate data for a site, and then `ground truth' the data
with observations in the vicinity. For your site at Buxton Beach, we utilize the offshore buoy data from
Diamond Shoals (Station 41025) for typical offshore and upstream wind and sea information, and then use
the wind and pressure data from the USCGC Station in Hatteras for a close coastal comparison. The
additional buoys to the north are also utilized if the information from those sources is suspect."
Period
Days
Occurrence of Waves Higher than 5 ft
Resulting
Weather Days
Historical Records at
Diamond Shoals
(NOAA 41025) (%)
StormGeo Peak Wave
Height at the Buxton
Borrow Area (%)
Difference
June 15-AG
15
0.15
0.58
0.42
6
July
31
0.17
0.64
0.47
15
August
31
0.14
0.56
0.42
13
September
30
0.48
0.98
0.40
12
October
31
0.41
027
0.46
14
Total
60
After reviewing Weeks Marine's request and also after considering the fact that the Contractor
should have already built in some weather delays in their original work plan, CSE determined that
a request for a time extension of 54 calendar days (out of 80 potential days of weather delays for
the entire project execution time period) had merit. Therefore, CSE recommended that Dare
County accept Weeks Marine's request and extended the project's final completion date to 7
February 2018 without any adjustment to the price of the contract.
Despite all the delays, the project was completed without any sea turtle takes or other
environmental incidents. Collaborations among the Owner (Dare County), regulatory agencies
(USACE, NPS, and NCDEQ), and the Contractor remained excellent during the course of
construction. The newly nourished beach withstood a series of nor'easters in March 2018 without
interruption of the passage of NC Highway 12 or any damage to the Buxton oceanfront properties.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 40 Buxton, Dare County, North Carolina

FIGURE 2.19a. Week's Marine's 30-inch Cutter suction dredge CR McCaskill with dimensions of
230'x62'x14' and 17,400 total horsepower (Thp) [Source: weeksmarine.com/equipment]. It was
onsite from 21 June to 23 August 2017 and completed approximately 1 million cubic yards of work
during the 2017-2018 Buxton beach nourishment project.
FIGURE 2.19b. Week's Marine's 4,000 cy trailing suction hopper dredge RN Weeks with dimensions
of 282.5'x54.1'x22.2' and 9,530 Thp [Source: weeksmarine.com/equipment]. It was onsite from 11
October 2017 to 28 January 2018 and completed approximately 1 million cubic yards of work during
the 2017-2018 Buxton beach nourishment project.
FIGURE 2.19c. Week's Marine's 8,550 cy trailing suction hopper dredge Magdalen with dimensions of
363'x79.5'x27.3' and 14,003 Thp [Source: weeksmarine.com/equipment]. It was onsite from 9 January
to 27 February 2018 and completed approximately 600,000 cy of work during the 2017-2018 Buxton
beach nourishment project.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 41 Buxton, Dare County, North Carolina

Weeks Marine Production Rates versus Wave Heights
Peak Wave Height (ft) • CR McCaskill • RN Weeks • Magdalen
30
70,000
• •
CR McCaskill Worked 37 days
25
ra
RN Weeks Worked 71 days
60,000
•
Magdalen Worked 34 days
t
• • • •
50,000
41
20
• • • f6
U
_
• a
• • N Y
40,000
0
—
15
• M '^
o
-1
• •
JL
`
30,000
0
10
•
•
• •
I •
• •
�+•
• • %
20,000
• to dL• • •
O
5
r ti •
10,000
• • �• • Dredging threshold* • • • j r
0
0�i 0 ••reams• ••
0
6/21 7/21 8/20 9/19 10/19 11/18 12/18 1/17 2/16
Date
FIGURE 2.20. Daily production rates of the three dredges in relation to the offshore wave climate at
Diamond Shoals (NDBC wave buoy Station 41025) during construction for the 2017-2018 Buxton beach
nourishment project.
Weeks Marine Cumulative Production versus Peak Wave Heights
Peak Wave Height (ft) —Cumulative Volume (cy)
30 3,000,000
CR McCaskill Completed -1 million cy
$ 25
RN Weeks Completed -1 million cy
2,500,000
Magdalen Completed-600,000 cy
T
s
ra
= 20
2,000,000 E
�
f6
0
>
15
------
- [ ----------- Y - ----- ---
1,500,000
O
th
0
Y
S
p 10
------------- ! - -
1,000,000 u
5
500,000
Dredgingthreshold
0
0
6/21
7/21 8/20 9/19 10/19 11/18 12/18 1/17 2/16
Date
FIGURE 2.21. Cumulative production rates of the three dredges in relation to the offshore wave climate
at Diamond Shoals (NDBC wave buoy Station 41025) during construction for the 2017-2018 Buxton beach
nourishment project.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 42 Buxton, Dare County, North Carolina

2.5.2 Year 1 (2018) Post -Project Volume Analysis
2.5.2.1 Post -Project Monitoring Requirements
To qualify for Federal Emergency Management Agency (FEMA) assistance, FEMA requires the
Project Ownerto establish a maintenance program involving post -project beach condition surveys
and periodic renourishment with imported sand to preserve the original design of an engineered
beach. The purpose of the post -project monitoring is to track the physical condition of the beach
after nourishment, quantify sand volume changes, and determine whether a maintenance
renourishment is needed to increase the life of the project. Another vital purpose of annual beach
monitoring is to evaluate whether the project area qualifies for FEMA's post -storm beach
restoration funding following declared disasters. The state and federal permits also required
specific physical condition monitoring after project completion, such as beach compaction tests
prior to turtle nesting season. The following tasks summarize the post -project monitoring work
that the Owner (Dare County) has been doing since the completion of the 2017-2018 nourishment
project.
• Beach compaction tests for two years (2018 and 2019) before the start of turtle
nesting season.
• Beach and inshore profiles at minimum 500-ft spacing at USACE/CSE stations
(including upcoast and downcoast areas) to track the project condition and the
spread of nourishment sand to adjacent areas.
• Data analysis to determine nourishment volumes remaining by reach and volumes
remaining relative to the quantity placed during construction.
• Sediment sample collection and analysis for monitoring the as -built quality of sand
on the visible beach every year.
• Aerial photography to document the general conditions of the shoreline each year
and periodic controlled vertical photography approximately once every three years.
Based on the above -stated monitoring requirements, beach compaction measurements were
made in February for Year 1 (2018) after the project. The methodology and results of the 2018
compaction tests are included in Attachment 3-A.
The first annual profile surveys were performed in June 2018 (forYear 1). The June survey provided
pre -storm condition data and served as the annual baseline for comparison with post -storm
condition surveys. In addition to the yearly pre -storm surveys, one post -storm survey was
conducted in October 2018 after Hurricane Florence.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 43 Buxton, Dare County, North Carolina

Volume losses caused by Florence are discussed in this section, and the volume changes relative
to the pre -project condition and nourishment sand remaining will be discussed in detail in Section
2.5.5.
2.5.2.2 Hurricane Florence (September 2018)
Hurricane Florence was a powerful and long-lived Cape Verde hurricane that caused severe
damage in the Carolinas in September 2018. As Florence approached the Buxton project area, a
wave buoy at Diamond Shoals (NOAA NDBC Station 41025), -17 miles southeast of Buxton,
recorded the maximum wave height as high as -30 ft at 8:40 pm on 13 September (Figure 2.22). A
FEMA Major Disaster Declaration was announced on 7 Octoberfor North Carolina (DR-4393), which
included Dare County.
Research Facility at Duck Pier (8651370)
5
Predicted
4 Measured
—Surge
3
2 r h A A 6 .
v
cu
v 1
�Y
cu
� o
-1
-2
Max Wave Height —30 ft
Occured at Diamond Shoals at 8:40 pm on 13 Sept
-3
8-Sep 10-Sep 12-Sep 14-Sep 16-Sep 18-Sep 20-Sep 22-Sep
Date
FIGURE 2.22. Predicted and measured water level at the USACE Field Research Facility at Duck
Pier (8651370) during Hurricane Florence in September 2018.
2.5.2.3 Pre- and Post -Storm Survey Results
Pursuant to Dare County's request, CSE conducted a comprehensive beach condition survey from
22-24 October 2018 to determine the sand volume within the calculated limits between the foredune
and the depth of closure at -24 ft NAVD. These calculation limits have been adopted to design and
track project performance in the Environmental Assessment by the USACE and the NPS (USACE-
USD01-NPS 2015). CSE used the volumes from June 2018 (ie - Year 1 post -project beach condition
survey) as the baseline condition and subtracted them from the results of the October survey (post -
Florence). This yields the volume change compared to conditions before the hurricane season.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 44 Buxton, Dare County, North Carolina

Figure 2.23 shows unit volumes in cubic yards per foot (cy/ft) of shoreline station by station
comparing June 2018 (pre -hurricane season) and October 2018 (post -Florence). Nearly all stations
lost sand in October 2018. There are several sections of the beach where the October lines are
above the June lines (ie - stations 1795+00 to 1805+00, and stations 1815+00 to 1825+00), meaning
those localities had more sand after the hurricane. Such differences are caused mainly by sand
migration alongshore during the storm.
Figure 2.24 provides total volume changes by reach relative to the pre -hurricane season
condition. Ali reaches lost a significant amount of sand after Florence. The Buxton project area
lost a total of 341,900 cubic yards of sand along the 15,500-foot project area (measured between
the foredune and the depth of closure at -24 ft NAVD). This loss represents -13 percent of the
total nourishment volume placed in the original 2017-2018 project. To isolate incident -related
damage, these losses were adjusted for background erosion that was predicted to occur during
the period between the pre -storm (June 2018) and post -storm (October 2018) surveys. The
annual background erosion was estimated to be 114,500 cy per year (cy/yr) (USACE-USDOI-NPS
2015). This is equal to 9,542 cy per month. During the four months between the pre- and post -
storm surveys, background erosion was estimated to be 38,168 cy. Therefore, the net sand
volume loss due to Hurricane Florence is 303,732 cy along the 15,500-foot project area
between the foredune and the FEMA depth limit.
Unit Volumes before and after Hurricane Florence
(From foredune to -24 ft NAVD)
1400
1200 Reach 2 - National Seashore i Reach 1- Buxton
600
400 1
1770+00 1790+00 1810+00 1830+00 1850+00 1870+00 1890+00 1910+00 1930+00
Station
FIGURE 2.23. Comparison of unit volumes by station along the Buxton project area between
the foredune and the depth of closure at-24 ft NAVD before and after Hurricane Florence during
Year 1 (2018) after project completion.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 45 Buxton, Dare County, North Carolina

Buxton Volume Changes by Reach from Foredune to -24
FT NAVD
(Oct 2018 Minus Jun 2018)
0
U
V �
LA U
� Ol
-100,000
N O
M
Ln
Inl
�
U
N n
ei
v
E
o-200,000
(p
U
N
O
H
N.
O
-300,000
O1
v
m
-400,000
R2 (National Seashore) RI (Buxton) Total
(11,000 feet) (4,500 feet) (15,500 feet)
FIGURE 2.24. Beach volume changes in October 2018 (after Florence) along Buxton's
engineered beach (NC) by reach relative to the June 2018 survey (before Florence) from the face
of the dune to the depth of closure at -24 ft NAVD.
2.5.3 Year 2 (2019) Post -Project Volume Analysis
2.5.3.1 Beach Monitoring
Based on the monitoring requirements listed in Section 2.5.2.1 for Year 2 (2019) after the project,
beach compaction measurements were obtained in March. The methodology and results of the
2019 compaction tests are included in Attachment 3-B. There was no need to conduct sediment
compaction tests after Year 2, but profile surveys in subsequent years continued to be performed
annually in summer (weather permitting). The Year 2 annual profile surveys were performed in
June 2019. The June survey provides pre -storm condition data and serves as the annual baseline
for comparison with post -storm condition surveys. In addition to the yearly pre -storm surveys,
one post -storm survey was conducted in November 2019 after Hurricane Dorian.
Volume losses caused by Dorian are discussed in this section. Additionally, volume changes
relative to the pre -project condition and nourishment sand remaining will be discussed in detail in
Section 2.5.5 together with Years 1 and 3 results.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 46 Buxton, Dare County, North Carolina

2.5.3.2 Hurricane Dorian (September 2019)
Hurricane Dorian was the first major hurricane of the 2019 Atlantic hurricane season and impacted
the Outer Banks on 6 September 2019. Dorian approached Dare County in early September, where
the Buxton project and the wave buoy at Diamond Shoals (NOAA NDBC Station 41025) are located
(-17 miles southeast of Buxton and -15 miles offshore of Cape Hatteras in -160 ft of water). The buoy
recorded the maximum wave height as high as -27 ft at 11:40 am on 6 September during high tide
(Figure 2.25). The surge level reached its first peak of -4 ft at 9:54 am and its second peak of 3.6 ft
at 2:42 pm on the same day based on tidal gauge records at the USACE Field Research Facility at
Duck (Station 8651370) (Figure 2.26). On 4 October 2019, FEMA made a Major Disaster Declaration
for North Carolina (DR-4465-NC NR 002).
Diamond Shoals Wave Buoy NOAA 41425
30
25
0
K o0
t 20 0
= c n
15 o
10
in a`� .a ceS
5 0
0
9/1/19 9/3/19 9/5119 9/7/19 9/9119 9/11119 9/13/19
Date
FIGURE 2.25. Significant wave heights measured by a wave buoy
at Diamond Shoals (Station 41025) during Hurricane Dorian in
September 2019.
2.5.3.3 Pre- and Post -Storm Survey Results
USACE Research Facility at Duck (Sta 8651370)
6
Predicted Tides
5—Mea5ured rides
—Surge Level
4
3
y 2
v 1
0
-1
-2
Max Wavc Height -27 tt at Diamond Shea is
-3
Occurred at 1L40 am on S Sept 2D19
09/01/19 09/03/19 09/05/19 09/07/19 09/09/19 09/11/19 09/13/19
Date
FIGURE 2.26. Predicted and measured water levels at the USACE
Field Research Facility at Duck (Station 8651370) during Hurricane
Dorian in September 2019.
At Dare County's request, CSE conducted a comprehensive beach condition survey from 24-27
November 2019 to determine the sand volume within the calculated limits between the foredune
and the depth of closure at-24 ft NAVD. These calculation limits are consistent with the ones used
in the volume loss estimates after Hurricane Florence. CSE used the volumes from June 2019
(ie - Year 2 post -project beach condition survey) as the baseline condition and subtracted them
from the results of the November survey (post -Dorian). This yields the volume change between
these two surveys. Background erosion between June and November was subtracted from the
volume change to isolate incident -related damages due to Hurricane Dorian.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 47 Buxton, Dare County, North Carolina

Figure 2.27 shows unit volumes in cubic yards per foot of shoreline (cy/ft) station along the
project area comparing June 2019 (pre -hurricane season) and November 2019 (post -Dorian).
Nearly all stations lost sand in November 2019. There were several sections of the beach where
the November lines are above the June lines (ie - stations 1815+00, 1845+00, 1865+00, and
1925+00), meaning those particular localities had more sand after the storm. Such differences
were caused mainly by sand migration alongshore during the storm, which was also observed
after Hurricane Florence.
1400
Unit Volumes before and after Hurricane Dorian
(From foredune to -24 ft NAVD)
1200
Reach 2 - National Seashore
Reach 1 - Buxton l
—Pre Dorian (Jun 2019)
T
—Post-Dorian (Nov 2019)
1000
w
E
a
+�
800
C
600
400
1770+00
1790+00 1810+00 1830+00 1850+00
1870+00 1990+00 1910+00 1930+00
Station
FIGURE 2.27. Comparison of unit volumes by station along the Buxton project area between the
foredune and the depth of closure at -24 ft NAVD before and after Hurricane Dorian.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 48 Buxton, Dare County, North Carolina

Figure 2.28 provides total volume results by reach relative to the June 2019 condition. Both
reaches lost a significant amount of sand after Dorian. The Buxton project area lost a total of
212,400 cubic yards of sand along the 15,500-foot project area (measured between the foredune
and the depth of closure at -24 ft NAVD). This loss represents -8 percent of the total nourishment
volume placed in the original 2017-2018 project.
To isolate incident -related damage, these losses were adjusted for background erosion that was
predicted to occur during the period between the pre -storm (June 2019) and post -storm
(November 2019) surveys. The annual background erosion was estimated to be 114,500 cy peryear
(cy/yr) or 9,542 cy per month (USACE-USDOI-NPS 2015). During the five months between the pre -
and post -storm surveys, background erosion was estimated to be 47,710 cy. Therefore, the net
sand volume loss due to Hurricane Dorian is 164,690 cy along the 15,500-foot project area
between the foredune and the FEMA depth limit. Reach 1 - Buxton lost-90,100 cy (20 cy/ft),
while Reach 2 - Seashore lost-122,300 cy (11 cy/ft) due to Hurricane Dorian.
Volume Changes by Reach from Foredune to -24 FT NAVD
(Post -Dorian in Nov 2019 minus Pre -Dorian in June 2019)
100,000
r
0 a
U
� O
U rl
Q
cu m �
E w
-100,000 N
m
o
0
a
N
a -I
-200,000 N
-300,000
R2 (National Seashore) R1 (Buxton) Total
(11,000 feet) (4,500 feet) (13,500 feet)
FIGURE 2.28. Beach volume changes in November 2019 (after Dorian) along Buxton's engineered
beach (NC) by reach relative to June 2019 survey (before Dorian) from the face of the dune to the
depth of closure at -24 ft NAVD.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D) 49 Buxton, Dare County, North Carolina

2.5.4 Year 3 (2020) Post -Project Volume Analysis
2.5.4.1 Beach Monitoring Efforts
The Year 3 annual profile surveys were performed in August 2020. The August survey provides pre -
storm condition data and serves as the annual baseline for comparison with post -storm condition
surveys. No declared hurricane directly impacted the Buxton area; therefore, no post -storm survey
was needed in 2020.
2.5.4.2 Years 1-3 Dune Volume Analysis (Lens 1)
As illustrated in Figure 2.9, Lens 1 represents the volume in the backshore and dune area. The
nourishment berm elevation was set to +7 ft NAVD in the 2017-2018 project (ie - little nourishment
sand was initially placed above the +7 ft contour in this lens). Unit volumes of Lens 1 from
properties* to +7 ft NAVD by station along the Buxton project area are shown in Figure 2.29. For
graphic clarity, unit volumes for the pre -project condition (May 2017) and the most recent
condition (August 2020) are plotted with thicker lines (red and black lines in the graphic,
respectively), and the rest of the survey dates are plotted with thinner lines.
*[Landward limit of this lens was originally determined of the properties of the time of project planning and design
(CSE 2013). It remains the some for most stations unless there are significant changes landward of o station (eg -
structure or fencing) thotprevent data collection. If the landward limit ofo station is changed, volumes at this station
will be re -calculated for oil survey dotes, so volume comparisons will be hosed on the some portion of the beach.]
As expected, the widened dry -sand beach served as a feeder for dune growth after nourishment.
However, with the beach along Reach 1 (Buxton) becoming narrower after hurricanes Florence
(September 2018) and Dorian (September 2019), dune growth reversed. As of August 2020, unit
volumes in Lens 1 have dropped below the pre -project condition, indicating that the dune and
backshore condition has deteriorated and became worse than in May 2017. Aerial photos and
ground photos in Figure 2.30 (taken on 15 April 2021) show that there was little dry -sand beach in
front of Buxton, and old sandbags (once were buried after the 2017-2018 nourishment project)
were re -exposed, and new sandbags were installed as a temporary shoreline protection measure.
NC Highway 12 was once again vulnerable to flooding and breaching.
Unit volumes along Reach 2 (National Seashore) have been steadily growing since project
completion. As of August 2020, the average unit volume along the Seashore was -6.8 cy/ft higher
than the pre -project condition of May 2017, equivalent to an increase of -2 cy/ft/yr.
Profile volumes will continue to be discussed in Section 2.5.4.5.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 50 Buxton, Dare County, North Carolina

FIGURE 2.29. Comparison of unit volumes along the Buxton project area from properties to +7 ft NAVD
before nourishment (May 2017) and after nourishment (Year 1 - June and October 2018, Year 2 - June and
November 2019, and Year 3 - August 2020).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 51 Buxton, Dare County, North Carolina

FIGURE 2.30a. Ground and aerial images taken 15 April 2020 - looking south. The Village of Buxton north limit is
located approximately at the end of the northernmost building showing near the bottom of the photo. Threeyearsafter
the initial2017-2018 beach restoration project, almost all nourishment sand shifted outside of the project limits beyond
the depth of closure. There was little dry -sand beach in front of the oceanfront, and sandbags were installed as a
temporary shoreline protection measure. NC Highway 12 was once again vulnerable to flooding and breaching.
M
FIGURE 2.30b. Ground and aerial images taken 15 April 2020 - from the Village of Buxton looking north into the Cape
Hatteras National Seashore.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 52 Buxton, Dare County, North Carolina

2.5.4.3 Years 1-3 Beach Volume Analysis (Lens 2)
Lens 2 in Figure 2.9 represents the active portion of the beach to low -tide wading depth (from +7 ft
to -6 ft NAVD). The majority of wave -breaking, uprush and backrush, and energy dissipation occur
over this zone. Unit volumes of this lens by station along the project area are shown in Figure 2.31.
The thicker red line represents the pre -project condition of May 2017, the thicker black line
represents the most recent survey results of August 2020, and the thinner brownish lines represent
the surveys in Year 1 (2018) and Year 2 (2019) after nourishment.
The results in Figure 2.31 show that the 2017-2018 project added sand in this portion of the beach,
as confirmed by the post -project survey in June 2018. The project area lost sand rapidly overtime,
particularly along Reach 1 - Buxton. The beach condition of Buxton has become worse than the
pre -project condition three years after project completion as of August 2020. The erosion rate
between May 2017 and August 2020 is -15 cy/ft (-5 cy/ft/yr) along Buxton. However, most stations
along Reach 2 - Seashore retained some nourishment sand. Compared to the pre -project
condition in May 2017, the Seashore gained -19 cy/ft (or -6.3 cy/ft/yr) as of August 2020.
2.5.4.4 Years 1-3 Underwater Volume Analysis (Lens 3)
Lens 3 in Figure 2.9 represents the underwater portion of the beach extending seaward of the bar
to the depth of closure, or the normal seaward limit of bottom change at this setting (from -6 ft to
-24 ft NAVD). It is the area over which waves of all sizes begin to break and to measurably
redistribute sediment. It includes the breakpoint of longshore bars, which trigger wave -breaking
in storms, and troughs between bars.
The results in Figure 2.32 show that the 2017-2018 project added sand in this portion of the beach
(mainly through profile adjustment after construction) as confirmed by the post -project survey in
June 2018. The project area lost sand rapidly over time, particularly along Reach 1 - Buxton.
Although most stations retained some nourishment sand, there are some stations in both Reaches
1 and 2 where volumes in August 2020 were less than volume in May 2017. As of August 2020, the
average unit volume along the Seashore and Buxton was -25 cy/ft higher than the pre -project
condition in May 2017, indicating some remaining nourishment sand from the 2017-2018 project.
2.5.4.5 Years 1-3 Cumulative Volume Analysis (Lenses 1-3)
Figure 2.33 shows the cumulated unit volumes along the beach from Lens 1 to Lens 3 (ie - from
properties to -24 ft NAVD — depth of closure and FEMA reference depth). The overall increase in
unit volume after the 2017-2018 nourishment is evident by comparing the graphic's red and dark
brown lines. The designed average fill density was -200 cy/ft for Reach 1 - Buxton and 155 cy/ft
for Reach 2 - Seashore.
The red line in Figure 2.33 represents the pre -nourishment condition in May 2017, and the black
line represents the most recent condition in August 2020. Measuring from the foredune to the
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 53 Buxton, Dare County, North Carolina

depth of closure, most stations along the Seashore retained some nourishment sand, as shown by
comparing the black line and the red line in Figure 2.33. The section of beach where the black line
overlaps with the red line or below the red line (ie - stations 1885+00 to 1930+00 in the Village of
Buxton) means there was less sand volume along that section in 2020 than in 2017.
Nourishment sand retained along the Seashore was -53 cy/ft, equivalent to an accretion of -17
cy/ft/yr between May 2017 and August 2020.
Unit Volumes Comparisons Along the Entire Project Area
(From +7 to -6 ft NAVD)
300
4 200
U
u
W
O
M 100
0
1770+00 1790+00
North
Reach 2 - National Seashore
Reach 1 - Buxton
1810+00 1830+00 1850+00 1870+00 1890+00 1910+00 1930+00
Station south
FIGURE 2.31. Comparison of unit volumes along the Buxton project area from +7 ft to -6 ft NAVD (Lens 3) before
nourishment (May 2017) and after nourishment (Year 1- June and October2018, Year2 - June and November2019, and
Year 3 - August 2020).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix Di 54 Buxton, Dare County, North Carolina

1000
04
800
u
j 700
O
7) 600
500
Unit Volumes Comparisons Along the Entire Project Area
(From -6 to -24 ft NAVD)
Reach 2 - National Seashore I Reach 1- Buxton
v
—Pre-Project (May 2017)
—Yr1-Pre Florence (Jun 2018)
Yr1-Post-Florence (Oct 2018)
Yr2-Pre Dorian (Jun 2019)
Yr2-Post Dorian (Nov 2019)
—Yr3 (Aug 2020)
400 L '
1770+00 1790+00 1810+00 1830+00 1850+00 1870+00 1890+00 1910+00 1930+00
North Station South
FIGURE 2.32. Comparison of unit volumes along the Buxton project area from -6 ft to -24 ft NAVD (Lens 2)
before nourishment (May 2017) and after nourishment (Year 1 - June and October 2018, Year 2 - June and
November 2019, and Year 3 - August 2020).
1400
1200
a
1000
v
E
0
800
600
Unit Volumes Comparisons Along the Entire Project Area
(From foreclune to -24 ft NAVD)
Reach 2 - National Seashore i Reach 1 - Buxton
—Pre-Project (May 2017)
—Yr1-Pre Florence (Jun 2018)
—Yr1-Post-Florence (Oct 2018)
Yr2-Pre Dorian (Jun 2019)
Yr2-Post Dorian (Nov 2019)
—Yr3 (Aue 2020)
400 '
1770+00 1790+00 1810+00 1830+00 1850+00 1870+00 1890+00 1910+00 1930+00
North Station South
FIGURE 2.33. Comparison of the cumulative unit volumes along the Buxton project area from the
properties to -24 ft NAVD (Lenses 1 to 3) before nourishment (May 2017) and after nourishment (Year 1- June
and October 2018, Year 2 -June and November 2019, and Year 3 -August 2020).
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 55 Buxton, Dare County, North Carolina

2.5.5 Updated Erosion Rate after the Initial 2017-2018 Nourishment (2018-2020)
By the most recent beach condition survey in August 2020, as shown in Figure 2.34, approximately
634,925 cy of the 2.6 million cubic yard nourishment sand remained in the project area, which is
equivalent to -25% of the volume placed during the 2017-2018 project (the orange bar on the right
in Figure 2.34). Sand loss along the Village of Buxton was greater than along the National Seashore.
Ninety-seven percent (97%) of nourishment sand placed along the Village of Buxton was lost
(represented by the first group of bars in Figure 2.34). In comparison, the project lost 65 percent
and retained 35 percent of the sand volume placed along the National Seashore.
Volume Changes by Reach from Foredune to -24 FT NAVD
(Relative to Pre -Nourishment Condition of May 2017)
2017-2018 Nourishment
2,500,000
Year 1 Pre -Florence (June 2018)
Year 1 Post -Florence (October 2018)
Year 2 Pre -Dorian (June 2019)
2,000,000
1 Year 2 Post -Dorian (November 2019)
Year 3 (August 2020)
a
U
E
1,500,000
— 00
�
o
0 0
0
~
1,000,000
0 `^
N
O 0
O
T
500,000
O
O Ln Ln Ln LA co W OLn O
u'1
N Ln
Ln
N M N N
O N V � n O� � nj N
c-i �O O1
Q1 en
R O .m-I l0 to
Ln M N N
00 rn
tD c00
O
R1 (Buxton) R2 (National Seashore)
Total
(4,500 ft) (11,000 ft)
(15,500 ft)
FIGURE 2.34. Cumulative total volume along the Buxton project area after the completion of the
2017-2018 beach restoration project relative to the pre -project condition in May 2017.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 56 Buxton, Dare County, North Carolina

Based on the survey results between May 2017 and August 2020, the average annual erosion rate
was between 650,000 and 700,000 cy/yr over the past three years. This number is several times
higher than the historical erosion rate estimated for the design of the 2017-2018 project (114,500
cy/yr or 7.4 cy/ft/yr as discussed in Section 2.4).
Surveys of the-5,000-ft downcoast area (Cape Point east shoreline) since nourishment indicate
that as much as 480,000 cy (or -50 percent) of the Buxton sand losses have accreted to (built-up)
the undeveloped beach of the National Seashore. This is equivalent to an accretion of 96 cy/ft (or
-38 cy/ft/yr).
A higher than normal erosion rate has also been measured at Nags Head (Dare County) in recent
years. Nags Head is located -60 miles northeast of Buxton in the northern Outer Banks. CSE's
beach condition surveys show that the average annual erosion rate at Nags Head has been over45
cy/ft/yr between November2017 and April 2019 (CSE 2019). That is much higherthan the historical
erosion rate of 5.2 cy/ft/yr along Nags Head.
Several factors caused the much -higher -than -normal erosion rate along the Buxton project area,
listed as follows.
(1) Normal nourishment sand spreading at the project boundaries (ie -End Loss) (Dean 2002).
(2) Four named hurricanes (Irmo, Jose, Kotio, and Maria) impacted the project area
in September 2017 during construction, increasing the sand deficit relative to
pre -construction conditions.
(3) A series of nor'easters impacted the project area in March 2018, soon after project
completion.
(4) Hurricanes Florence (September 2018) and Dorian (September 2019) impacted after
project completion.
(5) Sand shifted offshore beyond -24 ft NAVD due to the existence of a nearshore deep trough
(see Figure 2.4).
(6) Deterioration of sand -retaining structures (ie - groins) at the south end of the project
which lessen sand trapping.
The loss of function of the existing groins was discussed in the Environmental Assessment for the
project (USACE-USDOI-NPS 2015). It was not evaluated further because new groins are not allowed
along the northern Outer Banks under NC state Coastal Zone Management rules and regulations.
Due to the rapid loss of nourishment volume in recent years, CSE recommended in 2019 that the
groin conditions should be studied in greater detail while planning the renourishment project. The
initial inspection conducted by CSE in July 2019 showed that if the landward underground portion is
not included in the equation, less than 50 percent of the existing groin structure remained on the site.
Therefore, the repair or restoration work of the groins would be considered as construction of a "new
groin," and not permitted.
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58 Buxton, Dare County, North Carolina

3.0 COASTAL PROCESSES
Buxton is subject to coastal processes (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 along with hurricane surges. 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). Athorough coastal processes study was conducted for the 2017-
2018 Buxton nourishment project (USACE-USDOI-NPS 2015 -Appendix A- Littoral Processes). This
section updates the results and findings of littoral processes affecting the proposed project area
and addresses specific questions regarding the potential impact of the proposed project on these
processes (CERC 1984, Dean 2002).
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 sediment 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 potentially impact sediment transport
along other strategic locations. Closure depth (the approximate limit of measurable bottom
change over particular time scales) was examined in the Buxton area for the 2017-2018 project
because it is an important consideration in locating the borrow site (USACE-USDOI-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 the previous study for the 2017-2018
project, and was used in this study as well to evaluate the changes of wave patterns before and
after dredging of the proposed borrow area. The generalized model for simulating shoreline
change (GENESIS) was used in the previous study to evaluate the impact on sediment transport
caused by the placement of nourishment sand, and an updated shoreline evolution model
(GenCade) was used in this study. The GenCade model is a next -generation combination of
previous long-term planform evolution of beach models: GENESIS (GENEralized Model for
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[2403M-Appendix D] 59 Buxton, Dare County, North Carolina

Simulating Shoreline) and Cascade. GenCade is a regional model for calculating coastal sediment
transport, morphology change, and sand bypassing at inlets and engineered structures. 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 oscillations, and impact coastal
structures. The longshore and cross -shore gradients in wave height and direction can be as
important as the magnitude of these parameters 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 -17 miles offshore of Buxton with 18 years of wave records from 2003
to 2020, and the other is a hindcast wave station located -10.5 miles offshore with 40 years of
records from 1980 to 2019.
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 Buxton 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 2012. 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.n000.govlodcp_doto.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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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 Buxton project site. The station is located at 35.010' N
75.454' W, -15 miles directly offshore of Cape Hatteras (NC) and -17 miles from Buxton 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, -10.5 miles due east of Buxton 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 after2012 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
1 5.68
1 8.03
1 116
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 Buxton 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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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 observed 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 often
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 north.
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 due east of
Buxton 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 for40 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 8° 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 - 67.50 to 1350 band) occur 45.9 percent of the time, and waves coming 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 histograms 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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FIGURE 3.3.
Wave rose of WIS station 63230
showingthe occurrence frequency
of wave direction and wave height
based on the 40-year record
between 1980 and 2019. Buxton
shoreline azimuth is -8° from the
north as marked by the black solid
line in the figure.
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 Buxton project area is -8°
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
40-Year Record (1980-2019) at WIS 63230
from °True
Percentage of Occurrence
N
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
1 0.9
1 5.09
1 5.2
All Directions
1 100
4.62
5.8
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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. Buxton shoreline azimuth is -8° from the north as marked
by the black solid line in the figures.
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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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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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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 currents, 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 Buxton renourishment 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-
USDOI-NPS 2015).
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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 longforthe waves to attain fetch -limited orfully
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 coefficientto bring model results into alignmentwith 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) Linearradiation 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
GenCade was used in this study to evaluate longshore sediment transport during various
stages of the design life following the proposed beach renourishment project. Results were
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 tool. 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 multiple
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 managing 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 Coastal 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 longshore transport. Project sites
include stretches of coast in the United States such as Alaska, California, Louisiana, New Jersey,
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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
(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, Ekphisutsuntorn et al
2010, Kuang 2010, Kaczkowski & Kana 2012, USACE-USDOI-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 scientists interested in
nourishment performance" (reviewed by Marcel Stive, Chair of Coastal Engineering, Delft
University of Technology). Several project examples using this model are analyzed in this book.
As concluded by Dean (2002) and also addressed in numerous articles in the coastal engineering
literature, several key factors should be taken 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 point as required by the GENESIS model.
When the GENESIS model was used for the proposed Buxton project, the above -listed key factors
were satisfied. The model was calibrated and verified during the previous study for the 2017-
2018 Buxton nourishment project. Historic annual erosion rates were used to calibrate the
sediment -transport model. The model results were used to evaluate the relative impact of the
2017-2018 project on longshore sediment transport (USACE-USDOI-NPS 2015).
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. This internal model determined the breaking wave characteristics used to
calculate the actual longshore sediment transport.
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In the following sections, the details of model setup, applications for beach nourishment design
templates, and the impact of the proposed dredging and nourishment will be discussed. The
conclusions of the engineering study will be provided after the discussion. 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 application, 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
renourishment 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 parameters, 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 Mode! Grid
As discussed in previous sections, the proposed project area starts from station 1770+50 and
extends southward to station 1925+50, covering -2.94 miles (15,500 ft) from north to south. An
STWAVE grid extends about 1.5 miles beyond the north boundary and 2 miles beyond the south
boundary of the project site. Extensions of the model domain beyond 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 effects (USACE-USDOI-NPS 2015).
The STWAVE model grid was also extended seaward from the shoreline to a distance of about 3
miles. The seaward boundary is parallel to the general shoreline trend with an azimuth of 8° 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 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 932000 meters (m) East and 178600 m North in
North American Datum 1983 State Plane (NAD'83) North Carolina Zone 3200, and the grid
dimensions are 5,000 by 10,000 m in the x and y directions (respectively). The cell size is 100 m in
both directions.
3.4.2 GenCade Mode! Grid
The GenCade model boundary is parallel to they -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 3041000 ft East
and 586200 ft North (NAD'83), and the grid length is 25,000 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 August 2020, and the data provide sufficient
coverage for the model application in this study.
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3.4.3 Mode! 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 computation time. Model
sensitivity tests in the previous study with different spatial resolutions ranging from 50 ft to 500 ft
have been conducted (USACE-USDOI-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 Mode! Bathymetry
The setup of the STWAVE and GenCade 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 analyses 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 August 2020.
(2) CSE's bathymetric survey in an offshore sand search area in August 2020.
(3) CSE's bathymetric survey in the proposed borrow area in October 2020.
(4) NOAA navigation chart 11555 dated March 2012 shown as nearly continuous offshore
contours seaward of CSE's survey areas.
The shoreline used in this study is defined as the 0-m contour line relative to the NAVD'88 datum
extracted from the August 2020 beach condition survey.
Figure 3.7 shows the bathymetry across the grid after interpolating the scatter data to the model
domain before dredging. The proposed excavation depth is 10 ft below the existing grade in the
offshore borrow area. Figure 3.8 shows the assumed after -dredging scatter data (ie - the scatter
points within the proposed borrow area are expected to be excavated to 10 ft below the existing
grade). Figure 3.9 shows the bathymetry across the grid after interpolating the after -dredging
scatter data to the model domain. This is a conservative scenario for impact analysis because it
represents over 3.3 million cubic yards of sand excavated, whereas the proposed project will only
involve -1.2 million cubic yards. Grid origin and dimensions of the before -dredging and after -
dredging scenarios are the same and are represented in Figure 3.5. Water depth along the seaward
boundary increases from -13 m at the south to -16 m in the middle and slightly increases to
17-18 m to the north.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 75 Buxton, Dare County, North Carolina

Bottom Elevation (ft NAVD)
20
10 �;.� '••'� f
0 •' '
-10
-20
-30 i
-40
M
-50
�a:x::. • X 1.i•'•=•'�.• y.
-60
•, tip'• � t;�.�,; �•f� .
,� S s � • S�•9► w
e�a�cse- •�9-' • •i• jam'•••~ r •i.
e,.-"cos •.�A S •�'. � ����,,
r .�a•
• .••,,w ! "� + '.,'"'� 'w•:, ...,even
a: � � �' ,.• Nei?•.. ��. �,F '�• ,� � �:',��d `'.�„a,
4
FIGURE 3.6. Combined bathymetric data collected by CSE in August and October 2020
and the offshore contours digitized from NOAA navigation chart 11555 (dated March
2012). See Figure 3.5 for model origins,'Y' and "y" axes, and coverage.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 76 Buxton, Dare County, North Carolina

Bottom Elevation (m NAVD)
4
2 f
- 2 J 4
-6
-60
. -12
1� I
-14
-16-18 G
-20 a �C Mild,
aD l
{
G
d
a
c o
C
O
� l 4 � � IIVLLLrIIIIIIJ
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,'Y' and "y" axes, and coverage.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 77 Buxton, Dare County, North Carolina

Bottom Elevation (ft NAVD)
20
10
- 0
-10
--20
-30
- -40
50
-60
W
• �� ••;S� r a.
• •. •.� .
a tif.•�F:�tY.
�. • �Mr '••,• s �.
FIGURE 3.8. Combined bathymetric data collected by CSE in August and October 2020 and
the offshore contours digitized from NOAA navigation chart 11555 (dated March 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,'Y' and "y" axes, and
coverage.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 78 Buxton, Dare County, North Carolina

Bottom Elevation (m NAVD)
4
2 �r {
0 d
o
-8 f
-10 /
-12 fl��
-14 �1 i
-16 Q
-18 _
-20
G
0
a
�o
e�
�a
rc �
I I 1 r
11
1
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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 79 Buxton, Dare County, North Carolina

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 Buxton
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 Fig 3.1 for their
locations). Although the NDBC wave buoy has real-time measurements, this station was not used
because it is located -17 miles from Buxton to the southeast.
The WIS station 63230 is located -10.5 miles directly east of Buxton 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-USDOI-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 = 0.321 mm
• Average berm height = 7 ft NAVD
• Closure depth = -24 ft NAVD
Volumetric erosion studies at the project area show that average annual erosion rates are
estimated to be between-650,000 and 700,000 cy/yroverthe pastthreeyears afterthe completion
of the 2017-2018 nourishment project. The transport parameters Kl 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 -200 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 10 ft
below the present bottom. To determine if the total 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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 80 Buxton, Dare County, North Carolina

I
ff

�'�9
�1
uu�
_{
'd r'-
:�;}'
r
,� �
r
R
41
5 '. � ��

To evaluate wave pattern changes before and after dredging, 20 horizontal cross -sections were
chosen over the computational domain, as illustrated in Figure 3.10. 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.
0 STWAVE Model Origin: xo = 932000 m, yo=178600 m
• STWAVE Grid Length in "x" Direction equals: 5,000 m
0 STWAVE Grid Length in "y" Direction equals: 10,000 m
0 Project Northernmost Limit to STWAVE Origin: y = -2,000 m
• Project Southernmost Limit to STWAVE Origin: y = -6,700 m
• Borrow Area to Model Origin: 5,000 to 6,000 m in the "y" direction and 800-1,800 m
in the "x" direction
In Figure 3.12, 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 five plots in this graphic (ie - y-distance to origin =
100 to 3,000 m and 6,600 to 9,900 m) show almost no difference in wave height before and after
dredging. The intermediate eleven plots (ie - y-distance to origin = 3,500 to 6,400 m) show minor
differences in wave height between these two scenarios. Since the project area "y" distance to
origin is between 2,000 and 6,700 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 = 5,000 to 6,000
m), but the greatest difference was less than 10 percent of the height. In addition, the wave height
difference diminishes toward the shore.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 83 Buxton, Dare County, North Carolina

1.2
Y-Distance to Origin = 100 m
1.0
0.8
=
0.6
0.4
Before Dredging
--- After Dredging
0.2
0.0
X-Distance to Origin (m)
1.2
1.0
Y-Distance to Origin = 1000 m
0.8
=
0.6
m
'
0.4
Before Dredging
--- After Dredging
0.2
0.0
a a
o o 0
Lon
Ln 11 Q m m m m m N N N N N
X-Distance to Origin (m)
1.2
1.0
Y-Distanceto Origin = 2000 m
0.8
=
d
0.6
n
0.4
r
—Before Dredging
--- After Dredging
0.2
00
8$§ $§ 8$$§ 8$$§
�o Sc o§
8 $
� m
rl
X-Distance to Origin (m)
1.2
10
Y-Distanceto Origin = 3000 m
0.8
-
0.6
>
0.4
— Before Dredging
--- After Dredging
0.2
0.0
$ $ $ $ $ $ $ $ $
$ 8 $ o
ff,8
Uh - 'q 'q m m m m m N N N N N
con0
.--I rl .--I .m-I rl
X-Distance to Origin (m)
FIGURE 3.12. STWAVE simulated wave height comparisons at different horizontal cross -sections parallel to
the'Y' 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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D) 84 Buxton, Dare County, North Carolina

1.2
�
1.0
Y-Distance to Origin
= 3500 m
0.8
=
0.6
m
—Before Dr edging
04
--- After Dredging
0.2
0.0
—
Q O
O
Q
Q
n
Q
Q
in
O O
Q Q
m
Q O
Q
O Q Q
Q Q Q
m
O O O O
Q Q Q
n u4 m
Q Q
Q
Q
Q
R
O
Q
Q O O O O O
Q Q Q Qm
en
O
Q
,-i
u� 'It
It
���
-1R
m m
M
m m m
N N N N
,-i
N��
-1
-1
1--1 -1
x-Distance
to Origin (m)
1.2
�
1.0
Y-Distance to Origin
= 40M m
-
=
0.6
—Before Dredging
04
--- After Dr edging
0.2
0.0
—
O O
-1
O
0
O
O O
O O
0
O O O
O O O b
0
O O
O
O
O
O O O O O O
O
uni V
V
V
V V
cn m
m m M
cn
c l N rc+i CN
ci q_1
t
�-1-1
-1 4�
X-distance
to Origin (m)
1.2
E
1.0
Y-Distance to Origin
= 4500 m
=
m
0.6
04
—Before Dredging
--- After Dredging
0.2
0.0
a a
Q
a
O
h
a
Q
a v
O
a a
O
a a a
Ot.S O
a a a o
O O
a o
Q
0
Q
I�
o
O
v v o p vp a oa
O O
0
O
r1
Ln
u7
Q
t'A m
m m m
m
M N N N
ri
N -I
ri
ri
m ri t+ M
-1 -1
ri
X-distance
to Origin (m)
1.2
E
1.0
Y-Distance to Origin
= M m
0.8
----------
m
0.6
—Before Dredging
0.4
---After Dredging
0.2
0.0
Q o
O
Q
O
a
O
o 0
mf
0 0
0 0 0
O
0 0 0 o
Oi m
Q p Q
O
a
OLn
a
Q o p op Q o a 0a
m O
0
M d
d
d
d d
M m
tQri
m m m
N N N N
IN ri
ri
ri
OS M
ri ri
X-Distance
to Origin (m)
FIGURE 3.12(continued). STWAVE simulated wave height comparisons at different horizontal cross -
sections parallel to the'Y' 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.
Coastal Science & Engineering
[2403M-Appendix D]
Littoral Processes
85 Buxton, Dare County, North Carolina

1.2
1.0
Y-Distance
to Origin
= 54.00 m
=
m
0.6
—Before Dredging
04
---After Dredging
0.2
0.0
o
o
0
0
0
o
0 0
r
o
o
o
o
0
0 0 o
o
Ln
m
m
Ln cno
m m m
N
N
N N
f
N rl
rl
Ln
rl
M
rl rl
x-Distance to Origin (m)
1.2
�
1.0
Y-Distance
to Origin
= 56W m
0.8
=
0.6
m
—Before Dredging
04
---After Dredging
0.2
0.0
—
Q O
I
O
C
R
O
n
Li]
O
C
m
O
O
O
O
O
R
O O O
n G O
If] m
O
O
C
O O
m
m
O O
C
4
C
R
O
Ln
4 O O O O 4
m O G
m
O
C
Ln
In �
�
�
�
r-I
�
m
m
M
m m m
N
M
N
N N
r-I
N rl
rl
-1
rl
-1
rl rl
x-Distance to Origin (m)
1.2
�
1.0
Y-Distance
to Origin
= 58W m
0.$
=
0.6
m
— Before Dredging
0
---After Dredging
0.2
0.0
O pOp
p4p
pO
p4p
O
p4
O
pO p4 O
p4
d
o4 tO�
4 O
4
O
4 O pO p4 pO p4
d
Ln 4
4
tt
m
m
en m en
N
N
N N
N 1-1
1--1-11--1-1
x-Distance to Origin (m)
1.2
�
1.0
Y-Distance
to Origin
= 59W m
0.9
=
0.6
—Before Dredging
0.4
---After Dredging
0.2
0.0
O O
O
4
0
O
0
4
m
O
O
4
O
O
O 4 O
O
4
d
O
4 O
Ln 0
4 O
0
O
O
O
Ln
4 O O 4o oOp 4
m
d
Ln 4
Ln
m
m
n
M m m
N
N
e
ry cN
ni ,1
-1�-1
x-Distance to Origin (m)
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
hreak and diminish.
Coastal Science & Engineering
[2403M-Appendix D]
Littoral Processes
86 Buxton, Dare County, North Carolina

1.2
�
1.0
Y-Distance
to Origin = 6000 m
0.8
=
0.6
—Before Dredging
04
---After dredging
0.2
0.0
O
v,O
O
O
O
O
v
v
v
O
v
O
O
O
v v v
v 0
v v v O
O
O O
v
O
v
v
v O O O O O
v t7 O v v
O
v
Ln
m
m
M Mm
M
N N N N
-1
71
ri
r-i
-1 ri
x-Distance to Origin (m)
1.2
�
1.0
Y-Distance
to Origin = 6200 m
�
0.8
=
0.6
m
—Before Dredging
04
---After Dredging
0.2
0.0
-
--
-
v
�
Op
O
O
O
v
O�
v
v
rn
v
en
Q v Q
O M
v v Q O
m
v O
a �
v
O
v
v
v O O v O v
O 4 m
O
O
ul
d
d
't
d
d
d
m
m
M m M
N N N N
N ri
rl
Ln
rl
M
rl rl
X-Distance to Origin (m)
1.2
�
1.0
Y-Distance
to Origin = 6400 m
0.8
=
m
0.6
—Before Dredging
04
---After Dredging
0.2
0.0
v
v
v
v
o
o
0
v
0
v
0
v
cn
v
o
v v v
o v o
v v v v
c.4 o 0 o
v v
v
v
o
v
v
v v v v v v
o o o 0 0
o
0
Ul
m
m
m m M
N N [V N
N ri
rl
rl
rl rl
X-Distance to Origin (m)
1.2
�
1.0
Y-Distance
to Origin = 6600 m
0.8
=
m
0.6
—Before Dredging
04
--- After Dredging
0,2
0.0
0
O
o
v
0
n
v
0
in
a
v
m
v
v
o
a
0
a v v
C v v
Cn m
v v v v
v O m
R CN m
v v
v
v
G
R
v
v
v v v v v v
v v O O v
m n in m
v
0
�
LA
LA
�
�
�
�
,--i
�
m
m
m
m m m
N N N N
C- j
N rl
rl
1
rl
-1
rl rl
X-Distance to Origin (m)
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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 87 Buxton, Dare County, North Carolina

1.2
�
1A
Y-Distanoe to Origin
=70D0 m
0.8
0.6
—Before Dredging
04
---After Dredging
0.2
0.0
a a
O
a
O
a
a
a
s
a
a
a
a
O
a a a
a a
p a
a
a
a
m
a p a
O
a
a
a
m-1
a p o o ao O a[] p op
0
Lnd
d
d
d
m
m
Ln
m m m
N
N
[v
N
tli
N ri
ri
)
ri
ri
M1 Lri, [YI
ri
x-Distance to Origin
(m)
1.2
�
1.0
Y-Distance to Origin
= 8000 m
0.8
0.6
04
—Before Dredging
--- After Dredging
0.2
0.0
Ln
'n
cn
cn cn rn
C4
N
N
N
N rl
-1
-1
-1
-1
X-Distance to Origin
(m)
1.2
�
1A
Y-Distance to Origin
=90DO m
0.8
0.6
—Before Dredging
04
--- After Dredging
0.2
0.0
o p a
O
oo
a
p ap
a
a
a
o
O
a a a
a a
a
a
O
a
a
a
m
a a
a
a
O
a
a
a
m
a o a o o
a O
a
O
LA qT
M1
1,'Q
r�
d
_t
M
M
Ln
n7 M M
N
N
N
N
N ri
ri
Ln
-1
ri
-1
x-Distance to Origin
(m)
1.2
�
1.0
Y-Distance to Origin
= 99M m
0.8
=
0.6
—Before Dredging
04
---After Dredging
0.2
L
0.0
a o
0
o
a
a
a
o
0 0 0
o
a
a
a
o 0
o
a
a
o 0 0 0 0
0
Ln
d
d
d
d
m
m
m m m
N
N
N
N
N -1
ri
-1
_1
1--I
X-Distance to Origin
(m)
FIGURE 3.12(continued). STWAVE simulated wave height comparisons at different horizontal cross -
sections parallel to the'Y' 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.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 88 Buxton, Dare County, North Carolina

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 adjacent area. 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 10 ft will not significantly
alterwave 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 previous study for the 2017-2018 Buxton nourishment project. In that study, the
simulated rates fortotal volumetric erosion and net longshore sediment transport were compared
with historical erosion rates (USACE-USDOI-NPS 2015).
Three years after the 2017-2018 Buxton nourishment project, as of August 2020, approximately
only 25 percent of the 2.6 million cubic yard nourishment sand remained in the project area. The
average annual erosion rate was-655,000 cy per year (cy/yr). This number is several times higher
than the historical erosion rate estimated in the past, as discussed in Section 2. In light of this
higher -than -normal erosion rate over the past few years, the model parameters were adjusted.
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 positive transport rates
denote net sand movement to the south. Some key reference distances are listed below:
Project Northernmost Limit (sta 1770+50) to GenCade Origin: -5,000 ft
Project Southernmost Limit (sta 1925+50) to GenCade Origin:-20,500 ft
Figure 3.13 shows that the sediment transport rate varies along the shoreline, increases from north
to south, and is southerly, which is consistent with the historical trend of sediment movement and
spit accretion in this area. Transport rates increase markedly along the Village of Buxton, and
reach the highest near the south boundary of Buxton. The increasing transport rate explains the
higher erosion observed at Buxton. The simulated average annual net transport rate for the
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 89 Buxton, Dare County, North Carolina

project area was-700,000 cy. This average annual rate is consistent with the measured net annual
erosion rates over the past three years.
FIGURE 3.13. Average annual longshore net sediment transport rates over 3-year period after
completion of the 2017-2018 nourishment project. Positive rates denote net sand movement to the
cni ith
The series of images in Figure 3.14 illustrates the shoreline position over a three-year period after
completion of the 2017-2018 nourishment project. 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 images. Atthe end of the three-year period, the shoreline returned to the initial
2017-2018 pre -project position, indicating that most of the nourishment sand placed along that
section of oceanfront has been lost. This is consistent with observations in that area.
In conclusion, the net longshore sediment transport rate predicted by the GenCade model can
capture the higher -than -normal volumetric loss rates of-655,000 cy/yr over the past three years if
the WIS hindcast wave data between 2017 and 2019 is used. Shoreline evolution after the 2017-
2018 nourishment project agrees well with observations. 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 was not expected to be evaluated in greater detail.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix D] 90 Buxton, Dare County, North Carolina

T = 0 Days T = 98 Days
Before Nourishment End of Nourishment
T = 365 Days
Vr 1 Pnct_Prninrt
T = 730 Days
Yr 2 Post -Project
T =1,095 Days
Yr 3 Post -Project
FIGURE 3.14. Shoreline position at Oft NAVD contour before nourishment (red) and its evolution (green) over a three-year period after a 2.6
million cubic yard nourishment project.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 91 Buxton, Dare County, North Carolina

3.7 GenCade Model Results
The proposed project calls for pumping a maximum of 1.2 million cubic yards of beach quality sand
from the designated offshore borrow area onto 15,500 linear feet of ocean beach. The designed
berm height is 7 ft NAVD, and the average fill density is -77 cy/ft. Fill densities will vary from north
to south according to the historical erosion rates, with the center of the project receiving the
highest fill density upward of -150 cy/ft. 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.
Net sediment transport rate before and after the project are plotted in Figure 3.15. The average
annual net transport rate before the project for the shoreline segment between stations 1770+50
and 1920+00 (or a distance to GenCade origin between -5,000 ft and-20,000 ft) was-866,000 cy/yr;
the rate was-858,000 cy/yr after the project. These rates are -8,000 cy different before and after
the project. It indicates that renourishment and borrow area dredging will cause negligible
changes in the net longshore sediment transport rate. The rates will change locally where the
beach fill is conducted, but there will be no changes approximately 0.5 mile north or south of the
project area.
a
1,000,000
Y
L
Q
tA
LL 800,000
r
Y
v
E 600,000
v
Ln
w
L
❑ 400,000
t
nn
J
w 200,000
z
0
With Nourishment
—Without Nourishment
Seashore (11,500 ft)
Buxton
(4,500 ftj
Proposed Project Area (15,500 ft)
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 o a o a o a o 0 0 0 0 0
0 CL CL CL CL C CL 0
ri N M Ln l0 ^ cc a] 0 r1 N M Ln l0 r' 00 01 O r-1 N M �t Ln
r-1 r-I r-1 r-I r-1 r-I r-1 r-I r-1 r-I N N N N N N
Distance to GenCade Origin (ft)
FIGURE 3.15. Comparison of annual net longshore sediment transport rates before and after the proposed
renourishment project.
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[2403M-Appendix DI 92 Buxton, Dare County, North Carolina

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 rates before and after the proposed
renourishment project. Results were used to evaluate the impact of borrow area dredging and
beach fill on wave height and longshore transport rates along the study area at the Cape Hatteras
National Seashore, which includes a developed section in front of the Village of Buxton.
The STWAVE model results indicate that borrow area dredging will not cause any measurable 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. The
STWAVE model results show that the proposed excavations up 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 erosion rates of 655,000 cy/yr over the recent years after
completion of the 2017-2018 project. The calibrated model results yielded-700,000 cy/yr annual
net sediment transport rates, which are in close agreement with the measured 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 shoreline
segments resulted in only minor changes compared to the before -project condition (of the order
of thousands 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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94 Buxton, Dare County, North Carolina

REFERENCES
ACRE. 2008. Analysis of coastal processes for the Chatham south coastal between Mill Creek and Bucks Creek. Technical
Report, Applied Coastal Research and Engineering Inc.
Beumel, NH, and KE Beachler. 1994. Beach nourishment design within an existing groin field at Galveston, Texas. In Proc
1994 National Conference on Beach Preservation Technology, Alternative Technologies in Beach Preservation, 13 pp.
Birkemeier, WA. 1985. Field data on seaward limit of profile change. Jour Waterway Port, Coastal and Ocean Engineering,
Vol 111(3), pp 598-602.
Bodge KR, CG Creed, and AW Raichle. 1996. Improving input wave data for use with shoreline change models. Journal of
Waterway, Port, Coastal and Ocean Engineering, Vol 122(5), September/October, pp 259-263.
Booher, M, and L Ezell. 2001. Out of Harm's Way: Moving America's Lighthouse. Eastwind Publishing Company, Annapolis,
MD, 144 pp.
Bruun, P. 1962. Sea -level rise as a cause of shore erosion. Journal of Waterways and Harbor Div, ASCE, New York, NY, Vol
88(WWl), pp 117-132.
Byrnes, MR, RM Hammer, BA Vittor, SW Kelley, DB Snyder, JM Cote, JS Ramsey, TD Thibaut, NW Phillips, JD Wood, and JD
Germano. 2003. Collection of environmental data within sand resource areas offshore North Carolina and the
environmental implications of sand removal for coastal and beach restoration. US Dept Interior, MMS, Leasing Div,
Sand and Gravel Unit, Herndon, VA; OCS Rept MMS 2000-056, Vol I (main text 256 pp), Vol II (appendices 69 pp).
CERC. 1984. Shore Protection Manual. 4th Edition, US Army Corps of Engineers, Coastal Engineering Research Center, Ft
Belvoir, VA; US Government Printing Office, Washington, DC, 2 Vols.
CSE. 2011. Coastal Erosion and Solutions -A Primer. Second Edition, TW Kana, CSE, Columbia, SC, 38 pp.
CSE. 2013. Shoreline erosion assessment and plan for beach restoration, Rodanthe and Buxton areas, Dare County, North
Carolina. Feasibility Report for Dare County Board of Commissioners, Manteo, NC. CSE, Columbia, SC, 159 pp with
synopsis plus appendices.
CSE. 2014. Monitoring and analyses of the 2011 Nags Head beach nourishment project. Year 3 (2014) beach monitoring
report for Town of Nags Head, NC. CSE, Columbia (SC), 128 pp + appendices.
CSE. 2019. Final Construction Report - 2019 Nags Head Beach Renourishment Project. Coastal Science & Engineering Inc
(CSE), Columbia (SC), 74 pp + appendices (2458).
CSE. 2021. Geotechnical data analyses: Appendix A. Beach nourishment to Protect NC Highway 12 at Buxton, Dare County,
NC. Prepared for Dare County, NC. Coastal Science & Engineering Inc, Columbia, SC, 54 pp plus attachments.
Dean, RG. 1991. Equilibrium beach profiles: characteristics and applications. Jour. Coastal Research, Vol 7(1), pp 53-84.
Dean, RG. 2002. Beach Nourishment: Theory and Practice. World Scientific, NJ, 399 pp.
ERDC. 2005. GENESIS modeling study of Reefball Breakwater, Miami, Florida. Letter Report, The US Army Engineering
Research and Development Center.
Ebersole, BA, PJ Neilans, and MW Dowd. 1996. Beach -fill performance at Folly Beach, South Carolina (1 year after
construction) and evaluation of design methods. Shore & Beach, Vol 64(1), pp 11-26.
Ekphisutsuntorn, P, P Wongwises, C Chinnarasri, U Humphries, and S Vongvisessomjai. 2010. Numerical modeling of
erosion for muddy coast at Bangkhuntien shoreline, Thailand. International Journal of Environmental Science and
Engineering, Vol 2(4), pp 230-240.
Everts, CH, JP Battley, and PN Gibson. 1983. Shoreline movements: report 1: Cape Henry, Virginia, to Cape Hatteras, North
Carolina, 1849-1980. Technical Report CERC-83-1, Coastal Engineering Research Center, US Army Engineer
Waterways Experiment Station, Vicksburg, MS,111 pp.
Fisher, JJ. 1967. Development pattern of relict beach ridges, Outer Banks barrier chain, NC. PhD Thesis, University of North
Carolina, Chapel Hill, 325 pp.
Frey, AE, KJ Connell, H Hanson, M Larson, RC Thomas, S Munger, and A Zundel. December 2012. GenCade Version 1 Model
Theory and User's Guide. USACE ERDC/CHL TR-12-25, 187 pp.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 95 Buxton, Dare County, North Carolina

Hands, EB.1981. Predicting adjustments in shore and offshore sand profiles on the Great Lakes. CETA No 81-4, USACE-CERC,
Fort Belvoir, VA, 25 pp.
Hanson, H. 1987. GENESIS, A Generalized Shoreline Change Model for Engineering Use. Report No. 1007, Department of
Water Resources Engineering, University of Lund, Lund, Sweden, 206 pp.
Hanson, H, and NC Kraus. 1989. GENESIS, generalized model for simulating shoreline change. Tech Rept CERC 89-19,
Coastal Engineering Research Center, US Army Corps of Engineers, Vicksburg, MS,185 pp + appendices.
Hanson, H, NC Kraus, and LD Nakashima. 1989. Shoreline change behind transmissive detached breakwaters. Coastal
Zone'89, Proc of the Sixth Symposium on Coastal and Ocean Management, ASCE, New York, NY, Vol 1, pp 568-582.
Horikawa, K, and M Hattori. 1987. The nearshore environment research center project. In Coastal Sediments '87, Proc
Specialty Conf on Advances in Understanding of Coastal Sediment Processes, ASCE, New York, NY, Vol I, pp 756-771.
Inman, D, and R Dolan. 1989. The Outer Banks of North Carolina: budget of sediment and inlet dynamics along a migrating
barrier system. Jour Coastal Research, Vol 5(2), pp 193-237.
Juh, YS. 2008. An application of integrated coastal models on protection of sediment siltation at Taichung Harbor, Taiwan.
Master Thesis, Department of Hydraulic & Ocean Engineering, National Cheng Kung University.
Kaczkowski, HL, and TW Kana. 2012. Final design of the Nags Head beach nourishment project using longshore and cross -
shore numerical models. In Proc 33rd International Conference on Coastal Engineering (ICCE July 2012, Santander,
Spain, 24 pp).
Kana, TW. 1990. Conserving South Carolina Beaches Through the 1990s: A Case for Beach Nourishment. South Carolina
Coastal Council, Charleston, SC, 33 pp.
Kana, TW.1993. The profile volume approach to beach nourishment. In DKStauble and NC Kraus (eds), Beach Nourishment
Engineering and Management Considerations, ASCE, New York, NY, pp 176-190.
Kana, TW, and HL Kaczkowski. 2012. Planning, preliminary design, and initial performance of the Nags Head beach
nourishment project. In Proc 33'd International Conference on Coastal Engineering (ICCE July 2012, Santander, Spain,
12 pp).
Kana, TW, HL Kaczkowski, and SIB Traynum. 2015. An empirical approach to beach nourishment formulation. In YC Kim (ed),
Design of Coastal Structures and Sea Defenses, Vol 2, Chapter 4, World Scientific, Singapore, pp 105-144.
Komar, PD.1998. Beach Processes and Sedimentation. Second Edition, Prentice -Hall, Inc, Simon & Schuster, Upper Saddle
River, NJ, 544 pp.
Kuang, CP. 2010. Performance analysis and prediction of beach nourishment project in Zhongzhi 6th and 9th bathing places
in Beidaihe. Journal ofTongji University (Natural Science), Vol38(4), April, pp 509-514.
Lane, B. 2013. Hatteras Island economic impact. Report to Outer Banks Visitors Bureau. University of North Carolina,
Chapel Hill, NC, 22 pp.
Larson, M, N Kraus, and KJ Connell. 2006. Cascade Version 1: Theory and Model Formulation. USACE ERDC TN-SWWRP-
06-7, 26 pp.
NCDENR. 2012. North Carolina 2011 long-term average annual oceanfront erosion rate update study methods report. NC
Department of Environment and Natural Resources, Division of Coastal Management, Raleigh, 125 pp.
NOAA. 2008. Digital elevation model of Cape Hatteras, North Carolina: Procedures, data sources and analysis. NOAA
National Geophysical Data Center, January 2008, Boulder, CO, 17 pp.
NOAA-NOS. 1983. Cooperative shoreline movement study: Cape Fear, N.C, to Tybee Island, GA. National Oceanic &
Atmospheric Administration, National Ocean Survey, U.S. Dept. Commerce, Washington, D.C, 32 map plates.
NIPS. 1980. Cape Hatteras Lighthouse, Buxton, North Carolina. National Park Service, Authors: MTMA Design Group, JL
Machemehl, NPS,139 pp.
NMFS. 1997. Regional biological opinion concerning the use of hopper dredges in channels and borrow areas along the
southeast U.S. Atlantic coast. South Atlantic Regional Biological Opinion (SARBO). National Marine Fisheries Service,
Silver Spring, MID, 16 pp.
Coastal Science & Engineering Littoral Processes
[2403M-Appendix DI 96 Buxton, Dare County, North Carolina

NMFS. 1998. Recovery Plan for the Shortnose Sturgeon (Acipenser brevirostrum). Shortnose Sturgeon Recovery Team for
the National Marine Fisheries Service, Silver Spring, MD, 104 pp.
Olsen. 2006. Regional sand transport study: Morehead City Harbor federal navigation project. Olsen Associates Inc,
Jacksonville, FL, 110 p.
Overton, MF, and JS Fisher. 2003. The 1998 long-term erosion rate update for the North Carolina shoreline. Division of
Coastal Management, NC Department of Environment and Natural Resources, Raleigh.
Overton, MF, and JS Fisher. 2005. Bonner Bridge replacement: parallel bridge corridor with NC 12 maintenance: shoreline
change and stabilization analysis. Prepared for URS Corporation -North Carolina and NCDOT, Task Orders 18 and 20,
TPI No B-2500. FDH Engineering, Raleigh, NC, 39 pp.
Ravens, TM, and KI Sitanggang. 2007. Numerical modeling and analysis of shoreline change on Galveston Island. Journal
of Coastal Research, Vol 23(3), pp 699-710.
Tafun, M, SM Rogers, and J Lanfelder. 1979. A Method Report on delineation of an ocean hazard zone for North Carolina.
Department of Marine Science and Engineering, North Carolina State University, Raleigh.
USACE. 2001. STWAVE: Steady -State Spectral Wave Model User's Manual for STWAVE, Version 3.0, ERDC/CHL SR-01-1,
Coastal and Hydraulics Laboratory, USACE. 66 pages+ appendices.
USACE. 2010. Final environmental impact statement, beach nourishment project, Town of Nags Head, North Carolina.
USACE Wilmington District, Washington Regulatory Field Office, NC (Action ID SAW-2006-40282-182), 164 pp +
executive summary, references, and appendices.
USACE. 2013. Public notice for S-curves beach nourishment interim protection for NC Highway 12. Corps Action ID SAW-
2013-01129. US Army Corps of Engineers, Wilmington District, NC, 10 pp.
USACE-USDOI-NPS. 2015. Environmental assessment — beach restoration to protect NC Highway 12 at Buxton, Dare
County, North Carolina. US Army Corps of Engineers, US Department of Interior, National Park Service, NPS
603/129663, Volume I (204 pp) and Volume II (Appendixes A to G).
Verhagen, HJ. 1992. Method for artificial beach nourishment. In Proc 23rd Intl Coastal Engineering Conf, ASCE, New York, NY,
pp 2474-2485.
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