Oak Island Renourishment Project_Appendix C - Borrow Area Modeling

2019/2020 RENOURISHMENT PROJECT
OAK ISLAND, NORTH CAROLINA
JAY BIRD SHOALS BORROW AREA MODELING
Prepared for:
Town of Oak Island
Prepared by:
,"q moffatt & nichol

2019/2020 RENOURISHMENT PROJECT
OAK ISLAND, NORTH CAROLINA
JAY BIRD SHOALS BORROW AREA MODELING
M&N Project No.10128-01
Revision
Description
Issued Date
Author
Reviewed
Approved
A
Modeling report
September 6, 2019
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4700 Falls of Neuse Road, Suite 300
,", Raleigh, NC 27609
m o f f a t t & n i c h o l [phone] •:• Fax: [phone]
www.moffattnichol.com
EXECUTIVE SUMMARY
In order to investigate the potential effects of dredging material from a Jay Bird Shoals
borrow area identified for the 2019/2020 Renourishment Project on neighboring
shorelines of Caswell Beach and Bald Head Island, numerical models were developed to
investigate hydrodynamics, waves, and sediment transport using Deltares' DeIft3D
model suite. The hydrodynamics and wave models were successfully calibrated and
validated against available observed water levels, currents, discharges, and wave data.
The sediment transport model was not calibrated due to lack of measured data to
calibrate against.
Tidal current, wave, and sediment transport modeling was performed for the existing
and after -dredge bathymetry scenarios. Two borrow area after -dredge templates were
considered. Template 1 was designed to dredge 2.95 million cubic yards (mcy) and
Template 2 was designed to dredge 2.34 mcy of available beach compatible material.
For both after -dredge templates only part of the material, 1.034 mcy, will be dredged
for the 2019/2020 Renourishment Project. Thus, within the proposed borrow area, the
results from the DeIft3D model are believed to be a conservative overestimate of the
potential effects on the tidal current and wave climates.
The tidal current model results indicate that for both after -dredge bathymetry
templates effects on residual tidal currents would be localized and small. This implies
there would be no significant effects on sediment transport processes associated with
tidal currents. The figure below shows the effects of the two after -dredge bathymetry
templates on residual tidal currents over a spring -neap tidal cycle.
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 4 of 70
1
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x coordm.te —.
-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25
Residual Currents -(Template 1-Existing) (ftfs), magmt,de Residual Currents - (Template 2-E—ting)(flls), magmt,de
After -dredge bathymetry effects on residual tidal currents over a spring -neap tidal cycle
The wave transformation model results for the 2004 — 2018 average annual offshore
wave climates show that the two after -dredge bathymetry templates could result in a
slight redistribution of wave energy along the shoreline during moderate to severe
storm events.
Sediment transport modeling was also completed, to observe if the changes to wave
heights and wave directions would affect the longshore transport. The sediment
transport modeling results for both after -dredge bathymetry templates show that the
wave -induced longshore sediment transports could be reduced leeward of the borrow
area but could potentially increase on shoreline segments both east and west sides of
the borrow area. The net effect of these changes could result in localized adjustments
in shoreline erosion / accretion. Based on the model results of the longshore sediment
transport gradients as presented below, most of the potential increases in shoreline
erosion would be limited to discrete portions of Caswell Beach (between survey
transects 37+00 —60+00 and 150+00 — 170+00). Generally, both templates show results
close to existing conditions, with some areas above and below existing. There is no
strong evidence to choose one template over the other given the model results,
especially given that this is not a morphological model. The modeled sediment
transport inside the surf zone is greatly influenced by the imposed model bathymetry.
Thus, the model results only represent the bathymetric condition constructed based on
the available data sources. There will be an additional 0.6 mcy beach compatible
material available in Template 1. For this reason, Template 1 was chosen for the Town
of Oak Island's permit application for the 2019/2020 Renourishment Project. The Town
of Oak Island will monitor the Caswell Beach shoreline for nine (9) years post -project to
investigate any potential effects which might require mitigation.
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1/11
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+ Caswell Beach Transects
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 6 of 70
TABLE OF CONTENTS
1. INTRODUCTION....................................................................................................................... 10
2. MODEL DEVELOPMENTS......................................................................................................... 11
2.1 Model Grids....................................................................................................................... 11
2.1.1 Flow Model Grids....................................................................................................... 11
2.1.2 Wave Model Grids..................................................................................................... 12
2.2 Model Bathymetry............................................................................................................ 13
3. MODEL CALIBRATIONS............................................................................................................ 15
3.1 Calibration Metrics............................................................................................................ 15
3.2 Flow Model Calibration..................................................................................................... 16
3.2.1 Boundary Conditions.................................................................................................. 21
3.2.2 Calibration Results..................................................................................................... 22
3.3 Flow Model Validation...................................................................................................... 30
3.4 Wave Model Calibration.................................................................................................... 31
3.4.1 Model Inputs..............................................................................................................31
3.4.2 Calibration Results..................................................................................................... 34
3.5 Wave Model Validation..................................................................................................... 41
4. JAYBIRD SHOALS BORROW AREA MODELING....................................................................... 46
4.1 Tidal Currents.................................................................................................................... 48
4.1.1 Peak Tidal Flood Currents.......................................................................................... 48
4.1.2 Residual Tidal Currents.............................................................................................. 51
4.2 Waves................................................................................................................................ 54
4.2.1 Representative offshore waves.................................................................................. 54
4.2.2 Nearshore Wave Results............................................................................................ 59
4.3 Sediment Transport........................................................................................................... 64
5. SUMMARY AND CONCLUSIONS..............................................................................................68
6. REFERENCES............................................................................................................................70
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
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LIST OF FIGURES
Figure 1-1:
Jay Bird Shoals borrow area..................................................................................
10
Figure2-1:
Flow model grid....................................................................................................
11
Figure 2-2:
Wave model grids.................................................................................................
12
Figure 2-3:
Flow model bathymetry under existing conditions ..............................................
14
Figure 2-4:
Fine wave model bathymetry under existing conditions .....................................
14
Figure 3-1:
Locations of water levels and current measurements by RPS EH ........................
17
Figure 3-2:
Survey transects in Upper Wilmington area by RPS EH ........................................
18
Figure 3-3:
Survey transects in Lower Wilmington area by RPS EH ........................................
19
Figure 3-4:
Survey transects in Snow's Cut area by RPS EH ....................................................
20
Figure 3-5:
Survey transects in Southport area by RPS EH .....................................................
21
Figure 3-6:
Water level calibration results..............................................................................
24
Figure 3-7:
Depth -averaged current calibration results.........................................................
25
Figure 3-8:
Discharge calibration results (TR01— TR03).........................................................
26
Figure 3-9:
Discharge calibration results (TR04 — TR06).........................................................
27
Figure 3-10:
Discharge calibration results (TR07 — TR09).........................................................
28
Figure 3-11:
Discharge calibration results (TR10 — TR12).........................................................
29
Figure 3-12:
Discharge calibration results (TR13).....................................................................
30
Figure 3-13:
Water level validation results during Hurricane Matthew ...................................
30
Figure 3-14:
Offshore waves from NOAA Buoy 41013 during calibration period ....................
32
Figure 3-15:
Wind data at NOAA buoy 41013 and from CFSR during calibration
period....................................................................................................................
33
Figure 3-16:
Water level data from NOAA station 8658163 for model calibration ..................
34
Figure 3-17:
Significant wave height calibration results...........................................................
35
Figure 3-18:
Peak wave period calibration results....................................................................
36
Figure 3-19:
Peak wave direction calibration results................................................................
37
Figure 3-20:
Comparison of Bald Head ADCP wave energy spectrum: (up)
measured; (down) modeled.................................................................................
40
Figure 3-21:
Significant wave height validation results............................................................
42
Figure 3-22:
Peak wave period validation results.....................................................................
43
Figure 3-23:
Peak wave direction validation results.................................................................
44
Figure 4-1:
Jay Bird Shoals borrow area templates................................................................
46
Figure 4-2:
After -dredge bathymetry—Template 1................................................................
47
Figure 4-3:
After -dredge bathymetry—Template 2................................................................
47
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
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Figure 4-4:
Instantaneous peak flood current velocities — existing condition ........................
48
Figure 4-5:
Instantaneous peak flood current velocities — after -dredge Template 1.............
49
Figure 4-6:
Instantaneous peak flood current velocities — after -dredge Template 2.............
49
Figure 4-7:
After -dredge bathymetry effects on instantaneous peak flood current
velocities —Template 1.........................................................................................
50
Figure 4-8:
After -dredge bathymetry effects on instantaneous peak flood current
velocities —Template 2.........................................................................................
50
Figure 4-9:
Residual tidal currents — existing condition..........................................................
51
Figure 4-10:
Residual tidal currents — after -dredge Template 1...............................................
52
Figure 4-11:
Residual tidal currents — after -dredge Template 2...............................................
52
Figure 4-12:
After -dredge bathymetry effects on residual tidal currents —Template
1............................................................................................................................
53
Figure 4-13:
After -dredge bathymetry effects on residual tidal currents —Template
2............................................................................................................................
53
Figure 4-14:
Annual percentage of exceedance of significant wave height at the
offshoreboundary................................................................................................
54
Figure 4-15:
Wave rose of significant wave heights at the offshore boundary ........................
55
Figure 4-16:
After -dredge bathymetry effects on waves between 0 — 3 ft with
average height of 2.5 ft (top: Template 1; bottom: Template 2) .........................
60
Figure 4-17:
After -dredge bathymetry effects on waves between 3 — 6 ft with
average height of 4.5 ft (top: Template 1; bottom: Template 2) .........................
61
Figure 4-18:
After -dredge bathymetry effects on waves between 3 — 6 ft with
average height of 7.5 ft (top: Template 1; bottom: Template 2) .........................
62
Figure 4-19:
After -dredge bathymetry effects on storm waves comparable to
Hurricane Matthew in 2016 (top: Template 1; bottom: Template 2) ..................
63
Figure 4-20:
Caswell Beach transects........................................................................................
65
Figure 4-21:
Wave -induced net longshore sediment transports along Caswell
Beachshoreline.....................................................................................................
66
Figure 4-22:
Longshore sediment transport gradients along Caswell Beach
shoreline...............................................................................................................
67
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LIST OF TABLES
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 9 of 70
Table 2-1:
Model bathymetry data sources...........................................................................
13
Table 3-1:
Goodness -of -fit parameters for significant wave height calibration ...................
39
Table 3-2:
Goodness -of -fit parameters for peak wave period calibration ............................
39
Table 3-3:
Goodness -of -fit parameters for peak wave direction calibration ........................
39
Table 3-4:
Goodness -of -fit parameters for significant wave height validation .....................
45
Table 3-5:
Goodness -of -fit parameters for peak wave period validation .............................
45
Table 3-6:
Goodness -of -fit parameters for peak wave direction validation .........................
45
Table 4-1:
Representative wave conditions used as model inputs .......................................
56
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Jay Bird Shoals Borrow Area Modeling
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Page 10 of 70
1. INTRODUCTION
Moffatt & Nichol was retained by the Town of Oak Island for professional services to
execute the 2019/2020 Renourishment Project following Hurricane Matthew.
The Jay Bird Shoals borrow area shown in Figure 1-1 was identified as a potential
borrow area for this beach renourishment project. In order to determine if potential
adverse effects to the neighboring Caswell Beach and Bald Head Island shorelines could
be a possibility, numerical modeling studies were conducted.
DeIft3D, an open -source, fully integrated numerical modeling suite developed by
Deltares, Netherland, was selected as the modeling platform. DeIft3D can carry out
numerical modeling of flows, waves, sediment transport, morphological developments,
water quality and ecology in coastal, river, lake and estuarine areas. For the purpose of
this study, two modules in DeIft3D were used: DeIft3D-FLOW (Deltares, 2018a) and
DeIft3D-WAVE (Deltares, 2018b). DeIft3D-FLOW is the hydrodynamics and sediment
transport module; whereas DeIft3D-WAVE is the wave transformation module.
In this report, the effects of dredging material from a borrow area in Jay Bird Shoals on
waves, tidal current velocities, and sediment transport patterns were investigated.
Figure 1-1: Jay Bird Shoals borrow area
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Jay Bird Shoals Borrow Area Modeling
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Page 11 of 70
2. MODEL DEVELOPMENTS
In this section, the developments of flow and wave model grids and bathymetries are
discussed. The model horizontal coordinate is in North Carolina State Plane, and the
vertical datum is North American Vertical Datum (NAVD88).
2.1 MODEL GRIDS
2.1.1 Flow Model Grids
The flow model domain included the Cape Fear River estuary from upstream of the Cape
Fear, Black, and Northeast Cape Fear Rivers to 20 miles offshore from the mouth of
Cape Fear River near Southport, NC. The grid cell sizes were variable throughout the
domain. In the offshore area the resolution was approximately 90 meters. For the
upstream Cape Fear, Black, and Northeast Cape Fear River areas, the resolution was
approximately 30 meters. Along the channel the resolution was approximately five
meters. Figure 2-1 presents the flow model grid.
Figure 2-1: Flow model grid
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Jay Bird Shoals Borrow Area Modeling
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2.1.2 Wave Model Grids
Wave transformation from deep water to the shoreline was accomplished by nesting
three increasingly resolved model domains as shown in Figure 2-2.
The coarsest grid (gray) is comprised of approximately 20,000 cells with size of 500 m x
500 m. The offshore limit of the coarse grid is near the location of the National Oceanic
and Atmospheric Administration (NOAA) wave buoy 41013 from which offshore wave
conditions were derived.
The medium -resolved wave domain (blue) and the fine wave domain (red) were
developed based on the flow model grid. The fine wave model grid has approximately
5-meter cross -shore resolution in the surf zone region of Caswell Beach.
Figure 2-2: Wave model grids
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MODEL BATHYMETR`'
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 13 of 70
Bathymetric data from different sources were compiled and processed to cover the
entire computational domains. All bathymetric datasets were adjusted to NAVD88. The
data sources used for the development of the morphology model bathymetry are listed
in Table 2-1 from high priority to low priority. The most recent bathymetry data were
selected where available to create the model bathymetry.
The terminal groin constructed on the western tip of South Beach on Bald Head Island
between June and December 2015 was also included in the model.
Figure 2-3 and Figure 2-4 show the flow model bathymetry and the fine wave model
bathymetry under existing conditions, respectively.
Table 2-1: Model bathymetry data sources
Wilmington Harbor hydrographic surveys
USACE 2016 — 2017
Fugro channel bank surveys Fugro 2016 — 2017
Oak Island post Matthew beach profile surveys
TI Coastal 2016
(STA 210+00 — 700+00)
Bald Head Island beach profile surveys
USACE 2013
(STA 000+00 — 238+00)
Oak Island beach profile surveys
USACE 2012
(STA 005+00 — 210+00)
Cape Fear River 2010 surveys USACE 2010
NOAA hydrographic surveys
NOAA 1973 — 2007
NOAA Navigation Charts
MIKE C-MAP
ADCIRC bathymetry
NCDPS 2011
NC LiDAR
NOAA 2014-2016
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Figure 2-4: Fine wave model bathymetry under existing conditions
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3. MODEL CALIBRATIONS
3.1 CALIBRATION METRICS
Several goodness -of -fit statistical parameters were used to assess model calibration and
validation results. These include the mean error (ME), root mean square (RMS) error,
normalized RMS error, mean absolute error (MAE), correlation coefficient (R), index of
agreement (d), and time delay or lag (AT). These parameters are briefly described here.
If x and y are the measured and calculated data respectively, then the following
statistics can be calculated:
Mean error (ME):
ME= y—x (1)
Where "bar" denotes the sample mean.
Root mean square (RMS) error:
a
ERMS =r(XY) (2)
To reduce the effect of measurement error and possible outliers, a one -hour low-pass
filter was applied to the measured data to compute trend xf. Then the normalized error
is calculated as
£norm — ERMS • 100% (3)
Xf,max—xf,min
Where Xfmax and Xfmin are the maximum and minimum values of the trend xf. The
residual in the denominator defines the range of measured data.
The root mean square error of measured data was estimated as:
2
cmeas =F x f� (4)
Mean absolute error (MAE):
MAE= x-y (S)
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The correlation coefficient R was calculated using standard method and represents a
non -squared value.
The model prediction capability was estimated with an index of agreement between
measured and calculated data (Willmott et al., 1985):
d=1— (x—y)` 0<d<1
(x-x — Y-XT
The time delay AT shows expected time difference between corresponding events in
measured and calculated data. To estimate the delay, the cross -correlation function
between measured and calculated data is computed and the smallest time lag at which
a maximum occurs is found. Because the cross -correlation function is calculated from
discrete data, resulting time resolution may not be sufficient to accurately define the
maximum. Therefore, computed values of the cross -correlation function were
interpolated with a piecewise polynomial of 5' order, which was then used to
determine the maximum.
3.2 FLOW MODEL CALIBRATION
The flow model was calibrated for the period between March 27, 2017 and April 5, 2017
when RPS Evans -Hamilton (RPS EH) conducted water level, current, discharge, salinity,
and water quality measurements on the Cape Fear River (RPS Evans -Hamilton, 2017).
For the calibration period, water level measurements were available at Southport and
Wilmington (Figure 3-1); current measurements were available at Southport (Figure
3-1); and discharge measurements were available at the 11 transects between
Wilmington and Southport (Figure 3-2 through Figure 3-5). The model was calibrated to
match the measured water levels, discharges, and currents.
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
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Figure 3-2: Survey transects in Upper Wilmington area by RPS EH
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Figure 3-3: Survey transects in Lower Wilmington area by RPS EH
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Figure 3-4: Survey transects in Snow's Cut area by RPS EH
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Figure 3-5: Survey transects in Southport area by RPS EH
3.2.1 Boundary Conditions
The model has seven open boundaries as indicated on Figure 2-1: four offshore — West,
South, East, and North; and three upstream — NE Cape Fear River, Black River, and Cape
Fear River. The model was forced using tidal water levels at the offshore boundaries
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and river discharges at the upstream boundaries. Winds were applied uniformly over
the entire domain.
A) TIDAL BOUNDARY CONDITIONS
Astronomical tidal constituents for water levels were extracted from the Oregon State
University tidal database which is based on TOPEX/Poseidon satellite altimetry data
(Egbert and Erofeeva, 2002). The global model with a resolution of 1/6' along with high
resolution along coastal areas was used. North and West open boundary were specified
as Neumann boundaries, and South and East open boundary were specified as water
level boundaries.
B) RIVER DISCHARGES
The time series of discharges from the rivers measured at three United States Geological
Survey (USGS) stations (shown in Figure 2-1) were used at the three upstream open
boundaries: discharge data at Station 02105769 was used at the upstream boundary at
the Cape Fear River, Station 02106500 data was used at the Black River, and Station
02108000 data was used at the Northeast Cape Fear River. The discharges from the un-
gaged drainage areas between the USGS stations and the model upstream boundaries
were accounted for with appropriate scale factors based on the ratio of un-gaged
drainage area vs. gaged drainage area for each branch.
WINDS
From the analysis of available wind data, it was found that the wind field in the Cape
Fear River estuary is very seasonal in nature, i.e., predominant wind direction changes
according to the season, and wind speeds vary depending on the location of the station.
Stations that are offshore indicate higher wind speed than stations located on the coast
or on land.
Wind data from Station KILM (Wilmington International Airport) shown in Figure 2-1
was used to force the model. Station KILM is located on the land and is considered to
better represent wind over the estuary compared to the offshore stations.
3.2.2 Calibration Results
Water levels, currents, and discharges obtained from the model results were compared
with measurements available at various locations. Figure 3-6 shows the comparison of
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Jay Bird Shoals Borrow Area Modeling
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Page 23 of 70
water level time series. It can be seen that the model replicates the water levels well
with a small over prediction for most of the time (Station Wilmington (NOAA)). Figure
3-7 shows the comparison of depth -averaged currents and the model also replicates the
currents at Southport well.
Figure 3-8 through Figure 3-12 show comparisons of the discharge measurements. The
statistics shown in those figures were calculated by comparing the model and
measurement values at corresponding times. The positive and negative discharge
correspond to ebb current and flood current direction, respectively. The calibration
results match well at all transects in the main channel.
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 24 of 70
Measured
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E 1
. ..... ...... .. ..... m
Enorm--10.2%
ea
0.5
c - .0 . .
L
a�
0
I 2 e + 0
_ .15 s
-0.5
T 1 .0 in
d = 0.98
-1
23/Mar 25/Mar 27/Mar 29/Mar 31/Mar
2017
Figure 3-6: Water level calibration results
02/Apr 04/Apr 06/Apr
201912020 Renourishment Project
M&N Project No.10128-01

set04.v20 - Southport
Current Speed
2.5
2
0
28/Mar 29/Mar 30/Mar 31/Mar 01/Apr
2017
Current Direction
350
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 25 of 70
Measured
Calculated
02/Apr 03/Apr
300
ErmS = 21.209
rn
s = 19.927
meas
250
E
=
o o
CE
rn
541.3
200
m
as -
alc
m
a
=
.8
150
_
_ .
......
..
...
.......1
IAH.4
c
=
.9
100
Ic
0.
88
a
2.
71
U
LJ
alc
1
00
0•
ea
50
— -
_
sec
d = 1.00
0
28/Mar 29/Mar 30/Mar 31/Mar 01/Apr
2017
Figure 3-7: Depth -averaged current calibration results
02/Apr 03/Apr
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 26 of 70
set04❑ Measured
TR01 0 -multi Calculated
1500
1000
v 500
E
ai
2 0
cc
t
o -500
-1000
1500
e = 32.861
rms
cal - meas 61.51
❑ MA 103.44
R = 0.97
d = 0.98
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR02
1000
50
M
E
2I
<0
t
Aa
0
-50(
) �- J . `i . .... } ....x................. p ..x................./ ....� ................ .
8 = 13.550
rms
ca - me U- 52,41
: E 91.50
❑ R = 0.94
d = 0.96
-100G
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR03
300^
200
E 100
ai
rn
`m
t
0
w
-100(
.......... .......... .........
i
Erms — 8.784
ca c - meas 86.71
MAE 71.88
❑ = 0.95
d = 0.97
-2000
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
Figure 3-8: Discharge calibration results (TR01—TR03)
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 27 of 70
set04❑ Measured
TR04 0 -multi Calculated
3000
2000
M 1000
E
ai
21 0
cc
t
o -1000
-2000
3000
)mea
50.323
ca - 13.31
❑219.54
R = 0.99
d = 0.99
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR05
1000
500
M
E
6
rn 0
m
t
Aa
i]
-500
1
...... 00.. o❑..............:.�......... �....
8 - 113.631
rms
- ❑ - cal - mea = -33.96
o AE = 98.05
R = 0.98
d = 0.99
- 000
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR06
4000
3000
2000
M
E
ai 1000
rn
t 0
0
U)_
-1000
-2000
-3000
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
Figure 3-9: Discharge calibration results (TR04 — TR06)
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 28 of 70
set04❑ Measured
TR07 0 -multi Calculated
6000
4000
2000
ai
21 0
cc
t
o -2000
-4000
6000
crms413.691
_ - - -
ca - mea = -44.37
_.
M E = 330.34
R = 0.96
d = 0.98
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR08
1000
500
M
E
ai
2 0
An
t
An
0
-500
-1000
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR09
6000
4000
M 2000
E
ai
2 0
m
t
0
ij -2000
-4000
-6000
12:00 18:00 00:00 08:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
Figure 3-10: Discharge calibration results (TR07 — TR09)
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M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 29 of 70
set04.v2O - multi
❑ Measured
TR10 Calculated
400
300
v, 200
M
E
ai 100
m
cc
U
N
C, -100
-200
-300
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
x 104 TR11
1
0.5
W
ri
E
N
rn 0
f0
t
_N
-0.5
s 1070.629
. rms
Ic - me s 60.22
AE = 891.15
q ❑ R = 1.00
d = 0.99
-1
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
TR12
Lull]
200
M
E
ti
rn 0
m
t
U
N
-200
ter,
28.48
Rns
calc meas = 12.87 _
MAE = 21.48
R = 0.99
❑ ❑❑: d= 1.00
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
Figure 3-11: Discharge calibration results (TR10 — TR12)
201912020 Renourishment Project
M&N Project No.10128-01

x 104
1.5
1
42
M 0.5
E
ai
rn 0
m
L
U
0 -0.5
-1
set04.v2O - multi
TR13
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 30 of 70
❑ Measured
Calculated
12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00
30/Mar/2017
Figure 3-12: Discharge calibration results (TR13)
3.3 FLOW MODEL VALIDATION
For the flow model validation, the water level measurements at NOAA Wilmington
Station during Hurricane Matthew in October 2016 were used. The model was forced
with time series of measured water levels at Wrightsville Beach (NOAA station
8658163), and wind from the KILM station. It can be seen that the model captures the
more extreme water levels well during this hurricane event as shown in Figure 3-13.
set04.cl7 - Wilmington(NOAA) Measured
Wilmington(NOAA) Calculated
2
1.5 Erms - 0.132
s = 0.275
meas
E 1.0•..%q..
1 ........ orm
ID r es 6
0.5 al - 02
E _ .1
R = .96 0
calc = 0.8671 •meas+0.0499
-0.5 - .
calc = 0.9226•meas
AT =-12.0min
d = 0.98
-1
04/Oct 05/Oct 06/Oct 07/Oct 08/Oct 09/Oct 10/Oct 11/Oct 12/Oct 13/Oct 14/Oct
2016
Figure 3-13: Water level validation results during Hurricane Matthew
201912020 Renourishment Project
M&N Project No.10128-01

I11
WAVE MODEL CALIBRATION
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 31 of 70
There are six stations (as shown in Figure 2-2) with measured wave data available inside
the wave model domains: three NOAA National Data Buoy Center (NDBC) buoys —
41108, Ocean Crest Pier (OCP1), and Sunset Beach Nearshore (SSBN7); three United
States Army Corps of Engineers (USACE) Acoustic Doppler Current Profiler (ADCP) gages
— Eleven Mile, Bald Head and Oak Island. OCP1 and SSBN7 are owned and maintained
by the Coastal Ocean Research and Monitoring Program (CORMP). The NOAA buoy
41108 is at the same location as the USACE Eleven Mile ADCP. The following bulk wave
parameters are reported at both the NOAA buoys and the USACE ADCPs: significant
wave height, peak and average wave periods, and peak wave direction.
For the wave transformation modeling, in addition to the offshore wave data as the
boundary conditions, wind and water level inputs are also important especially during
storm events. Based on the contiguous data available at all wave stations along with
overlapping wind and water level data, the period of August 1st, 2008 to October 1st
2008 was selected for the wave model calibration purpose. Large waves generated by
Hurricane Hanna were included in this period; thus, the wave model's ability to replicate
both large and normal waves can be verified.
'A.4.1 Model Inputs
'A) OFFSHORE WAVE BOUNDARY CONDITIONS
The directional wave spectra from NOAA buoy 41013 were applied as spatially uniform
wave conditions at all three boundaries. The wave spectra were calculated based on the
spectral wave density, alphal, alpha2, r1 and r2 data using the extended maximum
likelihood method. The description of variables can be found in the NDBC website
(www.ndbc.noaa.gov/measdes.shtml), with the conversion method following Earle et al.
(1999) and Benoit et al. (1997). Figure 3-14 shows the offshore bulk wave parameters
for the calibration period. The maximum wave height of 8.4 m was observed on
September 6t", 2008 during Hurricane Hanna.
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 32 of 70
Significant wave height [m]
8.0
- --- ------------------ -- --- --- ---
4.0
2.0
0.0
August September
2008 2008
Peak wave period [sec]
20
15
10
5.
August September
2008 2008
Peak wave direction [deg]
360
240'14"�4
120
a
August September
2008 2008
Figure 3-14: Offshore waves from NOAA Buoy 41013 during calibration period
(B) WINDS
The spatially varying wind data from the National Centers for Environmental Prediction
(NCEP) Climate Forecast System Reanalysis (CFSR) were applied for the model
calibration period. The CFSR wind data interval is three hours. Figure 3-15 shows wind
data comparison between NDBC and CFSR at buoy 41013 with good agreements.
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 33 of 70
NDBC [mis]
CFSR [mis]
25
20
in
....................................... ---------
E
� 15
m
m
a
a 10
----- ---------
C_
5
August September
2C08 2008
NDBC [deg]
CFSR [deg]
360
it
30c
z
rn
m
� 240
C
O
u 180
N
a 120
C_
60
0
August September
2008 2008
Figure 3-15: Wind data at NOAA buoy 41013 and from CFSR during calibration period
(C) WATER LEVELS
A spatially uniform water level field was used for the model calibration. Due to the lack
of available measured water level data within the model domain, the data from nearby
NOAA Station 8658163 at Wrightsville Beach, NC (as shown in Figure 2-1) was used for
the model calibration. Figure 3-16 presents the water level data. However, it is
important to point out that Hurricane Hanna made landfall at the NC/SC border, so the
surge was much greater on Oak Island/Bald Head than at Wrightsville Beach. The
reported storm surge was about 5 ft at Wilmington, NC, and about 4 ft at Myrtle Beach,
SC, the back side of the storm. Thus, using the measured water level data at
Wrightsville Beach could adversely affect the modeled waves during Hanna.
Nonetheless, it's the closest available open coast water level station for the study area
and thus used for the wave model calibration without any adjustment.
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 34 of 70
2
—Water Level (m)
1.5
—Residual (m)
1
0.5
0
-0.5
-1
2008-08-0100:00 2008-08-16 00:00 2008-08-3100:00 2008-09-15 00:00 2008-09-30 00:00
Figure 3-16: Water level data from NOAA station 8658163 for model calibration
3.4.2 Calibration Results
Figure 3-17 through Figure 3-19 present the direct comparison between the computed
and measured time series of significant wave height, peak wave period, and peak wave
direction, respectively, at the gage locations of Eleven Mile ADCP, Bald Head ADCP, Oak
Island ADCP and OCP1. Based on the model bathymetry, the OCP1 ADCP location is at a
water depth of 5 m which is close to the wave breaking zone. Because the wave heights
during the peak of the storms were greatly under predicted, it is suspected that the
depth at the ADCP location was not correct (possibly due to the surge being higher) and
therefore the model output point for the OCP1 ADCP was moved offshore to a deeper
area of 7 m water depth.
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 35 of 70
Figure 3-17: Significant wave height calibration results
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 36 of 70
ElevenMile
20
• Measured
15
Modeled
0
«r..
►
d
10
_ _ _ _ • _ _ _ ► _ ■ «•• w w►+. «w�ww«• w ► •ww _ _
_ _ _ • _ _ ■ w�wn
5
..• •
�.... �.........«........�.....
Y
a�
a
0
Jul-31 Aug-10
Aug-20 Aug-30 Sep-09
Sep-19 Sep-29
Time
BaldHead
20
NMeasured
15
Modeled
•'
_ __ _
"�,
aie�■ . mna apt � • s
• ►- .. sr'.
d
10
14_ _ _*4004.
rw•► w.r ► •�wwrr .� •wiw _ _
_ _ ■ wwy«ru ter►
�«r■
Y
5��•i�.�,.....I......
■................�.i.. ,.....'..._
�" •_•_�. •tip...
a
0
Jul-31 Aug-10
Aug-20 Aug-30 Sep-09
Sep-19 Sep-29
Time
Daklsland
20
yMeasured
►
0
15
Modeled
•►
.tr
> • •«r w► ww•rww«• rwr
_.__
< wr �• �wr►
>
w. =.wr a•wwr w■ .. rw
_.__. __.. ■ ■w
w.
d
0
Jul-31 Aug-10
Aug-20 Aug-30 Sep-09
Sep-19 Sep-29
Time
OCP1
20
V
Measured
-a
15
Modeled
•'
'
O
. ! ■
Am
a
10
......... ..................
rrrr rrw rr wui
.......... Awww - _-11L ..........
■ • • wr rr r�
.... ... . �u. C.....
j
• r
••��►
r �■w► ■■5
pqnyr:■fir
`•fit ........ ..... . . • ....; is► ...........'; .
�
ww= �■ jI � %�7�
a�
a
0
Jul-31 Aug-10
Aug-20 Aug-30 Sep-09
Sep-19 Sep-29
Time
Figure 3-18: Peak wave period calibration results
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 37 of 70
ElevenMile
z
360
300
Measured
° 240
Modeled
-
0 180..
• ♦ •♦ _ • �♦ •♦ ' N w •i
0
a- Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29
Time
BaldHead
z 360
300
Measured
=0 240
Modeled
•�........ - - ___.. - -_L..
.♦ ..,•
IWO
Cz
60
............. ......... ........ . . ... . . . . . . . . . . . ....
co
0
a Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29
Time
Oaklsland
z 360
300
Measured
240-
Modeled
=0
AL
' 120
60
Co
0
• ,
a Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29
Time
OCP1
z 360
300
Measured .. ............. . �........., .......... .
240
Modeled
.
.2
.... ................. ..• .... .... ....... ... .. .
ALI
180
....... ....rN.'lr�i�. A4
> 120
............. ...... ......... .............
60
a Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29
Time
Figure 3-19: Peak wave direction calibration results
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 38 of 70
The calculated goodness -of -fit parameters for the wave calibration results are listed in
Table 3-1 through Table 3-3 for the significant wave height, peak wave period and peak
wave direction, respectively. The results suggest that:
• For the significant wave heights, the model predictions agree very well with the
measured data at all four ADCP locations, with MAE and RMS errors less than
0.2 m, and R and d values greater than 0.9.
• For the peak wave periods, the MAE and RMS errors are less than 2.5 s, and R
and d values around 0.7 and 0.8, respectively. The data indicates there are
periods when at least two wave systems exist — long period waves from offshore
and locally generated waves from onshore. In the presence of the two systems,
determination of peak period may not be consistent and may alternate between
two values. This negatively affects the statistics.
• For the peak wave directions, the model predictions have large deviations from
the measured values. It is more pronounced at the Bald Head Island ADCP during
period of September 17-26, when the reported ADCP peak wave directions are
from between 90 and 180°N, whereas most of the modeled values are from
between 330 and 360°N. Figure 3-20 presents both the measured and modeled
Bald Head ADCP wave energy spectrum at 1:00 am EST on September 24, 2008.
Two wave systems are evident from both the measured and the model predicted
spectra: waves coming from SSE —SSW (offshore) with the frequency of around
0.1 Hz; and waves coming from NNW—N (locally wind -generated) with the
frequency of around 0.4 Hz. The measured spectrum has some noise at higher
frequencies beyond 0.8 Hz. It appears that the peak wave direction from the
measured spectrum was calculated to be from offshore; whereas the peak wave
direction from the modeled spectrum was calculated to be from onshore. This
supports the fact that two or more wave systems can exist at the same time and
one can dominate the wave field, which can result in large peak wave direction
differences between the measurement and the model prediction. Per
communication with USACE personnel' who is familiar with the handling of ADCP
data, an upper cutoff frequency was used when post -processing the raw ADCP
data to the bulk wave parameters. The cutoff frequency was the lesser of the
two: when the wavelength is less than two times of the beam separation; or
when the pressure response correction for amplitude is 0.1.
1 Personal communication with Kent Hathaway from the USACE.
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 39 of 70
Table 3-1: Goodness -of -fit parameters for significant wave height calibration
Eleven Mile ADCP
0.14
0.19
4.3
0.96
0.97
Bald Head Island ADCP
0.11
0.15
5.3
0.91
0.95
Oak Island ADCP
0.10
0.13
4.6
0.92
0.96
OCP1 ADCP
0.08
0.11
3.5
0.94
0.97
Table 3-2: Goodness -of -fit parameters for peak wave period calibration
Eleven Mile ADCP
1.3
2.0
0.74
0.86
Bald Head Island ADCP
1.4
2.4
0.65
0.81
Oak Island ADCP
1.4
2.3
0.64
0.81
OCP1 ADCP
1.4
2.2
0.71
0.85
Table 3-3: Goodness -of -fit parameters for peak wave direction calibration
201912020 Renourishment Project
M&N Project No.10128-01

0
330
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
40
Figure 3-20: Comparison of Bald Head ADCP wave energy spectrum: (up) measured; (down) modeled
201912020 Renourishment Project
M&N Project No.10128-01

140
3.5 WAVE MODEL VALIDATION
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 41 of 70
Based on the contiguous data availability at all wave stations along with overlapping
wind and water level data, the period of July 1, 2009 to December 1, 2009 was selected
for the wave model validation purpose.
Similar to the wave model calibration period, the directional wave spectra from NOAA
buoy 41013 were applied as spatially uniform wave conditions; spatially varying wind
fields from CFSR were used as the wind inputs; and measured water level data from
NOAA station 8658163 were used as a spatially uniform water level field.
Figure 3-21 through Figure 3-23 present the direct comparison between the computed
and measured time series of significant wave height, peak wave period and peak wave
direction, respectively, at the gage locations of Eleven Mile ADCP, Bald Head ADCP, Oak
Island ADCP and OCP1. The goodness -of -fit parameters for the wave validation results
are listed in Table 3-4 to Table 3-6 for the significant wave height, peak wave period and
peak wave direction, respectively. The results suggest that:
• For the significant wave heights, the model predictions agree very well with the
measured data at all four ADCP locations except Oak Island ADCP, with MAE and
RMS errors less than 0.2 m. The wave heights were consistently over -predicted
at the Oak Island ADCP. The measured wave heights at Oak Island were lower
than OCP1 ADCP; whereas the predicted wave heights were similar. It is possible
that the deployment of the Oak Island ADCP during the validation period was in
a different depth than previous deployment periods.
• For the peak wave periods, the MAE and RMS errors are less than 2.6 s, and R
and d values around 0.6 and 0.8, respectively.
• For the peak wave directions, the model predictions have large deviations from
the measured values. After checking the measured and model predicted
directional wave spectra, the presence of a double peaked spectrum is what
caused the issue.
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 42 of 70
ElevenMile
3
Measured
-- Modeled
v� 2-1...... _....... A... ................. CD
'..
2 _
■
■ ' «
0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
BaldHead
3
Measured
L Modeled
CD
• t s ! # !
1 .4.
Ch
i
0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
Oaklsland
3
Measured
L Modeled
m2 ........ .........
a,
_ ;
■
m t
0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
OCP1
3
Measured
L Modeled
o) 2 ......... ..... ......... .
a
cn
0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
Figure 3-21: Significant wave height validation results
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 43 of 70
ElevenMile
20
NMeasured
■
-a 15
Modeled....... ........ ......... ! ......... .......... .......... ►....... . ..............
o
ass
♦ N W 1MNA�� M • ■ A ■
i111
AlkF
N
a 0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
BaldHead
- 20
• Measured ►
a 15
Modeled .....
o
♦
d 10
•: w. • ♦ M • : w • w w •wow
�
► •r t.. iW � • wi a� w w •i i � • � •
dw
a
0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
Oaklsland
20
i
Measured «
0 15
Modeled s s
•i
�
N
w ♦ ■ •� w wr w w w w: w w . r w rw
•: • :. ► ii r ►► r r r err li► r r�r r ■
d 1
• • • r� • w► • • .w ■ w r w rNr w ■•.•
..
. _
.01
a
0
.•
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
OCP1
20
N
L]
(
Measured
a 15
«
Modeled ""
a
10
•- �► • r< wwr ♦ « rrw•r •NMI : �
...... . ........... ......... ... _... �...N..•F�.wAw■... _. ........•►•..... ...... /•wIN•...f■..rwwN. w..
(D
a
0
Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
Figure 3-22: Peak wave period validation results
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 44 of 70
ElevenMile
z 360
•, ,
300
r
Measured �,
240
'•= j _
Modeled0—
.•4.f s
AAA -2
css
i ..
• • •
a 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
BaldHead
360
(D
'E� 300
Measured '
Modeled _: _ _ _ _ __� __ $s _
o
L7
•
♦ .. •..... .....
..;d� a
120
lr......*....!!l• • •i �1�....My ;!_+. .i3..�,. .del......... ........ ; . .......... .. ... IR! . ..
Y
ca
a)
0
a Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
Oaklsland
360
1�.
300
Measured
240
Modeled
0 180
* —
Y
0
- • • • • ♦ N
a Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
OCP1
z 360
=s
300
Measured
T° 240
Modeled
....... .
J;1
0 180
- � .
60
'
a 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23
Time
Figure 3-23: Peak wave direction validation results
201912020 Renourishment Project
M&N Project No.10128-01

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 45 of 70
Table 3-4: Goodness -of -fit parameters for significant wave height validation
Eleven Mile ADCP
0.14
0.18
8.7
0.88
0.93
Bald Head Island ADCP
0.12
0.15
8.6
0.87
0.92
Oak Island ADCP
0.19
0.22
20.3
0.88
0.77
OCP1 ADCP
0.09
0.13
8.2
0.90
0.94
Table 3-5: Goodness -of -fit parameters for peak wave period validation
Eleven Mile ADCP
Head Island ADCP
1.3 2.1 70.66
7827Bald
1.5
2.5
0
Oak Island ADCP
1.6
2.6
0.57
0.76
OCP1 ADCP
1.4
2.3
0.68
0.82
Table 3-6: Goodness -of -fit parameters for peak wave direction validation
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 46 of 70
1. JAY BIRD SHOALS BORROW AREA MODELING
To investigate the potential effects of dredging the identified Jay Bird Shoals borrow
area on tidal currents, nearshore waves, and sediment transports along the adjacent
shorelines, the existing model bathymetries were modified to reflect the after -dredge
conditions. Two borrow area templates were considered as shown in Figure 4-1.
Template 1 includes three zones with dredging elevation down to -26 ft-NAVD88 (Zone
1), -31 ft-NAVD88 (Zone 2), and -27 ft-NAVD88 (Zone 3), respectively. For Template 2,
the Zone 2 dredging elevation was reduced to -27 ft-NAVD88 (the same dredging
elevation as Zone 3 in Template 1) and its footprint was also reduced. The maximum
dredging scenario was considered for both templates, i.e. assuming to remove all the
available material identified as beach compatible (2.95 million and 2.34 million cubic
yards for Template 1 and 2 respectively). Only part of the available material, 1.034
million cubic yards, will be dredged for the 2019/2020 Renourishment Project. Figure
4-2 and Figure 4-3 illustrate the after -dredge bathymetries at the Jay Bird Shoals borrow
area for Template 1 and 2, respectively.
Figure 4-1: Jay Bird Shoals borrow area templates
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Figure 4-2: After -dredge bathymetry —Template 1
17
16
�r
15.-mom
m
c
a 14
� �
�v
o
13
12
694
695 696 697 698 699 700 701
x coordinate (km)
702 703
-40
-35 -30 -25 -20 -15 -10
After -dredge bathymetry - Template 2 (ft-NAVD88)
-5 0
Figure 4-3: After -dredge bathymetry —Template 2
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 48 of 70
The modeling results based on the after -dredge bathymetries were compared with the
modeling results from the existing bathymetry to identify the potential effects.
4.1 TIDAL CURRENTS
For the existing and the two after -dredge templates, the flow model was simulated for a
full spring -neap tidal cycle with astronomical tides and annual average river flows
without winds.
4.1.1 Peak Tidal Flood Currents
Figure 4-4, Figure 4-5, and Figure 4-6 present the instantaneous peak flood current
velocities during a spring tide under existing and the two after -dredge templates,
respectively. Figure 4-7 and Figure 4-8 show the peak flood current velocity differences
between the existing and the two after -dredge templates, respectively. The model
results indicate that Template 1 would have no measurable changes from existing,
Template 2 could cause a 1 ft/s increase of peak flood currents in highly localized areas.
Since the project peak current velocity magnitude in these localized areas is less than
1.5 ft/s under all conditions, effects on shorelines are expected to be negligible.
17
ED
Y
= -
-
U 14
a
13
12
694 695 696 697 698 699 700 701 702 703
x coordinate (km)
0 1 2 3 4 5 6
Flood Currents - Existing, magnitude (ftls)
Figure 4-4: Instantaneous peak flood current velocities —existing condition
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 49 of 70
_ rrrr/rr
cg
g'-
a
14ssi$s�
13
12
694 695 696 697 698 699 700 701 702 703
x coordinate —>
0 1 2 3 4 5 6
Flood Currents - Template 1, magnitude (fUs)
Figure 4-5: Instantaneous peak flood current velocities —after-dredge Template 1
16
I
-
irJJJ/�
T �JII/J
- -
Jr
i
O
' S { r 335
T
?'
tf :.
13
12
RON
694 695 696 697 698 699 700 701 702 703
x coordinate (km)
0 1 2 3 4 5
6
Flood Currents - Template 2, magnitude (fUs)
Figure 4-6: Instantaneous peak flood current velocities — after -dredge Template 2
201912020 Renourishment Project
M&N Project No.10128-01

17
16
15
T
is
-2 14
`0
0
V
13
12
11
694 695 696 697 698 699 700 701
x coordinate -�
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
702 703 704
-3 -2 -1 0 1 2 3
Flood Currents - (Template 1 - Existing), magnitude (fUs)
Figure 4-7: After -dredge bathymetry effects on instantaneous peak flood current velocities —
Template 1
17
16
T 15
E
Y_
N
14
`0
0
V
a 13
it
11
694 695 696 697 698 699 700 701
x coordinate (km) -+
702 703 704
-3 -2 -1 0 1 2 3
Flood Currents - (Template 2 - Existing), magnitude (fl!s)
Figure 4-8: After -dredge bathymetry effects on instantaneous peak flood current velocities —
Template 2
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 51 of 70
4.1.2 Residual Tidal Currents
Residual tidal currents over a spring -neap tidal cycle is the "net" flow that remains after
subtracting the flood flow vectors from the ebb flow vectors. The residual tidal current
pattern is an indicator of potential net movement of sediment over a tidal cycle. In
DeIft3D, the residual currents are calculated based on Fourier analysis for the current
velocities over a specified period.
Figure 4-9 to Figure 4-11 presents the residual tidal currents under the existing and the
two after -dredge templates, respectively. The difference of residual tidal currents are
shown in Figure 4-12 and Figure 4-13 for Template 1 and 2, respectively. The model
results indicate the two after -dredge bathymetry templates could cause negligible
residual tidal current increase (less than 0.05 ft/s).
17
15
0 14
T
'f
13
, ti:
694 695 696 697 698 699 700 701 702 703
x coordinate
0 0.2 0.4 06 0.8 1 1.2 1.4 1-6 1.8 2
Residual Currents - Existing (ff/s), magnitude
Figure 4-9: Residual tidal currents — existing condition
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 52 of 70
17
15
-
_ c
-= v
CU
a 14
•'•x
T
13
12
694 695 696 697 698 699 700 701 702 703
x coordinate
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Residual Currents - Template 1 (ftls), magnitude
Figure 4-10: Residual tidal currents — after -dredge Template 1
Figure 4-11: Residual tidal currents — after -dredge Template 2
201912020 Renourishment Project
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 53 of 70
17
�V
0
�'
C� Q
�
16
15
r
a
14
L�
o
U
�
T
13
12
11
694 695 696 697 698 699 700 701 702
703 704
x coordinate -4
-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15
0.2 0.25
Residual Currents - (Template 1 - Existing) (fVs), magnitude
Figure 4-12: After -dredge bathymetry effects on residual tidal currents — Template 1
16
15
�d
?
?3
Ca
P
}
oil
0
14
8
N,
a
13
12
11
694 695 696 697 698 699 700 701 702
703 704
x coordinate
425 42 -0.15 41 405 0 0.05 0.1 0,15
0.2 0,25
Residual Currents - (Template 2 - Existing) (ftls), magnitude
Figure 4-13: After -dredge bathymetry effects on residual tidal currents —Template 2
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Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 54 of 70
4.2 WAVES
As stated previously, there were concerns that any potential nearshore wave climate
changes caused by the project could affect the adjacent shorelines. For this study, a
representative wave approach was adopted to investigate this concern.
4.2.1 Representative offshore waves
The offshore wave data at the NOAA buoy 41013 from 2004 to 2018 was the primary
source for deriving the representative input wave conditions. The data gaps in the buoy
data were filled with available USACE Wave Information Studies (WIS) hindcast data and
NOAA WW3 hindcast data at locations close to 41013. The WIS hindcast data were only
available to 2014, so WW3 data were used to fill the data gaps afterwards. The
combined wave data were in an hourly time interval. Figure 4-14 shows the annual
percentage of exceedance of the significant wave height from the combined offshore
wave data. The annual mean significant wave height at the offshore location is about
4.4 ft. Figure 4-15 plots the wave rose for the significant wave height from the
combined wave records at offshore. It indicates that the dominant wave direction in
the offshore region of the project area is from the ESE. Wave heights less than 6 ft
comprise about 80% of the 15-year record.
Station 41013_comb_NDBC_WIS_WW3
Percent Exceedance (Annual)
100
N = 131478
µ=4.42,a=2.24
mode = 2.70
75
2.84
0
U
a
50
3.86
aD
U
X
W
25
5.44
10
7.38
1
8.73
0 1 2 3 4 5 6 7 8 9 10 11
12 13 14 15
Significant Wave Height, ft
Figure 4-14: Annual percentage of exceedance of significant wave height at the offshore boundary
201912020 Renourishment Project
M&N Project No.10128-01

Total
L
•CD 15
2
ro
12
c
9
6
W
in 3
0
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 55 of 70
Significant Wave Height (Annual)
Station 41013 comb NDBC WIS WW3
Period 01-Jan-2004 to 31-Dec-2018
N
NW, - - \ NE
20%
15% \
I \ �
! , , 10% v
i f f 1 I I
WI------L 0.00 -L- -- E
1 i L f
4 1 \
\
I /
SW v _ SE
\ � I
� I �
S
n
Ez15
❑ 12 - 15
�9-12
❑6-9
r73-6
No-3
Direction FROM is shown
Center value indicates calms below 0 ft
Total observations 131478, calms 0
About 0.01440% of observations missing
Percentage of Occurrence
0.60 1.77
4.31
9.39
12.76
17.84
14.62
9.15
9.75
8.62
6.16
1.66
0.99
0.95
0.72 0.72
100.00
0.23
0.10
0.11
0.11
0.12
0.13
0.80
0.15
0.55
0.33
0.22
0.34
0.37
0.49
0.37
0.24
0.12
3.34
0.36
1.14
2.56
1.20
1.45
1.34
1.44
1.88
1.32
1.05
0.33
0.30
0.24
0.18 0.19
15.07
0.42 1.17
2.57
4.53
5.91
8.30
7.23
4.55
5.33
5.10
3.74
0.85
0.47
0.52
0.42 0.43
51.5
0.22
0.44
1.64
5.17
7.73
5.56
2.67
1.89
1.74
1.10
0.36
0.16
0.14
29.03
N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW Total
Figure 4-15: Wave rose of significant wave heights at the offshore boundary
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 56 of 70
In order to derive representative wave conditions, the 15-year wave record was sorted
by peak wave direction and significant wave height. The sorting routine contained 24
direction bins (15 degrees each) and nine significant wave height bins (1 m each). Only
waves which would reasonably be expected to affect the project shorelines were
considered specifically including waves originating from between East (90 degrees) and
West (270 degrees) azimuth. This resulted in 86 wave cases used as model input and
which represent approximately 75.4% of the 15-year record by occurrence (waves from
east to north to west were excluded). The average wave parameters were calculated in
each wave case. Table 4-1 lists the characteristics of each wave case as they were
applied to the wave modeling.
Table 4-1: Representative wave conditions used as model inputs
MWD-Ibin
0-1 90 -105
Bin average sig.
2.5
Bin average peak
periodHs-bin
9.0
Bin average Wave Percentage
Occurrence
97.7 4.854
1-2
90 - 105
4.4
9.5
98.0 3.973
2-3 90 - 105 7.8
3-4 90 - 105 11.3
10.1
11.8
97.3 0.635
97.1 0.164
4-5
90 -105
14.2
12.4
98.0
0.054
5-6
90 - 105
17.5
13.9
99.0
0.016
6-7
90 -105
20.7
13.1
98.0
0.002
0-1
105 -120
2.4
8.9
112.5
6.297
1-2
2-3
105 - 120
4.4
9.4
112.4
112.8
5.030
0.714
105-120
7.7
9.6
3-4
105 -120
11.3
10.9
112.2
0.129
4-5
105 -120
14.1
12.2
112.0
0.038
5-6
105-120
17.6
11.2
115.9
0.005
6-7
105 -120
20.7
12.3
115.8
0.002
7-8
105 -120
23.3
15.3
115.1
0.002
0-1
120 -135
2.5
8.6
126.9
5.573
1-2
120 -135
4.4
9.0
127.3
4.728
2-3
120 -135
7.7
9.6
127.1
0.789
3-4
120 -135
11.1
10.1
128.1
0.135
4-5
120 -135
14.4
10.2
126.9
0.035
5-6
120-135
18.0
11.3
128.7
0.010
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6-7
120 -135
20.2
12.2
130.1
0.002
8-9
120 -135
26.8
14.8
128.6
0.002
0-1
135-150
2.5
8.0
141.6
3.391
1-2
135 -150
4.5
8.3
142.0
3.696
2-3
135 -150
7.8
8.9
142.5
0.646
3-4
135 -150
11.3
9.9
142.2
0.193
4-5
135 -150
14.1
10.4
142.1
0.054
5-6
135-150
18.3
11.1
142.9
0.011
6-7
135 -150
20.2
12.3
142.6
0.003
7-8
135 -150
25.2
15.9
141.2
0.002
8-9
135-150
27.6
14.8
143.3
0.001
0-1
150 -165
2.6
7.1
156.9
2.225
1-2
150-165
4.6
7.4
157.3
2.810
2-3
150 - 165
7.8
8.1
157.7
0.739
3-4
150 - 165
11.0
9.2
157.3
0.174
4-5
150 -165
14.6
9.7
157.6
0.035
5-6
150 -165
17.4
11.1
154.1
0.007
6-7
150 -165
20.5
11.9
154.8
0.003
7-8
150 -165
23.9
13.0
159.0
0.001
0-1
165 - 180
2.7
6.1
172.3
1.770
1-2
165-180
4.6
6.7
172.6
3.194
2-3
165-180
7.8
8.0
172.5
1.012
3-4
165 -180
11.1
9.0
172.9
0.204
4-5
165 -180
14.3
9.6
173.7
0.029
5-6
165 -180
17.6
11.2
169.7
0.004
6-7
165 -180
20.7
12.0
175.7
0.004
7-8
165 -180
25.8
13.8
169.7
0.002
8-9
165 -180
26.8
14.2
170.8
0.002
0-1
180 -195
2.7
5.5
187.0
1.607
1-2
180 -195
4.5
6.4
187.2
3.474
2-3
180 -195
7.9
8.0
186.7
1.063
3-4
180 -195
11.2
9.2
186.9
0.232
4-5
180 -195
14.2
10.0
186.9
0.050
5-6
180 - 195
17.6
11.2
186.6
0.005
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Jay Bird Shoals Borrow Area Modeling
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Page 58 of 70
6-7
180 -195
20.2
12.8
183.0
0.001
0-1
195-210
2.7
5.1
202.1
1.613
1-2
195 - 210
4.5
6.0
202.4
3.239
2-3
195 - 210
7.8
7.6
201.7
0.727
3-4
195-210
11.1
8.9
201.9
0.189
4-5
195-210
14.3
9.4
201.9
0.040
5-6
195 - 210
17.0
10.0
199.6
0.003
0-1
210-225
2.7
4.9
216.8
1.319
1-2
210-225
4.6
5.8
217.1
3.141
2-3
210 - 225
7.7
7.2
217.4
0.666
3-4
210-225
11.0
8.3
217.9
0.115
4-5
210-225
14.2
9.2
215.3
0.015
5-6
210-225
16.8
8.3
219.7
0.001
0-1
225 - 240
2.6
4.6
231.3
0.688
1-2
225 - 240
4.6
5.5
230.8
1.609
2-3
225 - 240
7.8
7.0
231.2
0.367
3-4
225 - 240
10.8
8.3
231.0
0.071
4-5
225 - 240
14.2
9.2
228.9
0.007
5-6
225 - 240
17.4
8.8
231.2
0.005
0-1
240 - 255
2.6
4.9
246.5
0.301
1-2
240 - 255
4.7
5.5
246.3
0.539
2-3
240 - 255
7.9
6.7
246.4
0.190
3-4
240 - 255
10.8
7.4
246.9
0.039
4-5
240 - 255
13.5
7.5
249.3
0.002
5-6
240 - 255
17.8
8.6
248.0
0.001
0-1
255 - 270
2.6
4.8
261.3
0.169
1-2
255 - 270
4.7
5.4
262.0
0.321
2-3
255 - 270
7.8
6.3
262.3
0.168
3-4
255 - 270
10.7
6.9
261.3
0.040
4-5
255 - 270
15.0
8.2
259.0
0.002
5-6
255 - 270
17.9
8.3
263.5
0.002
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Jay Bird Shoals Borrow Area Modeling
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Page 59 of 70
4.2.2 Nearshore Wave Results
Each of the 86 wave conditions listed in Table 4-1 were run for the existing bathymetric
condition and of the two after -dredge bathymetry templates. Winds and water levels
were not included in these model runs. For each discrete wave condition, the spatial
map of significant wave height (after -dredge Hs — existing Hs) was calculated. It is
expected and confirmed by the model results that nearshore waves would decrease
leeward of the Jay Bird Shoals borrow area due to wave refraction caused by the
excavated borrow area. At the same time nearshore waves could increase slightly on
both the east and west side of the borrow area. Some results from the 86 wave
conditions are presented below, all wave condition model results are included in
Appendix C1 and C2 for Template 1 and Template 2, respectively.
Figure 4-16 presents the model results for representative waves in the range of 0 — 3 ft
originating from Southeast (SE), South (S), and Southwest (SW). The waves in this range
comprise about 30% of the 15-year record. The average wave height is about 2.5 ft.
The two after -dredge bathymetry templates show that effects from these small wave
conditions are negligible. Vectors represent the modeled wave directions from the two
after -dredge bathymetry templates.
Figure 4-17 presents the model results for representative waves in the range of 3 — 6 ft
originating from SE, S, and SW. The waves in this range comprise about 50% of the 15-
year record. The average significant wave height is about 4.5 ft which is approximately
the annual average wave conditions in the offshore area. The two after -dredge
bathymetry templates could cause about 3 inches of wave height increase in highly
localized areas offshore of the shoreline.
Figure 4-18 shows the model results for representative waves in the range of 6 — 9 ft
originating from SE, S, and SW. The waves in this range comprise about 15% of the 15-
year record. The average significant wave height is about 7.5 ft. The two after -dredge
bathymetry templates induced show wave changes are mostly less than 0.5 ft.
Figure 4-19 shows the model results for storm waves originating from SE, S, and SW.
During Hurricane Matthew in 2016, significant wave height of 21 ft was observed
offshore. Similar to the model results under more frequent normal wave conditions, the
two after -dredge bathymetry templates could cause wave reduction leeward of the
borrow area and wave increases on both east and west sides. The magnitude of wave
change is mostly less than 1 ft in localized areas.
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16
E 15
14
kk 13
12
894 696 BBB 897 SOS 699 700 701 702 703
x d dinere{km1 —
.2 -1.5 -1 -0.5 0 0.5 1 1.5 2
5,9,R W Wave Height Odle— (Template 1 - Existing) (R)
17
16
? 15
E
p 14
kp 13
12
894 896 BBB 697 698 699 700 701 702 70
x doaldinare {km) —
In
694 695 696 697 698 699 700 701 702 703
x coordinate (km) —>
-2 -1 5 -1 -0.5 0 0.5 1 1.5 2
Sig,f.M We- Height DiRerenee (Templsle 1 - Ddsting) (R)
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 60 of 70
17
SW 2.5ft
incoming
wave
,
15
13
12
894
898
696
697
696
699
700
701
702 703
xfA dr te(km)
r
-1 -0.5 0 0.5 1 1.5 2
SigniticaM Wave Height pilfer (Template 1 - Existing) (R)
17
16
? 15
E
p 14
pd 13
12
894 898 696 697 698 899 700 701 702 703
xc dmate(km)-r
i
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 -2 -1.5 -1 -0.5 0 0.5 1 15 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Significant Wave Height D4lerenw ITemplate 2- Existing) (R) Sigm=rh Wave Height Meereroe (Template 2 - Existing) (R) SigmticaM Wave Height E),%.— (Template 2 - Existing) (R)
Figure 4-16: After -dredge bathymetry effects on waves between 0 — 3 ft with average height of 2.5 ft (top: Template 1; bottom: Template 2)
201912020 Renourishment Project
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In

17 S�sft mtn wave.. g wave -
14 1�}}"t`1r1S1t�4t1ty41114``�4y�4`Y`k4 (�` �l' r�)
13
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12 4 1 �} y�kll4Ykk4kl`Y4kYl}kLY4lYYtYY $t4`yY`4�yy"y�'4": ti,,, t { } V f .
4 l ) 1 ; 5 444k i S i �ii',';,•�:i ,', � 4 _
894 896 BBB 697 B98 699 700 701 702 703
x -din (km) —
-2 -1.5 - -0.5 0 0.5 1 1.5 2
Significant Wave Heigh[ rifle— (Template 1 - Existing) (R)
m
t 15
E
R 14
iS 13
694 695 696 697 698 699 700 701 702 703
x coordinate {km) —>
-2 -1 5 -1 45 0 0.5 1 1.5 2
Significant Wale Height hilt— (Template 1 - F ''sting) (11)
In
MY 719/1 ME FEN I MP
17
16
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 61 of 70
894 896 Bs$ 597 896 699 700 701 702 703
x could -le (km) -•
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
5191ificaW Wave Height l3iftem as (Template 1 - Existing) (R)
SO4
896 Bs8 697 696 899 700 701 702 703
x cohldin (km)--,
-2 -15 - -0.5 0 0.5 1 1.5 2 -2 -i.5 -1 -0.5 0 0.5 1 15 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Significant Wave Height title.— (Template 2 - Eaialing) (R) Sig,dx a R Wale Height MOerexe (Template 2 - Existing) (R) 5ignificam! Wave Height E),% — (Template 2 - Existing) (R)
Figure 4-17: After -dredge bathymetry effects on waves between 3 — 6 ft with average height of 4.5 ft (top: Template 1; bottom: Template 2)
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1e
? 15
E
14
13
12
894 896 BBB 697 898 699 700 701 702 703
x ooardinate(kml �
-2 -1.5 - -0.5 0 0.5 1 1.5 2
Significant Wave Height DiOer (Template 1 - Existing) (n)
17
1e
T 75
E
p 14
pd 13
12
e94 896 BBB 697 e9e 699 700 701 702 703
x—rdinete (kml �
m
694 695 696 697 698 699 700 701 702 703
x coordinate (km)
-2 -1 5 -1 -0.5 0 0.5 1 1.5 2
SignificaM wave Height Drffe—e (Template 1 - Basting) (8)
In
Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 62 of 70
1E
In
694 695 696 697 698 699 700 701 702 703
xc dinaia(km)->
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Sig,i—M Wave Height Difference (Template 1 - F 'sblg) (ti)
m
12 y�{TY�5g�4�'�I� �'�""+�4 l'F '' Az��j�I/���f���/�lfl.�f�j�f
Y r<r r 1 x :11/lll�i/li il���f� >� �ll.Gft f.U111�
694 695 696 697 696 699 700 701 702 703 694 695 696 697 698 599 700 701 702 703
x coordinate (km) - x coordinate (km) -
-2 -1 5 - -0.5 0 0.5 1 1.5 2 -2 -1.5 -1 -0.5 0 0.5 1 i 5 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Significant Wave Height Di$er (Template 2 - Exialing) (0) SignificaM Wave Height MOerexe (Template 2 - Exisfing) (R) SignificaM Wave Height Difference (Template 2 - Exisfing) (ff)
Figure 4-18: After -dredge bathymetry effects on waves between 3 — 6 ft with average height of 7.5 ft (top: Template 1; bottom: Template 2)
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894 695 3W 897 998 ego 700 701 762 703
x o dinete (km) �
.2 -1.5 -1 -0.5 0 0.5 1 1.5
SigMficenl Wave Height Ditlereiwa (pemplate 1 - Existing} tR)
11
�y9�fyyY4444147�y47r,✓r,"',i""r'✓f4'iyyy!{''i�111i�ti'�1'
}4.45 t r.r tR�,�,E1tF a?,��✓,i.,?!d7�r!J�r>'7 r�,4t'.e4�64{i��l;�.%/�I !✓/�iii$'i�5��lf llll, f .t !
694 695 696 697 696 699 700 701 702 703
x c—d—te (km) ,
2 2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Slgnilicant Wave Haight Oitterence (Template 1 - Ex h,g) (A)
T
5
tl
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 63 of 70
694 695 696 697 698 699 700 701 702 703
a coordinate {km) —>
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Signili-M Waue Height Diflerenee (Template 1 - Ewsting) (fl)
12 SS}•iti'Si'i}•1i1}1E44Qf+�1�i4'}t4+S;.SS,SS141i44i1'/d'�i'�•�•'�.:�;.;�������+�j�,e �'i1a 4ygif4'}'yf�tlryi41')'15�✓'r1,�rN�ri"`"Y,✓'r'§iiS°iJ{'ll�'11`illi''�'/�y����%/1'%'lrl
i4,t44,1�ti1Il,1�,14d tS.Slit}!4 titi,iS i (4.}4.4�li al.i{�t11 y j P216 , 9,Fi�Yrtrli a:.:+S 1i111 5 4iR4t4�, ;tfiFR:4 � 1 �i5l di �tr 4 l:S�e4 L{l{} ��flr`�J/fviii P �rf . (lf, � r� i'Jl /r f���f /ll l ir!all i}%lf!/����%r/dflTffU711
894 695 3W 897 698 699 700 701 762 703 094 695 696 697 898 899 700 701 702 703 694 695 696 697 698 699 700 701 702 703
x o di.te (km) x coordinate (km) —, x coordinate (km) —>
-2 -1.5 -1 -0.5 0 0.5 1 T 5 2 .2 -1.5 -1 '0.5 0 0.5 1 1.5 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Sig,i ce Wave Height Ditlererua (Template 2 - Existing) (fl) $IgMlicant Wave Haight OiHam—(Template 2 - ExlsOng) (fl) SignifiwM Wave Height DIfler— (Template 2- E)lsting) (fl)
Figure 4-19: After -dredge bathymetry effects on storm waves comparable to Hurricane Matthew in 2016 (top: Template 1; bottom: Template 2)
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 64 of 70
4.3 SEDIMENT TRANSPORT
Based on the model results demonstrated in section 4.1.2, it is reasonable to conclude that the
two after -dredge bathymetry templates have negligible effects on the residual tidal currents,
and thus upon the associated sediment transport processes along the Caswell Beach shoreline
due to tidal currents. Therefore, only wave induced sediment transport was considered for this
analysis.
For each of the 86 representative wave conditions in Table 4-1, the wave induced longshore
currents and associated sediment transport were estimated by coupling DeIft3D-FLOW and
DeIft3D-WAVE modules using only the fine wave model grid for the existing and the two after -
dredge bathymetric templates. There were no tide and wind inputs, and no morphology
update. A uniform median sediment grain size of 0.25 mm was assumed. No sediment
transport calibration effort was made due to lack of measured data.
The sediment transport rates through shore -normal transects along the Caswell Beach
shoreline (Figure 4-20) were extracted from the model results under each wave condition; and
were then subsequently weighted by the percent occurrence of each wave condition to
formulate the average annual potential sediment transport. Modeled sediment transport
inside the surf zone is greatly influenced by the imposed model bathymetry. Thus, the model
results represent only the bathymetric condition constructed based on the available data
sources listed in Table 2-1. In reality, the beach bathymetry tends to be smoothed out by
waves. Since this sediment transport study is not a morphological model, the sediment
transport results were smoothed through a 0.5 mile moving average.
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Jay Bird Shoals Borrow Area Modeling
September 6, 2019
Page 65 of 70
Figure 4-20: Caswell Beach transects
Figure 4-21 presents the modeled average annual "net" longshore sediment transport rates
along the Caswell Beach shoreline for both the existing and the two after -dredge templates.
Positive values represent westerly sediment transport direction. The model results indicate
potential sediment transport rate reduction leeward of the borrow area, and potential
sediment transport rate increases along both east and west shoreline segments away from the
borrow area.
The net longshore sediment transport gradients along the Caswell Beach shoreline are shown in
Figure 4-22. The net longshore sediment transport gradient is calculated as dQ/dx where dQ is
the transport rate differential between neighboring transects and dx is the alongshore distance
between transects. The transport gradient is a proxy to potential shoreline changes. Positive
and negative values in Figure 4-22 indicate potential localized adjustments in shoreline
accretion and erosion, respectively.
Based on the model results, it would appear that areas of concern for potential increases in
shoreline erosion would be limited to discrete portions of Caswell Beach (between survey
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Page 66 of 70
transects 37+00 — 60+00 and 150+00 — 170+00). Potential effects on shoreline erosion in other
areas seem to be minimal and some areas may experience increased shoreline accretion.
Generally, both templates show results close to existing conditions, with some areas showing
transport rates above and below existing conditions. There is no strong evidence to choose one
template over the other given the model results, especially given that this is not a
morphological model. The modeled sediment transport inside the surf zone is greatly
influenced by the imposed model bathymetry. Thus, the model results only represent the
bathymetric condition constructed based on the available data sources. There will be an
additional 0.6 mcy beach compatible material available in Template 1. For this reason,
Template 1 was chosen for the Town of Oak Island's permit application for the 2019/2020
Renourishment Project. The Town of Oak Island will monitor the Caswell Beach shoreline for
nine (9) years post -project to investigate any potential effects which might require mitigation.
200
150
100
50
0
-50
100
+ Caswell Beach Transects
I I I
I I I
—Net Transport - Existing
—Net Transport - After -Dredge Template 1
—Net Transport - After -Dredge Template 2
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I
I I I I I I
I I I I I
I I I I I I I
I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I
+ Westerly Transpdrt
00000 l000000 000000 0o00ooi0000000i000 00 l000 I I
00000
0th 00N �0N�[�LI00N0N 0N 0N 0N000000
00000 00
N 0N 0rl000l000000000 00�000��0+1+O+O10001�0 000
N 0�0'II00O 1O 10nOA0,Ol 0,0,0'r,OIOI0,:0, IONO' '0'JONO+++ i++ +1+ +++t+++
.CommMNoo,
NN N0.1~11mmm o.'.�-
N
- J I
I I I I I I
;Jay Bird Shoal l orrow Site
I I I I I I I I
Easterly Transpoq
I I I I I I
I I I I I ----- ---.
I I I I I
I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
692000 693000 694000 695000 696000 697000 698000 699000 700000 701000
Easting - NC State Plan (m)
Figure 4-21: Wave -induced net longshore sediment transports along Caswell Beach shoreline
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Town of Oak Island
1/11
100
80
60
a
r 40
U
c
a
20
O
Q 0
C
C
E -20
a
ai
o -40
L
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+ Caswell Beach Transects
—Transport Gradient - Existing
—Transport Gradient -After-Dredge Template 1
—Transport Gradeint- After -Dredge Template ---------;------------------;------------------
------------------------------------------------------------------------
_________,_________1_________ L_____ _ ________ _______1 _____L_________I_fm,
___
O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O I
Oo00oo,000000,000000Qoo 00 000000 0 00 0 000 000000Q
++++++++++++ +++++++++ +++ ++++++ + +++ ++ o0 000000
ONO M0 W 0M ON OM MOM OM0lh ON 0M 0MOM0 it N NO NO ++ + +++++
100 mm m W1,10 m N 't MNN H 0 0m mca I'W om �vt N 00 O NO1 ON ON ON
M ['!lM NN NN NN NN NN NNNv N Nr,'i.-,rl ei ei rl ei ei 'i ei 'i a100 DPI, I,W tom Vl�
1
' - ' - -
I I I I I I I
I I I I I I I
I I I I I I
I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
,Jay Bird Shoal borrow Site
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
I I I I I I I I
692000 693000 694000 695000 696000 697000 698000 699000
Easting - NC State Plan (m)
Figure 4-22: Longshore sediment transport gradients along Caswell Beach shoreline
700000 701000
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5. SUMMARY AND CONCLUSIONS
In order to investigate the potential effects of the Jay Bird Shoals borrow area identified for the
2019/2020 Renourishment Project on the neighboring shorelines of Caswell Beach and Bald
Head Island, numerical models were developed for hydrodynamics, waves, and sediment
transport using Deltares' DeIft3D model suite. The hydrodynamics and wave models were
successfully calibrated and validated against available observed water levels, currents,
discharges, and wave data. Sediment transport calibration was not conducted, due to lack of
measured data to calibrate against.
Tidal current, wave, and sediment transport modeling were performed for the existing and two
after -dredge bathymetric templates. The maximum borrow area dredge scenarios were
considered, i.e. assuming to remove the full 2.95/2.34 million cubic yards of available material
identified as beach compatible in Template 1 and 2, respectively. Only part of the material,
1.034 million cubic yards, will be dredged for the 2019/2020 Renourishment Project. Thus,
within the proposed borrow area, the results from the DeIft3D model are considered to be a
conservative overestimate of the potential effects on tidal current and wave climates.
The model results were analyzed to determine potential effects of the two after -dredge
bathymetric templates. The findings are:
• The two after -dredge bathymetric templates show that effects on tidal currents would
be localized and small, which implies no significant effects upon sediment transport
processes associated with tidal currents;
• The two after -dredge bathymetric templates could reduce waves leeward of the borrow
area; however, it could slightly increase nearshore waves on both east and west sides of
the borrow area in localized areas;
• and similarly, the two after -dredge bathymetric templates could reduce the wave -
induced longshore sediment transports leeward of the borrow area but could also cause
longshore sediment transport increases on shoreline segments both east and west sides
of the borrow area. The net effect of these changes could result in localized
adjustments in shoreline erosion / accretion. Based on the model results, it would
appear that most of the potential increases in shoreline erosion would be limited to
discrete portions of Caswell Beach (between survey transects 37+00 —60+00 and 150+00
— 170+00). Potential effects in other areas seem to be minimal. Generally, both
templates show results close to existing conditions, with some areas showing transport
rates above and below existing conditions. There is no strong evidence to choose one
template over the other given the model results, especially given that this is not a
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morphological model. The modeled sediment transport inside the surf zone is greatly
influenced by the imposed model bathymetry. Thus, the model results only represent
the bathymetric condition constructed based on the available data sources.
• Given that there will be an additional 0.6 mcy beach compatible material available in
Template 1; this is the chosen scenario for the Town of Oak Island's permit application
for the 2019/2020 Renourishment Project. The Town of Oak Island will monitor the
Caswell Beach shoreline for nine (9) years post -project to investigate any effects
predicted by the model which might require mitigation.
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Page 70 of 70
6. REFERENCES
Benoit, M., Frigaard, Peter, and H.A. Schaffer (1997) "Analyzing Multidirectional Wave Spectra",
Proceedings of the 27th IAHR Congress, San Francisco, 10-15 August 1997, IAHR Seminar
"Multidirectional Waves and their Interaction with Structures", Mansard, Etienne (ed.),
Canadian Government Publishing, Benoit.
Deltares (2018a), "Delft3D-FLOW, Simulation of multi -dimensional hydrodynamic flows and
transport phenomena, including sediments, User Manual".
Deltares (2018b), "Delft3D-WAVE, Simulation of short -crested waves with SWAN, User
Manual".
Earle, MA K.E. Steele, and D.W.C. Wang (1999), "Use of advanced directional wave spectra
analysis methods", Ocean Engineering, Volume 26, Issue 12, December 1999, Pages 1421-1434.
Egbert, Gary D., and Svetlana Y. Erofeeva (2002). "Efficient inverse modeling of barotropic
ocean tides." Journal of Atmospheric and Oceanic Technology 19.2 (2002): 183-204.
RPS Evans -Hamilton (2017). "Cape Fear current, water Level and water quality study".
Willmott, C.J., S.G. Ackleson, R.E. Davis, J.J. Feddema, K.M. Klink, D.R. Legates, J. O'Donnell, and
C.M. Rowe (1985), "Statistics for the evaluation and comparison of models". Journal of
Geophysical Research, 90(C5), 8995-9005.
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