Appendix D - JB Shoals Borrow Area Modeling

APPENDIX D
JAY BIRD SHOALS BORROW AREA MODELING

2021/2022 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
C Modeling report June 12, 2020 ZW KF JM

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page i

EXECUTIVE SUMMARY
In order to investigate the potential effects of dredging material from a Jay Bird Shoals
borrow area identified for the 2021/2022 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’ Delft3D model
suite. The hydrodynamics, wave, and morphology models were successfully calibrated
and validated against available observed water levels, currents, discharges, wave, and
channel shoaling data.
Tidal current, wave, and sediment transport modeling was performed for the existing and
two after-dredge bathymetry scenarios (Template 1 and Template 2). Each template was
divided into three zones, each zone has its own unique dredge elevation. The zones with
varying dredge elevations were intended to cause minimal disruption to the natural shoal
environment. Improvements to the borrow area template that was approved and
permitted for the 2020/2021 Renourishment Project (Template 2) were considered to
ensure dredging could be completed efficiently and effectively for the 2021/2022
Renourishment Project. These improvements were implemented in Template 1. Template
1 provides an additional 4 ft of dredging depth with a 2 ft overdredge depth allowance
(total 6 ft) from what was permitted in Zone 2 of Template 2. The dredge depth in Zone 2
was increased to a deeper depth to provide additional volume since it is the most offshore
in the shoal environment. Template 1 also incorporated a 2 ft overdredge depth
allowance in Zones 1 and 3 from what was permitted in Template 2.
Template 1 would contain 4.67 million cubic yards (mcy) of beach compatible material
and Template 2 (which was permitted and approved for the 2020/2021 Renourishment
Project) contains 2.95 mcy. Template 1 was developed to ensure that enough material
would be available for the 2021/2022 Renourishment project after the completion of the
2020/2021 Renourishment Project. Assuming 1.1 mcy is needed for the 2020/2021
Renourishment Project and 1.667 mcy is needed for the 2021/2022 Renourishment
Project this would mean 2.767 mcy is needed to complete both projects.
For Template 1 the amount of material available to be removed cost effectively (not
counting the volume associated with 2 ft of overdredge to account for dredging
inaccuracies) is 3.69 mcy of the 4.67 mcy available. For Template 2, the amount of
material available to be removed cost effectively is 2.08 mcy of the 2.95 mcy available.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page ii

Recall that 2.803 mcy is the volume needed to complete both projects. This is why the
originally permitted Template 2 was modified to result in Template 1 to ensure enough
quantity would be available for completion of the 2021/2022 Renourishment Project.
The maximum dredging scenario was considered for both templates, i.e. assuming to
remove all the available material identified as beach compatible (4.67 mcy and 2.95 mcy
for Template 1 and 2 respectively). This assumption is conservative since for each
template the dredge cannot remove this entire quantity of material in a cost-effective
manner. Thus, within the proposed borrow area, the results from the Delft3D 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 the improved Template 1 scenario, effects
on residual tidal currents would be localized and small, similar to the originally permitted
Template 2 scenario. This implies there would be no significant effects on sediment
transport processes associated with tidal currents implementing changes in depths for
Template 1. The figure below shows the effects of the improved template (Template 1)
and originally permitted template (Template 2) on residual tidal currents over a spring-
neap tidal cycle.

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 both after-dredge bathymetry templates could result in a slight
redistribution of wave energy along the shoreline during moderate to severe storm
events.
Sediment transport analyses were also completed, to observe if the changes to wave
heights and wave directions would affect the longshore transport. The sediment

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page iii

transport 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. Potential effects on shoreline erosion in other areas are minimal
although some areas may experience increased shoreline accretion. Based on the 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 – 185+00). 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 that the
improvements made to Template 2 in order to provide additional volume and efficiency
for completing the 2021/2022 Renourishment Project as shown in Template 1 would
cause any more significant impacts given the results, especially given that this is not a
morphological model. The sediment transport inside the surf zone is greatly influenced
by the imposed model bathymetry. Thus, the results only represent the bathymetric
condition constructed based on the available data sources.
In order to efficiently and effectively complete the 2021/2022 Renourishment project,
Template 1 will be used to allow for additional volume and efficiency given the dredging
process inaccuracies. The Town of Oak Island will monitor the Caswell Beach shoreline for
three (3) years post-project to investigate any potential effects which might require
mitigation.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page iv

Wave-induced longshore sediment transport gradients along Caswell Beach shoreline

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page v

TABLE OF CONTENTS
1. INTRODUCTION ...................................................................................................... IX
2. MODEL DEVELOPMENTS ......................................................................................... 2
2.1 Model Grids ....................................................................................................... 2
2.1.1 Flow Model Grids ....................................................................................... 2
2.1.2 Wave Model Grids ..................................................................................... 3
2.2 Model Bathymetry ............................................................................................ 4
3. MODEL CALIBRATIONS ............................................................................................ 7
3.1 Calibration Metrics ............................................................................................ 7
3.2 Flow Model Calibration ..................................................................................... 8
3.2.1 Boundary Conditions ................................................................................ 13
3.2.2 Calibration Results ................................................................................... 14
3.3 Flow Model Validation .................................................................................... 22
3.4 Wave Model Calibration .................................................................................. 23
3.4.1 Model Inputs ............................................................................................ 23
3.4.2 Calibration Results ................................................................................... 26
3.5 Wave Model Validation ................................................................................... 33
3.6 morphological Model calibration .................................................................... 37
3.6.1 Tide Schematization ................................................................................. 38
3.6.2 Wave Schematization .............................................................................. 38
3.6.3 Morphological Time Scale Factor (morfac).............................................. 44
3.6.4 River Flows ............................................................................................... 45
3.6.5 Sediments ................................................................................................. 45
3.6.6 Model Calibration Results ........................................................................ 46
4. JAY BIRD SHOALS BORROW AREA MODELING ..................................................... 53
4.1 Tidal Currents .................................................................................................. 56
4.1.1 Peak Tidal Flood Currents ........................................................................ 56
4.1.2 Peak Tidal Ebb Currents ........................................................................... 59
4.1.3 Residual Tidal Currents ............................................................................ 62
4.2 Waves .............................................................................................................. 65
4.2.1 Nearshore Wave Results .......................................................................... 65
4.3 Sediment Transport ......................................................................................... 71
5. SUMMARY AND CONCLUSIONS ............................................................................ 76
6. REFERENCES .......................................................................................................... 78

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page vi

LIST OF FIGURES
Figure 1-1: Jay Bird Shoals borrow area .................................................................. 1
Figure 2-1: Flow model grids ................................................................................... 3
Figure 2-2: Wave model grids ................................................................................. 4
Figure 2-3: Flow model bathymetry under existing conditions .............................. 5
Figure 2-4: Fine wave model bathymetry under existing conditions ..................... 6
Figure 3-1: Locations of water levels and current measurements by RPS EH ........ 9
Figure 3-2: Survey transects in Upper Wilmington area by RPS EH ...................... 10
Figure 3-3: Survey transects in Lower Wilmington area by RPS EH ...................... 11
Figure 3-4: Survey transects in Snow’s Cut area by RPS EH .................................. 12
Figure 3-5: Survey transects in Southport area by RPS EH ................................... 13
Figure 3-6: Water level calibration results ............................................................ 16
Figure 3-7: Depth-averaged current calibration results ....................................... 17
Figure 3-8: Discharge calibration results (TR01 – TR03) ....................................... 18
Figure 3-9: Discharge calibration results (TR04 – TR06) ....................................... 19
Figure 3-10: Discharge calibration results (TR07 – TR09) ....................................... 20
Figure 3-11: Discharge calibration results (TR10 – TR12) ....................................... 21
Figure 3-12: Discharge calibration results (TR13) ................................................... 22
Figure 3-13: Water level validation results during Hurricane Matthew ................. 22
Figure 3-14: Offshore waves from NOAA Buoy 41013 during calibration period .. 24
Figure 3-15: Wind data at NOAA buoy 41013 and from CFSR during calibration
period .................................................................................................. 25
Figure 3-16: Water level data from NOAA station 8658163 for model calibration 26
Figure 3-17: Significant wave height calibration results ......................................... 27
Figure 3-18: Peak wave period calibration results .................................................. 28
Figure 3-19: Peak wave direction calibration results .............................................. 29
Figure 3-20: Comparison of Bald Head ADCP wave energy spectrum: (up) measured;
(down) modeled .................................................................................. 32
Figure 3-21: Significant wave height validation results .......................................... 34
Figure 3-22: Peak wave period validation results ................................................... 35
Figure 3-23: Peak wave direction validation results ............................................... 36
Figure 3-24: Annual percentage of exceedance of significant wave height at the
offshore boundary .............................................................................. 39
Figure 3-25: Wave rose of significant wave heights at the offshore boundary ...... 40
Figure 3-26: Transects for OPTI-method ................................................................. 44

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page vii

Figure 3-27: Delft3D initial sediment layer thickness ............................................. 46
Figure 3-28: Condition surveys at the Cape Fear Entrance Inner Ocean Bar Channels
(USACE, 2011) ..................................................................................... 48
Figure 3-29: Delft3D 1-year channel shoaling patterns (d50=0.15mm) ................. 49
Figure 4-1: Jay Bird Shoals borrow area templates .............................................. 53
Figure 4-2: After-dredge bathymetry – Template 1 .............................................. 55
Figure 4-3: After-dredge bathymetry – Template 2 .............................................. 55
Figure 4-4: Peak flood currents – existing condition ............................................ 56
Figure 4-5: Peak flood currents – after-dredge Template 1 ................................. 57
Figure 4-6: Peak flood currents – after-dredge Template 2 ................................. 57
Figure 4-7: After-dredge bathymetry effects on peak flood currents – Template 1
............................................................................................................. 58
Figure 4-8: After-dredge bathymetry effects on peak flood currents – Template 2
............................................................................................................. 58
Figure 4-9: Peak ebb currents – existing condition ............................................... 59
Figure 4-10: Peak ebb currents – after-dredge Template 1 .................................... 60
Figure 4-11: Peak ebb currents – after-dredge Template 2 .................................... 60
Figure 4-12: After-dredge bathymetry effects on peak ebb currents – Template 1
............................................................................................................. 61
Figure 4-13: After-dredge bathymetry effects on peak ebb currents – Template 2
............................................................................................................. 61
Figure 4-14: Residual tidal currents – existing condition ........................................ 62
Figure 4-15: Residual tidal currents – after-dredge Template 1 ............................. 63
Figure 4-16: Residual tidal currents – after-dredge Template 2 ............................. 63
Figure 4-17: After-dredge bathymetry effects on residual tidal currents – Template
1 .......................................................................................................... 64
Figure 4-18: After-dredge bathymetry effects on residual tidal currents – Template
2 .......................................................................................................... 64
Figure 4-19: After-dredge bathymetry effects on waves between 0 – 3 ft with
average height of 2.5 ft (top: Template 1; bottom: Template 2) ....... 67
Figure 4-20: After-dredge bathymetry effects on waves between 3 – 6 ft with
average height of 4.5 ft (top: Template 1; bottom: Template 2) ....... 68
Figure 4-21: After-dredge bathymetry effects on waves between 3 – 6 ft with
average height of 7.5 ft (top: Template 1; bottom: Template 2) ....... 69
Figure 4-22: After-dredge bathymetry effects on storm waves comparable to
Hurricane Matthew in 2016 (top: Template 1; bottom: Template 2) 70

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page viii

Figure 4-23: Caswell Beach transects ...................................................................... 72
Figure 4-24: Wave-induced net longshore sediment transports along Caswell Beach
shoreline ............................................................................................. 74
Figure 4-25: Longshore sediment transport gradients along Caswell Beach shoreline
............................................................................................................. 75

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page ix

LIST OF TABLES
Table 2-1: Model bathymetry data sources ........................................................... 5
Table 3-1: Goodness-of-fit parameters for significant wave height calibration . 31
Table 3-2: Goodness-of-fit parameters for peak wave period calibration .......... 31
Table 3-3: Goodness-of-fit parameters for peak wave direction calibration ...... 31
Table 3-4: Goodness-of-fit parameters for significant wave height validation ... 37
Table 3-5: Goodness-of-fit parameters for peak wave period validation ........... 37
Table 3-6: Goodness-of-fit parameters for peak wave direction validation ....... 37
Table 3-7: Representative wave conditions used as model inputs ..................... 41
Table 3-8: OPTI wave schematization results and morfac ................................... 44
Table 3-9: Historical shoaling rates for the Inner Ocean Bar Channels from surveys
(USACE, 2011) ..................................................................................... 50
Table 3-10: Shoaling volume rate calibration results (cy/yr) ................................. 51

Appendix C1 - Model Parameters

Appendix C2 - Waves Template 1

Appendix C3 - Waves Template 2

Appendix C4 - Sediment Transports - Individual Wave

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 1 of 78

1. INTRODUCTION
Moffatt & Nichol was retained by the Town of Oak Island for professional services to
execute the 2021/2022 Renourishment Project following Hurricane Florence.
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.
Delft3D, an open-source, fully integrated numerical modeling suite developed by
Deltares, Netherlands, was selected as the modeling platform. Delft3D 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 Delft3D were used: Delft3D-FLOW (Deltares, 2018a) and
Delft3D-WAVE (Deltares, 2018b). Delft3D-FLOW is the hydrodynamics and sediment
transport module; whereas Delft3D-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

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 2 of 78

2. MODEL DEVELOPMENTS
In this section, the developments of flow and wave model grids and bathymetries are
discussed. The model horizontal coordinate system 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
Two flow model grids were developed: one for the full hydrodynamic (HD) model, and
the other for the entrance channel morphology model. The full HD flow model domain
(gray in Figure 2-1) 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. This model grid was used
for the hydrodynamics model calibration and providing boundary conditions for the
morphology model in an offline nested approach.
The entrance channel (local) morphology grid (red in Figure 2-1) is comprised of 575,113
cells with cross-shore resolution of ~10 m in the nearshore, covering the Bald Head Island
South Beach shoreline and half of the Oak Island shoreline. Its upstream boundary is in
the Upper Midnight channel range near the AIWW connection at Carolina Beach. Figure
2-1 presents the flow model grids.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 3 of 78

Figure 2-1: Flow model grids
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 in Figure 2-2) 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 in Figure 2-2) and the fine wave domain (red in
Figure 2-2) 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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 4 of 78

Figure 2-2: Wave model grids
2.2 MODEL BATHYMETRY
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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 5 of 78

Table 2-1: Model bathymetry data sources
Data Set Source
Wilmington Harbor hydrographic surveys USACE 2016 – 2017
Fugro channel bank surveys Fugro 2016 – 2017
Oak Island post Matthew beach profile surveys
(STA 210+00 – 700+00) TI Coastal 2016
Bald Head Island beach profile surveys
(STA 000+00 – 238+00) USACE 2013
Oak Island beach profile surveys
(STA 005+00 – 210+00) USACE 2012
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

Figure 2-3: Flow model bathymetry under existing conditions

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 6 of 78

Figure 2-4: Fine wave model bathymetry under existing conditions

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 7 of 78

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 (ΔT). 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):
xyME−= (1)
Where “bar” denotes the sample mean.
Root mean square (RMS) error:
()2yxRMS−=ε (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
%100
min,max,
⋅−=
ff
RMS
norm xx
εε (3)
Where xf,max and xf,min 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
fmeasxx−=ε (4)
Mean absolute error (MAE):
yxMAE−= (5)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 8 of 78

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):
()2
2)(1
xyxx
yxd
−−−
−−=,10≤≤d (6)
The time delay ∆T 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 5th 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. The model parameters in the FLOW
model are listed in Appendix C1.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 9 of 78

Figure 3-1: Locations of water levels and current measurements by RPS EH

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 10 of 78

Figure 3-2: Survey transects in Upper Wilmington area by RPS EH

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 11 of 78

Figure 3-3: Survey transects in Lower Wilmington area by RPS EH

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 12 of 78

Figure 3-4: Survey transects in Snow’s Cut area by RPS EH

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 13 of 78

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 and

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 14 of 78

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.
(C) 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

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 15 of 78

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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 16 of 78

Figure 3-6: Water level calibration results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 17 of 78

Figure 3-7: Depth-averaged current calibration results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 18 of 78

Figure 3-8: Discharge calibration results (TR01 – TR03)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 19 of 78

Figure 3-9: Discharge calibration results (TR04 – TR06)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 20 of 78

Figure 3-10: Discharge calibration results (TR07 – TR09)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 21 of 78

Figure 3-11: Discharge calibration results (TR10 – TR12)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 22 of 78

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.

Figure 3-13: Water level validation results during Hurricane Matthew

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 23 of 78

3.4 WAVE MODEL CALIBRATION
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. The model parameters in the WAVE model
are listed in Appendix C1.
3.4.1 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, alpha1, 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
6th, 2008 during Hurricane Hanna.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 24 of 78

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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 25 of 78

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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 26 of 78

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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 27 of 78

Figure 3-17: Significant wave height calibration results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 28 of 78

Figure 3-18: Peak wave period calibration results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 29 of 78

Figure 3-19: Peak wave direction calibration results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 30 of 78

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 1 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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 31 of 78

Table 3-1: Goodness-of-fit parameters for significant wave height calibration
Station MAE (m) RMS (m) RMSN (%) R d
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
Station MAE (s) RMS (s) R d
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
Station MAE (deg) RMS (deg)
Eleven Mile ADCP 33 46
Bald Head Island ADCP 32 56
Oak Island ADCP 15 23
OCP1 ADCP 15 22

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 32 of 78

Figure 3-20: Comparison of Bald Head ADCP wave energy spectrum: (up) measured; (down) modeled

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 33 of 78

3.5 WAVE MODEL VALIDATION
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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 34 of 78

Figure 3-21: Significant wave height validation results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 35 of 78

Figure 3-22: Peak wave period validation results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 36 of 78

Figure 3-23: Peak wave direction validation results

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 37 of 78

Table 3-4: Goodness-of-fit parameters for significant wave height validation
Station MAE (m) RMS (m) RMSN (%) R d
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
Station MAE (s) RMS (s) R d
Eleven Mile ADCP 1.3 2.1 0.66 0.82
Bald Head Island ADCP 1.5 2.5 0.60 0.78
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
Station MAE (deg) RMS (deg)
Eleven Mile ADCP 40 56
Bald Head Island ADCP 35 55
Oak Island ADCP 22 35
OCP1 ADCP 18 27

3.6 MORPHOLOGICAL MODEL CALIBRATION
The sediment transport and morphological model was calibrated against the annual
shoaling volumes in three Cape Fear River entrance channel reaches (Smith Island reach,
Bald Head reaches 1 and 2).
Modeling long term (1 year in this study) sediment transport and the resulting coastal
morphology in Delft3D using a real-time series of tides and waves as inputs would lead to
unsustainably long run times. In order to avoid this problem a tide and wave
schematization approach was adopted for the morphological calibration purpose.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 38 of 78

3.6.1 Tide Schematization
The schematized tide was based on a method developed by Lesser (2009). This method
creates a representative tide fluctuation based on input values of the M2, K1, and O1
constituents, where the resulting tidal time series is based upon the following
relationship: 𝜂𝜂=𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶∗𝑀𝑀2 cos(𝜔𝜔𝑀𝑀2𝑡𝑡+𝜑𝜑𝑀𝑀2 )+𝐶𝐶1 cos(𝜔𝜔𝐶𝐶1 𝑡𝑡+𝜑𝜑𝐶𝐶1 ) (7) 𝐶𝐶1 =√2 ∗𝑂𝑂1 ∗𝐾𝐾1 𝑎𝑎𝑎𝑎𝑎𝑎 𝜑𝜑𝐶𝐶1 =0.5(𝜑𝜑𝐾𝐾1 +𝜑𝜑𝑂𝑂1 ) (8)
Where, η is water surface elevation, ω denotes angular frequency of tidal constituents, ϕ
denotes phase offset of tidal constituents, M2 is the semi-diurnal tidal constituent, C1 is
the diurnal astronomical tidal constituent with amplitude and phase described as a
function of O1 and K1 constituents, and Corr = correction factor for M2 tide. The tidal
periods of the M2 and C1 constituents were set equal to 750 minutes (semi-diurnal) and
1500 minutes (diurnal), respectively for this study.
The purpose of the morphological tide is to represent the average currents and sediment
transport that occur during a spring-neap tide cycle. This requires a morphological tide
which is slightly above the mean tide given that the sediment transport attributable to
the spring tide is typically larger than that attributable to the neap tide. The application
of the correction factor, Corr, listed above accounts for the disproportionate spring-neap
contributions to sediment transport. A typical value of 1.08 (Lesser, 2009) was adopted
for this study.
3.6.2 Wave Schematization
The goal of wave schematization is to reduce the wave conditions into a few classes
without losing much accuracy in the morphological impact of these waves compared to
the full wave time series. The wave climate schematization for this study is based on the
OPTI-method (Mol, 2007). It is a tool developed for Delft3D usage, it ensures that the
same sediment transport formula is used for both the representative wave class selection
and the morphology modeling afterward.
The measured wave data from 2004 to 2018 at the NOAA NDBC Buoy station 41013 were
the primary source of wave conditions for the morphology modeling. The data gaps in the
buoy data were filled with available USACE WIS hindcast data and NOAA WW3 hindcast
data at locations close to Station 41013. The WIS hindcast data were available till 2014;

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 39 of 78

WW3 data were used to fill the data gaps afterwards. The combined wave data were
recorded in hourly time intervals.
Figure 3-24 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 3-25 plots the wave rose for the significant wave
height from the combined wave records at offshore. The data 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.

Figure 3-24: Annual percentage of exceedance of significant wave height at the offshore boundary

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 40 of 78

Figure 3-25: Wave rose of significant wave heights at the offshore boundary

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 41 of 78

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 in model, specifically 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 3-7 lists the characteristics of each wave case.
Table 3-7: Representative wave conditions used as model inputs
Hs_bin
(m)
MWD_bin
(degN)
Bin average sig.
wave height (ft)
Bin average peak
wave period (s)
Bin average Wave
Direction (degN)
Percentage
Occurrence
0 - 1 90 - 105 2.5 9.0 97.7 4.854
1 - 2 90 - 105 4.4 9.5 98.0 3.973
2 - 3 90 - 105 7.8 10.1 97.3 0.635
3 - 4 90 - 105 11.3 11.8 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 - 2 105 - 120 4.4 9.4 112.4 5.030
2 - 3 105 - 120 7.7 9.6 112.8 0.714
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
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

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 42 of 78

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
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

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 43 of 78

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

For each wave class, a coupled flow and wave model run with sediment transport, without
morphological updates, was conducted with a constant water level at Mean Sea Level
(MSL) for a half day period simulation so that a quasi-steady state sediment transport rate
condition could be achieved. Therefore, only the wave induced sediment transport was
considered when determining the representative waves. Two “target” datasets were
used for the OPTI-method in this study: net and gross annual transport rates through 40
predefined cross-shore transects as shown in Figure 3-26. These transects match the
profile monitoring transects for both the Bald Head Island and Caswell Beach periodic
surveys conducted by USACE as part of the Wilmington Harbor Sediment Management
Plan (WHSMP). After conducting the OPTI analysis, six wave classes were selected and are
listed in Table 3-8. These wave classes were used later for the 1-year morphology model
runs.
For the 1-year morphological simulations, the sequence of the wave classes in the model
was as listed in Table 3-8. A different sequencing of the waves might affect the model
results. However, since small Morphological Time Scale Factor (morfac) values were used,

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 44 of 78

the assumption was that the chance for irreversible bathymetric changes to happen
under each wave class was small.
Table 3-8: OPTI wave schematization results and morfac
Significant wave
height (ft)
Peak wave
period (s)
Peak wave
direction (°N)
Original
weight (%)
OPTI calculated
weight (%) Morfac
7.8 8.1 157.7 0.74 3.53 12.4
7.8 8.0 172.5 1.01 0.14 0.5
11.1 9.0 172.9 0.20 2.34 8.2
7.8 7.6 201.7 0.73 1.48 5.2
4.6 5.8 217.1 3.14 16.83 16.7
7.8 7.0 231.2 0.37 2.12 7.4

Figure 3-26: Transects for OPTI-method
3.6.3 Morphological Time Scale Factor (morfac)
Morphological developments take place on a time scale several times longer than typical
flow changes. For example, tidal flows change significantly in a period of hours, whereas

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 45 of 78

it may take weeks, months, or years for significant morphological changes of a coastline.
Simulating long term morphological changes in real-time is simply not practical from a
computational point of view. To address this problem, Delft3D adopted a technique called
“morphological time scale factor” whereby the speed of the changes in the morphology
is scaled up to a rate that it begins to have a significant impact on the hydrodynamic flows.
The implementation of the morphological time scale factor (morfac) is achieved by simply
multiplying the erosion and deposition fluxes to and from the bed by the morfac, at each
computational time-step. This allows accelerated bed-level changes to be incorporated
dynamically into the morphological calculations.
A time-varying morfac method was used in this study. During a morphological simulation,
each of the selected wave conditions in Table 3-8 was simulated for the duration of one
or more morphological tides (1500 minutes) in order to account for the random phasing
between waves and tides that occurs in nature. Morfac was then used to increase the
morphological changes occurring during this period to the changes that would occur
during the entire duration of the occurrence of that wave condition in one year. For each
wave condition, the morfac applied was dependent on the percentage occurrence of that
particular wave condition. This approach has the desirable effect that higher morfac are
applied to the more common, and generally smaller, wave conditions during which the
morphology is less active, and smaller acceleration factors are applied to the larger (and
less common) wave conditions (when the morphology is more active and large morfac
might cause a problem). The morfac applied to each wave condition is indicated in Table
3-8.
3.6.4 River Flows
For the upstream river flows, the annual average flows were used for the entrance
channel morphology modeling purpose. The flow rates were 148 m3/s (5,227 cfs), 22
m3/s (777 cfs), and 21 m3/s (742 cfs) from Cape Fear River, Black River, and Northeast
Cape Fear River, respectively.
3.6.5 Sediments
The native beach mean sediment sizes around the study area are between 0.20 mm –
0.25 mm (USACE, 2012). In this study, three sediment sizes were used in the model runs
separately to determine the potential shoaling volumes associated with the different
sizes: 0.15, 0.20, and 0.25 mm. The approach of multiple sediment classes within one

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 46 of 78

model was not considered. The current approach provides a sensitivity analysis of the
channel shoaling volumes related to different sediment sizes. As indicated by the model
results for the 0.25 mm sediment size, the shoaling volume in the Baldhead Shoal Reach
2 was much less than the actual value from the condition surveys (see Table 3-10). Thus,
coarser sediment classes than 0.25 mm were not considered because they are less
mobile, which would have resulted in even less shoaling volume in the channel.
The initial sediment thickness of the sediment layer throughout the model domain is
required by the morphological model. For this study, it was assumed that there was no
sediment available in the channel bottom initially, and the sediment thickness was 10
meter in the littoral zone for each sediment size. Figure 3-27 presents the final sediment
thickness map used in the final model calibration.

Figure 3-27: Delft3D initial sediment layer thickness
3.6.6 Model Calibration Results
For this study, the default non-cohesive sediment transport formulations in Delft3D based
on Van Rijn et al. (2000) were applied. The parameters for both hydrodynamics and waves
were determined during their calibration processes and were kept the same for the

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 47 of 78

morphological modeling. Values used for parameters not iteratively altered during the
calibration process were determined from the published literature and/or
recommendations from Deltares, the developers of Delft3D. The primary sediment
transport parameters adjusted in the calibration of the morphology model were: Sus, Bed,
SusW, BedW, SusW, and BedW are related to waves and were recommended to be close
to zero for the depth average Delft3D application. Sus and Bed are parameters related to
current induced sediment transport. The sediment transport magnitudes increase when
Sus and Bed become larger. The full parameters used in the models are included in
Appendix C1.
(A) CHANNEL SHOALING PATTERNS
Figure 3-28 presents the condition surveys for the three ocean entrance channel ranges
in January 2007, November 2008, and August 2010 which are near the end of the first,
second, and third maintenance dredging cycles, respectively, when the channels are
typically in their more shoaled condition. The SMP assumed that maintenance dredging
would be required on a 2-year basis based on historical dredging activity. For all three
periods, the surveys show very similar shoaling patterns for the channel areas of interest.
The predicted cumulative sedimentation and erosion patterns from the 1-year
morphology modeling results for a grain size of 0.15 mm is presented in Figure 3-29. The
model result shows the similar shoaling patterns as observed from the condition surveys
in all three channel reaches. The modeled results for the other two grain sizes indicate
similar shoaling patterns as well.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 48 of 78

Figure 3-28: Condition surveys at the Cape Fear Entrance Inner Ocean Bar Channels (USACE, 2011)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 49 of 78

Figure 3-29: Delft3D 1-year channel shoaling patterns (d50=0.15mm)
(B) CHANNEL SHOALING RATES
Historical channel shoaling rates were computed based on condition surveys for each of
the maintenance dredging periods and each of the three channel reaches in the SMP
reevaluation report (USACE, 2011) and presented in Table 3-9. The rates computed for
the last dredging cycle excluded the post Bald Head fill period so as to not bias the data
due to the influence of this locally performed project. An overall weighted average was
calculated for the entire maintenance period spanning the three cycles. As shown in Table
3-9, the results show fairly similar daily rates for each of the three channel reaches. The
total shoaling rate in all three channel reaches is 1,610 cy/d which results in a total of
volume of 587,470 cubic yards if this rate is used to project an average annual shoaling
volume.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 50 of 78

Table 3-9: Historical shoaling rates for the Inner Ocean Bar Channels from surveys (USACE, 2011)
Channel
1st Cycle 2nd Cycle 3rd Cycle
(Pre-BH Fill) Weighted Ave
Rate Days Rate Rate Days Rate Rate Days Rate Rate Days Rate
cy/d cy/yr cy/d cy/yr cy/d cy/yr cy/d cy/yr
Baldhead
Shoal
Reach 1
442.5 772 161,513 589.3 608 215,095 505.8 216 184,617 507.0 1596 185,055
Baldhead
Shoal
Reach 2
517.0 773 188,705 712.2 512 259,953 321.7 152 117,421 565.9 1437 206,554
Smith
Island 431.0 811 157,315 591.2 611 215,788 878.2 153 320,543 536.6 1575 195,859
Total 507,533 690,836 622,581 587,468

Between June and December 2015, a terminal groin was built on the west tip of the South
Beach on Bald Head Island. To check the impact of the terminal groin, condition surveys
in November 2015, November 2016 and December 2017 by USACE were used to compute
the shoaling volumes in these three channel reaches. The same approach as in USACE
(2011) was applied to calculate the volume changes above -46ft Mean Low Water (MLW)
channel prism, and the results are presented in Table 3-10. The total shoaling volumes
are 592,000 cy and 635,000 cy during the periods of November 2015 – November 2016
and November 2016 – December 2017, respectively. The magnitudes are similar to the
annual average shoaling volume of 587,470 cy/yr prior to the terminal groin construction
(USACE, 2011).
The predicted shoaling volumes were calculated from the 1-year morphology model
results in the same three channel reaches for the three sediment grain sizes. To account
for the sediment accumulation that would be dredged from the navigation channel, the
volume confined between the channel setback lines established by USACE (about 150 ft
along the Cape Fear Entrance Ocean Channels) can be seen as an adequate
approximation. The setback lines are indicated by the dash line on Figure 3-29. The
shoaling volumes calculated form the model results are included in Table 3-10.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 51 of 78

Table 3-10: Shoaling volume rate calibration results (cy/yr)
Baldhead Shoal
Reach1
Baldhead Shoal
Reach 2
Smith
Island Total
Modeled
d50 = 0.15mm 483,000 429,540 508,600 1,421,140
d50 = 0.20mm 207,570 176,730 395,760 780,060
d50 = 0.25mm 126,270 130,250 292,630 549,150
USACE (2011) 184,690 206,590 196,000 587,280
Condition survey
(11/2015 – 11/2016) 106,090 324,600 161,180 591,870
Condition survey
(11/2016 – 12/2017) 109,830 287,490 237,890 635,210

The modeled total shoaling volume of 549,150 cy within the three reaches with the grain
size of 0.25mm is within the range of the historical shoaling rates from condition surveys.
The predicted shoaling volume in Baldhead Shoal Reach 1 is close to that observed after
construction of the terminal groin. However, the predicted shoaling volume in Baldhead
Shoal Reach 2 is much less than was observed, whereas more shoaling was predicted in
Smith Island than observed from the surveys.
For a finer grain size of 0.20mm, modeled shoaling volumes in Baldhead Shoal Reach 1
are in line with the pre-construction surveys of the terminal groin. In Baldhead Shoal
Reach 2, the predicted shoaling volume, though, is lower than observed.
For the finest grain size of 0.15mm, the predicted shoaling volumes in all three channel
ranges are much larger than historical rates, which results in a total shoaling volume
about 140% more than the historical rates.
A plausible explanation is the sediment size decreases from the river entrance to offshore.
For sediments transported from Caswell Beach and Jay Bird Shoals to Smith Island range,
the grain size might be coarser than 0.25mm. Sediments feeding into Baldhead Shoal
Reach 1 are in the range of 0.25mm, mostly from Bald Head Island. Further offshore, the
grain size is finer (between 0.15 and 0.20mm) in Baldhead Shoal Reach 2 where sediments
mostly coming from Baldhead Shoal.
Another factor that could affect the shoaling volume calculations in Baldhead Shoal Reach
1 & 2 is periodic beach nourishments on the Bald Head Island beaches which provide extra
amounts of sediment to be transported back to the adjacent channels. Most of the
historical shoaling volumes calculated from the condition surveys are within 1 to 2 years

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 52 of 78

of post- beach nourishment. Beach nourishment was not incorporated in the model
bathymetry.
Other contributing factors to the model results include inherent model limitations such
as, nearshore and shoal bathymetry which influence both wave transformation and
sediment transport magnitude, and exclusion of potential storm impacts, etc.
In summary, the morphology model was calibrated against historical shoaling volumes
computed from condition surveys by USACE. The modeled shoaling patterns in the
channels are similar to the surveys. However, the shoaling volumes from the model were
found to strongly dependent on the sediment grain size within each reach. The model
results imply that the grain size reduces in the channel progressing from north to south.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 53 of 78

4. 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. Improvements to the borrow area template that was approved and permitted
for the 2020/2021 Renourishment Project (Template 2) were considered to ensure
dredging could be completed efficiently and effectively for the 2021/2022 Renourishment
Project. Template 1 includes three zones with dredging elevations down to -28 ft-NAVD88
(Zone 1), -37 ft-NAVD88 (Zone 2), and -29 ft-NAVD88 (Zone 3) as shown in Figure 4-1.
Template 2, follows the approved permit conditions for the 2020/2021 Renourishment
Project, includes three zones with dredging elevations down to -26 ft-NAVD88 (Zone 1), -
31 ft-NAVD88 (Zone 2), and -27 ft-NAVD88 (Zone 3) as shown in Figure 4-1.

Figure 4-1: Jay Bird Shoals borrow area templates
Template 1 would contain 4.67 mcy of beach compatible material and Template 2
contains 2.95 mcy. When considering what can be cost effectively dredged from each
template the volumes available for beach placement are reduced. Once the 2 ft
overdredge depth buffers are removed, which are added to account for dredging
equipment inaccuracies (not to account for additional volume), the 4.67 mcy available in
Template 1 reduces to 3.69 mcy that can be cost effectively dredged. For Template 2, the
amount of material available to be removed cost effectively is 2.08 mcy of the 2.95 mcy
available.
Assuming 1.1 mcy was removed from Template 2 during the 2020/2021 Renourishment
Project this would leave 0.98 mcy available for the 2021/2022 Renourishment Project
when 1.667 mcy is required. Therefore, Template 1 was developed to ensure that enough

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 54 of 78

material would be available for the 2021/2022 Renourishment project after the
completion of the 2020/2021 Renourishment Project.
To provide additional material so the dredge does not have to work in “clean up” mode
Zone 2 for Template 1 provides an additional 4 ft of dredging depth with 2 ft of overdredge
depth (total 6 ft) from what was permitted in Template 2. In “clean up” mode the dredge
would be forced to spend time collecting loads where not much material is being removed
at a time, which is not efficient. Zone 2 was chosen to dredge to a deeper depth to provide
additional volume since it is the most offshore in the shoal environment.
In addition, considering the dredge cannot control their equipment precisely enough to
remove all the quantity available without dipping below the permitted elevation in the
process a 2 ft overdredge depth buffer was incorporated to avoid incurring a permit
violation. Template 1 incorporated a 2 ft overdredge depth allowance in Zones 1 and 3
from what was permitted in Template 2. This overdredge allows for a comfortable buffer
for dredges to work as dredging could occur in these overdredge areas on an infrequent
basis during normal construction activities and operational procedures.
The maximum dredging scenario was considered for both templates, i.e. assuming to
remove all the available material identified as beach compatible (4.67 mcy and 2.95 mcy
for Template 1 and 2 respectively). This assumption is conservative as discussed earlier
knowing that the dredge cannot remove all of this material cost effectively. 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. The Bald Head Island terminal groin was included in
the bathymetries for these models.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 55 of 78

Figure 4-2: After-dredge bathymetry – Template 1

Figure 4-3: After-dredge bathymetry – Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 56 of 78

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 to Figure 4-6 present the peak depth-averaged flood currents 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 differences between the existing and the two
after-dredge templates, respectively. The model results indicate that both after-dredge
bathymetries would cause less than 0.2 ft/s increase/decrease of peak depth-averaged
flood currents adjacent to the borrow site.

Figure 4-4: Peak flood currents – existing condition

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 57 of 78

Figure 4-5: Peak flood currents – after-dredge Template 1

Figure 4-6: Peak flood currents – after-dredge Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 58 of 78

Figure 4-7: After-dredge bathymetry effects on peak flood currents – Template 1

Figure 4-8: After-dredge bathymetry effects on peak flood currents – Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 59 of 78

4.1.2 Peak Tidal Ebb Currents
Figure 4-9 to Figure 4-11 present the peak depth-averaged ebb currents during a spring
tide under existing and the two after-dredge templates, respectively. Figure 4-12 and
Figure 4-13 show the peak ebb current differences between the existing and the two
after-dredge templates, respectively. The model results indicate that both after-dredge
bathymetries would cause less than 0.2 ft/s increase/decrease of peak depth-averaged
ebb currents adjacent to the borrow area.

Figure 4-9: Peak ebb currents – existing condition

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 60 of 78

Figure 4-10: Peak ebb currents – after-dredge Template 1

Figure 4-11: Peak ebb currents – after-dredge Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 61 of 78

Figure 4-12: After-dredge bathymetry effects on peak ebb currents – Template 1

Figure 4-13: After-dredge bathymetry effects on peak ebb currents – Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 62 of 78

4.1.3 Residual Tidal Currents
Residual tidal currents over a spring-neap tidal cycle are 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
Delft3D, the residual currents are calculated based on Fourier analysis for the current
velocities over a specified period.
Figure 4-14 to Figure 4-16 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 4-17 and Figure 4-18 for Template 1 and 2, respectively. The model results
indicate the two after-dredge bathymetry templates could cause a negligible residual tidal
current increase (less than 0.05 ft/s).

Figure 4-14: Residual tidal currents – existing condition

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 63 of 78

Figure 4-15: Residual tidal currents – after-dredge Template 1

Figure 4-16: Residual tidal currents – after-dredge Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 64 of 78

Figure 4-17: After-dredge bathymetry effects on residual tidal currents – Template 1

Figure 4-18: After-dredge bathymetry effects on residual tidal currents – Template 2

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 65 of 78

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. The same
representative offshore waves listed in Table 3-7 were used.
4.2.1 Nearshore Wave Results
Each of the 86 wave conditions listed in Table 3-7 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 sides
of the borrow area. Some results from the 86 wave conditions are presented below; all
wave condition model results are included in Appendix C2 and C3 for Template 1 and
Template 2, respectively.
Figure 4-19 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 4-20 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 of the Jay Bird Shoals with water depths of 10 – 15 ft.
Figure 4-21 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.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 66 of 78

Figure 4-22 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 the east and west sides. The magnitude of wave
change is mostly less than 1 ft in localized areas.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 67 of 78

Figure 4-19: After-dredge bathymetry effects on waves between 0 – 3 ft with average height of 2.5 ft (top: Template 1; bottom: Template 2)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 68 of 78

Figure 4-20: After-dredge bathymetry effects on waves between 3 – 6 ft with average height of 4.5 ft (top: Template 1; bottom: Template 2)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 69 of 78

Figure 4-21: After-dredge bathymetry effects on waves between 3 – 6 ft with average height of 7.5 ft (top: Template 1; bottom: Template 2)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 70 of 78

Figure 4-22: After-dredge bathymetry effects on storm waves comparable to Hurricane Matthew in 2016 (top: Template 1; bottom: Template 2)

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 71 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

4.3 SEDIMENT TRANSPORT
Based on the model results demonstrated in Section 4.1.3, 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 3-7, the wave induced
longshore currents and associated sediment transport were estimated by coupling
Delft3D-FLOW and Delft3D-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.
The sediment transport rates through shore-normal transects along the Caswell Beach
shoreline (Figure 4-23) 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 five-point (~0.5 mile)
moving average.
The unweighted longshore sediment transport rates (in cy/yr) under each individual wave
case are presented in Appendix C4. These results can be misleading if misinterpreted. The
contributions of each wave case to the annual net sediment transport rates are the values
in Appendix C4 multiplied by the percentage of occurrence as shown for each wave case.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 72 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

Figure 4-23: Caswell Beach transects
Figure 4-24 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 (dash lines representing the unsmoothed transports; whereas solid lines are
smoothed). Positive values represent a westerly sediment transport direction. The model
results indicate potential sediment transport rate reductions leeward of the borrow area,
and potential sediment transport rate increases along both the east and west shoreline
segments away from the borrow area.
The smoothed net longshore sediment transport gradients along the Caswell Beach
shoreline are shown in Figure 4-25. 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 for potential shoreline changes. Positive and negative values in Figure 4-25
indicate potential localized adjustments in shoreline accretion and erosion, respectively.
Based on the model results, it appears that areas of concern for potential increases in
shoreline erosion would be limited to discrete portions of Caswell Beach (between survey

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 73 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

transects 37+00 – 60+00 and 150+00 – 185+00). Potential effects on shoreline erosion in
other areas are minimal although 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 that the improvements made to Template 2 in order to provide additional
volume and efficiency for completing the 2021/2022 Renourishment Project as shown in
Template 1 would cause any more significant impacts, 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. It is also
important to note the results modeled are a “worst case” approximation as only 2.547
mcy will ultimately be dredged for the completion of both the 2020/2021 Renourishment
Project and the 2021/2022 Renourishment Project.
In order to efficiently and effectively complete the 2021/2022 Renourishment Project,
Template 1 will be used to allow for additional volume and efficiency given the dredging
process inaccuracies. The Town of Oak Island will monitor the Caswell Beach shoreline for
three (3) years post-project to investigate any potential effects which might require
mitigation.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 74 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

Figure 4-24: Wave-induced net longshore sediment transports along Caswell Beach shoreline

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 75 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

Figure 4-25: Longshore sediment transport gradients along Caswell Beach shoreline

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 76 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

5. SUMMARY AND CONCLUSIONS
In order to investigate the potential effects of the Jay Bird Shoals borrow area identified
for the 2021/2022 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’ Delft3D model suite. The hydrodynamics, wave, and
morphology models were successfully calibrated and validated against available observed
water levels, currents, discharges, wave, and channel shoaling data.
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 4.67/2.95 mcy of available
material identified as beach compatible in Template 1 and 2, respectively. These are
“worst case” approximations as a total of 2.547 mcy is estimated to be removed from the
borrow area upon completion of both projects. Thus, within the proposed borrow area,
the results from the Delft3D 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 the
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 appears 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 – 185+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 that the improved Template 1 scenario has any more

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 77 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

significant impact than currently permitted Template 2 scenario, 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.
• In order to efficiently and effectively complete the 2021/2022 Renourishment
Project, Template 1 will be used to allow for the additional volume required and
to maintain efficiency given the dredging process inaccuracies. The Town of Oak
Island will monitor the Caswell Beach shoreline for three (3) years post-project to
investigate any effects predicted by the model which might require mitigation.

Town of Oak Island
Jay Bird Shoals Borrow Area Modeling
June 12, 2020
Page 78 of 78

2021/2022 Renourishment Project
M&N Project No.10128-01

6. REFERENCES
Benoit, M., Frigaard, Peter, and H.A. Schäffer (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, M.D, 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.
Lesser, G.R., 2009. “An Approach to Medium-term Coastal Morphological Modelling.”
PhD Thesis, TU Delft, Delft, Holland.
Mol A.C.S. (2007), “R&D Kustwaterbouw Reductie Golfrandvoorwaarden OPTI Manual.”
Research report H4959.10. WL|Delft Hydraulics, the Netherlands.
RPS Evans-Hamilton (2017). “Cape Fear current, water Level and water quality study”.
Rijn, L. C. van, J. A. Roelvink and W. T. Horst, (2000), “Approximation formulae for sand
transport by currents and waves and implementation in DELFT-MOR”, Tech. Rep.
Z3054.40, WL|Delft Hydraulics, Delft, The Netherlands.
USACE (2011), “Draft reevaluation report – sand management plan Wilmington Harbor
Navigation Project”, U.S. Army Corps of Engineers, Wilmington District, January 2011.
USACE (2012), “Draft integrated general reevaluation report and environmental impact
statement, coastal storm damage reduction, Brunswick County beaches, North Carolina”,
U.S. Army Corps of Engineers, Wilmington District, October 2012.
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.