The URL can be used to link to this page

Home My WebLink AboutAppendix D - Borrow Area ModelingAPPENDIX D BORROW AREA MODELING REPORT 2021/2022 RENOURISHMENT PROJECT OAK ISLAND, NORTH CAROLINA JAY BIRD SHOALS SORROW AREA MODELING Prepared for: Town of Oak Island N C A q� rrr4 87 _ 'gej Prepared by: �'''lY►l,d:,.N►'�� hdhq moffatt & nichol 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 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 1.703 mcy is needed for the 2021/2022 Renourishment Project this would mean 2.803 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. 16- r J- s 13 12- / 694 595 696 697 696 699 700 1W 702 703 -,,di, km) -025 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.,5 0.2 0.25 Re d-1 Curren,s-(Template 1- Existing)(ft'sl ,6- T ,5 - a14 12- 594 695 696 697 696 699 700 101 702 703 H coordi nate (km) -0.2, -0.2 -0.,5 -0.1 -0.0, 0 0.05 0.1 0.15 0.2 0.25 Re dust C1111 s-(Template 2-E—Ling)(h's) After -dredge bathymetry effects on residual tidal currents over a spring -neap tidal cycle The wave transformation model results forthe 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 sa 40 _6) _80 100 Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page iv ---------------- -rs-a-ter;-arsa acr ozrcaau; auaauu a is ovtsa A9, 0 of o�n o�n o�na v+o�n ov+ �n o�n ono ov+ r��n ono o�+��MMa'N �ry N.nrviO.rvN „''.�.ry N.�v N.�v .aoo� '::nniiOi `D �rnmw°Vn nm�v+ � -+ M - - fill I Ill --..... 5--.....�.---.... T--------r -----------.-T------.-r---- -r. -.}. ------ I I + SurveyTransects--"""'-"" ------- ".."'--"" Jay Bird Shoal Qorrow Site —Transport Gradient - Existing Transport Gradient - Aker -Dredge Template 1 _-_ —Transport Gradient - Aker -Dredge Template 2 691000 692000 693000 694000 695000 696000 69700D 698000 699000 700000 701000 Ea sting - NC State Plan (m) Wave -induced longshore sediment transport gradients along Caswell Beach shoreline Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 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 1Z 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: DeIft3D 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: DeIft3D 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 1Z 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: OPT[ 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. DeIft3D, an open -source, fully integrated numerical modeling suite developed by Deltares, Netherlands, 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. Bl2UNSWlCK COUNTY y Pro osetiPlaclit Area 580 00 - 688108 h pr rp�ae I Iryl 3rN4l I a p a,. $orrdw Ar 2POy0pi 300+0o from mPfaceo4li Area -- -- - y - Federa11SULO Eaters Bounda4. ry - - - _ '' e ✓'� Loge ntl Oak Islantl Monlrorrng Transacts 1 R. Oak Island Jay Bird Shoals Borrow Area ss� s 7r)!. Bald Head Island Jay Bird 6hoals Borrow Area '- _ Mlles 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 1Z 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 OOS+00 — 210+00) USACE 2012 USACE 2010 Cape Fear River 2010 surveys NOAA hydrographic surveys NOAA 1973 — 2007 NOAA Navigation Charts MIKE C-MAP ADCIRC bathymetry NCDPS 2011 NC LiDAR NOAA 2014 — 2016 1DD 10 0 8D -1❑ 00 6D -20 0 T z Y -s0 c 40 L; T o A0 � , a T O 20 -50 O LL -60 a -70 -20 1-80 660 680 700 720 740 x coordinate {km} Figure 2-3: Flow model bathymetry under existing conditions Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 6 of 78 10 0 20 10 18 m 0 1s -20 i' 14 � E d 12 ►: 30 itl m ` 10 a -40 `o m o a, 8- 6- -50 m c 4 LL -60 2 685 690 695 700 705 710 715 x coordinate (km) > -70 -80 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 (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: z ERMS =r(X YY (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 c„a n, _ £RMS • 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: _r=---z 6meas — x f Y (4) Mean absolute error (MAE): MAE=Ix-yl (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): d=1— (x—y)` ,0:�d<1 (6) (�X—x�—�y—YI)2 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 5t" 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. i AV,g Station iVilinington. Alt ;. � - 3 Station -Southport A r. 0 1 2 4 6 S . . Miles n Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 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 1Z 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 June1Z 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 June1Z 2020 Page 16 of 78 set04.v2O - multi Wilmington 2 1.5 E 1 aD 0.5 L m is 0 -0.5 -1 23/Mar 25/Mar 27/Mar 29/Mar 31/Mar 02/Apr 2017 Southport 2 1.5 E 1 a� 0.5 L 0 -0.5 -1 23/Mar 25/Mar 27/Mar 29/Mar 31/Mar 02/Apr 2017 Wilmington(NOAA) 2 Measured Calculated 04/Apr 06/Apr 04/Apr 06/Apr Erms - 0.164 1.5 . ... . . . :............. s = 0.093 meas E e 10 _ .2% g ea = 1. 0.5 c.. 0 � 4 8 0 1 n 2 al = .1 5 s -0.5 -- -- - -- .. - -- - -- T 1 0 in - d = 0.98 -1 23/Mar 25/Mar 27/Mar 29/Mar 31/Mar 02/Apr 2017 Figure 3-6: Water level calibration results 04/Apr 06/Apr Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 17 of 78 set04.v2O - Southport Measured Current Speed Calculated 2.5 2 28/Mar 29/Mar 30/Mar 31/Mar 01/Apr 02/Apr 03/Apr 2017 Current Direction 350 300 rn 250 c 0 200 (D C) 150 50 0 28/Mar 29/Mar 30/Mar 31/Mar 01/Apr 02/Apr 03/Apr 2017 Figure 3-7: Depth -averaged current calibration results Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 18 of 78 set04.v20 -multi ❑ Measured TR01 Calculated 1500 1000 M 500 E ti 2 0 m L i? -500 -1000 1500 sMS = 32.861 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 6 rn L U N -50( 100(, )I J "1....:..........: ...�...:.. p..\.......... /....\.:...........:. . s = 13.550 m1S UE3 ca - me 52.41 E 91.50 ❑ R = 0.94 d = 0.96 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 30/Mar/2017 TR03 3000 2000 N M E 1000 6 2) is cLi 0 -1000 2000 - - - - - e _ 8.784 rms ca c - meas 86.71 - MAE 71.88 U❑ = 0.95 d = 0.97 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) Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 19 of 78 set04.v20 -multi ❑ Measured TR04 Calculated 3000 2000 M 1000 E ti 2 0 m L i? -1000 -2000 -3000 s = 50.323 _ rme ca - mea = 13.31 M = 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 L U 0 -500 1000 m ............;.-� �.... E = 113.631 rms ......... ❑ .... _ . cal. - mea -33.96 . AE 98.05 R = 0.98 d = 0.99 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 6 1000 a) t 0 U M C, -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) Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 20 of 78 set04.v20 -multi ❑ Measured TR07 Calculated 6000 4000 M 2000 E ti 2 0 m L i? -2000 -4000 6000 1 1\ 11 1\ f Vmea413.691c-44.37 330.34 = 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 m rn 0 L U N -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 N M 2000 E N rn 0 L U 5 -2000 -4000 6000 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 30/Mar/2017 Figure 3-10: Discharge calibration results (TR07 —TR09) set04.v2O - multi TRIO ,+UU 300 42 200 C1> E 4i 100 21 M 0 An C, -100 -200 300 12:00 18:00 00:00 06:00 X 104 1 FT--- 0.5 E 6 21 0 A -0.5 4 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 21 of 78 El Measured — Calculated 12:00 18:00 00:00 06:00 12:00 30/Mar/2017 TR1 1 :13 .......... . I ---- ----- -- ---------- ------ -- ------- -- -- -- ----- ';rms 1070.629 rms me s -60.22 E 891.15 : Q .130 = 1.00 d = 0.99 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 30/Mar/2017 TR12 A rk^ -VV 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) x 104 1.5 F — 1 C3 0.5 E ai rn 0 M s U o -0.5 -1 15 set04.v20 - multi TR13 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 22 of 78 ❑ 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 /Al:..... Erms = 0.132 04/Oct 05/Oct 06/Oct 07/Oct 08/Oct 09/Oct 10/Oct 11/0ct 12/Oct 13/Oct 14/Oct 2016 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 15t, 2008 to October 111, 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, 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 61", 2008 during Hurricane Hanna. Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 24 of 78 Significant wave height [m] 8.0 ..... . . . .... . . . . . . . 6.0----------------- -------------.. 4.0 2.0 0.01 August September 2008 2008 Peak wave period [sec] 20 15 10 5 0 August September 2008 2008 Peak wave direction [deg] 360 240 120 0 August September 2008 2008 Figure 3-14: Offshore waves from NOAA Buoy 41013 during calibration period (- 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 NDBC [mis] CFSR [mis] 25 ^ 20 N v 15 ---____ Q1 Q Cn a 10 _.__._._---- 5 0 August September 2008 2008 NDBC [deg] CFSR [deg] 360 300 z rn m a 240 c 0 t 180 m a 120 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. Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 26 of 78 2 —Water Level [rnj 1.5 —Residual {mj 1 0.5 0 00 $Willi, -0.5 1 111 -1 2008-08-0100:00 2008-08-16 D0:00 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. Town of Oak Island ,", Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 27 of 78 5 4 Z 3 v > 2 0 Jul-31 r,— u Jul-31 r,— U — Jul-31 5 ElevenMile Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time BaldHead Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time Oaklsland Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time OCP1 Measured v 4 Modeled _ 3 _ . = w (> 2 .... ...... ......... ........ ................... ....'�..... ......... _ v� 1.... ...... .... ........ ... ........ _ ............. _ 65 0 Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time Figure 3-17: Significant wave height calibration results Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 28 of 78 ElevenMile 20 yMeasured 15 Modeled p r r rir �3 o • ' : .mow X t r 1r D �10 'r►NN"0 4" Nw.wN► •... �.. -i-q., �... ..... .. ..... s »ram 5 ... . �............. ........ ... _.. �..... w ...... ! a� a 0 Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time BaldHead 20 yMeasured 15 Modeled p «. wi►i�; sM• . main yr • 'T . •. .. a- a 10 ♦ . . .M/1�. .M/ ♦ .wIN.AY M wAl► f P► .. r ♦� w+r► 'rt ..... .. �. ... r '..� a 0 Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time Oaklsland 20 • Measured 15 Modeled ...........:........ ......... �� : _ ................................................................ p.x • .rr N wr.�.wu. •r a rw rr .r. .rr�. .......... ........w .. ...... _� ........ �. .... y... ......� w.... . .tN . ♦.. .w� . w wf ••� jj w ld 0 Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time OCP1 20 • Measured 15 Modeled •• ' .�Or�littr,��l1�� . !•� .A a 10 rr� ........... ......... ... .........._•...�........_"1:....�' e...t. �...*7flfi......... ...../'SNw.• .u'7►...... IL a� 0 0 Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time Figure 3-18: Peak wave period calibration results z 360 U aD a 300 T° 240 a� 0 180 c� 120 60 Y Cd I,- Jul-31 z 360 U aD a 300 T° 240 a� 0 180 > 120 c� 60 Y Cd N I,- Jul-31 z 360 U aD 300 240 a� 0 180 > 120 c� 60 m a� 0 I,- Jul-31 z 360 U aD -S� 300 240 a� 0 180 > 120 c� 60 m aD 0 ElevenMile Aug-10 Aug-20 Aug-30 Time BaldHead Aug-10 Aug-20 Aug-30 Time Oaklsland Aug-10 Aug-20 Aug-30 Time OCP1 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 29 of 78 Sep-09 Sep-19 Sep-29 Sep-09 Sep-19 Sep-29 Sep-09 Sep-19 Sep-29 a Jul-31 Aug-10 Aug-20 Aug-30 Sep-09 Sep-19 Sep-29 Time 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' 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 Joy Bird Shoals Borrow Area Modeling June 1Z 2020 Page 31 of 78 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 Station Eleven Mile ADCP MAE (deg) 33 RMS (deg) 46 Bald Head Island ADCP 32 56 Oak Island ADCP 15 23 OCP1 ADCP 15 22 0 330 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 32 of 78 40 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 OC131 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 June1Z 2020 Page 34 of 78 ElevenMile 3 Measured 2 .......Modeled. �. ..................... ,...... q ........ ..:... ...... ............. ... _Ca * _ > • • s « � 1 s 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time BaldHead 3 Measured t Modeled m 2 a� n w . ■ Q, t 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time Oaklsland 3 Measured L Modeled m2 m U) 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time OCp1 3 Measured L Modeled m2 _ • • ._ • _ 1 R...+.. .... r ...... .... i;...... ............ _m 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 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 35 of 78 ElevenMile 20 0 Measured a 15 Modeled . ......... o x. d 10 r: r ♦ • w •♦ ♦ w ► r ♦ :w w ., ram♦ w co 5 'r Tok N a i 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time BaldHead 20 a Measured ' Modeled r ■ r ■ r r ►► ►► :w r rr s a 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time Oaklsland 20 ► Measured r 0 15 w s r Modeled ... '...._ ....... ......' N ►► � ► t w w ■■ w w► w www 'li iti► w w� w► ► > coA ,_...�►.. �ac� .................. 1. .i. _i4�►w�w:S.... ,'�rww-i....w ....i�L...�r�,�i ..t...: �: w,,,,,.,,s..• co V 0 Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time OGP1 20 r • Measured Z (n v 15 « ► Modeled ,» ... «....... ......,.., ......... �, .., .. M .......... 10 rJ A A4 W WN jr-i -C W f .� • ► r� ■ --7��_ ti. �':'+' �.�.�... ♦ •.. _fir- ... .� _..,. �.._ ► . ..�..� �- i. 444 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 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 36 of 78 ElevenMile z 360 (D ,. 300 Measured �, • ... ......... •.' .. • '3" ........ ~ .t�.W... .. ° 240 180 Modeled .. 9 I •. ..... _ «l� mil'Ap •w.. . AAA *2 Y c15 0 a Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time BaldHead 360 , 300 Measured + • Modeled • 9 180 _' • •_ . �It, 120 «�,.• _ _~• • 60 .............. �....: ......... Y 0 i a Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time Oaklsland 2' 360 •. 300 Measured . Modeled 180 - +w .... , Y 60 _ _ _ • • �. _ , ..�. , 0 a Jul-06 Jul-26 Aug-15 Sep-04 Sep-24 Oct-14 Nov-03 Nov-23 Time OCP1 z 360 'E' 300 Measured 0 240 Modeled .. _, .� ..... .�, ........ ; ....... . 180 _ .c • 'i7t 'tee sc • _ ... j120 •.... > �.� ........• .......... •. ..........�... ........ .... 0 a 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 Town of Oak Island Joy Bird Shoals Borrow Area Modeling June 12, 2020 Page 37 of 78 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 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 Eleven Mile ADCP MAE (deg) 40 RMS (deg) 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 01 constituents, where the resulting tidal time series is based upon the following relationship: 77 = Corr * M2 cos(C)M2t + TM2) + C1 cos(wC1t + TC1) (7) C1 = vr2 * _01* K1 and cpC1 = 0•5(�Px1 + �001) N Where, q is water surface elevation, (o denotes angular frequency of tidal constituents, cp 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 01 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 DeIft3D 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 June1Z 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. 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 aS U 50 3.88 X W 25 5.44 10 7.38 8.73 12.07 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Significant Wave Height, ft Figure 3-24: Annual percentage of exceedance of significant wave height at the offshore boundary Total L 15 > 12 c� 9 cu �v- 6 rn Un 3 0 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 40 of 78 Significant Wave Height (Annual) Station 41013_comb_NDBC_WIS_WW3 Period 01-Jan-2004 to 31-Dec-2018 N NW/ - NE 20% 15% 10% I / 1 5% I I I 1 I I 1 I W! --!--L--L- 0.00 - - -'E I I I 1 1 I SW ` SE n N>-15 [-112-15 F-]9-12 F-16-9 03-6 N0-3 Direction FROM is shown Center value indicates calms below 0 ft Total observations 131478, calms 0 About 0.0144% of observations missing Percentage of Occurrence 0.60 1.77 4.31 9. .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 1.44 1.88 1.32 1.05 0.33 0.30 0.24 0.18 0. 0.42 1.17 2.57 4.53 5.91 8.30 . 3 4.55 5.33 5.10 3.74 0.85 0.47 0.52 0.42 0. 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 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 MWID-bin 0-1 90 - 105 Bin average sig. 2.5 Bin average peak periodHs-bin 9.0 Bin average Wave 97.7 Percentage Occurrence 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 1 105 - 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 1Z 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 1 210-225 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 ...... ��if �.�mm R.a,w��'sT1. �__9,�,,s fi►16� � �� i`�' �1 Te r �V d v�.. FP 33R 3 1z ryJ 11f g... - +.R[H 1�� ,_ ' Fr-y _ � �.., ;:►� Rr�i r _ _ � � 15n 9e ' jwo 6s 10m, g '• .. _ .. _ lPFI da 70n db1 jyppQ�S' .I�y19 .e ` 3 S6Ifi 21 R 8 �Li87�g1J}'�� s Ti1 . 12 _ iq •, - IBN� ' if7 •• s r� a e - a .16 a Le - 1• 1 —may � 8�-_ iy f r 3 1 3 r— P1 { 3 B @ naafi 1i ey f i0 '�. _, IR:14• 5 q � 5 9 . � 3 6 \ q � F Pys' 'N ,9 ,• 2nm L ?.-•B s yL• 2•2 24 201 ,6 S 50 8 7$.- `/ � x5 airs D'nfA S Y: kv Pi 89C gi f Wa>�Li nop 17 11 /$/ 14 lo�`• .::f¢...... / fY ` n ? p I 5 L 129 20u G'1,•' /%a9 a Bald Head PB 29 P R 4.1 fA`i2• 1 15 I i91- l7R n ,� z, 10 L , 31 20 19 1 ZO 27 1123 20��! /eon � �� ' 3 W �� 1y 2017 Ip 1 �! , aid Head Island 20 19 , t9/ 13 .... ii SJ . 1 10 31,• I 17 J 17 S 6 Legend 5 22 3 �t�17�7 '2 ..s �f �r v ,a S ,a 7 ,e � 17 16 p OPTI tran8 uts \ 32 22 19 �� 28 Fu A—ri,le�e, GeeE1b. EMnsler Geeprepnie,AC E3rRinus oB. 11S`L>i, Iy9GS, 78 18 r / %. Rb.4.tJ-]pP.w -.nn'3 an't'e 5 1Ir nIy iq 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. 5 30 4.5 4 25 3.5 E T 20 3 E m Y C Y 47 U t a 15 c m g 2 a T � N 10 1.5 1 5 0.5 6 690 695 700 705 710 x coordinate (km) Figure 3-27: DeIft3D initial sediment layer thickness 3.6.6 Model Calibration Results For this study, the default non -cohesive sediment transport formulations in DeIft3D 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 DeIft3D. 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 DeIft3D 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 JANUARY 2007 SURVEYS Baseline For Cross Sections SHOALING AREA a Fi isr W 30 Fr Q6 rr ^ �o FT Sllij]`T ,U FT ae Fr aarT +a FT zrT SHOALING AREA +6 FT ae FT w rr auF ixreer Depth Feet MLLW NOVEMBER 2008 SURVEYS Baseline For Cross Sections SHOALING AREA awMs ��"n 11T g ° I�IIIIF11T FT L1j^III} 11i r+ L� I SHOALING AREA --_ a Depth Feet MLLW AUGUST 2010 SURVEYS Baseline For Cross Sections SHOALING RBER FT �,i19 tpwA e�� Tv� OFe W �aR �I .4i R SHOALING AREA �* n FT i a mar• WAE,NF Depth Fen MLLW 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: DeIft3D 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 forthe entire maintenance period spanningthe 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) Baldhead Shoal Reach 1 Baldhead Shoal Reach 2 Smith Island Total 442.5 772 161,513 589.3 608 215,095 505.8 216 184,617 507.0 1596 185,055 517.0 773 188,705 712.2 512 259,953 321.7 152 117,421 565.9 1437 206,554 431.0 811 157,315 591.2 611 215,788 878.2 153 320,543 536.6 1575 195,859 507,533 690,836 622,581 587,468 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. Table 3-10: Shoaling volume rate calibration results (cy/yr) dso = 0.1-` Modeled dso = 0.2C dso = 0.2-` USACE (2011) Condition survey (11/2015 — 11/2016) Condition survey (11/2016 — 12/2017) Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 51 of 78 _7113aldhead mm mm mm Shoal 483,000 207,570 126,270 184,690 Baldhead Shoal 429,540 176,730 508,600 1,421,140 395,760 780,060 130,250 206,590 324,600 287,490 292,630 549,150 196,000 587,280 106,090 109,830 161,180 591,870 237,890 635,210 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 1.703 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 1Z 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. 17 m 15 0 14 7. r ll ^TFTr/� 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate 0 1 2 3 4 5 6 Flood Currents - Existing (ftls), magnitude 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 1 q� 1r/rs� a 14 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate (km) 0 1 2 3 4 5 6 Flood Currents - Template 2 (ftls), magnitude Figure 4-6: Peak flood currents— after -dredge Template 2 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 58 of 78 17 16 T 15 fl G E -2 14 00 U T 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate (km) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Flood Currents - (Template 1 - Existing) (ftls) Figure 4-7: After -dredge bathymetry effects on peak flood currents —Template 17 1s fl T 15 G y 41 fd L C -2 14 0 0 V T Z 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate (km) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Flood Currents - (Template 2 - Existing) (ftls) 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. 17 L L_" "i, T 15 0 14 12 694 695 696 697 698 699 700 701 702 703 x coordinate 0 1 2 3 4 5 6 Ebb Currents - Existing (f 1s), magnitude 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 15 fir!! 14 r o f 13 f/ b 12 s` 694 695 696 697 698 699 700 701 702 703 x coordinate (km) 0 1 2 3 4 5 6 Ebb Currents - Template 1 (ftls), magnitude Figure 4-10: Peak ebb currents — after -dredge Template 1 17 16 _ - c 13 12 ✓r 694 695 696 697 698 699 700 701 702 703 x coordinate (km) 0 1 2 3 4 5 6 Ebb Currents - Tempalte 2 (ftls), magnitude Figure 4-11: Peak ebb currents — after -dredge Template 2 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 2020 Page 61 of 78 17 16 T 15 fl G -2 14 00 U T 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate (km) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Ebb Currents - (Template 1 - Existing) (ft(s) Figure 4-12: After -dredge bathymetry effects on peak ebb currents — Template 1 17 1s fl T 15 G E p m id ti q =0 14 0 0 V T Z 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate (km) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Ebb Currents - (Template 2 - Existing) (ft(s) 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). 17 - f a 14 13 12 i 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 - Existing (ft/s), magnitude Figure 4-14: Residual tidal currents— existing condition Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 1Z 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 1Z 2020 Page 64 of 78 16 T 15 G p v � -2 14 � q T 13 12 694 696 696 697 698 699 700 701 702 703 x coordinate (km) -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) (ft/s) Figure 4-17: After -dredge bathymetry effects on residual tidal currents —Template 1 17 1s Il T 15 G E p m id ti q -2 14 0 0 V T Z 13 12 694 695 696 697 698 699 700 701 702 703 x coordinate (km) -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 Residual Currents - (Template 2 - Existing) (ft/s) 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. 694 No No 697 698 699 70C 701 702 703 N....climate (km) - 17 t6 � 1s a1a 13 12 694 695 696 697 898 899 700 701 702 703 x coordinate (km) - Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 67 of 78 89d 695 896 697 698 599 700 701 702 To x coordinate (km) ti -2 -1.5 -1 -95 0 0-5 1 IS 2 -2 .1.5 -1 -05 0 0-5 1 15 2 -2 .1.5 -1 -0.5 0 0-5 1 IS Signil Wave Height Difference (Template 1-E—fing) (h( Sign4fimnl Wave Height Difference (Template 1-Exbfing) (ft) Signiifil Wave Height Diff— (Template 1 - E.il(ft) 17.,;;1?,,,,;;;:,.r„•,, SE2.5ftincoming wave '7T'yy�44rS2.5ftincomingwave r5 15 4„"''%';;; ,•%' 7, ..•, ��1yr1,]Lrc11 �1✓�,l l�� / 16 l E E r�4 rs y E 4 r ti 1 74 ii} tt;f;a 1e 4 4 444r r 6� _ 1(�' y4j` �'` , 1�l+,}'try }�ll!1'V4�7!V1��•V''�,�1 jr r�l r f'� 131'4}�i' 1,'t, �',{E'l4 ly1={}F�i„yr7',r......;;,• 13 rr 1< l' rrl rfl 11r �NI r� 111 1 r /f,� ��!,�;(p', i}4't+)}4'ti}t '�41�,44��r'�}' 4t�'i •` ty44, yi !I r' t244}'y1,�L4`}}a1}i}}411+44}1V4+{t�l'14�+}ylltl�'y\�{I,}}V``',,{'I`;I�,t�k��5 ^,.. r ,,r 1 r { r 2 ' j' ;''' f y r 696 695 696 897 898 699 70C 701 702• I• Tij 694 695• BOB 697 698 699 700 701 702 ` 703 x coordinate (km) —• x coordinate (km1—s -2 -1.5 -1 -0.5 0 0.5 1 1.8 2 .2 .1.5 .1 -0.5 0 0.5 7 1.5 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Signifimnt Wane Height Difference (Template 2 - Existing] tm SignVfieant Wax Height Me— (Template 2 - Existing) (tti Significant Wave Height Difference (Template 2 - Existing) {N 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) 2 696 695 898 697 998 899 700 701 702 7W x coordinate (km) - 2 T x Egg_ tl 694 695 696 697 696 599 70C 701 702 703 x coordinate (km) - -1.5 -1 -11-5 0 0-5 1 1.5 2 Significant Wave Height Difference (Template 1 - E,,fing) (h) }kk}},rX':• •. SE 4.51t incoming wave 694 695 890 697 898 6% 700 701 702 7W x coordinate (km) -> .2 .1.5 .1 -05 0 0-5 1 16 2 Sign,fi®nl Wage Height Difference (Template 1 - Existing) (il) m T 15 E 1/ a 12 4}+'1V'}}+�V}tt�1VV1t1r{'�i.i1r+�1,VVt� � }I�t.,,,,;::t;� }iliil;� 1x � � y N' }Y r'�rr �P 1' 1jr4 }1ty'{• 3 wy' 4+1++4+1 1,1V11+tp5+++1V++)+++i++'t�++��r'+',�,`,;,.,,, ,• sl!�� ItISI+'�,�,r � �;,,;, r }!fr• rr' ' 4 1'' i �y'r, B96 695 696 697 BOB BOB 70C 701 702 T03 694 895 090 697 898 699 700 701 a702 a 703 x coordinate (km) -r x coordinate (km), -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 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Significant Wane Height Difference (Template 2 - Existing) tm Significant Ware Height Difference (Template 2 - Existing) (M Significant Wave Height Difference (Template 2 - Existing) CN 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 June1Z 2020 Page 68 of 78 89d 695 896 697 698 699 700 201 702 703 x coordinate flan) -2 .1.5 -1 -0.5 0 0-5 1 1.5 2 Sip,fi nt Wave Height Oft.— (Template 1 - Existing) {ft) WM 695 BOB 697 908 899 700 701 702 703 k inordinate (km) -r 694 We 096 597 698 599 70C 701 702 703 k....climate (km) - -2 -1-5 .1 -95 0 0-5 1 1.5 2 Significant Waue Height Difference (Template 1 - E..Imq) (h) 17 Yr y.,..,y incoming wave 17 m I 694 695 696 697 898 699 700 701 702 703 ...ordinate (km) - -2 .1.5 -1 -0.5 0 0-5 1 15 Signifimm Wane He lght Difference (Template 1 - Exbfing] (41 T E E T E QG tl Town of Oak Island Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 69 of 78 594 695 596 897 BOB 699 700 701 702 703 x coordinate (km) -. -1 -0.5 0 0-5 1 1.5 Significant Wave Height Difference (Template 1 - Existing) (8) 2 B94 %5 696 897 BOB BOB 70C 701 7W 703 894 695 696 697 698 699 700 701 702 703 694 695 896 697 698 699 700 701 702 703 x coordinate (km) -r x coordinate (km) -a x coordinate (km) -r -2 -1.5 -1 -0.5 0 0.5 1 U 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 Significant Wane Height Difference (Template 2 - Existing) tm Signficant Wave Height Difference (Template 2 - Existing) (tti Signficant Wave Height Oi0erence (Template 2 - Existing) (8) 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) 17 i�lr s11s� Sti, �} SE 20 ft incomiIs ng wave r tt{�i`r 45}I`i' x 14 +,+i�i;+'� !1 } }'+,fit f S }s;f(�;;;;-•':;','::'��>;:"'�/ 'i}1j�'y�i�r1'1'''�'{�4{}�{S4'S�r}��{'�j}��f}`S�I'i��,';;'i'•:�;;�,r��, ,/j�J. 12'4j'j1'IS4}iSll'4}}}iSS}i4}44SSSiS1}}isSIS4Si'SV14Pi'}Sy��}i'',','�S;i�•.t';'l`r/, S},t1�+111}41}S1y11!illtS�lS�tliSliitirl�el�V}515�1;44}il}.I}4e}k1S�:t.� t.l%-"�fi?��iii�'r,�rrrr%n,,..,.,. 696 695 696 697 598 699 700 701 702 703 x cooNina4e (km1 � -2 -1.5 .1 -05 0 0-5 1 15 2 Signifinni Wane Hight 091emrm (Template 1 - Existing) (N 17 SE 20 ft incoming wave 14 j 13 11,}, il�y' ,�t,,f'�, t� r�� r# y!'l;:y,,•s�;'•: ;r:::: � ; " ,;,t,' 12 1��1'i4lySlfii�}I�SlFilli�till{iSl}}i}44iV4�}SS'9ySf�ti,ff�'11'�'�'��''�'', 14 1.t1-u1d.1."411S.L�16U..'rA3,�ht'cLu�'�ir .'�i�i..e. vrrrnv.tr,''!'�' 696 695 698 697 698 699 700 701 732 703 x cppl0inale (kml v 17 16 ? 15 i i, 13 694 695 696 697 696 699 70C 701 702 703 x 000rtlln8te (km} -, Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 70 of 78 696 695 696 697 898 699 700 701 702 703 x_.dmM (km)-> -1,5 -1 -0,5 0 0,5 1 1.5 2 -2 -1-5 -1 -05 0 0-5 1 15 2 Signftin t Wage Heigh 011( tense (Template 1-ExMng) (ft) Signifimnt Wane Hight Oiflmenw (Template 1 -E..hfi g) (ft) 17 SW 18 ft incoming wave 13 12 f 'y r�, ,t• yam, . J/%��1�.� ram. _ ,{�/ � =<,r.,s�rA �4✓,.srrrr�a.,;r�1il� 694 695 696 697 ME 699 700 701 702 703 x —d iv. (km) -, 896 695 698 897 898 ME 700 701 732 703 x C.. 0mate (km) -a -2 .1.5 .1 -0.5 0 0.5 1 1.5 2 -2 -1,5 -1 -0,5 0 05 1 1.5 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Sigifi.nl Wane Hight Di%.,- (Template 2 - Exiefing) (ft) Signfir-an! W.- HeigM D ft,ence (Template 2 - Existingl (ft) Signofitant Wane Height DUN— (Templak 2 - E—Ung) (R) 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 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 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. 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. 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 72 of 78 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 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 73 of 78 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. 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island 20D Y 10D r a c F 50 v E v d d o o 0 iv -50 100 Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 74 of 78 I ■ ----J. ---------------------- ;11 !'I --- --- --�---------------- 'ii� t i { Westerly Transpon 1 t • o 0 of o000o aooaoo � a00000 oo oa oo' ooa ooa' o0 00g 000 0 0 0 00000 c ggc gg c gc gc q gc gc �c c gc �qc gg TT VgYl0000$ 00000�0000 o utv A----- ovan van van X.v++++ ++++++ m rm+r�-NNNNNN � NNNm NN�vNvom mmfo��la m��p'� ��v vm mm$ N�r�mmr�v 1 ' !1 1 Easterly Transport t I � ldd2 + SurveyTransectsNet Transport - Existing Net Transport - After -Dredge Template 1 Y �r —Net Transport - After -Dredge Template 2 ! I I ! 1� 1� 691000 692000 693000 694000 695000 696000 6970DD 698000 699000 700000 701000 Easting - NC State Plan (m) Figure 4-24: Wave -induced net longshore sediment transports along Caswell Beach shoreline 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island 60 40 _6D _80 100 Jay Bird Shoals Borrow Area Modeling June1Z 2020 Page 75 of 78 -rs-S? ter;-i! 1 c ozrcaau;<ui cC a -ts ovtsa A9, .vNrnmw°Vn nm�v+ --..... 5--.....�.---.... T--------r -----------.-T------.-r---- -r. -.}. ------ I I + SurveyTransects--"""'-"" """"'--"".."'--"" Jay Bird Shoal Qorrow Site —Transport Gradient - Existing Transport Gradient - Aker -Dredge Template 1 ___ —Transport Gradient - Aker -Dredge Template 2 691000 692000 693000 694000 695000 696000 6970DD 698000 699000 700000 701000 Ea sting - NC State Plan (m) Figure 4-25: Longshore sediment transport gradients along Caswell Beach shoreline 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 76 of 78 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' DeIft3D 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 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 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 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 77 of 78 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. 202112022 Renourishment Project M&N Project No.10128-01 Town of Oak Island Jay Bird Shoals Borrow Area Modeling June 12, 2020 Page 78 of 78 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, 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, Pages1421-1434. 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 Golf randvoorwaarden OPTI Manual." Research report H4959.10. WLI 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, WLI 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. 202112022 Renourishment Project M&N Project No.10128-01