20230797 Ver 1_Other_20230705

4700 Falls of Neuse Road, Suite 300
,", Raleigh, NC 27609
(919) 7814626 Fax: [phone]
m o f f a t t & n i c h o l www.moffattnichol.com
MEMORANDUM
To: Eugene Foxworth, Assistant County Manager, Carteret County
Cc: Douglas Huggett, Moffatt & Nichol
From: Mark Pirrello
Date: June 7, 2022
Subject: Flushing Analysis — Western Carteret County Boat Launch Project
Project Overview
The Western Carteret County Boat Ramp Facility (Project) is located on the north coast of Bogue Sound,
approximately 4.5 miles east of Cape Carteret. The project site is accessed through an entrance road on State
Highway 24, approximately 2,000 feet east of the intersection with J. Bell Lane. Carteret County received a
permit from the US Army Corps of Engineers (USACE) to construct an entrance road that provides access to
the boat launch facility and the adjoining North Carolina Coastal Federation (NCCF) property. Figure 1 shows
the project location.
Figure 1: Project Site
The proposed boat launch facility consists of excavating 1.5-acre launch basin to a minimum 5-foot water depth
@ Mean Lower Low Water (MLLW), a 450-foot-long x 50-foot entrance channel, six (6) concrete boat ramps,
4 queuing launch docks, and a 159-space trailer parking lot. The perimeter of the launch basin is stabilized with
riprap.
The geometry of the interior basin was guided by several considerations including safe and efficient launch
operations, existing site topography, minimization of impacts to coastal wetlands, the Outstanding Resource
Water (ORW) coastal line requirements, the Shellfish Sanitation water quality buffer setback, and water
exchange/flushing guidelines from North Carolina Department of Environmental Quality (NCDEQ). This

June 7, 2022 Memorandum
memorandum discusses the evaluation of flushing for the interior launch basin and the selection of final
geometry to meet the state's guidelines.
The County in conjunction with the North Carolina Department of Wildlife Resource (WRC) had developed
several interior basin configurations that considered the ORW coastal line requirements whereas the interior
basin was placed farther north with the overall geometry was more rectangular in the north -south orientation
as shown in Figure 2. Refinements to the site plan were performed to minimize impacts to coastal wetlands
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Figure 2: Initial Site Plans with Basin Geometry
and take advantage of the lower topographic relief in the central portion of the property. Figure 3 shows the
refined geometry that was selected for evaluation of basin water exchange in flushing. The basin in Figure 3
consists of a 1.5-acre basin with water depth of -5 feet relative to Mean Lower Low Water (MLLW) or elevation
-6.2 feet NAVD (land based vertical datum). The basin is approximately 470 feet long (measured from Mean
High Water (MHW) line and approximately 125 feet at center point. The dredge channel is 450 feet long and
approximately 50 feet wide and connects the basin to Bogue Sound.
The purpose of this study is to analyze the flushing within the proposed launch basin. As stated in North
Carolina Department of Environmental Quality's Water Quality Guidelines for Planning of Upland Marina
Development, an inland basin with good flushing characteristics will reduce the concentration of a non -
decaying conservative pollutant to less than 10 to 15% of its initial concentration within a 24-hour period. This
memorandum summarizes the hydrodynamic modeling that was developed by Moffatt & Nichol (M&N) to
evaluate flushing characteristics of the project..
Hydrodynamic and Flushing Modeling
Approach
The relative rate in water volume exchanged in the canal will be evaluated numerically by placing a non -decaying
conservative pollutant in the basin and evaluating the percent residual concentration of the pollutant at the
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June 7, 2022
Memorandum
Figure 3: Preferred Basin Configuration for Initial Evaluation
most north point in the basin over time. This situation is representative of a boat spill that would occur during
a launching or retrieval operation. The rate of concentration reduction within a 24 hour to 168-hour window
was calculated to assess how well the basin exchanges water with Bogue Sound.
M&N developed a depth -averaged two-dimensional hydrodynamic numerical model of Bogue Sound, Bogue
Inlet, and the project site as shown in Figure 4. The model was forced with the hydrodynamic conditions
extracted from TOPEX/POSEIDON global tidal model. The model mesh was reduced in size at the project
site to allow for a more thorough definition of the local topographic and bathymetric features and optimization
of the model runtimes. The MIKE21 models were used, which are a part of the MIKE by DHI suite of models.
The models are suitable for evaluation of changes in water levels and tidal currents for existing (baseline) and
proposed conditions (upland basin addition) at the project site. The MIKE21 Hydrodynamic (HD) model
simulates unsteady free -surface water levels and flows in water bodies where vertical stratification is negligible
(representative of hydrodynamic conditions at the project site). The model is designed to simulate tidal flows
at the project site, with water levels and depth -averaged flow velocities extracted at locations of interest. The
hydrodynamic model was then coupled to the MIKE21 Advection-Diffusion (AD) model to evaluate the
transport and mixing of the conservative pollutant in the basin. The AD model simulates the transport and
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June 7, 2022 Memorandum
dispersion of the conservative pollutant in water, with hydrodynamic forcing provided by the flow conditions
generated by the HD model.
The model includes all of Bogue Sound, Bogue Inlet and nearshore Atlantic Ocean area and the White Oak
River. The model is composed of triangulated mesh which have varying resolutions with cell sizes from 1000 ft
within the offshore -most areas of the Atlantic Ocean to 300 ft within Bogue Sound and 15 feet inside the
upland basin. The shoreline in the vicinity of the project site corresponds to the Mean High -Water Line
(MHWL) and was obtained from local bathymetry. The model bathymetry in the offshore areas of the Bogue
Sound was derived from the USACE data and data from C-MAP provided by Jeppesen and NAVIONICS.
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* Project site
4 2 0 4 Miles
Figure 4: Model Domain
Model Domain
Boundary Conditions
For calibration analysis, only tidal variations in water levels were applied in the model. Wind, waves, and other
freshwater inflows were not considered to evaluate the flushing characteristics of the project site under a more
conservative condition. The boundary conditions for the HD model consisted of a tidal signal along at the
boundaries of the MIKE 21 model (using Tidal Constituent data provided by TOPEX/POSEIDON).
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June 7, 2022 Memorandum
For the flushing analysis, an average tide was considered for the hydrodynamic analysis as it is reflective of
current velocities through the neap and spring ranges and, therefore, yields a representative assessment of
flushing performance of the basin. A Flather condition was used to capture tidal forcing and water currents to
drive the model and, therefore, were not included separately in the model. Further, a constant bed friction of
55 m1 /3/s (M = 1 /n), a Smagorinsky eddy formulation with a constant value of 0.28, and a Coriolis forcing
varying in the model domain were also specified for calibration. Horizontal dispersion was defined by a scale
eddy viscosity formulation. To capture the proposed upland basin, the topography and bathymetry were
adjusted to incorporate the basin and entrance channel.
The AD module specified an initial condition of a non -dimensional non -decaying conservative pollutant
concentration of 100 released over a period of 20 minutes at the northmost point in the basin (Point P01 in
Figure 5). The 20-minute period would be representative of a fuel tank spill as vessels are launched or hauled
out of the basin. This tracer concentration is a value that is relative to the remainder of the model domain. At
all other locations within the model, and at the boundaries, the initial concentration is specified to be zero.
U2
Bathymetry [ft-HAVD88]
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-12 -41
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-14--13
-15--14
Below -15
Undefined Value
Figure 5.-Basin Batbymetry and Observation Points
Calibration
Data to calibrate the model with historical recorded data was limited and the tide gauges within the model
domain were based on predicted tidal data and current and water level data collected during a three-day field
collection program in October 2020. Current and water level data was collected at several locations along the
south shore of Cape Carteret and the NC24 bridge at Swansboro as shown in Figure 6. For model calibration,
the high and low tidal predictions were downloaded and then interpolated to create an hourly time series and
compared to the field collected data. A total of five tidal constituents were used to generate Flather boundary
conditions. The Flather boundary conditions generate numerically stable offshore boundaries and are efficient
at downscaling models from coarse to finely resolved areas. Each of these constituents were used and then the
amplitude of each constituent was altered by the percentages shown to allow for calibration of specific tidal
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June 7, 2022 Memorandum
constituents. These data were then used to calculate tidal constituents which were then compared to model
output at each gauge location.
Figure 6: Location of Current and Water Level Measurements
A quantitative model calibration was performed, and it was determined the model adequately reproduces the
water level phases and amplitudes.
Model Results
The initial concentration of a non -dimensional conservative tracer was specified as 100 and dispersed over a
20-minute period in the proposed basin. The local hydrodynamic model forced with an average tidal signal for
a period of 7 days (Figure 7). The guidelines for upland basin indicate the residual concentration value after 24-
hours should be less than 10 to 15%, which would be indicative of the overall water exchange and quality for
a body of water. Model results were analyzed within a polygon over the entire basin area to analyze how the
tracer evolution behaves throughout the basin shown in Figure 7.
The results indicate that the residual concentration at the inner basin was greater than 20 percent after 24 hours
and did not effectively change after the 7-day period. The velocities were very low in this area and that mixing
was minimal with the change in the tidal prism. As noted in the state guidelines and discussed in the ASCE
Manual No 50 Planning and Design of Small Craft Harbors, the geometry of the basin heavily influences water
exchange in the basin. Upland basins that extend further inland and are narrower in width will not exchange
water as effectively as the basins that are wider and closer to the entrance.
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June 7, 2022
Memorandum
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Figure 7: Results of Initial Basin Geometry
3 a
Days after simulation starts
5 B 7
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June 7, 2022
Memorandum
X
Figure 8. Revised Basin Geometry
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June 7, 2022
Memorandum
100
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Days after simulation starts
Figure 9: Results of Modified Basin Geometry
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June 7, 2022 Memorandum
The basin geometry was significantly modified to improve water exchange. The basin was modified to a square
shape, with the distance to the north extent of the basin reduced from 470 feet to approximately 250 feet and
the basin width increased from 125 to 175 feet. The bottom width at the basin entrance with the navigation
channel was also widened from 50 feet to 65 feet, consistent with recommendations from industry and state
guidelines. Figure 8 shows the revised geometry of the basin.
The model was executed for the revised geometry. The results shown in FiguError! Reference source not
found.re 9 indicates that the residual concentration of the tracer was reduced to under 10 percent in a 17-hour
period at the north extent of the basin P01). The residual concentration at the basin entrance occurred in the
14th hour.
Summary
The hydrodynamic model developed by M&N simulated tidal propagation in Bogue Sound and the proposed
upland basin. The simulated water levels were compared to predicted data and acceptable agreement was
observed as shown by the calculated statistics. The hydrodynamic characteristics within a polygon covering the
area of the upland basin were analyzed using the model. The variation in the concentration of a tracer was
modeled for a period of 168 hours, considering average tidal forcing only.
The tracer concentration for final basin plan conditions initially reaches the concentration of 10 percent in
approximately 3 hours into the model simulation, increases above 10 percent then decreases and stays below
10 percent at 17 hours into the model run. The trendline computed from the time series of concentration shows
that on average, the concentrations reduce below 10% after 17 hours.
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