20230797 Ver 1_More Info Received_20240509 (2)

Model Plan
Western Carteret County Boat Launch Facility
Moffatt & Nichol will conduct a hydrodynamic and transport modeling analysis to
evaluate the flushing characteristics of the proposed Western Carteret County Boat
Launch Facility (BLF). The proposed BLF is located on the northern shore of Bogue
Sound, North Carolina approximately 7 miles east of Bogue Inlet. The location is shown
in Figure 1.
Figure 1 Site Location
The Environmental Fluid Dynamics Code (EFDC) will be used for the analysis. The
model is in the public domain, supported by the USEPA, and has been successfully
applied in 100's of studies worldwide.
Bogue sound is connected to the Atlantic Ocean by Bogue Inlet to the west and by
Beaufort Inlet to the east. Both inlets provide a connection to Bogue Sound but also
connect other water bodies to the north (White Oak River at Bogue Inlet) and to the
north and east (Newport River and Back Sound at Beaufort Inlet). There is no
measured tide or current data within Bogue Sound available from typical federal agency
sources (NOAA, USGS, NWS). NOAA does provide predicted tide data at stations
within and adjacent to the sound. These data conditions guide the model domain and
implementation.

Flushing in the proposed BLF is controlled by the tidal prism and dispersion both within
the BLF basin and adjacent to the BLF basin entrance. As there is no data in the
vicinity of the BLF to use as boundary conditions, a nested grid approach will be used.
A regional grid will be developed to simulate the tides and velocities within the sound,
and then those results will be used to provide boundary conditions for a local grid that
provides sufficient grid resolution in the vicinity of the BLF. The approximate extent of
the regional and local grid domains is shown in Figure 2. The regional grid domain, (red
polygon) consists of the majority of Bogue Sound. The local grid, approximated by the
black rectangle includes the proposed BLF site and will extend sufficiently far from the
site so that the boundaries do not impact the simulated transport at the site. The actual
site domain will conform to grid lines in the regional grid to facilitate the date transfer
between region grid solution and the local grid boundaries. The actual extent of the
local grid domain will be determined after the reginal grid simulation is calibrated and
the tidal excursion lengths in the vicinity for the BLF can be estimated.
Figure 2 Regional and Local Grid Model Domains
The regional calibration will be conducted using NOAA predicted tide data for both the
boundaries and calibration at interior points. The hydrodynamic situation is complicated,
not only due to the lack of data, but also due to there being two inlets to Bogue Sound.
The flushing characteristics at the site are dictated by the tidal prism (with the tidal
amplitude being a surrogate) and the tidal velocities adjacent to the site. We expect
some attenuation of the tide amplitude and subsequently the tidal velocities as one
moves from either inlet to the center of the sound. However, a quantitative assessment
of these processes is difficult because they are sensitive to the relative magnitude of the

flows occurring at each inlet. A strategy for calibrating the regional model and making a
reliable estimate of these conditions at the site is presented in Figure 3.
F
Brogue Inlet
(high/low)
C 1
50
F=
.2
3
11712024
Spooners Ireek
snmu
� main
NC State Fisheries
(High/low)
Beaufort Inlet
Channel Range
(High/low)
1/g/2024 1/91202A 1/10/2024 1/i112024 1/12/2024 1/13/2024 1/11/2024
—Rogue Inlet —Spooners—NCState Flshwles —Beaufort Inlet
Figure 3 Approach for Model Forcing and Calibration
There are four NOAA sites with precited tides that can will be used and are indicated in
Figure 3. The two inlet stations (Brogue and Beaufort Inlets) will be used as water
elevation boundary conditions. The other two stations are in the interior of the regional
model domain and will be used for calibration.
The time series plot within Figure 3 shows the NOAA predicted tides at the four
locations. The phases of the tides at those two locations are the same, but the
amplitude of the Beaufort Inlet tide is higher, on the order of 5 feet, compared to the
approximate 3.5 foot tide at Brogue Inlet. The two interior stations show both a phase
lag and amplitude attenuation as the tide propagates into Bogue Sound. It is assumed
that at these locations, the water is flowing from Beaufort Inlet, (as opposed to flowing
from Brogue Inlet) during the flood tide since they are located closer to Beaufort Inlet.
However, the model calibration will determine the actual contribution of each inlet to the
tides along Bogue Sound.
The model calibration will consist of simulating a two -week period using the regional
model and varying the bottom friction and lateral dispersion coefficients until good
agreement with the NOAA predicted tides at NC State Fisheries and Spooners Creek is
obtained.
Once the regional model is calibrated, the results will be used as boundary conditions
for the local model. Before applying the local model to simulated transport, the local

model simulated results at interior locations will be compared to those obtained with the
regional model at the same locations to assure that the local model is reproducing the
calibrated tidal flows correctly.
The local model simulations will consist of 3 scenarios each representing a different
starting time in the spring neap tidal signal to assure that the range of tidal flows is
considered in the analysis. The model will also include a conservative tracer transport
simulation for the purpose of accessing the flushing characteristics of the BLF basin. An
initial concentration of tracer will be set in the BLF basin, representative of a typical spill
that might occur. Once each scenario simulation is completed the flushing
characteristic will be evaluated. The basis of the evaluation is fitting a decay curve to
the time version of tracer concentration in the basin. A `generic' example of the fitting
process is shown in Figure 4. The coefficients in the decay curve will quantify the
flushing BLF rate.
100
90
J 80
70
bo
0
':P 50
L
= 40
30
0
U 20
10
0
0 1 2 3 4
Time [days]
Figure 4 Example of Flushing Analysis
The work will be documented in a final report, which will include a description of all data
and data sources, methodologies, key assumptions and limitations and any
recommendations if warranted.