NC0026611_Report_20231222

Modeling Report for Mixing Zone Analysis
of Calico Creek at the Morehead City, NC
Wastewater Treatment Plant Outfall
(NCO02661 1)
Dilution Evaluation with CORMIX Model
December 22, 2023
PREPARED FOR
North Carolina Department of
Environmental Quality (DEQ)
Division of Water Resources (DWR)
512 North Salisbury Street
1617 Mail Service Center
Raleigh, NC 27699-1617
OTETRA TECH
PREPARED BY
Tetra Tech
PO Box 14409
4000 Sancar Way
Suite 200
RTP, NC 27709
ON BEHALF OF
P [phone]
F [phone]
tetratech.com
Town of Morehead City, NC
1100 Bridges Street
Morehead City, NC 28557
P + 1 -252-726-6848
In Collaboration with
McDavid Associates
BT Solutions
Shealy Consulting
r
am EAD CITY H C A R O L I N A

Calico Creek Modeling Report for Morehead City, NC
EXECUTIVE SUMMARY
The Wastewater Treatment Plant (WWTP) that is owned and operated by the Town of Morehead City, North
Carolina, is subject to National Pollutant Discharge Elimination System (NPDES) permit limits for various metals,
which conservatively assume that no effluent dilution occurs within the receiving waterbody. These NPDES monthly
average metals limits are 42.6 micrograms per liter (pg/L) total zinc, 4.15 pg/L total nickel, and 1.85 pg/L total
copper. A dilution study was conducted on the receiving water, Calico Creek, to determine whether permitted limits
may be modified based on existing dilution within the near -field mixing zone of the WWTP outfall.
In October 2023, the North Carolina Department of Environmental Quality Division of Water Resources (NCDEQ
DWR) approved a near -field dilution plan entitled "Modeling Plan for Mixing Zone Analysis of Calico Creek at the
Morehead City WWTP Outfall (NC0026611)" to evaluate near -field mixing into the receiving tidal estuarine waters.
Best available information was incorporated to support this mixing zone analysis, including a recent EFDC model
of Calico Creek developed by NCDEQ DWR, data from the discharge facility, and additional field data collected for
this effort.
Using the Cornell Mixing Zone Expert System (CORMIX) mixing zone model, the near -field dilution factor at a
distance of nine meters (one-third of the 27-meter local channel width of Calico Creek) from the Morehead City
WWTP outfall was calculated to be six (6).
NTETRA TECH

Calico Creek Modeling Report for Morehead City, NC
TABLE OF CONTENTS
1.0 INTRODUCTION..................................................................................................................................................4
2.0 MODELING APPROACH.....................................................................................................................................6
2.1 Existing EFDC Model.....................................................................................................................................6
2.2 CORMIX Mixing Zone Model.........................................................................................................................7
3.0 CORMIX MODEL DEVELOPMENT.....................................................................................................................8
3.1 Modeling Period Determination......................................................................................................................9
3.2 Dilution Scenario Selection.........................................................................................................................
10
3.3 Ambient Conditions.....................................................................................................................................
11
3.3.1 Channel Geometry and Slope...........................................................................................................
12
3.3.2 Channel Roughness..........................................................................................................................
13
3.3.3 Wind...................................................................................................................................................
13
3.3.4 Ambient Velocity and Depth..............................................................................................................
13
3.3.5 Temperature, Salinity, and Density...................................................................................................
15
3.4 WWTP Outfall Parameterization.................................................................................................................
15
4.0 DILUTION EVALUATION.................................................................................................................................
18
4.1 Dilution Analysis Approach.........................................................................................................................
18
4.2 Modeling Results and Summary.................................................................................................................
18
5.0 REFERENCES..................................................................................................................................................
20
APPENDIX A: EFFLUENT PLUME VISUALIZATION...........................................................................................
21
N
TETRA TECH 2

Calico Creek Modeling Report for Morehead City, NC
LIST OF TABLES
Table 1. Monthly average water temperature and salinity with calculated ambient density (2015-2020).................9
Table 2. Scenario -dependent (seasonally independent) ambient conditions for CORMIX modeling scenarios..... 15
Table 3. Simulated dilution ratios in Calico Creek with distance from the Morehead City WWTP outfall............... 19
LIST OF FIGURES
Figure 1. Location of the Morehead City WWTP effluent discharge outfall to Calico Creek......................................5
Figure 2. Existing EFDC model grid (left) and velocity cross-section (right) for Calico Creek (NCDEQ 2021).........7
Figure 3. Location of Morehead WWTP outfall, AMS monitoring stations, and Duke Marine Lab tidal gauge ..........
8
Figure 4. Depiction of dilution scenario identification for Morehead City CORMIX modeling .................................
10
Figure 5. Graphical depiction of dilution scenario selection for Morehead City CORMIX modeling based on water
depth over time across the tidal cycle (summer or winter)......................................................................................
11
Figure 6. Topo-bathymetric LiDAR elevation data in Calico Creek.........................................................................
12
Figure 7. EFDC model grid of cells from NCDEQ EFDC model. Predicted values for cell 31,7 were used to
characterize ambient values for Calico Creek in the near -field mixing zone...........................................................
13
Figure 8. Predicted depth and ambient velocity in Calico Creek during a spring tide cycle with scenario depths..
14
Figure 9. Photo of partially submerged Morehead City WWTP discharge pipe with high velocity effluent .............
17
Figure 10. Mean WWTP dilution in Calico Creek across all scenarios by distance from outfall.............................
19
Figure 11. Visualization of the summer effluent plume three hours before high water slack (Scenario 1).............
21
Figure 12. Visualization of the summer effluent plume one hour before high water slack (Scenario 2).................
21
Figure 13. Visualization of the summer effluent dilution plume 1.2 hours after high water slack (Scenario 4).......
21
Figure 14. Visualization of the summer effluent plume three hours before low water slack (scenario 5)...............
22
Figure 15. Visualization of the summer effluent plume one hour before low water slack (scenario 6)...................
22
Figure 16. Visualization of the summer effluent plume 1.2 hours after low water slack (scenario 8).....................
22
Figure 17. Visualization of the winter effluent plume three hours before high water slack (scenario 9).................
23
Figure 18. Visualization of the winter effluent plume one hour before high water slack (scenario 10)...................
23
Figure 19. Visualization of the winter effluent plume 1.2 hours after high water slack (scenario 12).....................
23
Figure 20. Visualization of the winter effluent plume three hours before low water slack (scenario 13)................
24
Figure 21. Visualization of the winter effluent plume one hour before low water slack (scenario 14)....................
24
Figure 22. Visualization of the winter effluent plume 1.2 hours after low water slack (scenario 16).......................
24
NTETRA TECH

Calico Creek Modeling Report for Morehead City, NC
The U.S. Environmental Protection Agency (EPA) issued a new draft National Pollutant Discharge Elimination
System (NPDES) permit on August 19, 2019, for the wastewater treatment plant (WWTP) located in the Town of
Morehead City, in Carteret County, North Carolina (NPDES NC0026611). The Morehead City WWTP discharges
into the tidal estuarine waters of Calico Creek north of the town, which empties into Calico Bay and the larger
Newport River Estuary (part of the White Oak River Basin) (Figure 1). The permit language issued for the Morehead
City WWTP indicates the 2.5 million gallons per day (MGD) facility is required to meet new limits for metals. The
new metals limits and associated parameter codes include:
• Total nickel (01067): monthly average 4.15 micrograms per liter (pg/L), daily maximum 37.35 pg/L.
• Total copper (01042): monthly average 1.85 pg/L, daily maximum 2.90 pg/L.
• Total zinc (01092): monthly average 42.8 pg/L, daily maximum 47.6 pg/L.
Current NPDES permit language conservatively assumes that no dilution of effluent occurs in the receiving waters.
As described in the North Carolina surface water standards under 15A North Carolina Administrative Code 02B
.0204 (b), "A mixing zone may be established in the area of a discharge in order to provide opportunity for the
mixture of the wastewater with the receiving waters. Water quality standards shall not apply within regions
designated as mixing zones...". Per the mixing zone guidance from the North Carolina Department of Environmental
Quality (NCDEQ), the Division of Water Resources (DWR) will evaluate the feasibility and appropriateness of a
mixing zone if it is requested by the permittee (NCDEQ 1999). The Town of Morehead City has requested the
application of a mixing zone in their permit renewal.
Tetra Tech compiled a plan for developing and applying a mixing zone model to support regulatory permitting. In
June 2023, the plan document, "Modeling Plan for Mixing Zone Analysis of Calico Creek at Morehead City WWTP
Outfall (NC0026611)," was submitted to DWR and the Town of Morehead City for review and was approved in
October 2023 with one round of comments and responses, which were accepted. Upon DWR's acceptance of the
modeling plan, Tetra Tech completed the mixing zone analysis as planned in the near -field of the Morehead City
WWTP outfall.
Results from the modeling analyses were used to compute a representative dilution factor (or dilution ratio) that
may be used by decision -makers to inform NPDES permitting. The dilution factor quantifies the potential near -field
environmental impacts of WWTP effluent on the receiving waterbody while accounting for site -specific ambient
conditions, effluent characteristics, and mixing dynamics within the receiving water. DWR has the authority to
evaluate potential modifications to NPDES permit limits with respect to conservative parameters as modeled and
documented in this report (copper, zinc, and nickel). Whole effluent toxicity (WET) limits serve to protect receiving
waters from likely impacts of chronic or acute toxicity and are calculated as a function of the instream waste
concentration (IWC), which is the permitted wasteflow divided by the sum of the permitted wasteflow and critical
low -flow conditions. The IWC is typically expressed as a percentage, the inverse of which is the dilution factor (sum
of permitted wasteflow and critical low flow divided by the permitted wasteflow). Permit limits for individual toxicants
may be modified based on scientifically valid mixing zone analyses that indicate some level of dilution in the
receiving water will be protective of chronic and acute toxicity to wildlife.
However, the more typical approach to calculating IWC and/or dilution is not applicable for the Morehead City
WWTP discharge because Calico Creek is tidally influenced, and it is not representative to simulate conditions by
a traditional unidirectional flow of 7Q10 flow conditions (the lowest 7-day average flow that occurs approximately
once every 10 years). To best evaluate and calculate representative effluent dilution for the tidally influenced Calico
Creek, a robust modeling analysis was conducted. The Cornell Mixing Zone Expert System (CORMIX; Doneker
and Jirka 2021) was used to evaluate dilution under dynamic (nonsteady state) conditions in Calico Creek.
NTETRA TECH 4

Calico Creek Modeling Report for Morehead City, NC
According to the NCDEQ guidance on mixing zones, the size of the mixing zone is to be determined on a case -by -
case basis and will take several factors into consideration, such as the receiving water, extent of mixing, and outfall
configuration (NCDEQ 1999). For the purposes of this mixing zone modeling report, dilution is reported at
incremental distances from the outfall, with the proposed mixing zone boundary nine meters from the outfall,
equivalent to one-third of the width of the channel in the near -field.
Figure 1. Location of the Morehead City WWTP effluent discharge outfall to Calico Creek.
NTETRA TECH 5

Calico Creek Modeling Report for Morehead City, NC
Tetra Tech used the dilution model CORMIX for the mixing zone analysis of the Morehead City WWTP discharge
to Calico Creek. NCDEQ recently developed a hydrodynamic model of Calico Creek on the platform Environmental
Fluid Dynamics Code (EFDC) (Hamrick 1992) to assess nutrient -related impairments (NCDEQ 2020, 2021). The
existing EFDC model was used to establish some of the CORMIX model parameterization, particularly related to
ambient receiving water conditions, such as salinity and velocity.
2.1 EXISTING EFDC MODEL
In recent years, a Special Study was conducted by the NCDEQ DWR Water Planning Section, Modeling and
Assessment Branch, for Calico Creek (NCDEQ 2020, 2021). Calico Creek has been assigned a Category 5
impairment on the Clean Water Act Section 303(d) list since 2008; as of 2022, it remains on the list for nutrient -
related impairments (NCDEQ 2022). The upper approximately 0.8-mile-long segment of Calico Creek (assessment
unit [AU] 21-32a) is impaired for chlorophyll -a, dissolved oxygen, and turbidity, while the lower approximately 1.0
mile -long segment of Calico Creek (AU 21-32b) is impaired for chlorophyll -a and dissolved oxygen. A Special Study
was conducted to assess the water quality impacts of nutrient loadings to the Calico Creek Estuary. The study
incorporated datasets from the Ambient Monitoring System (AMS) program (as aggregated via the Water Quality
Portal, https://www.waterqualitydata.us/) and Intensive Survey Branch observations that were collected between
May 2017 and April 2019. Monitoring and modeling results from the Special Study are summarized in NCDEQ
(2020, 2021).
The hydrodynamic model grid for the EFDC model was developed using the National Oceanic and Atmospheric
Administration (NOAA) post -Hurricane Sandy topo-bathymetric light detection and ranging (LiDAR) data, which
covered a portion of the estuary (NCDEQ 2020, 2021). Additional cross -channel depth profiles were collected during
the Intensive Survey Branch along six transects in February 2018 and seven transects in August 2019. Acoustic
Doppler Current Profiler (ADCP) data consisting of neap tide and spring tide flow velocities with depth were also
collected in 2019 using a SonTek River Surveyor M9. Spatial and temporal patterns across Calico Creek were
evaluated for various water quality constituents, including the seasonal distributions of the vertical and longitudinal
variation in parameters such as water temperature, salinity, dissolved oxygen, and pH. Results from the data report
indicate Calico Creek is a "partially mixed" system with limited freshwater inflows that are primarily associated with
storm events. Tide gauging and ADCP data indicate the hydrodynamics in Calico Creek are largely controlled by
semidiurnal tides. This recent monitoring and modeling work was built on some of the historical modeling and
planning in the waterway and watershed conducted by the North Carolina Division of Environmental Management
(NCDEM 1990) and the Division of Water Quality (DWQ) (DWQ 1997). The modeling efforts focused on the
simulation of chlorophyll -a concentrations (as represented by primarily abundant diatoms) in Calico Creek in
response to changes in nutrient loadings (NCDEQ 2020, 2021). For the simulation, a curvilinear orthogonal grid
was developed, with individual grid cells varying from about 20 meters upriver to a coarser 90 meters near the
mouth of the estuary. The total 264 horizontal model cells were equally divided into three layers to capture the
vertical hydrodynamic variability occurring with depth (Figure 2). The model simulated the summers (June to
August) of 2017 and 2018 using the best available Special Study field data. The Morehead City WWTP is the only
point source present in the model extent, with additional inflows being attributed to (1) five small tributaries, and (2)
the open boundary (mouth) of the estuary.
NTETRA TECH 6

Calico Creek Modeling Report for Morehead City, NC
E
111 12 14 15 30
Figure 2. Existing EFDC model grid (left) and velocity cross-section (right) for Calico Creek (NCDEQ 2021).
2.2 CORMIX MIXING ZONE MODEL
To determine the dilution potential for effluent in the near -field, a mixing zone analysis is required, and CORMIX is
a modeling software that is frequently used for such evaluations.
CORMIX is a comprehensive software used for the analysis, prediction, and design of mixing zone dilution of
pollutants discharged into receiving water bodies (MixZon 2021; Doneker and Jirka 2021). The model can simulate
numerous types of diffusers under steady-state (fixed) or unsteady conditions. CORMIX was initially developed in
1985 and, over the past two decades, has been updated by several agencies, including the EPA, the U.S. Bureau
of Reclamation, Cornell University, Oregon Graduate Institute, the University of Karlsruhe (Germany), Portland
State University, and MixZon Inc.
Programming languages, including NEXPERT Object, an "expert systems shell," C++, and FORTRAN, are used to
write CORMIX to account for the diverse programming requirements. NEXPERT is used for knowledge
representation and logical reasoning, and FORTRAN is used for mathematical calculations (Doneker and Jirka
2021). The model can predict the geometry and dilution of the initial mixing zone to ensure compliance with the
water quality regulatory constraints. The program can also forecast the response of the effluent discharges' plumes
at longer distances. CORMIX has been verified by the developers, has undergone extensive peer reviews, and is
often used for regulatory dilution analyses. CORMIX v12.0 GTH was used for the Morehead City WWTP mixing
zone analysis.
Because the outfall configuration changes during the tidal cycle, it was determined to be appropriate that a more
complex and costly model such as the Jet Plume module coupled with EFDC (JP-EFDC, Hamrick 1992) would be
insufficient to capture the fine detail near the pipe for a higher end cost and complexity systemwide. The discharge
is relatively small (2.5 MGD permitted), and the receiving water is relatively shallow, with wind and tidal currents
facilitating mixing. Unlike JP-EFDC, CORMIX does not require an extensive amount of data, has short simulation
run-times, and includes dilution ratios in model outputs across the simulated tidal cycle.
NTETRA TECH

Calico Creek Modeling Report for Morehead City, NC
The CORMIX model simulation period was selected to cover the most relatively extreme dilution scenarios in the
model domain, bounding the simulated range of depth, temperature, density, and ambient current. Where receiving
waters are tidally dominated and unsteady by nature, CORMIX can simulate effluent plume dispersal at user -
specified times across the site -specific tidal cycle. Multiple scenarios set at different points in the tidal cycle are
necessary to provide a full picture of dilution (Doneker and Jirka 2021). Scenarios were developed for both summer
and winter simulation periods due to changes in water temperatures and densities that can impact the dilution
capacity of the waterway. Additional parameters that were subject to change between scenarios to capture impacts
across the full range of tidal conditions were the current direction (ebb and flood) and associated ambient velocities
and depths at various times relative to slack tide. Modeling efforts focused on spring tides (as opposed to neap
tides), where water levels reach their highest and lowest elevations, to capture the full extent of localized dilution.
At various times during the tidal cycle, the WWTP outfall pipe is either fully or partially submerged as water depths
change, which impacts dilution.
The following subsections detail the model inputs and parameterization developed for each simulated dilution
scenario evaluated. Data were collected from a variety of sources, including online databases, previous modeling
and data collection efforts, and field reconnaissance by the combined efforts of Tetra Tech, BT Solutions, Shealy
Consultants, Morehead City and WWTP staff, and NCDEQ personnel. Key model determinations were made with
respect to the simulation period (Section 3.1) and the selection of dilution scenarios to evaluate (Section 3.2). Model
inputs for the various scenarios were developed for both the ambient conditions in Calico Creek (Section 3.3) and
for the WWTP discharge (Section 3.4). Some of the local monitoring sites used during model parameterization are
depicted in Figure 3.
Morehead
City WWTP
'O
P8750000
Morehead City
WWTP Outfali
Calico Creek
P8B00000
Legend
Outrall Location
Duke Marine Lab
AMS Stators
Morehead City VIi
. Mo hood City -
Duke Mariner*
Lab Tidal Gage
_ Sources. Esri, HERE, Garmin, USGS, intermap. INCREMENT P NRCan, Esri
Japan, METE, Esri China (Hang Kang} Esd Korea, Esri (Thailand), NGGC. (c)
OpenStreetMap conlrihutors, and the GIS User Cammunity
V 0 025 0.5 1
Calico Creek Data Collection Points oKilometer TETRA TECH
Map produced by E. Kremer 6-7-2023
NAD 1983 StatePlane North Carolina FIPS 3209 Feet 0 025 0.5 C75 1
— — — — — — —I Mile
Figure 3. Location of Morehead WWTP outfall, AMS monitoring stations, and Duke Marine Lab tidal gauge.
NTETRA TECH 8

Calico Creek Modeling Report for Morehead City, NC
3.1 MODELING PERIOD DETERMINATION
For the CORMIX model simulation setup, a pollutant concentration was parameterized in the effluent to represent
a basic proxy for all three metals of interest. Because the fate of these metals (copper, nickel, and zinc) is considered
for this evaluation in the immediate near -field, only the impacts of physical mixing are of interest, as opposed to any
further transformations of the metals chemically or otherwise. A general value for excess discharge concentration
of metals was selected as 100 milligrams per liter (mg/L) pollutant. For the simulation, 20 output steps per CORMIX
module were selected for the intermediate modeling calculations.
The efficiency of effluent mixing and plume shape are influenced by the receiving water's ambient temperature and
salinity, which determine ambient density. Locally, conditions in summer tend to result in higher water temperatures
and higher salinity, while in winter, water temperatures and salinity are both generally lower. Monthly density and
salinity data measured by two AMS stations (Calico Creek stations P8800000 and P8750000) were included in the
model (NCDEQ 2023). The AMS station P8750000 is approximately 550 meters upstream of the WWTP outfall,
and station P8780000 is approximately 830 meters downstream of the outfall, so the averaged values were used
to estimate temperature and salinity at the outfall (Table 1). Water temperature and salinity data gathered at AMS
sites between 2015 and 2020 were used to select specific model simulation periods of relatively high and relatively
low densities.
Density was found to be, on average, lowest during December and highest in August. However, June was selected
for the modeling period because August had limited sampling data available across depths. The month of June had
the next highest density and a broader range of monitoring data to support model development.
Table 1. Monthly average water temperature and salinity with calculated ambient density (2015-2020)
16.2
9.0
1,012.6
18.0
12.4
1,013.5
19.7
16.4
1,014.1
23.0
22.5
1,015.1
20.2
26.9
1,011.8
24.4
30.3
1,013.8
21.2
30.2
1,011.5
27.5
29.8
1,016.3
22.6
27.0
1,013.5
18.6
21.4
1,012.1
18.1
18.0
1,012.5
16.0
15.1
1,011.5
Notes: ppt = parts per thousand; °C = degrees Celsius; kg/m3 = kilograms per cubic meter
NTETRA TECH

Calico Creek Modeling Report for Morehead City, NC
3.2 DILUTION SCENARIO SELECTION
Dilution scenarios were selected to best capture the extent of mixing that can occur across the tidal cycle and across
variable changes with seasons. Based on local conditions, data on Calico Creek tidal cycles (determined to be 12.4
hours), and exploratory modeling, it was determined that the combination of scenarios selected would reflect a
systematic evaluation based on the season; the times before, after, and during both high-water slack (HWS) and
low-water slack (LWS); and for both flood and ebb tides.
The suite of scenarios chosen are depicted in Figure 4 and graphically in Figure 5. Selected scenarios included
times at which maximum ebb and flood velocities occur (three hours before slack) and when water depths are at
their highest and lowest (slack conditions).
Note that due to the approximately 12-hour tidal period, a scenario at time 3 hours after HWS is equivalent to three
hours before L WS; therefore, those conditions were not duplicated as modeling scenarios. The selection of time 1
to 1.2 hours before and after slack was made based on the most reasonable results for model convergence around
that period in the tidal cycle.
Figure 4. Depiction of dilution scenario identification for Morehead City CORMIX modeling.
NTETRA TECH 10

Calico Creek Modeling Report for Morehead City, NC
:Itd .1
s
a
v
0
Time
• Scenarios
Lws
Figure 5. Graphical depiction of dilution scenario selection for Morehead City CORMIX modeling based on
water depth over time across the tidal cycle (summer or winter).
Although conditions during which the system is at slack were identified for simulation (scenarios 3, 7, 11, and 15),
the model results from CORMIX for these scenarios were inconsistent with reality and included significant warning
messages such as this for Scenario 7:
"WARNING: The LIMITING DILUTION (given by ambient flow/discharge ratio) is = 2.51. This value is below
the computed dilution of 7.42 at the end of the Near -Field Region (NFR). Mixing for this discharge
configuration is constrained by the ambient flow. Please carefully review the prediction file for additional
warnings and information."
Followed by:
"In this case, the upstream INTRUSION IS VERY LARGE, exceeding 10 times the local water depth. This
may be caused by a very small ambient velocity, perhaps in combination with large discharge buoyancy. If
the ambient conditions are strongly transient (e.g., tidal), then the CORMIX steady-state predictions of
upstream intrusion are probably unrealistic. The plume predictions prior to boundary impingement and
wedge formation will be acceptable, however."
These warnings messages from the CORMIX indicate that the lack of channel velocity occurring at the moment of
slack cannot be accurately simulated because no dilution can occur without ambient flow. However, because this
condition only occurs momentarily as the tide direction changes, scenarios for both one hour before and one hour
after slack were modeled as alternative representatives of similar depths with low (non -zero) ambient velocity.
3.3 AMBIENT CONDITIONS
CORMIX model inputs for representation of ambient conditions in Calico Creek include:
1. Static inputs that remain unchanged across tidal and season variations (e.g., localized channel geometry,
slope, and channel roughness)
2. Seasonal inputs that change only between summer and winter scenarios (e.g., wind speed, water
temperature, salinity, and density)
3. Scenario -dependent inputs that vary through time within the tidal cycle (e.g., water depth and tidal velocity).
How these various combinations of model inputs were developed for simulation of ambient conditions is
documented in the following subsections, followed by WWTP outfall parameterization in Section 3.4.
NTETRA TECH 11

Calico Creek Modeling Report for Morehead City, NC
3.3.1 Channel Geometry and Slope
Geometric properties in the vicinity of the discharge port were specified in CORMIX. The model requires the
receiving water be represented as a rectangular channel that is either laterally bounded or unbounded. Receiving
waterbodies constrained on both sides by banks, such as rivers, are considered bounded. When the interaction of
the plume is unlikely to reach the opposing bank (e.g., a wide estuary), the system is considered unbounded. The
bounded option is appropriate for the site and was used in the model. The model domain in CORMIX was
schematized as a uniform rectangular channel with a simulation region of length 10 times the width of the channel.
The width was measured as 27 meters at the outfall point based on a combination of LiDAR data (NGDS 2023) and
measurements obtained during a site visit by BT Solutions. Based on this data, the length of the model domain was
set to 300 meters.
Elevation data from the North Carolina Floodplain Mapping program (NC DPS 2016) and the NOAA post -Hurricane
Florence topo-bathymetric LiDAR data were used to determine the slope and inform the channel geometry and
boundaries along with in -situ measurements and data from the Duke Marine Laboratory tidal gage (Figure 3, Figure
6). Further discussion on the methods used to calculate depth at different tidal intervals is discussed in Section
3.3.4. The bottom slope is measured from the discharge to the center of the channel and is approximately 1.75%
(1 degree). From the top of the bank, the channel is approximately 2.2 meters deep at the thalweg and 33 meters
wide at the downstream AMS station. At the upstream monitoring station, the channel is 1.9 meters deep and 11.3
meters wide. The average width and depth were found to be 19.4 meters and 1.15 meters, respectively.
Morehead City N'
WWTP Outfall
Legend PP_ -
Elevation (ft)
Value + _
High: 24.4933 _ - -
- Low: -4.98
SourceEsri_ Maxar, Eadhstar GL" aphits, and the GIS User Community
Calico Creek Topobathymetric UDAR data N 0 0.075 015 0.K�See
Map produced by E—Kremer 6-8-2023 — TETRA TECH
NAD 1983 5tatePlane North Carolina FIP5 3200 Feet 0 0.05 0.1 0.2
M Ile
Figure 6. Topo-bathymetric UDAR elevation data in Calico Creek.
NTETRA TECH 12

Calico Creek Modeling Report for Morehead City, NC
3.3.2 Channel Roughness
In CORMIX, the roughness or friction of the channel bottom is represented by Manning's n (roughness coefficient).
Coefficients for open channel flow are typically selected from tables using waterbody characteristics, such as the
substrate type, vegetation and debris, and channel shape (meandering or straight). CORMIX internally converts
roughness inputs into a Darcy-Weisbach friction factor for the simulation of relevant mixing considerations. Calico
Creek was observed and documented to have sandy substrate, a relatively straight flow path, and be reasonably
clear of vegetation and debris through the main channel. A Manning's roughness coefficient of 0.025 was used to
represent the channel bottom based on site observations and recommended values from the CORMIX user manual
(Doneker and Jirka 2021). A sensitivity analysis varying the roughness coefficient had a negligible impact on the
model results.
3.3.3 Wind
The CORMIX model accounts for the impacts of wind on system hydrodynamics, such as influences on flow velocity
and potential for mixing due to aerial perturbation. The Duke Marine Laboratory continuously records wind data
approximately five miles from the model area, including speed and direction for both wind and gust (NOAA 2023a).
Model inputs for the wind were selected based on recent seasonal conditions, for which the average wind speed
for June in 2020-2022 was used during each summer scenario (3.64 meters per second), and the average wind
speed for December in 2020-2022 was used during each winter scenario (3.24 meters per second).
3.3.4 Ambient Velocity and Depth
Ambient channel conditions are some of the most important factors in determining near -field dilution within the
mixing zone. Measurements of both velocity and depth were taken in recent years as part of an NCDEQ Special
Study of Calico Creek (NCDEQ 2020, 2021). These measurements were not used directly as input into the CORMIX
model because of the distance downstream from where the measurements were recorded; however, model inputs
were supplemented by data collected by BT Solutions across various transects over the course of a spring tide
event on August 2, 2023.
The maximum ambient velocity in either direction for all scenarios was estimated at 0.283 meters per second based
on EFDC model output in the vicinity of the outfall (Figure 7).
Figure 7. EFDC model grid of cells from NCDEQ EFDC model. Predicted values for cell 31,7 were used to
characterize ambient values for Calico Creek in the near -field mixing zone.
NTETRA TECH 13

Calico Creek Modeling Report for Morehead City, NC
Due to the sinusoidal relationship between water depth and tidal current, the data observations were used to
approximate ambient conditions across the tidal cycle using a standard wave function (Figure 8).
During the Special Study conducted in 2020, NCDEQ-measured velocities approximately 76 meters (250 feet)
upstream of monitoring station P8800000 were generally observed to be less than 0.36 and 0.55 meters per second
during neap and spring tide, respectively (NCDEQ 2020, 2021). Depth -averaged velocities at this location were
between -0.33 meters per second (ebb) and 0.48 meters per second (flood) during spring tide, with stronger flow
speeds occurring during flood rather than ebb tides. These data, plus tidal data collected at the Duke Marine
Laboratory station approximately five miles from the outfall, were used to confirm predicted tidal velocities
throughout a spring tide using standard wave functions (NOAA 2023b).
Depth inputs were selected to correlate with different periods in the tidal cycle. The channel depths were estimated
using data collected during the recent NCDEQ Special Study of Calico Creek, available in -situ measurements, and
other relevant field observations (NCDEQ 2020, 2021). The Special Study found that approximately 76 meters (250
feet) upstream of P880000, the tidal range in water depth ranged from 0.69 meters during neap tide to 1.45 meters
during spring tide. At the Morehead City WWTP outfall pipe, the depth of water tends to range from approximately
the bottom of the 0.61-meter (24-inch) pipe to approximately 0.30 meters (1 foot) depth over the pipe. A site visit
conducted during low and high tide on July 25, 2023, involved measuring water depth at the outfall to be 0.66 meters
and 1.05 meters, respectively.
Scenario -dependent inputs represent those that are tidally and/or seasonally variable, for which a single static input
is not sufficient to capture the dynamic changes in the system. The range of possible values for scenario -dependent
inputs was determined from the best available data (such as the NCDEQ DWR Calico Creek EFDC model), field
measurements, and CORMIX manual and literature recommendations. Where possible, relationships between
specific variables and time were developed to predict model inputs at the relevant time for each scenario. Note that
the "time" modeled by scenario is representative of relative time within the tidal cycle since actual high and low tide
peak times change day-to-day.
1.8
1.6
1.4
E 1.2
a
1
0 0.8
0.6
0.4
0.2
] 5 10 15 20
Hours
—Depth Current
0.400
0.300
0.200
0.100
E
0.000
v
-0.100 u
-0.200
-0.300
-0.400
Figure 8. Predicted depth and ambient velocity in Calico Creek during a spring tide cycle with scenario depths.
Based on the breadth of scenarios selected across the tidal cycle, model inputs for depth and velocity were based
on the sinusoidal wave function that was constrained by observations (Table 2). Modeled ambient velocities were
very close to the observations made by BT Solutions during their conductivity evaluation in 2023.
NTETRA TECH 14

Calico Creek Modeling Report for Morehead City, NC
Table 2. Scenario -dependent (seasonally independent) ambient conditions for CORMIX modeling scenarios
Flood
3 hours before HWS
0.91
0.98
0.94
0.281
Flood
1 hour before HWS
1.35
1.41
N/Ab
0.172
Ebb
1.2 hours after HWS
1.32
1.39
N/Ab
0.142
Ebb
3 hours before LWS
0.83
0.89
0.86
0.283
Ebb
1 hour before LWS
0.37
0.44
0.41
0.142
Flood 1.2 hours after LWS 0.45 0.52 0.49 0.172
Notes: m = meters; m/s = meters per second
a Per Section 3.2, simulations at slack (Scenarios 3, 7, 11, and 15) were excluded from dilution calculations due to model instability at the
moment of null ambient velocity. Additionally, scenarios inputs are mirrored across summer and winter seasons.
3.3.5 Temperature, Salinity, and Density
Seasonally variable inputs for water temperature and salinity inform how the CORMIX pre-processing tool calculates
density of the receiving water. For the Calico Creek CORMIX model, ambient conditions were seasonally
represented using two different data sources:
• NC DEQ Calico Creek EFDC model output from cell 31,7 (NCDEQ 2021; see Figure 7)
• AMS monitoring sites P8750000 upstream of outfall and P8800000 downstream of outfall (see Figure 3)
Model output from the vertical layers of the NCDEQ EFDC model (cell 31, 7) were used to parameterize ambient
summer conditions for Calico Creek. Water temperature at the surface (top third of model cell) and bottom of the
water column (bottom third of the model cell) were 28.9 and 29.5 degrees Celsius (°C), respectively. Similarly,
salinity at the surface and bottom of the EFDC model grid cell output were 16.8 and 20.2 practical salinity units,
respectively. Using the CORMIX pre-processing Density Calculator, the summer surface and bottom densities were
1,008.5 and 1,010.8 kilograms per cubic meter (kg/m3), respectively.
Winter values could not be extracted from the EFDC model, as its simulation period was limited to summer. Average
values from the two AMS stations were used to characterize water temperature and salinity in winter based on
average December data from 2015 to 2020 measured above and below the 0.4-meter-deep pycnocline. Water
temperature at the surface (above the pycnocline) and the bottom of the water column (below the pycnocline) were
14.9 and 15.6 °C, respectively. Similarly, salinity at the surface and the bottom of the AMS sites were 13.4 and 23.8
parts per thousand, respectively. Using the CORMIX pre-processing Density Calculator, the summer surface and
bottom densities were 1,009.4 and 1,017.2 kg/m3, respectively.
3.4 WWTP OUTFALL PARAMETERIZATION
Parameterization of the Morehead City WWTP outfall pipe included some inputs that remained static between all
scenarios and others that varied based on how ambient conditions impacted the outfall pipe. Port configuration
features and dimensions (e.g., vertical and horizontal angles, distance to nearest bank, height above channel
bottom) were simulated based on a combination of existing engineering diagrams and additional field
measurements conducted by Tetra Tech, McDavid & Associates, BT Solutions, and Shealy Consultants in
partnership with the Town and WWTP facility staff. Within the CORMIX platform, the vertical angle of the discharge
pipe is measured between the port centerline and the horizontal plane, while the horizontal angle is measured
counterclockwise from the direction of the ambient current.
NTETRA TECH 15

Calico Creek Modeling Report for Morehead City, NC
As required for regulatory evaluation, one static input for all scenarios was that the effluent flow rate was input at
the maximum design flow of 2.5 MGD (approximately 0.109 cubic meters per second). The outfall pipe orifice is
located approximately 2.5 meters from the channel bank as measured using a geographical information system,
with a vertical angle flowing towards the water of approximately -1 degree as measured in the field. The effluent
discharge was simulated with no salinity, which impacts the model by modeling the effluent as a buoyant plume
over the denser seawater. The effluent temperature was estimated using the WWTP's discharge monitoring reports
(DMR). Average effluent temperatures for June and December were 25.6 and 17.8 °C, respectively.
The discharge pipe is 24 inches in diameter, made of high -density polyethylene (HDPE), with an extension of
corrugated metal piping beyond that. It was observed in the field that during average discharge conditions (when
the effluent flow is nearly half the limit at approximately 1.3 MGD), the 24-inch pipe is less than half full. CORMIX
model documentation recommends representing partially filled pipes as channels (Doneker and Jirka 2021). Depth
in the pipe was observed to be level with the tidal depth, so a channel set-up was used for scenarios in which the
pipe was partially submerged. A partially submerged outfall scenario simulated with a "channel" effluent discharge
is modeled using the "CORMIX 3" selection. Alternatively, when the outfall was fully submerged under the ambient
channel depth, the discharge is simulated as a "port" and modeled using the "CORMIX 1" selection.
The CORMIX model typically uses physical outfall configuration as static model inputs based on the precise
dimensions of the pipe. However, that approach did not produce reasonable modeling results based on the shallow
nature of Calico Creek paired with the 24-inch pipe that was never fully flowing with effluent. Given these challenges
and uncertainties, it was determined that the outfall parameterization would differ by scenario to meet variable input
requirements in CORMIX.
The dimensions of the pipe itself and the orifice varied by type of simulation (partially vs. fully submerged outlet)
and was constrained by valuable field information related to effluent discharge velocity, which was observed to be
relatively high velocity for its size and shape (Figure 9). If the design flow of 2.5 MGD were expelled from that 24-
inch pipe, mathematically, the velocity of that effluent would be approximately 0.38 meters per second under its
highest flows. However, in the field on 8/6/2023, velocities were measured at the outlet using a Global WaterFLow
Probe to be approximately 1.45 meters per second when the pipe was partially submerged during high tide and
approximately 0.69 meters per second when the pipe was fully submerged during low tide. Therefore, orifice inputs
for each scenario were based on which applicable dimensions at the pre -defined 2.5 MGD flow rate were able to
produce the observed velocities from the field. Because flow velocity increases as pipe diameter decreases, it is
assumed that the effluent may be conveyed by the smaller pipe or have a steeper incline from visible from the
terminal outfall.
To best capture in -situ field observations of effluent flow velocity, the partially submerged outfall was simulated as
a channel with a width of 0.3 meters and a depth (height from the bottom of the outfall channel to the top of the
water level) of 0.25 meters, which was applicable to scenarios 1, 5, 6, 8, 9, 13, 14, and 16. Similarly, when the
outfall was fully submerged, it was simulated as a port with a diameter of 0.45 meters and a port height (height of
port bottom to streambed) ranging approximately 0.93-0.94 meters depending on the point in the tidal cycle, which
was applicable to scenarios 2, 4, 10, and 12.
When the tidal current shifts direction, the directionality of the model is mirror imaged. While nothing changes about
the true geometry and placement of the outfall, in the modeling space, the tidal directionality and orifice
submergence include different requirements for assumptions of the nearest bank and angle of discharge. For fully
submerged port outfalls, flood currents are simulated with the nearest bank on the right-hand side with a horizontal
angle of 120 degrees, while ebb currents are simulated with the nearest bank on the left-hand side with a horizontal
discharge angle of 300 degrees. For partially submerged channel outfall conditions, flood currents are simulated
closest to the right bank, while ebb currents are simulated closest to the left bank, both of which have a horizontal
angle of discharge of 60 degrees.
NTETRA TECH 16

Calico Creek Modeling Report for Morehead City, NC
Figure 9. Photo of partially submerged Morehead City WWTP discharge pipe with high velocity effluent.
NTETRA TECH 17

Calico Creek Modeling Report for Morehead City, NC
In tidal conditions, the dilution ratios vary with changing conditions of ambient water depth, velocity, and
directionality throughout the tidal cycle. Instream dilution is determined by the shape of the effluent plume with
distance from the outfall, particularly the plume width and the rate of plume spread or dissipation. Variations in
seasonal ambient water densities also influence plume dilution, which is why scenarios were constructed for both
warm -water and cold -water conditions across the tidal cycle.
4.1 DILUTION ANALYSIS APPROACH
The concept of dilution is used to quantify the degree of mixing and the transport of the effluent discharge in the
tidal Calico Creek system. The dilution of the effluent discharge can be determined by the following equation, which
aligns with NCDEQ's mixing zone guidance (NCDEQ 1999):
Dilution Ratio = Effluent Concentration / In -Stream Concentration
A conservative tracer with a known concentration was introduced into the effluent discharge. As suggested in the
CORMIX manual (MixZon 2021), a concentration of 100 mg/L (or 100%) was used for the purpose of computing
the dilution ratios at the ebb, peak, and slack tides. Background levels of the conservative tracer were set to zero.
The mixing model was used to simulate plume geometry and tracer distribution in the system until a quasi -steady
state was achieved. Instream concentrations along the plume centerline were used to evaluate the dilution ratios at
multiple distances from the WWTP's outfall. CORMIX reports dilution along the plume centerline using X and Y
coordinates where the X-axis is perpendicular to the outfall (i.e., parallel to Calico Creek) and the Y-axis is parallel
to the outfall (i.e., perpendicular to Calico Creek). Geometric properties were applied to compute the distance of the
plume centerline relative to the outfall as: Distance = XZ+YZ. The effluent flow rate was equivalent to the permitted
design flow rate, which provides the dilution ratios for the expected highest discharge from the facility.
4.2 MODELING RESULTS AND SUMMARY
A CORMIX model was developed to predict and analyze the geometry and dilution of the Morehead City WWTP
effluent plume discharging into Calico Creek in the immediate near -field. The model includes scenario -specific port
geometry based on the effluent velocity; water surface elevation; and ambient velocity, salinity, and water
temperature inputs for each simulation period. After each of the modeling scenarios were initialized, the model was
reviewed to ensure numerical stability and reasonable results. Where appropriate, adjustments were made to some
model parameterization inputs as necessary to model achieve stability and prediction of reasonable results.
Following the model scenario development, the model was applied to simulate mixing and dilution of the WWTP
effluent in the immediate near -field of Calico Creek. Variation in simulated dilution factors between scenarios results
from dynamic changes of various scenario -dependent inputs, including channel depth, wind speed, ambient
velocity, and seasonal variability in ambient water temperature and salinity that impact the density differential
between the effluent and receiving water. Higher dilution was predicted during scenarios in which the outfall was
submerged in the channel and the ambient flow velocity was low (0.142 meters per second relative to 0.172), as
observed in model output for Scenario 2 compared to Scenario 4. When the outfall is submerged, the discharging
effluent velocity is lower; therefore, more diffuse dilution occurs near the pipe. Conversely, when the pipe is
exposed, discharging effluent velocity is higher relative to ambient conditions, which allows the plume to maintain
a more concentrated shape as it moves downstream. Dilution ratios were evaluated at incremental distances from
the outfall, with the primary dilution of interest being identified as one-third of the 27-meter-wide channel, which is
9 meters (Table 3).
NTETRA TECH 18

Calico Creek Modeling Report for Morehead City, NC
The average dilution predicted 9 meters downstream of the outfall across all scenarios was 6 (Figure 10).
This dilution factor may be used by NC DWR to inform permitting decisions regarding effluent limits.
Table 3. Simulated dilution ratios in Calico Creek with distance from the Morehead City WWTP outfall.
3h before HWS, flood tide, partially submerged
1h before HWS, flood tide, submerged
1.2h after HWS, ebb tide, submerged
3h before LWS, ebb tide, partially submerged
1 h before LWS, ebb tide, partially submerged
1.2h after LWS, flood tide, partially submerged
3h before HWS, flood tide, partially submerged
1h before HWS, flood tide, submerged
1.2h after HWS, ebb tide, submerged
3h before LWS, ebb tide, partially submerged
1 h before LWS, ebb tide, partially submerged
1.2h after LWS, flood tide, partially submerged
1
2
3
4
5
5
6
6
1 6 6 6
7 8 8
8 8 9
6 6 6
3 3 3
3 3 3
1
4
5
6
6
7
7
7
1
2
2
3
4
5
5
6
1
2
3
4
5
5
6
6
1
2
2
3
3
3
3
3
1
2
3
3
3
3
3
3
1
1
3
4
4
5
5
5
6
6 6
1
4
5
6
6
7
7
7
7 7 8
6 9 9
5 6 6
1
2
2
3
4
5
5
6
1
1
2
4
4
5
5
5
1
2
2
3
3
3
3
3
3 3
1
2
2
3
3
4
4
4
5 5
1
2
3
4
4
5
5
5
6 6
Notes: h = hours; m = meters
a Per Section 3.2, simulations at slack (Scenarios 3, 7, 11, and 15) were excluded from dilution calculations due to model instability at moment
of null ambient velocity.
Dilution with Distance Downstream: All Scenarios
8 i
i
7 i
i
A '
5
c
0
4
'o
3
2
1
0
0
1 2 3 4 5 6 7 8 9 10
Distance Downstream (meters)
Average of Scenarios — — — 9 meters
Figure 10. Mean WWTP dilution in Calico Creek across all scenarios by distance from outfall.
NTETRA TECH 19

Calico Creek Modeling Report for Morehead City, NC
BT Solutions. 2023. The Morehead City Wastewater Treatment Plant Specific Conductance Tracer Study for
Outfall #001, Final Report.
Doneker, R.L. and G.H. Jirka. 2021. CORMIX User Manual: A Hydrodynamic Mixing Zone Model and Decision
Support System for Pollutant Discharges into Surface Waters. Accessed June 30, 2023.
www.mixzon.com/downloads/.
DWQ (North Carolina Division of Water Quality). 1997. White Oak River basinwide water quality management
plan. Retrieved from https://www.deq.nc.gov/about/divisions/water-resources/water-planning/basin-
planning/river-basin-plans/white-oak.
Hamrick, J.M. 1992. Three -Dimensional Environmental Fluid Dynamics Computer Code: Theoretical and
Computational Aspects. The College of William and Mary, Virginia Institute of Marine Science (Special
Report 317). Accessed June 30, 2023. https://scholarworks.wm.edu/reports/715/.
MixZon. 2021. CORMIX downloads. Accessed June 30, 2023. http://www.mixzon.com/downloads/.
NCDEM (North Carolina Division of Environmental Management). 1990. A modeling evaluation of the water
quality impacts to Calico Creek from the Morehead City wastewater discharge. Retrieved from
https:Hfiles. nc.gov/ncdeq/Water%20Quality/Planning/TM DL/Special%20Studies/calicocreek/CalicoModel
Report_061721. pdf.
NCDEQ (North Carolina Department of Environmental Quality). 1999. Mixing Zones in North Carolina. Accessed
June 30, 2023. https://www.deq.nc.gov/coastal-managemenvgis/data/esmp-
data/2010/july/npdes/mixingzones-20090710-dwq-swp-npdes/download.
NCDEQ (North Carolina Department of Environmental Quality). 2020. Data Report for Calico Creek Estuary.
Accessed June 30, 2023.
https://edocs.deq. nc.gov/WaterResources/DocView.aspx?d bid=0&id=2715206.
NCDEQ (North Carolina Department of Environmental Quality). 2021. Calico Creek Hydrodynamic and Nutrient
Response Model. Accessed July 2, 2023.
https://edocs.deq. nc.gov/WaterResources/DocView.aspx?dbid=0&id=2715205&cr=1.
NCDEQ (North Carolina Department of Environmental Quality). 2022. 2022 Final 303(d) list. Retrieved from
https://www.deq. nc.gov/about/divisions/water-resources/water-planning/modeling-assessment/water-
quality-data-assessment/integrated-report-files.
NCDEQ (North Carolina Department of Environmental Quality). 2023. Ambient Monitoring System. Accessed
June 15, 2023. https://www.deq.nc.gov/about/divisions/water-resources/water-sciences/ecosystems-
branch/ambient-monitoring-system-ams.
NC DPS (North Carolina Department of Public Safety). 2016. Spatial data download: LiDAR, DEM, and GIS data
[Dataset]. North Carolina Department of Public Safety. https://sdd.nc.gov/.
NGS (National Geodetic Survey). 2023. 2019-2020 NOAA NGS Topobathy Lidar: Coastal VA, NC, SC, [Dataset].
National Oceanic and Atmospheric Administration National Centers for Environmental Information.
https://www.fisheries.noaa.gov/inport/item/66707.
NOAA (National Oceanic and Atmospheric Administration). 2023a. Meteorological Observation: Beaufort, Duke
Marine Lab. Accessed June 15, 2023. https://tidesandcurrents.noaa.gov/met.html?id=8656483.
NOAA (National Oceanic and Atmospheric Administration). 2023b. Tides & Currents [Dataset]. Accessed June
15th, 2023. U.S. Department of Commerce. https://tidesandcurrents.noaa.gov/.
NTETRA TECH 20

Calico Creek Modeling Report for Morehead City, NC
Included in this section are the output visualizations from the CORMIX model for the scenarios used to evaluate
dilution in the near -field of the Morehead City WWTP outfall to Calico Creek. Scenario visualizations are presented
for each modeled scenario across the tide cycle for both summer (Figure 11—Figure 16) and winter seasons (Figure
17— Figure 22). The only scenarios not depicted below are those at slack, for which the CORMIX model could not
be run (scenarios 3, 7, 11, 15).
I 3 4 6 10 r6 25 40 63 W
Figure 11. Visualization of the summer effluent plume three hours before high water slack (Scenario 1).
d
1 2 3 4 6 10 Ib 25 40 69 WO
Figure 12. Visualization of the summer effluent plume one hour before high water slack (Scenario 2).
i�NOW
7 7 3 �d b If] Id 25 40 63 I117
Figure 13. Visualization of the summer effluent dilution plume 1.2 hours after high water slack (Scenario 4).
NTETRA TECH 21

Calico Creek Modeling Report for Morehead City, NC
�8annwl-0E>4LNa
1 2 3 d h 10 Ib 25 40 63 �00 cuwnw,�w..
Figure 14. Visualization of the summer effluent plume three hours before low water slack (scenario 5).
16 25 40 69 700
Figure 15. Visualization of the summer effluent plume one hour before low water slack (scenario 6).
ScL_Su—r_Fluad_12aLW S
Discharge Excess (mg/q P—a-su o.W. waa SLO— --- P—
10 13 17 22 28 36 46 60 77 100 www � _ _ - — e d races r Plum Q1FF;
1. Px k4k rx•1.5 Z%•55 cmm.Moae Eour-0ay lMOo,
Wr
Nmdz�m W b %• I18m ¢%d P01 %• 11Bm)
Figure 16. Visualization of the summer effluent plume 1.2 hours after low water slack (scenario 8).
NTETRA TECH 22

Calico Creek Modeling Report for Morehead City, NC
1 2 3 4 6 10 16 25 40 63 700
Figure 17. Visualization of the winter effluent plume three hours before high water slack (scenario 9).
n
Okcno,pa Excees (m910 o,a, ,,,e,.yd, --- ��+. b..+•+Wwwrw»..rer v.+�r
1 2 3 4 6 10 16 25 40 63 100
Figure 18. Visualization of the winter effluent plume one hour before high water slack (scenario 10).
1
s.zzw�w.�nn_,wws w.��
OLenazys EiCssr Msp/p ie. o.r»ro o�wrt.. aona. ---n.r r.,r. a.w �,.o�. �er.a,ew�a u. e.w.v
1 2 3 4 6 10 16 25 40 63 100 m 1'1
o.,r,s�,..s z.•izz —c.». wsr �re.00,
Figure 19. Visualization of the winter effluent plume 1.2 hours after high water slack (scenario 12).
NTETRA TECH 23

Calico Creek Modeling Report for Morehead City, NC
Okchar9e Excros (m9/q
now s�i a.�wr se... __— Leda.-eewmsa.
f 2 3 4 6 10 16 25 40 63 107
Figure 20. Visualization of the winter effluent plume three hours before low water slack (scenario 13).
z
1
0�
----
s�r� w. r.�niwa
DYchwye Excna (m2
2 4 b IQ Ib 25
3 40 63 7017
axonm saw r.M.iR tx.ee
wrmwo x.8.pta wax.�.l
Figure 21. Visualization of the winter effluent plume one hour before low water slack (scenario 14).
DkchaW Excers (mUM S. o.. w EEAl 2.Lw 8
rn r3 �7 22 28 36 46 60 77 IW "==A'
o+R��s..r � •�e z,.ss
Figure 22. Visualization of the winter effluent plume 1.2 hours after low water slack (scenario 16).
aTETRA TECH 24