NC0004774_Thermal Mixing Zone Modeling Report_20220502

DUKE Duke Energy
ENERGY® Buck Combined Cycle
1385 Dukeville Road
Salisbury,NC 28146
April 25, 2022
North Carolina Department of Environmental Quality RECEIVED
Division of Water Resources MAY* U 2
1617 Mail Service Center 2022
Raleigh NC 27699-1617 NCDEQIDWRINPDES
RE: Duke Energy Carolinas, LLC
Buck Combined Cycle Station, NPDES Permit NC0004774
Part A. (19.) Clean Water Act Section 316(a) Thermal Variance
Dear Sir or Madam:
In accordance with the provisions of NPDES Permit NC0004774, Part A. (19.), enclosed
is our timely submittal of the requested Clean Water Act § 316(a) information for the
Buck Combined Cycle Station. Specifically, the Department has requested that the
submitted 316(a) information be provided by April 30, 2022. We believe that this report
completely satisfies this obligation.
As detailed in the enclosed report, a maximum daily temperature limit of 98°F satisfies
the water quality standard within a reasonable distance downstream and allows for safe
passage of aquatic organisms. The enclosed report details the model results and
defines a plume length for the summer and winter months.
Please contact Steve Cahoon ([e-mail], [phone] if
there are any questions regarding this submittal.
Sincerely,
ir
4,, l' 11I 1 —
Kris Eisenrieth,
General Manager II, Buck Combined Cycle Station
Attachment: Buck Combined Cycle Station 316(a) Thermal Mixing
Zone Modeling
Report
USPS: 7019 0140 0001 0794 0890 / 9590 9402 5350 9154 1985 45

Thermal Mixing Zone Modeling Report
Buck Combined Cycle Station, Salisbury, NC
Prepared for:
Duke Energy Corporation
Charlotte, North Carolina
April 13, 2022
Prepared by:
Water Environment Consultants
Mount Pleasant, South Carolina
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
Table of Contents
Executive Summary iv
1 Introduction 1
2 Outfall OO1A Discharge Location 3
3 EFDC Modeling 7
3.1 Field Data Collection 7
3.2 Model Setup and Calibration 9
3.3 7Q10 Model Inputs 14
4 Outfall 001A Thermal Mixing Zone Results 19
5 Permit Recommendations 22
6 References 23
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
List of Figures
Figure 1-1. Site Location Map(not current aerial imagery) 2
Figure 2-1. Outfall 001A discharge—August 14, 2019 4
Figure 2-2. Outfall 001A and wingwall—August 14, 2019 5
Figure 2-3. Shoal and small channel along upstream side of wingwall (photo taken August 14,2019) 6
Figure 2-4. Wingwall Breach connecting channel at property bulkhead(photo taken August 14, 2019) 6
Figure 3-1. River depths measured on August 14, 2019 (aerial dated February 3, 2018,shows higher
flow conditions) 8
Figure 3-2. Measured velocities(depth-averaged)on August 14,2019(aerial dated February 3,2018,
shows higher flow conditions) 8
Figure 3-3. EFDC model grid 10
Figure 3-4. EFDC model grid depths 11
Figure 3-5. Model grid and bathymetry converted to feet relative to NAVD88 12
Figure 3-6. Calibrated model and measured current velocities(upstream study area) 13
Figure 3-7. Calibrated model and measured current velocities(downstream study area) 14
Figure 3-8. Location of USGS station and catchment basins in relation to the project site. 16
Figure 3-9. Flow-stage relationship at USGS station 02116500 17
Figure 4-1. Summer 7Q10 thermal plume model results and 89.6°F standard mixing zone 20
Figure 4-2. Summer 7Q10 thermal plume model results and 5.04°F(delta-T)standard mixing zone 20
Figure 4-3. Winter 7Q10 thermal plume model results and 5.04°F(delta-T)standard mixing zone 21
List of Tables
Table 3-1. 7Q10 flow determination 16
Table 3-2. 7Q10 model water level adjustments 17
Table 3-3. 7Q10 model inputs 18
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
Executive Summary
Duke Energy Corporation (Duke) retained Water Environment Consultants(WEC)to prepare this report
for submission to the North Carolina Department of Environmental Quality(NCDEQ) in support of
existing NPDES permit NC0004774. Duke owns and operates the Buck Combined Cycle Station (Buck),
located in Salisbury, NC along the Yadkin River. In early 2019, Duke retained WEC to collect field data
within the Yadkin River and conduct a thermal mixing model of Buck's effluent discharge. WEC recorded
field measurements in August 2019, and in accordance with the original project scope,WEC investigated
the feasibility of conducting a thermal mixing model using CORMIX. Given the irregularities of the site
conditions, particularly around the discharge outfall, an accurate model of the effluent and river mixing
necessitated a different approach. Subsequently,WEC recommended using a two-dimensional
application of the Environmental Fluid Dynamic Code (EFDC) model instead,and Duke submitted a WEC-
prepared Thermal Modeling Study Plan(WEC 2021)to NCDEQ for approval. This report summarizes the
EFDC mixing zone model analysis and demonstrates that the proposed limitations and thermal mixing
zone meet the state surface water standards for temperature.
Buck's coal-fired steam station is retired,and all power is now generated at the Combustion Turbine
Combined Cycle (CTCC) plant. Under permit NC0004774, Duke's steam station discharged 395 million
gallons per day(MGD) of once-through noncontact cooling waters to the mainstem of the Yadkin River.
The new CTCC plant generates a drastically reduced thermal discharge,estimated to total 0.62 MGD,
which flows through Outfall 001A into the Yadkin River.
This section of the Yadkin River is categorized by NCDEQ as Lower Piedmont(15A NCAC 02B .0202,
Definitions)and is classified WS-V(also protected for Class C uses). The NCDEQ water quality standard
for temperature has two components: not to exceed 2.8°C(5.04°F)above natural background (referred
to herein as the delta-T standard)and not to exceed 32°C(89.6°F) (i.e.,the maximum standard). Both
conditions must be met at an acceptable distance downstream while also allowing safe passage of
aquatic organisms.
WEC set up the EFDC model using the water depths and water surface elevations measured in the field.
The model was calibrated to the field-measured currents. Once calibrated,the model was used to
evaluate mixing and dilution of the discharge plume within the Yadkin River under critical 7Q10 flow.
While this analysis does not consider heat loss to the atmosphere(a conservative assumption), it does
include summer and winter ambient water temperatures in addition to the seasonal 7Q10 (the lowest 7-
day average flow that occurs [on average] once every 10 years)flows.
Results from this mixing zone analysis indicate a year-round, daily maximum temperature limit of 98.0°F
satisfies the water quality temperature standards within a reasonable distance downstream and allows
safe passage of aquatic organisms. For the summer 7Q10 model,the maximum and delta-T standards
are met at thermal plume lengths of 204 feet and 266 feet, respectively. For the winter conditions
model,the delta-T standard is met at a plume length of 224 feet. Because the ambient water
temperature is cooler in the winter,the thermal plume dilutes below the 89.6°F maximum standard
faster than during summer(i.e.,the winter plume is smaller than the summer). In all cases,the cross-
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Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
shore plume width does not extend beyond the end of the wingwall, and thus,the thermal plume will
not inhibit the safe passage of aquatic organisms. Because the thermal mixing zone established by this
conservative modeling analysis satisfies both temperature standards,continuation of the Clean Water
Act(CWA) 316(a)thermal variance is not necessary. Therefore, Duke may request a permit modification
that includes the annual 98.0°F discharge limit and eliminates the CWA 316(a)thermal variance.

Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
1 Introduction
Duke Energy Corporation (Duke) retained Water Environment Consultants(WEC)to prepare this report
for submission to the North Carolina Department of Environmental Quality(NCDEQ) in support of
existing NPDES permit NC0004774. This report summarizes a thermal mixing zone analysis for Duke's
Buck Combined Cycle Station (Buck), located in Salisbury, NC(Figure 1-1).
Buck's coal-fired steam station is retired,and all power is now generated at the Combustion Turbine
Combined Cycle (CTCC) plant. Under permit NC0004774, Duke's CTCC plant discharges an estimated
0.62 million gallons per day(MGD)to the Yadkin River through Outfall 001A. WEC developed an
application of the Environmental Fluid Dynamics Code (EFDC) model to simulate thermal mixing of the
heated effluent with the ambient waters of the Yadkin River. This analysis can be used to propose a
new,year-round temperature limit for Duke's Buck station and remove the Clean Water Act(CWA)
316(a) requirement from the existing permit. The model results also indicate the size of the mixing zone
needed to satisfy state water quality standards for temperature and that also allows for safe passage of
aquatic organisms. WEC's analysis to support these conclusions is provided in the following sections:
• Section 2, Outfall 001A Discharge Location—provides a brief description of the outfall location;
• Section 3, EFDC Modeling—describes the field data collection, model setup and calibration, and
7Q10 modeling application;
• Section 4, Outfall 001A Thermal Mixing Zone Results; and
• Section 5, Permit Recommendations.
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Figure 1-1. Site Location Map(not current aerial imagery)
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Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
2 Outfall OO1A Discharge Location
As previously mentioned,the site's coal-fired steam station is retired,and all power is now generated at
the CTCC plant. Heated effluent from the CTCC plant discharges from Outfall OO1A into the mainstem
Yadkin River via a downward facing pipe along the property bulkhead (Figure 2-1). The outfall location is
impounded by a wingwall that extends from the shore, slightly upstream of the discharge, into the river
parallel to the shoreline (Figure 2-2). The wingwall is exposed during critical, low-flow, 7010 scenarios,
effectively separating the ambient waters of the Yadkin River from the heated effluent at Outfall 001A.
Where the wingwall meets the property bulkhead,the upper portion of the wingwall was breached so
that the area contained within the wingwall is partially connected to the ambient waters upstream of
the discharge. This partial connection occurs via a small channel that has formed along the upstream
side of the wingwall, between the wall and a sandy shoal. Figures 2-3 and 2-4 show photos of the
wingwall breach,the small channel, and shoal.
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Figure 2-2. Outfall OO1A and wingwall—August 14, 2019

Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Figure 2-3. Shoal and small channel along upstream side of wingwall (photo taken August 14, 2019)
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Figure 2-4. Wingwall Breach connecting channel at property bulkhead(photo taken August 14,2019)
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Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
3 EFDC Modeling
This portion of the Yadkin River is categorized as Lower Piedmont(15A NCAC 02B .0202, Definitions) and
is classified WS-V(also protected for Class C uses). The NCDEQ water quality standard for temperature
has two components: not to exceed 2.8°C(5.04°F)above natural background (referred to herein as the
delta-T standard) and not to exceed 32°C(89.6°F) (i.e.,the maximum standard) (15A NCAC 02B.0211
Fresh Water Quality Standards for Class C Waters). Both conditions must be met at an acceptable
distance downstream while also allowing safe passage of aquatic organisms. The purpose of the model
study is to simulate the thermal mixing of the effluent wastewater with the ambient river flow and
determine if the discharge will meet these standards. The study results may be used to propose a new,
year-round temperature limit for Duke's Buck Station and remove the CWA 316(a) requirement from
the existing permit. The following subsections describe: 1)field data collected to support the model, 2)
model setup and calibration, and 3) inputs used for modeling under 7Q10 conditions.
3.1 Field Data Collection
To develop a model that demonstrates compliance with the NCDEQ water quality standards for surface
water temperature,WEC collected field data including detailed measurements of current velocities,
water depths,and water surface elevations. These data were necessary for model setup and calibration.
Field measurements were recorded on August 14, 2019,when the river was at a low-flow condition, as
close to 7Q10 conditions as possible. The study area extended from just upstream of Outfall 001A to
approximately 1,800 feet downstream. While the river flow rate during the field measurements is
discussed below, please refer to Section 3.3 for discussion of 7Q10 conditions.
Water depth data were required to create a two-dimensional grid for the EFDC model. Measured water
surface elevations along the river were used to convert the depths to bottom elevations relative to a
fixed vertical datum. Current velocity measurements were required for comparison to the modeled
currents to verify that the model reasonably represents the hydrodynamics in the river.
WEC used a Sontek RiverSurveyor-M9 to collect water depths and current velocities within the Yadkin
River. The RiverSurveyor-M9 is an Acoustic Doppler Current Profiler(ADCP)that is equipped with a
differential GPS that provides horizontal positioning data. The ADCP measures depths and currents
through the water column as it traverses the river, providing a two-dimensional cross-section of current
velocities. Figure 3-1 shows the measured water depths along the ADCP transect paths. Figure 3-2
shows the depth-averaged current velocities. These velocities were scaled to a common reference
magnitude in both length and color. The arrows indicated flow direction and were anchored at the
point of measurement(i.e.,the arrow's base). The average river flow rate calculated by the ADCP was
54 cubic meters per second (m3/s)or 1,903 cubic feet per second (ft3/s).
Water surface elevations were measured using a Trimble Geo 7x Centimeter Edition GPS,capable of
±0.1-foot accuracy. WEC used the data to convert the measured water depths to bottom elevations
relative to the North American Vertical Datum of 1988(NAVD88). These bottom elevations were later
interpolated onto the model grid as discussed below. The water surface elevation during data collection
was measured as 189.3 meters NAVD88 (620.9 feet NAVD88).

Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
�Outfall 001A "' �
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Figure 3-1. River depths measured on August 14, 2019 (aerial dated February 3,2018, shows higher
flow conditions)
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Figure 3-2. Measured velocities(depth-averaged) on August 14,2019(aerial dated February 3,2018,
shows higher flow conditions)

Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
3.2 Model Setup and Calibration
WEC created a two-dimensional model grid of the Yadkin River, beginning just upstream of Outfall 001A
and extending approximately 1,800 ft downstream (Figure 3-3). The grid was comprised of 12,526 cells.
These cells were mostly uniform,2-meters by 2-meters in size. The wingwall was accounted for in the
model grid by de-activating the model cells that overlapped the wall location. Deactivated cells were
effectively removed from the model calculations blocking flow from passing through these cells. The
model domain included eleven active grid cells between the shoreline and the end of the wingwall to
resolve flow patterns in the sheltered region behind the wall.
The field-measured water depths and bathymetry were interpolated onto the model grid, illustrated in
Figure 3-4 and Figure 3-5. As noted during the field measurements and discussed above,the grid
bathymetry included the small channel and shoal along the upstream side of the wingwall. The breach
in the wall was included in the model. While high flow events could alter the shoal and shallow channel,
historical imagery and vegetation growth suggests the shoal has remained stable.
Boundary conditions for the calibration model included the river flow at the upstream boundary and the
measured water surface elevation at the downstream boundary. The ADCP measured flow within the
Yadkin River was distributed across the upstream grid cells. The effluent discharge rate was included in
the calibration model within the grid cell corresponding to Outfall 001A. Duke provided a timeseries of
Outfall 001A effluent flow rates that coincided with field measurements collection. The downstream
boundary was specified as an open boundary,set to the field-measured water surface elevation.
To confirm the EFDC model hydrodynamics,WEC compared the modeled and measured currents to
evaluate agreement in current magnitude and direction. Figure 3-6 and Figure 3-7 illustrate the
modeled and measured current velocities in feet per second (ft/s). In these figures,the field measured
currents are indicated by the red node. Both modeled and measured velocity vectors indicate flow
direction and are scaled to a common magnitude in color and length.
Overall,the modeled currents are in reasonably good agreement with the measured velocities,and the
model is sufficiently calibrated in this area for the purposes of evaluating the spatial extent of the
discharge mixing zones. Results from hydrodynamic model are typically smoother and show less
variation than direct measurements in rivers. The model is a snapshot in time whereas the field-
measured currents took place over several hours. Because the Yadkin River flow rate fluctuated slightly
over the course of field measurements,the model was expected to show some differences from the
measured data. For instance,the model does not show the isolated, individual peaks in currents in the
center of the river(as shown by the yellow vectors in Figure 3-6). This area of the river does not affect
flows behind the wingwall or near Outfall OO1A,though. In contrast,the model accurately
demonstrates an eddy formation on the leeward side of the wingwall as captured in field
measurements.
As mentioned in the Study Plan (WEC 2021), WEC modeled dilution of the Outfall 001A effluent plume
through advection only and did not include a full thermal model with heat loss from the water column
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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to the atmosphere. This is a conservative approach, and since a full thermal model was not conducted,
the model did not need to be calibrated for temperature. For a dilution analysis,the primary calibration
factors are current patterns and the dispersion coefficient. As a conservative assumption, WEC set the
dispersion coefficient to zero and based dilution only on advection.
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
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Figure 3-7. Calibrated model and measured current velocities (downstream study area)
3.3 7Q10 Model Input
As outlined in the Study Plan (WEC 2021), for this analysis, a 98.0°F daily maximum effluent temperature
was evaluated. The 98.0°F was determined by adding a minimal 3.0°F compliance margin to the existing
95.0°F permitted temperature limit. As done in other NCDEQ approved mixing zone models for Duke,
this compliance margin was determined to be a reasonable balance between the daily variability of grab
samples (versus a monthly limit and/or continuous temperature measurement) and a slightly larger
permitted mixing zone caused by the 3.0°F. Summer and winter ambient temperature conditions were
considered to ensure compliance throughout the year. WEC defined summer months as March through
October and winter months as November through February for the seasonal models as conservative
model inputs. These months reduce the ambient temperatures,therefore maximizing the temperature
difference between or"excess" (i.e.,the effluent temperature above ambient).
The EFDC model was run under two conservative assumptions: maximum permitted discharge rate of
0.62 MGD and temperature dilution from mixing only(i.e., no heat exchange with the atmosphere).
Adjusted inputs for this mixing zone analysis included the summer and winter ambient 7Q10 flow rates,
the downstream water surface elevation, and inclusion of the permitted discharge flow rate. The
discharge thermalplume was evaluated byinputting an effluent "dye" concentration to the
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
The original 7Q10 flow rate at the project site was determined in the 1980's and is outdated. In 2016,
however,the U.S. Geological Survey(USGS) published updated flow rates for upstream gauge stations
using data through 2012 (Weaver 2016). The appropriate 7010 flow rate for input to the model must
consider that the Yadkin River bifurcates and flows around an island at the project site, as seen in Figure
1-1, and the model domain is limited to the river channel on the south side of the island.
WEC estimated 7010 flow at the project site using two methods and used the more conservative
(lowest)value for the mixing analysis. The first method was to use 99%of the 2016 published summer
and winter 7010 values at the upstream USGS station 02116500. The 99%fraction was determined as
the ratio of WEC-measured flows of the south channel (1,903 cfs) and the reported flow during that time
at the USGS station (1,929 cfs). The second method of determining 7010 flow was to add the
incremental inflow between the USGS station and the project site to the published 7Q10 values,
proportional to the additional watershed area. This total river flow was split around the island based on
the relative channel widths on the north and south sides of the island. Based on channel widths,
approximately 71%of the total flow remains on the Buck Station (southern)side of the river.
Figure 3-8 shows the location of Buck Station, USGS station 02116500,and the respective watershed
basins as determined from USGS StreamStats(USGS 2019). Table 3-1 summarizes the results of the two
methods. The more conservative (lowest) results and the 7Q10 rates used for this analysis are 536 ft3/s
and 1,006 ft3/s for summer and winter, respectively,which were determined using the first method
described above.
The water surface elevation input to the model boundary was lowered from recorded field
measurements based on the change in water depths at USGS station 02116500 associated with similar
changes in flow rates. Figure 3-9 shows the measured depths and flow rate data at the USGS station
(USGS 2022). Based on a best-fit equation of this data developed by WEC, and similar stream
characteristics at both sites,WEC determined the water levels(or stage)at the project site associated
with the summer and winter 7010 flow rates. For the summer 7Q10 condition,the field-measured
water surface elevation was lowered by 1.1 ft(0.3 m),the difference between the stage elevations
during the 7Q10 flow and the flow during field measurements. For the winter 7010 condition,the
measured water level was lowered by 0.7 ft(0.2 m). Table 3-2 summarizes these adjustments. The
resulting 7010 water surface elevations at the downstream model boundary were 188.9 meters
NAVD88 (619.8 feet NAVD88)for the summer model and 189.0 meters NAVD88(620.2 feet NAVD88)for
the winter model.
Thermal mixing from the Outfall 001A discharge was modeled using EFDC's dye tracer module. This
method simulates thermal dilution from mixing only,excluding heat losses to the atmosphere as
previously mentioned, providing a conservative analysis. As the warm effluent mixes with the cooler
ambient waters of the Yadkin River,the excess temperature of the plume dilutes. WEC then identified
the downstream points where the plume dilution satisfied both the maximum and delta-T temperature
standards.
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
State F
USGS Basin Area:2280 mi2
Project Site Basin Area: 3452 mi2
v'nsto
Sate u
USGS 02116500
Yadkin River at High Pole
t• Yadkin College, NC
it-, di.
.,.,,,F,,I Legend
it II
* name
* Project Site
rt r AO- USGS Station
N
0 5 10 20 I I USGS Station Basin
Miles Q Project Site Basin
Figure 3-8. Location of USGS station and catchment basins in relation to the project site.
Table 3-1. 7Q10 flow determination
Variable: Value
7010 flow—Summer(USGS 02116500): 543 ft3/s
7Q10 flow—Winter(USGS 02116500): 1,020 ft3/s
Method 1
Average Measured Flow(August 14,2019): 1,903 ft3/s
Average USGS flow 12 AM-12 PM (August 14,2019): 1,929 ft3/s
Ratio of Measured to USGS Data: 99%
7Q10 flow—Summer: 536 ft3/s
7010 flow—Winter: 1,006 ft3/s
Method 2
Incremental Drainage Area: 1,172 mi2
incremental 7010 flow—Summer: 279 ft3/s
incremental 7010 flow—Winter: 524 ft3/s
71%x(7Q10+ incremental inflow)—Summer: 584 ft3/s
71%x(7Q10+ incremental inflow)—Winter: 1,097 ft3/s
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
Stage vs. Flow Rate
645
• USGS Data
644 • 7Q10-summer
643 7Q10-winter
642 • Field Days
I
00
•
0 638
637
R2=100
636
635
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Flow(ft3s)
Figure 3-9. Flow-stage relationship at USGS station 02116500
Table 3-2. 7Q10 model water level adjustments
7Q10-summer 7Q10-winter 8/14/2019
Flow(ft3/s): 536 1,006 1,903
USGS Station Stage(ft NAVD88): 638.4 638.8 639.5
Delta,as compared to 8/14/2019(ft): 1.1 0.7
Adjusted Stage,from field-measured 619.8 620.2
620.9(ft NAVD88):
WEC reviewed monitoring data from NCDEQ's Ambient Monitoring System on the Yadkin River,Station
Q4660000 just upstream of Buck,to determine the modeled ambient river temperatures during summer
and winter conditions (Water Quality Portal 2022). During the summer months, WEC analyzed the 95th
percentile ambient temperature (83.9°F)for compliance with the 89.6°F maximum standard. The 95th
percentile is used because it will take longer for the effluent to meet the maximum standard when the
ambient water is warm as compared to when it is cold (e.g., the 5th percentile). With a temperature
excess of 14.1°F(98.0°F minus 83.9°F),this standard is met once the plume excess dilutes to 5.7°F
(83.9°F+5.7°F=89.6°F). For compliance with the 5.04°F delta-T standard during summer months,WEC
analyzed the 5th percentile ambient temperature (50.0°F). This standard is met once the plume excess
dilutes to 5.04°F.The difference in the ambient and effluent temperatures is greater during winter
months than during summer. The thermal plume during the winter months will dilute below the 89.6°F
maximum standard at a shorter distance from Outfall 001A than during the summer months, even under
the warmest, ambient winter temperatures. Therefore, summer conditions were modeled to for
determine the worst-case scenario for compliance with the 89.6°F maximum standard.
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Thermal Mixing Zone Modeling Report — Buck Combined Cycle Station
Because the temperature excesses are greater than when evaluating against the maximum standard,
and will take longer to dilute downstream, it is more critical to examine where the delta-T standard is
met. WEC analyzed the 5th percentile ambient temperature during the summer and winter months,for
compliance with the 5.04°F delta-T standard. It is obvious that because the temperature excess is
greater in the winter, it will be the critical condition for determining compliance with the 5.04°F delta-T
standard.
Table 3-3 summarizes the 7Q10 model inputs.
Table 3-3. 7Q10 model inputs
Variable Summer Model Winter Model
7Q10 Flow Rate (m3/s): 15.2 (536 cfs) 28.5 (1,006 cfs)
Downstream Water Level (m NAVD88): 188.9 (619.8 ft) 189.0(620.2 ft)
Effluent Flow Rate (m3/s): 0.027 (0.62 MGD) 0.027 (0.62 MGD)
Effluent Temperature (°F) 98.0 98.0
95th Percentile Ambient Temperature (°F) 83.9 51.4
95th Percentile Temperature/"Dye" Concentration: 14.1 -
5`h Percentile Ambient Temperature (°F) 50.0 33.3
5th Percentile Temperature/"Dye" Concentration: 48.0 64.7
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Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
4 Outfal1001A Thermal Mixing Zone Results
The EFDC model was executed for a four-day simulation period,ample time for plume and
hydrodynamics to reach steady-state given the constant input boundary conditions. Figures 4-1 and 4-2
show the model results for the 0.62 MGD, 98.0°F discharge during the summer conditions for the
maximum and delta-T standards, respectively. Figure 4-3 shows model results for the winter condition
delta-T scenario. The color contours represent the thermal plume's temperature excess above
background in degrees Fahrenheit. In each plot,the dark red color represents the portion of the plume
that exceeds the temperature standard,for the 89.6°F maximum and 5.04°F delta-T standard. Thus,the
edge of the dark red delineates where the temperature standards are met.
In the summer conditions model,the 89.6°F maximum standard is met with a thermal plume length of
204 ft. Also,the summer conditions model 5.04°F (delta-T)standard is met with a plume length of 266
ft. In the winter model,the 5.04°F (delta-T)standard is met with a plume length of 224 ft. At the
downstream end of the wingwall,an eddy causes the discharge plume to recirculate behind the wall and
slowly mix with the ambient river flow. As a result,the cross-stream width of the thermal plume does
not extend beyond the end of the wingwall (approximately 27 percent of the river width), and it would
therefore allow safe passage for aquatic organisms that may be inhibited by a slight increase in
temperature.
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
ION"
4'it 'x it.rt .
41';•-•"'nil
r Legend
* Outfall 001A 2.1 -3-0
Temperature Excess(°F) 3.1 -4 0 »
i» � � 0.0-0.1 4.1 -5.0 z
0 125 250 500 ` °'u`~ 0.1 - 1.0 5 1 -5 7 :.
A
Feet g ) 1.1 -2.0 >5.7
r
Figure 4-1. Summer 7Q10 thermal plume model results and 89.6°F standard mixing zone
1 arm jn I
,Lr�
;' '
r ' " 1:4 • ems-' .
, - Legend
• Outfall 001A 1 01 -2 00
,, " ""' "' Temperature Excess(OF) 2-01 -3.00
a ,; - 00-001 301 -400
' Ak 0 125 250 500 111111 0.01 -0 50 4 01 -5.04
Feet _ 0.51 1.00 >5 04
. - —_u .^st...
Figure 4-2. Summer 7Q10 thermal plume model results and 5.04°F (delta-T) standard mixing zone
20
• V L' - -

Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
Legend
• Outfall 001 A 1.01 -2.00
.• Temperature Excess(°F) 2.01 -3.00
•,., 0.0-0.01 3.01 -4.00
A0 125 250 500 0.01 -0.50 4.01 -5.04
Feet 0.51 - 1.00 >5.04
Figure 4-3. Winter 7Q10 thermal plume model results and 5.04°F (delta-T) standard mixing zone
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Thermal Mixing Zone Modeling Report—Buck Combined Cycle Station
5 Permit Recommendations
Results of this modeling study indicate that a year-round 98.0°F thermal discharge would satisfy both
conditions of the NCDEQ water quality standards for temperature at a short distance downstream. The
plume size allows for safe passage of aquatic organisms,as the plume is confined to a small area
adjacent to the bank and does not extend across the river. Results from this analysis support Duke's
request for a year-round, daily maximum temperature limit of 98.0°F. Of course,the thermal plume will
be even smaller than what these conservative models predict because 1)actual discharge temperatures
and flow rate will be less than the maximum permit limits,and 2)this analysis considers extreme
ambient temperatures(both high and low) and excludes atmospheric heat loss. Because the actual
temperature excess will be far less and likely immeasurable within the DEQ-approved mixing zone,these
conservative models prove that instream monitoring/limit should not be required. Finally, because the
permit will include approval of this relatively small thermal mixing zone,continuation of the 316(a)
temperature variance should not be necessary,and this requirement may be removed from the NPDES
permit.
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Thermal Mixing Zone Modeling Report-Buck Combined Cycle Station
6 References
EPA 2021. https://www.epa.gov/ceam/environmental-fluid-dynamics-code-efdc. Accessed July 21,
2021.
Hill, David. "RE: [EXTERNAL] Duke Energy Buck Combined Cycle Plan Thermal Modeling".
Correspondence with NCDENR and Duke Energy. December 7,2021. E-mail.
USGS. 2019.StreamStats: Streamflow Statistics and Spatial Analysis Tools for Water-Resources
Applications. March 4, 2019,accessed 2022 at https://www.usgs.gov/mission-areas/water-
resources/science/streamstats-streamflow-statistics-and-spatial-analysis-tools
USGS. 2022. National Water Information System. USGS 02116500 Yadkin River at Yadkin College, NC.
Water Data for the Nation,accessed 2022 at
https://waterdata.usgs.gov/nwis/uv?site no=02116500
Water Quality Portal. 2022. National Water Quality Monitoring Council.Yadkin Riv at HWY 150 NR
Spencer(21NC01WQ-Q4660000)site data in the Water Quality Portal.accessed 2022 at
https://www.waterqualitydata.us/provider/STORET/21NC01WQ/21 NC01WQ-Q4660000/
Weaver C.J. 2016. Low-Flow Characteristics and Flow-Duration Statistics for Selected USGS Continuous-
Record Streamgaging Stations in North Carolina Through 2012.Scientific Investigations Report
2015-5001.Version 1.1, March 2016. Prepared in cooperation with the North Carolina
Department of Environment and Natural Resources, Division of Water Resources.
WEC. 2021.Thermal Model Study Plan. Buck Combined Cycle Station,Salisbury, NC.July 30, 2021.
Prepared for Duke Energy Corporation, Charlotte, NC.
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