NC0003468_Thermal Modeling Report_20200811

Thermal Modeling Report
Duke Energy – Dan River Combined Cycle

Prepared for:
Duke Energy Carolinas, LLC
Charlotte, North Carolina

August 11, 2020

Prepared by:
Water Environment Consultants
Mount Pleasant, South Carolina

Thermal Modeling Report – Dan River Steam Station

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Table of Contents
Executive Summary ...................................................................................................................................... iv
1 Introduction .......................................................................................................................................... 1
2 NPDES Outfall 001 ................................................................................................................................. 3
3 EFDC Modeling ...................................................................................................................................... 5
3.1 Field Data Collection ..................................................................................................................... 5
3.2 Model Setup and Calibration ........................................................................................................ 9
3.3 7Q10 Model Conditions .............................................................................................................. 14
3.4 Outfall 001 Mixing Zone Results and Proposed Permit Limits .................................................... 16
4 References .......................................................................................................................................... 18

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List of Figures
Figure 1-1. Site Location Map ...................................................................................................................... 2
Figure 2-1. Location of Dan River Combined Cycle Steam Station Outfall 001............................................ 3
Figure 2-2. Photo of discharge pipe at Outfall 001 ...................................................................................... 4
Figure 3-1. Photo of study area (outfall pipe is located at right as shown) ................................................. 6
Figure 3-2. Photo study area (downstream of outfall) ................................................................................ 7
Figure 3-3. Measured depths on August 13, 2019 (aerial at higher flow condition) ................................... 8
Figure 3-4. Depth-averaged, measured velocities on August 13, 2019 (aerial at higher flow condition) ... 9
Figure 3-5. Model grid and flow barriers ................................................................................................... 10
Figure 3-6. Depth grid ................................................................................................................................ 11
Figure 3-7. Model grid and bathymetry converted to feet relative to NAVD88 ........................................ 12
Figure 3-8. Calibrated model velocities and measured current velocities (upstream portion) ................. 13
Figure 3-9. Calibrated model velocities and measured current velocities (downstream portion) ............ 14
Figure 3-11. 7Q10 model results for summer (top) and winter (bottom) conditions ............................... 17

List of Tables
Table 3-1. 7Q10 model inputs .................................................................................................................... 15

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Executive Summary
Duke Energy Carolinas, LLC (Duke) retained Water Environment Consultants (WEC) to conduct a thermal
modeling analysis at Duke’s Dan River Combined Cycle Station in Eden, North Carolina. The coal-fired
steam station was retired and removed from the site, and a 620-MW combined cycle power plant was
constructed in 2012. The current discharge from Outfall 001 is process water and cooling water from
the combined cycle plant. A thermal mixing zone evaluation is required to determine the appropriate
permit limitations for the discharge without the ambient cooling previously provided by the wastewater
treatment basins. This report and the proposed limitations must be reviewed by the North Carolina
Department of Environmental Quality (NCDEQ) for incorporation into the next NPDES discharge permit.
WEC used the Environmental Fluid Dynamics Code (EFDC) to assess if the discharge will meet the 29°C
maximum and 2.8°C rise-above-background requirements (15 NCAC 02B .0211 Fresh Water Quality
Standards for Class C Waters) at an acceptable distance downstream. Per EPA’s Exposure Assessment
Models webpage, “EFDC is a state-of-the-art hydrodynamic model that can be used to simulate aquatic
systems in one, two, and three dimensions. It has evolved over the past two decades to become one of
the most widely used and technically defensible hydrodynamic models in the world.”
Local depths and current velocity measurements were collected using a Sontek M9 Acoustic Doppler
Current Profiler (ADCP), equipped with a differential GPS to provide horizontal positioning data. A
Trimble Geo 7x Centimeter Edition survey unit was used to collect water surface elevations along the
river bank. This data was used to set up and calibrate an EFDC model for the project. Once calibrated,
the model was used to predict thermal plume mixing under summer and winter 7Q10 conditions. The
proposed effluent temperature limits for summer and winter conditions were 99°F and 88°F,
respectively. The model was run under a maximum discharge flow rate of 1.33 MGD and with
temperature dilution from mixing only (no heat loss from evaporation). Both assumptions provide a
conservative estimate of how the effluent plume mixes with the ambient river flow.
The critical condition modeled for the summer was the 29°C maximum temperature standard. The
critical condition modeled for the winter was the 2.8°C exceedance over background conditions. The
model results show that a majority of the thermal plume is contained within the small channel beneath
the outfall pipe which connects to Dan River. As the plume mixes with the river, it remains attached to
same bank as the outfall. The results indicate the summer and winter limits will be met at a distance of
134 feet and 97 feet downstream, respectively.
Because the conservative modeled summer and winter temperatures of 99°F and 88°F respectively
support the ambient water quality standards, Duke should request these permit limitations be included
in the NPDES permit. Because actual discharge temperatures will be less than the maximum permit
limits, the actual instream plume will be smaller than the conservative model prediction results.
Therefore, there is no need for Duke to bear the expense of complying with upstream and downstream
temperature monitoring/limitations.
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1 Introduction
Water Environment Consultants (WEC) prepared this report for Duke to provide the results of a thermal
modeling analysis at Duke’s Dan River Combined Cycle Station (Dan River) in Eden, North Carolina
(Figure 1-1). The coal-fired steam station has been retired and removed from the site, and a 620-MW
combined cycle power plant was constructed in 2012. The current discharge from Outfall 001 is process
water and cooling water from the combined cycle plant. A thermal mixing zone will be required to meet
the discharge permit limitations issued by the North Carolina Department of Environmental Quality
(NCDEQ). WEC performed mixing zone modeling to assess if the discharge will meet the 29°C maximum
and 2.8°C rise-above-background requirements (15 NCAC 02B .0211 Fresh Water Quality Standards for
Class C Waters) at an acceptable distance downstream. A description of the work performed by WEC to
complete the assessment and the model results is provided in the following sections:
• Section 2, NPDES Outfall 001 – provides a brief description of the outfall;
• Section 3, EFDC modeling – describes the field data collection methodology, model calibration,
and 7Q10 modeling analysis and results.
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Figure 1-1. Site Location Map
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2 NPDES Outfall 001
Per the current NPDES Permit (NC0003468), the Dan River Outfall 001 discharges cooling tower
blowdown and plant collection sumps (low volume wastes) from the combined cycle unit directly to the
Dan River via Outfall 001.
WEC visited the Dan River site on August 13, 2019, to characterize the location and flow geometry at the
discharge location. The Outfall 001 discharge pipe is along the dam wall on the northern (downstream)
end of a short channel flowing into the Dan River (Figures 2-1 and 2-2). Water flowing from the outfall
pipe lands within a rectangular, containment basin before spilling into the channel and slowly flowing
into the Dan River. After mixing with the dam overflow, the discharge plume moves downstream within
the Dan River.

Figure 2-1. Location of Dan River Combined Cycle Steam Station Outfall 001

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Figure 2-2. Photo of discharge pipe at Outfall 001
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3 EFDC Modeling
This summary describes an application of the Environmental Fluid Dynamics Code (EFDC) for modeling
the Outfall 001 discharge into the Dan River. The purpose of the model study is to estimate the mixing
of the combined cycle wastewaters in the Dan River and determine if the discharge will meet the 29°C
maximum and 2.8°C rise-above-background requirements (15 NCAC 02B .0211 Fresh Water Quality
Standards for Class C Waters) at an acceptable distance downstream. This section describes: 1) the field
data collected to support the model, 2) the model setup and calibration, 3) the application of the model
under 7Q10 summer and winter conditions, and 4) model results that estimate where the NCDEQ water
quality standards are met.
3.1 Field Data Collection
Water depth and current measurements were collected to support the model study. Water depth data
is required in order to create a two-dimensional grid for the EFDC model. Current velocity
measurements are required for comparison to the modeled currents in order to verify that the model
reasonably represents the hydrodynamics in the vicinity of the mixing zone. Together, detailed water
depth and velocity data can be used to better select the ambient model geometry and the associated
portion of the 7Q10 flow.
Field data was collected on August 13, 2019. The study area extended from the discharge point to
approximately 880 feet downstream the Dan River. Photos of the site from the day of data collection
are shown in Figures 3-1 and 3-2.
A Sontek RiverSurveyor-M9 Acoustic Doppler Current Profiler (ADCP) was used to collect both water
depth and current velocity data. The Sontek M9 is equipped with a differential GPS to provide
horizontal positioning data. Water surface elevation was also measured using a Trimble Geo 7x
Centimeter Edition GPS with 0.1-foot accuracy. This data was used to convert the measured water
depths to bottom elevations relative to a fixed vertical datum (i.e., NAVD88). The measured depths are
shown in Figure 3-3 and the depth-averaged current velocities measured along transects are shown in
Figure 3-4.

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Figure 3-1. Photo of study area (outfall pipe is located at right as shown)

Outfall pipe
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Figure 3-2. Photo study area (downstream of outfall)
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Figure 3-3. Measured depths on August 13, 2019 (aerial at higher flow condition)
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Figure 3-4. Depth-averaged, measured velocities on August 13, 2019 (aerial at higher flow condition)
3.2 Model Setup and Calibration
WEC created a two-dimensional model grid of the Dan River extending from the dam approximately
1,000 ft downstream (Figure 3-5). Certain areas were known to be dry during low flow conditions,
which include several rock outcrops and a sandbar. These locations were noted as very shallow during
the field data collection, when flows were higher, and later verified as dry with low-flow aerial imagery.
Figure 3-5 uses this low-flow imagery where the dry areas are visible. Therefore, grid cells that
overlapped these locations were inactivated in the model in order to redirect flow.
The water depths measured with the ADCP were interpolated onto a grid, as shown in Figure 3-6. The
grid was then edited to account for shallow areas along rock ledges and the shoreline, as described
earlier. The water depths were converted to bottom elevations using the water surface elevations
collected with the Trimble survey unit. The bottom elevations are referenced to NAVD88. The
bathymetry grid was interpolated onto the EFDC grid for modeling as shown in Figure 3-7.
The model boundary conditions included the upstream flow and the downstream water elevation. The
upstream flow rate was determined from the ADCP measurements on the data collection day. The
average flow measured across the ADCP transects was calculated as 716.9 cubic feet per second (cfs)
and distributed across the upstream boundary along the
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Figure 3-5. Model grid and flow barriers
dam. Though US Geological Survey (USGS) gage 02071000, located approximately 13.0 miles upstream,
reports flow during the field measurement day, several rivers and stream merge into the Dan River prior
to the site, so the records could not be used. The downstream boundary was specified as an open
boundary with a constant water elevation. The downstream water elevation was measured during field
data collection as 479.3 ft NAVD88, roughly two feet lower than the water level just below of the dam.
The EFDC model was iteratively calibrated to match the current velocities measured on the field
collection day. Figure 3-8 and Figure 3-9 illustrate the modeled and measured current velocities in
meters per second (mps). The arrows indicate flow directions and are scaled to a common magnitude.
The modeled currents correlate with measured velocities. The measured data shows more local
variation in currents, such as a singular, larger peak velocity just north of the first rock outcrop. The
slightly lower peak modeled velocity, 1.0 mps, compared to the peak measured, 1.36 mps, is an artifact
of the model bathymetry being smoother than the actual river bathymetry. However, when comparing
areas on a whole, rather than a singular point, the velocities are much closer.
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Figure 3-6. Depth grid
Additionally, the model does well in estimating the flow paths, represented by the vector arrows,
around shoals and rock ledges. Both the modeled and measured velocities show a peak acceleration of
current along the northern (plant side) bank and part of the southern bank. On average, the model
tends to under-estimate the velocity magnitude in the rocky shoals. As a result, the model will likely be
conservative and under-estimate the amount of mixing and dispersion of the outfall effluent. Overall,
the model reasonably represents the current velocities in the river and the model is sufficiently
calibrated in this area for the purposes of evaluating the outfall discharge mixing zone.

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Figure 3-7. Model grid and bathymetry converted to feet relative to NAVD88

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Figure 3-8. Calibrated model velocities and measured current velocities (upstream portion)

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Figure 3-9. Calibrated model velocities and measured current velocities (downstream portion)

3.3 7Q10 Model Conditions
The 7Q10 model is used to verify whether the thermal mixing zone meets the discharge permit
limitations issued by NCDEQ. The NCDEQ water quality standard for temperature has two components:
not to exceed 2.8°C (5.04°F) above natural background and not to exceed 29°C (84.2°F). Inputs to the
calibrated EFDC model were modified to represent the critical 7Q10 conditions. Both summer and
winter conditions were considered in the analysis. The critical condition modeled for the summer was
the maximum temperature, and the critical condition modeled for the winter was the temperature
exceedance over background conditions.
The model was run under two conservative assumptions: maximum design discharge rate of 1.33 MGD
and temperature dilution from mixing only (no evaporative losses). The inputs adjusted from the
calibrated model included the summer and winter 7Q10 flow rates, the downstream water surface
elevation, and the discharge’s temperature excess as compared to the ambient temperature. The water
surface elevation measured during field monitoring was lowered based on change in water depths at
USGS station 02071000 associated with similar change in flow rates. Figure 3-10 shows the measured
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Figure 3-10. Flow-stage relationship at USGS station 02071000
depths and flow rate data points at the USGS station. Based on the best-fit equation of the data, and
similar stream characteristics at both sites, WEC determined downstream water levels at the project site
for the summer and winter 7Q10 conditions.
Thermal mixing from the discharge was modeled using EFDC’s dye tracer module, which simulates
dilution from mixing only and excludes evaporative losses as mentioned above. WEC reviewed four
years of ambient water temperature records near the outfall. The 95th and 5th percentile temperatures
were used as ambient conditions for the summer and winter critical conditions, respectively. During
summer months, the 95th percentile river temperature was 27.6°C (81.7°F). During winter months, the
5th percentile temperature was 1.4°C (34.5°F). Duke provided WEC with estimates of the maximum
projected effluent discharge temperatures for summer and winter months. The summer maximum
temperature was 35.6°C (96°F) and the winter maximum temperature was 29.4°C (85°F). WEC added a
3°F compliance margin similar to the thermal models approved by NCDEQ for Duke’s Rogers and
Asheville sites. The temperature excess (above background) used for modeling the summer condition
was 9.6°C (17.3°F), and the temperature excess for the winter condition was 29.7°C (53.5°F). Table 3-1
summarizes the inputs in SI units as used in EFDC.
Table 3-1. 7Q10 model inputs
7Q10 flow
rate (cms)
Downstream WL
(m NAVD88)
Effluent flow
rate (cms)
Effluent Temperature
Excess (°F)
Summer 8.89 145.9 0.058 17.3
Winter 16.4 146.0 0.058 53.5

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3.4 Outfall 001 Mixing Zone Results and Proposed Permit Limits
The model results for a 1.33 MGD discharge from Outfall 001 are shown in Figure 3-11. The colored
contours represent the temperature excess above background in units of degrees Fahrenheit for
summer and winter conditions. The downstream position where the water quality standard is met is
marked with a magenta line. For the summer condition, this line marks where the 29°C (84.2°F)
maximum temperature standard is met. For the winter results, the line marks where the discharge
meets the 2.8°C (5.04°F) exceedance above background standard.
As shown by the colored contours, the majority of the plume dilution takes place within the small
channel where the effluent is initially contained. Downstream, the plume remains narrow and strongly
bank attached where the water is deeper and the currents move faster. Near the boat ramp a small
rock outcrop pushes the plume away from the bank where it begins to spread. However, the plume
meets the summer and winter standards well prior to this point. The results for a 1.33 MGD discharge
at 99°F show the plume will meet the maximum temperature standard (84.2°F) approximately 134 feet
downstream of the dam. For a 1.33 MGD discharge at 88°F, the no-more-than 5.04°F exceedance above
background standard will be met approximately 97 feet downstream. In addition, the very narrow
plume widths also support safe passage of aquatic life.
Because the conservative modeled summer and winter temperatures of 99°F and 88°F respectively
support the ambient water quality standards, Duke should request these permit limitations be included
in the NPDES permit. These discharge limitations should be measured at the current compliance point,
noting that heat loss through the discharge pipe prior to entering the river should be minimal. Because
actual discharge temperatures will be less than the maximum permit limits, the actual instream plume
will be smaller than the conservative model prediction results. Therefore, there is no need for Duke to
bear the expense of complying with upstream and downstream temperature monitoring/limitations.

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Figure 3-10. 7Q10 model results for summer (top) and winter (bottom) conditions

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4 References
Martin, Joyce; D. H. Newcomb. “RE: Infor Needed – Future Summer & Winter Daily Max Permit Limits
(for modeling).” Correspondence with Duke Energy. June 3, 2020. E-mail
U.S. Geological Survey. 2020. National Water Information system. USGS 02071000 Dan River Near
Wentworth, NC. Water Data for the Nation, accessed 2020 at
https://waterdata.usgs.gov/usa/nwis/uv?site_no=02071000