NC0003468_Thermal Model Verificaiton_20240226

ste: DUKE Dan River Combined Cycle
ENERGY® Duke Energy Carolinas
864 South Edgewood Road
Eden,NC 27288
February 13, 2024 RECEIVED
Serge'Chernikov, Ph.D FEB 2 6 2024
NC Division of Water Resources
Water Quality Permitting Section-NPDES NCDEQ/DWR/NPDES
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
Subject: Duke Energy Carolinas, LLC
Dan River Combined Cycle
NPDES Permit NC0003468
Thermal Model Verification report
Dear Dr.Chernikov:
With reference to the study plan (March 28, 2023 letter to you) for verification of the CORMIX model Outfall 001
discharge, attached is the completed Thermal Model Verification report. Part I A. (6) (d) of referenced permit
requires after 12 months of temperature data is collected that a report be submitted to the Division to verify the
CORMIX model assumptions and predictions.
This report concludes that effluent mixing of Outfall 001 within the Dan River occurs in a short distance downstream.
Both the historical temperature monitoring and the verified mixing zone support the current NPDES permit
limitations.
Should you have questions or require additional information questions regarding the enclosed application, please
contact Joyce Dishmon at(336)623-0238 or email at [e-mail].
Sincerely,
Eisenrieth
GM-II Regulated Station
Dan River Combined Cycle
Attachment: Thermal Model Verification Report
Temperature Data
cc: Lon Snider, DWR,WRO
William Milam, Field EHS
Matthew Sumner,Sr. Engineering Technologist
1

Thermal Model Verification Report
Duke Energy - Dan River Combined Cycle
Prepared for:
Duke Energy Carolinas, LLC
Charlotte, North Carolina
January 2, 2024
Prepared by:
Water Environment Consultants
Mount Pleasant, South Carolina
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Thermal Model Verification Report- Dan River Steam Station
Table of Contents
Executive Summary iv
1 Introduction 1
2 Outfall Description 3
3 Field Measurements 5
3.1 Ambient Dilution Study 5
3.2 River Flow 5
3.3 Water Surface Elevation 7
4 Model Verification 11
4.1 Model Grid 11
4.2 Model Boundaries 11
4.3 Model Results Comparison 11
5 Regulatory Mixing Zone Analysis 16
5.1 Critical Flow Rates 16
5.2 Model Results and Proposed Permit Limits 16
References 20
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Thermal Model Verification Report- Dan River Steam Station
List of Figures
Figure 1-1. Site Location Map 2
Figure 2-1. Location of Dan River Combined Cycle Station Outfall 001 3
Figure 2-2. Photo of discharge pipe at Outfall 001 4
Figure 3-1. Dye injection setup (during pump calibration) 6
Figure 3-4. Embayment during dye study (arrows indicate observed circulation pattern) 8
Figure 4-1. Revised model grid bathymetry 12
Figure 4-2. Modeled dye concentrations for verification study conditions 13
Figure 4-3. Modeled and measured dye concentrations, 0 ft downstream 13
Figure 4-4. Modeled and measured dye concentrations, 50 ft downstream 14
Figure 4-5. Modeled and measured dye concentrations, 100 ft downstream 14
Figure 4-6. Modeled and measured dye concentrations, 150 ft downstream 15
Figure 4-7. Modeled and measured dye concentrations, 200 ft downstream 15
Figure 5-1. Flow-stage relationship at USGS station 02071000 17
Figure 5-2.7010 model results for summer(top) and winter(bottom) conditions 18
List of Tables
Table 3-1. Discrete dye concentration measurements 9
Table 3-2.Velocity and flow measurements 10
Table 5-1.7Q10 Model Inputs 17
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Thermal Model Verification Report- Dan River Steam Station
Executive Summary
Duke Energy Carolinas, LLC(Duke) retained Water Environment Consultants(WEC)to conduct a
verification of a thermal modeling analysis at Duke's Dan River Combined Cycle Station (Dan River) in
Eden, North Carolina. In 2020 (and revised in 2022),WEC developed an Environmental Fluid Dynamics
Code (EFDC)thermal model to analyze Duke's Outfall 001 effluent mixing with the Dan River and to
request permit limits for temperature.The North Carolina Department of Environmental Quality
(NCDEQ) accepted the analysis and issued an NPDES permit(NC0003468)that required field verification
of the modeling predictions used to set the daily maximum effluent and downstream temperature limits
of 37.2°C and 32.0°C. Part I.A.6.d of the permit states:
After 12 months of temperature data are collected, the permittee shall submit a report to the
Division to verify the CORMIX(should be EFDC(WEC])model predictions. The report shall include
field verification of assumptions used in the model and a summary of temperature data for effluent,
upstream and downstream...
WEC performed field data collection on August 22, 2023.This effort included measuring effluent dye
concentration, water depths, ambient flow,water surface elevations, discharge flow rate, and dye
concentrations in the river.The data collected was used to recalibrate the model inputs to better
correlate the model predictions with the field measurements.
The modeling and resulting permit limitations are based on a worst case, 7Q10 ambient flow as well as a
maximum discharge flow rate of 1.33 MGD. Neither of these conditions will occur simultaneously,
especially at the same time as a maximum, permitted effluent temperature, so these conditions provide
a conservative estimate for the thermal mixing zone.
The field data and the recalibrated model show that as the plume mixes with the river, it remains
attached to same bank as the outfall. The conservative model results indicate the summer and winter
limits will be met at distances of 99 feet and 192 feet downstream, respectively.These values are
slightly larger than the 2022 model distances of 43 feet and 97 feet, but these mixing zone lengths are
still very small. Because the model is conservative and the actual discharge temperatures will be less
than the maximum permit limits (summer and winter temperatures of 99°F and 88°F respectively),the
actual instream plume will be much smaller than the model prediction results. 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 that the existing permit limitations
remain 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 permit limits,the actual instream plume will
be smaller than the conservative model prediction results. Therefore,there is no need for the permit to
require Duke to bear the expense of routine river temperature monitoring as part of their permit
compliance.
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Thermal Model Verification Re ort- Dan River Steam Station
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1 Introduction
Water Environment Consultants (WEC) prepared this report for Duke Energy Corporation (Duke)to
verify an Environmental Fluid Dynamics Code (EFDC)thermal model created in 2020(and revised June
10, 2022)to analyze effluent mixing from Duke's Dan River Combined Cycle Station (Dan River's)Outfall
001.The North Carolina Department of Environmental Quality NCDEQ accepted the analysis and issued
the NPDES permit which requires field verification of the modeling predictions used to set the daily
maximum effluent and downstream temperature limits of 37.2°C and 32.0°C. Part I.A.6.d of the permit
states:
After 12 months of temperature data are collected, the permittee shall submit a report to the
Division to verify the CORMIX(should be EFDC(WEC])model predictions. The report shall include
field verification of assumptions used in the model and a summary of temperature data for
effluent, upstream and downstream...
To comply with the permit condition, WEC proposed to conduct a dye tracer study to determine the
instream dilution. A Study Plan for dye tracer work was submitted to NCDEQ and approved on July 14,
2023.The field work associated with the approved Study Plan was performed on August 22"d, 2023.The
measurements included effluent dye concentration,water depths, ambient flow,water surface
elevations, discharge flow rate, and dye concentrations in the river.
WEC used the measurements as input boundary conditions to the previously developed EFDC model to
predict the dilution of the effluent in the river.The EFDC's dye tracer module results were then
compared to the measured dye concentrations in the river to assess the model's accuracy.To improve
the model accuracy, WEC adjusted the model grid bathymetry in the vicinity of the outfall to better
represent the river geometry in this area.After making these improvements,WEC adjusted the worst
case, 7Q10 "permit" model to ensure that the instream standards are still achieved within an allowable
NCDEQ mixing zone.
A description of the work performed by WEC to complete the assessment and the model results is
provided in the following sections:
• Section 2,Outfall Description—provides a brief description of the outfall and how it enters the
Dan River.
• Section 3, Field Measurements—describes the dye study setup and measurements.
• Section 4, Model Verification—describes the model revisions and verification by comparison to
the field measurements.
• Section 5, Regulatory Mixing Zone Analysis—describes the modeling performed under worst-
case (regulatory) conditions and the proposed permit limitations for temperature.
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Thermal Model Verification Report- Dan River Steam Station
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Thermal Model Verification Report- Dan River Steam Station
2 Outfall Description
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 initially visited the Dan River site on August 13, 2019,to characterize the location and flow
geometry at the discharge location. Outfall 001 discharges into a short channel/embayment that flows
along the northern end (downstream)of a dam wall 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
embayment and slowly flowing into the Dan River. After mixing with the dam overflow,the discharge
plume moves downstream within the Dan River.
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Figure 2-1. Location of Dan River Combined Cycle Station Outfall 001
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Thermal Model Verification Report- Dan River Steam Station
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Thermal Model Verification Report- Dan River Steam Station
3 Field Measurements
WEC collected field measurements to determine the dilution of the effluent in the river and to provide
boundary conditions for the model verification.As explained in the previous model report (WEC 2022),
the model was conservatively run under the premise that effluent temperature effects on the river are
governed only by dilution with the receiving water.The model does not include all potential
temperature losses,such as exchange with the atmosphere,which would reduce the effect of the
effluent on the river temperature. WEC still measured temperatures as part of the field verification, but
dilution was the primary focus of the measurements.The sections below describe the dye study and the
measurement of river flow and water surface elevation. Note that the fieldwork followed the
procedures described in the NCDEQ-approved Study Plan.
3.1 Ambient Dilution Study
Rhodamine WT dye was injected into the wastewater discharge at a constant rate, under constant
discharge flow conditions.WEC coordinated with Dan River Staff to maintain and record a consistent
discharge rate during the dye study. A Pegasus Alexis peristaltic pump released the dye at a constant
rate into the effluent just before it flowed into the discharge pipe. Figure 3-1 shows the setup with the
dye reservoir and pump at the injection location.The dye injection started at midnight prior to the
instream measurements to ensure that the dye concentrations in the embayment at the outfall reached
steady-state conditions prior to measuring the dye concentrations in the river.
WEC employed a YSI EX01 Sonde with a Rhodamine sensor to continuously measure the instream dye
concentration (and temperature) downstream of the outfall at the boat ramp location.One purpose of
the measurements was to ensure a consistent dye concentration for the entire model reach during the
entire study period.The measured dye concentrations at the boat ramp are shown in Figure 3-2.
WEC navigated the river in a canoe and on foot using a second YSI EX01 Sonde to measure dye
concentrations near the water surface at Outfall 001 and at regularly spaced transects out into the river
until reaching the edge of the plume.The resulting data was used to generate a two-dimensional (aerial
view)graphic of the dye concentration (i.e., instream dilution), shown in Figure 3-3,for comparison to
the EFDC predicted dilutions.The discrete dye and temperature measurements are listed in Table 3-1.
Figure 3-4 shows the discharge outfall and embayment during the dye study.This figure shows the
circulation pattern in the embayment entrance observed by WEC during the field study.
3.2 River Flow
The model requires an input flow rate at the upstream boundary, and therefore,WEC measured the
river flow that occurred on the day of the field measurements. WEC measured current velocities across
the river just downstream from the boat ramp using a Marsh McBirney FloMate 2000 electromagnetic
current meter attached to a standard US geological survey(USGS)top set wading rod.The current meter
measured velocities at 60%of the water depth. Table 3-2 lists the measured velocities and flow rate
calculated using the USGS box method. The average river velocity was 1.6 feet per second (ft/s) and the
total flow rate was 990 cubic feet per second (cfs).
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Thermal Model Verification Report- Dan River Steam Station
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Figure 3-2. Continuous downstream (boat ramp) dye measurements

Thermal Model Verification Report- Dan River Steam Station
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A Feet • 1.1- 1.3 • 50.1- 100.0
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Figure 3-3. Discrete dye measurements
3.3 Water Surface Elevation
WEC measured the water surface elevation at the edge of the bank at the boat ramp using a Trimble
Geo 7x Centimeter Edition GPS.The GPS is capable of measuring with a vertical accuracy of±0.1 foot
and was used to establish the water surface elevation relative to the North American Vertical Datum of
1988 (NAVD88). Detailed depth and bathymetry data was collected by WEC when developing the
previous NCDEQ-approved EFDC model. That bathymetry data is also relative to the NAVD88 vertical
datum. WEC used the water surface elevation as a downstream boundary condition for the dye study
model.
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Thermal Model Verification Report- Dan River Steam Station
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Figure 3-2. Embayment during dye study(arrows indicate observed circulation pattern)
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Thermal Model Verification Report- Dan River Steam Station
Table 3-1. Discrete dye concentration measurements
Water
Longitudinal 1 Transverse Transverse SC RWT %
Date Time (ft) (yd) (ft) temp
( g )
dist. dist. dist. (uS/cm) u /L Effluent
I i
8/22/2023 09:54:50AM 200 7 21 23.9 I 94.2 1.2 0.7
8/22/2023 09:57:44AM 1 200 3 9 23.8 95.2 1.3 0.8
8/22/2023 09:58:52AM 200 1.5 4.5 23.9 99.3 2.2 1.4
8/22/2023 10:00:28AM 200 16 48 23.9 94.1 1.1 0.7
8/22/2023 10:03:05AM 200 19 57 23.8 1 92.8 0.8 0.5
-
8/22/2023 10:04:55AM 200 24 72 23.8 3.4 0.0 0.0
- - --
8/22/2023 10:06:44AM 150 27 81 23.8 88.8 0.0 0.0
8/22/2023 10:08:21AM i 150 j 19 57 23.8 93.9 1.1 0.7
8/22/2023 10:10:17AM 150 1 11 33 23.9 94.3 1.3 0.8
-8/22/2023 10:11:46AM 150 8.5 25.5 23.9 95.2 1.4 0.8
8/22/2023 10:13:22AM 150 5.5 16.5 23.9 98.7 2.2 1.3
8/22/2023 10:14:33AM 150 j 2 6 23.9 99.2 2.3 1.4
8/22/2023 10:16:35AM 100 2 6 23.9 99.8 2.6 1.6
8/22/2023 10:18:19AM 100 7 21 23.9 96.3 1.8 1.1
8/22/2023 10:20:16AM 100 12 36 23.9 95.6 1.5 0.9
8/22/2023 10:22:19AM 100 23 69 23.9 93.4 1.0 0.6
8/22/2023 10:24:53AM 100 27 81 23.8 90.6 0.3 0.2
8/22/2023 10:25:24AM 100 28 84 23.8 - 0.0 0.0
8/22/2023 10:25:55AM 50 28 84 23.8 87 0.0 0.0
8/22/2023 10:30:29AM 50 24 72 23.9 92.8 0.9 0.5
8/22/2023 10:32:09AM T 50 r 19 57 23.9 93.4 1.1 0.7
8/22/2023 10:34:42AM 50 14 42 23.9 96.7 1.9 1.2
8/22/2023 10:35:53AM 50 9.5 28.5 23.9 97.8 2.2 1.3
' 8/22/2023 10:37:31AM 50 5.9 17.7 24.0 100.5 2.8 1.7
8/22/2023 10:39:11AM 50 2 6 24.0 101 1 3.0 1.8
8/22/2023 10:42:38AM 0 1 3 24.1 102.9 3.6 2.2
8/22/2023 10:44:24AM i 0 3 9 24.1 1 107.9 L 4.5 2.7
8/22/2023 10:45:55AM I 0 5.1 15.3 24.1 1 107.9 I 4.6 2.8
8/22/2023 10:48:44AM L 0 9 27 24.0 J 101.1 2.9 1.8
8/22/2023 10:51:21AM 0 17 51 23.9 1 91.9 I 0.8 0.5
-_-. - -+ 1-.. _-r -_"-
8/22/2023 10:53:39AM YSI 1 0 0 26.4 367.8 79.8 49.1
8/22/2023 10:55:23AM , YSI 1 2 6 26.1 333.7 67.4 41.5
8/22/2023 10:59:46AM l Embayment center 0 27.0 408.1 92.5 ' 56.9
-r _
8/22/2023 11:01:33AM Embayment wall across from discharge 0 28.8 622 162.4 100.0 '
8/22/2023 11:04:57AM Embayment NE corner - 0 26.4 360.1 75.1 46.2
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Thermal Model Verification Report- Dan River Steam Station
Table 3-2.Velocity and flow measurements
Distance across river(ft) I Depth(ft) Velocity(ft/s) Flow(cfs)
0 -
_0
6 2.4 1.9 68.4
30 1 2.4 3.3 178.2
51 2.1 2.8 132.3
(.._ 75 I 1.5 2.4 75.6
93 ...... 1.8 2.6 91.3
114 1.5 2.6 70.2
129 j 2 2.5 90.0
150 J 2.8 2.1 97.0
162 , 3.1 1.76 106.4
189 1 3.5 -0.05 5.3
222 2.6 0.78 57.8
246 2 0.6 27.0
267 1.8 0.05 1.5
279 - - 0 0
TOTAL= 990
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Thermal Model Verification Report- Dan River Steam Station
4 Model Verification
WEC used the collected field data to recalibrate the EFDC model created in 2020 (and revised June 10,
2022).The recalibration included updating the model grid bathymetry, ambient flow rate and area,
effluent discharge rate and discharge location, and the downstream water level.These new conditions
were input into the EFDC model in order to model the measured dye concentrations.The predicted dye
concentrations from the model were compared to the measured dye concentrations to determine the
model's validity.The sections below describe this process in greater detail and the resulting conclusions.
4.1 Model Grid
Following the field data collection on August 22, 2023,WEC revised the model bathymetry to more
accurately represent the conditions that were observed on the day of the field visit.This mainly included
revising the bottom elevations in the vicinity of the outfall to include the dry land on the north side of
the embayment and a sloping bottom elevation that runs towards the river at a constant slope.There
were also a few grid cells that were determined to be dry along the riverbank, so these cells were also
inactivated to redirect the flow.The final grid bathymetry,following the revisions,can be found in Figure
4-1.
4.2 Model Boundaries
The recalibrated model boundary conditions were based on the field data collected on August 22, 2023.
The upstream flow was adjusted to match the measured flow of 990 cfs.As mentioned in the "Field
Measurements"section, WEC coordinated with Dan River staff to determine the effluent flow rate into
the Dan River.This value was determined to be 475 gallons per minute (gpm), or roughly 1.06 cfs.The
downstream, boat ramp water level was measured as 477.75 ft.
WEC also adjusted boundary conditions to better represent the actual conditions observed in the field.
The length of the river inflow boundary along the dam was shortened based on the aerial image, so the
flow rate was dispersed over a smaller number of cells.The effluent boundary condition was also moved
to a different cell to represent the outfall pipe location more accurately.
4.3 Model Results Comparison
WEC executed the EFDC model that represented the conditions present during the dye study.The model
employed a dye tracer module with the initial conditions set to the dye concentration injected into the
effluent stream.The model results provide a 2D map of the dye's dilution within the river(Figure 4-2).
These results were compared to the measured dye concentrations at specific points within the river,
beginning at the rocks near the downstream edge of the embayment,shown as the red line in Figure 4-
2. Several dye concentration measurements were taken at transects spread every 50 ft downstream
from this starting position.The graphs below show a comparison between the measured data and the
model results. Figure 4-3 shows the dye concentrations at the beginning transect,from both the
measured data and the model results.This transect has the highest variation, in terms of percentage,
between the model and the measured data. Moving further downstream,the results for the transects at
50 ft (Figure 4-4), 100 ft(Figure 4-5), 150 ft(Figure 4-6),and 200 ft(Figure 4-7), show a closer
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Thermal Model Verification Report- Dan River Steam Station
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` ft N A�/D$$ 476.1 477.0
471.5 4720 477.1 478.0
- 472.1-473.0 478.1-479.0
�. N. 473.1-474.0
479.1-480.0
N
Feet 474.1 475.0 480.1 481.0
0 50 100 200 300A
475.1-476.0lin 481.1-489.0
Figure 4-1. Revised model grid bathymetry
resemblance between the measured data and the model results.Overall,the measured condition and
the model show similar results,with the majority of data point concentrations showing a difference
ranging from 0 to 2%, and the greatest difference in dye concentration (as percent) is 4.6%at a 0 ft
longitudinal distance and a 3 ft transverse distance.The reason for this,and the reason that each
transect shows a higher percentage for the modeled dye concentration closer to the shore, is that the
measured data shows more lateral dispersion out into the river, away from the shoreline,than the
model. Because more mixing occurs than is predicted by the model,the model provides a conservative
estimate of effluent dilution in the river and is a suitable tool for establishing a thermal mixing zone for
the outfall.
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Thermal Model Verification Report- Dan River Steam Station
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Dye Concentration I%) 416,20-3.0
tl111, .1 eo.0- 100.0 1.0-2.0
N -24.0-60.0 -0.5- 1.0
Feet Q7.0-24.0 n0.0-0.5
0 75 150 225 300
in3,0-7.0
Figure 4-2. Modeled dye concentrations for verification study conditions
Dye Concentration % Field Collection vs. Model
(0 ft Longitudinal)
8.0
•
6.0
o
co
4.0 •
......•.:... •
2.0
0
u 0.0 • •...... _..
0 0 10 20 30 40 50 60
Transverse Dist. (ft)
• Field Collection • Model Results
Linear(Field Collection) Log.(Model Results)
Figure 4-3. Modeled and measured dye concentrations, 0 ft downstream
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Thermal Model Verification Report- Dan River Steam Station
Dye Concentration % Field Collection vs. Model
(50 ft Longitudinal)
5.0
4.0 •
3.0
c2.0 •....:::• ::......
a • •••..........•...........
1.0 ••...........
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v
O 0 20 40 60 80 100
Transverse Dist. (ft)
• Field Collection • Model Results
Linear(Field Collection) Log.(Model Results)
Figure 4-4. Modeled and measured dye concentrations, 50 ft downstream
%
Dye Concentration Field Collection vs. Model
(100 ft Longitudinal)
4.0
3.0 •
O
co 2.0
u• 0.0 • • .. • • ....... •
v
O 0 20 40 60 80 100
Transverse Dist. (ft)
• Field Collection • Model Results
Linear(Field Collection) Log.(Model Results)
Figure 4-5. Modeled and measured dye concentrations, 100 ft downstream
14
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Thermal Model Verification Report- Dan River Steam Station
Dye Concentration % Field Collection vs. Model
(150 ft Longitudinal)
1.5 •.. •
o 1.0
.� • ••••..•••
•
aci0.5
✓• 0.0 • •. .............__
_ •
0 10 20 30 40 50 60 70 80 90
Transverse Dist. (ft)
• Field Collection • Model Results
Linear(Field Collection) Log.(Model Results)
Figure 4-6. Modeled and measured dye concentrations, 150 ft downstream
Dye Concentration % Field Collection vs. Model
(200 ft Longitudinal)
2.5
2.0 :...
• 1.5• 1.0 :. .......
..................
v •
0.5 • ........�....
o •..........
v 0.0 • _ • . •_ •
0 0 10 20 30 40 50 60 70 80
Transverse Dist. (ft)
• Field Collection • Model Results
Linear(Field Collection) Log.(Model Results)
Figure 4-7. Modeled and measured dye concentrations, 200 ft downstream
15

Thermal Model Verification Report- Dan River Steam Station
5 Regulatory Mixing Zone Analysis
As explained previously,the model required adjustment to match the measured data.Therefore,WEC
used the recalibrated model to predict the instream mixing for temperature during conservative
conditions for permitting.The being said, after the model adjustments were made,the approach for
using the model results to justify the permit limits is the same as described in the WEC's 2022 report, as
is the use of the maximum design discharge flow rate of 1.33 MGD.
As described in the 2022 report, a conservative model analysis is used to verify whether the thermal
mixing zone meets the discharge permit limitations issued by NCDEQ under critical conditions.The
NCDEQ water quality standard for temperature has two components: not to exceed a maximum of 32°C
(89.6°F) and not to exceed a 2.8°C(5.04°F) change above natural background.To evaluate each
condition,the summer and winter models predict the mixing of the discharge's temperature excess for
comparison to the respective standards.The summer and winter models simply determine where the
downstream excess meets the 2.8°C standard.
Inputs to the recalibrated EFDC model were modified for both summer and winter critical 7Q10 river
conditions.The critical conditions for both the summer and winter models were based on the 2.8°C
temperature exceedance over background standard. In other words,the thermal mixing zone required
to meet the summer 32°C standard was smaller than the downstream distance required to meet the
2.8°C in the summer.
5.1 Critical Flow Rates
The summer and winter 7Q10 flow rates were entered into the recalibrated model, and this impacted
the downstream water surface elevations.The water surface elevation measured during field
monitoring was lowered based on the change in water depths at USGS station 02071000 associated with
similar change in flow rates. Figure 5-1 shows the measured 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 slightly adjusted (from the 2022 values)downstream water levels at the project
site for the summer and winter 7Q10 conditions.Table 5-1 summarizes the inputs in SI units (except for
temperature) as used in EFDC. Note that the Table 5-1 temperature excess values are the same as the
2022 models.
5.2 Model Results and Proposed Permit Limits
The model results for a 1.33 MGD discharge from Outfall 001 are shown in Figure 5-2. 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 standards are met are
marked with magenta lines. For the summer and winter conditions,this 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
embayment 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
16

Thermal Model Verification Report- Dan River Steam Station
Stage vs. Flow Rate
4 __.. _.._.._.-. __...
3.5 _ y=-2 -19x6+2 E-15x5-5E-12)0+9E-09x3-8E-06x2+0.0054x
R2=0.9985
2.5 — _ ---
cu
AO
m
1.5
1 —
0 1 I _� _
0 100 200 300 400 500 600 700 800 900 1000
Flow(cfs)
• USGS data 7Q10 Summer • 7Q10 Winter • Calibration day Best Fit Eq.(USGS data)
Figure 5-1. Flow-stage relationship at USGS station 02071000
Table 5-1. 7Q10 Model Inputs
7Q10 flow Downstream WL Effluent flow Effluent Temperature
rate(cms) (m NAVD88) rate(cms) Excess(°F)
Summer 8.89 145.9 0.058 17.3
Winter 16.4 146.0 0.058 53.5
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 no-more than 5.04°F
exceedance above background standard approximately 99 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 192 feet downstream. In addition,the very narrow plume widths also support safe
passage of aquatic life. Note that the newly recalibrated model shows an increase of the respective
thermal mixing zone lengths (from 43 to 99 feet and from 97 to 192 feet), but despite these increases,
the (conservatively modeled) mixing zone is very small.
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 remain 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 permit limits,the actual instream plume will be
17
L=�

Thermal Model Verification Report- Dan River Steam Station
-440 .
r_ - Legend
, Temperature Excess °F 20-3.0 •
N 0.0-0.1 3.0-4.0
A = I Feet IIM 0.1 1.0 F 4.0 5.0
0 75 150 225 300 1.0-2.0 MI 5.0-17.5
g '•
4 s arf
s:s>
,qs ,
‘ -- ..,- ' Legend
-!. - - Temperature Excess °F 4.0-5.0
s- li-
•
0.0-0.1 5.0-10.0 ':.;
*- - " - 0.1 - 1.0 10.0- 20.0
N � 1.0-2.0 20.0- 30.0
jk —=- -Feet 20 3.0 ® 30.0- 40.0
0 75 150 225 300 3.0 4.0 III40.0- 54.0
Figure 5-2. 7Q10 model results for summer(top)and winter(bottom) conditions
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Thermal Model Verification Report- Dan River Steam Station
smaller than the conservative model prediction results. Therefore,there is no need for the permit to
require Duke to bear the expense of routine river temperature monitoring as part of their permit
compliance.
(�..� 19
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Thermal Model Verification Report- Dan River Steam Station
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.Rov/usa/nwis/uv?site no=02071000
WEC. 2022. Thermal Modeling Report:Duke Energy—Dan River Combined Cycle. Prepared for Duke
Energy Carolinas, LLC.
_ 20
i�Y L �

Dan River Combined Cycle
Thermal Variance Verification Study
Temperature Data - ° C
Date Upstream Effluent Downstream Date Upstream Effluent Downstream
11/1/2022 15.07 24.8 15.07 6/1/2023 16.96 22.39 16.88
11/9/2022 14.93 26.6 14.66 6/8/2023 18.36 24 18.26
11/16/2022 8.48 24.4 8.52 6/15/2023 21 24.8 21.01
11/22/2022 4.31 26 4.73 6/22/2023 19.06 23.57 19.01
11/29/2022 8.9 23.7 9.26 6/29/2023 18.28 24.25 18.28 _
12/1/2022 8.81 24.2 15.48 7/3/2023 23.87 28.89 24.16
12/6/2022 7.21 24.2 7.41 7/10/2024 23.98 28.47 24.09
12/14/2022 8.23 24.4 4.73 7/17/2023 24.96 26.4 24.9
12/21/2022 4.38 24.5 6.08 7/24/2023 22.97 26.81 22.98
12/27/2022 1.1 18.5 1.98 7/31/2023 25.37 27.25 25.15
1/1/2023 8.1 22.3 8.18 8/1/2023 24.7 26.42 25.06
1/2/2023 8.48 21.6 8.62 8/8/2023 22.77 27.81 22.79
1/3/2023 9.34 21.6 9.45 8/15/2023 25.2 30.41 26.46
1/4/2023 10.57 20.7 10.47 8/22/2023 23.98 28.5 24.67
1/7/2023 7.69 23.4 7.69 8/28/2023 24.87 30.29 24.82
1/10/2023 5.81 24.1 5.98 9/7/2023 25.57 30.02 25.76
1/12/2023 6.26 25.3 6.61 9/12/2023 22.36 26.59 22.43
1/13/2023 8.58 25.3 8.52 9/19/2023 19.39 22.86 19.26
1/16/2023 4.92 23.2 5.19 9/25/2023 18.69 24.49 18.89
1/19/2023 8.84 24.2 8.97
1/25/2023 5.8 23.8 5.86
1/26/2023 5.88 23.5 5.99
1/28/2023 5.21 26.1 5.52 October- No Flow- Plant in Maintenance Outage
1/31/2023 7.29 23.8 7.26
2/6/2023 5.74 25.4 6.06
2/9/2023 9.69 23.9 9.81
2/12/2023 8.31 23.4 8.21
2/13/2023 7.12 25.3 7.07
2/22/2023 9.58 24.2 9.83
2/28/2023 1128 25.3 11.77
3/6/2023 11.39 24.4 11.6
3/13/2023 7.27 26.3 7.51
3/22/2023 11.56 21.8 9.83
3/28/2023 14.77 25 14.65
4/4/2023 14.42 24 14.01
4/10/2023 12.54 25 13.37
4/21/2023 18.11 23.9 18.28
4/25/2023 15.19 25 16.25
5/1/2023 13.69 23.09 13.7
5/9/2023 17.62 26.41 17.79
5/16/2023 19.79 25.33 19.83
5/24/2023 18.99 21.72 19.3
5/31/2023 16.97 23.66 16.96