NC0003468_Thermal Model Verification Study Plan_20230328

TC. - DUKE
ENERGY.
March 28, 2023
Dr. Sergei Chernikov
North Carolina Department of Environmental Quality
Division of Water Resources, Wastewater Branch
1617 Mail Service Center
Raleigh, NC 27699-1617
Subject: NPDES Wastewater Permit
Dan River Combined Cycle Station
Permit #NC0003468
Thermal Model Verification Study Plan
Dear Dr. Chernikov:
Dan River Combined Cycle
864 South Edgewood Road
Eden, NC 27288
Part I A.6.d of referenced permit requires after 12 months of temperature data are collected that a
report be submitted to the Division to verify the CORMIX model assumptions and predictions.
The Environmental Fluid Dynamics Code (EFDC) was used for the original thermal modeling
study at Dan River Combined Cycle, DRCC, and is the proposed method for the confirmation of
the original modeling assumptions and predictions.
To address this permit condition, the attached study plan has been developed. As outlined in this
plan, the approach is to use biodegradable dye in the discharge from Outfall 001 and measure the
downstream dye concentrations to understand the plume concentration levels. While the plume
temperatures near the outfall will be measured, the temperature will likely dissipate quickly.
Therefore, a dye study is necessary to measure the plume dilution further downstream.
The new permit term began on November 1, 2022. Therefore, the 12 months of temperature data
collection will be completed on October 30, 2023. A report using the study and the collected
temperature data will be compiled and submitted to your office by April 1, 2024.
Your review and concurrence of the attached study plan is requested. Should you have any
questions or require additional information, please contact Joyce Dishmon at 366-623-0238 or
'o ce.dishmon@duke-enera .com.
Sincerely
I
Kris Eisenrieth
General Manager II — Regulated Stations
Attachment: Thermal Model Verification Study Plan

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March 10, 2023
Ms. Joyce Dishmon
Permitting and Compliance, Carolinas
Duke Energy Corporation
2500 Fairfax Road
Greensboro, North Carolina 27407
Water Environment Consultants
P.O. Box 2221
Mount Pleasant, SC 29461
843-375-9022
Re: Dan River Thermal Model Verification Study Plan
Dear Ms. Dishmon:
Under contract with Duke Energy Corporation (Duke), and in support of the Dan River Combined Cycle
Station's (Dan River's) NPDES permit (NC0003468), Water Environment Consultants (WEC) developed a
thermal model verification Study Plan for the North Carolina Department of Environmental Quality
(NCDEQ) to review. Part I.A.6.d of the permit requires field verification of the modeling predictions used
to set the daily maximum effluent and downstream temperature limits. The goal of the Study Plan is to
document WEC's methodology for the data collection effort necessary to verify and/or adjust the
previously approved thermal mixing zone model for Outfall 001.
So that the required field work may be performed late spring or early this summer, WEC recommends
Duke provide the attached Study Plan to NCDEQ as soon as possible to obtain any comments and their
approval.
Sincerely,
John K. Durkee, P.E.
Principal
Enclosure: Attachment A
Water Environment Consultants I www.water-ec.com

ATTACHMENT A - Dan River Outfall 001 Thermal Model Verification Study
Plan
Introduction
Water Environment Consultants (WEC) prepared this Study Plan for Duke Energy Corporation (Duke) and
the North Carolina Department of Environmental Quality (NCDEQ) in support of NPDES permit
(NC0003468) for 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 effluent mixing from Dan River's Outfall 001 and to request permit limits for
temperature. 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 (EFDC) 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...
The previous EFDC model analysis showed the permitted effluent temperature limits met the 32°C
maximum and 2.8°C rise above -background temperature standards (Lower Piedmont/Coastal Waters as
defined in 15 NCAC 02B .0211 Fresh Water Quality Standards for Class C Waters) at a relatively short
distance downstream of Outfall 001. The permit limits and modeling were 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. In
addition, ambient and effluent temperatures are less important for the verification study, because the
EFDC model was conservatively executed assuming temperature mixes by dilution only.
For these reasons, WEC proposes to conduct a dye tracer study to determine the instream dilution.
Performing the field verification work during the spring or early summer months, when the ambient
flow is as close to 7Q10 as possible, is preferred. This also reduces the chance of thunderstorms
disrupting the study.
This Study Plan details the data collection effort and the following model adjustments:
• Ambient Dilution Study
• Ambient Currents
• Elevation Survey
• Outfall Characteristics
• EFDC Dilution Modeling
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WEC will update the previously developed EFDC model to reflect conditions measured in the field
(depths, ambient flow, water surface elevation, and discharge flow). The model will be executed using
EFDC's dye tracer module, and the results will be compared to the field -measured dye concentrations.
Based on the correlation of the modeled and measured dilution, WEC will adjust the worst -case 7Q10
model, if necessary, to ensure instream standards are still achieved within an NCDEQ allowable mixing
zone. The results will be summarized in a report that will be submitted to NCDEQ and will recommend
changes to the permit limits, if necessary. Permit condition I.A.6.f states:
This permit may be reopened to implement alternative temperature limits or requirements based
on the results of the data collected.
Ambient Dilution Study
As mentioned earlier, the previous EFDC model was conservatively run under the notion that the
effluent mixes only by dilution with the receiving waters. This assumption means the ambient
temperature is less important than the ambient flow because temperature changes within the region of
interest are primarily driven by mixing and not heat exchange with the atmosphere or by other means.
Temperatures will still be measured as part of the field verification, but the instream temperatures are
not expected to provide enough resolution (i.e., difference between the discharge and ambient
temperature) to accurately determine the downstream temperature reduction. Dilution, however, will
be the primary measurement.
To better determine instream mixing, WEC will conduct a dye tracer study using Rhodamine WT dye.
The dye will be injected into the wastewater discharge at a constant rate, under constant discharge flow
conditions. A peristaltic pump will be used to release the dye at a constant rate. Two EXO1 Rhodamine
Sondes will be deployed to continuously measure the instream dye concentration (and temperature).
One Sonde will be placed within the outfall basin, and a second sonde will be placed further
downstream within the plume.
A handheld display fluorometer will be utilized to measure the concentration at the point of discharge
and at regularly spaced transects across Dan River until reaching the edge of plume. These spot -
measured plume concentrations will be made at varying depths to determine an average concentration.
However, the depths are expected to be shallow if the field measurements are performed near 7Q10,
low flow conditions, and the concentrations are expected to be vertically mixed shortly downstream of
the outfall basin.
WEC will use the resulting data to generate a two-dimensional (aerial view) graphic of the dye
concentration (i.e., instream dilution) for comparison to the EFDC predicted dilutions.
Ambient Currents
WEC will use a SonTek RS5 Acoustic Doppler Current Profiler (ADCP), mounted on a Hydroboard, to
determine the ambient flow rate during the field study. The ADCP is equipped with a differential GPS to
A-2

Figure 1. Preliminary ADCP measurement transects
provide horizonal positioning data and measures depths and currents through the water column as it
traverses the river perpendicular to the shoreline, thereby providing a two-dimensional cross-section of
current velocities. WEC will make transects across Dan River, downstream of Outfall 001, as
preliminarily shown in Figure 1. The location is selected downstream because the bathymetry and
currents are known to be more regular downstream of the dam, rocks, sandbar, etc. The data will be
post -processed to calculate an ambient flow rate for input as an upstream boundary in the EFDC model.
The effluent flow contribution to the total river flow is expected to be minimal. However, WEC will
coordinate with Dan River to maintain and record a relatively consistent discharge rate during the time
of field measurements. WEC will subtract the effluent flow from the total river flow calculated from the
ADCP measurements. WEC will conduct ADCP measurements both before and after collecting dye
measurements to capture any changes to the ambient flow during the data collection effort.
A-3

Elevation Survey
WEC will measure water surface elevation at the edge of bank at various points using a Trimble Geo 7x
Centimeter Edition GPS. The GPS is capable of measuring with a vertical accuracy of±0.1 foot and will
be used to establish the water surface elevation relative to the North American Vertical Datum of 1988
(NAVD88). WEC previously collected detailed depth and bathymetry of the study site when developing
the previously approved EFDC model. That bathymetry data is also relative to the NAVD88 vertical
datum. Like current measurements, water levels will be measured both before and after collecting dye
measurements to capture any changes during the data collection effort. WEC will use the water surface
elevation as a downstream boundary condition in the EFDC model.
Outfall Characteristics
The Outfall 001 discharge pipe is located along the dam wall on the northern (downstream) end of a
short channel flowing into the Dan River (Figure 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.
WEC will coordinate with Duke for dimensions of the containment basin and update the model
bathymetry, if necessary, with ADCP measurements if it can be safely traversed.
Note that the report will not include a summary of the 12 months of temperature data required by the
NPDES permit, because it is not related to the worst -case modeling or the permit limit for temperature.
EFDC Dilution Modeling
WEC will execute an EFDC model representing the conditions present during the dye study. The model
will employ a dye tracer module with the initial conditions set to the dye concentration injected into the
effluent stream. The model results will provide a 2D map of the dye's dilution within the river. These
results will be compared to the field -measured dye concentrations within the river. If the measured and
model concentrations agree, the verification study will confirm the model inputs used to request the
existing NPDES limits. Should the model require significant recalibration, WEC will revise the model
inputs to match the measured data, and the re -calibrated model will be the basis for requesting
modified NPDES permit limitations for temperature.
A-4

Figure 2. Photo of discharge pipe at Outfall 001
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