NC0000752_Report_20231031

WestRock
Date: Tuesday, October 31, 2023
Subject: WestRock Roanoke Rapids Thermal Mixing Zone Evaluation, Permit NC0000752
To: Michael Montebello, NPDES Program Branch Chief
From: Ben White, Roanoke Rapids Mill General Manager (WestRock)
NPDES Permit No. (NC0000752) requires the WestRock Roanoke Rapids, North Carolina pulp and paper
mill (Mill) to conduct a thermal mixing zone study for its non -contact cooling water (NCCW) discharge to
the Roanoke River. The Mill's current NPDES permit does not include a mixing zone for temperature.
The Roanoke Rapids Mill discharges NCCW to the Roanoke River just downstream from the Roanoke
Rapids Dam. This section of the Roanoke River is Class C and is located in the Lower Piedmont and
Coastal Plain Waters area. The NCDEQ temperature standards (15A NCAC 02B.0211(18)) include a
maximum temperature of 32°C (89.6°F) and a maximum temperature rise above background of 2.8°C
(5.0°F). The NCDEQ regulations do not specify numerical limitations to mixing zone sizes and notes that
they will be determined on a case -by -case basis. In addition, a mixing zone shall not prevent free
passage of aquatic organisms around the mixing zone.
This memo presents the data obtained to develop a CORMIX dilution model, CORMIX model input setup,
and use of the model results to define a mixing zone for the Mill's NCCW discharge (Outfall 002) to the
Roanoke River.
RIVER DATA
Available data was obtained for the Roanoke River from the following sources.
• WestRock Discharge Monitoring Report (DMR) river temperature sampling upstream at the
Highway 48 Bridge and downstream at the Highway 158 Bridge.
• Flow and temperature data at the downstream USGS gage (#02080500) Roanoke River at
Roanoke Rapids.
• NCDEQ temperature data upstream at the Highway 48 Bridge (station N7300000).
• Flow and temperature data from Dominion Energy at the upstream Roanoke Rapids Dam.
• NCDEQ river temperature data collected on 8/3/2023 upstream and downstream from the Mill's
NCCW discharge.
Figure 1 presents a map of these monitoring station locations along with the location of Mill Outfalls 001
(treated wastewater) and 002 (NCCW). Figure 2 presents the DMR upstream and downstream river
temperature data. These data show little difference in temperature between the upstream and
downstream sampling locations. Figure 3 presents the river temperature and flow data from the
downstream USGS gage. There are few temperature data available at the downstream USGS gage and
river flows vary from about 1,500 cubic feet per second (cfs) to greater than 30,000 cfs. The Dominion
Energy FERC license indicates minimum release flows of 2,000 cfs from December through August, and
1,500 cfs from September through November. Figure 4 presents the NCDEQ upstream river temperature

WestRock
data. Figure 5 presents the flow and temperature data from Dominion Energy at the upstream Roanoke
Rapids Dam. The Dam flows and the USGS flows at the downstream gage are very similar and highlight
the minimum release flows that vary over the year.
On 8/3/2023 from about 10:15-11:30AM, NCDEQ collected near surface temperature data in the
Roanoke River upstream and downstream from the Mill's NCCW discharge. Figure 6 presents a map of
these river temperature data and the temperatures ranged from 27.9-51.5°C (82.2-124.7°F). The
upstream river temperature on this date was 29°C. In addition, the maximum temperature standard of
32°C (89.6°F) was met about 200 feet downstream from the mouth of the NCCW discharge channel and
about 100 feet offshore from the west riverbank. This dataset was used to test the CORMIX model as
discussed below.
The river temperature data from the three upstream locations (DMR, NCDEQ, Dominion) were compared
in Figure 7. All three datasets have similar temperatures, and the Roanoke Rapids Dam release
temperature was used to define monthly temperature inputs (90t" percentiles) for the CORMIX model
because this data source provided a continuous time -series record.
DISCHARGE DATA
Outfall 002 NCCW flow and temperature information were provided by WestRock based on a water
operations model that represents the various water flows and heat loads throughout the Mill. The NCCW
discharge flow was calculated monthly with this model using a maximum NCCW discharge temperature of
132.0°F (55.6°C), expected maximum Mill heat loads, and river intake temperature data from 2022-2023.
These Mill conditions used represent a conservative estimate of NCCW discharge flows and temperature.
Maximum monthly discharge flows ranged from 1.5-8.4 MGD with lower flows occurring in the winter
months and higher flows in the summer months.
CORMIX MODELING
The CORMIX dilution model was used to calculate the impact of the NCCW discharge in the Roanoke
River and to determine a thermal mixing zone size. The thermal mixing zone size was determined based
on meeting the NCDEQ temperature standards at the edge of the mixing zone. In addition, the NCDEQ
temperature data collected on 8/3/2023 was used for CORMIX model testing. The CORMIX dilution
model can analyze mixing and dilution associated with discharges from surface or submerged discharges
due to both jet and buoyancy momentum in addition to ambient mixing. The CORMIX3 surface discharge
program was used to evaluate the Mill's existing surface canal discharge to the river. The NCCW surface
discharge canal geometry was estimated as 50 feet wide and 2.5 feet deep.
HEC-RAS information from the North Carolina Flood Risk Information System was used to estimate river
depth in the river reach near the NCCW discharge at low river flow conditions. In addition, USGS water
depths at the Roanoke Rapids gage was also reviewed to assist in assigning a water depth for CORMIX
model input. A water depth near the discharge of 3 feet was used based on these data sources and also
based on the observation that the west side of the river is shallower than the east side. An average water
depth of 4.2 feet was used based on CORMIX input limitations. A river width of 550 feet was used based
on measurements made in Google Earth.

WestRock
CORMIX Model Testing
The surface temperature data collected on 8/3/2023 by NCDEQ were used to test the CORMIX model.
The following survey specific model inputs were used along with the river and discharge channel
geometry noted above.
• River flow on 8/3/2023 at the downstream USGS river gage (#02080500) of 2,342 cfs.
• Upstream river temperature on 8/3/2023 of 29°C.
• NCCW discharge flow on 8/3/2023 of 5.34 MGD.
• Maximum measured temperature on 8/3/2023 in the discharge channel of 51.5°C.
The CORMIX model results for these inputs reflecting the 8/3/2023 river and discharge conditions
indicate that the 32°C (89.6°F) maximum temperature standard occurs about 315 feet downstream from
the discharge channel and about 64 feet offshore from the west riverbank. In addition, the 2.8°C (5.0°F)
maximum temperature rise standard occurs about 327 feet downstream from the discharge channel and
about 66 feet offshore from the west riverbank.
The CORMIX model testing results showed a longer downstream distance (315 feet) and shorter offshore
distance (64 feet) was needed to meet the maximum temperature standard of 32°C (89.6°F) as compared
to the data (200 feet downstream and 100 feet offshore). These CORMIX model results compare
favorably to the data and are conservative in calculating the downstream distance to meet the maximum
temperature standard.
CORMIX Mixing Zone Modeling
The CORMIX dilution model was setup for monthly river and discharge conditions to evaluate the mixing
zone size needed to meet the NCDEQ temperature standards. Table 1 presents the river and discharge
inputs used for the CORMIX modeling. River flows of 2,000 cfs were used for the months of December
through August and 1,500 cfs for September through November based on the Roanoke Rapids Dam
minimum release flows. Table 1 also presents the required effluent dilution needed to meet the NCDEQ
temperature standards.
Table 1. CORMIX Model Inputs
Month
Upstream River
Temperature (°C)
NCCW Flow
(MGD)
NCCW
Temperature (°C)
Required
Dilution
January
10.0
1.5
55.6
16.3
February
9.4
1.7
55.6
16.5
March
12.8
2.4
55.6
15.3
April
17.8
3.6
55.6
13.5
May
23.3
4.8
55.6
11.5
June
26.7
6.6
55.6
10.3
July
29.4
8.1
55.6
10.1 *

WestRock
Table 1. CORMIX Model Inputs
Month
Upstream River
Temperature (°C)
NCCW Flow
(MGD)
NCCW
Temperature (°C)
Required
Dilution
August
29.4
8.4
55.6
10.1 *
September
27.8
7.3
55.6
9.9
October
25.0
5.2
55.6
10.9
November
18.3
3.3
55.6
13.3
December
12.7
2.1
55.6
15.3
* - Maximum temperature standard of 32°C controls required dilution, all other months the maximum
temperature rise of 2.8°C controls.
The CORMIX model was run for the inputs presented in Table 1 to determine the downstream distance
and plume width where the NCDEQ temperature standards are met. Table 2 presents the required
effluent dilution, and the plume length, width and percent of river width where the NCDEQ temperature
standards are met. The downstream plume length needed to meet the temperature standards ranges
from 274-687 feet and the plume width ranges from 53-190 feet. This results in a plume width that ranges
from 10-35% of the river width. The greatest downstream distances and plume widths to meet the
temperature standards occurred in September and October when the minimum Dam release flow is
reduced to 1,500 cfs.
Table 2. CORMIX Model Results
Month
Required
Dilution
Plume Length (ft)
Plume Width (ft)
Percent of
River Width
January
16.3
274
53
10%
February
16.5
298
59
11 %
March
15.3
352
72
13%
April
13.5
418
90
16%
May
11.5
452
101
18%
June
10.3
510
118
21 %
July
10.1 *
562
131
24%
August
10.1 *
574
134
24%
September
9.9
687
190
35%
October
10.9
597
160
29%

WestRock
Table 2. CORMIX Model Results
Month
Required
Dilution
Plume Length (ft)
Plume Width (ft)
Percent of
River Width
November
13.3
514
130
24%
December
15.3
325
66
12%
* - Maximum temperature standard of 32°C controls required dilution, all other months the maximum
temperature rise of 2.8°C controls.
THERMAL MIXING ZONE
The NCDEQ temperature standards include a maximum temperature of 32°C (89.6°F) and a maximum
temperature rise above background of 2.8°C (5.0°F). The NCDEQ regulations do not specify numerical
limitations to mixing zone sizes and notes that they will be determined on a case -by -case basis. In
addition, a mixing zone shall not prevent free passage of aquatic organisms around the mixing zone.
Based on the monthly CORMIX modeling, the month with the largest mixing zone downstream distance
and plume width is September. Rounding the September distances up to be conservative, a thermal
mixing zone with a downstream distance of 700 feet and width of 200 feet (36% of the river width) will
allow the NCDEQ temperatures standards to be met at the edge of the mixing zone under all NCCW
discharge and river conditions. In addition, this thermal mixing zone will provide for free passage of
aquatic organisms around the mixing zone.

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