NC0023965_Report_20081027

NPDES DOCUHENT SCANNING► COVER SHEET
NC0023965
Wilmington Northside WWTP
NPDES Permit:
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
("...°--6°-"*.
eey.,)
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
October 27, 2008
This document its printed on reuse paper - ignore any
content an the reYerse side

Mil Cape Fear
Public Utility Authority
Stewardship. 5usla inabit ity. Service.
October 27, 2008
Mr. Gil Vinzani. PE. Supervisor, Eastern NPDES Program
NCDEN R
DWQ
512 N. Salisbury Street
Raleigh, NC 27604
RE: CFPUA NPDES Permit NC0023965
Special Condition A.8 — Effluent Mixing Model
Dear Mr. Vinzani,
235 Government Center Drive
Wilmington, NC 28403
Phone: [phone]
Fax: [phone]
WASTEWATER TREATMENT
The cited condition requires the permittee to submit a CORMIX or equivalent model providing
additional information regarding end -of -pipe dilution at a permitted flow of 16.0 MGD for both the
existing and new discharge line diffusers. Tetra Tech, Inc. (TTI) was retained by the Cape Fear Public
Utility Authority (CFPUA) to address this issue.
TTI contacted DWQ to identify the product which would satisfy this permit requirement and
DWQ's program needs consistent with the ultimate application(s) and use(s) of this product. We are
pleased to submit the accompanying September 2008 document entitled "Technical Memorandum: Cape
Fear Public Utility Authority Northside WWTP Effluent Dilution Analysis" in satisfaction of NPDES
NC0023965 Special Condition A.B.
Upon your review, please don't hesitate to contact us should you have any questions or
comments. Once responding to all questions and comments, we would be pleased to meet with you and
your staff to discuss the report itself or any outcomes potentially arising from the application of its
findings. We await your notification on both these issues.
Sincerely,
Kenneth L. Vogt, Jr., PE, BCEE
Wastewater Treatment Superintendent
klv/sld
Attachments
cc: Nancy Gallinaro, Director of Operations, CFPUA
Beth Eckert, Environment/Safety Director, CFPUA
Jeff Cermak, NS WWT Plant Supervisor
Trevor Clement, TTI

TECHNICAL MEMORANDUM:
Cape Fear Public Utility Authority
Northside WWTP
Effluent Dilution Analysis
Prepared for:
Cape Fear Public Utility Authority
235 Government Center Drive
Wilmington, NC 28403
Prepared by:
TETRA TECH
3200 Chapel Hill -Nelson Hwy, Suite 105 • PO Box 14409
Research Triangle Park, NC 27709
Tel [phone] • Fax [phone]
September 2008

Northside WWTP Effluent Dilution Analysis Memo September 2008
1. INTRODUCTION
Special Condition A.8 Effluent Mixing Model within NPDES permit NC0023965 states that the City of
Wilmington shall submit a CORMIX Model (or equivalent) providing additional information regarding
end -of -pipe dilution no later than 12 months after an engineer's certification for the completion of 16.0
MGD expansion is issued. The model shall address dilution at the approved 16.0 MGD increased flow
rate into the Cape Fear River for both the existing and the new discharge -line diffusers.
Until recently, the City of Wilmington had acted as permittee for this permit which applies to the
Northside Wastewater Treatment Plant (WWTP). NPDES NC0023965 was transferred and reissued to
the Cape Fear Public Utility Authority (CFPUA) as permittee effective July 1, 2008. With this
transfer/reissuance, the CFPUA has assumed responsibility for addressing this permit requirement and has
caused the preparation of this report by Tetra Tech.
The required dilution analysis is to be used by the NC Division of Water Quality (DWQ) as the basis for
setting permit requirements for chronic whole effluent toxicity testing and water quality -based limits for
toxic substances. The Northside WWTP discharges in the proximity of the confluence between the
Northeast Cape Fear River and lower Cape Fear River mainstem. In the majority of discharge situations
throughout the state, DWQ is able to estimate instream waste concentration under critical conditions
simply by dividing the effluent flow by the sum of the effluent flow and 7Q10 stream flow. However,
because the receiving water for the Northside discharge is dominated by tidal influence, the simple
calculation is not representative of actual dilution. Therefore, a more complex modeling dilution analysis
is required to estimate instream waste concentration.
DWQ has typically addressed mixing zone determinations using the CORMIX model. However, this
model has some limitations for applications to energetic estuarine systems like the Cape Fear River and
Estuary. CORMIX does not incorporate a simulation of tidal currents; rather, these need to be specified
by the user, requiring the application of a separate model to calculate the hydrodynamics of the system.
In 2000 and 2001 Tetra Tech developed a three-dimensional (3-D) water quality model of the Lower
Cape Fear River for the City of Wilmington and New Hanover County using the Environmental Fluid
Dynamics Code (EFDC) model. As a part of that project, a general analysis of dilution of the Northside
and Southside WWTP discharges was conducted using EFDC's integrated "jet plume" code (JP-EFDC).
Although that analysis was not conducted specifically for establishing dilution ratios for NPDES permit
limits, the modeling tool represents a significant investment already made by what is now the CFPUA.
Importantly, as a true hydrodynamic simulator EFDC/JP-EFDC can generate accurate tidal currents
offering significant advantages over CORMIX in determining dilution at the edge of the mixing zone, in
the far -field region, and under a full range of environmental conditions. Therefore Tetra Tech requested
and was granted permission by DWQ to build upon the previous EFDC application to conduct the
required dilution analysis for the Northside facility (personal communication with Teresa Rodriguez, July
18, 2007; confirmed May 29, 2008 during meeting with Gil Vinzani and Toya Fields).
Tetra Tech met with DWQ Permitting Program staff on May 29, 2008 to review its proposed approach
and to discuss questions. Based on input received during that meeting and in follow up correspondence,
Tetra Tech conducted a modeling dilution analysis. The modeling approach and results are summarized
herein. It is anticipated that DWQ will use these results to establish the size of the chronic mixing zone
and instream waste concentration (or dilution ratio) upon which to base NPDES limitations.
TITRATIICH
Nct
1

Northside W v TP Effluent Dilution Analysis Memo September 2008
2. TECHNICAL APPROACH
The previously calibrated and validated EFDC model (Tetra Tech, 2001) was used as the starting point
for the analysis. A conservative tracer having concentration Ce was introduced into the plant effluent and
the hydrodynamic and transport model was then used to simulate the distribution of the tracer in the
system. A dye study performed in late 1999 provides the basis for comparison of model predictions. A
description of the model, model development, model validation, and scenario development for model
application are summarized below in sections 2.1 through 2.4.
2.1 MODEL DESCRIPTION
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 a widely used and technically
defensible hydrodynamic model. EFDC uses stretched or sigma vertical coordinates and Cartesian or
curvilinear, orthogonal horizontal coordinates to represent the physical characteristics of a waterbody. It
solves three-dimensional, vertically hydrostatic, free surface, turbulent averaged equations of motion for a
variable -density fluid. Dynamically -coupled transport equations for turbulent kinetic energy, turbulent
length scale, salinity and temperature are also solved. The physics of the EFDC model and many aspects
of the computational scheme are equivalent to the widely used Blumberg -Mellor model and U.S. Army
Corps of Engineers' Chesapeake Bay model.
In addition to the far field transport and fate simulation capability incorporated into EFDC code's water
quality and toxic contaminant modules, the code includes a near field discharge dilution and mixing zone
module. The near field model is based on a Lagrangian buoyant jet and plume model (Frick, 1984; Lee
and Cheung, 1990) and allows representation of submerged single and multiple port diffusers and buoyant
surface jets. The near field model provides analysis capabilities similar to CORMIX (Jirka and Doneker,
1991; Jirka and Akar, 1991) while offering two distinct advantages. The first advantage is that a more
realistic representation of ambient current and stratification conditions, provided directly by EFDC
hydrodynamic module, is incorporated into analysis. The second advantage is that multiple discharges
and multiple near field analysis times may be specified to account for varying ambient current and
stratification conditions. For example, the analysis of 10 discharges under 6 ambient conditions each
would require 60 executions of CORMIX, while the entire analysis of the 60 situations would be
produced in a single EFDC simulation. The Lagrangian jet -plume model has been extensively tested
against CORMIX1 by Davidson and Pun (1998) who conclude that the two models provide quantitatively
similar predictions of mixing and dilution from single port discharges over a wide range of conditions.
The near field simulation may be executed in two modes, the first providing virtual source information for
representing the discharges in a standard EFDC far field transport and fate simulation. The second mode
directly couples the near field and far field transport modes, again using a virtual source formulation,
during simultaneous near and far field transport and fate simulations (Hamrick, 1996).
2.2 MODEL DEVELOPMENT
The EFDC hydrodynamic model grid covers the portion of the Cape Fear River below Lock and Dam
No. 1 to the outlet to the Atlantic Ocean, along with the tidally -influenced portion of the Black River and
the Northeast Cape Fear River (Figure 2-1). Calibration of the EFDC hydrodynamic model was achieved
using available information. Numerous agencies and organizations have accumulated a vast amount of
data characterizing the Lower Cape Fear Estuary System. These information sources have been drawn
upon extensively to calibrate and verify the 3-D EFDC model during the 2000-2001 study.
Additionally, two extensive Rhodamine dye studies were conducted in December 1999. Samples were
collected for dye, salinity, and temperature throughout the estuary by boat during the two studies. The
2

Northside W /I TP Effluent Dilution Analysis Memo September 2008
dye study conducted on the Northside facility was used in this study to validate near field mixing and far
field transport of the existing effluent at the Northside outfall.
From a hydrodynamic modeling perspective, the Cape Fear exhibits sufficient three-dimensional features
to warrant a three-dimensional modeling approach. In the estuary proper, the combination of a narrow,
deep navigation channel bordered by wider shallow areas requires both horizontal resolution to properly
represent the channel and vertical resolution to represent the predominance of landward transport of
saltier water along the channel and seaward transport over the shallow areas. Although during portions of
the year stratification may not be significant and a depth average model might be appropriate, depth
average models are not capable of representing circulation associated with wind. For this reason, a
minimum two -layer vertical resolution is desirable. For the present study a four -layer vertical resolution
was selected to appropriately represent the ambient conditions for the near field model. LThe Northside facility will have two outfalls when construction is completed to support the facility's
anded wasteflow of 16 MGD (Figure 2-1). The outfalls are identical with three ports each, with the
wer outfall being located approximately 400 ft downstream of the existing outfall. A slightly larger
ount of flow is expected to be transported through the existing outfall than the new one (personal
mmunication, McKim & Creed, May 2008). 1-
The outfalls in the jet -plume model within EFDC were coded as a single discharge port. The approach
assumes that due to the proximity of the three ports, the effluent jet -plume merges into one and that the
total momentum is in the direction of the central port. The representative single port diameter was
selected to yield the same exit velocity to those expected in the actual diffuser design in order to represent
the near field jet -plume behavior correctly with the model setup.
2.3 MODEL VALIDATION
The model was validated by running the model for 20 days in December of 1999 to compare simulation
results to measurement done during the Rhodamine field study. Dilution was used to quantify the
agreement between the near and far field models and measurements during the dye study done in the
Northside existing outfall on December 15 and 16, 1999. The simulation run was started on December 1,
to allow for a model spin -up period before the dye study time. A conservative tracer was introduced into
the plant effluent at the concentration and flow rate of the dye study, and dilution was calculated at
different times and locations and compared to measure data. Several vertical profiles were measured and
also compared to model results.
3

Northside VVWTP Effluent Dilution Analysis Memo
September 2008
1 Grid
Shoreline
Discharges
- Existing outfall
• Proposed outfall
Figure 2-1. EFDC Model Grid of the Lower Cape Fear Estuary and River
4

Northside Effluent Dilution Analysis Memo September 2008
2.4 SCENARIO SETUP
The calibrated and validated hydrodynamic model was applied to evaluate near field and far field mixing
and transport of the existing and proposed outfalls for the Northside WWTP effluent. A conservative
tracer having concentration Ce was introduced into the plant effluent. The hydrodynamic and transport
model was then used to simulate the distribution of the tracer in the system.
The concept of dilution was used to quantify the degree of mixing and the transport of the effluent in the
Cape Fear system. The dilution was determined by the following equation:
D = Ce (equation 2.1)
C
where D is the dilution, Ce is the effluent concentration and C is the concentration at the location of
interest.
A 4-day average dilution value was used for the scenario analysis based on recommendations of EPA's
Technical Support Documents (TSD) for mixing zone analysis. DWQ validated this assumption for Tetra
Tech prior to conducting the analysis (personal communication with Toya Fields, June 23, 2008).
The first scenario considered the two outfalls discharging and the Northside plant operating at full
capacity. The discharged flow for the existing outfall was 9 MGD and for the proposed outfall 7 MGD.
The model was run for a month plus a spin -up period to cover a whole spring -neap tidal cycle under dry
weather critical conditions. Lower flows are usually registered during the summer, therefore the model
was run with constant flows to simulate summer dry weather conditions. These constant low flow
conditions were calculated using historical records at the USGS stations 02105769 Lock #1 near Kelly,
NC for the Cape Fear River, 02106500 near Tomahawk for the Black River, and 02108000 near
Chinquapin, NC for the Northeast Cape Fear River. The calculated flows were the 10th percentile of the
July average monthly flow for the period 1982-2007 when data was available for the three gages. The
resulting flows were 29.5 cms (1042.5 cfs) for the Cape Fear River, 2.5 cms (88.9 cfs) for the Black
River, and 1.6 cms (58 cfs) for the Northeast Cape Fear River. The model was run during June -July 2000
when data was available for the rest of the input variables (tides and meteorological condition). A model
spin -up period was included in the simulation to let tracer concentrations reach a quasi -steady state by
June 20.
A second scenario applied the same estuary conditions from the first scenario, but only the existing outfall
was assumed to be discharging, at a rate of 8 MGD. A third scenario was run with the same general
conditions, except that it assumed that only the proposed outfall was discharging, at a rate of 8 MGD.
A fourth scenario considered a wet weather condition. It was run during July and August 2000 with
measured daily flows at Lock #1, near Kelly, SC for the Cape Fear, USGS station 02106500 near
Tomahawk, NC for the Black River and USGS station 02108000 near Chinquapin, NC for the Northeast
Cape Fear River to include storm flows from July 24 to August 17. The existing outfall was discharging
9 MGD and for the proposed outfall 7 MGD.
5

Northside Effluent Dilution Analysis Memo September 2008
3. MODEL VALIDATION RESULTS
The hydrodynamic model was calibrated and validated during the 2000 and 2001 study. To evaluate the
near field and far field mixing and transport of the Northside WWTP effluent for the new and proposed
outfalls further validation was performed using the data from the Rhodamine dye study conducted in
December 1999 in the existing outfall.
A Rhodamine dye solution at a 20 percent concentration was injected into the Northside outfall from
10:30 a.m. until 4:00 p.m. on December 15, 1999. The rate of the solution was 220 mL/min. Rhodamine
concentrations throughout the system were measured on December 15 and 16, and were used to compare
to model results.
A conservative tracer coupled to the plant effluent was used to simulate the Rhodamine used in the field
study. The hydrodynamic and transport model was then used to simulate the distribution of the tracer in
the system for the period of the study with a spin -up period of 15 days.
Figure 3-1, Figure 3-2 and Figure 3-3 present the comparison between measured and simulated dilution
for the far field, near field and all data, respectively. The diagonal lines represent perfect agreement. It
can be seen that the numerical prediction reproduced the measured data with low systematic error
(R2=0.81). The point scattering is slightly higher for larger dilution, reflecting the possible difficulty of
measuring low concentration values.
Figure 3-5 through Figure 3-17 show vertical profile comparisons of measured and simulated data at
different locations in the Lower Cape Fear River and Estuary and at different times during and after the
dye discharge. The location of the different profiles is shown in Figure 3-4. The profiles show good
agreement between simulated and observed data and the capability of the model to simulate the vertical
structure of the system which is a critical component of a mixing zone study.
This validation together with the previous calibration and validation of the EFDC model performed in the
2000 and 2001 study show that the model is an appropriate tool for both far and near field analysis in a
mixing zone study.
® TWTRATROI
6

Northside Effluent Dilution Analysis Memo
September 2008
0
1000000
100000
10000
1000
100
10
10
Far -field data comparison
100 1000
Measured Dilution
10000
100000
1000000
Figure 3-1. Far Field Data Comparison
Simulated Dilution
10000
1000
100
10
10
Near -field data comparison
100
Measured Dilution
1000
10000
Figure 3-2. Near Field Data Comparison
7

Northside Effluent Dilution Analysis Memo
September 2008
Simulated Dilution
1000000
100000
10000
1000
100
10
Data comparison
♦• ♦
•
•
• ♦
♦
10
100 1000
Measured Dilution
10000
100000
1000000
Figure 3-3. Near and Far Field Data Comparison
® rrrnr►riai
8

Northside Effluent Dilution Analysis Memo September 2008
/ r
N
s
0
1 Miles
f �\
ti\ �
•
•
• Existing outfall
lst Power line Crossing in NE Fork
Hilton (RR) Bridge
Koch Berth
Memorial Bridge
Oxbow opening in NE Fork
Pak Tank Berth
Red Marker #8 in NE Fork
US 421 Bridge
Grid
Shoreline
Figure 3-4. Profile Locations
nTWTRATSCH

Northside Effluent Dilution Analysis Memo
September 2008
Vertical Profile at Red Marker #8 in NE Fork -12/15/1999 3:00 PM
Dilution (CdIC)
1.00 10.00 100.00 1000.00 10000.00 100000.00
0.0
5.0
10.0
15.0
sc 20.0
m
❑
25.0
30.0
35.0
40.0
-4- Measured -.- Simulated 1
Figure 3-5. Vertical Profile at Red Marker #8 in NE Fork on 12/15/1999 at 3:00 PM
1.00
0.0
5.0
10.0
15.0
5 20.0
0
w
25.0
30.0
35.0
40.0
Vertical Profile at 1st Powerline crossing in NE Fork - 12/15/1999 3:00 PM
Dilution (CdIC)
10.00 100.00 1000.00 10000.00 100000 00
1-4-Measured
�- Simulated
Figure 3-6. Vertical Profile at 1st Power line Crossing in NE Fork on 12/15/1999 at 3:00 PM
TIMATsw
10

Northside Effluent Dilution Analysis Memo
September 2008
Vertical Profile at Hilton (RR) Bridge - 12/15/1999 6:40 PM
Dilution (CdIC)
1,00 10.00 100.00 1000.00 10000.00 100000.00
0.0 •
5.0
10.0
15.0
r 20.0
a
a
25.0
30.0
35.0
40.0
—Measured —a—
Figure 3-7. Vertical Profile at Hilton (RR) Bridge on 12/15/1999 at 6:40 PM
1.00
0.0
5.0
Vertical Profile at Memorial Bridge - 12/15/1999 7:30 PM
Dilution (CdIC)
10.00 100.00 1000.00 10000.00 100000.00
10,0
15.0
x
L
20.0
a
in
25.0
30.0
35.0
40.0
•
--- --
—
Ft Measured —0—Simulated '
Figure 3-8. Figure 3-8 Vertical Profile at Memorial Bridge on 12/15/1999 at 7:30 PM
11

Northside Effluent Dilution Analysis Memo
September 2008
1.00
0.0
5.0
10.0
15.0
L 20.0
0
G
25.0
30.0
35.0
40.0
Vertical Profile at Pak Tank - 12/15/1999 8:00 PM
Dilution (CdIC)
10.00 100.00 1000.00 10000.00 100000.00
---�-Measured -F-Simulated
Figure 3-9. Vertical Profile at Pak Tank Berth on 12/15/1999 at 8:00 PM
1.00
0.0
5.0
10.0
15.0
E. 20.0
a
a
0
25.0
30.0
35.0
40.0
Vertical Profile near Koch Berth -12/15/1999 8:30 PM
Dilution (Cd!C)
10.00 100.00 1000.00
10000.00
100000.00 1000000.00
-�-Measured ---Simulated
Figure 3-10. Vertical Profile near Koch Berth on 12/15/1999 at 8:30 PM
TtRMTICH
12

Northside Effluent Dilution Analysis Memo
September 2008
Vertical Profile near Hilton (RR) Bridge on NE Fork - 12/16/1999 1:10 AM
Dilution (Cd/C)
1.00 10.00 100.00 1000.00 10000.00 100000.00
0.0
5.0
10.0
15.0
r
20.0
a
25.0
30.0
35.0
40.0
t Measured f Simulated
Figure 3-11. Vertical Profile near Hilton (RR) Bridge on NE Fork on 12/16/1999 at 1:10 AM
x
Vertical Profile near US 421 Bridge on Main Stem -12/16/1999 1:40 AM
1.00 10.00
0.0
5.0
10.0
15.0
25.0
30.0
35.0
40.0
Dilution (CdIC)
100.00 1000.00 10000.00 100000.00
Measured -a-Simulated)
Figure 3-12. Vertical Profile near US 421 Bridge on 12/16/1999 at 1:40 AM
® TIRRATIticH
13

Northside Effluent Dilution Analysis Memo
September 2008
Vertical Profile in NE Fork near first oxbow opening - 12/16/1999 1:50 AM
Dilution (CdIC)
1.00 10.00 100.00 1000.00 10000.00 100000.00
0.0
5.0
10.0
15.0
x
t 20.0
a
25.0
30.0
35.0
40.0
Y
�-Measured-a- Simulated 1
Figure 3-13. Vertical Profile near Oxbow Opening in NE Fork on 12/16/1999 at 1:50 AM
1.00
Vertical Profile near Pak Tank berth - 12/16/1999 12:40 PM
Dilution (Cd1C)
10.00 100.00 1000.00 10000.00 100000.00
0.0
5.0
-
10.0
-
15.0
r
ra
a
a
20.0
25.0
30.0
35.0
- -
40.0
Hs -Measured ': Simulated
Figure 3-14. Vertical Profile near Pak Tank Berth on 12/16/1999 at 12:40 PM
®17TRAT1Q1
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Northside Effluent Dilution Analysis Memo
September 2008
1.00
0.0
5.0
10.0
15.0
L 20.0
n.
❑
25.0
30.0
35.0
40.0
Vertical Profile near Hilton (RR) Bridge on NE Fork - 12/16/1999 1:50 PM
Dilution (CdIC)
10.00 100.00 1000.00
10000.00 100000.00
-6-Measured --- -Simulated
Figure 3-15. Vertical Profile near Hilton (RR) Bridge on NE Fork on 12/16/1999 at 1:50 PM
1.00
0.0
5.0
10.0
15.0
r
r 20.0
0.
m
25.0
30.0
35.0
40.0
Vertical Profile near Red Marker #8 on NE Fork -12/16/1999 2:00 PM
Dilution (Cd/C)
10.00 100.00 1000.00 10000.00 100000.00
-6- Measured Simulated
Figure 3-16. Vertical Profile near Red Marker #8 on NE Fork on 12/16/1999 at 2:00 PM
® 1*TRATICH
15

Northside Effluent Dilution Analysis Memo
September 2008
Vertical Profile near US 421 Bridge on Main Stem - 12/16/1999 2:30 PM
Dilution (Cd/C)
1.00 10.00 100.00 1000.00 10000.00 100000.00
0.0
5.0
10.0
15.0
F
20.0
o.
a
25.0
30.0
35.0
40.0
tMeasured —a—Simulated
Figure 3-17. Vertical Profile near US 421 Bridge on Main Stem on 12/16/1999 at 2:30 PM
® 17TRATOP1
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Northside Effluent Dilution Analysis Memo September 2008
4. MODEL APPLICATION RESULTS
The near field and far field dilution analysis was conducted to determine the mixing and dilution of the
effluents in the immediate vicinity of the plant discharges. Since the outfalls have a three -nozzle
submerged diffuser, the jet -plume sub -model was used. The JP EFDC model provides simulation for the
near field plume from the discharge until the buoyancy and jet momentum effects are lost, when the
transport and diffusion are provided by the far field. The EFDC model provides far field results. In order
to provide continuous results between the near and far field model, results were interpolated between the
last near field value and the EFDC model value of the cell where the near field model ended.
Concentrations were interpolated based on a first order dispersion equation
C = Coe
(equation 4.1)
where Co and Ro are the concentration and distance, respectively, from the discharge at the end of the near
field plume. C is the concentration at a distance R from the discharge and a is an attenuation coefficient
calculated using the far field concentration value. The distance was the radius of the EFDC cell,
calculated as R = dx.dy / 7r .
For the analysis of the different scenarios, 4-day average dilutions, calculated based on equation 2.1, were
computed at 10 m, 25 m, 50 m, 75 m and 150 m from the discharge. The first four distances reflect the
near -field distances agreed upon with DWQ (personal communication with Toya Fields, June 23, 2008),
and the 150 m distance reflects the beginning of the far field dilution.
Figure 4-1 and Figure 4-2 show continuous 4-day average dilutions over the full simulation period at the
stated distances for the existing and proposed outfall for the first scenario (i.e., assuming both outfalls
operating at full capacity). Figure 4-3 presents the water surface elevation variation at the location of the
existing outfall showing the spring -neap cycle of the tide during the simulation period. Table 4-1 presents
the average, 10th and 90th percentile dilution for the selected distances from the discharges. The 10th and
90th percentile values are provided to demonstrate the variability about the central tendency (average
dilution during the critical period).
The mean 4-day average dilutions for a distance equal to 1/3 of the width of the river at the discharge
location (50 m) were 20.7 and 22.7 for the existing and proposed outfalls, respectively. The 10th and 90th
percentile values of 18.8 and 23.5 demonstrate a relatively tight range of expected dilution during the
summer critical conditions.
Table 4-1. Dilution Statistics for the Dry Period Scenario when Both Outfalls are Discharging
Outfail
Existing
Proposed
Distance from Ouffall
10m
25m
50m
75m
150m
10m
25m
50m
75m
150m
Average
16.8
18.1
20.7
26.3
92.9
17.8
20.0
22.7
28.6
93.5
10th Percentile
15.0
16.1
18.8
24.4
89.9
15.4
17.2
19.9
25.8
90.3
90th Percentile
19.4
20.8
23.5
29.2
96.2
20.3
22.8
25.5
31.5
96.5
17

Northside Effluent Dilution Analysis Memo
September 2008
0
1000.0
1000
10.0
1.0
Existing outfall with both outfalls discharging -4 day average
6/24f2000 6/29/2000 7/4/2000 7/9/2000 7/14/2000 7/19/2000 7/24/2000 7/29/2000
Date
----10 m from discharge —25 m from discharge —50 m from discharge —75 m from discharge —150 m from discharge
Figure 4-1. Dilution from the Existing Outfall when Both Outfalls are Discharging for the Dry
Period Scenario
0
0
1000
100
10
1
6/24/2000 6/29/2000 7/4/2000 7/9/2000 7/14/2000 7/19/2000 7/24/2000 7/29/2000
Date
Proposed outfall with both outfalls discharging - 4 day average
� 1 �
—10 m from discharge —25 m from discharge —50 m from discharge —75 m from discharge —150 m from discharg�
Figure 4-2. Dilution from the Proposed Outfall when Both Outfalls are Discharging for the Dry
Period Scenario
I'�EI TITRAT1Q1
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Northside Effluent Dilution Analysis Memo
September 2008
1.2
0.8
-0.4
ql
i
i
di
Tide at discharge
111111i 1111 1111111111111
11
1111
q
-0.8
6/20/2000 6/25/2000 6/30/2000 7/5/2000 7/10/2000 7/15/2000 7/20/2000 7/25/2000 7/30/2000
date
Figure 4-3. Water Surface Elevation on Cape Fear River at the Existing Discharge Location
During the Simulation Period
Figure 4-4 presents dilutions for scenario 2 when only the existing outfall is discharging during the dry
period and Table 4-2 presents the results for this scenario in tabular form. Figure 4-5 presents dilutions
for scenario 3 for only the proposed outfall discharging during the dry period and 0 presents the results for
this scenario in tabular form.
The mean 4-day average dilutions for a distance equal to 1/3 of the width of the river at the discharge
location (50 m) were 24.8 and 25.4 for the existing and proposed outfalls, respectively. These results
show that both outfalls discharging (Scenario 1) is a more critical condition due to buildup and combined
effect of the discharges from both outfalls. Again, the variability about this mean is relatively low.
Table 4-2. Dilution Statistics for the Dry Period Scenario When Only the
Existing Outfall is Discharging
Outfall
Existing Only
Distance from Outfall
10m
25m
50m
75m
150m
Average
18.6
20.8
24.8
34.1
187.1
10th Percentile
15.8
18.4
22.3
31.4
178.5
90th Percentile
21.5
24.5
28.7
38.3
196.3
nTWTRATOCH
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Northside Effluent Dilution Analysis Memo
September 2008
0
0
1000
100
10
Existing outfall only - 4 day average
1
6/24/2000 6/29/2000 7/4/2000 7/9/2000 7/14/2000 7/19/2000 7/24/2000 7/29/2000
Date
—10 m from discharge 25 m from discharge —50 m from discharge —75 m from discharge —150 m from discharge
Figure 4-4. Dilution from the Existing Outfall Discharging Alone for the Dry Period Scenario
Table 4-3. Dilution Statistics for the Dry Period Scenario When Only the
Proposed Outfall is Discharging
Outfall
Proposed Only
Distance from Outfall
10m
25m
50m
75m
150m
Average
18.6
21.4
25.4
35.0
184.0
10th Percentile
16.1
18.6
22.5
31.7
176.3
90th Percentile
20.7
24.4
28.6
38.6
191.9
1Trt:A
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Northside Effluent Dilution Analysis Memo
September 2008
0
1000
100
10
Proposed outfall only - 4 day average
1
6/24/2000 6/29/2000 7/4/2000 7/9/2000 7/14/2000 7/19/2000 7/24/2000 7/29/2000
Date
-10 m from discharge -25 m from discharge -50 m from discharge -75 m Irom discharge -150 m from discharge)
Figure 4-5. Dilution from the Proposed Outfall Discharging Alone for the Dry Period Scenario
Figure 4-6 presents the hydrograph for Cape Fear River at Lock #1 for scenario 4 during July and August
of 2000, illustrating the wet weather conditions for the storm event(s) impacting flow from July 24
through August 17. Figure 4-7 and Figure 4-8 show dilutions at the stated distances for the existing and
proposed outfalls for the fourth scenario. Table 4-1 presents the average, 10th and 90th percentile dilution
for the selected distances from the discharges.
The mean 4-day average dilutions for a distance equal to 1/3 of the width of the river at the discharge
location (50 m) were 34.2 and 36.0 for the existing and proposed outfalls, respectively. These results
confirm that the critical conditions, lower dilutions, occur during dry weather periods. Note that lower
dilution values at 10 m result because under high flow conditions the effluent does not reach the surface
and mix at that distance away from the outfall.
Table 4-4. Dilution Statistics for the Wet Weather Scenario when Both Outfalls are Discharging
Outfall
Existing
Proposed
Distance from Outfall
10m
25m
50m
75m
150m
10m
25m
50m
75m
150m
Average
13.8
19.6
34.2
46.6
218.4
16.8
25.4
36.0
49.0
192.4
10th Percentile
11.0
14.1
24.2
32.4
156.9
15.5
22.5
26.3
34.6
154.4
90th Percentile
17.5
24.5
48.4
66.2
262.9
17.8
27.6
45.8
65.1
222.7
„T
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Northside Effluent Dilution Analysis Memo
September 2008
USGS 02105769 CAPE FEAR RIVER AT LOCK #1 NEAR KELLY, NC
300.00
250.00
200.00
g
150.00
3
0
LT.
100.00
50.00
0.00
7/13/2000
--
I
1
__
—1
1
—I-
- —
L
-
_I
-
1
-j--
—
1
I
1
I
—
7/18/2000
7/23/2000
7/28/2000
8/2/2000
8/7/2000
Date
8/12/2000
8/17/2000
8/22/2000 8/27/2000
9/1/2000
Figure 4-6. Cape Fear River Flow at Lock #1 Showing the July 24-August 17, 2000 Wet Weather
Scenario
c
0
1000
100
10
Existing outfall with both outfall discharging - 4 day average
1
7/25/2000 7/30/2000 8/4/2000 8/9/2000 8/14/2000 8/19/2000
Date
--10 m from discharge —25 m from discharge —50 m from discharge —75 m from discharge —150 m from discharge
Figure 4-7. Dilution from the Existing Outfall when Both Outfalls are Discharging for the Wet
Weather Scenario
® 17TRAT cH
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Norfhside Effluent Dilution Analysis Memo
September 2008
0
100
10
Proposed outfall with both ourfall discharging - 4 day average
1
725/2000
7/30/2000
8/4/2000 8/9/2000 8/14/2000 8119/2000
Date
1-10 m from the discharge —25 m from discharge —50 m from discharge m from discharge —150 m from discharge
Figure 4-8. Dilution from the Proposed Outfall when Both Outfalls are Discharging for the Wet
Weather Scenario
TVT1tAT1141
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Northside Effluent Dilution Analysis Memo September 2008
5. SUMMARY
The EFDC model development accounts for the dynamic processes and unique physical features affecting
the Northeast Cape Fear and Lower Cape Fear River Estuary system. Varying meteorological and tidal
conditions, as well as the unique bathymetry are well represented. This modeling effort allows a more
robust evaluation of conditions affecting the Northside WWTP effluent mixing zone by accounting for the
near field jet -plume dynamics and the far field transport and mixing. The calibration and the validation
using near and far field data from a field dye study validate that the model is simulating the characteristics
of the system appropriately. Thus, results from the dilution evaluation are credible.
Critical conditions were simulated when both outfalls were assumed to be discharging at the maximum
capacity of the wastewater treatment plant during dry weather conditions. The 4-day average dilutions for
a distance equal to 1/3 of the width of the river at the discharge location (50 m) were estimated to be
20.7:1 and 22.7:1 for the existing and proposed outfalls, respectively.
N =WWI
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Northside Effluent Dilution Analysis Memo September 2008
REFERENCES
Davidson, M.J. and K.L. Pun. 1998. Hybrid Model for Prediction of Initial Dilutions from Outfall
Discharges. Journal of Hydraulic Engineering, 124, 12, pp 1188-1197.
Frick, W.E., 1984. Non -Empirical Closure of the Plume Equations. Atmospheric Environment, 18,
653662.
Hamrick, J.M., 1996. User's Manual for the Environmental Fluid Dynamics Computer Code. Special
Report No. 331 in Applied Marine Science and Ocean Engineering, Virginia Institute of Marine Science,
Gloucester point, VA.
Jirka, G.H., and R.L. Doneker, 1991. Hydrodynamic Classification of Submerged Single -Port
Discharges. Journal of Hydraulic Engineering, Volume 117, pp 1095-1112.
Jirka, G.H., and P.J. Akar, 1991. Hydrodynamic Classification of Submerged Multiport-Diffuser
Discharges. Journal of Hydraulic Engineering, Volume 117, pp 1113-1128.
Jirka, G.H., R.L. Doneker, and S.W. Hinton. 1996. User's Manual for CORMIX: A Hydrodynamic
Mixing Zone Model and Decision Support System for Pollutant Discharges into Surface Waters. Office
of Science and Technology, US Environmental Protection Agency, Washington D.C.
Lee, J.H.W. and V. Cheung, Journal of Environmental Engineering, Volume 116, Issue 6, December
1990, pp 1085-1106.
Tetra Tech. 2001. 3-Dimensional EFDC Water Quality Model of the Lower Cape Fear River and its
Estuary. Prepared by Tetra Tech for City of Wilmington and New Hanover County. Research Triangle
Park, NC.
C:1 TIMULT■Q1
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