01_EFDC_Study_Plan

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TECHNICAL MEMORANDUM #1:
Tar River Flow Study – Final Study Plan
To: Copies:
Greenville Utilities Commission
Tar River Flow Study Technical Advisory Group
Neela Sarwar, DWR
Mary Sadler, ARCADIS
From:
Lauren Elmore and Paul Leonard, ENTRIX
Date / Rev:ARCADIS Project No.:
March 31, 2009 / May 27, 2009 NC706015.1000
Subject:
Tar River Flow Study
Greenville Utilities Commission
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Table of Contents
1.Introduction 7
1.1 Scoping Phase, Study Plan, and Technical Advisory Group 7
1.2 Study Goals and Objectives 8
1.3 Coastal North Carolina Water Management 9
1.4 Recent Drought and River Conditions 10
1.5 GUC Water Use and Interbasin Transfers (IBT) 11
1.6 Water Supply Planning and Management 11
2.Background and Current Conditions 12
2.1 Water Quality in the Lower Tar River Basin and Pamlico River Estuary 12
2.2 Hydrology of the Tar River 18
2.3 Tar River Water Management and Previous Instream Flow Studies 19
2.4 Tar River at Rocky Mount 20
2.5 Influence of Tar River Reservoir Operations on Tar River Flows 21
2.6 Tidal Influences in the Lower Tar River 22
2.7 Previous Hydrologic Analysis of GUC Withdrawals 23
3.Technical Approach 26
3.1 Study Area 26
3.2 Technical Approach 28
3.3 Study Components and Models 28
3.4 Range of Flows to be Considered (Indicators of Hydrologic Alteration – IHA) 30
3.5 Screening and Assessment of Water Quality Issues 33
4.Basin Hydrologic Analysis and Modeling 34
5.Hydrodynamic Modeling 35
5.1 Overview 35
5.2 Existing EDFC Model of the Pamlico River 35
5.3 Data Collection for Model Development 36
5.4 Tar River EFDC Hydrodynamic Model Development 37
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6.Habitat Modeling 40
6.1 Modeling Approach 40
6.2 Physical Habitat Modeling (Tidal Freshwater Segment) 42
6.3 Salinity Regime Modeling (Estuarine Transition Segment) 43
6.4 Water Quality Modeling/Screening (Tidal Freshwater Segment, Estuarine Transition
Segment, and Pamlico River Estuary Segment) 44
7.Water Quality Modeling 45
7.1 Water Quality Model Development 45
7.2 Model Calibration and Verification 46
8.Biological Data and Habitat Suitability Criteria 47
8.1 Aquatic Resource Information 47
8.2 Available Aquatic Survey Data for the Tar River and Pamlico River Estuary 47
8.3 Rare, Threatened, and Endangered Species Present in the Study Area 52
8.4 Supplemental Mussel Surveys within the Freshwater Tidal Segment 53
8.5 Criteria for Habitat Modeling 54
9.References 57
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Tables
Table 2-1: Water Quality Monitoring Locations and Statistically Significant Water Quality
Standards Violations in the Lower Tar and Pamlico Rivers 2003 to 2007 13
Table 2-2: Draft 2008 303(d) Impaired Waterbody List for Lower Tar and Pamlico River Basins 16
Table 2-3: Median Tar River Flows at Greenville (1931-2007) 25
Table 3-1: Tar River Flow Study Segments 26
Table 3-2: Tar River Flow Study Technical Analysis Approach to Four Study Segments 29
Table 3-3: Approximate Basin Withdrawals and Return Flows Including GUC for
Demonstrative Purposes 31
Table 3-4: Result of Indicators of Hydrologic Analysis (IHA) for Baseline (2002) and Four
Future Consumptive Use and GUC Withdrawal Scenarios 32
Table 6-1: Habitat Modeling Approach for Each Segment of the Tar River Flow Study 40
Table 8-1: Fish and Benthic Macroinvertebrate Sampling Locations in the Lower Tar Basin
Sampled by DWQ in 2007 49
Table 8-2: State and Federally Listed Species Observed in the Tar River between GUC Water
Treatment Plant Intake and Washington 52
Table 8-3: April 2009 Mussel Survey in Tar River near GUC WTP Intake 53
Figures
Figure 1-1: General Study Area, Tar River Flow Study
Figure 1-2: Tar River Basin and Central Coastal Plain Capacity Use Area
Figure 2-1: Flow Duration Curves Based on Average Annual Flow at Greenville Gage Station;
Comparison of Generated Flow Record Before (1932-1968) and After [with] (1972-
2006) Operation of the Rocky Mount Reservoir
Figure 2-2: Range of Tidally-influenced Flow Oscillations in the Tar River at Greenville for
three Different Ranges of Average Flow Condition (a) 150 cfs (August 20 – 23,
2007), (b) 1,500 cfs (December 18 – 21, 2006), and (c) 15,000 cfs (November 24 –
27, 2006), Fifteen minute flow data shown on plots
Figure 2-3: Schematic Diagram of the Tar River in the Vicinity of Greenville, NC Showing the
Relative Locations and Approximate Distances between Withdrawal and
Discharge Locations, USGS Gage Locations, and Hydrologic Model Output Points
(ENTRIX, 2008)
Figure 2-4: Average Annual Flows in the Tar River at Greenville Based on Analysis of
Historical Flow Records (1932-2006)
Figure 2-5: Flow Percentiles for Tar River at Greenville based on Modeled Data (1931 – 2007)
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Figure 2-6: GUC Average and Maximum Withdrawals as a Percentage of the Median Tar
River Flow
Figure 2-7: GUC Withdrawals as a Percentage of the 10th Percentile Monthly Flows
Figure 2-8: Average and Maximum GUC Permitted Withdrawals as a Percentage of the 7Q10
Flow
Figure 3-1: Proposed Study Area and Freshwater and Estuarine Segments
Appendices (on CD)
A Flow Exceedance Values at the Tar River at Greenville (Gage No. 02084000) from ENTRIX,
2008.
B Locations of Unique, Endemic, State, and Federal-Listed Aquatic Species within the Study
Area.
C Matrix of Agency Comments on Draft Study Plan and Responses.
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1. Introduction
The State of North Carolina typically limits water withdrawals to no more than twenty percent of the 7Q10
flow unless a flow study is conducted that determines additional withdrawals are acceptable. Greenville
Utilities Commission’s (GUC) water treatment plant (WTP) is permitted to withdraw 22.5 million gallons
per day (mgd) from the Tar River for public water supply and is one of the few tidally influenced public
water supply intakes in North Carolina. Because of this tidal influence, the State allowed GUC’s permitted
capacity to exceed twenty percent of the 7Q10 flow. When approving the permit that established GUC’s
22.5 mgd withdrawal allocation, the Division of Water Resources noted that any future plant expansion
requests would have to be accompanied by a flow study of the Tar River.
GUC has initiated this planning study to evaluate the issues associated with future flow conditions and a
range of water withdrawals. GUC does not intend to request a permit for a water treatment plant
expansion as a part of this instream flow study. The intent of this study is to develop an agency approved
approach for evaluating the Tar River’s instream flow needs. The study will develop models and methods
to assess water quality, water quantity, and habitat conditions over a range of flow and metrological
conditions. GUC has proactively initiated this Tar River Flow Study to understand the instream flow needs
of the river and the amount of water available to support long-term water supply and capital
improvements planning. Groundwater withdrawal reductions mandated by the Central Coastal Plain
Capacity Use Area (CCPCUA) Rules, recent droughts, encroachment of saltwater, and increasing
regional water supply needs have further emphasized the need for a better understanding of water
availability in the Tar River.
GUC is investigating the capacity of the Tar River to support additional future water withdrawal under
various flow conditions, particularly low flows. This investigation is not associated with a specific proposed
water withdrawal amount at this time. However, the study will be valuable for identifying the flow ranges
within which additional withdrawals are supported while maintaining beneficial habitat and water quality
conditions in the lower Tar River. The ultimate goal of the study is to provide a sound basis for planning
and to establish the scientific basis for GUC’s future water withdrawal permit applications.
This Study Plan presents background information about the river, the study goals and objectives, and
describes the proposed approach, study elements, and many technical details. GUC is committed to a
broadly collaborative study process, involving federal and state resource and regulatory agencies in a
Technical Advisory Group (TAG). The primary objective of this Study Plan is to serve as a basis for
discussion and agreement during scoping with government agency participants on the issues, study
design, Study Area, data collection, and methods of analysis.
1.1 Scoping Phase, Study Plan, and Technical Advisory Group
The Tar River Flow Study scoping is a collaborative process involving GUC, the consultant team, and the
resource and regulatory agencies constituting the TAG. The process involves periodic interactive
meetings with TAG participants, during which information will be exchanged so that TAG members can
reach a consensus regarding key elements of the study including the scope, design, and methods to be
used for the Tar River Flow Study.
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The TAG is responsible for evaluating and commenting on the proposed study design and methods and
will be consulted on methods development, analysis, and data interpretation throughout the duration of
the study. Coordination meetings, the study plan, and technical memoranda are the primary tools for
communicating and commenting on proposed approaches, methods, and analyses. Periodic meetings will
be held with the TAG to provide a forum for open communication on the scoping process, study
execution, and evaluation of the study results. The consultant team will summarize meetings and
document decisions and outcomes for each meeting. Meeting summaries will be distributed to the TAG
for review and comment, and will be placed on the project web site.
Elements of the Tar River Flow Study that continue to be refined in collaboration with the TAG include the
following:
Study goals and objectives
Geographic boundaries and important time periods for modeling and assessment
Data collection and analysis methods
Baseline river hydrology
Hydrologic, habitat, and hydrodynamic modeling approaches and model requirements
Representative and important species and resources for the assessment
Water quality parameters, species habitat indices, and appropriate biological criteria for
modeling and assessment
Study sites
Scenarios for analysis of potential habitat and water quality effects resulting from
increased water withdrawals
Identification of maximum withdrawal amount
This Final Study Plan provides the framework for the entire Tar River Flow Study, though some elements
of study site selection, biological criteria, and modeling details will be decided and documented separately
in Technical Memorandums after final publication of the Study Plan.
1.2 Study Goals and Objectives
GUC has initiated the flow study for the Tar River to better understand instream flow issues and the
amount of water seasonally available to support long-term water management projects. These include
possible future expansion of the water treatment plant and conveyance system and additional interbasin
transfers necessary to meet CCPCUA requirements and GUC’s existing and future water agreements.
The main goals of the Tar River Flow Study are to:
Quantify the amount of Tar River water, or withdrawal rate, available for future water
supply.
Identify environmental constraints to future withdrawals.
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Provide the analysis and documentation needed to support future permit applications for
expansion of GUC’s WTP.
Address the movement of the fresh-saltwater interface and the potential impact to
GUC’s water supply and management operations.
Allow GUC to respond to a wide range of water management issues in the basin.
The Tar River Flow Study objectives are to:
Identify the capacity of the Tar River to provide drinking water for GUC customers while
still supporting other critical instream water uses.
Provide a method for quantifying the relationship between flow, instream habitat, and
aquatic resources.
Identify the potential habitat and water quality constraints on flow withdrawals.
Develop potential flow management strategies to balance water availability and
protection of aquatic resources in the Tar Basin.
Characterize risk to public water supply associated with upstream movement of the
freshwater/saltwater interface, especially under critical drought periods.
The following sections provide a brief overview of some of the factors and circumstances providing the
impetus and need for this study, as well as how the findings of this study will fit into the context of GUC’s
water supply plans and programs.
1.3 Coastal North Carolina Water Management
The Tar River Flow Study is, in part, a direct result of the need to reduce groundwater use in coastal
North Carolina, and to use surface waters, such as the Tar River, as an alternative source to
groundwater. Groundwater is the primary water source in the central coastal plain of North Carolina,
comprising 67 percent (64 mgd) of the total publicly supplied water (95 mgd) in 1997 (Waters et al.,
2003). In the late 1990s, concern over declining groundwater levels, decreasing well yield, and saltwater
intrusion into these aquifers prompted the Environmental Management Commission (EMC) to designate
the CCPCUA, illustrated in Figure 1-1.
In order to reverse declining water levels and saltwater intrusion in the important Cretaceous aquifers, the
EMC passed rules in 2002 for groundwater use in the fifteen-county CCPCUA. The CCPCUA rules
mandated a reduction in groundwater withdrawals to decrease use of the Cretaceous aquifers in coastal
North Carolina to sustainable levels. The rules target up to a 75 percent reduction in annual groundwater
withdrawals by 2018. These rules have created the need for increased reliance on surface water sources,
such as the Neuse River, Tar River, Contentnea Creek, and Northeast Cape Fear River, as well as the
development of alternative water management strategies. According to Waters et al. (2003), the most
promising water supply alternatives are aggressive water conservation, development of under-used or
alternate aquifers and rivers, and regionalized water supply systems. Progress to date among suppliers
has included the investigation of alternative groundwater sources, formation of regional entities to make
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use of surface water sources, and greater reliance upon interconnection and purchase agreements
(DWR, 2004).
Unlike the surficial unconfined aquifers, Cretaceous aquifers are deep confined aquifers that are not
directly connected with the Tar River. Groundwater contributions from surficial aquifers, however, do
supply a considerable portion of the Tar River’s baseflow within the Study Area. O’Driscoll et al. (2008)
found that groundwater from the shallow water table aquifer contributes about 60 percent of the Tar
River’s baseflow. O’Driscoll et al. (2008) also noted that “groundwater flux into and out of the channel
varied between the north and south sides of the river” as well as over time and distance with most river
reaches gaining groundwater and others losing groundwater.
1.4 Recent Drought and River Conditions
Recent droughts have placed additional pressures on water resources in the Tar-Pamlico Basin. Two of
the most significant droughts on record have occurred in the past decade. During the droughts of 1998 to
2002 (Weaver, 2005) and 2007, recorded stream flows reached record low levels and greatly concerned
surface water suppliers. Rocky Mount reduced minimum release in 2007 and flows in the Tar River
downstream of the reservoir went below 40 cubic feet per second (cfs). The reduced reservoir releases
were the result of very low storage remaining in the reservoir due to a gaging error that caused their
release protocol to function incorrectly. Tables and figures in Section 2.7 provide average annual and
median monthly Tar River Flows at Greenville for flow comparisons and context for withdrawal quantities.
Climate cycles and possible long-term climate change also have the potential to increase variability in
precipitation and runoff, and may pose a significant long-term risk to North Carolina’s water resources,
particularly along the coast. Investigators have suggested that the State of North Carolina may expect
drier summers, when water is needed the most, and wetter winters, when water needs are lower
(USGCRP, 2004). Riggs et al. (2008), suggest that Coastal North Carolina will see increased storm
damage, higher flood levels and sea level rise and associated additional saltwater encroachment (Riggs
et al., 2008). Drought conditions can also affect how far saltwater moves upstream in the Tar River. GUC
has been closely monitoring the location of saline water in the river and in 2007, saltwater moved
upstream to within ten miles of the GUC WTP intake.
The TAG requested that climate change be considered in the scoping of the Tar River Flow Study.
Discussions related to climate change have been held with the TAG including recognition of possible
increased drought conditions and sea level rise. GUC has agreed that this issue will be addressed in the
future when there is additional consensus on the extent of the issue and the State has developed a
policy. Addressing climate change through modeling is not within the scope of the current study, and
accurate and accepted estimates of climate change on which those analyses would rely have yet to be
developed. It is noteworthy that a recently published article in the Journal of the American Meteorological
Society (Seager et al., 2009) demonstrates that droughts in the Southeast U.S. in recent decades are no
more severe than those in the historical record. Seager et al. (2009) also indicate that the predictive skill
of climate models to simulate future precipitation and evaporation patterns in the Southeast is low,
especially during the summer.
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1.5 GUC Water Use and Interbasin Transfers (IBT)
GUC’s WTP (Figure 1-2) is permitted to withdraw 22.5 mgd from the Tar River for public water supply and
one of the few tidally influenced public water supply intake in North Carolina. Currently, GUC’s annual
average daily water use is approximately 10 to 11 mgd with peak withdrawal rates of about 16 mgd.
Typical of many municipal water intakes, GUC’s intake is a run-of-river withdrawal with limited water
storage capacity, and so could be vulnerable to saltwater intrusion.
In addition to providing for the needs of its service area, GUC serves as a regional water supplier and
currently provides additional water to the Towns of Farmville, Winterville, Bethel, Stokes, and
communities in Greene County. GUC’s ability to supply water to Winterville, Farmville, and Greene
County is contingent upon obtaining an Interbasin Transfer (IBT) Certificate. An Environmental
Assessment (EA) was completed to evaluate potential effects on the Tar River and the receiving basins
(Neuse River and Contentnea Creek) associated with the proposed IBT (ARCADIS, 2007). The draft IBT
Petition was submitted to the EMC in May 2009. The IBT Certificate was issued in the fall of 2010.
1.6 Water Supply Planning and Management
GUC is proactively planning for long-term water supply and drought management. This planning has
resulted in the development of water conservation measures that include three water conservation stages
based on river level and salt wedge location. GUC is also conducting several studies that may provide
options for improving their ability to continue to provide reliable public water supply during drought
conditions. The use of water in existing sand mine borrow pits to supplement flows in the Tar River during
critical periods is also being evaluated.
In order to reduce the likelihood of Tar River water levels falling below the existing intake levels, and to
provide backup intake structures if the existing intake pipes were to become inoperable, GUC is installing
an additional water intake structure just downstream of the existing WTP intake (Brown and Caldwell,
2008). The new intake structures will be at a greater river depth than the existing intake and will provide
GUC redundancy and flexibility in water withdrawal location. Another study underway by GUC is an
evaluation of the use of a portable and temporary, submerged, saltwater intrusion barrier to prevent saline
water at the bottom of the river from moving further upstream and potentially reaching the existing WTP
intake under extreme low-flow conditions. The results of the Tar River Flow Study will be useful for this
evaluation; the modeling completed in the Tar River Flow Study will provide a better estimate of the
potential for upstream saltwater intrusion under critical low-flow periods. Model results will predict the
potential location of the saltwater wedge, and identify conditions when the barrier may be needed.
Aquifer storage and recovery (ASR) is also a potentially important component of the overall water supply
management scheme being studied and implemented by GUC. ASR is the use of a well for the injection
of potable water into an aquifer where it is stored until it is withdrawn for use. When implemented, GUC’s
ASR program will inject treated drinking water into the Black Creek and Upper Cape Fear aquifers for
storage during times of excess supply. This water will be recovered later to help meet peak demands
when river flows are typically low. GUC has received an underground injection control (UIC) permit from
the State of North Carolina.
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2. Background and Current Conditions
The Tar River Basin covers approximately 5,500 square miles and is the fourth largest basin in North
Carolina (Figure 1-1). Only four basins including the Tar Basin lie entirely within the State’s boundaries.
The Tar River originates in north central North Carolina in Person, Granville and Vance counties and
flows southeasterly until it reaches estuarine waters near Washington and becomes the Pamlico River
Estuary (DWQ 2004).
The upstream portion of the river from its headwaters downstream to the City of Washington is the Tar
River. Downstream from the City of Washington, the Pamlico River is a tidal estuary that flows into the
Pamlico Sound. Most of the Tar River is fresh water with some transitional estuarine waters, whereas the
Pamlico River is entirely estuarine. Major tributaries of the lower Tar River include Chicod Creek, Grindle
Creek, and Tranters Creek (Figure 1-2). The transition from Pamlico River Estuary to Pamlico Sound
occurs near the junction of the Pungo and Pamlico Rivers.
2.1 Water Quality in the Lower Tar River Basin and Pamlico River Estuary
2.1.1 Water Quality Monitoring Results and Waterbody Impairments
Water quality is monitored at a number of locations in the lower Tar River Basin and in the Pamlico River
by either the DWQ or the Tar Pamlico Basin Association (TPBA) (Table 2-1). Table 2-1 also identifies the
water quality classification of each location and whether any water quality parameters significantly
violated water quality standards in the period from 2003 through 2007 (DWQ, 2008). The DWQ stations
are monitored monthly for field parameters (temperature, DO, pH, and conductivity), fecal coliform,
turbidity, and nutrients (ammonia, nitrite plus nitrate, total Kjeldahl nitrogen, and total phosphorus). Total
suspended solids (TSS) and metals are monitored quarterly. Metals sampling has been suspended by
DWQ since 2007. The TPBA stations are sampled twice monthly for field parameters and monthly for
nutrients, turbidity, TSS, and fecal coliform. TSS is not measured at the two TPBA Flat Swamp Stations.
GUC collects field parameters, turbidity, and salinity at multiple depths when monitoring the location of
the freshwater/saltwater interface.
The DWQ considers water quality in the Tar River at Greenville to be “good” and “supporting”
downstream of Greenville (DWQ, 2004). Despite drought conditions in 2007, none of the water quality
samples collected by DWQ or the TPBA from 2003 through 2007 on the mainstem of the Tar River at
US 264 bypass upstream of Greenville violated state standards. Data for the Tar River at Grimesland,
about 10 miles downstream of the Tar River at US 264, indicate that dissolved oxygen (DO)
measurements fell below the water quality standard for a single measurement, 4 mg/l, two percent of the
time and pH fell below 6 salinity units (SU) two percent of the time. Fecal coliform levels in the Tar River
at Grimesland exceeded 400 colonies per 100 ml in five percent of the samples.
For the Pamlico River Estuary at the NC 17 Bridge at Washington, six percent of the DO samples fell
below 5 mg/l (saltwater DO standard), 16 percent of pH measurements were below 6.8 SU, zero percent
of the pH readings were above 8.6 SU. Also at the NC 17 Bridge, eight percent of the chlorophyll a data
were greater than 40 µg/l and one percent of the turbidity data were greater than 25 NTU. None of these
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measurements, except the low pH values on the Pamlico River at Washington, were significant enough
for DWQ to classify the waterbody as impaired.
Table 2-1: Water Quality Monitoring Locations and Statistically Significant Water Quality Standards
Violations in the Lower Tar and Pamlico Rivers 2003 to 2007
Organization Station ID Site Name
Stream
Classification*
Date
Established
Statistically
Significant Water
Quality Standard
Violations (2003 –
2007 Data)**
Lower Tar Basin
DWQ O6200000 Tar R. at NC
222 NR
Falkland
WS-IV NSW 10/10/1973
DWQ O6205000 Conetoe Ck at
SR 1409 NR
Bethel
C NSW 8/1/1984 32% of pH (<6 SU)
and 42% of DO
samples (<4 mg/l)
DWQ O6240000 Tar R. at
US 264 Bypass
NR Greenville
WS-IV NSW 11/16/2005
DWQ O6450000 Chicod Ck at
SR 1760 NR
Simpson
C NSW 8/1/1984
DWQ O6500000 Tat R. at
SR 1565 NR
Grimesland
B NSW 7/5/1968
TPBA O6700000 Grindle Ck at
SR 1427 near
Bethel
C NSW 3/1/2007
TPBA O6798000 Grindle Ck at
US 264 at
Pactolus
C NSW 3/1/2007
TPBA O7000000 Flat Swamp at
SR 1159
(Third St) at
Robersonville
C SW NSW 3/1/2007
TPBA O7100000 Flat Swamp at
SR 1157 NR
Robersonville
C SW NSW 3/1/2007
DWQ O7300000 Tranters CK at
SR 1403 NR
Washington
C Sw NSW 10/10/1973
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Table 2-1: Water Quality Monitoring Locations and Statistically Significant Water Quality Standards
Violations in the Lower Tar and Pamlico Rivers 2003 to 2007
Organization Station ID Site Name
Stream
Classification*
Date
Established
Statistically
Significant Water
Quality Standard
Violations (2003 –
2007 Data)**
Pamlico River Estuary
DWQ O7650000 Pamlico R. at
US 17 at
Washington
SC NSW 7/6/1968 16% of pH samples
(<6.8 SU)
DWQ O7680000 Pamlico R. at
CM 16 NR
Whichard
Beach
SB NSW 3/7/1992 17% chlorophyll a
(>40 ug/l) and 14%
pH (<6.8 SU) values
DWQ O7710000 Chocowinity
Bay Above
Silas Ck NR
Whichard
Beach
SC NSW 3/7/1992 31% chlorophyll a
(>40 ug/l)
DWQ O787000C Pamlico R. at
Mouth of Broad
Ck NR Bunyon
Mid Channel
SB NSW 6/13/1974 21% chlorophyll a
(>40 ug/l) and 10%
pH (<6.8 SU) values
DWQ O787000N Pamlico R. at
Mouth of Broad
Ck NR Bunyon
N Shore
SB NSW 6/14/1989
DWQ O787000S Pamlico R. at
Mouth of Broad
Ck NR Bunyon
N Shore
SB NSW 5/18/1999
DWQ O8495000 Bath Ck at NC
92 NR Bath
SB NSW 2/14/1974 21% chlorophyll a
(>40 ug/l) and 10%
pH (>8.5 SU) values
DWQ O8498000 Pamlico R. at
CM 5 NR Core
Point
SB NSW 5/31/1989 23% chlorophyll a
(>40 ug/l)
DWQ O865000C Pamlico R. at
CM 4 NR Gum
Point Mid
Channel
SB NSW 5/18/1999 20% chlorophyll a
(>40 ug/l)
DWQ O865000N Pamlico R. at
CM 4 NR Gum
Point N Shore
SB NSW 5/18/1999
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Table 2-1: Water Quality Monitoring Locations and Statistically Significant Water Quality Standards
Violations in the Lower Tar and Pamlico Rivers 2003 to 2007
Organization Station ID Site Name
Stream
Classification*
Date
Established
Statistically
Significant Water
Quality Standard
Violations (2003 –
2007 Data)**
DWQ O865000S Pamlico R. at
CM 4 NR Gum
Point S Shore
SB NSW 5/18/1999
DWQ O9059000 Pamlico R. at
Hickory PT NR
South Ck
SA HQW NSW 8/10/1977
DWQ O9755000 Van Swamp at
NC 32 NR
Hoke
C Sw NSW 8/1/1984 75% pH (<4.3 SU)
values
DWQ O982500C Pamlico R.
Between
Mouths of
Pungo R. and
Goose Ck Mid
Channel
SA HQW NSW 5/18/1999
DWQ O982500N Pamlico R.
Between
Mouths of
Pungo R. and
Goose Ck N
Shore
SA HQW NSW 5/18/1999
DWQ O982500S Pamlico R.
Between
Mouths of
Pungo R. and
Goose Ck S
Shore
SA HQW NSW 5/18/1999
* C = Freshwaters protected for secondary recreation, fishing, and aquatic life, SC = Saltwaters protected for secondary
recreation, fishing and aquatic life, B = Freshwaters protected for primary recreation which includes swimming and all
Class C uses, SB = Saltwaters protected for primary recreation which includes swimming and all SC uses, NSW =
Nutrient Sensitive Waters, Sw = Swamp Waters, SA = Suitable for commercial shellfishing and all other tidal saltwater
uses, HQW = High Quality Waters which are rated as excellent based on biological and physical/chemical
characteristics
**Statistically Significant Water Quality Standards Violations as determined by DWQ in Tar-Pamlico River Basin
Ambient Monitoring Report June 2008. Standards exceedances are considered significant if there is 95% or greater
confidence that a standard has been violated by 10% or more samples.
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Table 2-2 identifies the waterbodies within the Study Area that are listed on North Carolina’s 2008 Draft
303(d) list of impaired waterbodies. The most common reasons for impairment of these waters are when
more than ten percent of the monitoring results violate water quality standards, or biological monitoring
data indicates a negatively affected benthic or fish population. A segment of the mainstem, approximately
13 miles long, beginning at GUC’s WTP intake is currently impaired for fish consumption due to elevated
concentrations of mercury in fish tissue.
Table 2-2: Draft 2008 303(d) Impaired Waterbody List for Lower Tar and Pamlico River Basins
Waterbody
Impairment Segment
(Assessment Unit)
Impaired Use and
Parameter of Interest
303(d)
Listing Year
Lower Tar Basin
Tar River From GUC WTP Intake to point
1.2 miles downstream of Broad Run
Fish Consumption: Mercury in
Fish Tissue
2006
Chicod Creek From Source to Tar River Aquatic Life: Benthic sampling
indicates impaired biologic
health
1998
Hendricks Creek Source to Tar River Aquatic Life: Benthic sampling
indicates impaired biologic
health
2008
Cokey Swamp Source to Dickson Branch Aquatic Life: Benthic sampling
indicates impaired biologic
health
2006
Bynum’s Mill
Branch
Source to Town Creek Aquatic Life: Benthic sampling
indicates impaired biologic
health
2006
Conetoe Creek Source to 1350 meters north of
NC 42 (two assessment units)
Aquatic Life: Benthic sampling
indicates impaired biologic
health
1998
Conetoe Creek From Crisp Creek to Pitt County
SR 1404
Aquatic Life: Standards
Violation Low pH
2008
Ballahack Canal Source to Conetoe Creek Aquatic Life: Benthic sampling
indicates impaired biologic
health
2006
Greens Mill Run Source to Tar River Aquatic Life: Benthic sampling
indicates impaired biologic
health
2008
Pamlico River Estuary
Pamlico River
Estuary
G11 chlorophyll a 1996
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Table 2-2: Draft 2008 303(d) Impaired Waterbody List for Lower Tar and Pamlico River Basins
Waterbody
Impairment Segment
(Assessment Unit)
Impaired Use and
Parameter of Interest
303(d)
Listing Year
Pamlico River
(Upper Segment)
From NC 17 Bridge to line
0.75 miles downstream of Runyon
Creek and 0.5 miles downstream of
Rodman Creek
Aquatic Life and Recreation:
Standards Violation Low pH
and Elevated Enterococcus
2008
Pamlico River
(Upper Segment)
From the line 0.75 miles
downstream of Runyon Creek and
0.5 mile downstream of Rodman
Creek to 0.65 miles downstream of
Chocowinity Bay.
Aquatic Life: Standards
Violation Low pH
2008
Pamlico River
(Blounts Bay
Segment)
From 0.65 miles downstream of
Chocowinity Bay to line at east
mouth of Blounts Bay
Aquatic Life: Standards
Violation chlorophyll a
2008
Pamlico River
(Pamlico Bath
Segment)
From line at east mouth of Blounts
Bay to west mouth of Durham
Creek
Aquatic Life: Standards
Violation chlorophyll a
2008
Pamlico River
(Middle Segment)
From west mouth of Durham Creek
to line from Huddy Creek to Saint
Claire Creek
Aquatic Life: Standards
Violation chlorophyll a
2008
2.1.2 Relationship Between Inflows and Pamlico River Estuary Water Quality
The Pamlico River Estuary is considered impaired for chlorophyll a and DO and a Total Maximum Daily
Load (TMDL) was approved for total nitrogen and total phosphorus in 1995. Although nutrient inputs to
the estuary have decreased significantly over the last decade, the estuary still experiences episodes of
low DO (DWQ, 2003; DWQ, 2004) and a recent nutrient trend analysis indicates that nutrient
concentrations are increasing (DWQ, 2009).
In 2003, DWQ conducted an analysis of water quality data for the Tar River at Grimesland to look for
trends in nutrient loading from 1991 through 2002. The DWQ used a nonparametric statistical analysis,
the seasonal Kendall’s test, to identify trends in nutrient loading. After removing the variability in nutrient
concentrations associated with flow, DWQ found that there was an 18 percent reduction in total nitrogen
and a 33 percent reduction in total phosphorus concentrations at the Grimesland station from 1991 to
2002 (DWQ, 2003). In 2009, DWQ repeated this analysis for 1997 to 2008 data at the same station. This
analysis indicated that there was a 17 percent increase in total nitrogen and a 22 percent increase in
median total phosphorus (DWQ, 2009). The Pamlico River Estuary has continued to have problems with
summertime fish kills. The number of reported fish kills peaked in 2001 at 23 with significantly fewer fish
kills observed since then (eight in 2002, six in 2003, two in 2004, one in 2005, one in 2006 linked to high
water temperatures and bottom hypoxia, and zero in 2007). The number of fish kill events increased in
2008. According to DWQ’s 2008 Fish Kill Monitoring Report:
Salinity and DO profiles of the Pamlico River in early August [2008] reveal an infusion of
high salinities, water column stratification, and an apparent drop in DO levels at the
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location of the...kill. Conditions such as these have been repeatedly documented prior to
and during many events in the Neuse and Tar/Pamlico estuaries (DWQ 2008b).
Nutrient delivery and flows influence estuarine production. Certain inflow levels and meteorological
conditions produce the development of stratification in the estuary and the occurrence of low DO levels
particularly in the bottom layer of the water column next to the sediments. Relationships among bottom-
water DO, vertical stratification, and the factors responsible for stratification and destratification in this
shallow, low tidal-energy estuary were studied using a 15-year set of biweekly measurements (Stanley
and Nixon, 1992). Stanley and Nixon (1992) found that:
Hypoxia develops only when there is both vertical water-column stratification and warm
water temperatures (≥ 15°C). In July 1991, 75 percent of the DO readings were < 5 mg/l
and one-third were < 1 mg/l. Severe hypoxia occurred more frequently in the upper half
of the estuary than near the mouth. Both the time series data and correlation analysis
indicated that stratification events and DO levels are tightly coupled with variations in
freshwater discharge and wind stress. Stratification can form or disappear in a matter of
hours, and episodes lasting from one to several days seem to be common. Estimated
summertime respiration rates in the water and sediments were sufficient to produce
hypoxia if the water was mixed only every 6 to 12 days (Stanley and Nixon, 1992).
Stanley and Nixon saw no trend toward lower bottom water DO in the Pamlico River Estuary over the
15 years of data that their study evaluated. Lin et al. (2008) found a similar relationship between estuary
stratification and low DO levels. East Carolina University (ECU) and the United States Geological Survey
(USGS) conducted a 7-year water quality study on the Pamlico River system. ECU collected water quality
data bi-weekly at eight stations from 1997 to 2003. The USGS collected continuous water quality data at
three stations between 1999 and 2003. Analyses of the DO, salinity, temperature, and nutrient data were
published by Lin et al. (2008). Lin et al. (2008) used the water quality data collected by ECU and USGS to
associate estuary stratification levels and water quality conditions with patterns of freshwater discharge
and wind mixing events. Strong salinity stratification appears to be associated with higher freshwater
inflows and fewer wind mixing events. The greater nutrient loading and strong salinity stratification that
occur during high discharge years is associated with higher average chlorophyll a concentrations.
Chlorophyll a concentrations were significantly lower and DO concentrations higher in years with low
inflows.
Hypoxic conditions were observed mostly in the upper to middle Pamlico estuary, but the frequency of
hypoxic events varied between years. During June to October 1997–1999 bottom water hypoxia
(DO < 2 mg/l) was found in 8.7 percent of the observations. By contrast, during June to October in 2001
through 2003, 37.9 percent of the total measurements had DO concentrations less than 2 mg/l. The more
frequent and/or prolonged hypoxic conditions during 2001 to 2003 were closely associated with stronger
salinity stratification and greater loadings of nutrient and particulate matter (Lin et al. 2008).
2.2 Hydrology of the Tar River
The Tar River system is largely unregulated; total reservoir storage in the basin equals only a small
percentage of the annual flow of the river. The drainage area of the Tar River gage at Tarboro (Gage
No. 02083500) is 2,186 square miles. The drainage area of the Tar River at Greenville (Gage
No. 02084000) is 2,660 square miles. Historic flows in the Tar River were recently evaluated and
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characterized in detail by ENTRIX (2008) as a part of GUC’s recent application for an interbasin transfer
(ARCADIS, 2008). A hydrologic model was developed for the lower Tar River to predict river flows under
current and future water usage scenarios. The relationship between available flow records for the Tar
River at Greenville and the Tar River at Tarboro was estimated using hydrologic and statistical methods.
The analysis was based on available USGS flow records from the Tar River at Tarboro and the Tar River
at Greenville (USGS, NWIS). The flow record at the Tar River at Tarboro gage provides a long-term
record (1931 to 2007) of hydrologic conditions in the river, representing the majority of the entire drainage
area of the basin. The flow records at Tarboro are also the best available data for predicting downstream
flows at Greenville, where a more limited period of record is available (1997 to 2007). Monthly and annual
flow exceedance values at the USGS gage at Greenville, based on the hydrologic modeling, are provided
in Appendix A.
2.3 Tar River Water Management and Previous Instream Flow Studies
There have been no comprehensive studies of the instream flow or freshwater inflow needs of the Tar-
Pamlico River or Pamlico Sound. Flow management in the Tar-Pamlico River Basin to date has been
accomplished largely on a case-by-case basis in response to proposed projects, National Pollutant
Discharge Elimination System (NPDES) permits, stipulation of flows under the NC Dam Safety Act, and
one habitat-based instream flow study in the Upper Tar River near Louisburg. With the exception of
coordination of permits near the City of Rocky Mount for water supply and assimilative capacity, there are
no basinwide or stream segment-specific flow requirements.
2.3.1 Tar River at Louisburg
In 1995, Division of Water Resources (DWR) completed a flow study in the Tar River in conjunction with a
proposal by the Town of Louisburg to increase their withdrawal from the Tar River from 2 mgd to 3 mgd.
The study evaluated stream flows and aquatic habitat in the Tar River at a site just upstream of the
Louisburg wastewater treatment plant (WWTP) discharge and downstream of Fox Creek using a series of
models and procedures referred to as the Instream Flow Incremental Methodology (IFIM). The analysis
involved habitat mapping, hydraulic measurements and modeling, habitat preferences for aquatic species
including redbreast sunfish, creek chub, and the Tar River spinymussel, flow time series and habitat time
series. When the simulated physical conditions were merged with habitat preferences, the result was a
habitat versus flow relationship for the life stages of each species.
The habitat versus flow relationship was merged with stream flows to yield a record of the habitat
available over time for each species. This habitat record was analyzed to determine target amounts of
habitat, which should be maintained to prevent significant effects and to compare different project
scenarios to the pre-project baseline conditions. The pre-project flow record was modified to generate
different project flow and habitat scenarios. The first scenario involved a constant withdrawal of 3 mgd
"skimmed" from the pre-project flow record without any restrictions. Two other alternatives were
examined, including one alternative with an 11.5 cfs (or 7.5 mgd) minimum flow requirement.
A review of the effect of the 3 mgd withdrawal scenario on the habitat index for the life stages of all
species indicated that the greatest effects were on habitat available for the Tar River spinymussel. DWR
therefore limited the evaluation of other scenarios to this species. A constant, unrestricted withdrawal of
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2 mgd was modeled to evaluate the habitat effects under the existing maximum intake capacity. A third
scenario, referred to as "Alternative #1," was also modeled. This involved a constant withdrawal of 2 mgd,
with additional withdrawals up to a total of 3 mgd permitted as long as a minimum flow of 11.5 cfs was
maintained below the intake. The largest effect was a 10 percent loss in habitat occurring during July and
September under two scenarios. Because of the relatively small difference in withdrawal, and the
infrequent occurrence of such low flows, both alternatives had virtually the same effect on downstream
flows and habitat; however, Alternative #1 ensured that the 7Q10 flow for assimilation of the WWTP
discharge was maintained.
In conjunction with a 3 mgd withdrawal, agencies requested that flow between 9.0 and 11.5 cfs be
maintained at the stream gage just downstream of Highway 401. The Town of Louisburg later decided not
to pursue the expanded water withdrawal (Jim Mead, personal communication, 2007).
2.4 Tar River at Rocky Mount
Water resource developments on the Tar River near Rocky Mount include two dams, two water
withdrawals, and one wastewater treatment plant. The Tar River Reservoir, a 1,860-acre reservoir on the
Tar River, southwest of Rocky Mount, was completed in 1971, and is used for municipal water supply,
fishing, recreation, and other uses. Just downstream is the Rocky Mount Mills Dam, an unlicensed
hydropower facility. The Town of Rocky Mount also has a water intake in the small reservoir formed by
the Rocky Mount Mills Dam. Rocky Mount’s WWTP discharge to the river is approximately 6.3 river miles
downstream of the Mill Dam.
In the NC Dam Safety Act permit for the Tar River Reservoir Dam, there is a requirement for a continuous
downstream flow release (or minimum flow) of 80 cfs (DLR, 1999). The 80 cfs minimum flow was based
on flow rates needed to maintain assimilative capacity in the river for the Rocky Mount WWTP discharge
with some excess for future growth (Jim Mead, personal communication, 2007). During 1999, very low
flow occurred in the Tar River and negotiations between Rocky Mount and DENR resulted in protocols
that allowed Rocky Mount to reduce the minimum flow down to 60 cfs with certain conditions. A reservoir
management strategy and model were developed that identified different water conservation measures
and drought stages. The reservoir model identified when to initiate each drought stage based on the
amount of water in the reservoir and the probability that water levels will drop below specified heights.
The following operating conditions for Rocky Mount’s Tar River Reservoir were made permanent in 2002:
Stage I conditions involve a reduction in the minimum release from 80 to 75 cfs.
Stage II conditions (when the reservoir elevation is at or below 120 feet) allow a
reduction from 75 to 70 cfs.
Stage III conditions (the reservoir operations model indicates for two consecutive weeks
that the reservoir elevation will decrease to 115 feet or lower) allow a reduction from
70 to 60 cfs. Stage III requires Rocky Mount to impose mandatory water conservation
measures.
In addition to the minimum flow release requirements, the reservoir level must be
managed so that it does not fluctuate by more than one vertical foot between April 15
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and May 15 of each year. This management measure was put in place to protect
favorable fish spawning conditions.
Winter reservoir drawdowns are allowed under current management conditions.
In 1993, the City of Rocky Mount was given permission to allow the minimum release to go to 65 cfs and
in 1995, the minimum release requirement returned to 80 cfs. In 1999, Rocky Mount was allowed to
decrease reservoir releases to 60 cfs; and a 2002 letter from the Division of Land Resources (DLR)
reaffirmed the operating Stages (reservoir management Stages I through III) approved in 1999. Due to
drought conditions later in 2002, Rocky Mount was allowed to adopt a minimum release of 50 cfs at the
reservoir, but had to maintain 60 cfs at the WWTP discharge location downstream of Rocky Mount Mills
Dam. In 2007, reservoir releases below 50 cfs occurred as a result of the very low storage remaining in
the Tar River reservoir. This low storage volume was due to a gaging error that caused the Rocky Mount
reservoir model release protocol to function incorrectly.
Rocky Mount Mills Dam, under the NC Dam Safety Act, has a continuous, instantaneous minimum flow
requirement of 60 cfs in the natural channel directly below the dam, the “bypass reach” (DLR, 2003). The
dam is also required to have a calibrated staff gage on the dam crest or in the bypassed reach to monitor
the flow requirement.
The Rocky Mount WWTP discharge requirements are based on a minimum river flow of 60 cfs. The
WWTP permit conditions are based on the 60 cfs minimum release. DWQ may use the minimum release
rather than the 7Q10 for assimilative capacity determinations.
2.5 Influence of Tar River Reservoir Operations on Tar River Flows
The normal and low flow releases from Rocky Mount’s Tar River Reservoir are reflected in the historical
gage data. Reservoirs such as Rocky Mount’s often augment low flows in rivers because they store water
from higher flow periods and release it over extended lower flow periods. Flow duration curves for the
pre-reservoir period of record and post-reservoir period of record were developed and evaluated to
determine if this effect was present in the Tar River at Greenville flow record (Figure 2-1). In this case, the
Tar River Reservoir does not augment low flows observed at Greenville. It appears that the opposite is
true; however, the difference in pre-reservoir low flows and post-reservoir low flows is limited. This small
difference may be attributed to climatic or other natural differences between the two time periods and may
be within the error of the regression model used to estimate much of the period of record for the
Greenville gage.
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2.6 Tidal Influences in the Lower Tar River
The lower Tar River is influenced by tides to a point upstream of the USGS gage at Greenville. The
influence of tides on the Tar and Pamlico Rivers and their connection to the upper portion of Pamlico
Sound will factor significantly into the design of the flow study. The amount of tidal influence is variable
and depends on wind, tidal phase, and river flow (USGS, 2008; GMA, 2003). Tidal influence in the Tar
River at Greenville is more pronounced during low-flow periods. Figure 2-2 illustrates the daily range in
Tar River discharge levels at Greenville under different flow conditions and the greater relative influence
of tides at low flow levels (average daily discharge of about 150 cfs). The tidal influence is greater when
the daily average discharge is 150 cfs than 1,500 cfs or 15,000 cfs, (Figure 2-2). Instantaneous flow in the
Tar River at Greenville ranges from negative 570 cfs to 656 cfs in a single day. During low flow periods,
tidal fluctuations can significantly influence flow and aquatic habitat conditions. Tidal fluctuations will be
considered in the flow study.
Monitoring conducted by GUC in 2002, 2007, and 2008 has demonstrated that the salt wedge moves
further upstream during low flow conditions than during high flow conditions. Meteorological conditions,
especially wind, also play an important role along with downstream flow volume in determining the
location of the salt wedge. Wind strongly influences the magnitude of the tidal fluctuations in the lower Tar
River and promotes mixing and circulation within the Pamlico River and the Pamlico Estuary. Pamlico
Sound is a shallow estuarine system that is strongly influenced by wind-driven circulation and mixing
(Lin et al., 2008; Stanley and Nixon, 1992).
Under certain common conditions, the tidal influence determines flow and habitat conditions in the lower
Tar River. There are times when tidally-influenced flows in the river moving upstream and downstream
past the Greenville gage are much greater than the net amount of flow being delivered from upstream.
Groundwater Management Associates, Inc. (GMA) (2003) demonstrated that during low flow conditions
(about 100 to 300 cfs), instantaneous flow measurements for the Tar River at Greenville varied by over
600 cfs or more during a single day in response to changing tides. Flow was negative (upstream) and
positive (downstream) in response to tides, and were accompanied by a change in stage (water level) of
about one foot or less. Although the net downstream flow was negative, the water level at Greenville did
not substantially change (ENTRIX, 2008; GMA, 2003).
The GMA 2003 investigation, ENTRIX 2008, and ARCADIS 2008 reports, are the only available studies
that directly address the hydrology of the Tar River, tidal influences on Tar River flow and stage at
Greenville, and the potential effects of withdrawals. The GMA report characterized the magnitude of the
tidal influences, the volume of freshwater in the tidal portion of the Tar River in 2002 during a period of
very low flows, and average daily flow. The location of the salt wedge or brackish water boundary in the
lower Tar River was used to calculate the size of the freshwater volume in the lower river. The potential
effects of GUC WTP withdrawals were evaluated based on the freshwater volume of the lower river
during low flow conditions.
GMA concluded that tidal influences are a major factor in affecting the hydrology of the Tar River at
Greenville during low flow conditions. During periods of low flow, water levels at Greenville and the
Pamlico River at Washington are very similar. The tidal oscillations were found to be much higher than
net river flows under low flow conditions.
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The GMA study also concluded that the river downstream of Greenville could be thought of as a large
reservoir, which has the potential for holding over one billion gallons of water between the GUC intake
and the brackish water boundary of the Pamlico River. GMA concluded that even if there were no flow in
the Tar River, the City would have to pump for almost three days at the maximum permitted rate
(22.5 mgd) to produce an upstream migration of the brackish water boundary equal to that which
occurred naturally due to tides on a single day in August 2002. GMA calculated that with zero net
downstream flow and at GUC’s current peak pumping rate (then 16.4 mgd), it would take 62 days to
deplete the freshwater in the Tar River downstream of the intake. The study was a simple examination of
flows, stages, and volumes, but did not involve any predictive modeling that would allow the consideration
of other factors such as weather and wind effects, other flow conditions, different withdrawal rates, or
effects on water quality or ecological conditions. The ecological consequence of changes in the location
of the salt wedge or changes in other water quality parameters was not considered in the GMA report.
2.7 Previous Hydrologic Analysis of GUC Withdrawals
Extensive hydrologic analyses were conducted by ENTRIX as part of the EA (ARCADIS, 2008) evaluating
the potential effects of proposed interbasin transfers. To support the development of an EA for GUC’s
proposed transfer of water from the Tar River to the Neuse River Basin and the Contentnea Creek Basin,
a spreadsheet based hydrologic model was developed for the lower Tar River to predict river flows under
current and future water usage scenarios. This work is documented in the TM, “Analysis of Greenville
Utilities Commission’s Proposed Interbasin Transfer Withdrawals on Tar River Flows at Greenville, North
Carolina”, October 2007, revised April 2008 (ENTRIX, 2008).
The model was designed to evaluate the effect of GUC’s proposed IBT withdrawals on current and future
flows at Greenville. The model was based on available USGS flow records from the Tar River at Tarboro
and the Tar River at Greenville (USGS NWIS, 2008). The relationship between available flow records for
the Tar River at Greenville and the Tar River at Tarboro was estimated using hydrologic and statistical
methods. This relationship was used to generate a long-term flow record at Greenville, which was then
used in a spreadsheet model to estimate future flows at Greenville with and without the proposed IBT.
The model quantifies the relative differences in flow associated with current and projected water usage
and discharges. Tidal influences were not simulated in this model. Days may occur when the tidal
influence creates a net downstream flow of zero or a net upstream flow (“negative” flow).
The model was used to evaluate resulting flow in the river at two locations. The first location was the
USGS gage at Greenville, which is downstream of GUC’s water treatment plant intake, but is upstream of
GUC’s WWTP discharge (Figure 2-3). The 7.7 mile portion of the Tar River between the WTP intake and
the WWTP discharge is the reach that will have the lowest flows and is the reach of the Tar River most
affected by future GUC withdrawals. This 7.7 mile area between the WTP withdrawal and WWTP
discharge is proposed as the primary study area (Freshwater Tidal Segment) of the Tar River Flow Study.
The second location where flows were evaluated is the Tar River downstream of the GUC WWTP
discharge (Figure 2-3). This reach downstream of the Greenville WWTP will also be affected by upstream
water uses, but less so because the flows in that reach include the discharge from the WWTP. The flow in
this reach goes to the Pamlico River and Pamlico Sound. Both reaches are tidally influenced, especially
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at low flow. An assessment of standard flow statistics needs to consider that daily tidal fluctuations at
Greenville can be much greater than the net downstream flow of the Tar River arriving at Greenville.
The following scenarios were evaluated in the model at both locations:
Current flows with No IBT.
Current flows with 2030 Average Day IBT.
Current flows with 2030 Maximum Withdrawal IBT.
Predicted 2030 flows with No IBT.
Predicted 2030 flows with 2030 Average Day IBT.
Predicted 2030 flows with 2030 Maximum Withdrawal IBT.
Flow statistics for the Tar River were generated for each scenario at both river locations. In order to
evaluate the seasonal flow fluctuations, the statistics were based on daily average flow by month. A range
of flow statistics and comparisons were developed for each scenario in order to quantify and demonstrate
the effect of the proposed IBT withdrawals on current and future conditions at both Tar River locations.
The effects of projected growth and different IBT scenarios were evaluated by examining changes in
these statistics. Finally, a discussion of the importance of the influence of tidal fluctuations on the Tar
River at Greenville was provided to explain the potentially ameliorating effect of tides at Greenville on
river flow estimated for GUC’s proposed IBT.
Under some of the conditions where it was estimated that withdrawals and interbasin transfers have a
small effect on net downstream river flow, tidal influences may be greater than the net amount of flow
being delivered from upstream. The tidal influence during critically low periods may substantially
ameliorate the effects of IBT withdrawals. The tidal influence at Greenville was cited by GMA (2003) as
one factor that provides downstream aquatic habitat protection during low flows in the vicinity of
Greenville. However, this conclusion will be examined in much greater detail in this study.
The year-to-year variability in Tar River flows is illustrated in Figure 2-4, which identifies the average
annual flows for the period of record. Figure 2-5 identifies the range of flow percentiles observed in the
Tar River at Greenville based on the modeled data (1931 to 2007). The median monthly flows for the
period of record are represented by the 50 percent line (which means that the average monthly flow was
at or below this flow 50 percent of the time). To provide context for flow comparisons and relative
withdrawal quantities, Table 2-3 identifies the median annual and monthly flows for the Tar River at
Greenville for the period of record (1931 to 2007). As is typical of North Carolina streams and rivers, flows
are highest during winter and spring (median monthly flows range from about 1,700 cfs to 3,600 cfs) and
lowest during summer and fall (median monthly flows range from about 600 cfs to 1,300 cfs). Flows less
than 600 cfs can occur at any time of the year, but are far less frequent during December through April
(less than 5 percent of the time) than during May through November (occurring about 5 – 50 percent of
the time, depending on the month).
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Table 2-3: Median Tar River Flows at Greenville (1931-2007)
Annual or Monthly Value Median Flows at Greenville (cfs)
Annual 2,524
January 3,776
February 4,649
March 4,840
April 3,589
May 2,058
June 1,596
July 1,481
August 1,641
September 1,536
October 1,320
November 1,578
December 2,345
Monthly average and maximum withdrawals by GUC (based on 2005 records) are a relatively small
portion of median daily flows in the Tar River (Figure 2-6). The maximum permitted GUC withdrawal
ranges between approximately 0.6 percent and 4.2 percent of the mean monthly flows. However, during
low flow conditions, these GUC withdrawals are a larger portion (up to almost 15 percent) of the total river
flow (Figure 2-7). Figure 2-8 presents the monthly average and maximum GUC withdrawal amounts
(based on 2005 records) expressed as a percentage of the summer 7Q10 flow (108 cfs). The average
monthly GUC withdrawals range between 14 percent and 16 percent of the 7Q10 flow and the maximum
monthly withdrawals range between 16 percent and 22 percent of the 7Q10 flow. These percentages will
become larger in the future as flow withdrawals in the rest of the basin increase. It will be important for the
Tar River Flow Study to address those portions of the flow regime that may be affected by proposed
future withdrawals. The hydrologic analysis study, performed done in support of the EA for the IBT,
provides additional flow statistics.
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3. Technical Approach
This section of the study plan describes the general study area of the Tar River and defines analyses that
will be conducted in individual study area segments. The technical approach to modeling and analyses is
also described. Further details of the technical analyses and modeling are provided in subsequent
sections.
This section also describes the link between the study objectives and the technical analyses. Specific
analyses will be conducted based on the potential effects of GUC withdrawals and consumptive uses on
the river’s flow regime. The practicable scope of the analyses and the geographic extent of the Study
Area and its segments are identified.
3.1 Study Area
The Tar River Flow Study includes four segments of the Tar River in the general vicinity of Greenville.
These include the Freshwater Non-tidal, Tidal Freshwater, Estuarine Transition, and Pamlico River
Estuary segments (Figure 3-1). These segments have dynamically changing boundaries. The boundaries
are influenced by river discharge, tidal cycles, and meteorological conditions. Near Greenville, the river is
freshwater, but within the Estuarine Transition Segment, salinity levels typically increase in a downstream
direction to Washington. At Washington, the Tar River becomes the Pamlico River Estuary.
River flow, tidal cycles, and meteorological conditions (wind speed, direction, and barometric pressure) all
affect the boundaries of these segments. Superimposed upon these changing boundaries are the GUC
water infrastructure facilities, including the WTP and the WWTP. Together, the river tidal and salinity
boundaries, and the infrastructure elements define the logical study segments (Figure 3-1). Table 3-1
describes the different segments, their characteristics, and the analyses that will be conducted in each.
Table 3-1: Tar River Flow Study Segments
Tar River Segment Attributes and Approach
Tidal Freshwater
Segment
Primary study area
WTP to WWTP
Subject to total GUC withdrawals
Lowest net freshwater flows may occur in this segment due to withdrawals
Habitat conditions influenced by tidal fluctuations, especially at low flows
Estuarine Transition
Segment
Secondary study area.
WWTP to Washington.
Tidally dominated; fluctuating salinity levels and salt wedge location.
Subject to consumptive use, not total withdrawals.
Pamlico River Estuary
Segment
Tertiary study area
Estuarine circulation dominated; freshwater inflows can be important
Periodic summer bottom anoxia
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3.2.1 Non-Tidal Freshwater Segment
This segment of the Tar River extends from the approximate upstream extent of tidal influence, upstream
into the Tar River Basin. The upstream extent of tidal influence in the Tar River is variable. This boundary
occurs under most conditions in the vicinity of the US 264 Bypass Bridge, approximately 1.75 miles
upstream of the GUC WTP intake. Flow and water levels in this segment of the river are affected by river
discharge, but are not measurably affected by withdrawals at the WTP. This assumption will be evaluated
during the study using the hydrodynamic model.
A basin hydrologic model (Refer to Section 4, Tar River Basin Hydrologic Model) will be used to quantify
the flows delivered to the Non-Tidal Freshwater Segment of the river. Since this segment of the river is
not influenced by GUC withdrawals, no other habitat or water quality analyses are proposed.
3.2.2 Tidal Freshwater Segment
The Tidal Freshwater Segment extends from the GUC water intake located three miles upstream of the
USGS gage in downtown Greenville to the WWTP discharge location approximately eight miles
downstream of the water intake (Figure 3-1). This section of river experiences the effects of the total
withdrawals made at the WTP. At the downstream end of this segment, the WWTP discharge, the
majority of the water withdrawn at the WTP is returned to the river.
The downstream boundary of the Tidal Freshwater Segment of the river is highly variable. The boundary
can vary from approximately ten miles downstream of the WTP to near Washington, depending on river
flow. GUC monitors the saltwater boundary and during drought conditions in 2007, it moved as far
upstream as 10 miles from the WTP intake. The WWTP discharge is the location at which the majority of
the water withdrawn is returned to the river. Downstream of this point, the potential effects of GUC water
withdrawals are limited to GUC’s consumptive water use (e.g., withdrawals at the WTP minus discharges
at the WWTP).
The Tidal Freshwater Segment is different from the upstream riverine habitat, primarily due to tidally
induced physical processes, such as the increased residence time of the water, oscillating water levels,
and reversing current velocities and directions (Schuchardt et al., 1993; Wagner and Austin, 1999). This
eight-mile segment of the Tar River experiences the greatest quantity of flow depletion due to GUC
withdrawals and may have the lowest rate of net downstream freshwater flow. The term “net downstream
freshwater flow rate” is introduced at this point to address the fact that flow rates, due to tidal cycles in the
Tidal Freshwater Segment (upstream-downstream tidal flux), can be greater than the rate of flow being
delivered from the Non-Tidal Freshwater Segment of the river.
3.2.3 Transitional Estuarine Segment
The Transitional Estuarine Segment extends from the WWTP discharge, downstream to Washington
where the Tar River becomes the Pamlico River. Based on water quality data collected by the State of
North Carolina and GUC, this river segment is generally fresh water in its upper reaches and is
progressively more brackish or saline with distance downstream to Washington. Salinity levels and the
location of the salinity wedge are highly variable, depending on river flow, tidal cycles, and meteorological
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conditions (e.g., wind speed and direction, barometric pressure, etc.). This river segment is subject to
consumptive use only (e.g., withdrawals at the WTP minus returns at the WWTP).
3.2.4 Pamlico River Segment
The City of Washington, North Carolina is generally regarded as the boundary between the Tar River and
the estuarine Pamlico River. At this point, the Pamlico River contains higher salinity waters than the Tar
River, except under very high river flow conditions. This section of the river is subject to consumptive use
only (withdrawals at the WTP minus returns at the WWTP), WWTP discharges, and additional tributary
flow. The effects of any proposed withdrawal or change in withdrawal by GUC on the hydrodynamics of
the river in the Pamlico River Segment are greatly attenuated downstream of the NC 17 Bridge due to the
influence of the tidal dynamics of Pamlico Sound.
3.2 Technical Approach
The technical approach to the Tar River Flow Study is driven largely by two of the study objectives:
Quantify the relationship between river flows, instream habitat, and aquatic resources.
Identify potential water quality and habitat constraints on flow withdrawals.
The remaining study objectives (identifying available capacity, developing flow management strategies,
and characterizing risk during droughts) are also considered, but the basis for these objectives rely on a
synthesis of the results of the technical analysis.
Instream habitat and potential constraints on flow withdrawals, as defined in this study, may include
hydrodynamic conditions (e.g., river stage, depths, velocities, diffusion, and circulation), physical habitat
structure, temperature and water quality conditions, and salinity regime. The Tar River Flow Study design
primarily addresses the effects of flow rate manipulations, water withdrawals, but will also address the
analysis of the effects of water withdrawal on water quality. However, this study will not address the
detailed effects of future changes in WWTP discharge, its mixing zone, or provide a regulatory analysis
for a plant expansion or discharge permit modification.
3.3 Study Components and Models
The major activities that will be conducted in each of the four segments within the Study Area are
provided in Table 3-2. The Tar River Flow Study occurs within the context of a complex environmental
setting (e.g., riverine-tidal-estuarine transition), so a number of hydrologic, hydrodynamic and biological
approaches will be used address the issues and potential effects of greater water withdrawals.
All four segments of the Tar River will be addressed at various levels of detail, but the Tar River Flow
Study will focus principally on the potential effects of total flow withdrawals in the Tidal Freshwater (WTP
to WWTP) Segment and the potential effects of consumptive water use on the Estuarine Transition
Segment.
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In the Non-Tidal Freshwater Segment of the river, existing and future flows will be simulated using a basin
hydrologic model, the Water Evaluation and Planning System (WEAP) Model and previous modeling and
analysis of Tar River flows at Greenville (ENTRIX, 2008). WEAP is a comprehensive, straightforward
basinwide water planning and management tool that supports water resources planning efforts (ENTRIX,
2009). The WEAP modeling will be set up for the Tar River Basin based on historical flow data sets. The
model will be calibrated to enable simulation of existing and future inflow scenarios, so that flows arriving
at the upper end of the Study Area (at the WTP) are quantifiable. These simulated flows and flow
scenarios become the inputs for the modeling in the river segments downstream of the non-tidal to tidal
boundary.
Table 3-2: Tar River Flow Study Technical Analysis Approach to Four Study Segments
Study Segment
Hydrologic
Analysis
Habitat
Modeling
Hydrodynamic
Modeling of
Salinity Regime
Model Selected
Water Quality
Constituents
Non-Tidal Freshwater 
Tidal Freshwater (WTP to
WWTP)
Estuarine Transition
(WWTP to Washington)
Pamlico River Estuary
(Downstream of
Washington)

For the Tidal Freshwater Segment of the Tar River, the analysis will focus principally on:
The potential effects of water withdrawals on the physical habitat of fish and other aquatic
species (Habitat Modeling).
The potential for encroachment of salinity under various low-flow regimes, withdrawal and
metrological conditions (Hydrodynamic Modeling of Salinity Regime).
Modeling of selected water quality constituents (Water Quality Modeling), such as
temperature, and DO, will be completed to assess whether withdrawals during low-flow
conditions would result in degradation of water quality and aquatic habitat, or violation of
water quality standards.
The Environmental Fluid Dynamics Code (EDFC) hydrodynamic and water quality model will provide the
platform to perform all of the necessary modeling and analysis of hydrodynamic conditions and water
quality. The EFDC model is a state-of-the-art hydrodynamic and water quality model that has the ability to
simulate aquatic system hydrodynamics and water quality in multiple dimensions (EPA, 2009a). The
EFDC model will be the central model for all of the modeling analysis in the Tidal Freshwater, Estuarine
Transition, and Pamlico River segments. Its hydrodynamic modeling results will provide inputs for the
Habitat Model and the Water Quality Model. The EFDC model domain will extend from the GUC WTP
downstream to the Pamlico River.
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For the Estuarine Transition Segment of the Tar River, consumptive water uses are less likely to affect
river habitat than tidal influences or the variable salinity regime. The analyses in this segment will focus
on the hydrodynamic model results, potential effects on the salinity regime, and potential water quality
effects. Hydrodynamic modeling of this segment of the river will also help in the understanding of how far
and under what conditions the salt wedge (the front boundary between fresh and brackish water salinity
zones) may migrate upstream in the lower Tar River.
In the Pamlico River Estuary Segment, the primary concern is whether consumptive use by GUC would
result in any effects on water quality and attainment of water quality standards. The level of modeling and
detail needed to address potential water quality issues is not yet known. The first logical step in this
analysis is to determine the degree of modeling effort that is needed by performing initial model sensitivity
analysis. This analysis will determine if water quality conditions are responsive to flow withdrawals of the
magnitude being considered.
3.4 Range of Flows to be Considered (Indicators of Hydrologic Alteration – IHA)
Characterization of flows in the Tar River Basin and at Greenville was completed in advance of the Tar
River Flow Study. The hydrology of the Tar River will be evaluated and modeled in the current study.
Examination of the hydrologic data helps define the aspects of the Tar River flow regime over which GUC
withdrawals and discharges may exert an influence – most likely low flow magnitude and low flow
duration. It will also provide clarity for the elements of the flow regime that may remain unaffected
(e.g., high flow magnitude and timing, etc.) by potential future withdrawals.
In the Draft Study Plan, it was proposed that traditional hydrologic analysis combined with an analysis
with the Indicators of Hydrologic Alteration (IHA) could be used to identify the range of flow that would be
affected by GUC withdrawals. The IHA is a software program used widely to provide useful information for
managers to understand the hydrologic effects of human activities or trying to develop environmental flow
recommendations (TNC, 2009). The IHA can be used to analyze hydrologic data available from either
existing measurement points or model-generated data. It uses 32 different parameters, organized into five
groups, to characterize annual and inter-annual hydrologic variation that provide information on some of
the most ecologically significant features of flow regimes influencing aquatic, wetland, and riparian
ecosystems (Richter et al., 1996 and 1997). This program has the ability to compare historical flows with
present flows, regulated with unregulated flows, and offers a comparison of existing and future flows
under different management scenarios.
Since the Draft Study Plan, these analyses have been completed and presented to the TAG. Table 3-3
summarizes the current and projected future basin consumptive uses and GUC withdrawals. These
numbers are approximate and are for the purpose of evaluating the potential effects of cumulative
withdrawals.
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Table 3-3: Approximate Basin Withdrawals and Return Flows Including GUC for
Demonstrative Purposes
Scenario Year
GUC
WTP
(mgd)
Basin
Withdrawals
(mgd)
Basin
Discharges
(mgd)
Consumptive
Use
(mgd)
Consumptive
Use
(cfs)
1 2002 11.2 31 14 17 26
2 2030 22.5 52 24 27 42
3 2030 29 57 24 32 50
4 2048 32 64 28 36 55
5 1 40 1 72 28 44 68
1 Hypothetical increase for demonstrative purposes.
Table 3-4 provides a summary of the results of the IHA analysis comparing the baseline period (2002)
with future scenarios of approximate consumptive uses and GUC withdrawals. The results of the analysis
suggest that the effects of future increases in water basin consumptive use and GUC withdrawals will be
limited largely to the low-flow regime. The timing, magnitude, and frequency of moderate and high flows
will not be affected by cumulative withdrawals, and many of the monthly median flows will be essentially
unchanged by cumulative withdrawals. These results are expected because the magnitude of cumulative
withdrawals is a very small proportion of river flows during most conditions.
The results of the IHA analysis provide some insights about the range of Tar River flows potentially
affected by cumulative basin consumptive use and GUC withdrawals, and help to provide guidelines as to
how the analyses might be focused. The Tar River system is largely unregulated; total reservoir storage in
the basin equals only a small percentage of the annual flow of the river. Therefore, many of the natural
attributes of flow regime in the Tar River, including high flows, high flow pulse timing and duration,
monthly flow distributions, and other ecologically important elements of the flow regime are largely
unaffected by water use in the basin. Further evaluations of these findings will be conducted through the
modeling and analysis process described in this Final Study Plan.
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Table 3-4: Result of Indicators of Hydrologic Analysis (IHA) for Baseline (2002) and Four Future
Consumptive Use and GUC Withdrawal Scenarios
Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5
Watershed area (sq mi) 2660 2660 2660 2660 2660
Mean annual flow (cfs) 2564 2544 2534 2525 2513
Mean flow/area (cfs/sq mi) 0.96 0.96 0.95 0.95 0.94
Annual C.V. 1.27 1.28 1.29 1.29 1.3
Flow predictability 0.35 0.35 0.34 0.34 0.33
Constancy/predictability 0.67 0.66 0.66 0.65 0.65
% of floods in 60d period 0.28 0.28 0.28 0.28 0.28
Flood-free season 0 0 0 0 0
Parameter Group #1
October 100% 94% 92% 90% 88%
November 100% 97% 96% 95% 94%
December 100% 98% 98% 97% 96%
January 100% 99% 99% 99% 98%
February 100% 100% 99% 99% 99%
March 100% 99% 99% 99% 98%
April 100% 99% 99% 99% 98%
May 100% 99% 98% 98% 97%
June 100% 98% 97% 96% 95%
July 100% 97% 96% 94% 93%
August 100% 98% 97% 96% 94%
September 100% 96% 94% 92% 90%
Parameter Group #2
1-day minimum 100% 88% 83% 79% 73%
3-day minimum 100% 87% 83% 78% 73%
7-day minimum 100% 88% 84% 79% 74%
30-day minimum 100% 94% 91% 88% 85%
90-day minimum 100% 98% 96% 95% 93%
1-day maximum 100% 100% 100% 100% 100%
3-day maximum 100% 100% 100% 100% 100%
7-day maximum 100% 100% 100% 100% 100%
30-day maximum 100% 100% 100% 99% 99%
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Table 3-4: Result of Indicators of Hydrologic Analysis (IHA) for Baseline (2002) and Four Future
Consumptive Use and GUC Withdrawal Scenarios
Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5
90-day maximum 100% 100% 99% 99% 99%
Number of zero days 0 0 0 0 0
Base flow index 100% 93% 89% 85% 81%
Parameter Group #3
Date of minimum 100% 100% 100% 100% 100%
Date of maximum 100% 100% 100% 100% 100%
Parameter Group #4
Low pulse count 100% 100% 100% 100% 100%
Low pulse duration 100% 100% 100% 96% 100%
High pulse count 100% 100% 100% 100% 100%
High pulse duration 100% 100% 100% 100% 100%
Low Pulse Threshold 100% 97% 95% 94% 92%
High Pulse Threshold 100% 99% 99% 98% 98%
Parameter Group #5
Rise rate 100% 100% 100% 100% 100%
Fall rate 100% 100% 100% 100% 100%
Number of reversals 100% 101% 101% 101% 100%
3.5 Screening and Assessment of Water Quality Issues
An explicit part of the Tar River Flow scoping and study plan process will be the early screening of issues
and their sensitivity to flow and freshwater inflow. In this study, the level of effort needed to model certain
water quality issues, and to determine which water quality constituents might be sensitive to changes in
flow of the magnitudes considered, have yet to be determined. In this sense, initial sensitivity analysis will
be important to help define the important analysis to be completed, and to avoid spending effort on
modeling the full range of possible water quality interactions. For example, in addressing potential effects
on water quality in the Pamlico River Estuary Segment of the Study Area, an initial model sensitivity
analysis will be performed based on previous modeling efforts (Xu et al., 2008) and screening with the
EFDC model. If these analyses suggest that water quality changes are not responsive to flow withdrawals
of the magnitude considered, then additional modeling will not be conducted.
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4. Basin Hydrologic Analysis and Modeling
After discussions with the TAG, GUC, DWR, and the City of Rocky Mount, the Stockholm Environmental
Institute’s WEAP model has been selected to develop the Tar Flow Study Basin hydrologic model. A TM,
Tar River Flow Study: Selection of Basin Hydrologic Model, is available that describes the model selection
process. This model will be used along with previous hydrologic analysis and modeling.
The Tar River Flow Study requires an analysis of the basin projections of future flows arriving at
Greenville on an annual, seasonal, and daily basis. These projections will account for river hydrology,
future withdrawals upstream of Greenville, agricultural water use and other water management actions
that may affect the amount of flow. The basin hydrologic model and its results will be a central part of the
flow study and will provide information crucial to the other study elements, as well as information for
GUC’s water system planning and management and development of water conservation and drought
management responses.
The WEAP model will provide a time series of flows within each study segment associated with various
withdrawal scenarios. These time series will be used as inputs to the other models for the assessment of
effects on water quality and aquatic habitat. The WEAP hydrologic model for the Tar River Basin will be
developed, calibrated, and used to simulate existing and future flow regimes, including the effects of
increased or seasonally modified withdrawals (e.g., simulated flow time-series for various operational and
flow alternatives). The WEAP model will serve three basics functions.
1. Accurately simulates natural hydrological processes (e.g., rainfall, evapotranspiration and
infiltration, runoff, water gains and losses from various sources, and performs water
balancing within the system) to enable assessment of the availability of water within a
catchment.
2. Simulate basic anthropogenic activities superimposed on the natural system to influence
water resources and their allocation (i.e., consumptive and non-consumptive water
demands) to enable evaluation of the effects of human water use. The model must be
capable of performing a mass balance of flow sequentially down a river system, making
allowance for abstractions and inflows.
3. The model will be run with changing rules, constraints, flow requirements, and other
goals and objectives programmed within the system to simulate various operational
schemes and management alternatives.
Subsequent discussions with the TAG will address other important details related to the hydrologic
modeling effort such as basin water withdrawals and discharges, and consumptive use as well as
modeling assumptions, approach, and results.
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5. Hydrodynamic Modeling
This section describes the development of the hydrodynamic model for the Tar River Study Area. The
hydrodynamic model will be central to all of the analyses described in Section 3 (Study Area and
Technical Approach), because its simulations of riverine and estuarine circulation will be used in habitat,
salinity, and water quality modeling.
The Tar River hydrodynamic model will be built as an extension of an existing model previously
developed, calibrated, and used for hydrodynamic and water quality investigations of the Pamlico River
Estuary. This existing model will be modified and expanded to include a larger model domain
encompassing the Tidal Freshwater, and Transitional Estuarine study segments, and further calibrated
and validated.
5.1 Overview
Coastal hydrodynamic models, generally two-dimensional (2-D) or three-dimensional (3-D) models,
calculate the circulation of coastal water based on tides and wind, as well as the discharge from rivers.
More specifically, the models calculate tidal stage, water velocities (speed and direction), and water levels
for each cell within their computational model grid. The hydrodynamic models generally calculate salinity
as well as water temperature, based on heat balance equations. These last two parameters are
especially relevant for 3-D hydrodynamic calculations where density differences due to temperature and
salinity can play an important role in the circulation of water. Such models may be used to simulate
existing or historic conditions, and if calibrated and validated for a specific region, can predict future
conditions.
The hydrodynamic model is the starting point in the analysis. The hydrodynamic model determines the
water circulation, which in turn influences the transport and distribution of suspended materials and
nutrients. The hydrodynamic model simulates the water level, velocity, temperature and salinity data that
will be used in the habitat model to simulate habitat conditions and in the water quality model where they
are used for calculating the advective transport of dissolved and suspended material. The temperature
and salinity values are important in the simulation of many chemical and biological processes, as the
rates for many of these processes are temperature and salinity dependent.
This same model structure and grid that is used for the hydrodynamic modeling will also used for the
water quality and habitat models, and may be modified for specific purposes. Due to the importance of
the model’s representation of the physical structure of the Tar River and upper Pamlico Estuary, this
section begins with a description of the existing model and the development of the expanded model
configuration and grid.
5.2 Existing EDFC Model of the Pamlico River
The EFDC model is a public domain, multi-functional surface water modeling system, which includes
hydrodynamic, sediment-contaminant, and eutrophication components. The model can be used to
simulate aquatic systems in one, two, and three dimensions. It has evolved over the past two decades to
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become one of the most widely used and technically defensible hydrodynamic models in the world (EPA,
2009).
A coupled 3-D hydrodynamic and water quality EFDC model has been developed and tested for the
Croatan-Roanoke-Albemarle-Pamlico-Core Sounds estuarine system. This model was used to analyze
seasonal variation of water quality variables, and to predict the movement of the saltwater/freshwater
interface (salt intrusion). The EFDC model also predicts the vertical stratification of DO and salinity under
different river flow and wind conditions in the Pamlico River Estuary (Xu et al., 2008).
The EFDC model has been applied to several hydrodynamic and water quality numerical modeling
studies in North Carolina by researchers at North Carolina State University (NCSU) in the Department of
Marine, Earth, and Atmospheric Sciences (MEAS). Lin et al. (2007) applied a coupled 3-D hydrodynamic
and water quality model (EFDC) to the Croatan-Roanoke-Albemarle-Pamlico-Core Sounds estuarine
system (CAPE) to analyze seasonal variation in water quality parameters such as nitrite plus nitrate, total
phosphate, chlorophyll a, and DO.
Xu et al. (2008) applied this model to investigate current circulation, salt intrusion, and vertical
stratification under different river flow and wind conditions in the Pamlico River Estuary (PRE), the lower
end of the Tar River Flow Study Area. The domain of the PRE model extends from Washington
downstream to Pamlico Sound and includes the Pamlico River and its tributary, the Pungo River. The
model was calibrated and verified against water level, temperature, and salinity measured during 2001
and 2003. Eight sensitivity tests were conducted with different river flow and wind conditions specified in
the model. The EFDC model also predicts the vertical stratification of DO and salinity under different river
flow and wind conditions in the Pamlico River Estuary (Xu et al., 2008).
A copy of the PRE application of the EDFC Model (the Pamlico River EDFC Model) has been obtained by
ENTRIX and has been selected for use by the Tar River Flow Study. The models considered and the
rational for the selection of the Pamlico River EDFC Model are described in a TM, Tar River Flow Study:
Selection of Hydrodynamic and Water Quality Model.
5.3 Data Collection for Model Development
The existing Pamlico River Estuary application of the EDFC model will be modified and expanded to
include a larger model domain encompassing the Tidal Freshwater, Estuarine Transition, and Pamlico
River Estuary Segments of the Study Area. The following section describes the use of the existing model,
collection of bathymetric, meteorological, discharge, and other boundary condition information and how it
will be used for model development and calibration.
5.3.1 Hydrographic Data Collection
To extend the model domain upstream to GUC’s WTP, additional hydrographic data will be collected to
develop bathymetric maps. Sites for the hydrographic data collection will be from slightly upstream of the
GUC WTP downstream to the NC 17 Bridge in Greenville. Due to differences in needed resolution of the
hydrographic survey and data needs for the habitat modeling, surveys will be more intensive in the Tidal
Freshwater Segment of the river.
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5.3.2 Side Scan Sonar and Bathymetric Survey
A survey will be conducted from a shallow-draft survey vessel, positioned by a Trimble DSM 232 DGPS
system operating with HYPACK hydrographic survey software. Initially, bathymetric survey lines will be
run parallel to the course of the river. A minimum of two to three survey lines will be conducted in the
upper 20 miles of the Study Area where the bank-to-bank widths range from 150 to 250 feet. Additional
lines will be added at wider sections to keep the distance between survey lines at approximately 70 feet
or less. From approximately Grimesland down to the NC 17 Bridge at Washington, additional survey lines
will be added as needed to cover the width of the river at a minimum of 50 to 70 feet spacing. The
bathymetry data will be collected with an Odom CV100 echo sounder and an Ocean Data Equipment
Bathy 500 echo sounder operated simultaneously along two tracks separated by the length of the survey
vessel and up to 15 feet across the beam of the survey vessel. Both survey grade echo sounders will
utilize a 200 kHz three degree transducer to collect depth. Depth data will be collected at a minimum of
0.5 second intervals or approximately every two feet along the survey line at the expected survey speed
of two to three knots.
The side scan sonar survey will be conducted along with the bathymetry data using a frequency between
900 to 1800 kHz depending on the frequency that provides the best detail of the bottom. The sonar
records will be of sufficient detail to characterize and generally delineate the substrate types. Objects
such as snags, logs, wrecks and other debris exposed above the river bottom will also be identifiable.
Sonar investigations of side channels and branches of the Tar River will not be included. Sloughs
positioned along the main channel course will be included if water depth and conditions permit boat
access.
Each day, three sound velocity checks will be performed with a bar check. River water levels including
tidal fluctuations will be monitored and recorded each survey day relative to nearest NAV 88 vertical
datum in each river segment. River depths will be recorded with an expected vertical accuracy of
± 0.5 feet and a horizontal accuracy of a minimum ± 3 feet.
5.3.3 Hydrodynamic and Water Quality Data and Catalog
This task involves the assembly and analysis of existing data to support hydrodynamic and water quality
model calibration, verification and application. The objective will be to develop a central data set to
support the modeling project. Relevant water quality data will be compiled (location, river segment, date,
parameter, collecting agency, etc.), focusing on applicability of existing data to the current modeling effort.
Data sources are not limited to USGS, NOAA, National Weather Service (NWS), DWQ, and universities,
particularly ECU, will be reviewed and analyzed. A central data set will be developed to support the
modeling project.
5.4 Tar River EFDC Hydrodynamic Model Development
5.4.1 Hydrodynamic Model Configuration
Model configuration will include development of all input files required by the model for simulation of
historical and projected future conditions of the system. Model configuration is typically the most time
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consuming portion of model development since it requires the assembly and analysis of large quantities
of data into formats required by the model.
5.4.2 Model Grid Development and Bathymetric Interpolation
The EFDC hydrodynamic model operates by representing a water body by an assemblage of discrete
volumetric cells. The model’s horizontal grid defines the boundaries of these cells on a map projection.
The EFDC model uses a boundary fitted curvilinear-orthogonal horizontal grid to represent shoreline and
interior features of a water body. The model grid will be generated to resolve the shoreline and significant
interior features such as navigational channels and islands. The grid will be optimized to balance
tradeoffs between resolution and model run time.
The current PRE application of the EFDC model grid will be used for its portion of the model domain, and
the bathymetric survey results will be used to generate the model grid upstream to the GUC WTP.
Bathymetric information will be automatically interpolated to the grid and adjusted manually in critical
areas as required. Additional data sources that may be used in this process include depth and diameter
of the Tar River at various locations collected by GUC for depth studies; LiDAR data from North Carolina
Floodplain Mapping Program; and detailed digital elevation model (DEM) data (1.5 m by 1.5 m)
developed by USGS for their hydraulic modeling of areas around gage stations (Bales et al., 2007).
5.4.3 Open Boundary Conditions
Open boundary conditions at the ocean boundary and truncated open water interior boundaries are
required for water surface elevation, salinity and temperature. The 15-minute surface elevation data near
the mouth of PRE, and biweekly data of salinity and temperature from East Carolina University, will be
specified as open boundary condition as established by the existing PRE application of the EFDC model.
5.4.4 Inflow Boundary Conditions
Freshwater inflow time series from point and distributed non-point inflows will be developed during the
basin hydrologic analysis and basin model development. Point inflows include gaged tributaries and
permitted discharges (USGS gage data and National Pollutant Discharge Elimination System (NPDES)
permit records) and United States Environmental Protection Agency (EPA) records of the PCS Phosphate
Mine (PCS) discharges. Non-point inflows will include runoff from un-gaged areas.
Estimated daily river discharge at the Tar River gage at Greenville will be used as the upstream inflow
boundary condition. Flow records from the Chicod Creek gage at SR 1760 near Simpson will be used as
one of the tributary inflows. For un-gaged major tributaries such as Grindle Creek and Tranters Creek,
flow will be estimated from nearby gage station flow records and drainage area relationships.
5.4.5 Atmospheric Forcing Functions
Atmospheric forcing refers to forces that drive circulation in a particular waterbody, including tides, wind,
atmospheric pressure, and solar radiation. Data on atmospheric forcing are required to represent wind
driven circulation and to predict water temperature. Wind speed and solar radiation are also required by
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the water quality modeling component to predict re-aeration and primary production. Hourly wind data
observed at the PCS station on the southern bank of the PRE will be used in the model. Hourly
metrological data sets, including air pressure and temperature, relative humidity, rainfall, solar radiation,
and cloud cover for the N.C. State Climate Office at Aurora Station will also be applied.
5.4.6 Preliminary Testing
The configured hydrodynamic model will be tested for stability and run-time performance prior to
beginning calibration and verification. These tests will be primarily directed at identifying and correcting
aspects of the grid resolution, boundary conditions, and forcing functions that can cause unrealistic model
behavior or inhibit model performance.
5.4.7 Hydrodynamic Model Calibration and Verification Using Existing Data
Model calibration involves the adjustment of model spatial input data such as bathymetry and bottom
roughness, boundary conditions, and forcing functions to achieve a best fit between model predictions
and field observations over a specified period of time. Model verification involves using the calibrated
model to simulate a different period of time and achieve a model predictive performance similar to that
achieved over the calibration period.
The current PRE application of the EFDC model was calibrated and verified with observation data,
including salinity and temperature at various locations along the Pamlico River and surface elevations at
Washington Gage station, for year 2003 (a relatively wet year) and 2001 (a relatively dry year). The
extended lower Tar River and Pamlico River model developed for this study will simulate the same
periods, to confirm that model modifications have not violated earlier calibration and verifications.
5.4.8 Performance and Sensitivity Analysis
Performance of a simulation model requires quantitative substantiation of the model results, predictive
ability, and sensitivity to model parameters. A variety of performance analysis measures such as time
series error analysis, least squares harmonic analysis, regression analysis, and spectral analysis will be
used to quantitatively evaluate and document the hydrodynamic model’s performance in predicting water
surface elevation, currents, salinity, and temperature. These performance measures will be compared
with a sample of measures from other major estuarine modeling studies to document the quality of model
calibration and verification.
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6. Habitat Modeling
The availability of methods for establishing freshwater inflow requirements for estuaries lags behind those
for establishing flow requirements in riverine ecosystems. Approaches to instream flow studies in tidally-
influenced freshwater reaches and estuaries have borrowed heavily from free-flowing riverine methods,
typically from habitat-based methods that integrate hydraulic and habitat modeling. Instream flow studies
in tidal and estuarine systems have focused on salinity regimes and water quality issues as potentially
more important drivers of habitat. Some of the common modifications for lower river tidal systems include:
Modification of hydraulic and habitat modeling methods to accommodate tidal conditions.
Hydrodynamic modeling to address coastal circulation and flow-salinity relationships and
the potential effects of changes in flow regime on water quality.
Development of various biological-habitat indices related to tidal systems and their
salinity regimes. For the most part, these studies have been done in Texas and Florida,
and various approaches have been used in different river systems.
This section describes how these approaches will be integrated and adapted for the Tar River Flow
Study, how habitat will be defined in the Tar River Flow Study for each of the Study Area segments, and
the modeling and criteria that will be used for the assessment of potential habitat changes due to water
withdrawals and consumptive uses.
6.1 Modeling Approach
The approach to modeling in each Study Area segment is summarized in Table 6-1. Habitat will be
defined differently in each segment based on the potential effects in that segment and the magnitude of
withdrawals in that segment and the relative importance of physical habitat, salinity, and water quality.
Table 6-1: Habitat Modeling Approach for Each Segment of the Tar River Flow Study
Tar River Segment Habitat Definition and Modeling Approach
Tidal Freshwater
Segment
Habitat in this segment of the river will be defined by the combination of physical
habitat (depth, velocity, substrate, and cover), salinity regime, and water quality.
Habitat conditions may be influenced by tidal fluctuations, especially at low flows.
Physical habitat will be addressed by bathymetric, bottom type, and
cover/structure mapping together with a GIS model that integrates hydrodynamic
model results with habitat mapping in a GIS model to produce flow vs. habitat
relationships.
Potential for salinity encroachment is very low, based on historical data, but the
potential for this to occur will be addressed through sensitivity analysis using the
hydrodynamic model.
Water quality model to assess whether withdrawals at the WTP have the
potential to produce unsuitable habitat conditions or violations of water quality
standards.
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Table 6-1: Habitat Modeling Approach for Each Segment of the Tar River Flow Study
Tar River Segment Habitat Definition and Modeling Approach
Estuarine Transition
Segment
Habitat in this segment of the river will be defined by salinity regime and water
quality.
The hydrodynamic model will be used to address the greater potential for salinity
encroachment associated with higher GUC consumptive water uses.
Also to be addressed are the potential effects of consumptive water use on the
seasonal location and extent of salinity regimes and the amount of habitat area
and volume in defined salinity zones.
Water quality model used to assess whether withdrawals at the WTP have the
potential to result in unsuitable habitat conditions or violations of water quality
standards.
Pamlico River
Estuary Segment
Habitat in this segment of the river will be defined by water quality conditions.
The potential effect of consumptive water use on water quality will be addressed
through water quality modeling and sensitivity or screening analysis; this
approach is based on varying inflow parameters under different water use and
inflow scenarios, and observing water quality response.
This approach will be an extension of the approach used in Xu et al. (2008).
6.1.1 Scenario-Based Modeling
Approaches for evaluating riverine flow regimes often rely on the use of flow frequency analysis and flow
time series data. In particular, one approach that is commonly used in North Carolina and elsewhere is
“habitat frequency analysis.” In this approach, historical flow time series are combined with habitat versus
flow relationships to create habitat time series (Capra et al., 1995; Parasiewicz, 2008). These habitat time
series represent the history of habitat events experienced in the past based on flows. The habitat time
series are then divided into months and a habitat frequency analysis is developed, describing the levels of
habitat typically experienced in the past and their frequency of occurrence by month or season.
Although flow time series and flow frequency analysis will be important in the basin hydrologic analysis, a
time series approach to the habitat modeling will not be feasible in any of the study segments. This is
largely due to the tidal conditions, which makes this approach conceptually challenging, due to the
exceptional computational demands that it would create for the hydrodynamic and water quality models.
For example, Xu et al. (2008) reported a model run time of one half day to run the PRE application of the
EFDC model for one year for one scenario.
There are also other feasibility and data adequacy issues that limit the potential feasibility of long period
time series analysis, such as availability of needed boundary condition data sets for parameters other
than flow that extend for the same period of record. Even if the long-term boundary condition data were
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available, and the long run times could be accommodated, the resulting data sets would be massive and
unwieldy for analysis.
A more appropriate approach for modeling systems when a hydrodynamic model and other linked water
quality and habitat analyses are involved is the use of scenario-based modeling. Scenarios are defined by
a set of conditions characteristic of the system being modeled (tidal range, inflows, wind conditions) with
the values designed to cover the range of values experienced at some frequency. For example, in their
study of the factors contributing to salinity stratification in the PRE, Xu et al. (2008) defined eight
“scenarios” for investigative sensitivity analysis. Xu et al. created a base case and combinations of high
and low inflows, wind direction (upriver, downriver, northeast, southwestward, remote wind) and specified
tide and salinity settings, to address the relative effects of flow and wind conditions on salinity
stratification, salinity intrusion, and general circulation and vertical diffusivity within the Pamlico River
Estuarine Segment of the Study Area. Contrasting examination of these eight scenarios provided
considerable insight into the behavior and dynamics of the Pamlico River estuary, as well as a good
understanding of the relative importance of river discharge, wind direction, and other factors on the
salinity regime.
The scenarios to be evaluated in the Tar River Flow Study will be developed with input from the TAG and
will be documented in a subsequent technical memorandum.
6.2 Physical Habitat Modeling (Tidal Freshwater Segment)
Habitat in the Tidal Freshwater Segment of the river is defined by the combination of physical habitat
(e.g., depth, velocity, substrate, and cover), salinity regime, and water quality. The primary steps in
developing a physical habitat model of the Tidal Freshwater Segment of the Tar River will be as follows:
Collect hydrographic and habitat data (e.g., channel bathymetry, substrate, structure).
Complete habitat mapping and select representative study sites.
Build a geo-referenced river habitat model and hydrodynamic model of the study sites
with additional field data.
Perform hydrodynamic modeling for study sites under different scenarios.
Complete habitat modeling by combining hydrodynamic model simulation results with the
habitat structural model in a GIS environment.
Compute habitat values and relationships to flows for various scenarios.
6.2.1 Collect Hydrographic and Habitat Data
The procedures for collecting bathymetric data and side scan sonar data in the Tar River are fully
described in Section 5, Hydrographic Data Collection. The resultant data will be used to create a geo-
referenced GIS-based model of depths, bottom types, and structure/cover in the Tidal Freshwater Section
of the river.
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6.2.2 Habitat Data Collection and Site Selection
With the geo-referenced model, the general habitat types in the river will provide a characterization of the
different habitats present and sufficient to address habitat changes along the length and width of the
Study Area. The field survey will include confirmation of substrate and cover interpretations from side
scan sonar results.
Once the habitat characterization is complete, potential representative study sites will be identified. The
objective will be to select, in collaboration with representatives of the TAG, two study sites of sufficient
length to characterize the habitats present in the Tidal Freshwater Segment. It is anticipated that one
study site will be located between the GUC WTP and NC 13 and the other site will be located
downstream of the USGS gage at Greenville (Figure 1-2) and upstream of the WWTP. The length,
location, and intensity of habitat mapping and field checking are yet to be determined.
6.2.3 Hydrodynamic Model and Outputs
The EFDC hydrodynamic model that will be used for simulating tidal circulation, tidal height (stage), and
velocities was described in Section 5. For the study sites selected, higher resolution areas will be
developed within the overall EFDC model in order to create a more detailed computational model grid.
This will allow hydraulic and habitat conditions to be modeled at a suitable level of detail. For each
scenario, the hydrodynamic model will simulate the stage, depths and velocities within the model grid
cells for the time period simulated. The hydraulic outputs of the model will be provided for each cell grid.
6.2.4 GIS Habitat Model
In this final step, the hydrodynamic model results and the habitat structure models will be combined. The
depth and velocity results from EFDC model will be combined with substrate and cover attributes
identified during the field mapping. The integration will be completed in a GIS database to allow the geo-
referenced data to be assembled by grid cell, and to allow habitat values in each cell to be computed
using habitat criteria to be described in Section 8. Once cell habitat values are computed, the habitat
results can be displayed graphically on a map of the river as well as being summed together to produce
traditional habitat versus flow relationships.
6.3 Salinity Regime Modeling (Estuarine Transition Segment)
The potential effects on habitat in this segment of the river due to GUC water use will be defined by
salinity regime and water quality. The hydrodynamic model will be used to simulate flow-salinity
relationships, the location of ecologically important salinity boundaries, and the potential salinity
encroachment area. Analyses will include the spatial extent of salinity regimes and the amount of “habitat”
area and volume in each defined salinity zones.
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6.4 Water Quality Modeling/Screening (Tidal Freshwater Segment, Estuarine Transition
Segment, and Pamlico River Estuary Segment)
The EFDC water quality model for the Tar River will be used to address water quality relationships and
potential effects in the Estuarine Transition Segment and Pamlico River Estuary Segment due to GUC
consumptive water uses. The primary purpose of the analysis will be to assess whether withdrawals at
the WTP have the potential to result in unsuitable habitat conditions or violations of water quality
standards for modeled constituents. Water quality modeling is described in more detail in Section 7.
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7. Water Quality Modeling
In general, water quality models are designed to calculate the concentration and distribution of a
constituent, property, or parameter in the surface waterbody being evaluated. Water quality modeling
consists of simulating transport processes and constituent transformation processes. Simulation of
transport is accomplished with hydrodynamic modeling (Section 6) including advection, diffusion, and
dispersion. Transformation involves a quantification of sources and sinks to which a parameter is
subjected, and may include physical, chemical, or biological processes.
This section briefly outlines the general approach to water quality modeling envisioned at this time. In
contrast to the level of detail provided in other sections, the level of detail needed for water quality
modeling, the parameters to be modeled, and the sensitivity of hydrodynamic characteristics of the Tar
and Pamlico River Study Area have not yet been fully established. The final approach will be determined
after the extension of the hydrodynamic model into the Tar River, and initial sensitivity analysis of
withdrawals and consumptive uses. The modeling approach, evaluation of water quality constituents, and
other aspects of the modeling process for the Tar River Flow Study will be developed with input from the
TAG and will be documented separately.
7.1 Water Quality Model Development
The configuration of the water quality model will be based on the configuration of the hydrodynamic
model. That is, it will share the same model grid, boundary conditions, forcing functions, and be
dependent upon the hydrodynamic model results.
The first step in configuration of the water quality model is to select state variables, make decisions on the
use of sediment flux sub-models, and estimate different reaction rate coefficients for variables. The goal
is to choose a set of state variables that will achieve the level of modeling needed to meet the project
objectives, and determine the model’s predictive ability, consistent with the data requirements to define
loads and to support model calibration and verification. A minimum set of state variables is needed to
model specific parameters, such as DO. Decisions on sediment flux modeling, partitioning of state
variables, and other model parameters will also be made at this point.
Time-varying open boundary conditions are required for all water quality model state variables. Time
series of water quality open boundary conditions are typically developed using water quality monitoring
data from stations nearest the open boundaries. It is expected that bi-weekly water quality data from ECU
will be used (Section 5, Hydrodynamic Modeling), and then applied as open boundary conditions,
equivalent to the current Pamlico EDFC Model. The boundary condition estimation procedure used for
developing hydrodynamic open boundary conditions will be used, if necessary, for refining these
boundary conditions during water quality model calibration.
Loads for permitted discharges will be developed using NPDES data provided by DWQ and EPA. Non-
point source loads, including atmospheric deposition loads, will be estimated from the literature and other
relevant local sources. Boundary condition loading will be developed from available water quality and
gage discharge data.
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Atmospheric forcing information, including wind speed and solar radiation, required by the water quality
model component, will be provided from the hydrodynamic model simulation results, as will salinity and
temperature.
The configured water quality model will be tested for correct linkage with the hydrodynamic model,
stability, and runtime performance before beginning model calibration and verification. These tests will be
primarily directed at identifying and correcting any aspects of the reaction parameter set, boundary
conditions and loads that could result in instability and unrealistic model behavior.
7.2 Model Calibration and Verification
The water quality model with the expanded Tar River domain will be calibrated and verified with available
water quality data. The water quality model calibration process will include the adjustment of model
reaction parameters, boundary conditions, and loads to achieve a best fit between model predictions and
field observations over a specified period of time. Model verification involves using the calibrated model to
simulate a different period of time and achieve a model predictive performance similar to that achieved
over the calibration period. Data sources include the bi-weekly sampling by ECU along Pamlico River,
continuous data from the USGS, and additional data collected by DWQ along the Tar River.
A variety of performance analysis measures are available and will be used as appropriate. These include
time series error analysis, least squares harmonic analysis, regression analysis, and spectral analysis to
quantitatively evaluate and document the model’s performance in predicting observable water quality
state variables. These performance measures will be compared with a sample of measures from other
major estuarine modeling studies to document the quality of calibration and verification. Sensitivity
analysis is a methodology for determining the model’s response to input parameters. The most sensitive
parameters are those whose variation induces the greatest response and they correspond to the most
uncertain parameters. Minimizing uncertainty will be achieved by demonstrating that model performance
is insensitive to variation of uncertain parameters over their accepted ranges and provides additional
documented support for the model application, and estimation.
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8. Biological Data and Habitat Suitability Criteria
8.1 Aquatic Resource Information
Resident riverine aquatic species, estuarine dependent marine species, estuarine residents, and
migratory species use habitats of the lower Tar River and Upper Pamlico system. Over two-thirds of all
recreational fisheries and 90 percent of commercial fisheries in North Carolina are dependent on riverine
estuaries like the Tar-Pamlico system (APNEP Science & Technical Advisory Committee, 2004). A
number of important aquatic species and fisheries resources are present including state and federally
designated species, habitats of ecological importance, anadromous fish populations (e.g., sturgeon,
striped bass, and river herring), and estuarine-dependent marine species of recreational and commercial
importance.
The Tar and Pamlico Rivers and their tributaries provide spawning habitat for a variety of fishes that
migrate between saltwater and freshwater. These fish are ecologically, commercially, and recreationally
important and have a substantial economic effect within the State of North Carolina, and include:
Anadromous fish species, which live their adult lives in the ocean but move into
freshwater streams to reproduce or spawn, such as hickory shad (Alosa mediocris),
American shad (Alosa sapidissima), alewife (Alosa psuedoharengus), blueback herring
(Alosa aestivalis), Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus), and striped bass
(Morone saxatilis).
Catadromous species that live in freshwater and migrate to the ocean to spawn, such as
American eel (Anguilla rostrata).
A number of fish species that regularly migrate between saltwater and freshwater
habitats (e.g., white perch,Morone americanus).
The aquatic habitats of the Tar Pamlico River and Upper Pamlico Estuary support significant aquatic
resources. In 2000, the Tar-Pamlico River from the railroad bridge at Washington upstream to Rocky
Mount Mills Dam was designated by the North Carolina Wildlife Resources Commission (WRC) as an
Inland Primary Nursery Area (15A NCAC 10C.0503). Inland Primary Nursery Areas are inhabited by the
embryonic, larval, or juvenile life stages of marine and estuarine fish and crustacean species due to
favorable physical, chemical or biological factors (15A NCAC 10C.0502).
8.2 Available Aquatic Survey Data for the Tar River and Pamlico River Estuary
A number of agencies conduct regular sampling of the aquatic organisms present in the Tar River and
Pamlico River Estuary. Researchers and students at ECU have conducted a number of surveys and
studies in the Tar River Basin.
8.2.1 Tar River WRC Aquatic Survey Data
Annual spring surveys have been conducted by WRC for herring, shad, and striped bass since 1999 to
assess anadromous fish spawning in the Tar River between the Rocky Mount Mills Dam and Tarboro.
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Grindle and Chicod Creeks are also sampled for the presence of river herring. The numbers and species
of fish spawning in these areas are documented annually (data obtained from Kirk Rundle WRC in 2008
and 2009).
In 2004 through 2005, WRC conducted a creel survey to assess the types and numbers of fish caught by
anglers in the Tar River (WRC, 2005). The results indicated that most anglers in the Tar River are fishing
for striped bass and largemouth bass. Sunfish species (e.g., bluegill, redear, pumpkinseed, and
redbreast) and crappie are the game fish caught most often. In the spring of 2005 WRC, electrofishing in
the Tar River, collected 1,423 striped bass including multiple size classes (WRC, 2005).
8.2.2 North Carolina Division of Marine Fisheries (DMF) Data
Annual trawls, juvenile trawls, and gill netting have been conducted by DMF to characterize the fish and
benthic community in the Pamlico Estuary since 1978.
8.2.3 National Oceanic and Atmospheric Administration (NOAA) Studies
The overall objective of NOAA's Estuarine Living Marine Resources (ELMR) Program is to develop a
database on the distribution, relative abundance, and life history characteristics of ecologically and
economically important fish and invertebrates in the Nation's estuaries. The program focuses on 40
ecologically and commercially important estuarine species. In 1985, NOAA’s ELMR program sampled
extensively in Pamlico Sound and the Pamlico/Pungo Rivers (Nelson et al., 1991). NOAA typically
characterizes the species present in three salinity zones: tidal fresh (0.0 to 0.5 ppt), mixing (0.5 to 25 ppt)
and seawater (>25 ppt) and uses these zones when comparing estuary species composition. In the
Pamlico Sound, the biologic community was characterized by five different salinity zones. No seawater
zone (salinity >25 ppt) was found in the Pamlico or Pungo Rivers. The relative abundance of each
species and life stage are provided by month (Nelson et al., 1991).
8.2.4 East Carolina University Data
Researchers at ECU have completed a number of relevant fisheries studies and reports. Some of the
available data and reports include:
Larval fish data for Tar River, Jones and Overton (2004), documents the habitat use by
larval and juvenile fish as well as their community structure and spatial distribution;
Habitat use of early Alosa species and striped bass in the lower Tar River; Masters
Thesis, Eastern Carolina University (Smith, 2006).
Coggins and Rulifson (2007), River Herring Surveys in the Tar River.
Murauskas (2005), Patterns in Hickory Shad Spawning and Migration in Tar and Pamlico
River. Masters Thesis, Eastern Carolina University.
Salisbury (2008), Southern Flounder in the Tar-Pamlico River describes migration
patterns and habitat use (Salisbury et al., 2008).
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8.2.5 United States Geologic Survey (USGS) National Water Quality Assessment Program (NAWQA)
The Albemarle-Pamlico Drainage Study Unit (ALBE) National Water-Quality Assessment (NAWQA)
project has been through a number of cycles including the initial surface water, groundwater and
ecological data collection from 1991 to 1993, and groundwater and monthly surface water sampling from
1993 to 2001. A third cycle began in 2002 that focused on evaluating the influence of urbanization on the
water quality and ecology of the area (Effects of Urbanization on Stream Ecosystems (EUSE)). This third
cycle included nutrient source and delivery modeling (using Spatially Referenced Regressions on
Watershed Attributes (SPARROW) modeling) and analysis of water quality in the Castle Hayne aquifer
(Harned, 1994). No NAWQA fish data are available for locations within the Tar River Study Area, but data
is available for three sites outside of the Study Area: Tar River at Tarboro, Tyson Creek near Faulkland,
and Chicod Creek at SR 1760 near Simpson (http://nc.water.usgs.gov/albe/data/CycleI_ecology/
ecology.html).
8.2.6 DWQ Data Benthic and Fish Sampling Data
DWQ conducts benthic and fish sampling in the Tar River Basin every five years and site ecological
health is evaluated based on the North Carolina Index of Biotic Integrity (NCIBI) score determined for the
site. The fish community sampling methods involve wading-based techniques, so non-wadeable and
larger waterbodies are not monitored (DWQ, 2006a). The benthic community assessment methods allow
for sampling larger systems from a boat with use of a petite ponar dredge (DWQ, 2006b).
Benthic and fish data are available for a number of Tar River tributaries (Table 8-1), but no fish data are
available for the Tar River mainstem in the Study Area and fish community NCIBI rankings are not
provided for the tributaries. According to DWQ’s Standard Operating Procedure, Biological Monitoring,
Stream Fish Community Assessment Program, 2006, the NCIBI for the Upper Coastal Plain, where our
Study Area is located, is under revision (DWQ, 2006a).
Table 8-1: Fish and Benthic Macroinvertebrate Sampling Locations in the Lower Tar Basin Sampled by
DWQ in 2007
Waterbody Station County
Fish/
Benthic
Level IV
Ecoregion Date Observations
Bio-
classification
or NCIBI
Rating*
Ballahack
Canal
NC 42 Edgecombe Fish Southeastern
Floodplains &
Low Terraces
05/09/07 No darters or
intolerant species,
blue spotted sunfish
in snags and rip/rap,
BS and American
eel 66% of fish
Not Rated
Ballahack
Canal
NC 42 Edgecombe Benthic Southeastern
Floodplains &
Low Terraces
02/06/07 Very low habitat
score and algal
mats
Severe
Cokey
Swamp
NC 43 Edgecombe Benthic Rolling Coastal
Plain
02/08/07 Moderate
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Table 8-1: Fish and Benthic Macroinvertebrate Sampling Locations in the Lower Tar Basin Sampled by
DWQ in 2007
Waterbody Station County
Fish/
Benthic
Level IV
Ecoregion Date Observations
Bio-
classification
or NCIBI
Rating*
Cokey
Swamp
SR
1135
Edgecombe Fish Rolling Coastal
Plain
05/09/07 Eastern
mosquitofish is 40%
of fauna, 3 darter
and 7 sunfish
species
Not Rated
Conetoe Cr SR
1510
Edgecombe Fish Mid-Atlantic
Flatwoods
05/09/07 18 species present,
8 sunfish species,
pirate perch and
American eel
common
Not Rated
Conetoe Cr SR
1510
Edgecombe Benthic Mid-Atlantic
Flatwoods
02/06/07 Moderate
Conetoe Cr NC 42 Edgecombe Benthic Mid-Atlantic
Flatwoods
02/06/07 5 mollusk taxa in
2002 and 1 in 2007
Moderate
Crisp Cr SR
1527
Edgecombe Fish Mid-Atlantic
Flatwoods
05/09/07 low fish abundance,
4 species of sunfish
and American eel
common, low % of
tolerant fish
Not Rated
Crisp Cr SR
1527
Edgecombe Benthic Mid-Atlantic
Flatwoods
02/06/07 Moderate
Tyson Cr SR
1255
Pitt Fish Rolling Coastal
Plain
05/10/07 Good instream and
riparian habitats,
one intolerant
species sawcheek
darter, American eel
50% of fish
Not Rated
Otter Cr SR
1614
Edgecombe Benthic Rolling Coastal
Plain
02/07/07 Large number of
mollusks
Moderate
Otter Cr SR
1614
Edgecombe Fish Rolling Coastal
Plain
04/02/97 Not Rated
Town Cr NC 43 Edgecombe Fish Rolling Coastal
Plain
08/28/97 Not Rated
Town Cr SR
1601
Edgecombe Benthic Southeastern
Floodplains &
Low Terraces
06/27/07 Good
Chicod Cr SR
1777
Pitt Benthic Mid-Atlantic
Flatwoods
02/14/07 Natural
Grindle Cr US 264 Pitt Benthic Mid-Atlantic
Flatwoods
06/25/07 Good-Fair
Tranters Cr SR
1552
Edgecombe Benthic Mid-Atlantic
Flatwoods
02/13/07 Moderate
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Table 8-1: Fish and Benthic Macroinvertebrate Sampling Locations in the Lower Tar Basin Sampled by
DWQ in 2007
Waterbody Station County
Fish/
Benthic
Level IV
Ecoregion Date Observations
Bio-
classification
or NCIBI
Rating*
Flat Swamp SR
1157
Martin Benthic Mid-Atlantic
Flatwoods
02/13/07 Moderate
Horsepen
Swamp
SR1001 Beaufort Benthic Mid-Atlantic
Flatwoods
02/13/07 Moderate
Old Ford
Swamp
US 17 Beaufort Benthic Mid-Atlantic
Flatwoods
02/12/07 Moderate
Lathams Cr SR
1410
Beaufort Benthic Mid-Atlantic
Flatwoods
02/12/07 Natural
Beaverdam
Swamp
SR
1523
Beaufort Benthic Mid-Atlantic
Flatwoods
02/13/07 Moderate
Tar River SR
1565
(Grimesl
and
Bridge)
Pitt Benthic Mid-Atlantic
Flatwoods
06/26/07 Good-Fair
Cannon
Swamp
US 264 Pitt Fish Mid-Atlantic
Floodplains &
Low Terraces
05/10/07 Channelized and
0% canopy,
abundant
macrophytes.
Productive, 82%
tolerant species
Not Rated
Hardee Cr NC 33 Pitt Benthic Mid-Atlantic
Flatwoods
02/14/07 Natural
Juniper
Swamp
SR
1766
Pitt Fish Mid-Atlantic
Flatwoods
04/15/93 Not Rated
Parker Cr NC 33 Pitt Fish Mid-Atlantic
Floodplains &
Low Terraces
05/10/07 American eel,
redbreast sunfish
and bluegill most
abundant. Nine
sunfish and four
catfish species.
Third highest catch
rate of Tar Basin in
2007
Not Rated
Whichard
Branch
SR
1521
Pitt Fish Mid-Atlantic
Flatwoods
05/10/07 Not Rated
Whichard
Branch
SR
1521
Pitt Benthic Mid-Atlantic
Flatwoods
02/13/07 Moderate
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8.3 Rare, Threatened, and Endangered Species Present in the Study Area
Existing databases on unique, endemic, and state or federally listed aquatic species within the Study
Area have been obtained and summarized to serve as the basis for this and other project tasks. Species
occurrence data were obtained from the Natural Heritage Program (NHP), Natural History Museum, and
FWS. Species occurrence records from multiple agencies have been combined and mapped
(Appendix B). The state and federally listed species that occur within the Tar River Flow Study Area are
identified in Table 8-2. It is expected that WRC, DMF, and FWS will provide information and expert
opinion regarding these species and will participate in the development of their habitat criteria.
A recent mussel survey conducted for GUC by John Alderman (Alderman, 2009) in the Tar River
mainstem near the WTP intake found eight species of mussels, three species of which are listed as
Threatened by the State of North Carolina (Table 8-3).
Table 8-2: State and Federally Listed Species Observed in the Tar River between GUC Water
Treatment Plant Intake and Washington
Scientific Name Common Name Taxa
Federal
Status
State
Status
NOAA Fisheries
Status
Vertebrates
Trichechus manatus West Indian manatee Mammal E E None
Acipenser oxyrhynchus Atlantic sturgeon Fish T* None None
Ambloplites cavifrons Roanoke bass Fish FSC SR None
Alosa pseudoharengus Alewife Fish None None SOC
Alosa aestivalis Blueback herring Fish None None SOC
Necturus lewisi Neuse River waterdog Amphibian None SC None
Invertebrates
Lampsilis cariosa Yellow lampmussel Mussel FSC E None
Alasmidonta undulata Triangle floater Mussel None T None
Leptodea ochracea Tidewater mucket Mussel None T None
Lampsilis radiata Eastern lampmussel Mussel None T None
Elliptio Roanokensis Roanoke slabshell Mussel None T None
Oronectes carolinensis NC spiny crayfish Crayfish None SC None
Baetisca obesa Mayfly Mayfly None SR None
E = Endangered T = Threatened
T* = NC Stock recommended for listing as Federally Threatened
FSC = Federal Species of Concern
SR = State Rare, SC = State Species of Concern
SOC = National Oceanographic and Atmospheric Administration, Fisheries Division, Species of Concern
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Table 8-3: April 2009 Mussel Survey in Tar River near GUC WTP Intake
Scientific Name Common Name
Live
Mussels CPUE*
State
Status Federal Status
Elliptio
roanokensis
Roanoke
slabshell 10 2.22 T** None
Lampsilis radiata
radiata
Eastern
lampmussel 2 0.44 T** None
Leptodea
ochracea tidewater mucket 7 0.44 T** None
Elliptio
complanata
Eastern elliptio
complex 163 36.22 None None
Elliptio ictarina variable spike
complex 1 0.22 None None
Elliptio congarea Carolina
slabshell 29 6.44 None None
Elliptio
cistellaeformis box spike 85 ND None None
Corbicula
fluminea Asian clam ND ND None None
*CPUE = Catch per Unit Effort
**T = Threatened
8.4 Supplemental Mussel Surveys within the Freshwater Tidal Segment
The mainstem of the Tar River upstream of the Study Area is known to support a diverse freshwater
mussel assemblage. Although habitat for rare and protected mussel species is presumably present, there
is little recent collection information available for the Study Area other than the spring 2009 mussel survey
near GUC’s WTP intake (Alderman, 2009) and a few WRC surveys in the NHP database from the early
1980s.
A comprehensive freshwater mussel survey will be conducted as part of the Tar River Flow Study to
provide updated and more detailed information as to which species are present and their relative
abundance. Prior to conducting surveys field reconnaissance, aerial and topographical mapping,
bathymetry and side scan sonar data will be used to assist in targeting survey locations. Mussel surveys
will be conducted for three full days at a minimum of 15 sites within the Freshwater Tidal Segment of the
Study Area.
A range of habitat types will be surveyed at each site. The amount of time spent at each survey location
will be dependent on habitat quality and distribution across the river, with special attention to those that
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may support rare and/or listed State and Federal mussel species. Multiple sampling methods will be used
including SCUBA based, visual surveys, glass bottom buckets, and tactile methods. Tactile methods will
be employed particularly in the stream banks under submerged root mats.
Timed surveys will be conducted to provide catch per unit effort (CPUE) data. All species of freshwater
bivalves will be identified, recorded and returned to the substrate. Representative photographs will be
taken of each species and of the habitats where they are found. Habitat conditions for each site will be
recorded and will include substrate types, water depth range, location in channel, and bank conditions. All
individuals of species monitored by the NHP and WRC will be measured and checked for evidence of
reproduction, and a representative sample of species that are not monitored will be processed in the
same manner. The FWS and WRC will be contacted if any federally listed species are found.
8.5 Criteria for Habitat Modeling
In this portion of the study, we will use existing information and the results of fish and mussel surveys to
develop the biological and habitat criteria that will represent the basic ecological requirements and habitat
functions needed by freshwater and estuarine aquatic species and other flow-dependent habitats. These
criteria may include defined suitability values of depth, velocity, substrate, and cover; salinity thresholds,
criteria, or physical locations; and water quality standards. The development and use of these criteria is
highly dependent upon their availability in the literature, knowledge of the ecology of the Tar-Pamlico
system, and agreement with the TAG on their appropriate use.
Work under this task will include the compilation of available data and habitat approaches, assessing the
applicability of the criteria to the Tar River, developing a proposed set of criteria to be used, and
identifying any additional or supplementary data available through the TAG. The TAG will be provided
with a summary of the available criteria and our recommendations. Comments and input will be solicited
and used to develop a revised set of criteria and documented in a technical memorandum. The habitat
criteria to be evaluated in the Tar River Flow Study will be developed with input from the TAG and will be
separately documented. The final criteria will be used with habitat modeling to determine the effects of
flow changes on the different habitats needed by Tar River aquatic species.
8.5.1 Habitat Suitability Criteria for Tidal Freshwater Habitat Modeling
A set of suitability criteria for depth, velocity, substrate, and cover will be established for species and/or
functional groupings such as habitat-use groups or guilds, unique, endemic, and state or federal listed
aquatic species, and selected life stages of anadromous species. These criteria will define optimal and
suitable habitat for specific species or guilds. The development of biological and habitat criteria for the Tar
River Flow Study will begin by compiling the selected species and habitat suitability criteria from
published literature, previous studies, and available specialists with specific knowledge of key species.
The results will be a set of habitat criteria (e.g., depth, velocity, substrate, cover) for selected species or
habitat-use guilds that define optimal and suitable habitat for certain species and life stages. These
habitat criteria will be used in physical habitat modeling.
Evaluation of available habitat criteria will specifically include the species of special interest, including
yellow lampmussel, Atlantic sturgeon, Neuse River waterdog, and Roanoke bass. It is expected that the
55/60
DENR, NOAA fisheries, and FWS will provide information and expert opinion regarding these species and
will participate in the development of their habitat criteria.
8.5.2 Criteria for Evaluating Salinity Regime in the Freshwater-Tidal Transitional Segment
It is broadly recognized and documented that the salinity regimes of estuarine systems are highly
dependent upon the amount and timing of freshwater flows (Jassby, 1995; Alber, 2002; Estevez, 2002;
Mattson, 2002; Montagna, 2008) and that salinity has been identified as a key water quality characteristic
affecting species distribution and estuarine productivity (Longley et al., 1994; Flannery et al., 2002).
The approaches and criteria currently used to address salinity are variable among states and estuarine
systems, and generally include monitoring and simulation of circulation and salinity patterns and
assessment with biologically-defined salinity zones, salinity constraints, salinity refuges, and/or isohaline
position (position of an isohaline boundary near known important habitat areas). Many of these criteria
are river and estuary specific. Models are often used to predict changes in circulation and salinity for
various inflow scenarios, and strong correlations are found between freshwater inflows and the salinity
gradients in many estuaries. For example, desirable salinity regimes for different species are investigated
through multivariate flow-salinity regressions as a function of inflow for multiple sites in Texas estuaries
(Brandes et al., 2009).
Most often, these salinity criteria are based on long-term monitoring and the statistical characteristics of
salinity within the estuary combined with known salinity preferences and tolerance limits of the target
species. Long-term flow and salinity data sets and population levels of fish, invertebrates, and plants are
used to set constraints and to develop statistical relationships between flow and salinity to establish these
criteria.
While long-term research has been completed to support such studies in some estuaries in Florida,
Texas, and California, the availability of such information is far more limited for North Carolina. The
compilation of available biological data sets and flow records and meta-analysis has, with some limited
exceptions, not yet been conducted. These analyses are well beyond the scope and time frame of this
study, and may well take many years to be funded and accomplished.
However, the result of these available studies and the salinity criteria and guidelines that have been
developed may be applicable with some modification, to the Tar River Flow Study. It is also possible that
modeling approaches can be used to establish baseline conditions and sensitivity analysis can be used to
define historical and existing conditions that would serve as a baseline for potential effects on salinity
resulting from GUC’s consumptive water use.
As a part of the development of habitat criteria, an overview of potentially applicable salinity criteria and
recommendations will be provided to the TAG for review and discussion.
8.5.3 Water Quality Criteria
There is a wide range of available water quality criteria that could be used to assess the results of the
water quality modeling. These include state water quality standards and literature on the water quality
56/60
tolerances of various organisms. As the water quality modeling is completed, these issues will be
addressed with the TAG.
Existing information will be used to develop the biological and habitat criteria that will represent the basic
ecological requirements and habitat functions needed by select aquatic species and other flow-dependent
resources. The results will be a set of habitat criteria (e.g., depth, velocity, substrate, cover, temperature,
salinity, and water quality parameters) for selected species or habitat-use guilds that define optimal and
suitable habitat for certain species and life stages. These habitat criteria will be used in physical habitat
modeling. Verification of, and agreement on, the habitat criteria developed may require expert
consultation and literature research.
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9. References
Albemarle Pamlico National Estuary Program (APNEP). 2004. Science & Technical Advisory Committee
Meeting, November 2004. http://www.apnep.org/pages/stac_meetings.html
Alber, M. 2002. A conceptual model of estuarine freshwater inflow management. Estuaries 25 (6B): 1246-
1261.
Alderman Environmental Services. 2009. Freshwater Mussel Survey for Brown and Caldwell at
Greenville WTP, Pitt County, April 2009.
ARCADIS. 2008. Environmental Assessment, Greenville Utilities Commission Interbasin Transfer.
Bales, J. D., C. R. Wager, et al. 2007. LiDAR-Derived Flood-Inundation Maps for Real-Time Flood-
Mapping Applications, Tar River Basin, North Carolina. USGS Scientific Investigations Report
2007-5032.
Brandes, R. J., F, Heitmuller, R. Huston, P. Jensen, M. Kelly, F. Manhart, P. Montagna, G. Ward, and J.
Wiersema. 2009. Methodologies for Establishing a Freshwater Inflow Regime for Texas Estuaries
within the Context of the Senate Bill 3 Environmental Flows Process. Senate Bill 3 Science
Advisory Committee for Environmental Flows, Report # SAC-2009-03.
Brown and Caldwell. 2008. Draft Environmental Assessment, Greenville Utilities Commission Raw Water
Intake Improvements Project.
Capra, H., P. Breil, and Y. Souchon. 1995. A new tool to interpret magnitude and duration of fish habitat
variations. Regulated Rivers: Research and Management, (10): 69 – 387.
Coggins, T.C., and R. A. Rulifson. 2007. Habitat Use and Out-Migration Patterns of Juvenile Shad and
Herring in the lower Roanoke River, North Carolina.
Division of Land Resources. 1999. Letter from DLR to City of Rocky Mount specifying minimum release
requirements for Tar River Reservoir Dam.
Division of Water Quality Planning Branch. 2003. Trend-Analysis of Nutrient Loading in the Tar-Pamlico
Basin, May 2003.
Division of Water Quality Basinwide Planning Program. 2004. Tar-Pamlico River Basinwide Water Quality
Plan.
Division of Water Quality. 2006a. Standard Operating Procedure, Biological Monitoring, Stream Fish
Community Assessment Program, 2006.
Division of Water Quality. 2006b. Standard Operating Procedure for Benthic Macroinvertebrates,
Biological Assessment Unit, July 2006.
Division of Water Quality, Environmental Sciences Branch. 2008. Ambient Monitoring Report Tar-Pamlico
River Basin, June 2008.
Division of Water Quality, Environmental Sciences Branch. 2008b. Annual Report of Fish Kill Events,
2008. http://h2o.enr.state.nc.us/esb/Fishkill/documents/2008KillReport.pdf
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Division of Water Quality Planning Branch, Modeling and TMDL Unit. 2009. Trend Analyses in the Tar-
Pam River Basin in North Carolina.
Division of Water Resources. 2004. Central Coastal Plain Capacity Use Area Status Report.
http://www.ncwater.org/Reports_and_Publications/GWMS_Reports/ CCPCUAStatRep2004.pdf
Dominion Power. 2006. Technical Memorandum: Implementation of Article 414, Study Plan: Effects of
Within-Day Peaking on Fish and Benthic Macroinvertebrates, Roanoke Rapids and Gaston
Hydropower Project, FERC No. 2009. September 18, 2006.
ENTRIX. 2008. Analysis of Greenville Utilities Commission’s Proposed Interbasin Transfer Withdrawals
on Tar River Flows at Greenville, North Carolina, October 2007, revised April 2008. ENTRIX Inc.,
Raleigh, NC.
ENTRIX. 2009. Tar River Flow Study: Selection of Basin Hydrologic Model. ENTRIX Inc., Raleigh, NC.
ENTRIX. 2010. Tar River Flow Study: Selection of Hydrodynamic and Water Quality Model. ENTRIX Inc.,
Raleigh, NC.
Environmental Protection Agency. 2009.Environmental Fluid Dynamics Computer Code.
http://www.epa.gov/athens/research/modeling/efdc.html
Estevez, E. D. 2002. Review and assessment of biotic variables and analytical methods used in estuarine
inflow studies. Estuaries 25 (6B): 1291-1303.
Flannery, M. S., E. B. Peebles, and R. T. Montgomery. 2002. A percent-of-flow approach for managing
reductions of freshwater inflows from un-impounded rivers to southwest Florida estuaries. Estuaries
25: 1318-1332.
GMA. 2003. Groundwater Management Associates, Inc. Hydrological analysis report for Greenville
Utilities Commission, July 25, 2003.
Harned, D. A. 1994, A spatial characterization of nutrient concentrations for the Albemarle-Pamlico
Drainage Area, North Carolina and Virginia, American Geophysical Union 1994 Fall Meeting, Eos,
November 1, 1994, p. 247.
Jassby, A. D., W. J. Kimmerer, S. G. Monismith, C. Arbor, J. E. Cloern, T. M. Powell, J. R. Schubel, and
T. J. Vendlinski. 1995. Isohaline position as a habitat indicator for estuarine populations. Ecological
Applications 5:272–289.
Jones, N. and A. Overton. 2004. Habitat and cohort specific growth and mortality of larval River Herring in
Tar-Pamlico River.
Kimmerer, W. J. and J. R. Schubel. 1994. Managing freshwater flows into San Francisco Bay using a
salinity standard: Results of a workshop, p. 411–416.In K. R. Dyer and R. J. Orth (eds.). Changes
in Fluxes in Estuaries: Implications from Science to Management. Olsen and Olsen, Fredensborg,
Denmark.
Lin, J., L. Xie, L. Pietrafesa, J. Ramus, and H. Paerl. 2007. Water quality gradients across Albemarle-
Pamlico Estuarine System: seasonal variations and model applications. Journal of Coastal
Research 23(1): 213-229.
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Lin, J., H. Xu, C. Cudaback, and D. Wang. 2008. Inter-annual variability of hypoxic conditions in a shallow
estuary. Journal of Marine Systems 73:169-184.
Longley, W.L. (ed.). 1994. Freshwater inflows to Texas bays and estuaries: ecological relationships and
methods for determination of needs. Water Development Board and Texas Parks and Wildlife
Department, Austin, Texas, 386 p.
Murauskas, J. R., A. Rulifson, and Y. Zhu. Unpublished. Reproductive development during the
anadromous migration of adult hickory shad Alosa mediocris. Institute for Coastal and Marine
Resources and Department of Biology, East Carolina University, Greenville, NC 27858
Mattson, R. 2002. A resource-based framework for establishing freshwater inflow requirements for the
Suwannee River estuary. Estuaries 25:1333–1342.
Montagna, P. A., E. D. Estevez, T. A. Palmer, and M. S. Flannery. 2008. Meta-analysis of the relationship
between salinity and mollusks in tidal river estuaries of southwest Florida, U.S. A. American
Malacological Bulletin 24: 101-115.
Nelson, D. M., E. A. Irlandi, L. R. Settle, M. E. Monaco, and L. Conston-Clements. 1991. Distribution and
Abundance of Fishes and Invertebrates in Southeast Estuaries. ELMR Rep. No. 9. NOAA/NOS
Strategic Environmental Assessments Division, Silver Spring, MD. 167 p.
O’Driscoll, M., D. Mallinson, and P. Johnson. 2008. Surface Water/Ground Water Interactions Along the
Tar River, NC. Water Resources Research Institute of the University of North Carolina, Report No.
370.
Parasiewicz, P. 2008. Habitat time series analysis to define flow augmentation strategy for the Quinebaug
River. River Research and Applications 24: 439-452
Richter, B. D., J. V. Baumgartner, J. Powell, and D. P. Braun. 1996. A Method for Assessing Hydrologic
Alteration within Ecosystems. Conservation Biology 10:1163-1174.
Richter, B. D, J. V. Baumgartner, R. Wigington, and D. P. Braun. 1997. How Much Water Does a River
Need? Freshwater Biology 37:231-249.
Riggs, S. R., S. J. Culver, D. V. Ames, D. J. Mallison, D. R. Corbett, and J. P. Walsh. 2008. North
Carolina’s Coasts in Crisis: A Vision for the Future, East Carolina University Department of
Geological Sciences.
Rulifson, R. A. and C. S. Manooch, III. 1990. Recruitment of juvenile striped bass in the Roanoke River,
North Carolina, as related to reservoir discharge. North American Journal of Fisheries
Management 10(3):397-407
Salisbury, R. V., R. Spidel, and R. A. Rulifson. 2008. Critical habitat for Southern flounder: Do coastal
watersheds play an important role in life history and growth? North Carolina Sea Grant Fisheries
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Schuchardt, B., U. Haesloop, and Schirmer. 1993. The tidal freshwater reach of the Weser Estuary:
riverine or estuarine? Netherlands Journal of Aquatic Ecology 27:215-226
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Seager, R, A. Tzanova, and J. Nakamura. 2009. Drought and the Southeastern United States: Causes,
Variability over the Last Millennium and the Potential for Future Hydroclimate Change. Journal of
Climate 22:5021-5045. October, 2009.
Smith, M. C. 2006. Habitat use of early Alosa spp and striped bass Morone saxatilis in the lower Tar
River, North Carolina. Department of Biology, July 2006.
Stockholm Environmental Institute. 2009. Water Evaluation and Planning System (WEAP)
http://www.weap21.org/
Stanley, D. W., and S. W. Nixon. 1992. Stratification and bottom-water hypoxia in the Pamlico River
Estuary. Estuaries 15(3): 270-281.
The Nature Conservancy. 2009. Indicators of Hydrologic Alteration, IHA Software.
http://www.nature.org/initiatives/freshwater/conservationtools/art17004.html
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http://waterdata.usgs.gov/nc/nwis .
USGCRP (United States Global Change Research Program). 2004. US National Assessment of the
Potential Consequences of Climate Variability and Change. Regional Paper: The Southeast.
http://www.usgcrp.gov/usgcrp/nacc/education/southeast
Wagner, M. C., and H. M. Austin. 1999. Correspondence between environmental gradients and summer
littoral fish assemblages in low salinity reaches of the Chesapeake Bay, USA, Marine Ecology
Progress Series 177:197-212.
Waters, B., J. Crews-Klein, and J. Renner. 2003,.Water supply alternatives in the North Carolina Central
Coastal Plain Capacity Use Area, Proceedings of the 2003 Georgia Water Resource Conference,
April 23-24, 2003, Kathy J. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens,
Georgia.
Weaver, J. C. 2005. The drought of 1998-2002 in North Carolina -Precipitation and hydrologic conditions:
U.S. Geological Survey Scientific Investigations Report 2005-5053, 88 p.
Wildlife Resources Commission, North Carolina (WRC). 2006. Tar River Creel Survey.
http://www.ncwildlife.org/pg03_Fishing/pg3d_TarRiverWinners.htm
Xu, H., J. Lin, and W. Dongxiao. 2008. Numerical study on salinity stratification in the Pamlico River
Estuary. Estuarine, Coastal and Shelf Science 80:74-84.
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Tar

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CARTERET COUNTYDUPLIN COUNTY
PITT COUNTY
BEAUFORT COUNTY
ONSLOW COUNTY
CRAVEN COUNTY
WAYNE COUNTY
PAMLICO COUNTY
JONES COUNTY
MARTIN COUNTY
LENOIR COUNTY
EDGECOMBE COUNTY
WILSON COUNTY
GREENE COUNTY
WASHINGTON COUNTY
Louisburg
Rocky Mount
Greenville
Tarboro
Washington
Fishi
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Swift Creek
Town Creek Deep Creek
Marsh

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Figure 1-1. Tar River Basin and Central Coastal Plain Capacity Use Area
Tar River Flow Study
Greenville Utilities Commission
Date: xx/xx/xx Rev. Date: xx/xx/xx PM: XXX GIS Analyst: XXX Map Document: XXXXX.mxd Project Number: XXXX-XXX-XXX PDF Document: XXXXX.pdf Plot Size: 8.5 x 11
This map and all data contained within aresupplied as is with no warranty. ENTRIX, Inc.
expressly disclaims responsibility fordamages or liability from any claims that mayarise out of the use or misuse of this map. Itis the sole responsibility of the user to
determine if the data on this map meets theuser’s needs. This map was not created assurvey data, nor should it be used as such. Itis the user’s responsibility to obtain proper
survey data, prepared by a licensedsurveyor, where required by law.
Legend
Central Coastal Plain Capacity Use Area
Interstates
Streams
Tar River
Municipal Boundaries
County
Tar-Pamlico Basin
Coordinate System: North Carolina FIPS 3200 (NAD83)
www.entrix.com
3141 John Humphries Wynd
Suite 265
Raleigh, NC 27612
ph [phone]fx [phone]
0 20 4010 Miles1 inch = 22 miles
Greenville
Washington
Grindle Creek Tranters
Creek
Tar River
Bear Creek
Crawfo
r
d

C
r
e
e
kChicod Creek
BEAUFORT COUNTY
BEAUFORT COUNTY
MARTIN COUNTY
PITT COUNTY
Hardee CreekTar River at Greenville
Grindle Creek at US 264 at Pactolus, NC
Pamlico River at Washington
TAR RIV AT SR 1565 NR GRIMESLAND
Chicod Creek nr Simpson
Hardee Creek above NC 33 near Greenville, NC
Horsepen Swamp at Leggetts Crossroads
GUC WWTP Discharge
GUC WTP Intake
264
13
17
Tranters Creek Near Washington
USGS Gauges
USGS Gauge, Historical
USGS Gauge, Active
Stage Height Only, Active
Roads
Stream
Municipal Boundaries
Tar-Pamlico Basin
County
Pamlico River
Figure 1-2. General Study Area
Tar River Flow Study
Greenville Utilities Commission
This map and all data contained within are
supplied as is with no warranty. ENTRIX, Inc.expressly disclaims responsibility fordamages or liability from any claims that may
arise out of the use or misuse of this map. It
is the sole responsibility of the user todetermine if the data on this map meets theuser’s needs. This map was not created as
survey data, nor should it be used as such. It
is the user’s responsibility to obtain propersurvey data, prepared by a licensedsurveyor, where required by law.Coordinate System:
North Carolina FIPS 3200 (NAD83)
www.entrix.com
3141 John Humphries WyndSuite 265Raleigh, NC 27612
ph [phone]fx [phone]
0 2 41 Miles1 inch = 2.75 miles
cfs = Cubic feet per second.
Figure 2‐1. Flow Duration Curves Based on Average Annual Flow at Greenville Gage Station; 
Comparison of Generated Flow Record Before (1932‐1968) and After [with] (1972‐2006) Operation of 
the Rocky Mount Reservoir.
0
100
200
300
400
500
600
700
80%85%90%95% 100%
Percent of Time Flow ExceededFlow (cfs) Before After
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
0%5%10%15%20%25%30%35%40%45%50%55%60%65%70%75%80%85%90%95%100%Percent of Time Flow ExceededFlow (cfs)LOW FLOW DETAIL
ALL FLOWS
Figure 2‐2.   Range of tidally‐influenced flow oscillations in the Tar River at Greenville for three different 
ranges of average flow condition (a) 150 cfs (August 20‐23, 2007), (b) 1,500 cfs (December 18‐21, 2006), and 
(c) 15,000 cfs (November 24‐27, 2006), fifteen minute flow data shown on graphs
(a) Daily Average Flow of About 150 cfs
-500
-300
-100
100
300
500
700
8/20/07 8/21/07 8/22/07 8/23/07DaysFlow (cfs)(b) Daily Average Flow of About 1,500 cfs
1300
1400
1500
1600
1700
1800
12/18/06 12/19/06 12/20/06 12/21/06DaysFlow (cfs)(c) Daily Average Flow About 15,000 cfs
13000
14000
15000
16000
17000
11/24/06 11/25/06 11/26/06 11/27/06DaysFlow (cfs)

7.7 River Miles Tar River at Greenville
Gage
Tar River downstream of Greenville
WWTP Discharge
0.3 River Miles
2.7River Miles
4.7 River Miles
GUC WWTP Discharge
GUC WTP Discharge
GUC WTP Intake
Figure 2‐3. Schematic diagram of the Tar River in the vicinity of Greenville, NC showing the relative 
locations and approximate distances between withdrawal and discharge locations, USGS gage location, 
and hydrologic model output points 

Figure 2‐4. Average Annual Flows in the Tar River at Greenville based on Analysis of Historical Flow Records (1932‐2006)05001,0001,5002,0002,5003,0003,5004,0004,5005,00019321934193619381940194219441946194819501952195419561958196019621964196619681970197219741976197819801982198419861988199019921994199619982000200220042006YearAverage Annual Flow (cfs)
* Monthly median flows based on modeled data (1931 - 2007)cfs - Cubic feet per second.Figure 2‐5. Flow Percentiles for Tar River at Greenville based on Modeled Data (1931‐2007)90%75%50%25%10%02,0004,0006,0008,00010,00012,000Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonth Flow (cfs)
* Median monthly flow based on modeled data (1931 - 2007) for Tar River gage.cfs - Cubic feet per second.** Based on monthly average and maximum withdrawals for 2005.GUC - Greenville Utilities Commission.Figure 2‐6. GUC Average and Maximum Withdrawals as a Percentage of the Median Tar River Flow05001,0001,5002,0002,5003,0003,5004,000Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonth Flow (cfs)0.0%5.0%10.0%15.0%20.0%25.0%30.0%GUC Withdrawal as Percentage of Median MonthlyFlowAvg **50th Percentile Flows*Max **Avg **
* Monthly 10th percentile flows based on modeled data (1931 - 2007)cfs - Cubic feet per second.** Based on monthly average and maximum withdrawals for 2005.GUC - Greenville Utilities Commission.Figure 2‐7. GUC Withdrawals as a Percentage of the 10th Percentile Monthly Flows02004006008001,0001,2001,4001,600Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonthFlow (cfs)0%5%10%15%20%25%30%35%40%45%50%GUC Withdrawal as Percentage of 10th Percentile FlowMax **Avg **10th Percentile Flows *
* The summer 7Q10 flow is given as 108 cfs (GMA 2003).cfs - Cubic feet per second.** Based on monthly average and maximum withdrawals for 2005.GUC - Greenville Utilities Commission.Figure 2‐8. Average and Maximum GUC Permitted Withdrawals as a Percentage of the 7Q10 Flow0%5%10%15%20%25%30%35%40%Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonthGUC Withdrawal as Percentage of 7Q10 FlowAvg **Max **
Grindle Creek Tranters
Creek
Tar River
Bear Creek
Crawford Cree
kChicod Creek
BEAUFORT COUNTY
BEAUFORT COUNTY
MARTIN COUNTY
PITT COUNTY
Hardee CreekGUC WTP Intake
GUC WWTP Discharge
Tar River at Greenville
Hardee Creek above NC 33 near Greenville, NC
Grindle Creek at US 264 at Pactolus, NC
Horsepen Swamp at Leggetts Crossroads
Pamlico River at WashingtonChicod Creek nr Simpson
TAR RIV AT SR 1565 NR GRIMESLAND
Greenville
WashingtonTranters Creek Near Washington
Stream
USGS Gauge, Historical
USGS Gauge, Active
Stage Height Only, Active
Non Tidal Freshwater Segment
Tidal Freshwater Segment
Estuarine Transition Segment
Pamlico River Estuary Segment
Municipal Boundaries
County
Pamlico River
Figure 3-1. Study Area Segments
Tar River Flow Study
Greenville Utilities Commission
This map and all data contained within are
supplied as is with no warranty. ENTRIX, Inc.expressly disclaims responsibility fordamages or liability from any claims that may
arise out of the use or misuse of this map. It
is the sole responsibility of the user todetermine if the data on this map meets theuser’s needs. This map was not created as
survey data, nor should it be used as such. It
is the user’s responsibility to obtain propersurvey data, prepared by a licensedsurveyor, where required by law.Coordinate System:
North Carolina FIPS 3200 (NAD83)
www.entrix.com
3141 John Humphries WyndSuite 265Raleigh, NC 27612
ph [phone]fx [phone]
0 2 41 Miles1 inch = 2.75 miles