THE LIBRARY OF THE UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL THE COLLECTION OF NORTH CAROLINIANA PRESENTED BY Daniel Okun C387 G45h c. 2 UNIVERSITY OF N C AT CHAPEL HILL 111 00015566415 This book may be kept out one month unless a recall notice is sent to you. It must be brought to the North Carolina Collection (in Wilson Library) for renewal. Form No. A-369 Digitized by tiie Internet Arciiive in 2013 http://archive.org/details/hydrologyofmajorOOgies HYDROLOGY OF MAJOR ESTUARIES AND SOUNDS OF NORTH CAROLINA PREPARED IN COOPERATION WITH THE NORTH CAROLINA DEPARTMENT OF NATURAL RESOURCES AND COMMUNITY DEVELOPMENT BIBLIOGRAPHIC DATA 1. Report No. SHEET 3. Recipient's Accession No. 4. Title and Subtitle HYDROLOGY OF MAJOR ESTUARIES AND SOUNDS OF NORTH CAROLINA 5. Report Date July 1979 7. Author(s) G. L. Giese. H. B. Wilder, and G. G. Parker, Jr. 8. Performing Organization Rept. No. USGS/WRI-79-46 9. Performing Organization Name and Address U.S. Geological Survey Water Resources Division P. 0. Box 2857 Raleigh, N.C. 27602 10. Project/Task/Worlc Unit No. 11. Contract/Grant No. 12. Sponsoring Organization Name and Address U.S. Geological Survey Water Resources Division P. 0. Box 2857 Raleigh, N.C. 27602 13. Type of Report & Period Covered Final 1967-1978 15. Supple tary No Prepared in cooperation with the North Carolina Department of Natural Resources and Community Development 16. Abstracts Knowledge of the basic hydrology of North Carolina's major estuaries and sounds is necessary to help solve hydrology-related estuarine problems which include contamination of some estuaries with municipal and industrial wastes and drainage from adjacent intensively-farmed areas, nuisance-level algal blooms, excessive shoaling in some navigation channels, saltwater intrusion into usually fresh estuarine reaches, too-high or too-low salinities in nursery areas for various estuarine species, and flood damages due to hurricanes. Saltwater intrusion occurs from time to time in all major estuaries except the Roanoke River, where releases from Roanoke Rapids Lake and other reservoirs during otherwise low-flow periods effectively block saline water from the estuary. 17. Key Words and Dcument Analys 17a. Descriptors *Tides, *Saline water intrusion, *Saline water-freshwater interface, *Stratif ication , Salt marshes. North Carolina, *Flow characteristics. Wind tides, *Water chemistry, *Sedimentation. 17b. Identifiers/Open-Ended Terms *Albemarle Sound, *Pamlico Sound, *Cape Fear River, '"Northeast Cape Fear River, *Neuse River, *Trent River, *Tar River, *Pamlico River, *Chowan River, *Roanoke River, *Tidal hydraulics. 17c. COSATl Field/Group 18. Availability Statement No restriction on distribution 19. Security Class (This Report) UNCI ASSIFIFD 20. Security Class (This Page UNCLASSIFIED 21. No. of Pages 189 IS-35 (RE\ ENDORSED BY ANSI AND UNESCO. THIS FORM MAY BE REPRODUCED JSCOMM- DC 6265-F HYDROLOGY OF MAJOR ESTUARIES AND SOUNDS OF NORTH CAROLINA By G. L. Giese, H. B. Wilder, and G. G. Parker, Jr. U.S. GEOLOGICAL SURVEY WATER RESOURCES INVESTIGATIONS 79-46 P/iepo/Led In coopiiACitlon u)lth tko, Hohtk CoAollna and CommunJjty Vzv^Zopmznt July 1979 i UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. W. Menard, Director Von. additional '[n{^omatlon W^aXq. to: U.S. Giio logical SuAvny ?06t OiildZ Box 2^57 RalUgh, Hontk Carolina 27602 July 1979 ii CONTENTS Page Abstract 1 Introduction 2 CHAPTER 1— General hydrology 6 Ocean tides 7 Estuarine flow 9 Salinity 14 Estuarine types 15 Sediment 22 Effects of winds and hurricanes 26 The salt-marsh environment 30 CHAPTER 2 — Hydrology of the Cape Fear River estuarine system 33 The Cape Fear River estuary 34 Flow 35 Freshwater inflow 35 Tide-affected flow 37 Water quality 42 Sediment 44 Salinity . 45 Variations in time and space 45 Relation of salinity to freshwater inflow and tides 49 The Northeast Cape Fear River estuary 52 Freshwater inflow 55 Tide-affected flow 57 Water quality 61 Sediment 61 Salinity 63 Variations in time and space 63 "2 Relation of salinity to freshwater inflow 66 5 CHAPTER 3 — Hydrology of the Pamlico Sound estuarine system 70 Pamlico Sound 71 -> Water budget and flow 73 Water levels 81 £ Water quality 83 ^ Salinity 88 Page The Neuse-Trent river system 91 Effects of wind on water levels and specific conductance 93 Water quality 95 Spatial variations in salinity 98 Frequency of saltwater intrusion 99 Relation of salinity to freshwater inflow 109 The Tar-Pamlico river system Ill Water levels 113 Flow 117 Water quality 120 Sediment 122 Salinity 122 CHAPTER 4 — Hydrology of the Albemarle Sound estuarine system 129 Albemarle Sound and vicinity 129 Water levels 132 Freshwater inflow 135 Extent and duration of saltwater intrusion 137 The Chowan River estuary 147 Water levels 149 Flow 152 Water quality 156 Salinity 156 The Roanoke River estuary 159 Flow 161 Water quality 164 Summary and discussion 167 References 170 ILLUSTRATIONS Page Plate 1. Map showing major estuaries and sounds of North Caro- lina (In pocket) Figure A. Map showing drainage network of eastern North Carolina and approximate extent of estuaries and sounds 4 iv CHAPTER 1 Page Figures 1.1-1.3 Graphs showing: 1.1 Tide curve for Cape Fear River estuary at Wilmington for Aug. 9-22, 1977 10 1.2 Diurnal inequalities produced from interaction of semidiurnal and diurnal tidal components 11 1.3 Tide curves at spring tide and neap tide 11 1.4 Sketch showing behavior of an idealized tide wave in an estuary 12 1.5 Graph showing relation between specific conductance and chloride concentrations in North Carolina estuaries 16 Figures 1.6-1.9 Sketches showing: 1.6 A. Net circulation patterns in a highly stratified estuary. B. Salinity profile through section X-X'. C. Net velocity profile through section X-X' 18 1.7 A. Net circulation patterns in a partially mixed estuary. B. Chloride concentration profile through section Y-Y' . C. Net velocity profile through section Y-Y' 20 1.8 A. Net circulation patterns in a well-mixed estuary. B. Chloride concentration profile through section Z-Z ' . C. Velocity profiles through section Z-Z' 21 1.9 The effect of forward motion of a hurricane on wind velocities in each quadrant 29 CHAPTER 2 2.1 Graph showing concurrent discharge relation for esti- mating runoff from the ungaged drainage area be- tween the index stations and the mouth of the Cape Fear River estuary 36 2.2 Graph showing magnitude and frequency of annual mini- mum 7-day average net flow of the Cape Fear River estuary at the mouth, near Southport 38 2.3 Map showing Cape Fear River estuary upstream from Wilmington 39 Figures 2.4-2.11 Graphs showing: 2.4 Stage and discharge of the Cape Fear River estuary near Phoenix on March 8, 1967 41 V Page Figure 2.5 Variation of velocity with depth of the Cape Fear River estuary at Navassa on May 13, 1966 42 2.6 Longitudinal variations in chloride concentrations of the Cape Fear River estuary at high-slack tide, November 1, 1967 46 2.7 Variations in chloride concentrations in a cross section of the Cape Fear River estuary 1.5 miles upstream from Market Street, Wilmington, on June 5, 1962 47 2.8 Relation between chloride concentration and specific conductance at a known point in the Cape Fear River estuary and chloride concentration and specific conductance at other points either up- stream or downstream 48 2.9 Relation between chloride concentration near the bottom of the Cape Fear River estuary upstream from Wilmington at high slack tide and the number of hours the chloride concentration will exceed 200 mg/L 50 2.10 Magnitude and frequency of highest annual tide in the Cape Fear River estuary at Wilmington, corrected for component of river stage due to freshwater inflow 51 2.11 Interrelations of maximum encroachment of 200 mg/L chloride, outflow at the mouth, and tide heights in the Cape Fear River estuary 53 2.12 Map showing Northeast Cape Fear River estuary 54 Figures 2.13-2.16 Graphs showing: 2.13 Relation between flow measured at Northeast Cape Fear River at Chinquapin and net outflow from the Northeast Cape Fear River estuary at the mouth. . . 56 2.14 Low-flow frequency curves for the Northeast Cape Fear River estuary 57 2.15 Stage and discharge as measured continuously during October 23, 1969, in the Northeast Cape Fear River estuary, 6.4 miles upstream from the mouth 59 2.16 Average flushing time for a solute injected into the Northeast Cape Fear River estuary about 6.5 miles upstream from the mouth 60 2.17 Sketch showing longitudinal variations in specific conductance of the Northeast Cape Fear River estuary on November 9, 1966 64 vi Page Figures 2.18-2.20 Graphs showing: 2.18 Specific conductance type curve for the Northeast Cape Fear River estuary 65 2.19 Relation of the location of the saltwater front in the Northeast Cape Fear River estuary to the pre- ceding 21-day average discharge at Chinquapin.... 67 2.20 Frequency of intrusion of 200 mg/L chloride for various locations in the Northeast Cape Fear River estuary 69 CHAPTER 3 Figures 3.1-3.4 Maps showing: 3.1 Depth of Pamlico Sound 72 3.2 Texture of bottom sediments in Pamlico Sound 72 3.3 Oxidizable organic matter and organic carbon content of bottom sediments in Pamlico Sound 74 3.4 Calcium carbonate content of bottom sediments in Pamlico Sound 74 Figures 3.5-3.7 Graphs showing: 3.5 Magnitude and frequency of annual minimum 7 and 30 consecutive-day average inflow to Pamlico Sound from direct and indirect land drainage 77 3.6 Magnitude and frequency of annual maximum 7 and 30 consecutive-day average inflow to Pamlico Sound from direct and indirect land drainage 78 3.7 Schematic drawing showing estimated water levels in Pamlico Sound at 0200 and 0500 hours on September 12, 1960, during Hurricane Donna 82 3.8 Map showing areas subject to flood inundation caused by wind tides having A, 50 percent chance of being equaled or exceeded in any one year and B, 1 percent chance of being equaled or exceeded in any one year 84 3.9 Map showing sampling stations used in Pamlico Sound study by Woods (1967) 86 3.10 Graphs showing (A). Mean monthly water surface temperature of Pamlico Sound (B) . Relation between water surface temperature of Pamlico Sound and air temperature at Hatteras 87 3.11 Map showing average surface salinity of water in Pamlico Sound and vicinity for the month of April 89 vii Page Figure 3.12 Map showing average surface salinity of water in Pam- lico Sound and vicinity for the month of December 90 3.13 Wind diagram for the Neuse River estuary at New Bern 94 Figures 3.14-3.16 Graphs showing: 3.14 Change in water level of the Neuse River estuary at New Bern due to wind as recorded at New Bern Airport 95 3.15 Effect of wind on water levels and bottom specific conductance in the Neuse River estuary at New Bern 96 3.16 Range of water temperatures in Neuse River estuary at New Bern 98 3.17 Map showing lines of equal specific conductance for the Neuse River estuary, August 13, 1967 100 Figures 3.18-3.27 Graphs showing: 3.18 Change in surface specific conductance with upstream and downstream distance in the Neuse and Trent estuaries 101 3.19 Percentage of time specific conductances equaled or exceeded indicated values in the Neuse River estuary at New Bern, 1957-67 102 3.20 Percentage of time surface specific conductances were equaled or exceeded for several locations in the Neuse River estuary, 1957-67 103 3.21 Percentage of time integrated specific conductances were equaled or exceeded for several locations in the Trent River estuary, 1959-61 104 3.22 Percentage of time surface and bottom salinities were equaled or exceeded in the Neuse River estuary at Garbacon Shoals Light, 1948-68 105 3.23 Percentage of time surface and bottom salinities were equaled or exceeded in the Neuse River estuary at Wilkinson Point Light, 1958-67 106 3.24 Percentage of time surface and bottom salinities were equaled or exceeded in the Neuse River estuary at Hampton Shoal Light, 1958-60 107 3.25 Percentage of time surface and bottom salinities were equaled or exceeded in the Neuse River estuary at Fort Point Light, 1958-67 108 viii Page Figure 3.26 Percentage of time a specific conductance of 800 ymhos will be equaled or exceeded in the Neuse River at New Bern for various annual average discharges at Kinston 110 3.27 Percentage of time a specific conductance of 800 ymhos will be equaled or exceeded in the Trent River near New Bern for various annual average discharges at Trenton Ill 3.28 Wind diagram for the Pamlico River estuary 114 Figures 3.29-3.39 Graphs showing: 3.29 Water levels in the Pamlico River at Washington and effective component of wind speed at Cape Hatteras 115 3.30 Relation of rise in water level in the Pamlico River at Washington to effective wind velocity 116 3.31 Frequency curve of annual mean discharge of Tar River at Tarboro 117 3.32 Low-flow frequency curves of annual lowest mean dis- charge for indicated number of consecutive days for Tar River at Tarboro 118 3.33 Maximum 7 and 30 consecutive-day average discharges of the Tar River at Tarboro 119 3.34 Average monthly temperature of Pamlico River at Washington 120 3.35 Sediment-transport curve for the Tar River at Tarboro 123 3.36 Cumulative frequency curves of specific conductance and chloride, Pamlico River at Washington 124 3.37 Relation between surface specific conductance at one point in the Pamlico River estuary and surface specific conductance at other points either up- stream or downstream 125 3.38 Percentage of time a specific conductance of 800 ymhos will be equaled or exceeded in the Pamlico River at Washington for various annual average discharges at Tarboro 126 3.39 Relation between the preceding 30-day average dis- charge of the Tar River at Tarboro and the location of the saltwater front 127 CHAPTER 4 Figure 4.1 Map showing depth, in feet, of Albemarle Sound 130 4.2 Map showing texture of bottom sediments in Albemarle Sound 130 ix Figures 4.3-4.5 Graphs showing: Page 4.3 Wind speed at Elizabeth City and water levels at Chowan River near Edenhouse, Albemarle Sound near Edenton, and Pasquotank River at Elizabeth City, July 27-30 and Nov. 29-Dec. 3, 1960 134 4.4 Minimum 7 and 30 consecutive-day average inflow to Albemarle Sound from land drainage 138 4.5 Maximum 7 and 30 consecutive-day average inflow to Albemarle Sound from land drainage 139 4.6 Map showing average surface salinity of water in Albemarle Sound and vicinity during the month of March 140 4.7 Map showing average surface salinity of water in Albemarle Sound and vicinity during the month of December 140 Figures 4.8-4.15 Graphs showing: 4.8 Average monthly specific conductance of Albemarle Sound near Edenton, 1957-67 141 4.9 Cumulative frequency curve of specific conductance and chloride, Pasquotank River near Elizabeth City... 143 4.10 Cumulative frequency curves of specific conductance and chloride, Pasquotank River at Elizabeth City 143 4.11 Cumulative frequency curve of specific conductance and chloride, Perquimans River at Hertford 144 4.12 Cumulative frequency curves of specific conductance and chloride, Chowan River near Edenhouse 144 4.13 Cumulative frequency curves of specific conductance and chloride, Albemarle Sound near Edenton 145 4.14 Cumulative frequency curve of specific conductance and chloride, Scuppernong River near Cresswell 145 4.15 Cumulative frequency curve of specific conductance and chloride, Scuppernong River at Columbia 146 4.16 Map showing Chowan River, major tributaries, and adjoining swampland 148 Figures 4.17-4.23 Graphs showing: 4.17 Continuous water-level records for Chowan River near Eure and Chowan River near Edenhouse for December 6-12, 1974 150 4.18 Continuous water-level records for five gaging stations on the Chowan River for December 6, 1974 151 X Page Figure 4.19 Monthly average stages for Chowan River near Eure and Chowan River near Edenhouse 153 4.20 Frequency curve of annual mean discharges, combi- nation of Blackwater River at Franklin, Va. , and Nottoway River near Sebrell, Va 154 4.21 Low-flow frequency curves of annual lowest mean dis- charge for indicated number of consecutive days, combination of Blackwater River at Franklin, Va., and Nottoway River at Sebrell, Va 155 4.22 Monthly average discharge and maximum and minimum daily average discharge for each month of Chowan River near Eure and Chowan River near Eden- house 157 4.23 Percentage of time bottom specific conductances ex- ceeded 800 micromhos for various annual average discharges for Chowan River at Edenhouse, 1958-67 160 4.24 Map showing distributary system of the lower Roanoke River 162 4.25 Frequency curve of annual mean discharges of Roanoke River at Roanoke Rapids 163 4.26 Low-flow frequency curves of annual lowest mean dis- charge for indicated number of consecutive days, for Roanoke River at Roanoke Rapids 165 TABLES Page CHAPTER 1 Table 1.1 The ten most important partial tides 8 1.2 Primary production rates of various ecosystems 31 CHAPTER 2 2.1 Summary of chemical analyses of freshwater samples col- lected at key stations in the Cape Fear River basin 43 2.2 Summary of chemical analyses of freshwater samples col- lected at key stations in the Northeast Cape Fear River basin 62 xi CHAPTER 3 Page Table 3.1 Monthly and annual gross water budget for Pamlico Sound 75 3.2 Tidal flow and related data for Oregon, Hatteras, and Ocracoke inlets 80 3.3 Predicted tide ranges and maximum currents for locations at or near inlets to Pamlico Sound 81 3.4 Summary of chemical analyses of freshwater samples col- lected at key stations in the Neuse-Trent River system 97 3.5 Summary of chemical analyses of freshwater samples col- lected at key stations in the Tar-Pamlico River basin 121 CHAPTER 4 Table 4.1 Relative effects of wind on vertical movement of water levels at selected locations in and near Albemarle Sound 133 4.2 Monthly and annual gross water budget for Albemarle Sound 136 4.3 Maximum chloride concentrations and specific conductance of water at daily sampling stations in the Albe- marle Sound estuarine system 142 4.4 Summary of chemical analyses of freshwater samples col- lected at key stations in the Chowan River basin... 158 4.5 Summary of chemical analyses of freshwater samples col- lected at key stations in the Roanoke River basin 166 xii COVER: Harbor at Wanchese, North Carolina; drawing by Libby Giese from copyrighted photograph by Bill Patterson (by permission of Braxton Flye, editor HoKtk CaJiotlna Coaital fi^lving and Va- cation gLLtdd) INTERNATIONAL SYSTEM UNITS The following factors may be used to convert the U.S. customary units published in this report to the International System of Units (SI). Multiply U.S. customary unit By Length To obtain SI (metric) unit inches (in) feet (ft) miles (mi) 25.4 .3048 1.609 millimeters (mm) meters (m) kilometers (km) Area square feet (ft^) square miles (mi^) acre .0929 2.590 4,047 square meters (m^) square kilometers (km^) square meters (m^) Volume cubic yards (yd^) acre-feet .764 1,233 Flow cubic meters (m^) cubic meters (m^) cubic feet per second (ft3/s) cubic feet per second per square mile [(ft3/s)/mi2] .02832 cubic meters per second (mVs) .01093 cubic meters per second per square kilometer [(m-Vs)/km2] feet per second (ft/s) miles per hour (mi/hr) Velocity .3048 1.609 meters per second (m/s) kilometers per hour (km/hr) Mass pounds (lb avoirdupois) ton (short, 2,000 lbs) .4536 .9072 kilograms (kg) tonne (t) xiii HYDROLOGY OF MAJOR ESTUARIES AND SOUNDS OF NORTH CAROLINA By G. L. Giese, H. B. Wilder, and G. G. Parker, Jr. ABSTRACT Hydrology-related problems associated with North Carolina's major estuaries and sounds include contamination of some estuaries with municipal and industrial wastes and drainage from adjacent intensively- farmed areas, and nuisance-level algal blooms. In addition, there is excessive shoaling in some navigation channels, saltwater intrusion into usually fresh estuarine reaches, too-high or too-low salinities in nursery areas for various estuarine species, and flood damage due to hurricanes . The Cape Fear River is the only major North Carolina estuary having a direct connection to the sea. Short-term flow throughout most of its length is dominated by ocean tides. The estuarine reaches of the Neuse- Trent, Tar-Pamlico, Chowan, and Roanoke River systems are at least partly shielded from the effects of ocean tides by the Outer Banks and the broad expanses of Pamlico and Albemarle Sounds. With the probable exception of the Roanoke River, winds are usually the dominant short- term current-producing force in these estuaries and in most of Pamlico and Albemarle Sounds. Freshwater entering the major estuaries is, where not contaminated, of acceptable quality for drinking with minimum treatment. However, iron concentrations in excess of 0.3 milligrams per liter sometimes occur and water draining from swampy areas along the Coastal Plain is often highly colored, but these problems may be remedied with proper treatment. Nuisance-level algal blooms have been a recurring problem on the lower estuarine reaches of the Neuse, Tar-Pamlico, and Chowan rivers where nutrients (compounds of phosphorous and nitrogen) are abundant. The most destructive blooms tend to occur in the summer months during periods of low freshwater discharge and relatively high water tempera- tures. 1 Saltwater intrusion occurs from time to time in all major estuaries except the Roanoke River, where releases from Roanoke Rapids Lake and other reservoirs during otherwise low-flow periods effectively block saline water from the estuary. Salinity stratification is common in the Cape Fear and Northeast Cape Fear Rivers, but is less common in other estuaries which do not have direct oceanic connections and where wind is usually effective in mixing with depth. The greatest known upstream advance of the saltwater front (200 milligrams per liter chloride) in most North Carolina estuaries occurred during or in the aftermath of the passage of Hurricane Hazel on October 15, 1954. Hurricane Hazel ended an extreme drought when many North Carolina rivers were at or near mini- mum recorded flows. Consequently, saltwater intrusions in many North Carolina estuaries were at or near the maximums ever known to have oc- curred. When the hurricane struck, high storm tides along the coast drove saline water even further upstream in many localities. The proba- bility of two such rare events happening concurrently is not known, but the recurrence interval may be reckoned in hundreds of years. New shoaling materials found in the lower channelized reaches of the Cape Fear and Northeast Cape Fear Rivers are primarily derived, not from upstream sources, but from nearby shore erosion, from slumping of material adjacent to the channels, from old spoil areas, or from ocean- derived sediments carried upstream by near-bottom density currents. It is not known at this time whether this holds true for other estuaries discussed in this report. INTRODUCTION The estuaries and sounds of North Carolina are among the State's most valuable resources. They serve as routes for low cost transpor- tation by means of ships and barges, as sources of large amounts of water for industrial and, where not too salty, for municipal use. They also serve as both fisheries and nurseries for a wide variety of marine life, and as focal points for recreational activities. Planning for optimum use of the estuaries for these sometimes conflicting uses de- pends in part on having detailed knowledge of the physical and chemical processes at work in them. Problems which have arisen in connection with the uses of North Carolina estuaries and sounds include (1) con- tamination of some estuaries with municipal and industrial wastes and drainage from adjacent intensively-farmed areas, (2) excessive shoaling in some navigation channels, (3) in nursery areas, too-high salinities due to low fresh-water inflow or too-low salinities due to high fresh- water inflow, (4) occasional fish kills related to contamination or deoxygenation of estuarine waters, (5) nuisance-level algal blooms in some estuaries, and (6) flood damages from unusually high hurricane- induced tides. 2 The approximate extent of North Carolina's Sounds and estuaries Is shown in figure A, and plate 1 summarizes conditions of maximum upstream saltwater encroachment and tide effects for major estuaries. These estuaries are in an area which many feel is experiencing the leading edge of a wave of agricultural, commercial, and recreational develop- ment. To minimize possible adverse effects of development and maximize benefits, management decisions related to development should be predi- cated, at least in part, on a basic understanding of the present hy- drology of North Carolina's estuarine waters. Often, this information has been lacking or inconvenient to gather when decisions must be made. The purpose of this report is to summarize current basic knowledge of the hydrology of the major estuaries and sounds in North Carolina, not only for use in management decisions now, but also for use as a general information source for other current and future estuarine studies in hydrology and related fields. This report was prepared by the U.S. Geological Survey in cooper- ation with the North Carolina Department of Natural Resources and Com- munity Development and is based partly on data and interpretive reports originating from the Geological Survey and partly on data and interpre- tive reports originating from other Federal, State, local, and private sources. These sources are acknowledged in the text where appropriate. This summary report, while fairly complete with respect to work done by the Geological Survey, is much less so for work done by other public and private agencies. To summarize all the vast accumulated body of hydrology-related work done by others is beyond the scope of this report. Rather, the intent here is to present a basic picture of the hydrology of the major estuaries and sounds in North Carolina in terms of freshwater inflow, tide-affected flow, water levels, freshwater quality, salinity, and sedimentation - utilizing Geological Survey data where available, but filling gaps where possible with information from other agencies. This report is divided into four chapters. The first. General Hydrology , is primarily a discussion of basic hydrologic principles relating to tides, tidal flow, salinity, sedimentation, and the effects of winds and hurricanes. The other three chapters summarize present knowledge of the hydrology of individual sounds and estuaries in each of three estuarine systems. The estuarine systems are, in order of dis- cussion, the Cape Fear River system, the Pamlico Sound system, and the Albemarle Sound system. The comprehensiveness of the summaries for each sound or estuary is directly related to the availability of information on which the summaries are based. The Cape Fear River estuary, for example, has been much more thoroughly studied than the Roanoke River. In general, the larger the estuary, the more complete is the information available. 3 4 The Cape Fear River estuarine system includes the Cape Fear River istuary and the Northeast Cape Fear River estuary. These are unique among major North Carolina estuaries in that they have a direct con- nection with the ocean. The Pamlico Sound estuarine system comprises Pamlico Sound, the Neuse River estuary, the Trent River estuary, the Pamlico River estuary, and a number of lesser estuaries which enter Pamlico Sound. These are characterized by large channels, small lunar tides, and by flow which is greatly affected by winds. The Albemarle Sound system comprises Albemarle Sound, Currituck Sound, and those estuaries draining into Albemarle Sound, the largest of which are the Roanoke River and the Chowan River estuaries. These are also characterized by small lunar tides and, with the possible exception of the Roanoke Estuary, by the fact that wind exerts a dominant short- term influence on water levels and water movement. 5 CHAPTER 1 GENERAL HYDROLOGY A sound is formally defined as a relatively narrow passage of water, too wide and extensive to be called a strait, that connects two water bodies, or it can be a channel passing between a mainland and an island. A sound is also sometimes thought of as an inlet, arm, or recessed portion of the sea. This last definition comes closest to describing Pamlico and Albemarle sounds, but even it is not entirely satisfactory. Thus, it may be that the term sound as applied to North Carolina's sounds is a misnomer, but no more formally correct term is available. The term estuary , as used in this report, is that part of the lower course of a coastal river affected by ocean tides. Flow of water in an estuary can be described as the superposition of tidal flow on the otherwise unaffected freshwater discharge of the estuary. In the lower reaches of an estuary, average flow due to tides may be many times the flow due to freshwater inflow. However, flow due to tides is cyclic and in the North Carolina area is semidiurnal, changing direction every six hours and twelve minutes (6.22 hrs) . Over a number of tidal cycles, the flow component from tides averages out to be practically zero, whereas the flow component from freshwater inflow, although it may be smaller, always acts in a downstream di- rection and controls long-term average flow in an estuary. Saline or salty water in an estuary moves upstream and downstream in response to tidal action, freshwater outflow, and turbulent mixing. In discussing this movement, it is convenient to select an arbitrary value of salinity, the location of which represents the upstream limit of the zone of saltwater mixing, which we will refer to as the saltwater front . In this report a value of 200 mg/L of chloride is used to establish the saltwater front because that concentration clearly in- dicates the presence of some seawater, and water with less than this amount is usable for most purposes. Seasonal ranges in movement of the saltwater front are primarily caused by seasonal changes in freshwater inflow and commonly are greater than daily ranges due to tides alone. In the Cape Fear River, for instance, the seasonal range due to freshwater inflow is typically 15 miles or more, whereas the range due to tides is only about 3 to 6 miles. This seasonal movement of the saltwater front is basically the product of two opposing processes. The saltwater front tends to be displaced downstream by incoming freshwater. On the other hand, the more dense saltwater tends to move upstream by mixing and diffusion of 6 salty water with fresher water upstream. If these two processes are roughly in balance, there will be little net movement of the front. When freshwater inflows are high, then the tendency towards downstream displacement of the front by incoming freshwater overwhelms the tendency towards upstream movement due to mixing and diffusion. When freshwater inflows are low, the opposite is true. During and immediately after periods of low freshwater inflow, which typically occur in the late summer and fall in North Carolina, the landward encroachment of the saltwater front is usually at its greatest for the year. Another factor which can contribute somewhat to greater encroachment at this time of the year is that mean tidal level may be several tenths of a foot higher during November than, say, during July. In the winter and spring months, when freshwater inflow is high, dis- placement of the saltwater front seaward is usually at its annual maxi- mum. Ocean Tides The driving force in the regular, periodic fluctuations in estu- arine stage, discharge, and movement of the saltwater front is ocean tides. Although the word "tide" has often been used rather loosely to include water-level fluctuations caused by other forces, such as wind and barometric pressure, it is considered in this report to include only those water-level phenomena caused by the gravitational attractions of the moon and the sun acting on the earth. The movements of the earth, moon, and sun relative to each other occur in cycles which, while complex, are predictable and repetitive. This, in turn, results in tides wtiich occur in cycles that are also complex, but predictable and repetitive. Actually, the rise and fall of the water and the accompanying currents should be dealt with together, because they are only different manifestations of the same phenomenon, a tide wave. However, for practical reasons, they are often dealt with separately, and the common English usage is to refer to the rise and fall of the water level as the tide, and to the accompanying currents as tidal currents. The tidal forces can be separated into a number of components called partial tides. The principal partial tides are listed in table 1.1. The partial tides are characterized by their periods and a co- efficient directly related to the magnitude of the partial-tide pro- ducing force. The partial-tide forces, when plotted against time, produce sine-like curves of various magnitudes and periods which alter- nately reinforce, then interfere with, one another. The resultant of all these partial-tide forces is a series of alternating high and low tides of varying magnitudes having a period of 12.42 hours. 7 Tablz 7.J.--T/ie tun mo6t ImponXanZ paAtlat tidd^. [Adaptiid {^K.om SdhuAmcLYi, 1924.) Name of corresponding partial tide Semidiurnal : Principal lunar Principal solar Larger lunar elliptic Luni-solar Diurnal : Luni-solar Principal lunar Principal solar Long-period : Lunar fortnightly Lunar monthly Solar semi-monthly Period, in hours Coefficient 12.42 12.00 12.66 11.97 0.4543 .2120 .0880 .0576 23.93 25.82 24.07 .2655 .1886 .0880 327.86 661.30 2191.43 .0783 .0414 .0365 8 Figure 1.1 is a graph of predicted high and low tides for August 9- 22, 1911 y for the Cape Fear River estuary at Wilmington, North Carolina. It clearly shows diurnal inequalities in the heights of the two high and two low tides each day. These are caused principally by the interaction of the several semidiurnal (twice daily) and diurnal (daily) partial tides (fig. 1.2). The diurnal partial tide reinforces one of the semi- diurnal tides and interferes with the other, thus producing the diurnal inequalities . When the range between high and low tide is largest, the tides are called spring tides; when the range is smallest, they are called neap tides. The recurrence interval of spring tides and neap tides is 14.3 days. They are primarily caused by the interaction of the principal lunar and principal solar semidiurnal tides. These have slightly different periods (12.42 and 12.00 hours, respectively) which result in alternatingly reinforcing then interfering with each other in cycles which take 14.3 days to complete. The juxtaposition of the two tide components during spring tide and neap tide is illustrated in figure 1.3. During spring tide the two components are nearly in phase with one another; during neap tide the two are almost completely out of phase. The results of this interaction are also illustrated in figure 1.1, which shows that the range between predicted high and low tides during August 15-18 was greater than that during August 9-11. The long-period partial tides listed in table 1.1 are not as important as the semidiurnal and diurnal components in controlling tide heights but may make a difference of about 0.5 foot seasonally in tide heights. The effects of these long-period partial tides are incorpo- rated into tide predictions of the National Ocean Survey. In addition, allowances are made in these predictions for differences in seasonal tide heights due to increasing or decreasing freshwater inflow in estuaries. These differences may be more than a foot in some estuaries. The National Ocean Survey annually publishes tide height and current predictions for a number of North Carolina coastal locations. Estuarine Flow As mentioned earlier, flow of water in an estuary can be described as the superposition of tidal flow on the normal downstream river flow. Knowledge of the rates of movement of estuarine waters and how rates and direction of movement vary are of great importance in aiding navigation, predicting the manner of movement and dispersion of pollutants, and in understanding sedimentation characteristics. The purpose of this section of the report is to describe the most important aspects of estuarine flow. 9 10 3 12 TIME, IN HOURS SEMIDIURNAL COMPONENTS 24 diuAnal and dluAnal tidal compomnU . Adapted (^^om Svc^id^ap, Johnson, and Vlmlnq, 1942. 3 Spring tide SEMIDURNAL TIDES — L — -Principal lunar partial tide --S-- Principal solor portiol tide 12 2 4 TIME, IN HOURS flguAQ, 1.3.--Tid(i du/ivu at i>pnx.nQ tidn and map tide. Adapted i^om SvQJidAap, Jokn^on, and VlmlnQ, 1942. 11 The combination of river flow and tidal flow will be referred to as tide-affected flow in this report. Tide waves associated with the tidal component of flow may have wave lengths of several hundred miles or more. When we realize that most estuaries are much shorter than this, it is clear that most estuaries can be occupied by only part of a tide wave at any given time. The behavior of an idealized tide wave in an estuary is illustrated in figure 1.4, which shows how water velocity DIRECTION OF WAVE PROPAGATION EXPLANATION ►Flood flow ■< Ebb flow Length of arrow indicates relative velocity. Note: Drawing not to scale. Wave lengtti may be several tiundred miles. V.LQLxAd 1 .4. --BQ,kavlo^ 0|5 an Id^aLLzdd tidd u)aue In an Qj>tuaAy. varies along the profile of a tide wave propagating along a channel. The wave here is idealized in that it has assumed symmetry, is propa- gating in a fluid of homogeneous density, is free from the effects of internal and boundary friction, and is propagating without opposition from freshwater inflow, barriers or any other flow obstruction. Note that the direction of horizontal water movement at the wave crest is always in the direction of wave propagation; at the wave troughs it is in the opposite direction. Horizontal velocity is zero halfway between the crests and troughs, and is maximum at the crests and troughs. 12 Several other interesting facts may be derived from figure 1.4. First, times of zero velocity do not coincide with times of high and low tide, as common sense might suggest. Rather, times of zero velocity (slack tides) occur about halfway between times of high and low tides. Second, water in a tide wave may flow up-gradient, that is, the water surface may be sloping downstream where the flow is upstream. When the effects of freshwater inflow, friction, and any barriers (such as dams) are superimposed on the flow pattern of an idealized tide wave, velocity distributions relative to the crests and troughs are modified. Freshwater inflow, for example, will not affect the time of occurrence of high or low tide, but will result in high-water slack occurring earlier than otherwise and low-water slack occurring later than otherwise. Thus, as freshwater inflow increases, high-water slack will occur closer in time to high tide. Also, the closer to the head of tide (or to a dam) one is, the nearer in time are high water and high- water slack. Thus, in real estuaries, the above factors may consider- ably alter the velocity distribution along a tide wave from what might be predicted based on the idealized situation portrayed in figure 1.4. (See later discussion of measurements of tide-affected flow in the Cape Fear and Northeast Cape Fear Rivers) . The wave form itself propagates. The speed with which it propa- gates is referred to as wave celerity and is equal to /gh, where g is the gravitational constant and h is the depth of the water in which the wave is propagating. Thus, one can calculate the difference in time between occurrence of high or low tide at different points along a total reach of known depth. An important point here is that the wave celerity does not refer to the velocities of individual water particles, but only to the wave form. Actually, the wave form propagates much faster than individual water particles can move. Freshwater inflow to an estuary opposes the flood-tide flow and reinforces the ebb-tide flow. When freshwater inflow is larger than maximum flood-tide flows, net river velocities will be dowistream during all of a tidal cycle. Otherwise, there will be twice-daily reversals of flow direction. One important influence on estuarine flows which is not usually significant in ordinary river flow is the Coriolis Effect due to the earth's rotation. In the northern hemisphere, the Coriolis Effect tends to deflect a moving particle to the right of its direction of motion. On a flood tide, therefore, the water moving upstream tends to hug the left bank. (U.S. Geological Survey convention assigns right and left bank in the sense of facing downstream). On an ebb tide, the flow tends to hug the right bank. Because the net flow in an estuary is down- stream, there is an overall tendency for flows to hug the right bank. With regard to salinities, however, the observed tendency is for salini- ties to always be higher on the left bank than on the right during both 13 flood and ebb tides. Presumably, this is because flood tides, which carry saline water upstream, tend to hug the left bank, causing higher salinities there; ebb tides, which carry fresher water downstream, tend to hug the right bank. These observations have obvious value in locating water intakes for water supply or outfalls for waste water. All other factors being equal, the right bank would be the obvious choice for the location of freshwater intakes because water may be less saline (be of better quality) and for waste outfalls because flows on ebb tides would be greater (for better dilution and transport of wastes) . Salinity At this point, the term salinity needs to be more precisely defined and discussed. Salinity refers to the degree of saltiness of water, or more specifically, the concentration of dissolved solids in water. The generally accepted formal definition of salinity was given by Forch and others (1902) who defined it as "the total amount of solid material in grams contained in one kilogram of seawater when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized." Even though this formal definition refers to salinity as an amount, in practice salinity is generally expressed as a concentration, in parts per thousand of sea water (^/qq) or milligrams per liter of dissolved solids (1,000 mg/L is approximately equal to 1 part per thousand) . Average concentrations of the major constituents of seawater as determined by Jacobsen and Knudson (1940) are given below: Constituent Concentration (mg/L) Chloride (CI) 18,980 Sodium (Na) 10,556 Sulfate (S04) 2,649 Magnesium (Mg) 1,272 Calcium (Ca) 400 Potassium (K) 380 Bicarbonate (HC03) 140 These constituents account for 34,377 out of the total of 34,482 mg/L of dissolved solids in seawater. Although these values are the standard used in this report, it should be recognized that concen- trations of constituents in seawater vary from time to time and place to place. For example, the total dissolved-solids concentration of 17 samples of seawater collected near Wrightsville Beach, North Carolina, 14 by the U.S. Geological Survey between 1963 and 1965, ranged from 31,900 to 35,900 mg/L. However, these variations do not differ appreciably from the average of 34,482 mg/L of dissolved solids and thus they are of minor importance in accounting for salinity variations in North Carolina estuaries. Even where the total salinity of seawaters varies, the relative concentrations of major ionic species remains constant — and the salinity of a given water may be approximately determined if the concen- tration of any one of the major constituents of the water is known. For example, if the chloride concentration of a given water is 10,000 mg/L, then the salinity is HfgH >< 34.5 O/qq = 18.2 O/qq. Determinations of individual chemical constituents, such as chloride, can be time-consuming or otherwise impractical m some situ- ations. Salinity is often determined in the field by measurement of specific conductance of the water. The specific conductance, measured in micromhos (ymhos), is proportional to the dissolved-solids concen- tration of the water. Because the ratio of the concentration of a given major constituent dissolved in seawater to the total dissolved-solids concentration of seawater is almost constant, specific conductance may be used to estimate the concentration of any of the major constituents of sea water. Such a relation has been prepared for specific conduct- ance versus chloride and dissolved solids (fig. 1.5). Chloride is a particularly important constituent because it is often a limiting element in determining suitability of a water supply for public or industrial use. The National Academy of Sciences (1972) [1974] recommends an upper limit of 250 mg/L of chloride for drinking water, and water with 500 mg/L or more is unsuitable for a number of industrial uses. Thus, the 200 mg/L of chloride criterion used in this report (equivalent to a specific conductance of 800 ymhos in figure 1.5) to indicate the presence of saltwater is within the National Academy of Science's recommended upper limit for drinking water. Estuarine Types Mixing of freshwater and seawater takes place through turbulent mixing and molecular diffusion. The rates of mixing depend on channel geometry, the relative amounts of freshwater inflow and tidal flow, wind, and the differences in density between seawater and freshwater. (Seawater has a specific gravity of about 1.025 as compared to about 1.000 for freshwater.) Most mixing situations produce one of three "types" of salinity patterns — highly stratified, partially mixed, and well mixed. Estuaries are often classed according to which of these mixing patterns predominates. Among North Carolina estuaries, examples of each type can be found. It is worthwhile, therefore, to discuss the characteristics of each. 15 figuAn 1 . S . --RdZcition b^tw(L2.n 6p(Lai{)ic conduatcinc^ and ckJConldo.. concmtAoutioM in Uo^th CoAotlna utu- 16 In a highly stratified estuary, the freshwater, being less dense, tends to ride over the top of the seawater (Fig. 1.6). Viscous shear forces along the boundary between the freshwater and seawater cause much turbulent mixing, making this a zone of rapid transition between fresh- water and seawater. Turbulence patterns within this transition zone are such that the water has a net upward circulation and seawater becomes entrained with the freshwater moving downstream. This phenomenon of upward breaking waves has been discussed by, among others, Pritchard and Carter (1971) and Bowden (1967). Thus, seawater directly below the transition zone retains its sa- linity, while freshwater above becomes more mixed with seawater as it moves further downstream. Finally, at some downstream point, the water flowing near the surface becomes indistinguishable from seawater. The rapid salinity change in the transition zone is evident from examination of section x-x' in figure 1.6B. Net velocity under highly stratified conditions (fig. 1.6B) is downstream near the surface, is drastically less in the transition zone, is zero somewhat below the transition zone, then is upstream in di- rection at greater depths. If the velocities along this profile were integrated over a period of time, the resultant net flow would of course be downstream due to the energy gradient of the freshwater inflow. This energy gradient accounts for the net downstream flow in the upper levels of the river, while the net upstream flow near the channel bottom is from energy provided by gravitational convection. (Flows along the channel bottom due to gravitational convection are often referred to as density currents.) The saltwater wedge, which would otherwise move upstream to the limit of tidal influence due to density differences between freshwater and seawater, is constantly being eroded by contact and mixing with freshwater. This lost seawater is replaced by seawater moving upstream along the channel bottom. Thus, an equilibrium of the net position of the saltwater wedge may be maintained. A highly stratified condition may exist in an estuary only when the freshwater inflow is large in relation to tidal flow. A rule of thumb given by Schubel (1971) is that, in order to have highly stratified conditions, the volume of freshwater entering an estuary during a half- tidal period (6.21 hours) should be at least as great as the volume of water entering during a flood-tide. In other words, the ratio of the freshwater volume to the flood-tide volume should be at least 1.0. This ratio is called the "mixing index." The reliability of the mixing index in predicting the degree of estuary stratification is influenced by channel geometry. As width increases or depth decreases, an estuary tends to become less stratified for a given mixing index. 17 Sea DOWNSTREAM WATER SURFACE River I FRESHWATER ^ ^ ^£l^?5r:)>=. ^ I CHANNEL BOTTOM B. Section X-X T C. Section X-X 25 50 75 100 SALINITY, IN PERCENT OF SEA WATER ^ 20 o 40 1 1 on zone L Tronsiti 2 10 12 Downstreom Upstream VELOCITY, IN FEET PER SECOND f-tguAQ, J.6.--A, Wet c-iAculcutio n pcuU2Ayii> in a highly 6tA(itl{)i^d QJ^taoALj. B. SaLtnlty pn.oillz through i>zctA.on X-X'. C. hl^t KJoZociZy ph-oillo, th/iough i>(icJU.on X-X'. 18 If the freshwater inflow becomes smaller in relation to the flood- tide volume (within a mixing- index range of about 0.05 to 1.0) then partially-mixed conditions may prevail, as sketched in figure 1.7A. In this situation flow reversals will probably occur throughout the depth of the estuary during at least part of each tidal cycle. (By contrast, flow along the surface at some point in a highly stratified estuary may be downstream at all times and flow near the channel bottom in the saltwater wedge may be upstream at all times.) Under partially-mixed conditions tidal flow dominates and the added turbulence from this source provides the means for eradicating the saltwater wedge. Not only does seawater mix upward into what was the freshwater zone, but fresh- water mixes downward into what under highly stratified conditions was a zone of seawater. The sharp interface which separated the freshwater from the seawater in the highly stratified estuary is replaced by a much broader zone of moderate change in salinity. The saltwater front is shown in profile in figure 1.7A, and other lines of equal chloride concentration would have similar attitudes. The net changes in chloride concentration with depth through section Y-Y' (fig. 1.7B) are gradual. The lack of a sharp interface between fresh and saltwater is apparent, but some degree of stratification remains. The variations in net velocity with depth are not obvious from the circulation arrows shown on figure 1.7A, which show only that turbulent mixing is present throughout a wide region. However, a definite pattern does exist, as shown in figure 1.7C. As in a highly stratified estuary, net velocities in the upper layers of the channel are downstream and net velocities in the lower layers are upstream. However, instantaneous flow at a particular point in many partially-mixed estuaries may be upstream or downstream at a particular moment at any depth. An unusual feature of flow in a partially-mixed estuary is that the rates of flow in both the upper and lower layers may be an order of magnitude higher than river flow. For example, if F_ is the freshwater inflow, the upper-layer net seaward flow may be lOF . Since the estuary as a whole is neither filling nor emptying, then 9Y_ must be brought up the estuary from the sea in the lower layers. Examples of this phe- nomenon have been verified for the James River in Virginia and the Chesapeake Bay (Pritchard and Carter, 1971, p. IV 7). The third major type of estuary is termed well-mixed . Figure 1.8A shows the profile of the saltwater front in a well-mixed estuary. It is nearly vertical, which indicates that mixing forces are greater than in a partially-mixed situation. Schubel (1971) indicates that the upper limit for the mixing index is probably about 0.05 for well-mixed con- ditions to exist. In this situation, freshwater inflow is very small in relation to tidal flow. Because salinities are nearly homogeneous vertically, density currents are negligible. Thus, velocities are unidirectional from top to bottom at a given time in a given profile, as shown in figure 1.8C. With regard to salinity, although there may be 19 DOWNSTREAM Sea WATER SURFACE River o CHANNEL BOTTOM E 100 200 300 400 - NET CHLORIDE CONCENTRATION, ^ IN MILLIGRAMS PER LITER 2 10 12 Downstream Upstream NET VELOCITY, IN FEET PER SECOND VIquaq, /.7.--A. HqJ: CAAaaZcutlon pattoAM In a pa/itlalty mlxdd utu- cvty. B. ChloHldd CLoncdnXAotlon pK-O^lld thAoagh ^dction V - V . C. hldt velocity pKoiiZe, thAoagh i^ecition V - V\ 20 DOWNSTREAM Sea WATER SURFACE River OGIo o Or, CHANNEL BOTTOM C. Section Z-Z 10 20 30 40 100 200 300 400 x" NET CHLORIDE CONCENTRATION, ^ IN MILLIGRAMS PER LITER LU Q 1 1 1 ■o 1"< \ l\ •o o o _ 2 10 12 Downstream Upstream INSTANTANEOUS VELOCITY, IN FEET PER SECOND B. CkloH^do. conc^ntAcuUon p^oiiZz through. ^zcXlon Z-Z'. C. \/(ilocjXij pn.oiiJi^ thAoagk izction Z-Z'. 21 some slight changes from top to bottom in a well-mixed estuary (figure 1.8b), the changes are uniform, without the zone of more rapid change characteristic of partially-mixed and highly stratified estuaries (fig. 1.6B and 1.7B). The above discussions provide a framework for understanding sa- linity changes as related to circulation patterns in estuaries. Al- though three types of estuaries were discussed, it should be recognized that there is an almost continuous spectrum of salinity and circulation patterns and that there are gradual transitions between the three types In fact, a given estuary may be highly stratified in the spring during periods of high freshwater inflow, partially mixed in the early summer, and well mixed in the fall when freshwater inflow is minimum. Within the same estuary, for a given mixing index, the mixing type may change as the saltwater front moves into an area of changing channel geometry. The general rule with respect to channel geometry is that an estuary tends to shift from a highly stratified to a well-mixed condition with increasing width and decreasing depth (Schubel, 1971, p. IV-14) . Sediment The mechanics of transport and deposition of sediment in an estu- ary are far more complex than in ordinary streams. Yet, because of the impact of sediment deposition on aquatic life and on navigation and because large sums of money are spent in dredging and maintaining navigation channels and boat facilities, it is important to develop a clear understanding of the principles of estuarine sedimentation. Sediment, whether moving in a free-flowing stream or in an estuary, has two components — suspended sediment and bed load. Suspended sediment comprise particles that are held in suspension by the upward components of turbulent currents and finer particles held in colloidal suspension. Bed load consists of material too heavy to be held in suspension but which nevertheless moves by sliding, rolling or skipping along the bed of a stream or estuary. In a free-flowing stream, however, net flow (and, therefore, sediment discharge) is always downstream and usually changes in magnitude slowly, whereas in an estuary the tide-affected flow (and therefore sediment discharge) is almost always changing rapidly in magnitude and direction. Changes in chemical quality along streams are usually small and have negligible effect on sediment concen- trations, whereas quite dramatic changes in chemical quality occur within estuaries and these may profoundly influence sediment transport characteristics . In many estuaries, a characteristic zone of high concentrations of suspended sediment and high turbidity begins near the saltwater front and, in some estuaries, continues downstream for miles. This zone has been discussed by, among others, Ippen (1966) and Schubel (1971). 22 Upstream from this zone, in the freshwater portion of the estuary, concentrations are less; downstream from this zone, in the ocean, concentrations are also less. The probable explanation for this zone is that such estuaries act as sediment traps . This may be better under- stood by referring back to the net circulation patterns on figure 1.6C for a highly stratified estuary. Imagine suspended sediment being carried out to sea in the top layers of freshwater during ebb flow. As the time of slack water approaches where the estuary widens towards the mouth or as the freshwater spreads out over the bays and adjacent ocean, velocities decrease, thus allowing heavier sediment particles to settle. As they settle, they are entrained in water moving upstream along the channel bottom. At the upstream end of the saline water zone, flow circulates upward and downstream, and sediment particles may again be entrained upward and flow towards the sea in the upper freshwater layers. Again, the particles may settle as velocities decrease, and thus a sediment particle may be caught in a loop pattern several times. This sediment-trap phenomenon is found to some extent in highly stratified situations, but is even more pronounced in partially-mixed estuaries where net upstream velocities near the channel bottom and net downstream velocities near the surface are much greater than in highly stratified estuaries. This phenomenon is probably not found to any significant extent in well-mixed estuaries, where there is no signifi- cant net upstream flow along the channel bottom. The sediment trap zones in estuaries are, naturally enough, zones of high sediment deposition. Most of the deposited sediment in these zones is of clay or silt size. The particles tend to settle to the channel bottom wherever or whenever instantaneous or net velocities suddenly decrease or approach zero. The tip of a saltwater wedge is one area of rapid deposition because net velocity is zero in that vicinity. Other potential areas of deposition are where tributaries enter a slow- moving main channel, in bays, and in boat slips. Another potential factor that may account for some sedimentation in the sediment trap zone is f locculation and subsequent deposition of clay-sized particles in the water. This process depends on the presence of electrolytes, such as sodium chloride, which neutralize the electro- negative characteristics typically associated with sediment particles. Salt water is an electrolyte, and the settling of fine-grained particles is indeed observed in the saline water zone. However an additional or alternative binding mechanism brought about by filter feeding organisms has been advanced by Schubel (1971) . He describes the results of an ex- tensive size-analysis study of particles in suspension at all depths in Chesapeake Bay and the Susquehanna River. He reports on p. VII-20: "Many composite particles were observed, particularly in the lower layer, but careful microscopic examination showed that most were agglomerates weakly bound by organic matter and mucus and 23 probably produced by filter-feeding zooplankton. Preliminary experiments have indicated that suspension-feeding zooplankton probably play a major role in the agglomeration of fine particles in the water column, and in the subsequent deposition of those particles. The large population of filter-feeding zooplankton present in the Bay probably filter a volume of water equivalent to that of the entire estuary at least every few weeks, and perhaps every few days." Previous to this statement Schubel stated that his evaluation techniques failed to produce any evidence of f locculation. In view of this, it might be safer to refer to these composite particles as agglomerates rather than flocculates. An agglomerate is a more general term meaning a composite particle composed of two or more individual particles held together by any relatively weak cohesive force. A flocculate is an agglomerate bounded by electrostatic forces. Potential sources of silt and clay-sized sediment deposited in a sediment- trap zone are many. Studies on many United States estuaries have shown that sediment from upland discharge is inadequate in most cases to account for the shoaling rates that are observed in river channels and harbors. Ippen (1966, p. 654) lists other sources of shoaling material as follows: . ^ "a) marsh areas adjacent to the estuary with runoff draining into the tidewater, b) materials in larger estuaries being eroded from the shores by wave action and moving by density currents into the deeper portions , c) materials being displaced by dredging and propeller wash and moved by density or tidal currents, d) organic materials as a result of the biological cycles of estuarine plant and animal environment, e) industrial and human wastes discharged into the estuary, f) windborne sediment." - In addition to these sources mentioned by Ippen, sediment resus- pended from the channel bottom and later redeposited in shoaling areas may also be an important factor in high local shoaling rates in some estuaries. In some cases, the open ocean adjacent to an estuary may also be a significant source of sediment. 24 Regarding sediment deposition, and attempts to improve existing shoaling characteristics, Ippen (1966, p. 650) makes the following points : "a) sediments settling to the bottom zone in an estuary will on the average be transported upstream and not downstream, b) sediments will accumulate near the ends of the saltwater intrusion zone and form shoals. Shoals will also form where the net bottom velocity is zero due to local disturbances of the regime such as by tributary channels, c) the intensity of shoaling will be most extreme near the end of the intrusion for stratified estuaries and will be more dis- persed in the well mixed estuary. Therefore, with regard to human interference in existing estuary patterns, the following general rules may be derived: a) the major portion of sediments introduced from whatever source into an estuary during normal conditions will be retained therein, and if transportable by the existing currents will be deposited near the ends of the salinity intrusion, or at locations of zero net bottom velocity, b) any measure contributing to a shift of the regime towards stratification will cause increased shoaling. Such measures may be: structures to reduce the tidal flow and prism, diversion of additional freshwater into the estuary, deepening and narrowing the channel, c) dredging of channels should be accompanied by permanent removal of the sediments from the estuary. Dumping downstream is highly suspect and almost always useless. Agitation dredging falls into the same category, if permanent removal is desired." Although the principles discussed in this section are useful in understanding general aspects of estuarine sedimentation, the actual movement and deposition patterns of sediment in real estuaries are usually extremely complicated in detail, and may require hydraulic model studies to adequately define. Model studies of this kind are lacking for most sounds and estuaries in North Carolina. Nevertheless, some information about sedimentation in North Carolina sounds and estuaries is given in later sections of this report. 25 Effects of Winds and Hurricanes Most estuaries are thought of as freshwater inflow dominated, tide- dominated, or some combination of both. However, several major estu- aries in North Carolina fall into a less common category~wind-dominated . To understand how these wind-dominated conditions exist, first consider that the Outer Banks greatly weaken ocean tides in Pamlico and Albemarle Sounds and their tributaries. For example, the mean tide range in the ocean off Cape Hatteras is about 3.6 feet, according to the National Ocean Survey, while the range within Pamlico and Albemarle Sounds is less than half a foot. Consider also that the channels of many estuaries west of the Outer Banks are very large for the amount of water they carry and that, consequently, velocities due to freshwater inflow into them are often very low. In this situation of weak currents from both tides and fresh-water inflow, wind-generated currents take on a relatively more important role. In addition, the funnel effect of wind-generated currents flowing into estuaries from Albemarle and Pam- lico Sounds results in much stronger wind-generated currents in those estuaries than would otherwise occur. This combination of circumstances results in wind playing a much more prominent role in circulation and mixing patterns than would otherwise be the case. The physics of water movement in response to winds is extremely complex. It is sufficient for our purposes to consider that, owing to friction between moving air and the water surface, a certain amount of water will be "pushed" in the direction toward which the wind is blow- ing. The amount of water moved depends upon several factors, the most important of which are the velocity of the wind, the continuous distance along the water-surface over which the wind is effective (called the fetch) and the depth of the water. Movement of water by wind becomes important when water levels adjacent to shore lines are adversely af- fected. Obviously, on-shore winds cause water to pile up along the shore, and offshore winds cause a lowering of water levels. The interior shore lines of North Carolina have a complex configu- ration, and it is difficult to predict the effects of a given wind on water levels at a particular location. However, with certain modifi- cations, the following equation (Bretschneider , 1966, p. 240) may be applied with useful accuracy to some estuaries and sounds of North Carolina for predicting wind setup (change in water level) : (1.1) 26 where: S = wind setup, or change in water level, h = average depth, ~6 k = constant empirically evaluated at 3.3 x 10 , X = effective length of water surface over which wind is acting, or fetch, U = wind speed, n = constant, which is unity in a rigorous solution of the deriving equation. (Where calibration data are available for a particular location, n_ can be empiri- cally changed to obtain more precise estimates of S.), and g = acceleration due to gravity To use equation 1.1 to estimate the change in water level caused by winds at some point along the shoreline of the sounds, it is necessary to determine the component of wind that will be effective in producing the greatest change in water level at a point of interest. The ef- fective component will usually be the component acting along the longest continuous line of fetch to the point where the increase in water level is to be calculated. For a given wind the component is calculated from the angle of departure of the actual wind direction from the effective line of fetch. By simple trigonometry the effective wind speed is: U = U Cos a (1.2) e a where: = effective wind speed, = actual wind speed, and a = angle of departure of wind direction from the effective line of fetch. As an example of the use of equations (1.1) and (2.2), suppose the wind is blowing from the east at 30 mi/h (44 ft/s). The angle of de- parture of wind direction from a line perpendicular to the coast is 20°. The average depth along the fetch is 40 ft. The length of the fetch over which the wind is acting (distance perpendicular to the coast) is 50 miles (264,000 ft). Solving equation (1.2) first: U = (44 ft/s) (cos 20°) = (44 ft/s) (.9397) = 41.3 ft/s 27 Solving equation (1.1), for eventual wind setup, S, 40 40 2(3.3x10 S (264,000) ^ (41.3)^ + 1 32.2 (40)^ 1 [l.028439-lj = 40 028439]] = 1.13 ft In practice, a curve is plotted from equation (1.1) showing S versus U . Where possible, actual observations of S and U are used to adjust t^e constants in equation (1.1) and a final equation is developed which most accurately defines conditions at a particular location. A "wind compass" is developed on which the various angles of departure in equation (1.2) are plotted as adjustment coefficients. To determine setup for a given wind at a given location, it is only necessary to use the wind compass to determine the adjustment coefficient, multiply the actual wind speed by this coefficient, and consult the "S versus U " curve for that location. Like most coastal areas. North Carolina's shorelines are affected by a full range of winds, from gentle breezes caused by temperature differentials between land and ocean to violent winds associated with major storm systems. Predictable diurnal and seasonal shifts in wind direction cause daily and seasonal shifts in tide heights and currents. However, the most dramatic wind effects are those associated with hurri- canes, which seasonally threaten the eastern parts of the State. Since 1870, approximately one hundred storms of hurricane force have to some degree affected eastern North Carolina. One of the most spectacular and destructive was Hurricane Hazel which, on October 14, 1954, caused an estimated 100 million dollars worth of wind and water damage throughout the eastern one-third of the State and forced saltwater further upstream into the estuaries than ever recorded before or since. The effects of hurricanes on water levels along the coast are extremely complex. As illustrated on figure 1.9., hurricane winds in the Northern Hemisphere circulate in a counter-clockwise pattern around an eye, usually from 50 to 75 miles in diameter, within which winds are calm to very light. Depending upon the position of this eye relative to a particular location, winds associated with the storm may come from any direction. Should a part of the eye pass over the location, two periods of high wind velocity separated by a period of calm will be observed. Wind directions of the two high-wind periods may be out of phase with one another by nearly 180°. It is also worthy of note that effective wind speeds to the right side of these storms (with regard to the general direction of motion) are greater than those on the left side. In the North Carolina area, hurricane systems usually move at speeds of 5 to 40 miles per hour, and this forward motion is additive to wind speed due to the counterclockwise motion of the circulation system. 28 f^gu/LZ 1.9.--Tkz iiiizcX {^omoAd motion o{^ a kuAAicam on Mind vqZocJJxu In mck qaad/tant. 29 The overall effect of this complex pattern of hurricane winds is that resulting water-level changes are even more complex. At a given point on the shore, water levels may be either raised or lowered, or first raised then lowered, or first lowered then raised. The need to protect against hurricane damage has led to the development of sophisti- cated mathematical models to predict hurricane surge. Although a com- plete discussion of these models is beyond the scope of this report, the reader is referred to Amein and Airan (1976) for a presentation of a mathematical model of circulation and hurricane surge in Pamlico Sound. Later chapters of this report indicate the susceptibility of many of North Carolina's coastal areas to hurricane- induced flooding. The Salt-Marsh Environment Thus far, we have been concerned only with general principles of hydrology which will be applied in describing the major sounds and estuaries of North Carolina in following sections of the report. How- ever, the salt-marsh environment, a rather unique and important type of estuarine environment, deserves some general discussion here, also. Much of the coastal fringe areas of North Carolina's sounds and estuaries may be classified as a salt-marsh type of environment. These areas are ecologically very important for several reasons. First, they serve as nurseries for a variety of animals that are harvested in im- portant commercial and sport fisheries; these include shrimp, crab, scallop and many fish species. Secondly, salt marshes are very pro- ductive natural areas upon which many animals depend for food. An estimated 65 percent of the total commercial fisheries catch in coastal waters of the eastern United States is made up of species that depend directly in one way or another on salt marshes and estuaries during some phase of their life cycles (McHugh, 1966) . The 1973 dockside value of the North Carolina commercial fishery harvest was about $16 million, and the value is increasing every year (Thayer, 1975, p. 62). In addition, about $100 million is spent annually by North Carolina and out-of-state residents for sport fishing in coastal waters of the State. As of 1962, North Carolina contained about 58,400 acres of regu- larly flooded salt marshes and about 100,450 acres of irregularly flooded salt marshes (Wilson, 1962) . The most abundant plants in these salt marshes are saltwater cordgrass and needlerush, with grasswort and salt meadow cordgrass occurring in lesser densities (Frankenberg, 1975, p. 55). The primary productivity (plant production) of the salt-marsh environment is phenomenal — twenty times as productive as the open ocean and one to two orders of magnitude higher than most other ecosystems — they are rivaled in production only by tropical rain forests and highly cultivated land. Table 1.2 gives comparative production rates for the North Carolina salt marshes and various cultivated crops. Plant matter from these lush salt marsh areas is consumed either directly by some 30 marine animals (such as shrimp, mullet, and zooplankton) or indirectly by others which eat the direct consumers. Table, 1 .2.--FnAjfnaAy p^odacitlon n.cutu> voaIolu eco6y6tQ,m . [Adapted Odum, /959, and K^^^^, 1972.) System Net primary production rate, in pounds of plant material per acre per year Hay - U.S. Average Highest (California) 3,738 8,366 Wheat - World Average Highest (Netherlands) 3,497 11,867 Corn - World Average Highest (U.S.) 6,102 11,442 Rice - World Average Highest (Japan) 5,686 13,597 North Carolina Salt Marshes Saltwater Cordgrass (Spartina alternif lora) regularly flooded irregularly flooded 11,570 5,429 Needlerush ( Juncus roemarianus) regularly flooded irregularly flooded 9,834 6,034 Saltmeadow Grass (Spartina patens) regularly flooded irregularly flooded 11,534 8,837 The salt-marsh environment, important as it is in the life cycles of many marine (and freshwater) organisms, is a fragile one — subject to adverse effects from changes, not only from a wide variety of natural phenomena, but also from encroachment by man. Pollution, landfill and dredging, building, drainage of marshes, and alteration of freshwater flow regimes are some of the activities of man that either destroy or have adverse effects on the saltmarsh environment and marine organisms which are directly or indirectly dependent on it. Tihansky and Meade (1974) estimate that, nationally, we are losing for fishery production about 1 percent per year of our total estuarine environment. The annual percentage lost from salt-marsh areas may be even higher. 31 Many of the problems associated with maintaining a viable salt- marsh environment are related to changes in hydrology, either natural or man- induced. Heath (1975) reports on the hydrologic impact of agri- cultural developments in the Albemarle-Pamlico region, many of which affect the salt-marsh areas. He points out that drainage canals built in conjunction with large-scale corporate farming developments remove fresh-water runoff to the coast more quickly than the previous natural system. During periods of heavy runoff through drainage canals, salini- ties may be reduced in salt marsh areas to the point where young shrimp and other marine fishes sensitive to low salinities are forced out of the protective food-rich muck of the salt marshes into more-saline unprotective sandy-bottom areas where conditions are much less favorable for their survival. High sediment loads are another problem associated with high flows and also with construction activities. Clay-sized particles, particu- larly, may harm bottom-dwelling and filter-feeding organisms by clog- ging their feeding apparatus and hampering burrowing activities. Many salt-marsh areas are affected to some degree by pollution from agricultural areas. This pollution may be of several types — high levels of total coliform bacteria, fecal coliform bacteria from human and animal wastes, pesticides, and nitrogen and phosphorous (nutrients which may promote destructive algal blooms) . The Geological Survey is conducting several studies in cooperation with the North Carolina Department of Natural Resources and Community Development which bear on these issues. One, a study of the effects of land clearing and drainage canals on the hydrology of the Albemarle- Pamlico region, deals in part with effects on salt-marsh areas. Another seeks to determine drainage areas of streams throughout North Carolina - including the drainage areas of many small coastal streams which are at present undetermined. 32 CHAPTER 2 HYDROLOGY OF THE CAPE FEAR RIVER ESTUARINE SYSTEM Together, the lower Cape Fear and Northeast Cape Fear Rivers com- prise what we will term the Cape Fear River estuarine system. (See plate 1.) Actually, the Northeast Cape Fear River basin (drainage area - 1,740 mi^) is a subbasin of the Cape Fear River basin (total drainage area - 9,140 mi^) and the the Northeast Cape Fear River estuary may be thought of as a branch of the Cape Fear River estuary. Never- theless, for ease of analysis the Northeast Cape Fear River estuary will be discussed separately. The Cape Fear River and the Northeast Cape Fear River estuaries are the only major estuaries in North Carolina having a relatively free and direct access to the ocean, which results in significant tides and tide- affected flow within them. In the Cape Fear River estuary, tides extend up to Lock 1, about 65 miles upstream from the mouth near Southport. As shown in plate 1, the mouth is at a river cross-section extending from Fort Caswell east to the western tip of Smith Island. Tide effects in the Northeast Cape Fear River estuary extend to about 48 miles from its mouth at Wilmington, which in turn is located about 28 miles upstream from the mouth of the Cape Fear River estuary. The lower reaches of the Cape Fear River and Northeast Cape Fear River estuaries are subject to saltwater intrusion, sometimes rendering the water unsuitable for some uses. Plate 1 shows the approximate extent of saltwater intrusion in both estuaries, that is, the furthest upstream advance of water containing 200 mg/L of chloride ever known and the furthest upstream advance that has a 50 percent chance of being equaled or exceeded in any year. Rapid industrial growth has taken place along the banks of the two estuaries in recent years, and a number of industries use the water from them as process water and discharge industrial wastewater into them. The Cape Fear River and Northeast Cape Fear River estuaries are important navigable waters, and navigation channels are maintained to various depths by the U.S. Army Corps of Engineers. The channel di- mensions maintained for navigation will be discussed in more detail in following sections of this report. The summaries of the hydrology of the Cape Fear River and Northeast Cape Fear River estuaries are based on data and other information from various sources, primarily the Geological Survey. Plate 1 shows the location of key flow and water-quality stations operated in the Cape Fear River basin by the Geological Survey. These stations include the Cape Fear River at Phoenix (sta. no. 02107570), where the Geological 33 Survey generated records of volumes of water flowing upstream and down- stream during each tidal phase for the period April 1966 through March 1969. Data from these stations are published annually in the U.S Geo- logical Survey water-data report series for North Carolina. Wilder and Slack (1971a and 1971b) summarized data on chemical quality of streams in North Carolina from 1943 through 1967. Wilder and Hubbard (1968) reported on saltwater encroachment in the Cape Fear River estuary; Hubbard and Stamper (1972) reported on the movement and dispersion of soluble pollutants in the Northeast Cape Fear River estuary; the U.S. Corps of Engineers (1976) discussed sedimentation and related aspects of hydrology in the lower Cape Fear River estuary. These and other sources of information are acknowledged in the text where appropriate. The Cape Fear River Estuary The Haw and Deep Rivers, two major tributaries of the Cape Fear River, head in the Piedmont Province and flow southeast, joining at Moncure, to form the Cape Fear River (plate 1). From Moncure, the Cape Fear River flows southeast through the Coastal Plain to its mouth near Southport. The Cape Fear River drains a larger part of North Carolina than any other river; the drainage area at the mouth is 9,140 mi^, all of which is in North Carolina. Of this, the Deep River accounts for 1,422 mi^ and the Haw River accounts for 1,705 mi^. Other major tribu- taries include the Black River (1,563 mi^) and the Northeast Cape Fear River (1,740 mi^). Prior to the construction of Lock 1 about 37 miles upstream from Wilmington, river stage was affected by ocean tides possibly as far as 50 to 75 miles upstream from Wilmington. The construction of Lock 1 eliminated the effect of ocean tides above this point and this lock, therefore, marks the upstream limit of tide effect at the present time. The total length of the estuary from its mouth to Lock 1 is about 65 miles . The lower part of the estuary, beginning about three miles below Wilmington, averages about a mile in width and contains numerous scattered islands and a few tidal flats. It resembles an elongated bay only a few feet deep except along the ship channel which has been dredged and maintained in this reach to depths of 32-40 feet and widths of 300-500 feet. By comparison, the channel above Wilmington is narrow, with depths ranging from 20 to 60 feet. Tidal currents in the upper reach, particularly near the lower end, are strong and velocities may exceed 3 ft/s. 34 Flow Flow in the Cape Fear River estuary is strongly tide-affected. Except during periods of high freshwater inflow, regular reversals of flow occur with each tide. During periods of higher-than-average fresh- water inflow, outflow at some or all locations in the estuary may be high enough to overwhelm incoming flood tides, resulting in a period when no flow reversal occurs. Because the freshwater inflow is such an important component of flow in the estuary, it will be discussed at length before further considering tide-affected flow. Freshwater inflow . — The average discharge at the mouth of the Cape Fear River estuary is about 11,000 ft^/s. Because of the difficulty of accurately measuring flows in the estuary portion of the river, values for discharges were arrived at by summation of flows for gaged areas within the basin and estimates of flows for ungaged areas. The major part of the freshwater inflow is measured at the stream- gaging stations shown on plate 1. The stations closest to the estuary gage the runoff from 6,660 mi^ or 72 percent of the total 9,140 mi^ of drainage area above the mouth of the estuary. The three stations of greatest importance are the most downstream stations on the Cape Fear, Black, and Northeast Cape Fear Rivers. The runoff from 6,500 mi^ of the Piedmont and the inner part of the Coastal Plain is measured at the three stations listed below: Number Name Drainage area 02105769 Cape Fear River at Lock 1 near 5,220 mi^ Kelly 02106500 Black River near Tomahawk 680 mi^ 02108000 Northeast Cape Fear River near 600 mi^ Chinquapin These stations will be referred to as Lock 1, Tomahawk, and Chinquapin, individually, and collectively as index gages. The largest ungaged areas are in the lower part of the Coastal Plain, the area adjacent to the estuary itself. Most of the runoff from the total area of 2,640 mi^ below the index stations is ungaged. How- ever, a relationship between the runoff from this ungaged area and the combined runoff at the Tomahawk and Chinquapin stations was established by obtaining periodic base flow measurements at sites in the ungaged areas and relating these on a per square mile basis to concurrent dis- charges at Tomahawk and Chinquapin (fig. 2.1). These two stations were used in the relation because they drain only areas in the Coastal Plain. The net outflow from the estuary is the sum of discharges for the three index stations plus that contributed by the ungaged area, which can be estimated from figure 2.1. 35 aN0D3S d3d i33d 01900 Nl 'NIdVnONIHO ONV )(«VH\/WOi iV 39dVH0SIO 36 Because steady-flow conditions rarely occur throughout the Cape Fear River basin, lag-time corrections must be applied to streamflow records at the various gages in order to arrive at an estimate of freshwater outflow from the estuary at a given time. Lag times, com- puted from theories of wave celerity and rounded to the nearest whole day, are 2 days from Tomahawk to the mouth of the Black River and 3 days from Chinquapin to the mouth of the Northeast Cape Fear River. Combining all flow components, the following empirical equation was developed for estimating fresh-water outflow (Q^) from the estuary, Q, = Ql + Q2 + Q3 + Q4 (2 total outflow from the estuary, discharge at Lock 1 at day of Q^, discharge at Tomahawk two days prior to Q^, discharge at Chinquapin three days prior to Q^, and runoff obtained from figure 2.1 using Q2 + Q^* Equation 2.1 was used to calculate average 7-day outflows. The 7-day period was chosen because it was long enough to dampen the effects of minor, localized variations in the pattern of basin runoff and short enough to be a sensitive parameter for relating inflow to the position of the saltwater front. Insofar as water supply, sewage dilution, and saltwater encroach- ment in the Cape Fear River are concerned, the most critical flow periods are usually those of sustained low flow. Figure 2.2 shows the average recurrence interval, in years, at which various average 7-day minimum flows may be expected to occur. These values were generated by combining estimated annual 7-day minimum flows for the various flow com- ponents of equation 2.1. It is interesting to note that if the B. Everett Jordan Reservoir (plate 1) is filled and if releases from it are controlled according to the proposed operating schedule, then the mini- mum flow of the Cape Fear River at Lillington is expected to be no less than 600 ft^/s. Based on this, the minimum discharge at the mouth, near Southport, might be about 800 ft^/s, compared to an estimated 300 ft^/s at present for the 7-day, 100-year minimum net discharge near Southport. Tide-affected flow . — Flow of the Cape Fear River estuary is strong- ly influenced by ocean tides. The U.S. Geological Survey operated a tidal discharge gaging station near Phoenix (sta. no. 02107570) from April 1966 through March 1969. (See fig. 2.3.) The purpose of this station was to develop information on tide-affected flows for the estu- ary which could prove useful, for example, in better understanding the movement and dispersion characteristics of water substances discharged where % 37 VIqujiq, 2. 3. --Cape FeoA R^ueA 2J>taaAy up^tAnam {^K.om \}}iZminQton. 39 to the river or in better understanding the mechanics of sediment transport and deposition. Volumes of water flowing upstream and down- stream on each tidal phase were published in the annual data reports of the U.S. Geological Survey. Calibration measurements were made over full or nearly full tidal cycles on three occasions — May 11 and 12, 1966, March 8, 1967, and July 27, 1967. Partial results of the March 8, 1967 series of measurements are shown in figure 2.4. This series of measurements was made under fairly typical tide-affected flow conditions and provided, with other inform- ation, a means to calculate the minimum freshwater inflow required to prevent flow reversals in the vicinity of Phoenix. Figure 2.4 shows that the maximum measured discharge on March 8, 1967, was 15,400 ft^/s downstream at 1500 hours on ebb tide. By con- trast, the maximum discharge on flood tide was 6,690 ft^/s upstream at 1915 hours. The average flow over the tidal cycle was 6,240 ft^/s down- stream at the Phoenix gage. This value appears reasonable compared with the estimated freshwater flow at Phoenix for March 8 of about 6,500 ft^/s. From this information, it appears that little or no upstream flow should occur near Phoenix whenever the freshwater inflow exceeds about 13,000 ft^/s. Downstream from Phoenix, the tidal component of flow would be larger and a correspondingly larger freshwater inflow would be required to prevent flow reversal. Upstream, the tidal com- ponent would be less and the freshwater inflow required to prevent reversal would be less also. Two other facts from figure 2.4 are worth noting. First , times of high and low tides do not occur simultaneously with times of high and low slack water. Rather, slack water occurs from one-half to one-and-a- half hours later than high and low tides. Second, the duration of downstream flow is about 8 hours; the duration of upstream flow is less than 4 hours. These durations are more and less than the expected 6.22- hour duration of a tidal phase unaffected by freshwater inflow. As freshwater inflow increases, the duration of downstream flow also increases . Figure 2.5 shows typical variations of velocity with depth in the Cape Fear River. This particular profile was taken at the Atlantic Coast Line Railroad Bridge at Navassa (see figure 2.3) on May 13, 1966, during a series of discharge measurements on the Cape Fear River estu- ary. The manner of variation of velocity with depth is typical of hundreds of profiles made on this and other days. Velocities are very uniform with depth from within 5 or 10 feet of the surface down to within 5 or 10 feet of the channel bottom, then drop off sharply near the bottom. Velocities at the surface are usually only slightly less than at depths of 5 to 10 feet, but winds may cause near-surface veloci- ties to increase or decrease. Point velocities due to tides alone in 40 O) 'Z E o 3: _ - O llj tr i£) vo ^ ro cvj 133J Nl '39ViS wv3yiSNMoa wv3diSdn aN033S ySd 1333 DianD NI '39dVH3SIQ 41 the Navassa-Phoenix reach seldom exceed 2 ft/s. The particle distance traveled up or downstream due to tides alone is in the range of 6 to 8 miles in this reach. Velocities and travel distances due to freshwater inflow would have to be added to those due to tides to determine actual velocities and travel distances in the estuary. 10 5 20 o 30 40 50 60 I 1 1 1 o > J ) f <\ 1 CHANNEL 1 BOTTOM , "0 I 2 3 DOWNSTREAM VELOCITY, IN FEET PER SECOND VIquJlq. 2.5.--Ua/u.cutLon o{, vaZocMty MAXh d^ptk oi tko. Capo, fzcui Zive/L oj^tmcuiy at Navcu>i,a on May 13, 1966 [at me^eA station 20, 0759-0S03 kouAi] . Water Quality Summaries of water quality of inflowing freshwater at three key sites in the Cape Fear River basin are shown in table 2.1. Generally, minimum concentrations of dissolved constituents occur during high freshwater flows composed mostly of overland runoff, which is typically low in dissolved solids. Conversely, maximum concentrations of dis- solved constituents tend to occur during minimum streamflows, which are composed largely of more-highly-mineralized ground water. Except for color and iron, concentrations of major constituents of incoming fresh- water fall within limits for drinking water recommended by the Environ- mental Protection Agency (1976) [1978]. However, the North Carolina Office of Water and Air Resources (1972, p. 27-103) states that the 42 33R £SS 5SS If- OOO^ r^-H^ c.r.<0 ;^22 S-^;^ SS3 5?:S 11 D„08I ooo -.0^ o;o^ ^ r4 ^ -1 ^0^ c^or. ^o^. CO) apTaoTMD • • s-'s , • ■ ■ • (8h) mnTS8u3EW ^20. (3i) uoai 2°° , • ■ , III III III 1 f f H 1 5 i i 1 i 1 ' ^ i i !! f I li 43 rapid industrialization which has taken place along the Cape Fear River estuary, particularly by chemical industries, has resulted in a variety of chemical substances being discharged into it, rendering it unfit, even where fresh, for drinking and some other uses without expensive treatment. The report further states that synthetic organic compounds released into the estuary by petrochemical industries may be a par- ticularly difficult treatment problem because these compounds resist destruction by communities of micro-organisms used to treat ordinary sewage in waste treatment plants. Heating of the estuary water from power-plant operations and industrial-cooling operations may be a significant problem along some reaches. If not for contamination, the water, where fresh, would be suitable for most industrial, domestic, and agricultural uses. Superimposed on difficulties in freshwater use due to contamination are difficulties due to saltwater intrusion, which may at times affect water quality as far as 20 miles upstream from Wilmington. Details of saltwater intrusion in the estuary are given later. Sediment The Geological Survey has made suspended-sediment determinations from monthly samples collected at Lock 1 near Kelly (station number 02105769 on plate 1) since January 1973. The average suspended-sediment load there is about 920 tons/day or 336,000 tons/year. Particle-size analyses (Simmons, 1976) show that over 90 percent of this material is of silt or clay size (.062 mm or less). The fate of this sediment has not been completely studied, but it is known that some is deposited in the estuary. The U.S. Army Corps of Engineers estimates that, in order to maintain navigation facilities at the Military Ocean Terminal at Sunny Point (MOTSU) , about 18 miles downstream from Wilmington, 2,238,000 cubic yards, or about 3.5 million tons, of sediment must be removed annually at a cost of $2,169,000 (U.S. Army Corps of Engineers, 1976, p. 4). This gives some idea of the economic impact of sedimen- tation in the estuary. Note, from the above discussion, that the amount of material re- moved from the estuary through dredging far exceeds the amount entering by way of Lock 1. Additional fluvial sediment entering the estuary from the Northeast Cape Fear River and other sub-basins probably does not exceed 35,000 tons/year. Therefore, the primary source of the new shoaling in the Sunny Point area could not be new fluvial sediment, but must be derived from within the estuarine reach or elsewhere — from slumping along the channel, from shore erosion, from old spoil areas, or possibly from sediment derived from the ocean. 44 Salinity Variations in time and space . — The Cape Fear River may be classi- fied under some flow conditions as a partially mixed estuary. That is, turbulence is sufficient to prevent formation of a distinct saltwater wedge or tongue, yet there remains a definite salinity gradient with depth. This is illustrated in figure 2.6, which shows typical longi- tudinal variations in chloride concentrations of the Cape Fear River at a high-slack tide on November 1, 1967. The gradient is accounted for by the density differences between fresh and saltwater. The less dense freshwater tends to flow on top of the heavier saltwater. These density differences are also responsible for upstream density currents which may occur along the channel bottom. These have been observed at and near the Military Ocean Terminal at Sunny Point where a predominant upstream or flood flow exists at times in the lower portions of the river water column (U.S. Army Corps of Engineers, 1976, p. 2). A different view indicative of salinity stratification is shown in figure 2.7, which is a cross-section showing chloride concentrations of the Cape Fear River estuary 1.5 miles upstream from Market Street, Wilmington, on June 5, 1962. The top-to-bottom differences are very marked. The surface concentration of 150 mg/L of chloride is equivalent to less than one percent sea water while the bottom concentration of 3,000 mg/L or greater is at least 16 percent that of seawater. The salinity of the estuary is in a state of constant flux. Saline water moves in and out of the Cape Fear estuary regularly in response to tidal action, freshwater inflow, winds, and a number of other, less significant, factors. However, it has been found, at least for the part of the estuary above Wilmington, that the relative po- sitions of the various lines of equal chloride concentration are fairly constant. In other words, if we know the river mile position of 300 mg/L of chloride, then the position of 200 or 1,000, or 3,000 mg/L of chloride may be predicted with a fair degree of accuracy, provided that the locations are somewhere above Wilmington. This relation is shown in figure 2.8. The values given are for the channel bottom at high slack tide. Values at the surface would be somewhat less. The maximum upstream movement of saltwater in the Cape Fear River estuary probably occurred during the passage of Hurricane Hazel on October 15, 1954. The eye of the hurricane moved inland near the South Carolina border and proceeded in a northerly direction along a path that crossed the upper end of the estuary. The counterclockwise circulation of winds around the hurricane eye produced strong winds from the south- east which raised tides to the highest level ever recorded at Wilming- ton. The high tide measured at the National Ocean Survey's Wilmington gage was 7.9 feet above mlw (mean low water); the next highest recorded tide, corrected for river stage, is only 5.6 feet above mlw. (See fig. 2.10.) Although the position of the saltwater front during the passage 45 1 1 1 // tions in / hannel Bottoi aNATION de concentro rams per lit ;al scale gre leroted / 1 nf)illig Verti( exogc 1 ^^6000 o o o ro ■o o o CO , T 1 1 O Q O ro ^ lO 133d Nl 'Hld3a 4^ i33d Nl 'Hld3a 47 48 of Hurricane Hazel was not observed, it is estimated that it reached a position more than 20 miles above Wilmington (plate 1) . At the same time, it is estimated that the saltwater front on the Black River, a major tributary to the Cape Fear River, reached a position about 15 miles upstream from its mouth (plate 1) . The freshwater inflow to the estuary as measured at Lock 3 during the seven days preceding the hurricane averaged about 290 ft^/s. This is the lowest estimated inflow during the period of record (1938-1978) at Lock 3. This ac- counts, in part, for the extreme encroachment of the front which, it is inferred, must have taken place on October 15, 1954. The movement of the saltwater front in response to semidiurnal tides is of particular importance to industries and others withdrawing water from reaches of the river which may be subject to saltwater in- trusion. By carefully scheduling withdrawals, it is often possible to obtain freshwater from the estuary throughout much of each tidal cycle. High-slack tide, bottom-chloride conditions given in figure 2.8 repre- sent the highest chloride concentrations which usually occur during a tidal cycle and are of relatively short duration. Figure 2.9 shows the approximate number of hours during a tidal cycle that near-bottom chloride concentrations may be expected to exceed 200 mg/L for various maximum concentrations at high-slack tide. It is important to note that this relation applies only upstream from Wilmington. This figure shows that, unless the chloride concentration at high-slack tide exceeds approximately 5,500 mg/L, it is possible to obtain freshwater from the estuary for some part of the tidal cycle. For example, it indicates that with a maximum chloride concentration at high-slack tide of about 2,000 mg/L, it is possible to obtain freshwater for about 7 hours of the total 12.42-hour tide cycle. Relation of salinity to freshwater inflow and tides . — The distances upstream and downstream that the saltwater front moves in response to tides depends of course on the volumes of water transported by the tides. These volumes are reflected, in a general way, by the relative heights of high and low tides. Other factors being equal, the higher the tide, or series of tides, the farther upstream the front will move. Thus, tide heights can serve as useful indexes to semidiurnal saltwater movements . The National Ocean Survey (formerly the U.S. Coast and Geodetic Survey) has operated a tide gage at Wilmington since 1935. Data on tidal heights were also obtained by the Geological Survey at its stage stations on the Cape Fear River near Phoenix and Navassa, from April 1966 through March 1969. Figure 2.10 shows the number of years the observed highest annual tide equaled or exceeded indicated heights above mean low water at Wilmington during 1935-66. Neglecting the effect of serial correlation, the probability of occurrence of a highest annual tide of a given magnitude at Wilmington can be estimated from this curve. 49 3 oS2 iV SOHWOdOm \'l '30NVi3nON03 3ldl03dS y3iii U3d swvyomiw ni ;^ '30li X3\71S-H9IH IV NOIlVdl N33N0D 30iyO1H0 50 d31VM M01 Ny3W 3A0ev i33J Nl 'VSSVAVN HV3N 39\/9 S'O S n IV 3011 "IVflNNV 1S3H9IH g d31VM MOn NV3W 3A08\/ 133d Nl 'N019NIW1IM IV 3011 IVONNV 1S3H9IH 51 Wilder and Hubbard (1968) related the position of the saltwater front at high-water slack tide to the previous 7-day average freshwater discharge at the mouth, as determined from gaging station data and the use of fig. 2.1 and equation 2.1. They were able to adjust the relation for the effects of varying tide heights. Details of the development of these relations are contained in their report, but the final relation is presented in figure 2.11. This figure contains a family of curves for selected tide heights showing the estimated position of the saltwater front for different rates of inflow. Approximate results may be ob- tained by interpolation between the curves. Frequency of occurrence of minimum annual inflows and highest annual tides have been presented earlier in figures 2.2 and 2.10. Combining data from these figures to obtain the maximum annual encroach- ment of the saltwater front presents several problems. Two of the more salient of these are (1) the non-random distribution of tide height and inflow during a year, and (2) the possibility of the simultaneous occur- rence of high tides and low inflow, neither of which are annual ex- tremes, but which, in combination, may produce the maximum encroachment. However, if one of these parameters is held constant, probabilities may be determined with reasonable accuracy. For example, using figure 2.11 and assuming that the maximum tide will be 4.0 feet above mean low water during the period of annual minimum flow, it may be that encroachment to a point 10 miles above Wilmington will occur when the 7-day average outflow is about 820 ft^/s, and we estimate from figure 2.2 that such a flow condition will recur on an average of 3.8 years. It is apparent, from figure 2.11 that saltwater encroachment will not be a problem as far upstream as 15 miles above Wilmington, near the mouth of the Black River, without the simultaneous occurrence of both an exceptionally high tide and an exceptionally low inflow. Another important factor that may eventually affect the extreme annual position of the saltwater front is the B. Everett Jordan Reser- voir. Plans for this reservoir provide for a minimum flow of 600 ft^/s in the Cape Fear River at Lillington. This reservoir release, plus the minimum inflow that may be expected to occur about once in 100 years, on average, between Lillington and the mouth, will produce about 800 ft^/s of outflow from the estuary. This augmented flow may considerably reduce the maximum extent of saltwater encroachment in the estuary. The Northeast Cape Fear River Estuary The Northeast Cape Fear River (see plate 1 and fig. 2.12) heads in Wayne County, flows south through Duplin, Pender, and New Hanover Counties, and at Wilmington flows into the Cape Fear River, which empties into the ocean about 30 miles south of Wilmington. The total area drained is 1,740 mi^. The entire basin lies within the Coastal 52 Example: A 7-doy dischorge of i 1000 ftVs and a high tide of 3.0 ft will result in intrusion of the 200 mq/L isochlor to about 7 miles upstream from Market Street in Wilmington. J_i nil 4.5 DISTANCE 5 6 7 8 9 10 12 15 20 IN MILES UPSTREAM FROM MARKET STREET, WILMINGTON F^guAe 2.11 .--lnt2AAQj,cutloyii> maxJjnixm mcAoackrmnt o{, 200 mg/L aklonld^, oat^itotAJ cut tkd mouuth, and tldo, koyight^ In tkuA.Q,d at HoKtk- Q.cu>t Cape, Feo/L R-tueA at Cklnqaaptn and noX out{iloiO {^K.om the Hofvtkea^t Cape Vea/i Rtv2A eMtaoKij at the mouth. Adapted ^Kom HabboJid and StampuA (1972). 56 Tide-Affected Flow Tide ranges in the Northeast Cape Fear River estuary vary consider- ably depending on distance upstream from the mouth at Wilmington, as shown in the following table: Miles from mouth of Northeast Cape Fear River estuary Location Mean tide range in feet 6.4 at General Electric 3.4 Company plant about 23 near Castle Hayne 1.7 57 Flow due to tides is the dominant flow component in the lower reaches of the Northeast Cape Fear estuary. Strong flow reversals occur near the mouth with each tidal cycle. Here, river velocities due to tides average about 1-1.5 ft/s, and seldom exceed 2 ft/s. Although no specific information is available on velocities due to tides in the upper reaches of the Northeast Cape Fear River Estuary, presumably they would become insignificant near river mile 50. The average freshwater inflow to the estuary is about 2,100 ft^/s. At this rate, average velocities near the mouth due to freshwater inflow alone are only about 0.08 ft/s, or 5 to 8 percent of the average velocities attributable to tides. The U.S. Geological Survey made a continuous measurement of tide- affected discharge during one complete tidal cycle, 6.4 miles upstream from the mouth of the Northeast Cape Fear estuary, on October 23, 1969. The results of the measurement are shown in figure 2.15. The maximum discharge measured on October 23 was 22,250 ft-^/s. This occurred during flood tide at 1930 hours. The maximum discharge measured on ebb tide was 18,080 ft^/s at 1400 hours. These values are much larger than the estimated 420 ft^/s of freshwater inflow for that day. During ebb tide, a total of 315 million cubic feet of water flowed past the measuring section. Of this amount, only about 11 million cubic feet could be accounted for by freshwater inflow to the estuary. This represents about 3 percent of the total flow volume. Obviously, tides are the dominant short-term flow component near the mouth of the Northeast Cape Fear River estuary. A freshwater inflow of about 23,000 ft^/s, almost 11 times the average, would have been required to prevent flow reversals during the October 23 measurements, and flows as large or larger than 23,000 ft-^/s occur only about 0.02 percent of the time. Further upstream, the influence of tides on flow is less and becomes negligible about 50 miles upstream. As in the case of the Cape Fear River estuary, times of high and low tides in the Northeast Cape Fear River estuary do not coincide with times of slack water, as shown in figure 2.15. During the October 23 measurement, slack water occurred more than an hour after high and low tides . Detailed vertical velocity profiles are not available for the Northeast Cape Fear River estuary. However, based on some velocity observations made during the October 23 measurement, profiles would be similar to those observed during measurements of the Cape Fear River estuary, shown earlier in figure 2.5. During October 1969, a study of the movement and dispersion of dye in the Northeast Cape Fear River estuary was made by the U.S. Geological Survey. The results of this study are contained in the 1972 report by 58 0800 1000 1200 1400 1600 1800 2000 Ebb 315 m tide ilhon volume cubic eet High water slack^ i' ow wot er sloe k Fl ood 1 1 le Flo Dd tide ••• Higti / ide^ ..•J Low fide 1 - 3- 0800 1000 1200 1400 1600 TIME, IN HOURS OCTOBER 23, 1969 1800 2000 VIqujiq, 1.]S .--StcLQi and cLuckoAge. cu> mucauAiid (ioyvU.nuoLLi,ly duAlng OdtoboA 11), 1969, in tkn Ho'vtkz.(Li>t Capo, VdoA RivoJi utucviy, . 6.4 mAj,2J> upitA^am {^fiom tkklng time. (^on. a. potato, In- jzdtzd Into tkn hlonXh^cut Cape FeoA RIvqa utaoAy about 6.5 mtlD(J,CD OtMO. mr,^ Lved Is i " 3.081 (''oj) ajBMdsoqj (i) apTJonij (TO) apT^OTMO (OS) 33BJjns (BH) rnn-fpos (8W) omTsauSBH I'll III II. s s s ^ £• S ^ ° a 62 Obviously, the amount of sediment from upstream sources is not enough to account for the amount of maintenance dredging that is done in the estuary. The question then arises, what is the source of the sedi- ment that forms shoals in the navigation channel and harbor facilities in the Northeast Cape Fear River estuary? This question can be answered only speculatively because little actual work has been done to determine the exact sources. Probably only a part of the estimated 20,000 cubic yards of sediment that is carried into the estuary by freshwater inflow actually settles in the estuarine zone. Likely, a large proportion of it is carried out into the Cape Fear River estuary. However, sediment that is deposited from upstream sources would tend to settle in the deeper dredged areas. A second possibility is that some shoaling materials may be trans- ported upstream from the Cape Fear River. This could occur whenever the Northeast Cape Fear estuary is in a partially-mixed or stratified state. Then, net upstream flow might prevail near the bottom. The minimum mixing ratio needed to produce partially-mixed conditions is about 0.5. (Refer to discussion on estuarine types in Chapter 1 — GENERAL HYDROL- OGY.) Based on measurements of tide-affected flow in the estuary, the minimum freshwater inflow needed to produce partially-mixed conditions (and, hence net upstream velocities near the channel bottom) would be about 6,500 ft^/s. Although freshwater inflows of this magnitude or greater occur less than 5 percent of the time, it is possible that significant upstream migration of shoaling materials occurs due to this phenomenon . The third and possibly the major source of shoaling materials is within the estuarine reach itself. These sources could include ma- terials eroded from the shores, materials resuspended from the channel bottom and moved by tidal action to the shoaling areas, and slumping of materials adjacent to the navigation channel. Salinity Variations in time and space . — With respect to salinity, the Northeast Cape Fear River estuary may be classified as a well-mixed estuary, for most of the time, at least. This has been verified by several specific conductance surveys, one of which is summarized in figure 2.17. This figure shows lines of equal specific conductance along a channel profile of the Northeast Cape Fear River estuary, based on data collected on November 9, 1966. There was very little difference on that day between surface and bottom specific conductances in most of the reach portrayed here. The mixing index was estimated to be about 0.06 for that day, which is very close to the arbitrary upper limit of 0.05 for a well-mixed estuary. Although no extensive specific con- ductance data were collected within any cross-section on that day, it is not likely that any significant specific conductance differences existed within cross-sections. 63 WATER SURFACE 1 \ CHANNEL BOTTOM 1 r 1 1 \ 1 r 3 4 5 6 7 8 9 10 II 12 13 RIVER MILES UPSTREAM FROM U.S. 17 BRIDGE AT WILMINGTON F^guAe 2.17 .--LongAXudLlnaJi va/viatlon^ -in i>ptuaAy on Novmb2A 9, J 966. Vo^pth^ oAd voAlabld and should not be In^QAJtZ-d {^Kom tkz ^kztch. Historically, the maximum observed upstream intrusion of saline water into the Northeast Cape Fear estuary occurred during Hurricane Hazel on October 15, 1954, when chloride concentrations reached 1,450 mg/L at Castle Hayne. Based on this and information in fig. 2.18 (discussed later) , the saltwater front could have been 2 or 3 miles upstream from Castle Hayne on that date. The very extreme saltwater encroachment which took place due to Hurricane Hazel was in addition to the extreme encroachment which had already taken place due to record low river flows immediately preceding the hurricane. At Chinquapin, for example, the discharge averaged only 5.3 ft^/s on October 10-11, 1954, the all-time low for the 37-year period of record. On October 9 and 10, 1954, chloride concentrations were already about 500 mg/L at Castle Hayne, the greatest salinity intrusion of record, up to that time. The recurrence interval of two such rare events occurring in succession is not known, but it may be reckoned in centuries. The maximum seaward movement of the saltwater front occurs during times of high freshwater inflow, usually in the spring. At such times, the front may be displaced out of the Northeast Cape Fear River estuary altogether, leaving the estuary completely fresh for a short period. It is not economically feasible on a routine basis to survey the entire river to locate the saltwater front. However, based on previous salinity surveys, a type curve has been developed for the Northeast Cape Fear River estuary which may be used to estimate the specific conduct- ance at any point in the estuary, if the specific conductance is known at only one point (fig. 2.18). As an example of the use of this type curve, suppose that the specific conductance near the channel bottom at a high-slack tide is 6,000 ymhos at Cowpen Landing, 11.2 miles upstream from the U.S. Highway 17 bridge in Wilmington, (U.S. Highway 17 bridge 64 Vlqa/in l.H.--Sp2.(ilil(L conducXana^ typo. coAue ioh. tho. UonZhdo^t Cape 65 is about 0.85 miles upstream from the mouth of the estuary) and we want to know the location of the saltwater front (800 ymhos) . To determine this, first find the miles upstream value on the abscissa corresponding to the 6,000 ymho value of the ordinate. This value is 4.3 miles. Next, find the abscissa value corresponding to the ordinate value of 800 ymhos. This value is 8.0 miles. The distance upstream from the 6,000 ymhos location to the 800 ymhos location is 8.0 - 4.3, or 3.7 miles. Because Cowpen Landing is at the 11.2 river mile point we would expect on that date to find the saltwater front near the channel bottom at the 14.9 river mile point (11.2 + 3.7 = 14.9). Relation of salinity to freshwater inflow . — The relation of sa- linity to freshwater inflow to the Northeast Cape Fear River estuary is more complex than in most estuaries because it is affected by salinity conditions in the Cape Fear River estuary. Nevertheless, such a relation has been developed for the Northeast Cape Fear River estuary, which may be applied with useful accuracy to predict movements of the saltwater front. The relation (fig. 2.19) is based on the discharge at the Chinquapin gage and the location of the saltwater front during high- slack tide as observed during six salinity surveys made between August 1, 1955, and November 9, 1966. Of several flow parameters tried, the location of the saltwater front during high-water slack related best to the preceding 21-day average fresh-water discharge. The scatter of data points in figure 2.19 may be due to several factors. One is that unusually large or small tidal ranges for a given day may result in correspondingly greater or lesser upstream incursions of saline water on that day. Secondly, the relation is influenced by winds, which, if blowing upstream, may result in greater-than-normal saltwater advances, and, if blowing doxmstream, in lesser saltwater advances . Salinity conditions in the Cape Fear River estuary are a third factor influencing the scatter of points in figure 2.19. If, for ex- ample, high flows in the Cape Fear River basin due to a rainstorm oc- curring in that basin (but not in the Northeast Cape Fear River basin) displaced saline water downstream further than before the storm, this would also decrease salinities in the Northeast Cape Fear River basin to some degree. A fourth factor influencing the relation relates to a situation when the position of the saltwater front is not in equilibrium with freshwater inflow. This situation may exist when a period of high freshwater inflow is followed immediately by a period of much lower freshwater inflow. The saltwater front would immediately begin to move upstream in response to the diminished inflow, but may not reach an equilibrium position within the 21-day period used in developing the relation. v66 67 Regardless of these limiting factors, the relation can be used to roughly predict saltwater advances under a wide range of freshwater inflows as measured at Chinquapin. Such information could be useful, for example, in predicting whether or not saline water would reach a water-supply intake under prevailing inflow conditions. It may also be useful in locating future freshwater supply intakes where there is the least chance of saltwater intrusion. For some potential users contemplating a water supply in the lower reaches of the Northeast Cape Fear River estuary, a certain degree of risk of saltwater intrusion may be acceptable in return for advantages gained by locating near waterborne transportation. To help evaluate this risk, a frequency of intrusion relation has been developed and is shown in figure 2.20. The relation was developed by combining elements of the flow relation in figure 2.13, the 21-day low-flow frequency curve in figure 2.14, and the flow-salinity relation of figure 2.19. As an example of the use of the relation in figure 2.20, suppose that it was observed in one year that the maximum intrusion of the saltwater front was 18 miles upstream from the U.S. Highway 17 bridge in Wilmington. According to figure 2.20, we would interpret that the saltwater front would reach this far upstream, or farther, only once every 20 years, on the average. 68 1.03 l.ll 1.25 1.66 2 2.25 3.33 5 10 20 50 100 RECURRENCE INTERVAL, IN YEARS ViguAe. 1.20 .--V^dqanncij lyitALU>lon 0^5 200 mg/L Moftldz io^ va/Uoiu loccLtlo^i6 In the. No^the,(Ut Cape fnoA Riven. ej>tuaAy. CHAPTER 3 HYDROLOGY OF THE PAMLICO SOUND ESTUARINE SYSTEM For purposes of the report, the Pamlico Sound estuarine system (plate 1) includes Pamlico Sound, the Neuse-Trent and Tar-Pamlico river systems, and all other estuarine waters tributary to it. Technically, this includes Albemarle Sound and its associated estuarine waters to the north, but because of its large size and relatively minor degree of interaction with Pamlico Sound, the Albemarle Sound estuarine system is considered separately in this report. Core Sound to the south drains partially to Pamlico Sound, but it is not identified with the Pamlico Sound system for present purposes. The Pamlico Sound will be discussed first because the hydrology of the estuaries opening into it is inextricably related to the hydrology of the sound. Pamlico Sound is connected with the ocean through several relatively small openings in the Outer Banks, primarily Ocracoke, Hatteras, and Oregon Inlets. This limited access, in combination with the broad expanse of the sound, results in ocean tides being dampened to less than 0.2 foot, except near the inlets (Roelof and Bumpus, 1953). However, tidal ranges in the estuaries emptying into Pamlico Sound may be as much as a foot in some locations, due to funnelling effects. A second feature of the Pamlico Sound system is that, on a short- term basis, wind driven currents are often dominant in both the sound and adjoining estuaries. The large size of Pamlico Sound allows ample opportunity for wind setup over long fetches. Within the estuaries, the velocity of wind-driven currents may be increased because of funneling effects. A second factor which contributes to the relative importance of wind-driven currents in the system is that velocities due to fresh- water inflow are low. Pamlico Sound and its estuaries are drowned river valleys. Consequently, the river channels are oversized for the amount of water they now carry, resulting in low velocities due to freshwater inflow. In the long term, however, freshwater inflow is more important than wind in affecting net flow because the effects of winds blowing from various directions tend to cancel each other over time. This is true throughout the Pamlico Sound estuarine system. The data and information on which these discussions are based are from various sources. Plate 1 shows the location of key flow and water- quality data-collection stations operated by the Geological Survey with- in the Pamlico Sound system. These stations were used to help define freshwater inflow, freshwater quality, and salinity characteristics of the Pamlico Sound system. In addition to data from these sites, the Geological Survey has conducted a number of specific conductance surveys to determine the extent of saltwater intrusion, most notably six surveys 70 of the Tar-Pamlico and Neuse-Trent systems made between September 14, 1954, and June 1, 1955. In addition to the Geological Survey data, information acquired by the National Oceanic and Atmospheric Adminis- tration, the Office of Sea Grant of the National Science Foundation, the University of North Carolina, North Carolina State University, East Carolina University, and Woods Hole Oceanographic Institution was uti- lized in this study. Pamlico Sound Pamlico Sound (plate 1) covers an area of about 2,060 mi^, bounded on the west by the mainland and on the east by the Outer Banks. It is the largest sound formed behind the barrier beaches along the Atlantic Coast of the United States. The total volume of water contained in it amounts to about 920 billion cubic feet, or about 21 million acre-feet. In contrast to its great area, the average depth is only about 16 feet, and the maximum depth is only 24 feet. Pamlico Sound is an important commercial and sport fishery, and extensive shallow areas and salt marshes along its fringes serve as nurseries for a variety of commercially and recreationally important marine species, including shrimp, oysters, clams, scallops, blue crabs, spot, striped bass, croaker, and flounder. Pamlico Sound also is an important link in the Atlantic Intra- coastal Waterway. Water-related problems in Pamlico Sound include occasional fish kills due to anoxic conditions, contamination of some clam and oyster producing areas, property damage due to hurricane surge, too-low or too-high salinities in fish nursery areas due to both natural and man- induced causes, shoreline erosion, and sedimentation in shipping channels. The total area draining directly into Pamlico Sound is about 12,520 mi^, including the area of Pamlico Sound. In addition, water from Albemarle Sound and areas tributary to it (total of 18,360 mi^) enters Pamlico Sound indirectly through the Croatan and Roanoke Sounds. Thus, Pamlico Sound receives drainage from a total area of about 30,880 mi^,. The average freshwater inflow to Pamlico Sound from this area is about 32,000 ft^/s. At this rate, it would take about 11 months for the flow volume to equal the volume of the sound. The average inflow value accounts for precipitation on and evaporation from the sounds and wide open-water areas of the various estuaries. The average monthly inflow to Pamlico Sound ranges from about 55,200 ft^/s in February to about 21,000 ft^/s in June. As discussed by Folger (1972), Bluff Shoal (fig. 3.1) divides Pamlico Sound into two broad basins. Bottom topography in the northern area dips smoothly toward the center to the maximum depth of approxi- mately 24 feet. In the southern part, shoals project from the western 71 72 shore well into the sound. A tidal delta extends into the sound from Ocracoke Inlet. Folger notes that fine sand (fig. 3.2) covers most of the bottom, with silt present primarily in the deep areas of the northern basin and in the channels extending into the sound from the mouths of the Neuse and Pamlico rivers. Medium sand covers most shoals and extends sound- ward from inlets as tidal-channel deltas and from the barrier islands as washover fans. There is a general increase in oxidizable organic matter and organic carbon in the bottom sediments (fig. 3.3) toward the center of the northern part of the sound and toward the axes of the Neuse and Pamlico River channels where, according to Folger, the finer sediment is concentrated. Most of the organic material is apparently due to indig- enous biological activity, although he notes that some peat evidently underlies a thin veneer of sand at the southern end of the sound. The highest concentrations of calcium carbonate (fig. 3.4) in the bottom sediments are associated with the fine sediments in the northern basin and near the river mouths, and with the medium sands of tidal channel deltas at inlets. The calcium carbonate in these areas is mostly shell detritus. Water Budget and Flow The net freshwater inflow to Pamlico Sound may be determined by a simple accounting of freshwater according to the equation: P + I^ - E = I^ Equation (3.1) where P is precipitation on the sound, is freshwater inflow from land drainage, E is evaporation from the sound, and I is net freshwater inflow to the sound (change in storage in the sound is assumed to be zero) . Monthly freshwater budgets were calculated for Pamlico Sound utilizing equation 3.1 and these are shown in table 3.1. The major freshwater flow contributors to Pamlico Sound are the Neuse-Trent River system (average flow — 6,100 ft^/s from 5,598 mi^) , the Tar-Pamlico River system (average flow — 5,400 ft^/s from 4,300 mi^) . Indirectly, two other major rivers, the Roanoke (average flow — 8,900 ft^/s from 9,666 mi^) and the Chowan (average flow — 4,600 ft^/s from 4,943 mi^) drain into Pamlico Sound through Albemarle Sound. The monthly values for these rivers and other contributing areas were de- termined on the basis of discharge records at gaged locations, adjusted for ungaged areas. Precipitation values for Pamlico Sound and adjacent open water areas (2,064 mi^) were determined by averaging National Weather Service 73 ViguAQ. 3. 3. --Oxldlzable, oK.Qa.vilc moutt2A and o^QCLYvic. coAbon conto^nt ojj bottom 62.dl- rmyit in PamtLco Sound, Oxldtzablz oXQcmtc moutt<2A data. oAd ^n.om Ptckojtt (1965); oh-ganlc caAbon data aXd i^om HatkaL^ay ( 7 97 7 ). VtQiVtz 3.4. --Calcyim aaAbonate content o{i bottom 6 2.dtm2.nt6 in Pamtico Sound ({iKom PickM, 7 965). 74 00 1 1 I 1 J i i s i i 1 i 1 § § i § 1 § 2* ° 1 i i s 00 2 2 § § i s 1 § 1 1 s 1 00 i 1 1 > 5 2 ! 1 s § i i i § 1 3 2 § i . 1 2 § § 1 ! § 1 § till 1 1 2 2 i ! i 1 i Precipitation on Pamlico Sound Evaporation from Pamlico Sound 1 Freshwater Inflow to Pamlico Sound from land areas tributary to Pamlico Sound Inflow from Albemarle Sound to Pamlico Sound Net inflow to Pamlico Sound (or outflow to the ocean) station records at New Bern, New Holland, and Cape Hatteras. Precipi- tation on Albemarle Sound and adjacent open-water areas (934 ml^) was determined from monthly average values from National Weather Service stations at Elizabeth City, Manteo, and Plymouth. Evaporation values were derived for all major open water areas by applying a coefficient of 0.7 to monthly values of the Maysville pan evaporation station of the National Weather Service. Precipitation and evaporation values for Albemarle Sound, though not shown in table 3.1, are reflected in the inflow values from Albemarle Sound. It is interesting to note from table 3.1 that minimum net inflows to Pamlico Sound do not clearly occur in September, October, and No- vember, as is the case with many natural streams in North Carolina. Actually, minimum net inflows seem to occur in June, when evaporation rates are the greatest. As is often the case, extreme and unusual events are of greater interest than normal events. It is useful to speculate on what net inflows would be, say, in a month with little or no rainfall occurring during the low-flow period of June-October. In such a situation, fresh- water inflow from land drainage would be minimal (say at the 30-day, 10- year minimum flow range) and evaporation would be, if anything, somewhat greater than normal for such a month because incident solar radiation would be greater due to lessened cloud cover. Figure 3.5 shows low-flow frequency curves for 7- and 30-day periods for all inflow due to con- tributions from land areas draining directly or indirectly into Pamlico Sound. The minimum 30-consecutive day 10-year discharge derived from it is about 3,000 ft^/s. If we assume that this inflow occurs in June, when evaporation is at a maximum (about 15,300 ft^/s for Albemarle Sound, Pamlico Sound, and associated open-water areas), and if we further assume that direct precipitation on the sounds and associated open water areas is zero, then net freshwater inflow to Pamlico Sound, Albemarle Sound, and associated areas from equation 3.1 would be (0 + 3,000 - 15,300) ft3/s, or -12,300 ft^/s. In other words, the rate of loss of freshwater from the sounds and associated areas by evapo- ration would exceed gains from land drainage and precipitation by about 32 billion ft^. Of course, these evaporative losses would be made up by sea water entering Pamlico Sound through the ocean inlets, thus in- creasing the salinity of the Sound. High flow periods are also of great interest because the greater part of annual flow volumes occur during relatively short time periods and this is when most of the flushing of pollutants and saline water takes place. By inspection of Table 3.1, it is seen that the highest inflows generally occur from January-April, ranging from an average of 54,500 ft3/s in February to about 32,400 ft^/s in April. Figure 3.6 shows high-flow frequency curves of inflow from direct and indirect land drainage into Pamlico Sound for 7- and 30-day periods. As an example to contrast with the minimum 30-consecutive-day 10-year average flow of 3,000 ft^/s, the maximum 30-consecutive-day 10-year average flow from 76 77 78 figure 3.6 is about 106,000 ft^/s, about 35 times as great. If this inflow were to occur in February, when the excess of precipitation over evaporation is, on average, equivalent to an additional inflow of about 5,200 ft^/s, then the net inflow (or outflow) would be about 111,000 ft^/s, or an equivalent volume of about 269 million ft^ (29 percent of the volume of Pamlico Sound). At this rate, in a period of 14 weeks the water reaching the sound would equal the volume in storage, in contrast to the 11 months needed for average freshwater inflow to reach this volume . Discharges and resultant velocities due to freshwater flow into and out of Pamlico Sound are of small magnitude and usually overshadowed at any given moment by flows and velocities due to winds and/or tides. However, net flows due to tides and winds tend to approach zero over time, so that long-term average net flows at any point may be ascribed primarily to freshwater inflow. Consequently, flows due to freshwater inflow may have value in studies of long-term net transport of pol- lutants and nutrients into and out of Pamlico Sound. An idea of the magnitude of tidal exchange between the ocean and Pamlico Sound may be obtained from table 3.2, which indicates that com- bined maximum flood or ebb flows through Oregon, Hatteras and Ocracoke Inlets may be on the order of 200,000 ft^/s, far more than the net outflow of about 32,000 ft^/s due to freshwater inflow. Although total volumes associated with ebb flows (table 3.2) exceed over time those associated with flood flows, in the case of almost half of the indi- vidual measurements in table 3.2, the volumes associated with flood flows exceed those associated with the preceding or following ebbs. It may be that some or even all of these apparent exceedences of individual flood volumes over ebb volumes could be accounted for by measurement error or diurnal inequalities in tides, but just as likely the exceed- ences were real and caused by easterly winds prevailing during or before the measurements. It is also worthy of note that maximum flow rates occurred during flood tide in the majority of measurements, even in some cases where total ebb volumes exceeded those for floods. Table 3.3 gives predicted tide ranges and maximum currents for several locations at or near the inlets. The locations of these inlets are shown in plate 1. The predicted mean tidal ranges at the inlets are all similar, from 1.9 to 2.0 feet, though they are less than in the adjacent ocean, where predicted tide ranges vary from 3.2 feet at Kitty Hawk to 3.4 feet at Hatteras. It is interesting to note from comparison of the Ocracoke Inlet and Ocracoke stations how quickly tides damp out away from the inlets. At Ocracoke Inlet, the mean predicted tidal range is 1.9 feet, while at Ocracoke, located in Pamlico Sound only about 4.8 miles northeast of the inlet station, mean ranges are nearly halved, to 1.0 foot. 79 Table, 3, 2, --Tidal {iloM and fi2JiatQ,d data {^on. Oregon, HatteJuu and 0cAjac,o\i2, IviJLdti,, Except (^oK. mna^uAmint^ madz In KpnJX 1950, by Rodloib and Bimpa^ and on June. 1% , J 973, by Slng2A and KnoMloji, the. me^aiuAmznti oAd lh.om ^e.cioAdi tko, (J. S. knmy Con.pi> oi EngimeA^ Date Cross section (ft2) at mlw Maximum rate of flow (ft^/s) Flood ebb Total Flow (acre-f t. ) Flood Ebb Sept 9, 1931 Aug. 31, 1932 Oct. 11, 1932 Aug. 24, 1937 Aug. 14, 1939 Apr. 23, 1950 Sept 27, 1965 June 28, 1973 Apr . 25, 1950 39,000 44,400 56,000 28,000 66,800 68,800 Oregon Inlet 134,100 129,100 126,500 180,000 152,000 292,000 171,000 88,200 102,700 127,300 142,000 141,000 90,000 145,800 146,000 Hatteras Inlet Ocracoke Inlet Apr. 27, 1950 82,800 May 25, 1958 107,500 May 25, 1958 96,100 Oct. 14, 1962 94,100 Oct. 14, 1962 74,400 285,000 329,000 273,000 344,000 47,800 42,700 34,900 63,500 37,800 98,200 52,700 45,400 78,400 125,000 37,400 40,100 57,200 55,900 71,500 38,200 54,200 122,000 104,000 129,000 Of course, tide predictions are made on the basis of an analysis of predictable mutual gravitation forces of the sun, moon, and earth and actual tide heights and currents in Pamlico Sound often differ from predictions, primarily because of the unpredictable effects of winds. Detailed consideration of the complex effects of winds on circulation in Pamlico Sound is beyond the scope of this report, but several reports, including Singer and Knowles (1975) and Knowles (1975) discuss the effects of winds and tides on circulation at several locations in and near Pamlico Sound. 80 Idblo, 3.5.--?n.Z(iicX2.d tldz ^angu and ma>Umim cuAAtyvU {^o^ locations at OA. Yi2,an. IntoXi) to Pamtlao Sound, fxom MattonaZ Oczan Sa/ivzy Tidal CuAAznt Tabl2J> and Tide. Tables ioA. 1977. Location Tidal ranges in feet Average maximum currents in feet per second Average Average Lat. Long. mean spring flood velocity ebb velocity Oregon Inlet 35°46' 750321 2.0 2.4 Hatteras Inlet 35°12' 75°44' 2.0 2.4 3.6 3.4 Ocracoke Inlet 35°04' 76°01' 1.9 2.3 2.9 4.0 Ocracoke, Ocracoke Inlet 35°07' 75°59' 1.0 1.2 Water Levels The greatest water level fluctuations in Pamlico Sound occur during hurricanes, when land areas 10 to 15 feet above mean sea level are some- times innundated, causing great damage to buildings and croplands ad- jacent to the sound. Figure 3.7 shows water level configurations during Hurricane Donna at 0200 and 0500 hours on September 12, 1960. These contours were sketched by the U.S. Army Corps of Engineers from tide gage records and appeared in the 1961 publication "Report on the tropi- cal hurricane of September 1960 (Donna)." They illustrate not only the wide variations in stage which may exist from place to place at a given time, but also the great fluctuations which may occur at a given place in a relatively short period of time during hurricanes. It is important to note that the datum for figure 3.7 is 4.0 feet below mean sea level; thus, for example, a water level contour of 1.0 foot in figure 3.7 is actually 3.0 feet below mean sea level. An important tool for gaging potential flood losses from winds is knowledge of the frequency of flooding. The following three paragraphs on this subject are almost a direct quote from Wilder and others (1978, p. 58-59). Only the illustration numbers have been changed: "Prediction of the frequency with which a given locality is likely to be flooded with water from the estuaries or sounds is inexact because of the almost infinite number of possible combinations of wind direction and velocity, shoreline configuration, fetch, and the amount of vege- tation and man-made structures that might impede free advancement of a wave. Some idea of the severity of the problem can be obtained from 81 F^gu/ie 3.7 .--E^tancitzd Motdn. Idvol^ In Vcmtiao Sound cut 0200 and OSOO kouJa on SzptmboA 12, 1960, duAlng HuA/Licam Vonna. Vfiom Amexn and fiuAan [1916], aitoA U.S. Kmy CoA.p6 Englne,2A^ [1961]. 82 figure 3.8, on which are delineated approximate boundaries of wind-tide floods likely to be equalled or exceeded 50 percent of the years and 1 percent of the years. By "exceeded" we mean that Inundation of an area at least as great as that shown is likely every other year on the aver- age at the 50 percent probability, and once every hundred years on the average at 1 percent probability. These are average frequencies over long periods of time, and no specific time interval between two consecu- tive events is implied... However, all of the area with an equal chance of being flooded at a given frequency will seldom, if ever, be flooded by the same storm. For example, strong southerly winds cause inundation along northern shorelines of a body of water, but might actually lower water levels along the southern shorelines. "It is important also to qualify the accuracy of figure 3.8. The boundary outlining the area inundated by a flood with a 1 percent chance of exceedance was transferred directly from flood-prone area maps available from the U.S. Geological Survey. The lines in figure 3.8 are general and are not as detailed as those appearing on the large- scale flood-prone maps. These small-scale illustrations were prepared only to point out the potential flood problem. If more accurate data are desired, the large-scale flood-prone maps prepared by the Geological Survey and flood-plain information studies completed by the U.S. Army Corps of Engineers should be used. Generally the flood with a 50 per- cent chance of exceedance was sketched on the large scale floodprone area maps using a flood stage from 2.5 to 3.5 ft below the flood out lined as having a 1 percent change of exceedance and then transferred directly to the smaller scale maps of figure 3.8. "Because most of the areas adjacent to the shorelines presently contain dense vegetation or manmade structures, these sources of tidal- flooding information, all of which consider only wave height and land elevation, may tend to overestimate the extent of inundation." Water Quality Marshall (1951) pointed out the lack of data on water chemistry other than salinity in the open parts of Pamlico Sound and suggested this as an important area of study. His statement remains largely true today, although some valuable data have been and are being collected by various agencies, including the University of North Carolina Institute of Marine Sciences (summarized in Williams and others, 1967) and the North Carolina Department of Natural Resources and Community Develop- ment. However, much of these data were collected along the fringes of the Sound, not in the central part. The U.S. Geological Survey has collected chemical data for many years at various locations on rivers tributary to Pamlico Sound. These data will be discussed in later sections, but extrapolation of this information to infer water quality of Pamlico Sound would be very difficult for two primary reasons. 83 First, of course, Pamlico Sound water is everjwhere at all times at least partially mixed with ocean water. The second primary reason is that some chemical constituents, such as nitrate (NO3) and phosphate (PO^), are non-conservative and continue to interact chemically and biochemically with other substances or organisms in the water as the water moves into Pamlico Sound and, ultimately, into the Atlantic Ocean. The Pamlico River estuary, for example, acts as a trap for phosphate and nitrate during algal blooms, which occur there each late winter or early spring and each summer (Hobble, 1974, p. 2). The algae trap and utilize phosphorous and nitrogen for growth, then die and settle to the river bottom. Thus, phosphate and nitrate concentrations in the river water may be much decreased by the time the water reaches Pamlico Sound. Woods (1967), collected data on water quality at several sites in southern Pamlico Sound from June 1963-October 1966 (fig. 3.9). At one- month intervals at each site, vertical salinity and temperature measure- ments were made at one meter intervals and surface and bottom samples were analyzed for dissolved oxygen, plant pigment concentrations, and total phosphate. In his report, Woods indicated that both phosphorous and nitrogen are often present in high-enough concentrations in western Pamlico Sound to support algal blooms and that availability of nitrogen, rather than phosphorous, seems to be the limiting nutrient determining whether or not algal blooms may occur in Pamlico Sound, if other con- ditions for blooms are favorable. Dissolved oxygen concentrations in Pamlico Sound ranged from 4-11 mg/L through the course of Woods' study. The highest values occurred when water was cold and the lowest values when water was warmer. In terms of percent saturation, concentrations seldom went below 50 to 60 percent and during the winter months were normally close to 100 percent. Surface dissolved oxygen in Pamlico Sound varied little from place to place at any one time and vertical differences in the sound were slight, even when total oxygen depletion was noted at some bottom stations up- stream in the Pamlico River estuary. Many observers have noted that water temperatures in Pamlico Sound follow air temperatures closely but with some lag. Highest temperatures typically occur during late June, July and August and lowest tempera- tures occur during the winter months (fig. 3.10A). Figure 3.10B shows the relation between average surface temperature in the open sound and air temperature recorded by the National Weather Service at Hatteras. The air temperatures used in the graph represent the average of the mean temperatures on the day of water temperature observations and the previous day. This was done to allow for the lag in water temperature. Roelofs and Bumpus found the correlation coefficient between mean water temperature and air temperature to be 0.972, which is highly signifi- cant. 85 77° 76° VlQiVtz 5. 9. --Sampling 6tcuU.0Yii> a^zd In Vamtico Sound i>tady by i^ood^ (1967). 86 VIqlxah 3. jo. --(A). Mean moyithly mcUqa 6uA.^ac^ tmpoAd- tuAz oi Vamtido Sound. (B). Relation boJMddn u)cut2A iuA^^acQ. tmpd^ata/id oi Vamtico Sound and aJji t2jmpi2AatuAii at HaJX2Aa6. Adapted ^xom RooZoii, and BamptM (J953). 87 Thermal stratification in the open sound is slight year-round; surface to bottom differences rarely exceed 1 or 2°C. Areal differences are likewise small, rarely exceeding 3 or 4°C at any one time. Ap- parently, winds are effective in promoting vertical mixing throughout the relatively shallow depths of the sound. Salinity Figures 3.11 and 3.12 show the average surface salinity of waters of the Pamlico Sound system for the months of April and December, based on salinity data collected between 1948 and 1966 at a number of fixed locations in the system. April, on average, is the month of lowest salinities and December is the month of greatest salinities. The April-December differences are less at the ocean inlets (0-2 grams per kilogram) than they are near the mouths of the major estuaries where the April-December differences may be 4-5 grams per kilogram. The effect of high freshwater inflows during the late winter and spring months from the Albemarle Sound drainage and the Neuse-Trent and Tar-Pamlico river systems may be seen in the way the high-salinity water has been "pushed" further out into Pamlico Sound in April as compared to December. Actu- ally, maximum net outflows occur, on average, not in April but in February, and minimum outflows occur, on average, not in December but in Juhe or October. This lag in salinity response to changing outflows is due to the large volume of Pamlico Sound and the long time required to flush water out of the sound. Thus, the salinity distribution within the sound during any one month is due, not only to flows during that month, but to the flows during the immediately preceding months. Just as winds are the dominant influence on the short-term circu- lation of water in Pamlico Sound, they are also the dominant short-term influence on salinity distributions. It has been generally observed that easterly winds cause increasing salinities in the sound and wester- ly winds cause decreasing salinities. However, with regard to the effects of northerly and southerly winds, there is some confusion. Most observers agree that northerly winds will cause lower salinities in the northern part of Pamlico Sound as fresher water is pushed into this area from Albemarle Sound. They also agree that southerly winds will cause higher salinities in the northern part of Pamlico Sound as highly saline water from Hatteras and Ocracoke inlets is driven northward. It is the effect of northerly and southerly winds in southern Pamlico Sound which has been disputed. Wlnslow (1889) reported that southerly winds will cause decreasing densities (salinities) in the southern part of Pamlico Sound and Core Sound and northerly winds will cause increasing salinities there. However, Roelofs and Bumpus (1953) observed decreas- ing salinities in Core Sound during northerly winds as fresher Pamlico Sound water was "blown" into Core Sound. More study is needed to resolve these observational differences, but Whalebone Inlet, now closed, was open during Winslow's 1889 observations, and Drum Inlet, now 88 EXPLANATION . 19 SALINITY, IN GRAMS PER KILOGRAM. (Sea water contains about 3A.5 grams of dissolved solids per kilogram.) Contour interval variable. VIqulAz 3.11 .--Av2Aag(i ioA^^acz 6aJilYuJ:ij (ajcUqa in Vamtico Sound and vidlnAjty {^OK tho. month, o/j ApAJJi. ^hodt^^lo^d in.om WllLiam^ and othexi, (1 967). 89 EXPLANATION ,—-19 SALINITY, IN GRAMS PER KILOGRAM. (Sea water contains about 3A.5 grains of dissolved solids per kilogram.) Contour interval variable. figuAz 3. J2.--AueAa.ge ^uA^ace. i>CLLiviiZij o^^ watoA In Famtlco Sound and vlclniXy ^oA tk^. month o{, VzczmbeA. kdaptud ijAom \})UJiicmi, and otkdU {1967). 90 I open, was then closed. This would argue for accepting the more recent observations by Roelofs and Bumpus. Also it is probably too simplistic to say that higher salinity waters will always prevail at a given lo- cation with winds from a given direction. The wind speed and its duration are also important factors. Winds which have prevailed need also to be considered. For example, reversal in salinity trends may occur at a given location if wind has prevailed in a given direction for more than, say, 24 hours, due to water buildup on one side of the sound creating a return flow situation (Singer and Knowles, 1975). Salinity stratification in the open sound is usually slight, only 0.50 to 1.0 percent greater at the bottom than at the surface (Woods, 1967). Roelofs and Bumpus (1953) give the average difference in surface and bottom salinities as only 0.66 percent; this small difference they attribute to effective mixing by the winds throughout the shallow open- sound depths. Woods did observe larger open-sound surface-to-bottom salinity increases of as much as 6 percent from the surface to the bottom in 1964 during the spring, summer and fall. Woods thought that this stratification was due to increased freshwater inflows from the Neuse and Pamlico rivers which occurred approximately four to seven weeks prior to the observed stratification in the open sound. Magnuson (1967) indicated that salinities in parts of natural or man-made channels connecting to the inlets may be appreciably greater than salinities in adjacent shallower areas. However, this is partly speculative and needs better confirmation from data. The distribution of many aquatic organisms in Pamlico Sound and its estuaries and salt-marsh fringes is influenced greatly by salinity patterns. Some aquatic organisms can tolerate wide salinity ranges from almost fresh to almost sea water (Thayer, 1975, p. 64). Others can live and reproduce only within narrow ranges; still others require different salinity conditions at different stages of their life cycles. Thus, any modifications by man in the amount and annual distribution of freshwater inflow or in alteration of salinity patterns will exert some control on the plant and animal populations in Pamlico Sound and adjoining areas. The effects of upstream reservoirs (such as the proposed Falls of the Neuse reservoir), the effects of land clearing on freshwater runoff (such as is currently taking place on the Albemarle-Pamlico peninsula) , and the effects of creating new ocean inlets and navigation channels should all be carefully evaluated with regard to the above salinity considerations . The Neuse-Trent River System The Neuse River (plate 1) heads in Durham County, North Carolina, at the confluence of the Eno and Flat Rivers in the hilly Piedmont Province. The river flows southeast, enters the Coastal Plain near 91 Smithf ield, and empties into Pamlico Sound at Maw Point. The total length of the main stem of the river is about 250 miles, and its drain- age area is approximately 5,600 mi^, which is about 11 percent of the total area of North Carolina. The average annual precipitation ranges from near 45 inches in Durham County to about 54 inches at New Bern. The mean annual flow, measured at the most downstream gaging station, at Kinston (station 02089500 on plate 1), is about 2,900 ft^/s for a drain- age area of 2,690 mi^. The Trent River heads in Lenoir County, North Carolina, and flows almost due east to its confluence with the Neuse River at New Bern. This juncture is about 38 miles upstream from the mouth of the Neuse River at Maw Point on Pamlico Sound. The Trent drainage area of 516 mi^ is included in the 5,600 mi^ drainage area of the Neuse River. The total length of the Trent River is about 80 miles and the mean annual flow at the gaging station at Trenton (station 02092500 in plate 1) is about 200 ft^/s for a drainage area of 168 mi^. The estimated average annual discharge into Pamlico Sound at the mouth of the Neuse River (Plate 1) from the entire 5,600 mi^ drainage area of the Neuse-Trent river system is about 6,100 ft^/s. The esti- mated average monthly discharges at the mouth, in cubic feet per second, are as follows: Jan. - 8,400 Apr. - 7,700 July - 5,000 Oct. - 3,800 Feb. - 11,000 May - 4,200 Aug. - 5,300 Nov. - 3,800 Mar. - 10,000 June - 3,400 Sept. - 5,000 Dec. - 5,800 The upstream limit of tide effects in the Neuse and Trent Rivers has not been well established, but is thought to be near Fort Barnwell on the Neuse River (about 65 miles upstream from the mouth at Maw Point); on the Trent River the upstream limit of tide effects is thought to be about halfway between Pollocksville and Trenton, or about 35 miles upstream from its mouth at New Bern (about 73 miles upstream from the mouth of the Neuse River) . The Neuse River estuary varies from 6.3 miles wide and an average depth of 17 feet at its mouth near Maw Point to a width of about 0.9 mile and an average depth of about 8 feet at New Bern. From New Bern to the head of tide near Fort Barnwell, the estuary narrows considerably and maximum depths are at least 3 feet in any cross section. The Trent River estuary varies from 0.3 mile wide and about 10 feet deep at the mouth at New Bern to about 50 feet wide and 4 feet deep at the head of tide near Pollocksville. 92 Effects of Wind on Water Levels and Specific Conductance Water levels in the lower parts of the Neuse River and Trent River estuaries are primarily controlled by the direction and magnitude of the surface winds on Pamlico Sound. Because of the dampening effect of Pamlico Sound, tidal ranges are less than a foot at New Bern. Vari- ations in water levels due to freshwater inflow are also small because the surface areas of the lower parts of these estuaries are large rela- tive to freshwater inflow. Figure 3.13 is a wind diagram for resolving a given wind to the directional component which is effective in causing a change in water level at New Bern. The values on the circle are based on the cosine of the angle between the actual direction of the wind and the direction which causes the maximum effect. Winds blowing in the direction of the lower channel axis of the Neuse River (the lower 15 miles) have the greatest effect on the level of water in the estuary. This axis of maximum wind effect forms an angle of 60° with true north. Thus, a wind blowing from due north (cos 60° = 0.5) is only half as effective in producing high water levels at New Bern as a wind that blows from 60° east . Figure 3.14 is a curve that relates the wind component to the change in water level at New Bern. The curve shown was developed from Geological Survey water level records of the Neuse River at New Bern and wind speed and direction records from the National Weather Service station at New Bern. This relation might be used as a rough predictor of potential hurricane flooding and in assigning flood risks to stream- bank areas. To predict water level changes in the Neuse estuary at New Bern resulting from winds acting on Pamlico Sound, determine the di- rection of the wind, then multiply its velocity by the cosine of the angle formed between the actual wind direction and the direction of maximum effect, obtained from figure 3.13. The result is the wind ve- locity component. Then enter figure 3.14 with the wind velocity com- ponent and read the change in water level on the abscissa. Example: a 30 mi/hr wind from the east From figure 3.13, cosine of angle between actual wind direction and direction of maximum effect (30 ) = 0.87. Therefore, 0.87 x 30 = 26 mi/hr. And from figure 3.14, water level change = 4.6 ft rise. The highest recorded water levels at New Bern have been caused by a combination of hurricane winds, the associated low barometric pressure, and intense short-term rainfall. Hurricane lone (September 19,1955) passed about 15 miles east of New Bern on a northerly course and caused 93 FiguAz 3.1 3.--{)Jlnd diagram {^ofi tko, Hq^lx^z Zlv2A e^taoAy cut him BoAn, t 94 VIquah 3. 1 4 . --Change in woutoJi l^vel 0^5 tk^ Neoie Riv2A utaoAij cut Mm BeAn dad to Mind (U K -6.15 it MSL. site on each river upstream from any saltwater contamination. Gener- ally, maximum concentrations reflect the quality of the ground water entering each river and minimum concentrations are more reflective of surface runoff. The main difference in water quality between the two rivers is that concentrations of most dissolved constituents for the Neuse River average significantly more than those for the Trent River. The major exceptions are bicarbonate (HCO3) and calcium (Ca) which average higher for the Trent River. The higher concentrations in the Trent River are the result of significant ground-water inflow from the Castle Hayne Limestone. High color and iron are sometimes a problem with water in both estuaries. Some reaches of the Neuse River estuary occasionally undergo oxygen depletion due to algal blooms which may utilize all available oxygen, resulting in fish kills and the destruction of most bottom-dwelling 96 Hd (3„gZ soqnio -J3TK) 33UBq3np -UOD 3TJT33dS umTsauSBm mnioxBO psATOSSfQ (^ON) a.H.^N (J) sPTJontj (TO) 3PTJ0TM3 ^ ^ » - (''oS) sjejins ('■03H) 1 S S 2 3 (X) ninTSSEjOd (BO) mnTDXEO (^OTS) B3TTTS puE samsj^xH Isl III JO poTJaj about 3,900 168 U. H Jaqninu UOT3B3S 97 organisms. The role of nutrients in promoting these destructive algal blooms in the Neuse estuary is discussed by Hobbie (1975) . The temperature of the water in the estuaries is directly related to the seasons (fig. 3.16). Maximum temperatures usually occur in July and the minimums in January. Only small temperature differences are detectable laterally in any cross section and seldom does more than one degree Celsius temperature difference exists from the surface to the bottom. FiguA^ 3.1 6. --Range, MotoJi tzmp^h.cituA2yi, ■in WetMe Rlv^A utnoJiLj cut Hm BeAn {^K.om 0ctob2A 1963 thAougk Augui6t 1 967. Spatial Variations in Salinity Although water in Pamlico Sound is usually well mixed by wind and currents and is almost always uniform in salinity from the surface to the bottom, salinity stratification often occurs near the mouth of the Neuse River estuary during and following periods of sustained high freshwater inflow. This stratification is the result of the lighter freshwater overriding the more dense saltwater. Stratif iciation is even 98 more common further upstream in the estuarine portions of the Neuse and Trent rivers and at times may be quite pronounced. Samples have been collected where the surface-to-bottom salinity ratio approached 0.01. Usually, however, this ratio is not less than 0.8. Lateral salinity variations within a cross section of the wide portion of the estuary are also quite common. The salinity is usually higher on the left bank of the estuary (in the sense of facing down- stream), a phenomenon attributed to the Coriolis force, as discussed in the GENERAL HYDROLOGY section. In the northern hemisphere, this force tends to deflect a mass to the right of its direction of motion. Thus, the salty water intruding up the estuary tends toward the left bank and the fresh water flowing down the estuary tends toward the right bank. Figure 3.17 shows this effect for the Neuse River as observed during a survey of surface specific conductance on August 13, 1967. The specific conductance of the water at the left bank was more than 4,000 ymhos while on the right bank, directly across the channel, it was as low as 1,000 ymhos. This phenomenon has also been observed in the wide sections of other North Carolina estuaries. Field specific conductivity surveys have shown that, for the narrow reaches of the Neuse River and Trent River estuaries, relations between the specific conductance at one location and the specific conductance at other locations either upstream or downstream are fairly constant. Figure 3.18 shows such relations for the Neuse River estuary (valid up- stream from the U.S. 17 bridge at New Bern) and the Trent River estuary (valid throughout its length) . The relation for the Neuse upstream from New Bern is based on ten specific conductance surveys made by the Geo- logical Survey between September 1954 and September 1968. The relation for the Trent River estuary is based on nine surveys made during the same period. As may be deduced from figure 3.18, the change in specific conductance for a given distance is greater for the Neuse River estuary (874 ymhos/mi) than for the Trent River estuary (704 ymhos/mi) . This is primarily due to the greater volume of freshwater flowing down the Neuse River that resists the upstream intrusion of saltwater from Pamlico Sound. On the average, about 4,200 ft^/s of freshwater flows into the Neuse River estuary at Fort Barnwell compared to an average of only about 450 ft^/s at Pollocksville on the Trent River estuary. Frequency of Saltwater Intrusion Information on the frequency of saltwater intrusion at various locations in the Neuse and Trent River estuaries may have application in siting intakes for water supplies. Although no single water-quality criterion may be given, it was noted earlier that water with a chloride concentration of 250 mg/L is unsuitable for public supplies and water containing more than 500 mg/L is unsuitable for a variety of industrial uses . 99 77°00' 77°00' Qj>tuaAy, August 13, 7 967. 100 o 20,000 I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — r o 5 10 15 20 DOWNSTREAM DISTANCE, IN MILES VIqlxaq 3.1 S. --Change In iuA^^ace ip^dl^^lc donductoincLQ. Mlth up6tA^am and doMn^tA^am distance, in tkt Weu^e, and The U.S. Geological Survey collected water samples daily from the surface and bottom of the Neuse at the U.S. 17 bridge in New Bern (station 02092162 on plate 1) from 1957 through 1967. The specific conductance of these daily samples was checked against the record of a monitor that continuously recorded the specific conductivity of water at the bottom of the channel at the U.S. 17 bridge for a two-year period. In most cases there was less than 5 percent difference between the daily maximum recorded by the monitor and the conductivity of the daily sample. This indicates that once-daily sampling is sufficient to detect the presence of saltwater intrusion at this location. A frequency analysis of the specific conductivity data from the 11 years of daily samples at New Bern (fig. 3.19) shows that at least some saltwater (as indicated by a specific conductance of 800 ymhos) was present along the channel bottom 60 percent of the time and along the surface, about 45 percent of the time. 101 100 400 1000 4000 10,000 40,000 CONDUCTANCE, IN MICROMHOS AT 25° C FxLguAe 3.19.—FaAC2.ntag^ oi tunn ipe(U{ilc conductances equaled oA. exceeded Indicated values In the Weu^e RlveA estuan.y at Wew BeAn, 1957-67. The surface specific conductance-distance relation (fig. 3.18) and the surface specific-conductivity frequency curve (fig. 3.19) were used to estimate the frequency of occurrence of various specific conduc- tivities at points in the estuary upstream from the U.S. Highway 17 bridge (fig. 3.20). These estimates are the dashed curves above the solid curve labeled "surface specific conductance at New Bern" on figure 3.20. It should be emphasized that these dashed curves are generated and only the solid curve is based on measured data. As an example of the use of figure 3.20, suppose an industry desires process water which may have a conductance exceeding 800 ymhos not more than 5 percent of the time, with water during times of ex- ceedance being provided by emergency storage. Where is the most down- stream point along the Neuse River which could reasonably be expected to meet this criteria? From figure 3.20, the intersection of the 800-ymhos line and the 5 percent exceedance line falls between 6 and 8 miles up- stream from the U. S. Highway 17 bridge at New Bern. The interpolated value would be about 7.5 miles. The industry in this example would probably not want to locate its water intakes any further downstream than this. 1Q2 Flgitn-d 3. 20. --PeAcetitage turn iuA^ace. i>p(2,(iA^ic aondacXance^ mh^z nqnaldd dxct^dzd {)0K 2^tuaALj, 1957-67. In the Trent River estuary, daily samples were collected by the U.S. Geological Survey from 1959 through 1961 at a site 6.5 miles up- stream from its confluence with the Neuse River at New Bern (station 02092558 on plate 1) . The samples were integrated surface-to-bottom in a shallow part of the river where surface and bottom specific conduc- tivities were nearly the same. A frequency analysis of the daily conductivities is shown in figure 3.21. The added dashed curves repre- sent estimates of the frequency of occurrence of various specific con- ductivities at other points in the estuary. These dashed relations were generated by using the specific conductivity-distance relationship for the Trent River . estuary (fig. 3.18). ioa_ 100 400 1000 4000 10,000 40,000 SPECIFIC CONDUCTANCE, IN MICROMHOS, AT 25° C F^guAe 3.21 .--FoAce-ntagQ. oi tlmz lyitzg^aX^d ipzcyif^lc. conductance M;eAe equated on. exceeded ^oK i^eveAoZ locations, In tke J^ent Riven. e^tuaAy, 7 959-6 7 . Salinity data were collected at a number of other more downstream locations in the Neuse River estuary by the University of North Carolina Institute of Marine Sciences and the Carolina Power and Light Company. Data from four of the locations having the longest period of sampling have been analyzed to give some indication of the frequency of oc- currence of various salinities. The data were collected at varying intervals, from days to months apart, but with good year-round coverage. Figures 3.22-3.25 present salinity frequency curves for these 4 lo- cations, known as Garbacon Shoals Light, Wilkinson Point Light, Hampton Shoal Light, and Fort Point Light (plate 1). 104 Ol 1 1 Mil \ \ \ \ I I \ \ \ \ \ \ \ I 0.01 0.1 0.5 I 2 5 10 20 30 40 50 60 70 80 90 95 98 99.5 99 9 PERCENTAGE OF TIME INDICATED SALINITY WAS EQUALED OR EXCEEDED VlquJiz 3. 22. --VzAc^ntage, tim2. 6uA{^ci Light, 194S- 6S. Data aKo. {,A.om tho. Uni.vizA6ity NonXk Caxotina In^tltatu oi MaAyim- Scimce.^ , h\on.2,h2.ad City, N.C., and Can.oJU.na Pomqa and light. Company. 105 flguA^ 3.23 .--VoAc^ntagz o^j turn ^uJiiaco, avid bottom i^atiwitioji weAe eqaaXed 0^ zxc2.zd(id in tk Mill be equaXecf exceeded In tko. Meu^e R^ue^ at New; BeAn ion. vaJvioiu annual avQAaQii dLUchoAQQJ) at Kln6ton [d/iainag^ oAm - 2690 ml^] . Generally, conditions of minimum saltwater encroachment in the Neuse and Trent River estuaries occur during the month of April and maximum saltwater encroachment occur in December (figs. 3.11 and 3.12). This may seem surprising because maximum discharges of the Neuse River occur in February, on average, and minimums occur in June. However, changes in salinity due to changing freshwater discharge occur slowly in these estuaries because of the dampening effect of Pamlico Sound. 110 VlquAd 3.27 .--P^^c^/itag^ o{) turn a ^^pecx^jic con- daatancn oi SOO vrnkoi, Mill be zqualad 01 exceeded in tkz J^inZ RivoA maJi New Be-in (jO/L vcuvioiii annual avoAaqe. di^dhoAQdi) at iKiintan [dAalnagz aAm - 13^ mi^] . The Tar-Pamllco River System The Tar-Pamlico River system (plate 1) drains a total area of 4,300 mi^, with an average annual outflow of about 5,400 ft^/s. The upper part of the basin lies in the Piedmont Province and the lower part in the Coastal Plain. The largest tributary to the Tar-Pamlico River system is Fishing Creek (drainage area 860 mi^) . Other important tribu- taries include Cokey Swamp Creek, Conetoe Creek, Tranters Creek, and the Pungo River. Ill Actually, the Tar River and the Pamlico River are one and the same watercourse. Upstream from the mouth of Tranters Creek (plate 1), which is about 39 miles upstream from the mouth of the Pamlico River, it is known as the Tar River; downstream in the widening segment opening into Pamlico Sound, it is known as the Pamlico River. A navigation channel is maintained in the Tar-Pamlico River by the U.S. Army Corps of Engineers to 200 feet wide and 12 feet deep from the mouth of the Pamlico River to Washington (about 38 miles upstream from the mouth of the Pamlico River) ; 100 feet wide and 12 feet deep from Washington to a turning basin at the mouth of Hardee Creek, a tributary to the Tar River; and 75 feet wide and 5 feet deep from Hardee Creek to Greenville. The combined surface area of the Pamlico and Pungo Rivers is rather large, about 225 mi^. However, depths are shallow, averaging only about 11 feet. The total volume of the Pamlico River estuary (including the open-water segment of the Pungo River) is about 69 billion ft^. The influence of ocean tides extends upstream to Greenville on the Tar River, about 59 miles upstream from the mouth of the Pamlico River estuary. Due to the dampening effect of Pamlico Sound, tide ranges near the mouth of the Pamlico River are less than 0.5 foot. However, due to the funnelling effect of decreasing channel dimensions in the upstream direction, tide ranges at Washington approach 1.0 foot. Tide ranges in the Tar River have not been studied, but decrease to nothing near Green- ville. As is the case in the Neuse-Trent system, winds play a far more im- portant role than either lunar tides or freshwater inflow in generating currents and in changing water levels in the Pamlico River. For ex- ample, on Sept. 19, 1955, Hurricane lone produced surges of about 7.0 feet above mean low water at Washington, as recorded on a U.S. Army Corps of Engineers recording tide gage. Although saltwater intrusion in the Pamlico River occurs frequent- ly, in the Tar River saltwater rarely penetrates more than a few miles upstream from the mouth. The greatest known penetration of saltwater into the Tar River occurred on Oct. 15, 1954 following an extreme drought period and during a large influx of saltwater due to Hurricane Hazel; on this day a specific conductance of 15,600 umbos (5,800 mg/L chloride) was measured at Grimesland (sta. 02084171 on plate 1), which is about 4.7 miles upstream from the mouth of the Tar River. The likelihood of occurrence of an extreme drought followed immediately by a major hurricane is difficult to evaluate, but the recurrence interval of two such events occurring in succession is probably more than 100 years. If we assume that salinity gradients along the channel of the Tar are similar to observed gradients in the Pamlico, then the saltwater front (200 mg/L chloride) might have penetrated to about 16 miles upstream from the Grimesland station on that date. 112 Destructive algal blooms are a recurring problem in the Pamlico River estuary. Blooms of algae, predominantly dinof lagellates , occur each late winter or early spring and each summer (Hobble, 1974). Hobble attributes the winter blooms to high concentrations of nitrate in runoff from the Coastal Plain after crops are harvested and forest growth slows in the fall and winter. Summer blooms may be caused by a combination of moderately high concentrations of nutrients entering the estuary, uti- lization by algae of nutrients already present in the sediments in the estuary, and higher rates of biological productivity due to warmer temperatures . Summer dieoffs of all or nearly all bottom-dwelling organisms in the Pamlico River are common when salinity stratification is present. At such times, the decomposition of dead organisms (including algae) on the river bottom may utilize all available oxygen, which is normally re- plenished by mixing with the more oxygen-rich water higher in the water column. However, stratification prevents or greatly inhibits this mixing, thus contributing to the death of fish as well as clams, other bivalves, snails, and marine worms. Several investigators, including Hobble (1974), have established that nitrogen rather than phosphorus is the limiting nutrient in pro- ducing algal blooms in the Pamlico River - phosphorus always being present in sufficient amounts to produce blooms. Carpenter (1971) showed that phosphate in the effluent discharged to the Pamlico River estuary from phosphate mining operations near Beaufort (plate 1) was superfluous to requirements for algae production in the estuary. Davis and others (1978) found that substantial amounts of organic carbon and nutrients are trapped within the Pamlico River sediments and that these may be important contributors to algal blooms, particularly during the summer. Water Levels As previously mentioned, winds are the most important force af- fecting water levels in the Pamlico River. Winds from the east-south- east, blowing parallel to the channel axis, have the maximum effect in producing high water levels in the Pamlico River, while winds from the opposite direction, west-northwest, have the maximum effect in producing low water levels. The wind diagram (fig. 3.28) for resolving a given wind to the directional component effective in causing water level changes in the estuary is similar to the one prepared for the Neuse River estuary (fig. 3.13); values are based on the cosine of the angle between the actual direction of the wind and the direction causing the maximum effect. In the case of the Pamlico River, the axis of maximum effect forms an angle of about 110° from north. 113 Figure 3.29 provides an example of the sensitivity of water levels in the Pamlico River to winds. Actual wind speeds for the period Feb. 22 - Mar. 2, 1966, as recorded at the National Weather Service station at Cape Hatteras, were resolved to their effective componenents along the channel axis of the Pamlico River and plotted below a hydro- graph of water levels recorded by the Corps of Engineers for the same 114 115 period for the Pamlico River at Washington. The close correlation is apparent. The response of water levels to wind is strong and immediate and, except for a lull in the winds on Feb. 26, the influence of lunar tides on water levels is completely overshadowed by wind effects. Figure 3.30 relates the effective component of Cape Hatteras wind velocity to the change in water level of the Corps gage at Washington. The relation is of the same type presented earlier for the Neuse River at New Bern (fig. 3.14), for which an example of its use was given in the text. The relation may be used to predict the approximate rise in water level at Washington in response to east-southeast winds of various magnitudes. This information may prove of • value in hurricane warnings and in assigning flood risks to streambank areas. flgu/LZ 3. 30. --Rotation oi fvii,z in u)at^A. IzvoX In the Pamlico RlvoA cut lf}(Uhyington to e^j$ec;tci'C ivtnd voZocAXy. lib 116 Flow No flow measurements have been made either in the Pamlico River or in the tide-affected part of the Tar River. Therefore, some of what is said regarding flow in the Tar-Pamlico estuary is at least partly speculative. In the wide Pamlico River estuary, winds are undoubtedly the major short-term influence on flow, followed in importance by ocean tides and lastly, freshwater inflow. Freshwater inflow is probably more important in influencing short-term flows in the narrower Tar River than either winds or tides. However, freshwater inflow and the resulting net outflow are definitely the most important long-term influence on flow in both rivers. The channel of the Pamlico River, unlike that of the lower Tar Hi\/er, is vastly oversized for tlie amount of incoming freshwater it must larry. 'I'lieret ore , velocities due to freshwater inflow are very low. I'Or example, at. the mouth of the Pamlico River, the average velocity due Lo the average aiuiual freshwater outflow of 5,40U tt^/s is less tnan ().()/! It/s. On a monthly basis, estimated average freshwater outflows I t (Mil 111" I'ainlico River, in cubic feet per second, are as follows: Jan. - /,7U0 Apr. - 3,200 July - 3,300 Oct. - 3,700 i'Vh. - 10,000 May - 6,800 Aug. - 5,100 Nov. - 4,300 Mar. - 8,000 June - 2,400 Sept.- 3,200 Dec. - 4,900 Annual average flows vary considerably Iroiii year to year (fig. 3.31). For example, there is a 99 percent chance that the annual aver- a>ie flow at the Tar River at Tarboro (sta. 02083300 on plate 1) in any one year will be equal to or less than 1.8 (f t Vs) /mi-^, but only a one percent chance that it will be equal to or less than 0.39 ( f t ^/s ) /mi'^ . 0.3 Drainage area is 2140 mi^ at site Example There is a 50 percent 'chance in any one year that the annual mean discharge will be - equal to or less than 1.04 (ft'/s)/ mi -J J I I I L 99 98 95 90 80 70 60 50 40 30 20 PROBABILITY, IN PERCENT 10 flguAii 3. 51 .--FKuqumcy cuAve. oi annual mean cLuchoAQd oi Tan. Rlv^A at Tanbo/io. A^t^A lUlldM. avid otkaJU, 197S. 117 VIqixah 5.32.--Loijo-^Iom ^A.Q.quznay cuAvd^ annual loLOUt mmn diAc,kah.QQ. (^oK. indicated numboA con6a.CLJitivz daij6 io^ Tax RlvQA at Tafibofto. k{^ttAe.am on. doi^)n^tAtam, Significant saltwater intrusion (200 mg/L chloride or more) is present at Washington only about 23 percent of the time (fig. 3.36), but the frequency of significant saltwater intrusion quickly increases in a downstream direction. At the mouth of the Pamlico River, chloride concentrations are seldom less than 2,000 mg/L, even during periods of high freshwater runoff. The frequency of saltwater intrusion in the Pamlico River estuary is, of course, inversely related to freshwater inflow. Annual average discharges of the Tar River at Tarboro (sta. 02083500 on plate 1) were plotted against percent of time that a specific conductance of 800 ymhos or greater was recorded at Washington for each year during the period 1962-1967 (fig. 3.38). The interpretation of figure 3.38 is similar to that of figure 3.26 for the Neuse River estuary. The percent chance of occurrence (or recurrence interval) of a given annual average discharge and associated specific conductance conditions may be estimated from the right-hand ordinates. Of greater interest, perhaps, is the movement of the saltwater front (200 mg/L chloride) in response to changing freshwater inflow from the Tar River (fig. 3.39). The shape of the curve indicates that when the saltwater front is at or downstream from Washington, a large change in freshwater inflow is required to produce significant movement of the 125 flguAe. 3.3S.--P^^cpQ.di.(^l(i aondiic^tanc^ SOO \imko6 Mill be equaled Q,xd(izd Pi Pi Pi Pi Pi Pi > > ptS Pi Pi OS Pi Pi Pi M > Pi oi Pi Pi Pi Pi > > Pi Pi Pi Pi Pi Pi Pi Pi fa ns > > Pi Pi Pi Pi > > > fa MSS > > Pi Pi Pi > > > > fa > s > Pi Pi Pi Pi > > > > fa > ass > Pi Pi ffi Pi fa fa fa fa > 3S pc: Pi Pi fa fa fa fa > > 3S3 Pi &s Pi > fa fa fa fa > OS 3 Ci Pi > fa fa fa fa > Pi 3N3 pi Pi Pi > fa fa fa fa > oi 3N Pi Pi > fa fa > > pi pi m 1 3NN > > > > > Pi Pi pii OS rsi 1 n Pi Pi Pi Pi Pi Pi -Hi CO iH iH ?^ j-J m: •H C O or il or TP x: > c CU 4-J >-i 0) CU 01 Q CU hJ X) Cfi O 6 3 o C5 iH o 4-1 3 O 4J N u 4-1 QJ U .H X. !-i c o c 4J c o O c 0) o •r- 1 C d fa U CU p:: X fa C o CU CU •H fa ^-1 4-1 w z 4-1 o CU CU CU n3 o 4-J o; )-i S-i OJ >-l c CU CU a S-i u n3 >-l (U CU QJ 0) OJ 0) OJ -a c j-i > c > C > (U c c a 3 t3 > OS i-l OS U *pi o O C C 0) CO 3 3 Oi W) > > 0) O O tl > 0) CD CO u O OS n3 OS •H o -H o d E 4-1 Pi !-i -l cC O .H 3 x. OJ •H nJ o I— 1 O U >H a u Ph i-J 04 2: o OS O aJLiYiiXij i/JoutoA -in ktbrnaAtz Sound and vi-dtnAXy duJvinQ thd month 0^ MaJich [modA.{ii2,d {^K.om {JJllZlami and othoA^, 1967]. 50 60 K'LOMErERS Sea woler contains obout er kilogram of sea moler ) EXPLANATION < Solinily, in groms per kilogrom 34 5 grams of dissolved solids Salinity intemol vorioble figuAe. 4. 7. --AueAage -6uA^ac2. i>atinJjty oi McUeA In Klbmantz Sound and victnAXy duAtng the, month o(i Vo^dmboA {modt^^idd (,ftom Wttttam^ and othoAJi, 1967] . 140 pattern of saltwater intrusion in the Albemarle Sound near Edenton (station 02081155 on plate 1) is also evident from the specific con- ductance data in figure 4.8. F{(ja':L' 4 . S . --Avdiage. montkly ipamplz. See ploutd 1 {^o^ locations. Station No. Name Period of record Maximum chloride concentration in milligrams per liter Maximum conductance in micromhos 25°C 02043852 Pasquotank River near Elizabeth City, N. C. Oct. 57- Sept. 67 1,940 Oct. 15, 1961 6,380 Oct. 15, 1961 mn/. QQ(io UZUH jooZ Pasquotank River at Elizabeth City, N.C. Oct. 57- Sept. 67 o n o A / v> \ o , UzU (,B; Oct. 30, 1958 OA o r\r\ / T) \ 20,800 (B) Oct. 29, 1958 Perquimans River at Hertford, N.C. Oct. 57- Sept. 60 1,290 Dec. 25, 1958 4,290 Dec. 25, 1968 02050160 Chowan River near Eure, N.C. Oct. 67- Dec . 68 880 ymhos Dec. 19, 1967 02053244 Chowan River at Winton, N.C. Oct. 54- Sept. 67 398 Dec. 15, 1958 1,400 Dec. 13 and 15, 1958 02053652 Chowan River near Edenhouse, N.C. Oct. 57 Sept. 67 9,140 (B) Nov. 11, 1958 23,500 (B) Nov. 11, 1958 02081155 Albemarle Sound near Edenton, M r iM . . Oct. 57- Sept. 67 12,100 (B) Nov. 3-6, ly JO 30,600 (B) 02081166 Scuppernong River near Creswell, N.C. Oct. 59- Sept. 67 2,270 June 18, 1967 7,260 June 18, 1967 02081172 Scuppernong River at Columbia, N. C. Oct. 63- Sept. 67 2,980 June 5, 1967 9,300 June 5, 1967 142 H3in H3d SWVH3miH NI 'HaiHOlHD cj s; o ^ +^ CM d on 3619 dQily_ iservotions for o based on 3617 dail observations for t Oct. 1957 to Sept i; CD 1 O rve base ■face ob 3 period pt 1961 irve ' period - o \, =3 O «> cone 800 of tl Specific e o 1 1 Do S2 IV H3I3UIlNa3 H3d SOHHOHDIW NI 3DNV13nCIN0D 3I3I33dS H3in H3d swvyomiw NI 3aiyoiHD (X ^^vJ -o O o — -C3 ciJ O cJ O U- ::S 1:5 uu --^ CD <) CNT) o r> (X . r> o 5S IV H3I3WIINaD H3 to the next downstream segment. Because flows within the Chowan and"'"the lower reaches of its tributaries are tide- affected, flows occur in both upstream and downstream directions. By convention, upstream flow is indicated by a negative sign; downstream flow is positive. 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 PROBABILITY, IN PERCENT ¥i,QuJiz 4.21 .--Low- (iloM {^^^qumcy cuA-vu o^^ annaat toWQj:,t rman dU- choAgz ^on. indicatzd namboji coyi6zcuLtlvii day6 , comblncition 0|5 Elci(ikwcut2A Ziv2A at Vh-CinkLin, \J {^o^ vaAlouui annual. aueAage dAj,(ihaAg<2J> {)Ofi Chouian RiueA at EdmkoLiid, J 95^-67. 160 their mouths. The lower Roanoke, like the others are now, was once a drowned river valley. Now, however, it has been largely filled by sedi- ments. Within the delta thus formed is a fairly unusual system of distribitaries (fig. 4.24) which carry some water from the Roanoke into the Cashie River and, in the case of one large unnamed distributary, directly into Albemarle Sound. Maximum depths along the estuary vary from about 8 to 18 feet. A commercial navigation channel is maintained in the Roanoke River to Palmyra, 81 miles upstream from the mouth. The channel is maintained to 12 feet deep and 150 feet wide from Albemarle Sound to about 1 mile upstream from Plymouth, a distance of 10 miles; thence a channel 8 feet deep and 80 feet wide to Palmyra, a distance of 18 miles. Average annual precipitation over the basin is about 45 inches. The average annual outflow of the Roanoke River at the mouth is about 8,900 ft^/s, second only to the outflow of the Cape Fear River among North Carolina's estuaries. Flow of the Roanoke River is highly regulated, particularly by Roanoke Rapids Lake (details to be discussed later) . The combination of relatively high outflow, small cross-sectional areas, and low-flow augmentation by Roanoke Rapids Lake, effectively blocks saline water from the estuary. During 13 years (Oct. 1954 - Sept. 1967) of daily water sampling at Jamesville (sta. 02081094 on plate 1), the maximum measured chloride concentration was only 12 mg/L. Three specific con- ductance surveys by the Geological Survey during normally low-flow periods (10-6-54, 7-25-57, and 10-1-57) failed to reveal any significant saltwater encroachment, even at the mouth. Significantly, the survey of October 6, 1954, was made before increased low-flow augmentation from Roanoke Rapids Lake and at a time of record low streamflows in many parts of the State. At that time, near maximum-of-record saltwater en- croachments were being measured on other estuaries. Thus, it is not likely that any significant saltwater encroachment will occur in the future in the Roanoke River estuary, even under extreme drought con- ditions, as long as the current flow regulation patterns are maintained. Flow Flow in the Roanoke River estuary has not been studied in detail; thus it is not really known what role winds play in the flow or to what extent the flow is affected by tides. We can infer that winds and tides play a lesser role here than in any other major North Carolina estuary because of the relative narrowness of the channel and the lack of sig- nificant funnelling effects. Conversely, we can infer that freshwater discharges play a relatively larger role because of the greater magni- tude of the discharges in relation to channel cross-sectional areas. However, validation of these inferences awaits confirmation from water- level records and flow measurements. 161 VIquaz 4.24.--VU>tAlbuitaAy 6y6tm oi tkd Iomqa Roanoke RlveA. I'l-'-i':') 162' As previously mentioned, the average annual outflow of the Roanoke River at the mouth Is about 8,900 ft^/s, or about 0.92 (ft^/s) mi^, but average flows for a given year may range from about 0.50 to 1.50 (ft^/s) /mi^ (fig. 4.25). Actually, discussion of freshwater inflow to or outflow from the Roanoke River estuary is not really meaningful except within the context of a knowledge of the existing patterns of flow regulation. Flow of the Roanoke River is extensively regulated by Philpott Lake, John H. Kerr Reservoir, Roanoke Rapids Lake, Leesville Lake, Lake Gaston, and Smith Mountain Lake. All of these reservoirs were created primarily for hydroelectric power generation, but many also provide for flood control, low-flow augmentation, water supply, and recreation. Because it is the most downstream of these reservoirs, Roanoke Rapids Lake is most important from the point of view of its effects on flow in the Roanoke estuary. Pursuant to its license from the Federal Power Commission, the Virginia Power and Electric Company must maintain, subject to special provision, minimum instantaneous flow releases from Roanoke Rapids Lake (drainage area 8,395 mi^) according to the following schedule: ^ 2 Example There is a 50 percent chance in any one ~" year that the annual mean discharge will be equol to or less than 0. 85 ( ft Vs ) / mi ^ 99 98 95 90 80 70 60 50 40 30 20 PROBABILITY. IN PERCENT 10 2 I FiguAZ 4. 25. --F/Leqaenct/ cuAue o^j annu.cit mmn cUj,chaAgu ojj Roanokz RivM. at Roanoko. Rapids. A^teA W^deA and otke,^ {197S]. 163 Month Minimum instantaneous flow, in cubic feet per second January, February, March April 1,000 1,500 May - September 2,000 October 1,500 November, December 1,000 Usually, actual releases from Roanoke Rapids Lake far exceed these minimum requirements, as indicated by measured flows of the Roanoke River at Roanoke Rapids (sta. 02080500 in plate 1). Given below, in cubic feet per second, are estimated average monthly outflows at the mouth of the Roanoke River estuary for the period October 1965 - September, 1975. About 87 percent of these flow amounts are accounted for by controlled releases from Roanoke Rapids Lake: Jan. - 10,000 Apr. - 11,000 July - 8,000 Oct. - 6,500 Feb. - 12,000 May - 10,000 Aug. - 7,500 Nov. - 7,500 Mar. - 10,000 June - 8,500 Sept. - 6,500 Dec. - 8,300 The effects of high-flow regulation are reflected in the similar averages for January - May; flood flows are stored in the various reser voirs and released over long periods of time. Low-flow augmentation is apparent from the relatively high August - November flows; they average about 0.72 (ft^/s)/mi^ compared to only about 0.57 (ft^/s)mi^ in the un regulated Chowan River estuary for the same months. The effects of flow regulation are also apparent in the low-flow frequency curves of fig. 4.26 for the Roanoke River at Roanoke Rapids. At lower probabilities of occurrence, the consecutive-day low flows at Roanoke Rapids are much higher on a per-square-mile basis than are those, say, of the Blackwater and Nottoway Rivers (fig. 4.21); also, th expected range of values for a given consecutive-day discharge is much less for the Roanoke Rapids station. Summaries of the chemical quality of water at four key sites in th Roanoke River basin are given in table 4.5, including observed ranges and average values of major chemical constituents. Iron concentrations sometimes exceed the 0.3 mg/L upper limit recommended by the Environ- mental Protection Agency (1976) for public water supplies, but iron can be removed easily with treatment. Color sometimes exceeds the recom- mended upper limit given in the same report of 75 color units at all stations except Roanoke Rapids. Downstream from Roanoke Rapids, color Water Quality 164 increases in the Roanoke River due to inflows from swampy areas, which impart color from decaying vegetation and the leaching of humic acids. The Cashie River, in this regard, is typical of streams draining coastal swamp areas in the lower half of the Roanoke River basin, where even average color values may exceed recommended upper limits (table 4.5). ¥ igu/ic 26. --Low- ,yI'oi,o f^^icqimictj cuKvQi, anwaal Coa't^sf mean dts- clioA-gn f^oi indicated numbdK of^ con^dcutive daqb,, '(^o^ Roawoka Rivtn at Roanoke Rapids. KitdA {jJlldo.^ and otlicu, 197&. 165 sss sss sss if sss sss s^s sss SSS S3S SS5 SSS 3.08T SSS ^55 SSK SgS SgS S§§ SSS ... ... ... ... ^ ^ f^JO./1 cr.-H r... ^ .-; (i) sPT^ionii -^"i (ID) SPT^-OTMD ^S!^ .^'^ 3^-^ O. t-J. OO. JJr-i ^irlS S'^S (X) nmTSSE:o,j ,I>r~