• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Dedication
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Geomorphology of tidal inlets
 Methodology
 Evaluation of tidal inlet ebb shoal...
 Conclusions and future work
 Coastal inlets database
 Reference
 Biographical sketch














Title: Morphologic asymmetry of ebb shoals at tidal inlets
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Title: Morphologic asymmetry of ebb shoals at tidal inlets
Series Title: Morphologic asymmetry of ebb shoals at tidal inlets
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Table of Contents
    Front Cover
        Front Cover
    Title Page
        Page i
    Dedication
        Page ii
    Acknowledgement
        Page iii
    Table of Contents
        Page iv
        Page v
    List of Tables
        Page vi
    List of Figures
        Page vii
        Page viii
        Page ix
    Abstract
        Page x
        Page xi
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
    Geomorphology of tidal inlets
        Page 6
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    Methodology
        Page 52
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    Evaluation of tidal inlet ebb shoal asymmetries
        Page 83
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    Conclusions and future work
        Page 122
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        Page 125
        Page 126
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        Page 128
        Page 129
        Page 130
        Page 131
        Page 132
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        Page 134
        Page 135
    Coastal inlets database
        Page 136
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    Reference
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    Biographical sketch
        Page 171
Full Text



UFL/COEL-2002/016


MORPHOLOGIC ASYMMETRY OF EBB SHOALS
AT TIDAL INLETS






by






Erica Eva Carr-Betts


Master Engineer


2002













MORPHOLOGIC ASYMMETRY OF EBB SHOALS AT TIDAL INLETS


By

ERICA EVA CARR-BETTS















A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
ENGINEER

UNIVERSITY OF FLORIDA


2002





















This work is dedicated to Daniel con amor y amistad.













ACKNOWLEDGMENTS

I would like to acknowledge Dr. Nicholas C. Kraus for his enthusiasm and dedication

to this study and the US Army Corps of Engineers, Waterways Experiment Station,

Coastal Inlets Research Program for the support provided for this research. My gratitude

is also expressed to Dr. Robert J. Thieke for his assistance and for serving as my major

advisor. Additionally, my appreciation is extended to the members of my advisory

committee; Dr. Ashish J. Mehta, Dr. Robert G. Dean, and Dr. John Jaeger.

This work would never have come to fruition without the motivation of my entire

family including my husband, Daniel. With Daniel, sharing the adventure of education

has been just one extraordinary component of the journey of our lives.














TABLE OF CONTENTS
page

ACKNOWLEDGMENTS ................................................................................................. i

LIST OF TABLES................................................ ......................................... vi

LIST O F FIGU RES .............................. .................................................. .......... .. ...... vii

A B ST R A C T ...... ....... ................................................................................. .................... x

CHAPTER

1 IN TR OD U CTION .............................................................................................1

1.1 A rea of Study ............................................................................................ .......... 1
1.2 R research N eeds ......................................................................................... ........... 2
1.3 Objectives and Scope ................................ .............................. ................ 3
1.4 Overview of Thesis ................................................................................ ............ 4


2 GEOMORPHOLOGY OF TIDAL INLETS ...............................................................6

2.1 Tidal Inlets ............................................................ .............. ...... 7
2.1.1 Geomorphic Components of Tidal Inlets............ ................................. 7
2.1.2 Formation of Geomorphic Components.................................. ................ 14
2.2 Predictive R elationships................................... ................................................. 20
2.2.1 Relationships with Channel Cross Section ................................. ...... 20
2.2.2 Inlet Stability............................... .................................................................. 23
2.2.3 Relationships with Area and Volume ................................................. 24
2.3 Ebb Shoals and Tidal Channels ................................... ...... ............... 30
2.3.1 Asymmetries in Ebb Shoals ................................... ..... ............. 30
2.3.2 Asymmetries in Tidal Channels...................... ..................... 41


3 METHODOLOGY .................................................................................................52

3.1 Acquisition of Data.......................................................................................... 52
3.1.1 A sym m etry Indicators................................... ......................................... 52
3.1.2 Tidal Inlet Parameters .......................................... ........... ...... 60
3.1.3 The Federal Inlets Database.......................................................................... 68
3.2 Analysis of Data and Variability............................................................................. 77








3.2.1 Asymmetry Indicators........................................................................... 77
3.2.2 Tidal Inlet Parameters ........................................................................... 80


4 EVALUATION OF TIDAL INLET EBB SHOAL ASYMMETRIES ......................83

4.1 Findings of Direct Asymmetry Indicator Relationships....................................... 83
4.1.1 Distance Offshore.................................................... .............................. 84
4.1.2 Distance to the Updrift and Downdrift Attachment points....................... 88
4.2 Findings of Temporal Changes in Asymmetry Indicators.................................. 94
4.2.1 Case Study 1: St. Augustine Inlet, Florida............................... ........... 94
4.2.2 Case Study 2: Ocean City Inlet, Maryland ............................................ 96
4.3 Physical Considerations of Governing Parameters............................................ 98
4.3.1 Inlet Hydraulic Parameters ................................................ ............ 99
4.3.2 Inlet Geometric Parameters..................................................................... 103
4.3.3 Buonaiuto and Kraus Parameter ........................................................... 107
4.3.4 Carr and Kraus Param eter....................................................................... 109
4.3.5 Ratios of Ebb Shoal Attachment Point Planform Asymmetry.............. 112
4.4 Analysis of Correlations ............................................................................... 118


5 CONCLUSIONS AND FUTURE WORK...............................................................122

5.1 Findings........................................................................................................... 124
5.2 Application of Findings and Further Research................................................... 128


APPENDIX

A COASTAL INLETS DATABASE............................................................................137

LIST OF REFEREN CES .................................................................................................165

BIOGRAPHICAL SKETCH ...................................................................................171








LIST OF TABLES


Table page

2-1. Inlet morphologic variables (from Hubbard, Oertel, and Nummedal, 1979)............19

2-2. Forcing factors controlling tidal inlet ebb shoal morphology .................................31

3-1. Variability in tidal prism values ............................................................................82

3-2. Variability in cross sectional area values ..............................................................82

4-1. Coefficients of Equation 4-1 for trend lines in Figure 4-1 ......................................86

4-2. Coefficients of Equation 4-2 for trend lines in Figure 4-2 ......................................89

4-3. Coefficients of Equation 4-3 for trend lines in Figure 4-3 ......................................90

4-4. Coefficients of Equation 4-4...............................................................................104

4-5. Coefficients of Equation 4-5...............................................................................105

4-6. Coefficients of Equation 4-6...............................................................................106

4-7. Coefficients of Equation 4-7............................................................................108

4-8. Coefficients of Equation 4-8...............................................................................108

4-9. Coefficients of Equation 4-9...............................................................................109

4-10. Coefficients of Equation 4-10................................................................................110

4-11. Coefficients of Equation 4-11...........................................................................111

4-12. Coefficients of Equation 4-12................................................................................112

4-13. Coefficients of Equation 4-13 for trend lines in Figure 4-23 ..............................114

4-14. Ebb shoal attachment point distance ratios and proximity values........................ 16

4-15. Summary of the analysis results for inlet geometric parameters.........................120

4-16. F-test sum m ary .................................................................................................121

5-1. Averaged angle measurement values ....................................................................135








LIST OF FIGURES


Figure page

2-1. Components of an ebb shoal.................................................................................. 7

2-2. The ebb shoal com plex.......................................................................................... 8

2-3. Ponce de Leon Inlet, Florida (1992) with an r value equal to 345 ...........................10

2-4. San Francisco, California (1963) with an r value equal to 2....................................10

2-5. Gasparilla Pass, Florida with an r value equal to 110 .............................................11

2-6. M atanzas Inlet, Florida (1979) ................................................. .......................... 12

2-7. Inlet entrance morphology, East Pass, Florida (1990)......................................... 13

2-8. Coastline Classification (after Hayes 1979)......................................... .............. 18

2-9. Morphologic structures Georgia Bight barrier system (from Hayes 1994) ............18

2-10. Escoffier 1940 stability diagram.......................................................................24

2-1 la. Overlapping shoreline configuration (Galvin, 1970) Blind Pass, FL...................36

2-1 lb. Updrift offset shoreline configuration (Galvin, 1970) St. Lucie Pass, FL .............36

2-1 1c. Downdrift offset shoreline configuration (Galvin, 1970) Alsea Inlet, OR.............37

2-11d. Negligible offset shoreline configuration (Galvin, 1970) Stump Pass, FL ............37

2-12. Models of sediment bypassing from FitzGerald, Kraus, and Hands (2000) ...........47

2-12 Continued. Models of sediment bypassing from FitzGerald, Kraus, and Hands
(2000)............................................................. .............................. 48

3-1. Bogue Inlet, North Carolina (May 4, 1958) ..........................................................53

3-2. Ebb shoal complex planform shape outlines .........................................................54

3-3. Symmetric ebb shoal and measurements terminology ............................................56

3-4. Captiva Island, Florida (1963)................................... ...........................................59

3-5. Region identifiers of U.S. federally maintained tidal inlets ....................................69

3-6. N ew England region............................................................................................ 70








3-7. Central Atlantic Coast region ............................................................................ 70

3-8. South East region................................................................................................. 71

3-9. G ulf Coast region................................................................................................. 71

3-10. Great Lakes region............................................................................................. 72

3-11. W est Coast region..............................................................................................72

3-12. A laska region..................................................................................................... 73

3-13. Columbia River Inlet, Oregon/Washington (large) (1983) ....................................74

3-14. Venice Inlet, Florida (small) (1998) .....................................................................74

3-15. Government Cut, Florida (jettied) (1970)................................................................75

3-16. Moriches Inlet, New York (Atlantic) (1996)...................................................75

3-17. Pensacola Bay Entrance, Florida (no jetties) (1962)..................................... ..76

3-18. Tillamook Bay, Oregon (Pacific) (1958)..............................................................76

3-19. Colorado River Inlet, Texas (Gulf Coast) (1991)............................................77

4-1. D versus P ........................................................................................................... 84

4-2. D u versus P .......................................................................................................... 88

4-3. D d versus P ...........................................................................................................89

4-4. Temporal behavior of Do St. Augustine Inlet, Florida..........................................94

4-5. Temporal behavior of D, and Dd St. Augustine Inlet, Florida ..............................95

4-6. Temporal behavior of Do Ocean City Inlet, Maryland................................. ...96

4-7. Temporal behavior of Du Ocean City Inlet, Maryland................................. ...97

4-8. Temporal behavior of Dd Ocean City Inlet, Maryland................................. ...98

4-9. D o versus H .......................................................................................................... 99

4-10. D versus H ...................................................................................................... 100

4-11. D d versus H ....................................................................................................... 100

4-12. Ebb shoal attachment distance ratio versus transport rate ratio............................102








4-13. D versus W ............................................. ..................................................104

4-14. D versus W ..................................................................................................... 105

4-15. Dd versus W ...........................................................................................................106

4-16. Do versus (PH)1/4 .............................................................................................107

4-17. D versus (PH) 4 ............................................................................................. 108

4-18. Dd versus (PH) 4 ............................................................................................. 109

4-19. D versus (PW )1/4..................................................................................... 110

4-20. D versus (PW )14.......... ............................................ 11

4-21. Dd versus (PW )/4............................................................................................112

4-22. Ebb shoal attachm ent point symm etry................................................................. 113

4-23. Ebb shoal attachm ent point asymm etry.................................................................114

4-24. Du versus Dd ...................................... ..................................................................... 115

4-25. East Pass Destin, Florida D,/Dd = 0.91 ........................................................... 116

4-26. St. Lucie Inlet, Florida Du/Dd = 0.38 ................................................................. 117

4-27. Oregon Inlet, North Carolina Dd/Du = 0.74.......................................................... 118

5-1. Ebb shoal angle measurement definition sketch..................................................... 134















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Engineer

MORPHOLOGIC ASYMMETRY OF EBB SHOALS AT TIDAL INLETS


By

Erica Eva Carr-Betts

December 2002


Chair: Dr. Robert J. Thieke
Department: Coastal and Oceanographic Engineering

The ebb shoal is an integral and prominent morphological component of a tidal inlet.

Ebb shoal symmetry is related to sediment bypassing, and its symmetry, volume, and

depth exert control on inlet navigability. Specific identifiers of inlet ebb shoal

asymmetry, herein termed asymmetry indicators, have been measured from aerial

photographs and bathymetric charts and compared graphically to geometric and hydraulic

parameters of the associated inlet. The asymmetry indicators measured were distance to

the most offshore extent of the ebb shoal Do, and the distances Du and Dd from the inlet

edge to the updrift and downdrift attachment points, respectively.

The Coastal Inlets Database was developed to facilitate the collection of the tidal inlet

parameters. The database lists, in tabular form, coastal and tidal parameters, geometric

dimensions, and dredging data for 156 federally maintained inlets within the United

States. Once these parameters were available, the work of developing relationships that

quantitatively describe asymmetries at tidal inlets could proceed. The relationships









developed between the asymmetry indicators and the tidal inlet parameters indicate

strong correlations as power functions.

Given certain parameters of a tidal inlet, empirical formulas for asymmetry indicators

may provide guidance in the development of solutions to coastal engineering challenges.

For example, models presented may be utilized to identify such information as the growth

pattern of the ebb shoal, the orientation of the ebb channel, the bypassing rate of sediment

from the updrift shoreline to the downdrift shoreline, the maintenance dredging

requirements of the channel for navigational purposes, and the cause-and-effect

relationship ebb shoal mining has on the downdrift shoreline.

The results of this study, including sources of error and the methods used for the

analysis, are provided, together with a discussion of coastal engineering applications for

the relationships developed.














CHAPTER 1
INTRODUCTION

A tidal inlet or entrance can be defined as a channel connecting an ocean or lake to a

smaller water body and which experiences long-period water motion by tide or seiching,

together with a wave-induced longshore current. Inlet channels contribute to economic

vitality as commercial navigational waterways, are part of the military infrastructure of

the nation, are used by pleasure boaters, and are key components of the estuarine

ecosystem. Understanding the physical processes occurring at tidal inlets and entrances

is required for predicting the evolution of the inlet and adjacent beaches, both under

natural conditions and in response to engineering activities such as routine channel

maintenance, channel deepening, and mining of ebb and flood tidal shoals. One must

understand the evolution of tidal inlet morphology so that where engineering within the

tidal inlets and surrounding area becomes necessary, the solutions may be appropriate

ones.

1.1 Area of Study

Inlets are comprised of a channel, a flood shoal (an area of sediment deposition on the

bay-ward side of the inlet), and an ebb shoal complex (an area of sediment deposition on

the ocean-ward side of an inlet). These components vary in size and shape depending on

the hydrodynamic processes occurring at the inlet and its sedimentary environment.

Understanding of the physical processes at tidal inlets and the inlet's interaction with the

adjacent beaches is pertinent to the navigability of channels, the maintenance of water

quality for the productivity of larvae and fish species, and for sediment transport and









exchange within the influence of the inlet. Although engineers and geomorphologists

have studied tidal inlets, many questions remain because the processes occurring at these

inlets are complex, and specific inlet behaviors depend on the local environment. The

present study focuses on geomorphic symmetries of ebb shoals at tidal inlets. Ebb shoals

may be classified as either symmetrical or asymmetrical, where symmetrical ebb shoals

are shoals in their "ideal" form and asymmetrical ebb shoals are deviations from this

ideal. Asymmetries in tidal inlets are formed through the interaction of static and

dynamic processes occurring at the coast and within the back bay. The tidal inlet

components develop through processes both internal and external to the inlet.

Much research has focused on the development of predictive relationships between

tidal parameters and features of tidal inlets. Despite the critical role tidal asymmetries

play in the development of navigability of inlets and the coastline, predictive

relationships between tidal inlet parameters and ebb shoal asymmetries have not been

investigated systematically or quantitatively. It is, therefore, expected to be fruitful to

examine the causative factors of asymmetries and quantify asymmetries in tidal inlets to

advance understanding of the processes at tidal inlets and adjacent shores.

1.2 Research Needs

Increased sedimentation within an inlet ebb shoal and navigational channel may

impede flow through a tidal inlet by decreasing water depth and creating depositional

features. This deposition may result in a migrated channel, which in most cases must be

avoided. Additionally, ebb shoals may alter wave and current patterns near the inlet

entrance making transit through the inlet treacherous. With knowledge of the formation

of morphologic asymmetries at tidal inlets, predictive models may be developed which,

given certain parameters of the tidal inlet, may allow for the determination of such items









as the growth pattern of the ebb shoal, the orientation of the ebb channel, the bypassing

rate of sediment from the updrift shoreline to the downdrift shoreline, maintenance

dredging requirements of the channel for navigational purposes, and the cause and effect

relationship ebb shoal mining has on the downdrift shoreline among other things.

There is a need for greater understanding of the formation and development of inlet

asymmetries and shoreline reaction to these asymmetries. Additionally, there is a

demand for a single source for data on specific parameters at tidal inlets. Presently, there

appears to be no single source that provides a compilation of such data.

1.3 Objectives and Scope

The main objective of this research is development of relationships between

asymmetries observed at the ebb shoal complex and associated parameters at tidal inlets.

To fulfill the primary objective of the study, unambiguous indicators of asymmetry at

tidal inlets needed to be identified. These asymmetry indicators are applied to define

relationships with specific parameters at tidal inlets. To fully realize this objective, a

second objective, the creation of a Coastal Inlets Database, arose. The Coastal Inlets

Database, which was developed specifically to aid in the continuation of this research,

lists in tabular form, hydraulic and tidal parameters, geometric dimensions, and dredging

data for 156 federally maintained inlets within the United States. Once these parameters

were available, the work of developing relationships that quantitatively describe

asymmetries at tidal inlets could continue.

The implementation of empirical quantitative relationships for the prediction of the

characteristics of selected symmetries in morphological forms at inlet entrances enter in

applications such as









a. Formulation of sediment budgets at inlets, where detailed sediment pathways are

required;

b. Determination of the predominant or net direction of longshore sediment transport;

c. Determination of the natural causes of entrance channel migration and realignment

(both for maintenance of existing channels and for modification of channel alignment);

d. Estimating the consequences of construction of or modifications to jetties, such as

alteration of sediment pathways;

e. Understanding and estimation of the locations of potential areas of erosion and

accretion near new or modified inlets; and,

f. Development of guidance on effective areas of placement of dredged material for the

benefit of the downdrift beaches.

1.4 Overview of Thesis

Chapter 2 discusses the general characterization of the tidal inlets, including

formation, and the role of the ebb shoal complex in relation to the inlet and adjacent

shorelines. The inlet ebb shoal is then discussed and typical and asymmetrical forms

identified. Additionally, the chapter discusses the state of knowledge in the area of

predictive relationships developed for tidal inlets. Chapter 3 introduces the Coastal Inlets

Database and describes the procedures for the acquisition of data for this research. The

chapter continues with a description of the analysis of the data and the errors associated

with this analysis. Chapter 4 presents specific information about asymmetries at coastal

tidal inlets, the results of the present research, and the applications for the findings.

Chapter 5 gives conclusions of this study and describes potential future research activities

to build on the present understanding of the development of asymmetries at tidal inlets.





5


Appendix A presents the Coastal Inlets Database, which lists hydraulic, geometric, tidal,

and maintenance data on 156 federally maintained inlets within the United States.














CHAPTER 2
GEOMORPHOLOGY OF TIDAL INLETS

Tidal inlets are dynamic coastal features that have received considerable study by

coastal engineers and scientists. Some tidal inlets may serve two, often conflicting

purposes; that of a navigational waterway, which tends to restrict sediment transport

around the inlet, and that of sand bypassing system if left in a natural state. A number of

parameters including tides, currents, geologic setting, waves, geometry, and

anthropogenic intervention control the functioning of a tidal inlet. Additionally, the

morphologic features of the inlet themselves act to influence one another, increasing the

complexity of the processes and responses occurring at the inlet.

Asymmetry in the morphology of tidal inlets can be defined as the deviation from an

"ideal" form. The existence of an asymmetrical inlet does not imply the inlet functions as

a navigational pathway or sediment bypasser to a lesser degree, but simply that the

parameters influencing the shape of the tidal inlet and its components are not balanced.

The present study examines asymmetries found in the ebb shoal of the tidal inlet, for both

natural and engineered inlets.

The following sections present the components of tidal inlets and classical theory

regarding and their processes of formation. A conceptual ideal tidal inlet is introduced

and deviations from that ideal, or asymmetries, are presented together with thoughts into

the formation of these asymmetries. Additionally, this chapter examines existing

predictive relationships developed to more fully understand processes at tidal inlets.









2.1 Tidal Inlets

2.1.1 Geomorphic Components of Tidal Inlets

The components of the ebb shoal are shown in Figure 2-1 and include a main ebb

channel, channel margin linear bars, the terminal lobe, swash platforms, swash bars, and

marginal flood channels (Hayes 1979, 1980; FitzGerald and FitzGerald 1977; and

Gibeaut and Davis 1993).


Channel Margin
Linear Bars




f t
Marginal Flood Channel dominant Transport
SC Direction
Swash Bars'


Ebb Tidal Shoal terminal Lob

Model of Morphology of ebb-tidal deltas. Arrows Indicate
dominant direction of tidal Currents after Hayes 1980


Figure 2-1. Components of an ebb shoal



Figure 2-1 is representative of a mesotidal environment (Hayes 1979) with a tidal

range between 2 and 4 m, and relatively high wave energy such as along the New

England coastline. Ebb currents dominate over flood currents within the main ebb

channel. Channel margin linear bars are levee-like deposits located on either side of the

main ebb channel and are built through the interaction of tide-generated currents and

wave-generated currents (Hayes, 1980). The terminal lobe is a steep, seaward sloping,

deposit of sediment located at the end of the main channel. The swash platforms are









broad sheets of sand located on either side of the main channel. Swash bars are built by

wave action and are located on the swash platform. Marginal flood channels are located

between the swash platform and the adjacent updrift and downdrift beaches (Hayes

1980). Hayes (1980) described variations in this morphology for microtidal and

macrotidal environments. Inlets with a tidal range between 0 and 2 m are classified as

microtidal. The morphology of a microtidal inlet includes large flood tidal shoals that are

commonly coupled with areas of sediment washover and small to absent ebb shoals.

Inlets classified as macrotidal have a tidal range greater than 4 m, are dominated by tidal

currents, and are often associated with linear sand ridges, tidal flats, and small or absent

tidal shoals.

The ebb shoal complex is shown in Figure 2-2 with its individual components

identified. Notation in Figure 2-2 follows that of Kraus (2000b). The ebb shoal complex

is comprised of the ebb shoal proper, the updrift and downdrift attachment points, and the

updrift and downdrift bypassing bars.


Bay


(Inlet Throat)


Updrift Attachment Bar
. ....................
S.-..,.. Updrift Bypassing Bar

I ..-...... ::\ Ocean
-.-.-.-.-.-.-.-.-1-.-4-,-
SEbb Shoal Proper

S.. Downdrift Bypassing Bar

Downdrift Attachment Bar...........

Downdrift Attachment Bar


Figure 2-2. The ebb shoal complex









Bruun and Gerritsen (1958) described the process by which the submerged bar in front

of an inlet or entrance on a littoral coast can function as a sand bridge. In their 1958

work, Bruun and Gerritsen elucidated the bypassing of sand by natural action at coastal

inlets and other similar littoral barriers. They described the two main methods for the

bypassing of sand by natural action at improved and unimproved inlets, bypassing on an

offshore bar, and bypassing by tidal flow action, with a majority of inlets bypassing as a

combination of the two processes. The case of bar bypassing is one set in environments

of limited tidal action where, in the presence of a sufficient magnitude of littoral drift

material, sediment is moved downshore, depositing in a bar formation in front of the inlet

and eventually bypassing sediment to the downdrift shoreline. In the situation where

sediment bypasses an inlet through the mechanism of tidal action sediment transfer

occurs primarily through the migration of channels and bars and through the transport of

sand by tidal flow in channels.

Bruun and Gerritsen (1958) defined a ratio r of predominate littoral drift Mmean in

cubic meters per year to the maximum spring tidal flow Qm,, in cubic meters per second

(Equation 2-1) which determines the predominant mechanism for bypassing at an inlet.

mean
r = en Equation 2-1


For r greater than the range from 200 to 300, sediment is transferred through the

mechanism of bar bypassing, such as at Ponce de Leon Inlet on the east coast of Florida,

for which Bruun and Gerritsen calculated an r value of 345. For r less than the range

from 10 to 20, sediment is transferred through the mechanism of tidal flow bypassing as

with San Francisco, California with an r value of 2. Bruun and Gerritsen found that in

such cases where the value for r did not fall within these ranges, such as at Gasparilla









Pass, Florida with a value of r equal to 110, bypassing occurs by a third method

characteristic of mixed-energy inlets.


Figure 2-3. Ponce de Leon Inlet, Florida (1992) with an r value equal to 345


Figure 2-4. San Francisco, California (1963) with an r value equal to 2






























Figure 2-5. Gasparilla Pass, Florida with an r value equal to 110


The dual roles of inlets as sand bypassers and as the location of a navigational channel

may at times be in conflict, because a larger bypassing bar may decrease the suitability

for the inlet to be navigated as sediment stored in the ebb shoal complex blocks

navigation pathways. Bruun and Gerritsen (1958) found that natural inlets with

predominant bar bypassing are unfavorable to navigation and questioned the economic

justification for their dredging such as at Matanzas Inlet in Florida, which has a bar depth

of between 0.9 and 1.5 m inhibiting navigation.

Bruun and Gerritsen (1958) found that natural inlets with tidal flow transfer are

superior for navigation, provided the flow is utilized properly. At improved inlets that

exhibit bar bypassing and at improved inlets that exhibit tidal flow bypassing, the inlets









flushing and bypassing capabilities seem to conflict however; with proper jetty alignment

both bypassing and navigation requirements may be met simultaneously.


Figure 2-6. Matanzas Inlet, Florida (1979)



In this work, the terminology "ebb shoal" or "ebb shoal proper" refers to that portion

of the ebb shoal complex located within the ebb jet (Kraus 2000b). The combination of

the ebb shoal, bypassing bars, and attachment points is termed the "ebb shoal complex."

For discussion, it is convenient to define "inlet edge," labeled by dots in Figure 2-7,

which denotes the location where land and water join at the point of land encroachment

into the inlet. Two inlet edges exist, one on either side an inlet. If an inlet is stabilized

with jetties, the inlet edges are located at the intersection of the shoreline and jetty.







































Figure 2-7. Inlet entrance morphology, East Pass, Florida (1990)



Figure 2-2 represents an idealized ebb shoal complex with the ebb shoal represented in

its ideal form as arcuate, exhibiting symmetry about the entrance channel. For this

idealized inlet, the distances to the updrift and downdrift attachment points are equal

from the inlet edges. In this model, the main ebb channel is located in the center of the

inlet, halfway between the inlet edges, and functions as the navigational channel as it is

the safest and most hydraulically efficient passage from the back bay to the inlet. In








actual situations, inlets do not exhibit this degree of symmetry, as will be seen in

Chapter 3.

2.1.2 Formation of Geomorphic Components

Tidal inlets are formed through the natural or anthropogenic cutting of a pathway from

the ocean side of a land formation to a water body on the opposing side of the land

formation. The geomorphic components of the tidal inlet are formed through the

interaction of wave, tidal, and geologic parameters.

Ebb tidal shoals (also termed ebb tidal deltas or entrance bars) are formed through the

deposition of sediment on the ocean side of an inlet during ebb tide and through the

deposition of sediment moving alongshore. In a wave dominated coastal situation, the

formation of the ebb shoal complex at an inlet occurs, as sediment from the updrift

shoreline is transferred downdrift, through the mechanism of longshore sediment

transport (Hayes 1979). The sediment forms an attachment point at the updrift shoreline

from which the bypassing bar is formed. The updrift bypassing bar transfers sediment

from the attachment point to the ebb shoal proper.

Sediment entering the channel can move to the ebb shoal, flood shoal, or remain in the

channel. This idealized case of sediment transport and feature formation is presented in

Figure 2-2 and implies a single or highly predominant direction of sediment transport,

from updrift to downdrift, and assumes there are no reversals of sediment transport.

Concurrently, the ebb and flood currents exchange sediment between the ocean and the

bay and on ebb tide, sediment is deposited in the ebb shoal proper.

Dean and Walton (1973) compared the inlet to a nozzle where the ebb jet is a turbulent

jet issuing from the nozzle with the inlet throat acting as the nozzle end. The unique

shape of the ebb shoal is formed as ebb tidal currents carry sediment in a hydraulic jet out








of the inlet, past the throat, and into the ocean. The ebb jet (flow feature) also marks the

tidal channel (land feature), as it is the channel through which the ebb tide flows. The

ebb flow scours the channel and entrains sediments. Sediments carried in the ebb jet

settle out of the jet on the ocean side of the inlet as the water velocity in the jet decreases.

Acting to balance the ebb jet effect is flood flow where water converges toward the inlet

and back bay and is analogous to a radially symmetric concentric sink flow. Sediment

entrained in the flood flow waters that deposit on the bayward side of the inlet form a

depositional feature called the flood shoal. Sediment initially deposits into the "near

field" zone of the flood shoal located within the front portion of the back bay. Sediment

in this zone may become entrained in the ebb jet and be moved out of the inlet.

Conversely, sediment may remain in the near field feature or wash into the further

reaches of the back bay depositing into the "far field" zone. In this way, the back bay of

the inlet acts as a sediment sink (FitzGerald 1988).

The sediment contained within the ebb shoal is transferred downdrift to the downdrift

bypassing bar which joins the downdrift shoreline at a location termed the downdrift

attachment point. The attachment point is also called the "tie-in" or weldmentt" area.

FitzGerald (1982) noted the existence of these attachment points in his work

documenting sediment bypassing along six tide dominated coasts around the world. He

found that regardless of the mechanism, whether stable inlet processes or for ebb tidal

delta breaching, bypassing produces a bar complex that migrates onshore and attaches to

the downdrift shoreline. The attachment point location is controlled by the bypassing bar

and is dependent on inlet size, orientation of the main ebb channel, and wave versus tide

dominance of the shoreline. FitzGerald found bars ranging in width from 40 to 300 m









with lengths from 300 to more than 1,500 m and many containing more than 500,000 m3

of sand (FitzGerald 1988). FitzGerald stated from observation that the size of the bar

complex is proportional to the size of the inlet and the rate of longshore sediment

transport. The process of bypassing through the mechanism of bar attachment provides

large amounts of sediment to the downdrift shoreline and may dictate the shape of the

barrier island. The ebb shoal complex also acts as a natural submerged offshore

breakwater, altering the shape of the adjacent barrier islands by reducing wave energy

arriving to the beach adjacent to the inlet.

The ebb shoal planform shape at East Pass, Florida is shown in Figure 2-7, as inferred

by the pattern of breaking waves. The ebb shoal in the broad sense is comprised of the

ebb shoal proper, the updrift and downdrift bypassing bars, and the attachment points.

The ebb shoal proper forms primarily in the stream of the ebb jet, whereas formation of

the bypassing bars is more heavily influenced by wave action and wave-induced currents.

The attachment point develops from material bypassed around the ebb shoal complex,

either arriving to or leaving from the shore. Attachment points are the beginning and

final components in the sediment pathway around a tidal inlet. The downdrift attachment

point is a central feature in delivering sediment to the downdrift shoreline. In the absence

of an attachment point, the downdrift shoreline would become erosive, as sediment,

which in the absence of the inlet would have been delivered to the shoreline, is lost to the

inlet.

As previously described, the geomorphology of tidal inlets is controlled by wave

energy and tidal current energy (which is directly related to tidal range). Davies (1964)

classified depositional shorelines according to tidal range where microtidal coasts have a








tidal range TR from 0 to 2 m, mesotidal coasts have a tidal range of 2 to 4 m, and

macrotidal coasts have a tidal range above 4 m. Hayes (1979) detailed the features of

these three classifications of coastlines. Microtidal coasts are typically barrier island

chains, wave dominated with small to absent ebb tidal shoals and large flood tidal shoals.

Microtidal coasts exhibit numerous washover features with terraces and washover fans.

In contrast, mesotidal coasts typically form drumstick type barrier islands with shoreline

offsets with large ebb tidal shoals and flood tidal shoals that are moderate in size or

completely absent. Along mesotidal coasts, tidal inlets are numerous, whereas washover

features are minor and washover fans are rare. The third coastal type, macrotidal coasts,

have characteristic intertidal flats and salt marshes rather than barrier island formations

and sand deposits are restricted to offshore linear sand shoals. Tidal currents typically

dominate macrotidal coasts.

Hayes (1979) refined the Davies (1964) classification within the medium wave energy

environment (H=0.6-1.50 m) through his study of the barrier island morphology of the

east coast of the United States. Hayes presented the following system of shoreline

classification based on tidal range: microtidal (TR=0-1 m), low-mesotidal (TR=1-2 m),

high-mesotidal (TR=2-3.5 m), low-macrotidal (TR=3.5-5 m) and macrotidal coasts

(TR=5 m) in graphical form. The trend lines for five coastal classifications radiate from

the graph origin in Figure 2-8 and represent trends in the relationships between mean

tidal range versus mean wave height. Representative data points are also included in the

graphic and fall into the five morphological classes.

Hayes (1994) discussed the morphology of the Georgia Bight barrier system, which

extends along the east coast of the United States from Cape Hatteras to Cape Canaveral.






18



Along this stretch of barrier islands exist inlets that vary in regime from wave dominated


to tide dominated. Transitional inlets also exist along this stretch of shoreline. Hayes


presented representative graphical depictions of morphological structures found along the


Georgia Bight barrier system (Figure 2-9).


i.nn t I


500-



a 400
U
0

,v 300


z
S200



100


S25 5s 80 1620 10 1 40 160 140 2b02i02i 260
MEAN WAVE HEIGHT-CM


Figure 2-8. Coastline Classification (after Hayes 1979)


Figure 2-9. Morphologic structures Georgia Bight barrier system (from Hayes 1994)


S/ / / / TIDE.DOMINATED
B/ / / / / //. IHIG
////// .//, A TIDE-DOMINATED
/////// ////'ILOW
////// ///////, MIXEDENERGY
//7////7/, ////*//,/, // (TIDE.DOMINATED)
/// //..,..
////'/ 0 MIXED ENERGY
,'-, -, / /// ":;, IWAVE.DOMINATED
I""' ". -' / '" '*J .*"/ "'" / ,'./", I BAvE DOMINAIED
7-s / / / / f .t / -. / ,, I
"/I /'//e //1 le'

I 7,/ / " // /'7/./ ...
1//, / ' / /af' / S CRD
//, /,////





Ol7


ellP


U i








Hubbard, Oertel, and Nummedal (1979) reviewed variability in the morphology of

tidal inlets along the southeast shorelines of the United States. They identified the wave

current interaction process as a main process causing morphologic variability at the tidal

inlets they considered in their study. They identified three types of inlets: tide dominated,

wave dominated, and transitional. The following table (from Hubbard, Oertel, and

Nummedal 1979) identifies the morphologic features found at each of these inlet types.


Table 2-1. Inlet morphologic variables (from Hubbard, Oertel, and Nummedal, 1979)
Wave Dominated Transitional Inlet Tide Dominated
Variables
Inlet Type Type Inlet Type
Inside the bay, as Seaward of the inlet
Principal shoal multi-lobate flood In the throat as long linear channel
locations
shoal margin bars
Si Small and close to Large; extends far
Ebb tidal shoal bVanriable s
the beach from shore
Flood tidal Large; lobate or Poorly developed or Ge y
Generally absent
shoal digitate absent
Variable; often one
main channel and one T s
Tends toward
Channel Poorly defined; or more secondary sta .
S,1stability. Depths
character often multiple channels. Unstable in a
greater than 10 m
shallower portions 5-
10 m depths
Width/ depth Moderate Very large Small
ratio
SW Fringing marsh; marsh Marsh filled and
Lagoon Wide, open filled channelized
Swash bars Poorly developed Variable Variable
Swash
Swash Poorly developed Variable Well-developed
platforms
Channel
anne Absent Variable Large
margin bars
Sand body Tabular Variable "Pod" like; confined
character to near channels
Variable; often in Primarily by ebb
Sand bypassing Bar bypassing packets; channel currents in the main
abandonment channel and landward
significant transport by waves








2.2 Predictive Relationships

2.2.1 Relationships with Channel Cross Section

Predictive relationships for tidal inlets allow engineers and scientists to track and

anticipate the morphodynamic changes that occur at tidal inlets. Predictive relationships

involving the channel cross section have been in existence at least since 1905, when

LeConte (1905) from his observations of a small number of harbors along the coast of

California, found the relationship

AE = CP Equation 2-2

where AE is the equilibrium cross sectional area of the inlet, P the tidal prism, and C1 an

empirical coefficient (Equation 2-2). The empirical coefficient was found to be greater at

entrances with restricted sediment transport. Based on a larger data set, O'Brien (1931)

found the cross sectional area-tidal prism relationship of

A = C2P"1 Equation 2-3

and in 1969 refined the 1931 relationship and presented a linear relationship of

A = C3P Equation 2-4

(O'Brien 1969) (Equation 2-4). Nayak (1971) also found a linear relationship between

channel cross sectional area and tidal prism as did Johnson (1972). Jarrett (1976)

extended the data for use in the determination of relationships involving cross sectional

area and tidal prism in both breadth and depth. Jarrett found that unjettied and single

jettied inlets on the Atlantic, Pacific, and Gulf coasts of the United States, since they

differ in tidal and wave characteristics, have varied tidal prism versus area relationships.

Jarrett also concurred with the relationship found by O'Brien (1931).








The recognition that a tidal inlet has a stable, equilibrium cross sectional area is well

established (LeConte 1905; O'Brien 1931, 1969; Escoffier 1940; Bruun and Gerritsen

1960; Byrne, Gammisch, and Thomas 1980; Riedel and Gourlay 1980; and Hume and

Herendorf 1990). Of these, LeConte (1905), Riedel and Gourlay (1980), and Hume and

Herdendorf (1990) have, in their treatments of equilibrium cross sectional area and inlet

stability, considered the magnitude of the longshore transport rate at tidal inlets given that

tidal inlet stability is a function of the longshore sediment transport rate.

Shigemura (1981) studied the entrance characteristics of 312 natural bays on the major

coasts of Japan and developed a predictive relationship between minimum channel width

and associated tidal prism of the form

W= C4P2 Equation 2-5

Kraus (1998) developed a process-based model for determining the equilibrium cross

sectional area of a tidal inlet. The model he developed balances the longshore sediment

transport rate and tidal prism in the development of an equilibrium channel cross section

and includes a time-dependant component. Equation 2-6 takes the form of previous

relationships,

AC = CsP"3 Equation 2-6.

Although a great number of relationships have been developed relating channel cross

sectional area to the tidal prism, Gibeaut and Davis (1993) determined another

relationship that involved the ebb shoal area and the throat cross sectional area. They

found the ratio of the ebb shoal area to the throat cross sectional area that indicates the

ebb shoal is in equilibrium with the coastal processes acting at the inlet. The ratio of the

ebb shoal area to the throat cross sectional area was in range of 1x103 to 3x103. Gibeaut








and Davis found that unstable, wave dominated inlets have shoal area to throat area ratios

higher than that of stable mixed energy inlets indicating that shoals of wave dominated

inlets are greater than they would be if they were at equilibrium. Tide dominated inlets

that have small shoals with respect to the inlet cross sectional area. The shoals are

described as being undersized with respect to equilibrium conditions.

FitzGerald and Nummedal (1983) in their 3-year study of morphologic changes at

Price Inlet, South Carolina found that due to differential time lags between the ebb and

flood flows the inlet is highly ebb dominated. Their findings indicate that inlet cross

sectional area responds rapidly to flow changes, with variations in throat area as great as

8.3% over one tidal cycle as variations corresponded to tidal range. Over longer time

frames, changes of the morphology of the channel correlated to changes in the size of the

ebb shoal. Over the study period, growth of channel margin linear bars reduced wave

energy to the inlet's adjacent shoreline and to the landward ebb shoal platform thus

causing a reduction in sediment transport directed to the inlet. This reduction in sediment

transport resulted in an increase in the channel cross sectional area.

The morphodynamics of the inlet throat are determined by the inlet tidal prism and

wave energy, sediment supply, regional stratigraphy, and back barrier sedimentation

along with other controlling parameters. FitzGerald and FitzGerald (1977) documented

the factors that influence the geometry of tidal inlet throats and indicated the size and

depth of the throat at depositional tidal inlets is dependant upon the relative strength of

the wave regime and tidal energy. They provided the example of the shallower inlet

depths at inlets without extremely large bays found in North Carolina, Florida and the

Gulf Coast where wave processes predominate as compared to more tidal dominated








coasts like Georgia where the inlets are relatively deep. Additionally, FitzGerald and

FitzGerald (1977) reported that channel symmetry at the throat section of mesotidal inlets

is predominately controlled by the meandering of the channel thalweg, the inlet shoreline

configuration, and the dominant longshore transport direction. Their research at Price

Inlet in South Carolina indicated a relationship between both flow condition changes and

changes within the ebb shoal and erosion and deposition of the inlet throat.

FitzGerald (1996) explained that longer term changes of the inlet cross sectional area

and tidal prism are controlled by factors that include inlet migration, back bay

sedimentation, alterations in the size and shape of the back bay, and changes within the

ebb shoal. FitzGerald presented case studies to show that the relationship between the

inlet throat cross sectional area and tidal prism is adaptive and determined by a number of

processes that influence the inlet.

2.2.2 Inlet Stability

At all inlets, waves, wind, and currents produce change in inlet morphology. At

unstructured inlets in particular, response due to coastal forcing may maintain the inlet

open with little morphologic change over time, whereas other inlets may experience

instabilities and close or open in response to these forces. The determination of the

stability of an inlet is essential in the maintenance of the navigability at its entrance and

to preserve the inlet as a sand bypassing entity. Considering the early works of inlet

stability of O'Brien (1931) and others, Escoffier (1940) developed a method for

investigating the stability of tidal inlets that implements a stability diagram (Figure 2-10)

in which the inlet maximum velocity, Vm, and the inlet cross sectional area, Ac, are

plotted on the x-axis. Escoffier introduced the idea that the inlets maximum velocity

controls its cross sectional area and represented the maximum velocity curve at some








critical inlet cross sectional area graphically. As the figure indicates, at inlets with a

cross sectional area larger than the critical cross sectional area the velocity will decrease,

deposition of sediment inside the channel entrance will occur and cause the cross

sectional area to decrease until the inlet reaches a stable equilibrium condition. At inlets

where the cross sectional area is smaller than the critical cross sectional area, the velocity

inside the channel will increase, and sediment will be scoured from the channel until the

inlet reaches the stable equilibrium condition this describes a deviation from the point a

in Figure 2-10. Alternately, a decrease in the area at point b will cause the flow velocity

to increase causing scour and a movement toward point b on the curve.




Vm


Maximum Velocity Curve O'Brien's (1966) Equilibrium Velocity



b









0 a1 Area aE
Unstable Stable
Equilibrium Equilibrium

Figure 2-10. Escoffier 1940 stability diagram

2.2.3 Relationships with Area and Volume

Dean and Walton (1973) developed the "no inlet" contour calculation method for the

determination of volumes of sediment contained within an inlets ebb shoal. The method








involves calculating accumulated material above the offshore bathymetry in the absence

of an inlet. The offshore bathymetry in the absence of an inlet is assumed and is defined

along grid points and compared to depths at corresponding locations on the identified

existing with inlet bathymetry. This depth comparison method when multiplied by the

area of the ebb shoal, results in the quantification of the volume of sediment contained

within the ebb shoal. Dean and Walton (1973) presented ebb shoal volume quantities for

23 inlets in Florida with volumes ranging from one million cubic yards to two hundred

million cubic yards with a total volume of sediment contained within the ebb shoals in

the 23 inlets totaling approximately four hundred million cubic yards. Dean and Walton

(1973) then used the information from the 23 Florida inlets to estimate the quantity of

material within the ebb shoals of all the inlets in Florida and hypothesized that if the

sediment contained within Florida's inlet ebb shoals were to be dredged and used as

material for beach nourishment projects the ebb shoals contain enough volume to

forestall erosion for 76 years along 1,600 km of Florida shoreline assuming an average

erosion rate. The volume of sediment contained within the ebb shoal at some inlets along

mixed energy coasts can be of the same magnitude of the volume contained within the

adjacent barrier islands FitzGerald (1988). This means that relatively minor changes in

the ebb shoal sediment volume may have far reaching influence on the sediment budget

of the inlet and shoreline system as a whole.

Walton and Adams (1976) correlated ebb shoal volume, V with tidal prism, P and

found in areas with high wave activity there is a limiting relationship to the amount of

sand stored in the ebb shoal as a function of tidal prism. The correlation is less clear at

inlets with lower wave activity. Walton and Adams (1976) found that more sand is








stored in the ebb shoals of inlets on low energy coasts than in the ebb shoal of inlets on

high energy coasts. Walton and Adams (1976) suggested that an equilibrium ebb shoal

volume in a low energy environment may be a function of other forces than simply tidal

prism. From their investigation of 44 inlets they found the Equation 2-7

V = 10.7x10-5 pl.23 Equation 2-7

indicating increasing ebb shoal storage capacity with increasing tidal prism (where V has

units of cubic meters and P has units of cubic feet). The inlets were further segregated

into three categories based on their wave environment and the relationships that were

found as follows, highly exposed coasts:

V = 8.7x10-5 pl.23 Equation 2-8

moderately exposed coasts:

V = 10.5x10-5 P.23 Equation 2-9

and mildly exposed coasts:

V = 13.8x105 pl.23 Equation 2-10

Walton and Adams also found relationships between inlet cross sectional area and ebb

shoal volume for the three wave energy cases

highly exposed coasts:

V = 33.1A''28 Equation 2-11

moderately exposed coasts:

V = 40.7A.'28 Equation 2-12

and mildly exposed coasts:

V = 45.7A.128 Equation 2-13








Nummedal et al. (1977) investigated tidal inlets along the southeast coast of the

United States and presented ebb shoal area values for 27 inlets along the coastline. They

concluded from their investigations that wave dominated inlets have small ebb shoals

oriented close to the shore and with wide throats, multiple sand bodies and large flood

shoals. Tide dominated inlets have large ebb shoals that extend further offshore, have

well-defined and deep main channels and throats, and well-formed ebb shoals.

Marino and Mehta (1987) also developed relations for ebb shoal volume, Vwhich all

took the form:

V = CP" Equation 2-14

Gibeaut and Davis (1993) determined a relationship between the ebb shoal area AE and

the corresponding tidal prism P of the form

AE = C7P"5 Equation 2-15

Oertel (1988) investigated tidal prism-ebb shoal volume relationships at inlets where

the prevailing forces of the inlet currents are greater than the forces of the longshore

currents. Regression curves correlating delta volumes and tidal prism were similar to

those developed by Walton and Adams (1976). The relationship between ebb shoal

volume and mean tidal prism was

V = 0.096P'"3 Equation 2-16

and

V = 0.083P'.23 Equation 2-17

for the relationship between ebb shoal volume (in cubic yards) and spring tidal prism (in

cubic feet). They found a temporary disequilibrium condition when spring tidal prisms at

Georgia inlets are larger than their mean tidal prisms. It is this disequilbrium that causes








drainage and increased current speed from the back bay, scouring the inlet channel. With

this logic, it follows that the ebb shoals may increase in volume before reaching an

equilibrium volume consistent with the spring tidal prism. Marino and Mehta (1987)

indicated that as inlets deepen, relative to their width, the volume of the inlet ebb shoal

increases.

In their research into episodic shoal bypassing and determination of shoal volumes

Gaudiano and Kana (2001) collected data from nine South Carolina tidal inlets and found

relationships between the volume of sand contained within the ebb shoal and contained

within the individual bypassing shoals and developed a conceptual model to analyze the

data. They found the tidal prism is related to both the mean shoal bypassing event time

intervals and the mean bypassing shoal volumes. It was noted that larger inlets

underwent episodic shoal bypassing events less frequently than smaller inlets. The

relationship between the average shoal bypassing event I, (in years) and the tidal prism

(in cubic meters) is described by Equation 2-18

I = 0.046P + 4.56 Equation 2-18

Equation 2-19 describes the relationship between the average shoal volume S (in

thousands of cubic meters) and tidal prism (in millions of cubic meters)

S = 6.42P+113.4 Equation 2-19

Mehta, Dombrowski, and Devine (1996) corroborated the idea that one factor that

controls the growth of the ebb shoal in a micro tidal environment is the action of episodic

waves and under the action of episodic waves it may not be possible to assume the ebb

shoal tends to equilibrium. Mehta, Dombrowski, and Devine utilized the ratio of wave

power to tidal prism power, fi, from O'Brien (1971) and flood delta growth rate quantities








for four artificially opened micro tidal inlets in Florida (Equation 2-20). In Equation 2-20

Po is the shore-normal wave power per unit distance in the alongshore direction, and w, is

the entrance width with the total wave power equal to Po wo.


S= wT Equation 2-20
2apgP

Mehta, Dombrowski, and Devine (1996) concluded that when f is used as a measure of

long term wave and tidal conditions a quasi equilibrium ebb shoal volume may be

obtained. Additionally, it was found that the ebb shoal volume decreases under the

condition of increasing p.

Hayter et al., (1988) developed empirical relationships through laboratory tests

between shoal dimensions (length, width, and volume) and inlet geometry, tidal prism,

wave energy, and water depth. The equation for ebb shoal volume versus tidal prism for

the model condition that included waves was found to be

V P
S= 0.00048- -1340 Equation 2-21
G G

where V is the ebb shoal volume, and G is a parameter that includes the sediment size,

specific gravity, and average inlet area,

G = BhD, Equation 2-22
y

In the equation for G, B is the inlet width, h is the mean water depth in the inlet, D50 is the

median grain size of the sand, and ys and y are the specific weights of the sand and the

water respectively. The equation for ebb shoal volume versus tidal prism for the model

condition that did not include the effects of waves was found to be

V P
= 0.00069- 1870 Equation 2-23
G G








The developed two-dimensional equations between tidal prism and shoal volume

correlated positively to data from twelve Florida inlets.

Kraus (2000a) developed a mathematical model for calculating the change in the

volume and sand bypassing rate of ebb shoals. His so-called reservoir model treats

specific morphologic features of the ebb shoals as reservoirs, which can retain sediment

up to an equilibrium volume, at which time they bypass all of the sediment entering the

feature. The model is dependent upon longshore sediment transport rate and specified

equilibrium volumes of the ebb shoal, bypassing bar, and attachment point. The

analytical model explicitly predicts a delay in sand bypassing seen at tidal inlets. Kraus'

mathematical model is solved numerically for realistic situations. Results from the

numerical model, with input parameters from Ocean City Inlet, Maryland, were

compared to actual site data of the ebb shoal volume growth over time, bypassing bar

volume growth over time and attachment point volume growth over time. Kraus (2001)

extended the original reservoir model to include a greater number of sediment pathways

within the inlet system. These sediment pathways are introduced into the reservoir model

in the form of coupling coefficients that are required to be specified in the input and when

added together must be equal to unity for a given path.

2.3 Ebb Shoals and Tidal Channels

2.3.1 Asymmetries in Ebb Shoals

Asymmetries in the morphology of the ebb shoal and channel are caused by both static

and dynamic factors. Static factors identified in this study are the locations and

configurations of jetties, offshore and nearshore bathymetry, size and shape of the back

bay, and constraints as imposed by the local geologic structure such as hard bottom.

Dynamic factors identified are the magnitude and direction of net longshore sediment








transport, tidal prism, relict ebb shoal, offshore extent of the ebb jet, riverine sediment

supply, flood shoal evolution, dredging of the channel, and wave refraction and

diffraction over the offshore bathymetry and the ebb shoal. A symmetrically shaped ebb

shoal would tend to form if the left- and right-directed longshore sediment transport rates

were equal, the navigational channel dredged straight offshore, and the main static factors

of back bay configuration, jetties, shelf bathymetry, and geologic structure were

symmetric across the centerline of the inlet.

The distinction of the static and dynamic factors that control the development of

geomorphic features at tidal inlets is the focus of the present research. Table 2-2 lists the

primary factors identified in this study that control asymmetries in tidal inlet morphology.





Table 2-2. Forcing factors controlling tidal inlet ebb shoal morphology
Static Factors Dynamic Factors
location and Net longshore sediment
Jetty location and
Jt. transport magnitude Qnet and
configuration direction
direction
Offshore and Gross longshore sediment
nearshore bathymetry transport rate Qgross
Back bay size and Tidal Prism
Tidal Prism
shape
Local geologic Flood shoal evolution
structure
Channel dredging
Refraction and diffraction
Riverine sediment supply
Relic ebb shoal


Among the dynamic factors acting at ebb shoals at tidal inlets is the dissipation of

wave energy through the mechanism of the ebb shoal assuming the role of a submerged








breakwater for the landward inlet shorelines (FitzGerald 1988). The ebb shoal causes the

incoming waves to break further offshore thus diminishing the incident energy. Hales

and Herbich (1972) noted that the interaction of the ebb current with the incident waves

diminishes the wave height prior to reaching land. These properties of the ebb shoal and

tidal inlet provide wave sheltering to the adjacent shorelines thus reducing erosion to

these shorelines, but At mixed energy coasts, where the ebb tidal shoals are well

developed, wave sheltering is greater. The interaction of waves and currents occurring at

tidal inlets adjusts the development of the ebb shoal and tidal channel as this interaction

determines patterns of sedimentation at the tidal inlet. The accretion of sediment within

the ebb shoal is also governed by the components of the ebb shoal itself (Oertel 1972).

The location of the attachment point is determined by the width of the inlet, wave

versus tide dominance, and channel orientation. The area of the ebb shoal complex is

determined by the size of the inlet. It has been seen that larger inlets form larger bar

complexes with attachment points forming at greater distances from the inlet opening.

Bruun and Gerritsen (1958) and Hubbard, Barwis, and Nummedal (1977) indicated that,

at wave dominated inlets, sand is continuously transferred around the inlet by wave

action, whereas at tide dominated inlets sand is more typically bypassed in packets as a

number of bar complexes weld to the beach over time. The distance from the inlet

centerline alongshore at which the attachment point will form is related to the orientation

of the main ebb channel. The straighter the ebb channel, the closer the attachment point

will form to the inlet centerline. At an inlet with a deflected channel, the distance to the

attachment point will be greater along the shoreline in the direction of the channel

deflection. This increase in distance is due to formation of the ebb shoal proper at the








location along the ebb jet where sediment is no longer entrained. If there is an extension

and/or a deflection of the ebb channel, the ebb shoal proper will conform to this

translation and deposit at the terminus of the deflected ebb jet. Additionally, the

magnitude and direction of the net longshore sediment transport rate influences the size

of the bar complexes with larger bar complexes forming in the presence of higher rates of

longshore sediment transport (FitzGerald 1982).

Oertel (1976) endeavored to understand in what way changes in shoal morphology

brought about alterations in shoreline development through the analysis of a number of

tidal inlets along the Georgia coastline. He classified ebb shoals into four types based on

relative flood, ebb, and longshore current forcing. According to his classification system,

a Type A shoal exhibits equal forcing of currents and arcuate shoals result. A Type B

shoal forms as the Type A shoal matures, the shoal becomes elongated and the updrift

shoreline develops a spit, which restricts the tidal channel as the downstream shoreline

erodes and flow is restricted due to increased sedimentation.

As the Type B shoal matures, the spit becomes more elongated, flow is restricted to

one or two main channels at the distal end of the spit, erosion on the downdrift shoreline

at the distal end of the spit becomes more pronounced. At the later stages of the

evolution of the Type B ebb shoal, spill-over channels form within the spit, the shoreline

at the distal end of the spit becomes stable, and the Type B ebb shoal transitions into a

Type A shoal.

Oertel classified Type C ebb shoals as identical to Type B with the exception of

reversed longshore current dominance. Type C ebb shoals are similar to Type B shoals in








that the process occurring at them and the processes responsible for their formation and

maturity are identical but mirrored.

Type D shoals exist in situations where inlet currents are the predominant influence.

The ebb tidal currents produce a ebb shoal complex extending several kilometers offshore

with a shore-normal orientation. Erosion occurs along channel banks due to strong

currents and, as a result, the throat cross section increases in size. As shoals attach to the

shore, they function as shields and confine flow to the main ebb channel. The Type D

ebb shoal complex is attached and detached from the shoreline at various stages of its

developmental process.

In youth, the attached Type D ebb shoal is a broad, short, triangular feature with

elevated swash bars and swash platforms. As it matures, the attached Type D ebb shoal

elongates, and the portions of the shoal and shoreline nearest to the channel begin to

erode. During the mature stage of the process, several narrow spill-over channels

develop. As the ebb shoal enters old age, the erosion along the portions of the shoal and

shoreline nearest to the channel intensifies. When the narrow, segmented shoal feature is

separated from the shoreline, the ebb shoal complex has transitioned into a Type D

separated feature. It is in this beginning stage that the beach located at the proximal edge

of the shoal begins to accrete.

As the Type D ebb shoal complex matures, there is extensive development of the shoal

and the beach adjacent to the shoals. Spill over channels develop, and the ebb shoal

complex eventually forms into a broad, short, triangular sand body that is separated from

the barrier island by broad, shallow spillover channels. The beach adjacent to the shoal

continues to accrete sediment, as do the spillover channels. It is at this stage when the








Type D separated ebb shoal matures into the Type D attached shoal complex. The ebb

shoal complexes at tidal inlets studied by Oertel (1976) exhibit cycles of morphologic

development and asymmetries that recycle themselves over time through the interaction

of tidal currents, wave action, channelization, and shielding.

In their work of the classification of ebb tidal deltas of nine inlets along the west-

central coast of Florida, Gibeaut and Davis (1993) indicated that differences seen in ebb

tidal shoal planforms may be defined as (1) the amount of asymmetry of the planforms

and (2) the ebb shoal's seaward protrusion relative to alongshore length. Additionally,

they found shoreline offset amount and positioning to be an indicator of ebb shoal

planform asymmetry. Through statistical analysis of 71 ebb tidal delta planforms,

Gibeaut and Davis designated end points of a continuum of inlet planform variation with

wave-tidal current interaction classes (tide dominated, mixed energy, mixed energy with

large shoreline offset and large asymmetry, and wave dominated) forming a spectrum of

which the planform end points characterize all ebb shoal planforms of that wave-current

class. Their research progressed into the correlation of ebb shoal size and morphology,

which is discussed further in Section 2.3. Gibeaut and Davis found four primary factors

that cause variation in time and space of tidal inlet formation. These are time variation of

wave energy, time variation of tidal energy (tidal prism), space variation of tidal energy

(tidal prism), and the evolution of ebb tidal shoals and adjacent shorelines.

The configuration of the barrier islands adjacent to a tidal inlet is another factor that

dictates the asymmetry found within that inlets ebb shoal complex. Galvin (1970)

presented four groups of shoreline configurations. These configurations are presented in

Figure 2-11 (a, b, c, and d) and are (1) overlapping (Figure 2-11a), where the updrift








shoreline extends seaward and downdrift overlapping the inlet entrance; (2) significant

updrift offset (Figure 2-11b), where the updrift shoreline is seaward of the downdrift

shoreline and this offset exceeds the minimum width of the inlet; (3) significant

downdrift offset (Figure 2-11c), where the updrift shoreline is seaward of the updrift

shoreline and this offset exceeds the minimum width of the inlet; and (4) negligible

offset, where the offset, if any, is less than the minimum width of the inlet (Figure 2-1 d).

A shoreline offset will influence the width of the inlet throat and may shift the ebb

channel, creating an asymmetry in the tidal channel and in the ebb shoal complex.




Ocean



Bay .



Figure 2-11 a. Overlapping shoreline configuration (Galvin, 1970) Blind Pass, FL





Q Ocean
net



Bay


Figure 2-1 lb. Updrift offset shoreline configuration (Galvin, 1970) St. Lucie Pass, FL










Ocean -

Qnet Y .


Bay


Figure 2-11 c. Downdrift offset shoreline configuration (Galvin, 1970) Alsea Inlet, OR




Ocean


Bay




Figure 2-11d. Negligible offset shoreline configuration (Galvin, 1970) Stump Pass, FL

The processes influencing the development and adjustment of the ebb shoal complex

at natural inlets are similar to those occurring at jettied inlets. The process of ebb shoal

development is altered due to the implementation of structures as the jetties themselves

influence the morphology of the ebb shoal complex. In a conceptual model of ebb delta

and shoreline response to inlet stabilization, Pope (1991) analyzed morphologic

alterations in the ebb shoal structure of four stabilized tidal inlet systems located along

the east coast of Florida. The inlets were similar morphologically before stabilization,

each having a deep main channel, ebb flow distributary channels, marginal flood

channels, a main ebb delta lobe, swash bars, and an ebb delta lobe, additionally their

morphologies reacted similarly to their stabilization. Following stabilization, the main

channel increased in efficiency and became the primary channel at all of the inlets








examined. Sediment became trapped along the shoreline at the updrift jetty and jetty

adjacent fillets emerged. As waves scoured the previously existing ebb shoal, the

sediment contained within it was transported toward the shoreline. In the development of

the structured inlet, the platform of the new ebb shoal increased in steepness and

sediment from this feature began to be transported downdrift. The modification of an

inlet creates alterations in the morphology of the ebb shoal complex. Modifications may

induce far-reaching changes to the aerial extent of the ebb shoal, and produce changes to

the sediment budget along the adjacent shoreline. This modification to the sediment

budget may be far reaching and, as Pope (1991) points out, the entire shoreline must be

analyzed as a unit for complete understanding of processes prior to the stabilization of an

inlet within a sedimentary system.

Additional studies of the response of ebb shoal complexes to the stabilization of the

tidal inlet have been performed. Stauble (1998) evaluated the evolution of inlet shoals at

two inlets with similar morphologic features along the east coast of the United States,

Barnegat Inlet, New Jersey and Ponce de Leon Inlet, Florida to examine the evolution of

ebb shoal complex morphology at these inlets after their stabilization by jetty

implementation. The inlets examined exhibited a symmetrical ebb shoal complex

planform prior to the implementation of jetties and an asymmetrical ebb shoal planform

oriented in the downdrift direction subsequent to jetty construction. The ebb shoals at

both inlets grew in a seaward direction after the construction of the jetties. This seaward

movement of the ebb shoal following jetty construction has been seen at other inlets as

well.








A case study at Ocean City Inlet, Maryland (Stauble and Cialone 1996), confirmed

that after the 1934 placement of a north jetty (updrift) a fillet of sediment along the

updrift jetty developed as described by Dean and Perlin (1977). The case of Ocean City

inlet is a unique one since the United States Army Corps of Engineers stabilized this inlet

within one year of the inlets opening. Prior to the opening of the inlet, the offshore

contours in the vicinity were straight and parallel. Bathymetry data available from 1937

indicated a symmetrical ebb shoal extending 540 m seaward of the inlet, and bathymetry

data from 1976 shows the presence of an attachment point to the downdrift shoreline

approximately 600 m south of the inlet. Stauble and Cialone indicated the shoal has

reached an offshore distance of 1,300 m. Since the opening of the inlet, a symmetrical,

crescentic ebb shoal has developed in a southerly direction, the direction of the net

sediment transport.

Tomlinson (1991) followed the development of the ebb shoal complex at the Tweed

River Entrance in northern New South Wales, Australia before and after the extension of

the channel training walls i.e., construction of jetties. With the implementation of the

jetties, the updrift beach began to accrete sediment and form a fillet along the jetty. In

conjunction with the construction of the jetties, the ebb shoal underwent a period of

extensive dredging. Consequently, the inlet became strongly flood dominated, causing

sediment to form a large flood shoal feature. The downdrift shoreline did not begin to

erode immediately due to the existence of relict ebb shoals until such time as these

features were depleted by a series of storms. Tomlinson indicated that erosion still

occurs on the downdrift shoreline as sediment bypassing from the updrift shoreline has

not resumed. The morphology of the ebb shoal complex has been altered due to








alterations in the ebb jet hydrodynamics. The pre-jetty ebb shoal was skewed toward the

direction of net sediment transport while the post-jetty ebb shoal aligned itself at the

terminus of the ebb jet and is symmetrical about the channel centerline. The new ebb

shoal became orientated further offshore due to the flow constriction by the jetties. The

volume of sediment contained within the ebb shoal increased following the construction

of the jetties. As the wave processes increase in dominance and the ebb flow subsides,

the ebb jet becomes deflected in the direction of net sediment transport. It has been seen

that at the Tweed Entrance this process has begun to occur, and the morphology of the

ebb delta has once again resumed the characteristics of its pre-jetty state.

Hansen and Knowles (1988) described the response of ebb shoal complexes following

jetty construction at three South Carolina inlets (Murrells Inlet, Little River Inlet, and

Charleston Harbor). Following jetty construction, the natural main ebb channels, swash

platforms, and marginal flood channels were abandoned. Lack of substantial tidal flow

caused a wave dominated regime adjacent to the jetties creating landward sediment

transport of the relict ebb shoals. At Murrells Inlet and Little River Inlet, the construction

of the navigation channel isolated larger ebb shoal areas southwest of the channels, which

resulted in landward bar migration on the southwest sides. Charleston Harbor is a larger

inlet with a greater tidal prism, depth, and ebb shoal size. Here, although similar

landward migration of the swash platform and terminal lobe occurred, there was no

reported welding of material to the downdrift shoreline due to the greater water depths

throughout the ebb shoal complex. It may be that at Charleston Harbor, the greater depth

and overall size of the inlet and ebb shoal complex, in addition to the length of the jetties,








correlates to a slower migration rate of the sediment contained within these features than

has been seen at Murrells and Little River Inlet.

2.3.2 Asymmetries in Tidal Channels

Morphologic features and an inlet channel centerline that deviate from the ideal ebb

shoal (Figure 2-2) are categorized as asymmetrical. It is necessary to examine the factors

that lead to ebb shoal asymmetry to quantify asymmetries in ebb tidal shoals and to

advance understanding of the processes at tidal inlets as a system as a whole and

specifically to advance understanding in the realm of the design and modification of

jetties, breakwaters, and inlet training structures.

In their treatment of offset coastal inlets, Hayes, Goldsmith, and Hobbs (1970), found

the most common offset in their area of study (the New England and northern Gulf of

Alaska coasts) was the downdrift offset (Figure 2-11i), for which the downdrift side of

the inlet protrudes further seaward than the updrift side. Associated with the formation of

this type of offset is wave refraction around the ebb shoal in that a local littoral drift

reversal is created by this refraction allowing sediment to accumulate on the downdrift

side of the inlet "sheltering." Additionally, with this type of offset, a segregation of tidal

current flow within the inlet has been noted with a more channelized ebb flow over the

flood flow. Typically, in these cases there exists a main channel oriented perpendicular

to the shoreline with one dominant ebb channel with the flood flow issuing into the bay in

the manner of sheet flow or in a number of smaller flood channels that flank the main ebb

channel.

Hayes, Goldsmith, and Hobbs (1970) surmise this segregation of flow is due to a time-

velocity asymmetry of the tidal currents. In these cases, the maximum flood velocities

occur late in the flood tidal cycle between mid tide and high tide, and the maximum ebb








velocities occur close to low tide. Hayes, Goldsmith, and Hobbs (1970) found the most

common offset in their study area was the downdrift offset; however, offsets vary from

inlet to inlet and through their studies of the historical offsets at Hampton Inlet, New

Hampshire and Merrimack River Entrance, New Hampshire they documented shoreline

offsets that vary over time at the same inlet. The offset of a shoreline at the entrance of

an inlet causes deflection of the channel and leads to asymmetries in tidal channels and

consequently within the ebb shoals.

FitzGerald (1984) studied the interactions of the ebb shoal and the shoreline at Price

Inlet in South Carolina and found the shoreline adjacent to the inlet changes in

configuration cyclically. This periodic shoreline alignment change is controlled by the

orientation of the main ebb channel and the resulting ebb shoal morphology. Fitzgerald

found that at the time this mixed energy tidal inlet had a downdrift offset configuration

the ebb shoal was skewed along the updrift shoreline, causing accretion of the updrift

shoreline and erosion of the downdrift shoreline, eventually producing an updrift channel

offset. If the channel was deflected in the updrift direction, the ebb shoal became

repositioned along the downdrift shoreline, causing the downdrift shoreline to accrete as

the updrift shoreline eroded. These shoreline changes, while dramatic, (adjacent

shorelines have gained and lost hundreds of meters of beach) can be identified as cyclic

trends of erosion and deposition. Although Price Inlet has remained relatively fixed over

the past century, the main ebb channel has shifted as much as 90 deg with respect to the

gross shoreline trend of the region, and the ebb shoal configuration has likewise

fluctuated dramatically. The controlling factors of this type of cyclic change have been

identified to include the meandering pattern of the channel thalweg and the wave climate.








Fitzgerald and Nummedal (1983) found that while the channel of Price Creek is straight,

the channel thalweg at Price Inlet meanders relative to the inlet mouth. This meandering

governs the direction of ebb tidal flow and the orientation of the main ebb channel

FitzGerald (1984). The model for long-term changes presented by FitzGerald (1984) is

applicable to inlets with similar meandering channel thalweg patterns and whose main

channel orientation is primarily a function of tidal flow direction. FitzGerald (1984)

indicated there are a number of inlets that historically have stable offset configurations,

indicating that at some tidal inlets, factors other than the direction of ebb tidal flow

control the main ebb channel orientation. These factors may include the net rate of

sediment transport or the erodibility of underlying sediments.

FitzGerald and FitzGerald (1977), in their treatment of factors that influence tidal inlet

throat geometry, reiterated that wave energy versus tidal energy is one of the primary

forcing factors to the geometry of tidal inlets. Their study of central South Carolina inlets

determined that the meandering of the channel thalweg, the shoreline configuration, and

the dominant longshore transport direction along with the composition of underlying

sediments are the main factors in determining the symmetry of the inlet throat. The

inlets in their study have all narrowed over the past 100 years due to the development of

updrift and downdrift spits, and as a result the inlets have deepened. Cross sections at

Price Inlet, South Carolina from a three-year period indicate that the inlet responds

quickly to changes in flow and more slowly to changes in the ebb shoal. However,

changes within the ebb shoal significantly influence the sedimentation that occurs at the

inlet throat.








The process of sediment bypassing and the method by which sediment is bypassed

controls the symmetry of a tidal channel. Often, an asymmetrical orientation of the tidal

channel is caused by the preferential addition of sediment to one side of the ebb shoal.

Additionally, tidal creeks in the back bay approaching the inlet at an angle may cause an

asymmetrical tidal channel (FitzGerald 1982).

FitzGerald, Kraus, and Hands (2000) presented models of natural mechanisms of

sediment bypassing at tidal inlets. In certain cases at tidal inlets, natural sediment

bypassing can lead to asymmetries in the inlet's tidal channel. Model 1 through Model 6

relate sediment bypassing at natural inlets while Model 7 through Model 9 present

sediment bypassing at dual jettied inlets.

Model 1 is a tidal inlet with a symmetric ebb shoal complex where sediment is

bypassed through the formation, landward migration, and attachment of large bar

complexes to the downdrift shoreline. In Model 1 sediment bypassing occurs in the case

where the main ebb channel position is non-migrating. Often, however, sediment

bypassing at tidal inlets occurs through the migration of ebb shoals and the consequent

formation of asymmetrical tidal channels.

Model 2 describes ebb tidal shoal breaching occurring at tidal inlets with stable throat

positions but downdrift migrating main ebb channels. At these inlets, a dominant

direction of sediment transport causes an accumulation of sediment on the updrift side of

the ebb shoal. This in turn causes the main ebb channel to deflect downdrift, eroding the

downdrift shoreline as the main ebb channel encroaches upon the shoreline. If the main

ebb channel deflects to the point that it becomes hydraulically inefficient, a breach in the

ebb shoal may open thus creating a more hydraulically efficient channel. As this








happens, the relict channel begins to fill with sediment and the relict ebb shoal begins to

function as a bypassing bar and migrates onshore.

Model 3, called the inlet migration spit breaching model, follows the build up of

sediment on the updrift side of an inlet that forms a spit and begins to constrict the inlet

throat. To maintain stability, the constriction causes the velocity of the water in the

channel to increase, thus scouring the channel to a greater depth as indicated by Escoffier

(1940). Additionally, the opposite side of the inlet (the downdrift side) begins to erode,

and the inlet throat migrates accordingly. This process of migration produces beach

ridges and extends the inlet channel thereby reducing the tidal range within the bay. This

process will produce a less and less hydraulically efficient inlet and often it is the case

that a major storm event will cut a new, more hydraulically efficient channel along a

narrow portion of the barrier island. The old inlet entrance will close as the tidal prism is

diverted to the newly opened inlet.

Model 4 is similar to the ebb tidal shoal-breaching model but is limited to the seaward

end of the main ebb channel. Here, the inner portion of the main channel does not move

in position, as the outer channel is deflected downdrift due to the accumulation of

sediment on the seaward updrift side of the swash platform. The channel bends downdrift

and becomes hydraulically inefficient at which point a new more efficient channel may

breach during periods of high channel flow volume.

In Model 5, large quantities of sediment are bypassed downdrift as a spit forms,

extending the channel. Eventually, a new channel breaches through the spit platform

creating a more hydraulically efficient channel orientation (Model 6). For wave

dominated inlets with typically arcuate ebb shoal complexes, sediment transport occurs








continuously along the distal portions of the ebb delta. This bypassing has been

mathematically modeled by Kraus (2000a) and is similar to the conceptual model for

sediment bypassing at tidal inlets presented by Bruun and Gerritsen (1957, 1958).

Although all of the previously mentioned models are applicable to non-structured inlets,

Models 7 through 9 present sediment bypassing mechanisms that occur at jettied inlets.

Model 7 is termed the jetty-weir model for inlet bypassing because the bypassing of

sediment falling within this category occurs at jettied inlets containing one or two weirs

with no settling basin. Sand transported over the weir and into the inlet is entrained

within the ebb jet and is moved offshore, ideally being carried downdrift by longshore

currents, thus bypassing the inlet as the sediment deposits on the downdrift shoreline.

Model 8 is applicable at jettied inlets as sediment forms a fillet on the shoreward side

of the updrift jetty, and either deposits in the tidal channel or is moved offshore by rip

currents.

In Model 9, sediment bypassing occurs through outer channel shifting at jettied inlets.

In this case, the outer channel is directed downdrift due to sedimentation of the updrift

side of the main ebb channel. Eventually, a new pathway is cut in the ebb shoal and a

more hydraulically efficient channel is formed. This cutting of a new tidal channel

through the ebb shoal leaves the downdrift portion of the relict ebb shoal to attach to the

downdrift shoal under onshore-directed wave action. Figure 2-12 illustrates these models

of sediment bypassing and channel migration noted by FitzGerald, Kraus, and Hands

(2000).















MODEL 1
STABLE INLET PROCESSES


Main Ebb-Chanl ;____________
---a*n I-bt)'C *: '.-- - ,--- D i ...nt
Longshore










J In--cipnt Sp,

B. Gn**p. Glee

Swaeh Bar Fonnaio "'
and landward Migration

Ebb
Eventrany i n 7





a; . '- *.a.. . nor.'





MODEL 4
OUTER CHANNEL SHIFTING


TIME I




." Channel Margin
.Unear Bars

Outem Sd Sut ddal SholM
Channel "..po hane
Migtation

TIME 2 Ebb









.* . ... :rcharinnl


TIME 3
I .Channel h _.


heqSr Sal Unrlar.D





S- Sand
Depoition


MODEL 2
EBB-TIDAL DELTA BREACHING



-oanant
-~- LoDngshore
/ Transport


Channel Cnne
Migration

Ebb f






Landward Bar
M aliens


Bar Weling le






Channel Delection


MODEL 5

SPIT PLATFORM BREACHING
Ebb

Chenneh



Ek Dr...rL. l,
., ,a.,'/f e Lngshode
Iy ,' Plao nrm Transi oil


MODEL 3
INLET MIGRATION AND SPIT BREACHING









Ebb Tidal Dela
r ~ Tnllawa


IE
Indel FIal /



Spd A.1aliesn


TIME 3


Landward BaV" ,/
Mpilnace





MODEL

WAVE-DOMINATED INLETS





d | On-lnanl


Transport
S' Wae Crest


narBe~arldI /~
c-


Darninant




G e
Fwh< , Langhahna








Cha~r-nI

r ----


--- ;f,


Figure 2-12. Models of sediment bypassing from FitzGerald, Kraus, and Hands (2000)


TIM






48




MODEL MODELS MODEL
JETTY-WEIR SYSTEM JETTIED INLET BYPASSING OUTER CHANNEL SHIFTING
AT JETTIED INLETS
Dominant
torm Wa Ebb Currents TIME 1


? / T Longshore Jey T=r.sport I rppJng Tranoponn
t ed n Dbpit Sal d Ttraus por
Jetty-.. Sv Ebb Curents I Through .
S ,nd o rn- w \ C r el 1,'
/ jSandMoVement RipCunenls Deec0onr / n
Onehe Moving Snd











Hands (2000)n
Seabergh, Cialone, and Stauble (1996) studied the interactions between inlet structures
TIME 2




and channel location TatBarnegatInletinNewJerseythrough a review of the engi









history at the inlet and the documentation of the associated chang oes in hydraulics,












sedimentation, and channel location. Migration of the natural inlet from 1839 to 1930
followed by the construction of a sand dike in the adjacent bay in 1943 to redirect the
Nfw Chanee
Cut


Figure 2-12 Continued. Models of sediment bypassing from FitzGerald, Kraus, and
Hands(2000)




Seabergh, Cialone, and Stauble (1996) studied the interactions between inlet structures


and channel location at Bamegat Inlet in New Jersey through a review of the engineering


history at the inlet and the documentation of the associated changes in hydraulics,

sedimentation, and channel location. Migration of the natural inlet from 1839 to 1930


predicated the construction of two arrowhead jetties in the late 1930's at Barnegat Inlet


followed by the construction of a sand dike in the adjacent bay in 1943 to redirect the

flow within the channel. These jetties were low and functioned as weir jetties allowing


tidal flow, wave-induced currents, and sediment to be transported over the jetties. This


resulted in the development of sand spits within the inlet channel and caused the channel


to migrate. This low arrowhead jetty system allowed for sedimentation within the


channel that reduced the channel cross section area and correspondingly reduced the tidal


prism. The anticipated reaction of the sediment transport to the arrowhead jetties was








quite different from the actual response. It was predicted that the arrowhead jetties would

concentrate the ebb flows and push the sediment further offshore, past the existing ebb

shoal. Additionally, it was anticipated that the mean tide level jetties would cause the

less concentrated flood currents approaching the inlet to carry less sediment into the inlet.

It was not expected that sediment would be transported along the shoreline and through

the jetties. Subsequent to jetty construction, the interior navigation channel migrated to

its pre-structure alignment due to the sediment from the north jetty depositing within the

flood shoal and deflecting ebb currents from the bay in a southeasterly direction.

Following a series of physical model tests, the north jetty was sand tightened and raised

in the early 1970's. These construction activities caused sediment to be diverted offshore

and the ebb shoal to grow. Although there was a reduction of sediment form the north

beach, the channel maintained its minimum area due to the influx of sediments from the

south. Migration of the channel to a position along the north jetty, in addition to the

dredging of the channel, cut off sediment input to the flood shoal and redirected

sediments to the ebb shoal. A second south jetty was constructed in 1991 in an

orientation parallel to the north jetty. The addition of this jetty along with the dredging

of the channel, to increase the minimum channel area, and the prevention of sediments

entering the inlet from the south allowed for a larger tidal prism. Although the inlet is

still adjusting to the implementation of the second, parallel jetty, the existence of a more

stable channel has been identified along with a reduction in sedimentation.

Price (1951) discussed the reduction of maintenance in navigational channels through

the proper orientation of tidal inlet channels. From inspection of inlets along the Texas

coast, Price concluded that the possibility exists for there to be a stable position of a tidal








inlet and a stable, common orientation of the channel that may best serve as the ship's

channel outlet. The diversion of the inlet and channel from this orientation often is due to

sedimentation along the shoreline encroaching into and within the inlet. The

determination of the optimum channel and inlet alignment and the design of structures

and inlet geometry to conform to this orientation may reduce inlet maintenance costs.

Price (1951) indicated that the scarcity of symmetrical ebb shoals along the coasts of the

Atlantic and Gulf of Mexico points to the presence of a net longshore sediment drift. The

ebb shoal complex of the mouth of the Brazos River on the southwest coast of Texas is

asymmetrical to the southwest, consistent with the dominant direction of longshore

sediment transport. The natural inlets and their associated ebb shoals along this section

of the Texas coast migrate to the southwest as sediment is deposited on the updrift side of

the inlet. The natural north-south inlet orientation found along this coastline is the

equilibrium position of the inlet channels. The stable position of the shoreline, therefore,

is a downdrift offset. The southerly migration of inlets along this coastline continues

until they have reached a position of stability. Any reorientation of a tidal channel must

be in a configuration such that there is a strong offshore movement of water to fully scour

the inlet and maintain it open.

A case study at New Pass on the Gulf coast of Florida, McClung and Douglass (1999)

established data on the response of the ebb channel following the 1997 dredging of the

inlet. Prior to the 1997 dredging, which aligned the main ebb tidal shoal to the north, the

ebb shoal was migratory to the south. Nine months after dredging, the portion of the ebb

shoal south of the dredged channel began moving shoreward into the location of the old

channel. At this time the middle portion of new channel began to migrate south while the








outer portion of the channel did not migrate. The processes seen within the New Pass

ebb shoal were similar to the processes described in the ebb shoal-breaching model for

sediment bypassing at a tidal inlet presented by FitzGerald (1988). In the model, a

breach in the ebb shoal opens a more hydraulically efficient channel. Following the

breach the portion of the ebb shoal downdrift of the new channel, no longer held in place

by the tidal flow, migrates onshore and the new channel begins to migrate to its pre-

breach condition in equilibrium with the net longshore sediment transport. In the

sediment-bypassing model, FitzGerald (1988) describes the breaching of the ebb shoal by

a migrating channel by a storm or catastrophic event, at New Pass the dredging functions

as the catastrophic event (McClung and Douglass 1999).













CHAPTER 3
METHODOLOGY

Parameters defining geomorphic asymmetry, called indicators of asymmetry, were

sought and identified to quantify the morphology of the ebb shoal and the channel of

inlets. The asymmetry indicators were measured and related to forcing parameters

associated with the particular tidal inlet. To determine if relationships exist between

asymmetry indicators and tidal inlet parameters, a large data set encompassing the

Atlantic, Pacific, and Gulf of Mexico coasts of the United States was developed.

This chapter describes the acquisition of data on the asymmetry indicators and the

associated tidal inlet parameters, including procedures and error or uncertainty estimates

for the analysis.

3.1 Acquisition of Data

3.1.1 Asymmetry Indicators

As indicated in Chapter 2, ebb shoal complexes have three general shapes based on the

associated inlets wave: environment, tide dominated, wave dominated, and transitional

(Hayes 1994; Hubbard, Oertel, and Nummedal 1979). Inlets in areas with large waves

and comparatively small tidal range often display a characteristic crescentic planform

shape. This general outline can be identified in the aerial photograph of Bogue Inlet,

North Carolina shown in Figure 3-1. The planform shape of the ebb shoal complex has

been digitized and overlayed on top of the aerial photograph. The resultant curve from

the digitization process allows for identification of the cresentic shape of the ebb shoal.
































Figure 3-1. Bogue Inlet, North Carolina (May 4, 1958)



Analysis of aerial photographic for the determination of inlet morphologic features has

been successfully employed by a number of researchers. Vincent, Corson, and Gingerich

(1991). used aerial photographs to identify channel asymmetry and temporal channel

variability at tidal inlets, and Gibeaut and Davis (1991) incorporated the gross features of

shoal and channel morphologies, taken from vertical aerial photographs, for their work in

the development of an ebb tidal delta parametric model. Carr de Betts and Mehta (2001)

evaluated the area of accumulated material within the flood shoals within Florida. Here,

digitization of the ebb shoal features from the aerial photographs allowed for the

description of the seaward edge of an ebb shoal mathematically.

In this study, aerial photographs for 108 tidal inlets and entrances with clearly

identifiable ebb shoals were compiled, and their planform outlines were digitized. The








ebb shoal features that were digitized came from inlets that displayed characteristically

different ebb shoal outlines. Additionally, where available, photographs of the same inlet

over many years were also digitized to examine temporal changes in inlet ebb shoal

features.

Figure 3-2 shows a plot of a representative quantity of the ebb shoal planforms that

were digitized from the wave-breaking patterns observed in aerial photographs and

normalized, both in the offshore and alongshore distance, by the channel critical width to

determine the representative shapes these shoals may take. The shoreline served as the

baseline for the horizontal axis (independent variable), and the inlet centerline (Figure 2-

2) served as the vertical-axis (dependent variable) with positive values denoting the

shoreline located downdrift of the origin and the offshore.

12
Representative Ebb Shoal Outlines


I
1 -------------4-------------------------






8 ------ ------- ------ ---- -----
6 6---------------------- --------------


a) 4~ ---------------- -- --


o------------------
I I
0 - - - 1- - - -


-2
-12 -8 -4 0 4 8 12
Normalized Distance Alongshore

Figure 3-2. Ebb shoal complex planform shape outlines








In the figure, positive-x values (to the right of the origin) denote normalized distance

downdrift, and negative-x values (to the left of the origin) denote normalized distance

updrift defined at each inlet from literature or, in the absence of literary reference,

interpretation of aerial photographs. The measured offshore and alongshore distances

were normalized by their respective minimum or critical channel width to obtain

comparisons independent of the width of the inlet itself (inlet width varied from 0.25 to

1.7 km). Alignment of the digitized ebb shoal outlines to a common origin allows

comparison of the offshore and alongshore extents of the shoal. Nearly symmetrical

shapes of the ebb shoal complex outline as well as asymmetric shapes can be identified,

showing wide diversity in inlet morphology.

After the ebb shoal complex planform shapes were digitized, it became possible to

identify and compare variations in the planform shapes of the ebb shoals. It was

observed that a more symmetrically shaped ebb shoal would tend to form if the left and

right directed longshore sediment transport rates were approximately equal (based on

values obtained from the database and through visual observation of equal sediment

impoundment on the updrift and downdrift shorelines), the navigational channel was

dredged straight out, and the main static factors of back bay configuration, jetties, shelf

bathymetry, and geologic structure were symmetric across the centerline of the inlet.

Morphologic features and an inlet channel centerline that deviate from the ideal situation

are categorized as asymmetrical.

The question that arose from the examination of the ebb shoal outlines in Figure 3-2

was what forcing parameters cause inlets to acquire asymmetric morphologic

features? Relationships among forcing variables at tidal inlets and asymmetry indicators








were developed through examination of 108 tidal inlets in the United States to answer

that question and to promote the analysis of the ebb shoal planform outlines.

Three asymmetry indicators were identified which described the ebb shoal planform

shape asymmetry and which are controlled by specific tidal inlet parameters. Figure 3-3

is a definition sketch of the measurements discussed in this study that were made from

interpretation of aerial photographs. The asymmetry indicators are as follows:

a. Distance of the offshore extent of the ebb shoal measured from the shoreline

(labeled Do in Figure 3-3).

b. Distance to the updrift point where the ebb shoal complex attaches to the

shoreline (labeled Du in Figure 3-3).

c. Distance to the downdrift point where the ebb shoal complex attaches to the

shoreline (labeled Dd in Figure 3-3).


Shoreline Offset)

Bay
Channel


SAttachment Bar


SOcean

Do X
....... -- Shoal
i -Shoal

Bar


Figure 3-3. Symmetric ebb shoal and measurements terminology.










In Figure 3-3, the variable Do represents the distance from the shoreline (which was

identified as the water-beach interface for each individual measurement) to the seaward-

most point of the ebb shoal. The quantity We is the channel critical width, defined as the

narrowest point between the two landmasses on either side of the inlet. The variables Du

and Dd represent distances to the updrift and downdrift attachment points (where the

bypassing bars tie in to the shore), respectively. The quantities Du and Dd were

determined by reducing the measurement of the distance from the channel centerline by

half of the inlet critical width. This reduction allows the presentation of the distances to

the updrift and downdrift attachment points to be independent of the size of the inlet.

The inset diagram in Figure 3-3 represents an updrift shoreline position offset. The

shoreline, which defines the horizontal coordinate in the measurements, has been taken as

a line between the trends in updrift and downdrift shorelines. The line has been drawn to

connect the shorelines sufficiently far from the inlet to define a more regional trend.

Measurements from inlets with identified offsets were made in the same way as those

with more uniform shorelines. At inlets with jetties, jetty length did not enter the

measurement process. The jetties may alter the shoreline position, in which case the

preceding method was applied for defining the shoreline trend.

Both aerial photographs and nautical charts were analyzed to identify and measure

asymmetry indicators. The location of the ebb shoal at the inlet was visually determined

from each aerial photograph. For each inlet, a nautical chart was found of the area to be

measured within a year of survey closest to that of the aerial photograph, and a scale of

the photograph was obtained by reference to a fixed feature appearing on both the

nautical chart and the aerial photograph. Once a scale was calculated for the aerial








photographs, it was possible to measure the asymmetry indicators of the ebb shoal and

tidal channel. The asymmetry indicators of distance to the updrift and downdrift

attachment points and the offshore extent of the ebb shoal were also measured directly

from the nautical charts using bathymetry to identify the features of the ebb shoal

complex.

Identification of the net direction of transport, determining which shoreline lies updrift

and which downdrift, is central to applications such as development of a sediment

budget, bypassing requirements and, possibly, the alignment or realignment of the

navigational channel entrance. Here, identification of the net transport direction was

necessary in measuring Du and Dd. The direction of net longshore transport can usually

be determined by interpretation of the shoreline signature in aerial photographs.

Impoundment and erosion at jetties, growth of spits, asymmetry in the ebb shoal

complex, orientation of the channel, and the existence and location of attachment points

allow inferences of net transport direction to be made. Caution must be taken to account

for processes that may not be straightforward or readily apparent. Examples of

confounding processes are changes in shoreline orientation, which modify the direction

of transport locally; impoundment in the downdrift shadow region of a jetty, which may

make the downdrift side appear as an updrift side; seasonal sediment drift reversals

(Oertel 1975); and changes in the back bay that might realign the channel. Stauble and

Morang (1992) give additional guidance on determining net drift in complex systems.

Offshore distance was identified visually in the aerial photographs from the wave-

breaking pattern around the ebb shoal. The determination of the offshore extent of the

ebb shoal therefore depended on acquiring aerial photographs taken under environments








conducive to the production of breaking waves over the ebb shoal. Additionally, the

determination of the offshore extent of the ebb shoal was based upon the photographs of

tidal inlets with ebb shoals containing a large enough volume of sediment to cause

incoming waves to shoal as they reached the most offshore extent of the ebb complex.

The offshore distance was also calculated from nautical charts. The aerial photograph

served as a guide to identify the offshore contours of the ebb shoal complex on the

nautical chart.

The attachment points, or the updrift and downdrift locations where the ebb shoal

complex attaches to the shoreline, were recognizable from aerial photographs as a

continuation of the ebb shoal complex and as part of the natural sand bypassing system.

A clearly identifiable attachment point can be seen from the oblique aerial photograph in

Figure 3-4 at Captiva Island in Florida.


Figure 3-4. Captiva Island, Florida (1963)








The measurements of the distance to the updrift and downdrift attachment points were

taken from the inlet centerline (that is, at the midway point of the inlet critical width) to

the centroid of the attachment point. This measurement was then reduced by the value of

one half the inlet critical width. This reduction allows for the distances to the attachment

points to be independent of the size of the inlet. If shoreline bathymetry from nautical

charts showed the existence of an attachment point, the distance to the bar from the

channel centerline was measured and the reduction was taken. Once data of the

asymmetry indicators were obtained from the aerial photographs and nautical charts the

data was then available to be compared to the inlets associated tidal parameters.

3.1.2 Tidal Inlet Parameters

Tidal inlets are distinct one from another not only in their morphology and the degree

of asymmetry in the planform shape of their ebb shoal complex, but also in the

controlling factors of flow and sediment transport at the inlet. In this research, tidal,

geometric, and dredging parameters for each inlet examined served as a basis for

understanding of the patterns seen in the morphology at tidal inlets and the differences in

asymmetry of the ebb shoal complex as measured by the asymmetry indicators.

The parameters selected included basic information such as inlet name and location

and both static and dynamic factors that control the processes occurring at tidal inlets.

Those parameters selected and their associated definitions (USACE 2001) are presented

in Appendix A.

* Inlet Name; the published name of the inlet, as well as any alternate names or the

common name if one exists;








* State; the state in which the inlet is located; if the inlet is located at the border of two

states, both states are mentioned;

* Federally Maintained Inlet (Y/N); qualification denoting if the inlet is maintained by

the USACE;

* District; USACE District if federally maintained. The districts and the states for

which they are responsible are (acronyms below are those denoting the USACE

Division and District);

o New England (NAE): Maine, New Hampshire, Massachusetts, Rhode

Island, Connecticut

o New York (NAN): New York, New Jersey

o Philadelphia (NAP): New Jersey, Delaware

o Baltimore (NAB): Maryland

o Norfolk (NAO): Virginia

o Wilmington (SAW): North Carolina

o Charleston (SAC): South Carolina

o Savannah (SAS): Georgia

o Jacksonville (SAJ): Florida

o Mobile (SAM): Florida, Alabama

o New Orleans (MVN): Louisiana

o Galveston (SWG): Texas

o Los Angeles (SPL): California

o San Francisco (SPN): California

o Portland (NWP): Oregon, Washington








o Seattle (NWS): Washington

o Alaska (POA): Alaska

o Buffalo (LRB): New York, Ohio

o Detroit (LRE): Michigan, Wisconsin, Minnesota

* Latitude; eating of the location of the inlet;

* Longitude; nothing of the location of the inlet;

* Minimum Width (m), the width of the inlet at its narrowest point;

* Number of Jetties (0), (1), (2); a jetty is an engineering structure extending out from

the shore into a body of water, designed to direct and confine the current or tide, to

protect a harbor, or to prevent shoaling of a navigable passage by littoral materials,

Jetties are often built in pairs, one on either side of a harbor entrance, or at the mouth

of a river;

* Weir (Y/N); a weir is typically constructed in an updrift jetty. In this case, the jetty

will have a low section, or weir, over which littoral drift can pass into a deposition

basin, which is then dredged periodically;

* Location of Weir (N, S, E, W);

* Number of Breakwaters; a breakwater is an offshore structure that, by breaking the

force of the waves, protects a harbor, anchorage, beach, or shore area;

* Recent Spring Tidal Prism (m3); the tidal prism is defined as the volume of water that

flows in or out of a harbor or estuary with the movement of the tide, and excluding

any freshwater flow. It is computed as the product of the tide range and the area of the

basin at mid-tide level, or as the difference in volume at mean high water and at mean

low water;








* Date of Recent Spring Tidal Prism; the date the spring tidal prism was measured or

calculated;

* Average Tidal Range (m); a tidal constant representing the difference in height

between consecutive high water and low water at a given place; it is twice the tide

amplitude;

* Spring Tidal Range (m); the average semidiurnal range occurring at the time of spring

tides and most conveniently computed from the harmonic constants. It is larger than

the mean range where the type of tide is either semidiurnal or mixed, and is of no

practical significance where the type of tide is diurnal;

* Spring Discharge (m3/sec); spring tidal currents are tidal currents of increased

velocity occurring semi-monthly as the result of the moon being new or full, the

spring discharge is the rate of flow at a given moment, expressed as volume per unit

time measured during the time of the spring tidal current;

* River Average Discharge (m3/sec); the average rate of flow from an associated river

contributing to flow to the tidal inlet at a given moment, expressed as volume per unit

time;

* River Maximum Discharge (m3/sec); the maximum rate of flow from an associated

river contributing to flow to the tidal inlet at a given moment, expressed as volume

per unit time;

* Previously Documented Tidal Prism (m3); tidal prisms are often documented a

number of times, possibly by a number of methods of calculation and measurement;

* Previously Documented Cross Section (m2); the tidal prisms associated cross section,

or width and depth at the channels narrowest section;








* Representative Average Net Longshore Sediment Transport (m3/yr); a measure of the

average rate of net transport of sedimentary material parallel to the shore in the

littoral zone. The net transport rate is the difference in the sediment transport rates of

material transported toward the inlet from both longshore directions (updrift and

downdrift) measured in the direction of greatest transport;

* Direction of Representative Average Net Longshore Sediment Transport (deg); the

direction of the average longshore transport in degrees (0 to 360);

* Direction of Representative Average Net Longshore Sediment Transport (N, S, E,

W); the direction of the average longshore transport expressed in compass direction;

* Representative Minimum Net Longshore Sediment Transport (m3/yr); a measure of

the minimum rate of net transport of sedimentary material parallel to the shore in the

littoral zone. The net transport rate is the difference in the sediment transport rates of

material transported toward the inlet from both longshore directions (upcoast and

downcoast) measured in the direction of greatest transport;

* Representative Maximum Net Longshore Sediment Transport (m3/yr); a measure of

the maximum rate of net transport of sedimentary material parallel to the shore in the

littoral zone. The net transport rate is the difference in the sediment transport rates of

material transported toward the inlet from both longshore directions (upcoast and

downcoast) measured in the direction of greatest transport;

* Representative Average Gross Longshore Sediment Transport (m3/yr); a measure of

the average rate of gross transport of sedimentary material parallel to the shore in the

littoral zone. The gross transport rate is the sum of the rates of sediment transport

moving toward the inlet from both longshore directions (updrift and downdrift);








* Representative Minimum Gross Longshore Sediment Transport (m3/yr); a measure of

the minimum rate of gross transport of sedimentary material parallel to the shore in

the littoral zone. The gross transport rate is the sum of the rates of sediment transport

moving toward the inlet from both longshore directions (updrift and downdrift);

* Representative Maximum Gross Longshore Sediment Transport (m3/yr); a measure of

the maximum rate of gross transport of sedimentary material parallel to the shore in

the littoral zone. The gross transport rate is the sum of the rates of sediment transport

moving toward the inlet from both longshore directions (updrift and downdrift);

* Representative Wave Height (m); the vertical distance between a wave crest and the

preceding trough;

* Representative Wave Period (s); the time for a wave crest to traverse a distance equal

to one wavelength. The time for two successive wave crests to pass a fixed point;

* Median Grain Size (mm); an expression of the average particle size of sediment or

rock, obtained graphically by locating the diameter associated with the midpoint of

the particle-size distribution; the middlemost diameter that is larger than 50% of the

diameters in the distribution and smaller than the other 50%;

* Recent Minimum Channel Cross Sectional Area Below Mean Sea Level (MSL) (m2);

the minimum channel cross sectional is determined through the multiplication of the

width and depth of the channel below mean sea level (MSL) at the channels

narrowest location;

* Representative Average Annual Entrance Dredging (m3/yr); entrance dredging is the

process of excavating sediments and other materials from the entrance or avenue of

access or opening of a navigable channel or inlet, and includes the transportation and








disposal of the material for the purpose of constructing new waterways, maintaining

existing waterway dimensions, obtaining fill for land reclamation, beach nourishment,

dike and levee construction, creating wetlands and marshes, obtaining materials from

borrow areas, or other beneficial uses. The representative average annual entrance

dredging is a value determined from dredging logs and is the average quantity of

material removed from the inlet entrance on an annual basis;

* Representative Minimum Annual Entrance Dredging (m3/yr); the representative

minimum annual entrance dredging is a value determined from dredging logs and is

the least amount of material removed from the inlet entrance on an annual basis;

* Representative Maximum Annual Entrance Dredging (m3/yr); the representative

minimum annual entrance dredging is a value determined from dredging logs and is

the maximum amount of material removed from the inlet entrance on an annual basis;

* Maintained Channel Depth Over Bar Mean Low Low Water (MLLW) (m); the

authorized depth below MLLW to which the channel is dredged in maintenance

operations at its point of crossing the offshore bar;

* Maintained Channel Width Over Bar MLLW (m); the design width below MLLW to

which the channel is dredged in maintenance operations at its point of crossing the

offshore bar;

* Average Dredging Depth Over Bar (m); the average depth below MLLW to which

the channel is dredged in maintenance operations at its point of crossing the offshore

bar;








* Maintained Channel Depth Between Jetties MLLW (m); the authorized depth below

MLLW to which the channel is dredged in maintenance operations between the inlets

jetties;

* Maintained Channel Width Between Jetties MLLW (m); the design width below

MLLW to which the channel is dredged in maintenance operations between the inlets

jetties;

* Advance Dredging Depth Between Jetties (m); the average depth to which the

channel is dredged in maintenance operations between the inlets jetties;

* Orthogonal of Shoreline (N, S, E, W); the direction of the inlet in relation to its

surrounding shoreline;

* Comments; any additional comments about the inlets parameters.

The tidal inlet parameters were obtained from published reports and through personal

communication with specialists whose research and work allowed for an in-depth

understanding and knowledge of the processes occurring at individual inlets.

Tidal inlet parameters have been employed to describe the morphodynamic processes

at tidal inlets with great success. This research compiled specific tidal, geometric, and

dredging parameters into a database for 156 coastal inlets and associated them with the

asymmetrical identity of the inlet. Despite considerable effort, many tidal inlet parameter

quantities could not be identified, causing the database to be incomplete. Parameters for

which data are most complete include; main entrance channel width and depth, minimum

width, spring tidal range, number of jetties, and representative wave height. Parameters

that are more difficult to obtain, and for which there are not many values in the database,

include; magnitude of the average, minimum, and maximum gross and net longshore








sediment transport, maintained channel width and depth over bar, river discharge, and

dredging parameters such as maximum and minimum annual entrance dredging. Future

efforts, study, and resources will need to be employed to fully complete the database.

3.1.3 The Federal Inlets Database

The collection of the tidal inlet parameters transformed into a challenging task. To

provide organization to the search of tidal inlet parameters, a database was developed.

The inlets database is an extensive listing of 156 federally maintained United States inlets

and entrances and 51 tidal inlet parameters associated with each inlet. The Federal Inlets

Database is presented in Appendix A.

The collection of tidal inlet parameters was assisted by a number of inlet specialists at

regional USACE District offices. The District offices under the larger umbrella of the

USACE Headquarters maintain 156 Congressional authorized (federal) channels or

navigation projects within the continental United States and Alaska. The USACE

maintains inlet navigability by dredging the channels and through implementation and

maintenance of controlling structures. The inlets contained within the database are

located within 25 states and are maintained under the direction of 19 USACE Districts.

With the assistance of the USACE District offices and personnel of the Coastal Inlets

Research Program (CIRP), the Federal Inlets Database was developed. The Federal

Inlets Database developed was combined with the CIRP database of federal inlets. The

combined Federal Inlets Database consolidates inlet characteristics and statistics into a

conveniently accessible form.

The Federal Inlets Database was established to obtain information on many inlets over

a diverse range of locations. Inlets within the United States have widely differing wave

environments, tidal prisms, magnitudes and directions of longshore sediment transport,








structures, and physical geometries (as well as other parameters). The federal inlets

within the database are listed by location beginning with the USACE New England

District and continuing along the perimeter of the United States, and ending in the Detroit

district along the Great Lakes.

For ease in presentation, the federally maintained inlets of the United States have been

divided into regions as seen in Figure 3-5. The locations of the inlets of each region are

shown in Figures 3-6 to 3-12.


N




Alaska Great Lakes


Now England

SCentral Atlantic Coast
Central Atlantic Coast


2000 0 2000 Kilometers
i MI


I 1 State Outlines
* Inlet Identifiers
I I Regions


Figure 3-5. Region identifiers of U.S. federally maintained tidal inlets































Figure 3-6. New England region


Oregon


Figure 3-7. Central Atlantic Coast region































Govemment Cut


Figure 3-8. South East region


Figure 3-9. Gulf Coast region






























Figure 3-10. Great Lakes region


Newport Baf / Oceanside
San Diego Ba


Figure 3-11. West Coast region














Nome Harbo Aaska





Ninilchik






Figure 3-12. Alaska region



The Federal Inlets Database should be updated to complete it and modified over time

because inlets evolve. The Federal Inlets Database must ultimately function as a

continually evolving work to maintain accuracy.

The Federal Inlets Database sponsored by the CIRP has incorporated the database

developed for this research into its web-based inlet-structure database. This on-line

database includes downloadable aerial photographs of many United States federal and

non-federal inlets and is accessible to the public at the CIRP web site

http://cirp.wes.army.mil/cirp/cirp.html or may be found directly at the link

http://cirp.wes.army.mil/cirp/structdb/structdbinfo.html.

The inlets contained within the Federal Inlets Database include those with one, two, or

no jetties, are located along all coasts of the United States, and are of different sizes.

Examples of the inlet settings and locations are:























Figure 3-13. Columbia River Inlet, Oregon/Washington (large) (1983)


Figure 3-14. Venice Inlet, Florida (small) (1998)



























Figure 3-15. Government Cut, Florida (jettied) (1970)


Figure 3-16. Moriches Inlet, New York (Atlantic) (1996)






























Figure 3-17. Pensacola Bay Entrance, Florida (no jetties) (1962)


Figure 3-18. Tillamook Bay, Oregon (Pacific) (1958)






























Figure 3-19. Colorado River Inlet, Texas (Gulf Coast) (1991)



The parameters located within the Federal Inlets Database are a necessary set of tools

in the area of tidal inlet research and specifically, as is the focus of this research, in the

comparison of how the parameters influence the morphodynamics of tidal inlet features.

3.2 Analysis of Data and Variability

The data collection phase of this research had two components. The first was the

definition and measurement of asymmetry indicators, and the second component included

the collection of tidal inlet parameters. Both components of data collection have

associated errors, and the uncertainties in measurements were estimated to determine the

reliability or validity of the relationships arrived at in this study.

3.2.1 Asymmetry Indicators

The asymmetry indicators developed here to describe morphologic asymmetries in

tidal inlets are the distance to the most offshore extent of the ebb shoal and the distances








from the inlet edge to the updrift and downdrift attachment points. The asymmetry

indicators were measured from a number of aerial photographs and nautical charts.

Analysis of the process of measurement and the errors associated with the interpretation

of the asymmetry indicators follows.

The measurements of the distance to the most offshore extent of the ebb shoal and the

distances from the inlet edge to the updrift and downdrift attachment points from aerial

photographs necessitated the identification of the ebb shoal planform shape. From this

planform shape, the features of the ebb shoal could be identified and the distances to

them measured and recorded as indicators of asymmetry. A source of error associated

with the method of identification of ebb shoal outlines on aerial photographs is that the

ebb shoal complex is best identified by the pattern of breaking waves. On a calm day, a

minimal number of waves will break, and the aerial photograph would either show no

shoal or the shoal would not be as easily identified, thus increasing the possibility for

error. Such photographs were eliminated from analysis.

The distance to the farthest offshore extent of the ebb shoal complex was measured

from the water-beach interface. Therefore, a consistent interpretation of the location of

the shoreline from the aerial photographs and nautical charts was a main goal of the data

collection process. In aerial photographs however, the location of the shoreline depends

on the tide level at the time the aerial photograph was taken. This "wetted bound"

shoreline should not be confused with the mean high water shoreline, which depends on

the tidal datum (Kraus and Rosati 1997)

Unrectified aerial photos of different scales were analyzed for this work, and

individual photograph scales were determined through comparison of distance between








two stationary objects, such as jetties, to that same distance found on nautical charts.

Uncertainties introduced for the distance measurements are estimated to be 25 to 150 m

for the inlets examined, depending on the scale, distortion and parallax on the aerial

photograph.

Identification of asymmetry indicators from nautical charts eliminates visual error in

distinguishing the ebb shoal. National Ocean Service (NOS) nautical charts were

evaluated to identify the asymmetry indicators. Distances to the offshore extent of the

ebb shoal were identified on NOS charts through examination of the point at which the

contour lines were oriented similar to offshore contours far from the inlet. This distance

was visually clear and easily identified by assessment of the slopes of the contour lines.

In addition to the errors associated with the interpretation of the asymmetry indicators

from aerial photographs, the variation in the dates of the aerial photographs and nautical

charts available introduces further sources of error due to the dynamic nature of tidal

inlets. NOS nautical charts are comprised of measurements made at different times;

however, they were considered acceptable to obtain asymmetry indicators under an

assumption of quasi-equilibrium evaluated for each inlet. For some inlets, both aerial

photographs and nautical charts were available. In such case, measurements were taken

from both.

For each inlet, many aerial photographs of various years and various maturity stages

were examined, and the asymmetry indicator measurements made. Because the

morphology of mature inlets varies through time about an assumed dynamic equilibrium,

the asymmetry indicator measurements for each individual inlet were averaged.








3.2.2 Tidal Inlet Parameters

The tidal inlet parameters compiled for inclusion within the database were obtained

from a number of institutional sources such as from the 19 USACE Districts, from

university reports, and NOS nautical charts. Other sources accessed in the compilation of

tidal inlet parameters were consulting industry reports and personal communication with

coastal engineering professionals and with individuals conducting research at various

tidal inlets throughout the study region. The sources employed are recognized within the

coastal engineering community as reliable and in many cases are the entities responsible

for the data collection or construction at the inlet.

The tidal inlet information presented within the database is intended to serve as

representative. Most of the parameters reported, such as tidal prism, are not constant and

may vary over different time scales, implying the values should be taken as

approximations. In such cases, where more than one value for an inlet parameter was

found, the year of measurement was noted in the database. In many situations, however,

only one value has been reported without a corresponding date since that information was

not located.

The variability within the parameters represented in the database can be evaluated by

comparing the reported values when more than one value is presented. For example

Shinnecock Inlet, New York has reported tidal prisms of 33.20x106 m3 and 9.30x106 3.

The difference in values may be in response to dredging, to inlet scour, or simply due to

the measurement being made at spring or neap tide. The parameters within the database

may have more than one value due to natural and anthropogenic changes within the inlet

and surrounding coastal environment such as changes in back bay area, channel dredging,

gradual sea level rise, and the time in the tidal cycle that the measurement was made.








Dredging of the inlet or the back bay will create an increase not only of the cross

sectional area of the inlet, but also of the associated tidal prism. For example, at Mason

Inlet in North Carolina, where a new inlet channel was opened artificially in March of

2002 and the old inlet (that had migrated 900 m to the south within the previous 15 years)

closed mechanically, there was an increase in the percentage division of tidal prism

associated with the two main creeks behind Mason Inlet because of dredging of the creek

and reorientation of the inlet to give greater hydraulic efficiency. In addition to dredging

which causing hydraulic changes at tidal inlets, the construction or removal of inlet

training structures will alter an inlet's flow patterns and sediment transport. Tables 3-1

and 3-2 present the differences between tidal inlet parameters within the database for

which two or more values were reported.

The information presented in the database was limited to the availability of data that

directly related to the ease or complexity of the measurement process. An example of

this is the intricacy of the collection of longshore sediment transport data. There is no

convenient method to calculate the longshore sediment transport rate near an inlet

because of the complex process of the movement of sediment along the shoreline. Thus,

the data for transport rates are limited, and this is reflected in the database (Appendix A).

Although the dynamic environment that shapes the morphology of tidal inlets cannot

fully be evaluated by the measurements of specific tidal parameters due to the complex

morphology and variety in inlets and their environment (wave height, river dominance,

tidal prism, etc.), these measurements are of benefit to the coastal engineering community

as they provide an understanding into coastal response patterns and inlet morphology

change and development.








Table 3-1. Variability in tidal prism values
Previously Previously Previously
Documented Documented Documented
Tidal Prism Tidal Prism, Tidal Prism,
Inlet Name State x106 m3 x106 m3 x106 m3
Shinnecock Inlet NY 10.60 9.30 33.20
Moriches Inlet NY 19.20 3.48
Indian River Inlet DE 14.87 14.90
Oregon Inlet NC 113.00 61.20 113.10
St. Simon Sound / Brunswick GA 180.00 382.28
Ponce de Leon Inlet FL 17.53 15.30
Fort Pierce Inlet
(Fort Pierce Harbor) FL 17.30 13.50
St. Lucie Inlet FL 16.03 17.00 19.70
Johns Pass FL 14.00 14.20
Galveston Entrance
(Port Bolivar) TX 168.20 450.58
Tillamook Bay OR 59.77 70.51
Columbia River OR/WA 1081.70 1082.00
Gray's Harbor WA 481.39 535.00


Table 3-2. Variability in cross sectional area values
Previously Previously Previously
Documented Documented Documented
Cross Section, Cross Section, Cross Section,
Inlet Name State xl03 m2 xl03 m2 xl03 m2
Shinnecock Inlet NY 0.51 0.36 1.49
Moriches Inlet NY 1.73 0.14
Fort Pierce Inlet
(Fort Pierce Harbor) FL 0.98 0.98
St. Lucie Inlet FL 1.40 1.46
Johns Pass FL 1.26 0.82













CHAPTER 4
EVALUATION OF TIDAL INLET EBB SHOAL ASYMMETRIES

After data on the selected tidal inlet parameters (tidal prism, wave height, number of

jetties) and geomorphic asymmetry indicators (distance to the most offshore extent of the

ebb shoal, distances from the channel edges to the updrift and downdrift attachment

points) were assembled, empirical correlations were sought. First, morphologic

asymmetries were correlated with individual tidal parameters. These relationships were

evaluated and refined depending on the number of jetties at each inlet. Second, at

selected inlets, the temporal behavior of the asymmetry indicators was evaluated with the

intent of demonstrating that the construction of training structures and storms may be

identified through the morphodynamic response of the ebb shoal complex.

Subsequent to the analysis, a discussion is given on the physical considerations of the

parameters governing asymmetry relationships between the parameters that force inlet

asymmetries.

This chapter presents specific information about asymmetries at coastal tidal inlets, the

results of the investigation, and the applications for the findings.

4.1 Findings of Direct Asymmetry Indicator Relationships

The study of determining if relationships exist between the measured asymmetry

indicators and tidal inlet parameters was initiated with the correlation between the tidal

prism P and the three asymmetry indicators, distance to the most offshore extent of the

ebb shoal Do, distance to the updrift attachment point Du, and distance to the downdrift

attachment point Dd. As indicated in Chapter 2, past research has successfully related









tidal prism to a number of tidal inlet quantities, including channel cross section,

minimum channel width, inlet stability, and flood shoal area and volume (e.g., LeConte

1905; O'Brien 1931; Nayak 1971; Johnson 1972; Jarrett 1976; Shigemura 1981; Marino

and Mehta 1987; Gibeaut and Davis 1993; Carr and Mehta 2001). Here, tidal prism was

selected for initial comparison with the asymmetry indicators because of the large amount

of tidal prism data available for U.S. inlets, past successes with correlations between tidal

prism and inlet geomorphic properties, and the previously discussed identification of tidal

prism as a dynamic factor that influences coastal response at tidal inlets. In the figures

that follow the legends indicate the source of the data as information was taken from both

nautical charts (NC) and from aerial photos (AP).

4.1.1 Distance Offshore

Based on the data assembled in this study, the distance to the most offshore extent of

the ebb shoal complex Do is plotted against the corresponding tidal prism P in Figure 4-1.

The figure shows a relationship of increasing offshore distance with increasing tidal

prism for the inlets contained within the database.


20
No. of Jetties
0-NC
S1-NC
S2-NC
E 15 O 0-AP L _____ ______________
A 2-AP I
0 _ _O trendline
e 1 trendline A
S_ 2 trendline ,
S10 -- ----- ------- -----


5 -- --- - - - - - -
00 71





106 107 108 109 1010
Tidal Prism, m 3
Figure 4-1. Do versus P








The regression equation determined by the data in Figure 4-1 is

Do = ai *P Equation 4-1

where al, bl, the correlation coefficient R2, and N, the number of data points, are listed in

Table 4-1. All correlations presented here were obtained through a regression analysis

associated with the commercially available software package PSI Plot. The coefficient of

correlation R2 is an indication of how closely a data points are explained by the equation.

In most cases a strong correlation is identified with a R2 value greater than 0.8, a

moderate correlation is identified with an R2 value that ranges from 0.5 to 0.8 and a weak

correlation would have a correlation coefficient that is less than 0.5. These ranges hold

for the correlations identified here however these limits should not be considered

absolute. The transient nature of the coastal environment coupled with the great number

of parameters that force the morphology of tidal inlets should be noted when examining

the correlation coefficients determined for each relationship.

An additional method for determining if the correlations represented are

statistically significant is the F-Test. The F-Test was performed for each case in this

research. The F-statistic is determined by dividing the variation among the samples by

the variation within the samples. An F statistic that tends to one would indicate a chance

variation while a great value of F would indicate statistical significance. More

specifically, when the F statistic is compared to the F Test sampling distribution table the

significance can be evaluated. The F Test table is based upon the number of degrees of

freedom (1-sample size) of the sample and given at a certain significance level (for most

field measured analysis a 90% significance level is used). In the case where both the

numerator and the denominator of the F statistic have infinite degrees of freedom the F








value from the table is equal to one. For this research the F-test was performed through

the use of the PSI plot computer package. The F-Statistic summary is presented as Table

4-16. In most cases the F-test indicated no statistical differences at the alpha = 0.10

probability level providing validity to the R2 values reported.



Table 4-1. Coefficients of Equation 4-1 for trend lines in Figure 4-1
Number of Jetties al b, /2 N
Zero 2.5x10.3 0.404 0.839 64
One 1.5x103 0.399 0.536 10
Two 4.5x104 0.483 0.692 40
All inlets 1.203x10-3 0.438 0.814 114

The distance from the shoreline to the furthest seaward extent of the ebb shoal was

determined through examination of the breaking wave pattern in aerial photographs. On

nautical charts, the furthest seaward point of the ebb shoal was identified through

inspection ofbathymetric contours.

Magnitude of tidal prism, confinement of the ebb jet by jetties, and slope of the

nearshore shelf in great part determine the offshore extent of the ebb shoal. In principle,

sediment grain size would be another determining factor, but the database covers inlets

on sandy shores, so this parameter is essentially constant. Deposition of sediment carried

by the ebb current into the ebb shoal is enhanced through the refraction of the waves

around the ebb shoal complex, tending to generate a longshore current directed toward

the inlet on both sides. The ebb shoal shelters the area behind it from waves, creating a

zone of low wave energy where sediment can deposit (Dean and Walton 1973), so that

formation of an ebb shoal creates a self-preserving mechanism. Hubbard, Oertel, and

Nummedal (1979) found that an ebb shoal developed at wave dominated inlets (as








discussed in Chapter 2) would lie closer to the inlet opening than an ebb shoal that

developed at a tide dominated inlet. Ebb shoals formed on low-wave energy or tide

dominated coasts are longer and narrower with more defined ebb channels and terminal

lobes (Hayter et al. 1988).

As discussed in Chapter 3, static factors controlling the asymmetry of the ebb shoal

include the length and condition of the jetties. For a mature inlet exposed to a large gross

longshore sediment transport rate, it is expected that the greater the distance the jetties

extend offshore, the greater distance to the offshore terminus of the ebb shoal.

Additionally, it is noted the more seaward ebb shoal will produce a greater distance to the

downdrift and updrift attachment points. However, it is possible that in some situations

the jetties are sufficiently long and, possibly, relatively closely spaced such that the

resultant ebb shoal can never attach to the shoreline through the mechanism of bypassing

bar formation; material comprising the shoal is jetted so far seaward that wave action

cannot return it under typical wave environments. This is the situation at Grays Harbor,

Washington where seaward migration of the ebb shoal has altered (reduced) the amount

and location of sediment bypassing (Cialone and Kraus 2001).

The condition of the jetties also plays a role in determining asymmetry inlet

morphology. If the jetties are permeable or low, sediment can enter the entrance channel

laterally. At impermeable jetties, the sediment accumulates on the updrift side of the

structure until it can move around the tip of the jetty (jetty becomes fully impounded).

An impermeable jetty is expected to be a more effective sand bypasser to the downdrift

shoreline because more of the sediment will be transferred from the updrift shoreline

through the ebb shoal and ultimately deposit on the downdrift shoreline in the attachment









point. However, a permeable jetty can cause erosion of the adjacent downdrift beach,

because sediment can move alongshore (and will be lost) in both directions. For this

reason, the south jetty at Ocean City, Maryland was sand tightened in 1985 to prevent

sand from overtopping the south jetty and flowing into the channel located to the north

(Dean and Perlin 1977; Stauble and Cialone 1996).

4.1.2 Distance to the Updrift and Downdrift Attachment points

A trend of increasing distance to the maximum offshore extent of the ebb shoal Do and

increasing distance to the updrift Du and downdrift Dd attachment points with increasing

tidal prism was identified and quantified for each category of number of jetties as well as

for the entire data set.




10 I--i-
No. of Jetties
S0 NC
E 1 NC |
S A 2 -NC I
S8 0 AP -------- -----------
2-AP /
S --. 0 trendline
E ...1 trendline /
L 6 2 trendline ---------------------- ------


4 1. ---- ----_---- ----------------------
A I
Io





106 107 108 10 1010
Tidal Prism, m 3

Figure 4-2. Du versus P

The regression equation for the distance to the updrift attachment point versus tidal

prism, Figure 4-2, is:


D = a2 *Pb2 Equation 4-2




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