• TABLE OF CONTENTS
HIDE
 Front Cover
 Abstract
 Table of Contents
 List of Figures
 1. Introduction
 List of Tables
 2. Design of inlet model exper...
 3. Experiments and test result
 4. Sediment budget and sediment...
 5. Ebb tidal shoal evolution...
 6. Summary and conclusions
 References
 Appendix A
 Appendix B
 Appendix C






Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 95/021
Title: Laboratory mobile bed model studies on inlet sic
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00085026/00001
 Material Information
Title: Laboratory mobile bed model studies on inlet sic
Series Title: UFLCOEL-95021
Alternate Title: Ebb tidal shoal evolution process
Laboratory mobile bed model studies on inlets
Physical Description: vi, 117 p. : ill. ; 28 cm.
Language: English
Creator: Wang, Hsiang
Lin, Lihwa
Wang, Xu
Florida Sea Grant College
University of Florida -- Coastal and Oceanographic Engineering Dept
Publisher: Coastal & Oceanographic Engineering Dept., University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1995
 Subjects
Subject: Movable bed models (Hydraulic engineering)   ( lcsh )
Inlets -- Mathematical models   ( lcsh )
Banks (Oceanography) -- Mathematical models   ( lcsh )
Sediment transport -- Mathematical models   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 93-94).
Funding: Sponsored by Florida Sea Grant College.
Statement of Responsibility: by Hsiang Wang, Lihwa Lin, and Xu Wang.
General Note: Cover title.
Funding: This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
 Record Information
Bibliographic ID: UF00085026
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 34531775

Table of Contents
    Front Cover
        Front Cover
    Abstract
        Abstract
    Table of Contents
        Page i
    List of Figures
        Page ii
        Page iii
        Page iv
        Page v
    1. Introduction
        Page 1
        Page 2
        Page 3
    List of Tables
        Page vi
    2. Design of inlet model experiments
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
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        Page 20
        Page 21
        Page 22
    3. Experiments and test result
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
    4. Sediment budget and sediment flux analysis
        Page 34
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        Page 36
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        Page 39
        Page 40
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        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
    5. Ebb tidal shoal evolution process
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
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    6. Summary and conclusions
        Page 89
        Page 90
        Page 91
        Page 92
    References
        Page 93
        Page 94
    Appendix A
        Page 95
        Page 96
        Page 97
        Page 98
        Page 99
    Appendix B
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
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    Appendix C
        Page 108
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Full Text



UFL/COEL-95/021


LABORATORY MOBILE BED MODEL STUDIES ON INLET
PART II: EBB TIDAL SHOAL EVOLUTION PROCESS





by


Hsiang Wang
Lihwa Lin
and
Xu Wang



1995


Sponsored by

Florida Sea Grant College
Sea Grant Project No. R/C-S-33
Grant No. NA36RG0070





REPORT DOCUMENTATION PAGE
1. Report No. 2. 3. Iecipieat's Accomeloso o.


4. Title and Subtitle 5. Report Date
Laboratory Mobile Bed Model Studies on Inlet 1995
Part II: Ebb Tidal Shoal Evolution Process

7. Author(s) 8. Performing| Orgauzauio Report No.
Hsiang Wang, Lihwa Lin and Xu Wang UFL/COEL-95/021

9. Performing Organizatioon ae and Address 10. ProJect/Task/Work Unit No.
Department of Coastal and Oceanographic Engineering
University of Florida 11. coctrct or Grant o.
336 Well Hall NA36RG0070
Gainesville, FL 32611 13. Typ of port
12. Sponsoring Oiranization Nam and Address
Final
Florida Sea Grant College

14.

15. Supplementary Noes



16. Abstract


Laboratory mobile bed experiments were conducted to study ebb tidal evolution under storm
wave conditions. Three different inlet configurations were tested including a natural inlet case and
two cases of inlet with jetty structures one with rubble mound porous jetty and the other with
caisson type impervious jetty. The geometry and the hydraulics of the jetty was representative of
that of a small or median sized jetties along the east coast of Florida. Model run time varied from
480 minutes for the natural inlet case to over 4000 minutes for the impervious jetty case.
Formation of ebb tidal shoal was observed in all cases; the rate of growth and location of ebb tidal
shoal were different for the three cases. The natural inlet case has the fastest growth rate whereas
the porous jetty case has the lowest. The evolution process was documented for all cases. An
attempt was made by analyzing the sediment flux patterns in the shoal building process by a new
empirical eigen function approach. The ebb tidal shoal process is extremely complicated. The
present study showed the feasibility of utilizing mobile bed for such studies.







17. Originator's Key Uords 18. Availability Staument
Ebb-tidal shoal
Inlet
Mobile bed
Sediment transport

19. U. S. Security Classif. of the Report 20. U. S. Security Classi,. of This Page 21. No. of Pages 22. Price
Unclassified Unclassified 123








TABLE OF CONTENTS


LIST OF FIGURES....................................................... ii

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

1. INTRODUCTION .................. ..................................1

2. DESIGN OF INLET MODEL EXPERIMENTS ............................ 4

2.1 Basic Considerations and Constraints ............................... 4
2.2 Modeling Laws .............................................. 11
2.3 Inlet-Beach Model Design ................ .................... 20

3. EXPERIMENTS AND TEST RESULT ................................. 23

3.1 Test Conditions .............................................. 23
3.2 Test Procedures .............................................. 29
3.3 Test Results ................................................. 31
3.3.1 Natural Inlet (Experiment C1) ........................... 31
3.3.2 Inlet with Porous Jetties (Experiment C2) ................... 31
3.3.3 Inlet with Impervious Jetties (Experiment C3) ................ 31

4. SEDIMENT BUDGET AND SEDIMENT FLUX ANALYSIS ................. 34

4.1 Sediment Budget Computations .................................. 34
4.2 Sediment Flux Patterns ...................................... 43
4.3 Temporal Changes of Sediment Flux Patterns ........................ 47

5. EBB TIDAL SHOAL EVOLUTION PROCESS .......................... 53

5.1 Defining Ebb Tidal Shoal in the Laboratory ......................... 53
5.2 Ebb Tidal Shoal Evolution for the Natural Inlet Experiment ............ 54
5.3 Ebb Tidal Shoal Evolution for Jettied Inlets ........................ 61
5.4 Ebb Tidal Shoal Dynamics ................................... 75

6. SUMMARY AND CONCLUSIONS ................................... 89

REFERENCES ................... ........................... ..... . 92

Appendix A ........................................................ 95
Appendix B ..................................................... .. 100
Appendix C ........................................................ 108








LIST OF FIGURES

Figure 1: Tidal Prism Ebb-Shoal Volume Relationship for Florida's East Coast Inlets .. 6

Figure 2: Local Map of Nineteen Inlets along Florida's East Coast ................. 7

Figure 3: Josson's Flow Regime Chart ..................................... 9

Figure 4: Sediment Transport Modes Diagram after Shibayama and Horikawa (1980) 10

Figure 5: An Example from Sebastian Inlet Physical Model Testing ............... 12

Figure 6: Two Different Beach Profile Regions Scheme ......................... 15

Figure 7: The Final Profiles Comparison in Experiment B3. ..................... 18

Figure 8: Comparison Between Measured Profiles in B3 and GWK Data ........... 19

Figure 9: Initial Beach Profile and Equilibrium Profile in the Model. .............. 21

Figure 10: The Schematic Setup for Movable-Bed Inlet Model ................... 22

Figure 11: The Initial Topographic Contours for Experiment C. ................. 24

Figure 12: The Initial Topographic Contours for Experiment C2. ................. 25

Figure 13: The Initial Topographic Photo and Bottom Contours for Experiment C3. .. 26

Figure 14: The Current Measurements at Sebastian Inlet........................ 28

Figure 15: Dye Study of Current Pattern. .................................... 30

Figure 16: Natural Inlet Experiment Bathymetry Change Contours After 480 min .... 32

Figure 17: Porous Jetty Inlet Experiment bottom contour change after 1600 min ...... 33

Figure 18: Impervious Jetty Inlet Experiment bottom contour change after 4860 min .. 33

Figure 19: Sediment Budget Computation for Experiments C1, C2 and C3 .......... 36

Figure 20: Seven Zones for Sediment Budget Computation ..................... 37

Figure 21: The Sediment Budget in Seven Zones for Experiment C1 ............... 40








Figure 22:The Sediment Budget in Seven Zones for Experiment C2 ............... 41

Figure 23: The Sediment Budget in Seven Zones for Experiment C3 ............... 42

Figure 24: Sediment Transport Flux Pattern for the First Ebb Cycle in Natural Inlet
Case ....................................................48

Figure 25: Sediment Transport Flux Pattern for the First Flood Cycle in Natural Inlet
Case ........................................................ 48

Figure 26: Sediment Transport Flux Pattern after 480min. in Natural Inlet Experiment 49

Figure 27: Two Regions for the EEF Computation ............................. 49

Figure 28: Sediment Transport Flux Pattern for the First Ebb Cycle in Experiment C3 50

Figure 29: Sediment Transport Flux Pattern for the First Flood Cycle in Experiment C3 50

Figure 30: Sediment Transport Flux Pattern for 0 to 480 min. in Experiment C3 ..... 52

Figure 31: Sediment Transport Flux Pattern for 480 to 1600 min. in Experiment C3 .. 52

Figure 32: Description of Ebb Shoal Pattern Using net +0 cm Elevation in Experiment
C1....................................................... ... 55

Figure 33: Description of Ebb Shoal Pattern Using net +2 cm Elevation in Experiment
C 1 .... .................................. ................... 56

Figure 34: Accretive and Erosive Patterns during 0-40 minutes in Experiment Cl .... 57

Figure 35: Accretive and Erosive Patterns during 40-80 minutes in Experiment Cl ... 57

Figure 36: A Photo Showing the Model Topography after 480 min. in Experiment C1 58

Figure 37: Shoreline Change after 480 minutes in Experiment C. ................. 58

Figure 38: Description of Generation and Growth of EbbShoal in Experiment Cl .... 59

Figure 39: Accretive and Erosive Patterns during 0-40 minutes in Experiment C2 .... 63

Figure 40: Accretive and Erosive Patterns during 0-40 minutes in Experiment C3 .... 63

Figure 41: Accretive and Erosive Patterns during 40-80 minutes in Experiment C2 ... 64









Figure 42: Accretive and Erosive Patterns during 40-80 minutes in Experiment C3 ... 64

Figure 43: Accretive and Erosive Patterns during 0-1600 minutes in Experiment C2 .. 65

Figure 44: Accretive and Erosive Patterns during 0-4860 minutes in Experiment C3 65

Figure 45: Description of Generation and Growth of Ebb Shoal in Experiment C2 .... 66

Figure 46: Description of Generation and Growth of Ebb Shoal in Experiment C3. ... 70

Figure 47: Accretive Change of Ebb tidal Shoal Volumes in Experiments Cl, C2, and
C3........................................................77
Figure 48: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C1
(0-40 minutes)................................................. 78

Figure 49: Sediment Flux Pattern in the Initial Shoaling Region in Experiment Cl
(40-80 minutes)................................................ 80

Figure 50: Overall Sediment Flux Pattern in Channel Shoaling Region in
Experiment C1 (0-480 minutes)................................... 81

Figure 51: Overall Sediment Flux Pattern in Offshore Shoaling Region in Experiment
C1 (0-480 minutes). ........................................... 82

Figure 52: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C3
(0-40 minutes)................................................. 83

Figure 53: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C3
(40-80 minutes)................................................ 84

Figure 54: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C3
(0-480 minutes)................... ............................ 85

Figure 55: Sediment Flux Pattern in Channel Shoaling Region in Experiment C3
(480-1600 minutes)............................................. 87

Figure 56: Sediment Flux Pattern in Offshore Shoaling Region in Experiment C3
(480-1600 minutes)............................................. 88

Figure Al: Bottom Topographic Changes after 40 minutes ...................... 95

Figure A2: Bottom Topographic Changes after 80 minutes. .................... 96










Figure A3: Bottom Topographic Changes after 120 minutes..

Figure A4: Bottom Topographic Changes after 160 minutes .

Figure A5: Bottom Topographic Changes after 480 minutes.

Figure B 1: Bottom Topographic Changes after 40 minutes.

Figure B2: Bottom Topographic Changes after 80 minutes.

Figure B3: Bottom Topographic Changes after 120 minutes .

Figure B4: Bottom Topographic Changes after 160 minutes .

Figure B5: Bottom Topographic Changes after 200 minutes.

Figure B6: Bottom Topographic Changes after 480 minutes

Figure B7: Bottom Topographic Changes after 1120 minutes

Figure B8: Bottom Topographic Changes after 1600 minutes

Figure Cl: Bottom Topographic Changes after 40 minutes.

Figure C2: Bottom Topographic Changes after 80 minutes.

Figure C3: Bottom Topographic Changes after 120 minutes.


... ....... ...... .... 97

............. .. ..... 9 8

............ ... .... 99

................... 100

................... 10 1

................... 102

................... 103

................... 104

................... 105

................... 106

................... 107

................... 108

................... 109

....... ..... ....... 110


Figure C4: Bottom Topographic Changes after 160 minutes .................... 111

Figure C5: Bottom Topographic Changes after 480 minutes. ................... 112

Figure C6: Bottom Topographic Changes after 1120 minutes ................... 113

Figure C7: Bottom Topographic Changes after 1600 minutes ................... 114

Figure C8: Bottom Topographic Changes after 2240 minutes ................... 115

Figure C9: Bottom Topographic Changes after 3200 minutes. .................. 116

Figure C10: Bottom Topographic Changes after 4860 minutes. ................. 117








LABORATORY STUDY ON INLET EBB TIDAL SHOAL EVOLUTION


1. INTRODUCTION


Ebb tidal shoal is a common feature associated with tidal inlets in coastal area. It is
created by the combined deposition of littoral material diverted from adjacent beaches
together with the alluvial material carried out from the inlet by the tidal current. When inlets
are stabilized with training structures, ebb tidal shoals can become more prominent as littoral
material is now being diverted further offshore into deeper water. As a consequence, the
storage volume also increases. This causes further disruption of the normal longshore
sediment transport process and often results in severe downdrift shoreline recession. In
Florida, over 85% of the shoreline erosion is considered to be related to inlets, particularly
to those with training structures. Since ebb tidal shoal is formed mainly by material diverted
from the updrift beach it is a tempting source, and reasonably so, to tap for downdrift beach
nourishment. Yet, such practice is not common mainly because the formation of ebb tidal
shoal is part of the natural process and disturbing an ebb tidal shoal environment so close
to shoreline without knowing clearly its effect is unsettling.


The current knowledge on ebb tidal shoal dynamics is generally poor. There is no
known analytical formulation on ebb tidal shoal sedimentary dynamics. Available field
information related to inlets is generally limited and may not be suitable, both in temporal
and spacial resolutions, for studying ebb tidal shoal evolution which is a process affected by
local characteristics of the inlet, currents and waves. This represents a difficult task that
requires a large amount of instruments and survey effort for the required spatial resolution
and an extended period of deployment for documenting the morphological evolution. This
requires a substantial resource commitment over a long time period. For this reason movable-
bed hydraulic modeling in the laboratory provides an alternative that could shed insights on
the dynamic process on one hand and yield practical information on the other. One of the
major advantages of physical modeling is that there is really no prerequisite, even though








helpful, on the knowledge of the specific process to be studied. It can be carried out with
reasonable resource at accelerated time scales. It is, however, not without difficulty because
the limitations of the physical scale in the laboratory and the still questionable modeling laws
that might lead to unrealistic results or erroneous conclusions.


Information on movable-bed tidal shoal and inlet modeling is also scant. In the
United States, model studies of this type were mostly carried out by the U.S. Army Engineer
Waterways Experiment Station (WES) during the two decades before 1980 (Cain and
Kennedy, 1979). All of the WES models were constructed at fairly large prototype to model
ratio in the range of 300 to 500 with the vertical to horizontal distortion ratio around five.
All WES inlet models use sand as the bed material. The main focus of these model studies
was for navigation channel improvement although some of them also addressed the effects
on adjacent shorelines due to the proposed improvement measures mostly of jetties
structures. The topic of ebb tidal shoal evolution was not addressed in any of them. The issue
on model scaling although raised practically in all these studies no serious attempt was made
to construct the model based on any proposed modeling laws. Model calibrations were
carried out in an ad hoc manner mainly by adjusting current velocities in the vicinity of the
navigation channel and by observing scouring-accretion patterns in the nearshore zone. Sill,
(1981) and Hayter, (1988) investigated ebb tidal shoal dynamics in laboratory using a small
scale movable-bed inlet model with an inlet width of 0.3 m and a wave paddle of 3.1m wide.
Their model studies showed that the volume and shape of ebb tidal shoal obtained in the
laboratory bore. Certain resemblance to the field measured data. Their models mixed
prototype-scale sand and tidal period with laboratory-scale geometry and waves. Owing to
the small model scale and, hence, highly distorted model conditions, the question on
modeling laws cannot be addressed. Consequently, the morphological time scale on ebb tidal
shoal evolution could not be addressed either.








For this reason movable-bed hydraulic modeling in the laboratory provides an
alternative that could shed insights on the dynamic process on one hand and yield practical
information on the other. One of the major advantages of physical modeling is that there is
no prerequisite, even though helpful, on the fundamental knowledge of the process. It can
be carried out with reasonable resource at accelerated time scales. To achieve this goal,
modeling law is addressed first with the aid of laboratory model experiments. Inlet model
testing is then designed and carried out with the following specific objectives:
(1) Investigating ebb tidal shoal evolution process and the corresponding shoreline responses
for a natural, unimproved inlet.
(2) Examining the process of ebb tidal shoal evolution and the corresponding shoreline
responses for an inlet with jetty structures.
(3) Evaluating the effects on shoreline due to partial material removal from a matured ebb
tidal shoal and the regeneration process.


The investigation of modeling law is given in a separate report Movable-Bed
Modeling Law of Beach Profile Response" (Wang, et al., 1995). This report covers the
objectives (1) and (2) as listed above. The task on objective (3) is still continuing and will
be reported separately.








LIST OF TABLES


Table 1: Summary of Four Velocity Distorted Model Laws ...................... 14

Table 2: Model and Prototype Scale Quantities ................................ 14

Table 3: Modified Modeling Law .......................................... 17

Table 4: Test Conditions for Plane Beach Experiment .......................... 17

Table 5: Test Conditions of Inlet-beach System Experiment ..................... 23

Table 6: Sediment Volume Balance Computation .............................. 35

Table 7: Sediment Volume Change Rate in Different Zones ..................... 39

Table 8: Natural Tidal Inlet Ebb Shoal Characteristics ........................ .. 61

Table 9: Jettied Inlet Ebb Shoal Characteristics ............................. .. 76

Table 10: Comparison of Model and Prototype Scales for Experiment C3 ........... 86








2. DESIGN OF INLET MODEL EXPERIMENTS


In nature, tidal inlets appear in different shapes and sizes. The processes of
morphological changes including ebb tidal shoal evolution in the surroundings of an inlet are
very complicated involving numerous mutually interacting factors. In conducting laboratory
model experiment the first step is to narrow down the scope with due consideration of the
constraints and then determine the values or range of values of experimental parameters
based on modeling laws. The main experimental parameters are the inlet geometry, bottom
sediment material, magnitude of sediment supply from the inlet and the natural forces. The
important natural forces are recognized as that due to ocean waves, tidal currents and water
level changes. Ocean wave is clearly a dominant driving force which produces longshore
sediment transport known as littoral drift as well as cross-shore sediment transport that is
most prominent under storm wave conditions. The tidal current not only acts as a sediment
transport agency but also modifies the nearshore hydrodynamic condition by interacting with
waves and topography. The water level defines the boundary affected by the dynamic forces.
The design of experiment is discussed in this section with due considerations on
experimental constraints and modeling laws.


2.1 Basic Considerations and Constraints
The model inlet experiments are to be carried out in the wave basin located in the
laboratory of Coastal and Oceanographic Engineering Department, University of Florida. The
approximate dimensions of the basin are 28 m x 28 m x lm. The basin is equipped with a
snake-type wave-maker, which consists of 88 independent wave paddles of 24 cm in width
each. By adjusting the phase of each individual paddle motion it can generate waves of
various oblique angles. However, an oblique angle larger than 150 is deemed undesirable
because of the basin's lateral constraints. Depending upon the water depth which is limited
to about 75 cm, wave heights ranging between 1 to 15 cm and wave periods from 0.9 to 1.9
seconds can be produced without difficulty.








To study ebb tidal shoal evolution, the basic model scale, here defined as the
horizontal geometrical scale ratio of prototype to model, must be kept large enough such that
a reasonable portion if not all of ebb tidal shoal occurred in nature can be duplicated at the
expected laboratory scale. This requires preliminary information on the physical size of ebb
tidal shoal associated with inlets found in nature.


Nevertheless, inlets in nature vary greatly in size and shape so are ebb tidal shoal
volumes. The first task is then to select a test inlet configuration that is comparable to natural
conditions. Walton and Adams (1976), and Marino and Mehta (1986) compiled ebb tidal
shoal volumetric data for 15 inlets along the east coast of Florida and proposed different
empirical relationships between an ebb tidal shoal volume and tidal prism. The results from
Marino and Mehta (1986), with the locations of the inlets included in the analysis, are shown
in Figs. 1 and 2, respectively. The results showed that ebb tidal shoal volumes decrease from
north to south along this stretch of coast. The ebb tidal shoal volumes from the four
northernmost inlets appear to be clustered in one group whereas the ebb shoals of the rest 11
inlets located in the mid and south regions exhibit much smaller shoal volumes. It is decided
that the experiment should be representative of the latter group for two practical reasons: (1)
It includes more than two third of Florida's east coast inlets located in one of the most
densely populated coastal regions. (2) The physical dimensions of the test basin and the
associated scale effects impose a limit on the actual size of the ebb tidal that can be
adequately simulated. The shoal volumes associated with this group of inlets vary from 0.5
to 10 million m3, which are generally considered as small to modest sizes. The inlet model
is then designed based on an idealized inlet configuration that has the general hydraulic
characteristics of inlets in the shaded zone of Fig. 1 that covers the largest inlet population
in the group. Inlets in this group can generally be classified as mixed energy type in which
both waves and currents are important forces to cause inlet morphological changes in time
scales of engineering interest from days to decades. Among them, Matanzas Inlet is the only
natural inlet and the rest are all improved with jetties. At a horizontal geometrical scale in
the range of 40 to 80, the basin size in model can accommodate ebb tidal shoal associated








with smaller inlets in the group. And at a horizontal geometrical scale in the range of 100
or so, ebb tidal shoal associated with mid-sized inlets can be properly simulated. Figure 1

which plots tidal prism versus ebb shoal volume also provides a rough guideline on the range
of combinations of tidal current strength and the inlet cross-sectional area.


0.5 1.0 5 10

EBB SHOAL VOLUME, V


50 100

(x 106 m3)


Figure 1: Tidal Prism-Ebb Shoal Volume Relationship for Florida's East Coast Inlets
(after Marino, 1986).


100

50


0.1
0


I I I I I I I
V = 5.59 x10 4p.39 (Marino, 1986)
V = 6.08 x 10 3p123 (Walton and Adams, 1976)

St. Marys ./

x St. Augustine
x Ft. George -
/ Nassau St. Johns
Lake Worth x Sound
St. 0 x Ft. Pierce
Bakers Haulover x Lucie Ponce de Leon
Sebas x t X Matanzas
-Sebastian i
Boca Raton x
Juptex x South Lake Worth
A x Pt. Canaveral

/ I I I I I I I


.1


5001000


I I I i


.1












.St. Marys
Nassau Sound


S\ -' Matanzas




-^ Port Canaveral
jel


SSebastan
r-t. Pierce
Figure 2: Location Map of Nineteen Inlets along Florida's East. Lucoast
> ?-- Jupiter
Lake Worth
SSouth Lake Worth
Boca Raton
-'-'H lltsboro
L----Port Everglades
| f=\Bakers Haulover
SGovernment Cut






Figure 2: Location Map of Nineteen Inlets along Florida's East Coast.


Selection of experimental wave conditions is a more difficult task. In nature, wave

conditions are random in magnitude, period, and direction. In the model experiment, the

selection of wave conditions is rather limited owing to the facility and time limitation. In the

present study, a simple scheme is devised to select the experimental wave conditions by
examining the wave effects on sediment transport and the associated shoreline changes

which can be roughly divided into four categories: (1) Waves from dominant weather

direction causing beach erosion, (2) Waves from dominant weather direction causing beach

accretion, (3) Waves from non-dominant weather direction causing erosion, and (4) Waves

from non-dominant weather direction causing accretion. For instance, along the east coast
of Florida, the dominant weather direction is from north west whereas the non-dominant








weather direction is from south east. For the mid east Florida's coastal region (Wang, et al.,
1993) it is estimated that for about 75% of the time waves are from dominant direction and
for the rest 25% of the time waves are either from the non-dominant direction or negligibly
small in magnitude. Evidently, sediment transport is dominated by the extreme wave events

from either dominant or non-dominant direction. Therefore, it seems adequate to first test

the effects of storm waves from the dominant weather direction in the model experiments.
Under a storm wave event, suspended sediment transport usually dominates the bed load
transport and this mode must be preserved in the experiment.


A major constraint in movable-bed physical modeling is the consideration on the
compatibility of flow regime and modes of sediment transport between field and laboratory
scales. In nature, the flow is mainly turbulent which needs to be preserved in the laboratory.
The modes of sediment transport, on the other hand, could be a combination of suspended-

load and bed-load transport, and other subclasses such as sheet-flow transport. In this aspect
it is important to preserve the dominant mode of sediment transport in the model. Under

storm wave condition the logical choice is the preservation of suspended load transport as
observed earlier. In the case of finding flow conditions in the laboratory, Jonsson's(1966)

flow regime chart (Fig.3) is used as a guideline. The flow regime consists of three different
flow zones and three transition zones. The flow condition is determined by two parameters:

a roughness parameter

a/k


and Reynolds number

R bam
a V


where amand ubare the amplitudes of the fluid particle displacement and velocity,
respectively; v is the kinematic viscosity, and k, is the roughness length, generally considered
to be on the order of sand size.

















S .... '"" Rough turbulent




101





icf 104 1o" 1i id d
Re,Reynolds Number

Figure 3: Josson's Flow Regime Chart.




As to determining sediment transport modes, a diagram proposed by Shibayama and

Horikawa (1980) is used to classify the sediment transport conditions. The diagram is shown
in Fig.4 which consists of the use of two parameters: the relative particle fall velocity

ublW


and the Shields parameter


2
fU2
jm2sgd


where W is the particle fall velocity, f, is the bottom friction coefficient, ub is the bottom
flow velocity, s is the sediment specific gravity, d is the particle size and g is the gravitational
constant.






























1001
10


2


101
Shields Parameter


Figure 4: Sediment Transport Modes Diagram After Shibayama and Horikawa

(1980).



To achieve turbulent flow and maintain suspended mode of sediment transport in the

model while keeping the desirable horizontal scales in the range of 40 to 100, one has certain

freedom to select the combinations of bottom material and vertical geometrical scale.

Different materials have been proposed and used in movable model experiments but the most

common one is still the natural quartz sand for the obvious reason that it closely resembles

to, if not the same, the natural beach material and it is easier to obtain at low cost. To use

natural sand as bottom material, however, vertical geometrical scale distortion appears to be

necessary. The degree of distortion is addressed in the following section in Scaling Laws.



The last constraint in model testing is the consideration of time scale. In prototype,

ebb tidal shoal evolution and regeneration are of long-term morphological processes which

may take years or decades. In the model, these processes need to be accelerated in a different

time scale. Based on Froude number consideration such a time scale in the model can be


0 No movement
1 Bed load(BL)
2 Bed Load-Suspended load
Intermediate (BSI)
3 Suspended load(SL)
4 Sheetflow(SF)

\3
1 1-2 2
0



BL~
No movement BL BSI


Transi on

3-4




SL


'








shown to be in inverse proportion to the square root of the vertical scale. However, this time
scale appears not quite sufficient to describe the ebb shoal process in the model. For
instance, even for an undistorted model of vertical scales ranging from 40 to 100, one year
prototype time would require nearly two months model run time in a laboratory. The proper
accelerated time scale is augmented here by an equivalent storm approach.


Based on laboratory experiment and field observation, it is generally found that both
sediment transport rate and the associated bottom topographic changes in nearshore
environment are dominated by storm events. An example is shown here in Fig.5 from the
results of Sebastian Inlet movable-bed model testing (Wang, et al., 1992). In this example,
bottom contour changes were examined in the case for six-day NE storm wave attack (wave
height in 1.8 m and wave period in 8 second in prototype equivalents), followed by another
eight-day ENE swell condition (0.6 m and 16 second). The model scales used in the testing
were N =60, N8=41, NT=9.5, and Nt=6.3. The experimental results indicated that the six-
day storm waves produced a prototype sediment transport of 1,700 m3/day on the downdrift
side boundary as opposed to a mere 370 m3/day in the following eight-day swell period. It
is shown in Fig.5 that a marked ebbshoal topographic change (contour increment in 25 cm
in prototype equivalent) occurred only during the six-day storm wave event. This trend was
also observed for the net sediment loss into the inlet. The commonly accepted sediment
transport formulas also support this condition. Therefore, all the present model experiments
are conducted under storm wave conditions in order to accelerate the process.


2.2 Modeling Laws
The state of knowledge on nearshore movable-bed modeling is largely a mixture of
empiricism and art. Concerning modeling laws on an inlet-beach system, there hardly exists
any conceptional guideline. As reviewed earlier, the geometrical scales used in the inlet
models were mostly arbitrary. No serious attempt has been made to scale these laboratory
results to prototype values. Therefore, the task of scaling presents a new challenge.








(a) topographic change under a 6-day NE storm event ( --- erosion,


(b) topographic change under (a) and a 8-day ENE swell condition (contour in 0.6 cm).


Figure 5: An Example From Sebastian Inlet Physical Model Testing.


accretion).








Two prerequisites have been discussed in the previous Section that the nearshore flow
condition should be turbulent and that the suspended load should be the dominant mode of
sediment transport. To fulfill these requirements using natural sand as bottom material, the
vertical scale might have to be distorted in the model. Therefore, the modeling law must have
a flexibility to accommodate this requirement. In searching for past movable-bed modeling
practices, it is found that there has been reasonable success for modeling beach profile
responses under storm wave conditions. A summary on beach profile response modeling
laws can be found in Hughes (1993). All these modeling laws were based on the assumption
that suspended load is the dominant mode and most of them can be applied at distorted
scales. Therefore, the prerequisites given above can be satisfied. The present study represents
an initial attempt to extend'this class of modeling law to an inlet-beach system.


There are two main issues to be addressed. One is to select or modify, if necessary,
a modeling law among the existing ones for applications in the present study. The other is
more fundamental to see if a beach-response modeling law is actually suitable for inlet-beach
system. Of course, it is apparent that beach response under storm condition is mainly wave
driven, and it is more or less two dimensional for a straight shoreline whereas in an inlet-
beach system the sediment movement is due to the combined forces of waves and currents,
and the phenomenon is clearly three dimensional. Considerable effort has been spent on
addressing both issues although the second issue can only be examined after model
experiments and even then may not be answered for lack of verification.


Concerning the selection of a modeling law among the existing ones, a detailed
laboratory model experiment was carried out. The experiment entailed both two-dimensional
wave tank and three-dimensional wave basin tests on beach profile responses at different
geometrical scales. These test results were the compared with data from a prototype scale
experiment performed in German Large Wave Tank (GWK) test (Dette and Uliczka,
1986a,b). The comparison of wave tank test results with GWK data was reported separately
(Wang, et al., 1994).








Four different modeling laws, as shown in Table 1, were evaluated based on a series
of 2-D wave tank tests carried out at horizontal scales of 20, 30, and 40 with vertical
distortions specified by the modeling laws. These four modeling laws were proposed by Le
Mehaute (1970), Vellinga (1982), Hughes (1983) and Wang, et al. (1990), respectively.


Table 1: Summary of Fall Velocity Distorted Model Laws.

Author Geometric Hydrodynamic Morphological
Distortion Time scale time Scale

Le Mehaute(1970) N6 =(NwN )2/3 NT =_8 Ntm =N
044 078
Vellinga (1982) N6 =Nw N;8 NT= Ntm=FN

Hughes (1983) N,=(NwNX)2/3 NT=Nx/mN6 NtN,/ N6l

Wang, et.al(1990) N =(NN,0.4)N8 NT=N N, Nm


A parallel sets of experiments with undistorted scales were also conducted. It
becomes immediately evident that at undistorted scale extremely fine sand is required in the
model. For example, to model a prototype case with median sand grain size of 0.3 mm at
a horizontal scale of 40 requires laboratory sand grain size in the order of 0.09 mm. Even
finer sand has to be used for scale larger than 40. Table 2 illustrates the scale relationship
between prototype and model based on Wang's or Vellinga's criteria. Nevertheless, the
actual experiments of the undistorted model were not successful.


Table 2: Model and Prototype Scale Quantities.

Horizontal length scale NI=20

Vertical length scale N6 = 14.46

Fluid-motion time scale NT=5.26
Fall velocity scale N= 1.99

Morphological time scale N = 3.8












Dune Region Bar Region



- - ------------------ SWL



Initial Profile
Storm Profile





Figure 6: Two Different Beach Profile Regions Scheme.


Since the intent is to extend the beach profile modeling laws to also cover the
offshore shoal region, the model evaluation criteria should be extended to include that region.
This can be partially accomplished owing to the fact both the GWK tests and the present tests
in 2-D wave tank stopped at offshore bar which does not quite include the entire region of
ebb tidal shoal. In the present study, the evaluation of modeling laws is carried out in two
different beach profile regions: the dune region (shore region) and bar region (offshore
region) as shown in Fig.6. Five criteria: (1) dune erosion volume, (2) nearshore profile, (3)
bar volume, (4) bar crest location, and (5) bar geometrical location, are selected to evaluate
the modeling laws. The results from the 2-D wave tank tests indicated that:
(1) For dune erosion, all four existing modeling laws were reasonably adequate to predict the
final erosional volume but over predict the erosion rate before reaching the final
experimental stage.
(2) Wang's and Vellinga's modeling laws performed better for nearshore profile.
(3) All the modeling laws predicted the main bar location closer to shoreline than the
prototype data.








One probable cause for (3) is that all existing models treated wave height scale the
same as the vertical scale. However, in nearshore zone it is known that wave breaking is
affected by water depth as well as local beach slope. Thus, waves tend to break earlier at a
larger water depth on a more gentle slope than a steeper slope. A general breaking criterion
incorporating slope effect can be given as

Hb=y(m)hb


where Hb and h, are wave height and water depth at breaking location, respectively, y is
defined as the breaking index, here expressed as a function of slope, m. In general, the value
of y increases with increasing beach slope. In other words, when the slope becomes
exaggerated in a distorted model the wave height scale should also be enhanced accordingly
in order to preserve the surf zone width. Therefore, if the wave height is simply scaled
according to the vertical geometrical scale the surf zone width in the model when scaled up
to prototype will be narrower than that in nature. Hence the breaking bar location from the
model prediction will also be closer to shore than that occurs in nature. To rectify this
discrepancy, a modified modeling law was proposed with wave height scaling enhancement
as follows:

N8
NH = ()N6
NX


where N. and N are the vertical and horizontal scale ratios, respectively. The quantity in the
parenthesis can also be viewed as breaking index scale (Wang, et al., 1994). Accordingly,
some new sets of equations were established for the modified modeling law as shown in
Table 3.
This modified modeling law was found to adequately scale both nearshore and
offshore regions in the 2-D wave tank tests. This new modeling law was also tested in the
3-D wave basin for a 2-D beach at a horizontal scale of 20. Four sets of experiments were
carried out, three under normal incident wave conditions and one under oblique waves of
150 angle. The test conditions are summarized in Table 4.










Table 3: Modified Modeling Law.

Geometric Wave Height Hydrodynamic Morphological
Distortion Distortion Time scale time Scale


N8=(N)N,)0.4NY 8 NH=(N8/N)N6 NT Ntm= N


Experiments B 1, B2, and B3, with normal incident waves are used for verification
of modeling laws. The test wave height and period in both Experiments B1 and B2 followed
the original modeling law by Wang, et al. (1990), whereas B3 employed the modified
modeling law. In Experiments B and B2, two different initial profiles were used. The intent
is to determine the best modeling law and to test the model sensitivity. First of all, the two
dimensionality of the beach appeared to hold well in all three Experiments B1, B2, and B3
at the completion of the tests. Figure 7 shows the final profiles measured at five different
cross-sections in Experiment B3.


Table 4: Test Conditions for Plane Beach Experiment.

Test Incident Wave Condition Beach Slope
No. Wave period Wave height Wave angle Foreshore Offshore

Bl 1.14sec 10.5cm 0deg 1:24 1:17.4

B2 1.14sec 10.5cm Odeg 1:20 1:14.5

B3 1.33sec 12.5cm Odeg 1:20 1:14.5
B4 1.33sec 12.5cm 15 deg 1:20 1:14.5












BASIN EXPERIMENT AFTER 80 MINUTES
(H=12.5 cm, T=1.33 sec, D=34.6 cm)


50 100 150 200 250 300 350 400 450 500 550 600
OFFSHORE DISTANCE (cm)



Figure 7: The Final Profiles Comparison in Experiment B3.



The experimental results showed that the proposed new modeling law has the best

over all match with the GWK prototype data. Figure 8 compares the measured profiles in

Experiment B3 with the GWK data based on the new modeling law. The original modeling

law by Wang, et al. (1990) was also found to yield a reasonable match in Experiments Bl

and B2. This seemed to indicate that although the new modeling law has the best overall fit,

the model is not sensitive to slight variations of either wave period or height. Also since B 1

and B2 have different initial profiles the end profiles are similar. In other words, the model

is also not sensitive to initial profile shapes. This latter conclusion is somewhat expected

since the concept of equilibrium profile has been well established.


20



10




o

z
o -10
E-
^
r -20



-30



-40










3D MODEL RESULTS AND GWK DATA COMPARISON
(3D MODEL TEST AVERAGED PROFILE)


0. 10. 20. 30. 40. 50. 60.
OFFSHORE DISTANCE (m)


70. 80. 90. 100.


Figure 8: Comparison Between Measured Profiles in B3 and GWK Data.


Experiment B4 tested the beach model with oblique waves. The test result was

utilized to check the longshore transport rate by comparing with the longshore transport

formula given in SPM (1984). The measured transport rate was found to be considerably

smaller than that computed by the formula. The best fit transport coefficient, K, from the

laboratory data was found to be equal to 0.24, smaller than the recommended value of 0.77.

The experimental value, however, is much closer to that experienced along the east coast of

Florida and to some of the other laboratory experiments.


-6.1-
-10.








2.3 Inlet-Beach Model Design
The inlet-beach model was designed with due considerations of the constraints and
the modeling laws addressed in the previous two sections. An idealized inlet of rectangular
cross section was constructed cutting through a plane beach made of natural beach sand
with D50 =0.19 mm. The overall length of the beach from updrift end to the down drift end
is about 19 m. It is bounded on two sides by wave guides formed by concrete blocks. The
wave guides are semi-perforated to allow flows in and out of the test section. The downdrift
wave guide also has an opening near the beach end to allow downdrift littoral transport to
leave the test section and to be collected in the catch channel. The plane beach consists of
a flat back shore segment, a steep-slope foreshore segment and a mild- sloped offshore
profile which extends seaward to about 7 m from the shoreline beach face before merging
with the flat basin concrete floor. The beach profile approximates an equilibrium shape of
h=Axo08 (h is water depth, x is seaward distance from shoreline). Figure 9 shows this
composite profile together with the curve of the equilibrium shape. The inlet is a straight
rectangular channel with uniform width and depth of 1.75 m and 0.2 m, respectively. The
inlet is located offset from the center towards the updrift with the updrift beach length of 4.5
m and downdrift beach length of 12 m.


For this inlet model configuration the wave generator is located about 27 m from the
shoreline based on an average water depth about 0.35m. The model setup is shown
schematically in Fig. 10. As shown in this figure, tidal currents are generated by recirculating
water through the channels as depicted. The flow discharge is controlled by the weir boxes
located on the two sides of the basin. Water is supplied from the upper basin weir boxes
(flood flow weirs) for flood current and from the lower basin weir box (ebb flow weir) for
ebb current.


The design of sand supply from the updrift end presents a challenge. After trying
several techniques it is settled with forming a curved feeder beach section at the updrift end.
The sand supply to the downdrift is, therefore, purely due to wave-induced transport. This








design allows for continuous and more uniform sediment supply and the magnitude is
automatically adjusted for different incident wave angles. This feeder beach has to be
replenished from time to time during the intervals of conducting beach surveys.




INLET MODEL INITIAL AND EQUILIBRIUM PROFILES


0 10 20 30 40 50 60 70 80 90 100
OFFSHORE DISTANCE (m)


Figure 9: Initial Beach Profile and Equilibrium Profile in the Model.














MOVEABLE-BED


INLET MODEL


EBB FLOW
GATE <











FLOOD FL
GATE, C


EBB FLOW
GATE













NO TRAP
ANNEL


0 2 4 6 8 10
SCALE IN METERS


Figure 10: The Schematic Setup for Movable-Bed Inlet Model.








3. EXPERIMENTS AND TEST RESULT


3.1 Test Conditions
Three sets of experiments were carried out for the inlet-beach system. The test
conditions and test run times are given in Table 5.


Table 5: Test Conditions of Inlet-Beach System Experiment.

Test Water Incident wave condition Beaches slope Jetties Model
No. depth (Type) time
Wave Wave Wave Foreshore Offshore
(min)
period height angle

C1 35 cm 1 sec 8 cm 15 deg 1:20 1:14.5 none 480

C2 35 cm 1 sec 8 cm 15 deg 1:20 1:14.5 Riprap 1600

C3 35 cm 1 sec 8 cm 7.5 deg 1:20 1:14.5 Caisson 4860


Among these three test cases, Experiment Cl is to simulate a natural inlet; C2 is to
simulate a jettied inlet with riprap type jetties and C3 is to simulate a jettied inlet with
caisson type jetties. The jetty geometries are the same for Experiments C2 and C3. Both
updrift and downdrift jetties are straight and perpendicular to the shoreline. The length of the
updrift jetty measures 1.5 m from the shoreline. The downdrift jetty is about half the size
of the updrift jetty with a length of 0.7 m. The height of jetty top is about 5 cm above the
flood tide water surface and the jetty width is about 20 cm. This uneven jetty geometry is
common to the inlets with updrift and downdrift jetties in Florida. The major difference
between riprap and caisson type jetties is that the riprap is porous and not sand tight whereas
the caisson is impervious. Figures 11 to 13 show the initial topographic contours together
with photos of the three different test configurations.


















Natural Inlet Initial Contours


500


450


400

E
S350



0
300


a 250


:200
0


-100 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm




Figure 11: The Initial Topographic Contours for Experiment C1.







24


I I I I I I I i I I
.-.-.-. .-- .-. .-- .-+--33 -- -
-31-
---- ----- 317


-
-29
-27
. . . ... . . . . . . . . ..-- - -

. . . . .. . + . . . ... .. . . . ..-- - - .- ..


25- - - -








,77i23 21
`-







i.. IA


1. "I--1 1 5-------".--...


S2 i 15
,, 5 0 1 i15 ,


0
















Porous Jetty Inlet Initial Contours
I I I I
~----------------- -,---------
31
29. .......-----
500- -27 ---


400-- -- -- -------2---- --- -----


-100


0 100 200 300 400 500 600
Longshore Distance,cm


700 800 900


Figure 12: The Initial Topographic Contours for Experiment C2.






25


E



0
0300


o

200


.-- 21
...... ...... -- 1

S,"-17
7 - --' . . . .. -- - - - - - --: - - - -


7 ..-- ........ .._ ..

:/. .. ................ .
----------t- -----
/ 1 -- - - ;- - A- - .











(a) A photo showing the initial topography


(b) Bottom contours of the initial topography


300 400 500 600
Longshore Distance,cm


Figure 13: The Initial Topographic Photo and Bottom Contours for Experiment C3.


500



E 400


C,
U

5 300


200
o 200


-2 29 29
-25 -
-23---------.-----------
.-- -- -- -- ..... ... ..... .......... ._




.-23- --23 --2 --

- - 1,23


+-1-5 -151 .

.- ... F7 - ---- .- .... .-

.- -- - -_---
-- -. ----


WU








Incident waves utilized in the experiments are the storm waves generated by
wavemaker at the offshore boundary with 8 cm in height and 1 second in period. The
attacking wave direction is 150 in Experiment Cl and C2. After examining the results from
these two experiments, it was judged that this incident wave angle was too large causing
excessive channel shoaling and severe beach erosion and the experiments had to be stopped.
Therefore, the incident wave angle was reduced to 7.50 in Experiment C3.


The tidal currents are simulated in the experiment by alternating the ebb and flood
cycles at equal interval of 40 minutes. Based on Froude criterion, this time interval roughly
corresponds to a semi-diurnal tidal period at 1:80 geometrical scale ratio. The tidal current
condition can be simulated with a number of choices: equal flood-ebb discharge, equal
flood-ebb current strength, unequal discharges or current strengths at the inlet throat. Based
on field measurements at Sebastian Inlet (Wang, et al., 1991) and also at other inlets equal
discharge appears to be a reasonable choice and was adopted in the model test. This usually
will result in stronger ebb current which is often experienced in the field. In the present
study, the discharge is kept constant at 0.04 m3/sec within each ebb and flood period. The
cross-sectional averaged flood current in the inlet is 0.12 m/sec with an inlet water depth of
0.2 m. These values corresponding to ebb cycle, on the other hand, are 0.14 m/sec and 0.17
m, respectively. The ebb and flood currents were simulated alternatively in stepwise fashion,
instead of sinusoidal or other types. The stepwise changes of ebb and flood currents in model
experiment is deemed adequate as compared to the prototype data. Figure 14 shows the
current measurements at Sebastian inlet. It is seen that the current variations within each ebb
or flood can be reasonably approximated by uniform step function.


Water level is another important factor that affects beach erosion. In the present
study, no attempt was made to simulate storm surges. The periodical water level change due
to tidal cycles, however, was included in the test. During flood tide, water level was higher
as water feeds towards the inlet whereas during ebb tide, water level was lower as water jets
from the inlet. The simulated tidal range is 3 cm in the experiment.



























E

0
0


0I
a)
a
V





-0.5


10 10.5 11 11.5 12
January,1990



Figure 14: The Current Measurements at Sebastian Inlet..










3.2 Test Procedures
The model experiment is conducted according to the following procedures:
(1) Prepare model bathymetry.
(2) Survey initial profiles at twelve cross-sections as shown in Fig. 10.
(3) Adjust water level and discharge to the specified design values. Start up the current until
it stabilizes. All the tests start with ebb cycle first.
(4) Start wave generator with pre-calibrated settings. The experiment is interrupted at
intervals of every 40 minutes for the change of tidal conditions between ebb and flood.
(5) Conduct bottom profile surveys at selected time intervals. The time intervals are irregular,
shorter in the early stage of the experiments and progressively longer later. For example, the
surveys conducted for C3 are at the time marks of 20min, 40min, 80min, 120min, 160min,
480min, 1120min, 1600min, 2240min, 3200min and 4860 min, respectively.
(6) Collect sand cumulated outside the downdrift boundary and inside the inlet.
(7) Reshape the model to its initial bathymetry for the next experiment.


In addition to the normal operating procedures described above, dye and sand tracer
studies were also conducted from time to time. The dye studies were current observation and
were documented by video recordings. Sand tracers were mainly used for visual examination
on a sediment transport pattern. No quantitative analysis was attempted. Figure 15 shows a
picture of the dye study.


In the natural inlet case of Cl, the experiment was terminated at model time of 480
minutes when shoaling nearly closed the inlet entrance. In the case of C2, the experiment was
stopped at model time of 1600 minutes when both inlet shoaling and downdrift beach erosion
became too severe to continue. The large shoaling in inlet channel near the updrift jetty
entrance was partially due to sediment transport through the porous jetties and severe
downdrift erosion was judged to be caused by the large incident wave angle. In the case of
C3, the experiment was run under alternate flood and ebb tidal conditions up to 1600 minutes










(a) A photo showing dye test during ebb current condition (without waves)


--flu.


_ L 'A '-


(b) A photo showing dye test during ebb current condition (with waves)


- -~ J~i 1.


Figure 15: Dye Study of Current Pattern.


'' *F


-i
r 1~~
if~J~it








After 1600 minutes, the process of shoal development slowed down considerably. The
experiment was then continued to 4860 minutes under ebb tidal condition only to extend the
test duration as long as possible. After 4860 minutes the shoal began to move offshore
beyond the test beach and onto the concrete floor. The experiment was terminated.


3.3 Test Results
3.3.1 Natural Inlet (Experiment C1)
The natural inlet experiment was designated as Experiment C1 in the present study.
It is composed of a plane beach of straight shoreline with a rectangular tidal channel. The
experiment was conducted for a total of 480 minutes or six complete tidal cycles. The
experiment was not continued for a longer test time because both the downdrift erosion and
channel shoaling were too severe as large waves dominated the shore erosion process. Figure
16 displays the end condition after 480 minutes test time. Bottom topographic surveys were
conducted at 20, 40, 80, 120, 160 and 480 minutes, respectively. Detailed survey results
including bottom topographic changes at different time intervals and profile changes were
given in Appendix A.


3.3.2 Inlet with Porous Jetties (Experiment C2)
The porous jetties in C2 are of riprap type. The experiment was carried out for a total
model time of 1600 min and was stopped at the end of 1600 minutes. A significant amount
of sediment apparently had leaked through the porous jetties from the updrift side forming
a large local shoal just inside the updrift jetty. Topographic surveys were conducted in
model at every 40 minutes, i.e., at the end of each ebb or flood phase run, for the first 200
minutes. Afterwards, survey intervals were carried out at irregular and larger time intervals.
Figure 17 shows the bathymetry change contours at the end of 1600 minute. Survey results
including bottom topographic changes at different time intervals were given in Appendix B.


3.3.3 Inlet with Impervious Jetties (Experiment C3)
Experiment C3 is similar with C2 but with impervious jetties and a new incident









wave direction of 7.50. Bottom topographic surveys were again conducted at every 40

minutes in the early stage of the experiments and at larger time intervals in the later stage.

Limited dye study and sand tracer experiments were also carried out. The test was continued

for a total of 4860 minutes. Near the end, topographic changes became small and a more or

less stable ebb tidal shoal was formed, Fig. 18 shows the bathymetry change contours at the

end of 4860 minute. Survey results including bottom topographic changes during different

time intervals were given in Appendix C.





Natural Inlet Net Contours After 480min


500

450

400

S350

c 300

* 250
o
: 200
0
150

100

50


-100
-100


-9-- - /- --- -

.... ....... -"-" --

;i i111i,.; 7i -_ ..- -. -

0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 16: Natural Inlet Experiment Bathymetry Change Contours After 480 Minutes.










Porous Jetty Model Net Contours After 1600min


* -.-.-'' -' -_.. . .



19 4-7
"- f " -- - ^-

-. .. .. .---- - ,


200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 17: Porous Jetty Inlet Experiment bottom contour change after 1600 minute



Impevious Jetty Model NetContours After 4860min
600 5


500
04






S300 -

e 2oo
--2 .-. -. " -
0 200 -_7 -- -- -- - : .. ....
=- ~ ~ ~ Z --------------- -- -----. Z. "-..-

100 -11 ---------- --- --- ..
?'-^'-= --- ~? ^ ---_----7 T-- ." 'ZS'".^"Sa *^-S


0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 18: Impervious Jetty Inlet Experiment bottom contour change after 4860 minute.








4. SEDIMENT BUDGET AND SEDIMENT FLUX ANALYSIS


Before discussing the experimental results on ebb tidal shoal evolution, attempts were
first made here to compute the sediment budget and to establish the sediment flux patterns.
The sediment budget analysis will provide an overall spatial and temporal picture on the
sediment losses and gains in the region of interest. The sediment flux is a dynamic property
of fundamental importance towards interpretation of morphological evolution process.


4.1 Sediment Budget Computations
Two different analyses were performed for the sediment budget. The first was the
time history of the overall sediment budget for the entire tested model region as shown in
Fig. 10. The purpose of this analysis was to determine the updrift transport rate based on the
measured sediment quantities at the downdrift and channel ends and the measured net gain
or loss within the region. In this analysis, the sediment transport across the offshore boundary
was assumed to be equal to zero. The analyzed results for Experiments C2, and C3 are given
in Table 6. The results from all Experiment Cl, C2, and C3 are shown graphically in Fig.19.
It is seen that for all three test cases, the experiment can be roughly divided into two stages:
an initial adjustment stage followed by an evolution stage. In the first 160 minutes or so the
initial inlet environment apparently underwent a major adjustment in respond to the test wave
and current conditions. This is manifested by the rapid rate of changes in littoral transport
and in net volume change in the domain. Afterwards, the process was stabilized with rather
steady littoral drift environment. In Case 3, the cumulative net volume change inside the
basin reached a constant revealing a matured environment.


The second sediment budget analysis was performed by computing sediment volume
changes in seven different zones within the region as delineated in Fig. 20. Zone 1 was
defined as the updrift zone which covered the area of 2 m wide and 6 m long to the left of
the updrift jetty. Zone 2 corresponds to the area of 2 m wide and 3 m long inside the inlet
between two jetties. Zone 3 covers the area of 2 m wide and 3 m. long offshore between two










extended jetty lines. Zones 4 and 6, each comprising 4 m x 3 m area, together correspond

to the downdrift offshore region where ebb tidal shoals are generally located. Zones 5 and

7, each also comprising 4 m x 3 m area, together correspond to the downdrift nearshore

region where offshore bars are generally present under storm wave conditions.



Table 6: Sediment Volume Balance Computation.

C2: Porous Jetty Inlet Experiment

model updrift transport downdrift loss to inlet offshore transport net volume
time (min) (1) transport (2) (3) (4) (5)


40 0.011 -0.099 -0.000 0.000 -0.088

80 0.289 -0.524 -0.014 0.000 -0.249

120 0.316 -0.595 -0.014 0.000 -0.292

160 0.448 -0.835 -0.028 0.000 -0.415

480 0.463 -0.892 -0.040 0.000 -0.469

1120 0.581 -1.000 -0.062 0.000 -0.486

1600 0.597 -1.104 -0.079 0.000 -0.586

C3: Non-Porous Jetty Inlet Experiment

model updrift transport downdrift loss to inlet offshore transport net volume
time (min) (1) transport (2) (3) (4) (5)


40 0.038 -0.071 0.000 0.000 -0.033

80 0.038 -0.354 -0.003 0.000 -0.319

120 0.109 -0.396 -0.003 0.000 -0.290

160 0.318 -0.580 -0.006 0.000 -0.268

480 0.361 -0.637 -0.017 0.000 -0.293

1120 0.375 -0.750 -0.040 0.000 -0.415

1600 0.504 -0.850 -0.057 0.000 -0.402

3200 1.188 -1.416 -0.085 -0.099 -0.413



























S1.5
CO1

E 1

E 0.5

> o

-0.5

E
= -1

-1.5
0


Sediment Evolution--Volume Budget


500 1000


1500


2000


2500


3000


3500


1-
5

5 -. -__ .... 1---------------.--1-


o 3 porous jetty inlet

5 ................................... 4


1 2


0 500 1000 1500 2000 2500 3000 3500



5

5 .




/ impervious jetty inlet
5-3

54


2
5IIIIIII


0


500


1000


1500 2000
model time (min)


2500


3000


3500


Figure 19: Sediment Budget Computation for Experiments C1, C2, and C3.


- 1 --updrift longshore sand supply



I 3--measured loss to inlet natural inlet


4--net volume change inside the basin


2--downdrift measured longshore transport


CO
E
E .

0



-0.




1.
E


cc


-










2


4


.......................... j ..................................................

3 5


INLET


SINK


Figure 20: Seven Zones for Sediment Budget Computation.


SOURCE
SOURCE


6




7

SHORELINE


SINK


I i I








Table 7 tabulates the results for the rate of accumulated volume changes in seven
zones for Experiments C2 and C3. Figures 21, 22, and 23 exhibit the accumulated sediment
volume changes with reference to the initial bathymetry for Cl, C2 and C3, respectively. The
patterns of sediment volume changes for Experiments C2 and C3 are shown to be similar.
Using Fig.23 as an example, it is seen that the net sediment volume changes in Zone 1 were
rather small. After certain initial perturbation, this zone reached a rather stable state with very
little net gain or loss in the evolution process. This indicates that eventually updrift sediment
was simply passing through this zone to downdrift. Zone 2 roughly represents the inlet
channel within the confines of the jetties. Shoalings were rather localized and mostly
occurred in the vicinity of jetties. The shoaling rate decreased steadily and bottom
configuration seemed to have eventually approached an equilibrium state. Zone 3 extends
from the tips of the jetties to offshore which is a zone where both channel shoal and ebb tidal
shoal could occur. The total net volume change in this zone was in the same order of
magnitude as that of Zone 2. It also showed that in Zone 3 sediment from the updrift was
intercepted and deposited here at the initial stage and an equilibrium state appeared to have
been reached in the experiment at run time around 1200 minute; afterwards, most of the
sediment influx from updrift simply bypassed this zone. Zones 4 and 6 contain bulk of the
ebb tidal shoal in the matured stage. They clearly represent sediment storage with rather
significant volume increases in both zones, as resulted from the building and growth of the
ebb tidal shoal. None of these zones appeared to have reached equilibrium state at the end
of the experiment though a slower rate of volumetric increase has observed. Zones 5 and 7
represent downdrift nearshore region where both longshore sediment transport and on-off
shore transport are vigorous. These zones suffered heavy sediment losses owing to severe
nearshore erosion under the storm wave conditions. The rates of volume losses were quite
high initially but gradually slowed down in the later stage. Equilibrium state was also not
reached in these two zones at the end of the experiment. It was pointed out earlier from the
results shown in Fig. 19 that the cumulative volume within the entire region appeared to have
reached a constant in the end. The results given in Fig.23, on the other hand, suggested that
the dynamic process still caused material exchanges within the region and with the








surroundings. The patterns of exchange, however, cannot be determined without the
knowledge of sediment flux.


Table 7: Sediment Volume Change Rate in Different Zones.

Model C2: Porous Jetty Experiment, Volume Changes Rate (m3/min)
Time (min) Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7

40 0.090 -0.025 -0.090 0.035 -0.285 0.040 -0.115

80 0.303 0.013 0.015 0.035 -0.517 0.027 -0.280

120 -0.005 0.003 -0.080 0.123 -0.160 0.157 -0.145

160 0.153 0.040 0.140 -0.070 -0.335 -0.055 -0.180

200 0.155 0.012 0.005 0.092 -0.395 0.275 -0.413

480 0.004 0.039 0.000 0.016 -0.048 0.009 -0.020

1120 -0.003 0.001 0.016 0.018 -0.040 0.031 -0.020

1600 0.008 0.004 0.004 0.007 -0.016 0.012 -0.017

Model C3: Non-Porous Jetty Experiment: Volume Changes Rate (m3/min)

Time (min) Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7

40 0.000 0.025 0.037 0.093 -0.262 0.135 -0.044

80 -0.125 -0.043 -0.052 -0.058 -0.268 -0.003 -0.165

120 0.015 0.053 0.110 0.052 -0.067 0.017 -0.110

160 0.032 0.020 -0.012 0.043 -0.138 0.158 -0.047

480 -0.006 0.036 0.043 0.045 -0.097 0.042 -0.070

1120 -0.010 -0.002 -0.002 -0.005 0.016 -0.009 -0.010

1600 0.009 0.001 0.001 0.008 -0.005 0.002 -0.014

3200 0.005 0.011 0.007 0.025 -0.036 0.024 -0.036




















Sediment Volume Evolution in Different Zones for Case 1


100 200 300 400 500
model time (min)


Figure 21: The Sediment Budget in Seven Zones for Experiment Cl.


40


E
-0.2


,-0.4
E

0-0.6


















Sediment Volume Evolution in Different Zones for Case 2


0 200 400 600 800 1000 1200 1400 1600 1800
model time (min)


Figure 22: The Sediment Budget in Seven Zones for Experiment C2.



















Sediment Volume Evolution in Different Zones for Case 3


0 500 1000 1500 2000 2500 3000 3500
model time (min)



Figure 23: The Sediment Budget in Seven Zones for Experiment C3.


E
CO

a-
to

(U



S-0.5
o








4.2 Sediment Flux Patterns
Sediment flux is probably the single most important parameter in sediment dynamics.
However, it is generally difficult to directly measure this quantity whether in the laboratory
or in the field. No direct measurement was carried out in this experiment. Attempts were
made here to estimate the sediment flux using two hybrid methods relying partially on
bottom survey data.


Sediment flux is defined here as the sediment transport vector, q=(qx,,qy) in the
horizontal plane. The two dimensional equation of sediment mass conservation in discrete
form is:

Aqx Aq_ Ah
Ax Ay At


where h is the measured elevation, and t is the time. The quantity Ah/Atcan be calculated
from topographic surveys. To solve for qx and q, an additional governing equation is needed
which means certain relationship must be established for either the magnitude or the
direction of the flux, or certain relationship between the magnitude and the direction. This
type of information unfortunately is not available and, at present, there is no easy way to
acquire them through measurement. For lack of such additional information, an exploratory
technique based on empirical eigen function analysis is introduced for this estimation.


The idea of using Empirical Eigen Function (EEF) theory for deriving vectors of
sediment transport rates in the (x, y) plane is based on the technique of separation of
variables. It is assumed here that the components of the sediment flux gradient at any fixed
time can be expressed by the following series forms of the product of two functions f(x) and

g(y),








eq
qy = laf(x)g(y),
aq
Y = IPf)n8(Y)n
ay


where f(x), g(y) are orthogonal functions and a and P are weighting factors. Substituting the
above quantities into the mass conservation equation given in finite difference form,

Aq yn Ah(x,y) at ttk
Ax Ay At


The above Equation can be also written as:

xI= n Wf(x)ng(,y)n- Ah(x,y)
At


where

Wn = an + n

The Empirical Eigen Function technique is then employed here to obtainf(x)n, f(y)n and w,
for given Ah/At. For a finite grid size of I x J (a grid of I x J points in x and y directions,
respectively), the series expressions have to be truncated with the maximum number of eigen
vector, w, to be the smaller of the two grid sizes, say, for the present case to be I, i.e.,


- n=aIf(x)ng(y)n
aqy N
a- En=I nf(X)ng(Y)n
Dy


for N < I.
The x and y direction flux components at grid point (xi,yj) can be given as









qxi=, y) = anF(xi) g(yj )+d(y)
qy (xi, y) = EN Pnf(xi)nG(yj)n+e(xi)



with

F(xi) f(xi)dx, G(y) = fYg(y )dy



and is subject to

an + Pn =Wn

It should be noted here that the flux components so obtained are volumetric discharge per
grid width. The EEF method can be applied to the entire region as a whole or to different
subregions and then matching the subregions at the boundaries.


To explain the procedure, application to the entire region is used as an example. The
grid system for the entire region is I = 19 longshoree direction) with Ax=65.54 cm and J =80
(on-offshore direction) with Ay=7.62 cm. Therefore, in theory, a maximum of 19 pairs of
eigen values, f(x) and g(y), together with 19 eigen vectors, w, can be solved. To solve for a
and p, additional constraints must be specified through boundary conditions. The choices of
boundary conditions are the following:
1) No flux (impervious) or given flux (porous) normal to jetties.
2) Given flux distribution on updrift boundary (calculated from sediment budget
analysis presented in the previous Section).
3) Given flux distribution on downdrift boundary (measured in the laboratory).
4) No flux (shoreline) or given flux (inlet) on shoreward boundary.
To solve for all the an and [ and the integration coefficients of d(y) and e(xi) a total of
N + I + J conditions needs to be given. In EEF analysis, one routinely truncates the eigen
vectors to retain only a few dominant terms. In this case, the required conditions can also be








reduced. On the other hand, if all boundary conditions listed above are specified, the system
will be over constrained. For over constrained system, one either has to relax the system or
seek approximate solution.


It is natural to require no flux condition at the jetties and to specify the total sediment
influx at the updrift boundary and no flux at the shoreline while leave the downdrift and
offshore boundaries open. The sets of equations to be solved simultaneously are given in the
following:
1) Eigen function relationship:

an + pn = Wn n=1 to I

S= 1N: 1anF(x,)g(yj)n +d(y) =qin(xly)


2) Known updrift input flux:
3) Shoreline no flux and inlet known flux conditions:

E, nf(nG()+e(xi) = 0, for shoreline
EN =lf(Xi)nG(Ys)n+e(xi) =inlet, for inlet i=l to I



4) Jetties no flux condition:

C, n= F(Xjety)ngjett)n+d(y1) = j=a to b



where a and bmark the beginning and end of the jetty locations, respectively, and is assumed
to contain K grid points. Therefore, altogether there are I + J + K +1 equations for I + J +N
unknowns. If K + 1 is larger than N, the system is over constrained. One then must devise
a scheme to relax the constraints. A method used in the present analysis is to represent the
spatial distribution of the influx by a polynomial with K + 1 N degrees of freedom. The set
of equations can then be solved for all the a's, p's and the coefficients of the polynomial.










For a domain with no structure inside such as the natural inlet case, additional
boundary conditions must be specified to replace the missing no flux condition at the
structure. In the present study, the sediment influx at the updrift boundary is fitted with an
empirical distribution curve.


4.3 Temporal Changes of Sediment Flux Patterns
Two cases were examined in this section for temporal changes of sediment flux
patterns, the natural inlet case and the case with impervious jetties. For the natural inlet case,
the flux patterns for the first ebb and flood cycles obtained from EEF analysis are shown in
Figs. 24 and 25, respectively. In both cases, longshore sediment transport in the nearshore
zone dominated. The longshore drift was also seen to shift shoreward in the inlet region
causing rapid topographic changes near the entrance. The experiment was terminated at 480
minutes. The total resultant sediment flux pattern is given in Fig.26. Again, nearshore
longshore transport dominated. Sediment was also seen to be diverted to offshore due to the
combined ebb current and storm wave effects.


For the impervious jetties, the experiment was conducted for a total of 4860 minutes
which is equivalent to 40.5 days in prototype at a geometrical scale ratio of 80. A few cases
were examined here to reveal the sediment flux patterns at different stages in the experiment.


First, the flux patterns in individual tidal cycles were examined for the first ebb (0-40
minutes) and the first flood (40-80 minutes) cycles. In the EEF computation, the domain was
divided into two regions, one updrift region and one downdrift region as shown in Fig 27.
Computation was performed for the updrift region as a first step. The computation then
proceeded to the downdrift region using the output from the first region as the input
boundary condition. Figures 28 and 29 display the results of the computed flux patterns for
the first ebb and flood cycles. Figure 28 shows that during the first ebb cycle material from
updrift was carried around the updrift jetty, across the channel then towards offshore by the













OMIN O0MIN




















I i


0.00 M
0.00 M3


I -


0.3 Md
(Loboratory datao, COED, UF.)


Figure 24: Sediment Transport Flux Pattern for the First Ebb Cycle in Natural Inlet Case.



40MIN 80MIN


I






-* -* -~ -
-~ -~ S S ~
S -* -~* -


S-- -.


0.06 M3


(Laboratory data, COED, UF.)


0.3 M3


Figure 25: Sediment Transport Flux Pattern for the First Flood Cycle for Natural Inlet.



48


0.06 M3


0.2 M3













OMIN 480MIN

I I


t




-
* I ' > ,
I I, . , .


0>
0.23 M3


(Laboratory data, COED, UF.)


Figure 26: Sediment Transport Flux Pattern After 480 min. for Natural Inlet Experiment.


Figure 27: Two Regions for The EEF Computation.



49


1.1 M3
















OMIN 40MIN
I


0.00 M3


'0.05 M3
(Loboratory doLo, COED, UF.)


Figure 28: Sediment Transport Flux Pattern for the First Ebb Cycle in Experiment C3.


40MIN 80MIN
I I t ,
* \ \ \ I / I I
I I /
* ^
. \ \ / / / -
. i / I I I
I I I 1
* I I I *l

* *\ \ \ / I I I
t i \
I I I' '

/ i
\ N
I//
N ~ -* 1 ,
- \ -.


S0.21 M3
0.01 M3 (Laboratory dato, COED, UF.]


Figure 29: Sediment Transport Flux Pattern for the First Flood Cycle in Experiment C3.


I \ -
*r I
I
I I *

I ,' t I I

I
/ I. -



/:- -h-
ii
.. 4- .a -


0.07 M3


0.00 M3








ebb tidal current. Downdrift sediment motion is far more active than the updrift side owing
to the energetic fluid motion caused by current wave interaction. Material eroded from the
beach was largely carried offshore. Strong drift reversal was detected and material was
entrained into the ebb current towards offshore. Clearly, the strongest offshore transport was
offset to the downdrift side of the channel under the influence of the oblique incident wave
angle. Both updrift and downdrift material contribute to this offshore transport. Shoals were
developed in regions where the flux gradients were strongly negative. The validity of the flux
solutions can be checked to insure that input bottom contours can be recovered from the
computed flux distribution. Figure 28 shows the comparisons of the original input contour
map with the recovered one for the ebb cycle case. In the subsequent flood cycle, the results
given in Fig.29 showed strong downdrift components in the nearshore zone and over the
shoals formed during the ebb cycle. It appeared that ebb flow tended to build up shoals
whereas flood flow tended to destruct the shoals but promote longshore transport. Similar
situations were also observed in subsequent tests. The cumulative effect, however, was
growth of shoals.


To examine the cumulative sediment flux patterns, the test duration is artificially
divided into two stages: the adjustment stage covering the initial 4 to 6 tidal cycles and the
evolution stage covering the remaining period. The EEF analysis was applied to these two
stages. Figure 30 displays the computed sediment flux patterns from 0 to 480 minutes when
a rather stable ebb shoal can be identified. During this stage, net nearshore sediment transport
was more active than offshore region. Longshore transport was strong. Drift reversal near a
downdrift jetty was evident. Figure 31 displays the results from 480 to 1600 minutes. During
this period, the total magnitude of a transport rate was considerably smaller than the initial
480 minute periods. Net littoral transport slowed down significantly. Relatively speaking the
on/offshore flux component became more important than longshore component. Sand loss
too offshore was evident. Channel bypassing and channel shoaling were also evident during
this later stage.










OMIN 480MIN


I I /



' I i i
, r i
-. -


0.03 M3


I
'- ,




A~- -
NC


'0.63 M3
(LaboroLory data, COED, UF.)


Figure 30: Sediment Transport Flux Pattern for 0 to 480 minutes in Experiment C3.


480MIN 1600MIN
i t; t i t 1 1


I I 1 I


./// '

A/ /

I


j~~~


0.06 M3


. / t t / /
A / ./ t I
A / / / ',~ /,/ <
/ / / I /
/ / / / //
?// (/?$<
S/ \ t l t
- \ / / 1X \ \ 1
/ / I I / / \
I \ \ / / /
/ \ \ \ 1 / /
>.. t 1 I I I 1 I
,, / i \ I l


L. 15 M3
(LaboroLory daotlo, COED, UF.)


Figure 31: Sediment Transport Flux Pattern for 480 to 1600 minutes in Experiment C3.


0.36 M3


0.11 M3










5. EBB TIDAL SHOAL EVOLUTION PROCESS


The ultimate goal of the present study is to examine the ebb tidal shoal evolution
process. Current laboratory results concerning this process are discussed in this Chapter.


5.1 Defining Ebb Tidal Shoal in the Laboratory
In order to quantify the test results with regard to ebb tidal shoal growth it is
necessary to define the ebb tidal shoal first. In theory, ebb tidal shoal is the excess sediment
material accumulated in a zone under the influence of ebb tidal current. In reality, there exist
two difficulties to quantify an ebb tidal shoal; one is to determine the zone of influence and
the other is the selection of a reference bottom contour above which one defines as ebb tidal
shoal.


In examining the accretive pattern from the test results, a number of main accretive
features in topographic changes are identified. These features are defined here as: channel
shoals, ebb tidal shoals, breaking bars and downdrift accretion. Since they mutually affected
each other, separation of them at times could be subjective. For instance, in the early stage
shoaling could began in the channel. As the channel shoal grew, it might breakup in the
offshore direction to initiate ebb shoal. To separate them, the channel shoal is considered
as the material deposit inside the inlet channel or in the vicinity of the inlet entrance whereas
ebb tidal shoal is the material deposited in the offshore region roughly coincide with the ebb
tidal flow path. In the later stage, the down drift accretion which was influenced by the
downdrift boundary in the laboratory would gradually encroach into the ebb tidal shoal. In
this case, the contribution to ebb tidal shoal due to downdrift encroachment must be
separated. In the late stage, it was also possible that the ebb tidal shoal would expand into
the channel to link with the channel shoal. Therefore, the separation of these accretive
features became rather subjective at times.








Our main interest is to examine the ebb shoal and channel shoal development. To
define the boundaries of either shoal, reference planes must be defined. In the laboratory, the
initial bottom configuration would be a natural choice provided that the initial configuration
does not deviate too much from a quasi-equilibrium shape. The shoal is then the positive
elevation change with respect to the reference. In the present experiment, it is found that
using initial configuration as reference to define ebb tidal shoal is impractical as the outlines
of zero net elevation change was rather diffused as well as confused. This situation is
illustrated in Fig.32 using the case of ebb shoal generated in the natural inlet experiment.
Therefore, to identify and construct ebb tidal shoal in a consistent and manageable manner,
a net increase in elevation with more coherent contour can serve better as the reference plane.
In the natural inlet experiment, for instance, the net +2 cm elevation was selected as the
reference. The shoal below this level is extrapolated based on an assumed angle of repose
of 250 (for fine median sand). An improved picture describing ebb shoal pattern based on
this net +2 cm elevation in the natural inlet experiment is shown in Fig.33. In the jettied inlet
experiments, the experimental durations were considerably longer. In these cases, one could
find that even the +2 cm contours became too diffused. Under such condition one would
have to select a higher elevation to outline a coherent shoal. The procedure of extrapolating
to the zero contour remains the same as discussed using 250 angle of repose. In this manner,
the shoals are identified. Other derived quantities such volume, location, perimeter, areas,
etc., are then computed based on the constructed shoals.


5.2 Ebb Tidal Shoal Evolution For the Natural Inlet Experiment
In Experiment C1, the test was conducted under storm wave condition beginning with
an ebb current cycle. The beach responded immediately from the wave attack; beach and
foreshore material was rapidly carried offshore to form breaking bars. As expected, beaches
on both sides of the inlet entrance sustained severe erosion. Material from the updrift beach
was pushed towards the inlet and downdrift by the strong wave action. Inside the breaking
line, a small shoal began to form immediately near the entrance and inside the breaker line.
Outside the breaking line, where the wave motion was stronger, material was carried across
















Natural Inlet Net Contours


-100 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 32: Description of Ebb Shoal Pattern Using net +0 cm Elevation in Experiment
C1.


Natural Inlet Net Contours After 120min


--00 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 33: Description of Ebb Shoal Pattern Using net +2 cm Elevation in Experiment
C1.








the channel and deposited immediately downdrift of the channel outside the surf zone.
Meanwhile, downdrift erosion began near the entrance and gradually expanded towards
downdrift. Part of the eroded material was carried downdrift in the littoral zone but part of
the material was carried offshore to form breaking bars. Figure 34 shows the net sediment
erosive and accretive patterns at the end of the first ebb tidal cycle as compared with the
initial bathymetry.


In the following flood cycle, material from the updrift was pushed into the channel
by the combined strength of current and waves. The small channel shoal initiated during the
preceding ebb cycle was observed to grow both into the inlet and towards offshore. Shoreline
erosion was significant in the immediate updrift and downdrift beaches. On the downdrift
side the rate of littoral drift increased significantly as the flood current not only aided in the
incident wave to the downdrift but also caused more flooding on the dry beach. Figure 35
shows the sediment erosive and accretive patterns during this flood tidal cycle in the
experiment. In the subsequent cycles, one observed the breakup of shoals into what generally
referred to as channel shoals and ebb tidal shoals. The experiment was stopped after another
three complete tidal cycles (480 minutes) as both the channel shoaling and shoreline erosion
became excessively severe. Figure 36 shows a photo of the model topography at the end of
the experimnet. An inlet channel shoaling was clearly noticed. Figure 37 displays the final
shoreline configuration in the experiment. Both the updrift and the downdrift shoreline
erosions were significant with the most severe erosion occurred next to the inlet, and
shoreline erosive patterns were nearly symmetrical with respect to the inlet center.


Figure 38 displays the generation and growth of the ebb tidal shoal in Experiment
Cl using the net +2 cm as the base contour. It is seen that shoaling began at the channel
entrance and grew in both directions towards offshore and into the channel. The offshore
shoaling eventually broke off from the channel shoaling. After 120 minutes, channel shoaling




















Natural Inlet Net Contours After 40min


Figure 34: Accretion and Erosion Pattern during 0-40 minutes in Experiment C1.


Natural Inlet Net Contours 40-80min


0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 35: Accretive and Erosive Patterns during 40-80 minutes in Experiment C1.


500

450

400

I 350

S300

S250

200

150


500

450

400

S350

S300

S250

1200

150



100
50-L
-100


























- jS~


Figure 36: A Photo Showing the Model Topography after 480 minutes in Experiment C1.


n1 i


-20


-40


-60


-80


-100 -


Net Shoreline Changes Of Natural Inlet After 480min


advance


Inlet
4 0


retreat










downdrift direction


200


400 600
longshore distance, cm


800


Figure 37: Shoreline Change after 480 minutes in Experiment C1.


1000


An ..


5*_




















Natural Inlet Ebb Shoal Evolution


0 100 200 300 400 500 600 700 800 900


0 100 200 300 400 500 600
Longshore Distance,cm


700 800 900


Figure 38: Description of Generation and Growth of EbbShoal in Experiment Cl.


500
450
) 400
0
M 350
U)
300 -

S250
0
* 200
0 150.-
100
50
0
-100


500
450
) 400
0
M 350
4-'
300

C 250
C 200
S-I

S150
100
50

0
-10


2

-2 t=80 min




2 2 2


2 Shoren




shoreline


0


-

















Natural Inlet Ebb Shoal Evolution

500

S400- t=120 min
^ ^ -- f! '^' .2--3*
"S +2 '_ '
300 +

0a +
S200 -


01 shoreline

-100 0 100 200 300 400 500 600 700 800 900


a)
S400
C
S300
a-
200

0 0oo


200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 38: Description of Generation and Growth of Ebb Shoal in Experiment Cl
(Continue).



60








And ebb tidal shoal can be separately identified. The ebb tidal shoal began to shift towards
downdrift after this stage. At the end of 480 minutes, the ebb shoal was seen to grow to a
considerable size while the channel shoaling also grew to reconnect with the ebb shoal. A
number of parameters are defined here to quantify the ebb tidal evolution process; they are
the ebb tidal shoal volume, the volumetric centroid and the areal of spreading. The ebb tidal
shoal volume is the volume of the synthesized shoal above the initial profile. The geometrical
centroid is referenced to the Cartesian coordinate system with x- and y-axis coinciding with
the initial shoreline and the updrift jetty, respectively. The areal of spreading is simply the
area above the initial profile. Values of these parameters are given in Table 8 for the natural
inlet experiment.


Table 8: Natural Tidal Inlet Ebb Shoal Characteristics.

Experiment Cl: Natural Tidal Inlet Experiment

Ebb Shoal Characteristics
Model Time
volume Centroid of Volume spreading radius of
(min)
(m3) Xc (m) Y, (m) volume, R, (m)
40 0.0005 1.05 1.35 0.35

80 0.0085 1.05 1.75 0.75

120 0.0312 1.35 2.60 1.35

160 0.0520 1.95 3.05 1.65

480 0.2532 2.95 3.55 2.05


5.3 Ebb tidal Shoal Evolution for Jettied Inlets
Both Experiments C2 and C3 were conducted under storm wave conditions beginning
with an ebb current cycle. The wave incident angle in Experiment C2 was 150 which is twice
as large as in Experiment C3. Experiment C2 has riprap type porous jetties and C3 has
caisson type impervious jetties.








In the initial tidal cycle the general sediment transport patterns in both C2 and C3
were similar. Sand accretions occurred at the tips of both updrift and downdrift jetties. In
subsequent time, the transport patterns became different. In C2, the updrift jetty tended to
attract sediment owing to the structural porosity. Consequently, sediments were heavily
deposited on both sides of the updrift jetty around its tip. In C3, on the other hand, the updrift
sediment began to bypass the jetty and deposited in the channel. On the down drift side of
the inlet beach erosions were severe, particularly in C2 owing to the large incident wave
angle. Bars were formed in both cases due to offshore sediment transport. Figures 39 and 40
show the net contour changes after the first ebb cycle for C2 and C3, respectively. In the
following flood cycle, the porous jetty in C2 attracted more sediment and resulted in
substantial growth of shoaling around the tip of the updrift jetty. In C3, more sediment
bypassed the inlet and was transported downdrift into the littoral zone. Beach erosions were
seen to be more severe than ebb cycle on both sides of the inlet due to higher flood water
level. The sand eroded from the downdrift beach was carried out offshore as well as along
the shore direction. Unlike the natural inlet case, channel shoaling was not severe at this
stage. Figures 41 and 42 show the sediment erosive and accretive patterns during this flood
cycle interval for C2 and C3, respectively.


For C2, the porous jetty case, test was stopped at 1600 minutes when shoaling around
the updrift jetty grew so large that the channel was nearly blocked. This is shown in the net
contour change plot in Fig 43. Owing to this result, the testing wave angle in the subsequent
impervious jetty case, C3, was reduced to half that C2. For C3, the test was stopped at 4860
minutes. Figure 44 shows the net contours change pattern for C3 at 4860 minutes. It was
found that in C3 the bottom changes became less drastic and the development of shoals were
more orderly.


Based on the same procedures described earlier, the shoal evolution sequence can be
constructed for C2 and C3. The results are presented in Figs. 45 and 46 for C2 and C3,
respectively. It is seen that the porous updrift jetty behaved like a magnet that drew large


















Porous Jetty Model Net Contours After 40min


200 300 400 500 600
Longshore Distance


Figure 39: Accretive and Erosive Patterns during 0-40 minutes in Experiment C2.


Impervious Jetty Model Net Contours After 40min


E 400
C

= 300


S200
0 200


Figure 40: Accretive and Erosive Patterns during 0-40 minutes in Experiment C3.

















Porous Jetty Model Net Contours After 80min


Longshore Distance,cm


Figure 41: Accretive and Erosive Patterns during 40-80 minutes in Experiment C2.




Impervious Jetty Model Net Contours 40-80min
600



500 -.-

41 11--
1+
400 --1 1 1



2 300 - -3- .

S\ .- -------- -----
S .. __

100 1 =-- -


-100 0 100 200 300 400 500 600
Longshore Distance,cm


700 800 900


Figure 42: Accretive and Erosive Patterns during 40-80 minutes in Experiment C3.



64


E
Ci.

*; 300


o200.
c ..


I I I I I I I



















Porous Jetty Model Net Contours After 1600min


-7 --, --- ---.-
._._. -- .....

-7-
-. .v .-'- ,-- -* ",' '



200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 43: Accretive and Erosive Patterns during 0-1600 minutes in Experiment C2.


300 400 500 600
Longshore Distance,cm


Figure 44: Accretive and Erosive Patterns during 0-4860 minutes in Experiment C3.


E


jn 300



4 200


--4

---- ---- ------ --

--- ----- -- --- !;g- T--9 -- 7
















Porous Jetty Inlet Ebb Shoal Evolution


0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 45: Description of Generation and Growth of Ebb Shoal in Experiment C2.


0

400 -


. 300


0 200


o 100


0-
-100


















E ""u

400

.) 300

o 200
U)
0 100


Porous Jetty Inlet Ebb Shoal Evolution
I I I I I I I I


I I shoreline
) 0 100 200 300 400 500 600 700 800 900


-100 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 45: Description of Generation and Growth of Ebb Shoal in Experiment C2
(Continue).


t=120 min


2


4? 17

















Porous Jetty Inlet Ebb Shoal Evolution


E 500
E
o

400


O 300


0 200

,-0
S100


E 500
E

S400
U
0
.) 300
0

0 200
MC

o 100


300 400 500 600
Longshore Distance,cm


Figure 45: Description of Generation and Growth of Ebb Shoal in Experiment C2
(Continue).

















Porous Jetty Inlet Ebb Shoal Evolution


S500
40 0
( 400


U) 300


0 200
U)

0 100


0
-100







500-


S400
O
cc
"f 300


0200


0 100


0
-100


0 100 200 300 400 500 600 700 800 900


0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 45: Description of Generation and Growth of Ebb Shoal in Experiment C2
(Continue).













Impervious Jetty Inlet Ebb Shoal Evolution


200 300 400 500 600
Longshore Distance,cm


700 800 900


Figure 46: Description of Generation and Growth of Ebb Shoal in Experiment C3.


500


E
0
~ 400
o

t,
300

0
200
0


100


t=80 min


2 2


2 shoreline




shoreline


0 100


UJ\\










Impervious Jetty Inlet Ebb Shoal Evolution


-100 0 10 I2 I I I I I I
-100 0 100 200 300 400 500 600 700 800 900


E
0
~ 400


300

0
" 200
0


-100 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 46: Description of Generation and Growth of Ebb Shoal in Experiment C3
(Continue).














Impervious Jetty Inlet Ebb Shoal Evolution


600


500

E
- 400

u
S300

0
a-
S200
0

100


0
-100


600


500

E
o
6 400


300

0
200
0
100


0 100 200 300 400 500 600 700 800 900


300 400 500 600
Longshore Distance,cm


Figure 46: Description of Generation and Growth of Ebb Shoal in Experiment C3
(Continue).











Impervious Jetty Inlet Ebb Shoal Evolution


01 M I M I i i__
-100 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm


Figure 46: Description of Generation and Growth of Ebb Shoal in Experiment C3
(Continue).












Impervious Jetty Inlet Ebb Shoal Evolution
600 1 I


500 ^ 6 8 i


6 400


300
S300 t=3200 min

0O
S200
0
10 shoreline
100



-100 0 100 200 300 400 500 600 700 800 900





600


500 -


S400


300
t=4860 min
0
S200


100 - shoreline



-100 0 100 200 300 400 500 600 700 800 900
Longshore Distance,cm




Figure 46: Description of Generation and Growth of Ebb Shoal in Experiment C3
(Continue).










amount of sediment deposition. The rate of channel shoal was considerably slower than the
natural inlet case. However, as time progressed, the channel was eventually blocked when
shoaling from the updrift jetty expanded towards downdrift. The formation of ebb tidal shoal
was not evident until a later stage (outlined at 200 and 480 minutes). From the locations of
the ebb tidal shoal and the sequence of topographic changes, it is hard to judge how the ebb
tidal shoal was formed, whether it was directly from updrift bypassing, or from offshore
transport of nearshore material or from growth of downdrift accretion.


In Experiment C3, updrift sediment tended to bypass the imperious updrift jetty
instead of accumulating around the tip. The bypassed material first formed channel shoals
which eventually moved further downdrift and drew into a substantial ebb shoal. The ebb
shoal was a large elongated body of sand somewhat resembled to that observed in nature
downdrift of improved inlets.


The ebb tidal shoal volume, the volumetric centroid and the areal of spreading of the
ebb shoal were also calculated for C2 and C3. The results were given in Table 9. The ebb
tidal shoal volume changes as well as the areal of spreading for Cl, C2, and C3 are plotted
in Fig.47. In the early stage, the growth of ebb tidal shoal was unsteady for all the
experimental cases; first grew during ebb cycles but shrunk during flood cycles. After the
first few cycles, the ebb tidal shoal grew steadily, almost in a linear fashion. The process was
much rapid for the natural inlet case than the jettied case. In the case of porous jetties, the
smallest ebb shoal as the material was largely accumulated near the updrift jetty. The
impervious jetty experiment had the longest test time. The rate of growth apparently slowed
down at a later stage.


5.4 Ebb Tidal Shoal Dynamics
With the aid of the shoal evolution plots together with the sediment flux program
presented in Section 4.2, one is able to shed some light on the dynamic process of ebb tidal








Table 9: Jettied Inlet Ebb Shoal Characteristics.

Experiment C2: Porous Jettied Inlet Experiment

Ebb Shoal Characteristics
Model Time
volume Centroid's of A volume spreading radius of
(min)
(m3) XC (m) Y (m) volume, Rc (m)

40 0.0010 2.10 2.35 0.25

80 0.0029 2.10 2.50 0.55

120 0.0060 2.15 2.50 0.75

160 0.0050 2.10 2.55 0.60

200 0.0210 3.70 3.45 1.05

480 0.0329 4.25. 3.55 1.85

1120 0.0483 4.75 3.75 2.45

1600 0.0693 5.95 4.25 2.95

Experiment C3: Impervious Jettied Inlet Experiment

Ebb Shoal Characteristics
Model Time
volume Centroid of Volume spreading radius of
(min)
(m3) X(c m), Yc (m) volume, R, (m)

40 0.0092 2.00 2.75 0.45

80 0.0005 2.40 3.00 0.10

120 0.0130 1.00 3.00 0.95

160 0.0077 1.00 3.10 0.75
480 0.0597 1.05 3.50 1.95

1120 0.0942 1.10 4.35 2.05

1600 0.1709 2.55 4.50 2.30

2240 0.2346 2.95 4.75 2.95

3200 0.3908 4.05 5.05 3.95












Inlet Model Ebb Tidal Shoal Evolution


< 0.4
E

_ 0.3
> Natural Inlet Model Cas

Z 0.2


0.1
Porous Jetty Inlet Model Case 2

0 _I I II I IL I I I
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
model time,min




Figure 47: Accretive Change of Ebb tidal Shoal Volumes in Experiments C1, C2, and
C3.



shoal evolution. Experiments of natural inlet and the non-porous jetty case are used here for

discussions. The objective is to determine the source origins and the flux rate during the ebb

tidal shoal evolution process. The basic procedures used consist of the following two steps.

Step 1: Delineate shoal regions. The regions and grids of ebb tidal shoal and channel shoal

are constructed based on the results given in Section 5.3.

Step 2: Prepare a sediment flux diagram: The sediment flux program is applied to regions

of separate shoals to construct a sediment flux diagram.



The case of the natural inlet experiment is discussed first with a few representative

flux patterns. Figure 48 shows the sediment flux pattern around the initial shoaling region

just outside entrance. As can be seen this initial shoaling was formed by offshore sediment













Natural Inlet Model Net Contour After 40 min


E
0
400




0
= 200
u)
0
0


0 100 200 300 400 500 600 700 800 900


.ou
260

E 240
220

0
S200

2 180
160

o 140

^ 120
0


sediment transport flux pattern for the ebbshoal area
I I I I






S---









2
20cm /min
I I I II


longshore distance, cm


Figure 48: Sediment Flux Pattern in the Initial Shoaling Region in Experiment Cl
(0-40 minutes).








flux instead of directly from the updrift transport. In the subsequent flood tidal cycle, the
shoal growth was clearly diffused (Fig.49). In the ebb tidal shoal region, longshore drift
reversal was detected from the flux pattern. To examine the overall shoal growth patterns for
the duration of the experiment, flux patterns from two regions were constructed, one in the
channel region and one in the offshore regions on the downdrift side. The flux pattern for
the channel shoal is shown in Fig. 50. It appeared that there was a significant net onshore
component contributing to the shoal growth even though the original sand source might still
be from the updrift sediment. The longshore component was much larger but its net
contribution to shoal growth might not be more important than the on/offshore component.
The shoal growth in the offshore region, as shown in Fig.51, was clearly due to the material
from the nearshore region. It is suspected that it was simply a part of an over grown offshore
bar in the laboratory and was not an ebb tidal shoal in the true sense.


In the case of impervious jetty inlet experiment, both longshore and offshore
components contributed to the initial accretion at the jetty's tips during ebb tidal cycle as
shown in Fig. 52. It is seen that contributions came from various components in this
development stage. In the subsequent flood tidal cycle, the accretion at the updrift jetty
simply diffused (Fig. 53) similar to that in the natural inlet case. Sediment motion was more
active near the downdrift jetty. And the flux pattern was almost opposite to that obtained
during the ebb cycle and, again, caused shoaling diffusion. In these initial cycles sediment
transport was very active as can be judged by the magnitude of the mean flux values.


As discussed earlier that the topographic response in the experiment can be roughly
divided into an initial adjustment and response stage followed by a steady development stage.
Figure 54 displays the net pattern in Experiment C3 from 0 to 480 minutes which roughly
represents the initial stage. A clearly identifiable shoal was developed in this stage. The
supplies to the shoal region were from three main sources: updrift longshore influx, updrift












Natural Inlet Model Net Contour 40-80 min
600 i I I


500 -


400











0 100 200 300 400 500 600 700 800 900




sediment transport flux pattern for the ebbshoal area
450

400 -




250
100
0 t






450


0 35 I 0











250 _-_50c_/_min
-100 -50 0 50 100 150 200
longshore distance, cm




Figure 49: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C1
(40-80 minutes).













Natural Inlet Model Net Contour After 480 min


E
U
400


2 300

L_
0
'I-

200
U)
0
100


0







400


sediment transport flux pattern for the ebbshoal area


0 50 100 150 200 250 300
longshore distance, cm


Figure 50: Overall Sediment Flux Pattern in Channel Shoaling Region in
Experiment Cl (0-480 minutes).


0 100 200 300 400 500 600 700 800 900


350
E
300

C 250

200

150
O

r 100
0


10c/m I
10cn/min
I I I I I I I












Natural Inlet Model Net Contour After 480 min


0 100 200 300 400 500 600 700 800 900


sediment transport flux pattern for the ebbshoal area


500 F


E
450
0
5 400
5o
S350
0)
' 300

* 250
0


200 1


100 150 200 250 300 350 400 450 500
longshore distance, cm


Figure 51: Overall Sediment Flux Pattern in Offshore Shoaling Region in Experiment
Cl (0-480 minutes).


-


F


I-


F


5cm min
I I


I











Impervious Jetty Model Net Contour After 40 min


E
0
S400



V)
300


200-


100


0-
-








500

E 450
U
U)400

S350 -
:5
S300
0
U) 250
0
200


sediment transport flux pattern for the ebbshoal area
I I I I I I I I I


150 I


-150 -100


-50 0 50 100 150 200 250 300
longshore distance, cm


Figure 52: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C3
(0-40 minutes).


-100 0 100 200 300 400 500 600 700 800 900


50c----I
50cr /min
: i l i


l i s t


-











Impervious Jetty Model Net Contour 40-80 min


600


500
E
0
S400
U

(0
-0 300


0
200

100


0






500

E 450
U
6 400




0
B 350



C250

0 on2


sediment transport flux pattern for the ebbshoal area


150-" 'r "" .
-150 -100 -50 0 50 100 150 200 250
longshore distance, cm




Figure 53: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C3
(40-80 minutes).


-100 0 100 200 300 400 500 600 700 800 900


L


vuWI











Impervious Jetty Model Net Contour After 480 min


500
E

0400
C

S300


0
= 200
U=


0 I' I I I II
-100 0 100 200 300 400 500 600 700 800 900


sediment transport flux pattern for the ebbshoal area


450
E
0 400

C 350
CA
300 -
1.
O 250
CD
= 200
0


150 F


-100 -50 0 50 100 150 200 250 300
longshore distance, cm


Figure 54: Sediment Flux Pattern in the Initial Shoaling Region in Experiment C3
(0-480 minutes).


10cm$min








offshore influx and influx from offshore in the channel confinement. Drift reversal at the
downdrift end also contributed to the region but was mainly compensated by the loss into the
channel.The magnitude of the mean flux values was much smaller than those in the initial
cycles. In the following development stage the shoal formed in the initial stage began to grow
into two directions, one into the channel and the other towards downdrift offshore. Two shoal
regions were, therefore, delineated to examine the growth pattern for the duration from 480
to 1600 minutes. The computed net sediment flux pattern during this period for the channel
shoal region is given Fig.55. In this case, clearly the main contribution was from influx into
the channel which in a way provides the validation of the EEF method. The results for the
offshore shoal region are shown in Fig. 56. In this region, longshore influx almost balanced
by the outflows. The shoal growth appeared to be due to offshore influx from the channel
region and material supplied from downdrift nearshore zone. The magnitude of the mean flux
for the channel shoal growth was about one fourth of that for the offshore shoal indicating
that the channel shoal became matured much earlier.


The test results can be extrapolated to prototype values based on the proposed scaling
law. Result from C3 is used here for illustrative purposes. Values based on two horizontal
scales, 60 and 80, were computed and tabulated in Table 10. By comaring with the values
of ebbshoal volume in Fig.1, it is clear that the model tests have not reached the final shoal
development stage.


Table 10: Comparison of Model and Prototype Scales for Experiment C3.

Prototype to Model Inlet Wave Wave Tidal Ebb Shoal
Scale Ratio Width Height Period Prism Volume

N N N6 NH NT Nw (m) (cm) (sec) (m3) (m3)

1 1 1 1 1 1.7 8 1.0 98.4 0.5

60 35 20 7.7 2 102.0 163 7.7 1.24x107 63,000

80 44 24 8.9 2 136.0 193 8.9 2.77x107 140,800













Impervious Jetty Model Net Contour 480-1600 min
600] -------------------i-------------i--
600 I .



500
400

O
U) 300 -


S200-


100 -



0 100 200 300 400 500 600 700 800 900




sediment transport flux pattern for the ebbshoal area
350







0 100






1 cm /min
0 50 100 150 200
200










longshore distance, cm




Figure 55: Sediment Flux Pattern in Channel Shoaling Region in Experiment C3
(480-1600 minutes).
U)

100 2
1 cm /min

50L
0 50 100 150 200
longshore distance, cm




Figure 55: Sediment Flux Pattern in Channel Shoaling Region in Experiment C3
(480-1600 minutes).













Impervious Jetty Model Net Contour 480-1600 min


0 100 200 300 400 500 600 700 800 900


sediment transport flux pattern for the ebbshoal area


200 1-I I II I
0 100 200 300 400 500
longshore distance, cm


Figure 56: Sediment Flux Pattern in Offshore Shoaling Region in Experiment C3
(480-1600 minutes).


600


500
E

400
(u
. 300

0
= 200
U)
0
100


0


450

400

350

300

250


I 2 I
2
4cm /min










6. SUMMARY AND CONCLUSIONS


Laboratory model experiments were carried out in the present study aimed at
improving the fundamental understanding on ebb tidal shoal dynamics. The results from the
experiments are also useful for prototype applications through rational scaling laws. The
experiments consisted of two parts. The first part was for establishing modeling laws and
was conducted with a plane beach. The second part was for studying ebb shoal evolution
process and was carried out with three different inlet configurations, a natural inlet, an inlet
with porous jetties and an inlet with impervious jetties. All cases were tested under storm
wave conditions only.


In the first part, four different modeling laws as proposed by Vellinga (1982), Hughes
(1983), Wang, et al. (1990), and Wang, et al. (1994) were evaluated based on a series of 2-D
wave tank and 3-D wave basin tests on beach profile responses at different geometrical
scales. The experimental results were compared with data from a prototype scale experiment
performed in German Large Wave Tank. The modeling laws were then evaluated based on
five different criteria: (1) dune erosion volume, (2) nearshore profile, (3) nearshore bar
volume, (4) bar crest location, and (5) bar geometrical location. The results of evaluation
indicate that the modeling law proposed by Wang, et al. (1994) yields the best performance.
This modeling law is adopted for the inlet experiment. Detail of this part is reported
separately (Wang, et al., 1994).


The second part of the inlet experiments was all conducted in a 3-D wave basin with
an approximate dimension of 28 m x 28 m x lm. The inlet model consists of a single
idealized inlet on a simple beach configuration with straight and parallel bottom contours.
The idealized inlet has a rectangular cross section a straight channel normal to shoreline. In
all cases, the model was run with alternate ebb and flood tidal conditions. Each ebb and
flood tidal phases comprised a duration of 40 minutes in the experiment. Incident waves








were 8 cm in height and 1 second in period for all cases. The wave direction for Cases 1 and
2 was 150 but was reduced to 7.50 for Case 3.


For the natural inlet case, the inlet was clearly unstable under the test condition as
severe inlet shoaling and shoreline erosion occurred and test had to be terminated only after
a few tidal cycles. The presence of jetties significantly retarded the growth of shoaling and
pushed shoals further offshore as expected. Consequently the experiment can be continued
for much longer duration, particularly for the imperious jetty case. The experiment finally
had to be stopped when shoals began to move out of the test region.


The ebb shoal evolution processes were documented. A new method based on
empirical Eigen function analysis was developed to analyze sediment flux patterns. This
enables us to shed light on the dynamics of ebb tidal shoal evolution and the associated
nearshore sediment transport process. Specific findings of the present studies are summarized
in the followings:
(1) The laboratory model experiments of beach profile responses showed that the
results from the 3-D wave basin test are consistent with those from the 2-D wave tank tests
in simulating a near-prototype large scale laboratory test conducted in the Big German Tank
Experiment (Dette and Uliczka, 1986a,b). This formed the basis for applying the profile
scaling laws developed mainly from 2-D experimental results to the 3-D wave basin tests.
The modeling law is, however, rather restricted only suitable for erosive conditions under
storm waves.
(2) The longshore transport rate measured in the plain beach experiment under the
condition with storm waves attacking shoreline in an oblique angle is found to be about 30
percent of the value computed from the SPM formula (U.S. Army Corps of Engineers, 1984).
The smaller longshore transport rate obtained in the present model experiment, however,
appeared to be more consistent with other existing laboratory experimental results and was
also close to the values estimated along the east coast of Florida.








(3) Nature inlet experiment was stopped for only a short test run because of
significant shoreline erosion and inlet channel shoaling under the strong storm wave
conditions. It is evident that for tidal inlets in strong wave environment, jetty structures are
necessary.
(4) The experimental results showed that porous jetty attracts sediment deposition
whereas impervious jetty causes more bypassing.
(5) The formation and growth of ebb tidal shoal were observed in all the inlet
experiments. The location and rate of growth were different. For the natural inlet case, ebb
shoal was very close to the inlet entrance and the rate of growth was the fastest. In the
present study, the rate of growth for the natural inlet case was more than twice as much as
the cases with jetties. For porous jetty case, the jetty structure behaved like a sink that attracts
sand accumulation. Ebb shoal initiated on the down drift side with sand supplied from
downdrift beach. The growth of ebb shoal was the slowest. Impervious jetty caused updrift
sand to bypass the updrift jetty. Both updrift and downdrift sand sources contributed to the
formation and growth of ebb tidal shoal.
(6) A new method of establishing sediment flux patterns based on measured
topographic changes was developed. The method is based on Empirical Eigen Function
(EEF) analysis. Since there is insufficient information to uniquely determine the flux pattern,
the EEF provides the best estimate in the sense of least square error. Upon testing a number
of cases, the flux patterns as calculated appeared to be consistent with observation. One must
be cautioned, however, that although the solution by EEF under the assumed boundary
conditions is unique it is not the only solution to produce the required bottom changes.
(7) The EEF method enables us to determine the sediment transport patterns in ebb
tidal shoal development thus aids in the insight on the dynamic process. Generally speaking,
ebb current creates ebb shoal whereas flood current destroys it. The cumulative effect, on the
other hand, is the continued growth of ebb shoal. The flux contributing to ebb tidal shoal
came from three directions: updrift influx, downdrift offshore transport and onshore transport
in the zone influenced by the inlet.








In conclusion, the present study represents an exploratory laboratory experiment to
apply movable model to examine inlet evolution. The experiment was partially successful
in that ebb tidal shoals similar to that observed in nature can be produced in the laboratory.
The test wave conditions were by no means realistic. In addition, the modeling law is very
restrictive. Based on the experimental results, we gained certain fundamental knowledge on
ebb tidal shoal development forms and shapes of growth, the effects of structures and the
sand sources and flux patterns. The EEF is a promising technique for sediment flux analysis
with incomplete or insufficient information.


The issue of movable bed experiment remains a difficult one and the process of inlet
ebb tidal shoal is enormously complex. The present study demonstrated the feasibility and
usefulness of such type of experiment as well as provided frame work for future studies of
similar kind. Work is continuing with refined scopes.




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