• TABLE OF CONTENTS
HIDE
 Front Cover
 Report documentation page
 Preface
 Contents
 List of figures
 Introduction
 Model characteristics
 Experiments
 Experimental results
 Performance evaluations of structural...
 Sediment budget analysis
 Summary and recommendations
 References
 Appendix 1: Summary of bathymetric...
 Appendix II: Summary of bathymetric...
 Appendix III: Summary of bathymetric...
 Appendix IV: Summary of bathymetric...
 Appendix V: Summary of southside...
 Appendix VI: Histogram presentation...






Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 92/006
Title: Sebastian Inlet physical model studies: Final report - moveable bed model
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 Material Information
Title: Sebastian Inlet physical model studies: Final report - moveable bed model
Series Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 92/006
Physical Description: Book
Creator: Wang, Hsiang
Lin, Lihwa
Miao, Gang
Publisher: Coastal & Oceanographic Engineering Department, University of Florida
Publication Date: 1992
 Subjects
Subject: Sebastian Inlet (Fla.)
Sediment transport
Erosion
Spatial Coverage: Sebastian Inlet
 Notes
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: UF00080461
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

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Table of Contents
    Front Cover
        Front Cover
    Report documentation page
        Unnumbered ( 2 )
    Preface
        Unnumbered ( 3 )
    Contents
        Page i
        Page ii
        Page iii
    List of figures
        Page iv
        Page v
        Page vi
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
    Model characteristics
        Page 6
        Page 5
    Experiments
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
    Experimental results
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 12
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
    Performance evaluations of structural alternatives
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
    Sediment budget analysis
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
    Summary and recommendations
        Page 63
        Page 64
        Page 65
        Page 66
        Page 62
    References
        Page 67
    Appendix 1: Summary of bathymetric change figures for S1 to S8 in 6-Day NE Storm Process
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
    Appendix II: Summary of bathymetric change figures for S1 to S8 in 8-Day E recovery process
        Page 73
        Page 74
        Page 75
        Page 76
    Appendix III: Summary of bathymetric change figures for S2, S4, S6 in 8-Day NE moderate wave process
        Page 77
        Page 78
        Page 79
    Appendix IV: Summary of bathymetric change figures for Se and S5 in 6-Day SE storm process
        Page 80
        Page 81
    Appendix V: Summary of southside profile changes for S1 to S8 in 6-Day NE storm process
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
    Appendix VI: Histogram presentation of sand budget based on laboratory experiment results
        Page 98
        Page 99
        Page 100
Full Text



UFL/COEL-92/006


Sebastian Inlet Physical Model Studies
Final Report Movable Bed Model







by

Hsiang Wang
Lihwa Lin
Gang Miao


May, 1992




Submitted to:

Sebastian Inlet District Commission
Sebastian Inlet, Florida.





REPORT DOCUMENTATION PAGE
1. report No. 2. 3. Recip t'i Acuestioa no.


4. Title and SubCtile 5. Report Date
SEBASTIAN INLET PHYSICAL MODEL STUDIES May 10, 1992
Final Report -- Movable Bed Model 6.

7. Author(s) 8. Performing Oranization Report No.
Hsiang Wang, Lihwa Lin, Gang Miao UFL/COEL-92/006

9. Perform og Organiatioo Mame and Addresl 10. project/Task/nork Unit Io.
Coastal and Oceanographic Engineering Department
University of Florida 1. Contract or -rant so.
336 Weil Hall
Gainesville, F1.32611
13. Ty~p of Report
12. Sponsoring Organization Name and Addres Final
Sebastian Inlet District Commission
Sebastian Inlet Tax District Office
134 Fifth Avenue
Suite 103, Indialantic, FL 32903-3164 14.
15. Supplmentary Notes



16. Abstract
A movable bed model study was conducted to investigate nine structural configurations
to assess the sediment transport process in the vicinity of the Sebastian Inlet.
The model has a vertical to horizontal scale distortion of 2 to 3. A horizontal
scale of 1 to 60 was selected to cover approximately 2,500 ft of shoreline. The
vertical scale is 1 to 40. Natural beach sand of finer grain than the native sand
at the Sebastian Inlet was used to fulfill the scale requirement.


The nine tested structural configurations constitute various
extension, ebb shoal material removal and beach nourishment.
consisted of 6-day(prototype equivalent) storm waves from NE
waves from E and 8-day normal waves from NE direction.


combinations of jetty
The test conditions
and SE; 8-day recovery


The shoreline immediately south of south jetty (within 2000 ft south) suffers
recession for all tested configurations under storm waves from NE and SE. The
recovery is difficult in the vicinity of the south jetty. The ultimate shoreline
position south of the south jetty appears to be dictated by the length and
configuration of the south jetty. Significant bathymetric change also occurs in
the nearshore zone on the south side of the south jetty, which is due to strong
alongshore current.

Ebbshoal removal, extension of south jetty and beach nourishment all induce increased
rate of downdrift transport. The last case is seen to have the mild shore pro-innr
17. Originator's Key ords 18. Availability Sateent
Erosion
Sediment transport
Shore erosion
Structural alternative

19. U. S. Security Classif. of the Report 20. U. S. Security Classif. of This Pae 21. No. of Pages 22. Price
Unclassified Unclassified 108












PREFACE


This report presents results of the experiments of the existing inlet and eight
structural alternatives to the Sebastian Inlet from a movable bed model. It is intended
to find solutions for improvement of boating safety and protection of beaches adjacent
to the inlet. Based upon the experimental results from here and the fixed bed model
study, which is summarized in Part I report, an optimum structural modification plan
was then recommended providing a general frame of improvement scheme.

The research in this report was authorized by the Sebastian Inlet District Com-
mission of September 15, 1989. The University of Florida was notified to proceed
on November 14, 1989. The study and report were prepared by the Department of
Coastal and Oceanographic Engineering, University of Florida. Coastal Technology
Corporation was the technical monitor representing the Sebastian Inlet District.

Special appreciation is due to Mr. Michael Walther of Coastal Tech. for his
continuous technical assistance. Other personnel at Coastal Tech. and Inlet District
Office including Dr. Paul Lin, Ms. Kathy FitzPatrick and Mr. Raymond K. LeRoux
also provided their support at various stages of the experiment. Appreciation is
also due to Mr. T. Kim and Mr. J. Lee, both graduate assistants in the Coastal
Engineering Department, University of Florida, for their participation in laboratory
and field experiments.








Contents


1 Introduction

1.1 Authorization ..............................

1.2 Purpose . . . . . . . . .

1.3 Background ...............................

1.4 Scope . . . . . . . . .


2 Model Characteristics

2.1 Modeling Laws and Model Scales . . . . .

2.2 Model Construction . .. . .. .. . .


3 Experiments

3.1 Test Program . . . . . . . .

3.2 Test Procedures . . . . . . . .


4 Experimental Results

4.1 General Observations . . . .

4.2 Shoreline Response ........ . ...

4.3 Bathymetric Changes and Erosional Patterns

4.4 Ebb Shoal Movement . . . .

4.5 Sediment Balance Computations . .


5 Performance Evaluations of Structural Alternatives

5.1 Channel Shoaling and Sand Losses to the Inlet . . .

5.2 Downdrift Transport .........................

i









Volume Changes in Southside Nearshore Zone . . .

Ebb Shoal Volume Changes ......................

Updrift Volume Changes ..... ............. ......

Summary of Performance Evaluation . . . . .


6 Sediment Budget Analysis

6.1 Historical Shoreline Changes . . . .

6.2 Background Littoral Drift Environment . .

6.3 Ebb Shoal Volume ...................

6.4 Interpretation from Laboratory Results . .

6.5 Sediment Deficit Estimation . . . .

6.6 Sediment Budget ....................


7 Summary and Recommendations

7.1 Summary ........................

7.2 Recommendations .................... .


References


46

. . 46

. . 50

. . 55

. . 57

. . 59

. . 60


62

. . 62

. . 64


67


I Summary of Bathymetric Change Figures for S1 to S8 in 6-Day
NE Storm Process


II Summary of Bathymetric Change Figures for S1 to S6 in 8-Day
E Recovery Process


IIISummary of Bathymetric Change Figures for S2, S4, S6 in 8-Day
NE Moderate Wave Process




ii








IV Summary of Bathymetric Change Figures for S3 and S5 in 6-Day
SE Storm Process 80


V Summary of Southside Profile Changes for S1 to S8 in 6-Day NE
Storm Process 82


VI Histogram Presentation of Sand Budget Based on Laboratory Ex-
periment Results 98








List of Figures


1 Location of Sebastian Inlet, Florida. . . . . 3

2 Jetty configuration and shoreline changes since 1881. . . 4

3 A sketch of the movable bed model; short-dashed lines showing lo-
cations for monitoring the bottom profiles. . . . 7

4 Nine tested structural configurations. . . . . 9

5 Criterion of normal and storm profiles. . . .... 13

6 Locations and labels of bottom profiles surveyed in the model. 14

7 Initial profiles prepared in the model. . . . ... 14

8 Profile changes for SO under 6-day NE and SE storm processes. .. 16

9 Profile changes for SO in recovery and moderate wave processes. .17

10 Bathymetric changes for SO under 6-day NE and SE storm process. 18

11 Bathymetric changes for SO in recovery and moderate wave processes. 19

12 Shoreline changes in 6-day NE storm for Category 1,2,3 structures. 22

13 Shoreline changes in 6-day NE storm for Category 4 structures. 23

14 Shoreline changes in 8-day recovery condition. . . ... 25

15 Shoreline changes in 8-day NE moderate wave process. ...... ..25

16 Shoreline changes in 6-day SE storm process. . . ... 26

17 Sediment accretion and erosion pattern during flood. . ... 29

18 Sediment accretion and erosion pattern during ebb. . ... 29

19 Erosion pattern after 6-day storm for SO. . . . ... 30

20 Comparisons of surveyed profiles from J12 to J18 for SO. . 31

21 Comparisons of surveyed profiles from J20 to J30 for SO. . 32

iv








22 Five regional zones for sediment balance computations. ...... ..34

23 Sand budget in ft3/day (103yd3/day in prototype) from 6-day NE
storm experiment. .......................... .. 35

24 Sand budget in ft3/day (103yd3/day in prototype) from 8-day E
recovery experiment. .......................... 37

25 Sand budget in ft3/day (103yd3/day in prototype) from 8-day NE
normal wave experiment ......................... 38

26 Sand budget in ft3/day (103yd3/day in prototype) from 6-day SE
storm experiment. ........................... 39

27 Downdrift erosion history from storm and recovery processes . 43

28 A sketch of sand budget control box near inlet. . . .... 47

29 Shoreline change rates from 1929 to 1986. . . ... 49

30 Bathymetric Survey map for 1989. .. . . . ... 56

31 Contours of ebbshoal volume computed from 1989 survey. . 57

32 Orthographic plots of ebb shoal volume from 1989 survey. ..... 58

33 An estimate of annual sediment budget. . . ... 61








List of Tables


1 The input wave conditions ....................... .10

2 Testing program and experimental conditions. . . ... 11

3 Averaged shoreline changes(ft) in 6-day NE storm process. . 26

4 Averaged shoreline changes(ft) in 8-day E recovery wave process. .. 27

5 Averaged shoreline changes(ft) in 8-day NE normal wave process. 27

6 Averaged shoreline changes(ft) in 6-day SE storm process. . 27

7 Rate of volumetric change (103yd3/day) in 6-day NE storm process. 36

8 Adjusting factors for updrift volume changes in NE storm test ... 36

9 Comparison of volume change rates in 6-day NE storm process. 36

10 Volume change rate (103yd3/day) in 8-day E recovery wave process. 40

11 Volume change rate (103yd3/day) in 8-day NE normal wave process. 40

12 Volume change rate (103yd3/day) in 6-day SE storm process. . 40

13 Comparison of CDN and WIS wave data. . . .... 51

14 Estimated longshore transport (yd3/year) from 1956 to 1975. . 53

15 Monthly percentage of longshore transport direction from 1956 to
1975.................................. 54

16 Statistics of longshore transport rate (yd3/day) from 1956 to 1975. 54

17 Ebb shore volume computed for 1987 to 1989. . . ... 56








Sebastian Inlet Physical Model Studies
Final Report Movable Bed Model



1 Introduction


1.1 Authorization

This study and report were authorized by the Sebastian Inlet District Commis-
sion of September 15, 1989. On November 14, 1989, the "University of Florida"
was notified to proceed. This report was prepared by the Department of Coastal
and Oceanographic Engineering, University of Florida. Coastal Technology Cor-
poration was the technical monitor representing the Sebastian Inlet District.

On May 23, 1919, the original legislation establishing the Sebastian Inlet Dis-
trict (District) was passed by the State of Florida. In 1927 the Florida Legislature
passed Chapter 12259, Laws of Florida, which amend the original governing legis-
lation of the District. Chapter 12259 prescribes that "It shall be the duty of said
Board of Commissioners of Sebastian Inlet District to construct, improve, widen or
deepen, and maintain an inlet between the Indian River and the Atlantic Ocean..."
[1].


1.2 Purpose

The main objective of the study aims to find solutions improving the inlet
navigation as well as beach preservation at the Sebastian Inlet through physical
model experiments. The study was concentrated in testing the hydrodynamic and
littoral sediment drift properties for the existing inlet and several other structure
configurations, including modification of the jetties and excavation of the ebb shoal.

Two physical models were conducted in the study: a fixed bed model for the
hydrodynamic study and a movable bed model for the littoral sand process study.
The results of the fixed-bed model study were summarized in the Part I report. The
preliminary results of the movable-bed model experiment for the existing inlet and
two other alternative structural configurations, which modify the existing jetties,
were summarized in the Part II report. The final results of the complete movable-
bed model study, which includes testing another six structure alternatives, were
summarized in this report.








1.3 Background


Sebastian Inlet is located at the Brevard/Indian River County line approx-
imately 45 miles south of Port Canaveral entrance and 23 miles north of Fort
Pierce Inlet. It is a man-made cut connecting the Atlantic Ocean to the Indian
River Lagoon (Figure 1). Its coordinates are as follows:


Latitude Longitude
270 51' 35" N 800 26' 45" W


The First attempt to cut a man-made inlet in the Sebastian area was made in
1886 [2]. In the ensuing 60 years or so, the inlet closed, re-opened and shifted a
number of times. The present configuration was maintained after a major dredging
operation in 1947-48 to open a new channel. Since 1948, a series of dredging opera-
tions and jetty improvements have kept the inlet open in this existing configuration
[3]. In 1962, a channel of 11 ft deep was excavated.

In 1965, the A1A bridge across the inlet was completed (State Project Number
88070-3501) and navigation guides were installed in the open section under the
bridge which forms a natural throat of the inlet.

East of the bridge the dredged channel width was 200 ft and west of the bridge
the width was 150 ft. In 1970, the north and south jetties were extended to their
present configuration as shown in Figure 2, based on the results of a model study by
the Department of Coastal and Oceanographic Engineering, University of Florida
[4]. The present south jetty is a sand-tight rubble mound structure. The north
jetty, on the other hand, is of composite nature; the original section completed
before 1955 is rubble mound but the extension in 1970 with total length of 452
ft is a pier structure supported by concrete pilings. The rubble mound base only
extends to the mean sea level.

The channel has a rocky bottom of marine origin. The cross section in the
vicinity of the throat is about one-half that which would result in a stable inlet
with sandy bottom. In other words, the tidal prism is about twice the value
corresponding to the cross section. This has resulted in rather strong currents
through the inlet, over 8 ft/sec during both flood and ebb. So far, the channel
remains open with minimal maintenance dredging. Shoals were, however, gradually
forming on both sides along the banks of the inlet. The navigation channel becomes
narrower as a consequence. The ebb shoal from the south is also slowly encroaching
into the inlet creating a cross shoal near the mouth. This shoal enhances the
incoming waves and causes them to break. These combined effects have created a
hazardous condition for small craft in the vicinity of the inlet entrance.



































WATERSHED OF THE
INDIAN RIVER LAGOON.


Figure 1: Location of Sebastian Inlet, Florida.

















ATLANTIC OCEAN


OLD JETTIES (1924)
JETTY EXTENSION(1955)
0 500' ,.r RIPRAP (1959,1972)
SScale in: Feet t JETTY EXTENSIONS(1970)

Figure 2: Jetty configuration and shoreline changes since 1881.

In 1987, Coastal Technology Corporation carried out a "Comprehensive Man-
agement Plan" study for the Sebastian Inlet District Commission [5]. In which,
various engineering alternatives for maintenance and improvement of inlet naviga-
tion and beach preservation were presented. The present study is to evaluate these
alternatives through physical model experiments.



1.4 Scope


The general purpose of this study is to conduct physical model investigation
for inlet navigation improvement and sand transfer schemes; the former is a fixed-
bed model study and the latter a movable-bed model study. The fixed-bed model
experiments were completed in November, 1990, and the results were summarized
in the Part I Report. The movable bed model was first tested for the existing inlet
structure and two modified jetty configurations subject to the northeaster storm
waves and the results were summarized in the Part II report. The model was
then tested for another six structure alternatives subject to storm, normal and
swell-dominated waves. The final results for the completed movable-bed model
experiment are presented in this report.

The movable bed model was conducted in the three dimensional wave basin
at the Coastal and Oceanographic Engineering Laboratory, University of Florida.








This model study includes testing the existing inlet and eight other structure al-
ternatives. Through the model study, the littoral transport, onshore and offshore
sand transport, shoreline changes, entrapment of sand by the inlet and ebb shoal
system, etc., for the different structure alternatives were evaluated. The informa-
tion is then synthesized to estimate the sediment balance as well as the sediment
deficit of the inlet-beach system. From this analysis, the effects of different struc-
ture alternatives are evaluated.

In addition to the movable bed model study, a Sediment budget analysis was
perform based upon the combined information from historical data, hindcasting
model and the evidence produced in the movable model.



2 Model Characteristics


The movable bed model was constructed at the Coastal and Oceanographic
Engineering Laboratory, University of Florida, for the purpose testing the littoral
and ebbshoal sand drift processes at the Sebastian Inlet due to different alternative
structural configurations.


2.1 Modeling Laws and Model Scales


The movable bed modeling laws utilized here were developed by Wang et.al.
[6,7]. The basic hypotheses are that the similarities of both wave form and sus-
pended sediment trajectories are preserved between the model and prototype.
Based upon this hypothesis, the following modeling laws were derived:

S = W2/5A4/5 (1)
NT = A/Vs (2)
Nt = V (3)

where A is the horizontal length scale, 8 is the vertical length scale, W is the fall
velocity scale, NT is the fluid motion time scale, and Nt is the morphological time
scale. All the scales are defined as the ratios of prototype to model.

Since the modeling laws are based on suspended sediment mode, they seem
to be more applicable to storm conditions as opposed to mild wave environment.
In the model, natural fine sand with Dso=0.17mm was used as the bed material
whereas the beach sand in Sebastian Inlet has a D50=0.35mm. The corresponding
fall velocities to the two grain sizes at 10C or 50F are 0.6inch/sec and 1.8inch/sec,
respectively. Thus, W=3 in Eq.(1). A horizontal length scale of A = 60 was


j








selected, which permits to model the desired prototype range without encountering
significant scale effect. The proper vertical length scale was then calculated from
Eq.(1), which yielded S = 41. This vertical scale was rounded off to 40 in the
model. That is, the movable bed model constructed is, accordingly, scaled with a
horizontal to vertical distortion of 3:2.

The fluid motion time scale and the morphological time scale were computed
from Eqs.(2) and (3), which gave NT = 9.5 and Nt = 6.324. Thus, for examples, a
one-second wave in the model is equivalent to a 9.5-second wave in the prototype,
and the amount of sediment transport in a duration of one hour in the model is
equivalent to a duration of 6.324 hours in the prototype.


2.2 Model Construction

The movable bed model was conducted in the three dimensional wave basin at
the Coastal and Oceanographic Engineering Laboratory, University of Florida. It
models the study area approximately 2,000 ft of shoreline on either side of the inlet
entrance, landward of the 30 ft offshore depth contour to the A1A bridge (Figure 3).
The model was constructed by placing a layer of natural fine sand, Ds5=0.17mm.,
over the existing fixed-bed physical model. The sand layer is generally thick, at
least 2 inches in thickness to insure movable bed characteristics, in the beach and
ebbshoal area, and becomes thinner in the offshore region. The bed was then
molded by sections using templates to the scaled-down prototype topography of
1989 survey (provided by Coastal Tech. Corp.). Since the Inlet bottom is bed
rocks; it was simulated with small pebbles to attain the proper roughness. The
model bathymetries were leveled in with reference to 1929 N.G.V.D. The model
setup is shown in Figure 3.

The general north-to-south transport of sediment is also simulated in the model
where a sufficiently large amount of source sand was placed just inside the north
beach boundary before an experiment. Therefore, the original shoreline in model
is seen curving toward the sea near the north boundary.



3 Experiments


The existing inlet configuration and eight structure alternatives were tested in
the movable-bed model experiment. In each case, a combination of current/wave
conditions was tested. The survey program was also established in the experiment
to monitor the sediment drift in the model from different structure alternatives.








This model study includes testing the existing inlet and eight other structure al-
ternatives. Through the model study, the littoral transport, onshore and offshore
sand transport, shoreline changes, entrapment of sand by the inlet and ebb shoal
system, etc., for the different structure alternatives were evaluated. The informa-
tion is then synthesized to estimate the sediment balance as well as the sediment
deficit of the inlet-beach system. From this analysis, the effects of different struc-
ture alternatives are evaluated.

In addition to the movable bed model study, a Sediment budget analysis was
perform based upon the combined information from historical data, hindcasting
model and the evidence produced in the movable model.



2 Model Characteristics


The movable bed model was constructed at the Coastal and Oceanographic
Engineering Laboratory, University of Florida, for the purpose testing the littoral
and ebbshoal sand drift processes at the Sebastian Inlet due to different alternative
structural configurations.


2.1 Modeling Laws and Model Scales


The movable bed modeling laws utilized here were developed by Wang et.al.
[6,7]. The basic hypotheses are that the similarities of both wave form and sus-
pended sediment trajectories are preserved between the model and prototype.
Based upon this hypothesis, the following modeling laws were derived:

S = W2/5A4/5 (1)
NT = A/Vs (2)
Nt = V (3)

where A is the horizontal length scale, 8 is the vertical length scale, W is the fall
velocity scale, NT is the fluid motion time scale, and Nt is the morphological time
scale. All the scales are defined as the ratios of prototype to model.

Since the modeling laws are based on suspended sediment mode, they seem
to be more applicable to storm conditions as opposed to mild wave environment.
In the model, natural fine sand with Dso=0.17mm was used as the bed material
whereas the beach sand in Sebastian Inlet has a D50=0.35mm. The corresponding
fall velocities to the two grain sizes at 10C or 50F are 0.6inch/sec and 1.8inch/sec,
respectively. Thus, W=3 in Eq.(1). A horizontal length scale of A = 60 was


j








selected, which permits to model the desired prototype range without encountering
significant scale effect. The proper vertical length scale was then calculated from
Eq.(1), which yielded S = 41. This vertical scale was rounded off to 40 in the
model. That is, the movable bed model constructed is, accordingly, scaled with a
horizontal to vertical distortion of 3:2.

The fluid motion time scale and the morphological time scale were computed
from Eqs.(2) and (3), which gave NT = 9.5 and Nt = 6.324. Thus, for examples, a
one-second wave in the model is equivalent to a 9.5-second wave in the prototype,
and the amount of sediment transport in a duration of one hour in the model is
equivalent to a duration of 6.324 hours in the prototype.


2.2 Model Construction

The movable bed model was conducted in the three dimensional wave basin at
the Coastal and Oceanographic Engineering Laboratory, University of Florida. It
models the study area approximately 2,000 ft of shoreline on either side of the inlet
entrance, landward of the 30 ft offshore depth contour to the A1A bridge (Figure 3).
The model was constructed by placing a layer of natural fine sand, Ds5=0.17mm.,
over the existing fixed-bed physical model. The sand layer is generally thick, at
least 2 inches in thickness to insure movable bed characteristics, in the beach and
ebbshoal area, and becomes thinner in the offshore region. The bed was then
molded by sections using templates to the scaled-down prototype topography of
1989 survey (provided by Coastal Tech. Corp.). Since the Inlet bottom is bed
rocks; it was simulated with small pebbles to attain the proper roughness. The
model bathymetries were leveled in with reference to 1929 N.G.V.D. The model
setup is shown in Figure 3.

The general north-to-south transport of sediment is also simulated in the model
where a sufficiently large amount of source sand was placed just inside the north
beach boundary before an experiment. Therefore, the original shoreline in model
is seen curving toward the sea near the north boundary.



3 Experiments


The existing inlet configuration and eight structure alternatives were tested in
the movable-bed model experiment. In each case, a combination of current/wave
conditions was tested. The survey program was also established in the experiment
to monitor the sediment drift in the model from different structure alternatives.
























SEBRSTIRN


INLET MOVEABLE BED MODEL


SNAKE-TYPE WAVE MAKER


EBB FLOW
GATE


FLOOD FLOW
GATE





EBB FLOn
WEIR


FLOOD FLOW
WEIR


FLOOD FLOW
WEIR


OFFSHORE WAVE STATION

PROFILING LINES ..-'

I I I I I ..J i i I lI i I I II I IJ i I I 'III -

i I I l 11111 1 1 I II I -1 r1 I i I

I ---1-rfl1 I l i 1 i ll If 1 11 1 1 ll I

I I I I jllly I Il I I I IliIIII I III I



I I I 1111 1 i~i -+.+--1 I II111I 1111 I
ii i I IIIi i III I Ii tif 111 11 1 i I


I I
SEDIMENT
TRAP PLATE


(nIGHlMT AIR)
0 25

SCALE FEET


Figure 3: A sketch of the movable bed model; short-dashed lines showing locations
for monitoring the bottom profiles.


EBB FLOM
GATE






















ICENT
P CHANNEL


I








3.1 Test Program


The test program is designed to examine the following:


The effects of jetty alternations on shoreline and nearshore topography.

The effects of ebb tidal shoal removal (partial and total) on shoreline and
nearshore topography.

The effects of beach nourishment to nearshore environment.


The existing jetty configuration and eight other structure alternatives are selected
for the movable-bed model experiment. They are identified as SO, S1, S2, ..
to S8 structures. with the existing jetty configuration designated as SO These
nine structural configurations, one existing and eight alternatives, are described as
follows:

A brief summary of these structures are given below:


SO Existing jetty configuration.

S1 The SO configuration with the north jetty extended 250 ft southeastward
with a radius of approximately 900 ft.

S2 The S1 configuration with a hooked south jetty extension of 150 ft.

S3 SO configuration plus 150 ft south jetty extension but with no north jetty
extension.

S4 SO configuration with nearly 50% ebb shoal removal in the order of 500,000
yd3, herein referred as total or full removal.

S5 SO with about 25% ebb shoal removal in the amount of 230,000 yd3, herein
referred to as partial removal.

S6 S4 with both north and south jetties removed.

S7 SO with a 250,000 yd3 nourishment on the south beach. The width of the
beach is increased by approximately 150 ft.

S8 S7 with south jetty extended by 500 ft from the existing jetty configuration.


These nine configurations are shown in Figure 4.

The test wave conditions include storm waves, moderate normal waves, and
swell-dominated waves. The wave directions are from NE (100 left to shoreline

























so 51
-30S


S2

.30.- "


su SS


S6 S7 8

.30' .30. -30... .


20 -20


0.. .0 *10'.j -1 0

SA1A AlA A1A





Figure 4: Nine tested structural configurations.









Table 1: The input wave conditions.


Tested Case Wave Height Wave Period
6 Days NE Storm Waves 6 ft 8 sec
8 Days E Swells 2 ft 16 sec
20 Days NE Normal Waves 2 ft 6 sec
6 Days SE Storm Waves 6 ft 8 sec


normal), E (shoreline normal) and SE (100 right to shoreline normal). These
conditions are given in Table 1.

The test durations vary from 6 to 8 days prototype equivalent depending upon
whether the test is for storm erosion (6 days) or moderate erosion (8 days) or
swell-dominated recovery process (8 days). Within each test period, the maximum
flood and ebb currents for semi-diurnal tides (approximately two high, two low
tides in a day) are generated alternately based on the scaled time durations. The
corresponding current strength is 6.6 ft/sec for flood and 5.0 ft/sec for ebb. These
current strengths are determined from field measurements reported in the fixed
bed model study [3].

The survey program of the model experiment includes monitoring 34 bottom
profiles six profiles are located at the inlet with the spacings and ranges shown
in Figure 3. Along each line, the survey intervals are 3 inches (15 ft prototype
equivalent) from shoreline to the 10 ft contour line and one foot (60 ft prototype
equivalent) beyond the 10 ft contour line.

The bottom profiles were surveyed regularly before and after an experiment.
Due to rapid topographic changes in the case of storm erosion, the prototype
equivalent 2-day storm profiles were generally surveyed for all tested cases. Several
more intermediate surveys for the storm erosion case were also conducted for the
SO and S1 configurations; for the SO configuration these additional surveys were at
intervals of 1/4, 1/2, 1, and 2-day prototype-equivalent and for the S1 configuration
they were at 1/4, 1/2, 1, 2, 3, and 4-day prototype-equivalent.

Table 2 summarizes the test program described above.













Table 2: Testing program and experimental conditions.


Wave 6-day NEt 8-day E 8-day NE 6-day SE
Condition Storm Recovery Normal Storm
wave ht:6' 2' 2' 6'
structure* period: 8s 16s 6s 8s
(SO)
Existing 6-day run+ 6-day run+ 8-day run+ 6-day run+
configuration 3 surveys 1 survey 1 survey 2 surveys
(Sl)
Extension of same same -
N jetty
(S2)
Extension of same same same
N/S jetties
(S3)
Extension of same same -same
S jetty
(S4)
Total ebb shoal same same same
removal
(S5)
Partial ebb same same ame
shoal removal
(S6)
Removal same same same
of N/S jetties
(S7)
South beach same -
nourishment
(S8)
Jetty extension same# -
and beach fill

f Wave directions (from).
* North jetty is extended by 250 ft in S1 and S2, and south jetty by 150 ft in S2 and S3;
Ebb shoal is dredged in S4 and S5; South beach is nourished in S7 and S8.
# Also tested for the case when inlet is closed by a gate.








3.2 Test Procedures


The experimental procedures are:


Preparation of initial bathymetry.

Survey of initial profiles.

Establishment of the specified current and water level conditions following
the same procedures as the fixed-bed model experiments.

Generation of waves in model to start the experiment for the scaled duration.
For instance, a 6-day storm wave test is equivalent to a total 22.8 hours run
time in the model consisting of 12 complete semi-diurnal tidal cycles (one
complete tidal cycle consists of one high and one low tides).

Survey of bottom profiles at designated tidal cycles, say, at 1, 2, 4 or other
integer number of tidal cycles and the post-experiment profiles.

collections of sand transported to the inlet and those to the downdrift bound-
ary.

Reshape of the bottom to the initial bathymetry for the next experiment.
For recovery tests, the initial condition is the post-storm condition. And for
normal erosion tests, the initial condition is the post-recovery condition.



4 Experimental Results


The existing jetty configuration (SO) and eight structural alternatives (Sl to
S8) were tested subject to NE storm waves for a duration equivalent to six days in
prototype. For the SO to S6 configurations the tests were continued with E swells
intended for beach recovery for eight days prototype-equivalent. The SO, S2, S4,
and S6 structures were also tested with the NE normal waves; the test duration was
20 days prototype-equivalent for SO and eight days for the rest. For the SE storm
wave condition, only SO, S3, and S5 were tested for a 6-day prototype-equivalent
since the SE storms of this duration are rare events in this area.

The state of erosive or accretive beach against the tested wave conditions is
determined by the established criterion delineated in Figure 5 using the wave steep-
ness, 21rH/gT2 (where H and T stand for wave height and period, respectively),
and the non-dimensional sediment fall velocity, 7rW/gT (where W stands for the
fall velocity) as the parameters. If the plotting position of the test condition falls in
the region above the diagonal line, the condition should be accretional (or normal
















g T2/ I I / AUTHOR
I// I I F T_ 1 ERCSIVE
S/1 L L7g- ACCRETIVEE


0.001 -- --
0.001 0.010 0.100
7rW
gT
Figure 5: Criterion of normal and storm profiles.

profile); if the plotting position falls below the line it is erosional (or storm profile).
It is seen that the tested storm condition falls inside the zone of erosive profile and
the recovery condition falls in the zone of normal profile.


4.1 General Observations


Surveys were generally performed for the tested structure along the 34 profile
lines as shown in Figure 6. They were numbered from the north boundary at
JA (in the present case the updrift boundary) to the south boundary at J30 (the
downdrift boundary). The spacing between any two adjacent profiles ranged from
1.5 ft to 3.5 ft. The surveys for the S1 and S2 structural configurations, however,
included only 26 (J3 to J27) and 30 (JA to J27) profiles, respectively, as they were
selected in the earlier stage of the experiment. More survey lines were added in
the latter experiments to provide better spatial resolutions. The northernmost four
profiles (JA to J2) and the southernmost four profiles (J28 to J30) were judged
to be in the regions influenced by the model boundaries. Therefore, they were
excluded in most of the analyses. Along each profile, bottom was surveyed at a
regular intervals of 1 ft in the offshore region and 1/4 ft in the nearshore region.

The north jetty was located in the model at J6 and the south jetty was at J12.
The shoreline south of the inlet was about twice as long as the shoreline north
of the inlet in the model. The initial profile lines surveyed for the SO structural
configuration is shown in Figure 7. Ideally, these initial profiles should be the
same for all other configurations. In practice, they varied somewhat because of the
remolding process after each test configuration. All the surveyed data collected
















STRUCTURE:SO


JR JO J J2 J3 J 6 J5 J8J9 J10 J12J14 J16 JIB J20 J22 J24 J26 J27 J28 J29 J30



Figure 6: Locations and labels of bottom profiles surveyed in the model.


STRUCTURE:SO


20
INCHES
SFT


Figure 7: Initial profiles prepared in the model.


SFT

5FT








are digitized and stored in the VAX-8350 computer and the reduced data are
available in 3.5" floppy diskettes at the Department of Coastal and Oceanographic
Engineering, University of Florida.

Based upon the results of the profile survey, the shoreline and topographic
changes were determined as the differences between the survey and the initial
profiles. Figure 8 shows an example of the profile changes and shoreline positions
for the SO configuration under 6-day NE and SE storm waves; Figure 9 shows the
results corresponding to a 8-day swell-like waves from the east and to a 20-day
moderate waves from NE simulating normal wave conditions in this region. In
here, the positive values of elevation change correspond to accretion and negative
values indicate erosion. The shoreline in the model is defined as corresponding to
the 2 ft NGVD elevation in the prototype. These figures provide a visual display
of the erosional and accretional patterns.

The same data can be used to construct the contour maps of bathymetric
changes between surveys. Examples of the contour maps of bathymetric changes
for the SO configuration corresponding to the cases shown in Figures 8 and 9 are
given in Figures 10 and 11. The quantities of net sediment loss or gain in any
designated area can be determined from these maps by integrating the volume
differences within the area. The complete set of contour maps of the bathymetric
changes for all the tested configurations are given in Appendices I, II, III, and IV.

The general sediment transport patterns for all the tested configurations under
various wave conditions were similar to those for SO shown in Figures 10 and 11.
Beach erosion and nearshore sand losses were always noticeable on both sides of
the inlet near the jetties. Under NE storm waves, the overall sand transport direc-
tion was from north to south. Under SE storm waves, the overall sand transport
direction was then seen from south to north. Under E swell-like recovery waves
and NE moderate waves, sand transport in the longshore direction was negligible
on either side of the jetties except for S6 configuration where both jetties were
removed. Under this configuration, the inlet behaved like a sink drawing large
quantity of sand from both sides.

Under the NE storm wave condition, the sediment transport on the north side
near the inlet followed a path from nearshore to offshore and then to south with
most of the sand transported seaward of the north jetty, bypassing the inlet, to
the downdrift south side beach through the ebb shoal; a much smaller amount
of sediment was carried directly into the inlet following the flood currents in the
main channel. The southward sand bypassing was, however, weaker in S1 and
S2 in which the north jetty was extended which tends to block the passage of
the sediment. Nevertheless, this weaker southward transport could be a transient
phenomenon. As the bathymetry becomes gradually adjusted to the new jetty
configuration, the southward transport could also be gradually restored to the
existing condition.












STRUCTURE:SO
- INITIAL
..** 6-ORT (NE) STORM


5FT

> WCE


STRUCTURE:TO
- INITIAL
*... 6-OT (SE) STORM


5
INCHES


Figure 8: Profile changes for SO under 6-day NE and SE storm processes.














STRUCTURE:SO
- 6-ORT (NE) STORM
-.. 8-OAT RECOVER


-q17.
>--,-7.S


j INCHES


STRUCTURE: SO
INITIAL
.s ..-- 20-ORT (NE) WAVE
16.2
2.8

.7 3
7.6
-11.0


INCHES

5SFT


Figure 9: Profile changes for SO in recovery and moderate wave processes.








3.2 Test Procedures


The experimental procedures are:


Preparation of initial bathymetry.

Survey of initial profiles.

Establishment of the specified current and water level conditions following
the same procedures as the fixed-bed model experiments.

Generation of waves in model to start the experiment for the scaled duration.
For instance, a 6-day storm wave test is equivalent to a total 22.8 hours run
time in the model consisting of 12 complete semi-diurnal tidal cycles (one
complete tidal cycle consists of one high and one low tides).

Survey of bottom profiles at designated tidal cycles, say, at 1, 2, 4 or other
integer number of tidal cycles and the post-experiment profiles.

collections of sand transported to the inlet and those to the downdrift bound-
ary.

Reshape of the bottom to the initial bathymetry for the next experiment.
For recovery tests, the initial condition is the post-storm condition. And for
normal erosion tests, the initial condition is the post-recovery condition.



4 Experimental Results


The existing jetty configuration (SO) and eight structural alternatives (Sl to
S8) were tested subject to NE storm waves for a duration equivalent to six days in
prototype. For the SO to S6 configurations the tests were continued with E swells
intended for beach recovery for eight days prototype-equivalent. The SO, S2, S4,
and S6 structures were also tested with the NE normal waves; the test duration was
20 days prototype-equivalent for SO and eight days for the rest. For the SE storm
wave condition, only SO, S3, and S5 were tested for a 6-day prototype-equivalent
since the SE storms of this duration are rare events in this area.

The state of erosive or accretive beach against the tested wave conditions is
determined by the established criterion delineated in Figure 5 using the wave steep-
ness, 21rH/gT2 (where H and T stand for wave height and period, respectively),
and the non-dimensional sediment fall velocity, 7rW/gT (where W stands for the
fall velocity) as the parameters. If the plotting position of the test condition falls in
the region above the diagonal line, the condition should be accretional (or normal















Bothymetric Change Contours of SO after 6-Day(NE)Storm


--Erosion,-- ccretion(contours in 1/4 inch)


Bathymetric Change


Figure 10: Bathymetric changes for SO under 6-day NE and SE storm process.


j













Contours of SO after


--Erosion,-- Accretion(contours in 1/4 inch)


Figure 11: Bathymetric changes for SO in recovery and moderate wave processes.








Under the SE storm wave condition, the sediment transport pattern was similar
to that of the NE storm condition but with the reversed transport direction. Sand
was seen to move from the nearshore region south of the inlet towards the inlet
and then accumulated near the tip of the south jetty. A small amount of sand was
also observed to bypass the inlet to the offshore bars on the north side of the inlet:

Sediment came into the inlet in all tested wave conditions. It was apparent
that the majority of the sediment transport into the inlet was from south side
around south jetty. This was the case for all the tested configurations to the wave
directions. Consequently, the S2, S3 and S8 configurations with the extended south
jetty resulted in the smallest sand loss to the inlet.

The sediment movement in the ebbshoal region was found only active in the
upper layer. Visually, the ebbshoal appeared to be slowly moving towards south
under NE storm conditions. The pattern of ebbshoal change, however, was not all
that clear and not all consistent from run to run.

As far as the inlet channel change is concerned, it varied from case to case in
response to the different configurations. In all cases, accretion was visible inside
the jetties, particularly near the inlet entrance. Under the storm condition, erosion
in the main channel was also noticeable due to the often shoaling or accumulation
of sand near the south jetty. This was particularly the case for the S4 and S5
configurations where the ebb tidal shoal was fully or partially removed.


4.2 Shoreline Response


From the experiments, the northshore was rather stable for all tested cases
except the for case when the jetty was removed (S6). On the other hand, changes
along southshore were rather pronounced for all the test conditions. Therefore,
shoreline response was examined here mainly for the southshore.

The test configurations can be roughly grouped into 4 categories depending
upon the nature of the modifications in relation to the existing structures:


Category 1: Jetty structure modification only S1, S2 and S3.

Category 2: Ebb shoal removal S4 and S5.

Category 3: Removal of both jetties S6.

Category 4: Southside nourishment with or without jetty modification S7
and S8.








Shoreline changes are then examined in each of the four categories.

Figure 12(a) shows the net southside shoreline changes after the 6-day NE
storm experiment for Category 1 and that for the SO structure. The shoreline is
again defined as corresponding to an elevation of 2' NGVD in prototype. It is
seen that the experimental results were spatially irregular; however, the nature of
the changes as well as the magnitude of the changes were clearly similar for all
the structure configurations in this Category. Therefore, for the three structure
modifications, S1, S2 and S3. their effects on shoreline changes are most likely
limited to some local perturbation. Based on the faired-in mean curve (thick line)
shown in Figure 12(a), the magnitude of the shoreline retreat immediate to the
south jetty is in the order of 70 ft (50 ft) in prototype for an extended storm.
This magnitude decreases towards downdrift and becomes negligible around 2,000
ft from the south jetty. Within the 2,000 ft distance, S2 and S3 appeared to cause
slightly more shoreline retreat than SO and S1. This is the consequence of south
jetty extension which retards the downdrift northerly circulation that carries sand
towards the south jetty.

Figure 12(b) compares the net shoreline changes for Categories 2 and 3 con-
figurations for the 6-day NE storm case. Here, S4 is with full ebb shoal removal,
S5 is with partial ebb shoal removal, and S6 is similar to S4 but with removal of
both jetties, which is close to the situation when inlet is originally opened. The
solid line represents the mean curve shown in Figure 12(a). It appears that the
full removal of ebb tidal shoal causes a slight increase of shoreline retreat both in
magnitude and the extent. Partial removal of ebb tidal has negligible added effect
on downdrift shoreline changes other than local perturbation. The removal of both
jetties as in Category 3 (S6) configuration results in shoreline retreat on both sides
of the inlets and inlet becomes rapidly filled in.

In Category 4 structural configurations, the initial shoreline position was ad-
vanced approximately 2.5 ft in the model (150 ft prototype equivalent) due to
beach nourishment. The net shoreline changes for the 6-day NE storm case are
shown in Figure 13(a) together with the mean curve defined earlier. To obtain the
true position of the shoreline with respect to the existing (unnourished) shoreline
one simply add 150 ft to the ordinate such as shown in Figure 13(b). From this
Figure, one can see that downdrift beach nourishment without extending the south
jetty (S7) has lost significant amount of the added material in the vicinity of the
jetty. In fact, the shoreline near the jetty was retreated to the position close to
Category 1 configurations. For the case with jetty extension (S8) the overall net
shoreline change was similar to Category 1 structures but the shoreline retreat was
larger than the mean curve in Category 1 structures in the vicinity of the jetty.

From the above results it appears that under the NE storm condition the ulti-
mate shoreline position on the downdrift side is somewhat dictated by the down-
drift jetty configuration; too short a jetty will result in an unacceptably recessed












(a) Category 1 configuration:
2,


10 15 20 25
MODEL ALONGSHORE DISTANCE(FT)


I I I I
0 500 1000 1500

PROTOTYPE DISTANCE(FT)

(b) Categories 2 and 3 configurations:


u.


o

cJ


u
I



0
-2



K -3


-U


10 15 20 25 30 35
MODEL ALONGSHORE DISTANCE(FT) -' SOUTH


I I I
500 1000 1500

PROTOTYPE DISTANCE (FT)


Figure 12: Shoreline changes in 6-day NE storm for Category 1,2,3 structures.





22


w
0





-j
UJ
u
-2
v'-2
.I
0




-I


-* SOUTH


I
2000


I
2000













(a) Net Shoreline changes:


0 5 10 15 20 25 30 35
MODEL ALONGSHORE DISTANCE(FT) -- SOUTH


I I I I
0 500 1000 1500

PROTOTYPE DISTANCE(FT)

(b) Changes relative to existing shoreline:


2000
2000


-



z !
U-

w
4,
0
':
-J


(t-I
-J

0
= -2


MODEL ALONGSHORE DISTANCE(FT)
I I I
500 1000 1500

PROTOTYPE DISTANCE(FT)


180



120










- S7
UJ


0

357
a





-120 g
a-


I -180
35
-- SOUTH

2000
2000


Figure 13: Shoreline changes in 6-day NE storm for Category 4 structures.


u.

0

A-
w
-I





UJ


0
0








shoreline while too long a jetty may promote local erosion in its immediate vicin-
ity. Ebb shoal removals do not seem to have a significant effect on the ultimate
position of shoreline under storm events.

Also shown in Figures 12 and 13 is a general accretion of shoreline located
between 30 and 40 ft in model (1800 and 2400 ft prototype equivalent) south of the
south jetty. This shoreline accretion is, however, small, with the rate of accretion
less than 1 ft/day in model (10 ft/day in prototype) for all test configurations.

Figure 14 shows the southside shoreline position relative to the existing shore-
line from the 8-day E swell-dominated recovery waves for SO configuration and
for Categories 1, 2 and 3 configurations (Category 4 was not tested). It is seen
that the shoreline was generally in the status of recovery for all the tested con-
figurations. The recovery was seen to be spatially uneven but the mean position
appeared to be near complete recovery. The recovery patterns were similar for SO
and S3 due to the small differences in jetty configuration. The recovery patterns of
the other configurations were specially different. The foreshore beach face of the
recovered shoreline was considerably steeper than the pre-test condition. In the
previous report (7), the shoreline is defined from a level of +0 NGVD instead of +2
ft NGVD which is used in this report. The results showed that the 0 NGVD line
kept receding in most of the configurations. This beach steepening phenomenon
will be reflected in the results of the nearshore volumetric changes presented in
latter section.

Figures 15 and 16 show the shoreline changes relative to the existing shoreline
from the 8-day NE moderate wave and 6-day SE storm wave processes, respectively.
Only four configurations (SO, S2, S4 and S6) were tested for the 8-day NE moderate
wave condition and three (SO, S3 and S5) for the 6-day SE storm waves. From
the 8-day NE moderate wave experiments, all four tested structures sustained
mild shoreline recession, smaller than the recession rate in the 6-day NE storm
process. In the 6-day SE storm experiments, the three tested configurations showed
shoreline change patterns similar to the 6-day NE storm case in that shoreline
receded in the immediate vicinity of the south jetty but was progressional further
south. The point where shoreline progression began appeared to be closer to the
south jetty in the SE storm case. Apparently, sediment was transported towards
the inlet from the south but was sucked into the inlet.

Based on the surveyed profile data, averaged net shoreline changes with respect
to the initial shoreline were computed. The results for the 6-day NE storm, 8-day
E recovery, 8-day NE moderate waves, and 6-day SE storm process, are presented
in Tables 3, 4, 5 and 6, respectively.

These results showed that under storm condition, on the average the downdrift
shoreline receded up to J27 (or about 1,600 ft prototype distance southward from
the south jetty) under all tested configurations. Beyond J27, the shoreline actually













3 180
CHANGE DURING 8-OAT (E) RECOVERY PRO SS


2 2. 120 u.

-1-
U.U
S .. 0o




00 U
'U





-1 --S1 --60
-----2 S2
S....... S3
0-2 --St --120 c
---35

-3 SOUTH JETTY
0 5 10 15 20 2; 30 35
MODEL ALONGSHORE DISTANCE(FT) SOUTH
I I I I I





0 500 1000 1500 2000

PROTOTYPE DISTANCE (FT)


Figure 14: Shoreline changes in 8-day recovery condition.




2 120



1 60
060 0




U.j

-1 -- SO -60
S ". -- S2 g

-2 SS6 -120 uJ
0.





SSOUTH JETTY
z --


a-

-4 I ,I I I -240
0 5 10 15 20 25 30 35
NODEL ALONGSHORE DISTANCE(FT) --SOUTH
I I I I
I ----- I ----- I ----- ---------I-
0 500 1000 1500 2000

PROTOTYPE DISTANCE (FT)


Figure 15: Shoreline changes in 8-day NE moderate wave process.


















-.

z 0

- I
o





UJ
o
0


-3


SOUTH JETTI
-U.i --- 1 -- I -- I -------- -
0 5 10 15 20 25
MODEL ALONGSHORE ODISTNCE(FT)


30 35
-- SOUTH


30 1000 1!
PROTOTYPE OISTANCE(FT)


2000


Figure 16: Shoreline changes in 6-day SE storm process.





Table 3: Averaged shoreline changes(ft) in 6-day NE storm process.


Segment (range) Included in Computation*
Structure J12-21 J12-22 J12-23 J12-24 J12-25 J12-26 J12-27
(14.5') (16.2') (17.8') (19.5') (21.2') (23.0') (26.5')
SO -0.67 -0.67 -0.60 -0.51 -0.43 -0.39 -0.41
S1 -0.55 -0.49 -0.49 -0.48 -0.45 -0.41 -0.36
S2 -0.74 -0.68 -0.62 -0.60 -0.63 -0.62 -0.56
S3 -0.61 -0.56 -0.59 -0.63 -0.62 -0.60 -0.58
S4 -0.86 -0.81 -0.75 -0.72 -0.71 -0.69 -0.65
S5 -0.72 -0.68 -0.65 -0.63 -0.61 -0.57 -0.56
S6 -0.74 -0.71 -0.64 -0.62 -0.57 -0.52 -0.47
S7 0.69 0.80 0.86 0.96 1.07 1.18 1.24
S8 1.40 1.43 1.45 1.52 1.58 1.59 1.61

* Shoreline changes are relative to the existing shoreline; negative values indicate
shoreline recession.


I 1










Table 4: Averaged shoreline changes(ft) in 8-day E recovery wave process.

Segment (range) Included in Computation*
Structure J12-21 J12-22 J12-23 J12-24 J12-25 J12-26 J12-27
(14.5') (16.2') (17.8') (19.5') (21.2') (23.0') (26.5')
SO 0.25 0.21 0.10 -0.04 -0.16 -0.15 0.06
S1 0.01 -0.02 -0.01 0.02 0.05 0.08 0.09
S2 -0.02 -0.10 -0.16 -0.16 -0.14 -0.08 -0.02
S3 -0.31 -0.36 -0.33 -0.25 -0.19 -0.12 0.05
S4 0.27 0.22 0.16 0.11 0.10 0.10 0.10
S5 0.27 0.28 0.30 0.32 0.30 0.28 0.30
S6 0.46 0.38 0.31 0.27 0.21 0.15 0.13

* Shoreline changes are relative to the existing shoreline; negative values indicate
shoreline recession.



Table 5: Averaged shoreline changes(ft) in 8-day NE normal wave process.

Segment (range) Included in Computation*
Structure J12-21 J12-22 J12-23 J12-24 J12-25 J12-26 J12-27
(14.5') (16.2') (17.8') (19.5') (21.2') (23.0') (26.5')
SO -0.18 -0.13 -0.08 -0.04 -0.03 -0.04 -0.09
S2 -0.42 -0.38 -0.38 -0.36 -0.37 -0.38 -0.35
S4 -0.19 -0.17 -0.15 -0.13 -0.12 -0.12 -0.12
S6 -0.59 -0.52 -0.46 -0.39 -0.35 -0.32 -0.30

* Shoreline changes are relative to the existing shoreline; negative values indicate
shoreline recession.



Table 6: Averaged shoreline changes(ft) in 6-day SE storm process.

Segment (range) Included in Computation*
Structure J12-21 J12-22 J12-23 J12-24 J12-25 J12-26 J12-27
(14.5') (16.2') (17.8') (19.5') (21.2') (23.0') (26.5')
SO -0.77 -0.75 -0.70 -0.65 -0.59 -0.54 -0.50
S3 -0.79 -0.72 -0.69 -0.67 -0.62 -0.53 -0.45
S5 -0.56 -0.50 -0.50 -0.49 -0.45 -0.39 -0.34

* Shoreline changes are relative to the existing shoreline; negative values indicate
shoreline recession.








experienced mild advances in most cases. The extension of north jetty (S1 and S2
structure) appeared to have only minor effect on the southside shoreline response
as compared with the existing condition. As discussed earlier, lengthening the
south jetty increased shoreline recession in the immediate vicinity of the south
jetty. The absolute magnitude of the shoreline retreats were found to be quite
modest comparing with some of the open coast erosions experienced along the
Florida coast under comparable storm strength. For instance, the most severe
shoreline recession which occurred immediately south of the south jetty and was
in the order of 1.5 ft in six days (or less than 100 ft prototype). This modest
shoreline recession is due to the combined shielding effects provided by the jetties
and the ebb tidal shoal as well as due to the drift reversal which continuously feeds
sand to this region.

Under recovery wave condition, all configurations responded in shoreline recov-
ery. However, foreshore beach face was considerably steeper than the pre-tested
condition.

The shoreline response to storm waves from SE was found to have similar
pattern as that from NE.


4.3 Bathymetric Changes and Erosional Patterns


Under storm condition, the most visible bathymetric changes occurred in the
nearshore zone on the south side of the inlet including the region near the tip of
the south jetty. In the early stage of the storm, beach face was being eroded by
wave action and sand was being carried out towards offshore forming offshore bars.
Meanwhile, the ebb tidal shoal region also experienced vigorous sediment motion
due to wave breaking and sediment was carried both into the nearshore zone and
to the downdrift offshore area by wave induced currents. During flood cycle, a
portion of the ebb shoal material was carried directly into the inlet.

In the nearshore zone south of the inlet, it received material from both beach
face and ebb tidal shoal. During flood cycle, the sediment accumulated in the
nearshore zone was transported towards and into the inlet by the tidal currents
through marginal flood channels and around the tip of the south jetty creating
strong erosion zone in the vicinity of the inlet. During ebb tide, the sand in the
nearshore zone south of the inlet was also being carried towards the inlet and then
back to the ebb tidal shoal region following an ebb-current induced clockwise gyro
on the downdrift side. Figures 17 and 18 show the regions of accretion and erosion
with patterns of sediment motion based on sand tracer study during a single flood
and ebb cycles, respectively. These patterns of sediment motion were in close
agreement with the current patterns measured in the fixed-bed model.














STRUCTURE:SO
- ACCRETION
--- EROS ION


SFT


Figure 17: Sediment accretion and erosion pattern during flood.


STRUCTURE:SO
- ACCRETION
-- EROSION


5F1


Figure 18: Sediment accretion and erosion pattern during ebb.


L




























Figure 19: Erosion pattern after 6-day storm for SO.


The cumulative effect of bathymetric changes over a number of tidal cycles
is patched erosional and accretional patterns such as shown in Figure 19 for SO.
These patterns vary in detail for different configurations but in general agreement
with the current patterns.

For recovery runs, no bottom survey was made after a single flood or ebb. The
integrated effects appeared to be that the offshore bar created by storm waves
gradually diminished and moved towards the shore. However, the strong currents
in the vicinity of the south jetty appeared to prevent the offshore bar material
from reaching the nearshore zone to rebuild the beach. The shoreline recovery
addressed earlier appeared to be at the expense of the foreshore material causing
beach face to steepen. Since the onshore movement of the offshore bar material
was interrupted by the nearshore current and the foreshore material was moved
onshore to rebuild the shoreline, the nearshore zone continued to lose material even
under rebuilding cycle. The quantity of loss will be addressed in Section 4.5.

To examine the erosional and accretional patterns in further detail, the profile
lines surveyed at different time stages are plotted against each others. Figures 20
and 21, for instance, show the comparisons of the 12 profile lines located south
of the inlet for SO. Here, at each location 3 profiles representing the initial, the
6-day post storm and the subsequent 8-day post-recovery profiles are given. Also
plotted in these Figures is the quasi-equilibrium profile (thick line), which repre-
sents the beach in an equilibrium form away from the inlet. Similar plots for other













MEASURED PROFILES STRUCTURE, SO
0.5 INITIAL PROFILE
-----POST 6-DAY (NE) STORM
2 -----POST 8-DRY RECOVERY
0 J12


-0.5
0.5


0 J


-0.5
0.5


0 J18



0.5

F- 0





r 0 J15
cr


Fgr0 20: J1 o se p
S-0.5





-0.5




0 J18
-0.5








I I I
0 10 20 30

SEAWARD DISTANCE (FT)


Figure 20: Comparisons of surveyed profiles from J12 to J18 for SO













MEASURED PROFILES


STRUCTURE SO
-- INITIAL PROFILE
-----POST 6-DR (NE) STORM
-----POST 8-OAY RECOVERY


X, J_2J20


0.5


0


-0.5 L
0.5


0


-0.5
0.5


0


-0.5
0.5


J26


J28


0.5
I I I I
0 10 20 30
SEAWARD DISTANCE (FT)


Figure 21: Comparisons of surveyed profiles from J20 to J30 for SO.


J22







J2L4


-0.5 L


0


-0.5


0.5 r








configurations are presented in Appendix V.


4.4 Ebb Shoal Movement


The ebb tidal shoal was overall stable under both storm and recovery waves for
all the configurations. The sediment movement in the ebb shoal region was found
active mainly in the upper layer. Under NE storm condition, the ebb shoal as a
whole appeared to be slowly moving towards south in cases with jetty structures
and the existing ebb shoal configuration. In S4 and S5 where ebb shoal was either
partially or fully removed, the ebb shoal grew back fairly rapidly under both storm
and recovery conditions. The pattern of ebb shoal change, however, was not all
that clear and was not necessarily consistent from run to run.


4.5 Sediment Balance Computations

A scheme of sediment balance analysis was developed to quantify performance
evaluation for alternative configurations. The test region is divided into five zones
- Zone 1 to Zone 5 (see Figure 22). Zone 1 is defined as updrift zone which covers
the area north from the tip of the north jetty. Zone 2 is the inlet zone which covers
area inside the line connecting the tip of north jetty to the tip of south jetty. Zone 3
is the south nearshore zone which covers the area within the 6 ft prototype contour
line on the south side of the south jetty to the downdrift border. Zone 4 is the
ebb tidal shoal zone which is the area south of Zones 1 and 2 and east of zone 3.
Zone 5 is a small area south of the last surveyed profile (J30) where the downdrift
transport is estimated. The sediment balance analysis were then accomplished by
comparing volumes in each respective zone before and after the tests.

Figure 23 shows the results of the sediment balance computations in five re-
gional zones for the 6-day NE storm experiments. The values given are the rate
of volume change in the model scale in ft3/day; the values in the parentheses are
the equivalent prototype rate in 103yd3/day. The results are also summarized in
Table 7. Since SO, S3, S4, S5, S7 and S8 all have the same updrift boundary
condition, i.e., the north jetty configuration is the same, a same updrift response
is normally expected. In the actual tests, however, this updrift response varied
slightly from one experiment to the other. This is not desirable for comparison
purposes. To facilitate comparisons with the same reference, the averaged updrift
(Zone 1) volume change from these 6 configurations is used as the base value. The
values in zone 1 of these 6 configurations are all adjusted to this same base value.
The adjustment was also made to S1 and S2 configurations which have the same
north jetty configuration. The adjusting factors for these configurations are given
in Table 8.











STRUCTURE:SO


I FT


i1 / .---- --.-- .- s

2 3

JR JO J 32 J2 3 J4 JS J6 J8J9 J10 JI2JIl Ji 6 J20 J22 J2 2 26 J27 J28 J29 J30


Figure 22: Five regional zones for sediment balance computations.

The above results can also be presented with reference to those of SO config-
uration for comparison of the individual structural performance. This is given in
Table 9, where the data of volume changes over the ebb shoal (Zone 4) are not
shown since the extent of the ebb shoal is difficult to define. Results of sediment
balance computations for other test wave conditions are shown in Figures 24 to 26
and in Tables 10 to 12.

Based upon these results and the results of shoreline change presented earlier,
the performance of various tested configurations are then evaluated.

A summary of sand budget in south shore erosion, downdrift transport, and
the loss to the inlet, in terms of histogram presentation from the tests is given in
Appendix VI.

The experimental results of shoreline change and sediment balance presented
here are used in the next Section as the bases for evaluating the performance of
the different alternative configurations.

















-2.52
-3.q11 (-2.09)
(-2.82)/
-2.23
I -1.85)
1.71
(1.42)
S3

-2.32
-3.411 -* (-1.93)
(-2.82)(
-2-2.91
(-2.112)
0.78
(0.65)
S6

4.71
-8.42 -* (3.91)
(-6.98)
-6.60
I (-5. 7)
6.04
(5.01)


-9
2.62
(2.18)








-9
4.22
(3.50)









5.55
(4.60)


-1.07
-1.25 (-0.89)
(-1.04)
-1.24
(-3.52)
1.32
(1.09)
St4

3.36
-3.q1 (2.79)
(-2.82)
-7.63
L (-6.32)
2.78
(2.31)
S7

5.22
-3.141 -* (4.32)
(-2.82)
-7.36
| (-6.10)


1.33
(1.10)


3.00
(2.49)








5.89
(4.88)









3.26
(2.70)


-0.85
-1.25 (-0.70)
(-1.04)(
f -3.61
(-2.99)
1.00
(0.83)
5

2.36
-3.1 (1.95)
(-2.82)
-5.18
S(-4.30)
2.46
(2.04)
S8

5.03
-3.41 (q.17)
(-2.82 )
-7.22
-5,99)


0.26
(0.21)


Figure 23: Sand budget in ft3/day (103yd3/day in prototype) from 6-day NE storm experiment.


2.71
(2.25)








4.07
(3.37)









4.13
(3.43)











Table 7: Rate of volumetric change (103yd3/day) in 6-day NE storm process.

Rate of Volumetric Change*(103yd3/day)
Structure Inlet North Ebb Shoal Inlet South Downdrift Loss to
Region Region Shore Transport Inlet
SO -2.82 -2.09 -1.85 2.18 1.42
S1 -1.04 -0.89 -3.52 2.49 1.09
S2 -1.04 -0.70 -2.99 2.25 0.83
S3 -2.82 -1.93 -2.42 3.50 0.65
S4 -2.82 2.79 -6.32 4.88 2.31
S5 -2.82 1.95 -4.30 3.37 2.04
S6 -6.98 3.91 -5.47 4.60 5.01
S7 -2.82 4.32 -6.10 2.70 1.10
S8 -2.82 4.17 -5.99 3.43 0.21

Shown in prototype scale; negative values indicating the sand volume loss.




Table 8: Adjusting factors for updrift volume changes in NE storm test.

Structure SO S1 S2 S3 I S4 S5 S7 S8
Factor 0.93 0.98 1.02 1.03 1.01 0.98 0.86 1.19


Table 9: Comparison of volume change rates in 6-day NE storm process.

Rate of volumetric change relative to SO (10yd3/day)
Structure Updrift Southside Downdrift Loss to
Impoundment Volume Transport Inlet
S1 1.78 -1.67 0.31 -0.33
S2 1.78 -1.14 0.07 -0.59
S3 0.00 -0.57 1.32 -0.77
S4 0.00 -4.47 2.70 0.89
S5 0.00 -2.45 1.19 0.62
S6 -4.16 -3.62 2.42 3.59
S7 0.00 -4.25 0.52 -0.32
S8 0.00 -4.14 1.25 -1.25

















0.51
0.00 (0.43)

-2.97
(I-2.50)
0.16
(0.13)
S3

0.17
0.00 (0.15)
10.00)

Sf -2.51
(-2.12)
0.10
(0.09)
S6

0.85
-0.83 -- (0.71)
(-0.70)
I -2.10
S-1.77)
1.82
(1.53)


-0.50
0.00 (-0.42)
(0.00)
-0.62
(-0.53)
0.10
(0.09)


1.00
(0.85)


-0.54
0.00 (-O,45)




0.10
(0.09)
SS

0.81
0.00 (0.68)
(0.00)
-0.60
(-0.51)
0.03
(0.03)


Figure 24: Sand budget in ft3/day (103yd3/day in prototype) from 8-day E recovery experiment.

















0.41
0.00 (0.35)
(0.00)
{ -0.70
(-0.59)
0.10
(0.09)
S6

0.55
-1.11 -I (0. q7)
(-0.93)
-1.52
(-1.29)
0.92
(0.77)


0.57
(0.48)


0.58
0.00 (0. 9)
(0.00)
-0.99
(-0.83)
0.04
(0.03)


0.39
(0.33)


-0.28
0.00 (-0.23)
(0.00)
( -0.28
(-0.214)
0.04
(0.03)


0.51
(0.43)


Figure 25: Sand budget in ft3/day (103yd3/day in prototype) from 8-day NE normal wave experiment.


0.81
(0.68)

















0.40
0.27 (0.33)
(0.22) -

(-3.72)
1.09
(0.91)


0.99
0.27 (0.82)
(0. 22) f
-4.46
(-3.69)
0.47
(0.39)


2.44
0.27 (2.02)
(0.22)2-
S -6.00
(-4.97)
0.56
(0.46)


Figure 26: Sand budget in ft3/day (103yd3/day in prototype) from 6-day SE storm experiment.










Table 10: Volume change rate (103yd3/day) in 8-day E recovery wave process.


Rate of Volumetric Change*(103yd3/day)
Structure Inlet North Ebb Shoal Inlet South Loss to
Region Region Shore Inlet
SO 0.00 0.43 -2.50 0.13
S1 0.00 -0.42 -0.53 0.09
S2 0.00 -0.45 -0.46 0.09
S3 0.00 0.15 -2.12 0.09
S4 0.00 0.99 -2.01 0.85
S5 0.00 0.68 -0.51 0.03
S6 -0.70 0.71 -1.77 1.53


* Shown in prototype scale; negative values indicating the sand volume loss.




Table 11: Volume change rate (103yd3/day) in 8-day NE normal wave process.

Rate of Volumetric Change*(103yd3/day)
Structure Inlet North Ebb Shoal Inlet South Downdrift Loss to
Region Region Shore Transport Inlet
SO 0.00 0.35 -0.59 0.48 0.09
S2 0.00 0.49 -0.83 0.33 0.03
S4 0.00 -0.23 -0.24 0.68 0.03
S6 -0.93 0.47 -1.29 0.43 0.77

Shown in prototype scale; negative values indicating the sand volume loss.




Table 12: Volume change rate (103yd3/day) in 6-day SE storm process.

Rate of Volumetric Change*(103yd3/day)
Structure Inlet North Ebb Shoal Inlet South Loss to
Region Region Shore Inlet
SO 0.22 0.33 -3.72 0.91
S3 0.22 0.82 -3.69 0.39
S5 0.22 2.02 -4.97 0.46

Shown in prototype scale; negative values indicating the sand volume loss.








5 Performance Evaluations of Structural Alter-
natives


Five criteria are used to compare the performance of structural alternatives. They
are:


Sand losses to the inlet.

Downdrift transport.

Volume changes in southside nearshore zone.

Ebb shoal volume changes.

Updrift volume changes.


5.1 Channel Shoaling and Sand Losses to the Inlet


The experiments showed that inlet channel shoaling and net transport into the
inlet was dominated by storm events. For the existing SO configuration, the net
transport into the inlet during storm period was found to be about 1,400 yd3/day
for NE storm and about 910 yd3/day for SE storm. Under normal wave conditions
(2 ft height) this transport magnitude was found to be only about one tenth of the
above values. Of the eight alternative configurations five (S1, S2, S3, S7, and S8
configurations) showed improvement of channel shoaling and sand losses to the inlet
and three (S4, S5 and S6) worsened the condition. Evidently, major improvement
was always associated with south jetty extension. This was particularly the case
for the S8 configuration, when the jetty is extended beyond the existing marginal
flood channels; here the net transport into the inlet was cut down to 210 yd3/day
under NE storm, or 15% of the existing condition. The next best one was the
S3 configuration of which the transport rate was found to be about 46% of the
existing condition.

Extending north jetty (Sl) also cut down sediment transport into the inlet but
to a lesser extent (about 77% of the existing condition). Removal of ebb tidal
shoal, whether partially or fully, increased the channel shoaling rate by 40 to 60%
under storm condition. Removal of both jetties (S6) would drastically increase
the inlet transport. Under storm condition, the rate was found to be about 5,000
yd3/day, or about 3.5 times the existing rate of transport. At this rate, the inlet
likely will become impassable. This results demonstrated the necessity of the jetty
structures.








5.2 Downdrift Transport


Downdrift transport is defined here as the material passing through the test
region onto the south side, 2,500 ft beyond the south jetty. The test results showed
that removal of ebb shoal promotes downdrift transport by an increase of 50 to
100% initially. This was mainly due to fact that waves diffraction around the ebb
shoal was either eliminated or reduced. In either case, the littoral zone was re-
established to a normal condition of plane beach south of the jetty. Therefore, as
the ebb shoal rebuilds one expects the downdrift transport to reduce rapidly.

The removal of both jetties also increased the downdrift transport significantly.
The mechanism, however, is different from the ebb shoal removal situation. This
increase in downdrift transport was mainly due to the increase of sand supply from
the updrift as the northshore rapidly adjusted back to a plane beach form.

For cases with beach nourishment, S7 and S8, downdrift transport was also
increased. This increase was likely aided by the diffusion of sand to the two ends
of the nourished area.

For configurations with jetty modifications only(S1, S2 and S3), lengthening
north jetty has little effect on the downdrift transport away from the shadow zone
of the jetty. Extension of south jetty, on the other hand, improved downdrift
transport by reducing losses to the inlet. This is seen by comparing the downdrift
transport rates of SO with S3 and of S7 with S8 in Table 7.


5.3 Volume Changes in Southside Nearshore Zone

As discussed earlier, the bulk of the downdrift erosion took place within 2,000
ft from the south jetty. The results of volume change computation under storm
condition (see Table 7) indicated that all tested alternatives induced more south-
side erosion than the existing SO configuration. However, under recovery condition
the trend seemed to be reversed (Table 10). One explanation is that SO is the
closest to an equilibrium state under the current natural condition than the oth-
ers. Therefore, under storm condition the profile adjustment in SO was not as
pronounced as the others, hence the volume loss to offshore was also less. On the
other hand, during recovery, the test results showed the amount of sand returning
to the nearshore zone was also less than the other configurations. This can further
be shown in the time history plot of downdrift erosion (Figure 27). Here, at the
end of the storm period SO had the smallest total volume loss. However, at the
end of the recovery period, volume loss in SO was no longer the smallest. For
cases with beach nourishment, S7 and S8, this profile adjustment was considerably
larger under storm condition and more sand was being transported offshore. This















_j8
I-


U-
_,

UJ
o

0
r
-J


oJ

r2



0


PROTOTYPE TIME(DAY)
5 10 15

DOWNDRIFT EROSION HISTORY


so-
0

40 x
0,

30

.J
0
20 >
oJ

a-
0p
c-

0o


10 20 30 10 50
MODEL TIME(HOUR)


Figure 27: Downdrift erosion history from storm and recovery processes.


is revealed in Table 7 as the ebb shoal volume grew considerably in both cases.
Unfortunately, recovery test was not carried out for S7 and S8 to determine the
extent of beach recovery.


In the case of ebb shoal removal, complete shoal removal caused significantly
increase in the nearshore volumetric erosion whereas partial removal seemed to
promote both erosion during storm period and recovery during recovery period.
Again, the ebb shoal gained material under both wave conditions.


Finally, for the case with both jetties removed, the south shore volume loss was
large under storm condition owing to the increased transport into the inlet.




5.4 Ebb Shoal Volume Changes


The entire ebb tidal shoal volume is quite substantial in the order of 1.3 million
yd3. It is a difficult task to measure the change in short-duration tests as any small
inaccuracy in survey will lead to large error in volumetric computation. Therefore,
the shoal volume computations are restricted in an area in the immediate vicinity of
the inlet. Even in such restricted condition one should be cautious in interpreting
the results.


W-





SNE STORM RECOVERY
I I II


--e- SO
--- oS1
--- 52 -
A 53 x 56
* 5 + 57
SSS v SS8








Under NE storm condition, the sediment motion is very active in the ebb shoal
region owing to wave breaking over the shoal. The material was then transported
by currents induced by tides and waves. The tidal current carries the ebb shoal
material into the inlet during flood and re-circulates the material following a clock-
wise circulation in the vicinity of the south jetty during ebb. The wave induced
current transport part of the material into the nearshore zone towards the down-
drift direction. However, significant portion of the material simply moves in the
offshore region towards south. Therefore, under NE storms, ebb tidal shoal lost
material in the immediate vicinity of the inlet entrance but fed material into the
inlet and the downdrift nearshore zone; on the same time, the entire ebb shoal
appeared to shift to the south due to transport in the offshore region. This same
pattern held for S1, S2 and S3 where only jetty structures were modified.

From the fact that shoal volume changes were similar between SO and S3 (no
N jetty extension), and between S1 and S2(with N jetty extension), the volume
change in this region was mainly influenced by the north jetty configuration. Ap-
parently, the north jetty extension reduced the downdrift bypassing thus reduced
the volume change in this region.

For cases with ebb tidal shoal removal, the region gained instead of lost material
as part of the ebb shoal re-generating process. This was also the case for S6 with
the removal of the jetties, since the initial configuration in S6 has no ebb tidal
shoal; the material gaining was a normal ebb tidal shoal generating process in
the presence of an inlet. The patterns of ebb shoal growth were similar for the
three configurations of S4, 5 and 6 with material being accumulated near the inlet
entrance and gradually spreading towards downdrift.

For the two configurations with beach nourishment, the ebb tidal shoal region
also gained material. Since ebb tidal shoal was present in the initial condition
the pattern of growth of was somewhat different from the situation while the ebb
shoal was removed. Here, most of the gain was just outside the nearshore zone
in the form of an offshore bar, mainly as a consequence of profile adjustment. It
is unclear whether this material will become a permanent part of the ebb tidal
shoal or will be brought back to the beach during normal and/or recovery wave
conditions as no test were conducted under those conditions.


5.5 Updrift Volume Changes


Updrift volume changes were mainly associated with the change of north jetty
configuration. Extending the north jetty as in S1 and S2 resulted in additional
impoundment of sand at a rate of about 1,000 yd3/day during the extended storm
tests. This additional impoundment is very minor. Furthermore, this quantity
is expected to diminish as the north shore gradually adjusts to the new jetty








configuration. Removal of north jetty caused significant updrift volume loss as
expected. This loss is likely a transient phenomenon and probably will diminish
rapidly as the updrift profile adjusted to normal.


5.6 Summary of Performance Evaluation

Performance of different structural alternatives was evaluated from the point
of view of sediment budget. The main components in the eight alternative config-
urations tested in the laboratory consisted of extension of north jetty, extension
of south jetty, ebb tidal shoal removal and utilization, beach nourishment and the
combination of the above.

The extension of north jetty reduces the rate of inlet shoaling slightly but also
increases downdrift erosion slightly. Additional sand is initially impounded
on the north side of the north jetty but the rate of impoundment is likely to
slow down and eventually becomes negligible as the updrift shoreline adjusted
to a new equilibrium configuration. For the configuration tested the amount
of the additional impoundment should be small. The ebb shoal volume is to
grow modestly owing to the reduction of ebb shoal bypassing.
The extension of south jetty has the major benefit of reducing inlet shoal-
ing and loss of material to the inlet, as well as promoting more downdrift
transport. However, Erosion in the immediate vicinity of the south jetty also
increases; the degree of severity will depend upon the length and shape of the
extension. For the configurations tested (150 ft and 500 ft extensions), the
added erosions are modest. The final stable shoreline position will be largely
governed by the south jetty configuration. The current prevailing shoreline
recessed by a mount of 60 75 ft roughly corresponds to the mean final
position under extended NE storms. Under the present south jetty configu-
ration, a stable shoreline seaward of the current prevailing shoreline position
can not be maintained within reasonable cost and means. The effect on ebb
tidal shoal depends upon the extent of the jetty extension and it relationship
to the north jetty; for the configurations tested, the effect will be modest
as the south jetty still remains in the shadow zone of the north jetty under
prevailing northeasters. The south jetty extension shows little or no effect
on the updrift region.
Removal of both jetties is not a viable alternative as the inlet will become
innavigable.
Ebb tidal shoal removals cause increased channel shoaling and downdrift
erosion in the immediate vicinity of the inlet; the effect of the latter is likely
to be local. Downdrift transport increases initially but is likely to diminish
as the shoal re-generates. Ebb shoal re-generates at a fairly rapid pace.








Beach nourishment increases downdrift transport at the expense of the nour-
ished material. Under storm conditions a significant portion of the mate-
rial(larger than the portion transported downdrift) is transported offshore to
form a bar. This bar is located beyond the tip of the existing south jetty and
extends northward into the channel. This is likely to impair inlet navigation.
The problem can be contained with the south jetty extension.



6 Sediment Budget Analysis


Sediment budget is defined here as the annual net transport rate with reference
to a control area shown in Figure 28. To attempt a sediment budget analysis for
this control area one needs to establish six transport quantities:


Qi: net transport across updrift boundary.

Q2: net transport across downdrift boundary.

Q3: net gain (loss) from updrift shoreline erosion (accretion).

Q4: net gain (loss) from downdrift shoreline erosion (accretion).
Qs: net loss to the inlet.
AV: net ebb tidal shoal volumetric change.


It is not an easy task to establish these quantities with our current level of
knowledge on inlets. Information from different sources are pieced together here
to facilitate the best estimations. Unfortunately, as will be seen later, many of
these sources have very low reliability. The problem is further compounded by
the fact that these sources all have different spatial and temporal resolutions and
some of them have large annual variations. Therefore, the results are, at best, an
educated guesstimate and should be used with extreme discretion.

A detailed analysis was performed here for the existing condition (SO).


6.1 Historical Shoreline Changes

The quantities Q3 and Q4 which are associated with the long term gain(loss) due
to shoreline erosion (accretion) can be estimated by examining historical shoreline
changes in this region.

































.. \ Sea


5 I AV

Bay

S*Q4





Q4

i.
F f t


Figure 28: A sketch of sand budget control box near inlet.








The history of Sebastian Inlet can be divided into three stages. During the
first stage of 1886-1924, many unsuccessful attempts were made to open the inlet.
These attempts failed because the work was at a level that was too small in scale,
and consequently the minimum flow cross-section required for a stable inlet was not
met. During the second stage, 1924-1942, the inlet remained open but not stable.
This was due to insufficient flow cross-sections and insufficient jetty protection from
wave induced littoral drift. The third phase of the inlet begins with the reopening
of the inlet in 1948. A series of dredging operations and jetty improvements have
widened the flow area and reduced littoral drift so that the inlet has been stable
since then. However, this inlet stabilization also caused drastic shoreline changes.

Based upon historical maps, aerial photos and actual field surveys the shoreline
changes in the vicinity of the inlet were analyzed by Ahn et.al.[8]. Odd-even
analysis and Fourier analysis were used to separate the background erosion and
the effects of inlet. These two analysis practically yielded identical results. The
shoreline changes can be roughly divided into three stages. Prior to the inlet
stabilization in 1946, the shoreline in this region was relatively stable and mildly
accretional. After the stabilization, the shoreline apparently underwent significant
changes in the first 20 years. The rate of change reduced afterward as the inlet
matured. Figure 29 shows the rates of shoreline change during these three periods.
From 1929-1947, the shoreline was stable and mildly accretional. From 1947-1970,
the change showed extensive downdrift erosion and appreciable updrift accretion.
It appeared that the shorelines have been affected for approximately 5 miles on
either side of the inlet with the most significant changes occurring within 2 to
3 miles from the inlet. The net north and south shoreline offset, at present, is
close to 400 ft. The average downdrift shoreline retreat over the 5 miles distance
was about 120 ft., or an average rate of approximately 5 ft per year. The updrift
shoreline has accreted on the average of approximately 70 ft over 5 miles, or a rate
of 3 ft per year. During the period from 1970 to 1986, the rate of erosion decreased
to 1.5 ft per year and the rate of accretion also decreased.

In terms of annual volumetric rate of changes, the following values are obtained
by assuming equilibrium profile to -6 ft. contour:


Updrift: +1,300 yd3 per year from center of inlet to 7,000 ft updrift.

downdrift: -2,300 yd3 per year from center of inlet to 15,000 ft downdrift;
4,500 yd3 per year from center of inlet to 25,000 ft downdrift, and 6,800 yd3
per year from center of inlet to 40,000 ft downdrift.

















SHORELINE CHANGE RATE


0
DISTANCE(FT)


0
DISTANCE(FT)


0
DISTANCE(FT)


1929-1947








- -


25000


25000


25000


50000


Figure 29: Shoreline change rates from 1929 to 1986.


20.0


1 0.0


S0.0


S-10.0


-20.0


-50000


20.0


10.0


0.0


-10.0


-20.0


-50000


20.0


S10.0


0.0


S-10.0


-20.0


i 19q7-1970


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


-25000


-25000


-25000


50000


1970-1986









__I


-50000


50000








6.2 Background Littoral Drift Environment


The sediment transport at the updrift boundary, Qi, can be treated as the back-
ground littoral drift in the absence of the inlet. This quantity has been estimated
by a number of previous investigations:


300,000 cubic yard per year (Southward-USAE)

160,000 cubic yard per year (Southward-Walton)

157,000 cubic yard per year (Southward-Coastal Tech.)


An attempt was made here to estimate the littoral transport rate based on wave
hindcast data.

The Army Corps of Engineers Wave Information Study (WIS) contains 20
years (1956-1975) of hindcast wave data for the east US coast including Sebastian
Inlet area. In addition, approximately 5 years (1986-1990) of real wave data for
Vero Beach (located about 15 miles south of the Sebastian Inlet) was available
from the Coastal Data Network (CDN), Coastal and Oceanographic Engineering
Department at the University of Florida.

The WIS data was generated by using numerical hindcast models and offshore
meteorological records to calculate wave growth and wave transformation from
deep to shallow water. This data does not include the effects of tropical storms
or hurricanes. The wave height, period, and angle of approach were calculated
for the sea and swell conditions separately and then combined. The combined sea
and swell conditions were used for this study. The data was collected at 3-hour
intervals and the calculations performed for a shallow water depth of 10 m. For
this study, the WIS data was rearranged to twelve data sets of monthly statistics.

The CDN data which is real wave data was collected at 6-hour intervals (with
the exception of equipment downtime) at a depth of approximately 10 m off Vero
Beach CDN. As reported in the Part I Report the measured waves at Sebastian
Inlet were almost the same as that at the Vero Beach. Thus the CDN data could
serve the purpose of adjusting the hindcast data.

The CDN and WIS data compare relatively well, however, the WIS data shows
slightly larger wave heights and smaller wave periods. To account for this, the WIS
data was adjusted by first dividing the mean monthly CDN wave heights and pe-
riods by those from the WIS data to obtain correction coefficients. These monthly
correction coefficients were applied to the WIS data set. Table 13 compares the
CDN and WIS results and the tabulates the correction coefficients. The adjusted
WIS data was used as input for longshore sediment transport rate calculations.

















Table 13: Comparison of CDN and WIS wave data.

Mean Mean Mean Mean
Month HCDN Hwis TCDN TWIS
(m) (m) Coef. (sec) (sec) Coef.


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec


0.80

0.74

0.77

0.39

0.56

0.44

0.37

0.42

0.62

0.83

0.83

0.82


0.95 0.84


0.93

0.88

0.94

0.72

0.60

0.47

0.51

0.93


0.80

0.88

0.42

0.77

0.74

0.78

0.82

0.67


1.25 I 0.66


1.11

1.05


0.74

0.78


8.57

8.36

8.26

9.61

8.51

7.85

7.73

8.31

9.42

8.38

8.14

9.17


6.22

6.41

6.40

7.07

5.36

5.51

4.90

5.05

6.29

7.16

6.66

6.79


1.33

1.23

1.29

1.36

1.59

1.42

1.58

1.65

1.50

1.17

1.22

1.35


I








The volumetric longshore transport rate is computed here by [9]


K H~ Cb cosC sin a
8 (s- 1)(1 -p) (4)
where Hb is the breaking wave height, Cgb is the wave group velocity at wave break-
ing depth, and a is the angle between the wave ray and the shore perpendicular
line, p is the porosity of sediment and s is the specific gravity of sediment. The
coefficient K is empirically determined.

If the bathymetry is assumed as parallel to the shoreline and the energy losses
from deep water are neglected, Eq.(4) can be expressed in terms of deep water
conditions as follows:
K H24 T0.2 cos co sin co
0 = cosas (5)
8 (s 1)(1 p)21.47r0.20.4 coso.2 as )


Necessary assumptions for the derivation of Eq.(5) are the shallow water asymp-
totes, Cg, = 'g, Hb = Khb, with the deep water asymptote, Lo = gT2/2r, the
conservation of energy, and Snell's law.

For estimating the longshore sediment transport at Sebastian Inlet the following
values were used: cos0.2 ab ; 1, g=32.2 ft/sec2, s=2.65, p=0.4, K=0.78. Then, the
longshore sediment transport rate, Q, in ft3/sec was estimated as

Q = 0.3374 K H2.4 T.2 COS1.2 a sin a (6)

where K is a function of sediment size, among other factors. A value of K=0.77
was often recommended [9] in Eq.(6) as using the root-mean-square wave height
for Ho. Harris [10] suggested the value of 0.77 to be too high when he applied the
equation to Jupiter Inlet and adjusted the value downward to about 0.28. In the
present study, it was found that calculated littoral drift rate varied greatly from
year to year. Selecting K=0.28 would yield a 20-year averaged transport rate in
the same range as those estimated by previous investigators.

The root-mean-square wave height is related to the significant wave height by
the following equation assuming a Rayleigh distribution of wave heights:

H, ; zvH,.m.. 1.6Hm (7)

where H, denotes the significant wave height, Hr.m.s. denotes the root-mean-square
wave height, and Hm denotes the average wave height.

The longshore sediment transport rate was then computed at 3 hour intervals
for the 20 years period from 1956 to 1975. The statistics are then complied.
Table 14 shows the computed longshore transport values the percentage of drift









Table 14: Estimated longshore transport (yd3/year) from 1956 to 1975.

year Qsouth Qnorth Qnet Qgross Percentage of Drift
% south % north
1956 467,487 106,523 360,964 574,010 81.4 18.6
1957 304,377 196,522 107,855 500,900 60.8 39.2
1958 343,140 196,576 146,564 539,716 63.6 36.4
1959 508,983 248,095 260,888 757,077 67.2 32.8
1960 427,207 200,425 226,782 627,631 68.1 31.9
1961 389,891 190,606 199,285 580,497 67.2 32.8
1962 591,284 103,221 488,063 694,505 85.1 14.9
1963 496,666 130,036 366,630 626,701 79.3 20.7
1964 414,068 190,440 223,628 604,508 68.5 31.5
1965 318,070 195,616 122,454 513,685 61.9 38.1
1966 395,820 246,691 149,130 642,511 61.6 38.4
1967 483,036 94,026 389,009 577,061 83.7 16.3
1968 192,039 76,195 115,843 268,234 71.6 28.4
1969 400,836 198,553 202,283 599,389 66.9 33.1
1970 315,070 225,263 89,808 540,333 58.3 41.7
1971 342,872 139,837 203,035 482,708 71.0 29.0
1972 406,650 175,286 231,364 581,936 69.9 30.1
1973 655,430 100,284 555,146 755,715 86.7 13.3
1974 227,433 73,887 153,546 301,320 75.5 24.5
1975 207,392 105,227 102,165 312,619 66.3 33.7
Ave. 394,384 159,665 234,722 554,053 70.7 29.3


to the south and to the north for the 20 year period.


By convention, positive


transport denotes southward and negative transport denotes northward. The net
drift Qnet, is the difference between the positive and negative components, while
the gross drift, Qgros,, is the sum of the drift magnitudes.

The average value of annual net transport rate of 234,722 yd3 per year falls
in between those estimated by previous investigations. This value may be on the
high side as wave attenuation is neglected in the wave transformation from deep
to shallow water.

Table 15 shows the monthly littoral drift statistics based on daily longshore
transport values for the 20-year period from 1956 to 1975. Table 16 shows the
statistical parameters including the mean, / and the standard deviation, a, of
the daily transport rate, and also the mean and standard deviation of the daily
log-transport rate, plog and alog, respectively, for the same 20-year period.

The overall seasonal trend of the longshore transport is similar from the 20-year
data. During winter (from October to February), the net transport is southward












Table 15: Monthly percentage of longshore transport direction from 1956 to 1975.

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
%South 65.7 65.6 61.0 61.8 44.2 40.5 26.3 36.3 57.3 76.9 70.0 70.0
%North 34.2 34.2 39.0 37.7 54.8 58.7 71.6 62.1 42.2 22.9 30.0 29.8
% Zero 0.1 0.2 0.0 0.5 1.0 0.8 2.1 1.6 0.5 0.2 0.0 0.2


Table 16: Statistics of longshore transport rate (yd3/day) from 1956 to 1975.


Month P a piog t og
Jan (South) 3,318 5,190 3.013 0.804
(North) 1,477 2,211 2.671 0.808
Feb (South) 2,673 4,148 2.936 0.777
(North) 1,631 2.520 2.728 0.795
Mar (South) 2,476 3,708 2.951 0.728
(North) 1,741 2,927 2.744 0.752
Apr (South) 352 510 2.212 0.598
(North) 412 472 2.185 0.845
May (South) 1,136 2.236 2.546 0.827
(North) 1,111 1,599 2.505 0.886
Jun (South) 532 1,145 2.132 0.814
(North) 755 1,492 2.302 0.841
Jul (South) 351 453 1.992 0.867
(North) 503 857 2.199 0.831
Aug (South) 718 1,740 2.257 0.838
(North) 533 861 2.259 0.739
Sep (South) 1,407 2,701 2.626 0.706
(North) 961 1,891 2.473 0.787
Oct (South) 2,068 3,260 2.882 0.661
(North) 1,232 1,864 2.653 0.692
Nov (South) 2,767 3,981 2.974 0.744
(North) 1,452 1,890 2.741 0.753
Dec (South) 2,733 3,999 3.045 0.676
(North) 1,615 2,483 2.726 0.781








characterized by episodic large transport rate associated with Northeast Storm
events. During summer (May to August), the transport rate is much smaller and
the duration of northward transport becomes longer. Late spring and early fall
form the shoulder seasons with less predictable trend. Clearly, the bulk of the
annual cumulative transport rate is due to episodic events. Since episodic events
vary greatly from year to year, it helps to explain the large variance of the annual
transport rate as calculated from WIS wave data.


6.3 Ebb Shoal Volume


In the presence of an inlet, sediments which would have been transported to
the downdrift beach are instead being carried into the inlet by flood current as
well as being pushed offshore by ebb tidal currents. Therefore, the shoal formed
in the offshore region is known as the ebb tidal shoal. The rate of accumulation
of sediment in the ebb shoal thereby causes a depletion of sand at the downdrift
side of the inlet and needs to be accounted for in sediment budget analysis. From
the field experience, this quantity could be very large. Unfortunately, it is also
very difficult to estimate this volume and it is even more difficult to estimate the
annual volume change for lack of adequate long term data. Earlier estimates of
the ebb tidal shoal volume varied widely from 50,000 to 1,5000,000 yd3.

The ebb shoal volume is estimated here from 1987, 1988, and 1989 topographic
survey data using a quasi-equilibrium profile as the reference. Determining the
equilibrium profile that would exist in the absence of the inlet requires some judge-
ment and experience. It was determined that the northernmost profile for each
year was the best representation of the equilibrium profile and was used as the basis
for calculating the ebb shoal volume. The volume was first calculated by using the
Trapezoidal Rule as an estimate and then by Simpson's Rule with 100 ft interval
to obtain more accurate result. The ebb shoal was considered to extend from the
north jetty to the southernmost edge of the available survey data, and offshore to
an estimated closure depth of 36 ft(see Figure 30). The southern offshore limits of
the survey data fell slightly short of the full extent of the ebb shoal. To account
for the offshore portion of the ebb shoal, extrapolation from the existing survey
data was used to estimate the additional volume (about 5% addition of volume was
estimated). However, any additional volume to the south of the survey boundaries
was neglected due to the inaccuracy of extrapolating data in this region. The ebb
shoal volumes are shown in Table 17. Figure 31, for instance, shows contours of
the elevation differences for the 1989 survey. Three-dimensional images of the ebb
shoal were generated and shown in Figure 32 from two viewing angles. Figure 32
reveal two very interesting aspects. First, the shape of the ebbshoal with respect
to the reference plane appears to be very symmetrical even though it is crescent-
shaped in the prototype. Second, the sand deficit in the downdrift nearshore zone
and the ditch effect as discussed before are clearly demonstrated.










SEBASTIAN INLET CONTOUR MAP 1989


Region A: High Wave Ampfcatllon
Region B: Ebb Shoal Dominates the Wave Pattern
Region C: Sheltered Area, Waves Diverge to Banks


Depth Contour In teet


-8.00 0.00


8.00 16.00 24.00 32.00 40.00 48.00 56.00


X (ft) x 100

Figure 30: Bathymetric Survey map for 1989.


Table 17: Ebb shore volume computed for 1987 to 1989.


Volume From Extrapolated Total Estimated
Survey Data Volume Ebb Shoal Volume
Year (yd3) (yd3) (yd3)


1987 1,320,000 90,000 1,410,000


1988 1,390,000 90,000 1,480,000


1989 1,220,000 60,000 1,280,000


32.00-




24.00-

o
0

x 16.00-

>-


8.00-



0.00 -


34
-32

26--

22--


:14


-0c.uv





























Figure 31: Contours of ebbshoal volume computed from 1989 survey.

From these values, it appears that the ebb tidal shoal is rather stable in recent
years. Unfortunately, these values are usually large that even a small annual change
here could dominate other quantities in the sediment budget. For instance, the
volume difference between 1988 and 1987 is 70,000 yd3 which represented only
5% change. However, this 70,000 yd3 is about one third of the estimated annual
background transport rate. The difference between 1989 and 1988 is 200,000 yd3
which would certainly overshadow other transport quantities. Since computations
are carried out based on survey data over a large area, small survey inaccuracy will
also result in substantial error. Again, one must approach this value with extreme
caution.



6.4 Interpretation from Laboratory Results


The laboratory results suffer two drawbacks. First, they are for short term
extreme events and can not be easily extrapolated to long term sediment budget
application. Second, the updrift condition is significantly influenced by the bound-
ary effects. Therefore, one can not rely on the data to establish updrift transport
rate. The results are believed to be reliable for downdrift boundary longshore
transport, downdrift shoreline erosion, ebb tidal shoal changes and transport to
inlet as given in Section 4.5. These results do confirm the fact that sediment
transport and hydrographic changes are dominated by storm events.


/




















SOUTH
JETTY

































Figure 32: Orthographic plots of ebb shoal volume from 1989 survey.




4


The results from the SO structure configuration showed that a 6-day NE storm
of 6 ft wave produced about 2,200 yd3/day average transport rate on the downdrift
boundary whereas a moderate NE wave of 2 ft produced only 480 yd3/day average
downdrift transport. This is also true for net sediment losses into the inlet. For
storm waves this net loss amounted to 1,400 yd3/day and it reduced to less than
100 yd3/day for the moderate normal wave condition. The ebb tidal shoal behaved
differently under storm and normal wave conditions. During NE storm, the ebb
tidal shoal immediately downdrift of the inlet lost material to the downdrift and
offshore directions. Under moderate NE waves, the ebb tidal shoal region immedi-
ately south of the inlet gained material. Therefore, as discussed earlier that during
extreme events the ebb tidal shoal expanded towards downdrift and offshore at the
expense of drawing sediment from the existing shoal. Also part of the ebb tidal
shoal material was fed into the downdrift littoral zone which eventually ended in
the downdrift beach. On the other hand, during moderate NE waves, the material
lost in the existing shoal was being replenished by the updrift material.

For waves from east, the net littoral transport was nil. Material, however, was
continuously lost to the inlet mainly at the expense of downdrift shore erosion.
For the SE waves, the quality of data was not as good. The general pattern shows
continued loss of sediment into the inlet, erosion at the south side and modest
growth of ebb tidal shoal.



6.5 Sediment Deficit Estimation


Sediment deficit estimation is always an extremely difficult task for a number of
reasons. First of all, the annual longshore transport rate varies greatly from year
to year. Secondly, the change of ebb shoal volume is a dominant factor but current
information is insufficient to make a reasonable estimate. Finally and foremostly
there is simply a lack of long-term reliable field information to facilitate such an
estimate.

The sediment deficit estimation made here is extremely simplified. It is as-
sumed here that the northshore is stable, the ebb shoal is in a state of equilibrium
so that the net deficit is approximately equal to net loss of sand into the inlet. Fur-
thermore, it is assumed that the laboratory results obtained under storm events
can be extrapolated to annual application based upon the idea of equivalent storm
days.

The equivalent deepwater wave conditions for the storm waves used in the
laboratory can be established by the following equation:
H
Ho = (8)
K.K,


1








where K, and Kr are the shoaling and refraction coefficients, respectively. The
laboratory water conditions are H=6 ft. T=6s, 0 = 100 and h = 30 ft. The cor-
responding deepwater wave conditions are Ho = 6.5ft and 00 = 140. Substituting
these values into Eq.(6) using K=0.28, the longshore transport rate is calculated to
be 9,500 yd3/day. Based upon Table 14 the average annual southward transport
is about 394,000 yd3 and the average northward transport is 160,000 yd3. The
equivalent storm days are:


Equivalent storm days causing southward transport=394,000/9,500=42 days

Equivalent storm days causing northward transport=160,000/9,500=17 days


Since both southward transport and northward transport contribute to sediment
loss to the inlet, the total annual loss is obtained by:

Qd = Qin D. + Qis Ds (9)

where Qd is the annual sediment deficit; Qln is the net inlet loss per day due to
NE storm; Qi, is the net inlet loss per day due to SE storm; D, is the equivalent
transport days due to NE storm and D, is the equivalent transport days due to
SE storm.

From Figure 33, Q&l=1,420 yd3/day and Ql,=910 yd3/day. Therefore, the
average Qd based on Eq.(9) is

Qd = 1,420 x 42 +910 x 13 = 75,110

or about 75,000 yd3 a year. Since the littoral transport rate varies greatly from
year to year, the annual deficit is also expected to vary greatly. Once again, the
value calculated here only represents the net sediment loss to the inlet.


6.6 Sediment Budget


Based upon analysis give above the annual sediment budget can be constructed.
Figure 33 shows the balance for the control area given. This balance is based on the
average values presented earlier in Table 14. As one can see, the balance is a mix
of large and small quantities. Various quantities are obtained by various means at
various time scales. For instance, the beach erosion rate is based on the average
value of 23 years from field evidence during which time, numerous sand transfer has
been carried out; the background transport is based on empirical equation using
20 years hindcast data; the net sand transport into the inlet, on the other hand, is
based on laboratory data. Perhaps the weakest link of the entire balance act is the
assumption that the ebb shoal is in a state of equilibrium. As explained earlier,
































S**3 \ Sea
S" _75,000 yd3



Bay



6,800 yd-
40,000 ft








S164,500 yd3




Figure 33: An estimate of annual sediment budget.








the ebb shoal quantity is large, any fluctuation in this quantity may have a large
effect on the sediment budget. Unfortunately, the laboratory evidence shows that
although the entire ebb shoal remains reasonably stable visible changes will occur
under storm events, particularly in the top layer. Since the frequency of storm
events varies greatly from year to year as can be witnessed from the 20 years wave
hindcast data the annual ebb shoal volumetric fluctuation is also expected to be
large.



7 Summary and Recommendations


Movable bed experiments were carried out for Sebastian Inlet to assess the sediment
transport process. The main objective of the study is to explore various structural
alternatives aimed at improving inlet navigation as well as beach preservation in
the vicinity and, particularly, downdrift of the inlet. A total of nine configurations
were tested. These nine configurations constitute various combinations of jetty
structure modification, ebb shoal material removal, and beach nourishment. The
test conditions consisted of 6-day (prototype equivalent) storm waves from NE and
SE; 8-day recovery waves from E and 8-day normal waves from NE direction.

In addition, a sediment budget analysis for the study region was performed
aimed at determining the annual sediment deficit.


7.1 Summary

The findings from this study are summarized here:


The updrift zone north of the north jetty is relatively stable for all structural
configurations under all test wave conditions except for the case where both
jetties were removed. Under storm conditions, material in the nearshore
zone moves to offshore to form a bar as one expects. Extension of north
jetty induces sediment impoundment on the northside. The magnitude of
this impoundment is found to be modest. The location of the impoundment
is mainly in the offshore region, near the tip of the north jetty extension and
further updrift, and the area is rather diffused. Under normal NE waves,
the beach recovers by smoothing out offshore bar. Waves from east is less
effective in this updrift zone recovery.

The downdrift shoreline immediately south of south jetty (to approximately
2,000 ft south) suffers recession for all structural configurations under storm
wave conditions from NE and SE. It also suffers mild recession under normal








NE wave condition.Most the shoreline recession is confined within 2,000 to
2,500 ft from the south jetty. The shoreline recession is the smallest for
the existing structural configuration (SO). Extension of north jetty increases
downdrift shoreline recession. This is also true for the short south jetty
extension (150 ft). The magnitude of recession is estimated to be around 70
ft 50 ft for the 6-day NE storm. Of this magnitude, 50% or more takes place
within the first 24 hours. The rate of recession decreases almost exponentially
as the storm progresses. Also, the magnitude of recession decreases downdrift
and becomes negligible beyond 2,000 ft.
Full removal of ebb shoal causes further increase of shoreline recession but
partial removal has negligible added effect. Beach nourishment without cor-
responding extension of south jetty causes drastic shoreline recession with re-
spect to the nourished shoreline position. This large recession can be largely
eliminated by extending the south jetty beyond the nourished shoreline po-
sition.
Under east swell-like wave condition, the shoreline (+2 ft NGVD) recovers
but the foreshore slope steepens considerably resulting net loss of sand in the
nearshore zone.

* The ultimate shoreline position that can be maintained in the vicinity of the
south jetty appears to be dictated by the length and configuration of the
south jetty. Under the existing condition, the current shoreline appears to
be in a stable position.

* The most visible bathymetric changes occur in the nearshore zone on the
south side of the jetty, where sediment transport is active due to strong
nearshore current. This is also seen as a zone of sediment deficit which
attracts sand from both beach face and ebb shoal. Because of the strong
nearshore currents, post-storm recovery in this region appears to be difficult.

* For most of the structural alternatives (except S6 and S7), both shoreline
recession and nearshore erosion are modest, certainly no worse than open
coast erosion under storms of similar strength. Unlike plane beach on open
coast, recovery is difficult in the vicinity of the south jetty.

* There are two more regions where sediment movement is active, one in the
outer region around the north jetty and the other over the ebb shoal. In the
former region, it is erosional under storm condition on both side of the jetty
and accretional under recovery condition on the north side, mainly in the
offshore area outside the jetty. However, the rate of accretion or erosion does
not appear sufficient as a potential permanent bypassing site. The sediment
movement in the ebb shoal is active under storm condition. Sediment in this
region is diverted into the channel, fed into the downdrift littoral zone and
transported southward in the offshore zone. It appears that the offshore zone
transport is substantial.








The ebb shoal has a total volume of about 1.3 million yd3. It is overall a
stable shoal with most of the sediment movement occurring in the top layer
which moves southward.

The region between the jetties is, overall, accretional and the inlet behaves
like a sediment sink under all tested conditions. Sediment converges into the
inlet from three sources: updrift around the north jetty, downdrift from the
ebb shoal and nearshore material around the south jetty with the last one
being the major contributor. Consequently, shoaling mainly occurs near the
tip of south jetty. This shoal, from time to time, will spill into the navigation
channel. The navigation channel, however, is not threatened, due to strong
current. Adequate extension of south jetty could cut sand loss into the inlet
significantly. The test result, for instance, shows that for a 500 ft south
jetty extension sediment transported into the inlet was reduced to about
one-fourth of that of existing configuration under a 6-day storm.

Ebb shoal removal, extension of south jetty and beach nourishment all induce
increased rate of downdrift transport. The rate increase from the first two
sources may be at the expense of the accelerated shore erosion if no adequate
sediment supply is provided. Extension of north jetty retards downdrift
transport slightly as more sediment being diverted offshore.

Based upon historical shoreline survey information the influence of inlet is
estimated to be around five miles on each side of the inlet. The averaged
annual shoreline change and the associated erosion as well as accretion are
modest. The analysis, however, did not consider the numerous sand transfer
activities that took place in the past.

The average background littoral transport rate is around 394,000 yd3/year
southward and 160,000 yd3/year northward. This yields a net southward
transport of 234,000 yd3/year. These values may be on the high side owing to
the simplified assumption employed in computing the deepwater wave height.
The results clearly demonstrates that the sediment transport is dominated
by episodic events. Consequently, the variations of annual rates are large.

Sediment budget analysis was performed. The average sediment deficit is
estimated to be in the order of 75,000 yd3 a year. In this computation, the
ebb shoal is assumed to be stable with no net annual gain or loss. The actual
deficit will have large variations from year to year.


7.2 Recommendations


The recommendations made here are based on the experimental results from both
fixed bed model and movable bed model with the weighted considerations of im-
proving navigation as well as mitigating downdrift erosion. The recommendations








provide a general frame of improvement scheme and are given in order of prefer-
ence. A plan to reach the final design configuration is also presented.

Structural Modifications

It is recommended here that the south jetty be extended. The length of the
extension is estimated to be in between 300 to 500 ft. The shape of the south jetty
extension should follow the north jetty contour such that the extension remains in
the shadow zone of the north jetty and that the cross-sectional area of the inlet
channel is maintained at the existing condition. This extension is expected to
significantly reduce the net drain of sand into the inlet, reduce the shoaling rate
in the inlet region and promote downdrift transport. Again, a 500 ft extension
such as tested in the movable bed experiment is seen to cut the loss of sand into
the inlet over 75% under 6-day storm condition. In concurrence with the jetty
modification considerations should be given to nourish the beach in the immediate
downdrift of the south jetty to soften the impact of the sudden structural change.
This nourishment if deemed desirable is expected to be rather modest. One may
also wish to consider dredging the shoaling zones within the jetty so as to ease the
strong current-wave interaction near the entrance.

It is also recommended here to consider north jetty modification. There are
two viable options. The first one is to sand tight the existing jetty and monitor its
performance for a number of years. The second one is to extend the north jetty
in the order of 200 to 250 ft. The first one will likely to further cut down shoaling
inside the inlet. The cost is likely to be modest but the extent of benefit is not
clear. The second option improves the wave condition in the navigation channel.
For the 250 ft extension tested in the laboratory, the wave height reduction under
NE storm condition is found to be in the order of 25-50% under ebb current and
of 18-45% under flood current. The extension of north jetty will, however, reduce
downdrift sand transport initially. This reduction might be transient. A longer
test period is required to establish the real long term impact. The ebb shoal is
also expected to grow slightly under this jetty modification.

Finally, no jetty modification remains an option, though not a desirable one. In
this case, sandtrap dredgings, at least, at the current frequency need to be main-
tained and downdrift shoreline is likely to experience oscillations when compared
with the alternative of south jetty extension.

Sand Transfer Plans

Sand trapping in the inlet still appears to be the most efficient means. It is
estimated here that the average annual sand drain into the inlet is in the order of
75,000 yd3. The actual annual quantity, however, is expected to vary owing to the
fact that the transport is storm dominated. With the south jetty extension, this
quantity is expected to reduce significantly. It is recommended here to develop a








sand bypassing scheme based on inlet sand trapping to soften the impact of the
predicted large annual variations in sand deficit.

The sand storage in the ebb tidal shoal is substantial comparing with the
demand of sand deficit in the downdrift. The re-generating process after removal is
shown to be rather rapid but the area is diffused. It remains a viable option as sand
source for nourishment and renourishment. However, further studies are required
to establish the impact, the optimum utilization, and the economic viability.

The test results show that the accumulation of sand on the northside is at most
moderate and the area of accumulation is rather diffused and in the offshore zone.
Therefore, permanent sand bypassing plant on the northside is not recommended
at this stage.

Recommended Follow-up Actions

This study provides specific information on the current and wave conditions
in the inlet region, the process, deficit and budget of sediment transport, and the
effects of structural modifications. Also, to the extent possible the cause and effect
relationships are identified. The desirable follow-up actions are listed below:

Immediate Plan

Determine specific design configuration for structural modification. This
entails the determination on the optimum length and configuration of the
south jetty extension based on extended test period to determine the long
term benefit of channel dredging reduction and downdrift transport improve-
ment. The desirability and/or the extent of initial beach nourishment after
jetty extension as well as the benefit and/or the effect of initial dredging
of shoaling zones in the inlet should also be determined. With the south
jetty modification determined the incremental beneficial effect of north jetty
extension could be tested if deemed desirable.

Design and evaluate sand transfer scheme/schedule based upon inlet trap-
ping as the primary means of sand collection. This design should take into
consideration the anticipated annual variations and its effect on the down-
drift.

Long Term Plan

Explore the option of ebb shoal sand utilization in the long term planning of
inlet management.

Extend the study area downdrift 3-5 miles which is judged to be the limit of
influence zone by the Sebastian Inlet.








the ebb shoal quantity is large, any fluctuation in this quantity may have a large
effect on the sediment budget. Unfortunately, the laboratory evidence shows that
although the entire ebb shoal remains reasonably stable visible changes will occur
under storm events, particularly in the top layer. Since the frequency of storm
events varies greatly from year to year as can be witnessed from the 20 years wave
hindcast data the annual ebb shoal volumetric fluctuation is also expected to be
large.



7 Summary and Recommendations


Movable bed experiments were carried out for Sebastian Inlet to assess the sediment
transport process. The main objective of the study is to explore various structural
alternatives aimed at improving inlet navigation as well as beach preservation in
the vicinity and, particularly, downdrift of the inlet. A total of nine configurations
were tested. These nine configurations constitute various combinations of jetty
structure modification, ebb shoal material removal, and beach nourishment. The
test conditions consisted of 6-day (prototype equivalent) storm waves from NE and
SE; 8-day recovery waves from E and 8-day normal waves from NE direction.

In addition, a sediment budget analysis for the study region was performed
aimed at determining the annual sediment deficit.


7.1 Summary

The findings from this study are summarized here:


The updrift zone north of the north jetty is relatively stable for all structural
configurations under all test wave conditions except for the case where both
jetties were removed. Under storm conditions, material in the nearshore
zone moves to offshore to form a bar as one expects. Extension of north
jetty induces sediment impoundment on the northside. The magnitude of
this impoundment is found to be modest. The location of the impoundment
is mainly in the offshore region, near the tip of the north jetty extension and
further updrift, and the area is rather diffused. Under normal NE waves,
the beach recovers by smoothing out offshore bar. Waves from east is less
effective in this updrift zone recovery.

The downdrift shoreline immediately south of south jetty (to approximately
2,000 ft south) suffers recession for all structural configurations under storm
wave conditions from NE and SE. It also suffers mild recession under normal








References


[1] Law of Florida, Chapter 12259.

[2] Mehta, A.J., Wm.D. Adams, and C.P. Jones, 1976. "Sebastian Inlet Glossary
of Inlets, Report #3," Coastal and Oceanographic Engineering Laboratory, Uni-
versity of Florida. UFL/COEL-76-011.

[3] Wang, H., L. Lin, H. Zhong, and G. Miao, 1991. "Sebastian Inlet Physical
Model Studies: Part I Fixed Bed Model," Coastal and Oceanographic Engi-
neering Department, University of Florida. UFL/COEL-91-001.

[4] Engineering and Industrial Experiment Station, 1965. "Coastal Engineering
Hydraulic Model Study of Sebastian Inlet, Florida," Coastal and Oceanographic
Engineering Laboratory, University of Florida. UFL/COEL-65-006.

[5] Coastal Technology Corporation, Florida, 1988. "Sebastian Inlet District Com-
prehensive Management Plan".

[6] Wang, H., T. Toue, and H.H. Dette, 1990. "Movable Bed Modeling Criteria
for Beach Profile Response," Proc. 22nd Coastal Engineering Conf., Deft, The
Netherlands, pp.2566-2579.

[7] Wang, H., L. Lin, H. Zhong, and G. Miao, 1991. "Sebastian Inlet Physical
Model Studies: Part II Movable Bed Model, Interim Report" Coastal and
Oceanographic Engineering Department, University of Florida. UFL/COEL-91-
014.

[8] Ahn, K., A. Assaly, L. Wicker, M. Wrock, S. Samuel, T. Oh, and T. Kim, 1991.
"Downdrift Erosion Prevention Project at Sebastian Inlet, Florida," Coastal and
Oceanographic Engineering Department, University of Florida.

[9] Shore Protection Manual, 1983. Coastal Engineering Research Center, Corps
of Engineers, U.S. Army. Vicksburg, Mississippi.

[10] Harris, P., 1991. "The Influence of Seasonal Variation in Longshore Sediment
Transport with Applications to the Erosion of the Downdrift Beach at Jupiter
Inlet, FL." Coastal and Oceanographic Engineering Department, University of
Florida. UFL/COEL-91-015.


















APPENDIX I:
Summary of Bathymetric Change Figures
for S1 to S8 in 6-Day NE Storm Process.














Contours ofSl ofter 6-Day (NE)Storm


Figure I.1: Bathymetric changes for Sl and S2 in NE storm.


Bothymetric













Contours of S3 ofter


tion(contours in 1/4 inch)


Contours of S4 after 6-Day(NE)Storm


Figure 1.2: Bathymetric changes for S3 and S4 in NE storm.













Contours of S5 after


--Erosion,- Rccretion(contours in 1/4 inch)


Figure 1.3: Bathymetric changes for S5 and S6 in NE storm.












Bothymetric Change Contours of S7 after 6-


Contours of S8 after 6-Day(NE)Storm


Rccretion(contours


Figure 1.4: Bathymetric changes for S7 and S8 in NE storm.


















APPENDIX II:
Summary of Bathymetric Change Figures
for S1 to S6 in 8-Day E Recovery Process.











Bothymetric Change Contours ofSl after 8-Day Recovery


--Erosion, -Rccretion(contours in 1/4 inch)


Figure II.1: Bathymetric changes for S1 and S2 in recovery process.













Bothymetric Change Contours of S3 after 8-D(


--Erosion,- Rccretion(contours in 1/4 inch)


Contours of S4 after 8-Day Recovery


Figure 11.2: Bathymetric changes for S3 and S4 in recovery process.











Bathymetric Change Contours of 55 after


Figure 11.3: Bathymetric changes for S5 and S6 in recovery process.


















APPENDIX III:
Summary of Bathymetric Change Figures
for S2,S4,S6 in 8-Day NE Moderate Wave Process.













c Change Contours of S2 ofter


--Erosion,- Rccretion(contours in 1/4 inch)


Figure III.1: Bathymetric changes for S2 and S4 under moderate waves.





























Figure III.2: Bathymetric changes for S6 under moderate waves.



















APPENDIX IV:
Summary of Bathymetric Change Figures
for S3 and S5 in 6-Day SE Storm Process.











Contours of S3 ofter


Bathymetric Change Contours of S5 after 6-Day(SE)Storm


--Erosion,- Rccretion(contours in 1/4 inch)


Figure IV.1: Bathymetric changes for S3 and S5 in SE storm.

















APPENDIX V:
Summary of Southside Profile Changes
for S1 to S8 in 6-Day NE Storm Process.













MEASURED PROFILES STRUCTURE: Sl
0.5 INITIAL PROFILE
----POST 6-DAY (NE) STORM
S-----POST 8-DAY RECOVERY




-0.5
0.5


0 J16

I--
0


-0.5
o
> 0.5
z

C) 0 J 2 0




0.5


0 J22
0


-0.5
ir 0.5
z



I J2


0.5






-0.5

0 10 20 30

SEAWARD DISTANCE (FT)


Figure V.1: Comparisons of surveyed profiles from J12 to J27 for Sl.



83













MEASURED PROFILES


STRUCTURE tS2
INITIAL PROFILE
-----POST 6-DAY (NE)STORM
-----POST 8-DRY RECOVERY


J14








J15








J16








Jl8


SEAWARD DISTANCE (FT)



Figure V.2: Comparisons of surveyed profiles from J12 to J18 for S2.



84


-0.5 L

0.5r


-0.5L

0.5r


-0.5 L

0.5r


-0.5 L


0.5


0


-0.5


-J___ __ ------------------^f A


"L


I


T


0.5

0 -
0


-0.5 -

0.5r


q. I













MEASURED PROFILES STRUCTURE S2
0.5 INITIAL PROFILE
-----POST 6-DAY (NE) STORM
0 J20 --- POST 8-DRY RECOVERY
IN J20
0


-0.5
0.5


0 J22


-0.5

> 0.5


o 0 J23


- -0.5
CA







0 0 --- ^ ---- J26
0.5

Uj 0.5
-0








.---^
-j -0.5

z






0 'v_ J27
c 00


-0.5






-0.5

0 10 20 30

SEAWARD DISTANCE (FT)


Figure V.3: Comparisons of surveyed profiles from J20 to J27 for S2.














MEASURED PROFILES


STRUCTUREa S3
--- INITIAL PROFILE
-----POST 6-DAT (NE) STORM
-----POST 8-DRY RECOVERY


J12


0.5


0


-0.5

0.5


0


-0.5

0.5


0


-0.5

0.5


0


-0.5

0.5


0


-0.5

0.5


0


-0.5


SEAWARD DISTANCE (FT)


Figure V.4: Comparisons of surveyed profiles from J12 to J18 for S3.


J13


- _


-





J14








J15








J16








JIB













MEASURED PROFILES STRUCTURE: S3
0.5 INITIAL PROFILE
-----POST 6-DAY (NE) STORM
-----POST 8-DAY RECOVERY
0J20




-0.5







0



~ 0 J24
S0.5






J26
W 0


-0.5
o
z

















J28
S-0.5 -
,,











0.5
I-
















0 J.30
-0.5 -






0 10 20 30

SEAWARD DISTANCE (FT)

Figure V.5: Comparisons of surveyed profiles from J20 to J30 for S3.


87
-J



















87




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