Title: Short term impoundment of longshore sediment transport
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Title: Short term impoundment of longshore sediment transport
Physical Description: Book
Language: English
Creator: Bodge, Kevin R., 1958-
Copyright Date: 1986
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Bibliographic ID: UF00102766
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Full Text

SHORT TERM IMPOUNDMENT
OF LONGSHORE SEDIMENT TRANSPORT




By

KEVIN R. BODGE


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1986





ACKNOWLEDGEMENTS


This study was sponsored by the U.S. Army Corps of Engineers

Coastal Engineering Research Center (CERC) at the Waterways Experiment

Station, Vicksburg, MS, under Contract No. DACW-39-85-C-0062, Dr. Lee

Weishar, technical monitor. Additional support was provided by the

University of Florida Department of Coastal and Oceanographic Engineer-

ing, Gainesville, FL. The field work of the study was executed at the

CERC Field Research Facility (FRF) at Duck, NC, with the cooperation of

Curt Mason, Chief of the Facility. The laboratory phase of the study

was conducted at the University of Florida Coastal and Oceanographic

Engineering Laboratory. The support of Mr. Marc Perlin, Lab Director,

and the entire Laboratory staff was instrumental in the successful

completion of both the field and laboratory efforts of the study.

My thanks are extended to Dr. Michel Ochi, Dr. Joseph Hammack, Dr.

Charles Taylor, Dr. Anthony Randazzo, and Dr. Daniel Spangler, who

served as the doctoral committee, and especially to Dr. Robert Dean,

chairman. My six years of friendship and graduate work with Professor

Dean have been a challenging and always intense educational experience.

The support of many friends and colleagues, including David Mess,

Karyn Erickson, Jon Lott, Chris and Cheryl Jones, Janet Hearn, David

McGehee and Robin Hoban, Gregg Vontz, Wade Jefferey, and Barb Kirby, is

warmly appreciated. Special thanks go to my colleague Bill Buckingham

for enthusiastically introducing me to Gainesville, and to Tom Bolin,

who competently assisted with the laboratory work, often fromt dawn to

ii





well past dusk. The support and patient technical advice of my

colleagues lordanis Sahinoglou, Claudio Neves, Dr. James Kirby, Dr.

Sergio Schmidt, William Dally, David Kriebel and particular Dr. Peter

Nielsen, is much appreciated--especially as I recall the particularly

gloomy hours of research when science appeared impenetrable. Addition-

ally, Gail Terry and Lillian Pieter provided valuable assistance with

the manuscript and the figures, respectively.

Special thanks are extended to the Gainesville Community Playhouse,

which warmly welcomed me back to the stage when I was struggling to

regain the sanity I had lost to graduate school, and also to my '76 MGB

which patiently awaited me outside my office every night till 4 a.m.,

(and provided hours of shade while I labored underneath it on weekends).

As in so many similar circumstances before, Dr. Barry Anderson

provided the example for me to push through doctoral candidacy. Barry's

drive, compassion, ethical sensibility, and appreciation of life con-

tinue to serve as my daily source of inspiration. The world's babies

are in good hands for a while.

The assistance in the field of Paul Rodriguez and especially Danny

Brown is gratefully acknowledged. Very special thanks are given to

Brack Stovall and Diana Cronin whose friendship, philosophy, and dedi-

cated assistance made the field work possible. It was an unforgettable,

three month, existential experience in a 17-foot long trailer in a

parking lot behind a sand dune. Science will never be the same.

Finally, this work is dedicated to my beautiful roommate and my

wonderful mother and father. My deep appreciation for my parents is

reflected in every day which I labored through school, and my feelings

for Cristina are more important than any words which I have written in

this long paper.





TABLE OF CONTENTS



Pag~e

ACKNOWLEDGEMENTS .. .. .. .. ... .. .. .. . .. ii


LIST OF TABLES . .. ... ... .. .. ... .. .. .. viii


LIST OF FIGURES .. .. ... .. .. .. .. .. .. .. .. ix


ABSTRACT . . . . . . . . . . . . . . xviii


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


2. REVIEW OF LONGSHORE SEDIMENT TRANSPORT RELATIONSHIPS AND
CROSS-SHORE DISTRIBUTION DATA ... ... .. .. .. .. 6


2.1 Fundamental Expressions for Total Longshore Sediment
Transport .. .... .. .. .. .. .. .. .. 6
2.2 Existing Distributed Longshore Sediment Transport Models 19
2.3 Existing Field and Laboratory Data for the Distribution
of Longshore Sediment Transport Across the Surf Zone .. 32
2.4 Chapter Summary .. .. .. .. .. .. .. ... . 51


3. FIELD INVESTIGATION: EXPERIMENTAL METHOD AND DATA PRESENTATION 53


3.1 Introduction ...................
3.2 Experimental Method ...............
3.2.1 Overview .................

3.2.2 Groyne Construction and Removal ......
3.2.3 Profiling Techniques ...........
3.2.4 Groyne Deployment and Profiling Procedure .
3.2.5 Additional Measurements ..........
3.3 Description of the Experimental Conditions ....
3.3.1 Overview .................
3.3.2 Groyne #1 .................
3.3.3 Groyne #2 .................
3.3.4 Groyne #3 .................
3.3.5 Groyne #4 .................
3.3.6 Sediment Characteristics .........


4. TIDAL DECONVOLUTION AND
REMOVAL OF CROSS-SHORE TRANSPORT EFFECTS .......


4.1 Introduction ...................
4.2 General Description of Approach .........


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93
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100
108
113
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125

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127
127
127
133
144
152
156


4.3 Tidal Deconvolution Techniques . ...........
4.3.1 "Simple" Tidal Deconvolution ...........
4.3.2 Matrix Tidal Deconvolution ............
4.3.3 Illustrative Evaluation of Tidal Deconvolution
through Numerical Simulation ...........
4.3.4 Tidal Deconvolution and Field Data ........
4.4 Removal of Cross-Shore Transport Signals ........
4.5 Calculation of Longshore Transport Rate at a Depth Contour
4.6 Limiting Depth Contours of Barrier Effectiveness . ..
4.7 Total Longshore Transport Rate . ...........
4.8 Expressing the Longshore Transport Distribution
across the Surf Zone Width ...............

5. RESULTS FROM THE FIELD INVESTIGATION .............


5.1 Introduction .................
5.2 Distribution of Longshore Sediment Transport .
5.2.1 Groyne #2 ...............
5.2.2 Groyne #3 ...............
5.2.3 Groyne #4 ...............
5.3 Total Longshore Sediment Transport ......
5.4 Limiting Effectiveness of the Groynes ....
5.5 Offshore Distribution of Longshore Transport
for: the Two "Best" Data Sets .........

6. LABORATORY INVESTIGATION:
MODEL APPARATUS AND EXPERIMENTAL METHOD ......


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6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8


Introduction........ .....
The Physical Model.. .**.****
Profiling Apparatus.............
Wave Measurement .. ..........
Longshore Current and Wave Angle Measurement
Fluorescent Sand Tracer.. ........
Experimental Procedure ...... .....
Laboratory Data Analysis .... ......


7. LABORATORY INVESTIGATION: RESULTS..............

7.1 Introduction.................... ..
7.2 Plunging/Spilling Series--Field Experiment Conditions


Without Tide .................
7.2.1 Description of Experiment .......
7.2.2 Appropriate Groyne Profile ......
7.2.3 First Impoundment Interval ......
7.2.4 Second Impoundment Interval ......
7.3 Plunging Series ...............
7.3.1 Description of Experiment .......
7.3.2 No Groyne "Control" Interval .....
7.3.3 Impoundment Data Analysis Methodology .
7.3.4 First Impoundment Interval ("B to C") .
7.3.5 Second Impoundment Interval ("C to D")
and Smearing .............
7.3.6 Bed and Streamer Traps ........


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7.4 Plunging/Collapsing Series and Tests with
Simulated Tidal Fluctuations .........
7.4.1 Description of Experiment . ....
7.4.2 Tests with Tidal Fluctuation ......
7.4.3 Non-Tidal Tests ............
7.4.4 Effectiveness of the Tidal Deconvolution
7.5 Collapsing Series ...............
7.5.1 Description of Experiment .......
7.5.2 Impoundment Results ..........
7.6 Spilling Series ................
7.6.1 Description of Experiment .......
7.6.2 Impoundment Results ..........
7.7 Total Transport ................
7.8 Characteristic Features of the
Longshore Transport Distribution .......


201
201
204
210
217
220
220
221
224
224
227
231


. .. 232


8. MODELING THE LONGSHORE SEDIMENT TRANSPORT DISTRIBUTION....


8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
8.12


Introduction . ............
Bagnold Model ..............
Stress Model ..............
Alternate Model #1 . ....... ..
Alternate Model #2 ...........
Alternate Model #3 ...........
Alternate Model #4 ...........
Alternate Model #5 ............
The Proportionality Constants ......
Comparison to Field Data ........
The Preferred Model ...........
Some Considerations of Non-Linear Effects
8.12.1 Preliminary Remarks .......
8.12.2 Total Transport .........


236
236
242
245
248
251
254
257
259
260
264
265
265
265

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280
280
284
287

293
298
299


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8.12.3 Shoreward Convection of Longshore Current
by Wave Mass Transport ..........
8.12.4 Net Longshore Bottom Stress Induced by
Non-Linear Wave Orbital Motion ......

9. LONGSHORE CURRENT AND SEDIMENT TRANSPORT ACROSS A
SET-UP NON-SINGULAR CONCAVE-UP BEACH .........


9.1 Introduction ...................
9.2 Non-Singular Equilibrium Profile .........
9.3 Wave Induced Set-Up and Set-Down .........
9.4 Longshore Current ................
9.5 Comparison of the Predicted Longshore Current
with the Laboratory Observations .........
9.6 Swash on a Planar Beach .............
9.7 Longshore Sediment Transport ...........


10. SUMMARY AND CONCLUSIONS......... ..........










APPENDIX A: CONSTRUCTION OF PYRAMID-STYLE SAND BAG UNITS .....


320


APPENDIX B: EVALUATION OF ELEMENTS IN THE
TIDAL DECONVOLUTION MATRIX, A .. .. ... .. 323

APPENDIX C: LEAST-SQUARES SOLUTION OF THE OVER-CONSTRAINED
TIDAL DECONVOLUTION TECHNIQUE .. ... .. .. 325

APPENDIX D: LEAST-SQUARES FIT TO THE TOTAL TRANSPORT
FUNCTION ALONG A DEPTH CONTOUR ... .. .. .. 327

APPENDIX E: ESTABLISHING BED COORDINATES FROM
LABORATORY PROFILER DEVICE .. .. .. . .. ... 329

APPENDIX F: SWASH ON A PLANAR BEACH ... . .... .. .. 333

REFERENCES .. ... ... .. .. . ... .. .. .. .. 338

BIOGRAPHICAL SKETCH .. ... .. ... .. .. . .. 345
















LIST OF TABLES


Table Page


3-1: Summary of Field Experiments .. .. .. .. .. .. .. 62

3-2: Representative Surf Conditions for Field Experiments . .. 65

3-3: Sediment Grain Size Distribution--Field Data .. .. .. 89

5-1: Total Longshore Transport--Field Data . ... .. .. 153

6-1: Physical Model Design Parameters . ... ... .. .. 164

7-1: Representative Surf Conditions for Laboratory Test Series . 174

7-2: Total Longshore Transport Estimates
from the Laboratory Impoundment Data Sets .. .. .. .. 232


viii





LIST OF FIGURES


Figure pag

2-1: Comparison of total longshore sediment transport field
data with the CERC Formula . . .. . . . 8

2-2: Comparison of total longshore sediment transport field
data with the energeticc" model . .. .. .. .. 11

2-3: Variation of the CERC Formula proportionality constant K with
wave steepness as found from laboratory investigations .. 12

2-4: Comparison between median sediment diameter and the CERC
Formula proportionality constant K from field data and
laboratory data . ...... . . .. .. .. 14

2-5: Variation of the CERC Formula proportionality constant K with
similarity parameter for selected laboratory and field data 17

2-6: Variation of the proportionality constant K*/g (from
IQ=K* Sxy) with surf similarity parameter .. .. ... 18

2-7: Normalized distribution of longshore transport across a planar
beach from the Bagnold, Svasek, and Thornton models .. .. 22

2-8: Normalized distribution of longshore transport for a planar
beach calculated for the Komar and Madsen models . .. .. 25

2-9: Example of Bailard's distributed longshore transport model. 31

2-10: Prototype measurement of suspended sediment concentration,
longshore current, and tracer advection speed (from
Zenkovitch, 1960) ** ** ** ** ** ** ***. . .. 34

2-11: Laboratory measurement of distributed longshore transport
using downdrift traps (from Bijker, 1971) * ** *. 36

2-12: Field measurement of distributed longshore and cross-shore
transport by Sawaragi and Deguchi (1978) using circular
traps in the bed at Isonoura Beach .. .. ... ... 38

2-13: Field measurement of distributed longshore and cross-shore
transport by Sawaragi and Deguchi (1978) using circular
traps in the bed at Matshuho Beach .. .. . .. .. 39





2-14: Normalized distributed longshore transport from laboratory
measurements of Sawaragi and Deguchi (1978) .. .. .. .. 41

2-15: Average normalized longshore current and wave height across
the surf zone from laboratory measurements of Sawaragi and
Deguchi (1978) .. .. .. ... .. .. . ... . .. 41

2-16: Normalized longshore transport distribution as measured
by Tsuchlya (1982) ... .. . ... .. .. ... . 42

2-17: Normalized longshore transport distribution developed from
relative rotation of depth contours at a pocket beach after
a change in wave direction (from Berek and Dean, 1982) .. 44

2-18: Normalized longshore distribution developed from sediment
impoundment updrift of a barrier on a laboratory beach
(from Fulford, 1982) .. .. .. .. .. .. .. .. .. 46

2-19: Distribution of longshore transport calculated from field
measurements of longshore current and suspended sediment
concentration (from Downing, 1984) .. ... . ... .. 48

2-20: Distribution of immersed weight longshore sediment transport
calculated from field measurements of longshore current and
suspended sediment concentration (from Sternberg et al. 1984) 49

3-1: The pyramid-shaped sand bag unit .. .. ... .. .. 56

3-2: An individual sand bag showing the fill-flap design .. .. 56

3-3: Groyne #3 at low tide .. ... .. .. ... ... 58

3-4: Groyne #4, looking landward from updrift side at mid-tide . 58

3-5: Typical survey plan for the field impoundment experiments . 59

3-6: Typical nearshore bathymetry at the field investigation
site during the impoundment experiments . .... . 64

3-7: Approximate modal wave period and significant (unrefracted)
deep water wave height during the four field experiments . 66

3-8: Breaking wave angle estimated from HF radar imagery during the
fourth field experiment (Groyne #4) .. . ... .. .. 66

3-9: Beach profiles 2 hours and 15 hours after deployment of
Groyne #1 .. .. .. .. .. ... .. .. .. .. .. 69

3-10: Tidal fluctuation, beach profiling "loop" intervals, and
groyne deployment sequence for Groyne #2 .. . .. ... 70

3-11: Beach profiles 3 m up- and downdrift of the groyne for
Groyne #2 during groyne construction and approximately
16 hours after groyne completion .. .. ... .. .. 71





3-12: Beach profiles updrift and immediately downdrift of the
barrier for Groyne #2 during groyne construction and
approximately 16 hours after groyne completion . .. ... 72

3-13: Tidal fluctuation, beach profiling "loop" intervals, and
groyne deployment sequence for Groyne #3 . ... .. .. 74

3-14: Time history of surf zone location and mean longshore
currents during Groyne #3 .. .. .. .. .. .. ... 75

3-15: Beach profiles 3 m up- and downdrift of the groyne for Groyne
#3 before, 2 hours after, and 7 hours after groyne deployment 76

3-16: Representative beach profiles measured far updrift of the
barrier for Groyne #3 before, 2 hours after, and 7 hours after
groyne deployment ... .. .. .. ... ... .. 77

3-17: Visual observation of wave height and types, and Eulerian
measurement of longshore current across the surf zone during
the post-groyne deployment interval of Groyne #3 .. .. 79

3-18: Tidal fluctuation, beach profiling "loop" intervals, and
groyne deployment sequence for Groyne #4 .. .. ... 80

3-19: Time history of surf zone location and mean longshore
current during Groyne #4, first post-groyne deployment
survey interval ... .. ... .. .. .. .. .. 81

3-20: Time history of surf zone location and mean longshore
current during Groyne #4, second post-groyne deployment
survey interval .. .. .. .. .. .. .. ... .. 82

3-21: Beach profiles 3 m up- and downdrift of the barrier for
Groyne #4 approximately 6 hours and 20 hours after groyne
construction .. .. . ... .. ... .. . .. 83

3-22: Beach profiles 3 m downdrift prior to and after construction
of the barrier for Groyne #4 . .. .. .. .. 84

3-23: Beach profiles 3 m updrift prior to and after construction
of the barrier for Groyne #4 . .. .. .. .. .. ... 85

3-24: Beach profiles 91.4 m updrift prior to and after construction
of the barrier for Groyne #4 . .. ... .. . .. ... 86

3-25: Visual observation of wave height and type, and Eulerian
measurement of longshore current across the surf zone
during the first post-groyne deployment impoundment
interval of Groyne #4 ... .. .. .. .. .. .. .. 87

3-26: Visual observation of wave height and type, and Eulerian
measurement of longshore current across the surf zone
during th~e second post-groyne deployment impoundment
interval of Groyne #4 .. .. ... .. .. .. ... 88





4-1: The coordinate system adopted for a 3-dimensional beach with
shore-perpendicular barrier and fluctuating tidal water level 91

4-2: "Simple" tidal deconvolution . .. ... .. .. .. .. 95

4-3: "Matrix" tidal deconvolution ... .. .. . .. ... 97

4-4: Simulated profile changes over time for a beach of given
initial profile subjected to a prescribed total transport
function and fluctuating tidal water level .. .. .. .. 102

4-5: Comparison of the tidally deconvolved transport functions
and the actual prescribed transport function for simple,
N=M matrix, and least-squares matrix tidal deconvolution
schemes. (Numerical simulation.) .. .. .. ... .. 104

4-6: The transport functions deconvolved from "noisy" profile-
change data compared to the actual prescribed functions.
(Numerical simulation.) .. .. ... ... .. ... 105

4-7: The running filter used to smooth the transport functions
developed through the matrix tidal deconvolution schemes . 106

4-8: The smoothed transport functions compared to the actual
prescribed transport functions developed from N=M matrix and
least-squares matrix tidal deconvolution of "noisy" profile-
change data. (Numerical simulation.) ... . .. .. .. 107

4-9: Tidally deconvolved total transport functions A(z) developed
for profile change data from survey loops 4 and 5 of Groyne #4,
9 m updrift of the barrier .. .. .. .. .. . .. 109

4-10: Comparison of the tidally deconvolved total transport functions
A(z) developed using the simple, smoothed (N=M~) matrix, and
smoothed least-squares matrix techniques. (Field data.) . 110

4-11: Tidally deconvolved total transport functions A(z) developed
for profile change data from survey loops 1 and 2 of Groyne #3,
9 m updrift of the barrier .. .. .. .. ... . ... 112

4-12: Comparison of the measured profiles at survey time t2 and
the profiles calculated from the tidally deconvolved total
transport function at time t2 using the least-squares matrix
technique. (Field data.) .. .. .. ... .. .. .. 114

4-13: Fit of the total transport function along a depth contour
using non-cumulative and cumulative values . .. .. ... 118

4-14: Fit of the total transport function along a depth contour
using non-cumulative and cumulative values .. ... .. 119

4-15: Integration of the longshore component of the total transport
function A (z) updrift of the barrier in order to evaluate
the volumetric rate of impoundment along the contour . .. 120





4-16: An example illustrating the calculation of the longshore
trapping time At (z) for which contour a was "blocked" by
the presence of the barrier during the survey interval At . 123

5-1: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #2, loops 1-3 .. .. ... 128

5-2: Effective relative trapping time and time-weighted barrier
relief for each depth contour. Groyne #3, loops 1-3 . .. 129

5-3: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #2, loops 1-2 .. .. .. .. 131

5-4: The total transport function A(z) calculated for each of 11
profiles, and illustrated along various depth contours.
Groyne #r3, loops 2-3 ** ** ** *. . . .. 134

5-5: Effective relative trapping time and time-weighted barrier
relief for each depth contour. Groyne #3, loops 2-3 . .. 136

5-6: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #3, loops 2-3 .. ... .. 137

5-7: Effective relative trapping time and time-weighted barrier
relief for each depth contour. Groyne #3, loops 1-2 .. 139

5-8: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #3, loops 1-2 .. .. .. .. 140

5-9: Effective relative trapping time and time-weighted barrier
relief for each depth contour. Groyne #3, loops 1-3 .. 142

5-10: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #3, loops 1-3 .. .. ... 143

5-11: Total transport function A(z) calculated for each of 10
profiles, and illustrated along various depth contours.
Groyne #4, loops 4-5 ** ** ** ** ** ***. .. . 146

5-12: Effective relative trapping time and time-weighted barrier
relief for each depth contour. Groyne #4, loops 4-5 .. 147

5-13: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #4, loops 4-5 ... .. .. 148

5-14: Effective relative trapping time and time-weighted barrier
relief for each depth contour. Groyne #3, loops 1-3 .. 149

5-15: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #4, loops 1-3 .. .. ... 151

5-16: Distribution of longshore sediment transport rate for still-
water depth contours. Groyne #4, loops 1-5 .. .. ... 152


xiii





5-17: The longshore transport distribution, longshore current, wave
height, and average beach profile across the surf zone for
Groyne #4, loops 4-5 .. ... .. .. .. .. ... 157

5-18: The longshore transport distribution, longshore current, wave
height, and average beach profile across the surf zone for
Groyne #3, loops 2-3 .. ... .. .. .. .. ... 158

6-1: Plan view of the laboratory model . ... ... . ... 160

6-2: Oblique sketch of the support walls, rail system, heart, and
profiler used in the laboratory model .. .. .. .. .. 165

7-1: Profile of groyne, adjacent beach profiles, and beach profile
changes immediately downdrift and updrift of the groyne.
Plunging/spilling test series .. . ... . .. 176

7-2: The total transport function determined for the first impound-
ment interval of the plunging/spilling test series .. .. 179

7-3: Illustrative sketch of wave and current pattern around the
impoundment barrier in the laboratory model .. .. .. .. 180

7-4: Graphical description of variables used to calculate the
horizontal displacement Ax, between two beach profiles for
the general case of a non-uniquely defined contour .. .. 184

7-5: Idealized total transport function A(y) and associated
transport qty) along a given depth contour (or offshore
location) updrift of the barrier on the laboratory beach . 185

7-6: The longshore transport distribution, longshore current,
wave height, and average beach profile across the surf zone
for the first impoundment interval of the plunging/spilling
test series ** ** ** ** ** ** ***. . . ... 188

7-7: Comparison of the longshore transport distribution found from
the first and second impoundment intervals of the plunging/
spilling test series .. . .. .. .. *. .. . 190

7-8: Beach profile change before groyne deployment at the site of
the groyne for the plunging test series . .. *. .. 192

7-9: Beach profile changes at 5 m updrift and downdrift of the
groyne for the first and second impoundment intervals of the
plunging test series ** ** ** ** ***. .. .. . 195

7-10: The longshore transport distribution, longshore current, wave
height, and average beach profile across the surf zone for the
first impoundment interval of the plunging test series .. 196

7-11: Comparison of the longshore transport distribution found from
the first and second impoundment intervals of the plunging
test series ** ** ** ** ** *. . . 198


xiv





7-12: Comparison of combined bed and streamer trap sediment
accumulation rate, suspended sediment contribution, and
longshore sediment transport rate (from impoundment data)
across shore. Plunging test series .. ... .. .. .. 200

7-13: Time history of the mean water level fluctuation and survey
loops for the laboratory investigation involving simulated
tide effects ... .. .. .. ... . ... .. .. 203

7-14: Beach profiles measured immediately updrift and downdrift of
the groyne during the pre-, intermediate, and post-impoundment
surveys for the laboratory test involving simulated tidal
fluctuations .. .. .. .. .. .. . .. ... . . 205

7-15: Distribution of longshore transport rate for still water
depth contours derived from the rising, falling, and entire
tide impoundment intervals .. .. .. ... .. .. 07

7-16: Beach profiles measured immediately updrift of the groyne
site for the non-tidal plunging/collapsing test series .. 212

7-17: The longshore transport distribution, longshore current, wave
height, and average beach profile across the surf zone for
the plunging/collapsing (non-tidal) test series ... .. 213

7-18: Comparison of the longshore transport distribution across the
surf zone for the short-term and longer-term impoundment
intervals, and fluorescent tracer field after 18 minutes of
wave action. Plunging/collapsing (non-tidal) test series . 215

7-19: Comparison of the longshore transport distributions across
shore developed using tidal deconvolution. Plunging/col-
lapsing test series ... .. ... .. .. .. .. . 218

7-20: Comparison of typical beach profiles updrift of the groyne
for the field studies and the laboratory studies .. .. 220

7-21: Beach profiles at the groyne deployment site before impound-
ment, and immediately updrift and downdrift of the groyne
after the impoundment interval for the collapsing test series 221

7-22: The longshore transport distribution, longshore current,
wave height, and average beach profile across the surf zone
for the collapsing test series .. ... .. ... .. 223

7-23: Beach profiles measured before and after impoundment for
the spilling wave test series . .. ... .. .. .. .. 226

7-24: The longshore transport distribution, longshore current,
wave height, and average beach profile across the surf zone
for the spilling test series .. . . . .. .. 228





7-25: Longshore transport distribution developed from impoundment
data and fluorescent tracer field after 36 minutes of wave
action for the spilling test series .. .. .. .. .. 230

7-26: Normalized longshore transport distribution across the surf
zone developed from the best impoundment data sets of the
laboratory and field experiments .. .. .. .. .. .. 233

8-1: Comparison of normalized measured longshore transport
distributions from laboratory data with the Bagnold model
(no set-up, shoreline-discontinuous evaluation) . .. ... 238

8-2: Comparison of normalized measured longshore transport
distributions from laboratory data with the Bagnold model
(set-up, shoreline-continuous evaluation) . .. .. ... 241

8-3: Comparison of normalized measured longshore transport
distributions from laboratory data with the stress model . 244

8-4: Comparison of normalized measured longshore transport dist-
ributions from laboratory data with alternate model #1 .. 247

8-5: Comparison of normalized measured longshore transport dist-
ributions from laboratory data with alternate model #2 .. 249

8-6: Comparison of normalized measured longshore transport dist-
ributions from laboratory data with alternate model #3 .. 252

8-7: Comparison of normalized measured longshore transport dist-
ributions from laboratory data with alternate model #4 .. 255

8-8: Comparison of normalized measured longshore transport dist-
ributions from the two best field data sets with alternate
model #2 .. ... .. .. .. ... .. .. . ... 261

8-9: Comparison of normalized measured longshore transport dist-
ributions from the two best field data sets with alternate
model #3 ... .. .. ... .. .. .. .. .. .. 262

8-10: Comparison of normalized measured longshore transport dist-
ributions from the two best field data sets with the Bagnold
model ... ... .** **** **** 263

8-11: Ratio of the stream function and linear theory values of the
longshore wave energy flux PQ and the longshore radiation
stress Sxy for several cases evaluated at breaking .. .. 267

8-12: The uprush angle asw, for water particles entering the
swash zone with velocity us associated with a bore arriving
at the shoreline with angle as and in the presence of
a near-shoreline longshore current Vis...........27

9-1: The "non-singular concave-up equilibrium beach profile" .. 282





9-2: Comparison of average beach profiles from three of the labora-
tory test series with a non-singular equilibrium profile . 283

9-3: The ssculated total water depth across the surf zone for an
h=x2 3type beac profile compared with the approximation
d=ABx . . . . . . . . . . . . . 286

9-4: Coefficients Al and Bl of the longshore current expression
for the planar foreshore portion of the non-singular
equilibrium beach profile .. .. .. ... .. .. .. 291

9-5: Normalized (Lagrangian) measured longshore current across the
surf zone from the laboratory test series compared to the cal-
culated longshore current for a non-singular equilibrium beach
profile for various values of the mixing parameter, P . .. 295

9-6: Normalized (Eulerian) measured longshore current across the
surf zone from the two "best" field impoundment experiments
compared to the calculated longshore current for a non-
singular equilibrium beach profile .. ... . ... 297

9-7: Dimensionless longshore transport across the surf zone for a
non-singular equilibrium beach profile evaluated without
inclusion of swash .. .... .. .. .. .. . ... .. 301

9-8: Dimensionless longshore transport across the surf zone for a
non-singular equilibrium beach profile evaluated for shore-
ward-shifted longshore current profile, and linear decay of
wave height from the profile match point to the shoreward
limit of runup ... .. .. ... . ... .. .. 304

9-9: Dimensionless longshore transport across the surf zone for a
planar beach assuming linear wave decay from the breakpoint
to the still water line .. .. .. .. .. .. .. ... 305

A-1: Construction detail for an individual sand bag unit for use
in the pyramid-style groyne .. .. .. .. ... .. .. 321

E-1: Illustrative sketch of profiler apparatus geometry .. .. 330

E-2: Circuit diagram of profiler . ... .. .. .. . .... 332

F-1: Geometry of swash upon an inclined plane .. ... .. 334


xvii





Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fullfillment of the
Requirements for the Degree of Doctor of Philosophy

SHORT-TERM IMPOUNDMENT
OF LONGSHORE SEDIMENT TRANSPORT

By

KEVIN R. BODGE

December, 1986


Chairman: Dr. Robert G. Dean
Major Department: Engineering Sciences



The cross-shore distribution of longshore sediment transport is

investigated through the distribution of sediment impounded against a

shore-perpendicular barrier over short-term intervals in field and

laboratory environments. For each field experiment, a low-profile

groyne was deployed across a natural beach in less than eight hours and

profiles near the groyne were repeatedly surveyed for eight to twenty

hours thereafter. For each laboratory experiment, a low-profile barrier

was installed across a pre-equilibrated fine sand model beach, and

profile changes near the barrier were measured after five to forty

minutes of regular, obliquely-incident, unidirectional wave action.

Breaking wave angle, and longshore current and wave height across the

surf zone were also measured. The effects of cross-shore transport and

tidal fluctuation were addressed in the survey data, and the effective-

ness of the barriers as impoundment agents is discussed. Local down-

drift profile changes were found to be poor indicators of the local


xviii





updrift impoundment. In general, the longshore transport profiles were

found to be bimodal with peaks just landward of the breakpoint and near

the shoreline; the relative significance of the longshore transport

shifted from the near-breakpoint peak to the near-shoreline peak as the

wave condition varied from spilling to collapsing breakers. Alternately

stated, the longshore transport distribution appeared strongly beach

profile dependent, as transport was most pronounced over local regions

of high bed steepness. Between 10% and 30% of the total longshore

transport was observed seaward of the breakpoint for all cases. Long-

shore transport in the swash zone represented at least 5% to 60% of the

total transport, where the largest swash contributions were associated

with plunging/collapsing and collapsing surf conditions. A simple model

is proposed to describe the normalized longshore transport distribution

across the swash and surf zone as a function of the local longshore

current, beach slope, and dissipation of wave energy per unit surf

volume. The set-up, longshore current, and longshore transport are

described for an equilibrium beach profile which is finite-sloped at the

shoreline. The shoreward convection of longshore current by wave mass

transport is also discussed in relation to the longshore transport

distribution.


xix





CHAPTER 1

INTRODUCTION



Even the most casual beach observer will note that sediments do not

move only on and off the beach, but along it as well. The movement of

sand or other beach material along the shore is termed longshoree

sediment transport," or "littoral drift." Considerable effort has been

directed towards qualitative and quantitative descriptions of longshore

sediment transport because It plays a significant role in the process of

shoreline response to waves and currents. Although some progress has

been made in the determination and prediction of total longshore

sediment transport, the question of where the transport takes place on

the beach remains essentially unanswered. Knowledge of the distribution

of longshore sediment transport across the surf zone is central to the

effective design of groynes, jetties, and especially weirs, and to the

appropriate planning of pipeline landfalls and trenching across the surf

zone. In addition, insight to the distribution of longshore transport

aids in understanding spit development, the migration of prominent

natural or artificially placed shoreline features, and perhaps the

mechanism of the longshore transport process as a whole. Finally, a

parameterized description of the distributed longshore transport is an

integral part of any complete three-dimensional model of littoral

processes which the coastal geology and engineering communities strive

to develop.





From existing predictive models, it is generally thought that the

maximum longshore transport occurs in the seaward-half of the surf zone

and that transport vanishes towards the shoreline (see next chapter).

None of the models effectively treat longshore transport above the

shoreline (i.e., in the swash zone). Some of the models are discontinu-

ous at the breaker line, while some predict small contributions to the

transport outside of the breaker line. Available data indicate that

maximum longshore transport occurs in the landward-half of the surf zone

at least as often or more often than in the seaward-half, and that sig-

nificant levels of transport occur at or above the shoreline. Generally

less than a third of the total transport is observed to occur outside of

the breaker line. Few of the transport models explicitly or effectively

describe the effect of the dynamic nature of the beach or breaking wave

characteristics upon the longshore transport--whereas the data suggest

that these two parameters may be of importance to longshore transport

processes.

The present study represents an attempt to gain additional insight

to the cross-shore distribution of longshore sediment transport. The

study relies upon a "short-term sediment impoundment" experimental

technique which was developed and applied by the author in both field

and laboratory environments. The technique involves the rapid deploy-

ment of a shore-perpendicular barrier on an initially undisturbed

beach. Beach profile changes in the vicinity of the barrier are repeat-

edly surveyed as longshore sediment transport is impounded against the

barrier. Surf zone wave heights and currents are simultaneously moni-

tored. The measured beach profile changes are evaluated for cross-shore

and longshore components of sediment transport along particular depth





contours (where an adjustment for tidal fluctuation is made when

necessary). Ideally, after removal of the cross-shore transport sig-

nals, the average longshore transport rate between surveys along each

particular depth contour of interest can then be established. If bypas-

sing of the barrier occurs, then at least the significance of the long-

shore transport along any particular depth contour can be established

relative to the other depth contours. In either case, the shape of the

longshore transport distribution across shore can be achieved. The

experimental technique is termed "short-term" because the impoundment-

related beach profile changes are surveyed with the same time scale as

the directional wave event associated with the longshore transport

impoundment. Such an event may be thought of as a condition of quasi-

steady longshoree wave energy flux" or longshore radiation stress, for

example. Alternately, the beach profile changes are surveyed with the

same time scale of local beach geomorphologic changes. That is, profile

changes are surveyed only over the time for which the undisturbed beach

profiles are quasi-steady and/or for which the sediment impoundment is

not so large that severe bypassing and cross-shore smearing of the

impoundment occurs. Thus, the "short-term" impoundment approach allows

one to correlate specific directional wave events and beach morphologies

to the related longshore transport distribution.

Previous impoundment (or deposition) techniques have been generally

"long-term" in character (Johnson, 1952; Bruno and Gable, 1976; Bruno et

al., 1980; Dean et al., 1982). In these studies, weekly or monthly

surveys of the impoundment (or deposition) of sediment against a barrier

or in a basin were compared to integrated hourly or daily wave data such

that estimates of the longshore transport rate could not be correlated





to specific wave or beach conditions. Furthermore, the action of many

varied tidal and cross-shore processes smeared the distribution of the

impoundment (or deposition) of sediment and therefore obscured the

distribution of the longshore transport.

In the present study, four separate short-term impoundment experi-

ments were executed on the sandy Atlantic-coast beach of Duck, North

Carolina, during the summer of 1984. For each experiment, a shore-

perpendicular sand bag groyne was rapidly constructed over an eight hour

period and the subsequent sediment impoundment against the groyne was

surveyed over an eight to twenty hour period thereafter. Each experi-

ment was conducted during a quasi-steady, relatively low-energy direc-

tional wave event. Short-term impoundment experiments were then

executed on a fine sand beach with regular waves in the laboratory in

order to gain better understanding of the field results and the

variation of the distributed longshore sediment transport profile with

varying surf conditions.

Owing to the unknown trapping integrity of the barriers constructed

in the field and the poorly understood scaling relationships between

models and the prototype, the intent of this investigation is not to

present absolute magnitudes of total or distributed longshore trans-

port. Instead, this investigation emphasizes the relative contribution

of local longshore transport to the total transport; that is, its intent

is to present a normalized description of the distribution of longshore

transport across the surf zone as a function of beach and surf condi-

tions. In so doing, insight is gained to the mechanisms and signifi-

cance of various beach and surf conditions on the total longshore

sediment transport.





The following chapter of this paper highlights the existing total

longshore sediment transport literature and presents a state of the art

review of the predictive models and data base for the distribution of

longshore transport across the surf zone. Chapter 3 describes the field

impoundment experiments. Chapter 4 details the tidal and cross-shore

deconvolution techniques which were developed to analyze the field data

(and to some extent, the laboratory data). Chapter 5 presents the long-

shore sediment transport distributions developed from the field data

using the deconvolution methodologies described in Chapter 4. Addition-

ally, a preliminary discussion of the quality of the field data is

offered in Chapter 5. Chapter 6 describes the model and experimental

procedure utilized for the laboratory study. Chapter 7 presents the

longshore transport distributions developed from the laboratory experi-

ments for five different types of breaking wave conditions (collapsing

through spilling), including the scaled field conditions. The effects

of tidal fluctuation, groyne profile, and the impoundment duration--as

determined from the laboratory experiments--are also discussed in this

chapter. In Chapter 8, several existing and alternate models are tested

for agreement with the experimentally determined (normalized) longshore

transport distribution profiles. Some considerations of non-linear

effects are also discussed. A simple relation (employing linear theory)

is proposed as the best engineering model to describe the normalized

longshore transport distribution across the surf zone. In Chapter 9 is

presented the normalized longshore current and longshore transport

distribution profiles for a non-singular equilibrium beach profile (that

is, a profile composed of a planar foreshore matched to a concave-up

profile). Summary and conclusions are presented in Chapter 10.





CHAPTER 2

REVIEW OF LONGSHORE SEDIMENT TRANSPORT
RELATIONSHIPS AND CROSS-SHORE
DISTRIBUTION DATA



2.1 Fundamental Expressions for Total Longshore
Sediment Transport

Reviews of the total longshore sediment transport field and labora-

tory data base may be found in Savage (1962), Das (1971), King (1972),

Greer and Madsen (1978), Walton and Chiu (1979), Bruno, Dean, and Gable

(1980), and Hallermeier (1982). Sayao and Kamphuis (1983) offer a

review of total longshore transport relationships, several of which are

intercompared in Bakker (1970), van de Graaf and van Overeem (1979), and

Swart and Fleming (1980). The intent of the following pages is to

highlight several existing total longshore transport equations so as to

identify surf and beach parameters which may be of importance to the

investigation of the distributed longshore transport.

The early laboratory work of Krumbein (1944), and field work of

Watts (1953) and Caldwell (1956) suggested the following relationship

between the total longshore sediment volumetric transport rate QR, and

the so-called longshore wave energy flux factor PR,


Q~ = Kq PQ (2.1)


where Kq is a dimensional "constant," and


PA = ECg sina cosar (2.2)





In Eq. (2.2), a is the angle between the wave crest and the shoreline,

and E is the wave energy density,



E =T pg H2 (2.3)


where p is the density of the fluid, g is the acceleration due to

gravity, and H is the root mean square (rms) wave height. The wave

group celerity Cg is related to the wave (phase) celerity C, by


-f =n 1(1 + 2kh ) (2.4)
C2 sinh 2kh


In Eq. (2.1), the value of PQ is typically evaluated at the breakpoint

such that PQ is more completely written as P~b. Throughout this paper,

the subscript b denotes quantities evaluated at the wave breakpoint.

Inman and Bagnold (1963) expressed Eq. (2.1) in terms of an

immersed weight longshore sediment transport rate IQ, where



a = (ps p) g a' Qa (2.5)


and where ps is the density of the sediment, and a' is the ratio of sand

grain volume without voids to total volume with voids. Equation (2.1)

is then written



I, = K Pab (2.6)


where K is a dimensionless constant. Equation (2.6), in various forms,

is referred to as the "CERC Formula." This equation is widely accepted









and applied (primarily because of its simplicity) despite its empirical

nature and the uncertainty associated with the value of the constant of

proportionality, K. Since the introduction of this expression, the

recommended value of K has varied by a factor of four (Inman, 1978).

Based upon the range of values found for K from field studies, Dean

(1978) estimates that total longshore drift can be predicted to within

-67% to +200% error. At present, the recommended value of K is 0.78

(CERC, 1984). Figure 2-1 illustrates most of the total longshore trans-

port field data base compared with the recommended relationship.

Laboratory results (not shown) suggest K values much less than 0.78.





IO5


10 IO2 10 I

PAb=(ECn)b Sin"b Cosab (N/sec)


rd
u
a,
v,
z

w


-I =0.78Plbl







S1 1 tat1ni


/X
O

d~


O
p


r Watts (1953)
o Caldwell (1956)
o Komar & Inman (1970)
d Knoth & Nummedal (1978)
9 Inman et al. (1980)
IgBruno et al. (1980)
*Dean et al. (1982)
v Sternberg et al. (1984)
1 ) 1I111 11 I IIII1ll


o x
o


I I a li ttl


Figure 2-1: Comparison of total longshore sediment transport field data
with the CERC Formula.





Longuet-Higgins (1972) pointed out that the parameter PQ described

above is not the longshore component of energy flux, and, in fact, has

no obvious physical meaning. He re-expressed PQ as the product of two

"physically meaningful" quantities,



P, = (E -g cosa sina) C =S C (2.7)



where Sxy is the depth- and time-averaged alongshore momentum flux in

the onshore direction per unit longshore distance, or longshore radia-

tion stress, for short. At the break point,


Pll = SyCb (2.8)


where Sxy can be calculated anywhere outside the breaker line for a

coastline with straight and parallel depth contours if there is no wave

energy dissipation outside the breaker line. From Eq. (2.8) above, the

CERC Formula (Eq. 2.6) may be expressed


IQ = K SyCb (2.9)


or, alternately,


I = K* S (2.10)


where K* is a constant with dimensions of length per time. Vitale

(1981) indicates that Eqs. (2.6) and (2.10) are equally good (or equally

poor) indicators of total longshore sediment transport. The field data

of Bruno et al. (1980) indicate that Eq. (2.10) is a slightly better

predictor of total longshore transport than Eq. (2.6).








Inman and Bagnold (1963) and Komar and Inman (1970) presented an

alternative to the empirical CERC Formula based upon the energeticc"

approach of Bagnold (1963):



I, = K' (E C )b( cos a b VQ (2.11)



where K' is a dimensionless constant of proportionality, uo is the

maximum near-bottom horizontal orbital velocity, and VR is the average

longshore current. The subscript b refers to the breaking position, as

usual. The terms in parentheses represent the mean wave force applied

to the bed which mobilizes sediment for longshore transport by the

current VQ. An advantage of this model is that the source of VI is

unspecified. Komar and Inman (1970) found K'=0.28 based upon field

measurements of longshore transport and current. In the absence of

measured longshore current, VQ is often approximated by the planar

beach expression:


-5n m
V 1 (uo ) sin 2ab (2.12)


after Longuet-Higginsi (1970), where Cf is a bed friction factor, and m

is the beach slope (rise over run). Substitution of Eq. (2.12) into

(2.11) yields



Ie = K" (E C n)b cosab i~sin2ab (2.13)


Komar (1971) suggests that m cosab/Cf is a constant. If so, then Eq.

(2.13) reduces to the CERC Formula, where





5nm COSa
K = K' (2.14)
f6


Comparison of field data with Eq. (2.11) is shown In Figure 2-2.




10 -



II =0.28 (ECn)b 7 a



o of




o
-~ # raus et al. 0982)j
o IX Sternberg et al. (1984)j


11 IO I IO"IO

(ECn)b u ( Wa tts/nn)

Figure 2-2: Comparison of total longshore sediment transport field data
with the energeticc" model.



Several investigators have suggested that the constants of propor-

tionality, K and K* in Eqs. (2.6) and (2.10), respectively, are not

constants after all. Instead, K and K* may be dependent upon some wave

and/or sediment characteristics. From laboratory results, Saville

(1950), Shay and Johnson (1951), and Vicente (as cited in Sayao and

Kamphuis, 1983) suggested that the total longshore sediment transport,





























~ T-


YV_ _L _L I 1_


-MI 0


12

and therefore the proportionality constant K in the CERC Formula,

increases with increasing deepwater wave steepness up to H /Lo 0.01 to

0.025, and decreases thereafter. Overall, for waves of identical

energy levels, Saville suggested that lower steepness waves result in

greater total drift than higher steepness waves. The approximate rela-

tionship between wave steepness and the proportionality constant K is

illustrated in Figure 2-3 for six different laboratory investigations.


I : I


Soville (1950) 03 2.69
a S Wo (1951) 0.3 269
x Fairchild (1970) Q22 2.65-
oDelft (1976) 022 --
SKornphuis and
Readshow (1978) 0.56i 268
Vitole (1981) 0.22 265
Ozhon(1982) 0.83 265













SHo/Lo
eD
4 *0

**3


12R


1.0




08

K

06


04>-


III


002 00>5 .OI .02 .05 .




Variation of the CERC Formula proportionality constant K,
with wave steepness as found from laboratory investiga-
tions.


Figure 2-3:






13

The steepness H /Lo refers to the calculated wave height He at the

theoretical limiting water depth for appreciable longshore bedload

transport de, where



d (2.15)
e 0.03 (s-1) g


from Hallermeier (1978). The term U, is the maximum near-bed linear-

theory wave orbital velocity, and s is the specific gravity of the

sediment. The values of K were obtained through~


Il TQ(measured) IR(measured)
K P (2.16)
Plb tie pg( C i2
16 P( g~e sn e

Accordingly, K in Eq. (2.16) is not exactly identical to K in Eq. (2.6)

since PQe is only an approximation of P~b. Figure 2-3 was developed

from the compilation of laboratory data presented by Hallermeier

(1982). Although there is considerable scatter, a general decrease in K

is noted for an increase in wave steepness (and therefore decreasing

immersed weight longshore sediment transport with increasing wave

steepness for a given level of longshore wave energy flux). This

observation is substantiated by Ozhan (1982) who found from separate

laboratory investigation that


H'
K =0.007 (L~ ) (2.17)



where Ho' is the unrefracted deepwater wave height, and K is identical

to that expressed in Eq, (2.6). Ozhan's relationship is shown by the

solid line in Figure 2-3.





















































0.5 1.0 '

SEDIMENT DIAMETER, 050 (mm)


2.5 I


-


14

Castanho (1970), Bajournas (1970), van Hijum (1976), Swart (1976),

and Dean (1978) have suggested somewhat conflicting relationships

between longshore sediment transport and sediment size. In Figure 2-4

the mean values of the CERC Formula proportionality constant K (f one

standard deviation) found from several laboratory and field longshore

sediment present in each study. It is noted that the laboratory data


LABORATORY
D Saville (1950)
a Shay & Johnson (1951)
o Fairchild (1970)
r van Hijun (1976)
A Delft (1976)
o Kamphuis & Readshaw
(1978)
0 Vitale (1981)
hP Ozhan (1982)


FIELD
9 Watts (1953)
gl Caldwell (1956)
> Moore & Cole (1960)
A Johnson (1952)
*Komar & Inman (1970)


K


1.0




C.5


"


" l


l a i


O


6.0


Figure 2-4:


Comparison between median sediment diameter and the CERC
Formula proportionality constant K from field data and
laboratory data. The bars indicate one standard deviation
of K above and below the mean value of K.


"





15

points have not been scaled up to the prototype and that the laboratory

values of K are approximate (see Eq. (21).An obvious relationship

between sediment size and R is not apparent from the figure.

Dean (1973) suggested that longshore transport may be described by

considering the portion of wave energy flux dissipated by settling sand

grains. His suspension-dominant model indicates


/g Hb m cosab
K ac (2.18)
Cf Ws


where m is the beach bed slope, and ws is the sediment fall velocity.

Komar (1977) found no readily apparent relationship between K and we

after examination of available longshore transport data.

Much of the debate on the relationship between longshore transport

and sediment size depends upon whether the dominant mode of transport is

suspension or bedload. In general, however, one would expect Intu-

itively that larger sediments would be transported less readily than

smaller sediments. However, since larger sediments are associated with

steeper beaches, a given wave train will break closer to shore for

beaches of larger sediment and therefore the breaking wave energy or

radiation stresses will be concentrated over a smaller surf zone.

Hence, greater transport may be realized on beaches composed of

materials of larger diameter. These two opposing intuitive arguments

have yet to be reconciled.

Recently, from laboratory experiments on sand beaches, Kamphuis and

Readshaw (1978), Vitale (1981), Kamphuis and Sayao (1982), and Ozhan

(1982) each observed a relationship between total longshore transport

proportionality constants (K and/or K* or a similar constant) and the

surf similarity parameter (b, where





5 (2.19)
b / H./Lo


and Hb is the breaking wave height and Lo is the deepwater wavelength.

Since it is generally recognized that the (equilibrium) beach slope, m,

is related to the sediment size, the surf similarity parameter somewhat

combines the effect of both sediment size and wave steepness. (Choice

of the appropriate beach slope, m, is controversial; some recommenda-

tions are made by Sayao and Kamphuis (1983). In the present study, a is

taken as the average bed slope about the breakpoint--unless otherwise

noted.)

Figure 2-5 compares the surf similarity parameter and K for several

laboratory investigations and selected field data. Figure 2-6 compares

the surf similarity parameter and K* for the laboratory data of Kamphuis

and Readshaw (1978). In Figure 2-5, the values of K for the Kamphuis

and Readshaw (1978) and Romar and Inman (1970) data represent the actual

proportionality constant of the CERC Formula, while the values of K for

the remaining laboratory data are approximated by Eq. (2.16). The

laboratory data of Kamphuis and Readshaw and the field data of Komar and

Inman are represented by the surf similarity parameter (b, where m is

taken as an average surf zone bottom slope and local breaking bottom

slope, respectively, and significant wave height is utilized for the

field data. The remaining laboratory data are represented by a similar-

ity parameter:



( = (2.20)
e HI~





1 I 1








a
a
a
a


Yr


1 I


0.1 0.2


e n .. i l


17

where H_ is the wave height at the effective limiting depth for


appreciable longshore transport (Eq. (21). The values of He and m

are taken from Hallermeier (1982) for these data. Figures 2-5 and 2-6


indicate a logarithmic increase of K and K* with 5b. In general,

examination of relationships using the similarity parameter is more


appropriately done with a logarithmic scale since the similarity

parameter is logarithmic in character.


x Foirchild (1970) 0.22
o Delft (1976) 0.2
Komrphuis and
-Readshow(1978) 0.56
D Viotoe (1981) 0.22
Komor and
Inmon (1970):
* El Moreno 0.60
aSilver Strand O. I 7






*






.~~ a




a x

O
- o- D


lo~


0.4t


0 21.


05

~e b


1.0 2.0


Figure 2-5: Variation of the CERC Formula proportionality constant K
with similarity parameter for selected laboratory and field
data.







































0.5 1.0 2 O 5.0


O


117--TlT1 1


0.10 [


o o


0.08

4i


o
o


B o
0


006[


0.04 -02


0?.2


o


I I


III


Figure 2-6: Variation of the proportionality constant K*/g (from
IR =K*Sxy) with surf similarity parameter as observed by
Kamphus and Readshaw (1978).




Galvin (1968) and Battjes (1974) reported that the surf similarity

parameter may be used to describe breaker type. According to Battjes


Eb > 2.0
0.4 < (b (2.0

(b < 0.4


Surging (Collapsing)
Plunging
Spilling


(2.21)





19

From these limits and the trends shown by Figures 2-5 and 2-6, one may

conclude that plunging and collapsing breakers lead to greater longshore

transport than spilling breakers for a given level of longshore wave

energy flux or radiation stress.

The tendency of K and K* to increase with the similarity parameter

means that for a given wave steepness, larger sediment sizes exhibit

greater longshore transport rates (since larger sediment is associated

with steeper slopes and therefore larger values of m). This lends

credibility to the argument that longshore transport increases on

steeper beaches for a given level of longshore wave energy flux or

radiation stress.

In brief summary of this section, the proportionality "constants" K

and K* in Eqs. (2.6) and (2.10) are probably not constant at all, but

generally (i) increase with decreasing wave steepness; (ii) exhibit a

disputed trend with sediment size, density, and fall velocity; and (iii)

increase with the surf similarity parameter which combines beach slope

(and so partly sediment type) and wave steepness. Since the surf simi-

larity parameter is related to the wave breaker type, and the character

or structure of the surf zone varies for different wave breaker types,

one may expect that the total longshore sediment transport (and so

perhaps the distribution of longshore sediment transport across the surf

zone) should somehow depend upon the breaker type.



2.2 Existing Distributed Longshore Sediment
Transport Models

Bagnold (1963) proposed that wave orbital motion mobilizes beach

sands and wave power is expended maintaining the sand in motion so that

any mean local longshore current VR transports the sand. Bagnold





20

suggested a suspended and bedload model in accordance with this

energeticc" approach which may be written



d a
i =r kg -- EC ) (2.22)



for small angles of wave approach, where iQ is the local immersed weight

longshore sediment transport rate per unit offshore distance, and where

the x-axis is directed offshore with origin at the shoreline. The pro-

portionality constant k, is dimensionless. The term uo represents the

near-bottom wave orbital velocity.

Assuming linear theory, shallow water conditions, constant propor-

tion K between water depth h and wave height H, and further assuming

that the longshore current is given by Longuet-Higgins (1970) for a no-

mixing planar beach case (Eq. (2.12)), the Bagnold expression becomes



i 251r k pg3/2 2m sCb hh(23
1. 128 BC Jb h(.3



A simple distributed model based upon the concept of the CERC For-

mula was proposed by Svasek in 1969 (see Bakker, 1970). Svasek assumed

that the longshore component of sediment transport is proportional to

the loss of energy flux between the beach contours; i.e.,



i (z) = ~-- E C sina cosa) (2.24)
11 dh g


where iQ(z) is the local immersed weight longshore sediment transport

rate per unit depth. For the same assumptions as described above,

(ielinear theory, shallow water, H=Kh), the Svasek model may be

written





3 p3/2 2Z m sinab 2 (.5



where K is the constant of proportionality from the CERC Formula, and

use has been made of the relationship


ii = iQ(x) = m i (z) (2.26)

Throughout this paper, the notation iig refers to the local immersed

weight longshore transport rate per unit offshore distance, whereas

ig(z) refers to the transport rate per unit depth.
Thornton (1972) proposed a distributed longshore transport model

based upon the energetic approach of Bagnold. Specifically,


B1 V 1/2
iI l/s 8-( EE ) [ ] (2.27)



where Bs is a dimensionless cons tant For the s ame ass ump tions as

described above, the Thornton model becomes



iQ = B' pg3/2 m5/4 J;fi- h 1/4 h5/4 (2.28)



for a planar beach with no-mixing longshore current given by Longuet-

Higgins (1970). The proportionality constant Bs' is dimensionless.

Figure 2-7 illustrates the normalized longshore transport distribu-

tions for the Bagnold, Svasek, and Thornton models for a planar beach.

As described above, a no-mixing Longuet-Higgins longshore current

profile has been assumed for the Bagnold and Thornton models. The

normalization igxb 1,' where I1 is the total transport and xb is the





22

surf zone width, may be thought of as the local longshore transport ill

compared to a uniformly distributed transport across the surf zone,

Ig/xb. It is noted that none of the models represent transport landward

of the shoreline. Each model exhibits a sharp discontinuity In trans-

port at the breaker line since no transport is predicted seaward of the

breakpoint (assuming no energy losses outside of the surf zone).

Abdelrahman (1983) further developed Thornton's model by describing

the gradient in energy flux as a function of energy dissipation due to

breaking (modelled after a periodic bore) and due to bottom friction


. 2 .4 6 .8 1.0
x/xb


Figure 2-7: Normalized distribution of longshore transport across a
planar beach from the Bagnold, Svasek, and Thornton
models. A no-mixing Longuet-Higgins (1970) longshore
current profile is assumed for the Bagnold and Thornton
models.





(eventually neglected). The local longshore current VQ(x) was also

expressed through energy dissipation due to breaking after Liu and

Dalrymple (1978). Abdelrahman's final expression (which appears

dimensionally incorrect) predicts the peak longshore transport at about

eight-tenths of the distance from the shoreline to the breaker line for

the cases tested, and predicts transport which decreases to zero at the

shoreline. The model was developed and utilized for random waves.

Komar (1971, 1975, and 1977) also extended Bagnold's (1963) model,

likewise envisioning that breaking wave-induced stress at the bed

mobilizes sediment making it available for advection by a longshore

current. Therefore, Komar reasoned that the local longshore sediment

transport is related to the product of breaking wave-related stress and

longshore current. The wave-related stress is taken as a function of

the maximum horizontal component of wave orbital motion, uo, which Komar

suggested is greatest at the break point and decreases to zero at the

shoreline. His general expression for the local immersed weight

longshore sediment transport per unit distance is



iQ = -8 2g h VA (2.29)



where f, is a bed drag coefficient for wave motions and k1 is a

proportionality constant.

If the stress exerted on the bed by longshore current is also

included as a sediment "mobilizing" factor, Komar suggested the

following expression for distributed longshore transport


2 Pgw 2
1=k2 a [Cf V + 8 h] (2.30)





24

where k2 is another proportionality constant and Cf is a frictional drag

coefficient for the longshore current velocity. Since Komar assumed

that stress due to wave motion is greatest at the breaker line,

inclusion of the longshore current related stress suggests that the

distribution of longshore transport will shift seaward for small angles

of wave incidence since the contribution of stress due to waves will

dominate that due to longshore current. Large angles of incidence will

create stronger longshore current and a longshore transport distribution

profile which more closely follows that of the longshore current.

Breaking wave height should not affect the distribution of longshore

transport since changes in wave height will more or less equally affect

the longshore current and wave orbital velocities.

Figure 2-8 illustrates the normalized (by maxima) longshore trans-

port distributions from the Romar models for a planar beach and for wave

and longshore current conditions shown in the figure. Some transport is

predicted seaward of the breakpoint when lateral mixing is considered,

but neither model predicts transport above the shoreline in the swash.

Komar (1971) argues on a theoretical basis that the swash zone can be

modelled identically as the surf zone, but swash zone application of any

of the existing models is not obvious.

Bijker (1971) was among the first investigators to develop a

longshore transport model based upon river-borne sediment transport

studies. Bijker's expression for the bedload component of longshore

transport, taken after Frijlink, is of the form


q = transport parameter esirg amer (2.31)
E-bed

In classical fashion, Bijker's suspended load contribution, taken after



























Hb=1.m
gb=10 0 .-

0.4_ m =1/ 10 f O.8V

ma~c 0.6
V4 ma x" \V~f mx
0.2 -P -O4




0.5 I.0
x /xb

Figure 2-8: Normalized distribution of longshore transport for a pla-
nar beach calculated for the Romar and Madsen models using
the wave conditions and longshore current profile shown.





Einstein's work, is expressed in terms of the bedload component through

somewhat complicated integrals which depend upon the bed roughness or

ripple height, fall velocity, and bed shear velocity. Bijker's method

is cumbersome to apply, and it is also sensitive to the value of the

assumed thickness of the bedload transport. An example from Bijker's





26

model is shown in the next section (Fig. 2-11). Swart (1976) discussed

the Bijker model and suggested that a total load (i.e., bedload + sus-

pended load) model is more appropriate. Swart used a modified Ackers-

White (1973) approach and found a lengthy expression for the local

longshore transport which is not detailed herein.

Madsen (1978) suggested a distributed longshore transport model

which is based upon an experimentally verified expression for sediment

transport under oscillatory flow:

+ +3
*(t) = 40 9 (t) (2.32)


after Brown (1950), Einstein (1972), and Madsen and Grant (1976), where

#(t) and ~(t) are instantaneous values of the non-dimensional transport

function and Shields parameter, respectively. Specifically,



(t) = (2.33)
w D




'b(t)
(it) = (2.34)
pg(s-1)D


Here, q(t) is the instantaneous volumetric sediment transport rate per

unit width, and ws and D are the sediment fall velocity and grain size,

respectively. The instantaneous bottom shear stress Tb(t) is given by


+ 1 + + + +*
S(t) = -pf u (t) + V (u (t) + V) (2.35)



where u_ and V are unsteady (wave) and steady (current) velocities,

respectively, and f is a bed friction factor due to combined waves and





27

currents. Time-averaging in the longshore direction and employing

linear (shallow water) wave theory, Madsen found


w f
q, = 1.7 ~s ow~ )3 u 5 Vp (2.36)


The normalized (by maxima) longshore transport distribution across a

planar beach (for the same conditions shown for the Romar models) is

illustrated for the Madsen model in Figure 2-8. Evaluation of transport

landward of the shoreline is not straight-forward from Madsen's model.

However, the model does not exhibit a discontinuity in transport at the

breaker line if a lateral-mixing model for VQ(x) is assumed. It is also

noted that the model's dependence upon ws/D2 implies that the longshore

transport decreases with increasing sediment size for the case of

spherical sand grains.

The laboratory data of Sawaragi and Deguchi (1978), discussed in

the next section, suggests the following empirical description of

distributed longshore sediment transport

3.9
4q 9.1 VQ D5 F, (2.37)

where 2
uk u*c
F (2.38)
*~ g(s-1) D50

The friction velocity u 2 = b/P, where the bottom shear stress, rb, is

due to the combined peak wave orbital velocity and longshore current,

and the critical friction velocity, us is defined for various diameter

sands after Iwagaki's investigations of sediment transport threshold

velocities for open channel flow (see Sawaragi and Deguchi, 1978).





28

Walton and Chiu (1979) suggested a distributed longshore sediment

transport model of the form:



= Kw Pb X(x) (2.39)



which is similar in form to the CERC Formula as K, is a dimensionless

constant of proportionality, but where X(x) is a local modifying

function which details the bedload and suspended load components of

transport independently as functions of the local longshore current and

water depth. Besides basic uncertainty of the value RI as in the CERC

formula, there is difficulty in selecting the separate bedload and

suspended load transport coefficients in the modifying term X(x). Using

a Longuet-Higgins type longshore current profile, the model typically

predicts maximum longshore transport in the seaward half of the surf

zone for a planar beach. A discontinuity is present at the breaker line

and transport decreases to zero at the shoreline.

A series of simple expressions which individually reflect some of

the concepts described in the preceding pages was tested by Fulford

(1982) using laboratory data from Savage (1959) and Bijker (1971).

These models included


1: = K1 Va u (2.40a)

2: = Kz V u hx (2.40b)
1~~ ~ 2 hx
3: = Va [1K3 u + K 'hX 1X _d- C coso)] (2.40c)

4: =K [ +dE C cosa)] (2.40d)
a = 4 Va o dx g

5: = K5 a Tb (2.40e)





29

where the K's are proportionality constants, and Tb is the local bottom

shear stress induced by the longshore component of wave orbital motion

(Dean, 1977) or by maximum wave orbital/10ngshore current velocities

(Longuet-Higgins, 1970). Fulford evaluated each model using the expres-

sion for longshore current across a planar beach after Longuet-Higgins

(1970). Although Fulford concluded that Model 3 (Eq. (2.41c)) agreed

best with the laboratory data he tested against, none of the models

compared well with the laboratory results.

Hallermeier (1982) derived an expression for the local bedload

component of the longshore sediment transport based upon a laboratory

expression for bedload transport under oscillatory motion. His model

functionally appears as


a u 3 -l
q 0 tan a(2.41)
1 3/2
(g(s-1))


where a is the wave frequency. Hallermeter directly integrated this

expression across the surf zone to yield the total bedload longshore

transport and did not discuss the distribution of transport outright.

Tsuchiya (1982) considered the local longshore sediment transport

in the form


q1 = c h V (2.42)


where c is the average local concentration of sediment expressed as a

function of the local applied and critical shear stress. Tsuchiya's

model generally predicts the maximum longshore drift at about three-

quarters of the distance from the shoreline to the breaker line with

significant amounts of transport seaward of the breaker line for





decreasing critical shear stress values. This is not too surprising

since Tsuchiya's model is suspension-dominant in character.

Finally, Bailard and Inman (1981) and Bailard (1984) proposed an

(initially complicated) energetics-based expression for the distribution

of longshore sediment transport which independently describes the bed-

load and suspended load components of transport as functions of the

local longshore current and water depth. Accordingly, the model is

similar in structure to that of Walton and Chiu (1979) and therefore

involves the same difficulty of selecting separate bedload and suspended

load transport coefficients. Typical normalized distributed longshore

transport curves obtained from the model for a planar beach are shown in

Figure 2-9. The maximum transport is predicted to occur at about nine-

tenths of the distance from the shoreline to the breaker line. Trans-

port is described seaward of the breaker line with a slight gradient

discontinuity, but is not described above the shoreline.

Briefly summarizing this section, roughly a dozen expressions have

been suggested for the distribution of longshore sediment transport. In

general, most of the models assume that sediment is locally mobilized

(i) as a function of energy dissipation from the breaking waves, or

(ii) by the bed shear stress induced by the peak horizontal wave orbital

velocities alone or by the combined peak orbital velocities and long-

shore current. The mobilized sediment is then advected downdrift by the

local longshore current. Accordingly, knowledge of the distribution of

longshore current across the surf zone is important in most of the

existing distributed longshore transport models. Many investigators

have relied upon the planar beach longshore current model suggested by

Longuet-Higgins (1970).
































1.0
x/xb


Figure 2-9: Example of Bailard's distributed longshore transport model.
A Longuet-Higgins type longshore current profile is used
(as shown) with mixing parameter P = 0.2. Adapted from
Bailard (1984).




Most all of the existing models suggest that the longshore sediment

transport is greatest between the mid-surf zone and the breaker line for

a planar beach, and that the longshore transport tends to zero at the

shoreline and outside the breaker line. Models which do not include

bottom stress due to longshore current or non-breaking wave orbital

motion exhibit discontinuities in transport at: the breaker line with

zero transport seaward of the breaker line. None of the models explic-

itly describe nor are well conditioned to treat longshore transport in

the swash zone.





32

2.3 Existing Field and Laboratory Data for
the Cross-Shore Distribution of Longshore Sediment Transport

The data base which describes the cross-shore distribution of

longshore sediment transport is relatively limited. With few excep-

tions, relevant studies are fairly recent. Aside from the significant

level of longshore transport observed In the swash zone, the most

notable feature of the data base is a substantial and thus far

unexplained variation in the distribution profiles of the transport.

The earliest published prototype observations of the distribution

of longshore transport were by the Beach Erosion Board (1933). Measured

sediment concentrations from collected water samples beneath a pier were

used to indicate the relative magnitude of the suspended component of

longshore drift across the surf zone. The results suggested longshore

transport maxima at the breaker line and in the swash zone. Transport

decreased with increasing depth seaward of the breaker line. Wave

conditions during the tests were not well documented.

From a moveable-bed investigation in the laboratory, Saville (1950)

did not quantify the distribution of longshore transport across shore,

but noted the importance of the beach profile and wave conditions upon

the magnitude and mechanism of longshore transport. Most generally, he

observed that the bulk of the longshore transport occurred within the

surf zone and on the foreshore, while the significant levels of sediment

motion observed outside the surf zone were primarily on/offshore in

character. Saville emphasized that locations of concentrated longshore

transport might correspond to those of concentrated cross-shore

processes; that is, when a beach is in disequilibrium with the wave

conditions, greater amounts of sediment are mobilized for transport as

the profile adjusts to the wave climate. Saville suggested that this





33

not only increases the total longshore transport (substantiated by

laboratory measurements of Kamphuis and Readshaw, 1978) but may affect

where the transport occurs. Saville observed that on equilibrium storm

beach profiles, the longshore transport was primarily due to advection

of sediment by the longshore current within the surf zone. On equilib-

rium summer beach profiles, the transport was almost entirely due to

beach "drifting" along the foreshore (caused directly by the waves).

The transition between these two cases occurred abruptly at a wave

steepness value H /Lo = 0.03 (which corresponds to the transition

between spilling and plunging waves for Saville's laboratory beach slope

of 1:10). Most significantly, the total transport along summer beaches

was much greater than that along storm beaches for the same wave energy

levels. This suggests that foreshore "drifting," or longshore transport

in the swash zone, is associated with plunging waves and is at least as

significant as longshore transport associated with longshore currents

seaward of the shoreline.

Zenkovitch (1960) evaluated fluorescent tracer movements as an

indicator of longshore drift across beaches in the Soviet Union. A

typical result from his study is shown in Figure 2-10 which details the

measured distribution profiles of longshore current, longshore sediment

advection velocity, and suspended sediment concent ra tion. Like the

early Beach Erosion Board results, peak longshore drift was observed at

the breaker locations) and the shoreline/swash location. Generally,

longshore transport increased in areas of observed high turbulence

levels (bar breaks and shoreline) and decreased in lesser turbulence

areas (troughs). Rel~!atively large spilling breakers prevailed during

the collection of the data shown in Figure 2-10.





25 0.2



O -

-c 2 -
u~z
43


Figre2-1


05I
o




0.2
Ul


0.6

x /xb


Prototype measurement of suspended sediment concentration
tion, longshore current, and tracer advection speed (from
Zenkovitch, 1960). Each distribution is normalized by its
observed maximum value. Adapted from Walton and Chiu
(1979).


Ingle (1966) described a number of fluorescent tracer experiments

on prototype beaches which yielded longshore transport distribution

profiles similar to those of Zenkovitch, described above.

Thornton (1968) used several 20-em high traps operated from atop a

pier at Fernandina Beach, Florida, to collect the longhsore component of

sand transport. In general, he found that longshore transport increased





35

shoreward; and like the previous investigators, he found that greater

longshore transport was associated with bars rather than troughs.

Thornton rationalized (and susbtantiated through measurements) that

kinetic energy increases shoreward and therefore bed shear and associ-

ated mobilization of sediments increases shoreward. Thornton's

measurements were limited to the outer portion of the surf zone and

excluded mean water depths of less than about half a meter.

Bruun (1969) utilized the same technique as Thornton, but caught

fluorescent sand tracer in the traps. Bruun found several dissimilar

distributions of longshore transport and, unlike Thronton, observed

greater longshore transport in the beach troughs than over the bars.

Bruun suggested that this was likely due to an absence of wave breaking

over the bars and/or strong tidally-driven longshore currents which were

present in the troughs.

Bijker (1971) conducted laboratory experiments where sand trans-

ported along a model beach spilled into a cross-shore series of deposi-

tion bins located at the extreme downdrift end of the beach. Figure

2-11 illustrates the cross-shore profile of longshore transport derived

from the distribution of material deposited in the bins after about 10

hours of testing (Hb 16-17 cm, hb= 18-21 cm, T = 1.55 sec). Bijker

indicates that the distribution may be suspect because of transport

disturbances caused by the downdrift-end weir over which the sand flowed

in order to enter the bins. The longshore current, wave height, and

beach profiles 2 m updrift of the bins are shown in Fig. 2-11 and the

longshore transport computed from Bijker's model for these conditions is

also shown. Bijker reported that the distribution of sand caught in the

bins changed with time as the beach profiles slowly evolved: the peak





S20

a10
I



O 0


E .10


0.







So.oo
-1


F ue 21:


0 1 2 3
APPROXIMATE DISTANCE


4 5 67 8
E FROM SHORELINE (m)



distributed longshore transport
Beach, wave height, longshore
longshore transport profiles are
of traps. Adapted from Bijker


Laboratory measurement of
using downdrift traps.
current, and predicted
shown at 2 meters updrift
(1971).





longshore transport initially occurred seaward of the bar position and

gradually moved inside the bar position as the bar became more

pronounced. Bijker suggests that this may be attributed to a greater

concentration of the longshore current between the shoreline and the bar

as the bar grew in relief.

Sawaragi and Deguchi (1978) placed round traps divided into pie-

shaped sections into the bed in order to collect sediment transport from

several established directions simultaneously. Their findings of the

longshore transport and cross-shore transport distributions from two

field efforts each at Isonoura and Matsuho Beaches are shown in Figures

2-12 and 2-13, respectively. The approximate values of deepwater wave

steepness Ho/Lo and surf similarity parameter (~b, as calculated by this

author, are listed for each experiment. Most notably, the measurements

indicate four different longshore sediment transport distribution pro-

files: (1) maximum in the swash zone (Matsuho-a), (2) maximum towards

the shoreline (Isonoura-a), (3) maximum at the breaker line (Matsuho-b),

and (4) bimodal with maxima at the shoreline and at the breaker line,

(Isonoura-b). In each case, there is significant longshore transport

observed above the shoreline, and about 10% to 30% of the total trans-

port is observed seaward of the breaker line. There also appears to be

some correlation between the cross-shore and longshore distribution of

transport.

From inspection of the beach profile, breaker location, and surf

similarity parameter, it is noted that there was a slight terrace land-

ward of the breaker point at Isonoura Beach (b) for conditions of rela-

tively large, spilling waves. Under similar conditions, earlier inves-

tigators such as Zenkovitch (1960) found bimodal longshore transport



























!5 O 25 50 75 K(
DISTANCE FROM SHORELINE (m)


E O


w
03


O 50 100 1
DISTANCE FROM SHORELINE (m)


200


Figure 2-12: Field measurement of distributed longshore and cross-shore
transport by Sawaragi and Deguchi (1978) using circular
traps in the bed at Isonoura Beach. Adapted from Sawaragi
and Deguchi (1978).





E

E
u 25-

O

z
~ -




25-

S-
E O-


I -
a 2-


DISTANCE FROM SHORELINE (m)


E





E
u







o








Figure 2-13:


O 5 10 15
DISTANCE FROM SHORELINE (m)


Field measurement of distributed longshore and cross-shore
transport by Sawaragi and Deguchi (1978) using circular
traps in the bed at M~atsuho Beach. Adapted from Sawaragi
and Deguchi (1978).







40

distribution profiles similar to Sawaragi and Deguchi's. For Isonoura

Beach (a), waves were breaking landward of the terrace and were probably

also of a spilling type; the bimodal transport distribution of Isonoura

(b) is replaced by a single peak in the inner surf zone. Perhaps this

is because the smaller waves of Isonoura (a) broke closer to the shore-

line compared to the large waves of Isonoura (b). The surf zone at

Matsuho Beach was relatively small and so it is difficult to discuss the

distribution of transport. However, it is noted that the waves were

likely plunging/spilling almost directly onto the foreshore and, for

these cas es the reported swash zone contribution to the longshore

transport is significant.

Sawaragi and Deguchi (1978) subsequently used their trapping tech-

nique on a movable bed laboratory beach and also measured the cross-

shore and longshore components of the bed shear stress on a similar

fixed bed laboratory beach. Their findings of the longshore transport

distribution profile for two different sediment sizes are shown in

Figure 2-14. For each sediment size tested, the normalized distribution

profiles, qQ/qQmax, appear to be essentially independent of the deep-

water wave steepness. However, the average location of the peak long-

shore transport is shifted shorewards within the surf zone for the

larger sediment tests relative to the finer sediment tests. Sawaragi

and Deguchi similarly found that the distributions of normalized wave

height, H/Hmaxy, and longshore current, V /VQma,, were fairly independent

of the deepwater wave steepness values tested. The average distribution

profiles of these two parameters are shown in Figure 2-15 for comparison

to the longshore transport distributions. The laboratory data generally

indicate a peak in longshore transport at six-tenths to eight-tenths of





max


"O 0.4 0.8 1.2 1.6
x/xb
Normalized distributed longshore transport from laboratory
measurements of Sawaragi and Deguchi (1978); median sand
diameter of (a) 0.34 mm, (b) 0.68 mm. Adapted from Sawa-
ragi and Deguchi (1978).


Figure 2-14:


x /xb
Average normalized longshore current and wave height across
the surf zone from laboratory measurements of Sawaragi and
Deguchi (1978). Solid (broken) curves represent median
sand diameter of 0.34 mm (0.68 mm). Adapted from Sawaragi
and Deguchi (1978).


Figure 2-15:





42

the distance from the shoreline to the breaker line with 10% to 20% of

the total transport occurring seaward of the breaker line and some

unknown (but apparently non-zero) amount of longshore transport above

the shoreline. For the finer sediment, the longshore transport at the

shoreline, x/xb=0, increased with decreasing wave steepness (or

increasing surf similarity parameter). The measured laboratory distri-

bution of longshore transport across the surf zone was similar to the

measured distribution of cross-shore transport (not shown). This agrees

with the field observations.

Tsuchiya (1982) presents one case of distributed longshore sediment

transport data from an experimental investigation. His data are shown

in normalized form in Figure 2-16, where x/xb=0 is the mean shoreline

location. A primary peak is noted about eight-tenths of the distance

from the shoreline to the breaker line. Secondary peaks are noted in

the swash zone, inner surf zone, and just outside the breaker line.

Details of Tsuchiya's experiment are not known by this author.









0.1 mo P / s,. '


'^62 0.2 0.4 OS6 0.8 1.0 1.2 1.4 1.6 1.8

x /xb
Figure 2-16: Normalized longshore transport distribution as measured by
Tsuchiya (1982). Adapted from Tsuchiya (1982).





43

Berek and Dean (1982) analyzed the rotation of depth contours in a

pocket beach (Leadbetter Beach, Sta. Barbara, California) after a change

in wave direction. The authors hypothesize that the relative amount of

contour rotation can be interpreted as the cross-shore distribution of

longshore sediment transport so long as cross-shore transport does not

appreciably contribute to the contour changes. The local longshore

transport at particular depth contours of interest was calculated over

the two-month intervals between surveys through consideration of hourly

fluctuations in tide and longshore wave energy flux and, of course, the

measured contour changes along the pocket beach between surveys. Berek

and Dean's results are shown in Figure 2-17. The local longshore trans-

port has been normalized by the value at the mean shoreline. The

authors indicate less confidence in the October to December evaluation

because of presumed "leaks" in the pocket beach. Distribution models of

three other Investigators discussed in the previous section are shown

for comparison. These three other predictive models were evaluated over

the temporally fluctuating tide and longshore wave energy flux in a

manner similar to the evaluation of the original field data. Accord-

ingly, these models' predicted distribution, qll(x), moved across the

mean water depths, h(x), so that the local longshore transport at the

mean shoreline, qQ(h=0), for these models was non-zero. Berek and

Dean's investigation indicates that longshore transport increases

towards the mean shoreline, or at least is greatest in the inner surf

zone. The results also indicate significant levels of transport seaward

of the presumed mean breaking depth. Swash zone transport is implicitly

included in the calculated longshore drift because of the tidal fluctua-

tions; however, it is not explicitly described in Berek and Dean's

graphical results.






























b) 1ntersurvey Perlod October 23, 1980 to December 17, 1980

1.0 -~- Inferred from Field
.'s Measurements
--- Fulford
---- Komor
1 ------ Tsuchlyo

0.5


DEPTH RELATIVE TO MSL, h (m)


Figure 2-17:


Normalized longshore transport distribution developed from
relative rotation of depth contours at a pocket beach after
a change in wave direction. Dashed lines indicate various
predictive model results for the measured prototype surf
and tide fluctuations. From Berek and Dean (1982).


From the results of a major field effort using fluorescent tracers,

Kraus et al. (1982) identified four basically different longshore

transport distributions across the surf zone: (1) generally uniform,

(2) bimodal with peaks in the swash and breaker locations, (3) maximum





45

towards the breaker line, and (4) maximum towards the shoreline. These

four different distribution profiles are similar in type to the field

results of Sawaragi and Deguchi (1978). The surf similarity parameter,

(b, was approximated by this author for each of the six experiments from

which these distribution classifications were derived: type (1),

(b'0.12; type (2), 5 -0.11 to 0.15; type (3), b 10.11 to 0.23; and type

(4), 5b"0.18 to 0.35. From these data, a description of the longshore

transport distribution based upon surf similarity parameter seems

unlikely. However, it is noted that the highest value of Eb (which

indicates a greater tendency for wave plunging) is associated with

maximum longshore transport near the shoreline (type 4). The reported

wave conditions for each of the six experiments considered were mixed

spilling and plunging (Sunamura and Kraus, 1985). Kraus et al. suggests

that there is no reason to expect a "standard" distribution profile for

longshore sediment transport in the prototype given the variability of

longshore current distribution across real beaches and given the

variability of the dominant mode of sediment transport (i.e., suspended

load vs. bedload).

Fulford (1982) examined contour changes updrift of a high-relief

groyne after 10 hours of oblique wave attack on a laboratory beach

(Savage, 1959). The beach was in only approximate equilibrium with the

wave conditions before the test was begun. Fulford's work represents an

"impoundment technique" analysis, similar to that used in the present

study, where the local longshore sediment transport is calculated

through integration of measured profile changes updrift of a shore-

perpendicular barrier. From Savage's data, Fulford presented a long-

shore transport distribution profile as shown in Figure 2-18. The





cross-shore coordinate x/xb=0 corresponds to the zero-hour still-water

shoreline location. The distribution indicates peak longshore transport

at x/xb=0.35 and that 18% of the total transport occurs landward of the

setup shoreline location and another 18% occurs seaward of the average

breaker location. The updrift limit of barrier effect, y*, was found to

be about one groyne length (measured from the top of the berm to the toe

of the groyne). The surf similarity parameter for Savage's tests was

approximately 5b=0.23 to 0.28 (using rms wave parameters calculated by

Fulford). The nearshore peak in longshore transport agrees roughly with






I I

1.0 savage rest say7
0 10 hours

0.8 -


I I



II








-ID0 -05 O 0.5 1.0 1.5 2.0
RELATIVE DISTANCE FROM STILL WATER SHORELINE, x/xb




Figure 2-18: Normalized longshore transport distribution developed from
sediment impoundment updrift of a barrier on a laboratory
beach (Savage, 1959). Approximate breaking position im~me-
diately updrift of groyne after impoundment was x/xb=1.3.
Adapted from Fulford (1982).





47

with the Sawaragi and Deguchi (1978) and Kraus et al. (1982) field data

for this range of the surf similarity parameter.

It is noted that Fulford's distribution does not close to zero

seaward of the breaker line. This is partly because the groyne did not

extend significantly beyond the breaker line and partly because Fulford

did not account for cross-shore transport effects. Specifically, the

contours outside of the breaker line shifted considerably seaward during

the ten-hour test (out to a depth h=2.5hb). This shift, interpreted by

Fulford as a longshore transport signal, was relatively constant across

the entire length of the beach and was therefore mostly unrelated to the

impoundment of longshore transport by the barrier.

Abdelrahman (1983) evaluated beach profile changes out to wading

depths over several stormy days along Leadbetter Beach, Sta. Barbara,

California, in order to approximate the distribution of longshore

sediment transport. Abdelrahman assumed that cross-shore transport was

negligible during the survey period because of the absence of pronounced

berms or bars in the profiles. His data analysis technique was based

upon that of an impoundment approach; however, this requires that a

gradient of longshore transport exists along the beach. It appears that

Abdelrahman calculated the mean alongshore gradient in longshore trans-

port and assumed that its distribution was similar to that of the long-

shore transport. His results, not shown here, indicate considerable

shoreline/swash zone longoshore transport--or more correctly, consid-

erable longshore gradient in shoreline/swash zone transport.

Downing (1984) and Sternberg et al. (1984) measured vertical sedi-

ment concentration profiles simultaneously with the longshore current

across the surf zone on natural beaches, and developed local longshore













4q(x) = V (x;t) f hc(x,z:t) dz


48

suspended sediment transport values through


(2.43)


where c(x,z,t) is the local instantaneous sediment concentration

profile, and the over-bar indicates time averaging. Figure 2-19

illustrates Downing's findings for a wide, spilling, relatively planar-

bed surf zone at Twin Harbor Beach, Washington. The figure depicts the

normalized discrete distribution of suspended sediment concentration,


APPROXIMATE WATER DEPTH (m)
0.8 1.2 16


0.4


2.0


W
3 r.o

o
U
N 0.5

r


45 55 65 75 85 95 105 115
DISTANaCE FROM BASE LIN E (m)


Figure 2-19: Distribution of longshore sediment transport calculated
from field measurements of longshore current and suspended
sediment concentration. Each quantity is normalized by
its maximum value. Adapted from Downing (1984).





49

longshore current, and calculated longshore transport. Use of Eq.

(2.43) with the sediment concentration and longshore current data

indicates~ maximum longshore transport in the mid-surf zone which

corresponds to the maximum longshore current. However, the maximum

suspended sediment concentrations were found nearshore. Note that the

measurements do not include water depths less than about half a meter.

The results of Sternberg et al. (1984), using Eq. (2.43) with field

data from Leadbetter Beach, Sta. Barbara, California, are shown in


Figure 2-20. The value x/xb-0 indicates the mean shoreline position.


-to-













I le



0 02 04 06 QA 10

x/x


S02 0~ 04 08 10





0 2 0 06 0 o



0, Ot 0 0 8


Figure 2-20): Distribution of immersed weight longshore sediment trans-
port calculated from field measurements of longshore cur-
rent and suspended sediment concentration. From Sternberg,
Shi, and Downing (1984).







50

Cross-shore resolution of the longshore transport distribution is rela-

tively poor and few data points are available near the shoreline. Based

upon the investigators' extrapolations for each data set, the maximum

longshore transport is located at about the mid-surf position. The

authors report that considerable longshore transport was observed in the

swash zone but measurements could not be taken in this area.

Equation (2.43) implies that longshore current is uniform through

the water column. A more appropriate expression would be




q (x) =/ Oh (x'z;t) c(xIz;t) dz (2.44)



However, neither Downing nor Sternberg et al. measured vertical distri-

bution of the longshore current. Since both investigators found a loga-

rithmic decrease of sediment concentration above the bed, use of Eq.

(2.43) could appreciably alter the calculated longshore transport

distribution--if the longshore current is not uniform over depth as

originally assumed. It is also noted that the work of Downing and

Sternberg et al. neglects bedload transport.

Most recent ly as part of "DUCK-8 5 ," the Coastal Engineering

Research Center (CERC) sampled the longshore suspended component of

sediment transport using a series of "streamer traps" positioned through

the water column and across the surf zone (N. Kraus, personal communica-

tion; see also Mason, Kraus, and H~olman, 1985). Final results of the

data analysis were not available at the time of this writing; however,

preliminary results indicate that the greatest transport occurs near the

bed and decreases upwards through the water column. Transport was

measured above the mean water level as well. Longshore transport was









maximum near the breaker line, very small outside the surf zone, and

decreased towards shore despite the presence of a bar close to the

shoreline. No measurements were reported for water depths less than

about half a meter.

Briefly summarizing this section, distributed longshore transport

field and laboratory investigations indicate that (1) significant

levels of longshore transport occur above the shoreline (i.e., in the

swash zone), (2) contribution of the swash zone transport to the total

littoral drift increases as waves break near or upon the foreshore and

possibly increases with the surf similarity parameter, (3) about 10% to

30% of the total longshore transport is observed seaward of the breaker

line, (4) maximum longshore sediment transport is at least as likely to

occur within the shoreward half of the surf zone as within the seaward

half of the surf zone, (5) greater longshore drift is often associated

with shallower depths (i.e., break-point bars and the shoreline), and

(6) field measurements demonstrate great variability in the shape of the

longshore sediment transport distribution profile.



2.4 Chapter Summary

Several generalizations can be made regarding longshore sediment

transport based upon the literature review presented in this chapter.

First, total longshore sediment transport predictive models typically

lack sensitivity to the dynamic state of the beach geometry and to the

location and/or type of wave breaking. Second, existing distributed

longshore transport models predict the maximum transport in the seaward-

half of the surf zone for a planar beach. These models neglect swash

zone contributions and indicate that transport vanishes at or near the





mean shoreline. Formulation of the transport distribution is most

typically taken as the product of the wave orbital motion/longshore

current induced bed shear stress (mobilizing parameter) and the

longshore current advectionn parameter). Third, field (and some

laboratory) data often indicate maximum longshore transport in the

landward-half of the surf zone and significant levels of transport at

and above the mean shoreline. Since the position of the longshore

transport maxima predicted by the models for a planar beach often

conflicts with the field and laboratory data, one might conclude that

the existing models are poor predictors of the longshore transport

distribution, and/or that evaluation of the existing models for a planar

beach yield poor representations of the actual longshore transport dist-

ribution which exists in nature. Fourth, the total longshore transport

has been shown to increase with increasing value of the surf similarity

parameter (for a given longshore energy flux or radiation stress), and

increasing surf similarity parameter values indicate a tendency for

plunging or surging (collapsing) wave breaking. Some of the available

observations and data suggest that near-shoreline and/or swash longshore

transport dominates for plunging and surging (collapsing) conditions.

Since plunging and collapsing breakers are associated with both greater

total longshore transport and near-shoreline/swash transport, near-

shoreline and/or swash transport must be quite significant. In any

case, from the data, the longshore transport distribution profiles

appear to be related to the beach profile geometry and possibly to the

type and/or location of wave breaking.





CHAPTER 3

FIELD INVESTIGATION:
EXPERIMENTAL METHOD
AND DATA PRESENTATION



3.1 Introduction

An intensive field measurement effort was undertaken at the Coastal

Engineering Research Center (CERC) Field Research Facility (FRF) at

Duck, North Carolina, over the period July 16 through October 2, 1984,

to collect data which describes the impoundment of longshore sediment

transport against a shore-perpendicular barrier rapidly deployed across

an initially undisturbed beach, as well as relevant nearshore surf and

sediment data. The following pages describe the techniques utilized to

impound and measure the littoral drift. Beach profile, surf, and

sediment data collected during the impoundment experiments are also

described. A detailed description of the FRF experiment site is

off erred in Birkemeier et al. (1981).



3.2 Experimental Method

3.2.1 Overview

For each impoundment experiment, a shore-perpendicular sand-bag

groyne was constructed approximately 150 meters south of the FRF pier.

Visual inspection indicated that the beach and surf south of this

location were relatively free of pier effects for wave events from the

south. Although execution of the experiment much further from the pier





54

may have reduced further pier effects upon the investigation site, field

operation within 150 to 300 meters of the pier greatly facilitated

effective data collection. Beach profiles in the vicinity of the groyne

were usually surveyed before and after groyne deployment (over one or

two tide cycles). It was intended that these beach profiles over time

and space would indicate the total volume of sediment impounded against

the barrier as well as its distribution across the surf zone.

Coincident with the pos t-groyne-cons truction beach prof ailing, measure-

ments were made of the local longshore current ahd wave height at three

locations across the surf zone, sediment samples were taken updrift and

downdrift of the groyne, and tidal water level changes and breaking wave

angle were recorded.



3.2.2 Groyne Construction and Removal

The site whereupon each groyne was constructed was marked by two

ropes, separated by about 2.5 meters, which stretched from the top of

the berm to well past the low-tide breaker line. The groynes were built

using a series of sand-bag units laid end to end. Each sand-bag unit

was tied between the two shore-perpendicular ropes in order to hold it

in place while it was filled. The bags were filled by pumping slurry

from approximately 8 m downdrift of the groyne using a 600 gallon-per-

minute pump with 10-em hard-rubber suction hose and 10-cm flexible PVC

discharge hose. During filling, small slits were cut about the bags to

relieve the back-pressure caused by the great volume of slurry water

which was pumped into the bags. Filter cloth (Filter-X material) was

placed underneath the sand-bags on the foreshore in order to prevent

these bags from "jetting" and sinking into the soft, dry berm sediment





55

during pumping. Repetitive surveys of the groyne over a several day

period indicated that the groynes did not appreciably sink into the bed

after construction.

It was discovered that pyramid-shaped sand bag units were best

suited for an optimally high and quickly built groyne. Three long

cylindrical bags were sewn together in a triad to form a pyramid (Figure

3-1). Each bag was 3.7-meters long and 30 to 50 em In diameter.

(Larger diameter bags did not fill well and slumped to the same height

as the 50 cm diameter bags.) The top bag in the pyramid overhung the

bottom two bags by 1.2 meters. In this way, the groyne could be built

in a brick-laying fashion: the bottom bags were placed end-to-end to

their upshore neighboring bags while the top bag overlaid the seaward

1.2 meters of the neighboring upshore bags. Each bag was filled from

its landward end so that the landward ends were always well-filled

compared to the seaward ends. The "brick-laying" style of construction

ensured a more consistent groyne height because the landward (well-

filled) ends of the top bags compensated for the poorly-filled seaward

ends of the neighboring upshore bottom bags.

Each individual bag in a pyramid unit was constructed from a sheet

of Filter-X material sewn into a tube shape. Circular pieces of Filter-X

were sewn onto the ends of the tube shape. A 30-em overlap was sewn

into the tube shape and was termed the "flap" (Figure 3-2). The flap

served as a secondary or false ceiling to the inside of the bag. A slit

was cut across the top of the tube above the flap and another slit was

cut across the flap just below and slightly seaward of the top slit.

These two slits allowed entry of the discharge hose through which the

slurry was pumped. When the bag filled, the flap and the top surface of





Figure 3-1: The pyramid-shaped sand bag unit (not to scale).


Figure 3-2: An individual sand bag showing the fill-flap design.
(End pieces not shown).





the tube tightened together due to the weight of the sand pushing out on

the side walls of the tube. This served to close the bag and prevented

sand from leaking out of the slits. (It was important to almost

completely remove the discharge hose from the slits before the flap

tightened or else risk locking the hose inside the bag.) One-meter long

rope ties were sewn into the seams of the bags in order to secure the

bags while filling them on the beach face. The bags were stitched with

an industrial hand-held sewing machine using nylon-cotton thread.

Photographs of two groynes constructed with the pyramid-style units are

shown in Figures 3-3 and 3-4. Appendix A further details the construc-

tion of the sand bags.

The removal of the barrier after each experiment must be seriously

considered during the planning phase of the field effort. The updrift

impoundment as well as the relatively low wave activity required for the

execution of the experiments almost completely buried the groyne several

days after each experiment. This made immediate groyne removal all but

impossible. For this investigation, the top of the bags were slit open

and/or removed. Attempts at jetting the sediment out of the opened bags

proved fruitless. Instead, the remains of the bags were left to await

the action of erosive waves which quickly and effectively freed the bags

and swept the groyne site clear. The floating remains of the bags were

then retrieved by hand when possible.



3.2.3 Profiling Techniques

A survey baseline was laid at the base of the primary dune parallel

to and 71.65 m seaward of the FRF baseline. Stations were established

along the baseline at 9.15 m spacing, (Figure 3-5).-;r- Standard level,





~'~3h ; rr
J~Ft~--'
pk~_,,~
-rU


Figure 3-3: Groyne #3 at
low tide.
Photo by
D. Cronin.


Figure 3-4:


Groyne #4--looking landward from updrift side
at mid-tide. Photo by K. Bodge.





T South Railing of Pier


150m





-S-12

.N-14
-- -5-14Groyne
-S16
-S-18
-S-20


216m 1-s24





* Current Meters



Scale in Mrtersr
-- s37
Figure 3-5: Typical survey plan for the field impoundment experiments.




chain, and rod-man survey techniques were generally used for the pre-

groyne construction surveys. An OMNI Total Station Transit was used for

the night surveys. After dark, chemical light sticks were attached

around the OMNI prism to aid the transit operator in locating the rod-

man in the surf. Light sticks were also used as range markers for the

rod-man and to highlight hand-signal communication between the OMNI~

operator and the surf and beachface workers. The bullet site on the

OMNI was back-lit by a 12-V lamp in order to aid OMNI targeting. Mlost

of the surveys were executed to 1.5 meter water depths. During the last

impoundment experiment, the FRF's Coastal Research Amphibious Buggy





(CRAB) extended the profiles by making two shore-parallel transects

seaward of the rod-man's maximum offshore location.

Several surveys were made to test the repeatability and accuracy of

the OMNI survey system. Average vertical profile error was 1.5 em and

the maximum error was approximately 4 cm.



3.2.4 Groyne Deployment and Profiling Procedure

It was found that the most effective impoundment measurements could

be made when the groyne construction began at the berm, just above the

swash zone at high tide, and followed the falling swash zone through low

tide. Then, after a one to two hour lapse (while the pumping equipment

was secured and the survey gear deployed), beach profiling commenced as

the tide rose such that the waves were breaking within the end of the

groyne and the groyne extended completely across the surf zone. Beach

profiling continued until the wave break migrated back outside (seaward)

of the end of the groyne on the subsequent falling tide. In general,

one loop of profiles was taken on the rising tide (just after the break

moved inside the end of the groyne) and another loop was taken on the

falling tide and completed just as the break moved back outside the end

of the groyne. This resulted in two sets, or loops, of profiles which

described the sediment impoundment across the entire surf zone during

the high water portion of the tidal curve. Each loop of profiles

typically consisted of ten survey lines--2 downdrift, 6 within the trap,

and 2 "control" lines well updrift of the groyne. Spacing along the

beach between profile lines typically increased from about 9 m near the

groyne to about 60 m far updrift of the groyne. Usually, a complete

loop of profiles was also taken in the early morning or afternoon just





61

before or during each groyne deployment. The elevation along the top of

each groyne was surveyed immediately after each experiment.



3.2.5 Additional Measurements

Longshore current. Eulerian measurements of longshore current were

made at three locations across the surf zone approximately 185 meters

updrift of the groyne. Impellor-type current meters (aligned shore-

parallel) were mounted close to the bed on 5-cm diameter jetted steel

pipes. The data from each of the three current meters were recorded by

strip chart for 5 minutes once or twice an hour during the post-groyne-

construction profiling. Each current meter was calibrated in the

laboratory under steady flow conditions prior to the summer's field

work. The current meter amplifiers were typically set with a 10-second

response time in order to dampen short period gravity wave fluctuations.

Waves. Estimates of wave height and one-dimensional spectra were

available for the entire period of each experiment from Baylor gauges

located underneath the FRF pier. Directional spectra data were avail-

able from the FRF P-U-V directional wave gauge located south of the end

of the pier for the duration of each experiment. Observations of wave

type and height within the surf zone were made by visual reference to

red-and-white 15-cm graduated marks on the steel pipes which supported

the current meters. High-frequency (HF) radar imagery was utilized to

identify the breaker angle near the shoreline.

Sediment. H~and-grab surface sediment samples were collected near

the end of each experiment. Typically, samples were taken at three to

five cross-shore locations updrift and downdrift of the groyne.








62


3.3 Description of the Experimental Conditions

3.3.1 Overview


Four groynes, comprising four separate experiments, were estab-

lished during the period July 16 through August 30, 1984. (An attempted

groyne deployment on September 26 was aborted due to dangerous surf

conditions and the sudden appearance of an unexpected northeasterly

storm. ) The individual experiments will be referred to as Groyne #1,

#2, #~3, and #4. A summary of each of the four experiments is presented

in Table 3-1.


Of the four experiments conducted, the last two were by far the

most successful. The pyramid-style "brick-laying" bag deployment system

was developed for these experiments which enabled the rapid construction




Table 3-1: Summary of Field Experiments


EXPERIMENT NO. #1 #2 13

TOTAL I PROFILE LOOPS 3 4 3 5
L POST-DEPUYLOYMENT 1 POST-DEPLOYMENT 1 PRE-DEPLOYMENT I PRE-DEPLOYMIENT
1 MIDNIGHT- (PA~IAL) 2 HIDN:GHT (PARTIAL) RISING TIDE RINGTD
1NEXT MORNING 1 NEXT MORNING [1FLIG IE [ CALLING TIDE
RISING TEDE
1 CALLING TIDE

SURF ZONE UAVE HEIGHT
AND POSITION OBSEIRV.

CURRENT DATA I DYE STUDY 2 1/2 CURRENT METERS 3 CURRENT H~ETERS

SED. SAMrPLES J J

TRACER STUBT J

HF RADAR DATA HIGH-QUIALITY J

COMMeTES Low grayne. Low groyne. High-quality. Hihqlty
Leagshore features. Deployed on
Profiling loops not rising tide,
tied to tidal ProTfling looped
cycle. not tied to
t~idal cycle.





of a high-relief groyne with negligible gaps. Additionally, these two

experiments represent the most complete set of data consisting of all

supplementary measurements listed above. The first two impoundment

experiments were of relatively poor quality since field procedures were

still under development. These experiments did not employ the pyramid-

style groyne system, nor was the groyne deployment and survey sequence

coordinated with the tide. Accordingly, the groynes were of smaller

relief and considerable impoundment occurred during construction.

Each experiment was executed when the beach site was fairly regular

and surf conditions were acceptably small and steady. In the summer

months along North Carolina's Outer Banks the most consistent wave

events are small (less than about one meter breaking height), of

moderate period (7-9 seconds), and arrive at a relatively steady and

significant angle from the south (breaking angles between 50 and 15* to

the shoreline). These conditions--most favorable for the experimental

procedures described herein--result from a high pressure system over the

north Caribbean which dominates the southeastern U.S. coastal weather

pattern during the summer. Tropical storm activity (including Hurri-

canes Gloria and Isadora), as well as unexpected squalls from the north-

east, interrupted the ideal southerly wave climate for days or weeks at

a time. In general, however, the summer beach contours at the FRF

experiment site adjust to the dominant southerly wave climate; the

shoreline builds south of the FRF pier and recedes north of the pier.

Figure 3-6 illustrates typical nearshore bathymmetry at the FRF during

the Impoundment experiments. During the intensive beach profiling

intervals after groyne deployment, no longshore features (such as beach

cusps) were observed in the study area with the exception of Groyne #1.





64

Of possible interest, however, was a cusp field (18 to 26 m wavelengths)

which typically appeared about 24 hours after each groyne deployment and

extended updrift from a location beginning about five groyne-lengths

from the groyne. It was not clear whether the appearance of these cusp

fields was related to the presence of the groyne.

All of the experiments reported herein were executed during the

"ideal" southerly wave events described above. The approximate surf

conditions for each experiment are listed in Table 3-2. The values in

the table represent a synthesis of visual observation, Baylor gauge wave








-66



2 moo a a 85






,Fi- 181
w lr i -~l 172



-185
187

) 250450 60 85


PR BAHMER 11AU 8



CONTOURS IN METERS

Figure 3-6: Typical nearshore bathymetry at the field investigation
site during the impoundment experiments. Courtesy of
CERC Field Research Facility, Duck, N.C.





foreshore bed slope
typical breakpoint bed slope
average maximum vertical runup
PL=plunging, SP=spilling


65

data from the FRF pier, HF radar imagery, tidal data, and beach profile

data. The surf parameters listed most closely correspond to significant

values typical for each experiment. Figure 3-7 illustrates the time

series of (unrefracted) significant deep water wave height and modal

wave period for each of the experiments, as determined from Baylor

gauges along the pier. Hourly estimates of breaking wave angle (taken

from HF radar imagery) were available only for Groyne #4, and are illus-

trated in Figure 3-8. Figures 3-7 and 3-8 indicate that the directional

wave events were quasi-steady for each impoundment experiment (with the




Table 3-2: Representative Surf Conditions
for Field Experiments (1984)


mf 1 b2 R3

(m)


brkr

type


Hb hb ab

(m) (m) (deg)


Exp't.
& Date


Groyne#1
7/27-28

Groyne#2
8/03-04

Groyne~3
8/19-20

Groyne#4
8/29-30
0900-0500

Groyne#4
8/29-30
0900-1500


0.65 0.82 5 8.1 0.37 0.075 0.035 0.8 0.44 PL


0.52 0.64 6-7 9.0 0.27 0.072 0.038 1.0 0.59 PL/SP


0.60 0.78 5-6 9.0 0.37 0.085 0.042 1.0 0.61 PL



0.60 0.95 7-8 8.1 0.36 0.077 0.029 .85 0.38 SP/PL



0.45 0.70 5-6 8.2 0.24 0.077 0.030 .85 0.46 ~ PL
































































1600 2000) 0000 0400 0800 1200
TIME (E.D. S.T.) 8/29-8/30/84


Figure 3-8:


Breaking wave angle estimated from HF radar imagery during
the fourth field experiment (Groyne #4). Values represent
the angle between the breaking wave and the shoreline (in-
cidence from the south).


8/29-8/30/84 b0 d

-P


-


C)4C


I


'o0.2


0.4





0.2


a


F 8/19- 8/20/84


I


F V
8/3-8 /4/84


-- -- ---- --a- - -


-


-
/7 27-7/28/84


Ho
ST

0800 1200 1600 2000 0000 0400 08003 I:20
TIME (E.D.S.T. )


Figure 3-7:


Approximate modal wave period and significant (unrefracted)
deep water wave height during the four field experiments.


100 1200


8


6


4

08E





possible exception of Groyne #4--for which the directional energy was

somewhat less during the second impoundment interval than during the

first). The portions of the beach over which the groyne deployment and

surveying were executed were relatively monotonic for each experiment

and were characterized by a planar foreshore and a slightly concave-up

shape seaward of the low-water line. The raw beach profile data

collected during the last three experiments are tabulated in Bodge and

Dean (1985).

Meteorological data were collected routinely by the FRF during each

experiment. Wind and precipitation were light to negligible, and no

drastic changes in atmospheric pressure or temperature occurred during

any of the deployment or impoundment stages of the experiments.

Each of the groynes extended from the maximum extent of high tide

run-up to about 5 meters seaward of the low tide still water shoreline

(i.e., about one bag-diameter below the low tide mean water level). The

groynes were between 30 and 47 meters in length. The average relief

above the bed of the pyramid-style barriers (Groynes #3 and #~4) was

about 40 em. The average relief of the first two barriers (Groynes #1

and #2) was about 25 em. Construction of each groyne required the

pumping of 70 to 120 tons of sediment and was usually completed in an 8-

hour period between 0900 and 1700 E.D.S.T. by this author and three to

four other field hands. This same field crew also executed the round-

the-clock beach profiling and wave/current measurement tasks subsequent

to each groyne deployment. The work was extremely intense, exhaustive,

and often dangerous. The success of these experiments is directly

attributable to the competent and dedicated efforts of the seemingly

tireless field hands who assisted this author.





3.3.2 Groyne #1

The first impoundment experiment, Groyne #1, was viewed as a quali-

tative success inasmuch as a reasonable barrier was deployed in a short

period of time and updrift impoundment was observed. Beach profiles

updrift of the groyne taken just after and approximately 15 hours after

groyne deployment are shown in Figure 3-9. The integrity of the barrier

and the quality of the surveys associated with Groyne #1~ are suspect,

and the presence of a cusp field beginning about 60 meters updrift of

the groyne precluded the use of far-updrift profiles as a cross-shore

control signal. Accordingly, the data collected from this experiment

will not be discussed quantitatively. It is noted, however, that accre-

tion immediately updrift of the barrier was centered about the upper

foreshore.



3.3.3 Groyne #2

Construction of the second groyne utilized single, large diameter

sand bags laid end to end, and was initiated below the low-water

shoreline on a rising (lunar apogean) spring tide. The tide rose more

swiftly than the sand bags could be deployed, and so a gap in the groyne

construction resulted when pumping activity was forced to move

upshore. The upper portion of the groyne was then deployed along and

above the rising water level and completed as the tide crested. Bags at

and above the upper swash zone which were not placed upon a filter cloth

sheet settled into the bed from the jet-like action of the slurry pumped

into the bags. While waiting for the tide to fall so that the gap in

the groyne could be closed, impoundment rapidly occurred against the

portion of the groyne which was already constructed. The impoundment





iGROYNE cc1 I(~







-3m 36.5 m '1



lil I
2








> -9m I -54.9m


cusp bay









-18.3 m ~ -7 3.2 m
-25 45 65 85 20 40 60
DISTANCE FROM BASELINE (m)

Figure 3-9: Beach profiles 2 hours (solid lines) and 15 hours (broken)
lines) after deployment of Groyne #1. Relative location
updrift of groyne is indicated for each pair of profiles.



was most prominent in the upper swash zone where sediment was observed

to accrete against the groyne at a vertical rate of about 2 mm/minute.

Single beach profiles were executed up- and downdrift of the groyne

during this period. As the tide fell, the gap in the groyne was closed





and additional bags were laid atop the remainder of the groyne, where

possible, to make up for the barrier relief which was lost due to

impoundment during high tide. Figure 3-10 illustrates the groyne

deployment sequence and tidal fluctuation for Groyne #2. It became

obvious that the groynes must be deployed starting at the berm and

following the tide falling down the beach.




rof ile Loop Profile Loop rof ile Loop

I.6-

I.2 GROYNE #2 DEPLOYMENT
6 OS- TIDE



f 0.0-


z-04 -

-0.8
OEOO 1200 1600 2000 0000 0400 0800 1200
TIME ( E. D.S. T.) 8/3 8/4/84


Figure 3-10: Tidal fluctuation, beach profiling "loop" intervals, and
groyne deployment sequence for Groyne #2.



The profile of the completed groyne is shown in Figure 3-11

relative to the beach profiles immediately updrift and downdrift of the

groyne during construction and sixteen hours after completion of the

groyne. Several beach profiles measured updrift and immediately down-

drift of the groyne during these two times are shown in Figure 3-12 a

and b. In the figure, each profile has been horizontally displaced such

that the profiles just above maximum uprush (i.e., at and behind the

berm) are aligned. This is necessary for direct profile comparison along





71



1 5 -----Groyne #

1. *------ 3m Downdrif t





-ICO


(a)

15 .- Gr-Coyne #

10 *.l C----- 3m Downdrift





> -05


-0 (b) -
40 50 60 70 80 90
DISTANCE FROM BASELINE (m)

Figure 3-11: Beach profiles three meters up- and downdrift of the groyne
(bold line) for Groyne #2: (a) during groyne construction,
and (b) approximately 16 hours after groyne completion.



the beach because the survey baseline was not exactly parallel to the

shoreline. The horizontal shift for a given profile is the same in both

Figures 3-12 a and b. Considering that the beach profiles were fairly

uniform before groyne deployment commenced, it is observed from Figure

3-12a that the profile immediately updrift of the groyne (-3m), and to

some extent the profile just further updrift (-9m), accreted considera-

bly during the first stage of groyne construction--as described earlier.

The profiles further updrift and immediately downdrift are recessed

relative to these two profiles. After groyne completion (Figure 3-12b)

the foreshore continued to accrete on the updrift side, but eroded on













































I I r I I I I I I) 1


NOVD


the downdrift side. The profiles below NGVD remained relatively steady

during the experiment, although some accretion apparently occurred just


downdrift of the groyne.


The mean tidal level during the interval


between the two loops of profiles was 5 cm below NGVD.


o I



z


(Updrift)


+3m -'
(Downdrift)


`uli~,k


I 1


I I I I I I I
20 30 40 B{
HORIZONTAL DISTANCE(m)


Figure 3-12:


Beach profiles updrift and immediately downdrift of the
barrier for Groyne #2: (a) during groyne construction, (b)
approximately 16 hours after groyne completion. Profiles
are horizontally shifted for approximate berm alignment.





3.3.4 Groyne #3

The third experiment was the first to use the pyramid-style groyne

deployed during a falling tide. Construction began at the berm just

before high tide and rapidly reached the still water shoreline as the

water level crested. Unfortunately, the neap high tide lingered much

longer than was expected relative to the groyne-deployment time. Sedi-

ment impoundment began in the swash zone as the field crew awaited the

fall of the tide so that further deployment could proceed. (Bag-filling

in still water depths greater than about one bag diameter had earlier

proven inef fective. ) The groyne was finally completed to a depth of

about 20 cm below the low tide water level as darkness fell. The

difficulty experienced during Groyne #3 was that deployment proceeded

much more quickly than expected. Indeed, the pyramid-style construction

rapidly created a substantial and comely barrier of high integrity.

Figure 3-13 illustrates the tidal fluctuation and groyne deployment

sequence for Groyne #3.

In addition to a pre-deployment loop, two survey loops were taken

after groyne deployment: one each on the rising, then falling, tide

while the surf zone was bounded by the barrier. The movement of the

surf zone across the beach relative to the groyne and during the

interval between surveys is depicted in Figure 3-14. The Eulerian

measure of mean longshore current over time at each of three locations

is also shown in the figure. The distance offshore (extreme left

vertical axis) is referenced to the survey baseline. The approximate

corresponding location of the groyne is shown on the extreme right of

the figure. Since the baseline was not precisely shore-parallel, the

surveyed location of the groyne is not directly transferrable to other

















































Figure 3-15 comparatively illustrates the beach profiles immedi-

ately up- and downdrift of the barrier (a) before, (b) two hours after,

and (c) seven hours after deployment. From Figure 3-15a, it is readily

seen that the barrier was constructed on an initially undisturbed beach,

as desired. Considerable downdrift recession--immediately adjacent to

the groyne--occurred about the mean water level. Elsewhere along the

groyne, the downdrift side showed little net change except for some

accretion near the seaward end. Accretionary features are observed

updrift of the groyne well above the mean water level and towards the


74

profile stations. Accordingly, the groyne location shown in the figure

was determined by transferring the depth contours which the groyne

occupied to the contours measured at the station where the current and

visual surf-observation data were recorded.


Profile
Loop 2


Profi le
Loop 3


Profile


1200 1600o 2000 0000 0400
TIME (E.D. S. T.) 8/19-8/20/84

Tidal fluctuation, beach profiling "loop" intervals, and
groyne deployment sequence for Groyne #3.


0800


Figure 3-13:





TIME ( E. D.S.T.)


75

end of the groyne. A localized loss is observed just below the mean

water level immediately updrift of the groyne. The initially perplexing

appearance of a small loss on the updrift side--and some accretion on

the downdrift side--was later observed during the laboratory impoundment

experiments and is discussed in Chapter 7.


__~L_ _


3


4-'
eclk `
aD,,


I


40>-


2KK) 22-OC 2300 0000 OKX)0 0200 0300 0400 0500


Beg in End Begin End
2nd Loop 2nd Loop 3 rd Loop 3rd LDA)

Figure 3-14: Time history of surf zone location and mean longshore cur-
rents during Groyne #3; 8/19-20/84. Positive-valued long-
shore current indicates flow towards the north.


E0


. .2


CC-


.5

4E


C





1.6 2 hours after
E deployment



S04
i~ 0.8


a --.
z -0 4 ( b)




1.6 7 hours after
deployment


08


04


-0.4~ (C)
40 50 60 70
DISTANCE FROM BASELINE (m)


Figure 3-15: Beach profiles three meters up- and downdrift of the groyne
(bold line) for Groyne #3: (a) before, (b) two hours after
and (c) seven hours after groyne deployment.





Two sets of profile changes relatively far updrift of the groyne

(two or three groyne lengths away) are illustrated in Figure 3-16.

Assuming that the presence of the barrier did not affect these


40 50 60 70
DISTANCE FROM BASELINE (m)


Figure 3-16: Representative beach profiles measured far updrift of the
barrier for Groyne #3: before, two hours after, and seven
hours after groyne deployment. Location of profiles rela-
tive to groyne is indicated for each set of profiles.





78

locations (a reasonable assumption based upon laboratory results), the

updrift profile changes indicate that the beach was undergoing accretion

along the foreshore and just below NGVD by cross-shore processes during

groyne deployment, (0900 to 1930 hours). The far updrift profiles were

relatively stable after groyne deployment except for some profile

adjustments about NGVD. The mean water levels between the first and

second survey loops and the second and third survey loops were 0.30 m

and 0.42 m above NGVD, respectively.

Half-hourly wave observations of wave height and type were made at

selected offshore locations during the post-deployment survey interval

and are presented in graphical form in Figure 3-17a. The average

(Eulerian record) longshore current measured at these same offshore

locations is presented in Figure 3-17b. The longshore current was pre-

viously illustrated as a function of offshore location in Figure 3-14.

In Figure 3-17, the longshore current and wave data are shown as a

function of the still water depth. This presentation was accomplished

by noting the NGVD-referenced elevation of each offshore observation

point, a and the NGVD-referenced tidal water level, nl, during each

wave or current measurement. The still water depth corresponding to

each measurement was then calculated as z -n



3.3.5 Groyne #4

Groyne #4 was the most successful experiment of the four attempted.

A pyramid-style groyne was constructed atop the remains of Groynes #2

and #3, which surfaced the evening before the fourth experiment began.

The pre-deployment beach survey indicated that the groyne remnants had

not seriously altered the site bathymetry except for immediately updrift










In..,,..,,


04-

o

.2 o o

OE o
oo0
OO O
0.0 OOO
O O OO0 CO
o
OOo o


0 Breaking a
Breaking
xTranslotory
0Bore
8 Swash




- 0 1 ii11o g%


.i8 H 0 0


OA



Ol


-0.2 0-


0.4
STILL WATER DEPTH


(a) Visual observation of wave height and types, and (b)
Eulerian measurement of longshore current across the surf
zone during the post-groyne deployment interval of Groyne
#3; (8/19/84-1840 through 8/20/84-0500 EDST). Positive-
valued current indicates flow towards the north.


Figure 3-17:


of the old sand bags (mostly irregular scour holes). The groyne was

deployed from the berm to below the low water line on a falling (lunar

perigean) spring tide. Figure 3-18 illustrates the tidal fluctuation


and groyne construction sequence for Groyne #4.





Profile Prof ilIe Profile Prof ile Prof ile
Loopi Loop2 Loop 3 Loop4 Loop5

I .6
Groyne Deployment
z .2
p0.8



> -0.4
z-0.0
0800 1200 1600 2000 0000 0400 0800 1200 1600
TIME ( E.D. S.T. ) 8/29- 8/30/84

Figure 3-18: Tidal fluctuation, beach profiling "loop" intervals, and
groyne deployment sequence for Groyne #4.




In addition to the pre-deployment survey loops, profile loops were

taken after groyne deployment on the rising, then falling, tide when the

surf zone was bounded by the barrier. Another two loops were taken

during the subsequent rising, then falling, tide several hours later.

Inspection of the barrier before execution of the last two survey loops

indicated that the impoundment area was not yet filled to capacity.

Figures 3-19 and 3-20 illustrate the movement of the surf zone relative

to Groyne #4 during the two survey loop impoundment intervals after

groyne deployment.

Figure 3-21 illustrates the groyne profile compared to the beach

profiles immediately up- and downdraft during the third and fifth survey

loops. (These loops may be each thought of as the termination of an

ideal surveyed-impoundment interval.) It is seen that the updrift side

generally accreted all along the profile--relative to the downdrift

side--during the survey period.





80r


70 >


O


sot-


S2
E .I


50E-


'eseoawod 0\\


nsla84 TIME (E.0 S.T) ersn241
000 2100 2200 2300 0000 0100 0200 0500


Begin End Begin End
Ind Loop 2 nd Loop 3 rd Loop 3 rd Loop


Figure 3-19: Time history of surf zone location and mean longshore cur-
rents during Groyne #4, first post-groyne deployment survey
interval (8/29-30/84). Positive-valued longshore current
indicates flow towards the north.



Figures 3-22 and 3-23 illustrate the beach profile changes over

time immediately down- and updrift of the barrier, respectively, for the

impoundment intervals of profile loops 1-2-3 and 4-5. Th~e scour

observed on the updrift side before groyne deployment rapidly filled in


XL a
21



2




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