• TABLE OF CONTENTS
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 Front Cover
 Title Page
 Copyright
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Background
 Preliminary experiments
 Final large basin experiments
 Results and discussion
 Summary and conclusions
 Complete beach profiles
 Overhead digital photographs
 Even-odd comparisons














Title: Physical modeling of nearshore response to offshore borrow pits
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Title: Physical modeling of nearshore response to offshore borrow pits
Series Title: Physical modeling of nearshore response to offshore borrow pits
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Table of Contents
    Front Cover
        Front Cover
    Title Page
        Page i
    Copyright
        Page ii
    Acknowledgement
        Page iii
    Table of Contents
        Page iv
        Page v
    List of Tables
        Page vi
    List of Figures
        Page vii
        Page viii
        Page ix
        Page x
    Abstract
        Page xi
        Page xii
    Introduction
        Page 1
        Page 2
    Background
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
    Preliminary experiments
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
    Final large basin experiments
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
    Results and discussion
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
    Summary and conclusions
        Page 60
        Page 61
        Page 62
        Page 63
    Complete beach profiles
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
    Overhead digital photographs
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
    Even-odd comparisons
        Page 92
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
Full Text




UFL/COEL-2002/005


PHYSICAL MODELING OF NEARSHORE RESPONSE TO
OFFSHORE BORROW PITS







by




Brian P. Williams


THESIS


2002












PHYSICAL MODELING OF NEARSHORE RESPONSE TO
OFFSHORE BORROW PITS












By

BRIAN P. WILLIAMS


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

UNIVERSITY OF FLORIDA


2002


wppw-


























Copyright 2002

by

Brian P. Williams














ACKNOWLEDGMENTS

This work is the result of many dedicated individuals in the Department of Civil and

Coastal Engineering at the University of Florida. Dr. Robert G. Dean, my advisor and

committee chair, deserves special thanks for his guidance and assistance during the past

two years. I would also like to thank the other members of my committee, Dr. Robert J.

Thieke, and Dr. Ashish J. Mehta. In addition, Becky Hudson deserves special

recognition for keeping me on track and informed during my graduate studies. Finally,

this thesis would not have been possible without the efforts of the staff at the Coastal

Engineering Laboratory. James Joiner, Vernon Sparkman, Vic Adams, and Sidney

Schofield were always around to lend a helping hand during my laboratory experiments.

The project under which this study was conducted was sponsored by the Bureau of

Beaches and Coastal Systems of the Florida Department of Environmental Protection;

this support is appreciated greatly.















TABLE OF CONTENTS
pagge

ACKNOW LEDGM ENTS ........................................................ ................................. iii

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

LIST OF FIGURES .................................................................................................... vii

ABSTRACT................................................................................................................. xi

CHAPTERS

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

Problem Statement.................................................................................................... 1
Objectives ....................................................................................................................... 2

2 BACKGROUND ................................................. ................................................ 3

Grand Isle, LA ....................................................... .................................................. 3
Previous Investigations ............................................................ ................................. 5
Horikawa et al. (1977) ................................... ................. .......... ..... 5
M otyka and W illis (1974) ............................................................. ........................ 7
Kojima et al. (1986) ........................................................... .... ....................... 8
Price et al. (1978)................... ....................................................... ......................... 10

3 PRELIM INARY EXPERIM ENTS................................................. ..........................12

Small Basin Experiments ............................................................. ................ 12
Experiment Setup and Equipment................................................................... 12
Borrow Pit M odels.............................. ......... .............................................. 14
Results of Small Basin Experiments..................................................................... 16
Preliminary Large Basin Experiments.............. ........................................... ................ 18
Experiment Setup and Equipment.......................................................................... 18
Profile M easurements ............................................................ .......................... 20
Shoreline M easurements ........................................................................................ 21
Results of Large Basin Experiments....................... ..... 21

4 FINAL LARGE BASIN EXPERIMENTS................................................................24

Accuracy of Laser Level ............................................. ............................................ 24








Experiment Setup and Equipment ........................................................................ 25
Profile Measurements .................................................................................... 28
Bathymetric Measurements............................................................................ 30
Shoreline Measurements ................................................................................ 31
Measurement Procedure and Sequencing of Experiments........................ ............ 31

5 RESULTS AND DISCUSSION............................................................................. 33

Shoreline Change Trends ........................................................................................ 33
V olum e Change Trends ................................................................................................ 36
Partial Explanation for Volume Change.............................................................. 45
Even-O dd A analysis .................................................................................................. 45
V olum e Change ...................................................................................................... 51
Effective Vertical Dimension of Active Profile...................................... ........... 52
Nearshore Bathymetric Trends ................................................................................ 53
Comparison of Current Results to Previous Investigations........................................ 57

6 SUMMARY AND CONCLUSIONS ..........................................................................60

APPENDICES

A COMPLETE BEACH PROFILES................................................................... ......64

First Final Large Basin Experiment ..................................................................... 64
Second Final Large Basin Experiment ................................................................ 69
Third Final Large Basin Experiment ................................ .............................. 73

B OVERHEAD DIGITAL PHOTOGRAPHS........................ .............................. 78

First Final Large Basin Experiment......................................................................... 78
Second Final Large Basin Experiment ............................................................... 85

C EVEN/ODD COMPARISONS....................................... .............................................92

First Final Large Basin Experiment................................................................... 93
Second Final Large Basin Experiment ..................................... ... .......... 94
Third Final Large Basin Experiment ................................................... ................... 95

LIST OF REFERENCES ................................................................................................... 96

BIOGRAPHICAL SKETCH ............................................................ ...........................97














LIST OF TABLES


Table page

4.1: Results of repeated measurements............................................................................25

4.2: Physical characteristics of final large basin experiments............................................27

5.1: Comparison of relative influence of odd components of shoreline and volume
change to shifted even components of shoreline and volume change.....................51

5.2: Comparison of current results to Horikawa et al. (1977) and Motyka and Willis
(1974)....................................................................................................................................58















LIST OF FIGURES

Figure page

2.1: Aerial photo of salients and erosional hot spots at Grand Isle, LA. in December,
1986...................................................................................................................... 5

3.1: Small basin setup for preliminary experiments.....................................................13

3.2: Cross-sectional view of plastic cylinder setup. .....................................................15

3.3: Results of preliminary small basin experiment using trimmed plastic cylinder. .........17

3.4: Results of preliminary small basin experiment using rectangular wooden box............17

3.5: Plan view of large basin setup. ................................................................................. 19

3.6: Cross-section view of large basin setup................................................................. 19

3.7: Summary of shoreline changes for the preliminary large basin experiments. ..............22

3.8: Center profile of the first experiment of the second large basin setup........................23

4.1: Plan view of laboratory setup for final large basin experiments.................................26

4.2: Cross-sectional view of laboratory setup for final large basin experiments ................27

4.3: Locations of survey stations ...................................................................................... 29

4.4: Example of measured beach profiles for one survey station.......................................30

4.5: Experiment progression timeline.................................... ...........................................32

5.1: Shoreline change for the first final large basin experiment.........................................34

5.2: Shoreline change for the second final large basin experiment....................................35

5.3: Shoreline change for the third final large basin experiment..................................35

5.4: Measurement setup for an example profile .................................................................37

5.5: Volume change for first final large basin experiment........................................ ...38








5.6: Volume change for second final large basin experiment ............................................39

5.7: Volume change for third final large basin experiment..................................................39

5.8: Cumulative volume change per unit length for first final large basin experiment........41

5.9: Cumulative volume change per unit length for second final large basin experiment. ..42

5.10: Cumulative volume change per unit length for third final large basin experiment.....43

5.11: Total volume change estimates for all final large basin experiments .......................44

5.12: Shifted even component of shoreline change for the first final large basin
experim ent. ...........................................................................................................47

5.13: Shifted even component of shoreline change for the second final large basin
experim ent. .................................................. ........................................ ............ 47

5.14: Shifted even component of shoreline change for the third final large basin
experim ent ...................... ... ........................................ ..........................................48

5.15: Shifted even component of volume change per unit length for the first final large
basin experim ent..................................................................................................49

5.16: Shifted even component of volume change per unit length for the second final large
basin experiment.................................. .................................................... 49

5.17: Shifted even component of volume change per unit length for the third final large
basin experim ent......................................................................... ..........................50

5.18: Determination of the effective vertical dimension of the active profile for the final
large basin experim ents. ..................................................................... .................. 53

5.19: Initial contours of the first large basin experiment..........................................54

5.20: Contours of the first large basin experiment after six hours with the pit covered.......55

5.21: Contours of the first final large basin experiment after six hours with the pit
uncovered............................................... ...................................................................55

5.22: Initial contours of the second final large basin experiment.................................... 56

5.23: Contours of the second final large basin experiment after six hours with the pit
covered.................................................................................................. ...............56

5.24: Contours of the second final large basin experiment after six hours with the pit
uncovered....................................................................... ......................................56

A.1: Profiles for -120 cm survey station of first final large basin experiment. ...................64








A.2: Profiles for -80 cm survey station of first final large basin experiment ....................65

A.3: Profiles for -40 cm survey station of first final large basin experiment ....................65

A.4: Profiles for -20 cm survey station of first final large basin experiment ....................66

A.5: Profiles for 0 cm survey station of first final large basin experiment ........................66

A.6: Profiles for +20 cm survey station of first final large basin experiment....................67

A.7: Profiles for +40 cm survey station of first final large basin experiment....................67

A.8: Profiles for +80 cm survey station of first final large basin experiment....................68

A.9: Profiles for +120 cm survey station of first final large basin experiment ..................68

A.10: Profiles for -120 cm survey station of second final large basin experiment..............69

A. 11: Profiles for -80 cm survey station of second final large basin experiment................69

A. 12: Profiles for -40 cm survey station of second final large basin experiment................70

A.13: Profiles for -20 cm survey station of second final large basin experiment................70

A.14: Profiles for 0 cm survey station of second final large basin experiment ..................71

A.15: Profiles for +20 cm survey station of second final large basin experiment ..............71

A. 16: Profiles for +40 cm survey station of second final large basin experiment ..............72

A.17: Profiles for +80 cm survey station of second final large basin experiment ..............72

A.18: Profiles for +120 cm survey station of second final large basin experiment ...........73

A. 19: Profiles for -120 cm survey station of third final large basin experiment.................73

A.20: Profiles for -80 cm survey station of third final large basin experiment .................74

A.21: Profiles for -40 cm survey station of third final large basin experiment..................74

A.22: Profiles for -20 cm survey station of third final large basin experiment ..................75

A.23: Profiles for 0 cm survey station of third final large basin experiment .....................75

A.24: Profiles for +20 cm survey station of third final large basin experiment...................76

A.25: Profiles for +40 cm survey station of third final large basin experiment ..................76

A.26: Profiles for +80 cm survey station of third final large basin experiment...................77








A.27: Profiles for +120 cm survey station of third final large basin experiment ................77

B.1: Photo of first final large basin experiment at 0.0 hr time step....................................78

B.2: Photo of first final large basin experiment at 1.5 hr time step....................................79

B.3: Photo of first final large basin experiment at 3.0 hr time step....................................80

B.4: Photo of first final large basin experiment at 6.0 hr time step....................................81

B.5: Photo of first final large basin experiment at 7.5 hr time step....................................82

B.6: Photo of first final large basin experiment at 9.0 hr time step....................................83

B.7: Photo of first final large basin experiment at 12.0 hr time step..................................84

B.8: Photo of second final large basin experiment at 0.0 hr time step...............................85

B.9: Photo of second final large basin experiment at 1.5 hr time step...............................86

B.10: Photo of second final large basin experiment at 3.0 hr time step.............................87

B. 11: Photo of second final large basin experiment at 6.0 hr time step.............................88

B.12: Photo of second final large basin experiment at 7.5 hr time step.............................89

B.13: Photo of second final large basin experiment at 9.0 hr time step.............................90

B.14: Photo of second final large basin experiment at 12.0 time step................................91

C.1: Odd and shifted even components of shoreline change for first final large basin
experim ent. ..........................................................................................................93

C.2: Odd and shifted even components of volume change for first final large basin
experim ent. ..........................................................................................................93

C.3: Odd and shifted even components of shoreline change for second final large basin
experim ent. ..........................................................................................................94

C.4: Odd and shifted even components of volume change for second final large basin
experim ent. ................................................ ......................................................94

C.5: Odd and shifted even components of shoreline change for third final large basin
experim ent. ..........................................................................................................95

C.6: Odd and shifted even components of volume change for third final large basin
experim ent. .......................................................................................................... 95














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

PHYSICAL MODELING OF NEARSHORE RESPONSE TO
OFFSHORE BORROW PITS

By

Brian P. Williams

August 2002


Chair: Dr. Robert G. Dean
Cochair: Dr. Robert J. Thieke
Department: Civil and Coastal Engineering

Beach nourishment has become the shore protection method of choice for many

communities and states in recent years. Despite all that is currently known about the

planning and scheduling of a successful beach nourishment project, questions remain

concerning some of the possible impacts of dredging and placing sand. Dredging

compatible, "beach quality" sand from offshore sources is the most widely implemented

procedure for obtaining the large volumes of sediment required. The dredging process

leaves depressions in the seabed called offshore borrow pits. These offshore borrow pits

can alter the waves approaching the shoreline and thus alter the shoreline itself. While

the complete relationship between dredged offshore borrow pits and the adjacent

shoreline is not currently understood, it is recognized that the position, dimensions, and

configuration of the pit, along with the site specific wave characteristics, are important








factors in determining whether the pit will cause adverse effects or not and, if so, the

magnitude of the effect.

To investigate the nearshore response to borrow pits, a physical model study of a

beach and borrow pit system was conducted in the Coastal Engineering Laboratory at the

University of Florida in Gainesville, Florida. A wavemaker produced shore-normal

waves and the model beach was subjected to waves both with and without the presence of

an offshore borrow pit. Numerous experiments were carried out from June 2001 to

March 2002, but only three contained information specific and complete enough to be

included in the final data set. Measurements of the shoreline location relative to a pre-

established, fixed baseline, nine beach profiles, and digital photographs of nearshore

contours were collected during the experiments. Upon completion of the experiments,

data analysis focused on the shoreline changes due to the presence of the borrow pit and

volume changes as determined by beach profiles. Reviewing the results of the shoreline

changes revealed shoreline advance due to the presence of the borrow pit on the leeward

shore. Volume gains, from the profile data, were generally experienced in the same areas

as shoreline advance.

It is clear from the data collected that under the conditions of the experiments, the

existence of the borrow pit was responsible for both shoreline advance and volume gains

on the beach in the lee of the pit. These results agree with previous experiments using

physical models but are contrary to previous results based on numerical modeling.













CHAPTER 1
INTRODUCTION


Problem Statement

Since the 1970s, beach nourishment has been a popular alternative to hardened

structures such as seawalls, breakwaters, and groins for communities seeking to maintain

the natural character of their beaches. As more Americans continue to relocate to coastal

communities or choose beaches as vacation destinations, the popularity of beach

nourishment will continue to grow. Millions of dollars are spent on beach nourishment

projects each year in the United States. The State of Florida alone spends roughly $30

million each year. That total increases to approximately $100 million considering that

most of the projects within Florida receive matching funds from both the federal and

local governments. In view of the vast amount of resources spent on beach nourishment,

there continues to be a need to better understand the results of nourishment projects.

The majority of beach nourishment projects are dependent on the availability of large

offshore deposits of "beach quality" sand. The performance of a beach nourishment

project can be correlated directly to the compatibility of the fill with the native sediment.

When deposits meeting compatibility requirements are located, dredging equipment is

required to relocate the sediment to the beach where it can be placed to increase the beach

width and volume. The complete effects of the offshore borrow pit, created during

dredging, on the landward wave climate and surrounding beach are still unknown.

However, it is clear that the pit size, configuration, and location offshore will influence








the magnitude of its effects. Some researchers suggest that borrow pits are responsible

for erosion of the leeward beach in question, while others contend that accretion results.

Objectives

This study employs physical models to examine the relationship between offshore

borrow pits and the adjacent nearshore environment. Through the course of

experimentation it became clear that, in the laboratory environment, shoreline response

alone was not sufficient to understand the complex nature of the interaction between the

offshore borrow pit and the nearshore environment. Therefore measurements of beach

profiles and nearshore bathymetry were undertaken in addition to shoreline observations.

Beach profiles, taken at specific intervals during experimentation, will allow calculations

of volume per unit length changes at specific locations along the model beach. The

volume changes and shoreline changes will then be used to better understand the results a

borrow pit can have on adjacent nearshore environments. Wave field measurements were

not obtained during these experiments due to the fact that a previous student, Christopher

Bender (2001, p. 105) (in his MS Thesis), had already undertaken a wave field

investigation within the same laboratory set-up. Mr. Bender noted the following:

The data (wave height) obtained were very erratic and inconsistent. While
the general trends of wave height reduction inside and behind the pit were
seen, other areas showed trends that were conflicting and unexpected.
Also, consistent and repeatable wave height values for the case of no pit
were elusive for most incident waves trials.

The inability to quantify the effects of the test pit on the wave field is believed to be

due to the somewhat irregularity of the wave field and the small changes induced in the

wave field. Thus, the experiments reported here did not include any investigation of the

wave fields in the vicinity of the pit.













CHAPTER 2
BACKGROUND



The effects on the adjacent shoreline of borrow pits created for beach nourishment

remain a subject of considerable debate. At this time, it is unclear whether borrow pits

are responsible for shoreline advance or retreat or a combination of both. It is possible

that offshore borrow pits can have either effect depending on the dimensions and shape of

the pit, its location relative to the shoreline, and the dominant wave climate at the site.

The analysis is further complicated due to the four different wave transformation

processes an offshore borrow pit can cause: wave refraction, wave diffraction, wave

reflection, and wave dissipation (Bender, 2001). The combination of these processes

could result in sheltering of the shoreline leeward of the pit and sediment transport

towards the centerline of the borrow pit (Dean and Dalrymple, 2001). Wave refraction

can also be used to support theories of shoreline retreat leeward of offshore borrow pits.

As waves encounter the borrow pit, the wave celerity increases due to the deeper water

and a convex shoreward wave front forms. The resulting refraction pattern would cause

the waves to break at larger angles at the shore and create sediment transport divergent

from the centerline of the pit (Komar, 1998).

Grand Isle, LA

Grand Isle, Louisiana is the site of one of the most well known examples of

documented interaction between a borrow pit and the surrounding beach. The

development of the salient features at Grand Isle was one of the motivating factors behind








the research presented in this thesis. A nourishment project completed in the summer of

1984 resulted in a complex borrow pit shaped much like a dumbbell. The borrow area

was located approximately 914 m offshore and measured nearly 2,740 m in the longshore

direction by 460 m in the cross-shore direction (Combe and Soileau, 1987). The "bells"

were excavated to approximately 6 m below the seabed and the middle portion was

excavated to 3 m below the seabed (Combe and Soileau, 1987). The centroids of the two

bell shaped pits were located about 1,370 m apart (Combe and Soileau, 1987). By

August of 1985, the beach experienced growth of two salients and three erosional hot

spots. It appears that the borrow pits are responsible for the development of both the

salients and erosional hot spots. Figure 2.1 below provides an aerial view, taken in

December of 1986, of the nourishment area and the aforementioned features.



































Figure 2.1: Aeral photo of salients and erosional hot spots at uirano Isle, LA. in
December, 1986.

Previous Investigations

Borrow pit influence on adjacent nearshore environments has been investigated

previously. This section summarizes the experiments undertaken by three earlier groups

of researchers.

Horikawa et al. (1977)

Horikawa et al. (1977) investigated, through the use of mathematical and laboratory

models, the effects on shoreline shape due to removal of submarine deposits of sediment.

The mathematical model was designed to simulate the shoreline changes of a beach

subjected to an altered topography and wave climate resulting from offshore dredging.

Dredged borrow pits have the ability to change the pattern and magnitude of the waves








approaching the shoreline. The changes in the waves can be manifested by alterations in

the wave height and direction. The assumptions of the mathematical model were as

follows:

The initial shoreline is infinitely long and straight.

The beach is composed of the same profile everywhere before dredging
(straight, parallel contours).

Only longshore sediment transport is taken into account.

Erosion or accretion has no effect on the beach profile shape in the nearshore
region.

Only wave refraction due to the pit was considered.

The mathematical "one-line" model first calculated breaking wave conditions by

refracting given offshore wave conditions over the altered offshore topography. It then

calculated rates of longshore sediment transport on the beach and determined changes in

shoreline shape. The model was used to predict shoreline changes for six different cases

of dredge pit location. The cross-shore position and length of the pit were varied, as was

the duration of testing.

The mathematical model results showed accretion behind the dredge pit and slight

erosion on either side of the pit. The prediction of accretion is somewhat puzzling since

no wave diffraction, reflection, or dissipation were taken into consideration. The model

simulations revealed the following trends:

The effects on a shoreline are reduced, as the pit is located farther seaward.

As the cross-shore length of the pit increases, the amount of accretion increases,
but the increase in erosion is only minor.

The erosive areas display rapid changes for approximately half a year and then
undergo slower changes. The area of accretion experiences large increases in
magnitude for about one year and shows little change thereafter.








The laboratory studies were conducted in a 6 m long, 1.2 m wide, and 0.15 m deep wave

basin. The model beach was constructed with a 1/10 slope from mesalite (specific

gravity 2.4 and median diameter 0.66 mm) and the rectangular borrow pit was 40 cm

long, 30 cm wide, and 1.25 cm deep. The pit was located approximately 100 cm from the

stillwater line of the beach. Waves with a period of 0.41 sec and constant offshore wave

height of 1.3 cm were generated over a filled pit for 5.5 hours, to approximate an

equilibrium condition. Then the pit was opened and the model was subjected to waves

with the same characteristics for a period of 3 hours. The shape of the shoreline and the

distribution of wave height were measured after periods of 1, 2, and 3 hours.

The results of the laboratory experiment showed accretion behind the pit and erosion

on either side. Thus qualitative agreement between the mathematical and laboratory

models existed. However, the magnitude of accretion during the laboratory model was

much greater than that predicted by the mathematical model with otherwise identical

characteristics. Both the magnitudes of accretion and erosion were greater in the

laboratory model than predicted by the mathematical model.

Motyka and Willis (1974)

Motyka and Willis (1974) also employed a mathematical model to investigate the

effects of borrow pits on adjacent coastlines. The mathematical model consisted of two

parts:

1. A wave refraction model to calculate the paths of wave orthogonals over the
nearshore topography.

2. A beach plan shape model that calculated time dependent changes in beach shape
produced by longshore sediment transport.

The model first calculated breaking wave conditions using the refraction model and then

calculated the rates of longshore sediment transport due to the breaking waves and








determined changes in the beach plan shape. An 880 m long and 305 m wide rectangular

pit was selected for the mathematical model. The pit's depth and its cross-shore position

on the model beach were varied during the model testing. The model beach

configuration was selected from a composition of numerous profiles on Great Britain's

Great Yarmouth coast. The wave climate was also selected from typical conditions on

the North Sea and English Channel coasts of Britain.

Preliminary tests of the mathematical model were analyzed for a period equivalent to

10 years. However, it became clear that the results were reasonably stable after a period

equivalent to 2 years. The results of the mathematical model showed erosion behind the

borrow pit and accretion on either side. These results are contrary to those presented by

Horikawa et al. (1977). The mathematical model also revealed the following trends:

Erosion magnitude increased with increasing pit depth.

Erosion magnitude also increased as the pit's location was moved closer to the
shoreline and into shallower water depths.

The reasons behind the diametrically opposed results of these two previous

investigations are not clear. Both mathematical models used a range of wave heights,

periods, and durations. The magnitude of the wave heights and periods were also very

similar. Hopefully, the results from the physical models being presented in this thesis

will serve to reduce some of the confusion surrounding the conflicting findings presented

above.

Kojima et al. (1986)

Since the early 1970s, sediment has been removed from offshore locations along the

coast of the Genkai Sea in the northern part of Kyushu, Japan, for use as aggregate in

concrete. This study was undertaken due to public concern that dredging of sand and








gravel from the sea bed was causing erosion of beaches previously considered stable.

The goals of this study were as follows:

1. Identify historical shoreline changes and their characteristics;

2. Investigate causes of significant beach erosion and accretion through
considering wave and climatic characteristics of the area as well as human
activities like offshore dredging and coastal structures;

3. Evaluate the relationship between beach erosion and offshore dredging;

4. Propose guidelines on how offshore mining should be conducted.

In order to complete the defined objectives, the following studies were undertaken from

1981 to 1985.

1. Meteorological surveys including assemblage of wind data since 1896 and
the number of typhoon attacks.

2. Assemblage of offshore wave data from 1975 to 1984.

3. Assemblage of permitted volume of sand removed from the sea bed
between 1972 and 1983.

4. Analysis of aerial photographs taken from 1947 to 1982 to determine
historical shoreline change.

5. Hydrographic surveys to obtain profile changes in beach and dredged
holes during the study period.

6. Fluorescent tracer studies and sea bed level measurements to obtain data
on sediment movement.

This study found the major cause of severe beach erosion within the study area to be

the results of an abnormally high frequency of destructive wave attacks, which were

inferred from the storm-wind data. However, the offshore dredging areas seemed to be

responsible for some degree of shoreline recession. This assertion was aided by the fact

that dredged holes in water shallower than 30 meters were found to be refilled with sand.

The main source of sand filling the holes originated from the onshore environment.








Therefore, the dredged holes were trapping sand that naturally moves in either cross-

shore or longshore directions. Despite the inability of this study to establish a cause-and-

effect relationship between offshore dredging and beach instability, it recommended that

dredging be discouraged within the water depth where drastic beach profiles changes

occur. This depth is commonly referred to as the critical or closure depth.

Price et al. (1978)

In the United Kingdom in 1976, roughly 11% of the total sand and gravel production

was provided by offshore dredging. Therefore, the Hydraulics Research Station sought

answers to the following questions before licensing a dredging application.

1. Is the area of dredging far enough offshore to prevent beach drawdown into the
deepened area?

2. Is the dredging going to take place in deep enough water so as not to affect
onshore movement of shingle?

3. Does the dredging area include features like bars and banks that might provide
protection from wave attack?

4. Is the area to be dredged far enough offshore and in deep enough water that
refraction of waves will not cause significant changes to the pattern of sediment
transport?

The Hydraulic Research Station referenced previous studies in California and instituted

the criteria that dredging must occur at a minimum depth of 10 meters and a minimum

offshore distance of 600 meters. The results of field tracer studies lead to the criterion

that shingle not be removed in depths shallower than 18 meters. They also established

that offshore banks should not be modified by dredging unless special circumstances are

met and the quantity of dredged material is limited. Finally, a mathematical model using

wave refraction and sediment transport yielded that, in general, the effects of wave





11


refraction due to dredging that takes place in water depths greater than 14 meters are

insignificant.













CHAPTER 3
PRELIMINARY EXPERIMENTS



All physical modeling during this investigation was conducted in the Coastal

Engineering Laboratory on the campus of the University of Florida in Gainesville

Florida. Two different wave basins were used during this investigation. Every effort was

taken during experimentation to document all findings accurately and to maintain

consistency in measurement procedures.

Small Basin Experiments

These initial experiments were intended to provide insight into the processes due to

borrow pit interaction with the nearshore environment. It was necessary to become

familiar with and develop testing procedures and alternative methods of quantifying

nearshore response. Thus, these experiments were undertaken as training exercises

before the main experiments began.

Experiment Setup and Equipment

Initial investigations began in a relatively small wave basin measuring 50 feet (15.24

m) by 50 feet (15.24 m). The model measured 11 feet (3.35 m) in the longshore

direction, 18 feet (5.49 m) in the cross-shore direction, and was bordered by rows of

concrete blocks as wave-guides. The offshore portion of the model consisted of small

rocks, varying in size, and was separated from the sandy beach by a wooden partition that

ran the width of the model. Figure 3.1 shows a plan view of the basin setup.






























Figure 3.1: Small basin setup for preliminary experiments.

This setup was established by a previous student and since these experiments were meant

to be initial investigations, no effort was made to replace the rocks with sand.

Several methods were implemented to simulate the existence of a borrow pit. A

hollow plastic cylinder, cinder block, and rectangular wooden box were all used to model

the borrow pit. Each model was tested at different cross-shore positions and wave

characteristics. The only constants during each experiment were the water depth and the

angle of approach of the waves. Realistically, the water depth was not truly constant

since leaks in the basin and evaporation during the summer months were responsible for

lowering the water level slightly. However, the water depth was established at the proper

level prior to each new phase of experimentation.








Waves were generated by a paddle-type wavemaker, hinged at the floor, with both an

adjustable stroke and period. Other equipment consisted of a stopwatch, a meter stick,

and a shovel.

Each experiment consisted of a control phase and a test phase. The control phase

began by smoothing the beach to provide straight and parallel contours and was followed

by an interval of waves to establish an equilibrium condition. The test phase consisted of

introducing the borrow pit model which was subjected to waves of the same height,

period, and duration as the equilibrium condition. The results of each phase were

recorded in the form of shoreline response. The shoreline response was determined by

recording the distance from a stationary baseline to the stillwater line at six-inch (15.25

cm) intervals along the width of the project beach. The longshore locations of these

measurement intervals remained constant throughout testing in this basin to allow

shoreline changes to be calculated.

Borrow Pit Models

Each physical model of the borrow pit underwent different testing conditions. The

wave period, wavemaker stroke, and position of the physical model relative to the

baseline were all varied during these preliminary small basin experiments. Initially, a

12.75 inch (32.4 cm) diameter, hollow cylinder, extending through the entire water

column was used to test the nearshore response to an obstruction. This condition

simulates a detached breakwater rather than an offshore borrow pit, but the results could

be helpful in determining whether a borrow pit behaves similar to a breakwater. Figure

3.2 below provides a cross-sectional view of the setup.





























Figure 3.2: Cross-sectional view of plastic cylinder setup.

For the next set of experiments, the plastic cylinder was trimmed to a height of 4.5

inches (11.43 cm) so that it could be buried with its top rim level with the surrounding

sand. During control phases, the cylinder was filled with rocks and the rocks were

excavated prior to test phases. Later, the plastic cylinder was replaced by a concrete

block measuring 4 inches (10.16 cm) wide, 16 inches (40.64 cm) long, and 7.5 inches

(19.05 cm) high. Like the full height plastic cylinder, the concrete block modeled a

breakwater rather than a borrow pit. Next, a rectangular box was nailed to the partition,

such that the inside of the box was buried in the sand. The inside dimensions of the box

were 35 inches (88.90 cm) in the longshore direction and 9.5 inches (24.13 cm) in the

cross-shore direction. During control phases, rocks filled the interior of the box and were

then removed for test phases. Two additional experiments used the rectangular box in a








partially filled condition. Rocks were piled against each short side during test phases,

reducing the area and introducing tapered sides to the borrow pit.

Next, the model beach was reshaped in order to more closely approximate the

conditions at Grand Isle. The wave period, wavemaker stroke, and water level were

adjusted in order to scale the model as closely as possible to the natural conditions at

Grand Isle. The rectangular box was also relocated closer to the baseline to achieve a

better representation of Grand Isle conditions.

After reviewing the results of the previous experiments and after having made

numerous visual observations during the experiments, sediment mobility emerged as a

concern. Efforts to increase sediment mobility included inserting a "soaker hose"

underneath the sand portion of the model and replacing the existing sediment with much

smaller diameter sediment. After several attempts with the soaker hose and the smaller

sediment, it became clear that the limitations of this wave basin were such that adequate

shoreline response could not be obtained apparently due to the small forces available for

sediment movement.

Results of Small Basin Experiments

Experiments using the full height plastic cylinder and concrete block exhibited an

overall trend towards shoreline advancement in the lee of the cylinder. However, since

these were not tests of borrow pit interaction with the nearshore environment, they were

conducted for general purposes only. In contrast, experiments with the trimmed cylinder

and rectangular wooden box failed to exhibit clear trends towards either shoreline

advance or retreat. Introduction of the soaker hose failed to produce any definitive

results, as did replacing the larger sand with finer sediments. Figures 3.3 and 3.4 present






17


representative results of experiments with the trimmed cylinder and rectangular box

respectively.


Figure 3.3: Results of preliminary small basin experiment using trimmed plastic
cylinder.

rtitlon


--Base(ne


Period =0.53 sec ----Contro Phase
Stroke = 4.0 In -Test Phase

Figure 3.4: Results of preliminary small basin experiment using rectangular wooden box.

Unfortunately, the results from these preliminary experiments were inconclusive as to

whether any relationship between the borrow pit models and shoreline existed. Because

of this, the experiments were relocated to a larger scale wave basin where it was believed


Plastic Cylinder Pit


- 30
o
25
0

15 2 ------------------------
2 15
o
au 10
0 20 40 60 80 100 120
Longshore Position (in.)

I -- Control Phase ...... Test Phase


c








a better and more energetic wave field could be obtained and thus more sediment

transport and a greater possibility for measurable results.

Preliminary Large Basin Experiments

A model similar to that used by Horikawa et al. (1977) was constructed in the large

wave basin of the Coastal Engineering Laboratory at the University of Florida by Bender

(2001) for a previous experiment. The experiments discussed in this section were all

carried out in this model.

Experiment Setup and Equipment

The dimensions of the model were 15 m in the cross-shore direction and 3 m in the

longshore direction. The offshore portions of the model consisted of a constant depth

section of concrete floor leading to a sloped concrete section. The nearshore portion of

the model consisted of another constant depth section of concrete floor. The rectangular

offshore borrow pit was located in the center of the nearshore portion and had a depth of

12 cm relative to the adjacent bathymetry. Control phases (no pit) of these experiments

were performed by inserting a weighted, wooden cover into the borrow pit. The model

beach was placed "landward" of the rectangular pit using sand sized sediment. Figure 3.5

shows a plan view and Figure 3.6 shows a cross-section view of the model.





19


Pr





















. w First


Second


Figure 3.6: Cross-section view of large basin setup.


evious


;,








The equipment used during the experiments consisted of a snake-type wavemaker,

Lasermark Rotary Laser Level, microcassette recorder, meter stick, metric measuring

tape, tripod, and a small survey rod. The wavemaker had individually controlled paddles

with adjustable phase and stroke. Each paddle was set to the same phase and the stroke

was constant throughout the experiments. The period could also be regulated and was

kept constant throughout. The first two experiments utilized a baseline 270 cm from the

landward edge of the borrow pit. Following the second experiment, the setup was

changed in order to advance the beach closer to the borrow pit by adding sand such that

the stillwater line was advanced seaward by approximately 60 cm. In addition, the

baseline was relocated 90 cm towards the borrow pit.

Profile Measurements

Beach profiles were measured using the laser level, measuring tape, and survey rod. A

metal beam was placed with a shore-parallel orientation across the concrete blocks

directly above the landward edge of the borrow pit. The measuring tape was fixed to a

sliding sleeve on the metal beam. The other end of the measuring tape rested on a chair

beyond the baseline. This allowed the tape to stretch the entire length of the survey area

and be relocated between the various survey stations. The first experiment in this series

used three survey stations across the width of the beach, while the next three experiments

used five survey stations. During the first experiment, one survey station was located

above the centerline of the borrow pit, while the two remaining stations were spaced

relative to the center as follows, -56 cm and +64 cm. The convention used places

negative values to the left of the pit and positive values to the right as an observer looks

from the beach toward the wavemaker. The next three experiments had survey stations at








0 cm, -28 cm, -59 cm, +31.5 cm, and +59.5 cm relative to the centerline of the borrow

pit. Recordings of the elevations along the beach profile were taken at 5 cm intervals

beginning at the baseline and ending at the last position of sand on the model floor.

During the surveys, which were conducted by one person, the coordinates of each survey

point were read aloud and recorded via a microcassette recorder. The coordinates were

later entered into a spreadsheet for analysis. After draining the model, the measurements

were made by standing on the model beach with the survey rod.

Shoreline Measurements

Shoreline or stillwater line positions were obtained for all experiments except for the

first. These measurements were obtained by measuring the distance from the baseline to

the stillwater crossing for the entire longshore distance of the model at 10 cm intervals.

The cross-shore distances were obtained with a meter stick and can be used only as

approximations.

Results of Large Basin Experiments

This model allowed greater water depths and thus larger wave heights. The possible

wave periods were still limited; however, overall it was believed this setup could provide

the necessary sediment mobility to produce better results. It was apparent both by visual

inspection and from the results, that this new setup in the large basin provided increased

sediment mobility.

Shoreline analysis of the second experiment using the first baseline showed accretion

in the lee of the borrow pit and erosion on both sides. The first and second experiments

of the second baseline did not demonstrate agreement between their locations of shoreline

retreat and advance. The first experiment showed erosion of the shoreline in the lee of

the borrow pit and trends toward accretion on both sides, while the second experiment









showed the opposite. The figure below depicts the shoreline change between the control

phase (no pit) and the test phase (pit open) for the second experiment (#2) of the initial

setup (first baseline) and both the first (#3) and second (#4) experiments of the second

setup (second baseline).


Shoreline Change (Preliminary Large Basin)





&/



6.0/







Longshore Posiion (cm)
-----Eperimert#2 - Eperime #3 Eeriment #4


Figure 3.7: Summary of shoreline changes for the preliminary large basin experiments.

After examining the beach profiles in concert with the shoreline responses, it became

clear that the results did not always agree with the assumption that shoreline retreat

would correspond to volume loss or that shoreline advance would correspond to volume

being gained. This is illustrated in Figure 3.8 below, where sediment has been re-

distributed to advance the shoreline, but the profile has experienced an overall loss in

volume.
Expenmet #2 - Expenmert #3. Expenment #4


Figure 3.7: Summary of shoreline changes for the preliminary large basin experiments.

After examining the beach profiles in concert with the shoreline responses, it became

clear that the results did not always agree with the assumption that shoreline retreat

would correspond to volume loss or that shoreline advance would correspond to volume

being gained. This is illustrated in Figure 3.8 below, where sediment has been re-

distributed to advance the shoreline, but the profile has experienced an overall loss in

volume.











Center Profile (First Experiment, Second Setup)

4.0

2.0

0.0
2 1 i 40 60 80 100 1:10

S-2.0
.0

C -8.0
o


LU -12.0

-14.0
Cross-shore Distance (cm)

Control Phase ----- Test Pha

Figure 3.8: Center profile of the first experiment of the second large basin setup.


Again, preliminary experiments in the large basin focused on establishing whether a


relationship exists between the borrow pit and the adjacent shoreline. When these


experiments failed to yield repeatable results, it was concluded that a more complete data


set was needed. Therefore, the number of profile survey stations was increased to nine


and nearshore bathymetry measurements were included in the survey procedure. These


changes were instituted during the final two preliminary large basin experiments and


carried on through all three final large basin experiments. These experiments are


presented in detail in the following chapter.













CHAPTER 4
FINAL LARGE BASIN EXPERIMENTS



The experiments presented in this chapter are similar to those large basin experiments

presented in the previous chapter, except that these experiments included the addition of

bathymetric observations in addition to shoreline measurements, thus allowing volume

changes to be calculated. Each experiment included measurements of the shoreline and

profiles and digital photographs ofnearshore bathymetric contours. In addition, the data

analysis for these experiments was more extensive than those presented previously.

Shoreline changes and volume per unit length changes were determined for each survey

station and were subjected to even-odd analysis. The results of the even-odd analysis

were used in the determination of the value of effective vertical dimension of active

profile, (h. + B), for the model studies.

Before collecting what would become the final three data sets, two preliminary large

basin experiments were carried out in order to test both the accuracy of the data and the

validity of the procedures more completely. The results of these experiments were not

used as data sets for the final results.

Accuracy of Laser Level

The instruction manual for the Lasermark Rotary Laser Level states that the device

is accurate to 5 mm at 30 m distance and 3 mm at 15 m distance. Despite the manual's

assurance of accuracy, the accuracy of the measurements being taken was evaluated. The

farthest distance surveyed was approximately 5 m from the laser level. Therefore, during








the preliminary large basin experiments immediately prior to the three final large basin

experiments, some of the profile measurements were repeated. During the two

preliminary experiments a total of 11 profiles were duplicated. A total of 324

measurements were taken along the 11 profiles. The following table summarizes the

analysis of the repeated measurements.



Table 4.1: Results of repeated measurements.
Difference (cm) Cumulative
D (+ cm) Number of Occurrences Percentage (%) percentage (%)
(+/-) Percentage (%)

0.0 111 34.26 34.3

0.1 133 41.05 75.3

0.2 67 20.68 96.0

0.3 10 3.09 99.1

0.4 3 0.92 100.00



By examining the table above, it is clear that the measurement techniques used for the

laser profiles were accurate and repeatable. Ninety-nine percent of the repeated

measurements fall within the 3 mm published accuracy of the laser for a distance of 15

meters and 75.3 percent are within 0.1 cm, which is the scaled accuracy for the 5 meter

distance encompassing the measurements.

Experiment Setup and Equipment

The experiments discussed in this section were all carried out with a model similar to

that used by Horikawa and presented in the previous chapter. A model beach was






26


constructed with a 1/9 slope from quartz sand (specific gravity 2.65 and median diameter

0.24 mm) and the rectangular borrow pit was 80 cm long, 60 cm wide, and 12 cm deep.

The first two final large basin experiments had a baseline 175 cm from the landward edge

of the borrow pit and an average distance from the borrow pit to the shoreline of 160 cm.

The third experiment had a baseline 245 cm from the landward edge of the borrow pit

and an average distance from the borrow pit to the shoreline of 200 cm. The data

collection procedure was identical for all three experiments. Figure 4.1 shows a plan

view and Figure 4.2 shows a cross-sectional view of the model.









Basellne For
a "q----- 'thi expertnent










Baseine for
f t o experiments


Figure 4.1: Plan view of laboratory setup for final large basin experiments.































Figure 4.2: Cross-sectional view of laboratory setup for final large basin experiments.

Table 4.2 provides some of the physical characteristics of the three final large basin

experiments.


Table 4.2: Physical characteristics of final large basin experiments.
Characteristic First and Second Final Third Final

Wave Period (sec) 1.35 1.35

Approx. Wave 6.0 6.0
Height (cm)

Beach Slope 0.112 0.122

Distance to borrow 160 200
pit (cm)
Water depth
immediately 15 15
landward of pit (cm)


y I emaker








The equipment used during the experiments consisted of a snake-type wavemaker,

laser level, microcassette recorder, meter stick, metric measuring tape, digital camera,

and a small survey rod. Both the stroke and period of the wavemaker remained constant

throughout all experiments. The water depth was also held constant. Assuming the

stroke, period, and water depth remained constant, the approaching waves should, under

ideal circumstances, have had identical characteristics.

Profile Measurements

Profiles of the beach were measured using the laser level, measuring tape, and survey

rod. The same setup and procedure were used during these final three experiments as

was used during the initial large basin experiments presented in the previous chapter.

The only difference was that the number of survey stations across the width of the beach

was increased from five to nine. One survey station was located along the centerline of

the borrow pit, while the remaining eight stations were spaced relative to the centerline as

follows, -120 cm, -80 cm, -40 cm, -20 cm, 0 cm, +20 cm, +40 cm, +80 cm, and +120 cm.

As for the previous experiments, positive and negative values indicate positions to the

right and left of the borrow pit, respectively as an observer looks from the beach toward

the wavemaker. Figure 4.3 presents the survey station locations.






























Baseline for
third ,xpgrlnent


Survey







BaseIne for first
twao experients


-120 -80 -40 0 40 80 120 L

Figure 4.3: Locations of survey stations.

Recordings of the vertical position of the beach profile were taken at 5 cm intervals

beginning at the baseline and ending at the cross-shore position of the landward edge of

the borrow pit. To ensure that the sand bed was not disturbed during the measurements,

the investigator conducted these measurements while standing on a wooden plank,

supported on the concrete block wave guides, above the sand bed and oriented in the

cross-shore direction. Figure 4.4 provides an example of the beach profile recordings for

one of the survey stations.


I~lli

I I I I I 1





-20 20
i ,~ ~ I I













0 10 cm
Profile at -120 cm 0 1


0-




cn

Previous Open (0.0 hr)
Covered .1.5 hr1___ Covered (3.0 hr) Covered (6,0 hr)
Open (7.5 hr) Open (9.0 hr) Open (12.0 hr)




Figure 4.4: Example of measured beach profiles for one survey station.

Bathymetric Measurements

Nearshore contours were quantified as follows. Upon completion of each control and

test phase, the water surface was allowed to become calm. A length of yarn was then laid

at the stillwater location on the beach face. A release valve was then opened to allow

water to escape from the basin. This process of laying lengths of yarn at the stillwater

crossing was then repeated for every 0.5 cm change in the water level. This was done

until the bathymetry became too complex for a single length of yarn. Usually five

contour lines could be laid before the bathymetry became too complex. Following the

completion of establishing the contours, an overhead digital picture was taken of the

model. Using the baseline and sidewalls as reference points, the individual digital photos

could then be scaled and compared. These photos were used for qualitative comparisons

only.








Shoreline Measurements

Shoreline measurements were taken at the same time as the profiles. Instead of

recording shoreline data at 10 cm intervals as during the preliminary large basin

procedure, the cross-shore distance to the stillwater line was recorded only for each

profile survey station. This reduced the number of data points for the shoreline

comparisons, but was not a concern due to the amount of data being collected on the

profiles.

Measurement Procedure and Sequencing of Experiments

Preliminary experiments began from a smoothed beach with straight and parallel

contours. During the final experiments, however, there were no modifications made to

the beach before another experiment began. The first experiment of this final group

began after the completion of the test phase (with the pit uncovered) of the last

preliminary large basin experiment. This process allowed the investigation to monitor

whether the model reverted back to original conditions after experiencing control phases

and test phases. All the profiles and volume calculations were based on relative measures

to the test phase of the previous experiment, hereafter referred to as the 0.0 hour

observation. The second experiment began after the test phase of the first experiment.

Each phase consisted of three different time steps. The duration of the time steps were as

follows: 1.5 hours, 1.5 hours, and 3.0 hours. Each time step was followed by

bathymetric, profile, and shoreline measurements. Thus, the total testing time for each

phase was 6 hours and each total experiment time was 12 hours. Figure 4.5 provides a

timeline representation of the experiment sequence.













Complete Experiment



Current Control Next Control
Phase Phase
Previous Test (covered pit) Current Test (covered pit)
Phase Phase
(open pit) 1 (open pit)

I I I I I I i I

6,0 7,5 9.0 0,0 1,5 3.0 6,0 7,5 9,0 12.0 1,5 3.0 6.0
(12.0) (0.0)

Time Step (Hours)


Figure 4.5: Experiment progression timeline.

Hereafter, each phase will be referred to by its completion time after the 0.0 hour

observation (example: The final control phase was the 6.0 hour time step and the final

test phase was the 12.0 hour time step).

The third experiment involved reshaping the model and shifting both the baseline and

stillwater line landward. In order to maintain the procedure of the previous final large

basin experiments, the initial straight and parallel contours were subjected to 6 hours of

waves in the presence of the borrow pit prior to initiation of the control phase. The full

battery of measurements then commenced and the control and test phases followed.













CHAPTER 5
RESULTS AND DISCUSSION



Shoreline Change Trends

In order to obtain shoreline change magnitudes, the difference between the cross-shore

position at the time step in question and the cross-shore position at the 0.0 hour time step

was obtained. The following equation illustrates the process.

Ay(x)= y(x,t)- y(x,0) (5.1)

where: Ay(x) is the shoreline change at survey station "x" between times "t"

and t = 0.

y(x,t) is the cross-shore distance to the stillwater line at time step "t"

y(x,0) is the cross-shore distance to the stillwater line at time step 0.0

hours

"x" denotes the longshore survey station position (-120, -80, -40,

-20, 0, 20, 40, 80, 120 cm)

and

"t" indicates the time step (1.5, 3.0, 6.0, 7.5, 9.0, 12.0 hr)

Figures 5.1 through 5.3 below present the shoreline changes for the first, second,

and third final laboratory experiments respectively. The line labeled "0 to 6 Hours"

denotes the shoreline change between the end of the last preliminary large basin

experiment (0.0 hour) and the end of the control phase of the first final large basin

experiment (6.0 hours). Note that retreat of the shoreline during this phase amounts to re-









equilibration of the shoreline when the borrow pit was not present. Similarly, the line

labeled "6 to 12 Hours" denotes the shoreline change between the end of the control

phase of the first final large basin experiment (6.0 hours) and the end of the test phase of

the first final large basin experiment (12.0 hours). Figure 4.5 in the previous chapter

presents a timeline representation of the experiment progression and may assist in better

understanding the relationship between the control and test phases.


Shoreline Change (First Final Large Basin)

25







-140 120 -100 .40 20 20 40 60 80 "100 120 1
o -.-


20


Longshore Position (cm)

..---.. 0 to 6 Hours 6 to 12 Hours


Figure 5.1: Shoreline change for the first final large basin experiment.











Shoreline Change (Second Final Large Basin)


Longshore Position (cm)

....... 0 to 6 Hours 6 to 12 Hours


Figure 5.2: Shoreline change for the second final large basin experiment.


Shoreline Change (Third Final Large Basin)



20


40
20
16


1 0 -1 -100 -80 -60 "40 -20 20 40 ...100 120 10



20


Longshore Position (cm)

- - Oto6 Hours 6to 12 Hours

Figure 5.3: Shoreline change for the third final large basin experiment.

During the control phase, in the absence of the borrow pit, all of the final large basin


experiments exhibited shoreline retreat in the lee of the borrow pit. In each case, the


magnitude of retreat was greatest at the centerline of the borrow pit, survey station 0.0


cm. Each experiment also experienced shoreline retreat adjacent to the boundaries of the








borrow pit. The third large basin experiment was the only one not to experience

shoreline advance at the edges of the survey area (-120 cm and +120 cm).

Each of the final large basin experiments also demonstrated shoreline advance in the

lee of the borrow pit during the test phases (when the borrow pit was uncovered). Only

the first experiment did not show the maximum advance located at the centerline of the

borrow pit. Almost every survey station that experienced shoreline retreat during control

phases experienced advance during test phases. In each experiment, the magnitude of

largest retreat was closely matched by the magnitude of largest advance. With this

evidence, one can conclude that under the conditions being tested, the presence of the

borrow pit resulted in shoreline advance for the area of the model beach landward of the

borrow pit. The positive effect of the borrow pit may also extend to the adjacent areas,

however the results do not provide enough conclusive evidence to establish definite

limits.

Volume Change Trends

Volume change between consecutive time steps was obtained after the recorded

measurements were converted to coordinates relative to the initial, 0.0 hr time step.

Because the ends of the measuring tape were fixed at each survey station and the baseline

position was constant, the cross-shore position (y coordinate) of each survey point

remained constant along each profile. In order to track changes in the cross-shore

position of the stillwater point at each survey station in a time-dependent manner, all

cross-shore measurements were adjusted to be relative to the stillwater point of the initial,

0.0 hr time step. This results in survey points landward of the stillwater point having

negative cross-shore positions and survey points seaward of the stillwater point having

positive cross-shore positions.








The vertical positions of each survey point were read from the intersection of the laser

with the survey rod. In order to make the plotting of the profiles easier, the measured

vertical positions (z coordinate) of each survey point were adjusted to be relative to the

vertical position of the stillwater point of the initial, 0.0 hr time step. Figure 5.4 provides

the measurement setup for an example profile. This resulted in positive vertical positions

for survey points landward of the stillwater point and negative positions for those points

seaward of the stillwater point.








z














Figure 5.4: Measurement setup for an example profile.

After obtaining relative coordinates for all the survey points along each survey station,

the coordinates were plotted using AutoCAD. The volume change per unit length for

each time step at each survey station was determined from the plotted profiles using the

built-in area determination function of AutoCAD to evaluate the differences in area of

consecutive profiles at the same survey station. The area difference corresponds to the










volume change per unit length. Next, the volume change per unit length was plotted for

each time step across the width of the model, one volume change for each survey station.


Again, AutoCAD was used to determine the area under this plot, giving an estimate of


total volume change for each time step. Figures 5.5 through 5.7 provide the volume

change per unit length versus longshore position plots for all three final large basin

experiments.


Volume Change (First Final Large Basin)


-j
E





E
0


- .120 .100 .-S -60 -40 -20 20 40 60 80 .-O*1 120 1





Longshore Position (cm)

S - 0 to 6 Hours 6 to 12 Hours


Figure 5.5: Volume change for first final large basin experiment.












Volume Change (Second Final Large Basin)


15







40 -12 -80 -60 -40 -20 20 40 60 80 1 120 1



00
130


Longshore Position (cm)


--....... 0 to 6 Hours 6 to 12 Hours


Figure 5.6: Volume change for second final large basin experiment.



Volume Change (Third Final Large Basin)


10 -1 100 -80 -60 40 -2 20 40 60 80 00 120
S %. 60

100' -,


110
Longshore Position (cm)

- - to 6 Hours 6 to 12 Hours


Figure 5.7: Volume change for third final large basin experiment.


With the exception of a few survey station locations, the entire model beach lost


volume after 6 hours of wave interaction with the borrow pit covered. Most of the


locations that experienced a gain in volume were located at the edges of the survey area.


0 U


E-S


C.)
0
E
.5


0B
C




EE
0-


(u--
C)

0








Losses for all of the experiments were of the same order of magnitude, with the second

and third experiments exhibiting the largest losses.

After six hours of testing with the borrow pit present, most of the survey stations that

previously experienced losses regained some or all of the volume lost. The second final

large basin experiment was the only experiment that did not recover approximately the

same magnitude of volume that was lost. For this experiment, the average magnitude

regained was less than the magnitude lost by approximately 50 cm3/cm. The second

experiment also continued to lose volume at the -120 cm survey station while the borrow

pit was present. This was the only experiment to exhibit this behavior. In addition, most

of the survey stations that gained volume in the absence of the borrow pit lost volume in

the presence of the pit. Although accurate conclusions cannot be drawn as to where the

volume gained originated, it is evident that the presence of the borrow pit has a positive

volumetric relationship with the adjacent beach.

By integrating curves of cumulative volume change per unit length versus longshore

position for each testing period, it was possible to determine an estimate for the total

volume change as a function of time for the survey area. Figures 5.8 through 5.10

provide plots of the cumulative volume change per unit length versus longshore position

of each testing period for the final large basin experiments.







41






50 -
S 40 '
S30--








-a o..

-60. ./ -
0-701 \ 0 4 80 /
-so.. -






\ /
:' -10. V



l, Hour __ 3,0 Hours 6 Hou 9, Hous 12,0 Hours




Figure 5.8: Cumulative volume change per unit length for first final large basin

experiment.
CL





















experiment.



































-All
1 -. p a
















He. 30 Ver. 6 -- ft HM f HeO Hourr t -

Figure 5.9: Cumulative volume change per unit length for second final large basin
experiment.



















4T II?
I I I.i








ll.




1.5 Htourr 3 3 Hoxs -. i &0 Ho --- "" "*Or; H % Hu _. 12.0 H.our
Figure 5.10: Cumulative volume change per unit length for third final large basin
experiment.

By examining the cumulative volume change plots, it is evident that volume is lost at

the survey stations within and immediately adjacent to the boundaries of the borrow pit

during phases when the borrow pit was covered. Recall that these volume changes are

relative to the volumes present at the beginning of each experiment. Since each

experiment begins at the conclusion of a test phase, the greatest changes would be

expected at the 6.0 hour and 12.0 hour time steps. Ideally, one would expect that the

cumulative volume change would return to zero at the 12.0 hour test phase.

The cumulative volume change curves were integrated for each experiment and the

results were plotted versus time in order to produce the results in Figure 5.11 below.










Total Volume Change vs. Time




E 0o 1.5 . 30 4.5 6.0 7.5 9.01 1 .0
| -000---0 ---- *
- - -









F Epen .Second E ent - ThExperint

Figure 5.11: Total volume change estimates for all final large basin experiments.

The results presented in Figure 5.11 should be considered as estimates as there were

only nine survey stations across a 3.0 m wide beach. These were the most complete

results that could be calculated with the data available from these experiments. This

figure reinforces the fact that volume was recovered due to the presence of the borrow

pit. Note the increases in total volume change immediately after the 6.0 hour period in
.20000









Timeeach experiment.(Hours)
S-- Fft Expenm. t ...... *Second Expermt Thr E.pamet |









Figure volume losses can be attributed to several final actors, consolidation of sediments.

infilling of the results presented in Figure 5.oss-shore transport of sbe considered as estimates as the survey are were

In addition, volume changes were calculated at the survey stations across a 3.0 m wide beach. These werone and no data exist

results the model n be caltween these locations. The cumuavailative volume change curvriments. This

Figures 5.8reinfo through volume was recoverely connecting the presvalues at the survey
pit. Note the increases in total volume change immediately after the 6.0 hour period in

each experiment.

The volume losses can be attributed to several factors, consolidation of sediment,

infilling of the borrow pit, and cross-shore transport of sediment outside the survey area.

In addition, volume changes were calculated at the survey stations alone and no data exist

for the model in between these locations. The cumulative volume change curves in

Figures 5.8 through 5.10 were produced by linearly connecting the values at the survey

stations. The assumption of linear interpolation between survey stations is believed to

introduce only secondary errors in the results.








Partial Explanation for Volume Change

During the course of experimentation, significant amounts of sediment were observed

to deposit in the borrow pit. By visual inspection, it was evident that approximately three

times the amount of sediment was deposited in the borrow pit during the second final

large basin experiment as during the first. The amount of sediment removed from the

borrow pit during the second experiment was estimated to be between 2500 and 3000

cm3. This may explain the difference in the total volume change estimates between the

first and second experiments. Since the water depth, wavemaker stroke, and wave period

were constant throughout these experiments, a change in test conditions cannot be

responsible for the increased infilling of the borrow pit. The amount of sediment

deposited in the borrow pit during the third final large basin experiment was far less than

the previous two experiments. This is most likely due to the fact that the model beach

was located farther away from the borrow pit during the third experiment. During each

of the three final large basin experiments, sediment deposited inside the borrow pit was

removed after each testing phase and was permanently removed from the system. In

hindsight, the volume of sediment removed from the pit should have been quantified for

each experiment. This volume could have then been compared to the estimates for total

volume change.

Even-Odd Analysis

In an effort to isolate the changes (shoreline or volumetric) due to the presence of the

borrow pit from other background processes, an even-odd analysis was performed on

both the shoreline and volumetric change observations. The process decomposes

longshore distributions of shoreline or volumetric changes into an even and an odd

function about the origin of a longshore coordinate system where the origin is located at








the feature being considered (Dean and Dalrymple, 2001). Normally, an even-odd

analysis is used in situations where inlets or coastal structures are present, but in this

case, an even-odd analysis was used to examine shoreline and volume changes associated

with the borrow pit. The total volume or shoreline change (AVs) can be written as the

sum of the even and odd components of change.

AV, =AVe,()+ AVo(x) (5.2)

When using this technique for a groin, the even function can be interpreted as the

ongoing changes in the absence of the feature and the odd function interpreted as changes

due to the feature alone. However, since the waves in the laboratory model approach the

shoreline nearly shore-normal and the borrow pit is a symmetric feature, the even

function represents the shoreline and volumetric signals due to the borrow pit. Reflection

of the waves due to the sidewalls of the model can affect the signal. However, for

purposes of this investigation, these effects were assumed to be negligible. The even and

odd components are determined through Equations 5.3 and 5.4 respectively.


AV,(x) = (AVs (x)+ AVS (- X)) (5.3)
2

AVo (x)= I (A V, (x)- AV, (-x)) (5.4)


The even function is symmetric about the axis and the odd function is anti-symmetric.

The even function should result in equal positive and negative areas. This was not the

case for the laboratory results. Therefore, the results were adjusted so that the areas were

equal. This was accomplished by integrating the even function curve, thus obtaining an

area, and dividing the area by the longshore distance (240 cm). This result was then

subtracted from the original even function values to produce new values resulting in an







47



integral equal to zero. The shifted/offset curves of the even components of shoreline


change are presented in Figures 5.12 through 5.14 below.



Shifted Even Component of Shoreline Change (First Final Large Basin)


4)
0


0


2O

15

10

------ ---


4I .120 .100 EN .60 '0 20 20 Af 60 0o 100 120

5
13


Longshore Position (cm)

-- 0 to 6 Hours - - 6 to 12 Hours


Figure 5.12: Shifted even component of shoreline change for the first final large basin
experiment.


Shifted Even Component of Shoreline Change (Second Final Large Basin)





OW
C I ---15-I





SC -- ---------
o c I 5
2 Cc... --
S. 120 -100 80 "' -40 20 200 0 100 120 1.0






Longshore Position (cm)

------- O to6 Hours ----6to 12 Hours



Figure 5.13: Shifted even component of shoreline change for the second final large basin
experiment.






48



Shifted Even Component of Shoreline Change (Third Final Large Basin)

Pg -


0

Sc.
E

-o,
U)
00


Longshore Position (cm)

S....... 0 to 6 Hours ----6 to 12 Hours

Figure 5.14: Shifted even component of shoreline change for the third final large basin
experiment.

For each experiment, the shifted even component of shoreline change for the period


from 6 hours to 12 hours is positive for the longshore position ranging from


approximately -40 cm to +40 cm and negative outside this range. For the period from


0.0 Hours to 6 Hours, the shifted even component of shoreline change is approximately


opposite (a mirror image). These results reinforce the interpretation that the borrow pit


was responsible for shoreline advancement on the leeward shore.


Figures 5.15 through 5.17 present the shifted/offset even component of volume change


per unit length for each final large basin experiment.


\
\

................. . o 7- ..... ...............
10 .120 .100 gW .60 .40 '-20 "' 40 60 ~"-,W 100 120 1.







49




Shifted Even Components of Volume Change (First Final Large Basin)


0
E

O -
0 OM

o E




>>
a
0.0

2U
* .


Longshore Position (cm)

- - 0 to 6 Hours - - 6 to 12 Hours



Figure 5.15: Shifted even component of volume change per unit length for the first final
large basin experiment.



Shifted Even Component of Volume Change (Second Fnal Large Basin)



125

E W
C
0 E7
oE




,U .140 .120 .100 80 .'o ., .20 2 \ 100 120 1

00



Longshore Position (cm)

- ---- 0 to 6 Hours - -6 to 12 Hours



Figure 5.16: Shifted even component of volume change per unit length for the second
final large basin experiment.


125





-5g-----* I--


___________________- ^) '------------------
-- -
. 2.. . 20 2 00- ... . .
40 1'0 1c i o


AA











Shifted Even Component of Volume Change (Third Final Large Basin)


E
0

oE

CU
o E
0 u
0

5-


126






/ \

0 -
10 -120 -100 -80 3 s.o 1 120




100
----------------------------------------------------------------


Longshore Position (cm)

- - 0 to 6 Hours - 6 to 12 Hours


Figure 5.17: Shifted even component of volume change per unit length for the third final
large basin experiment.

For the period from 6 hours to 12 hours, each experiment exhibits positive values for


the shifted even component of volume change in the lee of the borrow pit and negative


values to the sides of the pit. The magnitudes and the ranges for which they are positive


varied from one experiment to the next. Conversely, the shifted even component of


volume change is negative in the lee of the pit and positive to the sides for the period


from 0.0 hours to 6 hours. Again, these results support the earlier findings that the

borrow pit was responsible for volume gains of the leeward beach.

The odd components of shoreline and volume change are not presented within this


chapter because the even components provide the shoreline and volumetric signals due to


the borrow pit. However, since it was still possible that the odd components contributed

significantly to the total shoreline or volume change, they were compared to the shifted

even components of shoreline and volume change. Comparisons were made both


graphically and numerically. Graphical comparisons of the shifted even and odd








components of shoreline and volume change for each experiment in the final data set

appear in Appendix C. Numerical comparisons were made by taking the mean value of

the shifted even and odd components squared for each experiment. Only the shifted even

and odd components of shoreline and volume change due to the presence of the borrow

pit were used for this analysis. The relative influence of the odd components can be

quantified by calculating the ratio defined in Equation 5.5.


Ratio = Ay(5.5)
Ay2


Table 5.1: Comparison of relative influence of odd components of shoreline and volume
change to shifted even components of shoreline and volume change.
Shoreline Changes
First Experiment Second Experiment Third Experiment
Shifted Shifted Shifted
Even Odd Ratio Even Odd Ratio Even Odd Ratio
(cm2) (cm2) (cm2) (cm2) (cm2) (cm2)

32.27 7.28 0.23 55.35 25.17 0.45 25.00 6.06 0.24

Volume Change
First Experiment Second Experiment Third Experiment
Shifted Shifted Shifted
Even Odd Ratio Even Odd Ratio Even Odd Ratio
(cm4) (cm4) (cm4) (cm4) (cm4) (cm4)

525.3 543.6 1.03 1756 227.4 0.13 1931 1008 0.52



The ratios from the table above indicate that only the contribution of the odd component

of volume change for the first experiment equaled or exceeded the contribution from the








shifted even component. All of the other ratios indicate ratios of odd to even component

mean square values ranging from approximately one-eighth to one-half.

Effective Vertical Dimension of Active Profile

The sum of the depth of the active profile, h., and the berm height, B, is an extremely

useful quantity for coastal engineers. Application of the Bruun Rule is perhaps the most

common use for this quantity. The Bruun Rule is the earliest relationship between

increased water level and profile response (Dean and Dalrymple, 2001). The form of the

Rule being applied here is to relate the volume change to a horizontal profile change.

The volume is the product of the horizontal profile recession or shoreline change and the

vertical dimension of the profile out to the width of the active profile. The resulting

formula is presented below.

AV = Ay(h, + B) (5.6)

The vertical dimension of the profile out to the width of the active profile was

determined for the first two final experiments and the third final experiment. Using the

values of the shifted/offset even components of shoreline change and volume change per

unit length, a plot was generated with shoreline change on the x-axis and volume change

per unit length on the y-axis. A linear regression line was then plotted through the data

points. The slope of the regression line corresponds to the (h. + B) value for the model

beach in question. Figure 5.18 presents the data points and the regression line used to

determine the (h. + B) value of all the final large basin experiments.










E Volume Change vs. Shoreline Change
S150
E Linear Regression
o A
0 First 0 to 6
E First 6 to 12
2 100 A Second 0 to 6
-'Ex Second 6 to 12
O Third 0 to 6
-i- Third 6 to 12
c 50-
.2 +
4) x

E x0


0 O









Shifted Even Component of Shoreline Change (cm)
Sl l 4.78
0



w 0
S-100(effective
and -15 -10 -5 0 5 10 15 20
wthn Shifted Even Component of Shoreline Change (cm)

Figure 5.18: Determination of the effective vertical dimension of the active profile for
the final large basin experiments.

The slope, (h. + B), of the linear regression line is 4.78 cm. In order to conform to the

Bruun Rule, the line should pass through the origin. Therefore, the slope (effective

vertical dimension of the active profile) was determined using a least squares technique

and found to be about 5.6 cm. This value seems too small, especially considering that

with an approximate wave height of 6.0 cm, the depth of the active profile (h*) would be

approximately 7.7 cm alone. The addition of a berm height between 2 and 3 cm would

increase the effective vertical dimension of the active profile to about 10 cm, almost

twice the amount determined from the data shown above.

Nearshore Bathymetric Trends

Overhead digital photographs taken of the contour lines were used to qualitatively

compare successive phases of each experiment. Photographs were taken after each








control and test phase of the first two final large basin experiments. Originally, the

photographs were planned for use to quantitatively determine volume changes in the

nearshore, but the lack of an effective method for digitizing the contours resulted in a

qualitative analysis. For this reason, the digital photos were discontinued during the third

final large basin experiment. After converting all the photos to the same scale, analysis

could begin. Figures 5.19 through 5.21 provide the contours for the 0.0, 6.0, and 12.0

hour time steps of the first large basin experiment. For each figure in this section, the

first contour is the stillwater line and the elevation of each subsequent contour is 0.5 cm

lower. If Figure 5.19 is used as an example, the first contour is the stillwater line (0.0

cm), the elevation of the second contour is -0.5 cm, the third is -1.0 cm, and the fourth is

-1.5 cm. The intermediate photos are not presented here, but are available in Appendix


Figure 5.19: Initial contours of the first large basin experiment.





















Figure 5.20: Contours of the first large basin experiment after six hours with the pit
covered.


Figure 5.21: Contours of the first final large basin experiment after six hours with the pit
uncovered.

These overhead photos reinforce the previous shoreline change observations in that

they show the 0.0 contour moving towards the baseline after six hours of waves without

the pit present. In addition, Figure 5.21 shows the shoreline approaching the initial

configuration. Figures 5.22 through 5.24 provide the contours for the 0.0, 6.0, and 12.0

hour time steps of the second large basin experiment.




















Figure 5.22: Initial contours of the second final large basin experiment.


Figure 5.23: Contours of the second final large basin experiment after six hours w
pit covered.


Figure 5.24: Contours of the second final large basin experiment after six hours with the
pit uncovered.

Figure 5.21 and 5.22 are the same since the second final large basin experiment began

directly following the end of the first experiment with no modifications to the model

beach. Comparing Figures 5.22 and 5.23 further reinforces shoreline retreat in the lee of





57


the pit and Figure 5.24 clearly illustrates the shoreline approaching a configuration

similar to the initial.

Comparison of Current Results to Previous Investigations

The results of the current laboratory models show that the leeward beach generally

experiences shoreline advancement and volume increases, which agrees with the results

presented by Horikawa et al. (1977), but conflicts with the results of Motyka and Willis

(1974). Table 5.2 below summarizes the significant elements of each model.








Table 5.2: Comparison of current results to Horikawa et al. (1977) and Motyka and
Willis (1974).
Horikawa et al. Horikawa et al. Motyka and Willis
Physical Current (Physical (Mathematical (Mathematical
Property Experiments Model) Model) Model)
Wave Period Variable Variable
1.35 0.41
(sec.) (6-13) (5-8)
Beach Slope 0.11 / 0.12 0.10 Natural Natural
Wave Height .Variable Variable
(cm) (25 -270) (40- 200)
Pit Distance
from shoreline 160 / 200 100 Variable Variable
(cm)
Pit Length 80 40 Variable 880
(cm) (2 4 km)
Pit Width (cm) 60 30 2.0 km 305 m

Pit Depth (cm) 12 1.25 300 Variable

Water Depth
adjacent to pit 15 4.1 Variable 18 m
(cm)
Sediment
edin Quartz Mesalite
Median 0.24 mm 0.66 mm Unknown NA
Diameter
(Sp. Gravity) (2.65) (2.4)
(Sp. Gravity)

Max Shoreline 1/ /
+13/+19/
Response in +13 +6 cm +2.7 m -30 m
Lee of Pit (cm)



The physical model presented in this thesis used longer period and larger waves, a

deeper borrow pit with twice the plan area, a larger water depth, and a greater density

sediment with a smaller median grain size than the physical model used by Horikawa et

al. (1977). Despite these differences, the models produced similar results. That the two

physical models resulted in similar positive shoreline responses is encouraging.

However, that encouragement is tempered by the results presented by the mathematical





59


model of Motyka and Willis (1974). The differences between the two mathematical

models represented above is interesting and perplexing considering that both models

relied on wave refraction as their only wave transformation process.













CHAPTER 6
SUMMARY AND CONCLUSIONS



Following preliminary experiments to establish methodology, three sets of final

laboratory experiments were conducted on a physical model of a beach and offshore

borrow pit system. The results of the nearshore response to the borrow pit for these three

experiments form the basis for this thesis. Each of the final experiments exhibited

shoreline advancement and volume increases on the leeward beach due to the presence of

the borrow pit. This agrees with the results of physical and mathematical models

presented by Horikawa et al. (1977), but conflicts with the results of mathematical

models by Motyka and Willis (1974).

Overall, shoreline response trends for each of the three final large basin experiments

were consistent. The leeward shoreline advanced after 6 hours under the influence of the

borrow pit. The longshore limits of shoreline advance varied for each experiment, with

+80 cm and -80 cm in the longshore direction as average limits of advance compared to

the pit width of 60 cm. There was no conclusive evidence to suggest that altering the

distance between the pit and the beach, from 160 cm to 200 cm, had any effect on the

magnitude or width of the area of shoreline advance. With the exception of four survey

stations, two in each of the first and third experiments, all of the survey stations that

experienced shoreline advance also experienced an increase in volume due to the

presence of the borrow pit. The exceptions exhibited shoreline retreat, but actually








gained volume. That the majority of locations exhibiting shoreline advance also gained

volume demonstrates an overall agreement with the Bruun Rule.

The even/odd analysis revealed similar signals for all three experiments. The even

function displayed strong signals towards shoreline advance in the lee of the pit and

shoreline retreat at both edges due to the presence of the pit. Again, relocating the beach

farther from the borrow pit did not affect the magnitude of change or its longshore limits.

The even component of volume change also displayed volume increases leeward of the

borrow pit for each experiment. The even components of shoreline and volume change

for the first and third experiment behaved in a similar fashion to the total shoreline and

volume change results, in that the limits of shoreline advance in the longshore direction

were greater than the limits of increased volume.

It was relevant to determine whether or not the odd components, of both shoreline and

volume change, constituted a large percentage of the overall change. The graphical

comparisons between the shifted even and odd components of both shoreline and volume

changes for each experiment are presented in Appendix C of this thesis. It was

determined that the ratios of odd to shifted even mean squared values ranged from

approximately one-eighth to one-half. Inspection of the results in Appendix C suggests

that the "true" center of the model was not the physical or geometric center; rather it was

offset due possibly to an angle of wave approach other than normal.

Of particular interest is the difference between the magnitude of the estimated total

volume change for the second final large basin experiment (-11,900 cm3) when compared

to the estimates for the first (+2,200 cm3) and the third (-2,900 cm3) experiments. This

can be partially explained due to the fact that more sediment was deposited in the borrow








pit during the second experiment than during the other two experiments. However,

attributing the entire volume loss during the second experiment to borrow pit infilling

would require a uniform infilling depth of approximately 2.5 cm. Although

measurements of the depth were not taken during experimentation, it is not believed that

this amount of infilling occurred. Therefore, the remaining discrepancy in total volume

change could be explained in part by sediment transported outside of the survey area. It

was noticed that the amount of sediment seaward of the borrow pit appeared to increase

from the beginning of the first final large basin experiment to the end of the second final

large basin experiment.

The physical experiments undertaken during this investigation were all performed

with constant wave conditions; i.e. period, wave height, and direction were not varied

during the course of the experiments and thus, the results cannot be interpreted as being

applicable for all situations. It is unknown what differences may have occurred by

changing the period or wave height for the same borrow pit configuration.

Future model studies should include different wave periods and heights for the same

borrow pit configuration; size, distance to shoreline, relative depth, and beach slope.

Perhaps then, a more complete understanding of the relationship between the borrow pit

and the leeward nearshore environment can be provided. In addition, with more

advanced survey equipment and techniques, additional profiles could be obtained in a

shorter time, allowing for a more complete data set across the model beach. This would

help to provide more complete definitions of the measured shoreline and volume

changes. Although Bender (2001) noted that the spatial variations in wave height in this

model were so small that they were inconsistent and not repeatable, future efforts should





63


attempt to modify the model in such a way as to allow consistent measurements of the

effects of the wave guides and the wave field adjacent to the borrow pit.
















APPENDIX A
COMPLETE BEACH PROFILES




First Final Large Basin Experiment


Profile at -120 cm


0 10 cm
---


cmn

Previous Open (0.0 hr)
Si--, oer;d ,,5 ihr-_ Covered (3.0 hr) Covered (6,0 hr)
Open (7,5 hr) D Open (9.0 hr) Open (12.0 hr)






Figure A.1: Profiles for -120 cm survey station of first final large basin experiment.
















Profile at -80 cm


0
51
cm

Previous Open (0.0 hr)
Cve'.rcd ,1,5 Ir ___ Covered (3,0 hr) Covered (6,0 hr)
Open (7.5 hr) Open (9,0 hr) _. Open (12.0 hr)






Figure A.2: Profiles for -80 cm survey station of first final large basin experiment.


Profile at -40 cm


0 10 cm
k---


0
51
cm

Previous Open (0,0 hr)
C, e '.5 h.__ Covered (3.0 hr) Covered (6,0 hr)
S Open (7,5 hr) -- Open (9.0 hr) Open (12,0 hr)


Figure A.3: Profiles for -40 cm survey station of first final large basin experiment.


0 10 cm


















Profile at -20 cm


0
51
cm

S Previous Open (0.0 hr)
__ C.er' r hr-___ Covered (3,0 hr) Covered (6.0 hr)
S Open (7,5 hr) Open (9,0 hr) _.. Open (12.0 hr)



Figure A.4: Profiles for -20 cm survey station of first final large basin experiment.


Profile at 0 cm


0 10 cm
I--


51
cm

Previous Open (0,0 hr)
-o C e 1,5 h-_ Covered (3,0 hr) Covered (6,0 hr)
-- Open (7.5 hr) Open (9,0 hr) _ Open (12,0 hr)



Figure A.5: Profiles for 0 cm survey station of first final large basin experiment.


0 10 cm
















Profile at +20 cm


0 10 cm
K---


51
cr,

Previous Open (0,0 hr)
__ Coered I h___ Covered (3.0 hr) Covered (6,0 hr)
pen (7.5 hr) O- pen (9,0 hr) .._ Open (12,0 hr)






Figure A.6: Profiles for +20 cm survey station of first final large basin experiment.


Profile at +40 cm


0 10 cm
P-1


0
51


Previous Open (0,0 hr)
C c.er 5 hr___ Covered (3.0 hr) Covered (6.0 hr)
Open (7,5 hr) Open (9,0 hr) O pen (12.0 hr)



Figure A.7: Profiles for +40 cm survey station of first final large basin experiment.




















Profile at +80 cm


0 10 cm
---


0
51
cm

Previous Open (0,0 hr)
Covered ,1.5 r '___ Covered (3.0 hr) Covered (6.0 hr)
S Open (7,5 hr) O Open (9,0 hr) Open (12,0 hr)




Figure A.8: Profiles for +80 cm survey station of first final large basin experiment.


Profile o +120 cm 0 0 cm


0



Previous Open (0.0)
L:, .-d '15 hr ___ Covered (3.0 hr) Covered (6.0 hr)
Open (7,5 hr) Open (9,0 hr) Open (12,0 hr)




Figure A.9: Profiles for +120 cm survey station of first final large basin experiment.


I ~ ~ I I I I


I Iu i






69


Second Final Large Basin Experiment


ProPile at -120 cm


0 10 cm
H---


5
CM

Previous Open (0.0 hr)
Covered l'15 hr___ Covered (3.0 hr) Covered (6.0 hr)
S Open (7.5 hr) o Open (9.0 hr) Open (12.0 hr)






Figure A.10: Profiles for -120 cm survey station of second final large basin experiment.


Profile at -80 cm


0 10 cm
K---


0


Previous Open (0,0 hr)
cver-ed : 5 Ir ___ Covered (3.0 hr) Covered (6.0 hr)
Open (7.5 hr) Open (9.0 hr) Open (12.0 hr)


Figure A. 11: Profiles for -80 cm survey station of second final large basin experiment.














0 10 cm


Profile at -40 cm


0

cm

Previous Open (0.0 hr)
-_- Co, red (1.5 Ihri? Covered (3.0 hr) Covered (6,0 hr)
Open (7.5 hr) Open (9.0 hr) O_ pen (12.0 hr)









Figure A. 12: Profiles for -40 cm survey station of second final large basin experiment.


Profile at -20 cm


0 10 cm


CM

Previous Open (0.0 hr)
:--Covered .1,5 hr,__ Covered (3.0 hr) Covered (6,0 hr)
S Open (7,5 hr) -, Open (9,0 hr) Open (12,0 hr)


Figure A. 13: Profiles for -20 cm survey station of second final large basin experiment.













Profile a t 0 cm


0 10 cm
I--


0I



Previous Open (0,0 hr)
Covered (1.5 hr)__ Covered (3,0 hr) Covered (6.0 hr)
O pen (7.5 hr) o Open (9,0 hr) _._ Open (12.0 hr)









Figure A. 14: Profiles for 0 cm survey station of second final large basin experiment.


Profile at +20 cm


0 10 cm
H---


0
51
cm

S Previous Open (0.0 hr)
SCo.er 1ed 1.5 hr___ Covered (3.0 hr) Covered (6,0 hr)
S Open (7.5 hr) Open (9,0 hr) Open (12,0 hr)


Figure A. 15: Profiles for +20 cm survey station of second final large basin experiment.


















Profite at -40 cm


0 10 cM
P---


0
51
CM

Previous Open (0,0 hr)
S Covered '.5 h___ Covered (3.0 hr) Covered (6,0 hr)
S Open (7.5 hr) Open (9.0 hr) Open (12,0 hr)





Figure A. 16: Profiles for +40 cm survey station of second final large basin experiment.


Profile at +80 cm


0 10 cm


51


Previous Open (0.0 hr)
..overe,-cl .5 hr___ Covered (3.0 hr) Covered (6.0 hr)
S Open (7.5 hr) Open (9.0 hr) Open (12.0 hr)


Figure A. 17: Profiles for +80 cm survey station of second final large basin experiment.















Profile at +120 cm


0
I~


Previous Open (0.0)
Co, ered (1.5 hr Covered (3.0 hr)
O pen (7,5 hr) Open (9,0 hr)


Covered (6.0 hr)
. Open (12,0 hr)


Figure A.18: Profiles for +120 cm survey station of second final large basin experiment.

Third Final Large Basin Experiment








Pr-oPe at -120 cm 0 0 cr
I---


Previous Open (0.0 hr)
Coeret: il. hr Covered (3.0 hr) _ Covered (6,0 hr)
S Open (7.5 hr) Open (9.0 hr) Open (12.0 hr)


Figure A.19: Profiles for -120 cm survey station of third final large basin experiment.


0 10 cm
---















Profile at -80 cm 0 0 cm


I I


Previous Open (0.0 hr)
Covered (1,5 h-r_ Covered (3.0 hr) Covered (6.0 hr)
Open (7.5 hr) Open (9.0 hr) _._ Open (12.0 hr)








Figure A.20: Profiles for -80 cm survey station of third final large basin experiment.








ProFle at -40 cm 10 cn


Previous Open (0.0 hr)
Coverid 1.5 hr._ Covered (3.0 hr) _ Covered (6.0 hr)
S Open (7.5 hr) Open (9.0 hr) ..- Open (12.0 hr)


Figure A.21: Profiles for -40 cm survey station of third final large basin experiment.


i ~---




















ProFLe at -20 en


0 10 cm
I--I


Previous Open (00 hr)
Covered (1.5 hr___ Covered (3.0 hr) Covered (6.0 hr)
SOpen (7.5 hr) Open (9.0 hr) ..- Open (12.0 hr)










Figure A.22: Profiles for -20 cm survey station of third final large basin experiment.


Profile t 0 cn


0 10 cm
---H


Previous Open (0,0 hr)
Cover-,d i5 h_.__ Covered (3X. hr) Covered (6.0 hr)
Open (7.5 hr) Open <9.0 hr) Open (12.0 hr)


Figure A.23: Profiles for 0 cm survey station of third final large basin experiment.























Profie ot +20 cm 0 0 cm
--H


Previous Open (0.0 hr)
Covered <(15 hr.,__ Covered (3M hr) Covered (6.0 hr)
S Open (7.5 hr) Open (9.0 hr) ..- Open (12.0 hr>








Figure A.24: Profiles for +20 cm survey station of third final large basin experiment.


Profie at +40 cr


0 10 cm
I--I


Previous Open (0.0 hr)
- Coer ed .1,5 h- Covered (3.0 hr)
SOpen (7.5 hr) Open (9,0 hr)


Covered (6.0 hr)
S Open (12.0 hr)


Figure A.25: Profiles for +40 cm survey station of third final large basin experiment.


























Profile at +80 cn


1010 c
I--


Previous Open (0.0 hr)
SCcer6d (1.5 ht-' Covered (3.0 hr) Covered (6.0 hr)
Open (7.5 hr) Open (9.0 hr) .._ Open (12.0 hr)





Figure A.26: Profiles for +80 cm survey station of third final large basin experiment.


Profile at +120 cm


0 10 cm
I---


Previous Open (0.0)
SCoered I. hr Covered (3.0 hr) Covered (6.0 hr)
O pen (7.5 hr) -o Open (9,0 hr) O pen (L2,0 hr)


Figure A.27: Profiles for +120 cm survey station of third final large basin experiment.













APPENDIX B
OVERHEAD DIGITAL PHOTOGRAPHS



First Final Large Basin Experiment


Figure B.1: Photo of first final large basin experiment at 0.0 hr time step.




































Figure B.2: Photo of first final large basin experiment at 1.5 hr time step.





































Figure B.3: Photo of first final large basin experiment at 3.0 hr time step.









































Figure B.4: Photo of first final large basin experiment at 6.0 hr time step.




































Figure B.5: Photo of first final large basin experiment at 7.5 hr time step.





































Figure B.6: Photo of first final large basin experiment at 9.0 hr time step.



































Figure B.7: Photo of first final large basin experiment at 12.0 hr time step.





85


Second Final Large Basin Experiment


Figure B.8: Photo of second final large basin experiment at 0.0 hr time step.




































Figure B.9: Photo of second final large basin experiment at 1.5 hr time step.




































Figure B.10: Photo of second final large basin experiment at 3.0 hr time step.




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