Citation
Beach profile response to abrupt volumetric perturbations

Material Information

Title:
Beach profile response to abrupt volumetric perturbations
Series Title:
UFLCOEL-96013
Creator:
Goodrich, Matthew S., 1971-
University of Florida -- Coastal and Oceanographic Engineering Dept
Place of Publication:
Gainesville Fla
Publisher:
Coastal & Oceanographic Engineering Dept., University of Florida
Publication Date:
Language:
English
Physical Description:
xii, 115 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Beach erosion ( lcsh )
Coast changes ( lcsh )
Beach nourishment ( lcsh )
Shore protection ( lcsh )
Dissertations, Academic -- Coastal and Oceanographic Engineering -- UF ( lcsh )
Coastal and Oceanographic Engineering thesis, M.S ( lcsh )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (M.S.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 112-115).
Statement of Responsibility:
by Matthew S. Goodrich.

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University of Florida
Holding Location:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
36800130 ( OCLC )

Full Text



UFL/COEL-96/013


BEACH PROFILE RESPONSE TO ABRUPT VOLUMETRIC PERTURBATIONS by



Matthew S. Goodrich Thesis


1996
















BEACH PROFILE RESPONSE TO ABRUPT
VOLUMETRIC PERTURBATIONS













By

MATTHIEW S. GOODRICH


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


1996














ACKNOWLEDGEMENTS


I wish to express a great deal of thanks to my advisor and supervisory committee chairman, Dr. Robert G. Dean, for his support and guidance during my stay at the University of Florida. I would also like to thank the members of my supervisory committee, Dr. Hsiang Wang and Dr. Robert J. Theike.

My most sincere appreciation and gratitude belong to my parents, Ruth and Robert Goodrich, for their unfaltering love and support that has enabled me to exceed in all my endeavors.


ii














TABLE OF CONTENTS


ACKNOWLEDGMENTS ............................................ ii

LIST OF FIGURES ................................................. v

LIST OF TABLES .................................................. x

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

CHAPTERS

1 INTRODUCTION ................................................ 1I

1. 1 Descriptive Terms ......................................... 2
1.2 Profile Classification ....................................... 4
1.3 Fall Velocity ............................................. 4
1.4 Equilibrium Beach Profile................................... 6
1.5 Empirical Models ......................................... 6
1.6 Large Wave Tank Experiments................................ 8

2 METHODOLOGY ............................................... 10

2.1 Test Facilities ............................................ 10
2.2 Laboratory Experiments .................................... 14

3 RESULTS AND DISCUSSION ..................................... 19

3.1 Expected Results .......................................... 19
3.2 Data Analysis............................................. 22
3.3 Experimental Results ....................................... 29
3.4 Perturbation Half-life .................................... 92
3.5 Peak Transport Evolution.................................... 97
3.6 Nature of Profile Response to Volumetric Perturbations ..............98

4 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS FOR FURTHER
STUDY ..................................................... 108

4.1 Summary ............................................... 108
4.2 Conclusions ............................................. 109








4.3 Recommendations for Further Study ........................... 110

BIBLIOGRAPHY ................................................. 112

BIOGRAPHICAL SKETCH......................................... 116













LIST OF FIGURES


1.1 Definition sketch of the beach profile, from Larson and Kraus (1989) ........3 2.1 Schematic of the Air-Sea Tank facility .............................. 11

2.2 Cumulative sand size distribution .................................. 12

2.3 Fall velocity of spherical grains as a function of size, Rouse (1937) ......... 13 2.4 Spectrum of an irregular wave train at the toe of the beach................ 17

2.5 An irregular wave train at the toe of the beach ........................18

3.1 Expected response of an equilibrium profile to deposition of a volume of sand 21 3.2 Expected response of an equilibrium profile to removal of a volume of sand 21

3.3 Comparison of adjusted and unadjusted average sediment transport rates (Case
E,l120to 125min) ............................................ 27

3.4 Summary of profile changes in Case A .............................. 31

3.5 Summary of average sediment transport rates in Case A .................31

3.6 Summary of profile changes in Case B .............................. 35

3.7 Summary of average sediment transport rates in Case B.................. 35

3.8 Summary of profile changes in Case C .............................. 37

3.9 Summary of average sediment transport rates in Case C .................. 37

3.10 Summary of profile changes in Case I ............................... 40

3.11 Summary of average sediment transport rates in Case I .................. 40


v









3.12 Summary of profiles at an elapsed time of 120 min for experiments with
regular waves (H = 0. 14 m, T = 1.65 sec) ............................ 42

3.13 Summary of profile changes in Case D .............................. 44

3.14 Summary of average sediment transport rates in Case D .................44

3.15 Comparison of average sediment transport rates for Case D and Case I (unperturbed case) for the period 120 to 180 min............................ 46

3.16 Comparison of profile changes for Case D and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 180 min................. 47

3.17 Profile evolution with time for the period from 120 to 180 min for Case D ..48 3.18 Case D: evolution of bar volume with elapsed time.................... 49

3.19 Summary of profile changes in CaseE .............................. 51

3.20 Summary of average sediment transport rates in Case E.................. 51

3.21 Mean profile evolution after the profile approached an equilibrium during
Experiment MTO1. Elapsed times = 0, 242,297, 352, 407 and 476 min. Note the substantial erosion of the area seaward of the bar and the deposition of the
area immediately landward of the bar trough. Note also the landward movement of the bar (Oh, 1994)....................................... 53

3.22 Profile evolution of Case E at elapsed times = 140 and 170 min.............53

3.23 Comparison of average sediment transport rates for Case E and Case I (unperturbed case) for the period 120 to 180 min............................ 55

3.24 Comparison of profile changes for Case E and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 180 min................. 56

3.25 Summary of profile changes in Case E .............................. 58

3.26 Summary of average sediment transport rates in Case E.................. 58

3.27 Comparison of average sediment transport rates for Case E and Case I (unperturbed case) for the period 120 to 180 min ........................... 59


Vi








3.28 Comparison of average sediment transport rates for Case E and Case I (unperturbed case) for the period 180 to 240 min............................ 59

3.29 Comparison of profile changes for Case E and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 240 min................. 61

3.30 Summary of profile changes in Case G .............................. 63

3.31 Summary of average sediment transport rates in Case G .................63

3.32 Comparison of average sediment transport rates for Case G and Case I (unperturbed case) for the period 120 to 180 min............................ 65

3.33 Comparison of average sediment transport rates for Case G and Case I (unperturbed case) for the period 180 to 240 min............................ 65

3.34 Comparison of profile changes for Case G and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 240 min................. 66

3.35 Summary of profile changes in Case H .............................. 69

3.36 Summary of average sediment transport rates in Case H .................69

3.37 Comparison of average sediment transport rates for Case H and Case I (unperturbed case) for the period 120 to 180 min............................ 71

3.38 Comparison of average sediment transport rates for Case H and Case I (unperturbed case) for the period 180 to 240 min............................ 71

3.39 Comparison of average sediment transport rates for Case H and Case I (unperturbed case) for the period 240 to 300 min............................ 72

3.40 Comparison of profile changes for Case H and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 300 min................. 73

3.41 Summary of profile changes in Case J ............................... 76

3.42 Summary of average sediment transport rates in Case J1.................. 76

3.43 Summary of profiles at an elapsed time of 120 min for experiments with
irregular waves (Pierson-Moskowitz spectrum, H, = 0. 16 m, Tk = 1.65 sec) 77 3.44 Summary of profile changes in Case K .............................. 79


vii









3.45 Summary of average sediment transport rates in Case K .................79

3.46 Comparison of average sediment transport rates for Case K and Case J (unperturbed case) for the period 120 to 180 min............................ 82

3.47 Comparison of average sediment transport rates for Case K and Case J (unperturbed case) for the period 180 to 240 min............................ 82

3.48 Comparison of average sediment transport rates for Case K and Case J (unperturbed case) for the period 240 to 300 min............................ 83

3.49 Comparison of profile changes for Case K and Case J (unperturbed case) for
the interval from 120 mini (after perturbation) to 300 min ................84

3.50 Summary of profile changes in Case L .............................. 87

3.51 Summary of average sediment transport rates in Case L.................. 87

3.52 Comparison of average sediment transport rates for Case L and Case J (unperturbed case) for the period 120 to 180 min ........................... 89

3.53 Comparison of average sediment transport rates for Case L and Case J (unperturbed case) for the period 180 to 240 min ........................... 89

3.54 Comparison of average sediment transport rates for Case L and Case J (unperturbed case) for the period 240 to 300 min ........................... 90

3.55 Comparison of profile changes for Case L and Case J (unperturbed case) for
the interval from 120 mini (after perturbation) to 300 min................. 91

3.56 Evolution of bar height and bar volume versus time for Case L ............ 93

3.57 Solution of Equation (5), from Dean and Zheng (1994) .................. 96

3.58 Evolution of peak offshore sediment transport rate and best-fit empirical
expression for Cases A and B ..................................... 99

3.59 Evolution of peak offshore sediment transport rate and best-fit empirical
expression for Cases C and D.................................... 100

3.60 Evolution of peak offshore sediment transport rate and best-fit empirical
expression for Cases E and F .................................... 101

3.61 Evolution of peak offshore sediment transport rate and best-fit empirical
expression for Cases G and H.................................... 102

viii







3.62 Evolution of peak offshore sediment transport rate and best-fit empirical
expression for Cases I and J ..................................... 103

3.63 Evolution of peak offshore sediment transport rate and best-fit empirical
expression for Cases K and L.................................... 104


ix













LIST OF TABLES


1.1 Profile classification, from Oh (1994).............................. 5

2.1 Summary of regular wave test conditions and profile modifications ... 16 2.2 Summary of irregular wave test conditions and profile modifications ... 17

3.1 Average required change in elevation between profiles, in mm, in order to
achieve sediment volume conservation (change due to addition/removal
of sand at t = 120 min not included)............................... 25

3.2 Average net change in elevation, in mm, over given intervals required to
achieve sediment volume conservation (change due to addition/removal
of sand at t = 120 mini not included)............................... 26

3.3 'Half-life' of increase in sediment transport due to perturbation, magnitude
of increase due to perturbation and description of perturbation............94

3.4 Summary of least-squares fit parameters q and a for both before and after
perturbation of the experiment................................... 105


x













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 BEACH PROFILE RESPONSE TO ABRUPT VOLUMETRIC PERTURBATIONS

By

MATTHEW S. GOODRICH

May 1996

Chairman: Dr. Robert Dean
Major Department: Coastal and Oceanographic Engineering

This study was conducted to develop an improved understanding of beach profile morphology, more specifically, beach profile response to volumetric perturbations. Twelve moveable bed studies were performed in a narrow wave tank that involved running waves on an initially planar profile for 120 minutes, perturbing the profile by adding or removing a volume of sand, and continuing the tests. The experiments included both regular and irregular waves, and included both addition and removal of volumes of sediment.

The time scale of the response of the profile to the perturbations was analysed using both a 'half-life' parameter as well as a best-fit expression for the time evolution of the peak offshore transport, and it was found that with increasing perturbation volume, a decreased profile response time scale resulted. Also, the data show that the response of a beach profile to a volumetric perturbation is non-diff-usionary in nature, although the response is complex,


xi








the entire profile appears to respond in unison to the perturbation. Other conclusions are that perturbations altering wave breaking cause the beach profile to evolve towards a new equilibrium shape (the re-equilibrated profile is not simply the previous equilibrium profile translated some uniform distance landward or seaward); a three-dimensional horizontal circulation system may form in a narrow wave tank and produce a very significant impact on cross-shore sediment transport rates; and a perturbation in the shape of a peaked bar introduced to an irregular wave profile may help position the break point and result in a bar form being maintained. The data resulting from the method employed in this study appear useful for evaluating and improving cross-shore transport algorithms.


xii













CHAPTER 1
INTRODUCTION





The prediction of beach profile evolution has been of paramount interest in the coastal engineering field. The regulation and design of coastal construction requires a quantitative understanding of profile evolution. For instance, coastal structure design requires estimates of shoreline recession, and beach nourishment projects require estimates of project lifespan, which are dictated, in part, by profile response of the projects.

However, the calculation of shoreline evolution is a difficult problem due to the complex nature of the sediment transport processes that govern profile morphology. At present, knowledge of the nonlinear unsteady hydrodynamics of the surf zone and sediment fluid interaction is limited, and even if the hydrodynamics were known precisely, the sediment transport mechanics are complex and poorly understood. Much remains to be understood before an accurate physics-based model of sediment transport on the microscale can be developed. When viewed on a macroscale, though, beach profile morphology appears smooth, and thus, presents the most likely option for developing a working model of beach profile evolution. This report endeavors to advance the understanding of beach profile evolution at the macroscale through the use of laboratory model experiments.


1







2

The experiments conducted and described here are designed to study the response of beach profiles to volumetric perturbations (the addition or removal of a volume of sediment on a beach profile subjected to wave action). To simplify the problem, the experiments are focused on sediment transport in the cross-shore direction, and attempt to achieve a twodimensional system by conducting the tests in a long, narrow wave flume. Through these experiments it is desired that some fundamental questions concerning beach profile morphology may be answered: On what time scale does the profile respond to the perturbation in the cross-shore direction? Does the entire profile respond to the perturbation simultaneously, or does the region affected by the perturbation increase with time (as it does in the longshore direction)? Does the profile respond by reforming a similar shape to the preperturbation profile, only translated seaward or landward, as simple equilibrium beach profile theory predicts?

The remainder of this chapter presents the terminology associated with beach profiles and a review of previous studies on beach profiles. The literature on the subject of beach profile morphology is vast, however, this section will provide only a brief review of the major contributions. For a more extensive review of published works on beach profile change, see Larson and Kraus (1989).

1. 1 Descriptive Terms

The beach profile is a cross-section of the beach taken perpendicular to the shoreline (Figure 1. 1). The profile is generally divided into four sections: the offshore, the nearshore, the beach and the coast. The profile is shaped by waves that propagate from the offshore and break in the nearshore zone. The waves are gradually shoaled by the sloping bottom of the







3


Figure 1. 1 Definition sketch of the beach profile, from Larson and Kraus (1989)



profile and break when the wave height is approximately 0.8 times the water depth. The submerged profile landward of wave breaking is denoted as the surf zone, and seaward is the offshore. A bar feature is often formed in the region of wave breaking. The broken waves continue to propagate landward and the remaining wave energy is dissipated in the swash zone, or foreshore, as waves rush up on this steep portion of the profile. On the backshore, accretionary features known as bermns may exist, which are created by sediment deposited by wave runup. The landward boundary of the beach is often defined by a line of dunes, which are large ridges of sand transported by wind from the beach.


COASTAL AREA


NEARSHORE ZONE COAST- BEACH OR SHORE '
BACKSHORE INSHORE OFFSHORE DUNE -FORESHORE

BERM STEP HWL BREAKERS






HWL: HIGH WATER LEVEL LWL: LOW WATER LEVEL







4

1.2 Profile Classification

Much of beach profile research has been focused on bar properties, the classification of beach profiles based on bar and berm features, and the development of a criterion that can delineate between bar and berm profiles.

Early field studies (Evans 1940, King and Williams 1949, and Shepard 1950) as well as laboratory studies (Waters 1939, Keulegan 1948, Rector 1954, and Saville 1957) show that breaking waves are the main cause of bar genesis. Based on these studies, as well as later investigations (Hands 1976, Kriebel et al. 1986, Larson and Kraus 1989, etc.), it is generally accepted that storm wave conditions result in offshore directed sediment transport, profile erosion and the genesis of an offshore bar, while normal wave conditions result in onshore sediment transport, and the development of a berm profile without an offshore bar.

Many criteria have been proposed to delineate bar and berm profile response. The first criteria were based only on wave steepness (Johnson 1949, Saville 1957). Later criteria included sediment size, fail velocity and beach slope. Table 1. 1 summarizes some of these criteria.

Since the formation of bar and berm features is closely related to the cross-shore sediment transport direction, some criteria used to predict bar formation have been also used as indicators of sediment transport direction (Rector 1954, Dean 1973).

1.3 Fall Velocity

Dean (1973) hypothesized that sediment was suspended during the passage of a wave crest and if the fall time were less or greater than one half wave period, the net transport would be landward or seaward, respectively. Considering the height of suspension to be







5

Table 1. 1 Profile Classification Criteria, from Oh (1994)
Researcher Bar Formation Criteria Scales

Johnson (1949) H0/IL,.> 0.03 small Rector (1954) HIL > 29.4(D50/L,)08 small Saville (1957) H0/L, > 0.025 small H(/L0 > 0.0064 large Dean (1973) H01,T > 0.85 small

Sunamura and Horikawa (1974) HO1LO > C(tanP)-027(D5L) 0.67 small

Kriebel et al. (1986) H0/L0 > Ant&IgT small, large

Larson and Kraus (1989) H0/L0 < 0.007(H0/G))3 large

Dalrymple (1992) gH3'G(3T > 9000 -10400 large

where, in this table
HO = Deep water wave height
L. = Deep water wave length
T = Wave period
g = Gravitational acceleration
tan P3 = Initial beach slope
D50 = Median sediment diameter
W = Sediment fall velocity
A,C = Constants



proportional to the breaking wave height, the fall velocity parameter was defined as Hb w ,T
whereHb is the breaking wave height, wL)is the fall velocity and T is the wave period. Wave tank tests conducted by Kriebel, Dally and Dean (1986) and Hughes and Fowler (1990) support that the fall velocity parameter is a valid modeling parameter as a dynamic similarity constraint. The fall velocity is scaled by the length scale as W,- (Lr)"12, which is consistent with Froude modeling laws.







6

1.4 Equilibrium Beach Profile

One of the most basic assumptions in coastal engineering is that of the equilibrium beach profile: given time, a beach under constant forcing will evolve to an equilibrium shape and will cease to continue changing. Bruun (1954) developed a predictive equation for the equilibrium profile with the form


h(y) = A(D)ym



where h is the depth, y is the distance offshore, and A is the scale parameter, which is a function of sediment diameter D. The scale parameter was evaluated by Dean (1977) and 24 D.
Moore (1982) and is given as A(D) = ]m, where D. is the equilibrium energy
5 pg 32.K
dissipation rate per unit volume, p is the fluid density, g is the acceleration due to gravity and icis the breaking index. Dean (1991) presents the scale parameter as a function of sediment fall velocity.

Dean (1977) explained the power law profile given by Bruun (1954) on physical grounds by assuming the profile was in equilibrium if the energy dissipation per unit water volume due to wave breaking was uniform across the profile. Also, Dean (1977) analyzed 502 beach profiles on the United States Atlantic and Gulf of Mexico coastlines, and concluded that the power law given by Bruun (1954) was the optimal function to describe equilibrium profile shape.

1.5 Empirical Models

Many numerical models have been developed to predict profile evolution. A model based on wave breaking was developed by Daily (1980) and Daily and Dean (1984). Models







7

based on the equilibrium concept have been developed by Kriebel (1982, 1986, 1989, 1990), Kriebel and Dean (1985) [EDUNE], Larson and Kraus (1989) [SBEACH], Chiu and Dean (1984, 1986) [CCCL], and Dean and Zheng (1994) [CROSS]. Zheng and Dean (1995) provides brief descriptions of five equilibrium based models.

This report reviews only the concept used in Kriebel (1982), Kriebel and Dean (1985) and Kriebel (1986, 1989, 1990), and first proposed by Dean (1977), which is an empirical model in which the cross-shore transport rate is proportional to the degree of disequilibrium, given by

Q = k(D(y)-D.)



where Q is the sediment transport rate [volume/time], and k is an empirical constant. D is energy dissipation per unit water volume, and the value of D is given by D = g -pl2 (gh) 1/2 L
16 dy


where h is the water depth and y is the distance offshore. Since D is proportional to the product of the square root of the local water depth and the bottom slope, a profile with steeper or milder slope than the equilibrium will result in sediment transport offshore or onshore, respectively.

The continuity equation is used to close the system, which is given by ay =
at ah


where t is time.







8

In order to satisfy model scaling requirements, Dean and Zheng (1994) propose the following transport model


Q = k(D-D.)ID-D.1 n'



where n = 3. This value was supported by comparison with large wave tank data.

1.6 Large Wave Tank Experiments

One of the most useful tools in studying beach profile morphology is large wave tank (LWT) experiment data. Many small scale laboratory experiments have been performed and have been useful in identifying potential parameters related to beach profile change. However, scaling distortion is a problem and generally applicable scaling laws for interpreting small scale experiments have yet to be determined. The usefulness of field data is diminished by our inability to extract cause and effect relationships between waves and profile change due to: lack of high resolution in time and space of morphology and wave data, spatial and temporal variability of waves, and the three-dimensional character of nearshore bathymetry. Thus, the best alternative for the study of beach profile change is the use of large scale wave tank experiments. Such experiments allow the control of wave and sediment parameters, and can approximate a two-dimensional system. Also, scaling problems are avoided, repeatability can be evaluated, and a high resolution of data can be acquired.

LW'1' experiments performed with monochromatic waves include experiments performed by the US Army Corps of Engineers (CE) in the years 1956-1957 and 1962 (Saville 1957, Caldwell 1959, Kraus and Larson 1988a) at Dalecarlia Reservation,







9

Washington, DC, experiments performed at the Central Research Institute of Electric Power Industry (CRIEPI) in Chiba, Japan (Kajima et al. 1983a, b), and experiments performed in a large German wave flume in Hannover (Dette and Uliczka, 1987a). Irregular wave LWT experiments have also been performed (Vellinga 1986, Dette and Uliczka 1987b, U.liczka and Dette 1987).













CHAPTER 2
METHODOLOGY

This chapter provides a description of the facilities and procedures used in the experiments completed for this study.

2.1 Test Facilities

All experiments were conducted in the "Air-Sea Tank" at the University of Florida's Coastal and Oceanographic Engineering Laboratory facility in Gainesville, Florida. The tank measures approximately 37 m long, 2 m wide and 1.9 mn deep (Figure 2. 1). The test section has been divided into two parallel test sections 0.9 m wide by a concrete block wall placed along the centerline of the tank. The experiments used only the eastern half of the tank, of which the outer wall is constructed of glass panels, thus enabling direct observation of the experiments.

The tank is equipped with a hydraulically driven wave paddle measuring 1.8 mn wide by 1.2 mn high. The wavemaker bulkhead is mounted on a carriage and is driven by two hydraulic rams. The apparatus is configured such that independent control of the rams may provide either piston or flap type motion, or any linear combination. For the experiments in this study only piston type wavemaker motion was used to generate waves.

The wavemaker was controlled by a Seasim programmable spectrum signal generator and a Pegasus Servo Controller/Amplifier capable of producing both regular and irregular


10









I1I


/Flow Wall Sa2nd Seacni



wave-i PLAN VTEWN C Paritio
-%= wai


sasin? rC



Wave
~Abscr;


-n Thck wal
A RaiLs ( I 83V e-I -,nick Plate Gras SW!U Water Lwel CRCSS-SEC77 CN ....in Tank~


Figure 2.1 Schematic of the Air-Sea Tank facility.







12

waves, both of which were used in the present study. Directly in front of the wavemnaker is an array of wave screens designed to prevent cross-tank variations in the generated waves.

The sand beach used for the experiments was located at the opposite end of the tank from the wavemaker in the east bay. The material used was a fine brown sand with a median grain diameter of 0. 1 mm (Figure 2.2), and the size distribution was nearly uniform across the profile. The mean fall velocity of this sediment is approximately 0.64 cm/sec, which was estimated from the values of fall velocity of spherical grains given by Rouse (1937) (Figure

2.3).



100



80










40




0.01 0.1 1
Grain Size (mm)


Figure 2.2: Cumulative Sand Size Distribution







I13


Ma


-;.1 7


1 1 1 1 1


u~. --
oc~g -


iii' ~ ~ tt77 iI
*it)~**~~ I



usK 002 -1114 006 0. 2 ILI -3605 as a Ao M io so to =0 -c a a


Figure 2.3: Fall Velocity of Spherical Grains as a Function of Size, Rouse (1937)


I


WVI







14

The experimental profiles were measured using a mobile cart system equipped with a profiling rod. The cart rides along level rails mounted on top of the walls of the east bay of the Air-Sea tank. The top of the eastern-most wall is graduated at 2 cm intervals to provide a horizontal reference scale. The profiling rod mounted on the cart is graduated in millimeters and was used to measure the elevation of the beach profile during surveys.

The wave conditions during each experiment were measured using a capacitance type wave gage mounted to the mobile cart. The gage was connected to an IIBM compatible PC and surface elevation data was recorded at a sampling rate of 20 Hz using the data acquisition module of a Global Lab software package.

2.2 Laboratory Experiments

The objective of the laboratory experiments was to measure, analyze and interpret the response of beach profiles to instantaneous additions or removals of sediment volume. Ideally, the beach profile would be in equilibrium for the given wave condition before the profile was perturbed. Thus, the evolution of the profile following the perturbation could be attributed exclusively to the deposition/removal of the volume of sediment. However, achieving an equilibrium shape by running waves for a long time on the beach profile is not practical for this study because of the large amount of time that would be required for each experiment. In large wave tank (LWvT) experiments performed by the Corps of Engineers

(CE) at Dalecarlia Reservation, Washington, DC, and experiments performed at the Central Research Institute of Electric Power Industry (CRIEPI) in Chiba, Japan, experiments using waves of similar steepness to those in this study, significant volume changes were still occurring after 100 hours of wave action (Larson and Kraus 1989). Therefore, the model







15

tests were not designed to be in equilibrium before the profiles were perturbed. Instead, the experimental profiles were subjected to waves for a sufficient amount of time for the sediment transport rate to subside enough such that following the perturbation of the beach profile, the increase in sediment transport would be very noticeable.

2.3 Experimental Procedure

First, the initial beach profile of uniform slope 1:20 was constructed in the following manner: (1) the initial profile was drawn on the outside of the glass wall of the tank with a marker; (2) the sediment in the wave tank was redistributed to conform to an elevation slightly higher than the initial slope dictated by the drawn line; (3) the material was compacted using a 30 pound tamper; (4) the profile was then scraped with a board equal in length to the width of the tank to conform to the line on the glass wall while special attention was given to keeping the profile uniform across the tank width, which was aided by the use of a level. The tank was then filled with water to the required depth and the initial profile was surveyed.

The beach profile was subjected to waves for 120 minutes, while being stopped at intervals to allow the completion of surveys after elapsed times of 10, 20, 40, 80, 110 and 120 minutes. A volume of sand was then deposited or removed at various location on the beach profile, and the newly perturbed profile was surveyed. The waves were then continued for another 60 minutes while being stopped at total elapsed times of 125, 130, 140, 170 and 180 minutes to allow for profile surveys. The later experiments ran for longer durations: 240 minutes and 300 minutes with additional surveys conducted at 30 minute intervals. Shorter durations of wave action were run between surveys at the beginning of the experiment and







16

immediately before and after the perturbation to allow higher resolution of sediment transport

information at these points of increased profile disequilibrium.

2.4 Test Conditions

A total of twelve experiments was conducted with various wave conditions and

profile modifications. The first nine experiments used regular waves, and the remaining

three used random waves (Tables 2.1 and 2.2). Tests A, B and C were subjected to larger

wave heights and the resulting profiles had broad, diffuse bar formations. The subsequent

regular wave cases used smaller waves to produce a more peaked bar formation. The

irregular wave cases were modeled by a Pierson-Moskowitz spectrum with a peak wave

period of 1.65 sec and significant wave height of 0.16 mn (Figures 2.4 and 2.5). A brief

description of the profile modification for each experiment is also provided in Tables 2.1 and

2.2.

Table 2. 1: Summary of Regular Wave Test Conditions and Profile Modifications
Test Wave Wave Water Grain Test Profile
height period depth size dura- modification
(in) (sec) (in) (mm) tion
_ _ _(min)
A 16.5 0.465 180 bar removed B 16.0 0.45 180 bar removed
C 16.0 0.45 180 seaward section of bar removed
D 14.0 1.65 0.45 0.10 180 landward section of bar removed
E 14.0 0.45 180 volume deposited in trough F 14.0 0.45 240 volume deposited on beach face
G 14.0 0.45 240 volume deposited just
landward of trough
H 14.0 0.45 300 volume deposited just
seaward of beach face
1 14.0 1___ 0.45 1___ 1 300 unperturbed case







17

Table 2.:Smayof IrglrWave Test Conditions and Profile Modifications Test Significant Peak Water Sand Test Profile wave height period depth size duration modification
______ (in) (sec) (in) (mmn) (min) __________J unperturbed case
K 0.16 1.65 0.45 0.10 300 deposition at mid-surf zone
L deposition at break
____ __ ___ ___ __ ___ ___ ___ __ ___ ___ ___ Point


CO) C-)


ciD


2.5



2



1.5



1



0.5


0


2


4


6


8


10


frequency [Hz]
w


Figure 2.4 Spectrum of an Irregular Wave Train at the Toe of the Beach











10








-10

-15
0 510 15 20 253
Time (sec)

Figure 2.5: An Irregular Wave Train at the Toe of the Beach












CHAPTER 3
RESULTS AND DISCUSSION

This chapter presents the results of the wave tank experiments and attempts to interpret these results to provide meaningful conclusions relating to cross-shore sediment transport processes. Included is a discussion of the expected results, according to equilibrium beach profile theory, as well as a description of the procedures used in the analysis of the experimental data.

3.1 Expected Results

The concept of the equilibrium beach profile is widely accepted and commonly used in coastal engineering. A comparison of the experimental results with the expected results according to equilibrium beach profile theory would provide insight aiding the analysis and interpretation of the data. It may also provide a basis for evaluating the existing cross-shore sediment transport relationships and proposing new relationships. This section discusses the general expected response of an equilibrium profile to a deposition or removal of a volume of sand.

Dean (199 1) asserts that when a volume of fill is added to a beach profile, and the sediment characteristics of both are identical, it is assumed that it will equilibrate eventually to the pre-fill equilibrium profile shape translated a uniform horizontal distance seaward across the entire active profile. If this assumption is true, it raises the question: in what manner is the deposited fill volume redistributed?


19







20

Pelnard-Considere (1956) described the evolution of a rectangular planform fill using an equation similar to a heat diffusion equation, where the planform is gradually smoothedout over time as the sand diffuses along an increasing length of shoreline. Perhaps this applies to a perturbation in a two-dimensional beach profile system in a similar manner: the perturbation is gradually smoothed-out, with the affected region of the profile gradually increasing. The expected responses of the profiles to the perturbations are based on this assumption.

The expected response of an equilibrium profile to a deposit of a volume of sand is shown if Figure 3. 1. Included are both the short-term and long-term expectations of the beach profile shape, as well as the approximate sediment transport rate curves for the short and long-term profile changes. In the short-term forecast the perturbation begins to be smoothed-out by the wave action, and sand is transported both landward and seaward of the perturbation. On a longer term basis, the same transport form will occur, but will be smaller in magnitude and distributed over a larger portion of the profile. The predicted equilibrium is the result of redistribution of the deposited sand over the entire active profile resulting in the translation of every point of the initial active equilibrium beach profile a uniform distance seaward.

The impact of the removal of a volume of sediment is presented in Figure 3.2. The short- term profile change is the smoothing-out of the depression by the transport of sediment into the removal area from both onshore and offshore directions. The equilibrium response is the uniform translation of the equilibrium profile landward. The rates at which different elevations on the active profile approach equilibrium will depend on the elevation.







21
re-equilibrated profile '~initial perturbation ,short-term equilibrium profile



Q /short-term










Figure 3.1 Expected response of an equilibrium profile to deposition of a volume of sand



re-equilibrated profile

/equilibrium profile initial perturbation>logtr

,short-term long-term


Figure 3.2 Expected response of an equilibrium profile to removal of a volume of sand







22

The anticipated response of each of the experimental cases is presented and compared to the results for each case in the experimental results section.

3.2 Data Analysis

The measured experimental data consist of profile data and water surface elevation data. To assist interpretation of these results, various quantities are calculated using these data sets. The most important of these quantities is the average sediment transport rate between surveys, which is the variable indicator used to assess the impact of the perturbation on the beach profile system. The procedures used in calculating the transport rates and some other quantities are given below.

Sediment Transport Rate Calculation

The equation describing the relation between the profiles and sediment transport rate is given by the two-dimensional sediment conservation equation, which is ah aq(31
at ax(31

where h is the profile elevation at a given point x and time t, x is the offshore distance, and q is the time-averaged sediment transport rate per unit length of shoreline.

Integrating Equation (3. 1) with respect to x from the landward end of the profile, x0, to any point x, and setting q(x0,) = 0 as the landward closure yields q(x) f hd (3.2)
x,0a







23


with


ah [h(x,t2) h(x,t1)]

at t2-t(33



Since h, t and x are measured in each experiment, the time-averaged sediment transport rate between the times of any two profiles may be calculated using Equations 3.2 and 3.3.

Application of these equations to the experimental data presents a problem: the transport rates are not necessarily found to be zero at the offshore closure depth as they should be theoretically. This is the result of three effects. First, sediment may not necessarily be conserved, because there may have been some transport of material beyond the offshore measurement depth. However, from visual observations it was concluded that this effect was very small. Second, in attempts to model a two-dimensional system in a long, narrow wave tank, the assumption of a two-dimensional system is often not strictly correct. Three-dimensional morphology was present at times to some degree in every experiment. Thirdly, although sand mass is conserved, sand volume may not be. Transported sand may be deposited in a state of increased or decreased porosity relative to its initial condition.

To facilitate interpretation and comparison of the sediment transport curves, the curves were adjusted to obtain closure at the offshore end of the profiles. This was accomplished by subtracting uniformly across the entire profile the average change in elevation between profiles from the later profile used in the transport rate calculation. Thus, the average sediment transport rates were adjusted by a uniformly increasing/decreasing rate across the entire profile. Table 3.1 tabulates the average change in elevation between







24

adjacent profile surveys that were used for the closure adjustments. Table 3.2 summarizes the net change in elevation required to achieve closure over the intervals 0 to 120 min, 120 to 180 min, 120 to 240 min and 120 to 300 min. For the interval 0 to 120 min, the net change in elevation required was negative in all experiments, which indicates that the system gained sediment volume during the first 120 min of each experiment. For the interval 120 to 180 min, seven of the ten perturbed experiments and both of the control experiments required negative net adjustments.

The adjustment was made uniformly across the profile because the sources of nonclosure could not be isolated. The location of the three-dimensional areas could not be isolated by visual observations (due to the high turbidity associated with the fine sediment used in the experiments), and, unfortunately, no additional profiles were taken to document the three-dimensionalities. However, when the wave tank was drained (when the perturbation was added or at the end of an experiment), it was noted that the primary location of the 3-D effects was the region seaward of the beach face and landward of the bar/trough formation. Oftentimes trough-like formations occurred along the walls of this region, with maximum depths up to approximately 0. 1 in below the centerline elevation.

These 3-D effects are similar to the findings documented by Oh (1994), who conducted regular wave experiments in a narrow wave tank with 2-D initial conditions. After four hours of testing, Oh found 3-D features associated with fairly strong horizontal cellular circulation and rapid net landward sediment transport and shoreline advancement. Later in the testing, the profile returned to nearly 2-D conditions with the exception of a deep and narrow return channel near one of the tank walls. Oh concluded that there is a relatively








25


Table 3.1 Average required change in elevation between profiles, in mm, in order to achieve sediment volume conservation (change due to addition/removal of sand at t = 120 min not included)


survey:
30 40 60 80 110 120 125 130 140


Time
10



-0.71
-1.64
-0.6
0.306
-0.56
-2.28
-3.5
-2.42
-1.05
-1.38
-1.51
-0.4


-0.52
0.648
-1.07
0.41 0.44 0.85


-1.41
-2.06
-0.33
0.12
-0.59
-1.41
-0.51


-0.78
0.16


-0.1 -0.17
-0.48 0.16


-0.14
-1.4
-0.58
-1.01
-0.25
-0.62
0.55
-0.52
0.67
-0.34
0.198
-0.25


1.32 0.856
0.02
-1.31
-0.1
0.8 17
0.147



0.244 0.457


0.216
-0.3
--0.44
-0.5
-0.16
0.459
-0.51
0.156



-0.14
-0.2


1.17 1.106
-1.33
-0.63
0.03
-0.61
-0.1



-0.18
-0.49


150 160 170 180 200 210


230 240 270 300


0.359 -0.11
-0.92 0.358
-1.71
-0.72
-0.71
-0.37
0.225


-2.15 -2.77
-0.1 -0.47
0.79
0 0.02
-0.43 0.129


Case:


-0.88
-0.1
-1.05
-0.1
-0.3
0.5
-0.37
-0.87



0
-0.48


of
20


-1.12
-1.79
-1.36
-0.66
-0.53
-0.72
-0.51
-0.34



0.695
0.04


A
B
C
D
E
F
G
H



K
L


-1.21
-0.28


A
B
C
D
E
F
G
H



K
L


-2.55


-1.22
-1.84
0.5
-1.49


1.465 0.395
-0.72



-0.27
0.164


-0.77
0.577 0.3 15
-0.85
0.27
-0.43
0.22
-1.4
-0.91
-0.61
-0.1
0.104


-0.44
0.22


-1.67



0.219
-0.36







26

Table 3.2 Average net change in elevation, in mm, over given intervals required to achieve sediment volume conservation (change due to addition/removal of sand at t = 120
min not included) _______ _____Case 0 120 min 120 180 minl 120 240 min 120 300 min

A -2.85 -3.10 B -6.34 1.55 C -6.17 -0.003 D -1.15 -2.16 E -2.54 -3.32 F -3.30 1.42 1.23

G -4.80 0.3 12 -3.03

H -3.81 -1.92 -5.30 -10.22 1 -2.37 -0.91 -1.63 -2.20 J -1.84 -0.61 -1.32 -0.53 K -0.887 -0.446 -0.597 -0.577 L -1.41 0.035 -0.10 -0.401


slow feedback between the hydrodynamics and the morphology that leads to the initiation and growth of 3-D features.

An extreme example of the resulting change in the transport curves due to the closure adjustment is shown in Figure 3.3. This particular case required a large sediment transport rate adjustment to achieve offshore closure, and it was selected to illustrate the difficulties associated with interpreting profile changes presented in two-dimensions when, in fact, it is evident that the beach system had very significant three-dimensional changes.








27


0.003


0.002 0.001
E

i2
0


0-0.002


-0.002


-2 -1 0 1 2 3 4 5 6 7 8 9 10


Offshore distance (in)


-adjusted -unadjuse


Figure 3.3 Comparison of adjusted and unadjusted average sediment transport rates







28

It should be noted that although the closure adjustments sometimes had a large affect on the sediment transport curves, the closure adjustments were generally on the order of 1 mm and thus are reasonably small.

Calculation of Other Quantities

Various other quantities were calculated and used, when helpful, to aid in the evaluation of the impact of the perturbation on the beach profile system. They include: change in profile elevation, maximum time-averaged sediment transport rate, shoreline position, bar height, bar volume, 'half-life' of perturbation and best-fit of empirical expression to peak transport data. A brief description of each and the procedures used to calculate them are described below.

The change in profile elevation was calculated for the last profile available in each experiment. The change in elevation was defined as the difference in the final profile elevation from the profile elevation at an elapsed time of 120 min (after the volume of sand was added/removed).

The shoreline position was plotted versus time for each experiment. The shoreline position was taken as the interception of the still water line with the beach profile. This quantity, however, was found to have poor repeatability between tests, and thus, was judged to be a poor indicator of profile response.

The bar height was calculated as the difference in elevation between the minimum elevation in the trough adjacent to the bar and the maximum elevation at the peak of the bar. The bar volume in the regular waves experiments was taken as the volume of material between the interception points of the bar feature with the initial profile.







29

In order to evaluate the time scale of the changes in sediment transport rate induced by the perturbation, the 'half-life' of the changes was calculated for each case. The 'half-life' was defined as the time required for the maximum sediment transport rate in the experiment to decay to a magnitude that was equal to half the increase in maximum transport rate due to the perturbation plus the pre-perturbation rate. In order to find this value it was necessary to assign the maximum sediment transport rate for each interval to the midpoint of that time interval, and to interpolate between the measured maximum sediment transport rates to solve for the half-life value.

The maximum time-averaged positive sediment transport rate was plotted versus elapsed time for each experiment to provide an approximate gage of the magnitude of the offshore sediment transport at a given time. The adjustments made to the sediment transport rates to achieve closure have a significant influence on the maxima, and thus, these results are only approximate.

An empirical expression was least-squares fitted to the time evolution of the peak sediment transport rate data. The expression selected was q. q,,, which was found 1+ at
by Larson and Kraus (1989) to have the best general agreement with CE and CRIEPI data.

3.3 Experimental Results

This section describes the changes made to the experimental profile and presents a brief description of the expected response of the profile to the perturbation. Also, both a general statement of evaluation following the perturbation is given as well as a more detailed account of the results.







30


3.3.1 Case A

3.3.1.1 Volume removed and experiment duration

The experiment in Case A was modified by the removal of the volume of sediment that composed the broad offshore bar after an elapsed time of 120 min (Figure 3.4). The experiment was run for 180 min.

3.3.1.2 Expected response

The expected immediate response of the profile was an increase in the energy dissipation rate per unit volume landward of the perturbation, since the removal of the bar would allow more wave energy to propagate past the location where the bar previously existed and be dissipated closer to the shoreline. This would increase the offshore directed sediment transport rate landward of the perturbation, and it would cause increased shoreline recession and increased deposition in the region where the bar was removed. The longerterm expected resp onse was the reformation of the bar and the development of a profile somewhat similar to the pre-perturbation profile translated slightly landward.

3.3.1.3 General results

The general response of the beach profile to the perturbation was a significant increase in offshore transport landward of the perturbation and the deposition of sand in the region where the bar was removed (Figure 3.5). The experimental results agree with the anticipated results, in that the perturbation resulted in an increase in offshore directed sediment transport rate. However, it was not expected that the perturbation would induce a dramatic increase in erosion of the beach face as rapidly as occur-red in Case A. Also, most of the profile experienced a shift landward as expected. However, the bar position did not shift landward, but instead, the bar reformed at the same location as the pre-perturbation bar.








31


-1 0 1 2 3 4 5 Offshore Distance (in)


6 7 a 9


t =0min t- t=120min, before -t =20min, after -t = 180min


Figure 3.4 Summary of profile changes in Case A


0.0016

0.00140.00120.001E 0.0008'E 0.0006a0.00040.00020

-0.0002


-2 -I 4 5 6 7 8 9 1


Offshore distance (in)


t=40-20min -t= 120-30min -t= 130-150min -t= 150-8min]


Figure 3.5 Summary of average sediment transport rates in Case A


0.1


0



o 0.2



*-0.4







32

3.3.1.4 Detailed description and discussion of results

The sediment transport rate for the interval immediately following the perturbation (120 to 130 min) shows an increase in offshore sediment transport both in the region immediately landward of the perturbation and also, to an even greater extent, in the region from -0.5 to 2.3 m offshore. This suggests that instantaneous profile alterations are not necessarily just smoothed out across the profile, with the perturbation affecting adjacent areas of the profile that increase in size with time. Instead, from this experiment it appears that the removal of a volume of sand may have an immediate influence on the sediment transport at all points of the profile landward of the perturbation.

The sediment transport curves for the next intervals though 180 min generally show a slight increase in offshore transport over the profile from -0.5 to 4.6 m and decrease in transport from 4.6 to 8.5 m offshore, relative to the pre-perturbation rates. During this time the bar reformed in a shape similar to that of the pre-perturbation form, but by the end of the experiment it had not yet regained the size of the pre-perturbation bar. Also, the location of the bar was the same as that of the pre-perturbation bar.

The experiment in Case A was performed as a preliminary experiment, and this experiment established that more frequent surveys were necessary in order to quantify the perturbation induced transport. Thus, in subsequent experiments more frequent profiles were measured, particularly for the times immediately before and after the perturbation of the profiles.







33


33.2 Cs

3.3.2.1 Volume removed & experiment duration

Similar to Case A, but with slightly decreased water depth and wave conditions, Case B also was altered by removal of the bar formation near the toe of the beach (Figure 3.6). The experiment was run for 180 min.

3.3.2.2 Expected response

The expected response of Case B was the same as that of Case A, except the magnitudes of the sediment transport rates were expected to be slightly smaller because of the smaller wave height used in this case.

3.3.2.3 General results

The response to the removal of the bar was a large increase in offshore sediment transport rate over the profile from 0.9 m offshore to the seaward end of the profile, and onshore directed transport in the beach face region, which accelerated the growth of the berm feature. In the 60 min following the removal, the sand being transported seaward was deposited in the region where the bar previously existed. In comparison to Case A, the experimental results agree better with the expected results, as there was not such a dramatic increase in the erosion of the beach face region immediately following the perturbation. Also, the magnitude of the offshore transport following the perturbation in Case B was found to be much larger than that of Case A, which was due to the more frequent sampling intervals.







34

3.3.2.4 Detailed description and discussion of results

The adjusted average sediment transport rates are presented in Figure 3.7. For the interval from 120 to 125 min, the average change in profile elevation was 1.32 mm, and thus, the transport rate for this interval required a large negative adjustment. The post-perturbation curve shows onshore directed transport in the beach face area, which resulted in the growth of the berm. Dominant is the large increase in offshore directed transport from 1 to 9.5 mn offshore induced by the removal of the offshore bar.

The subsequent intervals through 180 min generally showed offshore transport over most of the profile with the peak offshore transport much closer to the pre-perturbation peak rate. It was surprising to find that in the region -1.5 m to 0.8 m offshore the sediment transport rates were always less than the pre-perturbation rates for this region. This is counter-intuitive, since it was anticipated that the removal of the bar would increase erosion in this region, as it clearly had in Case A.

Further comparison with Case A shows similar magnitudes of maximum sediment transport rates immediately preceding the removal of the bar volumes. However, the sediment transport rate immediately following the perturbation was much larger in Case B than in Case A, with the increase in maximum average sediment transport following the alteration approximately twice that of Case A. Also, the shape of the transport is much more peaked in Case B. These variations are due, in part, to the increased resolution of profile data in Case B immediately following the removal of the bar. The larger increase in Case B may also be due to the perturbation resulting in a large shift away from the equilibrium profile shape. Note that after the sand was removed in Case A a small bar formation still remained approximately 5.75 m offshore, where in Case B no bar formation remained at all.








35


o.c o.c


Eo.(

0.


-0


C
0
CU,


0 1 2 3 4 5 6
Offshore Distance (in)


7 8 9 10


-t = 110-120 min t = 120-125 min t = 125-130 min
I- t =130-4mmn t=140-70min t =170-80min Figure 3.7 Summary of average sediment transport rates in Case B


0.1


0


-0.1


-0.2


-0.3


-0.4


-0.5


-2 -1 0 1 2 3 4 5 6 7 8 9 10 Offshore distance (in) t1= 0min t t=120min, before
-t =20min, after = 180min

Figure 3.6 Summary of profile changes in Case B )025




Oil 8




)004


)003-


-2 -1


-0.(


.001







36


3.3.3.1 Volume removed & experiment duration

The profile in Case C was altered by the removal of a seaward section of the bar formation while the peaked portion of the bar remained undisturbed (Figure 3.8). The experiment was run for 180 min.

3.3.3.2 Expected response

The expected response of the profile was an increase in sediment transport rate landward of the perturbation and a decrease in transport seaward of the perturbation, as in Cases A and B, except smaller in magnitude. The bar was expected to regenerate its previous form, and the position of the bar was expected to be slightly landward of the location of the pre-perturbation bar.

3.3.3.3 General results

The results of Case C are summarized by the sediment transport rates plotted in Figure 3.9. The perturbation resulted in a large increase in sediment transport rate over the seaward half of the profile and a reduction in sediment transport rate over the landward half of the profile. The largest increase in sediment transport rate occurs just landward of the perturbation, which agrees with the expected results. However, the reduction in transport over the landward half of the profile and the increase in transport seaward of the perturbation are inconsistent with the expected results.

3.3.3.4 Detailed description and discussion of results

The immediate response of the profile (t = 120 to 125 min) is difficult to interpret due to the magnitude of the closure adjustment. In the region x = -I to 1 m the transport curve








37


0.1


0



-0.1
C
w. -0.2



-0.4


-0.5------2 -1 0 1 2 3 4 5
Offshore distance (in)


6 7 8 9


I- t=0min - t =l120min, before -t =20min, after t= 80min


Figure 3.8 Summary of profile changes in Case C


0.002 0.0015


0.001


-~0.0005


0


-0.0005


-2 -1 0 1 2 3 4 5 Offshore distance (in)


6 7 8 9


t =110-20min t 120-25 min t 125-30min
I- t =130-40min -t 140-70min t =170-180min Figure 3.9 Summary of average sediment transport rates in Case C


10


10







38

shows onshore transport, which is counter-intuitive for the perturbation imposed. This may have been caused by the negative transport rate adjustment used to achieve closure. An inspection of the unadjusted profiles shows a recession of the profile in this area, and the profile in this region was relatively two-dimensional. This suggests that the sediment transport rate was actually directed offshore over the region in question. Thus, the transport rate was probably directed offshore over the entire profile.

In comparison with the pre-perturbation transport rate, this transport curve shows an increased offshore transport rate in the region of x = 4.5 9.5 m. The subsequent profiles display a gradual reduction in the transport rate across the profile.

It is likely that the increased sediment transport rates in the bar region were caused by the steepened seaward slope of the bar, which resulted in more sudden shoaling and breaking of the waves. Also, one may hypothesize that since the maximum elevation of the bar was not reduced by the perturbation, increased wave energy would not have been transmitted beyond the wave breaking zone. Thus, the shoreward half of the profile did not experience increased erosion, and the offshore transport in this region decayed with time as it would have, were the profile undisturbed.

Also, it was expected that seaward of the perturbation the sediment transport would be directed landward. However, this did not occur for the interval 120 to 125 min, but it did occur for the intervals 125 to 130 and 140 to 170 min.



This experiment was conducted as the control for the experiments subjected to regular waves with a wave height of 0. 14 m and a wave period of 1.65 sec. The experiment was run for 300 min without any removal or deposition of sediment.






39


3.3.4.1 General results

The experiment conducted in Case I produced a barred, erosional profile. The profile evolution and the calculated sediment transport rates are shown in Figures 3. 10 and 3. 11, respectively. A bar developed in the region approximately 3 m offshore, similar to those developed in the other regular wave cases prior to being perturbed. Also, on a longer time scale than the mid-profile bar, an offshore bar developed in the region approximately 7 m offshore. As this offshore bar grew, it caused a reduction in wave energy passing the bar, thus resulting in a reduction in size of the mid-profile bar.

3.3.4.2 Detailed description and discussion of results

After being subjected to waves for 120 min, the profile developed into a barred profile with a peaked bar at x = 3.5 m. A comparison of the profiles shows the bar crest moving in the offshore direction over the first 120 min.

For the next 60 min (t = 120 to 180 min) the profile was re latively stable over the region landward of the bar, with the exception of the berm still continuing to build up. The bar shifted landward approximately 0.3 m. The region seaward of the bar experienced offshore transport, and deposition of material near the seaward end of the profile began to build a broad bar feature about 7 in offshore. This offshore bar caused some waves to begin spilling earlier, and thus, allowed less wave energy to reach the inner bar. This was likely the cause for the landward shift of the bar.

From t = 180 to 240 min, the profile experienced increased offshore transport over most of the profile, deposition built up the broad offshore bar, and the landward bar shifted further landward.







40


0.1


0


-0.1


E


-0.3


-0.4 -


-2 -1 0 1 2 3 4 5 Offshore distance (in)


6


7 8


9


10


-t=0min -t=20min -t=80min -t=240min -t=300min


Figure 3. 10 Summary of profile changes in Case I


0.002. 0.0015 a0.001 0.0005


0


-0.0005-


0 1 2


3 4 5 4 offshore distance (in)


6 7 8 9


I- t= 0 to 120


-t= 120 to 180 t= 180 to 240 t =240 to 3001


Figure 3. 11 Summary of average sediment transport rates in Case I


2 -1


10


-U.U







41

From t = 240 to 300 min the profile experienced even more increased offshore transport over the profile, and the continued build-up of the offshore bar, but the inner bar shifted slightly seaward.

3.3.4.3 Assessment of repeatability

In order to use this experiment as a control to which the perturbed experiments are compared, it must be assumed that had the other experiments not been perturbed, they would have experienced a similar profile evolution. Therefore, to evaluate the validity of this assumption, it is necessary to evaluate the repeatability of the experiments. Figure 3.12 illustrates a comparison of the profiles after an elapsed time of 120 min for all the regular wave experiments subjected to the same wave conditions as Case I. All of the profiles have the same basic shape, with a few exceptions. Most notably, the bar in Case E is located farther seaward than the other 5 cases. Also, there is a significant amount of variation in the volume of material deposited near the toe of the beach.



Beginning with Case D the wave height was reduced in order to produce a smaller, more peaked bar formation.

3.3.5.1 Volume removed & experiment duration

The profile in Case D was altered by the removal of a landward section of the bar formation which included the peak of the bar (Figure 3.13). The experiment was run for 180 min.








42


0.1


0


-0.1-


C
'=-0.24


-0.31


-0.4+


-0.5


0 1 2 3 4 5 6 7 Offshore distance (in)


8


9


10


11


12


I- test D test E test F test G test H test II


Figure 3.12 Summary of profiles at an elapsed time of 120 min for experiments with regular waves (H = 0. 14 m, T = 1.65 sec)


4 ~







43


3.3.5.2 Expected response

The expected impact of the perturbation was an increase in offshore sediment transport rate landward of the perturbation and a decrease in transport rate seaward of the perturbation. The bar was expected to regenerate its previous form, and the position of the bar was expected to be slightly landward of the location of the pre-perturbation bar.

3.3.5.3 General results

The impact of the perturbation on the profile landward of the perturbation was a large increase in offshore directed sediment transport rate, as anticipated. Surprisingly, the bar did not begin to rebuild immediately. Instead, the bar system collapsed first, and then began to reform approximately 0.5 m further landward than the previous bar position.

3.3.5,4 Detailed description and discussion

In the first 5 min following the perturbation (t = 120 to 125 min) the profile experienced increased erosion over most of the profile landward of the bar (Figure 3.14). In the region of the perturbation and approximately 0.5 m seaward of the perturbation there was considerable negative transport as the bar collapsed and was transported landward into the trough formation.

For t = 125 to 130 min, the profile continued to erode over the region x = -1.5 to 3.4 m. The onshore transport in the bar region was more pronounced as the bar began to reform landward of its location prior to the perturbation.

The remainder of the experiment up to t = 180 min showed a gradual reduction in the magnitude of the sediment transport rates. The bar was rebuilt at a position approximately 0.5 m landward of the pre-perturbation position, and was still shifting landward during the interval from t = 170 to 180 min.








44


0.1

0



o 0.2



0.


-0.5



















0.0015 0.001 0.0005


2 3 4 5
Offshore distance (in)


6 7 8 9 10


t= 110-20min -t= 120-25 min t =125-30 min -t = 130-140 min t = 140-170 min t = 170-180 min


Figure 3.14 Summary of average sediment transport rates in Case D


-2 -1 0 1 2 3 4 5 6 7 8 9 iC Offshore distance (in)



t = 0 min t = 120min, before
it =l120min, after -t=80min


Figure 3.13 Summary of profile changes in Case D


E 0 S-0.0005



-0.0015

-0.005


-2


-1


0 1







45

A comparison with the unperturbed case is presented in Figures 3.15 and 3.16. These show that the impact of the removal of the landward section of the bar was a considerable increase in the erosion of the profile landward of the perturbation. This agrees well with what one would anticipate intuitively for the same reasons as in the previous three cases: a removal of sediment from the bar increases the amount of wave energy transmitted past the bar and dissipated in the region landward of the bar/trough system.

An interesting occurrence is the collapse and reformation of the bar system following the removal of sediment. Inspection of the profile morphology in Figure 3.17 and the evolution of bar volume in Figure 3.18 shows a continued reduction in the size of the bar for the 10 minutes following the perturbation. The bar then grows over then next 40 minutes, and the last 10 minutes show a slight reduction in bar volume.



3.3.6.1 Volume added & experiment duration

Case E was altered by the deposition of a volume of sediment in the trough region of the profile (Figure 3.19). The experiment was run for 180 min.

3.3.6.2 Expected response

The expected response of the profile to the perturbation was the transport of the deposited sediment both landward and seaward. The bar/trough formation was expected to regenerate a form similar to its pre-perturbation form, and the profile was expected to be shifted slightly seaward from the unperturbed case.








46


0.001


0.0008 0.00060E00
E
0.00040

-0.0002


2 -1


0


1


2 3 4 5 6


Offshore distance (in)


- CaseI ase D


Figure 3.15 Comparison of average sediment transport rates for Case D and Case I
(unperturbed case) for the period 120 to 180 min


7


8


9


10







47


0.1 0.05




0




-0.05





0.1


2 4 6 Offshore distance (in)


8


10


........ Casel1, t 120min
Case D, t = 120 min o ...0Case1, t =l180min
+-+ Case D, t = 180 min


L I I


0


4
Offshore distance (in)


6


8


10


Figure 3.16 Comparison of profile changes for Case D and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 180 min


.....Case I


Case D


0


0


2





r


1


2


1-0.

C 0
=. -0.

w


-A


-~ I


-2


-U.13


-0.4-








48


i I I T I T i I .
-2 -1 0 1 2 3 4 5 Offshore distance (in)


6


7 8 9


t = 2min, after t= 25fin -t=30 min t= 40 min -t = 80min


Figure 3.17 Profile evolution with time for the period from 120 to 180 mini for Case D


0.1


0


o -0.2 'U -0.3


-0.4


10


-%J.U








49


















0.05




0.04 E 0.3Idue to a) removal
E
6 0.02


0.01


0
0 30 60 90 120 150 180 Elapsed time (min)


Figure 3.18 Case D: evolution of bar volume with elapsed time







50


3.3.6.3 General results

The general response to the perturbation appeared to be the redistribution of the deposited sediment both landward and seaward, as anticipated. However, the exact response of the profile to the perturbation was difficult to determine in Case E. The results were obscured by both a large closure adjustment required for the interval from 120 to 125 min and an apparent 3-D circulation that dominated sediment transport during the interval from 140 to 170 min.

3.3.6.4 Detailed description and discussion

The immediate response of the profile (t = 120 to 125 min) is difficult to interpret from the calculated sediment transport rates due to the large adjustment required to achieve closure. The transport curve in Figure 3.20 for this interval shows offshore transport seaward of the perturbation and onshore transport over the 3 m landward of the perturbation as one would anticipate. However, it also shows increased offshore transport over the region -1.5 to 0.25 m offshore, which is counter-intuitive (one does not expect an offshore deposition of sand to increase beach face erosion). This increased offshore transport may be the result of the adjustment made to the transport rate to achieve offshore closure (a plot of the adjusted and unadjusted transport curves is shown in Figure 3.3). This region was not erosional according to the unadjusted profiles, and since the region was nearly 2-D, and the deposited volume of sand was not near this region, the closure adjustment probably does not apply to this region. Thus, it is plausible that the perturbation did not induce increased erosion of the beach face area. Also, the profiles at t = 120 and 125 min show a large amount of erosion in the perturbation area due to the reformation of a trough. It appears that most of the eroded volume was deposited just seaward of the new trough, which increased the bar height.








51


-2 -1 0 1 2 3 4 5 Offshore distance (in)


-t = 0 min t = 120 min, after


6 7 8 9 10


t = 120Omin, before t = 180 min


Figure 3.19 Summary of profile changes in Case E


0.0025


0.0018 E0.0011


__0.0004 -


-0.0003


-0.001


-2


-1


0 1 2 3 4 5 6 7 8 9 10
Offshore distance (in)


-t =110-120 min t =120-125 min t =125-130 min -t =130-140 min t =140-170 min t =170-180 min



Figure 3.20 Summary of average sediment transport rates in Case E


0.1


0





C
-0.2



-0.4


-0.5


1 ____________________________________







52

The interval from t = 125 to 130 min shows erosion in the region x = -1.5 to 2.5 m offshore. Onshore transport occurred in the regions x = 2.5 to 3.7 m, which was the transport of some of the deposited material to the area immediately landward of the deposition site. Seaward transport occurred from x = 3.7 to 4.5 m and onshore transport occur-red from x

4.5 to 6 m, which deepened the trough feature and built the bar formation.

From t = 130 to 140 min there was a small amount of onshore transport in the region of the trough (x = 2.7 to 3.75 in), and seaward transport occurred over the remainder of the profile. The elevation of the bottom of the trough remained the same, and the bar shifted slightly seaward.

The changes in the profile between t = 140 and 170 min were very different than the trends in profile morphology shown from t = 120 to 140 min. During this interval there was little change in the profile from -1.5 to 2 mn, but there was a considerable amount of onshore transport over the remainder of the profile. The size of the bar dramatically reduced while much of the bar material was apparently deposited over the 2 mn length of profile just landward of the trough formation. Material was also transported shoreward from the region 6.5 to 8.5 mn offshore. The effects of a 3-D current system were documented by Oh (1994), and a comparison of Oh's results with the current results in figures 3.21 and 3.22 suggest that the onshore transport may have been caused by a 3-D current system.

The final 10 min of the experiment showed offshore directed transport dominating over most of the profile. The bar formation continued to reduce in size as sand was transported seaward from the bar region and deposited near the toe of the beach.












53


0.1 5 0.10


cJ 3
-I

C-,




3






3-


0.05



-0.00



-0.05



-0.10



-0.15



-0.20



-0.25



-0.30


478 miwn.
- 407 zIir%. .... 352 rnizx.
297 min.
-- 242 miii. N -ins'i-l prnfila Id. S. L.























0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 8.0

DISTrANCE FROM TANK END (Mn)


Figure 3.21 Mean profile evolution after the profile approached an equilibrium during Experiment MTO 1. Elapsed times = 0, 242, 297, 352, 407 and 476 min. Note the substantial erosion of the area seaward of the bar and the deposition of the area immediately landward of the bar trough. Note also the landward movement of the bar (Oh, 1994).


0.1




0









o2-0.2 LUJ -0.3




-0.4




-0.5


-2 -1 0 1 2 3 4 5 Offshore distance (in)


6 7 8 9


10


- t=140min-t=70minI


Figure 3.22 Profile evolution of Case E at elapsed times = 140 and 170 min.


-- --1- - - - - - - - - - - - - -






54

A comparison with Case I of the average transport rates for the interval t = 120 to 180 min is presented in Figure 3.23, and a comparison of the changes in profile elevation is shown for the same interval in Figure 3.24. Unfortunately, the large apparent 3-D effect in Case E reduces confidence in the interpretation of the graph, and it is difficult to draw conclusions from this plot concerning the impact of the perturbation.

3.3.7 CaseF

3.3.7.1 Volume added & experiment duration

The profile in Case F was altered by the deposition of a volume of sand in the beach face region (Figure 3.25). The experiment was run for 240 min.

3.3.7.2 Expected response

The expected response of the profile was the transport of most of the deposited material seaward and only a very slight amount of sand transport landward. The profile was expected to evolve toward the form the active profile would have had if unperturbed, but translated slightly seaward.

3.3.7.3 General results

The immediate response of the profile to the sand deposit was a small onshore transport of sand and a large offshore transport of sand from the fill, as anticipated. A comparison of sediment transport rates with the unperturbed case shows that the deposit of sand induced greater offshore transport rates over most of the fill region (except for the berm region). A comparison of the final profile and the profile elevation changes with the unperturbed case shows that at the end of the experiment the fill resulted in the shift of the beach face seaward and the bar position was also shifted seaward relative to the unperturbed case. The perturbation did not appear to have a major effect on the remainder of the profile.







55


111I1~1~


0 1 2 3 4 5
Off shore distance (in)


3 7 8 9


-Case E, 120-1 80 unadjusted -Case E, 120-180, adjusted Case 1, 120-180, adjusted


Figure 3.23 Comparison of average sediment transport rates for Case E and Case I
(unperturbed case) for the period from 120 to 180 min


0.0004 0.0002


C 0

c-0.0002
E
(3-0.0004


-0.0006 -0.0008


-2


-1


I I I I I


10


I V -- -







56


4


Offshore distance (in)


~--0.
E
a 0
.= -0.
as
wi


1 F


-U.%:) r-


-0.4 0 -...


-0.5'
-2


Case 1, t =120 min'G Case E, t =120 min Case 1, t = 180 min Case E, t =180 min


I I IIII


0


2


4


Offshore distance (in)


Figure 3.24 Comparison of profile changes for Case E and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 180 min


.......CaselI


Case E


.A h~A..


0.


0.06[


*0.02

0

-0.02

-0.04

-0.06


0.1


0


0


2


6


8


10


6


8


10


nR.







57


3.3.7.4 Detailed description and discussion

The transport in the 5 min following the perturbation exhibited increased offshore transport across nearly the entire profile, except for the beach face (Figure 3.26). Onshore directed transport occurred over a landward section of the beach face region, which resulted in a build up of the berm. The seaward portion of the sand deposit eroded substantially. Also, note the landward shift in the bar formation.

In the interval from t = 125 to 230 min the profile continued to experience increased erosion with offshore transport occurring everywhere except near the peak of the berm. The shoreline showed a large recession, as one would expect. The bar position oscillated in its horizontal position around 3 m offshore.

The final 10 min (t = 230 to 240 min) showed a minimal accretion in the beach face area as the shoreline advanced slightly. Also, the bar/trough formation shifted landward.

A comparison with the unperturbed case for the interval t = 120 to 180 min (Figure 3.27) shows greatly increased offshore transport in the region 0 to 4 m offshore. Also, the comparison displays an increased onshore transport rate in the region from -0.3 to -1.2 m offshore, thus, resulting in a larger berm than in the unperturbed case.

For the period t = 180 to 240 min, the comparison with Case I shows much smaller differences (Figure 3.28). Case F experienced slightly greater offshore transport over the entire profile, save the region -1 to 0 m offshore, which had greater onshore directed transport than Case I. Obviously, the deposit had a considerably lessened impact on the transport rate after it had been somewhat smoothed out over the 60 min following the deposition.








58


0.1


0


--0.1E

a

-0.3


-0.4 -


-2 -1 0 1 2 3 4 5 6 7 8 9 Offshore distance (in)


0.001 0.00 0.000


E

cr


-0.000.


-0.00


t=0min t = 1~20Omin, before -t= 20min, after
t= 80 min -t =240 min

Figure 3.25 Summary of profile changes in Case F


5.


1


5






5


1. . . . .


-2 -1 0 1 2 3 4 5 6 7 8 9 10 Offshore distance (in)


-t = 110-120 min t = 120-125 min t = 125-130 min t = 130-140 min
-t = 140-160 min t = 160-180 min t = 180-230 min t = 230-240 min

Figure 3.26 Summary of average sediment transport rates in Case F


-0.5








59


0.0004 0.0003


0.0002 E0.0001


0


-0.0001


-2 -1 0 1 2 3 4 5 6 Offshore distance (in)


7 8 9


I- Case F - CaselI


Figure 3.27 Comparison of average sediment transport rates for Case F and Case I
(unperturbed case) for the interval from 120 to 180 min


0.0005


0.00040.00030.00020.0001 t


0


-0.0001


0 1 2 3 4 5 4
Offshore distance (in)


6 7 8 9 10


[- Case F- CaselI

Figure 3.28 Comparison of average sediment transport rates for Case F and Case I
(unperturbed case) for the interval from 180 to 240 min


10


E
E


I:


1.1 I I I I I I I


-2 -1







60

A comparison with the unperturbed case of changes in profile elevation over the interval from 120 to 240 min is presented in Figure 3.29. Here it can be seen that the deposition caused berm growth over the landward third of the fill area and greatly increased erosion of the seaward two-thirds of the fill area. The perturbation seemed to have little effect on the profile between the fill area and the bar/trough formation, but the perturbation did seem to have an impact on the bar. The bar in Case F shifted landward less than half the distance the bar in Case I shifted landward. Also, the bar crest elevation increased, whereas the bar crest elevation decreased in the unperturbed case. Overall, the perturbation caused a seaward shift in the shoreline and bar formation relative to the unperturbed case, and appeared to have little effect on the remainder of the profile.



3.3.8.1 Volume added & experiment duration

A volume of sand was deposited just landward of the trough formation in Case G (Figure 3.30). The experiment was run for 240 min.

3.3.8.2 Expected response

The general expected response of the profile to the perturbation was the transport of the deposited sediment both landward and seaward. The immediate expected result was the reduction in wave energy propagating beyond the fill area, and thus, a decrease in sediment transport rate landward of the fill, as well as an increase in offshore sediment transport rate in the region of the fill. The long term expected result was the redistribution of the material across the profile, resulting in a seaward shift of the profile from the unperturbed case.







61


0


4 6 Offshore distance (mn)


......Casel1, t 120min
Case F, t = 120 min +-+Case F, t =240 min


0


4
Offshore distance (in)


Figure 3.29 Comparison of profile changes for Case F and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 240 min


0.15 0.1 0.05


0


Ec


......CaselI
Case F


K.


-0.05*





-0.1


8


10


C


1 L


2-


,-0.
a
2


-0.5'
-2


3


8


10


.A.


I


2







62


3.3.8.3 General results

The sediment transport curves calculated for the experiment show that the immediate response of the profile to the sand deposit was a small onshore transport of sand and a large offshore transport of sand from the fill, as anticipated. A comparison of sediment transport rates with the unperturbed case shows that the deposit of sand induced greater offshore transport rates over most of the profile seaward of the perturbation, as expected, but for the interval 120 to 180 mini the sediment transport curves did not show any noticeable difference in the region landward of the perturbation. However, for the interval from 180 to 240 mini there was a reduction in offshore sediment transport rate relative to the unperturbed case for the region landward of the fill area. A comparison of the final profile and the profile elevation changes with the unperturbed case shows that at the end of the experiment the sand deposit appeared to be smoothed out over the profile and produced a seaward shift in most of the profile relative to the unperturbed case, as expected.

3.3.8.4 Detailed description and discussion

The sediment transport rates for the experiment are presented in Figure 3.3 1. It is important to note that the closure adjustments for the intervals from 120 to 125 mini and 125 to 130 mini are relatively large. The average change in elevation for the two intervals were

0.817 mm and -0.51 mm, respectively.

For the interval immediately following the perturbation (120 to 125 mini) the transport curve is directed onshore in the region -2 to 1 m offshore. However, the large negative closure adjustment may be responsible for much of this apparent onshore sediment transport. Despite the large adjustment of the transport curve, it is clear that the perturbation induced








63


0.1


0


--0.1
E
a

-0.3 -


-0.4 -


I I I I I II I i I i I I
-2 -1 0 1 2 3 4 5 6 7 8 9 Offshore distance (in)


t=0min t = 1~20Omin, before- t =120, after t =80min -t =240min


Figure 3.30 Summary of profile changes in Case G


0.00,1


0.001 E a 0.001


0.000E

CY


-2 -1 0 1 2 3 '4 5 Offshore distance (in)


6 7 8 9


-t = 110-120 min t = 120-125 min t = 125-130 min t = 130-140 min
-t=140-1 60 min t = 160-200 min -- t = 200-230 min t = 230-240 min


Figure 3.31 Summary of average sediment transport rates in Case G


-V.Z)







64

strong offshore transport over the portion of the profile seaward of the landward edge of the deposited sediment volume. This is confirmed by a comparison of the profiles at elapsed times of 120 and 125 mini, which show erosion of the deposited volume, a reduction in bar height, a shift of the bar landward and deposition offshore.

For the interval t = 125 to 130 mini the deposition area eroded further, the bar continued to shift landward and there was some deposition in the offshore area.

For the next 100 mini, up to t = 230 mini, the sand deposited at t = 120 mini continued to erode and be transported seaward until the perturbation was mostly smoothed-out at t = 200 mini. Then, the profile landward of the region where the sand deposit previously existed began to erode. Also, the bar position oscillated as the bar grew in height.

In the last 10 mini, t = 230 to 240 mini, the bar suddenly decreased in height, apparently becoming unstable after reaching its maximum height at t = 230 mini.

A comparison with the unperturbed case for t = 120 to 180 mini shows increased offshore transport over the profile seaward of 1.5 m offshore (Figure 3.32). This is reasonable, but the lack of shoreward transport (relative to the unperturbed case) landward of the perturbation is surprising.

The comparison with Case I for t = 180 to 240 min is more intuitively consistent, in that the perturbation did appear to cause some shoreward transport landward of the deposition area, although the beach face experienced slightly increased erosion (Figure 3.33). Seaward of the deposition region showed increased offshore transport, as expected.

A comparison with the unperturbed case of changes in profile elevation over the interval from 120 to 240 mini is presented in Figure 3.34. This plot shows a large negative








65


0.0006 0.00050.0004


F= 0.0003+ E 0.0002


0.0001

0

-0.0001


0 1 2 3 4 5
Offshore distance (in)


6 7 8 9 10


I- Case G -- Case I


Figure 3.32 Comparison of average sediment transport rates for Case G and Case I
(unperturbed case) for the interval from 120 to 180 min


1 2 3 4 5
Offshore distance (m)


6 7 8


I- case G- Casel1-


Figure 3.33 Comparison of average sediment transport rates for Case G and Case I
(unperturbed case) for the interval from 180 to 240 min


-2 -1


-~ I ri I I I I I


0.0005


0.0004 -


E-=

E


0.0003


0.0002 -f


0.0001+


0


-0.0001


-2


-1 0


.1..111i1.f I I I I I I I I I


9 10


I







66


0.

0.0 0.0 0.0 0.0


.-2


0


2


Offshore distance (in)


0.1 r-


0


11.


4-


......Case1, t 120Omin
~Case G, t = 120 min 0o...0Case1, t =240Omin 2I---- Case G, t = 240 min


-2


0


2


4


Offshore distance (in)


6


8


10


Figure 3.34 Comparison of profile changes for Case G and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 240 min


81 ......... CaselI

16

4

2


'4j


-0.0

-0.0'


4


6


8


10


~-.-0.
E

a
0w0


-0.3







67

change in the Case G profile elevation in the region where the fill was deposited, which was caused by the smoothing out of the perturbation, as expected. Landward of the sand deposit site the Case G profile showed almost no change, whereas the unperturbed case showed erosion in this region (except the berm area). Thus, the perturbation induced a positive elevation change in this region, relative to the unperturbed case. Also shown in this figure, the bar in Case G shifted landward, but only about half the distance that the bar shifted landward in Case I. This suggests that the perturbation affected the bar by shifting it seaward relative to the unperturbed case. Seaward of the bar, up to an offshore distance of 7 m, the figure shows a positive elevation change in Case G relative to the unperturbed case. Overall, the sand deposit appeared to be smoothed out over the profile and produced a seaward shift in most of the profile relative to the unperturbed case, as expected.



3.3.9.1 Volume added & experiment duration

In Case H a volume of sand was deposited at a location between the deposition sites chosen in Cases F and G (Figure 3.35). The experiment was run for 300 min.

3.3.9.2 Expected response

As in the previous deposition cases, the general expected response of the profile to the perturbation was the transport of the deposited sediment both landward and seaward. The immediate expected result was the reduction in wave energy propagating beyond the fill area, and thus, a decrease in sediment transport rate landward of the fill, as well as an increase in offshore sediment transport rate in the region of the fill. The long term expected result was the redistribution of the material across the profile, resulting in a seaward shift of the profile from the unperturbed case.







68


3.3.9.3 General results

The results of the experiment in Case H are very similar to those of Case G, and it appears that the change in position of the fill area between these experiments did not have a large effect on the results. The sediment transport curves calculated for the experiment show that the immediate response of the profile to the sand deposit was a small onshore transport of sand and a large offshore transport of sand from the fill, as anticipated. A comparison of sediment transport rates with the unperturbed case shows that the deposit of sand induced greater offshore transport rates over most of the profile seaward of the perturbation and decreased offshore sediment transport landward of the perturbation, as expected. A comparison of the final profile and the profile elevation changes with the unperturbed case shows that at the end of the experiment the sand deposit appeared to be smoothed out over the profile and produced a seaward shift over most of the profile relative to the unperturbed case, as expected.

3.3.9.4 Detailed description and discussion

The transport curve for t = 120 to 125 min, shown if Figure 3.36, required only a small adjustment for closure (there was only a 0. 147 mm average change in elevation over the active profile) and may be a fairly accurate description of the sediment transport rate for the 5 min interval. It shows some onshore directed transport in the berm and beach face region, as one would anticipate due to a deposit of sand this size so close to the beach face. Dominant is the large increase in the offshore directed sediment transport rate over the region 1 to 7 m offshore. A slight decrease in the bar height as well as a shift landward in the bar position occurred. Also, note the onshore transport at the toe of the profile.








69


-2 -1 0 1 2 3 4 5 6 7 8 9 10 Offshore distance (in)


0.00


0.001


-~0.00 E'

E0.000
a


-0.000


t =0min t = t120Omin, before t= 20min, after
t =80min -t =240min t 300min


Figure 3.35 Summary of profile changes in Case H


2


5


1


5-


-2 -1 0 1 2 3 4 5 Offshore distance (in)


3 7 8 9


-t = 110-120 min t =120-125 min t =125-130 min t =130-140 min t = 140-1 80 min t =160-180 min t =21 0-240 min t =270-300 min Figure 3.36 Summary of average sediment transport rates in Case H


0.1

0


--0.1
E

02 -0.2 Cz


-0.4 -


10


I I I .


I


-U.0







70

The period from t = 125 to 180 min showed much of the same behavior. There was offshore directed sediment transport over most of the profile, though of much lesser magnitude, and there was slight onshore transport at the landward end of the perturbation. The bar position oscillated and the bar height varied slightly. The offshore bar approximately

7 m offshore continued to develop and become more pronounced.

For t = 180 to 240 min, offshore directed sediment transport over the entire profile occurred. The offshore bar formation became more developed and an inner bar formed about

1 m offshore.

The final 60 min of the experiment experienced the same trends as the previous 60 min. The transport over the entire region was directed offshore. The inner bar at 1 m offshore became more peaked, the main bar at 3.5 m offshore remained relatively stable and the offshore bar continued to build.

A comparison with the unperturbed case for t = 120 to 180 min, shown in Figure 3.37, displays greatly increased offshore directed sediment transport from 0.5 m to 6.5 m offshore. There is little change in transport landward of the deposit area, except for a small increase in onshore transport immediately adjacent to the landward edge of the perturbation. For t = 180 to 240 min, a comparison with Case I displays a reduction in offshore sediment transport in the region -i to 1.5 m offshore and an increase in offshore transport 2.5 to 9 m offshore (Figure 3.38). For t = 240 to 300 min, the offshore sediment transport in Case H is less than that of Case I over most of the profile (Figure 3.39).

A comparison with the unperturbed case of changes in profile elevation over the interval from 120 to 300 min is presented in Figure 3.40. This plot shows a large negative








71


0.0006



0.0004 S0.0002
E


0



-0.0002


-2 -1 0 1 2 3 4 5 6 7 8 9 10 Offshore distance (in)


I- Case H- case i


Figure 3.37 Comparison of average sediment transport rates for Case H and Case I
(unperturbed case) for the interval from 120 to 180 min


0.0006



0.0004 S0.0002
E

0




-0.0002


0 1 2 3 4 5
Offshore distance (in)


6 7 8 9 10


I- case H- Caset1-


Figure 3.38 Comparison of average sediment transport rates for Case H and Case I
(unperturbed case) for the interval from 180 to 240 min


-2 -1


-----------








72


0 1 2 3 4 5 6
Offshore distance (in)


7 8 9


I- Case H -- CaselI


Figure 3.39 Comparison of average sediment transport rates for Case H and Case I
(unperturbed case) for the period 240 to 300 min


0.0006



0.0004 -


E

E
cr


0.0002 -


0



-0.0002


-2 -1


10







73


4


Offshore distance (in)


-~-0.
E

0.
*.c-0
wU


1


2h


-0.4 [22...


-0.02

-0.04

-0.06







0.1


0


51 1


0


2


4


Offshore distance (in)


6


8


10


Figure 3.40 Comparison of profile changes for Case H and Case I (unperturbed case) for
the interval from 120 min (after perturbation) to 300 min


0.1 0.08

0.06

0.04 0.02

0


Eo


.. . C as I

CaselH


Cas H







I I I


Case 1, t =120 min Case H, t = 120 min Case 1, t = 300 min Case H, t = 300 min


0


2


6


8


2.


10


-2


-U.;j r


-0.







74

change in the Case H profile elevation in the region where the fill was deposited, which was caused by the smoothing out of the perturbation, as expected. Landward of the sand deposit site the Case H profile showed only small changes, whereas the unperturbed case showed erosion in this region (except the berm area). Thus, the perturbation induced a positive elevation change in this region, relative to the unperturbed case. Also shown in this figure, the bar in Case H shifted landward, but only about half the distance that the bar shifted landward in Case L. This suggests that the perturbation affected the bar by shifting it seaward relative to the unperturbed case. Seaward of the bar, up to an offshore distance of 7.5 m, the figure shows a positive elevation change in Case H relative to the unperturbed case. Overall, the perturbation in this case produced results very similar to those of Case G: the sand deposit appeared to be smoothed out over the profile and produced a seaward shift over most of the profile relative to the unperturbed case, as expected. This suggests that the change in position of the fill area between Cases G and H has little influence on the results.



Case J was conducted as the control experiment for the irregular wave experiments. Random waves (HS = 0. 16 m, T-k= 1.65 sec) were run on the 1:20 slope initial profile for 300 mini without any perturbation introduced to the profile by the deposition or removal of sand.

3.3.10.1 General results

The summary of profile evolution for the experiment is presented in Figure 3.4 1. The profile developed a shape similar to a power law profile in the region -I to 4 m offshore. Also, a small "step" feature formed approximately. 0.6 m seaward of the still water line.







75


3.3.10.2 Detailed description and discussion

In the first 120 min of the experiment, the sediment transport rate was directed onshore over x = -i to 0 m which built up a berm at -1 m (Figure 3.42). The transport was directed offshore for x = 0 to 6 m and onshore for x = 6 to 10 m. In the region -i to 4 m offshore, where most of the waves broke, the profile developed an exponential shape similar to that of the hi = Ax Bpower law profile proposed by Bruun (1954) and Dean (1973).

For the next 180 min, the experiment continued along very similar trends with only slightly modified sediment transport rates across the profile. At an elapsed time of 240 mini a small step in the profile developed about 0.6 m offshore and became more pronounced at t=300 mini.

3.3.10.3 Assessment of repeatability

Since this experiment is used as a control to which the perturbed cases are compared, it is necessary to assess the repeatability of the experiments with irregular waves. Figure 3.43 shows the profiles of Cases J, K and L at an elapsed time of 120 mini. The profiles are very similar, showing only minor differences, with the exception of Case J showing a greater erosion near the beach face and a slightly lesser beach face slope.

3.31 ase~

3.3.11.1 Volume added & experiment duration

The experimental profile in Case K was altered at t = 120 mini by the deposition of a volume of sand in the area 2 to 3 m offshore (Figure 3.44). The experiment was run for 300 mini.








76


0.2 0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5


0 1 2 3 4 5 6 7 8 9 10
Offshore distance (in)


t=0min -t=20min -t=80min -t=240min -t=300mini


Figure 3.41 Summary of profile changes in Case J


0.0006


0.00040.0002


0


-0.0002


-0.0004-


2 -1


0 1 2 3 4 5 6 7 8 9
Offshore distance (in)


I-t=180 to 240 min t = 240 to 300 min


Figure 3.42 Summary of average sediment transport rates in Case J


10


-2 -1


0


w


E)
1-







77


2 3 4 5 6 7 8 9
I I I I I I I I I


Offshore distance (in)


fcaseJ case K case LI


10 11


12


Figure 3.43 Summary of profiles at an elapsed time of 120 min for experiments with
irregular waves (Pierson-Moskowitz spectrum, Hs = 0. 16 m, Ta= 1 .65 sec)


0.2

0.1

0



C
(D -0.2wi


-0.3 -1


-0.4

-0.5


) 1







78


3.3.11.2 Expected response

As in the previous deposition cases, the general expected response of the profile to the perturbation was the transport of the deposited sediment both landward and seaward. The immediate expected result was the reduction in wave energy propagating beyond the fill area, and thus, a decrease in sediment transport rate landward of the fill, as well as an increase in offshore sediment transport rate in the region of the fill. The long term expected result was the redistribution of the material across the profile, resulting in a seaward shift of the profile from the unperturbed case.

3.3.11.3 General results

For the 60 min following the perturbation the results were mostly consistent with expectations. A comparison of sediment transport rates with the unperturbed case shows that the deposit of sand induced greater offshore transport rates over most of the profile seaward of the perturbation and decreased offshore sediment transport landward of the perturbation, as expected. However, for the subsequent intervals the perturbation did not induce any onshore transport relative to the unperturbed case. A comparison of the final profile and the profile elevation changes with the unperturbed case shows that at the end of the experiment the sand deposit appeared to have a negative impact on the profile from -0.2 to 0.9 m offshore, which is contrary to what was expected.

3.3.11.4 Detailed description and discussion

The average sediment transport rates are presented in Figure 3.45. An odd occurrence in this experiment was the sediment transport directed onshore for the 10 min preceding the perturbation. Although relatively small, this did not occur in the other random waves case








79


0.2

0.1

0


c-0.1

> -0.2

-0.3

-0.4

-0.5












0.0c


0.001


E~00 a0.00
E



-0.000


-2 -1 0 1 2 3 4 5 6 7 8 9 iC


Offshore distance (in)






Figure 3.44 Summary of profile changes in Case K






5


1i


5


0*


'5


-2 -1 0 1 2 3 4 5 6 7 8 9 Offshore distance (in)


10


-t = 110-120 min t = 120-125 min t = 125-130 min t = 130-140 min t =140-160 min t = 160-180 min t = 210-240 min t = 270-300 min Figure 3.45 Summary of average sediment transport rates in Case K


77 L






80

in which transport was measured for the same interval (Case L). This is due, in part, to the negative closure adjustment made for that interval.

In the 5 min following the perturbation there was moderate offshore sediment transport from -2 to 1.9 m offshore. It is surprising that the addition of the volume of sediment caused this increase in offshore sediment transport in this region. There was a 'spike' of onshore directed transport from 1.9 to 2.2 m offshore, the position of which corresponded approximately to the landward edge of the fill area. Also, there was a large amount of offshore sediment transport over the profile 2.25 m offshore and seaward.

During the next 5 min, from t = 125 to 130 min, offshore sediment transport occurred seaward of 1 m offshore, but unlike the previous 5 min, onshore transport occurred from -2 to 1 m offshore.

Through an elapsed time of 180 min the perturbation was smoothed out. Offshore directed transport occurred over x = I to 6 m and slight onshore transport occurred over the beach face region.

From 180'to 300 min the same trends of sediment transport continued: offshore transport in the middle of the profile and onshore directed transport at the ends of the profile. The berm continued to build as well as the 'bar' at 4.5 m offshore. Also, the same feature as noted in Case J, a rather abrupt drop-off about 0.5 m offshore, developed during this interval.

The comparison with Case J for t = 120 to 180 min displays considerable change in sediment transport rates (Figure 3.46). The perturbation induced a negative impact on the transport rate landward of the deposition area, and a large positive impact on the transport






81

rate seaward of the deposition area. Note that the magnitude of the maximum transport rate in Case K is twice that of Case J.

For the interval t = 180 to 240 mini, the sediment transport rates for Cases J and K are very similar (Figure 3.47). During this interval the perturbation is well smoothed out, but the added volume has altered the profile in the region of the deposit by flattening the slope compared to the unperturbed case. It is odd that this is not reflected by greater differences in sediment transport rates.

For t = 240 to 300 mini, the comparison of sediment transport rates (Figure 3.48) yields results that are more explainable than those in Figure 3.47. Case K exhibits increased offshore transport from 2 to 9 m offshore, and from this plot it is clear that the added volume does, in fact, still influence the sediment transport rates.

A comparison with the unperturbed case of changes in profile elevation over the interval from 120 to 300 mini is presented in Figure 3.49. This plot shows a large negative change in the Case K profile elevation in the region where the fill was deposited, which was caused by the smoothing out of the perturbation, as expected. Landward of the sand deposit site the Case K profile showed both expected and unexpected changes. Case K showed positive profile changes relative to Case J at the berm and from about 0.9 m to 2 m offshore. However, unexpected and unexplainable changes occur in the region from 0 to 0.9 m offshore, where Case K shows a negative profile change relative to the unperturbed case. Seaward of the deposit area the figure shows a positive elevation change in Case K relative to the unperturbed case over most of the profile. Overall, the sand deposit appeared to be smoothed out over the profile and produced a seaward shift in the most of the profile relative








82


0.00060.0004C? 0.0002E

0




-2 -1 0 1 2 3 4 5 6 7 8 9 10 Offshore distance (in)


I- Case K- case iI


Figure 3.46 Comparison of average sediment transport rates for Case K and Case J
(unperturbed case) for the interval from 120 to 180 min


0.0006



0.0004

E
a .00
E


0




-0.0002


-2 -1 0 1 2 3 4 5 6 7 8 9 10 Offshore distance (in)


r-Case K - CaseJ I


Figure 3.47 Comparison of average sediment transport rates for Case K and Case J


I.







83


0


2 4 Offshore distance (in)


........ Case J, t 120min
Case K, t= 120 min 0o...0Case J, t 300 min
-I----+Case K, t =300 min


0


2


4


Offshore distance (in)


Figure 3.49 Comparison of profile changes for Case K and Case J (unperturbed case) for
the interval from 120 min (after perturbation) to 300 min


0.08 0.06

0.04 0.02


......Case J


Case K


Ec


0


-0.02

-0.04

-0.06

-0.08,





0.2 0.1


0

w


6


2


8


10


-0.3




-2


6


8


10


I I I -


-U. 1

-0.21







84


-2 -1 0


1 2 3 4 5 6
Offshore distance (in)


7 8 9


I- case K- caseJi















Figure 3.48 Comparison of average sediment transport rates for Case K and Case J
(unperturbed case) for the period 240 to 300 min


0.0006



0.0004

E

S0.0002
E

0



-0.0002


-


10






85

to the unperturbed case, with the exception of the region from -0.7 m to 0.9 m offshore. Thus, the experimental results generally agreed with the expected results for most of the profile, except for the beach face and the region immediately adjacent offshore, which gave unexpected results.

3.3.12 Case L

3.3.12.1 Volume added & experiment duration

The purpose of the experiment conducted in Case L was to test the hypothesis that a peaked bar formation created during a storm by somewhat regular waves may act to position the break point and thus maintain the bar in a similar shape under irregular waves. Thus, a volume of sand was deposited at t = 120 min in the area 4 to 5 m offshore in a shape similar to that of the bars formed during the regular wave cases (Figure 3.50). The location of the deposition was selected as the point where most of the plunging breakers occurred and where a peaking in the profile had formed under the irregular waves. The experiment was run for 300 min.

3.3,12.2 Expected response

The general expected response of a profile to a deposit of sediment is the transport of the deposited sediment both landward and seaward as the sediment is redistributed across the profile. However, in this particular case it was expected that the deposited volume might be maintained in a bar formation by positioning a break point at the location of the fill. It was not expected that the exact form of the deposit be preserved, but instead, reformation of the deposited sand while retaining a bar form was expected. The extent of the redistribution of the fill material to be expected was unknown. It was anticipated that the redistribution of







86

the fill might cease at some point while a bar formation still existed at the location of the perturbation, and that bar formation might be maintained.

3.3.12.3 General results

The volume of sand that constituted the emplaced bar was largely redistributed both landward and seaward, though primarily seaward. However, a large amount of the sand was not redistributed far from the fill area, but rather was redistributed to form a broader, flatter bar than that which was emplaced. The evolution of the bar characteristics of volume and height suggest that the broad, flat bar was stable and may have continued to grow with the continuation of the experiment.

3.3.12.4 Detailed description and discussion

The perturbation caused waves to break in a much more rapid, plunging manner over the newly introduced bar. An inspection of the average sediment transport rates for the interval t = 120 to 125 min shows surprising results (Figure 3.5 1). The transport rate greatly increased in the region 0 to 3 m offshore, which is counter-intuitive. One would expect that the bar would reduce the amount of wave energy propagating past the bar and reduce the erosion of the nearshore area.

For the interval t = 125 to 130 min, a comparison with the pre-perturbation transport curve (t = 110 to 120 min) shows the bar inducing a 'spike' of increased offshore transport over a 1 m length of the profile corresponding to the bar location, and a region of increased onshore transport over a 1.5 m length of the profile just seaward of the bar.

Up to t = 180 min the sediment transport was directed predominantly offshore over the entire profile. The bar was smoothed out considerably, and the 'step' feature began to form about 0.6 m offshore, as it had in the previous irregular wave experiments.








87


-2 -1


0 1 2 3 4 5
Offshore distance (in)


6 7 8 9 10


t =0min t = 1~20Omin, before -t =20min, after
t=80min -t =240min t =300 min


Figure 3.50 Summary of profile changes in Case L


8 6


4


2


0


2


-2 -1 0


1 2 3 4 5
Offshore distance (m


6 7 8 9 10


-t = 110-120 min t =120-125 min t =125-130 min t 130-140 min
-t =_140-1 60_mn- t =160-180 min t =21 0-240 min t =270-300 min


Figure 3.51 Summary of average sediment transport rates in Case L


0.2 0.1

0




.> -0.2

-0.3

-0.4

-0 .5


I I I'


0.000 0.000 0.000




a


-0.000