Beach profile and seawall interaction during severe storm conditions

UFL/COEL-94/023

BEACH PROFILE AND SEAWALL INTERACTION
DURING SEVERE STORM CONDITIONS

by

Paul L. Miselis

Thesis

1994

BEACH PROFILE AND SEAWALL INTERACTION
DURING SEVERE STORM CONDITIONS

By

PAUL L. MISELIS

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

1994

DEDICATION

The following work and all that it represents is dedicated to my parents, Vytas and

Dana, because of their unfaltering faith in me, even during my lowest times. I try to be

like you and will never forget your lives in me.

ACKNOWLEDGMENTS

I would like to express my sincere appreciation to the members of my supervisory

committee Dr. Robert G. Dean (chairman), Dr. Ashish J. Mehta, and Dr. Robert J. Thieke

for their guidance of my studies at the University of Florida. I extend thanks to the

Coastal lab crew and to the Coastal secretaries for their skill at their work. I would also

like to thank my fellow students, especially Chris, Eric, Tom, Mark, Kenny, and Al, for all

the help academically and otherwise. A special thanks goes to Sue for shining a new light

on an ancient sun. I am grateful for my sister, Daiva, and for her reminding me with her

free spirit of what living really is.

iii

TABLE OF CONTENTS

ACKNOWLEDGMENTS................. ...................... .... ..................... iii

L IST O F F IG U R E S ......................................................................... ....................... vi

L IST O F T A B LE S............................................................................... .............. ix

LIST O F SY M B O L S......................................................................................... x

A B S T R A C T ................. ........................................................................................ xii

CHAPTERS

1 IN TR O D U CTION ....................................... ........................... ........ 1

1.1 Need for Study..................... .................... .............. 1
1.2 Description of the Study Area......................... ... ................. 2
1.3 Review of Past W ork................... ............................... 4
1.3.1 Review of Past Literature ............................................. 5
1.3.2 Seawall Overtopping Studies......................... ......... 6
1.3.3 Beach Profile Response Studies................................... 9
1.3.4 Scour Studies..................... .. ...................... 12
1.3.5 Summary of Literature........................... ................. 14

2 M ETHODOLOGY................................................................................ 15

2.1 T he F acility............................................... ............................ 15
2.1.1 The W ave Tank............................... ........................ 15
2.1.2 Wave Data Collection ..................................... 17
2.1.3 The Survey Technique........ ....................... 18
2.2 Scaling the Model............................ ................ 20
2.2.1 Modeling Goals..................... ..................... 20
2.2.2 Froude Scaling..................... ...................... 21
2.2.3 The Resulting Model Scale.......................... ........... 21
2.3 Beach and Seawall Models.................................... 23
2.4 Storm Surge Modeling.............................................. 24
2.5 Test Cases Considered.......................... ....... ............... 25

iv

2.5.1 Tests W without A Seawall .............................................. 26
2.5.2 Effects of Seawall Elevation.................. ................ 26
2.5.3 Seawall Failures........................ .... ..................... 26
2.5.4 Seawall With Toe Protection...................................... 28
2.5.5 Overtopping Volumes..................... ............. 28
2.5.6 Effects of Different Sand Sizes................. ............ 29
2.5.7 Effects of Different Wave Periods................................... 30
2.6 Advantages and Disadvantages of Modeling................................ 30

3 RESULTS AND DISCUSSION........................................................ 33

3.1 The Basic Profile with Seawall............................ ................ 33
3.1.1 Profile Evolution of Case B3....................................... 34
3.1.2 Profile Evolution of Case B4.......................................... 43
3.2 Variations of the Basic Seawall Tests........................................ 49
3.2.1 Changes in Seawall Elevation.............................. .......... 49
3.2.2 Addition of Seawall Toe Protection................................. 54
3.2.3 Changes in Sand Grain Size .......................................... 57
3.3 Beach Profile Response in the Absence of a Seawall....................... 59
3.3.1 Profile Evolution Trends of Beach Without Seawall........ 59
3.3.2 Transport Comparison................................................ 62
3.4 Seaw all F ailure................................................... ....................... 66
3.4.1 Total Failure....................................... 67
3.4.2 Half Failure.................................... 70
3.4.3 Three-Quarter Failure............................ ............... 72
3.5 Seawall Overtopping Volume Flux...................... ........... 73
3.5.1 Effects of Seawall Elevation........................ ........... 74
3.5.2 Effects of Storm Surge Level.................... ... .......... 76
3.5.3 Effects of Sand Grain Size..................... .... .......... 77
3.5.4 Effects of Wave Type and Wave Period........................... 78
3.6 W ave Reflection................................................................... ........ 79
3.7 Summary of Scour Depths and Beach Recession......................... 83

4 C O N CLU SIO N S.......................................................... .................... 85

4.1 Profile Evolutionary Trends.......................................................... 85
4.2 Seawall Toe Scour and Beach Recession...................................... 86
4.3 W ave Overtopping......................... .... ... ..................... 87
4.4 W ave Reflection .... .............................................. ................. 87
4 .5 G general R esults........................................................ .................. 88
4.6 Future Studies........................................................................... 88

R E FE R EN C E S ........................................................................................... 90

BIOGRAPHICAL SKETCH............................................................ 93

LIST OF FIGURES

Figure Page

1.1 Location of Study Area ............... ...... ......... ................. 3

2.1 Schematic of the Wave Tank Facility......................... ................................. 16

2.2 Wave Envelope Schematic with Reflection Bars......................... .................. 18

2.3 The Cart System for Surveys and Wave Measurement....................................... 20

2.4 Prototype and Initial Model Beach Profiles........................ .................... 24

2.5 Comparison of Prototype and Model Storm Surge.................. ................ 25

2.6 Sand Grain Size Distributions............................... ............... .................... 30

3.1 Profile Evolution of Case B 3.............................................. ....... ................ ..... 35

3.2 Profile Elevation Difference Between Initial and 21 Hour Profile.................... 37

3.3 Profile Recovery Trends After the Storm.............................. ................. 39

3.4 Comparison of Prestorm and Poststorm Profiles........................................... 40

3.5 Volume Transport for Case B3 ................................................. ................. 42

3.6 Profile Evolution of Case B4.................. .... .................... 44

3.7 Profile Elevation Difference Between Initial and 21 Hour Profile.................... 45

3.8 Profile Recovery Trends After the Storm..................................... 46

3.9 Comparison of Prestorm and Poststorm Profiles ............................................ 47

3.10 Volume Transport for Case B4...................................................................... 49

3.11 Profile Evolution of Case B ............................................... ....................... 51

vi

3.12 Profile Evolution of Case B2................................................................. 52

3.13 Profile Evolution of Case B5................................................... .................. 53

3.14 Profile Evolution of Case B6................... ............... 53

3.15 Profile Evolution of Case B10 .................................................. .. ... ......... .... 55

3.16 Volume Transport of Case B10..................... .................. ....... .................. 56

3.17 Profile Evolution of Case A2........................................... ........... 58

3.18 Profile Evolution of Case N 1........................................................ ................... 60

3.19 Profile Evolution of Case N2.......................................................................... 60

3.20 Prestorm and Poststorm Profiles of Case N1................................ ................. 62

3.21 Prestorm and Poststorm Profiles of Case N2............................................... 62

3.22 Volume Transport of Case N1............................................. .... ...................... 63

3.23 Volume Transport of Case N2.................................................................. 64

3.24 Comparison of Volume Transport, Cases B3 and N2............... .... ........ 65

3.25 Comparison of Volume Transport, Cases B4 and N2....................................... 65

3.26 Profile Evolution of Case Fl.................................................................... 69

3.27 Profile Evolution of Case F2......................................................................... 70

3.28 Profile Evolution of Case F3............................................................ ................ 71

3.29 Profile Evolution of Case F4.......................................................... ................... 71

3.30 Profile Evolution of Case F5......... ........................................... ................. 73

3.31 Profile Evolution of Case F6........................... ......... ........................ 73

3.32 Overtopping Flux for A-, B-, and R-Series Tests ............................................ 75

3.33 Overtopping of C-Series Tests.................................................. ................ 78

3.34 Wave Trace in Front of Seawall From 0 to 6 m......................... .......... 81

3.35 Continued Wave Trace in Front of Seawall From 6 to 12 m............................. 81

LIST OF TABLES

Table Page

2.1 Scaling of the M odel........................................................... ................... 22

2.2 Experimental Conditions............................................................................. 27

3.1 W ave R eflection........................ ... ......................................................... 82

3.2 Scour Depths and Beach Recession............................................................... 84

LIST OF SYMBOLS

A Antinode height of partially standing wave envelope

B Node height of partially standing wave envelope

C, Reflection coefficient

C, Dimensionless overtopping coefficient

d, Water depth at seawall toe

ds5 Median sediment grain size

E Elevation

F Freeboard, vertical distance from still water level to seawall crest

F' Dimensionless freeboard

g Acceleration of gravity

HI Incident wave height

H-o Zero-moment wave height

Ho Deep water wave height

H, Reflected wave height

L Wave length

m Model parameter, subscript, or mean

max Maximum, subscript

N Ratio of model parameter to corresponding prototype parameter

p Prototype, subscript

Q Overtopping volume flux (m3sm'1)

Qo Overtopping volume flux coefficient (m3s-'m')

S Storm surge or scour depth

T Wave period or time scale

W Sediment fall velocity

WL Water Level

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 AND SEAWALL INTERACTION
DURING SEVERE STORM CONDITIONS

By

Paul L. Miselis

December 1994

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

An investigation into storm effects on beach profile evolution patterns in the

presence of a vertical seawall is described. Although the test configuration was based on

an existing seawall in Highland Beach, Florida, the test results may also be applied to

similar seawall configurations. The seawall fronts the Atlantic Ocean where there is a 40

m wide dry beach in front of the wall during calm wave conditions. This study is based on

laboratory tests that subject the seawall to normally incident wave conditions and storm

surge levels typically generated by a 100-year return period hurricane. Variations of the

seawall tests include several seawall crest elevations, seawall failure, and the addition of

toe protection. Both regular and irregular waves were run for most test cases considered.

Features of the analysis include wave overtopping, wave reflection, scour around the

seawall, profile evolutionary trends, sand bar formation, and beach recession.

Generally, the beach profile exhibited erosive trends throughout the duration of the

storm surge. The evolution of the profile may be broken down into three main parts: (1)

pre-peak storm surge profile development, characterized by the formation of reflection

bars near the seawall and a break-point sand bar farther off shore; (2) peak storm surge

profile development, characterized by scour hole development at the seawall toe and the

smoothing-out of the sand bars; and (3) post-peak surge profile development,

characterized by the redevelopment of the break-point bar and the filling-in of the toe

scour hole. The development of break-point bars occurs most dominantly for regular

waves and is associated with times of near-constant water levels such that the breaking is

fixed at a particular point.

The beach recession in front of the seawall was found to be on the order of 15 m,

but the level of erosion behind the seawall was relatively small by comparison. Wave

overtopping of the 104 m long seawall during the peak storm surge is about 0.001 m3/s.

The results indicated that wave overtopping rates during the peak of the storm could be

reduced by 50% if the present seawall crest elevation was increased by 50 cm, and could

be entirely eliminated if the seawall crest was raised by 90 cm. The expected scour hole

depth at the seawall toe is about 2 m, reaching a maximum during the peak storm surge

and subsequently filling back in as the storm surge recedes. Wave reflection in front of the

seawall may be expected to be about 25% to 30%. The addition of rubble toe protection

reduced wave reflection by about a factor of 2 and also reduced the erosion behind the

seawall. Severe profile erosion -- about 20 m of berm erosion behind the seawall -- may

be expected should the seawall fail during the storm.

CHAPTER 1
INTRODUCTION

1.1 Need for Study

A seawall as defined here is a shore-parallel coastal hard structure built with the

intent to provide protection to upland properties or structures against natural forces,

particularly wave attack. In many cases, coastal protection methods have employed the

use of seawalls to reduce damage from major storm events. Additionally, seawalls have

been used as a measure to eliminate further erosion of properties fronting beaches.

According to some researchers, the existence (or addition) of a seawall reduces the

erosional threat due to a major storm event in front of beach front properties. However,

the effects of a storm event on the beach in front of a seawall are not well understood and

only rough predictions of the effects can be obtained from computer models.

Furthermore, many previously published reports are limited to generic, non-site-specific

seawalls and, thus, are good for qualitative results only. There is also a lack of field

documentation regarding seawall and beach interaction during storm events. Therefore,

an investigation into storm effects on beach profile evolution in the presence of

site-specific seawalls is required to achieve a better understanding of the local beach

response.

This study describes a model investigation of a 100-year return period storm and

its effects on the beach profile's accretional and erosional trends in the presence of a

vertical seawall. Specifically, a seawall located in Highland Beach, Florida, was selected

as representative of such structures in a study to address recent concerns regarding the

re-establishment of the Coastal Construction Control Line (CCCL) in Palm Beach County.

Special attention was given to beach profile evolution, dune erosion (erosion behind the

seawall), seawall elevation, wave overtopping volume, wave reflection, and seawall toe

scour. Additionally, the effects of seawall toe protection were examined for a case in

which maximum scour was encountered. These data were then used to identify the trends

in beach response during the 100-year storm. Several seawall failure cases (including

total, half, and three-quarter failure) were also examined to determine the extent of

erosion behind the seawall should a failure occur. Further, the model results were

extended to the prototype scale in order to provide an idea of what might actually be

expected if such a storm were to occur in the area of Palm Beach County. The results of

the study were then compared to the recommended location of the CCCL to examine the

feasibility of relocating the line. The model results are believed to be reliable even though

field data sets of beach surveys during the course of a 100-year storm are not available for

comparison.

1.2 Description of the Study Area

Highland Beach, Florida is located on the southeastern coast of Florida fronting

the Atlantic Ocean. Highland Beach is a barrier island resort community located just north

of Boca Raton and is separated from mainland Florida by the Intracoastal Waterway. The

width of the island at this point is about 200 m and the maximum dune elevation is about 5

m above mean sea level (USGS Topographic Map, Delray Beach, FL, 1986). The

o3$.
4,. /3

Figure 1.1 Location of the Study Area

s*
O0
C16

C%

seawall itself is located in the northernmost part of Highland Beach at the Department of

Environmental Protection (DEP) monument R-192 in Palm Beach County. See Figure 1.1

for the study area location. The seawall is vertical, about 104 m long, and is a concrete

sheet pile with steel reinforcement. The sizable building protected by the seawall is

located approximately 6 m behind the structure at the closest point. The beach in front of

the seawall consists of fine sand and broken shell (Dean et al., 1992).

1.3 Review of Past Work

Seemingly all the research regarding seawalls can be divided into five major topics:

(1) water level and wave conditions, (2) coastal processes and geotechnical stability, (3)

hydraulic aspects of design, (4) forms of construction, and (5) administrative aspects (after

Allsop, 1986). The primary interest of this thesis is actually a combination of the above,

i.e., beach movement under high water levels and severe wave conditions, and the origin

of the project has its roots in the administration of the Coastal Construction Control Line.

Even though seawalls have been researched quite extensively in the laboratory (as

well as in the field), relatively few laboratory studies examined site-specific seawall effects

on the fronting beach profile. Quantitative results on the interaction between beaches and

seawalls are fewer still (Kraus, 1987). Often, the extent of the laboratory studies is limited

to generic vertical seawalls with non-site-specific initial beach profiles and waves forms

not representative of storm conditions. Therefore, the results can only contribute to a

qualitative, although valuable, understanding of seawall and beach interactions. On the

other hand, this project models a very site-specific seawall (in Highland Beach, Florida at

range R-192) subject to a 100-year storm predicted specifically for the Highland Beach

area (Dean, et al., 1992). Furthermore, there is a lack of field data (for obvious reasons)

regarding the evolution of the beach profile during major storm events, although there has

been a number ofprestorm and poststorm field studies conducted.

1.3.1 Review of Past Literature

Several extensive seawall literature reviews have been conducted including one by

Kraus (1987) and a much more exhaustive one by Allsop (1986) in the United Kingdom.

Their reviews cover a very broad range of subjects.

Kraus (1987) presents an extensive summary of the effects of seawalls on the

beach and includes laboratory, field, and conceptual studies. He reviews 70 previously

published papers and concludes that although several trends in beach movement are

identified, there is still a lack of quantitative results. Furthermore, Kraus states that some

laboratory work has been needlessly repeated. Kraus summarizes that the maximum scour

tends to occur "if a seawall is located in the middle to outer third of the surf zone." This

would correspond to a seawall subject to waves during a fairly high storm surge.

Maximum scour tends to be on the order of the incident wave height. Additionally, for

laboratory tests, the equilibrium slope of the beach profile tends to be about the same with

or without a seawall. Kraus also mentions that in high reflection situations, the sand bed

tends to develop an undulating pattern that follows the form of partially standing waves.

These patterns are generally known as reflection bars, but they are not reported in the

field. However, reflection bars were found to readily occur in this laboratory study,

especially for the case of regular waves.

Allsop (1986) performed a much more extensive and general literature search of

seawall topics dealing with a listing of over 500 publications primarily from the United

Kingdom. Allsop includes the following topics relevant to seawalls: administrative

aspects, summary of design and construction practices, construction materials and forms

of construction, water levels and wave conditions, coastal processes and geotechnical

stability, and hydraulic aspects of design.

1.3.2 Seawall Overtopping Studies

Overtopping of coastal structures has been a concern since at least the early 1950s

(Saville and Caldwell, 1953) and has continued as an interest to even some of the largest

research groups including the U.S. Army Corps of Engineers (Ward and Ahrens, 1992).

The aim of much of this research is the development of a general overtopping equation to

predict the overtopping volume flux based simply on the wave conditions and the seawall

configuration. Such is the goal of Tsuruta and Goda (1968), Ahrens, Heimbaugh, and

Davidson (1986), Ward and Ahrens (1992), and others. Other research, though limited in

quantity, includes field research of seawall overtopping, for example, Inman and Jenkins

(1989), and Kriebel, Dally and Dean (1986).

An early laboratory investigation of seawall overtopping due to wind waves is that

of Paape (1960) in the Netherlands. These experiments were concerned with the

performance of sloped, earthen dikes commonly found in the Netherlands. Paape states

that most damage to seawalls is due to overtopping. A common practice in the design of

the seawalls was to construct the seawall such that no more than 2% of the waves overtop

the seawall crest. One goal of this paper was to identify the relationship between the

significant wave height and the overtopping rate for a series of planar sloping seawalls.

The results of the laboratory study indicate that the overtopping rate is primarily a

function of the seawall elevation above the mean water line. Paape also found that

wind-generated waves created more overtopping than regular waves of the same mean

wave height and wave period, thus concluding that the overtopping rate depends largely

on the irregularity of the waves. Additionally, the overtopping rate is a function of wave

height, wave period, wave length, and dike slope.

Tsuruta and Goda (1968) described a laboratory investigation of overtopping of

vertical walls due to irregular waves. The investigation yielded two relations stating that

the overtopping flux is not dependent on the wave period, but is more a function of the

wave height and the seawall crest elevation above mean water. The first relation was

developed for vertical seawalls and the second relation was developed for seawalls with

concrete blocks for toe protection. The authors concluded that the total overtopping of

irregular waves can be treated as a linear summation of the overtopping of each individual

wave as if it were a regular wave.

More recently, Ward and Ahrens (1992) present a general, empirical overtopping

equation that states that the overtopping rate is an exponential function of a dimensionless

freeboard parameter. The dimensionless freeboard is related to the water level, wave

conditions (including wave height and period), and the relative elevation of the seawall

crest. This work is an extension of previous work conducted by Ahrens et al. (1986).

Predictions from the overtopping equation are then compared and fit to the measured

laboratory results for various seawall configurations such as vertical and stepped seawalls

with and without toe protection. Once calibrated to a specific data set, the equation

reasonably predicts the overtopping rate. However, the equation seems to be of limited

use when a "blindfolded" approach is taken, i.e., when trying to predict overtopping

without any prior knowledge of the overtopping rate. The authors also summarize a

danger-level comparison originally presented by Fukuda, Uno, and Irie (1972). For a

person walking 3 m behind a seawall, an overtopping rate greater than about 1.9x10"

m3sl'm was dangerous. Similarly, an overtopping rate greater than about 1.9x10"

m3s' mr' would prohibit high-speed vehicular traffic and that damage to a house would

occur for rates greater than 6.5x105- m3S-lm'. The rates can be increased by a factor of 10

for a location 10 m behind the seawall.

An earlier version of an empirical wave overtopping equation, presented by

Weggel (1976), was based on a series of laboratory tests conducted at the Waterways

Experiment Station in Vicksburg, Mississippi. Weggel concludes that the overtopping is

related to the wave height, wave length, and wave period. However, this work is limited

to sloping seawalls.

A field study by Inman and Jenkins (1989) examines a well-documented

overtopping event of the San Malo rubble mound seawall in Oceanside, California. They

conclude that infragravity waves (surf beat) are an important component in the wave

runup and thus the wave overtopping. Additionally, the combination of high waves and

high water at the crest of the infragravity waves coincided with the overtopping event.

1.3.3 Beach Profile Response Studies

A qualitative description of the sand transport characteristics in the surf zone is

presented by Dean (1973). In this paper, he identifies the longshore and cross-shore sand

transport mechanisms and derives semi-empirical expressions for the transport based on

previously-collected data. These expressions are able to predict reasonably well the

correct onshore or offshore direction of sand transport. From this, one can classify the

waves as. either accretional or erosional. Specifically, Dean's heuristic model of the

expression for bar formation is as follows:

Ho>1, _W
HZ>1.7xt-
Lo < gT

where Ho and Lo are the deep water wave height and wave length, T is the wave period,

and W is the sediment fall velocity through water. When the left-hand side (LHS) is

greater than the right-hand side (RHS), an offshore bar is predicted. A barless profile is

predicted to occur for the opposite scenario (when the LHS is less than the RHS).

In applying Dean's heuristic model to our wave tank models, the expected outcome

is an offshore motion of sediment because the model represents storm conditions which

are erosive in nature. The model's results are as follows:

0.16 > 0.0064
9.81xl.652 < 9.81xl.65
2xx

0.0376 > 0.0012

LHS >RHS, therefore offshore motion of sand and bar formation.

The above suggests that the model's profile evolutionary trends will be more erosional

than accretional. (This is verified in Chapter 3.)

In another paper by Dean (1986), a discussion of coastal armoring and the

associated two-dimensional effects are presented. In the discussion, an approximate

principal of sediment conservation is introduced which states that in a two-dimensional

situation, the volume of scour in front of a seawall will be less than or equal to the volume

of sediment lost from the same profile without the armoring. Dean states that there is a

lack of factual data supporting the notion that coastal armoring causes profile steepening,

increases longshore sediment transport, or slows profile recovery after an erosional event.

However, local scour and increased sediment transport may be associated with the

armoring.

Barnett (1987, 1988) conducted a series of laboratory tests at the University of

Florida investigating the seawall effects on the beach profile response and from this,

identified several trends. Barnett concludes that wave reflection, often thought to be a

major cause of toe scour, actually did not appear to be a significant contribution.

Conversely, the water level appeared to play a major role in the erosional trends. He also

noted that a greater volume of sand was recovered on a seawalled beach than on a

corresponding natural beach, but did not support the idea of placing a seawall on an

eroded beach in the hopes of recovering lost sand to rebuild the beach. Experimental data

indicated that about 60% the volume of sand that would have been eroded from behind the

seawall had it not been present was eroded from in front of the seawall. Recall that Dean's

approximate principle (1986) suggests that the eroded volumes be nearly the same (in a

two-dimensional case). A counterintuitive conclusion is that the seawall's presence had

little influence on major transport processes. Barnett also considered field data but to a

limited extent and with little success due to the lack of an appropriate overlap between the

wading surveys and boat surveys.

A study of beach response in front of a seawall that did include field data, as well

as a number of laboratory model tests, is that of Kriebel et al. (1986). This report

considered the beach recovery process following an erosional event (Hurricane Elena). It

seems that the beach recovery process is much more sensitive to water level and wave

conditions than the beach erosion process. This is because the erosion process is generally

dominated by intense turbulence in the storm surf zone, whereas the recovery process is

dominated by narrow-banded waves that produce a distinct break point. Additionally, the

prominent features of a recovering beach, such as sand bars and berms, tend to form in the

shallow nearshore area or in the intertidal region and generally grow to the limit of the

swash uprush. The field data consisted of 5 profiles at Clearwater, Florida, surveyed 11

times within the first two months after the passage of Hurricane Elena. Two of the

profiles were located in front of an exposed seawall. The authors noted that all of the

profiles developed a swash bar within one or two days after the storm and that it

continued to build throughout the duration of the study. Furthermore, about 72% of the

total volume of sand recovered was transported back to an emergent sand bar within the

first two days after the peak of the storm. This occurred even in the presence of the

seawall on two of the profiles, suggesting that seawalls have little effect on the recovery

process.

1.3.4 Scour Studies

Scour at the seawall toe is often a concern to those designing the structure because

of possible foundation undermining and ultimate failure of the structure. Many

investigators have addressed the problems associated with scour. To avoid redundancy,

only a select few reports are addressed in this review and a limited amount of scour data

are examined in the experiments.

Two thorough summaries of scour prediction methods are presented by Fowler

(1992, 1993). The 1992 report deals strictly with scour in front of vertical seawalls while

the 1993 report encompasses more general coastal scour problems. The vertical seawall

scour prediction methods range from a rule-of-thumb technique to semi-empirical

methods. The rule-of-thumb states that the maximum expected scour depth at the seawall

toe will be less than or equal to the deep water wave height. However, the rule-of-thumb

seemed to work better for irregular waves than it did for regular waves in Fowler's

experiments. Fowler also developed a dimensionless equation based on irregular wave

tests for application to vertical seawalls. The equation states that the scour depth is a

function of the deep water wave length, Lo, the deep water wave height, Ho, and the

pre-scour depth of water at the seawall toe, d,. From Fowler:

Smax 22.72d,4 025
= +0.25 .
Hj7.- Lo

Furthermore, it is recommended that for design purposes, the conservative rule-of-thumb

be used for vertical seawalls. Fowler supported Dean's (1986) approximate principal of

sediment conservation.

An earlier version of a theoretical scour equation was presented by Herbich and

Ko (1968). They addressed scour from the standpoint of shallow water wave theory and

boundary layer equations. In addition to the consideration of wave characteristics as in

Fowler's analysis, Herbich and Ko included wave reflection, boundary layer currents, and

nonvertical seawalls. One immediate limitation is the added complexity of the analysis due

to the extra terms. In contrast, Barnett (1987) suggested that wave reflection has little

influence on the ultimate scour depth. Herbich and Ko also include a theoretical analysis

of reflection bars (termed wave scour in their paper) and attribute their major spacing

influence to be the incident wave length. As in the experiments within this study, the

troughs of the reflection bars developed at the node of the partially-standing wave system

generated by interference of the incident and reflected waves. Similarly, the crests of the

bars developed under the antinode of the partially-standing wave system.

Kadib (1963) conducted a laboratory test considering a range of vertical seawall

elevations and concluded that a higher seawall would generate more toe scour than a

lower seawall which permits overtopping. The tests of this paper were mostly concerned

with the effects of seawall elevation on toe scour and scour behind the seawall. The

greatest toe scour depth was encountered when the crest of the seawall elevation was one

wave height above the still water level, and that the smallest overtopping was encountered

for this seawall elevation. On the other hand, the smallest toe scour and the greatest wave

attack on the area behind the seawall occurred for a seawall elevation of one-half wave

height below still water level. Kadib also found that by increasing the mean sediment

diameter by a factor of four, the scour decreased by about 15%. The mechanism of

sediment transport in the immediate vicinity of the seawall seemed to be a combination of

the wave action and a vortex structure created by the backflow of water over the seawall.

1.3.5 Summary of Literature

In summary, previous laboratory work on seawalls seems to be limited to generic

or non-site-specific studies. However, there are many engineering studies that investigate

site-specific matters other than major storm effects on beach response. Much of the past

research is aimed at local scour at the toe of the seawall. Other efforts include feasibility

studies of seawalls as beach erosion control measures and seawall construction techniques

(which are not necessarily the focus of this report). There is a general agreement on the

part of researchers developing overtopping equations; the overtopping of seawalls is in

some manner dependent upon the wave height, wave period, and the seawall elevation.

Scour is dependent upon wave height and wave length. Additionally, the presence of the

seawall does not significantly alter the recovery process of an eroded beach, yet according

to some investigators, the seawall does affect the total amount of sediment removed from

the area during an erosional event.

CHAPTER 2
METHODOLOGY

This chapter describes the Highland Beach model, the goals and approach of each

test series, and the facilities used to achieve the goals.

2.1 The Facility
2.1.1 The Wave Tank

All of the model seawall tests were conducted in the "Air/Sea Tank" at the

University of Florida's Coastal and Oceanographic Laboratory facility in Gainesville,

Florida. Constructed in 1955 (and updated since then), the wave tank's twin parallel test

sections measure 36.6 m long, 0.9 m wide, and 1.2 m deep. Only one test section was

used in the tests. The tank is equipped with a hydraulically-driven, piston-type

wavemaker and is also equipped with a 25 hp centrifugal fan for the generation of

wind-waves. Refer to Figure 2.1 for a schematic representation of the wave tank facility.

The wavemaker is controlled by a Seasim programmable spectrum signal generator

which is capable of creating irregular waves and regular waves, both of which were used

in the experiments. The Seasim signal generator can also generate a wide range of wave

periods, but the scope of the modeling herein used only two wave periods which were

based on the anticipated sea state typical of a 100-year storm. Opposite the

wave-generating end of the tank is the sand bed and seawall installation detailed below

under the Beach and Seawall heading.

/m"30~I.2rn

I 1 1i 11 11 I I LI i I L

L WAVEMAKER
ACTUATORS

SAND BED 1.WINOWS r.Zmxl.2m \

CROSS- SECTION

LOCATIONS OF SEAWALL
I MODEL

WAVE GAUGE

WAv4m+AKE

WAVE MAKER.

" H('S-HAIFf

HYONo
POWER UNIT

CARRIAGE FOR WAVE
-i WAVE SCREENS /GAUGiF ANn PROFilFR

1 ET

TANK DIVIDER

VARAIN
VALVES

BASIN

79m
I

II
-1.11.1000

PLAN VIEW

Figure 2.1 Schematic of Wave Tank Facility

--30
E
- i
a

S1 1 1 111 1 1 I I' 1 1 1 I I 11 I 1

j- VWINDOWS 0.6 M x 1. 2 M

-

_

I~-~~~ ---~~~ '~~~~ -~ ~~ ~

2.1.2 Wave Data Collection

During the tests, the wave conditions were monitored by two capacitance-type

wave gages connected to the Global Lab data acquisition system. The purpose of the

gages is simply to sense the free surface elevation. The seaward wave gage (Gage No. 1)

is located in deep water 18.3 m (457.5 m prototype) seaward of the Seawall. The second

wave gage (Gage No. 2) is mounted on a motorized mobile cart system that rides on level

rails on top of the wave tank walls. During normal wave data collection, Gage No. 2 is

placed inside the surf zone 5.3 m (132.5 m prototype) seaward of the seawall. The

locations of both wave gages during normal data collection is depicted in Figure 2.1.

Gage No. 2 was also used to record the wave envelope along the length of the

wave tank. Measurement and analysis of the wave envelope yields gives the reflection

coefficient, Cr, which is defined as the ratio of the reflected wave height, Hr, to the

incident wave height, Hi, i.e., C, = H, / Hi. The reflection coefficient, around 30% for a

vertical seawall without overtopping, is a measure of the amount of wave energy

dissipated by the seawall and by other means such as scour and turbulence. The wave

envelope was recorded during the peak storm surge of a run with regular waves. This

was accomplished by starting at the seawall and running the mobile cart seaward at a rate

of about 2 cm/s while simultaneously recording the wave trace from Gage No. 2. The

resulting plot gives a trace of the wave envelope along the length of the wave tank.

Referring to the wave envelope schematic in Figure 2.2, the reflection coefficient is

approximated from the wave envelope as:

Cr = (A-B)
(A+B) Hi

where A is the antinode wave height, and B is the nodal wave height. Additionally, this

method of measuring the envelope gives a record of the wave decay after breaking.

Another way of estimating the wave reflection is to physically measure the amplitude of

the wave envelope at the antinode, A, and at the node, B. In this case, the reflection

coefficient is defined in the same way.

A WL
Sand ------------------------- ---------- -------------

Wave Envelope
SScour Hole

Seawall Sand Reflection Bar

Figure 2.2 Wave Envelope Schematic with Reflection Bars

2.1.3 The Survey Technique

In order to conduct a meaningful series of surveys, a consistent reference

elevation and baseline must be established. This is accomplished by using the mobile

cart system as a gage for the reference distances (see Figure 2.3). The cart rides along of

the wave tank on level rails fixed to the top of the tank walls. One of the rails that the

cart rides on is graduated every two centimeters to provide a reference scale for the

measurement of distance across the beach profile. The cart is equipped with an indicator

from which the cross-shore distance is read. The zero for the cross-shore distance is an

arbitrary baseline and is not related to the baseline monuments set by the Department of

Environmental Protection (DEP) (previously the Department of Natural Resources) in

1971. In the model, the cross-shore distance zero datum is set at the seawall. The cart is

also equipped with a vertical point gage, graduated in millimeters. The point gage was

used to measure the elevation of the sand bed during the surveys. The prototype vertical

datum is the National Geodetic Vertical Datum (NGVD). The model vertical datum is

also referenced to the scale equivalent of NGVD. Beach profiles were obtained by using

the combination of the cart's cross-shore distance indicator and the elevation point gage.

There is a 0.68 m horizontal offset between the point gage and the distance indicator

which is accounted for in the data reduction. For example, the seawall was installed in

front of the 2 m mark on the rail. This corresponds to 2.68 m as read by the cart's

distance indicator when the point gage rests on the seawall. The cross-shore distances

mentioned herein are as read from the cart's distance indicator unless specified otherwise.

There were seven surveys conducted for each test; the initial survey at storm-time

equal to 0 hours, and surveys at 9, 15, 18, 21, and 24 hours prototype, and the final

survey at 39 hours prototype (the end of the storm). Survey data points were taken every

5 cm across the profile up to 4.32 m in front of the seawall, then points were taken every

10 cm thereafter. The sand bed elevation was read to the nearest millimeter. The

surveyed cross-shore distance ranged from 0.5 m to 20 m, covering the length of the sand

bed. Each survey took approximately 30 minutes to complete. Because the profile

changes most rapidly around the peak storm surge, the surveys were concentrated

throughout this period. (The storm surge model is detailed below.)

Elevation Reading 4 Cart
S() Level Rail

Level acrtl Ti tate
Survey Rod

Sand

Figure 2.3 The Cart System for Surveys and Wave Measurement

2.2 Scaling The Model

2.2.1 Modeling Goals

The main goal of selecting the appropriate scaling laws is assuring that the

governing physical phenomenon are modeled accurately. This ensures that the model

may be extrapolated reasonably reliably to the prototype scale, however, it is not always

possible to correctly scale every dynamic process involved. Thus, the most important

processes must be preserved while the less important processes are compromised as little

as possible. For example, in the case of seawall/beach interaction modeling, the focus is

on the preservation of the beach profile evolution characteristics. Therefore, the

preservation of the fluid motion that transports the sand must be maintained in order to

obtain meaningful results in the model. Furthermore, (from Kriebel, Dally, and Dean,

1986) the model should have undistorted horizontal and vertical length scales, and should

use the Froude criterion because of the free surface in water wave problems. Thus,

Froude scaling was chosen for all the seawall tests in the interest of maintaining the beach

evolution characteristics.

2.2.2 Froude Scaling

Kriebel, et al. (1986) present a summary of the Froude scaling law that was used

in these experiments. For the sake of preserving the sediment transport characteristics,

the modeling argument is based on the ratio of the sediment fall velocity in the model,

W,, to the fall velocity in the field, Wp. If a model sand particle is raised to some

elevation, E,, and a prototype sand particle is raised to a corresponding elevation, Ep,

then the ratio of these elevations is the length scale. Furthermore, the ratio of the time, T,

it takes both cases to fall through a quiescent water column to the bottom is the time

scale. Thus, the time scale and the length scale are dependent upon the ratio of the

sediment fall velocities. The Froude scaling law can be written as:

Frm = Frp

Note that the ratio of fall velocities, Nw, is essentially the length scale ratio NE, divided by

the time scale ratio, NT, i.e., N=NE/NT. Thus, the time scale ratio is given by solving for

time, T, in the Froude relation:

NT = JN = JE = Nw.

2.2.3 The Resulting Model Scale

A general rule-of-thumb of physical modeling in coastal engineering states that

the larger the model, the more accurate it can be, thus the better it is. Therefore, the

physical dimensions of the model are limited by the size of the wave tank facility. Based

on the Froude scaling laws and the wave tank size, the resulting model-to-prototype

length scale was selected as 1:25, and thus the time scale ratio is 1:5. This states that 1 m

in the model is equal to 25 m in the field and that 1 minute in the tank equals 5 minutes in

the field. Refer to Table 2.1 for a list of the model and prototype dimensions.

Table 2.1 Scaling of the Model
Length Scale (Model : Prototype) 1:25
Time Scale (Model : Prototype) 1:5
Deep Water Wave Height Model 16.0 cm
Prototype 4.0 m
Wave Period No. 1 Model 1.65 s
Prototype 8.25 s
Wave Period No. 2 Model 1.30 s
Prototype 6.50 s

Two wave periods were considered in the tests: 1.65 s and 1.3 s. The 1.65 s

period was based on an 8.25 s wave of a 100-year storm (Dean, et al., 1992) and was

scaled down according to the Froude scaling laws. The 1.3 s wave corresponds to a 6.5 s

wave in the prototype scale but was used for comparison purposes only. Similarly, the

deep water wave height in the model was 16 cm which was based on a 4.0 m prototype

storm wave. In the case of regular waves, the wave profiles appeared as a sinusoidal

pattern of fixed period and wavelength. The random waves followed a

Pierson-Moskowitz spectrum based around a median wave period of either 1.65 s or 1.3 s

and a maximum deep water wave height of 16 cm (4.0 m prototype).

2.3 Beach and Seawall Models

The model seawall tests described herein are based on an actual seawall and beach

located in Highland Beach, Palm Beach County, Florida. The model beach profile is

based on the Department of Environmental Protection beach profile from 20 February

1991 at range number R-192. Although the beach profile in the field approximately a

smooth curve, it is modeled in the tank by a series of representative straight lines. This is

done for the sake of simplicity and for the ease of hand-leveling the initial model profile

in the wave tank. See Figure 2.4 for a comparison of the prototype profile and initial

model profile.

The prototype seawall is a vertical concrete and steel wall which is modeled in the

wave tank by a one-inch thick plywood board that extends across the width of the tank

and down into the sand to a sufficient depth to provide support without failure, even at

the point of maximum toe scour. Three seawall elevations are considered in the test

series. (Seawall elevation is measured from NGVD to the top of the seawall.) The

elevation of the actual prototype seawall (referred to as Seawall No. 1) measures 5.28 m

above NGVD, corresponding to 21.12 cm in the model. The other two seawall

configurations are identical to Seawall No. 1 with the exception of the elevation. The

elevation of Seawall No. 2 is 22.34 cm in the model, equivalent to 5.86 m prototype. The

elevation of Seawall No. 3 is 19.90 cm in the model (4.98 m prototype). This translates

into a 2.44 cm elevation range in the wave tank or 0.61 m prototype.

Figure 2.4 Prototype and Initial Model Beach Profiles

2.4 Storm Surge Modeling

A combined total storm tide hydrograph has been established by Dean, Chiu, and

Wang (1992) for each of the twenty-four, sandy-beach coastal counties in the state of

Florida. Furthermore, each county is considered as several representative reaches in

which a more site-specific storm hydrograph may be developed. This combined total

storm tide prediction is based on a numerical model of a 100-year frequency storm and

takes into consideration storm surges, astronomical tides, and wave set-up occurring

within the surf zone. The resultant storm tide hydrograph is a smooth curve which is

approximated in the model in a stepwise fashion as shown in Figure 2.5. Each individual

step is labeled as a "run." Each test consisted of all the runs comprising the entire storm

surge hydrograph, thus a test consisted of 13 runs. The stepwise hydrograph model was

required due to the limitations of the wave tank facility. Specifically, simple,

manually-operated gate fill- and drain-valves were not able to provide a smooth, timely

response in water level changes. As previously discussed, the model-to-prototype length

Palm Beach County, R-192
20 February 1991
10
(> 5 ---------------------
z 0

-15
I -10 .--- .--------------^'*~-
-1 . . . . . . . . . . ..
-100 0 100 200 300 400 500
Distance From Seawall (m)

Typical Model Initial Profile
Scaled to Prototype Size
10
S5----------------------
- 0-5______________

S-10
I1 -15 .-......................

-100 0 100 200 300 400 500
Distance From Seawall (m)

scale is 1:25 and the time scale is 1:5. This also applies to the scaling of the storm

hydrograph.

100-Year Combined Total Storm Tide
o Palm Beach Co., Range 186 227 -

E 31 -4-:*-4-- 12

W.. 1 1 I I I I
4 . . .
I I I I I I I I I I I I

0 3 6 9 12151821242730333639
o ,*

STime (hours)
Q.

Prototype --Model

Figure 2.5 Comparison of Prototype and Model Storm Surge

2.5 Test Cases Considered

There were a total of 25 tests conducted for various combinations of seawall

configuration, wave period, or sand grain size while using only one storm surge model as

described above. This was done for the sake of profile evolution comparisons and to

observe the effects of modified seawall configurations. The following listing briefly

describes the purpose of the main test variations. Table 2.2 is a comprehensive

presentation of the test cases examined and the assigned case names.

2.5.1 Tests without A Seawall

The Highland Beach Profile, R-192, was tested in the absence of a seawall

(N-Series tests) to examine the extent to which erosion might occur had the seawall not

been constructed. These tests were also used to check the approximate principal

presented by Dean (1986) that states that in a two-dimensional case, the volume of

erosion is roughly conserved regardless of the seawall's presence or absence.

2.5.2 Effects of Seawall Elevation

By modifying the seawall elevation, different amounts of erosion might be

expected in front of the seawall as well as behind the seawall, especially for a 100-year

storm. These are the base tests (B-Series) and include the model of the actual seawall.

The B-Series tests also serve as a comparison to the seawall failure tests and to the

no-seawall tests.

2.5.3 Seawall Failures

Seawall failures (F-Series tests) were considered solely for the case of the scale

model of the actual seawall (i.e., for Seawall No. 1 at an elevation of 21.12 cm, 5.28 m

prototype). The model seawall was modified for the failure cases by simply cutting

horizontally across the width of the seawall. The modified seawall was held together by

two bar stock aluminum guides until the predetermined time of failure. At failure, the

upper portion of the cut seawall was manually removed from the guides for the remainder

of the test. Six seawall failure cases were modeled by total or partial seawall removal at

the time in the test corresponding to the occurrence of maximum storm surge (3 hours, 54

Table 2.2 Experimental Conditions
Experiment Seawall Elevation Sediment ize Wave Characteristics Overtopping
Name No. 1 No. 2 No. 3 No. 1 No. 2 Wave Type Wave Period Measured
21.12 cm 22.34 cm 19.90 cm 0.18 mm 0.09 mm Regular Random 1.65 s 1.30 s
Al X X X X
A2 X X X X
A3 X X X X X
A4__ X X X__ X X
B1 X X X X
B2 X X X X
B3 X X X X
B4 X X X X
B5 X X X X X
B6 X X X X X
B10 X _X X X
C3 X X X X X
C4 X X X X X
C5 X X X X X
C6 X X X X X
R3 X X X X X
R4 X ______X X X_ X
N1 X X X
N2_____ X X X_
F1 X X X X
F2 X X X X
F3 X X X X
F4 X X X X
F5 X X X X
F6 X X X X

minutes model time or 19.5 hours prototype time). Specifically, three characteristic

seawall failure cases were examined: (1) total failure, (2) half failure, and (3)

three-quarter failure. Total seawall failure was characterized by the complete removal of

the seawall from the wave tank at the time of the peak storm surge. Half failure was

characterized by removal of the top half of the seawall as measured relative to NGVD.

Specifically, the original seawall crest elevation was 21.12 cm, NGVD and was

subsequently reduced to 10.56 cm, NGVD at the time of failure. Similarly, three-quarter

failure reduced the seawall crest to an elevation of 5.28 cm, NGVD at failure. All three

failure types were conducted for both regular and random waves of the same wave period

and deep water wave height.

2.5.4 Seawall with Toe Protection

Seawall toe protection effects were evaluated for the case in which a significant

amount of toe scour was encountered, specifically, the case of regular waves with the

seawall at the actual scale level (21.12 cm). This test, B10, is identical to test B3 except

for the addition of 5 cm diameter rocks at the toe of the seawall. The objective of this test

was to determine how scour patterns, wave reflection, and beach profiles might differ

from the seawall without the toe protection.

2.5.5 Overtopping Volumes
Wave overtopping volumes, often a concern of beach house owners, were

measured in the interest of determining the amount of water passing over the seawall

during peak storm surge periods. (See the B-Series and C-Series tests.) Seawall

overtopping is often expressed in terms of a volume flux per unit length of seawall (i.e.,

m3'm-'). Measurement of the overtopping volume was accomplished by installing a

208-liter catch basin immediately behind the seawall. The test was conducted as usual

and the time required to fill the basin was recorded for each run. If the surge was not

sufficiently high to fill the entire basin during the course of one run, the actual volume

collected was then measured. Overtopping volume was measured for all three seawall

elevations and for both random and regular waves.

2.5.6 Effects of Different Sand Sizes

Two sand grain sizes were considered for the sake of evaluating effects on model

beach profile evolution and seawall toe scour. However, the primary objective of using

the smaller sand grain size was to better approximate the scale sediment fall velocity.

Modeling sand grain size proves to be a great difficulty because cohesive effects are

encountered for grain sizes smaller than about 0.02 mm. Additionally, the basis the

correct modeling approach matches the sediment fall velocity parameters. The first grain

size, measuring 0.18 mm median diameter, was used in the A-Series experiments only.

The fall velocity of this sediment through 20C water is about 2.2 cm/s. The remaining

tests were conducted with 0.09 mm sand which approximated the scaled sediment fall

velocity better than the coarser sand. The fall velocity of this sediment is 0.64 cm/s. The

size distribution for both sand samples is shown in Figure 2.6. In all test cases, the sand

size distributions were nearly uniform across the length of the model beach profile.

Figure 2.6 Sand Grain Size Distributions

2.5.7 Effects Of Different Wave Periods

The overtopping volume flow rate is dependent upon the wave period as well as

the storm surge level relative to the seawall elevation. An overtopping equation proposed

by Ahrens, et al. (1986) indicates that longer wave periods generate smaller overtopping

fluxes. Thus, the purpose of altering the wave period was to investigate such differences.

The two wave periods considered were 1.65 s (8.25 s prototype) and 1.3 s (6.5 s

prototype).

2.6 Advantages And Disadvantages Of Modeling

The use of a wave tank model has several advantages over conducting a field

experiment. The most obvious advantage in this case is the ability to obtain accurate

beach profiles during the course of a raging 100-year storm. Other advantages include

Sand Grain Size Distribution
1

0.8-----------
0.7 ---- -- ---------------------------------
.0.6 ---- --- --------------------------------
S 0.5 .---- --- -------------------------------
S0.7

0.54 ---- -----------------------------

-5 0.3 .----- ---- -----------------------------
E 0.3 ---

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Diameter (mm)

-d50=0.18 mm-d50=0.09 mm

the ability to easily control the wave conditions, the storm surge level, seawall

configuration, and the initial beach profile. Furthermore, it is possible to stop the waves

in the model while a beach profile is taken at a predetermined time during the storm.

The primary disadvantage of using a model is the impossibility or impracticality

of exactly modeling all relevant dynamic processes. However, the compromises are

small considering the advantages. The classic example in modeling with a moveable bed

is the difficulty of scaling down the sand grain size or fall velocity while maintaining

similarity for all other pertinent dynamic properties. If an attempt was made to scale the

sand grain median diameter linearly by down-sizing, the Highland Beach grain size

would have to be reduced 25 times from about 0.17 mm to 0.0068 mm. In this case,

problems would occur due to cohesion between the sand particles. Therefore, as

previously discussed, similarity is maintained by basing the time and length scales on the

ratio of sediment fall velocities (Kriebel, et al., 1986).

Another less obvious limitation in the model is the assumption that the beach

profile has a uniform, cross-shore sand grain size distribution. On the other hand,

prototype profiles typically have coarser sand closer to the beach and finer sand farther

offshore. This, however, probably has little consequence on profile evolution

considering the energetic nature of a 100-year storm which in some cases can move even

the largest of rocks in a jetty.

An advantage of using a model study would seem to be the scaled-down storm

time. However, this may not be the case. Although the actual storm time in the tank is

only one-fifth of that in the field (8 hours model time and 40 hours prototype time), it still

32

requires at least 16 to 18 hours to conduct the entire experiment (one test), including time

for leveling, surveying, and filling the tank with water.

CHAPTER 3
RESULTS AND DISCUSSION

This chapter describes and quantifies the test results and general trends including

beach profile evolution, overtopping volume fluxes, and wave effects regarding the

Highland Beach, Florida, R-192 seawall fronting the Atlantic Ocean during a 100-year

return period storm. Frequently occurring features in this analysis include the

development of a pronounced break-point sand bar, a scour hole at the seawall toe,

reflection bar occurrences around the peak storm surge, beach distance retreat, and the

peaks in the volumetric transport corresponding to such characteristics.

3.1 The Basic Profile with Seawall

This section describes the results of model tests (B3 and B4) used as a standard of

comparison throughout the experiment series. The primary objectives are to identify

profile evolutionary trends, and to relate the trends to what might be expected on the

prototype scale. Additionally, beach recession due to severe storm conditions is

considered. Other items of note include the development of sand bars and seawall toe

scour. The first test discussed is Case B3 which used the scaled-down version of the

Highland Beach seawall (Seawall No. 1), and regular waves with a 1.65 s period and 16

cm deep water wave height. The second case is B4 which is the same as B3 except that

irregular waves were used with a maximum deep water wave height of 16 cm. Both B3

and B4 are run with Sand No. 2 (0.09 mm diameter). All of the profile elevations are

relative to NGVD on the model scale unless otherwise noted.

3.1.1 Profile Evolution of Case B3

All seven profiles for B3 are shown in Figure 3.1 with a vertical offset for clarity.

Each profile is shown with the corresponding NGVD for reference. The associated

prototype time of the profile survey is listed on the right-hand side. The general evolution

of the profile shape is readily seen when all the surveys are presented together. In Figure

3.1, a prominent feature is the development of a sand bar about 14 m seaward of the

seawall. The development starts from the initial profile (0 hours) up to about 15 hours.

The bar is then smoothed out from 18 to 24 hours. Subsequently, the bar redevelops

between 24 and 39 hours. Not unexpectedly, the break point of the regular waves

corresponds to the location of the bar. The peak storm surge level occurs between 18 and

21 hours (refer back to Figure 2.5) so the profile is expected to experience the most

erosion (scour and beach recession included) during this time. However, the sand bar is

smoothed during this time, probably due to the fact that the waves are breaking closer to

shore, thus eliminating the sustaining mechanism of the bar. The sand bar redevelops only

after the water level starts to recede (after 21 hours). The time of development of the

sand bar also corresponds to extended periods of elevated water levels. Since the break

point is generally dependent upon water depth, the maintenance of a relatively constant

water level allows the sand bar more time to develop.

Figure 3.1 Profile Evolution of Case B3

Seawall toe scour changes are also readily apparent in Figure 3.1, and as expected,

the maximum scour occurs during the peak storm surge (profile at 21 hours). (The depth

of scour will be discussed later with references to Figure 3.2.) The item of note here is

that toe scour increases to a maximum with the maximum storm surge level and then

partially fills in when the water recedes, but does not fully recover to its original elevation.

This is probably due to the fact that the surge level falls rapidly after the peak, thus not

allowing the waves enough time to completely fill the scour hole. An ideal situation for

totally filling the scour hole requires that the limit of the maximum wave runup just barely

reaches over the seaward limit of the scour hole. In any case, the sand from the scour hole

is not lost from the system but can be accounted for in the accretion in the beach profile at

about 3 m seaward of the seawall or in the formation of offshore sand bars.

Profile Evolution of Case B3
2

E" 1.50 r

S. .15

24
39
-0.5 .. .. . .
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Another feature of interest in Figure 3.1 is the beach recession and poststorm

recovery of the beach profile relative to NGVD. While the beach/NGVD intersection is

located fairly well seaward of the seawall in the initial profile (prestorm condition), it is by

no means a fixed point. The intersection is seen to move shoreward during the increasing

storm surge and, once the water starts to recede after 21 hours, some recovery is seen to

occur in the 24-hour profile. Additionally, Kriebel, at al. (1986) monitored a Gulf of

Mexico beach for 5 weeks after the passing of Hurricane Elena and found that of the total

volume recovered, about 70% of the beach returned within 48 hours after the peak storm

surge level. Therefore, the model beach may be expected to continue to recover after the

storm (after 39 hours) even though this was not physically tested in the wave tank.

On the other hand, Figure 3.1 indicates the opposite of what might be expected.

For example, the beach/NGVD intersection moves seaward (not landward) during the

peak of the storm. (Compare the profile at 18 hours to that at 21 hours.) Similarly, in the

39-hour profile the beach/NGVD intersection moves landward (not seaward) after the

storm. (Compare profiles at 24 hours and 39 hours.) This is contrary to what is expected

and can be explained by considering sand volumes rather than the profile intersection with

NGVD. For example, the volume of sand may actually be increasing above the water line

although the beach/NGVD intersection is receding. The volume transport of sand at and

after the peak storm surge is discussed below with reference to Figure 3.5.

Figure 3.2 shows the elevation change of the 21-hour profile relative to the initial

profile. Erosion is seen to occur from the seawall toe out to about 2 m seaward of the

seawall. This erosion is associated with the seawall toe scour. The actual toe scour depth

is about 11 cm, which fits in with the rule-of-thumb which predicts that the maximum

scour depth will be less than or equal to the deep water wave height (Fowler, 1992).

Fowler also presents an empirically derived equation for the prediction of scour at a

vertical seawall as follows:
Smax /22.72d,
7_0- 2=.7 +0.25,
Ho / Lo '

where Sm, is the maximum depth of scour, Ho and Lo is the deep water wave height and

wave length, and d, is the water depth at the seawall toe. Using Ho=16 cm, Lo from linear
gTz 9.1(1.65)2
wave theory = = 1 = 425 cm, and d,=4.5 cm, the predicted maximum scour is

S, = 11.2 cm. As previously stated, the actual seawall toe scour depth for Case B3 is

about 11 cm, or only 2% smaller than Fowler's prediction.

Figure 3.2 Profile Elevation Difference Between Initial and 21 Hour Profile

In Figure 3.2, there is also a slight accretional trend between about 2 m to 4 m

seaward of the seawall, suggesting that the sand lost from the seawall toe may have been

deposited in this region of the profile. Furthermore, there is an amount of smoothing of

the humps in the initial profile that is probably associated with the waves reworking the

Elevation Diff. from Initial Profile
Case B3 After Max Storm Surge
0.4
' Elevation

> 0 .2 --------- ----. ......... ....... . D iff r ...

-0.4
-0.6 .
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance from Seawall (m)

-Initial Profile 21 Hour Profile

profile into an equilibrium shape. Accretion at the very end of the profile (at about 16 m

and seaward) is attributed to the sand sliding down the steep slope to the wave tank

bottom. Because the main interest is in the profile closer to the seawall, the sliding effects

at the termination of the profile are of little consequence to the rest of the profile. As

stated previously, the beach/NGVD intersection moves seaward (not landward as

expected) at the peak of the storm. The intersection point is about 2 m seaward of the

seawall. In Figure 3.2, there is a general erosive trend from the seawall out to about 2 m.

Therefore, the profile is in fact losing sand in this area even though the NGVD intersection

point moves seaward.

Figure 3.3 is a plot similar to Figure 3.2 because it displays changes in the profile

elevation. However, in Figure 3.3 the elevation changes in question occur after the peak

storm surge. Specifically, the elevation difference between the 21 and 24 hour profiles

and between the 24 and 39 hour profiles are examined. The objective of this plot is to

show beach recovery trends. For instance, the filling-in of the scour hole at the seawall

toe is easily seen and appears as a positive (deposition) spike in the heavy line. The results

also indicate that landward of the seawall, the profile changes are relatively small as

compared to those seaward of the seawall. The landward migration of the beach/NGVD

intersection at the end of the test can also be explained with Figure 3.3. Even though the

landward migration is the opposite of what is expected, the beach is accreting from the

seawall to about 2 m. (Examine the thin line representing the elevation difference between

the 24 hour profile and the 39 hour profile.) Another prominent feature in the plot is the

erosional spike of the thin line at about 14 m corresponding to the development of the

plunge-point trough landward of break-point sand bar. Additionally, there is an erosional

trend prevalent in both profiles from about 7 m to 10 m from the seawall and an

accretional trend seaward of 10 m (with the obvious exception of the plunge-point trough

development).

Profile Recovery Trends of Case B3
S Elevation Changes After Storm Surge
% 0.04

0.02 ----------------- --------------- -- -----
O 0
0

-0.06
2-0.06 --- i-- ---- i1111 -- --+------
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

-21-24 hr Diff-- 24-39 hr Diff

Figure 3.3 Profile Recovery Trends After the Storm

Referring to the prestorm and poststorm profile comparisons in Figure 3.4, it is

evident that the humps in the initial profile are leveled to some extent and that the profile

seems to be taking on a smooth, equilibrium-type shape. Furthermore, similar trends in

the profile shape are recognizable in Figure 3.4 as they were in Figure 3.2. For example,

there is a persistent erosional trend between the seawall toe out to about 2 m, and there is

the development of the sand bar at about 14 m. It is important to note that the generation

of an offshore sand bar is often associated with storm conditions and that the bar material

is usually expected to migrate back onshore when the storm waves subside. However, the

test was not run in the tank for conditions extending beyond the storm surge. The beach

retreat at the end of the storm is about 10 cm, equivalent to 2.5 m on the prototype scale.

A 2.5 m retreat is probably small for the prototype case and probably should be on the

order of tens of meters.

Basic Seawall, Case B3
Pre- and Poststorm Profiles

-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance from Seawall (m)

-Time = 0 hrs -Time = 39 hrs

Figure 3.4 Comparison ofPrestorm and Poststorm Profiles

By examining the beach evolution process in terms of volume of sand lost or

gained in a particular area, a better appraisal of the quantity of sand movement can be

made. In the ideal two-dimensional case of profile evolution in a wave tank, a change in

the volume of sand is directly related to a change in the elevation of the profile. In this

0.4

0.2

0

-0.2

-0.4

_n

Elcvafion
Difference

case the volume change or transport is given in terms of a sand volume flux per unit length

of beach. The resulting units are m's-f'. In other words, the elevation change across an

incremental span of the profile represents the gradient in volume transport across that

same span for a given time step and cross-shore length of beach. A disadvantage to this

analysis is that it does not show beach recession lengths in front of the seawall, however,

the volume of sand taken to or from a particular area along the profile is readily illustrated.

In the volume transport analysis, a positive transport represents offshore sand movement

whereas a negative value represents onshore sand movement.

Figure 3.5 presents the volumetric transport of sand for the 4 surveys spanning the

maximum storm surge (from 3 hours (prototype) before the peak surge to the end of the

storm at 39 hours). Notable features include the relatively small transport values behind

the seawall, the generally increasing transport followed by a decreasing transport across

the profile, and the relative positions of the local maxima in transport. In the plot of the

15 to 18 hour transport, a local maximum slope occurs near the seawall toe and again at

13 m seaward of the seawall. The first maximum slope (positive) corresponds to the

erosion of the sand berm in front of the seawall and the second maximum slope

corresponds to the location of the break-point sand bar. The movement of sand at these

locations is also evident by referring back to Figure 3.1. Additionally, the minimum

transport between 7 and 10 m suggests that the sand in this area is moving shoreward.

The positive trend of the transport shows that the sand is generally moving offshore. An

offshore movement, as discussed in Chapter 1 with reference to Dean's heuristic sand

transport model (1973), is not unexpected because the storm surge is increasing to the

maximum at 18 hours. Further, a generally offshore trend might be expected while the

water level is elevated during the storm. Though the magnitudes are different, the general

shapes of the two curves spanning 18 to 24 hours are similar suggesting that the sand

transport characteristics across the length of the profile are alike around the peak of the

storm. The largest transport values occur during the three hours following the peak storm

surge at a location two-thirds of the way across the profile.

Volume Transport for Case B3

o 0.06

S0.04 -----------------------
0.02 ---------- -- ------------- ---------

S-0.02
0 V

a~ -0.04 -------------
o -0.04 -------- -------------------------------------- --------
E
S2 -0.06 .. .. . . . .
> -4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

-15, 18 hrs- 18, 21 hrs-21, 24 hrs--24, 39 hrs

Figure 3.5 Volume Transport for Case B3

Because this is a wave tank test, the sand mass in the tank is conserved, thus, the

plot of the volume transport across the profile must start and end with a zero value

(assuming a constant sand porosity). This is termed as closure. However, the lack of

closure at the offshore portion of the profile is simply attributed to the early termination of

the beach profile data in which the surveys did not extend to the end of the active beach.

3.1.2 Profile Evolution of Case B4

In contrast to Case B3, Case B4 uses irregular waves with a maximum deep water

wave height of Ho=16 cm and a mean wave period of Tm=1.65 s. Other parameters

remained the same, such as the seawall elevation (Seawall No. 1), and initial beach profile.

The analysis of Case B4 is presented in the same order as that for B3 for the sake of easier

comparison.

The effects of the irregular waves in Case B4 become apparent in Figure 3.6

where the development of a sand bar is not nearly as prominent as it was for Case B3.

This can be attributed to the wider range of break points associated with irregular waves,

whereas regular waves always broke at a common location. Another interesting item is

the relative lack of reflection bars in the profile during the peak hours of the storm surge

(i.e., 15, 18, and 21 hours). There still is, however, a considerable scour hole at the

seawall toe in the 21 hour profile that is filled with sand by the time of the 24 hour profile.

Beach recession is evident throughout the storm and continues even after the peak of the

storm (after 21 hours). The beach recession for Case B4 is about 70 cm (17.5 m

prototype).

Figure 3.6 Profile Evolution of Case B4

Figure 3.7 presents the 21 hour profile elevation change relative to the initial

profile. There are two main points of interest in the figure. The first point is the toe scour

depth. In this case, the scour depth is about 7 cm (1.75 m prototype), or just less than half

of the maximum deep water wave height, and less than occurred for the regular wave

case. This was found to be consistent throughout the experiment series. The second point

is that the erosion immediately behind the seawall (back to -1 m) is rather small as

compared to the erosion immediately in front of the seawall. The erosion behind -1 m

(-25 m prototype) is associated with the overtopping run-off on a steep end-slope. The

steep end-slope behind the seawall was necessary in the model due to a lack of available

sand, and does not exist in the prototype dune configuration. Therefore, this run-off

erosion should be discounted. In any case, the limited erosion immediately behind the

Profile Evolution of Case B4

S0 hrs

' "/ --- -- s 15
0 0.5 -

21

5 .39
-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

seawall suggests that the structure is an effective means of protection for the land behind

the wall.

Elevation Diff. from Initial Profile
Case B4 After Max Storm Surge
0.4
o Elevation
> 0.2 -------- ---------------------- ---- -----Dif&-
0 /
z 0
0 ------ t-"------------r^ -..__. r------ ^----------
0-0.2 -----

| -0.4 ---------------

-0.6 ,, ,
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)
Initial Profile 21 Hour Profile

Figure 3.7 Profile Elevation Difference Between Initial and 21 Hour Profile

As in the B3 case, the B4 profile also recovers to some extent from the erosion

generated during the peak storm surge. Specifically, in the thick line of Figure 3.8,

representing elevation changes between the 21 hour and 24 hour profile, the significant

peaks at the seawall correspond to the filling-in of the scour hole. Landward of the

seawall, relatively little change in the profile takes place. On the other hand, the elevation

changes between the 24 and 39 hour profiles (the thin line) are fairly significant, especially

in the area between 12 and 16 m from the seawall. Again, this is the area where the sand

bar is formed. Note that in this case, the irregular waves generated a somewhat longer but

lower sand bar than the regular waves did. (Use Figure 3.3 for comparison.)

Furthermore, the profile seems to take on a generally erosive trend (excepting the sand bar

area) as shown by the 24 to 39 hour curve. The elevation changes at the seawall toe are

about the same for both B3 and B4, but in the offshore portion of the profile, the elevation

changes are about twice as great for the B3 case than for the B4 case. As before, the

difference in the profile elevation changes is attributed to the type of wave. Specifically,

the regular waves in B3 generated greater elevation changes than the irregular waves in

B4.

Profile Recovery Trends of Case B4
Elevation Changes After Storm Surge
E
0.04
c0.03 --------------------------------------------- --------------
o
0.0 ---------------------------------------
0.02
0.01
0

S-0.0 ---------------- ---- ---------------- -----
W -0.02
= -0.03
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

-21 & 24 Hrs Diff- 24 & 39 Hrs Diff

Figure 3.8 Profile Recovery Trends After the Storm

The sand bar developed at the end of the storm is evident in Figure 3.9 between 12

and 15 m from the seawall. As in the B3 case, the humps in the initial profile are

smoothed and the profile takes on an equilibrium-type beach profile. The scour hole is

completely filled and its previous existence is not even noticeable. There is about a 70 cm

beach retreat equivalent to 17.5 m on the prototype scale. (Recall that the beach recession

due to regular waves in Case B3 was about 10 cm.) The beach recession is greater for the

irregular wave case than the beach recession for the regular wave case. A 17.5 m

(prototype) beach retreat in Case B4 is probably a more realistic outcome than the 2.5 m

retreat as Case B3.

Figure 3.9 Comparison of Prestorm and Poststorm Profiles

Figure 3.10 shows the sediment volume transport across the profile for Case B4.

Again, a positive transport corresponds to a seaward movement of sediments and a

negative value is a landward movement of sediments. (The corresponding case for

Basic Seawall, Case B4
Pre- and Poststorm Profiles
0.4
Elevation
C Difference
> 0.2 ----------------------
z 0

-0.2 ---------- ---------------------------- --- --------------
-o/-

-0.46 ... ----------------

-0.6
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

--Time = 0 hrs -Time = 39 hrs

comparison is B3 as shown in Figure 3.5.) As in Case B3, the local maximum positive

slopes in Figure 3.10 correspond to sand removal due to scour at the seawall toe between

0 and 2 m seaward of the seawall, and to the development of a break-point sand bar

between 11 and 14 m. A local maximum corresponds to an area of no elevation change.

The transport behind the seawall is insignificant as compared to the volume transport in

front of the seawall, suggesting that the structure is functioning well as protection from

wave attack to the upland properties. The generally negative trend in the transport

between 15 and 18 hours indicates that the sediment is moving landward at this time. The

magnitude of the maximum transport values (0.04 cm3s'cm') is about the same for both

B3 and B4, but the maxima occur at different times during the storm. The location of the

maximum transport slope in Figure 3.10 occurs at 13 m seaward of the seawall, whereas

the maximum of B3 occurs slightly landward. This is probably due to the fact that the

irregular waves of Case B4 do not share a common break point, thus spreading the effects

of the breaking waves over a longer portion of the profile and subsequently changing the

location of the maximum transport across the profile. In general, more sand volume is

transported by the regular waves of Case B3 than by the irregular waves of Case B4. The

lack of closure at the seaward end is due, in part, to termination of the survey before the

physical end of the profile.

Volume Transport for Case B4

0.06
0.05 -------------------------------------- -- ---------------
S0.05 -------- ------

0.03 ----------- ----- --------- ------------------ -------
S0.04
2 0.03 ---------- ------ -------------

a 0.02 --- ---------------- -------- --------

0
E -.0 12 -.------- ----------- .. .....
2 -0.02
> -4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

15, 18 hrs- 18, 21 hrs-21, 24 hr 24, 39 hrs

Figure 3.10 Volume Transport for Case B4

3.2 Variations of the Basic Seawall Tests

The variations to the basic seawall tests discussed below include changes in the

seawall elevation, changes in sand grain size, and the addition of seawall toe protection.

3.2.1 Changes in Seawall Elevation

The following section identifies profile evolutionary trends related to changes in

the seawall elevation. The tests discussed in this section are B1, B2, B5 and B6. The

elevation of Seawall No. 2 is 23.34 cm, NGVD and is presented under Cases BI and B2.

The lowest crest elevation is that of Seawall No. 3 at 19.90 cm, NGVD and is presented in

Cases B5 and B6. The seawall used for comparison is Seawall No. 1 (21.12 cm, NGVD)

and was previously discussed above in Section 3.1. The focus of these tests includes

differences in scour depths, differences in profile evolution, and differences in beach

recession. The expected results are that the highest seawalls generate the greatest toe

scour, partly because of greater wave reflection associated with a higher seawall (Kadib,

1963). Additionally, the beach recession is probably greatest for the cases with the highest

seawalls. However, the general profile evolutionary patterns are probably about the same,

regardless of seawall elevation.

Figures 3.11 and 3.12 present the profile evolution sequences for Cases B1 and

B2, respectively. These tests were run early in the experimental series and are part of the

model calibration, or fine-tuning, of the test configuration. Cases B1 and B2 differ at the

peak storm surge level, otherwise they are essentially the same. The peak storm surge

level for Case B1 is 16.8 cm, NGVD (model), and the peak level for Case B2 is 14.4 cm,

NGVD. The storm surge hydrograph of B2 is that used throughout the rest of the

experiments and was presented in Chapter 2, Figure 2.5.

Referring to Figure 3.11, the beach recession for Case B seems to be a continual

process where the largest beach width (relative to the NGVD intersection) is associated

with the initial profile and the narrowest beach width is associated with the 38 hour

survey. The distance of the beach retreat is about 0.4 m model or 10 m prototype.

Similarly, the beach recession for Case B2 (Figure 3.12) is about 0.5 m model or 12.5 m

prototype.

Profile evolutionary trends for both B1 and B2 are similar. For example, features

such as the development of the break-point sand bar and the development of a scour hole

at the seawall toe in the 21 hour survey are evident in both tests. These features are also

found in Case B3 (Figure 3.1). Additionally, reflection bars form across the profile around

the peak storm surge (from 15 to 21 hours). The reflection bars form during the time that

the storm surge level intersects the exposed seawall front.

As in Case B3 (the middle seawall elevation), the scour hole develops during the

highest storm surge (18 to 21 hours) and subsequently fills in as the storm surge recedes.

The depth of the scour hole is 7.6 cm (1.9 m prototype) for Case B1, and is about 7.0 cm

(1.75 m prototype) for Case B2. The scour for case B3 is somewhat greater at 11 cm

(2.75 m prototype). This is opposite of the expected outcome in which a higher seawall

was expected to generate greater scour. According to the rule-of-thumb (Fowler, 1992),

the maximum scour depth will be less than, or equal to, the deep water wave height

(H=16 cm in this case). The measured scour depths stated above do match the

rule-of-thumb and are actually on the order of one-half the deep water wave height for the

cases with the highest seawall. The rule-of-thumb still is valid for the test with the middle

seawall elevation (Case B3), although the scour depth is greater for the lower seawall.

Profile Evolution of Case B1
2 -

0 ho
-- 9

U 12
0 0.5 18
21
S0 24

-0.5 .6
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.11 Profile Evolution of Case B

Profile Evolution of Case B2
2-

S0.5 18
S- 21
0 24

-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.12 Profile Evolution of Case B2

Cases B5 and B6 represent tests with the lowest seawall (see Figures 3.13 and

3.14). For Case B5, the maximum scour depth is 7.2 cm (1.8 m prototype) and the

distance of the beach retreat is 0.6 m (15 m prototype). For Case B6, the scour depth is

10.3 cm (2.6 m prototype) and the beach recession is also about 0.6 m (15 m prototype).

The general profile evolutionary patterns are about the same as those for Case B3. For

example, a scour hole develops at the peak storm surge and fills back in when the surge

level recedes. A well-defined break-point sand bar develops at 14 m seaward of the

seawall. The profile elevation changes behind the seawall are relatively small, although

Cases B5 and B6 are tests with the lowest seawall configuration.

Figure 3.13 Profile Evolution of Case B5

Figure 3.14 Profile Evolution of Case B6

Profile Evolution of Case B5
2

ca 1 0 hrs
- 1 o 15
o0 0.5
18

L------ --_ 24

-0.5 39
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Profile Evolution of Case B6
2

1.5

-4 2 2 46811-------- 1621
Ds 1 a l0 b (hrs

I 0
24
39
-0.5 39
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

In contrast to Kadib's results (1963), the results of this test suggest that the seawall

elevation has little to do with the profile evolution, beach recession, and toe scour depth.

Support to this claim is given by an empirical equation developed by Fowler (1992) for

calculation scour at the toe of vertical seawalls. Fowler's scour equation, previously

introduced in Sections 1.3.4 and 3.1.1 predicts that the maximum scour for these tests will

be 11.2 cm. Fowler's empirical equation simply states that the maximum expected scour is

a function of the wave characteristics and water depth in front of the seawall, and does not

include a term for seawall elevation, explicitly or otherwise. Yet, Fowler's scour equation

seems to work fairly well for test cases with irregular waves.

3.2.2 Addition of Seawall Toe Protection

In many cases in the State of Florida and throughout coastal territories around the

world, the use of some kind of seawall toe protection is not uncommon. However, the

Highland Beach, Florida seawall is one without toe protection. In this experiment (Case

B10), toe protection in the form of granite rock rubble of approximately 5 cm mean

diameter (1.25 m prototype) was added at the base of the seawall modeling the actual one

in Highland Beach. The rocks were laid upon plastic geotextile that was in turn placed

directly on the usual initial profile. The rocks extended 37 cm in the cross-shore direction

from the seawall toe. Wave conditions identical to those ofB3 (regular waves, T=1.65 s,

Ho=16 cm) were chosen because they had previously generated the deepest seawall toe

scour. It is anticipated that because the toe protection affects the scour at the seawall, it

will also affect other matters such as wave reflection and the way that the beach profile

adjusts to the altered wave conditions. Furthermore, the toe protection may also act as a

wave energy absorber, thus the addition of toe protection may reduce the volume of sand

transported, especially near the seawall.

Figure 3.15 Profile Evolution of Case B10

Figure 3.15, shows the beach profile evolution throughout the experiment. The

corresponding plot for comparison is Figure 3.1 of Case B3. As in Case B3, a prominent

feature in the profile is the development of a sand bar at 14 m from the seawall. The

persistence of this feature is attributed to the common breakpoint location of each wave.

Another feature is the development of significant reflection bars between 1 m and 5 m

from the seawall as evident in the 21 hour profile corresponding to the end of the peak

storm surge. Surprisingly though, wave reflection was reduced by about a factor of two,

from approximately 30% in case B3 (without toe protection) to about 14% in case B10

(with toe protection). A reduction in wave reflection would seem to suggest that

Profile Evolution of Case B10
Seawall with Toe Protection
2
2 --:= -'--,,---,-----.-

0 hrs

'0)
" 1. n- o15
00.5 8
>---"q -- 21
WJ 0

-0.5 39
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

development of reflection-related features would be reduced as well. However, this is not

the case. Although there are reflection bars in the 21 hour profile of Figure 3.1, they are

not as well developed as they are in Case B10. Additionally, Figure 3.15 indicates that toe

protection seems to help retain sand in the region immediately in front of the seawall, but

at the cost of a slightly greater beach recession at the end of the storm. Specifically, the

beach recession for Case B10 is 60 cm (15 m prototype), whereas the recession for Case

B3 is 10 cm (2.5 m prototype).

Volume Transport of Case B10
E Seawall with Toe Protection
a 0.08
c 0.06 --------------------------------------- --- ------
E
S 0.04 ---------------------------------- ----------------------
2o 0.02-- ---------------- ---------

0 -

-0.04 .. .. .1,
> -4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

-15, 18 hrs- 18, 21 hrs-21,24 hrs -24, 39 hrs

Figure 3.16 Volume Transport of Case B 10

As previously stated, wave reflection was reduced by approximately a factor of

two. This is probably due to wave energy dissipation during runup onto the rock toe

protection. Because other configurations of toe protection were not tested, it is difficult

to state that such a reduction in wave reflection is a normal outcome with the addition of

seawall toe protection. Quite conceivably, the opposite might happen where wave

reflection is actually increased because toe scour is reduced, thus more wave energy is

available for reflection. (Wave reflection is detailed later in Section 3.6.) Unfortunately,

wave overtopping was not measured for Case B10, but from a visual analysis of the test, it

is believed that the overtopping flux was reduced with the addition of the toe protection.

The volume transport across the profile for various times during the test is shown

in Figure 3.16. One feature of this plot is the relatively small transport in the region

seaward of the seawall toe out to about 0.5 m. This area of the profile corresponds to the

location of the toe protection. Therefore, the sand transport in this area is limited to that

which is carried into this region from offshore.

The results of the seawall toe protection test suggest that the addition of toe

protection with proper wave energy dissipation characteristics benefits the beach in the

immediate vicinity of the seawall by reducing wave reflection and wave overtopping, and

by retaining sand volume in the area. However, slightly greater beach recession may be

expected with the toe protection than without the toe protection.

3.2.3 Changes in Sand Grain Size

Thompson, et al. (1994) of the University of Florida present a comparison of two

identical tests, Cases A2 and B1, differing only in the sediment grain size used in the wave

tank. Case A2 used a quartz sand of d,5=0.18 mm and Case B2 used sand of d50=0.09

mm. Refer to Figure 3.17 for the profile evolution of Case A2 and to Figure 3.11 for Case

B1. Several conclusions were presented:

(1) There was a general seaward transport of sediments in both Case A2 and Case

Bl. This was as expected and is in accordance with Dean's heuristic model (1973) of

cross-shore transport presented in Section 1.3.3.

(2) For Case A2, the beach at the seawall toe recovered after the peak surge level

to an elevation higher than the sand level before the storm. The opposite was true for

Case B 1 where the beach at the seawall toe did not recover to the original sand level.

(3) The maximum scour at the seawall toe was greater for the finer sediments than

for the coarser sediments. Specifically, Case A2 generated a 5.2 cm (1.3 m, prototype)

scour hole and Case B1 generated a 7.6 cm (1.9 prototype) scour hole.

(4) Both Cases experienced erosion at the peak surge level but the erosion trends

started about 1 hour (prototype scale) earlier in B (fine sand) than in A2 (coarse sand).

These results concur with the established fact that, in general, finer sand is more readily

mobilized than coarser sand (within the non-cohesive limits).

Figure 3.17 Profile Evolution of Case A2

Profile Evolution of Case A2
2

-1.5

I 4 1 4 oh 2h.
9
0.5 18
21
1 0o 24
42
-0.5 .. . . . .
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

3.3 Beach Profile Response in the Absence of a Seawall

This section presents and discusses the results of the two model tests (Series N1

and N2) that were conducted for the Highland Beach profile without the protective

seawall. One objective is to observe and quantify any general trends that might occur as a

result of the seawall's absence. This includes erosional or accretional trends, the distance

of beach retreat, and especially the volume of sand transported in the vicinity of where the

seawall is usually located. Additionally, the without-seawall profiles were compared to

the corresponding with-seawall profiles (Test Series B3 and B4) in an attempt to verify the

approximate principal presented by Dean (1986).

Like B3 and B4, the N-Series experiments consisted of two tests run for different

wave types. The N1 test, corresponding to the seawalled Case B3, was run with regular

waves, and the N2 test, corresponding to Case B4, was run with irregular waves. All tests

were run a 1.65 s wave period which, in the case of irregular waves, represented the

modal period. The initial profiles were the same as in previous tests, except that the

N-Series tests did not include a seawall.

3.3.1 Profile Evolution Trends of Beach without Seawall

Figure 3.18 displays the profile evolution for Case N1. The corresponding

with-seawall test for comparison is Case B3 with Figure 3.1. As before, the most notable

feature is the sand bar at 14 m. However, a more critical feature is the 0.7 m (17.5 m

prototype) berm erosion behind the location of where the seawall is usually located.

(Berm erosion is measured as the retreat distance of the beach crest relative to its initial

position.) Since a berm erosion of greater than 6 m prototype may start to undermine the

60

foundation of the building at R-192, it appears that the seawall is required in order to save

the structure from wave attack. Similarly in Figure 3.19, the total recession behind the

seawall's former location increases to 1 m (25 m prototype). Additionally, erosion is

shown in each of the plots in the area of the former location of the seawall toe. While the

maximum erosion occurs at the peak of the storm, general erosional trends are the same

across the profile for both cases as shown in Figures 3.18 and 3.19.

Profile Evolution of Case N1
2

1 0.5 --o- _- --_ is
S0 24

-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.18 Profile Evolution of Case N1

Profile Evolution of Case N2
2

1.5
0 hrs

- 15
0.5- 18;
U) -21
w 0
24
39
-0.5 --
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.19 Profile Evolution of Case N2

Maximum erosion of the beach profile occurs during the peak storm surge level

(18 to 21 hours prototype time). Therefore, most of this analysis is concentrated on the

profiles taken around that time. Typically, erosion tends to occur in the area of the

seawall toe to about 2 m seaward (50 m Prototype) of the seawall. This appears to be

true regardless of whether or not a seawall is present. This is evident in all the prestorm

and poststorm profile comparisons including those of the B3 and B4 Series, and those of

Cases N1 and N2 depicted below in Figures 3.20 and 3.21. The curve of interest in this

case is the elevation difference of the poststorm profile relative to the initial profile. The

elevation difference can be related to the quantity of sand transported to or from the area

in terms of a volume flux per unit length of seawall. Furthermore, in the cases without the

seawall (N1, N2), erosion occurs, as expected, behind the site where the seawall would

normally be located. In contrast, relatively little erosion occurred immediately behind the

seawall in Cases B3 and B4. The beach recession (measured as the recession of the beach

profile/NGVD intersection) in the no-seawall cases is 0.1 m (2.5 m prototype) for Case

N1, and 0.7 m ( 17.5 m prototype) for Case N2. These recession values are the same as

those obtained for Cases B3 and B4, thus implying that the seawall has little effect on the

ultimate distance of recession of the beach at the end of the storm.

No Seawall Case N1
Pre- and Poststorm Profiles
0.4
Elevation
0 Difference
> 0.2 -- ---------------------------- -----...
z0

-0.4 -------

-0.6
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)
Initial Profile 39 Hour Profile

Figure 3.20 Prestorm and Poststorm Profiles of Case N1

No Seawall Case N2
Pre- and Poststorm Profiles
0.4
Elevation
> 0.2 -------------------/ .

-0.2 ----- ---- ----- --------------------------
-0.4

c:-0.2 ---- ------------- ----------------- -- ------- --------
-0

-0.6
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)
Initial Profile 39 Hour Profile

Figure 3.21 Prestorm and Poststorm Profiles of Case N2

3.3.2 Transport Comparison

Figures 3.22 and 3.23 show the volume transport rate of sand across the profiles

for the no-seawall Cases N1 and N2. A feature that is unique to the no-seawall cases is

the spike in the transport at the site where the seawall is usually located, otherwise the

curves follow similar shapes. The similarities include the maximum transport occurrence

63

about two-thirds of the way across the profile and a relative minimum at about 2 to 4 m

seaward of the seawall. The transport spike for both N1 and N2 is on the order of 0.03

cm3s'cmn', and is largest for the time corresponding to the maximum storm surge. The

volume eroded at this location is related to the magnitude of the berm erosion discussed

above.

Volume Transport of Case N1
8 No Seawall, Regular Waves
o 0.08

o 0.04 ---------- ---------------- ---------
0
0.02 -- -- ----- ---- -- -- --------

F--

E-0.04 ........--- ----
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

-15, 18 hrs- 18, 21 hrs-21, 24 hrs-24, 39 hrs

Figure 3.22 Volume Transport of Case N1

For example, the volume transport immediately in front of the seawall's location is greater

for Case N2, much as the berm recession was greater for Case N2. On the other hand, the

volume transport value around the break-point sand bar at 14 m is greater for Case NI

than it is for Case N2. These results also support the notion that irregular waves generate

a greater beach recession than regular waves, probably due to the occurrences of higher

waves in the series.

64

Volume Transport of Case N2
No Seawall, Irregular Waves
E 0.05

0.02
O.0 ---------- ---------------------------- ----------------
0 0.02 ---------- -- ----------------------------- -----------
S 0.03 ----- ----------- ---- ---------
C 0.02 .--------- ----- --------

I. *
> -4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

--15, 18 hrs-18, 21 hrs-21, 24 hrs-24, 39 hrs

Figure 3.23 Volume Transport of Case N2

Figures 3.24 and 3.25 display the difference in the volume transport between the

no-seawall Cases N1 and N2 and the with-seawall Cases B3 and B4. The volume

transport differences are given in model units. The purpose of these graphs is to compare

the effects of a seawall on the volume transport across the length of the profile. The

plotted curves represent the transport of the B-Series subtracted from that of the N-Series,

such that a positive value implies that the transport was greater for the N-series test. The

primary difference in transport is in the area of where the seawall is usually located. In the

with-seawall cases the sediment transport behind the seawall is essentially zero, whereas in

the no-seawall cases the transport is a finite value although of considerable magnitude.

This manifests itself in Figures 3.24 and 3.25 as a positive spike around the usual location

of the seawall.

-0.01
-n n9

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

Volume Transport Differences
I between Case B3 and N1
0.1
E
8 0.08 ------ ------------ -----

0.04
o 0.02 ------- ---- --------- -- -----
| 0.04 -----------------
g 0.02 ----------------------------

0.02

o -4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)
15, 18 hrs-18, 21 hrs-21, 24 hrs-24, 39 hrs

Figure 3.24 Comparison of Volume Transport, Cases B3 and N1

-- Volume Transport Differences
.3 between Case B4 and N2
E 0.03

S0.01 ---------- --------- -- ------------------
A 0.01 --------

0 02 - -- --- -----
2 -.02-------------- ------------ -----------------------
-0.

E -0.03
S -4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)
15, 18 hrs- 18, 21 hrs-21, 24 hrs-24, 39 hrs

Figure 3.25 Comparison of Volume Transport, Cases B4 and N2

In Figure 3.24 seaward of the seawall, the transport differences range between

-0.01 cm3s- cm1 and 0.02 cm3s-cm-' for three of the four curves. The curve for the

sediment transport between 21 and 24 hours is by far the highest and indicates that near

the seawall, the transport is greater for the B3 test. The negative value in the 21 to 24

hour transport at the seawall toe indicates that the transport is greater for the seawalled

case. This dip corresponds to the filling-in of the scour hole generated at the seawall toe

during the peak storm surge. Beyond about 2 m in front of the seawall, the sediment

transport between the 21 to 24 hour profiles is greater for the no-seawall case NI, yet the

remaining curves show a relatively small difference in transport. On the prototype scale,

the transport values are on the order of 12.5 cm3s'cm'. The similarities between sediment

transport quantities for the with-seawall and no-seawall tests suggest that the presence of

the seawall has little effect on the beach profile, except for the area immediately adjacent

to the seawall.

Figure 3.25 indicates that the irregular wave sediment transport is generally greater

for the with-seawall tests than the transport for the no-seawall tests. The obvious

exception is the transport in the vicinity of the seawall where the no-seawall transport is

larger. Additionally, the sediment transport between the 15 and 18 hour profiles is greater

for the no-seawall tests across the entire length of the profile. As in Figure 3.24, the

sediment volume transport differences between the no-seawall and the with-seawall cases

are small, suggesting that the seawall has little influence. These results support Dean's

approximate volume conservation principal (1986). The absence of closure at the offshore

end of the profile is due to the termination of the survey before the end of the beach

profile.

3.4 Seawall Failure

Seawall failure is sometimes a consequence of a violent storm and is usually due to

wave attack causing either toe scour or scour behind the seawall. In the toe failure

situation, the supporting sand is scoured from around the foundation of the seawall

causing failure. In the scenario of erosion behind the seawall, enough sediments are

removed from around the deadman retaining structures to cause the seawall to fail. The

following section describes the model results of three failure scenarios: (1) total seawall

failure, (2) half seawall failure, and (3) three-quarter seawall failure. All of the model

seawalls are assumed to remain intact up to the point of the maximum storm surge (19.5

hours prototype), at which time the seawall fails and remains in the failed state for the

remainder of the test. Profile survey times, wave conditions, and initial beach profiles

were identical to the B-Series tests (the base seawall configuration). Unlike the scour in

the preceding tests, seawall toe scour for the total failure cases was measured at the end of

the 18 hour profile, not at the end of the 21 hour profile. Toe scour for the half and

three-quarter failure cases was measured at the end of 21 hour profile.

3.4.1 Total failure

Total seawall failure is represented by Case F1 (regular waves) and Case F2

(irregular waves). Total seawall failure is characterized by the complete removal of the

seawall at 19.5 hours (prototype) such that no components of the seawall remain within

the area of active sand transport. The original elevation of the seawall was 21.12 cm,

NGVD (Seawall No. 1).

Figures 3.26 and 3.27 display the beach profile evolution for Cases Fl and F2.

The most important feature in these plots is the post-failure erosion behind the former

location of the seawall. Judging from the surveys at and after 21 hours, the resulting berm

erosion distance (as measured from the original location from the seawall) is about 1 m on

the model scale, or about 25 m prototype, for both Case Fl and F2. On the other hand,

the berm erosion with an intact seawall (Cases B3 and B4, Figures 3.1 and 3.6) is

relatively insignificant. The magnitude of the berm erosion distance is fairly consequential

because the distance between the seawall and the building it protects is only about 6 m

(prototype). Therefore, the results of both regular and irregular wave tests suggest that

the building behind the seawall could suffer a significant amount of structural damage due

to beach erosion.

An interesting comparison can be made with the results of the test conducted

entirely without a seawall in place, namely Cases Ni and N2. (Refer back to Figures 3.18

and 3.19.) The berm recession for the no-seawall cases is between 0.8 to 1 m, whereas

the total seawall failure case generated a berm recession of about 1 m. Even though the

berm recessions would be expected to be about the same or greater for the no-seawall

cases because the beach is exposed to wave attack for a longer time, they are, in fact, less

than the berm recessions for the total failure case. This is especially true of the regular

wave cases (Fl and Ni). The differences in the erosional distance may be attributed to the

non-seawalled beach adjusting more readily to an equilibrium-type shape. Additionally,

the no-seawall case initial profile may have been closer to an equilibrium shape. On the

other hand, the seawalled beach during a failure case suddenly exposes a large quantity of

sand to the attacking waves, thus promoting greater erosion. This is consistent

throughout the seawall failure series.

The beach recession (distance of beach profile/NGVD intersection retreat relative

to its initial position) is 0.4 m (10 m prototype) for Case Fl and 0.2 m (5 m prototype) for

Case F2. In contrast, the recession for Cases B3 and B4 is 0.1 m (2.5 m prototype) and

69

0.7 m (17.5 m prototype), respectively. The scour depth (measured at the end of the 18

hour survey, prior to seawall failure) for Case Fl is 3.3 cm (0.83 m prototype). Oddly,

Case F2 showed an accretion at the seawall toe of 3.2 cm (0.8 m prototype). Depositional

sand in the area of the former seawall is considered to be due to the transport of sand from

behind the seawall.

Inter-comparison of the final profiles of the B-, N-, and F-Series shows that they

all share similar features. Specifically, for the regular wave cases (B3, Fl, N1), a well

defined break point sand bar forms at 14 m. The main difference is the profile changes

near the seawall where, in the seawall failure cases, large amounts of erosion occur at the

peak of the storm.

Profile Evolution of Case F1
Total Seawall Failure @ 19.5 hrs
2

S1.5

9

-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From seawall (m)
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.26 Profile Evolution of Case Fl

70

Profile Evolution of Case F2
Total Seawall Failure @ 19.5 hrs
2
1.75
E 1.5 -- 18

0 1.25 ----
O 1 9
07 0.75

0 21
W -0.25 --24
-0.5 39
-0.75
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.27 Profile Evolution of Case F2

3.4.2 Half Failure

Half seawall failure was characterized by the removal of the top half of the seawall

(as measured from NGVD to the top of the seawall). Specifically, the original seawall

elevation was 21.12 cm, NGVD and was subsequently cut to 10.56 cm, NGVD. The half

failure series consisted of Case F3 which was run with regular waves and Case F4 which

was run with irregular waves.

The results of this test include a berm recession of about 1 m (25 m prototype) in

both Case F3 and Case F4. Beach recession for the half failure cases, 0.5 m (12.5 m

prototype) for F3 and 0.6 m (15 m prototype) for F4, was slightly greater than those for

the total failure tests and the base tests B3 and B4. Seawall toe scour was reduced to 1.7

71

cm (0.42 m prototype) for Case F3. Like F2, Case F4 showed an elevation increase of 2.6

cm (0.66 m prototype) at the seawall toe.

Profile Evolution of Case F3
Half Seawall Failure @ 19.5 hrs
2

0.5

-4 -2 0 2 4 6 8 10 12 14 16 18 20

2 -
E1.5- Oh

S 1 15

0.5-
24
ED 0 2
39
-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.28 Profile Evolution of Case F3

Profile Evolution of Case F4
Half Seawall Failure @ 19.5 hrs
2 --------------

E 1 "b r

WJ 0 24--- __4
39
-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.29 Profile Evolution of Case F4

3.4.3 Three-Quarter Failure

For the three-quarter seawall failure tests, the original seawall elevation was 21.12

cm, NGVD and was cut to 5.28 cm, NGVD at 19.5 hours prototype. The resulting berm

recession was again on the order of 1 m (25 m prototype) for both Cases F5 and F6.

Beach recession was 0.2 m (5 m prototype) for F5 and 0.6 m (15 m prototype) for F6.

The scour depth in the three-quarter failure tests, among the greatest of all the tests, was

13.6 cm (3.4 m prototype) for Case F5 and 6.5 cm (1.63 m prototype) for Case F6.

Consistent features of the seawall failure tests include a 1 m (25 m prototype)

berm recession and an average beach recession of 0.4 m (10 m prototype). Seawall toe

scour, on the other hand, varied from 3.2 cm elevation increase to 13.6 cm scour. Beach

profile evolution during the seawall failure cases was similar to the base tests B3 and B4 in

the offshore portion. For example, the regular wave cases developed a pronounced

break-point sand bar near 14 m seaward of the seawall, and reflection bars sometime

developed while the seawall was still intact. Conversely, profile evolution behind the

seawall differed notably due to the seawall failure. In the cases where the seawall

remained intact, very little profile change was observed behind the seawall, whereas the

seawall failure cases were associated with the consistent berm recession, mentioned above.

Figure 3.30 Profile Evolution of Case F5

Profile Evolution of Case F6
Three-Quarter Failure @ 19.5 hrs

S 1.5- 0 oh,

S0- ---- 24
39
-0.5 . . .
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

Figure 3.31 Profile Evolution of Case F6

3.5 Seawall Overtopping Volume Flux

Overtopping of seawalls is often of concern to the property owners located

directly behind the seawalls. Furthermore, a significant amount of the total storm damage

incurred by a beach front structure can be due to inundation, thus the resulting interest in

seawall overtopping. The following section presents the results of the model tests that

Profile Evolution of Case F5
Three-Quarter Failure @ 19.5 hrs
2

E 1.5 "-- "--

"15
0 0.5 18
21
S0 "- 24
39
-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20
Distance From Seawall (m)

measured the overtopping volume flux. Overtopping is usually given as a volume flux per

unit length of seawall (i.e., m'3sm'). The results were then scaled up to the prototype

size. Specifically, there were ten tests in the overtopping series: A3, A4, B5, B6, C3, C4,

C5, C6, R3, and R4. The following analysis is broken down into four parts: (1) the effects

of seawall elevation, (2) the effects of storm surge level, (3) the effects of sand grain size,

and (4) the effects of wave type and period. In general, the overtopping flux tended to be

more dependent upon the storm surge level and seawall elevation than upon wave type or

wave period. Additionally, there seemed to be a threshold storm surge before overtopping

started to occur and that the overtopping flux increased exponentially with the storm surge

level to a point where each wave simply poured over the seawall. This is detailed below.

3.5.1 Effects of Seawall Elevation

Three seawall elevations were considered in the overtopping tests including the

scaled-down model of the actual seawall in Highland Beach represented by Seawall No. 1

at 21.12 cm, NGVD (5.28 m prototype). The other two seawalls are Seawall No. 2 at

22.34 cm, NGVD (5.585 m prototype), and Seawall No. 3 at 19.90 cm, NGVD (4.975 m

prototype). The A-Series tests were run with Seawall No. 2, the B-Series tests used

Seawall No. 3, and the R-Series tests used Seawall No. 1. Table 2.2 presents a

comprehensive list of the test conditions. Each test was run under the same wave period

(Tm=1.65 s), with similar wave conditions (except regular or irregular waves), and with

similar beach profiles. Case A4, however, included the use of small wind-generated waves

along with regular, piston-generated waves.

Overtopping Volume Flux Comparison
I 0.0035
S.-0.4
E.0.0028 ----------------------------------- ------- -

u- 0.0021 -------- -----------------. -- ---
0)A /
S0.0014 --------------------------- --- ---- 0.2 .

| 0.0007 -------_------0.1
o.oooo ------------------------------

o 0.0000 0
9 11 13 15 17
Model Storm Surge Level (cm, NGVD)

--- A3 A4 -*- B5 -a- B6
-- R3 -o- R4 -Ahrens' Eq.

Figure 3.32 Overtopping Flux for A-, B-, and R-Series Tests

Figure 3.32 compares the cases of A3, A4, B5, B6, R3 and R4, and also includes

the empirical Ahrens' overtopping equation. As expected, the overtopping volume flux

decreases as the seawall elevation increases. Specifically, the overtopping flux decreases

from a maximum associated with the B-Series tests (the lowest seawall) to a moderate

overtopping with the R-Series (the middle seawall), and finally to a minimum overtopping

associated with the A-Series tests (the highest seawall). Furthermore, the overtopping

flux increases exponentially with an increase in storm surge. The upper limit of the

overtopping curves corresponds to the point at which the storm surge level is equal to the

elevation of the seawall crest. A similar trend was realized by Ahrens, Heimbaugh and

Davidson (1986) when they related the relative freeboard (the distance between the

seawall crest to the still water level) to the overtopping flux. Additionally, Ahrens et al.

suggested that the overtopping flux is basically a function of the seawall elevation above

the surge level (the freeboard), the wave height, and the wave period. Another

observation from Figure 3.32 is that the regular wave tests (B6, and R3) produced greater

total overtopping volumes than the irregular wave tests (B5, and R4).

On the model scale, the overtopping flux seems small. However, when related to

the prototype scale (Figure 3.32), the overtopping flux is fairly significant. Quantitatively,

the maximum expected overtopping of the entire 104 m long Highland Beach seawall

during the peak storm surge amounts to 0.001 m3/s. According to the results of Figure

3.32, if the overtopping volume flux was to be reduced by a factor of 2 during the peak of

the storm by simply raising the seawall elevation from its present configuration (Seawall

No. 1), the current seawall would have to be raised by about 50 cm on the prototype

scale. Raising the seawall 90 cm would effectively eliminate all wave overtopping during

a 100-year storm.

3.5.2 Effects of Storm Surge Level

As previously stated, the overtopping volume flux is a function of the seawall

configuration, the wave conditions, and especially the storm surge level as is readily

apparent in Figure 3.32. The most obvious trend is the exponential increase in the

overtopping flux with an increase in the storm surge level. Ahrens et al. (1986) present a

simple overtopping equation based on the freeboard, F, the zero-moment wave height,

Ho, measured near the seawall toe, and the local wavelength, L,, at the seawall toe. The

inclusion of the storm surge level is implicit in the definition of the freeboard. The wave

period also effects the wave overtopping. From linear wave theory, a longer wavelength

corresponds to a longer wave period. Thus, a longer period will generate greater

overtopping volume fluxes. With this, Ahrens, et al. defined a dimensionless fieeboard F'

as:
F'- F

The resulting overtopping equation is simply written as:

Q = Qoe(cl),

where Qo and C, are coefficients determined from a regression analysis of the data. The

equation was developed using English units with Q and Qo in ft3 ft-', and the lengths

freeboardd, wavelength, and wave height) in feet. The coefficients that approximately fit

the data in these experiments are Qo=70 ft3sl'Rft and Cl=-30. The resulting overtopping

curve prediction is plotted as Ahrens' equation in Figure 3.32. The main disadvantage to

using this curve is that prior knowledge of the overtopping rate is required before Ahren's

overtopping equation can be used. In other words, the empirical coefficients in the

equation are tailored to fit available data and cannot be used dependably to predict

overtopping in a "blind-folded" situation. However, once Ahren's equation is applied to a

particular seawall, it can probably be extended to other seawalls of similar configurations.

3.5.3 Effects of Sand Grain Size

Referring back to the overtopping curves for the A-Series tests (d5o=0.18 mm) to

the B- and R-Series tests (d50=0.09 mm) in Figure 3.32, there seems to be no appreciable

78

effect of sand grain size on the overtopping flux. However, if extremes in grain size were

compared, for example small rocks versus fine sand, there might be a variation in the wave

characteristics as the waves approach the seawall, thus resulting in different overtopping

rates. The presence of a beach made of rocks might act as an effective wave energy

dissipater and, therefore, reduce the overtopping rate. Furthermore, wave shoaling on

different beach profiles resulting from different sands at different equilibrium profiles may

produce different overtopping rates. In any case, the tests considered herein showed that

there were no appreciable differences in the overtopping volume flux due to a change in

the sand grain size from 0.18 mm to 0.09 mm.

SOvertopping Volume Flux Model Scale
E C-Series Experiments
S0.00030 C5
'0.00025 --------- ----- --- ----------
i 0.00020 -------..-C3.------- -C.------ -
4)

S0.00005 -------------
o.ooooo --
0.00000
( Series Name
j*C3 C4E C5 MC6|

Figure 3.33 Overtopping of C-Series Tests

3.5.4 Effects of Wave Type and Wave Period

According to the data presented in Figure 3.32, the regular waves produced a

greater overtopping volume flux than the irregular waves for the same seawall

configuration. On the other hand, the data in Figure 3.33 suggests the opposite; that

irregular waves generate more overtopping for the same wave period. The regular wave

height in the model is taken as the significant wave height of the storm prediction by Dean,

et al. (1992). By the nature of the waves, the mean wave height of the irregular waves

(5.3 cm) is less than the wave height of the regular waves (16 cm). Thus, the larger,

singular irregular waves may give more overtopping than an individual regular wave, but

when considered as a system, regular waves generate more overtopping. Additionally,

regular waves generate more overtopping because the wave system has a higher total

energy than an irregular wave system of the same mean period and maximum wave height.

Furthermore, Figure 3.33 suggests that longer wave periods (1.65 s) generate more

overtopping volumes than the shorter wave periods (1.30 s). This is in agreement with

what Ahrens et al. (1986) proposed in their overtopping equation (discussed above).

Additionally, greater wave heights produce greater overtopping fluxes. More generally,

wave systems of greater energy will produce greater overtopping rates.

3.6 Wave Reflection

Wave reflection is usually associated with subaqueous features referred to as

reflection bars. The development of the reflection bars probably has little effect on the

actual design of the seawall, however, their presence is affected incident wave forms,

particularly the wave envelope. One such envelope is shown in Figure 3.34 and is

continued over into Figure 3.35. The figures show a trace of the water surface elevation

across the length of the beach profile out to just past the break-point located at 10.76 m

seaward of the seawall. The wave envelope associated with wave reflection usually takes

a form of partially-standing waves where there is a system of nodes and antinodes. The

partially standing wave form starts at the seawall with an antinode and extends seaward.

The distance between sequential node and antinode is equal to one-half wave length. The

reflection bar under the partially-standing wave forms with the troughs under the nodes

and the crests under the antinodes.

The wave reflection coefficient, C,, can be measured from the wave trace or by

directly measuring the nodal and antinodal wave heights along the tank side walls. (Refer

back to Figure 2.2 for a diagram of the wave envelope.) The reflection coefficient is equal

to the difference between the nodal and antinodal heights divided by the sum of the nodal

and antinodal heights as follows:
C = (A-B) Hr
r(A+B) H,'

where A is the antinode height, B is the node height, H is the reflected wave height, and

H. is the incoming wave height. The first node and antinode pair seaward of the seaward

was measured. The reflection coefficient found from the wave trace in Figure 3.34

(regular waves) is approximately 27%.

Another detail that can be obtained from the wave envelope trace is the decay rate

of the wave height after breaking. The break-point is located at 10.76 m from the seawall

and is shown in Figure 3.35. After breaking, the wave height is seen to rapidly decrease

across the surf zone to a minimum at the seawall in Figure 3.34.

Wave Reflection
In Front of Seawall

10
Et
-1------------------------
8 -.. . . .- . .

2

) 4 --- -
-6B A

0 1 2
Distance

I - - - - - - - - - - - - - - - - - - - -

II' I

3 4
From Seawall (m)

5 6

Figure 3.34 Wave Trace in Front of Seawall From 0 to 6 m

Wave Reflection
In Front of Seawall

7 8 9 10
Distance From Seawall (m)

11 12

Figure 3.35 Continued Wave Trace in Front of Seawall From 6 to 12 m

Breakpoint

U
0

.i2

C.)

a)

10 ----L---------------- ----------------

5

S-..-...--......---............- --- 4 -0.

4 r\ . . t . . i

-IU

i i i i I

' ' ' ''L'

15

The reflection coefficients for several tests are listed in Table 3.1 and seems to be

between 25% and 30% for most cases. (As a side note, the D-Series tests were run with a

stepped seawall configuration of a similar seawall crest elevation and a similar storm surge

profile, yet they still yielded comparable reflection coefficients. Every other test listed in

Table 3.1 was run with a vertical seawall.) The test listed as B 10 is the obvious exception,

however, Case B10 was run with the addition of toe protection in front of the seawall.

The reduction in the wave reflection by nearly 50% with the addition of toe protection

suggests a strong correlation between the beach configuration immediately in front of the

seawall and the resulting wave reflection. However, it is difficult to determine if such a

dramatic reduction in reflection is a typical result because only one case with toe

protection was tested. However, it seems reasonable that a modification to the beach

profile will invariably change the wave form in some way. This could appear as a

reduction of the wave reflection or wave runup, or as an increase in the wave energy

dissipation in front of the seawall.

Table 3.1 Wave Reflection

Case Name Reflection (%) Comments

Wave Trace 27 S=14.4 cm
Pressure Test 30 Different Test Series, S=14.5 cm
D3, Run 5 31 Different Test Series, S=12.0 cm
D3, Run 6 26 Different Test Series, S=14.4 cm
B3 30 Base Test Series, S=14.5 cm
B10 14 Seawall w/ Toe Protection, S=14.5 cm

3.7 Summary of Scour Depths and Beach Recession

Table 3.2 lists the scour depth and the beach retreat relative to the initial position

of the beach/NGVD intersection for the test cases discussed in the preceding sections.

Based on the results of this study, the Highland Beach seawall may expect a maximum toe

scour depth of about 2 m in the event of a 100-year storm. The seawall toe scour was

generally greater for regular waves than for irregular waves. On the other hand, scour

depths behind the seawall were relatively small in comparison to the toe scour depths.

Additionally, the distance of beach retreat ( measured from the initial position of the beach

profile/NGVD intersection) will be about 15 m for an intact seawall. A greater beach

recession was generated by irregular waves. Should the seawall fail, the berm erosion

(measured from the initial location of the berm) will be on the order of 25 m behind the

seawall.

Table 3.2 Scour Depths and Beach Recession

Case Name Maximum Scour Depth Beach Retreat Comments
Model (cm), Proto. (m) Model (m), Proto. (m)
Al 3.4, 0.85 Accretion 0.3, 7.5 Seawall 2, Reg. waves
A2 5.2, 1.23 0.3, 7.5 Seawall 2, Reg. waves
B1 7.6, 1.9 0.4, 10 Seawall 2, Reg. waves
B2 7.0, 1.75 0.5, 12.5 Seawall 2, Reg. waves
B3 11, 2.75 0.1, 2.5 Seawall 1, Reg. waves
B4 7.0, 1.75 0.7, 17.5 Seawall 1, Irreg. waves
B5 7.2, 1.8 0.6, 15 Seawall 3, Reg. waves
B6- 10.3, 2.6 0.6, 15 Seawall 3, Irreg. waves
B10 N/A 0.6, 15 Seawall w/ Toe Protection
Fl 3.3, 0.83 0.4, 10 Total failure, Reg. waves
F2 3.2, 0.8 Accretion 0.2, 5.0 Total failure, Irreg. waves
F3 1.7, 0.42 0.5, 12.5 1/2 failure, Reg. waves
F4 2.6, 0.66 Accretion 0.6, 15 1/2 failure, Irreg. waves
F5 13.6, 3.40 0.2, 5.0 3/4 failure, Reg. waves
F6 6.5, 1.63 0.6, 15 3/4 failure, Irreg. waves
N1 N/A 0.1, 2.5 No seawall, Reg. Waves
N2 N/A 0.7, 17.5 No seawall, Irreg. waves

Scour depths behind the seawall for the above cases was on the order of 1 cm (25

cm prototype).

CHAPTER 4
CONCLUSIONS

This chapter assembles and summarizes the test results that were presented in

Chapter 3. The following results are based on a site-specific laboratory study of beach

and seawall interaction during severe storm conditions. The measurements below are

presented in prototype units so that they might be directly applied to the existing vertical

seawall in Highland Beach, Florida.

4.1 Profile Evolutionary Trends

In general, the Highland Beach profile experienced an erosive trend when

subjected to a 100-year storm surge and wave conditions typically generated by a

hurricane-type storm. The process of profile evolution can be divided into three distinct

phases based on the consistent development of specific features in the beach profile.

(1) The first phase occurred before the time of the peak storm surge and was

often characterized by the growth of a break-point sand bar and the development of a

reflection bar system in front of the seawall.

(2) The second stage occurred during the time of the peak storm surge level and

was marked by the development of a scour hole at the seawall toe, and the smoothing-out

of the reflection and break-point sand bars.

(3) Finally, the third phase of profile evolution occurred while the storm surge

receded from the maximum level. The third phase was characterized by the filling of the

85

toe scour hole, and the redevelopment of the break-point and reflection sand bars. The

seawall's presence seemed to have little effect on profile recovery during the third phase

of the storm.

Another general occurrence was that when the water level intersected the dry

beach, a small amount of accretion occurred at the landward limit of the swash zone. On

the other hand, the profile exhibited the most erosive trends when the water level

intersected the seawall. The subaerial features at the end of the storm included a

reduction of the dry-beach width, minor erosion behind the seawall, and considerable

flooding of the properties behind the seawall. This is summarized below.

4.2 Seawall Toe Scour and Beach Recession

The maximum scour at the seawall toe varied with the wave type, but always

occurred during the peak storm surge level. Generally, the toe scour hole was deeper for

regular waves than it was for irregular waves and was on the order of 2 m deep. On the

other hand the scour hole behind the seawall was small by comparison and was generally

on the order of 25 cm deep. The addition of toe protection reduced toe scour to a

minimum of all the test cases considered but did little to reduce the distance of beach

recession.

The beach recession at the end of the storm was measured relative to the initial

beach profile, with the location of the profile/NGVD intersection point as the reference.

Unlike toe scour, the beach recession was generally greater for irregular waves than it

was for regular waves. A beach recession of about 15 m may be expected in front of the

seawall, assuming that the seawall remains intact. If, on the other hand, the seawall fails

MILLISECOND	CLASS.METHOD	MESSAGE
0	sobekcm_page_globals.constructor
0	sobekcm_page_globals.constructor	Application State validated or built
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.constructor	Navigation Object created from URI query string
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.display_item	Retrieving item or group information
0	sobekcm_page_globals.get_entire_collection_hierarchy	Retrieving hierarchy information
0	sobekcm_assistant.get_entire_collection_hierarchy
0	cached_data_manager.retrieve_item_aggregation
0	cached_data_manager.retrieve_item_aggregation	Found item aggregation on local cache
0	item_aggregation_builder.get_item_aggregation	Found 'all' item aggregation in cache
0	system.web.ui.page.page_load (ufdc.page_load)
0	sobekcm_page_globals.constructor.on_page_load
0	html_echo_mainwriter.add_style_references	Adding style references to HTML
0	html_echo_mainwriter.add_text_to_page	Reading the text from the file and echoing back to the output stream
34	html_echo_mainwriter.add_text_to_page	Finished reading and writing the file