Estimated wave energy dissipation by natural and artificial reefs via video imaging techniques

UFL/COEL-96/007

ESTIMATED WAVE ENERGY DISSIPATION BY
NATURAL AND ARTIFICIAL REEFS VIA VIDEO
IMAGING TECHNIQUES

by

Renji Chen

Thesis

1996

ESTIMATED WAVE ENERGY DISSIPATION BY NATURAL AND ARTIFICIAL
REEFS VIA VIDEO IMAGING TECHNIQUES

By

RENJIE CHEN

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

UNIVERSITY OF FLORIDA

1996

ACKNOWLEDGMENTS

I am sincerely thankful and appreciative to my advisor and supervisory committee

chairman, Dr. Robert Dean, for his long-standing inspiration and support. Also, I am very

grateful for his thoughtful discussions relative to my thesis, and much of the theoretical

understanding of this problem which has resulted from these discussions. I would like to

express my appreciation to the members of my supervisory committee, Dr. Robert J.

Thieke, Dr. Hsiang Wang, and Dr. Ashish J. Mehta, for their meticulous review of the

manuscript and innovative suggestions. I would also like to thank Robert Sloop and

Erdman Video Systems for setting up the video monitoring system on the south elevator

service room roof of the Breakers Hotel in the Town of Palm Beach, Florida. I wish to

thank Mr. Viktor Adams and Mr. George Chappell for measuring the camera elevation in

the field. Special thanks go to Dr. Robert J. Thieke for providing the video image

digitizing equipment. Special thanks also go to Ms. Cynthia Vey for assistance with the

final revision. Many thanks go to Mr. Lihwa Lin, Mr. Subarna Malakar, Mr. John Davis,

Ms. Becky Hudson, Ms. Sandra Bivins, and Ms. Julie Wang. This study was sponsored by

the Florida Department of Environmental Protection and the Town of Palm Beach,

Florida.

TABLE OF CONTENTS

ACKNOW LEDGMENTS ........................ ...................... ii

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

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

ABSTRACT ......................................................... x

CHAPTERS

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

2 PROJECT BACKGROUND AND ENVIRONMENT ..................... 3

2.1 Introduction .................................................. 3
2.2 Description of the Artificial Reef Project ............................ 4
2.3 Profile Survey Results ...................................... .... 6
2.3.1 Definition of Study Zones .................................. 8
2.3.2 Volume Changes by Zones ............... ................ 8
2.3.3 Interpretation .......................................... 11

3 LITERATURE REVIEW .......................................... 12

4 DATA COLLECTION ........................................... 21

4.1 The Video Monitoring System ................................... 21
4.2 Deployment Description ....................................... 21
4.3 Data Description ............................................. 24

5 IMAGE ANALYSIS .............................................. 28

5.1 Understanding Image Processing and Analysis ....................... 28
5.1.1 Basic Digital Image Concepts .............................. 29

5.1.2 Mathematical Background for Image Processing ................
5.2 Video Digitization Procedure ....................................
5.3 Analysis Techniques ........................................
5.3.1 H istogram Plotting .......................................
5.3.2 Simple Pixel Operations .................................
5.3.3 Pixel Intensity Plotting ..................................
5.3.4 Im age M asking ..........................................
5.4 Im age Rectification ........................................
5.5 Error in Rectification Process .................................

6 RESULTS AND DISCUSSION ......................................

Introduction .................................
Theoretical Background ........................
Image Area of Interest .........................
Average Images Under Storm Conditions ..........
Im age R results .............................
An Example of Oblique Image Analysis ...........
Ancillary Results From In Situ Data and Model Studies
6.7.1 W ave Attenuation ........................
6.7.2 Currents ...............................
6.7.3 Laboratory Tests .........................
6.7.4 Numerical Model Results ..................

6.8 Summary ..................
6.9 Possible Future Applications of the

7 SUMMARY AND CONCLUSIONS ..

7.1 Summary .................
7.2 General Conclusions ..........

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

BIOGRAPHICAL SKETCH ...........

Video Installation .

........... 10 1

6.1
6.2
6.3
6.4
6.5
6.6
6.7

LIST OF FIGURES

Figure page

2.1 Location of the artificial reef relative to Port of Palm Beach Entrance ........ 4

2.2 Schematic of the artificial reef project. ................................ 5

2.3 Profile monitoring plan relative to the artificial reef installation. ............. 7

2.4 Description of nine zones defined for quantification of sediment volume changes 9

2.5 Volumetric changes by zones for the period of August 1993 to June 1995 .... 10

4.1 The Palm Beach fixed view cameras (Camera 1 in foreground) ............. 22

4.2 A quad image from four video cameras, April 11, 1994 .................. 23

4.3 Typical views from four cameras ................................... 25

5.1 Pixel coordinate system ...... ................................. 31

5.2 Video digitization procedure ...................................... 34

5.3 The SVIP mainmenu ............................................ 35

5.4 A typical histogram from the Palm Beach images ....................... 37

5.5 A pixel intensity plotting along a horizontal line without the artificial reef
intensity spike, original image dated April 11, 1995 ..................... 40

5.6 A pixel intensity plotting along a horizontal line with the artificial reef
intensity spike, original image dated April 11, 1995 ..................... 40

5.7 An average background image in calm weather conditions dated April 10,
1995, one day before the storm events on April 11 and April 12, 1995 ....... 41

5.8 A corrected event image created by subtracting the difference between
calm water background and storm event images from the event image
dated April 11, 1995 .......................... ... ............. 43

5.9 A masked image created by subtracting the background image dated
April 10, 1995 from the storm event image dated April 11, 1995 ........... 43

5.10 The total intensity values of the offshore and nearshore areas on an hourly
basis during a storm event dated April 11, 1995 ........................ 44

5.11 The geometry and labeling conventions used in the rectification process
(modified from Lippmann and Holman, 1989) ......................... 46

5.12 An oblique image of storm event dated April 11, 1995 with three white
reference lines marked at the approximately locations of the natural and
artificial reefs and seawalls on a visual basis ........................... 49

5.13 A rectified image obtained from Figure 5.12 ........................ .. 49

5.14 Comparison of image intensity and beach profile .................. .... 50

6.1 Energy dissipation by reefs with crest near the still water level as a function
of the relative reef width (Ahrens, 1987) ............................. 55

6.2 Profile ( Monument R97C) at approximate mid-length of the artificial reef
...... .................. ................................ 55

6.3 A time averaged image of 39,600 images dated from July 18, 1995 to
August 16, 1995 ........... . ... .. .... ........ ...........57

6.4 Bathymetry in the vicinity of camera view for June 1995 survey ............ 59

6.5 A rectified image obtained from an average image of oblique images as
shown in Figure 6.3 ............................................. 59

6.6 Average images of storm events listed in Table 6.1 ................... .. 62

6.7 Rectified images of areas of interest corresponding to Figure 6.6 ........... 63

6.8 Three-dimensional view of the June 1995 beach survey .................. 64

6.9 (a), (b), (c), (d), (e), (f) 3-D perspective plots of image intensities of
rectified images corresponding to Figure 6.7 (a), (b), (c), (d), (e), (f) ........ 65

6.9 (g), (h), (i), (j), (k), (1) 3-D perspective plots of image intensities of
rectified images corresponding to Figure 6.7 (g), (h), (i), (j), (k), (1) ........ 66

6.10 An example of perspective plot of image intensities above background of
rectified image, April 11, 1995 ..................................... 68

6.11 An example of image intensity analysis, April 11, 1995. .................. 69

6.12 An intensity profile along a specific oblique line of the average oblique
image dated April 11, 1995 ....................................... 78

6.13 Wave transmission coefficients based on significant wave heights, January-
October 1994 .................................................. 82

6.14 Wave transmission coefficients based on significant wave heights,
January-June 1995 ........................................... 82

6.15 Schematic of model basin arrangement for artificial reef testing (Dean et
al.,1994b) ................................................. 85

6.16 Circulation patterns documented in model studies. Showing result of net
flow of water over the reef and induced longshore currents (Dean et al.,
1994b) .............. ....... ......... .................. 86

6.17 The transmission coefficients related to the freeboard and the incident wave
height by the numerical model compared with the Kt values presented in
Table 6.15 ................... .................... ........... 89

6.18 A typical view from Camera one which presents wave activity at the Clark
Avenue public beach, July 31, 1995. ................................. 91

6.19 A typical view from camera two before, during and after beach nourishment .. 92

6.20 Shoreline changes corresponding to the storm period of March 8, 1996
though March 14, 1996, images are from Camera four ................... 93

6.21 The average image of March 12, 1996 storms, before and after rectification ... 94

LIST OF TABLES

Table pge

3.1 The cited applications of image techniques ............................ 13

6.1 Times, number of digitized images and wave conditions for average images of
storm events ................... ................................ 60

6.2 Estimated wave energy dissipation based on intensity values, April 11, 1995 72

6.3 Estimated wave energy dissipation based on intensity values, April 12, 1995 72

6.4 Estimated wave energy dissipation based on intensity values, May 10, 1995 ... 72

6.5 Estimated wave energy dissipation based on intensity values, June21, 1995 ... 73

6.6 Estimated wave energy dissipation based on intensity values, July 27, 1995 ... 73

6.7 Estimated wave energy dissipation based on intensity values, July 28, 1995 ... 73

6.8 Estimated wave energy dissipation based on intensity values, July 29, 1995 ... 74

6.9 Estimated wave energy dissipation based on intensity values, July 30, 1995 ... 74

6.10 Estimated wave energy dissipation based on intensity values, July 31, 1995 ... 74

6.11 Estimated wave energy dissipation based on intensity values, August 1, 1995 75

6.12 Estimated wave energy dissipation based on intensity values, August 2, 1995 75

6.13 Estimated wave energy dissipation based on intensity values, August 3, 1995 75

6.14 The percentages of wave breaking related dissipation by various locations
for storm events ............... ........................... 76

6.15 Monthly averaged significant wave height and dissipation percentage ........ 81

6.16 The historical changes of the unit settlement, distance to the shoreline and
depth seaward of the units ....................................... 88

CHAPTER 1
INTRODUCTION

The nearshore coastal zone is generally a wave dominated environment. Various

techniques have been employed historically to study nearshore processes, including profile

surveys, hydrographic surveys, wave and current gages, aerial photography, remote

sensing, satellite imaging, etc. During the last two decades a number of laboratory, field,

and analytical studies have been carried out to study wave energy dissipation across the

surf zone. Studies of waves in the surf zone have been conducted chiefly through spectral

analysis of current meter and/or wave sensor records. However, these instrument arrays

are vulnerable in high-energy situations and rather expensive to install and maintain.

Furthermore, the spectral structure of these records depends upon the location of the

instrument, and large arrays are necessary to provide good coverage of the surf zone.

Recently the availability of low cost video hardware and the capability of extracting

valuable data from video records by image processing techniques have proven that video

monitoring is a simple and economic method for studying the nearshore. Although this

technique may be of lower accuracy and only provides information indirectly describing

the water surface, measurements can be made at low cost and over long periods of time

using video techniques.

2

The primary purpose of this thesis is to investigate the application of video image

processing to the study of wave energy dissipation by natural and artificial reefs occurring

during storm events in the Palm Beach area, Florida. Chapter 2 introduces the

background and environment of the project. Chapter 3 provides a literature review of

previous applications in coastal engineering using video imaging techniques. Chapter 4

describes the video monitoring system and the field deployments of the video imaging

system. Chapter 5 examines the elements of video imaging processing and the video

digitization procedure. Examples of image analysis are presented associated with the

image rectification technique. Also presented are remote sensing techniques that allow the

visualization and subsequent quantification of wave energy dissipation based on the

patterns of incident wave breaking and the profile survey bathymetry within the video

camera view. Chapter 6 discusses the incident wave energy dissipation features based on

the results of image analysis. The discussion of the image results focuses on cross-shore

intensity profiles in the rectified images at prescribed longshore distances scaled to known

beach profile surveys. Finally, summary and conclusions of this study are provided in

Chapter 7.

CHAPTER 2
PROJECT BACKGROUND AND ENVIRONMENT

2.1 Introduction

In an attempt to provide a wider beach and reduce wave impact on a protective

seawall, an experimental proprietary submerged breakwater, the Prefabricated Erosion

Prevention Reef (PEP Reef or artificial reef), was installed as a shore protection project of

4176 feet overall length in a water depth of approximately 9.5 feet off the Town of Palm

Beach, Florida. The system consists of 330 units, 57 of which were installed in the

Summer of 1992 with installation of the remaining 273 units commencing in May 1993

and completed in August 1993. To evaluate the performance of the Reef installation, a

comprehensive field monitoring program was carried out and included wave

measurements, beach and offshore profiles, settlement of the units, local scour data,

remote video observations and information related to the background coastal processes.

Additionally, biological studies including sea turtle nesting on the beaches in the vicinity

and fish populations and invertebrates on the Reef were conducted (Dean and Chen,

1996). The main body of this thesis is directed toward providing quantitative estimates of

wave energy dissipation by natural and artificial reefs using video imaging techniques.

This study is restricted to waves breaking both offshore and nearshore simultaneously due

to storm events. For more information about the full scale monitoring project, the reader

4

is referred to the following references: Browder (1994), Dean et al. (1994 a and b),

Browder et al. (1994), Dean (1995), Dean and Chen (1995 a, b, c, d, and 1996).

2.2 Description of the Artificial Reef Project

The artificial reef project was located approximately 4.5 miles south (downdrift) of

the Port of Palm Beach Entrance (Figure

2.1). A total of 330 proprietary precast
Net Longshore
Sediment Transport
interlocking units were placed in

approximately 9.5 feet of water about 240 ft
Blue Heron Port of
Palm Beach
from the seawalls, resulting in an overall Entrance
SBypassing location
length of 4,176 feet including a gap of 216 F

feet near the north end through which a fiber

optics cable transited offshore. Although
Okwecho, _
almost all of the shoreline landward of the asva EP Reeef4176ft
-Y
artificial reef is seawalled, a beach averaging
Southern ei9
approximately 26 feet in beach width was

present in July 1992 when the first 57 reef

units were installed. The individual units are

6 ft hi, 1 f l a 1 f wie Figure 2.1 Location of the artificial reef
6 feet high, 12 feet long and 15 feet wide o P B Etr
relative to Port of Palm Beach Entrance
and weigh 25 tons in air (Figure 2.2).

Project installation commenced in July 1992 and 57 units had been emplaced when

Hurricane Andrew occurred in August 1992. Surveys conducted subsequent to the

Seawall
Approximately 240 feet

HH
.V* L-
... .VSWL.
.* *..
..* **. '. "*. Approximately 3.5 ft

*. .** *. : :: Approximately 9.5 feet
*.*.Sand . ..... *.* ..'4 P e L
S'*.'.Sd : *.: PEP Seaward
* ***. .. ..... .....:. -...///,,;/anut/fi//u//a/uwuT//u///l/

Figure 2.2 Schematic of the artificial reef project.

hurricane documented substantial settlement of the units resulting in a hiatus in the

installation. The original design called for the direct placement of the units on the sea

floor without any scour prevention. During the hiatus, various foundation designs were

considered. However, due to the added costs, it was decided to continue placing the units

without scour prevention and installation of the remaining units commenced in July 1993

and was completed in August 1993.

The stated objectives of the artificial reef were twofold, including wave height

reduction and increase in beach width. Because the State of Florida and the Town of Palm

Beach were interested in documenting the performance of the project, an extensive

monitoring program was carried out. The monitoring program began with the installation

of the first 57 units in summer 1992 and continued through June 1995, two months before

removal of the experimental artificial reef units.

6

2.3 Profile Survey Results

To examine the sediment volume changes in the vicinity of the artificial reef, a

profile survey plan was carried out as shown in Figure 2.3, including a total of 75 profile

lines originating from a monumented baseline configured for this study. The baseline

incorporates the Department of Environmental Protection (DEP) monuments which are

numbered consecutively from north to south and spaced at approximately 1000 feet

intervals. Intermediate profile locations are denoted by a letter, e.g. 95A and 95B are the

first and second profile lines south of DEP Monument 95. Most of these lines were

surveyed on a quarterly basis (every three months) by a combination of land surveying

techniques, swimming surveys in which level and rod techniques are employed and farther

offshore, using standard fathometer measurements. The swimming portion of the surveys

extended at least 50 feet seaward of the reef units for greater accuracy. The quarterly

surveys extended 1200 feet seaward as compared to 5,500 feet for the annual surveys. At

stations where the DEP monuments are located, the profile lines extended 6,500 feet

seaward of shore. In addition to surveying the profiles, elevations were taken on the

north, middle and south of the top of each artificial reef unit to document any settlement.

Ten profile and settlement surveys are available during the monitoring period

between July 1992 and June 1995: July 1992, April 1993, August 1993, December 1993,

March 1994, July 1994, November 1994, December 1994, March 1995, June 1995.

Tropical Storm Gordon affected the Palm Beach area on November 14 and 15, 1994.

Figure 2.3 Profile monitoring plan relative to the artificial reef installation.

2.3.1 Definition of Study Zones

For purposes of overall comparison of sediment volume changes, the study area is

divided into nine zones as shown in Figure 2.4. Zones one through six are in the

immediate vicinity of the artificial reef. The inner three zones and the outer six zones are

landward and seaward of the artificial reef, respectively. Each of the inner three zones is

approximately 240 feet in width, the mean offshore distance of the 330 artificial reef units

from the seawalls. Zones seven through nine are located landward of the natural reef with

an approximate width of 480 feet. Zones one, four and seven are between monuments

94A and 95E, zones two, five and eight between monuments 95E and 99B and zones

three, six and nine are defined by monuments 99B and 101A. The northern and southern

regions extend roughly 2000 feet north and south of the Reef and the central region is

within the approximate 4000 feet artificial reef confines.

2.3.2 Volume Changes by Zones

As described in the First Six Month Report (Dean et al., 1994a), the volume

changes per lineal foot are converted into volume change (yds3) by the "average end area

method."

August 1993 to June 1995 (overall basis for all units present period). Figure 2.5

depicts the overall volumetric changes that have taken place during the monitoring period

of the fully installed artificial reef, from August 1993 to June 1995. The overall changes

of the nine zones ( zones 1 through 9) for this period are erosional with a total loss of

Zone Designation

Zone 1 zone 4 zone 7

.............................j I ............................ ... ....................................................
I.I

oo zone 2 zone 53 zone 8

z

---240' --"- 2401'-'.]-- 480'

Figure 2.4 Description of nine zones defined for quantification of sediment volume
changes
changes

August 1993 to June 1995

Figure 2.5 Volumetric changes by zones for the period of August 1993 to June 1995

- 240' -- -- 240' 0 -- 480'

11

107,700 yds3. The greatest loss (- 85,200 yds3) occurred landward of the artificial reef.

The second greatest loss (-35,300 yds3) occurred south and landward of the artificial reef.

It is noted that zone eight gained 17,600 yds3 of material.

2.3.3 Interpretation

Generally, the artificial reef has caused a removal of sand landward of its alignment

and had a deleterious effect on the stability of the beach and nearshore profile. The beach

profile surveys indicate an overall erosional trend in the vicinity of the artificial reef.

These results are interpreted as due to water carried over the artificial reef by the waves

and since the water can not return as readily as it would normally due to the presence of

the artificial reef, a portion of this water is diverted in an alongshore direction and

transports sand from the lee of the artificial reef in both the north and south directions,

causing substantial erosion landward of the artificial reef (Dean and Chen, 1996).

CHAPTER 3
LITERATURE REVIEW

As early as the 1920s, vertical images from aircraft and balloons have been used to

study shoreline changes. In order to study nearshore processes such as breaking waves,

longshore currents and beach changes, the techniques using oblique images have been

developed during the past several decades associated with image digitizing systems.

However, there are only limited references available due to the small range of applications.

Table 3.1 lists the cited applications of image techniques. Many applications were

conducted by R. Holman of Oregon State University, Corvallis, Oregon.

Maresca and Seibel (1976) used single-image and stereoscopic-image analysis to

study breaking waves, water levels, and currents within the surf zone. They stated that the

photographic technique is simple to install, reliable, accurate, and inexpensive compared to

other techniques in coastal monitoring programs. They pointed out that the oblique

images taken from a 35-mm camera mounted 8 m high and capable of viewing a 250 m

offshore range are suitable for the measurement of breaking waves under storm

conditions. Accuracies of measured distances were within 1% in the horizontal plane and

better than 10% vertically. The accuracy of the estimate of the plunging-breaker height is

better than 10%. They also noted that no special equipment is required for taking

Table 3.1 The cited applications of image techniques

Authors and Date Title of Article Importance to the Field

Maresca, J.W. and E. Terrestrial Photogrammetric Used 35 mm camera to measure breaking waves
Seibel (1976) Measurements of Breaking Waves and and currents in the nearshore region.
Longshore Currents in the Nearshore
Zone

Holman, RA. and Longshore Variability of Wave Run-up Used time-lapse photography to measure wave
A.H. Sallenger,Jr. on Natural Beaches run-up.
(1984)

Holman, RA. and Measuring Run-up on a Natural Beach Introduced longshore-looking time-lapse
R.T. Guza (1984) photography to study longshore variability of wave
run-up on natural beaches.

Holman, RA. and Remote Sensing ofNearshore Bar Developed a technique for modeling and
T.C. Lippmann Systems-Making Morphology Visible measuring 3D morphology of sand bar.
(1987)

Aagaard, T. and Digitization of Wave Run-Up Using Used video records to measure wave run-up.
J.6Holm Video Records
(1989)

Lippmann, T.C. and Quantification of Sand Bar Morphology: Developed a remote sensing technique to measure
RA. Holman (1989) A Video Technique Based on Wave the scales and morphology of natural sand bars
Dissipation based on the patterns of incident wave breaking.

Lippmann, T.C. and Wave Dissipation on a Barred Beach: A Demonstrated a technique to remotely measure
RA. Holman (1989) Method for Determining Sand Bar natural sand bar scale and morphology change.
Morphology

R. A. Holman et al. Observations of the Swash Expression of An automatic digitization system was applied to
(1990) Far Infragravity Wave Motions improve upon the previous run-up experiments
conducted by Holman and Guza (1984) during
SUPERDUCK'86 experiment.

K. T. Holland et al. Estimation of Overwash Bore Velocities A video technique was applied that allowed the
(1991) Using Video Techniques quantification of overwash bore celerity vectors
along several cross-shore transacts.

Lippmann, T.C. and Phase Speed and Angle of Breaking The phase speed and incident angle of breaking
RA. Holman (1991) Waves Measured with Video Techniques surface gravity waves in the surf zone were
measured using video processing techniques.

Lippmann, T.C. and Wave Group Modulations in Cross-shore A Video based technique was presented which
RA. Holman (1992) Breaking Patterns accurately quantifies temporal modulations in
wave breaking across the width of the surf zone.

R. A. Holman et al. The Application of Video Image Showed several examples of bathymetry
(1993) Processing to the Study of Nearshore measurements that can be made at low cost and
Processes over long periods of time using video.

T. P. Mason (1993) Video Monitoring Techniques in the Used video to monitor the Hollywood Beach
Coastal Environment renourishment of summer 1991.

R. V. Sloop (1995) Beach Cusp Analysis and the Dry Beach Used video techniques to study beach cusp
Evolution of Longboat Key, Flonda formation and shoreline change in Longboat Key,
Using Video Monitoring Techniques Florida.

14

the oblique images and no reference stakes are required for image analysis if the horizon,

focal length, and camera elevation are known.

Holman and Guza (1984) performed field experiments to evaluate and

intercompare two techniques for measuring wave run-up on natural beaches: dual-

resistance wire and time-lapse photography. Both techniques showed accurate results in

the mean swash elevation, but up to 83% difference in swash variance. The distances

from camera to the set-up area range from 100 m to 250 m. Video images were collected

along the shoreline. They pointed out that the main advantage of the photographic

technique was low cost, simple logistics, potential for digitizing, ability to "see" the

phenomenon, and effectiveness during storms, although the process of manual digitization

is tedious and subjective.

Holman and Sallenger (1984) introduced an image technique, longshore-looking

time-lapse photography to study longshore variability of wave run-up on natural beaches.

Eight video cameras were mounted approximately 13 m above mean sea level to collect

run-up data. Several markers were placed in the cross-shore direction which served as

reference points for the beach profile grid and the film images. For normally incident

waves, a ten minute exposure was believed to give an adequate averaging due to

modulations generally occurring on the time scale of minutes. The results of this study

showed that this technique is not only useful to determine dominant variabilities in wave

statistics, but also to image offshore sand bar morphologies. Holman and Lippmann

(1987) applied this technique to remotely image and quantify 3-D nearshore morphology

during the DUCK'85 field experiments at Duck, North Carolina. Cameras with a 35-mm

15

focal length lens were set up on a 14 m scaffold. The cameras recorded nearshore wave

activities in all daytime weather conditions. This application depends on ten-minute

photographic time exposures in which white foam due to breaking waves produces images

wherein bands of light intensity are interpreted as dissipation of the incident waves

breaking over a sand bar system. Image photogrammetry has been developed to allow the

quantification of distances and locations of points on the oblique images. The initial tests

showed the distance errors of the photogrammetry are less than 2% in the longshore

direction from the camera and up to 15% in the offshore direction with increasing tide.

Aagaard and Holm (1989) used video records to measure wave run-up by applying

an updated method of the time-lapse photography technique described in Holman and

Guza (1984). The camera was mounted in a transparent box and protected against

moisture and salt spray on a tripod on the foreshore. The camera view is about 75-100 m.

Stakes were set at specific intervals for later conversion of run-up length and run-up

height. The video records were digitized by a PC-Vision frame grabber. The film is

displayed on a monitor, and the computer scanned the specified picture line between

reference stakes at discrete time intervals. The digitization procedure takes about 2.5

times as long as the method described by Holman and Guza (1984); however, it was

believed to be more precise, especially in the determination of the backwash position. The

authors noted that this method can permit an immediate visual inspection of incident wave

and swash periods.

Lippmann and Holman (1989) developed a remote sensing technique that allows

visualization and subsequent quantification of nearshore morphology based on the patterns

16

of incident wave breaking. The application of video images was primarily set up as an

improvement to the time-lapse photographic technique. They assumed that the light

intensity recorded on the film is simply proportional to the wave energy dissipation

(modeled theoretically by dissipation of a random wave field), although they did not

actually test this hypothesis by measuring wave dissipation. The theoretical dissipation

model was found to work best for small waves that just break over the bar. A video

camera was mounted on a 40-m-high tower with a tilt angle of 85 degree and recorded 20

minute records each hour. In order to remove the bias from images, time exposures were

created digitally by mathematically averaging successive video frames over a 10-min

period using an image processing system. Oblique images were rectified to quantify the

offshore and longshore dimensions of the sand bar. From the rectified view, the image

processing system can generate cross-shore intensity profiles at prescribed longshore

distances. The image intensity profile lines showed that there are local maxima in the

vicinity of the shoreline and sand bar. The results of an analysis yielded errors in the

estimate of the sand bar offshore distance less than 35% during large wave conditions and

errors were less than 5% of the longshore distance to the camera. This time averaging

technique provided a good method of underlying morphology and determining cross-shore

and longshore length scales.

Holman et al. (1990) investigated infragravity wave motions using videotapes.

The cameras were mounted on a 43.2 m-high tower. Video records were 1 hour 55

minutes long and were taken to coincide with in-situ current meter data runs as nearly as

possible. An Imaging Technology Series 150 image processing package was used to

17

digitize the video frames. This automatic digitization technique eliminated the tedious

process of manual digitization and minimized the operator subjectivity. The geometric

variables were solved using a set of known ground control points with the image adjusted

to conform to the known positions in a least squares sense. The beach profile transects

were placed at 10 m intervals based on the now-known geometric variables along the 380

m-long beach. The pixel values for each transect were read into a computer, and the

computer searched the run-up and located swash excursion along each line every second.

The resolution of the technique was found to depend on distance from the camera and

focal length of the individual lens. Typical horizontal resolutions varied from 20 cm for

close ranges to 73 cm for the most distant cases.

Holland et al. (1991) applied a video technique to quantify the overwash bore

celerity vectors along several cross-shore transects on a barrier island off the coast of

Louisiana. A cross-beach instrument array and a video recording system were deployed

between August 1987 and January 1990. The video camera was mounted on top of a

tower and triggered to record overwash activities by a salt water sensor during daylight

hours. The camera setup recorded more than 20 overwash events including a most

significant event which occurred during Hurricane Gilbert on September 14, 1988. A 16 x

16 m grid was centered in the camera's view. Grid points were placed every 2 meters.

Pixel values at each grid point were digitized to determine the celerity vectors using an

image processing system. The maximum error in the celerity magnitude measurements

was found to be 15%. Maximum celebrities were found to exceed 2 m/s based on an ability

to quantify wave presence in terms of pixel intensities.

18

Lippmann and Holman (1991) measured the phase speed and incident angle of

breaking surface gravity waves in the surf zone using video processing techniques. The

technique is based on the premise that the foam and bubbles resulting from breaking waves

are bright in contrast to the darker unbroken surrounding water. The breaking wave

crests are visually identified by a local maximum in image intensity recorded on the video

film. Up to eight video cameras were set to record data from the dune crest to 200 m

offshore, but only one particular camera view covered the entire study area and was used

to conduct the image analysis. The time series of records were collected for two hours

sampled at 10 Hz frequency, and later resampled at 8 Hz to allow best comparison with

other in-situ instrumentation. Phase speeds and wave angles of individual breaking waves

were estimated from image intensity time series. The results from video and in-situ

pressure gauge data were similar and showed reasonable agreement in terms of wave

spectra, angle, and speed.

Lippmann and Holman (1992) presented a video based technique to quantify

temporal modulations in wave breaking across the width of the surf zone. The cameras

were mounted on top of a 44 m high tower and viewed from the dune crest to 400 m

offshore. Video image intensity series were sampled across the width of the surf zone

from just outside the shore break to the far reach of the surf zone at 10 m increments.

Image locations of interest are established using known photogrammetric transformation

equations (Lippmann and Holman, 1989). Image time series were sampled at 6 Hz for 2

hour records. Resolution in image pixels is within 1 m in the offshore direction and 2.5 m

19

in the longshore direction. This study found wave group modulations in cross-shore

breaking patterns consistent with simple shoaling expectations.

Holman et al. (1993) discuss the basic elements of video image processing as the

determination of image intensity into a two-dimensional array of picture elements. The

authors showed several examples of bathymetry measurements using video techniques.

To convert the relationship between image data and geophysical features of interest, the

transformation algorithms developed by Lippmann and Holman (1989) are highly

recommended. The authors also pointed out that the primary disadvantage of video

techniques is the often unknown relationship between visual signals and geophysical

variables of use in testing the theory. Finally, the authors suggested developing long-term

large databases for future study.

Mason (1993) used video monitoring procedure to study nearshore turbidity on

the influences of beach nourishment which occurred during Summer of 1991 in

Hollywood Beach, Florida. A video camera was mounted in a building at 55 m above

mean sea level. The camera view was in the northeast direction, extending 6.5 miles.

Video records of 0.7 seconds each were sampled at 30 minute or 1 hour intervals from

April 1991 through November 1991. Mason used the equations presented by Lippmann

and Holman (1989) for geometrical transformation from image to ground coordinates.

Accuracies were found to be within 5% of estimates of general length scales and shoreline

position. Cross-shore and longshore velocities of observed turbidity plumes were similar

to in-situ currents. This study found the turbidity plumes at Hollywood Beach to

propagate well outside the surf zone and form under relatively low wave conditions.

20

Sloop (1995) conducted an analysis of beach cusps and the dry beach evolution of

Longboat Key, Florida using video monitoring techniques. Beach cusp formation and

spacing prediction theories were investigated using data obtained from video images and

in-situ wave gauges. A video capture device was used to obtain data for the monitoring

program of the Longboat Key beach nourishment project. This device consisted of a

video camera, a video tape recorder, a timer, a pan/tilt mechanism and one 486/33

personal computer with corresponding image digitization software. In addition, the

computer was employed to control the monitor to view up to 34 different scenes and

provide telephone access via modems for image retrieval, real time viewing or system

reprogramming. The video monitoring system was installed on top of the Longboat

Harbor Towers. The system was operated during daylight hours from May 1993 through

July 1994. More than eighty thousand images were digitized for the analysis. Based on

observations and analysis from the video images, Sloop found that edge wave theories do

not accurately predict cusp spacing, but are accurate in prediction of maximum wave

heights for cusp formation.

CHAPTER 4
DATA COLLECTION

4.1 The Video Monitoring System

The video monitoring system (VMS) employed for the monitoring of the Town of

Palm Beach artificial reef project was designed and installed by Erdman Video Systems

(EVS) of Miami Beach, Florida on December 2, 1994. EVS has developed computer

controlled camera systems that offer a cost-effective method for monitoring and

documenting complex projects. A single camera system can monitor up to 80 different

views and allow access to images, complete reprogramming and real time viewing over

standard telephone lines. The major components of the VMS are the camera, the tilt and

pan mechanisms, the timer, and the video capture devices with corresponding software.

The camera can be mounted on a roof with variable zoom, automatic exposure control,

automatic focus, and a polarizing filter.

4.2 Deployment Description

The system used in this study consists of four fixed view cameras mounted on the

southeastern corner of the roof on top of the elevator service room of the Breakers Hotel

in Palm Beach, Florida ( Figure 4.1). The room is reached from a stairwell off the fifth

Figure 4.1 The Palm Beach fixed view cameras (Camera 1 in foreground)

floor of the Breakers Hotel. All cameras have fixed focal lengths and are contained in

weatherproof housings. Cables run from the camera platform, across the roof and into the

elevator service room. The cameras are controlled by a multiplexer and a timer that allow

for sequential images to be taken at user-specified intervals. In addition, the timer records

the time of collection directly on the images. The multiplexer feeds this information to an

SVHS video recorder for archiving. The resulting tapes must be decoded by EVS to

separate the individual camera views. Decoded tapes in standard VHS configuration are

delivered by EVS on a monthly basis. Initially, the multiplexer was set to sample all

cameras every twelve seconds. At this sampling rate, a standard VHS tape was full after

two weeks, and required changing on-site. Later, the multiplexer was set to sample every

23

thirty seconds during daylight hours, allowing for a full month between tape changes with

a negligible reduction in information conveyed by the data.

The system as a whole requires limited maintenance; however, the elevator room is

not air conditioned, and is hot and humid at all times. This environment has led to failures

in both the multiplexer and SVHS video recorder at different times and has led to the loss

of data. The camera heads must be hand cleaned at every tape change, and the multiplexer

must be tested to insure proper function. The viewing ports of the environmental camera

housing have been coated with RAIN-X to encourage raindrops to bead and run off.

The cameras are located at an elevation of 114 feet above NGVD and are

positioned approximately 1560 feet north and 180 feet west of the artificial reef

installation, and the views are oriented in a southeastward direction and encompass 2500

feet of cross-shore distance and up to 8500 feet of longshore distance. The cameras focus

on different areas in the vicinity of the artificial reef area. Figure 4.2 shows typical quad

views from the four cameras. Camera 1 was a zoomed view of Clark Avenue public

beach, and had the only lens which has not been replaced. As such, the view provided by

Figure 4.2 A quad image from four video cameras, April 11, 1995

24

camera 1 was the only semi-continuous record of the events from December 2, 1994 to

April 1, 1995. Early attempts were made to sample at night using a low light lens, but the

images were washed out by bright lights near the beaches of interest and the lens was

replaced. Two of the other original lenses were replaced because their focal lengths were

too small, and created extremely wide angle images, unsuitable for this deployment.

Camera 1 was a black and white camera and remained as the zoomed view of the Clark

Avenue public beach (DEP monument R-96). It had a focal length of 12 mm. Camera 2

was a color camera with a focal length of 6 mm, and provided the clearest and best over

all images. Camera 3 was a black and white camera with a focal length of 6 mm, and

mimicked the view of Camera 2. Camera 4 was a black and white camera with a focal

length of 50 mm, and zoomed in on a seawall at the extreme southern end of the visible

shoreline (DEP monument R-97). At this point, unfortunately, the shoreline and the

artificial reef curve away toward the west, limiting the far ranging views. Figure 4.3

shows the typical views from the four cameras as they were configured.

4.3 Data Description

The purpose of this VMS deployment was to document the approximate wave

energy dissipation due to the artificial reef project, which begins slightly south of the

Breakers Hotel and extends south beyond the view of all four cameras. The deployment

system provides one year of continuous monitoring data collection, beginning on April 1,

1995 and ending on April 1, 1996. The Town of Palm Beach commenced removal of the

artificial reef in late July 1995, and initialed and completed construction of a beach

Figure 4.3 Typical views from four cameras

nourishment project in October and December 1995, respectively. In this regard, the

cameras were well positioned to document these events and provided a "before and after"

view of the area.

As mentioned, the data were only continuous for all four cameras beginning in

April 1995. Previously collected data contained gaps between January 23, 1995 to

February 15, 1995 and from February 20, 1995 to February 25, 1995. On almost every

tape, at least one camera suffered some sort of failure in operation or alignment.

Beginning in April 1995, however, it was believed that the problems in the system had

been worked out to allow for a year's worth of high quality, continuous data collection.

Qualitatively, the series of analog tapes captured nearly all of the events associated

with coastal phenomena at Palm Beach. Evident in the tapes were waves breaking on the

beach, waves breaking on the offshore reef, low and high tide, beach response to storms,

swash activity, rip currents, and wind conditions. The activity of removal of the artificial

reef was captured during July 19, 1995 and October 19, 1995. The artificial units were

removed from north to south. Hurricanes and storms caused several hiatuses in the

removal. Also, the activities of the beach nourishment project were recorded in all

weather conditions. In addition, the system taped interesting meteorological events along

with automobile, boat, airplane, blimp, pedestrian, and bather traffic. On very calm days,

the artificial reef can be seen underwater on the video monitor, as can the buoy that marks

its northern limit. The time lapse nature of the collection scheme lends itself well to

27

viewing long term phenomena and patterns in a reasonable amount of time. This type of

data would also be effective in public presentations.

The analog tape, however, does not provide an optimum platform for quantitative

analysis. Almost all types of numerical evaluations require digital information. Black and

white scenes can be digitized and stored as large matrices with the values at each x, y

location representing the brightness or intensity of the corresponding pixel. The intensity

range is from 0 (black) to 256 (white). Color images can be represented in a similar

fashion, although they require three matrices to describe the amounts of red, green, and

blue in each pixel, which exponentially increases the complexity and memory usage of

even simple analytical routines. The color images created by Camera 2, however can be

used as black and white images, and produce digitized results that are far superior in terms

of sharpness and clarity than the three black and white cameras. Currently only images

collected by Camera 2 have been used in the image analysis due to the high quality and full

coverage of the study area.

CHAPTER 5
IMAGE ANALYSIS

5.1 Understanding Image Processing and Analysis

According to Green (1989), "Most digital image processing techniques fall into

one of two broad categories: subjective and quantitative. Subjective image processing is

designed to improve human visual interpretation of an image. Subjective processing, often

called image enhancement, is usually performed in an adaptive, interactive, and iterative

manner. Quantitative image processing is usually performed on imagery in a nonadaptive,

noninteractive manner" (p. 50). Quantitative processing is based on predefined

mathematical algorithms, and success in processing is based on the correction for camera

system-induced geometric distortion or computation of a two-dimensional Fourier

transform of an image (Green, 1989).

This section gives a brief introduction to the principle of the applications of digital

image analysis. In this section a conceptual and mathematical framework is presented for

understanding digital image processing and analysis. It helps to understand the method of

image processing and have a review of mathematical tools which are relevant to image

processing. In general, the purpose of digital image processing is to enhance or improve

the image in some way, or to extract information from it. Typical geometric operations

are (1) Image preprocessing, (2) Image enhancement, (3) Image processing, and (4) Image

29

analysis. For a more comprehensive information the reader is referred to the references

provided in this thesis. This section describes techniques that assist the analyst in the

quantitative interpretation of images.

5.1.1 Basic Digital Image Concepts

The following definitions are useful in the ensuing discussions.

Object: Three-dimensional shape that is the subject of analysis by a vision system.

An object forms the basis for creating an image (Zuech, 1988).

Image: A two dimensional functionf(x,y) of brightness (or gray level) values

which give a visual representation of an object or scene in digital image processing, where

x and y denote spatial coordinates and the value off at any point (x,y) is proportional to

the brightness of the image at that point. This numerical representation of images permits

the application of a wide assortment of computer processing techniques of the data

(Schowengerdt, 1983). The results of this computer processing are new arrays of

numbers representing enhanced images, which are then converted to an analog

representation for display. Often images are square, and typical image sizes are

256 x 256, 512 x 512, and 1024 x 1024.

Pixel: Acronym for picture element. Spatial resolution element; smallest

distinguishable and resolvable area in an image (Zuech, 1988).

Intensity (or Gray Level): Pixel values or brightness. The intensity range is from 0

(black) to 255 (white).

30

Digitizing: Process of conversion an analog video image into digital brightness

values that are assigned to each pixel in digitized image (Niblack, 1986).

Image Preprocessing: The initial processing of the raw data to smooth out noise,

improve the contrast or other visual properties, correct geometric distortions, and then

submit the corrected images to enhancement (Niblack, 1986).

Image Enhancement: Any one of a group of operations that help clarify details

within an image. These operations include, but are not limited to, histogram equalization,

edge finding, density slicing, image adjustment, and filtering (Niblack, 1986).

Image Processing: Encompasses all the various operations that can be applied to

image data. These include, but are not limited to, image compression, image restoration,

image enhancement, preprocessing, quantization, spatial filtering, and other image pattern

recognition techniques (Niblack, 1986).

Image Analysis: Process of generating a set of descriptors or features on which a

decision about objects in an image is based (Niblack, 1986).

5.1.2 Mathematical Background for Image Processing

In order to provide a form suitable for computer processing, an image function

f(x,y) must be digitized both spatially and in amplitude (Gonzalez, 1977). Digitization of

the spatial coordinates (x,y) will be referred to as image sampling, while amplitude

digitization will be called gray-level quantization. Usually a continuous imagef(x,y) is

approximated by equally spaced samples arranged in the form of a square array as shown

in Equation (5.1) (Gonzalez, 1977)

f(O,O) AfO,1) ... JfO, N-1)
AI,O) /1,1) ... (1, N-1)

fxy) (5.1)

(N-1,0) f(N-1,1) ... fAN-l, N-1)

where the array at the right of this equation represents an image, while each element of the

array is referred to as a pixel or image element. The terms "image" and "pixel" will be

used throughout the following discussions to denote a digital image and its elements.

The pixel coordinate system (see Figure 5.1) is the most common coordinate

system for digital image processing. Despite its wide acceptance, there is no standard

notation for this system. For pixel coordinates, the first component x increases to the

right, while the second component y increases downward. It is similar to the Cartesian

coordinate system, except they direction is reversed. The origin is the upper-left corer.

The pixel coordinates are typically integers but can be real numbers.

Figure 5.1 Pixel coordinate system

O (1.1) x
1 2 3 ...
-1

-2

-3 */(3,3)

5.1.2.1 An image model

Gonzalez (1977) presented the image functionf(x,y) characterized by Equation

(5.2) by the amount of source light incident on the scene being viewed, denoted by i(x,y)

and the amount of light reflected by the objects in the scene, denoted by r(x,y).

f(xy) = i(xy)r(xy) (5.2)

where

0
and

O
Since light is a form of energy, i(x,y) must be nonzero and finite, i.e., O
reflectance function r(x,y) is bounded by 0 (total absorption) and 1 ( total reflectance).

5.1.2.2 Image formation

An image forming system may be treated as a "black box" that operates on an

input signal to produce an output signal. The input signal is the scene radiance and the

output signal is the image irradiance (Schowengerdt, 1983). Generally, this system of the

input and output signal is a linear system, and it can be expressed as Equation (5.3),

g(x) = ft(xy) +f2(xy)]
= 9[f,(xy)] + [f2(xy)] (5.3)
= g1(x>)+g,(xy)

wheref(x,y) is the scene radiance and g(x,y) is the image irrradiance (Schowengerdt,

1983).

33

5.1.2.3 Point processing and neighborhood processing

Point processing consists of a transformation of each original image pixel value

into a new value for the output image, without regard for neighboring pixel gray levels.

Neighborhood processing performs a transformation on each pixel in a way that depends

not only on the gray level of the pixel being processed but also on the gray levels of pixels

in the vicinity of the pixel being processed and includes techniques such as edge

enhancement and interpolation (Schowengerdt, 1983). For convenience, the point

processing is selected in the transformation between oblique image and rectified image in

the rectification process which is described in Section 5.4. Future effects may include

rectification comparing the point processing method with neighborhood processing.

5.2 Video Digitization Procedure

To digitize images, a VCR, monitor, computer, and appropriate "frame-grabber"

software are required. A high quality SVHS VCR with at least four heads and the

capability to play tapes frame-by-frame is preferred. The computer should have adequate

memory to store images and perform calculations, as each uncompressed image typically

occupies more than 180 Kbytes of memory. Figure 5.2 shows the basic elements of the

image digitization procedure. When the tape is played on the VCR, the Frame Grabber

converts the analog TV-signal input to digital values and stores the data in the frame

memory. Subsequently, the system reconverts the digital data to an analog TV-signal

Figure 5.2 Video digitization procedure

output and the two-dimensional picture is displayed on the video monitor frame by frame.

The hardware of the system used in this study includes Sony Hi8 FX 710 cameras,

standard VHS tapes, one Panasonic AG-1970 VCR, one Panasonic WV-5470 video

monitor and one Micron Pentium PC. Most of the images digitized in this study with a

size of 751 x 239 pixels are saved as tagged interchange format file (TIFF) which is an

industry standard for personal computers and image processing packages. Each TIFF file

contains an intensity value matrix ranging from 0 to 255 and a header with information

about the image size and other characteristics. A 751 x 239 image occupies over 180

Kbytes. The TIFF data can be converted to other format data by some software programs

like MATLAB, Math Works, Inc., etc.

The software used in this analysis is SILICON Video Image Processing (SVIP)

version 6.7, produced by MicroDISK, Inc. Figure 5.3 shows the main menu of the SVIP

package. The SVIP program provides basic video operations to digitize and display

images with adjustable resolution and video format, to select image buffers, to manipulate

SVIP SILICON VIDEO Image Acquisition, Display, Processing, Analysis
V67 Copyright C 1984 1992 EPIXINC A d

! COMMAND >>

> Video Formats
> Video Resolution

> Video Digitize/Display
> Motion Sequence Capture/Display
> Special Operations & Modes

>Contrast & Lookup Tables
>Pixel Peek, Poke & Plot
>Image Zoom & Pan

>MIPX Scripts
> Custom Menu

< Quit Menu

? Help Key Usage
> Image Test Patterns & Sequences

> Image Processing
> Image Printing
> Image File Load/Save

> Image Measurements
> Paint, Draw & Text Overlay
> Feature Finders

> DOS Escape
> Obscure Menus

Figure 5.3 The SVIP main menu

lookup tables, to save and load images with optional data compression, to alter spatial

relations between pixels, and to examine or modify image size and shape and extend the

operations with a wide selection of image processing operations, image measurements,

timed capture of image sequences, halftone printing of images, image drawing, image

painting, and text overlay. SVIP resides in its own subdirectory on the hard drive and is

run in a MS-DOS environment.

5.3 Analysis Techniques

The following is a summary of the analysis tools and techniques (with examples),

which have been developed in SVIP to analyze Palm Beach data. These techniques are

universal and should be available in other programs with similar capabilities.

5.3.1 Histogram Plotting

The image histogram describes the statistical distribution of 256 gray level scales in

an image in terms of the number of pixels (or percentage of the total number of pixels)

having each gray level. It is analogous to the familiar probability density function in

statistics. Histograms for many images tend to be Gaussian in shape, but often have an

extended tail toward higher gray levels (higher scene radiances). It is important to

remember that an image histogram only specifies the total number of pixels at each gray

level; it contains no information about the spatial distribution of gray levels throughout the

image. The SVIP programs can allow for a horizontal or vertical presentation of the

histogram for the entire image, a selected region, or a selected line. In addition, the data

can be saved to an ASCII file for use in other programs (i.e., spreadsheet or database).

Statistics, including mean, standard deviation and max/min values can be calculated. The

histogram can yield details concerning the overall appearance of an image and an

indication of how much of the intensity range is being utilized. If the histogram is narrow,

it can be expanded, effectively increasing the contrast of the image. Image contrast is the

range of difference between light and dark values in an image. If the histogram is shifted

to either the black or white end of the spectrum, it can be centered, changing its

brightness. If an image histogram is concentrated in some ranges, histogram equalization

techniques can be used to spread out the middle intensities and reduce the contrast in very

light or dark areas, making the image easier to analyze. It rearranges the intensity values

so that the image's cumulative histogram is approximately linear.

37

Figure 5.4 depicts a typical histogram from the Palm Beach images. It is evident

from this histogram that the image has a high percentage of pixels that are nearly black or

nearly white.

5.3.2 Simple Pixel Operations

The pixel intensities can be modified by multiplication/division or

addition/subtraction. (brightness modifications) This is useful for standardizing the

exposures on multiple images taken under different lighting conditions. A "negative" of

an image can be created by subtracting the intensity value of a pixel from 255, with white

changing to black and vice-versa.

The contrast of an image can be improved associated with transformation of each

pixel's gray level into a new gray level. To change the contrast of an image, (i.e., increase

HISTOGRAM
3000

2500
_j
LI
X 2000
a.
UL
O 1500

m 1000

500

0
C4C o rW T- CM VOO- OV- W- ri V- q. N CMI N N 14 N
PDXEL INTENSITY

Figure 5.4 A typical histogram from the Palm Beach images

38

histogram range), for simplicity, all pixels with intensities lower than that specified value in

the image areas of interest can be set to 0 (black) and/or all pixel intensities higher than

that specified value can be set to 255 (white).

Usually image data contain superimposed broadband noise. In order to reduce

these finely structured erroneous intensity fluctuations, linear smoothing operations such

as spatial filtering are frequently performed on the distorted image signal.

Spatial filtering is a pixel-by-pixel transformation that alters the gray level of a

pixel according to its relationship with the gray level of other pixels in the immediate

vicinity. According to Schowengerdt (1983), "Spatial filters used in image processing are

based on three basic types, low, high, or band-pass. Low-pass filtering smooths the detail

in an image and reduces the gray level range and high-pass filtering enhances detail at the

expense of large area radiometry and produces an image with a relative narrow histogram

centered at zero gray level" (p. 73). The low -pass filter is used to reduce noise for some

of the Palm Beach images.

5.3.3 Pixel Intensity Plotting

The SVIP can provide the intensity values for an individual, line, or region of

pixels. The pixel value can be changed by entering a different numerical value. This is

useful for marking reference points and objects (i.e., the artificial reef buoys) that are

visible in some images and not in others. A 17 x 17 section of the actual intensity matrix

in the region of interest can be displayed on the monitor screen. This is helpful for finding

the edges of regions of different intensities, and picking out small objects of different

39

intensities such as the buoys attached to the artificial reef. The SVIP can plot the intensity

values of the pixels along a horizontal line at a specific user-specified location. This

feature is extremely useful and informative for determining breaking wave characteristics

at Palm Beach. Specifically, the artificial reef and the buoy at its northern end can be

drawn in on an image from a calm day, and transferred to an image of a rough day, to

determine the changes in breaking on the artificial reef. The drawing can be made

permanent, with a pixel intensity of 255 and then used to pick out the artificial reef area in

a histogram, as an intensity spike (=255) will appear. An example of this technique is

shown as the following figures. Figure 5.5 presents the pixel intensities along a specified

horizontal line on the original image without the artificial reef intensity spike. However,

Figure 5.6 is exactly as same as Figure 5.5, except a line was drawn with an intensity of

255 (white) on the original image, to denote the exact location of the artificial reef.

5.3.4 Image Masking

This technique has been developed in an attempt to quantify the amount of wave

breaking from the pixel intensity levels. In essence, a false image is created which

represents only the pixels where wave breaking is occurring. This is accomplished by

masking the common areas from a background image to the event image. The following is

an example of a step-by-step method for masking the images.

* Create an average background image (see Figure 5.7). For a particular wave breaking

event, images from the days either proceeding or following the event with limited or no

breaking should be used. An average image can be created from these that represents

100 200 300 400 500 600 700

Pixel X Coordinate

Figure 5.5 A pixel intensity plotting along a horizontal line without the artificial reef
intensity spike, original image dated April 11, 1995

0 100 200 300 400 500 600 700

Pixel X Coordinate

Figure 5.6 A pixel intensity plotting along a horizontal line with the artificial reef intensity
spike, original image dated April 11, 1995

Figure 5.7 An average background image in calm weather conditions dated April 10,
1995, one day before the storm events on April 11 and April 12, 1995

the typical background conditions. By selecting days near the event, differences in

tidal cycle (spring vs. neap) and seasonal lighting (winter vs. summer) can be reduced.

* Determine the average intensity of the background image. Select an area in the water

to determine the average intensity using the histogram plotting for a user-specified area.

Insure that the region selected does not contain breaking waves in either the

background and event images. Note the pixel coordinates of the region selected.

* Create an average event image. Typically, 5-10 images from the same hour will yield a

good average.

* Determine the average intensity for the event image using the same pixel coordinates

for the defined region.

42

* Correct the event image for ambient lighting. Using the simple pixel operations, add or

subtract the correct pixel intensity as determined from the difference between the

averages of the background and event images. A corrected event image results as

shown in Figure 5.8.

* A masked image (Figure 5.9) will be created by subtracting the background image from

the event image, with black (0) in the locations where the images were the same, and

the difference between the background water color and the wave breaking in various

shades of gray.

* Compare the background, event and masked images to determine which pixels

represent the wave breaking of interest. Typically, there will be some scattered gray

pixels in locations where wave breaking is impossible, either on land, in the sky or on

the timecode information. This information is noise, and should be ignored by simple

pixel operations.

This mask clearly shows the regions from the event which are different from those

in the background image, along with some noise in the timecode region. In reviewing the

event image, it is apparent that there are two distinct areas of breaking, near the shoreline

and offshore over the natural reef. The total intensity values of the two areas can be

determined by the SVIP operations. Figure 5.10 plots the total intensity values of the

three areas on an hourly basis during a storm event.

Figure 5.8 A corrected event image created by subtracting the difference between calm
water background and storm event images from the event image dated April 11, 1995

Figure 5.9 A masked image created by subtracting the background image dated April 10,
1995 from the storm event image dated April 11, 1995

800000
-4- Wave breaking Over the Natural Reef
700000 -.--- Wave Breaking near the Shoreline
700000
-..

600000 /
"i ". i s/
500000 '

5 400000

300000 /

200000 /

100000

0
6 8 10 12 14 16 18
Time of Day (April 11, 1995)
Figure 5.10 The total intensity values of the offshore and nearshore areas on an hourly
basis during a storm event dated April 11, 1995

5.4 Image Rectification

One of the major stumbling blocks in the image analysis to date is the

determination of areas and distances. The nature of oblique photography, however, does

not easily lend itself to direct distance measurements. The oblique image recorded onto

the film has been passed through a spherical lens, thereby distorting the distances from the

center of the focal plane in both the horizontal and vertical directions. The simplest way

to avoid these complications is to compare objects along a given horizontal or vertical line

that are equidistant from the centerline of the camera (Sloop, 1995). Vertical structures,

such as groins, seawalls, and trees can be scaled to provide estimates of wave heights and

45

surf zone locations at the same longshore distance from the camera in the Palm Beach

images. By measuring along lines parallel to these structures, offshore distances can be

estimated if a known distance can be determined. A grid of large stakes can be placed on

the beaches of interest to aid in distance estimates. For the applications of Palm Beach

images, image locations of interest are determined using known photogrammetric

transformation equations, assuming the vertical coordinate to be still water level

(Lippmann and Holman, 1989). Mason (1993) presented a detailed application of image

rectification process using the transformation equations given by Lippmann and Holman

(1989) .

The location of any object in the image is a function of the spatial orientation of

the camera in relation to ground topography. This can be expressed as Equation (5.4)

(Mason, 1993).

( x,y) = g ( X, Y, Z, f,, r, (, s, E) (4)

where x and y are the image coordinates, X, Y, and Z are the corresponding ground

coordinates to be imaged, fe is the camera focal length, t is the camera tilt (upward from

vertical), 4 is the camera azimuth, s is the swing angle of the camera, and E is the

elevation of the camera. If the ocean surface is assumed to be a plane, the relationship

between two-dimensional oblique image and the three-dimensional ground coordinates can

be transformed by a simple analysis of the geometry. Lippmann and Holman (1989)

developed the equations (Equations (5.5) through (5.7)) for geometrical transformation

from image to ground coordinates. Figure 5.11 shows the geometry and labeling

O o
N

Figure 5.11 The geometry and labeling conventions used in the rectification process
(modified from Lippmann and Holman, 1989)

47

conventions used in the rectification process ( modified from Lippmann and Holman,

1989). The camera is located at point O with elevation of Ze above ground plane origin

N, called the nadir point. The optic axis of the camera intersects the center of the focal

plane at point p, called the principal point, and forms an angle T (the camera tilt), upward

from vertical. For the simple case, the ground location of any point Q is determined from

its image coordinates by

XQ = Zsec(t+a)tan(y)
YQ = Zctan(t+a)

where the angles

a = tan-'( )

(5.6)
y = tan-( q (5

By simple inverting and combining, Equations (5.5) and (5.6) yield

yq = ftan(tan-1 ) T)
22 1 (5.7)
x =( 2 2 X2
z2 +Y y
c Q

Equations (5.7) express the geometrical relationship between image coordinates and

ground coordinate systems. These equations are functions of the camera tilt, t, and focal

length, fc, however, field measurements of the camera tilt and focal length are difficult and

48

can be inaccurate. Moreover, the ground coordinate system defined by the principal line is

not convenient. The transformed ground points need to be rotated into a more traditional

coordinate system with the x-axis and y-axis directed offshore (eastward) and longshore

(southward), respectively. Let (p be the angle between the two coordinate systems, then

the ground point positions are given by

X = -E-cos(- ) + S-sin(- p)
72 2 (5.8)
YQ = Esin(- (p) + S'cos(- )
2 2

where E and S are the offshore and longshore distances from the nadir point.

Unfortunately, the field measurement of (p is again difficult. However, the

unknowns, T, f,, and p, can be solved accurately using iterative methods from Equations

(5.7) and (5.8) by knowing both the ground and image coordinates of reference points.

For the Palm Beach conditions, with the camera elevation of 114 ft, the t, f., and cp are

found to be 86.04, 623 pixel units and 18.5 respectively.

Figure 5.12 shows a time-averaged oblique image of a storm event in April 1995.

To test the rectification program, three bright lines are marked at the approximate

locations of the natural and artificial reefs and seawalls on a visual basis in this oblique

image. The intensity values of the three lines are set at 245, 235 and 255, respectively.

Figure 5.13 presents a rectified image obtained from Figure 5.12 using the rectification

program based on Equations (5.7) and (5.8). These three reference lines are approximate

parallel in the rectified image consistent with beach surveys, although a large distortion of

Figure 5.12 An oblique image of storm event dated April 11, 1995 with three white
reference lines marked at the approximately locations of the natural and artificial reefs and
seawalls on a visual basis

Figure 5.13 A rectified image obtained from Figure 5.12

50

the image is evident in the far field of the camera due to the large camera tilt (86.04*). A

maximum camera tilt of approximately 85 degrees is recommended for large oblique

images. Figure 5.14 plots the cross-shore intensity profile and bathymetry for the profile

line R96G which is approximately 2960 feet south of the camera. The locations on the

intensity values at 245, 235 and 255 indicate the positions of the artificial and natural reefs

and shoreline which match the bathymetry for March 1995 very well. An error in the

estimated offshore distance of less than 10% was found in the rectified image compared

with the distance measured by beach survey. It is believed that the rectification program is

reliable in the application to the Palm Beach images.

Profile R96G

200 ---- ----------------------------- --------------------

S: Intensity
100 ----- ----------
__ ------------------------
C

0
0 500 1000 1500

20

S 10-------------------------- --------------
CO
0 ------ ------------------- 7-------------------------- 1 ---------- ---- ----------
e :Bathymetry for March 1995
W -10 ..------ .-- J...-----.-- ---------- ------------. - --. ---... ---. .-
A0
Artificial reef Natural reef
-20
0 500 1000 1500
Distance from monument R96G (ft)

Figure 5.14 Comparison of image intensity and beach profile

51

5.5 Error in Rectification Process

As discussed, the rectification process which transforms points in the oblique

image to real ground locations is based on estimates of the camera height, the focal length,

the camera tilt and the angle between the camera and ground coordinate system. The

accuracy of the technique then relies on the precision of each of these estimates.

Lippmann and Holman (1989) found theoretical errors in the estimates of t, fe, and (p with

rectification method to be within 0.250, 5% and 0.5, respectively. The primary sources

of error of these estimates are measurement errors.

In image rectification, it is assumed that all points lie at the mean water level.

Since the water surface fluctuates around its mean depending on wave and tide conditions,

some errors in estimates of horizontal distance will error. The error in the offshore

distance is larger than in the longshore distance and will increase with increasing distance

from the camera. Larger waves and higher tides tend to cause errors in the offshore

direction, while residual foam tends to skew errors onshore (Lippmann and Holman,

1989). Also tidal variations will influence the location of breaking waves. Estimates of

cross-shore scale in the rectified image are good for small waves just breaking over the

natural reef and at low tides.

The measurement error includes the effects of measuring the locations of the

camera nadir and reference points on the map, outlining the image frame, locating the

principal point and reference points in the image coordinate system with some uncertainty.

52

Measurement errors are the primary source of errors in the image analysis, because these

measurements are done by hand on a visual basis or by simple equipment.

There are other sources of error for example, due to camera horizontal tilt,

camera elevation surveying error, the distortions in the oblique image caused by the large

camera tilt angle of 86.04, the resolution limit of pixel for rectification processing

(since no better than /2 pixel can be resolved), uncertainty in image size (usually image

size is square, but this image processing system ( described in Section 5.2) represents the

image as a 751 x 239 array of pixels) and pixel transformation error in point processing

without regard for neighboring pixel gray levels (described in Section 5.1.2.3). However,

these effects are negligible relative to the measurement error.

Using the rectification technique, it is believed that errors of estimated longshore

and offshore distance to the camera from the rectified image are found to be less than 3%

and 10%, respectively.

CHAPTER 6
RESULTS AND DISCUSSION

6.1 Introduction

One year of video images, extending from April 1, 1995 to April 1, 1996, were

collected as described briefly in Chapter 4. The cameras recorded beach and offshore

activities in all weather conditions during daytime. Images of interest under storm

conditions were analyzed with the rectification method and other techniques mentioned in

Chapter 5. This chapter quantifies the incident wave energy dissipation features by the

artificial and natural reefs based on image intensity values above background obtained for

calm water levels without wave breaking activity.

The imaging technique encompasses the horizontal positions of the shoreline and

the natural and artificial reefs over a large spatial area. As stated previously, in image

rectification, it is assumed that all points lie on the mean water levels. Some errors in

estimates of horizontal positions will occur due to the water surface fluctuation around its

mean depending on tide and wave conditions. However, an average of images over a full

tidal cycle would reduce the effect of the tidal and swash motions. The longshore

direction error is not significantly affected by the water surface compared with the

offshore direction error. The error in the location of a breaking wave over the natural reef

54

in the cross-shore distance is more dependent on the water surface. Moreover, this error

increases with increasing distance from the camera. Larger waves and higher tides tend to

cause errors in the offshore direction, while residual foam tends to skew errors onshore.

For a worst case with a tide range of 3 feet, then the error is approximately 30 feet in

the offshore distance at 1000 feet from the camera. However, the errors resulting from

the tidal fluctuation are within the range of image-based estimates (10%). Estimates of

cross-shore scale in the rectified image are good for small waves just breaking over the

natural reef and at low tides.

Lesnik (1979) presented an annotated bibliography on detached breakwaters and

artificial headlands. A reasonable large body of literature is available for the development

of reliable design procedures for detached breakwaters, including many references dealing

with topics on wave diffraction, reflection, transmission, overtopping and wave dissipation

characteristics of submerged breakwaters.

Ahrens (1987) conducted a series of model tests to determine the wave

transmission, wave reflection, and energy dissipation characteristics of low-crested reef

breakwaters. It was found about 35 to 70 percent of the wave energy would be dissipated

by reefs with crests near the still water level, and dissipation was strongly dependent on

the relative reef width (At/(dLp)) as shown in Figure 6.1, where At is cross-sectional area

of reef breakwater, d, is total depth of water, and Lp is Airy wave length.

Referring to Figure 6.2, the artificial and natural reefs are approximately 250 and

1000 feet east of the seawall location, respectively. It is noted that the crests of the

artificial and natural reefs are almost at the same elevation; however, the base width of the

0.8

0.7

.o!
a. 0.6
(o

0.5

w
0.4

0.3

0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32

Relative Reef Width, A,/(dLp)

Figure 6.1 Energy dissipation by reefs with crest near the still water level as a function of
the relative reef width (Ahrens, 1987)

20

15

>10
(9
Z,

0
0
Q.

a0
w-il

0 150 300 450 600 750 900 1050 1200 1350 1500

Distance from monument 97C (ft) seaward --

Figure 6.2 Profile (Monument R97C) at approximate mid-length of the artificial reef

---'------- --- --- ----I------------......-.......-- -------..................---'---I---'-................---'-----
l l I I I I I

... -...... . ......... ........... ......... ... ....... - -....... ................. ........ .

: ::
co

0; 0

. . . .......... . .. .. ... ... ...... ... .... ..... . .... ..
0
...... ........... ................................ ........... ............ ......... ..................... ......................

I I I I I I "
-Seawall

--- Bathymetry forAugust 1993
-...... ...... ... Bathymetry for June 1995 ..................

A

Artificial Reef
-------. -- -- \ . i .. .

Nat ural :Reef

56

artificial reef is 15 feet with a crest width of 1 foot. Comparatively, the natural reefs crest

width in this area varies from 400- 600 feet, a significant difference.

The average images of storm waves are digitized and rectified. The analysis of

video images focuses on cross-shore intensity profiles in the rectified images at prescribed

longshore distances corresponding to available beach profile surveys. Image light intensity

profiles are assumed proportional to the wave dissipation profiles of the incident waves.

During storm periods, wave dissipation over the natural and artificial reefs and near the

shoreline is estimated based on the intensity profiles. The percentage of wave energy

dissipation by the artificial reef estimated by image intensities compares with in-situ wave

gage data and wave transmission losses conducted by a numerical model developed by

Dean and Bootcheck (1996, unpublished).

6.2 Theoretical Background

The time averaged image will be dominated by the white foam of breaking waves

for the case of storm waves breaking both offshore and nearshore. It will essentially be

an image of average wave energy dissipation. During high waves, persistent surface foam

obscures the relationship of the visible signal of image intensity to wave dissipation with

some uncertainty. The actual patterns of image intensity recorded on the video film are a

result of the bubbles and foam of breaking waves. It has been hypothesized and proven

under some conditions that the intensity value above the background value digitized from

the rectified image is approximately proportional to the wave energy dissipation

(Lippmann and Holman, 1989). Previous work by other investigators had established that

57

the degree of "whiteness" of the white foam on the images could be considered as a

measure of the wave energy dissipation. With this as a basis, the relative effectiveness of

the various areas in wave energy dissipation is illustrated by referring to storm events in

Section 6.5. This technique relies on the foam whiteness generated by wave breaking

relative to the surrounding non-breaking waves; thus the method best represents the

energy dissipation due to wave breaking. In particular, for the artificial reef, some

additional energy dissipation will occur due to wave structure-interaction.

Figure 6.3 shows a time averaged image of 39,600 images dated from July 18,

1995 to August 16, 1995. This image yields a much clearer view in the offshore and

nearshore areas, and thus removes the "noise" signal from the image. It is representative

of the background of oblique images. Using histogram operations, it is found that the

mean intensity value for calm seas under Palm Beach conditions is 96.

Figure 6.3 A time averaged image of 39,600 images dated from July 18, 1995 to August
16, 1995

58

6.3 Image Area of Interest

As previously described, the camera is mounted at an elevation of 114 feet, is

located approximately 1560 feet north and 180 feet west of the artificial reef installation,

and the view is oriented in a southeastward direction (approximately 18.50) and

encompasses the natural and artificial reefs and the shoreline. Figure 6.4 illustrates the

bathymetry in the vicinity of camera view for the June 1995 survey.

The rectified image area of interest is almost the same size as the areas plotted in

Figure 6.4. The size of the rectified image is 1500 feet east-west and 6000 feet north-

south. Figure 6.5 presents a rectified image obtained from an average image of oblique

images as shown in Figure 6.3. Here, the maximum resolution of the rectification

technique is limited to 10 feet in east-west and north-south directions, since the image is

rectified to 10 feet/pixel. This is within the range of measurement errors and can give

accurate results, although the image could be rectified at 5 feet/pixel or less.

6.4 Average Images Under Storm Conditions

Due to the current study focus on wave energy dissipation by the artificial and

natural reefs, only images which show wave breaking both offshore and nearshore under

storm conditions are analyzed, although one year of video data are available. Table 6.1

lists times, number of digitized images and wave conditions for average images of storm

events which are of major interest between April 1995 and August 1995, when the

removal of the artificial reef commenced. During the storm events, those images

1200 ... .l 14.o
1-00 .'"- ... .... .--... .- -...-... ... ...... ; ... ...... 1.. . -
1100 12".0. . .." .. "
1000 .. ..- Approxirr ately Natural Reel Position
1000 "... .* " ,. ""'"' i -"- | .. '
900
Soo800
S" ... I ... .. '

E 700 ..
"* .' '.. : i..' .- i: .:.i : "' 5
S oo00 : : i .' i -
S 00 .- ..,. -W
400
S- .. -. ......... ficil: ".....:. .-

200 R96B .......R96G .R97D r.8E ....

-100 .I
0 1000 2000 3000 4000 5000 6000
300 North
Distance South of Camera (ft)

Figure 6.4 Bathymetry in the vicinity of camera view for June 1995 survey
Figure 6.4 Bathymetry mn the vicinity of camera view for June 1995 survey

Figure 6.5 presents a rectified image obtained from an average image of oblique images as
shown in Figure 6.3

60

Table 6.1 Times, number of digitized images and wave conditions for average images of
storm events

Number of
Date Time digitized Wave conditions
images

Large wave breaking both
April 11, 1995 6:00 am 19:00 pm 245 in e and offsho
inshore and offshore

Reduced wave breaking
April 12, 1995 6:00 am 13:00 pm 145 ofr
both inshore and offshore

Limited wave breaking
May 10, 1995 8:00 am 14:51 pm 105 of
both inshore and offshore

Small wave breaking both
June 21, 1995 5:00 am 19:30 pm 275 o h
inshore and offshore

Small wave breaking both
July 27, 1995 11:35 am 14:53 pm 70 S re
inshore and offshore

Large wave breaking both
July 28, 1995 15:23 pm 19:25 pm 85 inshoe and offshore
inshore and offshore
Large wave breaking both
July 29, 1995 5:33 am 16:09 pm 215 Largi ve breaking both
inshore and offshore

Reduced wave breaking
July 30, 1995 9:22 am 19:13 pm 200 ofhr
both inshore and offshore

Large wave breaking both
July 31, 1995 5:50 am 19:19 pm 270 insh
inshore and offshore

Reduced wave breaking
August 1, 1995 5:53 am 18:58 pm 25 a ofhr
both inshore and offshore

Medium wave breaking
August 2, 1995 15:38 pm 16:38 pm 260 a ofh
both inshore and offshore

Medium wave breaking
August 3, 1995 5:58 am 9:53 pm 85 bh in re ad o re
both inshore and offshore

61

documented substantial breaking over the natural reef and near the shoreline, but very

little in the vicinity of the artificial reef. Waves broke over the natural reef by

plunging/spilling as well as breaking nearshore and continued as dissipative bores up the

beach.

Figure 6.6 show the average images of each storm event listed in Table 6.1. These

time averaged images were created digitally by mathematically averaging successive video

frames over the storm period using an image processing system. Wave breaking is evident

on the beach face and the offshore natural reef as seen from those average images. Using

the rectification technique, the average images can be rectified to produce a plan view with

known scaling. The rectification process involves mapping the oblique image intensities

pixel by pixel onto the scaled ground grid. From the rectified view, cross-shore intensity

profiles at prescribed longshore distance are readily found. Figures 6.7 presents the

rectified images of areas of interest of the average images as shown in Figures 6.6. Figure

6.8 shows a three-dimensional view of June 1995 survey corresponding to Figure 6.4. The

approximate locations of the artificial and natural reefs are shown as well as the Clarke

Avenue Beach which has no seawall. It is noted that the overall length of the artificial reef

is 4,176 feet including a gap of 216 feet near the north end in the vicinity of the Clarke

Avenue Beach. Figure 6.9 presents three-dimensional maps of image intensity values of

rectified images shown in Figure 6.7. This 3-D plotting gives the visual representation of

intensity distribution in relation to the bathymetry. Clearly, there are local maxima in the

intensity distribution in the vicinity of the shoreline, and the second highest intensity

distribution is located over the natural reef. It is also seen that the distribution of wave

(b) April 12, 1995

(c) May 10, 1995

(d) June 21 1995

(e) July 27, 1995

(t) July 28, 1995

(g) July 29, 1995

(h) July 30, 1995

(i) July 31, 1995

(j) Augustl, 1995

(k) August 2, 1995

(l)August 3, 1995

Figure 6.6 Average images of storm events listed in Table 6.1

(a) April 11, 1995

(a) Anril 11. 1995

(c) May 10, 1995

(d) June 21, 1995

(e) July 27, 1995

(f) July 28, 1995

(g) July 29, 1995

(h) July 30, 1995

(i) July 31, 1995

(j) August 1, 1995

(k) August 2, 1995

(1) August 3, 1995

Figure 6.7 Rectified images of areas of interest corresponding to Figure 6.6

(b) April 12, 1995

10

> 0
o Natural Reef
z
-10
0
> -20
.

-30
6000
o/4i 4000

o) 2000
o',

Pr)

0 1500

S-500
1000 era 0)
East o0 Ca
DistanCe

Figure 6.8 Three-dimensional view of the June 1995 beach survey

(a) April 11, 1995

200
150
100

0
6000

4000

o, 2000
S1500 1 00
(4 0 2000 ODto c E '

(c) May 10, 1995

(b) April 12, 1995

o, 2000 0 00
S1500
ol 0 2000 ot ..o ,C"

(d) June 21, 1995

200
150
S1oot

50

6000

4000
'P 2000 '. 500
1000 Eto to
So',, - .... 1500 oltofC "
0 2000 1snc

(e) July 27, 1995

150500
0 2000 t500 E o
0 2000 DI$kSAG

(f) July 28, 1995

Figure 6.9 (a), (b), (c), (d), (e), (f). 3-D perspective plots of image intensities of rectified

images corresponding to Figure 6.7 (a), (b), (c), (d), (e), (f)

200 200
150 150
0 io100j
0 50.

0 0
6000 6000

o00 0
0 20004 1500 50 c1 0
100 O Oes 1500 O G.* taes
90 2000 005 *o 2000 DnOO (4*S

200
150
100
S 50

0]
6000

(g) July 29, 1995

200
150
100
S50.

0
6000

500
S 1500 me t
4 0 2000 O Stae *(Go

(i) July 31, 1995

(h) July 30, 1995

0 0 500
1500 1 (Ca'mear'a0
"e 0 2000 ost,,c' L East'

(j) August 1, 1995

200.
150

S100.
50

0
6000

4000

10 i'500
I- r ^1500 --c e** '
*, O 2000 0 I'st s e

(k) August 2, 1995

200
150
S100
8 50

0
6000

44, 4000

S2000 1000 5
1500 5 0."
(1)0 2000 gust 3 ,

(1) August 3, 1995

Figure 6.9 (g), (h), (i), (j), (k), (1). 3-D perspective plots of image intensities of rectified

images corresponding to Figure 6.7 (g), (h), (i), (j), (k), (1)

200 200

150 150
100 100
S50, E 50

0 0
6000 6000

o 444000 0 4000 0

22000 15 0

S 0 2000 ",,sto 0ee% 0 2000 D2S.c
10 o 1000 ___o_________

200
150

c 50

0,
6000

4000

67

heights with some dissipation arises offshore while nearly all waves are breaking near the

shoreline. Offshore breaking occurs for the more energetic waves. Referring to Equation

(5.2), the image intensity is characterized by the amount of source light incident on the

scene and the amount of light reflected by the objects in the scene. For adverse weather

conditions such very cloudy or rain, less reflectance will occur and thus lower intensity

recorded on the video film with increasing distance from the camera, so there is lower

intensity distribution in the south areas compared with north areas. Figure 6.10 presents

an example of perspective plot of image intensities above background of rectified image.

This figure demonstrates that the natural reef causes a substantial loss in wave energy

while very little wave energy dissipation occurs due to the presence of the artificial reef.

But most wave energy is dissipated near the shoreline.

6.5 Image Results

Based on the hypothesis that the intensity value above background digitized from

the rectified image is approximately proportional to wave energy dissipation, the

dissipation intensity values can be represented by four representative beach profiles (at

Monuments R96B, R96G, R97D, R98E) indicated on Figure 6.4. Those four profiles are

located approximately 2,000, 3,000, 4,000, and 5,000 feet south of the camera,

respectively. Similar profiles could be taken at various locations. Profile R96B represents

the Clarke Avenue Beach area where a natural beach of 650 ft length is present. Profiles

R96G and R97D are approximately at the one-third and one-half distance along the

artificial reef. Those three profiles start at the seawall that fronts most of this shoreline

Figure 6.10 An example of perspective plot of image intensities above background of
rectified image, April 11, 1995

within the artificial reef confines. Profile R98E is representative of the profile lines in the

south area of the artificial reef confines without seawall effects.

To illustrate the general quantitative aspects resulting from the time averaging

images, comparisons of image intensity and beach profile are developed with the use of an

image processing system. Figure 6.11 shows an example of image intensity analysis of

storm waves dated April 11, 1995. The intensity plot and corresponding surveyed beach

profile were taken at the same longshore coordinate. The intensity profile magnitude

80
0
L.
o 60

e0
> 40
0

S 20

c0
c 6000

S 4000
-a0

0 500
1500 00 (c e
1500 Vast ot CaOer

,^^S%

1F

P ro file R 9 6 B

200

0 In tens ity

0
0 500 1000 1500

20

10

0
0 Bathym etry for M arch 1995
-1 0

-20
0 500 1000 1500
D vista n c e from m o n u m e n t R 96 B (ft)

Profile R96G

Breaking nerabore , Breakling over the natural reef

200 (11iI

SIntensity
S 100- --

Brek.ing over the PEP Reef

0 500
O 500

-- A

1000

1500

Bathymetry for March 1995

500 1000
Distance from monument R96G (ft)

1 500

P ro file R 9 7 D

200

100

500

1000

1500

B athym etry for M arc h 1995

0 500 1 000 150

D instance from m on u m ent R 97D (ft)

Figure 6.11 An example of image intensity analysis, April 11, 1995.

20

1-0
o
LL -10

-20
O0

/ In te n s ity

_L

0

(

70

was scaled to fit the range of the survey profile. It is noted that the intensity plot was

taken from a rectified image on April 11, 1995 and the survey profile was performed on

March 28, 1995. Based on the video records, there was only minor wave activity during

the period from March 28, 1995 to April 11, 1995. A general comparison can be made

since only minor bathymetric changes are expected due to low wave activity. By

examining this figure, there are five features evident: (1) The apparent correlation of the

magnitude of the image intensity and beach survey profile lines is high, (2) The maximum

intensity values are in the vicinity of the shoreline where wave dissipation is complete, (3)

The second highest intensity values, located over the natural reef, depict the wave

breaking dissipation due to its shallow and wide crest, (4) No significant wave breaking

occurred due to the artificial reef, and (5) The lowest intensity values landward and

seaward of the offshore natural reef correspond to the trough area and little or no incident

wave breaking. Tables 6.2 through 6.13 list the estimated wave energy dissipation values

due to wave breaking over the natural and artificial reefs and nearshore beach areas for the

four profile lines for the storms presented in Table 6.1. Since the absolute magnitudes of

intensity vary with ambient light and camera apertures, only relative intensity values above

the background value are interpreted as representative of wave energy dissipation. By

examining Profile R96B from Tables 6.2 through 6.13, the percentage dissipation over the

natural reef and near the shoreline are approximate 30.7-60.3% and 39.1-68.4%,

respectively. The percentage dissipation over the artificial reef ranges from 0.2% to 0.9%.

This profile starts at the Clarke Avenue Beach through the artificial reef unit immediately

south of the artificial reef gap. Wave breaking near the shoreline at the Clarke Avenue

71

Beach is most evident associated with large dissipation in this area. The low percentage

dissipation over the artificial reef at this profile line is believed to be partially due to the

absence of the artificial reef unit at the gap which commences only 24 feet north of this

line.

Based on Tables 6.2 through 6.13, the average dissipation percentages over the

natural reef, the artificial reef, in the vicinity of shoreline and the average percentage

dissipation by the artificial reef of the energy passing over the natural reef are summarized

in Table 6.14.

Referring to Table 6.14, for small storm waves ( such as on May 10, 1995,

June 21, 1995, July 27, 1995 and July 30, 1995), more than 50% percent of the energy is

dissipated near the shoreline due to shore break, while for large storm waves (for example,

on April 11, 1995), most of the dissipation located over the offshore natural reef. For the

storm waves from August 1 through August 3, 1995, more than 50% of the dissipation

occurred offshore. For the period July 29, 1995 through August 3, 1995, the percentages

of wave energy dissipation by the artificial reef varied over a small range from 0.6% to

0.9%, and lower than those before July 29, 1995. This may be due to the high tides

during this period.

Overall, the ranges of dissipation percentages over the natural reef, the artificial

reef, and near the shoreline are 35.7% to 65.6%, 0.6% to 1.3%, and 33.7% to 63.1%,

respectively. Moreover, the dissipation percentages by the artificial reef of the energy

passing over the natural reef vary from 1.1% to 2.5%.

Table 6.2 Estimated wave energy dissipation based on intensity values, April 11, 1995

Background Breaking near the Breaking over the Breaking over the
roil i shoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 96 11543 51.9 208 1.0 10499 47.1

R96G 94 11433 45.1 361 1.4 13557 53.5

R97D 97 6607 37.2 218 1.2 10967 61.6

R98E 98 8428 43.8 172 0.9 10646 55.3

Table 6.3 Estimated wave energy dissipation based on intensity values, April 12, 1995

Background Breaking near the Breaking over the Breaking over the
P e vae shoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 90 9048 64.6 140 1.0 4829 34.4

R96G 89 9999 57.7 309 1.7 7051 40.6

R97D 92 4864 50.2 143 1.4 4698 48.4

R98E 91 6939 53.1 114 0.8 6034 46.1

Table 6.4 Estimated wave energy dissipation based on intensity values, May 10, 1995

Background Breaking near the Breaking over the Breaking over the
Proe le ve shoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 73 11710 68.6 126 0.7 5249 30.7

R96G 75 10589 66.5 196 1.2 5152 32.3

R97D 78 5199 54.9 131 1.3 4148 43.8

R98E 81 4386 54.4 172 2.1 3511 43.5

Table 6.5 Estimated wave energy dissipation based on intensity values, June 21, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line value shoreline artificial reef natural reef
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 105 7363 59.4 103 0.8 4946 39.8

R96G 106 7047 57.8 204 1.6 4955 40.6

R97D 108 3791 50.8 101 1.3 3576 47.9

R98E 103 7482 43.2 185 1.1 9678 55.7

Table 6.6 Estimated wave energy dissipation based on intensity values, July 27, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line value shoreline artificial reef natural reef
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 94 6893 65.6 54 0.5 3559 33.9

R96G 94 7605 57.9 168 1.2 5369 40.9

R97D 98 3745 56.7 64 1.0 2797 42.3

R98E 99 4267 51.6 119 1.4 3891 47.0

Table 6.7 Estimated wave energy dissipation based on intensity values, July 28, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line value shoreline artificial reef natural reef
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 75 15667 54.1 239 0.8 13065 45.1

R96G 75 16079 51.0 220 0.7 15256 48.3

R97D 79 9976 45.9 293 1.3 11486 52.8

R98E 82 10522 49.5 428 2.0 10328 48.5

Table 6.8 Estimated wave energy dissipation based on intensity values, July 29, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line valueshoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 84 13318 57.6 94 0.4 9708 42.0

R96G 85 11228 52.6 208 1.0 9890 46.4

R97D 87 7710 48.2 139 0.9 8150 50.9

R98E 88 9143 50.2 189 1.0 8891 48.8

Table 6.9 Estimated wave energy dissipation based on intensity values, July 30, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line value shoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 103 11394 60.6 54 0.3 7368 39.1

R96G 105 9946 61.2 136 0.8 6180 38.0

R97D 106 5952 55.3 97 0.9 4709 43.8

R98E 105 6431 51.5 75 0.6 5972 47.9

Table 6.10 Estimated wave energy dissipation based on intensity values, July 31, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line value shoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 89 11985 50.3 63 0.3 11759 49.4

R96G 90 10794 50.5 136 0.6 10441 48.9

R97D 91 7126 43.6 131 0.8 9099 55.6

R98E 92 8785 48.0 128 0.7 9377 51.3

Table 6.11 Estimated wave energy dissipation based on intensity values, August 1, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line vashoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 106 10287 40.4 49 0.2 15143 59.4

R96G 110 7284 39.0 192 1.0 11216 60.0

R97D 111 4826 35.8 116 0.9 8538 63.3

R98E 112 6003 39.6 176 1.2 8971 59.2

Table 6.12 Estimated wave energy dissipation based on intensity values, August 2, 1995

Background Breaking near the Breaking over the Breaking over the
Profile line value shoreline artificial reef natural reef
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 106 8204 39.1 126 0.6 12627 60.3

R96G 108 5216 31.8 192 1.2 11003 67.0

R97D 110 2740 24.1 124 1.1 8493 74.8

R98E 101 9581 34.6 57 0.2 18031 65.2

Table 6.13 Estimated wave energy dissipation based on intensity values, August 3, 1995

Background Breaking near the Breaking over the Breaking over the
roil lin v shoreline artificial reef natural reef
Profile line value
(intensity) Intensity Percentage Intensity Percentage Intensity Percentage

R96B 101 9559 49.0 94 0.5 9848 50.5

R96G 102 7935 51.1 176 1.1 7404 47.8

R97D 103 4261 38.4 150 1.4 6686 60.2

R98E 100 7509 40.9 172 0.9 10694 58.2

Table 6.14 The percentages of wave breaking related dissipation by various locations for
storm events

Dissipation
Dissipation percentage of the total wave percentage
energy on the
artificial reef
Date of storm events of the wave
energy
Nearshore Artificial reef Natural reef passing over
the natural
reef

April 11, 1995 44.9 1.1 54.0 2.5

April 12, 1995 57.0 1.3 41.7 2.2

May 10, 1995 63.1 1.2 35.7 1.9

June 21, 1995 52.0 1.2 46.8 2.3

July 27, 1995 58.4 1.1 40.5 1.8

July 28, 1995 50.5 1.1 48.4 2.2

July 29, 1995 52.6 0.8 46.6 1.5

July 30, 1995 57.8 0.6 41.6 1.1

July 31, 1995 48.5 0.6 50.9 1.2

August 1, 1995 39.0 0.7 60.3 1.9

August 2, 1995 33.7 0.7 65.6 1.9

August 3, 1995 45.4 0.9 53.7 2.0

77

6.6 An Example of Oblique Image Analysis

The rectification process is somewhat time-consuming. The SVIP system can

permit an immediate visual inspection of intensity distribution along a user-specified

horizontal or oblique line. Also, an intensity profile of an image of interest can be plotted

at the specific location in order to provide a "rough" measurement from the original

oblique images. The intensity plot would be displayed on the video monitor with

horizontal axis representing pixel x coordinate and the vertical axis indicating the intensity

value. The locations of images of interest can be estimated by measuring the distances

between the known references such as seawalls, groins, buoys, stakes and trees. This

method is not intended to be exact. However, it may be useful in identifying for further

study from the large number of digital images, before applying the rectification process

that is tedious and requires a considerable amount of time and effort. Figure 6.12 presents

an intensity profile at a specific oblique line of the time-averaged oblique image of

April 11, 1995. The location of this profile is somewhat near the profile R96G as

estimated by scaling the distance of seawall alignments. Notice that this plot is non-

dimensional and the vertical intensity values are not related to the offshore distance, so

only relative values are analyzed. Based on this intensity profile, the estimated dissipation

percentages over the natural reef, the artificial reef, and near the shoreline are 53.4%,

1.2%, and 45.4%, respectively. Of the energy passing over the natural reef, 2.6% was

dissipated by the artificial reef. By contrast, these values are 53.5%, 1.4%, 45.1%, and

200

180

160

140

120

100

80

60

40

20

0

200 300 400 500

Pixel X Coordinate

600

Figure 6.12 An intensity profile along a specific oblique line of the average oblique image
dated April 11, 1995

2.6%, respectively, according to the intensity profile line R96G in the Table 6.2

corresponding to the rectifiedimage of storm waves on April 11, 1995.

6.7 Ancillary Results From In Situ Data and Model Studies

6.7.1 Wave Attenuation

To monitor the reduction in wave energy, two subsurface pressure-velocity (PUV)

gages were installed approximately 75 feet on either side of the artificial reef and

Dissipation over the Artificial Reef

I _

Dissipation over the Natural Reef

79

somewhat south of the artificial reef centerline. The nearshore and offshore gages were

installed in approximately 6 and 13 feet water depths, respectively. The gages collected

data hourly, recording average pressure and two horizontal velocity components. Every

sixth hour, a full 1,024 second pressure/velocity record was recorded. These gages began

operation in mid-October, 1993 and reported via a telephone modem to the Coastal &

Oceanographic Engineering Laboratory (COEL) of the University of Florida in

Gainesville, Florida. The data have been analyzed in the Florida Coastal Data Network

(FCDN) format and stored in the COEL database.

In order to quantify more completely the wave height reduction characteristics

caused by the artificial reef, from September 20, 1994 through October 7, 1994 two

additional gages were installed adjacent to the longer term gages which are located

landward and seaward of the reef. This served as a check on the gage calibrations. From

October 7 through October 23, the additional gages were relocated to approximately 500

feet south of the reef in approximately the same water depths and the same separation

distance as the longer-term wave gages. The longer-term gages were damaged by storms

in late October 1994, were refurbished in April 1995 and removed in June 1995.

To quantify the reduction of wave height, transmission coefficients were defined as

follows:

S Hs nearshore
K, = (6.1)
Hs offshore shoaled

where Kt = transmission coefficient, Hs nearshore = nearshore (landward of the artificial

reef) significant wave height, and Hs offshore shoaled = the offshore (seaward of the

80

artificial reef) significant wave height shoaled by linear wave theory to the location of the

nearshore gage. Therefore, assuming no wave reflection, the dissipation percentage, rl,

over the artificial reef can be written approximately as

r = (1 K)*100% (6.2)

where rI = dissipation percentage and Kt = transmission coefficient. Table 6.15 lists the

average monthly significant wave height, transmission coefficients, and dissipation

percentages by the artificial reef for the period December 1993 through October 1994, and

January 1995 through March 1995. Figures 6.13 and 6.14 present the transmission

coefficients for the available data of 1994 and 1995, respectively.

For the period October 1993 through July 1994, the monthly average Kt for each

month varies over a fairly small range, from 0.76 to 0.87. These values are smaller (show

greater wave attenuation) than those predicted by theory and measured in laboratory tests.

It was believed that a portion of the wave height attenuation was due to wave energy

dissipation between the gages that occurs naturally in the absence of the artificial reef. To

evaluate this possibility, two additional gages were placed 500 feet south of the artificial

reef in approximately the same water depth and with the same separation distance as the

artificial gages (Browder, 1994). Data from these gages indicated that there was a

reduction in wave height between the two gages of about 10%, thus establishing that the

transmission coefficients resulting from the presence of the artificial reef range from 85%

to 95%(Dean and Chen, 1995c), considerably larger than those determined from analysis

of the two longer term gages alone (76% to 87%, October 1993 to July 1994) as shown in

Monthly averaged significant wave height and dissipation percentage

Month Hs Offshore Hs Nearshore Hs Offshore Shoaled Kt 11
(ft) (ft) (it)
December 2.10 1.84 2.43 0.76 42.24

January 94 2.30 2.07 2.66 0.78 39.16

February 94 1.97 1.77 2.26 0.78 39.16

March 94 1.67 1.51 1.94 0.78 39.16

April 94 1.84 1.67 2.00 0.84 29.44

May 94 1.28 1.12 1.44 0.78 39.16
June 94 0.79 0.69 0.85 0.81 34.39

July 94 0.98 0.89 1.02 0.87 24.31

August 94 1.05 1.08 1.11 0.97 5.91

September 1.12 1.21 1.18 1.03* *
October 94 1.77 1.84 1.90 0.97 5.91

November N/A N/A N/A N/A** **

December N/A N/A N/A N/A** **

January 95 1.48 0.98 1.57 0.62 61.56

February 95 1.51 1.34 1.59 0.84 29.44
March 95 1.84 2.26 1.93 1.17*** ***

April 95 1.80 1.75 1.93 0.91 17.19

May 95 1.28 1.28 1.32 0.97 5.91

June 95 1.71 1.70 1.78 0.96 7.84

*One of the gages is believed to have lost calibration
**No data available due to inoperable wave gages
** *Believed to be affected by wave reflection from seawall
Note that the reported Kt values are the averages of individual transmission coefficients and may
differ slightly from the values obtained by dividing the monthly averaged Hs Nearshore by
Hs Offshore Shoaled in the table above.

Table 6.15

1.2 i I It: *- -
+ + ++ + : -j

1.0 ... +-.... ................ .... +... + ..... ...... ..... ^^ M .-
+ +++ -+ +

+ -H- +
S+
10.8 + ++

0.4
0 .2 ......... ....... .... : ...
0 .2 ......... ........ .... ......... .................................. .......... ......... ....... ..

JanuaryFebruaryMarch April May June July August SeptemberOct.
0 .0 I 1 I I I I
0 30 60 90 120 150 180 210 240 270 300
Days
Figure 6.13 Wave transmission coefficients based on significant wave heights, January-
October 1994

1.2 .- + i I' i -H: : I: + I:

1 0+4: + + + : :

... : .. .. ..... ..-. ... .. .. :-. .. ... .+ -.. .. ... .......... .......... ....

70.6 _
+- + :
0.4 i ^ +

o .< ...... .......4 .......... .......... .......... .......... .......... ........ ....
0 .2 - --- --- .............
0.2
January ebruaryMarch April May June July :August Sept
0.0 I I I I" I I L
0 30 60 90 120 150 180 210 240
Days
Figure 6.14 Wave transmission coefficients based on significant wave heights,
January-June 1995

270 300

83

Table 6.15. The transmission coefficients for August 1994 through June 1995 are

generally significantly higher than for previous months. It is believed that either the

nearshore gage had lost calibration or the data at both gages were being affected by the

increased wave reflection from the seawall due to the significant loss of the fronting beach.

Referring to Figures 6.13 and 6.14, it seen that there is considerable scatter in the

transmission coefficients. The reasons for this scatter are threefold. First, greater wave

height transmission over the artificial reef occurs during high tides. Secondly, the effect of

wave heights as noted previously is apparent in the transmission coefficient data with the

larger wave heights resulting in the smaller transmission coefficients and vice versa.

Thirdly, the effect of the loss of beaches landward of the artificial reef has resulted in

greater reflection coefficients from the revetment and seawall, especially during periods of

high tides. This wave reflection can be observed during energetic conditions and causes

areas of wave reinforcement and cancellation between the incident and reflected waves,

the location of which depends on wave period and tide elevation. Thus, for some

conditions, the locations of wave reinforcement would occur at one of the gage locations

and possibly cancellation at the other gage and vice versa for other conditions. Reflection

from the seawall contributes to the scatter in the transmission coefficients although it is

believed that it does not affect the long-term average results. Wave reflection from the

seawall and interference with incident waves were evident visually in this field.

The PUV gages also measured current magnitude and direction near the artificial

reef centerline. During the summer months, the average current was approximately 0.23

ft/s, directed north along the beach. During periods of higher wave activity, i.e., the

winter months, the currents average approximately 0.10 ft/s, and are southerly directed.

The natural currents in the area may be a result, in part, of the tidal flows in and out of the

Port of Palm Beach Entrance, located approximately 4.5 miles to the north.

6.7.3 Laboratory Tests

A laboratory test reported by Dean et al. (1994b) and Browder (1994) has

evaluated the hydrodynamic performance of the artificial reef in the three-dimensional

wave basin at the COEL of the University of Florida. This model study was carried out to

investigate the installation of the artificial reef at Vero Beach, Florida; however, because

of the general nature of the results, these are reported briefly here. Forty-eight individual

1:16 model scale concrete units were tested in various configurations on a fixed horizontal

bed. Periodic waves were used in the study. Figure 6.15 shows a schematic of the

laboratory setup (Dean et al., 1994b).

Evaluation of the artificial reef consisted of wave attenuation measurements of the

periodic waves as well as current measurements via dye and drogues. Capacitance type

wave gages were used to measure wave heights from which transmission coefficients, Kt,

were determined as described previously. Current measurements were based on

Reef Model

Figure 6.15 Schematic of model basin arrangement for artificial reef testing (Dean et al.,
1994b)

videotaping dye movement and drogue displacement on the floor of the fixed concrete

bed.

This study predicts transmission coefficients of greater than 0.95 for general Palm

Beach conditions, much greater than those obtained from field data based on the two

wave gages adjacent to the artificial reef, but more consistent with the field measurements

where the previously noted non-reef induced dissipation was taken into consideration.

Figure 6.16 presents bottom drogue trajectories induced by the reef system which

indicate the existence of a pumping mechanism over the artificial reef system (Dean et al.,

1994b). It was found that especially for small freeboards (still water to the artificial reef

crest elevations), a strong current pattern generated by flow over the structure caused a

longshore current to the ends of the artificial reef and then offshore. These flows would

1:8 Gravel Gridded
Beach Test Area

Paddle
Wavemaker

Figure 6.16 Circulation patterns documented in model studies. Showing result of net flow
of water over the reef and induced longshore currents (Dean et al., 1994b)

play a key role in sediment transport and the cause is interpreted as follows. The transport

of water over a submerged breakwater creates a water level elevation termed "ponding" in

the lee of the structure. This ponded water cannot return seaward as readily as it would

without the presence of the artificial reef and a portion of the water transport over the

artificial reef is directed alongshore. The ponding elevations are believed to be relatively

small for the Palm Beach installation, and although neither the ponding nor associated

currents were documented directly in the field, it appears that the associated effects on

erosion landward of the artificial reef were substantial.

6.7.4 Numerical Model Results

Dean and Bootcheck (1996, unpublished) have developed a two-dimensional

numerical model quantifying the breakwater overtopping and increased water levels

behind the structure that modifies both wave and current fields landward of the

breakwater. These modified waves and currents are related to the freeboard (still water to

structure crest elevation), the length of the structure, the proximity of the structure to the

beach, and wave height. It is evident that all of these are relevant design parameters.

For the Palm Beach installation, settlement of the individual units, the shoreline

changes and the sediment considerations in the vicinity of the structure have been

documented since installation commenced in July 1992. Table 6.16 presents the history

of average settlement of the 330 units, the distance from the structure to the shoreline and

water depth immediately seaward of the artificial reef based on the beach surveys.

Figure 6.17 presents the predicted and measured transmission coefficients as

related to the freeboard and the incident wave height. Note that the field Kt values do not

account for the non-reef dissipation and should be increased on average by 0.05 to 0.18.

This figure indicates that the transmission coefficient value increases with the freeboard,

while it decreases with the incident wave height. Referring to Table 6.16, for the Palm

Beach installation, the freeboards were mostly in range of 5 to 6 feet. As seen from Table

6.15, for Palm Beach conditions, the incident significant wave heights varied from 0.7 to

2.3 feet. Based on those conditions, transmission coefficients larger than 0.90 are

predicted by this numerical model.

Table 6.16 The historical changes of the unit settlement, distance to the shoreline and
depth seaward of the units

Distance Water
Cumulative from the Average depth
settlement Freeboards depth
Survey date months units to the seaward of
of the 330 (feet) ,.
since 8/93 shoreline o t the units
units (feet)
(feet) uts (f) (feet)

8/93 0 180.9 0 3.50 9.50

12/93 5 183.6 1.59 5.09 10.67

3/94 8 188.7 1.77 5.27 10.34

7/94 12 200.1 1.80 5.30 10.27

11/94 16 198.0 1.81 5.31 10.51

12/94 17 202.8 2.02 5.52 10.65
3/95 20 204.5 2.11 5.61 10.67

6/95 23 205.7 2.11 5.61 10.29

6.8 Summary

The wave transmission coefficient values obtained from the two wave gages

located approximately 75 feet on either side of the artificial reef and somewhat south of its

centerline were found to be low (76% to 87%, indicating greater dissipation and/or

reflection) compared to the numerical model results and laboratory measurements. This

was investigated further by placement of two wave gages south of the installation in the

same approximate water depths and at the same separation distances as those on either

side of the artificial reef. The natural reduction of wave height without the artificial reef

effects was found to be in the range of 5% to 15% and formed a basis for correction of the

89

1.0 i- I I
........... .. ..... .' .. .... ... .. ....... .. ................ ... ........... .......... ......

-*- Freeboard 4 ft
0.9 Freeboard 5 ft
0 .9 .... ...... ..........

0 . . . .. .. . . ..7 . . ..-- -- - - - --- - ... . ... . .. . . . . . .. . .. . .. . . . . .. .. .. .. .. .. . .. .. ... . . ... . .

Figure 6.17 The transmission coefficients related to the freeboard and the incident wave
height by the numerical model compared with the Kt values presented in Table 6.15

results from the gages adjacent to the artificial reef. These corrected transmission

coefficients ranged from 85% to 95%.

The numerical model predicted that the transmission coefficients increase with
0.8 .--

0.7

0.5
0 1 2 3 4 5 6
Incident Wave Height (feet)

increasingly The transmission coefficients related to the freeboard and decrease with increasing incident wave height. Based on
height by the numerical model compared with the Kt values presented in Table 6.15

results from the gages adjacent to the artificial reef. These corrected transmission

coefficients ranged from 85% to 95%.

The numerical model predicted that the transmission coefficients increase with

increasing freeboard and decrease with increasing incident wave height. Based on

numerical model results, for higher relative crest elevations, wave heights and currents in

the breakwater lee are both reduced; however, at the Palm Beach installation, the low

crest height may allow so much water to flow over the artificial reef that resulting

longshore flows will outweigh the benefits of a small reduction in wave height.

90

Based on video image analysis, for larger storm waves, most of the energy

dissipation occurred over the offshore natural reef while for smaller storm waves more

than 50 percent of the energy was dissipated near the shoreline due to shore break.

Generally, the ranges of dissipation percentages over the natural reef and near the

shoreline are 35.7% to 65.6% and 33.7% to 63.1%,respectively. Only a relatively small

amount of wave energy dissipation occurred due to the presence of the artificial reef,

ranging from 0.6% to 1.3% of the total wave energy. Moreover, the dissipation

percentages by the artificial reef of the energy passing over the natural reef vary from

1.1% to 2.5%. In interpreting these results, it is important to note that those estimates are

based on the image intensities above background of the video records and thus subject to

some uncertainty. However, they are certainly supportive of the wave gage results of 5%

to 15% as an upper limit range for the wave height reduction.

6.9 Possible Future Applications of the Video Installation

The video imaging system has provided results in addition to the quantification of

wave energy dissipation of primary interest here. These results include both possible

research and practical application areas, several of which are illustrated below.

One year images from different scenes were collected by four cameras. Figure

6.18 from Camera 1 illustrates the wave swash activity at the Clark Avenue public beach,

which could be expanded further as a research subject. Also, rip currents induced by

structure have been documented. Cameras 2 and 3 captured the beach nourishment

commencing in October 1995 and completed in December 1995. Images collected can be

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