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

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Title:
Estimated wave energy dissipation by natural and artificial reefs via video imaging techniques
Series Title:
UFLCOEL-96007
Creator:
Chen, Renjie, 1969-
University of Florida -- Coastal and Oceanographic Engineering Dept
Publication Date:
Language:
English
Physical Description:
xi, 101 leaves : ill. ; 29 cm.

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Subjects / Keywords:
Dissertations, Academic -- Coastal and Oceanographic Engineering -- UF ( lcsh )
Coastal and Oceanographic Engineering thesis, M.E ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (M.E.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 98-100).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Renjie Chen.

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University of Florida
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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
35777915 ( OCLC )

Full Text
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 LEDGM ENTS ............................................... ii
LIST OF FIGU RE S .................................................... v
LIST OF TABLES .................................................. viii
A B STR A C T ......................................................... x
CHAPTERS
I INTRODU CTION ................................................. 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 M onitoring System ................................... 21
4.2 Deployment Description ....................................... 21
4.3 D ata D escription ............................................. 24
5 IM AGE 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 A nalysis 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 R ectification ...........................................
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 esults ...............................
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 ............

6.1 6.2 6.3
6.4 6.5 6.6 6.7

................
Video Installation .

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




LIST OF FIGURES

Figure P=
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 (Carnera I 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 m ain m enu ............................................ 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 I I 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 A pril 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 fines 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
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5
6.3 A time averaged image of 39,600 images dated from July 18, 1995 to
A ugust 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
show n 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), 0), (k), (1) 3-D perspective plots of image intensities of
rectified images corresponding to Figure 6.7 (g), (h), (i), 0), (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, JanuaryO ctober 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
T able 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

Ighk Pqu
3.1 The cited applications of image techniques ............................ 13
6.1 Times, number of digitized images and wave conditions for average images of
stonn 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, June2l, 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 3 1, 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 Introductio
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 Not 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 _d Entrance
Bypasslng location
length of 4,176 feet including a gap of 216 F ah
0."7
feet near the north end through which a fiber
0
optics cable transited offshore. Although
Okw~mho,_.
almost all of the shoreline landward of the ,v4 PEP Reeef4176ft
artificial reef is seawalled, a beach averaging Southern ilv
approximately 26 feet in beach width was
present in July 1992 when the first 57 reef
units were installed. The individual units are
6 feet high, 12 feet long and 15 feet wide Figure 2.1 Location of the artificial reef 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
* ..Approximatel 3.5 ft .V
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 10 IA. 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."
Auzust 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 I through 9) for this period are erosional with a total loss of




Zone Designation
4i
zone 1 zone4 zone 7
I. I
Zone 2 zone 5: zone 8
. . ........................... ............................ .......................................................
o zone 3 zone zone9
Li
-"-2 4 0' 1 240'-.]' 480'
Figure 2.4 Description of nine zones defined for quantification of sediment volume changes




August 1993 to June 1995

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

240' -- --a- 240' 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, R.A. 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, R.A. 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, R.A. 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.Homin Video Records
(1989)
Lippmann, T.C. and Quantification of Sand Bar Morphology: Developed a remote sensing technique to measure
R.A. 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 R.A. 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 R.A. 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: dualresistance 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-rn-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-mmn 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.
Hlman et al. (1990) investigated infragravity wave motions using videotapes. The cameras were mounted on a 43.2 in-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 3 80 rn-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. (199 1) 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 mn 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 celerities were found to exceed 2 m/s based on an ability to quantify wave presence in terms of pixel intensities.




18
Lippmann and Holman (199 1) 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 1{z 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 mn




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 199 1. 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 ines. 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 I 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 environent 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 RAJN-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 artifical 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 I 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 fights 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 I 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, zip 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 Imasze 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 functionffxiy) 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 (xty) 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 x512, 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(0,0) A(O, 1) ... f(O, N-l1)
1(1,O) 1(1,1) ... 1(1, N-i)
fxy) (5.1)
((-1,0) f(N-l,1) ... J(N-l1, 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 comer. The pixel coordinates are typically integers but can be real numbers.

Figure 5.1 Pixel coordinate system

0(1,1) x
I I I "
1 2 3 ...
-1
-2
-3 .f(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
0o and
O Since light is a form of energy, i(x,y) must be nonzero and finite, i.e., O 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(xyV) = g [f (xyV) +f2(xV)]
= g[f1(xy)] + g[f(xyv)] (5.3)
-= g1(x~v) +g2(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 Mi FX 7 10 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 (SVIIP) version 6.7, produced by MicroDISK, Inc. Figure 5.3 shows the main menu of the SVIP package. The SVJP 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
V6 7 Convriaht (0 19R4 19992 PIX INC All rights reserved.

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 Hstoaram 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. 11istograms 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 fight 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 fight 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
x 2000
0.
LIL
O 1500
z 500
,e ... - ,4 O 3 ', .. ...... .. .. . ...... .O 0 O 1 3 " U D '
PIXEL 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 SVLP 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 fine 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 Ima2e MaskinQ
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.
9 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 fine 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 I I 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. e 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 fr~om.
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 SVLP 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 ave breaking Over the Natural Reef
700000 -. U-Wave Breaking near the Shoreline I
A-.
600000
>%'5000001' .
S400000
300000 ,
200000
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).
where x and y are the image coordinates, X, Y, and Z are the corresponding ground coordinates to be imaged, f, is the camera focal length, 'r 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




0 0
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 0 with elevation of Z, 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 -r (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(+ 'a)tan(y) YQ = Zctan(+t+a)
where the angles
c = tan-' () y = tan-( Xq (5.6)
22
By simple inverting and combining, Equations (5.5) and (5.6) yield YQ
yq = f~tan(tan-'(_Q) t) zc
2 1 (5.7)
X Yq + c )-QX
2 2
Equations (5.7) express the geometrical relationship between image coordinates and ground coordinate systems. These equations are functions of the camera tilt, -r, 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
XQ= -Ecos(2 )+ S-sin(2 p)
2 2 (5.8)
YQ= E-sin(2 (p) + Scos(-
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, -, 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 p are found to be 86.040, 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 -- ----- ---- ------------------------ ----- -------------------Intensity
.~100 -- -------------01
0 500 1000 1500
20
10 ------------------------.-------------AwA1
Artificial reef Natural reef
-20 1 i
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 (19 89) found theoretical errors in the estimates of r, f, and (9 with rectification method to be within 0.250, 5% and 0.50, 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 1 pixel for rectification processing (since no better than 1/2 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 AN'~D 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 (N(d.Ld) as shown in Figure 6. 1, where At is cross-sectional area of reef breakwater, ds is total depth of water, and L P 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 CL ~ LM0. (
. 0.6
0.3
0.3

0.10 0.12 0.14 0.18 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32
Relative Reef Width, At/(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
CI > 10
0
4.9
cc-5
0w-in

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

.........- .......... ............ .......... -- .......... .......... .......... : .......... ..........
0:
0 0::
.. .. .. ... .. ..,-.. .. .. .........., .......... ........ ... .. .......... ......... .......... -- -- .......... ..........
.. .. .. .. .. .. .. .. .. .. .. .. .. .......... .... ... ......-- -- -- .. .. . .... . ... .....--00
coo
..................... .......... ............................... ........ ............ . . . . . . . . .
0 0
0
.......... 0 0.. ... ..... ......... ... .
.......... ...... .... .... 5................ ........... .................... .......... .......... .......... .......... ..........
0
......... .......... .......... .....I..... .......... .....-----

I I I I I I "
-Seawall
BathymetryforAugust 1993 .............. ... ... Bathymetry for June 1995 ...................
A
. Artificial Reef
- \_ i i i . .,




56
artificial reef is 15 feet with a crest width of 1 foot. Comparatively, the natural reef s 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 Imagze 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 northsouth. 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 Averagze Imagzes 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




2000

3000

4000

Distance South of Camera (ft)

Figure 6.4 Bathymetry in 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

1200 1100 1000 900 800 700 600 500
400 300
200 100
0
-100

.I .. .0. . i4 0
. ...." .o ... .... .. .... """: ....i .. .... ..... i ... ':! .... ..........I . .
2 .0- .......... ...... ..... .........
"" ~~ .;.."...... L i r . -l .. .
Approxirroately, Natural Reel Position -.
S ... .........
-. .. .......Artificial R .e
- .' '- : i : 'i . . .. . . . . ".. ,,
'.. ~~ ~ ~ ~ t ...... ....: : i ..- o..:.
....... ... ....... ....... ....... I ... ..
.. .. .. I. ....."' --- i ..... .. 0 .
.o :.. .. .. : : i : : i ; ........
. . . . . . .. . . . . -.."- : ' L
a-Camera Nadir P r 7
R9 B '- R96G I R97 , R ,98E

0 1000
* North

5000

6000




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 _____________Apri 11 195 600 a 9:0 pm245 Large wave breaking both
Apri 11 195 600 a 9:0 pm245inshore and offshore
Apri 12 195 600 a 3:0 pm145 Reduced wave breaking
Apri 12 195 600 a 3:0 pm145 both inshore and offshore
May 0, 195 :00 m 4:5 prn 105Limited wave breaking
May 0, 195 :00 m 4:51 pm105 both inshore and offshore
June21, 995 5:00am -19:0 pm275 Small wave breaking both
June21,1995 5:0am-19:3 pm275inshore and offshore
Juy2,1995 11:35 am 14:53 pm 70 Small wave breaking both
July 27,inshore and offshore
July28, 995 15:2 pm 1925 p 85 Large wave breaking both
July28, 995 15:2 pm 1925 p 85inshore and offshore
July29,199 5:3 am- 1:09pra 215 Large wave breaking both
July29, 995 5:33am -16:9 pm215inshore and offshore
July30,199 9:2 am- 1:13pra 200Reduced wave breaking July30,1995 9:2 am 19:3 pm200 both inshore and offshore
Large wave breaking both
July 31, 1995 5:50 am 19:19 pm. 270 isoenofhr
Augut 1 195 553 a 8:5 pr 25Reduced wave breaking Augut 1,199 5:5 am 1858 p 25both inshore and offshore
Augut 2,199 15:8 pa 6:3 prn 260Medium wave breaking Augut 2 195 1538 m -16:3 pm 260 both inshore and offshore
Augut 3 199 5:8 am- 953 p 85Medium wave breaking
Auut3,19 55 m :3Im8 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 3 1, 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 I I 1995 ()Arl1,19

(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 3 1, 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
a Natural Reef
F-10
z
S-20 ILl
-30 6000
t 4000
o04t 2000
of

0 1500

1000
east o1

500 caMera 0t)

Figure 6.8 Three-dimensional view of the June 1995 beach survey




200 150 100, 50,
0
6000
44000
204 2000 1 500
1000 (test)
*', OA. 0 2000 Ol est
(a) April 11, 1995
200 150 100 5 50
0
6000
0. 4000
0 o 12000 1000 500t)
- 1500 o ares *6
0O 2000 Oltc E
(c) May 10, 1995
200 150
100 50
0
6000
0.
4, 4000
o,2000 1000 a0 01
4 1500 ato t -'
, 0 2000 tae
(e) July 27, 1995

0

65
200 150 100 50 0
6000

0 500
1000 (test)
1500 10*tS*"
0 2000 o Ce 6. 01t
(b) April 12, 1995
(b) April 12, 1995

200 150
100 S50
6000
4000
'$'1"4o, 2000 500
' 1000
- 1500 t oi GM@*
% ".. 0 2000 s 6 .tfato'
(d) June 21, 1995

200 150
100
& 50
0
6000

4000

411 111!!N!I 1 I PI 500
710 1000
1500 E6% !
0 2000 0itwo

(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, 150 100
5 50
0
6000
0Z
'194 4000
*44) 2000 500
5 1000 ,.se % 0 2000 OlstAce
(g) July 29, 1995
200 150 100 50,
0
6000 S e01 40000
4 2000 51000
0
0 2000 1500 a e'1
(i) July 31, 1995
200 150 100 50
0*
6000
0
%. 4000 2000 1 0 500
oil 1000Lf.
O 0 1500 ses
& 0 2000 Ost st o
(k) August 2, 1995

0

200 150 100
5SO S50
0
6000

o 2000 500
o~p 1000 ra e**
1500
, O 2000 D.t
(h) July 30, 1995

200 150
100 50
0
6000

4000

0 500
0 1000
1500 uCam*1,
*(;,e9 0 2000 0 stan,4 ost o
(j) August 1, 1995

200 150 1 00 5 50 S o.
0
6000

4000

5 00 a00 a 500
0 o 2000 0c es
(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)




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 Iman 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




C 0
> 40,
0
0
-o
S6000
0.
't6 4000
15000
000 150 1000 Le'
(4e 0 2000 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




P ro file R 96 B 200
100 In tens ity
00
0 500 1000 1500
20
10
0
Bathym etry for M arch 1995
-1 0
-20
0 500 1000 1500
Distance from m on u m e n t R 96 B (ft)
Profile R96G Breaking ne.rmbore ', Breaking over the natural reef

~,200 A1111"i In
Intensity 100
0
Breking over the PEP Reef
0
0 500
O 500

- 'A

1000

1500

Bathymetry for March 1995

500 1000
Distance from monument R96G (ft)

1500

P ro file R 9 7 D

200 100

500

1000

1500

B athym etry for M arch 1995 05 00 1 000 150

Distance from m onum ent R97D (ft) Figure 6.11 An example of image intensity analysis, April 11, 1995.

20
2-- O -10
- iO LUI -10
-20
0

In te n s ty

- -- L.

00




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 fines 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 I 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
Profile line value shoreline artificial reef natural reef
(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 1 98 1 8428 43.8 172 0.9 10646 1 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
Profile line value shoreline artificial reef natural reef
(intensity) Intensity Percentage intensity Percentage Intensity Percentage R96B 90 9048 64.6 1 140 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 Profile line value shoreline artificial reef natural reef
(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
81 4386 -r 4.4 1 172 2.1 3511 43.5
R98E I




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 value shoreline artificial reef natural reef
(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 1 50.2 189 j 1.0 j 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
(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 1 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
(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
r ;F 1 92 8785 48.0 128 0.7 j 9377 j 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 value shoreline artificial reef natural reef
(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 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 t 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
Profile line value shoreline artificial reef natural reef
(intensity) Intensity Percentage intensity Percentage Intensity P- tage 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 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 2 1, 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 3 1, 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 nondimensional 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

Disspaton verthe abial eef Dissipation over the Artificial Reef

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:
Kt Hs nearshore(61
Hs offshore shoaled(61
where Kt = transmission coefficient, I-Is nearshore =nearshore (landward of the artificial reef) significant wave height, and H~s 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, il, over the artificial reef can be written approximately as
'= (1 Kt/)*100% (6.2)
where 71 = 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) (f0) (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 Nearshoe by Hs Offshore Shoaled in the table above.

Table 6.15




1.2 "'. I" I : : ".
S+, + : + + -+ +"+ + -+ + 4
+ + + +
+4+ +
1 0. . ... . .. .. .. .. . .. ... . ... + + . -+ .. . . . . ..... .. . . .. .. .. . .
++ + ++
1-46 + + +
+ +H
+
0 .6 .......... ::......... !. . . ".. . .i . . . . ............. : ....... :......... : .......
0 .4 ... . ... . . .i .. .. .. : . . .. . . .. . .. . . .. . . .. .. . . . .. . .. . .. .. i. ... . ..
0 .2 - - - - - -- - - -- --- I . . .
JanuarebruaryMarch April May June July :August SeptemberOct.
0 .0 I 1 i I 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, JanuaryOctober 1994

1.2 .- : +*h. -H: + I f I: I
1+: ++_ + + +:
-.- r- .. :.... -- .. .. ..: -- . .+ -- -!.......... !.......... . .
0.6
0 . . + 4 . . . . . . .. . .. . . . .. . . . . ... . . . . . . . . .
.. .. .++ Z ...i ......... .......... :........... !.......... i.......... ----------i ...
0.4 .*
0 .4 ...... ....... .......... .......... ... ........ ..... -- -- ---------- .......... . .
0 .2 - - - . . .
Januarlebrua.yMarch: April May June July :August Sept
0 .0 = I I I I' I ,
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 fi7om 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 fls, 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. (I1994b) 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 Pdl
Beach Test Area aeae

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 Average Water
Cumulative from the settlement Freeboards depth
Survey date months units to the of the 330 (feet) seaward of
since 8/93 shoreline the units
(feet) umts (feet) (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
0r eb2ard 5 6
Figure~~~~~~~~~~~~~~~Freor 6.1 Th trnmsincefcetfeae t h reor n hniet wv
heigt b thenumricl moel ompred iththe t vlue preentdoi Table 6.15
reslt from ....... the--------- gage adacn.t.heariic. re. These.. corrected.... transmission...............
coefficients ranged.. fro 85%. to.... ...5%....
The nume... cal moe predicted- that---- the... trnmiso coeficent-icrese -it
increasing ..freeboard ..and.decrease..with.increasing. incident.wave.height...Based .on
numerical ~ ~ ~ ~ ~ ~~ model............ reut,.o.ihe.eaiv.rsteeaios.ae.egtsadcrrnsi
the.... breakwater le are.... both... reduced;.. ho evr at..... th Palm... Beach.. intlaIon the.....low...
cres height .. ma alo.s.uh.aert.lo.ve.h.atfiilrefthtrsutn g....... flow s ... w ill .outw eig the.... be I t of.... a..... sm all ......... reduction in .... w av h i .... ---




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 3 5.7% to 65.6% and 3 3.7% 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 I 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