identifying and analyzing coastlinedigilib.library.usp.ac.fj/gsdl/collect/usplibr1/index/... ·...
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IDENTIFYING AND ANALYZING COASTLINE CHANGES ALONG THE CORAL COAST, SOUTH-
WEST VITI LEVU, FIJI ISLANDS, VIA MULTI-TEMPORAL IMAGE ANALYSES
by
Prerna Bharti Chand
A thesis submitted in partial fulfilment of the requirements for the degree of
Master of Science
School of Islands and Oceans Faculty of Science, Technology and Environment
The University of the South Pacific
Copyright © 2010 Prerna Bharti Chand
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ACKNOWLEDGEMENT
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I would like to present my sincere acknowledgement to those who have aided and
inspired me to accomplish the goals of this research. Firstly, I would like to thank my
supervisors; Dr. Susanne Pohler, (Principal Supervisor) who has guided and
encouraged me throughout my research, Dr. Gennady Gienko, for his constant
guidance, inspiration, support and advice in achieving the appropriate methods for
the multi-temporal data analyses and Professor Patrick Nunn, for guiding me through
the initial stages of my research.
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I am grateful to Mr. Shingo Takeda who taught me the mapping techniques needed
for the final presentation of the results. I would also like to thank Mr. Laisiasa
Cavakiqali, and Miss Yashika Nand for assisting me with the field work and Mr.
Laisiasa Cavakiqali for driving me to my study sites in the Coral Coast, area. My
acknowledgement to all the villages and resorts along the Coral Coast who
accommodated me for my field work, including, Beach House, Hideaway Resort,
Tabakula Resort, Outrigger Resort, Navutulevu, Namatakula, Tagaqe, Yadua and
Vatukarasa Villages and Korolevu Settlement.
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I would like to show my appreciation to Mr. Rinel Ram, and Ms Shirleen Bala who
provided constant words of encouragement, advice and unfailing support and aided
me with the final formatting of the thesis. I am also grateful to my fellow post-
graduate friends, Ms. Simita Singh, Mr. Naushad Yakub, Mr. Zulfikar Begg, Mr.
Pranesh Kishore and Ms Kirti Lal, for their generosity, friendship and support.
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Finally, I would like to thank my parents, who have been a constant pillar of support
throughout my life, I will always be grateful to my mother and father who are my
tower of strength, for their firm believe in me. I thank them for their consistent words
of encouragement and moral support which inspired me to complete my thesis.
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ABSTRACT �
This research identifies the nature of temporal coastline changes along the Coral
Coast area in Fiji Islands. Prograding, resilient and eroding coastlines were identified
by comparison of historical aerial photographs and satellite images. For this study
1967 aerial photographs and 2002 IKONOS satellite images were compared to
observe and classify the coastline changes over the 35 year period. Subsequently, a
ground truthing exercise along the Coral Coast was carried out to re-evaluate the
results obtained from the desktop study of historical aerial photographs and satellite
images.
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The desktop comparison did not reveal any prominent coherent coastline change
patterns. However, a slight pattern is evident; the far west (Fijian Resort area) and
the far east (Naboutini Village area) sides of the study area generally indicate
resilient and prograding coastlines. Along the shoreline in the central region it was
found that stretches of prograding coastlines alternate with resilient and eroding
coastlines. The rates of coastline change for prograding and eroding coastlines were
quantified in terms of area (in square meters) and maximum distance (in meters) of
landward and seaward movement. For prograding coastline segments the area
advancement ranged from 1 564 ± 6 m2 to 97 285 ± 6 m2 and the maximum distance
of progradation ranged from 13 ± 3 m to 400 ± 3 m. For eroding coastline segments
the area recession ranged from 840 ± 6 m2 to 21 487 ± 6 m2 and the maximum
distance of coastline recession ranged from 14 ± 3 m to 40 ± 3 m. The area and
maximum distance values for progradation and erosion indicate that the Coral Coast
coastline change is influenced more by progradation than erosion. However, the
ground truthing exercise revealed only two sections of the study area to be naturally
prograding; the region at the head of Sovi Bay and Namatakula Village front. All
other prograding and resilient coastlines had been fortified by coastal engineering
structures; indicating artificial progradation and foreshore reclamation. Beach
erosion was prominent on most of the beaches with fortified coastlines.
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The wave energy along the Coral Coast, set up by the dominant southeast trade
winds is relatively high. Since this area only possesses a narrow fringing reef system,
interrupted by numerous channels, the wave energy does not dissipate significantly
upon reaching the shore. Hence, the Coral Coast coastline is a “High Energy
Coastline”. Due to fortification of the coastline by the construction of seawalls, a
large portion of the Coral Coast coastline is showing signs of artificial progradation.
Coastal processes, erosion, progradation and resilience are depended on the local
wind, wave and geomorphological characteristics of an area. Therefore, the dominant
coastal process at any given area would be specific to that area, respective to the
local characteristics.
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Keywords: Temporal coastline change, coastline erosion, coastline progradation,
resilient coastline, remote sensing methods.
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TABLE OF CONTENTS �
ABSTRACT..............................................................................................................................ii�
TABLE OF CONTENTS.........................................................................................................iv�
LIST OF FIGURES ................................................................................................................vii�
LIST OF TABLES ....................................................................................................................x�
LIST OF APPENDICES ..........................................................................................................xi�
CHAPTER 1 – INTRODUCTION .......................................................................................... 1�
1.1 INTRODUCTION ......................................................................................................... 1�
1.2 THESIS RATIONALE .................................................................................................. 3�
1.3 THESIS OBJECTIVES ................................................................................................. 5�
1.4 STUDY AREA .............................................................................................................. 5�
1.5 THESIS FRAMEWORK ............................................................................................... 7�
CHAPTER 2 – COASTLINE CHANGE PROCESSES IN THE PACIFIC ISLANDS.......... 9�
2.1 INTRODUCTION ......................................................................................................... 9�
2.2 SHORELINE CHANGES USING THE REMOTE SENSING METHOD .................. 9�
2.2.1 Remote Sensing Methods and Shoreline Change.................................................. 10�
2.2.2 Role of GIS in Coastline Change Studies.............................................................. 11�
2.2.3 Mapping Shoreline Changes ................................................................................. 12�
2.3 COASTAL PROCESSES ............................................................................................ 15�
2.3.1 Waves, Tides and Currents ................................................................................... 17�
2.3.2 Sediment Budget and Longshore Drifting............................................................. 19�
2.3.3 Coastal Accretion Processes................................................................................. 22�
2.3.4 Coastal and Beach Erosion Processes.................................................................. 23�
2.4 CHARATERISTIC COASTAL ZONES OF THE PACIFIC ISLANDS .................... 26�
2.4.1 Coastal Geography of Pacific Islands .................................................................. 26�
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2.4.2 Sediment Sources of the Coastal Zones ................................................................ 29�
2.5 CAUSES OF COASTAL EROSION IN THE PACIFIC ISLANDS .......................... 31�
2.5.1 Natural Causes of Coastal Erosion ...................................................................... 32�
2.5.2 Human-induced Causes of Coastal Erosion ......................................................... 42�
2.6 SUMMARY ................................................................................................................. 44�
CHAPTER 3 – METHODOLOGY ....................................................................................... 45�
3.1 INTRODUCTION ....................................................................................................... 45�
3.2 WORKFLOW IN DETERMINING THE RANGE OF COASTLINE CHANGE ...... 45�
3.2.1 Phenomenal Range of Spatial Change.................................................................. 48�
3.2.2 Identification of Source Data ................................................................................ 48�
3.2.3 Range of Coastline Change Methods .................................................................... 49�
3.2.4 The Error Budget and A-priori Accuracy Estimation........................................... 50�
3.3 THE STUDY SITE ...................................................................................................... 51�
3.4 RESEARCH METHODOLOGY................................................................................. 53�
3.3.1 Source Data used for Geographical Information Systems Analysis ..................... 54�
3.3.2 The Range of Coastline Changes .......................................................................... 54�
3.3.3 Coastline Categorization ...................................................................................... 61�
3.3.4 Quantification of Coastline Changes .................................................................... 61�
3.5 FIELD INVESTIGATION .......................................................................................... 62�
3.6 SUMMARY ................................................................................................................. 64�
CHAPTER 4 – RESULTS ..................................................................................................... 66�
4.1 INTRODUCTION ....................................................................................................... 66�
4.2 COASTLINE CATEGORIZATION ........................................................................... 66�
4.2.1 Fijian Resort, Naevuevu Village and Yadua Village ............................................ 69�
4.2.2 Yadua Village and Sigatoka Sand Dunes.............................................................. 70�
4.2.3 Sigatoka Sand Dunes and Korotongo Village....................................................... 72�
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4.2.4 Outrigger and Tabakula Resorts........................................................................... 73�
4.2.5 Sovi Bay, Vatukarasa Village and Namada Village.............................................. 75�
4.2.6 Tambua Sands Resort, Hideaway Resort and Tagaqe Village.............................. 79�
4.2.7 Nagasau Village, Votualailai Village and Naviti Resort ...................................... 82�
4.2.8 Votua Village, Korolevu Settlement and Warwick Resort..................................... 85�
4.2.9 Komave Village, Navola Village and Beach House Resort .................................. 88�
4.2.10 Namatakula Village, Navutulevu Village and Naboutini Villagea ..................... 90�
4.3 QUANTIFICATION OF ERODING AND PROGRADING COASTLINES ............ 92�
4.5 SUMMARY ............................................................................................................... 103�
CHAPTER 5 – DISCUSSION AND CONCLUSION ........................................................ 104�
5.1 INTRODUCTION ..................................................................................................... 104�
5.2 DISCUSSION OF THESIS FINDINGS .................................................................... 104�
5.2.1 Coastline Changes .............................................................................................. 104�
5.2.2 Quantification of Eroding and Prograding Coastlines....................................... 112�
5.2.3 Coastal Geomorphology of the Coral Coast....................................................... 118�
5.3 RECOMMENDATIONS AND IMPLICATIONS .................................................... 121�
5.3.1 Recommendations ............................................................................................... 121�
5.3.2 Implications of Thesis Research ......................................................................... 123�
5.4 CONCLUSION .......................................................................................................... 126�
REFERENCES .................................................................................................................... 128�
GLOSSARY LIST ............................................................................................................... 138�
APPENDICES ..................................................................................................................... 141�
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LIST OF FIGURES �
Figure 1. 1: Map of study area. Source: Media Centre, 2008, the University of the South Pacific. ........................................................................................................ 6
Figure 1. 2: Fringing reef systems associated with the study area along the Coral Coast. Source: Google Earth, 2006....................................................................... 7
Figure 1. 3: Thesis framework.Figure.......................................................................... 8 �
Figure 2. 1: A schematic coastal embayment in (a) plan form and (b) profile. Some of the key definitions based on proximity to the shore, wave characteristics, and substrate are shown. (Source: Woodroffe, 2002) ............................................... 16
Figure 2.2: Holocene sea-level changes in the Pacific. A. Sea-level changes during the Holocene in Fiji; B. Late Holocene sea-level changes in the Tuamotu Islands, French Polynesia. Source: Nunn, 2002. ................................................ 27
Figure 2.3: Monthly sea-level record from SEAFRAME at Lautoka, Fiji. Source: Gray, 2009. ......................................................................................................... 34
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Figure 3. 1: An illustration of the order of processes involved in the examination of coastline changes by a comparison of historical aerial photographs and satellite images. ................................................................................................................ 47
Figure 3. 2: The study area situated along the south-west coast of Viti Levu. Source: Pacific Maps Pty Limited, 2002. ........................................................................ 52
Figure 3. 3: A representation of the processes involved in determining the range of coastline changes along the Coral Coast. ........................................................... 53
Figure 3. 4: A schematic representation of the method and technique development in the determination of the range of coastline changes; 3 trial methods leading to the ulimate method of range of coastline change analyses. (Note:AP – aerial photographs; GE – Google Earth images.) ......................................................... 55
Figure 3. 5: The study areas for the ground truthing exercise along the Coral Coast............................................................................................................................. 63
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Figure 4. 1: The Coral Coast; the area in each black frame corresponds to the labelled figure which gives details of the coastline categorization of the respective area. Map background: 1967 aerial photograph mosaic.............................................. 68
Figure 4. 2: Coastline categorization along the Fijian Resort, Naevuevu Village and Yadua Village area. ............................................................................................ 69
Figure 4. 3: Coastline categorization along the Yadua Village and Sigatoka Sand Dunes. ................................................................................................................. 70
Figure 4. 4: Damaged seawall and debris along the Yadua Village beach front. ...... 71
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Figure 4. 5: Coastline categorization from the eastern edge of Sigatoka Sand Dunes to Korotongo Village. ......................................................................................... 72
Figure 4. 6: Coastline categorization along the Outrigger Resort and Tabakula Resort area...................................................................................................................... 73
Figure 4. 7: Seawall separating the Outrigger Resort from the beach front. ............. 74 Figure 4. 8: Coastline categorization along Sovi Bay, Vatukarasa Village and
Namada Village area........................................................................................... 75 Figure 4. 9: Wide beach at the head of Sovi Bay....................................................... 77 Figure 4. 10: Low tide on a steep beach face in front of Vatukarasa Village............ 77 Figure 4. 11: Exposed tree roots (left) and beach rocks (right) in front of Vatukarasa
Village................................................................................................................. 78 Figure 4. 12: Gabion Baskets; east of Vatukarasa Village; adjacent to the Queens
Road. ................................................................................................................... 78 Figure 4. 13: Coastline categorization of the Tambua Sands Resorts, Hideaway
Resort and the Tagaqe Village area. ................................................................... 79 Figure 4. 14: Seawall along the east of Hideaway Resort.......................................... 81 Figure 4. 15: Exposed tree roots and beach rock in front of Tagaqe Village. ........... 81 Figure 4. 16: Coastline categorization from Nagasau Village, to Votualailai Village
to Naviti Resort................................................................................................... 82 Figure 4. 17: Narrow stretch of beach in front of the Naviti Resort. ......................... 84 Figure 4. 18: Artificial Island in front of Naviti Resort; surrounded by a seawall and
connected to the mainland by a causeway. ......................................................... 84 Figure 4. 19: Coastline categorization along Vouta Village, Korolevu Settlement and
Warwick Resort area........................................................................................... 85 Figure 4. 20: Sloping embankment separating the western side of Korolevu from the
beach front. ......................................................................................................... 87 Figure 4. 21: Coastline categorization along Komave Village, Navola Village and the
Beach House area. .............................................................................................. 88 Figure 4. 22: Exposed tree roots (left) and beach scarp (right) in front of the Beach
House .................................................................................................................. 89 Figure 4. 23: Coastline categorization along Namatakula Village, Navutulevu Village
and Naboutini Village. (Note: the eastern end of Namatakula and Naboutini Villages could not be classified since these sections of historical aerial photographs were missing.) ................................................................................ 90
Figure 4. 24: River meandering through the beach front of Namatakula Village...... 91 Figure 4. 25: A comparison of the eroding and prograding coastlines in terms of the
maximum distances of change in coastlines over the years 1967 to 2002 for the Coral Coast area.................................................................................................. 93
Figure 4. 26: Maximum distance of landward movement (m) at each erosion hotspot along the Coral Coast. The erosion spots correspond to the red lines in Fig. 4.25, from west to east. ................................................................................................ 94
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Figure 4. 27: Average rate of erosion at each erosion hotspot along the Coral Coast. The erosion spots correspond to the red lines in Fig. 4.25, from west to east.... 94
Figure 4. 28: Maximum distance of seaward movement (m) at each progradation spot along the Coral Coast. The progradation spots correspond to the green and blue lines in Fig. 4.25, from west to east. ................................................................... 95
Figure 4. 29: Average rate of progradation at each prograding spot along the Coral Coast. The prograding spots correspond to the green and blue lines in Fig. 4.25, from west to east. ................................................................................................ 95
Figure 4. 30: Map illustrating 13 erosion hotspots identified from the comparison of historical aerial photographs (1967) and IKONOS satellite images (2002) along the Coral Coast. .................................................................................................. 97
Figure 4. 31: The area eroded at each of the 13 spots identified as erosion hotspots (Fig. 4.17) over a 35 year period, from 1967 to 2002. Each area reading has an error margin of ± 6m2 (Section 3.3.4, Chapter 3). .............................................. 98
Figure 4. 32: Average rate of erosion at each of the 13 spots identified as erosion hotspots (Fig. 4.17) over a 35 year period, from 1967 to 2002. Each rate of erosion reading has an error margin of ± 6m2 (Section 3.3.4, Chapter 3). ......... 98
Figure 4. 33: Map illustrating 21 prograding spots identified from the comparison of historical aerial photographs (1967) and IKONOS satellite images (2002) along the Coral Coast. ................................................................................................ 100
Figure 4. 34: The area prograded at each of the 21 spots identified as prograding spots (Fig. 4.20) over a 35 year period, from 1967 to 2002. Each area reading has an error margin of ± 6m2(Section 3.3.4, Chapter 3)................................... 101
Figure 4. 35: Average rate of progradation at each of the 21 spots identified as prograding spots (Fig. 4.20) over a 35 year period, from 1967 to 2002. Each rate of progradation reading has an error margin of ± 6m2(Section 3.3.4, Chapter 3)........................................................................................................................... 101
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Figure 5. 1: An illustration of the level of erosion (in m2) along the coastlines of the Coral Coast with and without reef barriers; the level of erosion is related to the presence and absence of shoreline armor in each case. .................................... 107
Figure 5. 2: Headland bay beaches in dynamic and static equilibrium. Source: Hsu, 2005. ................................................................................................................. 112
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LIST OF TABLES �
Table 2. 1: List of currents generated in various ways, and some currents of multiple origins. Source: Bird, 2008. ................................................................................ 19
Table 2 2: The Budget of Littoral Sediments............................................................. 20 Table 2 3: Indications of coastal and/or beach erosion. ............................................. 24 Table 2. 4: The causes of coastal and/or beach erosion. ............................................ 25 Table 2. 5: A list of cyclones affecting Fiji Islands between the 1969/70 to 2007/08
seasons. Source: Fiji Meteorological Services, 2008. ........................................ 37 �
Table 3. 1:Details of available source data. ............................................................... 48 Table 3. 2: A comparison of the multiple and the two ground control point methods.
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Table 5. 1: Division of erosion hotspots into high, moderate and low level clusters. The numbers in brackets correspond to the sites in Figure 4.17. ..................... 115
Table 5. 2: Division of prograding spots into high and moderate level clusters. The numbers in brackets correspond to the sites in Figure 4.20.............................. 116
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LIST OF APPENDICES �
Appendix 1. 1: Map illustrating the eroding area on the east of Yadua Village...... 142 Appendix 1. 2: Map illustrating the eroding area along the Sigatoka River spit and
the east of the Sigatoka River spit. ................................................................... 143 Appendix 1. 3: Map illustrating the eroding areas on the east of Tabakula Resort. 144 Appendix 1. 4: Map illustrating eroding area in front of Vatukarasa Village and the
east of Vatukarasa Village. ............................................................................... 145 Appendix 1. 5: Map illustrating the total area eroded in the east of Tambua Sands
Resort. ............................................................................................................... 146 Appendix 1. 6: Map illustrating the total area eroded in front of Tagaqe Village... 147 Appendix 1. 7: Map illustrating the total eroding area along Korolevu Settlement and
on the west of Warwick Resort......................................................................... 148 Appendix 1. 8: Map illustrating the total eroding area in the west of Komave Village.
.......................................................................................................................... 149 Appendix 1. 9: Map illustrating the total eroding area in the Beach House area. ... 150 �
Appendix 2. 1: Map illustrating the total prograded area in the Fijian Resort area. 152 Appendix 2. 2: Map illustrating the total prograded area along the Sigatoka River
spit..................................................................................................................... 153 Appendix 2. 3: Map illustrating the total prograded area at the Korotongo bridge,
Korotongo roundabout and in front of Outrigger Resort.................................. 154 Appendix 2. 4: Map illustrating the total area prograded in the east of Tabakula
Resort. ............................................................................................................... 155 Appendix 2. 5: Map illustrating the total prograded area in the west and at the head
of Sovi Bay. ...................................................................................................... 156 Appendix 2. 6: Map illustrating the total area prograded at the east and near the
Vatukarasa Village............................................................................................ 157 Appendix 2. 7: Map illustrating the total area prograded in the east of Tambua Sands
Resort. ............................................................................................................... 158 Appendix 2. 8: Map illustrating the total area prograded at the Hideaway Resort and
the east of Tagaqe Village. ............................................................................... 159 Appendix 2. 9: Map illustrating the total area prograded in Naviti Resort area. ..... 160 Appendix 2. 10: Map illustrating the prograding area in the Votua Village area.... 161 Appendix 2. 11: Map illustrating the total area prograded in the Korolevu Settlement
area.................................................................................................................... 162 Appendix 2. 12: Map illlustrating the total area prograded in the Komave Village
area.................................................................................................................... 163 Appendix 2. 13: Map illustrating the total area prograded in the Navola Village area.
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Appendix 2. 14: Map illustrating total areas prograded in the Namatakula, Navutulevu and Naboutini Village areas. ......................................................... 165
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CHAPTER 1 – INTRODUCTION
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1.1 INTRODUCTION
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Generally, it is accepted that there is geographic diversity in coastal landforms. The
reason for such diversity is the differences and combined effects of coastal processes
and antecedent geology; shoreline change and coastal configuration are the
integrative result (Galgano and Leatherman, 2005). Complex physical processes
encompassing a number of natural and human-induced factors determine the changes
in shoreline and coastal configuration. The natural factors accountable for spatial and
temporal coastline changes include sea-level rise, tidal variations in wave energy,
and the episodic influence of storms. Whereas, human influences on shaping
coastlines include building structures (for example, groynes and jetties), dredging,
damming rivers, and beach nourishment (Galgano and Leatherman, 2005).
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The determination of shoreline trends is one of the fundamental objectives of coastal
geomorphology (Galango and Leatherman, 2005). Monitoring techniques for coastal
geomorphology have been divided into three classes; remote sensing methods, in situ
instrument methods and sampling methods (Morang and Gorman, 2005). Although a
comprehensive study may employ data and instrument from all three classes
(Morang and Gorman, 2005), a number of coastal states use historical shoreline
change data (remote sensing methods) to project shoreline positions for application
in land use policies, which mostly concentrate on determining building set-back lines
and insurance zones (Heinz Center, 2000).
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Since development along the world’s coasts has risen dramatically over the past few
decades, the ability to forecast future coastal positions has taken on increased
importance (Crowell et al., 2005). Therefore, it is essential to have a precise
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understanding of the shoreline change using accurate shoreline change models and
geomorphic characteristics. The shoreline has been defined as the high water line or
the wet-dry boundary of a coastal zone (Galgano and Leatherman, 2005). On the
other hand, when considering coastal erosion, other coastline reference features, such
as, erosion scarp, crest of washover terrace, or the vegetation line are more
diagnostic of long-term change than the high water line. However, using these
reference features limits the temporal span of source data, as these features cannot be
easily depicted in most of the source data (Crowell et al., 2005).
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The remote sensing methods use technologies such as aerial photography, satellite
imaging systems and laser bathymetry. The images produced from remote sensing
methods use tools which remotely image the seafloor or the strata below. A depiction
of the subsurface geology, a mathematical model based on varying acoustic
impedances of air, water, sediment and rock is the outcome of using remote sensing
methods to attain data (Morang and Gorman, 2005). The limitation of this way of
data acquisition is that the interpretation of the data from the model is based on
numerous assumptions, and the real earth may be very different from what is
represented in the remotely sensed image. Taking the limitation into consideration,
this method has been proven to be an extremely powerful tool in numerous coastal
applications (Morang and Gorman, 2005).
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Accurately determining the long-term behavior of shorelines based on sparse data
sets can have many difficulties. In addition, if engineering changes (seawalls and
groynes) have been made to the shorelines and/ or if the shorelines are exposed to
storm surges, the complexity of shoreline analyses increases. Furthermore, complex
episodic cycles of erosion and accretion are displayed by spits which are unrelated to
storm events (Crowell et al., 2005). Hence, in order to obtain quality results for the
rate of shoreline changes, meteorological and geomorphic factors and the role they
play in shoreline evolution need to be considered.
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1.2 THESIS RATIONALE
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The Pacific Island countries, for example, Fiji, Tonga, Western Samoa, have large
portions of their populations concentrated on narrow, low-lying areas fringing the
mountain along the coast. The economic activities of these Island countries are
concentrated on the coastal regions as their capital cities are situated on the coast.
Hence, effects of inundation and flooding would be a major problem for these
Islands (Mimura, 1999). The coastal areas of small island states are the most
vulnerable to climate change problems as these countries have limited resource bases
and are not well equipped to handle existing environmental problems (Leatherman
and Beller-Simms, 1997).
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When compared with the continental coasts, the population densities along most
Pacific Island coasts are low. However, many Islanders utilize coastal resources far
more than their continental counterparts. This is characteristic of predominantly
subsistence lifestyles on most inhabited islands, and the comparatively rich bounty of
near shore areas, particularly reef and mangrove ecosystems (Nunn, 2000). Also, in
the Pacific Island countries 90% of the population live along the coast.
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Over the past several decades, the South Pacific Island countries have experienced an
increased rate of shoreline retreat (Mimura and Nunn, 1997). For Fiji Islands, coastal
erosion was not a prominent problem until 40 years ago (Mimura, 1999; Mimura and
Nunn, 1997). According to Mimura (1999), human activities are considerably
responsible for beach erosion in the South Pacific region. Mangroves have been
cleared off in many places in the past to be used as fuel wood and land reclamation.
The intensive use of beach sand as construction material and for decoration of tombs
as a social tradition has resulted in depletion of many beaches. On atolls, the main
sources of sand are the biological activities of the coral reefs; hence, the supply of
sand is very much limited. Recent retreat of shorelines, therefore, can be partly
attributed to such human pressure (Mimura, 1999).
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This research is based in the Coral Coast area, in Fiji Islands (Fig. 1.1).The Coral
Coast, is located in the south-west of Viti Levu, the largest island in the Fiji
archipelago (with an area of 10,388km2) (Nunn, 1998). The coastal population in the
Coral Coast has intensified over the past few years due to increasing tourism
development and population growth. This has resulted in conflicts for coastal
managers in dealing with beach erosion. The shoreline changes of an area can be
detected by the analyses of historical aerial photographs and satellite images with
respect to coastline retreat and advancement (Crowell, et al., 2005; Morang and
Gorman, 2005). This data, together with a study of the geomorphology and climate
of an area can help to assess chronic and acute erosion. There have been numerous
studies carried out in the Coral Coast area; researches on reef health and ecology,
socio economic surveys, oceanography, geology, water quality (Aalbersberg and
Mosley, 2003; Aalbersberg and Thaman, 2003; Lomax, 2004; Tokalauvere, 2007),
and shoreline change studies (Pitman et al., 2000; Tawake, 2007). Shoreline change
issues have also be researched as student projects. However, most of these studies
were limited to selected areas. This study, on the other hand, aims at examining
coastline changes by means of historical aerial photographs and satellite images for a
continuous stretch of coastline along the Coral Coast.
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Being in the windward side of the Island, the southern coast of Viti Levu experiences
the effects of the strong southeast trade winds (Mataki et al., 2006). The Coral Coast
area is fringed by a narrow fringing reef flat which is interrupted by numerous reef
passages. The following predictions have been made with respect to these factors.
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i) Since breaks in the reef systems can bring in strong ocean currents and
intensive waves in times of storms and cyclones in lagoons (Wolanski and
Pickard, 1983), it is predicted that overall the Coral Coast coastal zone would
be influenced by erosion processes.
ii) Reef flats which are dissected by channels would have prominent erosion
spots opposite the channel entrance.
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1.3 THESIS OBJECTIVES
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This research examines temporal coastline changes along the Coral Coast (Fig. 1.1).
The goal of the study was to develop a technique to determine the range of coastline
change by comparison of historical aerial photographs and satellite images in the
Coral Coast area, and to determine the dominant coastal process at work in the Coral
Coast area.
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The principal objectives of this study were to:
i) Identify and classify the Coral Coast coastline under the following categories:
(1) erosion hotspots (coastal erosion has threatened shoreline development
and infrastructure), (2) erosion watch spots (coastal environments will soon
be threatened if shoreline erosion trends continue), (3) resilient shorelines (no
coastal retreat or advancement observed in the time frame of the image
overlays) and (4) prograding shorelines (shoreline is advancing sea-ward).
ii) Further identify and classify the prograding coastlines after field
investigations as (a) natural progradation (coastlines advancing seaward
through sediment accretion) and (b) artificial progradation (coastlines
advancing seaward through engineered structures such as a sea wall).
iii) Quantify the range of coastline change due to erosion, (areas identified and
classified as erosion hotspots) and progradation.
iv) Observe the general coastal geomorphology along the Coral Coast.
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1.4 STUDY AREA
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The study area is located on the southern coast of Viti Levu, the largest island in Fiji
(Fig. 1.1). The climate in Viti Levu is more predictable than most of the other Pacific
Islands (such as Samoa, Solomon Islands and Vanuatu) due to its location within the
trade wind belt for most of the year (Nunn, 1998). Viti Levu’s location in the trade
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wind belt sets up the dominant southeast trade winds in the Coral Coast area. The
Coral Coast area is fringed by a narrow band of fringing reef system which is
interrupted by numerous reef passages (Fig. 1.2). The primary sources of sediments
in the area are from the reef system (oceanic sediments) and the river systems
(terrestrial sediments).
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A number of villages and tourist resorts are situated along the coastal zone of the
Coral Coast, which would be affected by the coastline retreat induced by short- and
long-term coastal events.
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Figure 1. 1: Map of study area. Source: Media Centre, 2008, the University of the South Pacific.
Study�Area�
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Figure 1. 2: Fringing reef systems associated with the study area along the Coral Coast. Source: Google Earth, 2006.
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1.5 THESIS FRAMEWORK
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This chapter gave an introduction to the thesis; outlining the goals and objectives of
the thesis and the rationale behind it. Chapter 2 discusses and reviews the literature
associated with the study topic. Chapter 3 delineates the methodological approach
and techniques used to manipulate historical aerial photographs and satellite images
to examine the coastline changes along the Coral Coast area. Chapter 4 presents the
results obtained from the desktop analyses of historical aerial photographs and
satellite images and the results obtained from the ground truthing exercises. Chapter
5 reviews and discusses the findings of Chapter 4 and finally draws conclusions from
the findings. The following flow diagram indicates the flow of chapters in this thesis
(Fig. 1.3).
Study Area
WideFringingReefs
NarrowFringingReefs
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Figure 1. 3: Thesis framework.Figure
Chapter 1
Introduction
Chapter 2
Literature Review
Chapter 3
Methodology
Chapter 4
Results
Chapter 5
Discussion & Conclusion
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CHAPTER 2 – COASTLINE CHANGE PROCESSES IN THE
PACIFIC ISLANDS
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2.1 INTRODUCTION
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Sea-level rise has been identified to be the most dominant long-term cause of
coastline changes worldwide, while severe storms has been known to cause most
rapid short-term coastline changes (Dean, 2005). In the duration of severe storms
beaches may retreat on the order of 50 m in several hours. “The global rate of sea-
level rise is approximately 12 cm per century” (Dean, 2005). There is concern that
global climate warming may increase this rate substantially in future through melting
of ice and thermal expansion (Dean, 2005). According to studies concerning past
driving forces, the present sea-level trends and possible future variability, indicates a
moderate sea-level rise of 10cm (± 10 cm) per century (Mörner, 2005).
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2.2 SHORELINE CHANGES USING THE REMOTE SENSING METHOD
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Large amounts of shoreline change data are required for coastal engineering and
research, management of natural resources, beach and wetland restoration and
navigation improvement purposes (Morang and Gorman, 2005). To obtain and
analyze data for these purposes three main techniques are applied. These include
remote sensing techniques, in situ instruments and sampling methods. This section
focuses on shoreline change data analyses using the remote sensing method. The
remote sensing method involves acquiring information about the land and sea from a
distance, without physical contact, such as aerial photographs and satellite images
(Morang and Gorman, 2005).
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2.2.1 Remote Sensing Methods and Shoreline Change �
Remote sensing techniques to map coastal geomorphology, analyze shoreline
changes and coastal process interpretation require the use of both historical and
recent air photographs and/or satellite images (Gillie, 1992). This technique offers
broad area coverage, covers a large area in a short time and is suitable for analysis of
hostile environments. However, the interpretation acquired from this technique is a
model and must be verified with field data or a priori knowledge, often the
resolution is too coarse for shoreline mapping or morphology studies and
occasionally it produces challenging or conflicting interpretations; results produced
are a representation over time and need to be extrapolated to determine spatial
changes (Morang and Gorman, 2005).
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It is essential to use a proper statistical approach in order to determine shoreline
changes or to do shoreline position forecasting using the remote sensing method. An
appropriate forecast model would be one which would be in reasonable agreement
with the actual physical situation. The most common method used to calculate long-
term coastline changes is the end-point-rate method (Crowell et al., 2005). This
method uses two representative shorelines (usually, the earliest and the most recent)
to calculate the amount of shoreline change. The distance of shoreline change is then
divided by the time elapsed between successive shoreline positions (Crowell et al.,
2005). This technique is simple to apply and it only requires two shorelines to obtain
a rate of change. However, this method can be misleading for determining future
shoreline positions, since one or both of the end point shorelines may be aberrant due
to acute events such as storms (Crowell et al., 2005). In addition, potentially useful
information on determining the rate of change associated with the data between the
end points is ignored.
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Another technique to determine the rate of shoreline change is the linear regression
technique (Crowell and Leatherman, 1999; Fletcher et al., 2003). This method
determines a best fitting line that minimizes the sum of the squares of the lengths of
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vertical line segments drawn from the individual data points to the fitted line
(Crowell et al., 2005). Three advantages of using this method are: (1) except for
post-storm data points, all other data points are used in the rate calculation, thus
reducing the influence of spurious data points; (2) linear regression can be performed
by with most spreadsheet programs and scientific pocket calculators and is easily
understood and (3) the quality of the straight-line fit can be tested and measured
against a summary and related statistical techniques to estimate the variance of the
data (Crowell et al., 2005; Kleinbuam and Kupper, 1978).
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2.2.2 Role of GIS in Coastline Change Studies
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Numerous natural resource applications have used GIS and Remote Sensing
techniques to analyze and interpret data. GIS and Remote Sensing play an important
role in coastline change studies. Features such as vegetation line, sandy coasts,
shallow sea, medium sea and deep sea can be classified by image classification
performed on satellite imageries. The different forms of classifications that can be
applied in a shoreline change study include pattern recognition, spectral
classification, textural analysis and change detection. The changes in coastal
geomorphology can clearly be brought out with the aid of GIS and Remote Sensing
techniques (Sundaravadivelu, 2005).
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Using overlay analysis techniques of GIS applications a quantitative as well as
qualitative analysis with respect to the coastal geomorphology (mostly two
dimensional) can be accomplished. For instance, using historical and recent aerial
photographs and/or satellite imageries by the means of the overlay analysis
techniques of GIS applications, both long- and short-term coastline change rate can
be estimated (Koukoulas et al., 2006; Sundaravadivelu, 2005; Fletcher et al., 2003;
Pitman et al., 2000 and Gillie, 1992; 1993a). These coastline data can then be
complemented with other temporal and spatial datasets to interpret the causes and
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implications of a particular coastal issue; for example the cause of a high erosion rate
and its implications to infrastructure development (Fletcher et al., 2003).
The Geographical Information Systems (GIS) can be used to visualize erosion
predictions and analyze the implications, including possible responses, by integrating
with other spatial datasets for impact estimation and decision support (Koukoulas et
al., 2006). When visualizing coastline predictions, the use of GIS technology
provides a powerful means to understand coastal changes and their impact in local
and regional scales (Brown et al., 2004).
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2.2.3 Mapping Shoreline Changes
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Understanding the relative importance of the different contributing factors, both
natural and anthropogenic, is essential for management and preservation of beaches
and coastal development. Geographical Information Systems (GIS) can be used as a
tool to effectively manage development in the coastal zone. There have not been
many studies concentrated on shoreline changes using the remote sensing method in
the Pacific Islands; however, numerous studies on coastline changes have been
carried out across the globe. Following are brief descriptions of some studies around
the globe related to evaluating modifications in coastal zones by mapping shoreline
changes using GIS.
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Maui Island, Hawaii shoreline changes (Fletcher et al., 2003) – Past shoreline
positions on Maui Island, Hawaii have been documented by the means of digital,
aerial orthophotomosaics with 0.5-3.0 m horizontal accuracy, used with NOAA
topographic maps. The research focused on determining the annual erosion hazard
rate to be used by shoreline managers. Three regions of Maui Island were examined;
Kihei, West Maui and North Shore coasts. The annual erosion hazard rates, an
average end point rate, amount of beach loss and the percentage of beach narrowing
for each of these regions were calculated using a least median of squares regression.
This research indicated that overall, the mean island-wide end point rate of eroding
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shorelines was 0.24m/yr, the average erosion hazard rate of eroding shorelines was
0.3m/yr, shoreline change rate was 0.21m/yr and beach width showed a 19 percent
decrease over the period 1949/1950 – 1997/2002.
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GIS Modelling of sea-level rise (Hennecke, 2004) – A GIS-based coastal-behavior
model has been developed to formulate simple algorithms for simulating the
potential physical impacts of rising sea level on the coastal environment, focusing on
coastal re-entrants. This model has been designed based on work undertaken for the
Dutch Wadden Sea. The first estimates of potential shoreline change based on readily
available information, can be provided by this GIS model. The GIS model gives
further enhanced outputs; that is, the rate of shoreline change analyzed in greater
detail using a spreadsheet-based hazard probability model. The combination of both
the GIS model and the hazard probability model provides a rapid assessment of the
probability of shoreline changes instead of a single impact zone, as would be
provided by the GIS model alone. The spreadsheet-model returns the hazard
probability rates to the GIS to be displayed as a grading of risk instead of a single
impact zone. This model has been used to test the shoreline changes at two sites in
southeastern Australia.
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GIS-based vulnerability assessment (Szlafsztein and Sterr, 2007) – In the last 25
years, there have been numerous evidences of natural impacts of flood and erosion
processes in the northeast coastal zone of the State of Para (Brazil). A GIS-based
composite coastal vulnerability index (CVI) has been used to identify, assess and
classify natural and socio-economic vulnerabilities of this coastal zone. The CVI
score used to classify, weigh and combine 16 separate natural and socio-economic
variables to create a single indicator provides a reliable measure of differences
among the regions and communities, despite the problems and shortcomings of
ESRI’s Arcview 3.2 program. The results have been portrayed in the form of maps
referred to as Natural, Socioeconomic and Total Vulnerability. Finally, there is
analyses and discussion on the confidence associated with the results, the need to
utilize another variable and to frequently update the ones used already.
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Beach erosion along Tottori Coast – Yasumoto et al., (2007) used aerial photographs
and bathymetric survey data were used to investigate the beach erosion of the Tottori
coast, Japan, based on long-term shoreline changes. There is an indication of excess
accretion and severe erosion occurring simultaneously in the waver-shelter zone over
the years 1947 and 2003. Considering grain size changes, a predictive model for
shoreline changes was applied to the coast. The main factors which triggered beach
erosion were by construction of wave-sheltering structures and by dredging of
navigation channels and offshore disposal/land reclamation using dredged sand.
Hence, in order to reduce/control Tottori coastal erosion three policies have been
developed: 1. “No more offshore sand dumping”, 2. “No more offshore sand
dredging for land reclamation”, and 3. “Sand dredged from accretion zones must be
dumped near-shore where depth is smaller than the depth of closure”. It was also
found that local measures taken to reduce/control sand loss are not effective.
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Environmental changes in Mediterranean coastal landscape – Alphan and Yilmaz,
(2005) focused on change detection analyses in Cukurova, an extensive coastal plain
in the southeast of Mediterranean coast of Turkey; a remote sensing approach has
been used to monitor temporal land use/cover changes in the area. Using digital
interpretation of remotely sensed satellite data, temporal changes in the coastal
landscape between 1984 and 2000 were evaluated. Multi-temporal Landsat TM and
ETM+ images were used to do a pair-wise comparison of methods used to quantify
changes from 1984 to 1993 and 1993 to 2000. From 1984 to 1993 the total change in
area was 2448 ha and there was an increase by two folds to 6072 ha from 1993 to
2000. Information obtained from individual change detection outputs of different
periods were used to determine the change trends. Agriculture, urban, and natural
vegetation cover were estimated to be the most prominent changes that have
occurred in the area.
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Shoreline change analysis and its application to prediction – Maiti and
Bhattacharya, (2008) carried out shoreline change analysis and prediction by the
means of satellite remote sensing images and statistics. Shoreline positions have been
demarcated using multi-date satellite images from which shoreline change rates have
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been estimated using linear regression. Based on a 113.5 km coastal stretch in the
Bay of Bengal in eastern India, this study was performed over the time interval 1973
and 2003. Two time periods of short and long terms in three modes, transect-wise,
littoral cell and regional were used to estimate the past and future shoreline positions.
Overall, the results indicate that 39% of transects have uncertainties in shoreline
change rate estimations near cell boundaries and 69% of transects lie between
estimated and satellite based shoreline positions. The cells dominated by natural
processes have lower root mean square errors (RMSE) when considered for long
term period, whereas, cells affected by anthropogenic interventions show better
agreement for short term period. There is no significant difference in RMSE values
on regional considerations. The results are corroborated by geomorphologic
evidence. This study confirms the combined use of satellite imagery and statistical
methods to be reliable when doing shoreline related studies.
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Coastal processes mold and shape the coasts following certain basic natural laws.
These processes occur at a variety of time and space scales. Therefore, shoreline
changes are extremely variable from location to location and cannot be generally
predicted from single-site studies. The above cases amplify the significance of
Geographical Information Systems (GIS) tools in temporal and spatial coastal
researches. Some benefits of using GIS and remote sensing in the monitoring and
protection of coastal zones include information accessibility and dissemination,
efficient and accurate map storage and updating, monitoring terrain surface and
coastline changes, extraction of sediment movement and shift of coastlines,
computation of sediment volume change and presentation of outputs. Hence, GIS
plays a significant role in the analysis and study of coastline changes researches.
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2.3 COASTAL PROCESSES
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Carter (1988) describes the coastal zone as a broad transitional area in which
terrestrial environments influence marine environments and in which marine
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environments influence terrestrial environments. The coast comprises the interface
between the land and the sea, including areas below and above the water line, such as
shoals, dunes and cliffs. The actual margin of the land and the sea is termed
‘shoreline’ (Woodroffe, 2002).
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Figure 2. 1: A schematic coastal embayment in (a) plan form and (b) profile. Some of the key definitions based on proximity to the shore, wave characteristics, and substrate are shown. (Source: Woodroffe, 2002) �
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Explaining landforms in the coastal zone by examining the form, sediments and
depositional history of a modern shoreline is known as coastal geomorphology
(Woodroffe, 2002). According to Woodroffe (2002) the geomorphology of the coast
can be examined in plan form (also called shore-parallel, or long-shore) or profile
(also called cross-section, cross-shore, shore normal or orthogonal) (Fig. 2). The
shape of coastal landforms is dependent on the materials available at the coast and
the processes acting on these materials.
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Each of the numerous controlling factors that shape a particular stretch of coastline
need to be determined in order to understand its coastal processes. Winds, waves,
tides and currents are the major processes at work in coastal waters. The
combinations of all these processes provide the energy that shapes and modifies a
coastline by eroding, transporting and depositing sediments (Bird, 2008).
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2.3.1 Waves, Tides and Currents
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Waves are the principal source of energy in the coastal zone for erosion and
deposition (Woodroffe, 2002). In theory, provided the land area remains tectonically
stable, erosion by waves can ultimately reduce the world’s land areas to a planed-off
surface (Bird, 2008). Waves are formed as the result of the pressure contrast between
their driven (upwind) and advancing (downwind) slopes; initiated by the stress and
pressure variations on the water surface from the turbulent flow of the wind blowing
over the water (Bird, 2008). Hence, the formation of waves is due to the transfer of
wind energy to water particles; resulting in undulations on a water surface (Segar,
1998).
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Tide is a giant wave which influences coastal processes worldwide. Tides can be
defined as the cyclic rising and falling of the ocean’s surface due to the gravitational
forces exerted by the sun and the moon on the ocean. The variations in water level
produced by the varying tides are important to processes acting on the beach. The
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waves move sediments landward. As the tide comes in, the sea level rises, potentially
leading to recession of the beach profile and withdrawal of sediment seaward.
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However, the effect of tidal currents may not be prominent on most coastal areas due
to other controlling factors such as effects of longshore drift, variability of wave
action and rates of sediment input and output (Bird, 2008). Tides can produce strong
currents which prevent the building up of sediments across bay and lagoon entrances.
Furthermore, tides aid in the daily flushing of coastlines, estuaries, and harbors
which aid in the purging of pollutants. In certain parts of the world, exceptional tidal
currents are capable of significant coastal erosion; for example tidal currents off the
citadel of Mont-Saint-Michel, Normandy, France and in the Bay of Fundy, Canada
(Komar, 1976).
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Currents are generated in various ways and from multiple origins (Table 1). The
various types of currents relating to the coastal zone are responsible for the
deposition or erosion of sediments in the coastal zone.
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Table 2. 1: List of currents generated in various ways, and some currents of multiple origins. Source: Bird, 2008.
Rip currents flows back into the sea through breaking waves at intervals along the shore
Wave-generatedcurrents
flow alongshore when waves arrive at an angle to the shoreline
Tidal currents are ebb and flow (flood) currents generated by falling and rising tides
Ocean currents are slow mass movements of water in response to variations in water temperature salinity, atmospheric pressure and wind stress
Wind-generated currents
flow in the direction of the wind
Fluvial currents are the discharge where a river flows into the sea
Density currents occur where water of higher specific gravity (colder or more saline) moves to displace water of lower specific gravity, but these have no direct effect on the coasts
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2.3.2 Sediment Budget and Longshore Drifting
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Komar (1976) explains the budget of sediments to be an application of the principle
of continuity of conservation of mass to the littoral sediments. According to Bird
(2008) coastal sediment budgets deal with the volumes of sediment supplied to a
particular sector by onshore and longshore drifting and yields from the hinterland
and the volumes of sediment lost offshore, alongshore or landward over a specific
period. Beaches which have a balanced budget, where the net influx of sediment
equals the net loss of sediment are classed as healthy beaches. The net gain or net
loss of sediments from a coast can be determined by making repeated surveys along
and across a beach, using conventional methods to measure variations in the plan and
profile of the beach. Beach profiles together with information from series of dated air
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photographs can be used to monitor the advance or retreat of the coastline (Bird,
2008).
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A coastline has various sediment sources and sinks. The table below shows some
sediment sources and sinks prominent at a coastline.
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Table 2 2: The Budget of Littoral Sediments.
CREDIT DEBIT BALANCE Longshore transport into area
Longshore transport out of area
Beach deposition or erosion
River transport Wind transport out Sea cliff erosion Offshore transport Onshore transport Deposition in submarine
canyonsBiogenous deposition Solution and abrasion Hydrogenous deposition Mining Wind transport onto beach Beach nourishment �
(Source: Komar, 1976)
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In the nearshore zone, there are two wave-induced current systems which dominate
the water movements in addition to the to-and-fro motions produced by the waves
directly. These include a cell circulation system of rip currents and associated
longshore currents produced by an oblique wave approach to the shoreline. The main
cause of sediment movement along the shore is the wave-induced longshore currents;
the other currents are effective only under exceptional circumstances. For instance,
the tidal currents near a river mouth of a bay could be strong enough to cause
significant sediment transport on a beach (Komar, 1976).
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Longshore currents are accompanied by waves that arrive at an angle to the coastline
producing a transverse swash, running diagonally up the beach, followed by a
backwash that retreats directly down into the sea. The currents induced by obliquely
arriving waves result in the zigzag movement of beach material alongshore, that is
accompanied by sediment flow along the nearshore zone. The combined effect of
these processes is known as longshore drifting of sediment to beaches and spits
downdrift. There is rapid longshore drifting when wave crests approach the shore at
an angle of between 40o and 50o, where the coastline is straight or gently curved and
unbroken by headlands, inlets or estuaries, and where the nearshore sea floor profile
is smooth. Longshore drifting increases with wave energy and is aided by a small
tide range, as this gives rise to a more continuous and concentrated wave action than
where the zone of breaking waves rises and falls over a substantial tide zone (Bird,
2008).
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Accretion alongside headlands, groynes, breakwaters or landslides, migration of
beach lobes, deflection of river mouths and lagoon outlets or growth spits could be
indicators of longshore drifting. However, these features could also result from other
coastal processes. For instance, patterns of beach accretion may result partly from
sediment movement in from the sea floor, rather than alongshore (Bird, 2008).
Hence, accretion at the above mentioned places is due to the combined effect of
longshore drifting and shoreward drifting of sand and gravel by waves arriving
parallel to the coastline. According to Bird (2008) natural tracers in beach sediments
can be used to determine longshore drift patterns. For example pebbles of an unusual
rock type or specific mineral sands, may act as mineral tracers indicating longshore
drifting from a source area such as cliff outcrop or river mouth.
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In order to determine the net longshore movement of sediment along a beach, the
sum of the transport under all individual wave trains arriving at the shore from
various wave-generating areas needs to be taken into account. The accumulation of
sediments at jetties and breakwaters reflect the long-term net sediment transport
along a beach (Komar, 1976). Studies by Johnson (1956, 1957) indicate that the
littoral drift can produce a movement of up to nearly a million cubic meters of sand
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along the beach in a single year. According to Komar (1976) the application of the
budget of littoral sediments has proved to be an extremely useful approach in
evaluating the relative importance of the various sediment sources and losses to the
nearshore zone and in accounting for regions of beach erosion or deposition.
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2.3.3 Coastal Accretion Processes
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Coastal accretion is the advancement of the coastline where the deposition of
sediment exceeds the rate of erosion, or where there is emergence due to uplift of
land or a fall of the sea-level (Bird, 2008). There are various natural sources of
sediment supplements to the coastline. These include sediment supplements from
rivers, materials derived from erosion, the seafloor, by winds blowing from the
hinterland and by longshore drifting processes (Section 2.3.2; Bird, 2008). Sand and
gravel are washed down to the coastline by rivers. The weathered sandstones and
conglomerates eroded from nearby cliff and foreshore outcrops are brought to the
coastline by wave action. Sand and gravel are washed from the seafloor by waves
and currents. For oceanic coasts, many of the coastline sediments are calcareous
derived from marine organisms (Bird, 2008; Komar, 1998).
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The introduction of structures such as groynes and breakwaters intended to stabilize
features that were changing in unacceptable ways, particularly where erosion
threatened seaside towns, ports, or other coastal development have modified the
coastline (Bird 2008). Coastlines associated with land reclamation activities have
been advanced seaward by several kilometers (French 1997, cited in Bird 2008).
Seaward advancement of the coastlines have been induced in a number of places
around the world including the Netherlands, southeast Asia – Tokyo Bay, Hong
Kong and Singapore (Bird, 2008). The Southeast Asia countries have increased their
land area by 10% in the recent decade to accommodate their densely populated
coasts (Bird, 2008). Artificial progradation can also result from groyne construction.
For instance, construction of causeways between an island and mainland would
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induce progradation in the wind ward side of the causeway due to reduced wave
energy (Goodman et al., 2008). Finally, seaward advancement of the coastline can
also be induced by artificial beach nourishment and/or replenishment by seaside
resorts (Bird, 2008; Komar, 1998).
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2.3.4 Coastal and Beach Erosion Processes
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Coastal erosion is prominent in areas which loose more sediment alongshore,
offshore or to the hinterland, than they receive from the various sources (Bird, 2008).
Destructive wave action in stormy periods and the depletion of beach sediments by
weathering and winnowing, as well as a reduction in inputs by rivers, cliff and shore
erosion, spilling dunes and drifting from the sea floor are some of the processes that
lead to coastal erosion (Bird, 2008; Komar, 1998).
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Table 2.3 shows characteristic features of eroding coasts and beaches.
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Table 2 3: Indications of coastal and/or beach erosion.
Characteristic features of coasts and /or beaches under the influence of erosion 1. A concave beach profile, especially with a micro cliff. 2. Cliffed backshore dunes (whereas prograding beaches are backed by beach
ridges and incipient fore dunes). 3. Truncated vegetation zones (whereas prograding beaches are backed by tree
canopies descending to beach level, or by shrub and grass zones on recently formed sandy terrain).
4. Patches of sand adhering to rock outcrops indicating the previous higher level of the beach.
5. Exposures of beach rock that formed within the former beach. 6. Exposure of rocky or muddy substrate.
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(Source: Bird, 2008)
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Upon assembling evidence of coastline changes around the world between 1976 and
1984 for the preceding century, the Commission on the Coastal Environment
(International Geographical Union) found that beach erosion had become
widespread. Over this period more than 70 per cent by length of beach-fringed
coastlines had retreated, less than 10 per cent having advanced (prograded), the
balance having either remained stable or shown alternations with no net gain or loss
(Bird, 1985).
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Table 2.4 indicates the causes of coastal and/or beach erosion compiled by the
Commission on the Coastal Environment. The listed factors have been identified as
having initiated or accelerated beach erosion, their relative importance varying from
one coast to another. Intensification of erosion on a particular coast is usually the
result of more than one of these factors. When attempting to explain coastal and/or
beach erosion, each of the possible factors should be considered and ranked in
importance in relation to the rate of erosion.
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Table 2. 4: The causes of coastal and/or beach erosion.
The Causes of Coastal and/or Beach Erosion As Proposed by the Commission on the Coastal Environment
1. Submergence and increased wave attack 2. Reduction of fluvial sediment supply 3. Reduction in sediment supply from cliffs 4. Reduction of sand supply from inland dunes 5. Reduction of sediment supply from the sea floor 6. Extraction of sand and shingle from the beach 7. Increased wave energy 8. Interception of sediment supply by longshore drifting 9. A change in the angle of incidence of waves 10. Intensification of obliquely incident wave attack 11. Increased losses of beach sediment to the backshore 12. Increases storminess 13. Attrition of beach material 14. Beach weathering 15. Increased scour by wave reflection from a sea wall 16. Migration of beach lobes 17. A rise in the beach water table 18. Removal of beach material by runoff 19. Diminished tide range 20. Abrasion by driftwood 21. Removal of mangroves and other coastal vegetation
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(Source: Bird, 2008)
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By comparing dated sequences of maps and charts, or air and ground photographs,
the rate of retreat of high tide shoreline, which is often also the seaward boundary of
terrestrial vegetation communities, can be measured (Bird, 2008).
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2.4 CHARATERISTIC COASTAL ZONES OF THE PACIFIC ISLANDS
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The combined effect of a number of variables determines the characteristics of a
coastal zone; intensity of upwash and backwash, wave steepness, sediment
characteristics and underlying substrate type (Komar, 1976).
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2.4.1 Coastal Geography of Pacific Islands
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Shorelines in the southwest Pacific during the Last Interglacial, which peaked about
125,000 years ago, were mostly within a few meters of the modern shorelines (Nunn,
2002). The southwest Pacific sea-level at the height of the Last Glacial was about
130 meters below its present level. Ice melt since the Last Glacial maximum, which
occupied most of the first half of the Holocene, was the principal cause of sea-level
rise in this period (Fig. 2.2). Due to the Earth’s rheological response to the melting of
the last continental ice sheets and subsequent redistribution of melt-water, a +1 to +3
meter(s) relative sea-level highstand on oceanic islands has been predicted by
Grossman, et al. (1998). The rising sea level had profound effect on the climate and
vegetation along island coastlines (Nunn, 2002). Prior to 5,000 to 7,000 years ago,
most coastlines in the Pacific compromised sheer cliffs, composed of Pleistocene
coral reef, with steeply dipping offshore and little erosional shoreline development
(Gibbons, 1984; Nunn, 2002). The sea-level in most of the Pacific region rose above
its present level and stabilized for the first time in about 12,000 to 13,000 years
during the Holocene Climate Optimum, allowing lateral erosion to commence in
earnest (Nunn, 1991, 1994, 2002; Pirazzoli, 1978). Lateral erosion lead to the
production of shore platforms at low-tide levels which became exposed subsequently
when sea-level fell in the late Holocene. Coastal plains and flats that most islands
occupy today are formed from the accumulation of alluvium, colluviums, and
marine-derived material on emerged shore platforms (Nunn, 2002).
27�
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Figure 2.2: Holocene sea-level changes in the Pacific. A. Sea-level changes during the Holocene in Fiji; B. Late Holocene sea-level changes in the Tuamotu Islands, French Polynesia. Source: Nunn, 2002.
�
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The processes responsible for island formation dominate the southwest quadrant of
the Pacific Ocean and this is where most of the Pacific Islands are distributed (Nunn,
2005). Another prominent feature of most of the Pacific Islands is the occurrence of
coral reefs (Nunn, 2005). Corals and associated reef organisms of many nearshore
and shallow areas are a result of recolonization during the postglacial warming of the
ocean surface waters (Nunn, 2005). The coral reefs managed to “keep-up” with the
rising sea level, so that the reef surface remained within the photic zone (Nunn 1994;
Woodroffe 2002). This is evident in Tarawa, Kiribati and in parts of French
Polynesia (Nunn, 1994). In most parts of the tropical southwest Pacific the coral
28�
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grew at a slower rate and only managed to “catch-up” with the sea-level when its
postglacial rise slowed around 6,000 – 5,000 years ago (Nunn 2005; Woodroffe
2002). Some coral reefs are believed to have been established at the sea level during
the early postglacial. These were unable to grow upwards fast enough and are not
visible at the surface today, for example coral reefs to the west of the islands of
Samoa (Nunn, 2005). In nomenclature of Neumann and McIntyre (1985) these reefs
are classified as “give-up” reefs.
�
Coral reefs are significant in the dynamics of modern coastlines of the tropical
Pacific Islands. The beaches in the Pacific Islands are fossil beaches. The sediments
created from coral reefs form a major or the main component of many beaches and
sand islands. Beaches and sand islands can become severely eroded where the
sediment production cease due to coral degradation (Nunn, 2005).
�
The Pacific region is comprised of islands from various origins; including islands of
continental origin (for example islands in New Caledonia and New Zealand), of
island arc origin (for example islands in the Fiji, Solomon and Vanuatu groups), and
islands that form as a product of hotspot volcanism, occurring in lines that mark the
passage of the Pacific Plate across a fixed mantle hotspot (Nunn, 2007). Many small
Pacific Island nations are atolls (for example Kiribati and Tuvalu), where the primary
sediment source is marine derived sediment. Carbonate particles are generally
coarser with high percolation rates in comparison to continental and high volcanic
islands (Nunn and Mimura, 2007).
�
Smaller Pacific Island coasts are either cliffed and commonly hard rock, for instance,
those found around high limestone islands such as Vava’u in Tonga and volcanic
islands like Nuku Hiva in the Marquesas (French Polynesia) or low lying and
composed of partly unconsolidated sediments, such as atoll islands in Kiribati, the
Marshall Islands, Tokelau and Tuvalu. The older, more denuded coasts of larger
island also possess soft shorelines, especially around river mouths, as in the Rewa
29�
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Delta of Viti Levu Island in Fiji, or where broad fringing coral reefs exist (Nunn and
Mimura, 2007).
�
Low-lying coasts are vulnerable to most environmental changes. In order to
comprehend the impact of varying environmental conditions on low-lying coasts it is
essential to analyze the sub-components of low-lying coasts; onshore, shoreline and
nearshore environments. Vulnerability of onshore environments of low-lying coasts
is dependent on the elevation and land use. Extensive coastal lowland may appear to
be the most vulnerable; however, under certain conditions they may offer
considerable resilience. Low-lying shorelines exposed to high wave conditions may
have swash processes having large run-up levels, which lead to the steepening of
shorelines and creation of berms through overwash processes (Hughes and Turner,
1999). Heightened vulnerability may be exhibited by narrow strips of coastal
lowland because threats such as storm surges are focused on comparatively small
areas (Nunn and Mimura, 2007).
�
The amplitude and frequency of wave energy conditions are highest on the shoreface
and this is where profile changes are the greatest (Cowell et al., 1999). The width of
the upper shoreface is able to adjust its profile rapidly to a range of wave energy
forces. These forces may be operating episodically, for instance storm events or on a
regular basis, such as tidal, seasonal events (Bird, 2008). With minor adjustments to
small scale geomorphological components of the system, this adjustment process
tends to restore equilibrium of the shoreline (Komar, 1976; Bird, 2008).
�
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2.4.2 Sediment Sources of the Coastal Zones
�
The sediments on a beach can be classified into primary and secondary sources. In
contrast to primary sources of sediments, secondary sources of sediments have
undergone weathering and initial sorting, prior to entering the coastal environment
30�
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(Carter, 1988). In the case of low-lying coasts of Pacific Islands, majority of the
primary sediment is produced on the fringing and barrier reef edge and rim as
biogenic sediment (Nunn and Mimura, 2007; Woodroffe, 2002). After main
producers some of the sources of biogenic sediments may be forminifera, faeces of
invertebrates and products of bio-erosion by borers, etchers and grazers. These
organisms generate and/ or break down coarser particles of carbonate produced by
the principal reef builders (Woodroffe, 2002). The reef crest and fore-reef slope are
the most important areas where calcareous sediment is produced. Reef systems are
important sources of both lagoonal and shoreline (beach) sediments (Nunn and
Mimura, 2007).
�
Throughout episodic and long-term coastal events, sediment production, which is
linked to climate and reef productivity, is essential for maintaining shoreline
stability. The reef crest and reef front are zones of high calcification and sediment
production, due to abundant coral growth and this is also where high energy waves
occur (Nunn and Mimura, 2007; Woodroffe, 2002). Initially these sediments are
poorly sorted and texturally immature with the biological components in the
sediments influencing the sediment size (Woodroffe, 2002). With the aid of waves
and bioerosion these sediments are modified and transported as calcium carbonate
sediments to the shore, reef flat, lagoon and offshore. Beach rocks are formed when
sediments are cemented by microbially mediated precipitation of carbonates (Turner,
2005). It forms by rapid cementation of sandy beach sediments over several decades
and its formation is limited to the intertidal zone (Neumeier, 1999). The cement
involved in beach rock formation consists of high magnesium calcite (HMC) or
aragonite (Bricker, 1971; Neumeier, 1999). Different cements of various habitus and
mineralogy are produced by several diagenetic phases following one another.
Usually, diagenesis begins with micritic cement and is followed by prismatic cement
(Holail and Rashed, 1992; Meyers, 1987; Neumeier, 1999; Taylor and Illing, 1969).
�
The rate of erosion and deposition on a shoreline depends on how much sediment is
available to allow the shoreline to remain in equilibrium. Beaches in most islands
with stream and river outlets consist of both terrigenous and carbonate sediments, for
31�
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instance, Tuvu and Korotogo Villages along the southern coast of Viti Levu in Fiji
Islands (Kench and Cowell, 2002). If sediment supply to the coast increases from one
or both sources, shoreline progradation may occur. However, if sediment supply
decreases coastal erosion can occur.
�
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2.5 CAUSES OF COASTAL EROSION IN THE PACIFIC ISLANDS
�
According to Bird (1986) and Nunn, (2000), over the past century, most tropical
Pacific Islands have experienced shoreline erosion and lateral coastline inundation.
�
There are multiple causes of coastal and beach erosion; no single factor (Table 2.4)
can account for the modern prevalence of erosion of the world’s coasts, or the onset
of acceleration of erosion on any particular beach (Bird, 2008). Coastal erosion is
driven by sea level rise, wave and current impacts (2.3.1) and sediment deficiencies
(2.3.3). Other causes of eroding shorelines include sudden events such as cyclones,
storm surges and tsunamis. Building of coastal engineering structures can also
modify the coastline resulting in beach erosion. This section attempts to explain the
major causes of shoreline retreat in the Pacific Island nations.
�
Erosion can be described as acute and chronic erosion (Anthony, 2005). Acute
erosion occurs as a consequence of a single event while chronic erosion is a
continuous phenomenon. Chronic erosion may be evident on coastal lands due to
long-term erosion under certain conditions. On the other hand, Gillie (1997)
describes coastal erosion for the purpose of coastal management by dividing it into
two main types: natural and human-induced causes. For the purpose of this study the
causes of coastal erosion in the Pacific Island nations is discussed according to the
natural and human induced types.
�
�
32�
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2.5.1 Natural Causes of Coastal Erosion
�
In the tropics, most natural beach systems are in a state of dynamic equilibrium. The
beach adjusts to a new shape in equilibrium each time a new condition is introduced
due to a change in the wave and/or current patterns and sediment transport patterns.
In the Pacific Island nations the natural causes of coastal erosion includes, changes
and/or cycles of long-term weather patterns, natural shoreline evolution or re-
alignment, sea level rise and catastrophic geohazards in the coastal zones (Gillie,
1997).
�
�
Changes and/or Cycles in Long-term Weather Patterns
�
The wave direction approaching a shoreline when examined over a relatively long
period (20-30 years) may fluctuate over a certain value due to annual and inter-
annual cycles; changes in seasonal wind patterns (Gillie, 1997). According to
Gourlay (1988), over longer-time scales, the average wind direction and resultant
wave directions may vary over 10 degrees from the mean long term average
direction. This variation may be due to decadal changes in cyclone frequency,
oceanographic conditions, and/or climate change. Sand spits and cays may have a
significant impact due to this change in wave direction.
�
Sea level changes and wind and wave patterns associated with El Niño Southern
Oscillation (ENSO) events, which range in period from a few to eight years in
duration, have a major influence on coastal processes, especially shore erosion
(Gillie, 1997). In some South Pacific countries the cycles of beach profile changes
are associated with ENSO events; for example beaches on Tarawa Atoll, Kiribati
(Harper, 1989; Howorth, 1983; 1985; Sallenger Jr. et al., 2002).
33�
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Sea-Level Rise
�
Higher sea levels could affect the coastal zone in a variety of ways, including greater
shoreline retreat, increased coastal erosion, property destruction, and salt water
intrusion into bays, rivers, and underground water sources (Primo, 1997). Currently,
retreating shorelines and increased coastal erosion are considered to be major
problems induced by the rising sea level. The Bruun rule states that if there is a rise
in sea level, the shoreline will recede not only to the position of the water level
increase but also to the position by wave action (Brooks et al., 2006). This signifies
an increase in the rate of coastal erosion with an increase in sea level. However,
studies of sequence stratigraphy suggest that the rate of sediment supply can keep
pace with the rate of relative sea-level rise (Donovan, 2005). Hence, sea-level rise is
not necessarily the cause of coastal erosion.
�
Over the past two centuries the Pacific sea level has been rising at 1.0-1.5mm/year
(Pirazzoli, 1986; Hannah, 1998; Wyrtki, 1990; Nunn, 1993). According to
Intergovernmental Panel for Climate Change (IPCC), (2007), the 3.1mm increase in
eustatic seal level over the years 1993 to 2003, has been faster than the sea level rise
over the years 1967 to 2003 (1.8mm). Apart from the larger millennial scale due to
global sea level rise, there can be rapid variations in sea level. Examples of these
would be occurrences as an outcome of a significant drop in atmospheric pressure
typical of intense short-term events such as storms and tropical cyclones, at internal
timescale as with ENSO (El-Niño Southern Oscillation) events (Diaz, 2005).
�
The sea-level throughout the Pacific is monitored by a network of 12 high resolution
SEAFRAME (Sea Level Fine Resolution Acoustic Measuring Equipment) stations.
These are located in Cook Islands, Federated States of Micronesia, Fiji, Kiribati,
Marshall Islands, Nauru, Papua New Guinea, Samoa, Solomon Islands, Tonga,
Tuvalu and Vanuatu (Gray, 2009).
34�
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Installed in Lautoka, Fiji in October 1992, the SEAFRAME gauge indicates the sea-
level trend to date to be +3.5 mm/year. To provide the relative net rate of sea-level
trend, effects of vertical movement of the tide gauge platform and the inverse
barometer effect from the observed rate of relative sea-level change are removed
(Pacific Country Report, 2006). Considering the precise levelling results and inverted
barometric pressure effect, the sea-level trend for Lautoka, Fiji is estimated to be
2.9mm/year (Gray, 2009). Monthly mean sea-levels variations were affected by
moderate seasonal cycles, El Niño (in the years 1997/1998) and tropical cyclones (in
the years 1993, 1997, 2003) (Fig. 2.3).
�
�
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Figure 2.3: Monthly sea-level record from SEAFRAME at Lautoka, Fiji. Source: Gray, 2009.
�
�
According to Nunn (1991), there is evidence of shoreline changes over the last
century in parts of Fiji, Tonga and Western Samoa, although largely anecdotal,
which generally points to an increase in shoreline erosion and a rise of high-tide
level. Studies along the coasts of Verata and Tailevu (eastern of Viti Levu, Fiji),
indicate the landward movement of high-tide level of 130m, representing a sea-level
35�
�
rise of 10-30cm in the last forty years (Nunn, 1987). As indicated in Fig. 2.3 (above),
the current rate of sea-level rise, after compensating for land movement and
barometric pressure effect, recorded by SEAFRAME at Lautoka, Fiji, is 2.9 mm/year
(Gray, 2009). Gray, (2009) also states that according to Figure 2.3, there has been no
change in sea-level between 2000 and halfway through 2007. The above mentioned
+3.5mm/year “long-term trend” is influenced by the El Niño period in 1998 and
1999 and should not be used to conceal the stability of the next seven and half years.
�
For the coasts fringed by reef systems the coral reefs can either “keep-up” with the
sea level rise in order to remain in the photic zone, or “give-up” due to events such as
thermal expansion, coral bleaching and ocean acidification and become submerged
reefs (Nunn, 2005). The removal of the protective effects of offshore reefs, due to
submergence, would allow shoreline attack by waves of much greater amplitude than
present under a given set of environmental conditions, enhancing shoreline erosion
and the rate of landward movement of high-tide level. Furthermore, over the next
century the effects of storm surges on the shorelines may possibly increase in
severity (Nunn, 1991).
�
�
Tropical Cyclones and Storm Surges
�
Tropical cyclones which have wind speeds greater than 61 km/hr generate extreme
elevations of the mean sea level producing storm surges. This results in erosion,
flooding and inundation of the coastal margin (Smith and Jackson, 1990). When
compared with waves generated by the trade winds, cyclonic waves are much more
dangerous as they are characterized by higher amplitudes, longer periods, and
changing directions with cyclonic movement (Durand, 1997). The inverted
barometer effect and wind set up which produce temporary elevations in sea surface
height, with wave run-up often leading to the overtopping of berms and dunes far
above the mean sea level, are the main components of a storm surge (Aung, 1998).
Since, the maximum wave value superimposed on a high storm surge can reach the
36�
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coastal zone without breaking, its energy is retained. Hence, a high rate of littoral
transport, beach erosion, and overwash deposition, which results in accumulation of
storm debris on the beach backshore. During trade and cyclonic periods, waves are
the main dynamic agent of sediment transport on the insular self (Durand, 1997).
Except for a relatively narrow zone (5-10 degrees) on either side of the equator, all
parts of the tropical and subtropical Pacific Island region experience the effects of
tropical cyclones occurring with a variable seasonal frequency (Gillie 1997). In the
South Pacific, the year to year variability in tropical cyclone numbers is mostly
related to non-seasonal fluctuations in the Pacific ocean-atmosphere system (Terry
2007). A major influence is exerted by El Niño-Southern Oscillation (ENSO) events.
During strong Southern Oscillations tropical cyclone activity is 28% above average
(Basher and Zheng 1995).
�
In the South Pacific, majority of the tropical cyclone coincides with the hot and wet
summer season, and traditionally lasts for 6 months from November to April (Terry
2007).Tropical cyclones are most likely to form at the height of the Southern
Hemisphere summer, in January and February. A small number of cyclones develop
just before and after the traditional hot season since the beginning and end of hot and
wet conditions marking the summer season are naturally variable on an inter-annual
basis (Terry 2007).
�
On the low-lying coastal zones of Fiji, tropical cyclones pose a serious threat;
causing infrastructure damage, loss of land, and a threat to the health and well being
of the people. Between 1964 and 2001, 50 cyclones have transversed the Fijian
waters which amounts to approximately 15 cyclones per decade (Terry, 2002). Seven
of these tropical cyclones, (including Joni, Kina, Oli, Gavin, Dani, Ami and Paula)
and numerous storms with different intensities, have hit the Fiji Islands since 1993.
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Table 2. 5: A list of cyclones affecting Fiji Islands between the 1969/70 to 2007/08 seasons. Adapted from: Fiji Meteorological Services, 2008.
CYCLONE NAME YEAR MONTH DAY WINDS Km/hr Knots
Nora 1970 October 29 63-87 34-47 Priscilla 1970 December 17-18 63-87 34-47
Bebe 1972 October 23-26 >117 >64 Henrietta 1973 February 1-2 88-117 48-63 Juliette 1973 April 3-4 88-117 48-63 Lottie 1973 December 9-10 >117 >64 Pam 1974 February 1 63-87 34-47 Tina 1974 April 26 63-87 34-47 Val 1975 Jan-Feb 31-2 >117 >64
Betty 1975 April 5-6 88-117 48-63 Anne 1977 December 25-26 88-117 48-63 Bob 1978 January 4-5 >117 >64
Ernie 1978 February 18-19 88-117 48-63 Fay 1978 December 28-29 88-117 48-63 Meli 1979 March 26-27 >117 >64 Peni 1980 January 2-5 63-87 34-47 Tia 1980 March 23-24 88-117 48-63
Wally 1980 April 3-5 63-87 34-47 Arthur 1981 January 12-16 >117 >64 Oscar 1983 March 1-2 >117 >64 Sarah 1983 March 25-28 >117 >64 Cyril 1984 March 17-18 63-87 34-47 Eric 1985 January 14-19 >117 >64
Nigel 1985 January 16-20 >117 >64 Gavin 1985 March 5-7 63-87 34-47 Hina 1985 March 16-18 88-117 48-63 Keli 1986 February 9-10 63-87 34-47
Martin 1986 April 11-13 88-117 48-63 Raja 1987 December 26-31 >117 >64 Bola 1988 March 3-4 63-87 34-47 Eseta 1988 December 23-24 63-87 34-47
Unnamed 1989 February 11-12 63-87 34-47 Kerry 1989 Mar-Apr 30-2 63-87 34-47 Rae 1990 March 18-24 63-87 34-47 Sina 1990 November 27-29 >117 >64 Fran 1992 March 7-9 63-87 34-47 Joni 1992 December 8-12 >117 >64 Kina 1993 January 1-4 >117 >64 Mick 1993 February 7-8 63-87 34-47 Oli 1993 February 17-18 63-87 34-47
Gavin 1997 March 6-9 >117 >64 Ian 1997 March 17-19 63-87 34-47
June 1997 May 3-5 63-87 34-47 Lusi 1997 October 10-12 63-87 34-47
Susan 1998 January 7-8 63-87 34-47 Cora 1998 December 24-25 63-87 34-47
Jo 2000 January 24-26 63-87 34-47 Neil 2000 April 14-16 63-87 34-47 Paula 2001 March 1-3 63-87 34-47 Zoe 2002 December 30-31 63-87 34-47 Ami 2003 January 12-15 >117 >64 Cliff 2007 April 4-6 63-87 34-47
Daman 2007 December 5-9 >117 >64 Gene 2008 January 27-29 88-117 48-63
38�
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Tropical cyclones and associated depressions in Fiji approach from a range of
directions, with the most common direction being West to North-West (Fiji
Meteorological Services, 2008). Table 2.5 indicates the tropical cyclones
experienced by Fiji Islands in the years 1969 to 2008. The Fiji Meteorological
Services places the speed of the winds given in knots and km/hr into 3 grades: Gale
(63-87 km/hr or 34-47 knots), Storm (88-117km/hr or 48-63 knots) and Hurricane
(>117 km/hr or >64 knots). The frequency of cyclones in the Fiji region varies with a
minimum interval between two cyclones of a few months to four years. In the year
1985 Fiji experienced four cyclones within the period of a few months (Table 2.5).
�
In 1972, Mele Bay, (in Port Vila, Vanuatu) experienced the effects of Cyclone
Carlotta which produced north to northwest winds; blowing at gale force for 34 hours
and at hurricane force for 9 hours. Cyclone Carlotta destroyed sections of the seawall
near the Hotel Rossi in Port Vila. For the Mele Bay area, cyclone data indicate that
significant damage to the coastal environment is due solely to climatic events
expected to occur every 10 years. Cyclone activity has caused two major river
mouths (Tagabe and La Colle) in Mele Bay to change their positions along a
coastline length of approximately 500m at least 3 times in the last 50 years.
(Howorth and Greene, 1991).
�
In 1986, the north of Guadalcanal coast (Solomon Islands) experienced extensive
economic losses from infrastructure and crop damage by cyclone Namu. Cyclone
Namu was characterized by heavy precipitation and flooding instead of strong winds,
hence, it was predicted that longer term effects of the cyclone would include an
enhanced rate of buildup of beaches from the enormous amounts of sediment which
flooding brought to the coast (Danitofea and Baines, 1991).
�
There was extensive foreshore erosion along 20 kilometers of the northern coastline
of Rarotonga, Cook Islands caused by Cyclone Sally in 1986. The high energy waves
induced by the strong winds of the cyclone destroyed buildings and uprooted trees
along the foreshore. Also, there was recession of the vegetation line of up to 30
39�
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meters. Heaps of coral boulders and rubble, visible at low tide, accumulated in an
approximately 30m x 10m area along the fringing reef on the northeast coast of
Raratonga. These materials were eroded from the beach fronts or dislodged from the
seabed and transported laterally along the lagoon by the sea (Cowan and Utanga,
1991).
�
The effect of storms and cyclones on atoll islands (where atolls experience
hurricanes/ typhoons and tropical cyclones) are both constructional and erosional
(Woodroffe, 2008). The atolls in the storm belt are different from those that are
unaffected or occasionally affected by storms. Storms are responsible for
transporting larger materials such as boulders which aid in the construction of
conglomerate platforms. The presence of a conglomerate platform is also depended
on the sea-level history and suitable conditions to cement the materials. The atoll
islands which are not in storm areas are predominantly sandy and conglomerate on
the islands is rare (Woodroffe, 2008). The indication of erosion on the oceanward
beaches is nearly ubiquitous on atoll islands; for example the presence of beach
scarps, undercutting of vegetation and outcrops of beach rocks from removal of
unconsolidated sediments by waves (Stoddart and Steers, 1977). However, a number
of factors indicate that the oceanward beaches of atoll islands are net sinks of
sediments. These include radio carbon chronologies indicating incremental accretion;
sediment produced on or near the reef crest is transported by the unidirectional
progression of waves across reef flats implying accumulation; and multi-temporal
image and photographic analyses indications of shoreline progradation (Biribo, 2008;
Woodroffe, 2008).
�
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Tsunamis
�
Tsunamis are generated when seismic events cause a section of the seafloor to move
vertically or to slump. The sudden vertical movement either lowers the seafloor and
overlying water to form a wave trough or pushes the seafloor and the overlying water
40�
�
to form a wave crest (Segar, 1998). These long infragravity waves cannot be detected
by a boat in the ocean or by observation from an airplane and occur through a
number of ways; a submarine landslide, earthquakes or volcanic eruptions (Aung,
1998). Tsunamis have the ability to temporarily raise the sea-level to extreme levels
very rapidly with the potential to devastate large tracts of the coastline (Segar, 1998).
There is little information on the frequency and magnitude of tsunamis (Gillie,
1997); however, it has been found that earthquakes of magnitude less than 6.5 on the
Richter scale generally do not generate tsunamis (Singh, 1991).
�
According to Aung (1998) the major cause of tsunamis in the Pacific are
earthquakes. In the year 1996 there were two significant earthquakes in the Pacific
Ocean which caused significant rise in sea level near Vanuatu; Richter scale readings
for these two earthquakes were 7.7 (in the vicinity of Alaska) and 6.9 (in the vicinity
of Samar, Philippines). However, Ripper and Moaina (1991), state that the most
devastating Papua New Guinea tsunami was not generated by an earthquake but by a
volcanic eruption, which occurred in the Huon Peninsula/West New Britain region.
In contrast, Cooke (1981) suggests that this 50 feet high tsunami was as a result of
submarine slumping and had no association with the volcanic eruption.
�
Irrespective of the causes of tsunamis, they have devastating impact potentials. In
case of the tsunami in Papua New Guinea, all villages in the northern and eastern
coasts of Umboi Island and Sakar Island were destroyed. The path of the tsunami
was marked by a sharply defined strip about 40 to 50 feet above sea level; running
parallel with the coastline of Umboi and Sakar Islands (Ripper and Moaina, 1991;
Cooke, 1981).
�
A total of eleven tsunamis over a 100-year period, 1877 to 1977, have been
catalogued for the Fiji Islands (Everingham, 1987). The cause of the largest three
tsunamis (with an average maximum amplitude of two meters) out of the eleven
catalogued was earthquakes off the coastlines of northern Vanua Levu (1881), and
southern Viti Levu (1953) (Singh, 1991). It is only the 1953 tsunami which caused
41�
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significant damage and life loss as the barrier reefs and surrounding shallow seas
protect most of Fiji from local and distant tsunamis. The 1953 tsunami, was caused
by slope failure near Suva harbor triggered by an earthquake with a Richter scale
reading of 6.7, damaged the Suva waterfront before breaking over the seawall. Coral
blocks up to three meters in diameter were dislodged; the hulk of a wrecked vessel
was thrown up on the reef about seven kilometers southwest of Suva, sunken logs
and miscellaneous debris littered the foreshore after the wave receded and sand
banks were created in various reef passages while some existing banks were scoured
out along the south coast and in Suva harbor (Houtz, 1962). Apart from this damage,
over the 100-year period; 1877 - 1977, tsunamis have not had any prominent effect
on the coastlines of Fiji Islands (Singh, 1991).
�
Rahiman and Pettinga (2006) reveal that the seafloor in southeast of Viti Levu
(which forms part of the Viti Levu Seismic Zone (VLSZ)), indicates a complex
network of linear submarine canyons and numerous submarine slides. Complex co-
seismic faulting through the fault mesh of the submarine canyons are due to low
occurrence of large earthquakes and a diffused pattern of seismicity currently
observed in the VLSZ. The VLSZ has abundance of submarine slides which typically
occur on the outer barrier reef edges, as well as at the head of submarine canyons and
walls, and mid slope. This implies seismic shaking to be a significant factor for
triggering slope failure. Morphometric analysis and empirical modeling of slides
indicate the largest near-field tsunami amplitude to be produced by slides triggered in
shallow water depths, within 5 Km of the coastline at the outer barrier reef edge and
submarine canyon heads (Rahiman and Pettinga, 2006). According to Rahiman and
Pettinga (2006) a significant tsuami hazard can be created by such submarine slides.
�
�
42�
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2.5.2 Human-induced Causes of Coastal Erosion
�
In the Pacific Island nations human-induced coastal erosion include beach sand
extraction, effects of sand trapping structures, and reclamation of shorefront and
construction of seawall (Gillie, 1997).
�
�
Sand Extraction
�
In the tropics the supply of carbonate sediments is often very low due to the limited
supply rates and rapid formation of beach rock. Sand and beach rocks are mined
from beaches and coral boulders are removed from reefs for traditional construction
activities and minor shoreline reclamation purposes prior to European contact (Gillie,
1997). These beach mining activities were minimal due to the absence of modern
equipment; hence, the supply of sand from the reef and other sources was sufficient
to compensate for the material removed. Today, modern mining equipment has
enhanced the rate of removal of beach material to such an extent that the natural
sediment supply is unable to keep up with the rate of sand removal. For instance, the
beaches provide the sand used in the construction industry on all major islands in
Tonga. Extensive sand extraction from two of the islands has led to shoreline
recession. If the rate of sand extraction on these islands continues, its beaches will be
completely destroyed (Tappin, 1993).
�
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Sand Trapping Structures
�
Usually, in order to stabilize a segment of the coast, groins or other barriers are
constructed perpendicular to the shoreline to prevent sand from being moved along a
beach by longshore currents. Often there are downdrift impacts of structures that
induce sediment blockage of littoral drift. Causeway construction between sand islets
43�
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on the atoll rim in some Pacific island countries results in the disruption of natural
currents and sediment transport through the inter-islet channel (Maragos, 1993). Due
to this there is both erosion and re-alignment of adjacent shorelines; for instance in
the Gilbert Group, Kiribati (Gillie, 1993b).
�
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Reclamation of Shorefront Land and the Construction of Seawalls
�
Backshore and beach areas have often been reclaimed and developed due to a local
shortage of land area induced by uncontrolled population growth and development
pressures as well as a lack of coastal management guidelines or legislation. The sand
that was previously part of the active beach system and was available as a buffer
against storms is fixed upon the reclamation of shorefont land as this encroaches on
the dynamic coastal zone width. Shore zones adjacent to the reclaimed area often
experience erosion as the removal of sand from the active beach system increases the
likelihood of erosion on adjacent or downdrift beaches (Gillie, 1997).
�
When vertical concrete, rock or wood piling walls are constructed behind a beach
there is an increased likelihood of beach erosion. Under average wave conditions,
such seawalls may not be subjected to wave action, however, the sand in front of the
wall will tend to be moved in an offshore direction during storms. The re-
establishment of sand once a seawall is exposed to waves becomes difficult, because
even mild breaking on the wall will induce turbulence that will start sand movement.
There could be complete structural failure of the seawall as a result of undermining
of the seawall foundation by toe erosion (Gillie, 1997).
�
Theoretically, if a coastal zone is only exposed to the effects of acute erosion, there
would be no detrimental long term effects (Maharaj, 2000). In this case, there would
be no need for any other measures except for the building of a protective structure,
for instance a heavy revetment. However, the onset or intensification of erosion on a
44�
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particular coastal area is often due to a combination of both acute and chronic
erosion.
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2.6 SUMMARY
�
This chapter outlines the processes related to coastline erosion in the Pacific Islands.
To begin with, it discusses the appropriateness of the remote sensing method when
examining coastline changes. It is more appropriate to use the linear regression
method than end-point-rate method when analyzing multi-temporal data. However,
the end-point-rate method is commonly used due to its simplicity in application and
limited data requirements. Today, a number of researches on shoreline change and
coastal geomorphology are carried out using remote sensing methods due to its
suitability in examining temporal changes and ability to manipulate large data sets.
Coastal processes are dominated by the action of waves, tides and currents. It is these
factors which determine the geomorphology of a coastal setting. The stability of
coastal zones are depended on the sediment budget; sediment supply (example, from
coral reefs and rivers), sediment sinks (example, by wave and wind transport out of
an area) and sediment distribution (example, longshore drift). Most of the Pacific
Islands lie in the southwest quadrant of the Pacific Ocean and are surrounded by
various types of coral reef systems; fringing, barrier and atolls. The major sediment
supply for many of the Pacific Islands is the coral reefs, while some larger volcanic
islands have sediment supplements from river systems. The causes of erosion in the
Pacific Island countries are both natural and human-induced. The natural causes
include: changes in long-term weather patterns, natural shoreline evolution, sea-level
rise, tropical cyclones, storm surges and tsunamis. The human-induced causes
include: sand extraction, sand-trapping structures, shorefront land reclamation and
construction of shoreline protection structures such as seawalls and groynes. The
remote sensing method together with field studies (in-situ and sampling methods)
are appropriate techniques for examining coastlines for research and/or planning
coastal development purposes.
45�
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CHAPTER 3 – METHODOLOGY
�
�
3.1 INTRODUCTION
�
The methodological approach in this study has been designed to examine temporal
changes along a coastline. The research design focuses on identifying and classifying
the shorelines as prograding, resilient and eroding coastlines; identified by
comparison of historical aerial photographs and satellite images. Subsequently,
carrying out a ground truthing exercise along the study coast to re-evaluate the
results obtained from the desktop study of historical aerial photographs and satellite
images. This chapter delineates the techniques and processes involved in
investigating coastline changes by separating them under the following headings:
workflow in determining the range of coastline change – outlines a theoretical
approach to analyzing coastline changes, the study site – location, description and the
selection of the study site, research methodology – discusses the techniques involved
in the development of an appropriate method to examine coastline changes and
finally, leading to the ultimate method used to analyze coastline changes and field
investigation – ground truthing exercise to re-examine the results acquired from the
comparison of historical aerial photographs and satellite images.
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3.2 WORKFLOW IN DETERMINING THE RANGE OF COASTLINE
CHANGE
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This section outlines a hypothetical approach to analyze the range of coastline
changes by comparing historical aerial photographs and satellite images. In doing
so, it looks at the phenomenal range of coastline changes, the identification of source
46�
�
data, the techniques involved in analyzing coastline changes and the various types of
error that are likely to arise from image analysis.
The following flow chart outlines the order of the processes involved in the
examination of coastline changes by comparing historical aerial photographs and
satellite images.
47�
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�
�
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Figure 3. 1: An illustration of the order of processes involved in the examination of coastline changes by a comparison of historical aerial photographs and satellite images.
Phenomenal range of spatialchange
Identification of source data
Range of coastlinechangemethods
Error budget
Relative data referencing Geo-referencingaerialphotographs
Coastline Digitization
Online – GoogleEarth
Offline – GIS Desktop
Individualphotograph
Photographmosaic
Aerialphotographto satellite image
Satelliteimage to aerialphotographmosaic
48�
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3.2.1 Phenomenal Range of Spatial Change
�
The range of spatial change of a coastline is governed by details of the local
topography controlling incident wave forcing. Generally, each coastal zone has a
unique topographic setting and wave exposure making it necessary to assess long-
term changes on a case by case basis (Norcross et al., 2002). Conventionally,
coastline change analysis and prediction are performed by field and aerial surveys
(Maiti and Bhattacharya, 2008). The range of spatial change of coastlines can be as
little as 7-15cm/yr (with an error margin of 10cm) over a time interval of 41 years
(Moore and Griggs, 2002). A study by Fletcher et al. (2003) along the coast of Maui
and Kehei, in Hawaii, found the rate of beach loss to be 0.2 m/yr over a period of 30
years using the end point rate method (Section 2.2). Hence, it is essential to have
studies on spatial coastline changes, spread over a relatively large time interval.
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3.2.2 Identification of Source Data
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Since the range of spatial change of a coastline is only evident over a large time
interval, the earliest and the latest possible data were selected for the research. Three
sets of suitable data were identified for the Coral Coast area, along the south-west
coast of Viti Levu (Fig.3.2). These were aerial photographs and satellite images:
IKONOS and Google Earth images (Table 3.1).
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Table 3. 1:Details of available source data.
Data Type Year Spatial Resolution Aerial photographs 1967 1.1m
IKONOS images 2002 4.0m
Google Earth images 2006 0.5m
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3.2.3 Range of Coastline Change Methods
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Since there are only two sets of data available with significant time interval (earliest:
1967 aerial photographs and the recent satellite images), the end-point-rate method is
considered to be appropriate for this research. The end-point-rate method uses two
representative shorelines (often the earliest and the most recent) to calculate the rate
of shoreline change (Crowell, et al., 2005) (Section 2.2).
�
While doing quantitative analysis of changes, unless absolute geographic location is
required, the changes are usually relative. Absolute geo-referencing is usually
desired for mapping purposes, however, it is not required. For the purpose of this
research, relative referencing techniques are used to geo-reference the source data.
Two possible approaches to do relative geo-referencing of the source data are (a)
aerial photographs to satellite images and (b) satellite images to aerial photograph
mosaic. In order to geo-reference the aerial photographs two probable techniques can
be applied: (a) each aerial photograph can be mapped onto the satellite image
individually or (b) an aerial photograph mosaic can be created and then the mosaic
can be mapped onto the satellite image. The advantage of the aerial photograph
mosaic technique is that fewer ground control points (GCP) will need to be identified
in both the satellite image and the aerial photograph. On the other hand, employing
this technique would introduce errors, which would accumulate from the creation of
mosaics and geo-referencing.
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The final process, after geo-referencing the aerial photographs and the satellite
image, involves coastline digitization. Coastline digitization can be carried out online
via Google Earth or offline, by the means of Desktop Geographical Information
Systems (GIS) software. Using an online system such as Google Earth provides the
benefit of using high resolution geo-referenced satellite images. While, using an
offline system such as ArcGIS software, would make it possible to import the aerial
photographs into the GIS software rather than digitizing and importing only the
coastline, as in the online system.
50�
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3.2.4 The Error Budget and A-priori Accuracy Estimation
�
A number of possible errors need to be considered while undertaking this type of
research. These include phenomenal error, data error, image error, human error,
image distortion and image displacement. Phenomenal errors are the error margins
calculated by analysts involved in prior researches associated with the rate of
coastline change. Data error is initiated by spatial resolution of the image. Image
error is associated with identification of the location of a GCP in relation to pixel
distribution across the GCP. It is difficult for the human eye to identify the exact
pixel designated to a GCP; hence, usually an approximate instead of the precise
position of the GCP is selected. Human error is related to incorrect identification of
objects while locating GCPs and errors due to image distortion and displacement.
Image distortion is the physical distortion of an image due to atmospheric conditions,
for instance humidity. Finally, image displacement is the geometrical displacement
of objects in an image due to relief. In this case there is zero geometrical
displacement since this research concentrates on geo-referencing the coastal zone. At
sea level there is no relief; therefore the geometrical displacement is zero meters. The
estimated error budget (below) has been calculated taking into consideration the
above mentioned errors.
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Error Budget Estimation =
= =
[Phenomenal Error] + [Data Error] + [Image Error] + [Human Error] + [Image Distortion Error] + [Image Displacement Error] 0.1m + 4.0m + 1.1m + 0.5m + 1.0m 6.7m
51�
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3.3 THE STUDY SITE
�
The only readily available data to study coastline changes along the coast of Fiji
Islands was found to be for the southern part of the island of Viti Levu. Hence, the
Coral Coast area, was chosen as the study site (Fig. 3.2).
�
Located in the south-west coast of Viti Levu, Fiji Islands, the Coral Coast covers a
stretch of approximately 50-60 kilometres of coastline. It is situated within the
following coordinates 18o04’59.97” S, 177o 18’14.90” E and 18o17’28.81” S,
177o57’42.34” E and encompasses developed areas including villages, hotels and
resorts and a great many coastal ecosystems including reef ecosystems, mangrove
ecosystems and sea-grass beds.
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Impacts in the coastal zone becomes a hazard when it threatens the human well being
(Mimura, 1999). Apart from hosting numerous resorts and hotels, the Coral Coast is
also a home to many local village settlements. The Coral Coast is a low-lying area
exposed to the impacts of wave action generated by normal wind conditions as well
as those generated by storms and cyclones. Hence, the results of a research on the
patterns of coastline change for this area would be an essential tool for integrated
coastal zone management.
52�
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1 15’ 2 30’� 3 45’ 4 178o 5 15’ 6 30’
15’
H
30’
I
45’
J
18o
S
K
15’
10km
Figure 3. 2: The study area situated along the south-west coast of Viti Levu. Source: Pacific Maps Pty Limited, 2002.
Study Area
53�
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3.4 RESEARCH METHODOLOGY
�
This section discusses the techniques involved in the examination of the range of
coastline change along the Coral Coast (Fig. 3.2). The order of processes as
represented by the flow chart below (Fig. 3.3), are as follows: identification of source
data, determination of the range of coastline changes, coastline categorization and
finally, the quantification of coastline changes.
�
While determining the range of coastline change, the reference line used to digitize
the coastline was the vegetation line. It is difficult to differentiate the low and high
tide marks from aerial photographs and/or satellite images. However, the line of
vegetation along the coast can easily be differentiated from other features such as
sand and water. Furthermore, the vegetation line often reflects coastal erosion
processes, thus can be used as an indicator of coastal erosion. Therefore, in this study
the vegetation line is used as a reference line for the coastline.
Figure 3. 3: A representation of the processes involved in determining the range of coastline changes along the Coral Coast.
Source Data
Range of Coastline Changes (Figure 3.4)
Coastline Categorization
Quantification of Coastline Change
54�
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3.3.1 Source Data used for Geographical Information Systems Analysis
�
The two sets of raw data used for the Geographical Information Systems (GIS)
analysis in this study were historical aerial photographs (year: 1967), and satellite
images (2002, IKONOS and 2006, Google Earth) of the study area (Fig. 3.2). A set
of ten historical aerial photographs in paper form, for the year 1967 were originally
obtained from the Fiji Lands and Survey Department. The IKONOS satellite source
images for the year 2002 were acquired from Pacific Islands Applied Geoscience
Commission (SOPAC). The IKONOS satellite image had a spatial resolution of 4
meters for multi-spectral channels and the historical aerial photographs have a scale
of 1:24 000, scanned at 600dpi, producing 1.1m spatial resolution.
�
�
3.3.2 The Range of Coastline Changes
�
This section discusses three trial methods leading to the development of an
appropriate method to examine coastline changes along the Coral Coast.
�
The flow chart represents the order of the processes and techniques involved in each
trial method leading up to the final method. Each set of trial is associated with the
following processes: type of source data used, historical aerial photograph geo-
referencing techniques, coastline digitization techniques and the predicament
produced after each trial.
55�
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Figure 3. 4: A schematic representation of the method and technique development in the determination of the range of coastline changes; 3 trial methods leading to the ulimate method of range of coastline change analyses. (Note:AP – aerial photographs; GE – Google Earth images.)
Method Development
Trial 1 Trial 2 Trial 3 Final Method
1967 A.P. to 2006 G.E. image
Multiple GCPs from G.E. plotted onto A.P. using
ArcGIS
1967 coastline digitized in
ArcGIS; viewed/ compared online
(G.E)
Improper coastline
alignment, limited GCPs
1967 A.P. mosaic to 2006
G.E. image
Multiple GCPs from G.E. plotted onto A.P. mosaic using ArcGIS
1967 coastline digitized using ArcGIS;
viewed/compared online (G.E)
Improper coastline alignment; image error in mosaic formation,
limited GCPs
1967 A.P. mosaic to 2002 IKONOS image
Image to image warping using multiple GCPs in
HyperCube
1967/ 2002 coastlines digitized in ArcGIS; viewed/ compared
online (G.E)
Improper coastline aligment; uneven
distribution of GCPs
1967 A.P. mosaic to 2002 IKONOS image
Image to image warping using two/ multiple GCPs in HyperCube
1967/ 2002 coastlines digitized/ viewed offline in ArcGIS
Reasonably proper coastline alignment achieved using two
GCPSs
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Trial Method 1
�
The source data used for the first trial method were the 1967 aerial photographs and
Google Earth images; the software used were ArcGIS and Google Earth.
�
Geo-referencing – The 1967 aerial photographs were geo-referenced with respect to
Google Earth. A total of three and/ or more ground control points (GCPs) similar to
both Google Earth and the 1967 aerial photographs were identified. The coordinates
for the GCPs were identified using Google Earth. These coordinates were then
plotted onto the 1967 aerial photographs using the Geo-referencing tools in ArcGIS.
�
Coastline Digitization – Once the 1967 aerial photographs were geo-referenced, the
coastline was digitized using ArcGIS tools. The shapefile containing information on
the digitized coastline was converted to a kml file and imported in Google Earth. The
changes in the coastlines between the years 2006 (Google Earth image) and 1967
(geo-referenced aerial photograph) was compared by overlying the 1967 coastline
onto the Google Earth image.
�
Predicament – The 1967 digitized coastline overlaid on the 2006 Google Earth
image indicated improper alignment of the coastlines. There was lack of similar
ground control points (GCPs) on the 1967 aerial photographs and the Google Earth
image. Insufficient similar ground control points (GCPs) lead to uneven distribution
of the GCPs. Hence, there was greater Root Mean Square Error (RMSE), indicating
inappropriate geo-referencing and finally, misalignment of the two images.
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Trial Method 2
�
The source data used for the second trial method were the 1967 aerial photographs
and Google Earth images. The software used to process the raw data were
HyperCube, ArcGIS and Google Earth.
57�
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To overcome the problem of evenly distributed ground control points (GCPs) in Trial
Method 1, photo mosaics of the 1967 aerial images were created using the
HyperCube software. To generate photo mosaics, ground control points (GCPs) on
overlapping portions of the images were matched. Three aerial photographs were
used to create photo mosaics in order to get greater number of evenly distributed
similar ground control points (GCPs).
�
Geo-referencing – The generated 1967 aerial photo mosaic was geo-referenced with
respect to the Google Earth image. The method of geo-referencing was the same as
that used in Trial Method 1.
�
Coastline Digitization – The coastline for the geo-referenced 1967 aerial photo
mosaic was digitized using ArcGIS tools; as in Trial Method 1. The shapefile for the
digitized coastline was converted to a kml file and imported in Google Earth. The
1967 and 2006 coastlines were compared by overlying the 1967 coastline shapefile
over the 2006 Google Earth image.
�
Predicament – As in Trial Method 1, when the 1967 kml coastline shapefile was
overlaid on the 2006 Google Earth image, the coastlines were not properly aligned.
The 1967 aerial photographs for the Coral Coast region was not a complete set. A
number of aerial photographs were missing from the 1967 the Coral Coast aerial
photograph sequence. Adjacent aerial photographs only had 10–20% overlapping
areas, instead of the 60–80% common in historical aerial photographs. Therefore,
there were insufficient overlapping areas to form the mosaic. Furthermore, the
mosaic itself had the same problem as in Trial Method 1; uneven distribution of
ground control points (GCPs) on the far right and left of the mosaic image.
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Trial Method 3
�
The source data used for this method were the 1967 aerial photographs, the 2002
IKONOS image and Google Earth image. The software used for analyzing these
images were HyperCube and Google Earth.
To avoid the problems encountered in Trial Methods 1 and 2, a different method of
geo-referencing was used in this trial. The HyperCube software was used to geo-
reference the 1967 aerial photographs.
�
Geo-referencing – By the means of the HyperCube software, the image to image
warping technique was used to geo-reference the 1967 aerial photographs. The 1967
aerial photographs were geo-referenced with respect to the 2002 IKONOS images by
warping multiple similar ground control points (GCPs) identified on the 1967 aerial
photographs and the 2002 IKONOS image.
�
Coastline Digitization – Once the 1967 aerial photographs were geo-referenced, the
coastline for both the 1967 and 2002 images were digitized in ArcGIS using the
same method as in Trial Methods 1 and 2. The 1967 and 2002 coastline shapefiles
were overlaid onto the IKONOS image in order to gauge the accuracy of coastline
digitization and geo-referencing. After this the coastline shapefiles for both years
were converted to kml files and imported in Google Earth.
�
Predicament – When overlaid on the Google Earth image, the digitized coastlines for
1967 and 2002 showed a shift of approximately 15 meters to the east. The
coordinates of the IKONOS image were changed such that the whole image shifted
15m to the west. After this, a second set of coastline was digitized using the
IKONOS image with the changed coordinates. The coastline shapefile was converted
to a kml file and imported in Google Earth which indicated a relatively better
alignment of the coastlines then the previous methods. However, this coastal
alignment was not adequate for the purpose of studying coastline changes. The
difference in the alignment was due to uneven distribution of the ground control
59�
�
points (GCPs) with respect to the whole image, since, the GCPs were concentrated
along the coastline.
�
�
Final Method
�
Ultimately, the following method was decided to be the most appropriate for this
study. The raw data used in this analysis were the 1967 aerial photographs and the
2002 IKONOS image.
�
Geo-referencing – The 1967 aerial photographs were geo-referenced with respect to
the 2002 IKONOS image by image to image warping using the HyperCube software.
Similar ground control points (GCPs) were identified in both corresponding set of
images. Then the images were warped using the orthogonal transformation at a
warping scale of 4; to preserve spatial resolution of the aerial photograph. The
images were warped using two methods: using multiple ground control points
(GCPs) along the coastline and using two ground control points (GCPs) along the
coastline. Although the multiple GCPs method produced low Root Mean Square
Error (RMSE) values, the coastline alignment for the two images (1967 and 2002)
using the two GCPs method was more precise than the coastline alignment generated
using the multiple GCPs method (Table 3.1). With only 2 GCPs, the image is geo-
referenced with respect to the linear line created by the two points. More than 2
GCPs introduce greater differences among a greater number of points which
accumulates to a larger RMSE value. While, the two ground control points method
uses only 2 GCPs for geo-referencing, thus the RMSE value is 0. Hence, the two
GCPs method was favoured over the multiple GCPs method.
60�
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Table 3. 2: A comparison of the multiple and the two ground control point methods. �
Image�#� Multiple�GCPs�Method� Two�GCPs�Method�A� B� C� D� E� F� G�
�RMSE�Value�
Coastline�Difference�(meters)�
Number�of�GCPs�
RMSE�Value�
Coastline�Difference�(meters)�
Number�of�GCPs�
1� 2.91� 25�26� 6� 0.00� 10� 2�
2� 3.06� 10�15� 6� 0.00� 2.6�9� 2�
3� 3.03� 48�50� 7� 0.00� 4�10� 2�
4� 3.85� 8�10� 9� 0.00� 5�10� 2�
5� 3.02� 25�60� 7� 0.00� 2.6�20� 2�
6� 2.23� 33�35� 11� 0.00� 0� 2�
7A� 2.73� 28�30� 9� 0.00� 1.2�2� 2�
7B� 2.77� 15�30� 8� 0.00� 2�15� 2�
8� 3.91� 150� 7� 0.00� 2.7�20� 2�
9� 3.67� 35�50� 5� 0.00� 4�15� 2�
10� 3.08� 5�10� 8� 0.00� 2.6�3� 2��
�
The coastline difference in meters (columns C and F) have been determined by the
average minimum and maximum distance separating the 1967 and 2002 digitized
coastlines for each image.
�
The multiple GCPs method (column B) have root mean square error (RMSE) values
between 2 to 4 while, the two GCPs method has an RMSE value of 0, since only 2
points were used as GCPs. The total error budget of 3m (Section 3.3.4) calculated
from the digitized coastline was lower than the estimated error budget of 6.7m
(Section 3.2.4).
61�
�
Coastline Digitization – After geo-referencing the 1967 aerial photographs using the
two ground control points (GCPs) method the coastline for the 1967 and 2002
images were digitized by the means of ArcGIS tools. For the 2002 IKONOS images;
infrared images were generated using the infrared, blue and red channels. This
separated the vegetation from all other features by giving it a distinct red colour,
hence, making it easier to digitize the vegetation line as the coastline.
�
�
3.3.3 Coastline Categorization
�
Once the 1967 and 2002 coastlines were digitized they were overlaid on the 2002
IKONOS image. The coastline was categorized under the following categories after a
comparison between the 1967 and 2002 coastlines. Erosion hotspots (Sea Grant,
1997), erosion watch spots (Sea Grant, 1997), resilient coastlines and prograding
coastlines. The prograding coastline were categorized further into artificial
progradation and natural progradation after a ground truthing exercise (Section 1.3).
For the classification of erosion hotspots and erosion watch spots; the eroding areas
covering an area greater than 20 m2 were categorized as erosion hotspots and those
covering an area less than 20 m2 were categorized as erosion watch spots.
�
�
3.3.4 Quantification of Coastline Changes
�
Using the Hawths Tools in ArcGIS the level of the erosion and progradation were
calculated in meters (maximum distance of erosion and progradation) and meters
squared (m2). An average per year of erosion and progradation was worked out for
each segment of the coastline which was eroding and/or prograding. The total error
budget of 3m (and/or 6m2) for the eroding and prograding areas, was calculated by
averaging the areas of the polygons introduced while digitizing the coastline along
resilient coastlines. The coastline categorization and the eroding and the prograding
areas were finally presented using maps (Figs. 4.2 – 4.11 and Appendix 1 and 2).
62�
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3.5 FIELD INVESTIGATION
�
After the desktop (GIS) analyses phase of the research a ground truthing exercise
was carried out along the Coral Coast. Selected study areas based on the output of
the first (GIS) phase, were visited over a two-week period. Combinations of all
coastline categories were included in the field study; eroding, prograding and
resilient. Out of the thirteen areas chosen for the ground truthing exercise there were
seven resort and six village settlement areas (Fig. 3.5).
�
The general coastal geomorphologies at each study site were observed. Particular
emphasis was placed on the sediment characteristics (beach sediment material
composition and origin, grain size), natural beach features (coastline type, beach
width, beach steepness, channels, freshwater systems), coastal structures, the state of
erosion and wind and current directions. The prograding coastlines were further
categorized as shoreline that is advancing towards the sea due to sediment accretion
(naturally prograding) and engineered structures such as seawall (artificially
prograding).
�
After the ground truthing exercise the documented results were recorded, tabulated
and finally presented as maps indicating coastal erosion, beach erosion, progradation
due to sediment accretion and engineering structures and resilient coastlines. The
general geomorphology along the Coral Coast was documented.
63�
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Figure 3. 5: The study areas for the ground truthing exercise along the Coral Coast.
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64�
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3.6 SUMMARY
�
An a-priori analysis of errors was conducted to estimate the accuracy of an
appropriate method to examine coastline changes using historical aerial photographs
and satellite images. The details considered during this hypothetical analysis were
the phenomenal range of spatial change, identification of source data, the techniques
involved in analyzing the range of coastline change and the estimated error budget.
�
Since, the only readily available data for studying coastline change in Fiji Islands,
was found to be for the southern coast of Viti Levu, the Coral Coast (Fig. 3.2) was
chosen as the study site.
�
An appropriate methodology to examine temporal coastline changes was developed
based on historical aerial photographs, high resolution proprietary satellite images
and online geo-spatial resources. Since the root mean square error (RMSE) values
obtained from geo-referencing the 1967 aerial photographs and the 2002 satellite
images were lower than the estimated error budget of 6.7m, this technique of geo-
referencing was concluded to be appropriate for this research.
�
Subsequently, the Coral Coast coastline was categorized as eroding, prograding and
resilient coastlines. The eroding and prograding areas were quantified by the means
of the Hawths Tools in ArcGIS. The error margin for the amount of erosion and
progradation was calculated by averaging the areas of the polygons created from
resilient coastlines.
�
The second part of the research involved a ground truthing exercise along the Coral
Coast over a period of two weeks. The study sites for the ground truthing work were
selected based on the output of the coastal change identificaiton analyses in the first
part. At each study site the general coastal geomorphologies were observed. The sites
categorized as prograding coastlines were further categorized as progradation due to
(a) sediment accretion and (b) engineered structures such as seawall. Finally, the
65�
�
analyzed data were presented in the form of maps, illustrating categorized coastlines
and eroding and prograding areas and the observed geomorphology of the Coral
Coast were documented.
66�
�
CHAPTER 4 – RESULTS
�
�
4.1 INTRODUCTION
�
The techniques and processes described in methodology (Chapter 3) have been
utilized to produce maps indicating the range of coastline changes along the Coral
Coast. Temporal coastline changes, (areas of coastal retreat, progradation and
stability) have been illustrated as maps over an interval of 35 years (1967 to 2002) by
the comparison of historical aerial photographs and satellite images. In addition, the
maps also illustrate beach erosion and the subdivision of prograding coastlines into
natural and artificial progradation, after a ground truthing exercise. The outcomes of
these analyses have been presented in the form of maps under the following
headings: coastline categorization – illustrates observed beach erosion, the various
coastline categories and presents a general description of the observed coastal
geomorphology at selected study sites along the Coral Coast; quantification of
eroding and prograding costlines – indicates the rate of change of the coastlines as
the maximum distance and total area of coastline change.
�
�
4.2 COASTLINE CATEGORIZATION
�
An overview of the Coral Coast area indicates the areas presented by the figures,
following Figure 4.1 (Fig. 4.1). Each map figure illustrates the various categories of
coastlines in specific areas along the Coral Coast. The coastline areas have been
ranked as (1) erosion hotspots (coastal erosion has threatened shoreline development
and infrastructure), (2) erosion watch spots (coastal settlements will soon be
threatened if shoreline erosion trends continue), (3) resilient shorelines (no coastal
retreat or advancement observed in the time frame of the image overlays) and (4)
prograding shorelines (shoreline is advancing sea-ward); with areas investigated in
67�
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the groung truthing exercise subdivided into (a) natural progradation (due to
sediment accretion) and (b) artificial progradation (due to engineered structures).
The subdivision of prograding coastlines into artificial and natural progradation has
been determined by a ground truthing exercise of selected areas along the Coral
Coast. Areas with a combination of coastline segments (resilient, eroding and
prograding) were selected from the desktop study for the ground truthing exercise
(Fig. 4.1). The structures responsible for artificial progradation along the Coral Coast
were identified to be seawalls and groynes. The areas influenced by resilient
coastlines also have seawalls and/ or groynes along its shoreline. Most areas
investigated during the field study had exposed tree roots, beach scarps, narrow
beaches and exposed beach rocks, indicating beach erosion.
�
The general coastal geomorphology of the various study sites along the Coral Coast,
(Fig. 3.5) have been described under each area heading. The different study areas for
the ground truthing exercise are being referred to as per their location on the
respective area maps. The various features described under each area heading include
the sediment characteristics, natural beach features, coastal structures, and the state
of erosion. The Coral Coast area is fringed by a narrow band of fringing reef system
which is interrupted by numerous passages. These passages give entrance to waves
induced by the dominant southeast trade winds which are a prominent factor in
determining the coastal geomorphology of the Coral Coast. The current direction of
the coastal waters along the Coral Coast is in accordance to the southeast trade
winds; towards the northwest direction.
�
68�
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Figure 4. 1: The Coral Coast; the area in each black frame corresponds to the labelled figure which gives details of the coastline categorization of the respective area. Map background: 1967 aerial photograph mosaic.
Fig 4.2
Fig 4.3
Fig 4.5
Fig 4.4
Fig 4.11
Fig 4.6
Fig 4.7
Fig 4.8
Fig 4.9Fig 4.10
69�
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4.2.1 Fijian Resort, Naevuevu Village and Yadua Village �
�
Figure 4. 2: Coastline categorization along the Fijian Resort, Naevuevu Village and Yadua Village area.
�
The map indicates that most of the coastline along the Fijian Resort to Yadua Village
is resilient. Some portions of the western side of the area show prograding coastlines
and there is an erosion watch spot on the west of Yadua Village. The western side of
the area is fringed by a relatively wider reef flat when compared with the eastern side
of the area. A river system and a narrow reef passage are prominent on the east of
Naevuevu Village.
70�
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4.2.2 Yadua Village and Sigatoka Sand Dunes �
�
Figure 4. 3: Coastline categorization along the Yadua Village and Sigatoka Sand Dunes.
�
Except for the stretch of coastline on the east of the Yadua Village indicating an
erosion hotspot and beach erosion, this area shows the dominance of a resilient
coastline system. A wide fringing reef flat is evident in front of Yadua Village, in
contrast with no reef systems in front of the Sigatoka Sand Dunes. There are two
prominent reef passages in the area, one opposite the small creek mouth on the east
of Yadua Village and the other on the western edge of the Sigatoka Sand Dunes. A
tributary of the bigger Sigatoka River system is evident on the far eastern side.
71�
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Observed Coastal Geomorphology in the Yadua Village Area
�
Two sections of Yadua area were examined; the Yadua Village front and the
Sigatoka Sand Dunes front, located on the east of Yadua Village. The study areas are
separated by a narrow creek which merges into the shallow fringing reef lagoon. The
fringing reef system is interrupted by two channels. Gently sloping and narrow
beaches are characteristics of both Yadua Village and Sigatoka Sand Dune fronts.
Sections of a damaged seawall (Fig. 4.23) and exposed tree roots are evident in front
of Yadua Village. Majority of the beach material on Yadua Village front are coarse
sized rocks and rubble. The beach sediments comprise of sea wall debris and oceanic
sediments. Majority of the beach material on Sigatoka Sand Dune front are fine
grained terrestrial sediments.
�
Figure 4. 4: Damaged seawall and debris along the Yadua Village beach front.�
72�
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4.2.3 Sigatoka Sand Dunes and Korotongo Village �
�
Figure 4. 5: Coastline categorization from the eastern edge of Sigatoka Sand Dunes to Korotongo Village.
�
The western side of the Sigatoka River system illustrates the dominance of a resilient
coastline in front of the Sigatoka Sand Dunes. The eastern edge of the Sigatoka Sand
Dunes indicates a prograding coastline. A spit is evident on the east of Sigatoka
River; the western side of the spit is prograding while the eastern side is eroding. The
stretch of coastline from the spit to Korotongo Village indicates a combination of
erosion hotspot, erosion watch spots, artificially prograding coastlines and resilient
coastlines. A wide fringing reef flat interrupted by a narrow channel is evident in
front of the Korotongo area in contrast with no reef systems in front of the Sigatoka
Sand Dunes.
�
73�
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4.2.4 Outrigger and Tabakula Resorts �
Figure 4. 6: Coastline categorization along the Outrigger Resort and Tabakula Resort area.
�
This area indicates a combination of erosion watch spots, artificially prograding
coastlines and resilient coastlines with two erosion hotspots on the east of Tabakula
Resort. The coastline along the Outrigger Resort and Tabakula Resort is mostly
resilient except for a segment of artificially prograding coastline in front of Outrigger
Resort. Beach erosion is prominent in front of Outrigger and Tabakula resorts. The
eastern side of the map illustrates alternating segments of erosion watch spots,
resilient coastlines and artificially prograding coastlines. The area is fringed by a
narrow reef flat interrupted by a wide passage situated opposite the river system on
the east of the resorts.
74�
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Observed Coastal Geomorphology of the Outrigger and Tabakula Resort Areas
�
The beach fronts of Outrigger and Tabakula Resorts were observed. The continuous
stretch of beach in front of Outrigger and Tabakula Resorts is interrupted by a small
creek flowing through the buildings of Outrigger Resort. Both resorts have gently
sloping and relatively narrow beaches; however, the stretch of beach in front of
Outrigger Resort is narrower than that in front of Tabakula Resort. The fringing reef
system enclosing a shallow lagoon is interrupted by two channels, one opposite each
resort. Outrigger Resort is separated from the beach area by a stretch of 2 – 5 meters
high seawall (Fig. 4.24). Exposed tree roots and rocky outcrops are prominent along
the Outrigger Resort beach front. Majority of the beach sediments in Outrigger
Resort area is composed of rubble, pebbles and coarse sand while the Tabakula
Resort front mainly comprises of coarse sand. The origins of beach materials in both
areas are oceanic and terrestrial.
�
Figure 4. 7: Seawall separating the Outrigger Resort from the beach front.
�
75�
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4.2.5 Sovi Bay, Vatukarasa Village and Namada Village �
Figure 4. 8: Coastline categorization along Sovi Bay, Vatukarasa Village and Namada Village area.�
�
Naturally prograding coastlines are dominant in the Sovi Bay area, with alternating
segments of resilient coastlines and an erosion watch spot on the west of the Bay and
erosion watch spot segments on the east of the Bay. Vatukarasa Village has a
segment of resilient coastline on the village front with a short segment of naturally
prograding coastline on the eastern end of the village with relatively long segments
of erosion hotspots on either side of this prograding spot. Beach erosion is prominent
in the Vatukarasa Village area. An artificially prograding coastline segment is
prominent on the far east of Vatukarasa Village whereas, the west of the village
indicates alternating segments of erosion hotspot, erosion watch spots and resilient
coastline. Namada Village front shows a prominent stretch of erosion watch spot
76�
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with segments of resilient coastlines on either side of this erosion watch spot. The
area is fringed by narrow reef flats interrupted by two wide channels opposite river
systems at the head of Sovi Bay and on the eastern side of Vatukarasa Village.
�
�
Observed Coastal Geomorphology of Vatukarasa Area
�
Three sections of Vatukarasa area were examined; Sovi Bay, Vatukarasa Village
front and the east of Vatukarasa Village. A large river system is evident at the head
of Sovi Bay, while Vatukarasa Village and the eastern section of Vatukarasa Village
are separated by a smaller river system. The fringing reef system in front of the study
sites are interrupted by two wide channels, one of which forms the Sovi Bay, while
the other forms the Vatukarasa Bay. The stretch of beach from Sovi Bay to the east
of Vatukarasa Village is relatively wide (Fig. 4.25 and 4.26). Vatukarasa Village has
a steep beach with massive beachrocks dipping seaward at the water line (Fig. 4.27)
while, the Sovi Bay area and the east of Vatukarasa Village have gently sloping
beaches. Gabion Baskets have been fixed on the east of Vatukarasa Village adjacent
to Queens Road (the major highway of Viti Levu) (Fig. 4.28). Exposed tree roots and
rocky outcrops (Fig. 4.27) are prominent on all beaches of the study areas except for
the head of Sovi Bay. The major beach materials of Vatukarasa Village front and the
eastern end of the village are coarse sand, pebbles and rubbles with both terrestrial
and oceanic origins. On the other hand, the major beach material of the Sovi Bay
area is fine sand of a terrestrial origin.
77�
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Figure 4. 9: Wide beach at the head of Sovi Bay.�
�
�
Figure 4. 10: Low tide on a steep beach face in front of Vatukarasa Village.
78�
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Figure 4. 11: Exposed tree roots (left) and beach rocks (right) in front of Vatukarasa Village.
�
�
Figure 4. 12: Gabion Baskets; east of Vatukarasa Village; adjacent to the Queens Road.
79�
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4.2.6 Tambua Sands Resort, Hideaway Resort and Tagaqe Village �
�
Figure 4. 13: Coastline categorization of the Tambua Sands Resorts, Hideaway Resort and the Tagaqe Village area.
�
The area indicates alternating segments of erosion watch spots, erosion hotspots,
resilient coastlines and artificially prograding coastlines. The Tambua Sands Resort
front is mostly resilient except for alternating short segments of artificially
prograding coastlines. Adjacent to Tambua Sands Resort (eastern end) is an erosion
hotspot with alternating short segments of resilient coastline and erosion watch spot.
The stretch between Tambua Sands Resort and Hideaway Resort comprises a
segment of artificially prograding coastline, an erosion hotspot and a segment of
resilient coastline, from west to east, respectively. The Hideaway Resort front is
mostly resilient, except for a short segment of artificially prograding coastline on the
western end of the resort and two erosion watch spots on both the western and
eastern ends of the resort. Tagaqe Village front has an erosion hotspot with a stretch
of artificially prograding coastline on the eastern side of the village. Beach erosion is
80�
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prominent along the entire coastline irrespective of the coastline type. The area is
fringed by a narrow reef flat interrupted by 3 channels. The two wider channels are
situated in between the Tambua Sands and Hideaway stretch and opposite the river
on the eastern end of Tagaqe Village, and a much smaller channel is situated
opposite the Hideaway Resort.
�
�
Observed Coastal Geomorphology in the Tambua Sands Resort, Hideaway Resort
and Tagaqe Village Areas
�
All three study areas, Tambua Sands, Hideaway Resort and Tagaqe Village have
small creek mouths merging into the shallow lagoon formed by a fringing reef
system. The fringing reef system in front of Tambua Sands and Taqage Village are
interrupted by channels; one in front of each site. The channel in front of Tagage
Village is relatively wide, while Tambua Sands has a narrow channel. There is a
continuous fringing reef system with a narrow reef flat in front of Hideaway Resort.
In addition, all three study areas have gently sloping and narrow beaches. Hideaway
Resort is separated from its beach front by a seawall (Fig. 4.29). Exposed tree roots
and beachrocks are prominent along the beaches of Tambua Sands and Tagaqe
Villge (Fig. 4.30). Majority of the beach materials in front of Tambua Sands and
Tagaqe Village are coarse sand, rubbles and pebbles with an oceanic origin.
Hideaway Resort front mostly has fine to coarse sand, with few rubble pieces; the
sediments are of oceanic origin.
81�
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Figure 4. 14: Seawall along the east of Hideaway Resort.
�
Figure 4. 15: Exposed tree roots and beach rock in front of Tagaqe Village.
82�
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4.2.7 Nagasau Village, Votualailai Village and Naviti Resort
Figure 4. 16: Coastline categorization from Nagasau Village, to Votualailai Village to Naviti Resort.
�
The stretch of coastline between Nagasau Village and Votualailai Village is mostly
resilient. The Votualailai Village and Naviti Resort fronts illustrate artificially
prograding coastlines. Despite the prograding coastlines, the Votualailai Village and
Naviti Resort area are subjected to beach erosion. This area is fringed by a narrow
reef flat which is interrupted by 3 channels. The two wider channels are located
opposite river systems on the eastern ends of Tagaqe Village and Naviti Resort,
while the narrower channel is located on the eastern side of Nagasau Village.
�
�
83�
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Observed Coastal Geomorphology of Naviti Resort Area
�
Naviti Resort has a gently sloping and narrow beach (Fig. 4.31), with a narrow creek
mouth on the eastern end of the resort. The fringing reef system in front of the resort
is interrupted by a wide channel. This channel opens in front of an artificial island in
front of the resort. The artificial island is surrounded by a circular stretch of seawall
and connected to the mainland by a causeway (Fig. 4.32). Another artificial island is
under construction. The resort is separated from the beach front by a continuous
stretch of sea wall. Majority of the beach material is composed of fine to coarse
sands of oceanic origin.
84�
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Figure 4. 17: Narrow stretch of beach in front of the Naviti Resort.
�Figure 4. 18: Artificial Island in front of Naviti Resort; surrounded by a seawall and connected to the mainland by a causeway.
85�
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4.2.8 Votua Village, Korolevu Settlement and Warwick Resort �
Figure 4. 19: Coastline categorization along Vouta Village, Korolevu Settlement and Warwick Resort area.
�
The stretch of coastline in the Votua Village area is artificially prograding. The
eastern side of Votua Village indicates alternating stretches of artificially prograding
and resilient coastlines. The coastline in front of Korolevu Settlement indicates a
long stretch of artificially prograding coastline with shorter stretches of erosion
hotspot and resilient coastline. The western side of Warwick Resort illustrates an
erosion hotspot and an erosion watch spot. The Warwick Resort front is mostly
resilient; however, there are 3 small land masses in front of the resort indicating an
artificially prograding coastline. The eastern end of Warwick shows resilient
coastlines. All areas indicate beach erosion irrespective of the coastline processes.
The area is fringed by a narrow reef flat interrupted by 3 channels; two wider ones
located opposite Naviti Resort and opposite the river system on the eastern side of
86�
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Korolevu Settlement, and a narrower one located on the eastern side of Votua
Village. Another river system is situated on the western end of Votua Village.
�
�
Observed Coastal Geomorphology of the Korolevu Settlement and Warwick Resort
Areas
�
Both the Korolevu Settlement and Warwick Resort area have gently sloping and
narrow beaches with two small creeks. The fringing reef system in front of Korolevu
Settlement is interrupted by a wide channel. A sloping embankment (sea wall)
separates the western side of Korolevu Village from the beach front (Fig. 4.33).
Similarly, a continuous stretch of seawall separates the Warwick Resort from the
beach front. There are three artificial islands constructed in the shallow lagoon
formed by the fringing reef in front of Warwick Resort. These islands are connected
to the mainland by three causeways. Exposed tree roots and beach rocks are
prominent on the eastern side of Korolevu and the western side of Warwick Resort.
Majority of the beach materials in both study areas are coarse sand, pebbles and
rubbles from both terrestrial and oceanic origin.
87�
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Figure 4. 20: Sloping embankment separating the western side of Korolevu from the beach front.
88�
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4.2.9 Komave Village, Navola Village and Beach House Resort �
Figure 4. 21: Coastline categorization along Komave Village, Navola Village and the Beach House area.
�
The western side of Komave Village is mostly resilient except for two erosion
hotspots located close to the village. The Komave Village area is dominated by
artificially prograding coastlines, with the eastern side of the village indicating a
resilient coastline. Navola Village front illustrates artificially prograding coastlines.
The Beach House front shows a long stretch of erosion hotspot, with stretches of
resilient coastlines on either side of the eroding spot. Beach erosion is prominent in
the Beach House area. The area is fringed by a narrow reef flat interrupted by 3
narrow channels; located on the west and front of Komave Village and in front of
Navola Village. The map indicates 3 river systems situated on the eastern ends of
Korolevu Settlement and Navola Village, and on the western end of Komave Village.
89�
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Observed Coastal Geomorphology of the Beach House Resort Area
�
The Beach House area has a gently sloping and narrow beach with one creek mouth
emptying into the shallow lagoon created by the fringing reef system, at the eastern
end of the resort. The fringing reef system is interrupted by two channels, located on
either ends of the resort. A small jetty (constructed in the year 2006) sits on the
western end of Beach House. Exposed tree roots and scarps along the vegetation line
are prominent in front of the resort (Fig. 4.34).
�
�
Figure 4. 22: Exposed tree roots (left) and beach scarp (right) in front of the Beach House
90�
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4.2.10 Namatakula Village, Navutulevu Village and Naboutini Villagea �
Figure 4. 23: Coastline categorization along Namatakula Village, Navutulevu Village and Naboutini Village. (Note: the eastern end of Namatakula and Naboutini Villages could not be classified since these sections of historical aerial photographs were missing.) �
The Namatakula Village area indicates naturally prograding coastlines and beach
erosion. The Navutulevu Village shows artificially prograding coastlines with two
short stretches of resilient coastlines between Namatakula and Navutulevu villages.
Prograding coastline is prominent on the western side of Naboutini Village. This area
is fringed by a narrow reef flat interrupted by two channels located opposite river
systems situated on the eastern end of Namatakula Village and on the western end of
Navutulevu Village.
91�
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Observed Coastal Geomorphology of the Namatakula and Navutulevu Village Areas
�
Namatakula Village has a gently sloping and wide beach with a river on the eastern
end of the village. The river meanders through the beach area, flowing towards the
western end of the village, dividing the beach into two sections, before finally
merging into the sea (Fig. 4.35). Exposed tree roots are evident along the river
meandering through the beach. The fringing reef system in front of the village is
interrupted by a channel situated on the eastern side of the village. Majority of the
beach sediments in both study areas are from fine sand to coarse sand and pebbles
with both terrestrial and oceanic origins.
�Figure 4. 24: River meandering through the beach front of Namatakula Village.
�
Navutulevu Village has a gently sloping and wide beach with a river system on its
western end. The fringing reef system along the front of the village is interrupted by
a wide channel. Navutulevu Village is separated from the beach front by a stretch of
seawall. Majority of the sediments is from fine to coarse sand with both terrestrial
and oceanic origins.
92�
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4.3 QUANTIFICATION OF ERODING AND PROGRADING COASTLINES
�
Two approaches have been used to quantify the eroding hotspots and prograding
coastlines along the Coral Coast. Firstly, the eroding and prograding coastlines have
been compared in terms of maximum distance (in meters) eroded and prograded over
the years 1967 to 2002. The rate of erosion and progradation (in meters per year) at
each site has been illustrated by means of line graphs. Secondly, the total areas (in
square meters) eroded and prograded at each site over the years 1967 to 2002 have
been presented using line graphs. The rate of erosion and progradation (in square
meters per year) at each site has been illustrated by means of line graphs.
�
�
A Comparison of Eroding and Prograding Coastlines
�
The maximum distances (in meters) eroded and prograded along the Coral Coast,
have been illustrated by the means of vertical colour coded lines (Fig. 4.25). For the
purpose of a visible comparison of the eroding and prograding distances, each line
has been magnified one hundred times.
�
Figure 4.25 shows the distribution of eroding and naturally and artificially
prograding coastlines along the Coral Coast. Overall, when compared to eroding
spots, the prograding spots are greater in number and distance (in meters) of change.
Out of the 21 prograding spots, only 2 spots are naturally prograding; at the head of
Sovi Bay (130 ± 3 m) and at Namatakula Village front (30 ± 3 m). The range of
artificially prograding coastlines is from 13 ± 3 m (round about on the east of
Korotongo Village) to 100 ± 3 m (Naviti Resort area) and that of eroding coastlines
is from 14 ± 3 m (west of Vatukarasa River) to 40 ± 3 m (west of Hideaway Resort).
93�
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Figure 4. 25: A comparison of the eroding and prograding coastlines in terms of the maximum distances of change in coastlines over the years 1967 to 2002 for the Coral Coast area.
20�meters
Fig.
4.2
Fig.
4.3
Fig.
4.2
Fig.
4.5
Fig.
4.5
Fig.
4.5
Fig.
4.5
Fig.
4.6
Fig.
4.6
Fig.
4.6
Fig.
4.6
Fig.
4.6
Fig.
4.8
Fig.
4.8
Fig.
4.8
Fig.
4.8
Fig.
4.8
Fig.
4.13
Fi
g. 4.
13
Fig.
4.13
Fig.
4.13
Fi
g. 4.
13
Fig.
4.13
Fig.
4.16
Fig.
4.19
Fig.
4.19
Fi
g. 4.
19
Fig.
4.19
Fig.
4.21
Fi
g. 4.
21
Fig.
4.21
Fi
g. 4.
23
Fig.
4.23
94�
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�Figure 4. 26: Maximum distance of landward movement (m) at each erosion hotspot along the Coral Coast. The erosion spots correspond to the red lines in Fig. 4.25, from west to east.
�Figure 4. 27: Average rate of erosion at each erosion hotspot along the Coral Coast. The erosion spots correspond to the red lines in Fig. 4.25, from west to east.
95�
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�Figure 4. 28: Maximum distance of seaward movement (m) at each progradation spot along the Coral Coast. The progradation spots correspond to the green and blue lines in Fig. 4.25, from west to east.
�Figure 4. 29: Average rate of progradation at each prograding spot along the Coral Coast. The prograding spots correspond to the green and blue lines in Fig. 4.25, from west to east.
96�
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Overall the Coral Coast graphs show more progradation than erosion (Fig. 4.25). For
prograding coastline segments the maximum distance of progradation ranged from
13 ± 3 m (0.37m/yr) to 400 ± 3 m (11.43 m/yr) (Figs. 4.28 and 4.29), while for
eroding coastline segments the maximum distance of erosion ranged from 14 ± 3 m
(0.40 m/yr) to 40 ± 3m (1.14 m/yr) (Fig. 4.26 and 4.27). The upper limits of the
range of coastline change for maximum distance for progradation and erosion, 400 ±
3 m and 40 ± 3m, respectively, indicate that the Coral Coast area is influenced ten
times more by progradation than by erosion.�
�
�
Eroding Coastlines
�
A total of 13 spots along the Coral Coast were identified to be erosion hotspots.
Figure 4.30 illustrates the location of these spots along the Coral Coast. Line graphs
have been used to indicate the rate of erosion over the 35 year period, from 1967 to
2002, at each site. The graphs show the total area eroded in square meters, (Fig. 4.31)
and the rate of erosion in square meters eroded per year, (Fig. 4.32), at each of the 13
sites. The eroding hotspots (13 spots; from west to east) indicated in Figure 4.30
correspond to the erosion quantities in Figures 4.31 and 4.32. For detailed maps of
erosion hotspots at each site refer to Appendix 1.
97�
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Figure 4. 30: Map illustrating 13 erosion hotspots identified from the comparison of historical aerial photographs (1967) and IKONOS satellite images (2002) along the Coral Coast.
Appendix 1.1
Appendix 1.2
Appendix 1.3
Appendix 1.4
Appendix 1.5
Appendix 1.6Appendix 1.7
Appendix 1.8
Appendix 1.9
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Figure 4. 31: The area eroded at each of the 13 spots identified as erosion hotspots (Fig. 4.17) over a 35 year period, from 1967 to 2002. Each area reading has an error margin of ± 6m2 (Section 3.3.4, Chapter 3).
�
Figure 4. 32: Average rate of erosion at each of the 13 spots identified as erosion hotspots (Fig. 4.17) over a 35 year period, from 1967 to 2002. Each rate of erosion reading has an error margin of ± 6m2 (Section 3.3.4, Chapter 3).
99�
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The line graphs (Figs. 4.31 and 4.32) illustrate relatively high erosion levels in spots
1 (Yadua Village area), 2 (Sigatoka River area) and 6 (Vatukarasa Village area).
Over a period of 35 years (1967 to 2002), these spots indicate erosion levels of more
than 11 000m2 (314m2/yr); with the maximum level of erosion to be 21 487 m2 (614
m2/yr), in the Yadua Village area. Spots 8 (Tambua Sands area), 9 (Tagaqe Village
area) and 13 (Beach House area) with eroding area values between 5 214m2 (149
m2/yr) and 6 086m2 (174 m2/yr), indicate moderate erosion levels over the years
1967 to 2002. The erosion levels, over the 35 year period, in all other eroding spots
are low; falling between the interval 840m2 (24 m2/yr) – 2 116 m2 (60 m2/yr).�
�
�
Prograding Coastlines
�
Altogether, there were 21 spots identified to be prograding coastlines. Figure 4.33
indicates the location of these spots along the Coral Coast. The prograding coastline
data has been analyzed in a similar manner as the erosion hotspots data. Line graphs
have been used to indicate the rate of progradation over the 35 year period, from
1967 to 2002, at each site. The graphs below show the total area prograded in square
meters, (Fig. 4.34) and the rate of progradation in square meters prograded per year,
(Fig. 4.35), at each of the 21 sites. The prograding area (21 spots; from west to east)
indicated in Figure 4.33 corresponds to the progradation quantities in Figures 4.34
and 4.35. For detailed maps of prograding coastlines at each site refer to Appendix 2.
100�
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Figure 4. 33: Map illustrating 21 prograding spots identified from the comparison of historical aerial photographs (1967) and IKONOS satellite images (2002) along the Coral Coast.�
Appendix 2.1 Appendix 2.2
Appendix 2.3
Appendix 2.4
Appendix 2.5
Appendix 2.6
Appendix 2.7
Appendix 2.8
Appendix 2.9
Appendix 2.10
Appendix 2.11
Appendix 2.12
Appendix 2.13
Appendix 2.14
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�Figure 4. 34: The area prograded at each of the 21 spots identified as prograding spots (Fig. 4.20) over a 35 year period, from 1967 to 2002. Each area reading has an error margin of ± 6m2(Section 3.3.4, Chapter 3).
�Figure 4. 35: Average rate of progradation at each of the 21 spots identified as prograding spots (Fig. 4.20) over a 35 year period, from 1967 to 2002. Each rate of progradation reading has an error margin of ± 6m2(Section 3.3.4, Chapter 3).
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Significantly high progradation levels are evident in spots 1 (Fijian Resort area), 3
(Sigatoka River area), 10 (Sovi Bay area), 16 (Naviti Resort area) and 21
(Navutulevu and Naboutini Village areas) (Figs. 4.34 and 4.35). The line graphs
indicate that over the years 1967 to 2002, the progradation levels in these areas are
greater than 24 000m2 (686m2/yr). The highest progradation level is apparent in spot
1 (Fijian Resort area); with a progradation area of 97 285m2 (Fig. 4.34) and a
progradation rate of 2 780m2/yr (Fig. 4.35). All other areas indicate lower
progradation levels; with all the progradation values lying between 1 564m2
(45m2/yr) and 18 349m2 (524m2/yr).
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4.5 SUMMARY
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This chapter was aimed at presenting the outcomes of the comparison of historical
aerial photographs and satellite images to analyze temporal coastline changes along
the Coral Coast. The coastline categorization from field investigation and the desktop
study of 1967 aerial photographs and 2002 IKONOS satellite images revealed there
to be no coherent coastline change patterns. However, a slight pattern is evident; the
far west (Fijian Resort area) and the far east (Naboutini Village area) sides of the
study area generally indicate resilient and prograding coastlines. Along the coastline
in the central region it was found that stretches of prograding coastlines alternate
with resilient and eroding coastlines. There were only two areas with naturally
prograding coastlines; the head of Sovi Bay and Namatakula Village areas and most
of the regions along the Coral Coast were found to be subjected to beach erosion.
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Attempts were made to quantify the erosion and progradation levels in three different
ways; calculation of the maximum distance of erosion and progradation in meters,
the total area eroded in square meters and the rate of coastline change in square meter
per year. The maximum distance of erosion at the various sites along the Coral Coast
ranged from 14 ± 3m (0.40 m/yr) to 40 ± 3m (1.14 m/yr), while the maximum
distance of progradation ranged from 13 ± 3m (0.37 m/yr) to 400 ± 3m (11.43 m/yr).
The total area of erosion ranged from 840 ± 6m2 to 21 487 ±6 m2 and the total area of
progradation ranged from 1 564 ± 6m2 to 97 285 ±6 m2. The rate of erosion ranged
from 24 ± 6m2/yr to 614 ± 6 m2/yr and the rate of progradation ranged from 45 ±
6m2/yr to 2 780 ± 6m2/yr.
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The Coral Coast area is fringed by a narrow stretch of fringing reef system which is
interrupted by numerous reef passages. The Coral Coast waters are subjected to
northwest currents which are set up by the dominant southeast trade winds. Overall,
progradation is the dominant process in most of the areas along the Coral Coast.
However, site specific observations reveal that the dominant coastal process at any
given area is seen to be specific to that area, with respect to the local characteristics.
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CHAPTER 5 – DISCUSSION AND CONCLUSION
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5.1 INTRODUCTION
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The findings from the previous chapter (Chapter 5) are discussed in this chapter. This
chapter restates the principal objectives outlined in Chapter 1, discussing the range of
coastline changes along the Coral Coast, coastline. It evaluates the various coastal
processes evident along the Coral Coast, determined from the comparison of the
1967 aerial photographs and the 2002 IKONOS satellite images. The state of the
Coral Coast coastline is further discussed by assessing the results from field
investigations. The chapter moves on to reflect on the implications and
recommendations of the study and finally draws conclusions on the findings of the
research.
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5.2 DISCUSSION OF THESIS FINDINGS
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5.2.1 Coastline Changes
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The objectives of section 5.2.1 are to identify and classify the coastline along the
Coral Coast, under the categories erosion hotspot, erosion watch spot, resilient
coastline and prograding coastline; artificial and natural progradation and to quantify
the range of change for coastline segments identified as eroding and prograding
coastlines. This was achieved by evaluating the temporal coastline change along the
Coral Coast by comparison of 1967 aerial photographs and 2002 IKONOS satellite
images and field investigation.
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5.2.1.1 Overall Coastline Changes
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Based on the digitized coastline of the Coral Coast, the total distance of the coastline
studied was 57.7 km; out of which 28.1 km was resilient, 12.7 km was artificially
prograding, 7.8 km was naturally prograding, 5.1 km was identified as erosion
hotspot and 4.0 km was identified as erosion watchspot. Visual inspections during
field investigations and historical aerial photographs and satellite image analyses
revealed long stretches of resilient and artificially prograding coastlines to be caused
by coastal engineered structures (seawalls and groynes). Seawalls and groynes halt
the retreat of the coastal areas; usually the area placed behind these structures (Bird,
2005, Charlier, 2005 and Gillie, 1997).
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All coastal regions investigated with artificially prograding and resilient coastlines
showed signs of beach erosion (Figs. 4.2–4.3, 4.6, 4.13, 4.15–4.16, 4.19, 4.21, 4.22,
and 4.23). The artificially prograding and most of the resilient coastlines possess
vertical seawalls, built parallel to the beach. The seawalls do not have any armor
protection or wave dissipation blocks near and in front of their toes (Figs. 4.4, 4.7,
4.11, 4.14, 4.17–4.18 and 4.20). Hence, the incoming high energy waves are not
dissipated and cause scouring to the bottom at the toe of the seawall (Hsu, 2005). In
addition, the increased water turbulence at the seawall prevents sediment deposition
along the beach during swell conditions (Dingler, 2005). Furthermore, the seawall
may have reduced or completely blocked the upcoast sediment supply sources (Bird,
2005). This caused the land behind the seawall structures to prograde, while the
beaches in front of it eroded, (other causes of beach erosion are presented in Table
2.4). Continuous beach erosion along the Coral Coast has resulted in loss of high and
wide beaches. Over time, continuous direct impact of waves on seawalls would
damage the seawalls as well (Dingler, 2005).
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Contrary to the prediction that the Coral Coast coastal zone would have overall
eroding coastlines, results from the desktop analyses and the ground truthing exercise
indicate the Coral Coast coastlines to be mostly influenced by prograding processes.
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In total, there were 21 prograding stretches of coastline (Figs. 4.25, 4.33) and 13
eroding stretches of coastline (Figs. 4.25, 4.30) along the Coral Coast shoreline. The
slight pattern that the far west (Fijian Resort area) and the far east (Naboutini Village
area) sides of the study areas are resilient and prograding, while the central region
has stretches of prograding coastlines alternating with resilient and eroding coastlines
(Fig. 4.25), may be due to the different wave environments influencing these areas
The resilient and prograding coastlines at the far west and east of the study site may
be a result of sediment supplements from the various sources in these areas. The
sediments may have been transported to the coastlines by rivers, materials derived
from nearby cliff and outcrop erosion, washed from the sea floor and/or from
artificial beach nourishment by seaside resorts (Bird, 2008). Furthermore, the
fringing reef system is a source of oceanic sediments for the beaches in this area
(Nunn, 2005). There is also terrestrial sediment supplement by the Naevuevu River
in the far west of the region. Consequently, the coastlines in the far west and east of
the study area are mostly resilient and/ or prograding.
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The prediction that erosion spots would be prominent opposite the channel entrance,
for reef flats which were dissected by channels, appears to be incorrect. There are a
number of spots along the Coral Coast situated opposite channels influenced by
erosion processes; including Yadua (Fig. 4.3), Korotongo (Fig. 4.6), Tagaqe (Fig.
4.16) and Vatukarasa (Fig. 4.8) Villages, Korolevu Settlement (Fig. 4.19), east of
Tambua Sands (Fig. 4.13) and Tabakula (Fig. 4.6) Resorts and Beach House (Fig.
4.21). However, there are numerous spots along the Coral Coast situated opposite
channels with no apparent signs of erosion processes; including Naevuevu (Fig. 4.2),
Votua (Fig. 4.19), Komave (Fig. 4.21), Navola (Fig. 4.21), Namatakula (fig. 4.23)
and Navutulevu (Fig. 4.23) Villages, Naviti Resort (Fig. 4.16) and Sovi Bay area
(Fig. 4.8). It is impossible to conclude that these areas are naturally not affected by
erosion since most of these areas, except for Sovi Bay and Namatakula Village, are
armoured by sea walls.
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�Figure 5. 1: An illustration of the level of erosion (in m2) along the coastlines of the Coral Coast with and without reef barriers; the level of erosion is related to the presence and absence of shoreline armor in each case.
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Out of the 13 spots identified as coastal erosion hotspots only 3 lie within regions
with reef barriers. The number of erosion hotspots for both armored and unarmored
coastlines are approximately the same. For regions with no reef barriers there are 5
erosion hotspots along armored coastlines and 5 along unarmored coastlines. For
regions with reef barriers there is 1 erosion hotspot along armored coastlines and 2
along unarmored coastlines. However, the level of erosion is higher along unarmored
coastlines for both regions with reef barriers and without reef barriers (Fig. 5.1).
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The above graph (Fig. 5.1) indicates that the coastline stretches with no reef barriers
are subjected to more erosion processes than the coastline stretches with reef barriers.
High erosion levels are prominent along areas with no reef barriers and no shoreline
armors. High energy waves can cause erosion and deposition well above the highest
tides (Bird, 2008). It appears that in this case coastline stretches with no reef barriers
are exposed to high energy waves which cause erosion. In addition, there are no
artificial barriers to sustain the wave actions in areas without shoreline armor, hence,
these places are subjected to high erosion levels. The increased momentum in the
waves may be caused by the dominant southeast tradewinds, storms and cyclones
during the cyclone season, and fluctuations in wind patterns due to ENSO (El Niño
Southern Oscillation) events (Sallenger Jr. et al. 2002; Smith and Jackson 1990;
Terry 2007).
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5.2.1.2 Coastline Categorization
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The coastlines along the Coral Coast area were either straight over a large length or
curved in plan separated by headlands, natural and/ or man-made. However, except
for the prominent headland-bay coasts in the Sovi Bay and Vatukarasa area (Fig.
4.8), the Coral Coast area mostly had straight coastlines over large lengths.
Compared with their straight counterparts, curved beaches are more stable (Hsu,
2005).
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Eroding Coastlines
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A number of areas along the Coral Coast are subjected to erosion processes. A total
of 13 spots along the Coral Coast have been identified as coastal erosion hotspots
including Yadua Village (Fig. 4.3), Sigatoka River (Fig. 4.5), East of Vatukarasa
Village (Fig. 4.8), West of Vatukarasa Village (Fig. 4.8), adjacent to Tambua Sands
Resort (Fig. 4.13), East of Tambua Sands Resort (Fig. 4.13), Tagaqe Village (Fig.
4.13), Beach House Resort (Fig. 4.21), Korotongo Village (Fig. 4.5), Tabakula
Resort (Fig. 4.6), , Korolevu Settlement (Fig. 4.19), Warwick Resort (Fig. 4.19) and
Komave Village (Fig. 4.21). There are numerous other regions along the Coral Coast
which have been identified as erosion watch spots. These areas have lost less than 20
m2 of land area over a 35 year period (1967 to 2002).
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The eroding regions along the Coral Coast loose more sediment alongshore and
offshore than they receive from the various sources (Table. 2.2; Bird, 2008). There
may be a number of natural and human-induced causes of coastline erosion along the
Coral Coast area (Section 2.5.1 and 2.5.2). The natural causes may include
fluctuations in wave directions due to changes and/or cycles in long-term weather
patterns (Gillie, 1997), sea level rise (Primo, 1997), tropical cyclones and storm
surges (Smith and Jackson, 1990), and tsunamis (Segar, 1998). Beach profile
changes in the South Pacific countries have a significant association with wave
directions related with ENSO (El Niño Southern Oscillation) events (Sallenger Jr. et
al., 2002). According to the SEAFRAME (Sea Level Fine Resolution Acoustic
Measuring Equipment) gauge, the sea-level in Fiji is calculated to be rising at a rate
of 2.9mm/year (Gray, 2009). Fiji experiences tropical cyclones and associated
depressions that approach from a range of directions between the months November
to April (Fiji Meteorological Services, 2008). The common human-induced causes of
coastal erosion in the South Pacific include sand extraction from beaches for
construction and shoreline reclamation purposes (Gillie, 1997), sand trapping
structures such as groynes and other barriers constructed perpendicular to the
shorelines (Maragos, 1993), and reclamation of shorefront land and the construction
of seawalls (Gillie, 1997). Out of these causes the two prominent human-induced
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causes of coastal erosion along the Coral Coast are groyne (causeways connecting
artificial islands to the mainland (Fig. 4.18)) and reclamation of shorefront land and
sea wall construction (Fig. 4.7, 4.14, 4.17 and 4.20). The causeways linking the
Warwick Resort’s artificial islands to the mainland act as groynes which may be
disrupting the longshore sediment transport along the coast (Dingler, 2005 and Hsu,
2005). The disruption of the longshore drift causes the shoreline downdrift of the
groynes (coastline stretch between Warwick Resort and Korolevu Settlement) to lose
its sand and retreats (Figs. 4.18 and 4.19).
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Artificially Prograding Coastlines
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Out of the 21 prograding spots identified along the Coral Coast area, 19 are
artificially prograding. These areas include the West of Fijian Resort (Fig. 4.2), the
East of Fijian Resort (Fig. 4.2),Sigatoka River (Fig. 4.5), Naviti Resort (Fig. 4.16),
Navutulevu – Naboutini (Fig. 4.23), West of Korotongo Village (Fig. 4.5), East of
Korotongo Village (Fig. 4.5), Outrigger Resort (Fig. 4.6), West of Tabakula Resort
(Fig. 4.6), East of Tabakula Resort (Fig. 4.6), Vatukarasa area (Fig. 4.8), Tambua
Sands Resort (Fig. 4.13), Hideaway (Fig. 4.13), Tagaqe Village (Fig. 4.13), Votua
Village (Fig. 4.19), Korolevu Settlement (Fig. 4.19), Komave Village (Fig. 4.21),
Navola Village (Fig. 21).
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Progradation is prominent in coastal areas which receive more sediments from
upcoast, alongshore and offshore areas than they lose to the various sediment sinks
(Table 2.3; Komar, 1998; Bird, 2008). A common feature along all artificially
prograding areas is the presence of shoreline armor (Figs. 4.7, 4.12, 4.14, 4.17, 4.18
and 4.20). Shoreline protection structures may have stopped and/or minimized the
retreat of coastal areas behind the armor (Bird, 2005; Charlier, 2005; Goodman et al.,
2008). These structures may have also stopped the upcoast sediment transport from
dispersing into sediment sink zones (Komar, 1998). All of the above mentioned areas
have been fortified by sea walls. The Warwick and Naviti Resorts have causeways,
linking the artificial islands in front of the resorts to the mainland, which act as
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groynes (Fig. 4.18). Groynes may induce progradation in the wind ward side of the
causeway due to reduced wave energy (Goodman et al., 2008). Most of the shoreline
armor, including sea walls and gabion baskets (Figs. 4.7, 4.12, 4.14, 4.17 and 4.20)
and shorefront reclamation along the Coral Coast were constructed in association
with the building of the main highway (Queens Road) which runs along the coastline
along the Coral Coast.
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Naturally Prograding Coastlines
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Two naturally prograding coastlines were identified along the Coral Coast; in the
Sovi Bay and Namatakula Village areas. The Namatakula Village area has an
upcoast source of sediment supply. The river on the east of the village transports
sediment to the Namatakula area (Komar, 1998).
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The head of Sovi Bay was dominated by prograding coastlines (Fig. 4.8). The Sovi
Bay area has a curved planform which is produced by the persistent swell waves
diffracted from the tip of the eastern headland, combined with wave refraction and
nearshore current circulation system in the lee of the headland (logarithmic spiral
beach – Fig. 5.1) (Hsu, 2005 and Moreno, 2005). Since the littoral drift of the
logarithmic spiral beach is being supplied from a river system (upcoast source), this
coastal area is rendered stable but in a dynamic equilibrium (Hsu, 2005).
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Figure 5. 2: Headland bay beaches in dynamic and static equilibrium. Source: Hsu, 2005.
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Usually the general retreat of coastlines associated with erosion processes are not
uniform because of variations in rock resistance. The resistant rocks recede slowly
and remain as headlands, stacks and offshore islands. Coastal features associated
with depositional coastlines include spits, bays, lagoons, cuspate foreland, tombolo
formations, and barrier islands (Komar, 1998). Rocky cliffs and headlands are
evident along the southern coast of Viti Levu, while spits, bays, lagoons and barrier
islands are evident along the northern coast of Viti Levu (Google Earth 2006).
Therefore, it can be deduced that the southern coast of Viti Levu is an erosional
coastline while the northern coast of Viti Levu is a depositional coastline.
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5.2.2 Quantification of Eroding and Prograding Coastlines
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The erosion and progradation levels along the Coral Coast have been quantified by
the comparison of the 1967 aerial photographs and the 2002 IKONOS satellite
images. The level of erosion and progradation over the 35 year interval (1967 to
2002) were quantified using two approaches; (i) the maximum distance eroded and
prograded in meters and (ii) the total area eroded and prograded in square meters.
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The average rates of erosion and progradation for both techniques were calculated
and discussed. Finally, the results obtained from the two approaches were compared.
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(i) Maximum Distances Eroded and Prograded in Meters
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The comparison of the maximum distance eroded and the maximum distance
prograded revealed the Coral Coast area to be influence more by prograding than by
eroding processes (Fig. 4.25). As mentioned in section 4.2.2, the upper limits for the
maximum distance eroded (40 ± 3 m/ 1.14 m/yr) and prograded (400 ± 3 m/ 11.43
m/yr) indicate the Coral Coast area to be influenced ten times more by progradation
than by erosion (Figs. 4.26 and 4.27).
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The maximum eroded distances at specific sites, ranged from 14 ± 3 m (0.4 m/yr) (at
Vatukarasa) to 40 ± 3 m (1.14 m/yr) (at Tagaqe Village), (Figs. 4.25, 4.26 and 4.27).
The lowest erosion rate of 0.4 m/yr along the Coral Coast coastline is higher than the
significantly high erosion rate of 0.3 m/yr along the Maui, Hawaii shorelines
(Fletcher, et al., 2003). Hence, over an interval of 35 years (1967 to 2002), the Coral
Coast shorelines were subjected to high erosion rates.
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The maximum prograded distances ranged from 13 ± 3 m (0.37m/yr) (front of
Outrigger Resort) to 400 ± 3m (11.43 m/yr) (west of Fijian Resort), (Figs. 4.25, 4.28
and 4.29). The only prominent pattern in the maximum prograding distances is that
the spit in the east of the Fijian Resort (Appendix 2.1) indicates a much higher
prograded distance (400 ± 3m) than the other spots. However, the maximum
prograding distances in all other sites lie in a shorter distance bracket of 13 ± 3 m to
130 ± 3 m and do not show any coherent prograding patterns (Fig. 4.28). The
absence of any prograding patterns in these areas may be because; except for the
Sovi Bay and Namatakula Village areas, all other areas are subjected to artificial
progradation associated with shoreline armors.
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(ii) Total Areas Eroded and Prograded in Square Meters
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Most studies related to shoreline change quantify the rate of coastline change in
meters per year. For instance an average rate of shoreline change of 0.3 m/yr along
the Maui, Hawaii, shoreline (Fletcher, et al., 2003), shoreline change rate of 4.75
m/yr along the Bay of Bengal in eastern India (Maiti and Bhattacharya, 2008), and
0.07 – 0.15 m/yr coastline change along the central shores of the Monterey Bay
(Moore and Griggs, 2002). However, in addition to representing the rate of coastline
changes in meters per year, this research attempts to quantify the coastline rate of
change in square meter per year. This technique of erosion and progradation
quantification was applied to gauge entire areas influenced by erosion and
progradation processes instead of only considering the maximum distance of
landward and seaward movement of the coastlines.
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As with the maximum distances eroded and prograded, there are no coherent patterns
observed among the various eroded and prograded areas along the Coral Coast. The
total areas eroded ranged from 840 ± 6 m2 (Komave Village area) to 21 487 ± 6 m2
(Yadua Village area), (Figs. 4.30 and 4.31). The 13 erosion hotspots and the 21
prograding spots identified along the Coral Coast were divided into high, moderate
and low levels for eroding spots, (Table 5.1) and high and moderate levels for
prograding spots (Table 5.2).
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Table 5. 1: Division of erosion hotspots into high, moderate and low level clusters. The numbers in brackets correspond to the sites in Figure 4.30.
Levels Erosion Clusters (m2)
Erosion Spots and Areas
High > 11 000 Yadua Village (1), Sigatoka River (2), Vatukarasa Village (6)
Moderate 5 214 – 6 086 Tambua Sands Resort (8), Tagaqe Village (9), Beach House Resort (13)
Low 840 – 2 116 Korotongo Village (3), Tabakula Resort (4), Vatukarasa Village (5), Tambua Sands Resort (7), Korolevu Settlement (10), Warwick Resort (11), Komave Village (12)
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Table 5.1 indicates the division of erosion hotspots into high, moderate and low
levels. The different levels have been determined from the rough erosion clusters
evident in Figure 4.31. The erosion clusters for the three different levels of erosion
have been calculated for an interval of 35 years, over the years 1967 and 2002 and
have an error margin of ± 6m2 (Section 3.3.4). Although the total eroding areas per
site have been classed as high, moderate and low levels, these erosion levels are
relatively high. The erosion quantities determined by this research indicate the Coral
Coast area to be influenced by relatively high erosion processes.
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Table 5. 2: Division of prograding spots into high and moderate level clusters. The numbers in brackets correspond to the sites in Figure 4.33.
ProgradationClusters (m2)
Progradation Spots
High > 24 000 Fijian Resort (1), Sigatoka River (3), Sovi Bay (10), Naviti Resort (16), Namatakula – Naboutini (21)
Moderate 1 564 – 18 349 Fijian Resort (2), Korotongo Village (4,5), Outrigger Resort (6), Tabakula Resort (7,8), Vatukarasa (9,11,12), Tambua Sands Resort (13), Hideaway (14), Tagaqe Village (15), Votua Village (17), Korolevu Settlement (18), Komave Village (19), Navola Village (20)
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Table 5.2 indicates the division of prograding spots into high and moderate levels.
The different levels have been determined from the rough progradation clusters
evident in Figure 4.34. The progradation clusters for the two progradation levels
have been calculated for an interval of 35 years, over the years 1967 and 2002 and
have an error margin of ± 6m2 (Section 3.3.4). Except for Namatakula Village and
Sovi Bay area, all other areas have artificially prograding coastlines. Although the
total prograding areas per site have been classed as high and moderate, these
progradation levels are relatively higher than the erosion levels (Table 5.1). Overall,
the total prograding area was 363 300 m2 out of which 297 712 m2 was artificial
progradation and 65 588 m2 was natural progradation, while, the total eroding area
was 76 673 m2. Therefore, the artificial progradation quantity determined by this
research indicate the Coral Coast area is more influenced by artificial progradation
than by natural progradation and erosion.
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Rates of Erosion and Progradation
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The rate of erosion and progradation values (Figs. 4.27, 4.29, 4.32 and 4.35) at each
site (Figs. 4.30 and 4.33) indicate the same properties as shown by the corresponding
maximum eroding and prograding distances in (i) and the total areas eroded and
prograded in (ii) above. In order to calculate the rate of erosion per year, the
maximum distance and total area at each site has been divided by the same constant,
that is, 35; the interval from 1967 to 2002. The rate of erosion and progradation
(m2/yr and m/yr) can be used to gauge the average distance and area eroded and
prograded per year over the years 1967 to 2002. However, these rates have been
determined using the end-point-rate method (Sections 2.2 and 3.2.3), where two
representative shorelines, the earliest (1967) and the most recent (2002), have been
used to calculate the amount of shoreline movement. Hence, it does not take into
consideration erosion and progradation caused by acute events such as cyclones and
storms (Crowell et al., 2005). In order to obtain more accurate data which could be
used in future coastline prediction, it would be appropriate to use the linear
regression method of determining the rate of change (Section 2.2). This method
considers shorelines from shorter time intervals, leaving out the post-storm shoreline
data. Thus, the rate of coastline change attained from this method would be in
reasonable agreement with the actual physical situation of long-term coastline
change; excluding short term coastline change episodes such as erosion due to storms
followed by extended periods of recovery (Crowell et al., 2005).
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The Maximum Distances and Total Areas of Erosion and Progradation
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Both the maximum distance and total areas of erosion and progradation indicate
higher levels and more numbers of prograding spots along the Coral Coast
shorelines. Although the artificial progradation levels are much higher than the
erosion levels, both the maximum eroding distance and total eroding area indicated
that the Coral Coast area was influenced by significantly high erosion rates. The
quantification of eroding and prograding coastlines indicates the Coral Coast area to
be more influenced by prograding coastlines than by eroding coastlines.
Neither of the quantification techniques show any coherent patterns of erosion and/
or progradation. Hence, the dominant coastal process, whether erosion or
progradation, is site specific and depended on local factors.
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5.2.3 Coastal Geomorphology of the Coral Coast
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The objective of this section was to observe the general coastal geomorphology
along the Coral Coast. Since the field study of all sites along the Coral Coast was not
feasible, the objective was achieved by field investigation of selected sites (a
combination of resilient, prograding and eroding coastlines) along the Coral Coast
(Fig. 3.5).
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The Coral Coast area is fringed by a narrow length of fringing reef system which is
interrupted by numerous reef channels. These channels give entry to waves induced
by the dominant southeast tradewinds (Terry 2007); the major factor manipulating
the various coastal processes and geomorphology of the Coral Coast area. For most
of the year the southeast tradewinds are persistent, however, they tend to be weaker
in the summer season (from November to April), and stronger in the winter (May to
October) (Terry 2007). The dominant current set up by the southeast tradewinds is in
a northwest direction. In addition, the Coral Coast area is influenced by tidal currents
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set up by the semidiurnal-micro tidal system; two highs and two lows per day,
(Segar, 1998).
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Most of the study sites had beach sediments from both, terrestrial and oceanic
origins, with majority of the sediments in the coarse sand to rubble range. Finer
sediments were observed close to the vegetation line (Figs. 4.4, 4.17 and 4.18).
Coarse sediments are a characteristic of coastal environments influenced by high
energy waves (Inman and Jenkins, 2005). The channels along the Coral Coast area
are inlets to high energy waves that do not dissipate before reaching the shorelines.
The high energy waves are able to transport coarse sediments to the beaches. The
Hideaway (Fig. 4.12) and Naviti (Fig. 4.15) resorts had fine to coarse grade
sediments along the beaches. Since, the fringing reef system in front of these resort
areas are relatively narrow and there are channels providing an entrance to high
energy waves, these areas were expected to have coarse sediments. However, both
the resorts provide beach nourishments to the beach area, hence, the prominent finer
grade sediments. The Sovi Bay (Fig. 4.9) and Namatakula Village (Fig.4.22) areas
have fine beach sediments. The Sovi Bay area has fine sediment supplements from
an upcoast river source which are distributed in the bay area (section 5.2.1.2). The
Namatakula area has a river meandering through the beach in front of Namatakula
Village (Fig. 4.24). The river deposits fine sediments from an upcoast source on the
Namatakula beach.
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A number of regions along the Coral Coast have coastline protection structures;
mainly seawalls. Seawalls are a common structure along the coastlines of majority of
the resorts along the Coral Coast area. Resorts with seawall as coastline protection
structures include; Outrigger (Fig. 4.27), Hideaway (Fig. 4.12), Naviti (Fig. 4.15 and
4.17) and Warwick. The Naviti and Warwick resorts also have artificial islands (with
coastline protection structures around the islands) in the fringing reef flat area. The
artificial islands are connected to the mainland by causeways which act as groynes.
In the case of Warwick Resort, the groynes act as sediment traps and disrupt the
longshore drift (Hsu, 2005). As a result coastal erosion is evident on the west of the
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resort (Fig. 4.19). Gabion baskets and rip rap (Davis, 2005) structures have been
placed at random coastal stretches along the Coral Coast area to prevent the main
highway (Queens Road) from eroding (Fig. 4.11).
�
In many countries around the world, human intervention in and interception of
sediment supply have been considered two of the most detrimental factors causing
beach erosion (Hsu, 2005). The interference in sediment supply to the Coral Coast
beaches by seawalls was one of the major causes of beach erosion in this area.
Almost all beaches affected by erosion (Figs. 4.3, 4.6, 4.8, 4.13, 4.14, 4.16, 4.19,
4.20, and 4.21) have two major erosion indicators; exposed tree roots and beach
rocks. All beach erosion sites had exposed tree roots which are considered to be one
to the major indication of severe beach erosion (Anthony, 2005). Numerous beach
rocks were exhumed at various sites along the Coral Coast as indications of beach
erosion (Turner, 2005). The Tagaqe (Fig. 4.14) and Vatukarasa areas had
significantly high areas of exposed beachrock, while the Tambua Sands and
Korolevu areas had lower areas of exposed beachrocks. However, the beach rocks
may be fossils exposed by the late Holocene sea level fall (Pirazzoli, 2005).
�
All the study sites had gently sloping narrow beaches except for the Vatukarasa (Fig.
4.10) and Namatakula areas. Despite showing a high number of beach erosion
indicators, the Vatukarasa area had a significantly wide and steep beach (Fig. 4.10).
This may be because the length of rocky outcrops dipping seaward in front of the
Vatukarasa area act as wave breakers. The steepness of the beach also helps to break
the waves (Hsu, 2005), hence, the resilient coastline in front of the Vatukarasa
Village (Fig. 4.8). The Namatakula area has a gently sloping and wide beach. The
wide beach is as a result of low wave energy environment due to a wider fringing
reef flat and the sheltering effect by the wide fringing reef flats in the eastern side of
the village (Fig. 5.2).
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5.3 RECOMMENDATIONS AND IMPLICATIONS
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5.3.1 Recommendations
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Generally, there are three monitoring techniques which can be applied to coastline
change studies; remote sensing methods, in situ instruments and sampling methods
(Morang and Gorman, 2005). For the purpose of this research, the first type of
monitoring technique (remote sensing methods) was used to examine coastline
changes along the Coral Coast. In relation to this technique, it may be relevant to
base future studies on both short-term and long-term coastline changes. Examination
of shorter yearly intervals would take into account the effect of sudden acute events,
such as storms and cyclones, on coastlines. Refined coastline data may be achieved if
this research technique is coupled with in situ instruments and sampling methods
techniques.
�
In addition, this research has been conducted along a narrow fringing reef flat. Future
studies can be concentrated on both coastlines with wide and narrow reef flats. It
may be useful to look at the coastlines adjacent to both the west and east ends of the
study site (Fig. 1.2). In future it may be useful to investigate the orientation of the
reef systems and the island (Viti Levu) in relation to the dominant southeast
tradewinds and the sheltering effect of the Beqa lagoon on the east end of the study
site.
�
This research indicates the Coral Coast coastlines to be affected by high rates of
erosion processes (Figs. 4.25 – 4.27, 4.30 – 4.32). Considering the identified erosion
hotspots along the Coral Coast, the villages which require urgent attention in terms
of coastal protection include Yadua, Vatukarasa, and Tagaqe villages and the east
end of Korolevu Settlement. Korotongo and Namada villages may require coastal
protection structures later. The significance of coastal erosion has only been
identified at places where it has threatened populations, infrastructure and
development. In addition, there is limited knowledge on prograding coastlines.
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Hence, in future, it may be useful to locate and assess coastlines associated with
progradation together with eroding coastlines.
�
In comparison to eroding coastlines, there are a lot more areas affected with high
rates of progradation (Figs. 4.25, 4.28 – 4.29, 4.33, 4.34 – 4.35). Most of the
progradation is caused by coastline protection structures and associated land
reclamation. Hence, hard defense structures, such as seawalls and groynes, can be
used as a response to erosion processes affecting the coastlines along the Coral
Coast. In order to compensate for the beach loss caused by the construction of
coastline protection structures (Dingler, 2005), artificial beach nourishment could be
provided (Charlier, 2005). This approach to sustain beaches around the globe has
been used since the 1920s. However, it is not an inexpensive operation to undertake
since beaches would require regular additions of sand. In addition, storm events
could transport major parts of the artificial deposit back to the sea (Charlier, 2005).
�
Another approach that can be taken to overcome land loss due to coastline retreat is
the headland-control concept of shore protection (Moreno, 2005). First proposed by
Silvester (1976), the headland control concept of shoreline protection was described
as “a combination of groynes and breakwaters at along-shore and seaward spacing
such to create long lengths of equilibrium-bay beaches” (Fig. 5.1) (Silvester and Hsu,
1993). Best applied in sediment deficient areas, headland beaches compartmentalize
the coastline and reorient it in the local compartments to be parallel to the wave
crests of the predominant wave direction. Headland beaches would become unstable
or be destroyed if there is substantial change in annual wave directions and if there is
no tombolo formation behind the anchoring headland (this would result in sediment
movement to adjacent compartments). The major characteristic of headland beaches
is the creation of pocket beaches, where there is no communication of sand
alongshore (Moreno, 2005). Thus, headland beaches create complete barriers to the
longshore drift, and can only be considered a means of coastline protection if the
adjacent beaches are not threatened by sediment deficiency problems. The headland-
control concept is considered to be appropriate if the design goal is to stabilize a
regional extent by multiple pocket beaches (Moreno, 2005). Hence, in future it may
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be useful to compare differently oriented coastlines; straight coastlines and pocket
beaches.
�
Finally, the results from this research could be used as a baseline for future studies
involving historical shoreline examinations for shoreline forecasts; which would aid
in coastal land-use plans. Usually, the average annual erosion rate (AAER)
multiplied by a specific number of years (commonly 30 and 60 years) are used to
decide building setbacks of an area (Crowell et al. 2005).
�
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5.3.2 Implications of Thesis Research
�
The findings from the comparison of historical and modern data can be
communicated to and/ or used by engineers, coastal managers, and policy-makers.
Morang and Gorman (2005) outlined a list of phenomena which can be examined by
the study of historical charts, modern maps, aerial photographs, and LIDAR or
topographic data. These include:
� “Long- and short-term advance or retreat of the shore. Shoreline change data
are critical for coastal managers tasked with establishing setback lines and
guiding growth in the coastal zone, especially in low areas subject to
flooding.
� The impact of storms, including barrier island breaches, overwash, and
changes in inlets, vegetation, and dunes.
� Human impacts caused by coastal construction, dune destruction, or
dredging.
� Compliance with permits, illegal filling, and dumping.
� Biological conditions of wetlands, estuaries, and barrier islands.
� Susceptibility of urban areas to storm flooding and catastrophic events (e.g.,
hurricanes) by means of storm surge models.”
According to Morang and Gorman (2005), quantitative information can be provided
by volumetric comparisons between the old and modern surveys if historical 3D
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(bathymetric and topographic) data are available. The quantitative information that
can be computed includes:
� “Longshore sediment movement.
� Shoaling or siltation associated with tidal inlets, river mouths, estuaries, and
harbours.
� Sediment changes on ebb and flood shoals and in inlet channels.
� Nearshore bathymetry changes over time.
� Migrations of channel thalwegs.”
Volumetric comparisons are often carried out by coastal engineers to compute
amounts of sediment trapped by structures, examine the growth of shoals in
navigation channels, determine dredging contract payment, and evaluate post-
dredging channel conditions and to monitor the performance of beach nourishment
projects (Morang and Gorman, 2005).
�
Land-use planning is the primary reason for calculating long-term coastline change
rates. The purpose could range from providing information to property owners to
establishing regulatory setback lines for coastal construction (Crowell, et al., 2005).
Most Pacific Island countries have significant development in the coastal regions; for
example, village settlements and tourist hotels/resorts. Studies such as this can aid
people in making important decisions on coastal development by incorporating
environmental, economical and social factors.
�
A significant amount of land of many small island countries may be lost due to high
levels of erosion while there may be others who may have an extension of their coast
lines due to accretion. This loss and gain of coastal land plays a significant role in the
coastal ecosystems as well. Coral reefs are one of the major ecosystems affected by
sedimentation problems. Eroded sediments contribute to sediment stress on corals
which has been known to cause drastic decreases in coral species diversity and coral
species percentages (Acevedo et al., 1989).
�
Most small island countries are developing countries and cannot commit the
necessary resources to prevent coastal land loss in the face of rising sea level
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(Leatherman and Beller-Simms, 1997). Therefore, student oriented researches such
as this, would be a means of complimenting the country’s coastline data sets. One of
the major accomplishments of this research was the deduction that remote sensing
techniques alone can only be useful in identifying general progradation processes.
However, other methods (Section 5.3.1) would need to be incorporated with the
remote sensing technique in order to gauge whether the progradation is natural or
artificial. Moreover, this research identified the naturally prograding areas (Sovi Bay
and Namatakula) along the Coral Coast. Areas of natural progradation are significant
to development strategies.
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5.4 CONCLUSION
�
Majority of the coastline along the Coral Coast was found to be resilient; straight-line
distance of 28.1 Km. Similar to the artificially prograding coastlines (12.7 Km), field
investigations revealed this resilience to be caused by shoreline protection structures
(seawall and groynes) and associated processes. A stretch of 7.8 Km was identified
to be naturally prograding as a result of upcoast sediment supply. The total erosion
hotspot stretch was 5.1 Km, while the erosion watch spots were distributed over 4.0
km of the coastline. The erosion processes were manipulated by wave actions, which
in turn were induced by the dominant southeast tradewinds.
�
The level of erosion and progradation along the Coral Coast were quantified in terms
of maximum distance (meters) and area (in square meters) of landward and seaward
movement, respectively. The maximum distance of landward and seaward movement
revealed the Coral Coast area to be affected ten times more by artificial progradation
than by erosion. The total prograding and eroding area technique of quantification
was used to gauge the entire area influenced by progradation and erosion processes
instead of only considering the maximum linear distance of coastline change. Upon
examination of the erosion and progradation area graphs, the level of erosion and
progradation were categorized as high (> 11 000 m2), moderate (5 216 – 6 086 m2),
and low (840 – 2 116 m2), and high (> 24 000 m2), and moderate (1 564 – 18 349
m2), respectively.
�
The coastline categorization and quantification by comparison of the 1967 aerial
photographs and the 2002 IKONOS satellite images indicate that the Coral Coast
coastline is influenced more by progradation than erosion. However, field
investigation revealed only two areas along the Coral Coast to be naturally
prograding; the head of Sovi Bay and the Namatakula area. The head of Sovi Bay
was in a state of dynamic equilibrium, and supplemented by a continuous supply of
sediment from an upcoast river; hence, the dominance of prograding processes.
Namatakula area on the other hand, has an upcoast river sediment source; therefore a
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prograding coastline. All other prograding and resilient coastlines were influenced by
artificial progradation processes due to fortification by coastline protection
structures; indicating artificial progradation and reclamation. Beach erosion was a
prominent occurrence along the artificially prograding and resilient coastline. It may
have been induced by seawalls interrupting terrestrial sediment supply coupled with
the increased water turbulence at the seawall that may prevent sediment deposition
along the beach during swell conditions.
�
Most of the eroding hotspots occur along coastlines which do not have any reef
barriers. The number of eroding hotspots is not related to the presence or absence of
shoreline armor. However, the erosion levels are higher in the absence of shoreline
armor and no reef barriers. The high erosion levels may be caused by high energy
waves which impact the coastlines with no reef barriers and shoreline armor. The
presence of seawalls in most places along the Coral Coast indicates that this area is
experiencing high levels of erosion. The only exception may be seaside resorts,
which may have shoreline armors to extend their land area.
�
Finally, the method of coastline change analyses used in this research was the end-
point-rate method which does not take into consideration the effect of acute events
such as storms, on the state of the coastlines. Hence, it would be appropriate to base
future studies on other methods of shoreline change analyses such as the linear
regression method which takes into account both short- and long-term coastline
changes. The impact of acute events (such as storms and cyclones) can be taken into
consideration if remote sensing monitoring methods are used to examine shorter
yearly interval data sets. For detailed studies of particular areas it may be useful to
couple the remote sensing method with the in situ instruments and sampling methods
techniques.
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GLOSSARY LIST
�
Backwash – downward flow of water on beach face (Dingler, 2005).
�
Coastline – the vegetation line along the coastal area.
�
Desktop study – offline coastline analyses using the Geographical Information
Systems (GIS) by means of the ArcGIS software.
�
Dynamic equilibrium of bay beaches – when the littoral drift is still being supplied
from upcoast and/ or from sources within the bay (Hsu, 2005).
�
Erosion hotspots – coastal erosion has threatened shoreline development and
infrastructure (Sea Grant, 1997).
�
Erosion watch spots – coastal environments will soon be threatened if shoreline
erosion trends continue (Sea Grant, 1997).
�
In situ instruments – monitoring technique whereby instruments are placed in the
media being studied, such as current meters moored in the ocean (Morang and
Gorman, 2005).
�
Logarithmic spiral beach – a curved or embayed beach developed in the direction
under which it is sheltered by a headland. Its planform is a characteristic of persistent
swell waves diffracted from the tip of a headland, combined with wave refraction
and a nearshore current circulation system in the lee of the headland (Hsu, 2005).
Also known as zeta curved bays, half-heart shaped bays, crenulate shaped beaches,
headland-bay beaches, pocket beaches, and offset coasts (Silvester and Hsu, 1993).
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Prograding shorelines – shoreline is advancing sea-ward; either by sediment
accretion (natural progradation) or by engineered structures such as seawalls
(artificial progradation (Bird, 2005)).
�
Remote sensing methods – monitoring technique whereby the instruments provide
information about the land and the sea from a distance without being in physical
contact (for instance, aerial photography, laser imaging) (Morang and Gorman,
2005).
�
Resilient shorelines – no coastal retreat or advancement observed in the time frame
of the image overlays.
�
Run-up – upward flow of water on beach face (Dingler, 2005).
�
Sampling methods – monitoring technique whereby devices retrieve a sample of the
material being examined (i.e., water, ice, sediment, biological material) so that the
scientist can conduct more detailed examination in a laboratory ((Morang and
Gorman, 2005).
�
Static equilibrium of bay beaches – the littoral drift on a bay beach is negligible or
supply from upcoast is nonexistent (Hsu, 2005).
�
Swash zone – the area on the beach face exposed to the upward (run-up) and
downward (backflow) flow of water (Dingler, 2005).
�
Temporal coastline changes – changes along a coastline over a time interval.
�
Tombolo – deposit of unconsolidated material that connects an island to another
island or to the mainland (Sverdrup, 2005).
�
140�
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Unstable bay beaches – implies potential retreat due to imbalance of sediment input
and output (Hsu, 2005).
141�
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APPENDICES
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APPENDIX 1 – Eroding Coastlines
�
This section represents the details of eroding coastlines areas, analyzed over the
period 1967 to 2002, along the Coral Coast in the form of maps (Appendices. 1.1 –
1.2). The overall map of the Coral Coast area indicating the location of the eroding
sites along the Coral Coast is presented in Figure 4.17. Detailed maps of each
marked area are illustrated in the corresponding appendices shown by arrows.
�
Each figure indicates the erosion levels as the total area eroded and the maximum
linear distance of erosion over the years 1967 to 2002. The total area eroded at each
site has an error of ± 6 m2 (Section 3.3.4). For more details on each area refer to Figs.
4.17 – 4.19. For details on the maximum linear distance of erosion at particular sites
refer to Figs 4.12 – 4.14.
142�
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Appendix 1. 1: Eroding area on the east of Yadua Village.
�
�
Maximum linear distance eroded 30 ± 3 m (0.86 m/yr).
(614m2/yr)21487 m2
143�
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Appendix 1. 2: Eroding area along the Sigatoka River spit and the east of the Sigatoka River spit.
�
�
Maximum linear distance eroded along the spit:33 ± 3 m (0.94 m/yr)
Maximum linear distance eroded on east of spit:21 ± 3 m (0.60 m/yr)
(498m2/yr)17446 m2
(29m2/yr)1007 m2
144�
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�
Appendix 1. 3: Eroding areas on the east of Tabakula Resort.
�
�
Maximum linear distance eroded: 20 ± 3 m (0.57 m/yr).
(29m2/yr)1024 m2
145�
�
�
Appendix 1. 4: Eroding area in front of Vatukarasa Village and the east of Vatukarasa Village.
�
�
Maximum linear distance eroded in Vatukarasa Village front: 14 ± 3 m (0.40 m/yr)
Maximum linear distance eroded in east of Vatukarasa Village: 38 ± 3 m (1.08 m/yr)
(40m2/yr)1417 m2
(317m2/yr)11081 m2
146�
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�
Appendix 1. 5: The total area eroded in the east of Tambua Sands Resort.
�
�
Maximum linear distance eroded adjacent to Tambua Sands: 40 ± 3 m (1.14 m/yr)
Maximum linear distance eroded in the east of Tambua Sands: 26 ± 3 m (0.74 m/yr)
(53m2/yr)1871 m2 (174m2/yr)
6086 m2
147�
�
�
Appendix 1. 6: The total area eroded in front of Tagaqe Village.
�
�
Maximum linear distance eroded in Tagaqe Village front: 32 ± 3 m (0.91 m/yr)
�
(168m2/yr)5865 m2
148�
�
�
Appendix 1. 7: The total area eroded along Korolevu Settlement and on the west of Warwick Resort.
�
�
Maximum linear distance eroded along Korolevu Settlement: 23 ± 3 m (0.66 m/yr)
Maximum linear distance eroded in west of Warwick Resort: 32 ± 3 m (0.91 m/yr)
(35m2/yr)1219 m2
(60m2/yr)2116 m2
149�
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�
Appendix 1. 8: The total area eroded in the west of Komave Village.
�
Maximum linear distance eroded in west of Komave Village: 19 ± 3 m (0.54 m/yr)
�
(24m2/yr)840 m2
150�
�
�
Appendix 1. 9: The total area eroded in the Beach House area.
�
�
Maximum linear distance eroded in the Beach House area: 27 ± 3 m (0.77 m/yr)
(149m2/yr)5214 m2
151�
�
APPENDIX 2 – Prograding Coastlines
�
This section represents the details of prograding coastlines areas along the Coral
Coast in the form of maps (Appendices. 2.1 – 2.14). The overall map of the Coral
Coast area indicating the location of the prograding sites along the Coral Coast is
presented in Appendix 2.1. Detailed maps of each marked area are illustrated in the
corresponding appendices shown by arrows.
�
Each figure indicates the progradation levels as the total area prograded and the
maximum linear distance of progradation over the years 1967 to 2002. The total area
prograded at each site has an error of ± 6 m2 (Section 3.3.4). For further details on
each area refer to Figs. 4.20 – 4.22. For details on the maximum linear distance of
erosion at particular sites refer to Figs 4.12, 4.15 – 4.16.
152�
�
�Appendix 2. 1: The total area prograded in the Fijian Resort area.
�
�
Maximum linear distance prograded west of Fijian Resort: 400 ± 3 m (11.43 m/yr)
Maximum linear distance prograded east of Fijian Resort: 22 ± 3 m (0.63 m/yr)
(969m2/yr)33921 m2
(325m2/yr)11368 m2
153�
�
�
Appendix 2. 2: The total area prograded along the Sigatoka River spit.
�
�
Maximum linear distance prograded at the Sigatoka River spit: 50 ± 3 m (1.43 m/yr)
(702m2/yr)24565 m2
154�
�
�Appendix 2. 3: The total area prograded at the Korotongo bridge, Korotongo roundabout and in front of Outrigger Resort.
�
�
Maximum linear distance prograded at Korotongo Bridge: 26 ± 3 m (0.74 m/yr)
Maximum linear distance prograded at Korotongo roundabout: 36 ± 3 m (1.03 m/yr
Maximum linear distance prograded at Outrigger Resort: 13 ± 3 m (0.37 m/yr)
(289m2/yr)10112 m2
(116m2/yr)4074 m2
(69m2/yr)2416 m2
155�
�
�Appendix 2. 4: The total area prograded in the east of Tabakula Resort.
�
�
Maximum linear distance prograded east of Tabakula Resort: 21 ± 3 m (0.6 m/yr)
Maximum linear distance prograded east of the river: 16 ± 3 m (0.46 m/yr)
(118m2/yr)4122 m2
(156m2/yr)5471 m2
156�
�
�Appendix 2. 5: The total area prograded in the west and at the head of Sovi Bay.
�
�
Maximum linear distance prograded west of Sovi Bay: 22 ± 3 m (0.63 m/yr)
Maximum linear distance prograded; head of Sovi Bay: 130 ± 3 m (3.71 m/yr)
(132m2/yr)4611 m2
(1540m2/yr)53884 m2
157�
�
�
Appendix 2. 6: The total area prograded at the east and near the Vatukarasa Village.
�
�
Maximum linear distance prograded at Vatukarasa Village: 67± 3 m (1.91 m/yr)
Maximum linear distance prograded east of Vatukarasa Village: 20 ± 3 m (0.57
m/yr)
(125m2/yr)4372 m2 (143m2/yr)
4994 m2
158�
�
�Appendix 2. 7: The total area prograded in the east of Tambua Sands Resort.
�
�
Maximum linear distance prograded east of Tambua Sands Resort: 20± 3 m (0.57
m/yr)
(143m2/yr)4989 m2
159�
�
�Appendix 2. 8: The total area prograded at the Hideaway Resort and the east of Tagaqe Village.
�
�
Maximum linear distance prograded at Hideaway Resort: 23 ± 3 m (0.66 m/yr)
Maximum linear distance prograded east of Tagaqe Village: 17 ± 3 m (0.49 m/yr)
(157m2/yr)5482 m2(45m2/yr)
1564 m2
160�
�
�Appendix 2. 9: The total area prograded in Naviti Resort area.
�
�
Maximum linear distance prograded at Naviti Resort: 100 ± 3 m (2.86 m/yr)
(157m2/yr)44318 m2
161�
�
�Appendix 2. 10: The prograding area in the Votua Village area.
�
�
Maximum linear distance prograded Votua Village: 25 ± 3 m (0.71 m/yr)
(306m2/yr)10707 m2
162�
�
�Appendix 2. 11: The total area prograded in the Korolevu Settlement area.
�
�
Maximum linear distance prograded at Korolevu Settlement: 28 ± 3 m (0.8 m/yr)
(524m2/yr)18349 m2
163�
�
�Appendix 2. 12: The total area prograded in the Komave Village area.
�
�
Maximum linear distance prograded Komave Village: 18 ± 3 m (0.51 m/yr)
(232m2/yr)8112 m2
164�
�
�Appendix 2. 13: The total area prograded in the Navola Village area.
�
�
Maximum linear distance prograded east of Vatukarasa Village: 30 ± 3 m (0.86
m/yr)
(245m2/yr)8584 m2
165�
�
�Appendix 2. 14: The total areas prograded in the Namatakula, Navutulevu and Naboutini Village areas.
�
�
Maximum linear distance prograded at Namatakula, Navutulevu and Naboutini
Villages: 40 ± 3 m (1.14 m/yr)
(2780m2/yr)97285 m2