Coasts & Ports Australasian Conference 2003
A PROCESS BASED ASSESSMENT OF ENGINEERED STRUCTURES ON REEF
ISLANDS OF THE MALDIVES
Paul Kench1 , Kevin Parnell2 and Rob Brander3
1
School of Geography and Environmental Science, The University of Auckland, Private Bag 92019, Auckland.
[email protected]
2
School of Tropical Environmental Studies and Geography, James Cook University, Townsville QLD 4811.
[email protected]
3
School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW
2052, Australia. rbrander @unsw.edu.au
Abstract: This paper examines the use and environmental consequences of engineered
structures on reef platform islands in the Maldives. The Maldives comprises 1 500 islands
which are mostly low-lying sand cays. These cays are inherently unstable, changing their size,
and position on reef platforms in response to short-term and seasonal adjustments in wind,
wave and current patterns. Island instability combined with high population densities has
resulted in the proliferation of engineered structures to combat erosion and to provide boat
access. In many instances introduction of hard engineered structures has exacerbated island
erosion and degraded reef productivity. Reasons for the negative environmental consequences
are twofold. First, the design, materials used in construction and the mode of construction
contravene most standard measures of sound engineering design. Constraining sound design
is a total lack of environmental information, particularly wave and current data. Second,
structures are often inappropriate with respect to natural coastal processes. This is illustrated
by wave and current measurements on Hulhudhoo Island in Baa Atoll. Results show the
nearshore process regime is dominated by strong alongshore current gradients. Furthermore,
seasonal shifts in monsoon winds produce reversals in wave, current and sediment transport
patterns. The unique circulatory nature of coastal processes around islands has a number of
implications for use of engineered structures. First, conventional engineering practices
resulting from an understanding of onshore/offshore and alongshore processes are not
necessarily appropriate. Second, it requires reconsideration of notions of passive erosion and
placement loss. Effects of structures that are usually transferred alongshore are contained
within the 360o island coastline and act to compound island instability and erosion.
Keywords: Reef islands, Maldives, Hydrodynamics, Alongshore drift, Island stability
Engineering structures
INTRODUCTION
Islands located on coral reef platforms throughout the Pacific and Indian oceans are typically
low- lying (< 4m maximum elevation), small in area and are composed of bioclastic sediment
generated from the surrounding reef platform (Stoddart, 1977; McLean and Stoddart, 1978;
McLean and Hosking, 1991). Consequently, island growth, maintenance and change are
reliant on the transport of sediments from the reef source to the island sink (Gourlay, 1988,
Kench and Cowell, 2002). As noted by Hopley (1982) the most influential factors in
controlling these sediment transfers are waves and currents operating on the reef surface.
Reef islands are morphologically unstable, changing their size, shape, elevation and position
on reef platforms in response to short-term and seasonal adjustments in wind, wave and
current patterns (Flood, 1982; Ali, 2000) and medium to long-term shifts in sea level
(Leatherman, 1997; Kench and Cowell, 2001). The limited land area of reef islands and the ir
morphological instability pose significant management problems for island communities.
Shoreline erosion is perceived as one of the most pervasive environmental problems in small
island nations and is considered to be exacerbated with the spectre of sea-level rise
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(Leatherman, 1997). Of note, island shoreline erosion is manifest not only as a net loss of
island materials, but due to low elevation and small island size, sediments can be reworked to
the island surface or alongshore to promote island migration on reef platforms (Kench and
Cowell, 2001, 2002). Consequently island change does not necessarily imply smaller islands.
Confronted with unstable island shorelines management responses have included the standard
suite of hard engineering solutions to stabilise shorelines and maintain island size and volume.
Development pressure on islands has also introduced a range of structures (causeways,
dredged boat channels, reclamation) to the coastal zone that interact with reef top processes.
Introduction of such structures is known to result in adverse environmental effects including
island erosion and reef degradation (Maragos, 1993). However, few studies have examined
how structures interact with reef platform processes that affect island shorelines.
This study presents wave and current measurements for one island in the Maldives
archipelago and identifies some of the broad scale coastal processes controlling island
morphology. This process regime is evaluated in light of how processes are likely to interact
with typical engineering structures. Findings are discussed with respect to the validity of
using existing standard engineering structures in reef platform settings.
FIELD SETTING
The Maldives archipelago, situated southwest of India in the central Indian Ocean, is
comprised of 22 atolls and four reef platforms that stretch 750 km from Ihavandiffolhu
(6o 57’N) to Addu just south of the equator (0o 34’S, Fig. 1). To the west and east of the
archipelago the sea floor falls steeply to depths of 2500-3000m and 2800-4200m respectively,
while the chain is dissected by several major west to east channels with water depths around
1000m.
More than 1200 islands are located on the atoll and reef platforms. Only 300 of these islands
are inhabited, either by villages or resorts. The islands are geologically young having been
deposited on the reef flats in the late Holocene (Woodroffe, 1993). While the islands are
typically low- lying and have limited land area there is great variation in their morphological
characteristics. In particular the islands differ in sediment texture (sand or gravel), island
shape (circular, oval, elongate), size, proportion of reef flat occupied, orientation and position
in an atoll (windward or leeward reef rim or central lagoon; Ali, 2000). While such
differences have not been systematically studied for the entire atoll chain, recent work
suggests these differences in part reflect variations in exposure to major island building
processes (waves, currents, sediment production) through the archipelago (Woodroffe, 1993;
Ali, 2000). Indeed, Woodroffe (1993) notes that significant changes in climate from south to
north and the switching of the monsoon seasons (Southwest to Northeast) are likely to exert
an indelible imprint on the morphology of islands.
The focus of detailed hydrodynamic measurements in this study is Hulhudhoo Island, located
on a lagoonal reef platform in Baa atoll in the northern third of the archipelago (Fig.1). The
island is circular in shape, approximately 200m in diameter (Fig. 1C) and is uninhabited.
Climate and Oceanographic Setting
A pervasive feature of the Maldives climate is a predictable change in monsoon conditions.
Between March and November the islands are influenced by the southwest monsoon that
generates westerly winds. Between December and February the archipelago experiences
north-northeast winds generated by the northeast monsoon (Woodroffe, 1993). Consequently
the archipelago experiences predictable seasonal shifts in climate, which also control wave
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energy impacting on reefs. The significant wave height of oceanic swell incident at Baa atoll
peaks in June/July at 1.8m and is lowest in February/March at 0.8m, (Young, 1999). How
waves are transformed within the atolls is unknown.
Figure 1. A;B) Field location, Maldives, Indian Ocean; and C) Hulhudhoo Island, Baa atoll.
ENGINEERED STRUCTURES IN THE MALDIVES
Beach erosion is prevalent on many islands in the Maldives and island communities in the
Maldives have confronted coastal erosion throughout the history of occupation of the islands
(Maniku, 1990). Seawalls, groynes, boat harbours and dredged channels are ubiquitous on
inhabited islands in the Maldives (Fig. 2). However, field inspection of numerous structures
indicates that their life span is short, they do not stop the erosion problem, and they often
exacerbate island erosion and can degrade reef productivity. Reasons for the poor
performance and negative environmental consequences of structures are twofold.
Design and Construction Methods
The design, materials used in construction and the mode of construction contravene most
standard measures of sound engineering. In a review of erosion management processes in the
Maldives Kench (2001) found that few structures are formally designed. Rather, structures are
designed and built ad hoc by local communities using locally available materials (coral block,
coral sand blocks or other solid debris, Fig. 2). This approach has copied typical techniques
used in non-reef environments without regard to the unique process characteristics of reef
islands.
Further constraining sound design is a total lack of environmental information, particularly
wave and current data (Kench, 2001). For example, no data exist on wave heights impacting
on island shorelines or sediment flux in the nearshore. Consequently, the choice of structure,
its physical dimensions and construction materials are selected without knowledge of detailed
coastal processes and in particular extreme energies at the shoreline. Furthermore, although
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the process relationship between seawalls and erosion on the fronting beach remains unclear
(Kraus and McDougal, 1996), the process interactions between seawalls and carbonate
sediments are virtually unstudied.
Figure 2. Examples of coastal engineering structures in the Maldives: A) Seawall with
sediment leakage behind wall; B) coral boulder seawall cemented onto reef surface; C)
Breached boat harbour block wall 1- month after construction; D) promulgation of groynes to
stabilise islands; and E) Male island (note multiple coastal structures).
Failings of Coastal Structures
Outside of the capital island (Male) coastal protection structures are commonly composed of
coral rock (Fig. 2b), coral sand cement and blocks, or any other solid debris that can be
sourced by local communities (e.g. 44 gallon drums). Coral fragment shape and density mean
that individual clasts are frequently too small to remain immobile under even relatively low
energy conditions, and cements are either of poor quality or not used (Figs. 2b and 2c).
Appropriate grading of material used in construction is rare. Except where erected on beach
rock, toe protection is rarely considered and appropriate foundation materials are rarely used.
The absence of suitable natural structures to which seawalls and groynes can be fixed, leads to
structures liable to flanking at the lateral extent of seawalls or on the inshore end of groynes.
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Construction methods and materials mean that in many cases water and sediments can move
freely around, and occasionally over and through seawalls. The net effect of failed structures
in many cases is exacerbation of erosion problems. This leads to promulgation of engineering
structures with examples of entire shorelines being armoured (Fig. 2D, E).
FIELD STUDY
Investigation of wave and current processes affecting the shoreline of Hulhudhoo Island (Fig.
1) were undertaken in short four-day deployments in June 2002 and February 2003. These
experimental periods were chosen in order to document key coastal processes influencing the
island under both the southwest and northeast monsoons.
During each experiment four InterOcean S4 wave and bidirectional current meters were
deployed approximately 5 m seaward of the toe of the beach around the island (Fig. 1C).
Current meters were mounted on pods and fixed to the reef surface with sensors located 0.2m
above the reef surface. Water level and currents were recorded at 2Hz except on one
instrument that sampled at 5Hz. Burst length varied depending on the memory capacity of
instruments with a minimum of 18 minutes data collected every hour. Raw current data was
averaged at two- minute intervals and current speed and direction plotted as progressive
vectors at the shoreline. Summary wave statistics from each instrument were processed using
WAVEWIN.
Morphological changes in shoreline position were surveyed using global positioning system
surveys of the island edge and toe of beach. The toe of beach was depicted as the point where
the sand beach slope intersects the solid reef surface. Surveys were repeated in January and
June 2002 and February 2003 representing a complete monsoon cycle.
Selected water level, summary wave and current records from the southwest and northeast
monsoon measurement periods are presented in Figure 3. Wave records show that incident
wave height at the windward shoreline was small in magnitude during both measurement
periods. Significant wave height peaked at 0.39m and 0.16 m for the southwest and northeast
shoreline respectively (Fig. 3). Wave energy at the shoreline is clearly modulated by tidal
elevation peaking at high water levels (Fig. 3). Mean wave period incident at the shoreline
was 5.3 s and 6.2 s for the June and February experiments respectively.
In contrast to wave records, peak mean current velocity occurred on the lateral flanks of the
island during each measurement period (Fig. 3). Of note, these currents exhibit inverse tidal
modulation. Peak currents (approximately 0.37 ms -1 ) occur at the low to mid-water level
phases in both measurement periods. Progressive vector plots of mean current records for
each measurement period indicate that shorelines are controlled by unidirectional alongshore
flows under both monsoon seasons. In June, wave propagation onto the southwest portion of
the reef drives flow northward (along the western shoreline) and eastward (along the southern
and northern shoreline; Fig. 4A). In contrast, results from February indicate a general reversal
in flow direction (Fig. 4B). The progressive vector on the windward (NE) side of the island
displays oscillation in current flow toward the west and southeast. This is thought to reflect
subtle shifts in direction driven by changes in wave approach to the current meter. Elsewhere
flows are oriented oblique to the shoreline and flow southward (western side of island).
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Figure 3. Windward water level, wave records and northwestern velocity records from the
Hulhudhoo reef platform. A) June 2002; B) February 2003.
The beach surrounding Hulhudhoo shows marked fluctuations in position between surveys.
Maximum beach width (80 m) is located on the eastern side of the island in June 2002 (Fig.
4A). However, in February 2003 maximum beach width is located on the western side of the
island (approximately 70m wide). Of note, the January 2002 survey (not shown) is virtually
identical to that shown in Fig. 4B and indicates the beach assumed similar configuration over
the 12 month period. This indicates an approximate 180 degree oscillation in beach position
corresponding to changes in monsoon conditions.
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Figure 4. Four-day progressive vector plots at current meter locations on Hulhudhoo. A) June
21-24, 2002; B) February 25-28, 2003. TOB = toe of beach position for each time period.
DISCUSSION
Shoreline processes on the Hulhudhoo reef platform exhibit significant changes controlled by
seasonal shifts in direction of incident wind and wave energy. Wave energy is shown to be
tidally modulated. Highest waves recorded in both measurement periods coincide with high
water stages and result from wave reformation across the reef platform (Gourlay, 1988) and
propagation of unbroken waves directly across the reef crest. Waves of this magnitude are
capable of entraining and transporting sand-size material on the island shoreline.
Currents at the shoreline are directed alongshore and reach speeds in excess of 0.35 ms-1 (Fig.
4). These currents appear out of phase with maximum wave input to the shoreline and are
likely forced by wind and wave-generated set-up at the reef edge. Symonds et al. (1995)
propose that set- up gradients peak when the wave height (hs ) to water depth (hr) ratio is hr =
hs/2. For the wave records on Hulhudhoo this maxima is achieved during low tide periods
when water depths at the reef edge are 0.2 m. When water depths exceed 0.4 m (hr = hs) at the
reef edge, wave driven flows cease. Unlike modelling scenarios of wave-driven flow
(Symonds et al., 1995; Hearn, 1999) the reef width is narrow and backed by an island
shoreline. This presents the possibility that reformed reef flat waves are able to generate
secondary set-up at the shoreface. Furthermore, the shoreline of small reef islands is convex.
Consequently subtle variations in wave energy around the reef platform (particularly away
from the windward margin) are likely to generate variations in magnitude of wave –driven
flow promoting alongshore currents.
The unidirectional nature of nearshore currents on the Hulhudhoo reef platform supports the
assertion that currents are not tidally driven. Of significance to this study, sediment transport
at the shoreline, during each season, is likely dominated by observed unidirectional
alongshore currents. During the southwest monsoon southwest to westerly wind driven waves
drive sediment clockwise northward and along the northern edge of Hulhudhoo, depositing
sediment on the eastern side of the island (Fig. 4). Sediment is effectively blocked from
transport around to the southeast side of the island due to weak northward flow from this zone
(Fig. 4A). During the northeast monsoon, northeast wind-driven currents transfer sediment
counter-clockwise (Fig. 4B). Such transfers indicate that different parts of the Hulhudhoo
shoreline experience markedly different sediment flux processes. The western and eastern
sides of the island act as short-term sediment stores as reflected in seasonal beach accretion.
Driven by predictable shifts in dominant incident energy, the northern shoreline experiences
large alongshore and seasonally reversing sediment fluxes. The southern shoreline does not
appear to play a major role in the mass transfer of sediment between seasons.
Lessons for Use of Engineering Structures
Seasonal oscillations of the monsoons provide a uniquely different process mechanism by
which island beaches and shorelines exhibit morphodynamic adjustments. It is clear that the
conventional models of beach behaviour developed on continental coastlines, which depict
seasonal on-offshore movements of beach material are not applicable to islands in the
Maldives. Rather, these shorelines are dominated by alongshore current and sediment
transport processes that drive temporary shifts in the sediment budget. Consequently, the
Hulhudhoo shoreline possesses a 3-dimensional envelope of change with shifts in planform
being spatially greater than changes in individual beach cross-section.
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The alongshore dominated process regime identified on Hulhudhoo island is likely to operate
in a wide variety of reef islands. As islands are small in area and are able to be affected by
wave and currents on all sides, sediment is likely to be trapped at the shoreline and will
respond to temporal shifts in the imbalance of wave and current energy around the island. The
precise magnitude of shoreline change, however, will be dependent on a range of factors
including the relative magnitude of energy at the shoreline and temporal variations in wind
and wave energy.
Findings of this study have implications for the use and application of engineering structures
on island shorelines. Conventional engineering practices resulting from an understanding of
on/offshore and alongshore processes are not necessarily appropriate. In settings where there
are large shifts in beach position, shore parallel structures experience periodic exposure and
burial. In the Maldives, seawalls have commonly been constructed on one side of the island in
response to seasonal removal of sediment. Avoidance of flanking in this situation would
necessitate extension of structures around the entire dynamic zone, which on Hulhudhoo
would encompass the entire shoreline. This has been done on some resort islands and on Male
(Fig. 2F).
The circulatory nature of sediment movement requires reconsideration of the concept of
placement loss. Small islands have a finite volume of sediment. This sediment can be
continually reworked to promote island migration and change. Consequently, placement of
seawalls not only prevents material from contributing to beach change in front of the
structure, but also inhibits contributions to temporal accretion of the beach on other parts of
the island shoreline. This depletes the sediment volume available to the beach and increases
the period of time unprotected island shorelines are exposed to wave energy. Furthermore, the
process of island migration is prevented.
Shore perpendicular structures also interfere with oscillating sediment transfers around reef
islands. Groynes that trap alongshore fluxes of sediment deplete the volume of sediment
available for seasonal beach development around an island. Consequently the beach is
stripped of sediment more rapidly in the alternating monsoon period (by alongshore
processes) exposing the island to erosive action for a greater period of time. Furthermore,
groynes prevent the redeposition of sediment. In the Maldives such effects have led to the
promulgation of groyne fields around islands (Fig. 2D).
CONCLUSIONS
There has been little validation of whether the coastal process regime of reef platform islands
is conducive to the use of standard engineered structures. This is attributed to two factors; i)
the absence of detailed information of coastal processes in reef environments; and ii) poor
understanding of how structures interfere with the process regime and shoreline morphology
of reef islands.
Small islands face inherent constraints in managing erosion given limited materials,
resources, background environmental data and access to suitable expertise. This study shows
that reef islands have unique coastal process regimes. Due to the dominance and seasonal
reversal of alongshore drift both shore parallel and perpendicular structures interfere with the
natural dynamics of reef island shorelines and result in islands being out of equilibrium with
the process regime. Consequently, conventional structural designs presently used in the
Maldives are largely inappropriate and do not solve the problem of shoreline (island)
instability. Alternative methods are required that reflect the unique process regimes of reef
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platform islands. These methods must be developed alongside detailed investigations of reef
platform processes in a range of different island settings.
ACKNOWLEDGEMENTS
This research was funded by the Royal Society of New Zealand, Marsden Fund to the
principle author. We acknowledge the logistical and field support of staff of the Environment
Research Center of the Government of the Republic of Maldives. In particular, Dr Mohamed
Ali, Mahmoud Riyaz, Amjad Abdulla, Admiral Lateef. The University of New South Wales
and University of Waikato for use of S4 current meters and Sarah MacDonald for preparation
of Figure 4.
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KEYWORDS
Reef islands, Maldives, Hydrodynamics, Alongshore drift, Island stability Engineering
structures
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