Coastal erosion and management of Amatuku Island, Funafuti atoll

Coastal erosion and management of Amatuku Island, Funafuti atoll

COASTAL EROSION AND MANAGEMENT OF
AMATUKU ISLAND, FUNAFUTIATOLL, TUVALU
Chunting Xue
SOPAC Secretariat
August 1996
SOPAC Technical Report 234
This project was funded by the Government of the
People's Republic of China
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TABLE OF CONTENTS
Page
SUMMARY.................................................................................................................................. 6
ACKNOWLEDGMENTS............................................................................................................. 7
INTRODUCTION........................................................................................................................ 7
STUDY METHODS .................................................................................................................. 11
DEVELOPMENT HISTORY ..................................................................................................... 11
REEF FLAT AND BEACH........................................................................................................ 12
COASTAL EROSION............................................................................................................... 19
BORROW PIT AND CHANNEL ............................................................................................... 27
COASTAL MANAGEMENT....................................................................................................... 31
CONCLUSIONS ....................................................................................................................... 31
RECOMMENDATIONS ............................................................................................................ 32
REFERENCES ........................................................................................................................ 32
APPENDIX 1............................................................................................................................. 33
APPENDIX 2............................................................................................................................. 34
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LIST OF FIGURES
1.
Location of Amatuku.
2.
Air photographs, taken on 7 July 1943 (left photo) and on 19 July 1984 (right photo).
There was a dark shadow belt of trees on the lagoon shore on 1943 air photo. The
channel on the reef flat and borrow pit northwest of Amatuku are seen on the air
photograph of July 1984 but not on the air photograph of July 1943.
3.
Map showing the channel on the lagoon reef flat, and the causeway and borrow pit
between Mulitefala and Amatuku in the Second World War (excavated or built in 1943 or
the first half of 1944). The outline of Amatuku is based on the air photographs of July
1943.
4.
Coastal geological map of Amatuku, Funafuti Atoll, Tuvalu.
5.
Remains of the causeway, connecting Amatuku and Mulitefala in the Second World
War, which was composed of big reef rock blocks (0.5-2 m). The northwest end of the
causeway(closest the camera) has been washed away. Smaller rubble has been
washed onto southwest of remains of the causeway. The sand beach in the foreground
is at the southeast end of Mulitefala. The dark blue colour of water left (ocean side) of
the remains indicates the long borrow pit parallel to the causeway. View 1350 from
Mulitefala.
6.
The opening (passage) between Mulitefala (right) and the remains of the causeway
(left), and reef rock blocks on the lagoon flat, moved by waves and currents. View 236° .
7.
Types of beach on Amatuku.
8.
Beach profile (TU96204) on the stable coast, southwest lagoon coast.
9.
Beach profile (TU96202) on the erosional coast, east part of the north ocean coast.
10. Beach profile (TU96206) on erosional coast, the northeast ocean coast. The location
map and profile data are given in Appendix 1.
11. Beach profile (TU96203) on erosional coast, northwest lagoon coast. The bare
beachrock demarcates the usual location of the beach. For location map and profile
data are given in Appendix 1.
12. Coastal erosion and dominant longshore sediment transport of Amatuku.
13. The shoreline change from 1979 to 1996, based on the October 1979 topographic map
and bearings and distances between shoreline and houses or concrete foundations
measured during the May 1996 field survey.
14. Erosion scarp of 0.6 m high and shoreline retreat of at least 2 m, at TU96202, north
ocean shore, Amatuku. View 115° .
15. A tree with landward extending roots, indicating 4.3 m shoreline retreat, north ocean
shore, Amatuku. View to southwest.
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16. Erosion scarp of 1.3 m high on the north ocean shore, close to the northwest tip of
Amatuku, where the shoreline has retreated at least 4 m, based on distribution of soil
among gravel. View 150° .
17. Coconut tree trunks lying on lagoon reef flat, removed from the northwest tip of
Amatuku. View 220° .
18. The northwest lagoon shore at the northwest end of Amatuku with erosion scarp of 0.51.5 m. View 100° .
19. The northwest lagoon shore near the jetty, The beach is composed almost entirely of
beachrock (conglomerate and sandstone) with little loose material. View 110° .
20. Erosion scarp of 0.9 m high with gravel on top of the scarp and consolidated breccia at
the base. Location is on the shore behind the channel, southwest of the Jetty. View
110° .
21. Two trees with landward extending roots, indicating 4.5 m of shoreline retreat on the
southwest lagoon shore. The beachrock exposed in front of the trees and concrete
blocks were put in front of the trees and the house (student dormitory). View 0° .
22. Southeast tip of Amatuku showing erosion. View 325° .
23. The present configuration of the borrow pit between Mulitefala and Amatuku excavated in
the Second World War measured and observed on 24 May 1996. The upper diagram is
a map showing sediment distribution; R-rubble; S-sand; Cr-branching coral. The middle
diagram shows water depths of the pit along the centre line (lower than the reef flat).
The lower diagram is a cross- section of the pit. Measured and observed on 24 May
1996.
24. The depths (lower than the reef flat) of the channel on lagoon reef flat of Amatuku,
measured on 24 May 1996,
25. Funafuti wind data for 1950-1984 (after Carter, 1986).
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SUMMARY
A six-day field investigation of Amatuku, a small island on the northeast rim of Funafuti Atoll,
Tuvalu was carried out 18-24 May 1996. Observations on the shoreline, reef flat, a borrow pit
and a channel were made to determine the extent of erosion and the causes.
Although Amatuku is small in size it is important to the Tuvalu economy since it is the site of
the Tuvalu Marine School for training seamen.
The principal conclusions of the investigation were as follows:
• The ocean reef flat is composed of five zones: spur and groove, outer pavement, rubble
and pavement, beachrock, and beach. The lagoon reef flat is composed of three zones:
branching coral, pavement, and beach.
• The beach includes three types: sand beach, gravel beach, which is further divided into
two subtypes: gravel beach and gravel/beachrock beach ( gravel on upper beach and
beachrock on lower beach), and bare beachrock replacing the beach.
• Most parts of the coast have been eroded. The most severe erosion has occurred on the
north ocean coast. Since 1944, one small peninsula and two small islets have been
eroded away, and the shoreline retreated at least 4.3 m. Severe erosion has also occurred
on the lagoon coast near the channel.
• The ocean coast erosion and lagoon coast erosion have been mainly caused by the
trapping of sediment by a borrow pit between Mulitefala and Amatuku islands and by a
channel on the lagoon reef flat of Amatuku. Both the borrow pit and the channel were
excavated during the Second World War.
It is recommended as follows:
• Fill the borrow pit between Mulitefala and Amatuku, and the fill channel on lagoon reef flat,
and replace with a pier extending from shore to the edge of the lagoon reef flat.
• Mining loose sediments should be prohibited.
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ACKNOWLEDGMENTS
Funds for this project were contributed by the Government of the People's Republic of China.
The work was carried out in collaboration with the Tuvalu Government. Particular assistance
was received from the Land and Survey Department, the Ministry of Natural Resources and
Tuvalu Maritime School. I am grateful for the assistance and valuable information from Mr
Kenneth J. Barnett, the captain superintendent of Tuvalu Maritime School.
Appreciation is extended to Mr S. Solomon for the useful comments on the drafts of the report
and Dr R. Howorth for his revision on the final manuscript.
INTRODUCTION
Amatuku is a small island of only 5.86 hectares (0.0586 km 2), located on the northeast rim of
Funafuti Atoll (Figure 1). It is a triangular island, more than 700 m long with the maximum
width of 180 m, extending northwest-southeast (Figure 2). Geographically, the Amatuku coast
is divided into five parts: the north ocean coast (from the northwest tip to the northeast point);
northeast ocean coast (from the northeast point to the southeast tip); northwest lagoon coast
(from the northwest tip to the channel); southwest lagoon coast (from the channel to the
southeast tip); and the coast behind the channel (Figure 3). Mulitefala Island is located to the
northwest and Tengako Island to the southeast; the reef flat connects both islands with
Amatuku.
Amatuku was occupied by US troops during the Second World War at which time major
changes were made to the coastal zone including the dredging of a borrow pit and a boat
channel.
The Tuvalu Maritime School for training seamen is located on Amatuku. Employment as
seamen on foreign merchant ships is a major source of income for Tuvaluans. Therefore,
although small, Amatuku Island is of importance.
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Figure 1 - Location of Amatuku.
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Figure 2 - Air photographs, taken on 7 July 1943 (left photo) and on 19 July 1984 (right
photo). The channel on the reef flat and borrow pit northwest of Amatuku are seen on the air
photograph of July 1984 but not on the air photograph of July 1943.
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Figure 3 - Map showing the channel on the lagoon reef flat, and the causeway and borrow pit
between Mulitefala and Amatuku in the Second World War (excavated or built in 1943 or the
first half of 1944). The outline of Amatuku is based on the air photographs of July 1943.
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STUDY METHODS
The field survey was conducted from 18 to 24 May, 1996 (6 working days).
A base map was traced from a 1984 air photograph (MNS 360, run 1, photo 1), which was
scaled using an October 1979 topographic map (scale 1:750). This photo was compared to
July 1943 air photos taken by the US military (VD1-5, photos 54, 56).
The reef flat and beach were mapped onto the 1984 air photo (1:4000). Observations were
also made of coastal processes at the time of the survey. Beach profiles were measured
using a level (type C3E) and tape measure.
The borrow pit dimensions were measured and bottom sediments described. Dimensions
were also measured on the 1979 topographic map.
Interviews with islanders were an important source of information on the timing of human
activities, especially causeway construction and borrow pit excavation during the Second
World War.
DEVELOPMENT HISTORY
There had been no residents on Amatuku before the Second World War. The US troops
arrived on Funafuti Atoll in October 1942 and developed Fongafale and Amatuku as forward
support bases. "Amatuku was developed as a supply depot with storage sheds, wharf, and a
small-boat harbour blasted out of the reef. Nearby Mulitefala served as a fuel dump."
(McQuarrie, 1994).
The US troops excavated the small-boat harbour as a channel, about 90 m wide and 4 m
deeper than the adjacent reef flat, extending to the base of the beach on the lagoon reef flat.
At the time of the survey rusty piles used as boat moorings and some relicts of trucks were
still evident on the lagoon reef. Mr Barnett (captain superintendent of the Tuvalu Maritime
School) said that this harbour was an American Patrol Torpedo (PT) boat base. It is possible
that this was both a supply depot and PT boat base, because Amatuku is close to the north
passage of the Funafuti Lagoon, through which boats can enter at any level of the tide. A
causeway connecting Mulitefala and Amatuku was built by excavating a 144 m long and 15 m
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wide borrow pit, which is adjacent to the site of the causeway on the ocean reef flat (Figure
3).
These coastal engineering activities occurred during the second half of 1943 or the first half of
1944, because there was no channel on the air photo taken on June 4 1943, and by the
middle of 1944, fighting in the Pacific had moved further north towards Japan and the
Americans began to withdraw from Tuvalu (Telavi, 1983). These activities greatly changed
the pattern of coastal processes both on ocean and lagoon sides, causing severe erosion.
.
The Tuvalu Maritime School was set up in 1979. Gravel, rubble and sand were collected from
both the beach and reef flat for building houses, concrete foundations for sport fields, and for
paving paths. This mining of the beach and reef flat caused further deterioration of the coastal
environment of Amatuku.
REEF FLAT AND BEACH
The reef flat is divided into two; the ocean side, and the lagoon side.
The ocean reef flat is commonly composed of five zones (from ocean to shore); spur and
groove, outer pavement, rubble and pavement, beachrock, and beach (Figure 4). There are a
few living corals on the outer pavement. The rubble and pavement zone is not continuous,
with 30-70% rubble (including a few big blocks) overlying the pavement. The beachrock is
breccia, extending from the base of the beach to the rubble and pavement zone and is very
wide on the ocean reef flat between the Mulitefala and Amatuku. A rubble area is also present
along the north shore of Amatuku.
The lagoon reef flat is composed of three zones (from lagoon to shore); branching coral,
pavement and beach. The branching coral zone is commonly 30 m wide, and living corals are
usually occurring on the outer part of this zone and dead coral on the inner part. The
pavement zone, occupies most of the lagoon reef flat. On the pavement rubble is scarce and
sand nearly absent except on the flat southeast of the artificial channel, where there is a
narrow rubble zone, and close to the base of the beach northwest of the channel, where reef
rubble excavated from the channel is found.
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Figure 4 - Coastal geological map of Amatuku, Funafuti Atoll, Tuvalu
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At the northwest tip of the island the remains of the causeway constructed during the Second
World War are still present in the form of a zone of blocks of reef rock ( 0.5-2 m diameter).
Smaller rubble has been washed onto the southwest of the block zone (Figure 5) and some
blocks have been transported to the lagoon flat southwest of the passage opening between
Mulitefala and the remains of the causeway (Figure 6).
Figure 5 - Remains of the causeway, connecting Amatuku and Mulitefala in the Second
World War, which was composed of big reef rock blocks (0.5-2 m). The northwest end of the
causeway(closest the camera) has been washed away. Smaller rubble has been washed
onto southwest of remains of the causeway. The sand beach in the foreground is at the
southeast end of Mulitefala. The dark blue colour of water left (ocean side) of the remains
indicates the long borrow pit parallel to the causeway. View 1350 from Mulitefala.
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Figure 6 - The opening (passage) between Mulitefala (right) and the remains of the causeway
(left), and reef rock blocks on the lagoon flat, moved by waves and currenst. View 2360.
The beach is divided into three types (Figure 7); sand beach, gravel beach and bare
beachrock.
The sand beach is distributed in limited areas. In the middle of north ocean coast, where a
gravelly sand beach is 24 m wide; and southwest of the student dormitory, where there is a
gravelly sand beach. Sand beaches were present on most of the north ocean coast and the
middle of the lagoon coast (around the jetty), but they are poorly defined on 1943 aerial
photographs.
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Figure 7 - Types of beach on Amatuku.
The gravel beach is subdivided into two subtypes. Subtype 'A' consists primarily of surficial
gravel. Subtype 'B' is composed of a gravel upper beach but the lower beach consists of bare
beachrock. Subtype 'A' is developed on most parts of the lagoon coast (Figure 8) and the
east part of the north ocean coast (Figure 9); and Subtype 'B' is present on most of the ocean
(Figure 10) and the southwest end of the lagoon coast.
Bare beachrock is distributed on the lagoon coast northwest of the jetty. The erosion scarp at
the upper edge of the beach consists of loose sediment or loose sediment overlying
beachrock. The entire beach surface is composed of bare beachrock (conglomerate and
sandstone) with scattered coarse gravel (Figure 11).
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Figure 8 - Beach profile (TU96204) on the stable coast, southwest lagoon coast.
Figure 9 - Beach profile (TU96202) on the erosional coast, east part of the north ocean coast.
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Figure 10 - Beach profile (TU96206) on erosional coast, the northeast ocean coast. The
location map and profile data are given in Appendix 1.
Figure 11 - Beach profile (TU96203) on erosional coast, northwest lagoon coast. The bare
beachrock demarcates the usual location of the beach. For location map and profile data are
given in Appendix 1.
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COASTAL EROSION
The most severe erosion has occurred on the north ocean coast (Figure 12). There was a
bow-shaped small peninsula with coconut trees and two small islets separated from the main
island by beach, present on the July 1943 air photo (Figure 2). These were eroded away
before 1979, since they were not shown on the 1979 topographic map and nor were they
present on the July 1984 aerial photograph (Figs. 2 and 3). Most buildings and the sport field
were built in 1979, as illustrated on the map which was surveyed in October 1979.
Figure 12 - Coastal erosion and dominant longshore sediment transport of Amatuku.
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Distances measured during the field survey from houses or concrete foundations to the
shoreline (vegetation line and top of the erosion scarp), indicate 2-3 m of shoreline retreat on
the north ocean shore and 6 m at the middle of the northwest lagoon shore from 1979 to 1996
(Figure 13).
Figure 13 - The shoreline change from 1979 to 1996, based on the October 1979 (dashed
lines) topographic map and bearings and distances between shoreline and houses or
concrete foundations measured during the May 1996 field survey.
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Erosion scarps and undercutting are common features on these coasts. The trees in front of
eastern most house shown on Figure 13 indicate the shoreline retreated at least 2-3.5m
(Figure 14). Other measurements of exposed tree roots or soil suggest erosion of at least 4.3
m elsewhere on the north ocean shore (Figure 15) and 4 m close to the present northwest tip
of the island (Figure 16). Assuming 4 m retreat along the 280 m north ocean coast and the
beach to be on average 1.8 m high, about 2000 m 3 of sediment has been lost.
Figure 14 - Erosion scarp of 0.6 m high and shoreline retreat of at least 2 m, at TU96202,
north ocean shore, Amatuku; View 1150.
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Figure 15 - A tree with landward extending roots, indicating 4.3 m shoreline retreat, north
ocean shore, Amatuku. View to southwest.
Figure 16 - Erosion scarp 1.3 m high on the north ocean shore, close to the northwest tip of
Amatuku, where the shoreline has retreated at least 4 m, based on distribution of soil among
gravel. View 1500.
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On the northeast ocean coast, a 0.5-1 m high erosion scarp is common. Some beachrock
with a white surface colour indicates that gravel has been removed in recent years (if the
beachrock is exposed for long periods, the surface colour become dark); Pandanus trees
indicate 2.2 m of shoreline retreat. However, the erosion on the northeast ocean coast is not
as severe as on the north ocean coast.
The position of the northwestern tip of Amatuku appears to have been relatively stable
between 1944 and 1979, and has retreated dramatically by as much as 45 m between 1979
and 1996 (Figure 13). This observation is based on the difference between the position of the
tip on 1979 map compared with at the time of the survey in May 1996. This inference is based
on the position of the northwestern tip of Amatuku compared with the southwest corner of the
borrow pit; (1) in 1944, (ii) as shown on the 1979 map, and (iii) as measured during the field
survey in May 1996. Erosion of the northwestern tip of Amatuku has occurred along both
sides (Figure 2) and eventually the link between the island and the remains of the causeway
(large blocks of reef rock) was breached. This permitted washover to occur through the
narrow gap on storm tides, accompanied by strong currents, which caused rapid erosion of
the unconsolidated island sediments. About ten scattered coconut tree trunks lay on the
lagoon reef flat in May 1996 (Figure 17) and are presumed to be the remains of the tip of the
island.
On the lagoon side, the coast is stable only in the middle of the southwest part of the
southwest lagoon coast (Figure 12) with eroding shorelines immediately adjacent. Severely
eroded coast is distributed along the northwest lagoon coast, the coast behind the channel
and the north part of the southwest lagoon coast. On the north part of northwest lagoon
coast, the erosion scarp is 0.6-1.5 m high (Figure 18). On its southwest part, nearly all of the
loose sediment has been washed away, leaving bare beachrock on the lower portion of the
scarp, with loose sediment commonly exposed on the upper portion of the erosion scarp
(Figure 19). There is almost no sand on the reef flat northwest of the channel. A 0.9 m high
erosion scarp is developed along the shoreline behind the channel, with gravel on the upper
part and beachrock on the lower (Figure 20). Large trees with roots extending landward
indicate at least 4.5 m of shoreline retreat. Abundant concrete blocks have been put on the
beach in front of the student dormitory to protect the shore on the north part of the southwest
lagoon coast (Figure 21). The southeast tip of Amatuku has also been eroded, there is a little
loose sediment on the beach (Figure 22).
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Figure 17 - Coconut tree trunks lying on lagoon reef flat, removed from the northwest tip of
Amatuku. View 2200.
Figure 18 - The northwest lagoon shore at the northwest end of Amatuku with erosion scarp of
0.5-1.5 m. View 1000.
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Figure 19 - The northwest lagoon shore near the jetty. The beach is composed almost
entirely of beachrock (conglomerate and sandstone) with little loose material. View 1100.
Figure 20 - Erosion scarp of 0.9 m high with gravel on top of the scarp and consolidated
breccia at the base. Location is on the shore behind the channel, southwest of the jetty. View
1100.
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Figure 21 - Two trees with landward extending roots, indicating 4.5 m of shoreline retreat on
the southwest lagoon shore. The beachrock exposed in front of the trees and concrete blocks
were put in front of the trees and the house (student domitory). View 00.
Figure 22 - Southeast tip of Amatuku, showing erosion. View 3250.
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BORROW PIT AND CHANNEL
In order to build the causeway between Mulitefela and Amatuku, a large borrow pit was
excavated in 1943 or the first half of 1944. It is 144 m long and 15 m wide. The southeast end
is close to 50 m northwest of the present northwest tip of Amatuku Island, and the opposite
end is 68 m southeast of the beach base and 85 m southeast of shoreline of Mulitefala Island
(Figs. 4 and 5). The present depths have been measured (Figure 23). The longitudinal section
shows that the pit shallows towards both ends, because of rubble accumulation. The floor of
the pit is highest towards the centre-northwest, suggesting that it might have been originally
shallower. The cross-section in most parts narrows towards the base. The rubble
accumulated on the ocean side slope in the pit came from ocean reef flat and the rubble on
lagoon side slope came from the causeway. Branching corals (the surface corals are living)
grow in the middle part. The deepest part in the pit is 3.5 m lower than the surrounding reef
flat. The original depths are inferred to have been 3.5-3.7 m and volume was about 6000 m 3
(Appendix 2). Sand is only distributed on the surface of the southeast part. The shape of the
pit and the sediment infill indicate that sediment from reef flat, causeway and shoreline have
been trapped in it. Roughly estimated, half of the borrow pit volume has been filled.
In order to land supplies on the island the US troops excavated a channel about 90 m wide,
extending from the lagoon reef edge to the base of the beach at middle of Amatuku lagoon
coast. Now, sand with some rubble is distributed on the floor of most of the channel,
extending to the northwest boundary; rubble was accumulated in the area close to landward
boundary. Branching corals grow on the bottom in the zone close to the southwest boundary,
where sand accumulation is much less than in the area along the northwest edge (Figure 4).
The cross-section is like that of a basin, becoming gradually shallower towards both
boundaries. The depths in the middle part are 3.7-4.3 m deeper than the surrounding reef flat
(Figure 24).
Funafuti is in the trade wind belt in the South Pacific. A wind rose reproduced from Carter
(1986) is based on the wind record from 1950 to 1984 (Figure 25) and illustrates the
dominance of the easterly trades (34%). Winds rarely exceed Beaufort force 7 or, 28 to 33
knots. April through October are the months when the stronger east and southeast winds
dominate. During December through March stronger west and northwest winds appear
(Carter, 1986). Under natural conditions prior to the 1943 excavations, the loose sediments
moved back and forth, northwest transport dominating on the ocean side and southeast
transport dominating on the lagoon side (Figure 12). However, the dynamic equilibrium of
sediment transport was disrupted by the construction of the borrow pit between Mulitefala and
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Amatuku and the channel on lagoon reef flat adjacent to Amatuku. Both act to trap sediments
being transported by waves and longshore currents.
Figure 23 - The present configuration of the borrow pit between Mulitefala and Amatuku
excavated in the Second World War measured and observed on 24 May 1996. The upper
diagram is a map showing sediment distribution; R-rubble; S-sand; Cr-branching coral. The
middle diagram shows water depths of the pit along the centre line (lower than the reef flat).
The lower diagram is a cross- section of the pit. Measured and observed on 24 May 1996.
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Figure 24 - The depths (lower than the reef flat) of the channel on lagoon reef flat of Amatuku,
measured on 24 May 1996.
Figure 25 - Funafuti wind data for 1950-1984 (after Carter, 1986).
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During and after construction the borrow pit acted as a trap for sediments being transported
from the ocean shore and reef flat. Ocean waves driven by the predominantly easterly winds
would transport sediments driven from the ocean coast and by northeasterly winds transport
from north ocean coast into the pit, where the sediments were effectively removed from the
sediment pathway. The result of the sediment trapping by the pit was a dramatic increase in
erosion, especially along the north ocean coast. In comparison with the north ocean coast,
the northeast ocean coast can still receive some sediment from the northeast point
transported by north, northwest and northeast waves, so the coastal erosion is not so severe
as on the north ocean coast. Sand was transported initially, but rubble was deposited at a
later stage.
Based on direction and strength of winds, most of the sand deposited in the channel blasted
on the lagoon reef flat was driven from the northwest lagoon coast beach and reef flat. A
narrow branching coral zone is distributed on the reef flat at the northwest edge of the
channel, connecting the coral zone along the reef flat edge. Gravel in the channel was
transported from the northwest coast by waves and longshore currents, and from the coast
behind the channel by backwash, where it accumulated in the area close to the northeast
end. The southwest lagoon coast close to the channel was deprived of sediment supply from
the northwest and occasionally served as a sediment supply to the channel during south and
southeast winds. Therefore the coast from the northwest tip of the island to southeast of the
channel (the part close to the channel) is severely eroded. The middle of the southern part of
the southwest lagoon coast still maintains in dynamic equilibrium and has become a stable
coast, in contrast to the erosional coasts on either side.
Furthermore, removal of gravel, rubble and sand for building houses, concrete foundation,
and paving paths has increased erosion. However, this action has had much less dramatic
effect than the construction of the borrow pit and the channel.
In the 50 years, since the Second World War, initial high rates of erosion removed most
sand, fine gravel and rubble from the beaches. Erosion rates decreased (except at the
northwest tip), because most remaining gravel and rubble were too coarse to be removed in
common wave energy and the bare beachrock resisted further erosion. However, the erosion
on the ocean coast will proceed albeit at reduced rate, until the borrow pit is filled up. Erosion
on the lagoon coast will proceed indefinitely, because wave set up on either side of the
channel induces rip currents within the lower channel, which transport sand off the beach,
especially when the channel becomes shallow.
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COASTAL MANAGEMENT
In order to decrease erosion, the borrow pit and channel must be filled and beach mining
must be stopped. The borrow pit can be filled with the remains of the causeway. The method
for filling should be considered carefully since removal of the large blocks first could result in
loss of the smaller rubble by washover. It is suggested that filling begin at the ends and
proceed towards the middle of the pit.
The channel should be filled and replaced by a pier (timber deck on piles) to allow boats to
land in the deeper water of the edge of the lagoon reef flat. There is no local material to fill the
channel except material left over from filling the pit.
On Fongafale, wartime activities have caused similar erosion problems, however, the
construction was only on the lagoon side and erosion only occurred on the lagoon coast (Xue
and Malologa, 1995). Since Amatuku is an isolated island with little infrastructure, it could
serve as an experiment in coastal management and provide a trial area for Fongafale
restoration work.
CONCLUSIONS
• The ocean reef flat is composed of five zones: spur and groove, outer pavement, rubble
and pavement, lower beachrock, and beach.
• The lagoon reef flat is composed of three zones: branching coral, pavement, and beach.
• The beach includes three types: sand beach, gravel beach (further divided into two
subtypes: gravel beach, and gravel on the upper beach with beachrock below), and bare
beachrock on the common beach face.
• Most part of the coast has been eroded. The most severe erosion occurred on the north
ocean coast. Since 1944, one small peninsula and two small islets have been eroded
away, and the shoreline retreated at least 4.3 m. The severe erosion also occurred on the
northwest, southwest lagoon coasts and the portion of coast behind the channel.
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• On the ocean coast, erosion is mainly caused by trapping of sediment by the borrow pit
between Mulitefala and Amatuku excavated during the Second World War.
• The lagoon coast erosion is caused primarily by trapping and diversion of sediment by the
channel excavated during the Second World War.
RECOMMENDATIONS
• It is recommended that the borrow pit between Mulitefala and Amatuku be filled.
• It is recommended that the channel on the lagoon reef flat be filled and a pier (timber or
concrete deck on piles) extending from shore to the edge of the lagoon reef flat be built.
• Mining loose sediments should be prohibited.
REFERENCES
Carter R., 1986. Wind and Sea Analysis, Funafuti Lagoon, Tuvalu. SOPAC Technical
Report 58.
McQuarrie P., 1994. Strategic Atolls, Tuvalu and the Second World War. Macmillan Brown
Center for Pacific Studies, University of Canterbury and Institute of Pacific Studies,
University of the South Pacific.
Telavi, M., 1983. War. In Laracy Hugh (editor): Tuvalu, a History. Institute of Pacific
Studies and Extension Services, University of the South Pacific.
Xue C. and Malologa, F., 1995. Coastal Sedimentation and Coastal
Management of Fongafale , Funafuti Atoll, Tuvalu. SOPAC Technical Report 221.
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APPENDIX 1
Beach profile measuring data and location (Only the following two profiles have permanent
base stations)
1. Profile TU96206
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APPENDIX 2
1. Calculation of original volume of the borrow pit
Length of the top of the borrow pit: 144 m (measured); length of bottom of the borrow pit: 140
m (inferred); average length: 142 m
Width of the top of the borrow pit: 15 m (measured); width of bottom of the borrow pit: 11 m
(inferred); average width: 13 m
Volumes of separated parts
The central-north higher part 12x13x1=156 (m3)
The north higher part 5x13x1.8=117 (m3)
The rest (most) part (142-12-5)x13x(3.5+3.7/2)=5850 (m3)
Total 156+117+5850=6123 (m3)
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