COASTAL SEDIMENTATION, EROSION AND MANAGEMENT OF SOUTHWEST ATOLL,

Chunting Xue SOPAC Secretariat

September 1996 SOPAC Technical Report 238

This project was funded by the Government of the People's Republic of China [3]

TABLE OF CONTENTS

Page

SUMMARY ...... 5

ACKNOWLEDGMENTS...... 6

INTRODUCTION ...... 7

STUDY METHODS...... 8

COASTAL GEOLOGY...... 9

SEDIMENT SUPPLY AND TRANSPORTATION ...... 17

COASTAL EROSION, ACCUMULATION AND CAUSES ...... 22

CURRENT MEASUREMENTS IN THE CHANNEL ...... 30

CONCLUSIONS ...... 37

RECOMMENDATIONS FOR COASTAL MANAGEMENT...... 38

REFERENCES...... 40

APPENDIX Beach Profile Data...... 40

ENCLOSURE Xue, C. (1996) Coastal Geology of Tuvalu - Southwest Nukufetau . SOPAC Coastal Series Map 8. (Inside back cover).

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LIST OF FIGURES

Figure 1 Location of the investigation area ...... 8 2 1943 air photo of the southwest corner of Nukufetau Atoll...... 10 3 1984 air photo of the southwest corner of Nukufetau Atoll...... 11 4 Breccia and rubble ridges on the reef flat northwest of Savave ...... 13 5 Reef crest rubble with a large block of reef rock (6 m long, 1.8-2.4 m high), ocean reef flat, south of Temotuloto ...... 13 6 Lagoon reef flat pavement with sand beach in background, northeast coast of Savave ...... 14 7 Tidal channel on the reef flat between Savave and Fale ...... 15 8 wind data ...... 18 9 Dominant sediment transport...... 18 10 Sand accumulation at the north corner of Savave...... 19 11 Sand accumulation at the southwest end of the concrete blocks paved seawall, northeast coast Savave ...... 19 12 Sand deposition at the corner between two seawalls, northwest coast, Savave ...... 20 13 The sand beach at the southwest end of Savave showing sediment transport towards the south ...... 20 14 Prograding shore with wider sand beach and reef flat sand in front of it, at the point west of the Hospital, Savave ...... 21 15 Summary of coastal stability of the southwest Nukufetau Atoll...... 22 16 Historical change of the shoreline southwest of the present artificial channel (passage), lagoon coast, Savave ...... 23 17 Erosional shore with 1 m erosion scarp, close to the southwest end of the artificial channel, Savave ...... 24 18 Sediment distribution at the southwest end of the artificial channel, Savave...... 25 19 The 1.6 m high erosion scarp with marked undercutting, northwest ocean prograding shore of Fale ...... 26 20 The west ocean prograding shore of Fale ...... 27 21 A cross section of the prograding shore at the north point of Fale...... 27 22 Prograding shore at the north point of Fale...... 28 23 Prograding shore at the east corner of Fale ...... 28 24 Profile of the prograding coast at the north point of Fale...... 29 25 Prograding shore at the northeast point of Fale...... 31 26 Erosional shore with erosion scarp ...... 31 27 Location of current measuring site and directions of water moving on reef flat in falling and rising tides ...... 33 28 The current speed, direction and water depths measured in the channel...... 34 29 The channel at the beginning of the stage 2...... 35 30 The channel in the middle of stage 2 ...... 35 31 The channel in the stage 3, close to lowest tide...... 36 32 Schematic diagram of the recommended wharf and pier...... 38

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SUMMARY

Field investigations on southwest Nukufetau Atoll were made from 23 April to 8 May, 1996. Observations on the coast and reef flat of Savave, Fale and Temotuloto and three small adjacent islets were made and beach profiles were measured. The bottom currents in the artificial channel were also determined over one tidal cycle.

The principal conclusions are as follows: · The reef flat can be divided into five areas. There are reef flat ridges on the reef flat northwest of Savave and south of Savave and Temotuloto. · Erosion is occurring on Fale, but the coast is little influenced by human activities. · Savave is an area of sand accumulation. It received sediment from reef flat and the lagoon and is protected by Fale to the southwest. · Erosion on the northwest ocean coast of Savave was caused by mining of the beach and reef flat. · Erosion on the lagoon coast of Savave was caused by the artificial channel, which not only traps sediment but also transports sediment to the lagoon. · The high speed lagoonward ebb currents in the artificial channel last much longer than the high flood currents in the opposite direction. Therefore the net sand transport in the channel is always towards the lagoon.

The following are recommended: · Stop mining gravel and sand from the coast of Fale, Temotuloto and three small islets and mine only from the inland parts of Fale. · Fill part of the artificial channel (passage) at the nearshore end and build a pier in order to avoid interfering with longshore movement of sand.

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ACKNOWLEDGEMENTS

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 Council of Nukufetau. I am grateful for the assistance and valuable information from Mr Meauma Maeaga. Appreciation is extended to Mr S. Solomon and Mr R. Howorth for their useful comments on the drafts of the report.

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INTRODUCTION

Nukufetau Atoll (8o 00’S; 178o 23’E) is situated in the centre of the group of nine atolls aligned NW - SE that make up Tuvalu. Its nearest neighbour is 67 km to the northeast, while the capital, Funafuti, is some 167 km to the southeast. The atoll of Nukufetau is rectangular; its long axis oriented NE-SW is 14 km long, whilst its width is 8.25 km (Figure 1). Within the outer reef edge perimeter, there is a total area of approximately 116.5 km2, of which 22 % comprises the reef platform and 78 % the enclosed lagoon. The bulk of the reef platform consists of reef flat (85 %), the vegetated islands and adjacent beaches account for the remaining 15 % (McLean et al., 1991). Southwest Nukufetau comprises four islands; Savave, Fale, Temotuloto and Motumua, (Table 1), together with four small islets; Teafuatakalau, Teafuasaufili, Teafuafalenui and Teafualoto.

Table 1. Areas and percentages of the four largest islands (including beach) of southwest Nukufetau (after McLean et al., 1991).

Island Area (km2) Percentage of the total land area of Nukufetau

Savave 0.2128 6.43 Fale 0.3595 10.86 Temotuloto 0.0594 1.79 Motumua 0.1069 3.23

According to Lafita (1983), the people lived on three islands: Lafanga, Motulalo (situated at the northeast and southeast corners of the atoll) and Fale before the arrival of the missionaries (in the middle of 19th century, Kofe, 1983). In later years, the whole population reassembled at Fale, before shifting to nearby Savave. The 1983 census was 739. Now almost all of residents live on Savave, only one family on Motulalo and two families on Tematuloto.

During the Second World War, Nukufetau was one of three bases for American troops in Tuvalu. However, the engineering activities were limited to the Motulalo Island, where a long airfield and a jetty were constructed. In comparison with Funafuti, there was very little reef blasting

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(McQuarrie, 1994). Since only one family now lives on Motulalo Island, there is little concern over its coastal environment.

Figure 1. Location of the investigation area.

The field survey described in this report was conducted from 23 April to 8 May, 1996, on Savave, Fale and Temotuloto Islands and reef flat at the southwest corner of Nukufetau.

STUDY METHODS

The reef flat and beach were mapped onto a computer-drawn base map from a 1:25000 topographic map published in 1979 and which was based on 1971 air photography by the Department of Lands, Mines and Survey of Fiji, with additional information supplied in 1974 and

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published in 1979 by the Directorate of Overseas Survey, Ministry of Overseas Development, British Government. The shoreline on the geological map produced during this study has been altered to incorporate obvious change compared with a more recent 1984 air photo and the present field survey. The 1984 air photo was compared with a 1943 air photo (Figures 2 and 3). Beach profiles were measured using a level (model C3E) and tape measure. Observations of coastal processes were made at the time of the survey.

Interviews with islanders were an important source on the timing of human activities, especially the construction of the artificial channel, seawalls, and sediment transport by cyclones.

Bottom currents were measured in the artificial channel for 11 hours and 14 minutes with a current meter (model SD-2). This counts the revolutions of the rotor during a 50 seconds sampling period and gives directly the average speed in cm/s during this period. The Savonius rotor needs a 2-3 cm/s current speed to overcome friction. The current direction is measured at the beginning, middle and end of the 50 seconds sample period. The compass accuracy is ±10 degrees.

COASTAL GEOLOGY

The geomorphology and sediment distribution are shown on the coastal geology map of southwest Nukufetau (Xue, 1996) and a copy of which is included with the report (enclosure). For convenience and discussion the reef flat is divided into five areas.

The ocean reef flat west and south of Fale

This area is composed of four zones (from ocean to shore): a shallow spur and groove zone, the survey only examined the inner boundary of this zone; a pavement zone occupied most of the area with rubble scattered on its surface; a beachrock zone consisting of cemented breccia; and a beach zone composed of gravel or rubble. In addition, a limited patch of rubble occurred near the southwest extremity of the atoll reef rim.

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Figure 2. 1943 air photo of the southwest corner of Nukufetau Atoll.

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Figure 3. 1984 air photo of the southwest corner of Nukufetau Atoll.

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The reef flat northwest of Savave

This area is composed of six zones: a spur and groove zone; an outer pavement zone; a zone of pavement alternating with reef flat ridges; an inner pavement zone; a beach zone, and an algae and coral zone. Some living and recently dead corals occur on the outer pavement. In the zone of pavement alternating with reef flat ridges, there are nine ridges which are streamlined, extending and narrowing landward or lagoonward, extending in one case to the shore. These ridges are usually 0.5-1 m higher than the surrounding reef flat pavement and consist of cemented breccia with a covering of rubble mostly on the lagoonward sections or in the middle (Figure 4). This rubble cover coalesces in the northernmost group of ridges. The reef flat ridges occupy about half of this zone. The breccia was formed from cemented older reef flat rubble, which was transported from the ocean by waves. This material was then partly covered by rubble which was later removed. Scattered rubble is found on the inner pavement. The beach zone is gravel on the north shore of Fale and mainly sand on the northwest shore of Savave. The latter is now behind a seawall. The narrow algae and coral zone marks the lagoon reef edge.

The ocean reef flat south of Savave and Temotuloto

This area is composed of five zones: a spur and groove zone; and outer pavement zone; a reef crest rubble zone; a zone of pavement alternating with reef flat ridges; and beach. The reef crest rubble zone is higher than the adjacent pavement and consists reef rock blocks and rubble. Some of the blocks are very large, more than 2-3 m in dimension (Figure 5). There are five reef flat breccia and/or rubble ridges extending lagoonward. These ridges have been present for a considerable amount of time. However in 1972 Cyclone Bebe increased the amount of the rubble covering.

Lagoon reef flat northeast of Savave and Temotuloto

In the northwest part this area is composed of four zones (from lagoon to land): an algae and coral zone; a pavement zone; a reef flat sand zone; and a beach zone. The algae and coral zone is 20-30 m wide and gradually deepens towards the lagoon from the pavement zone. There are

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algae (Halimeda) and coral (living and dead) with bare reef rock in this zone. Seaward of this zone is an area of reef sand with patch reef, which is separated from the algae and coral zone

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Figure 4. Breccia and rubble ridges on the reef flat northwest of Savave. View to 300o.

Figure 5. Reef crest rubble with a large block of reef rock (6 m long, 1.8-2.4 m high), ocean reef flat, south of Temotuloto. View to ocean.

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with an approximately 1 m high break of slope. The pavement zone is characterised by scattered rubble with a little sand, and abundant sessile benthic bivalves which live on the surface. The reef flat sand zone is a hundred metres wide (and covered with a 25-30 cm thickness of sand. A sand beach occurs along the northeast lagoon coast of Savave (Figure 6).

The reef flat, from the southeast corner of Savave to northeast of Temotuloto is more complicated than the northwest part. The outer algae and coral zone changed to sand and coral zone, which is present on the northeast of Temotuloto. The sand is more than 30 cm thick and comprises 60% or more of the area. This zone gradually slopes down to the lagoon, with no break of slope. A sand beach is present along most of the beach zone. An outer pavement zone and an inner sandstone beachrock zone are present between the beach of Temotuloto and the algae and coral zone. A pavement zone east of Temotuloto extends from the reef crest rubble zone on the ocean side to the algae and coral zone on the lagoon side. Another pavement area

Figure 6. Lagoon reef flat pavement with sand beach in background, northeast coast of Savave. View to 205o.

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extends from the ocean reef crest towards the northeast (between Savave and Temotuloto) but does not reach the sand and coral zone. There is a unique area composed of rubble overlying sand north of Temotuloto. In addition, sand and rubble are distributed to the east of that pavement area.

The reef flat between Savave and Fale

The reef flat is 0.7-0.8 m higher than the lagoon reef flat (based on measurements of the reef flat surface southwest of the Council Office compared with the lagoon reef flat surface northeast of the Council Office). The highest part is on the flat close to Teafuasaufili and Teafuatakalau, which is 0.42 m higher than the surface between the west coast of Savave and the northeast

Figure 7. Tidal channel on the reef flat between Savave and Fale. The relative heights are marked at some places of the tidal channel.

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corner of Fale. A curved natural tidal channel has formed on this reef flat and extends onto the ocean reef flat to the northwest, the end of the channel in the spur and groove zone becomes a small fishing boat passage (Figure 7). Most of the channel bottom is pavement. With the exception of this tidal channel, the reef flat is composed of reef flat sand, patches of cemented breccia or sandstone distributed NW -SE and pavement.

The beach zone can be divided into two types; sand beach, and gravel beach. The sand beach (including gravelly sand beach) is distributed on most parts of Savave, the northeast coast of Fale, and the northeast lagoon coast and the north end of the west coast of Temotuloto. The sand beach is 20 m wide on the north, lagoon shore of Savave (Appendix 1-1 and 1-2). On the south coast of Savave the sand beach is commonly mixed with some gravel or gravelly sand beach and is only a few meters wide (Appendix 1-3). Here the reef flat is higher than the lagoon reef flat and the loose beach sediment layer is thinner.

The gravel beach (including rubble beach) is mainly distributed on the coast of Fale and Temotuloto and the three islets: Teafuasaufili, Teafuatakalau and Teafuafalenui.

SEDIMENT SUPPLY AND TRANSPORTATION

The main sediment supply sources to the reef flat and beach are from the ocean reef, the lagoon reef, and the reef flat itself.

Sediment transport from the ocean reef to the reef flat occurs mainly during cyclones. The streamlined reef flat ridges clearly indicate the sediment transport route from ocean towards the lagoon. The present reef crest rubble zone along the southern ocean coast was mainly formed by Cyclone Bebe in 1972. On the reef flat close to the south part of the east coast of Fale, there was a natural passage to the ocean from Savave which allowed access for small fishing boats before 1972 (based on the information from local residents and also shown on the 1943 air photo). This passage was filled by rubble during Cyclone Bebe. The rubble covering the sand layer on the reef flat north of Temotuloto was also mainly transported by Cyclone Bebe. The original rubble of the reef flat crest rubble zone and on the rest of reef flat was also partly moved landward or lagoonward. In addition, the prograding beach on the south coast, which is composed of rubble not gravel, resulted from the rubble migration from the reef flat.

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There is a 1 m high break of slope along most of the lagoon reef edge, and the waves in the lagoon are much weaker than in the ocean. Therefore, it is usually difficult for sand from the lagoon to be transported onto the reef flat. However, the sand and coral zone northeast of Temutoloto slopes from the reef flat towards the lagoon, so lagoon sand could be moved into this zone and onto the reef flat and beach during storms with east and northeasterly winds. The area of sand on the reef flat before 1972 was probably larger than present, since part of the sand area was covered by rubble transported from the ocean by storm waves.

Sediment derived from Algae (Halimeda) and coral living in the outer zone of the lagoon reef flat is transported landward. A few coral fragments are also transported from the outer pavement onto the ocean reef flat. Foraminiferal tests are one of the dominant sand sources, especially on the lagoon coasts. Sessile benthic bivalves are common on the surface of the pavement but usually do not easily break up. Small gastropod shells also contribute a small proportion of the sediment.

The reef flat between Fale and Savave receives sediments both from the ocean to the southeast and from the lagoon reef flat and lagoon to the north. As a result, the reef flat between the two islands is higher than the adjacent lagoon and ocean reef flats.

The winds control the dominant sediment transport. Nukufetau is in the tradewind belt of the South Pacific. There are no weather records for Nukufetau, however, the climate in Nukufetau is very similar to nearby Funafuti. A wind rose for Funafuti from 1950 to 1984 (Figure 8; Carter, 1986) illustrates the dominance of the easterly trades (34%). Winds rarely exceed Beaufort force 7, (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 occur.

The dominant longshore sediment transport is northwestward along the northeast lagoon coast of Savave because of the dominant easterly winds (Figures 9 and 10) and to the southwest along the northwest coast because of a northerly wind. Sand has accumulated at the intersection of two seawalls at the southwest end of the northwest coast of Savave (Figures 11 and 12). A sand ridge extends to the northwest of the intersection of the seawalls because of the effects of the ebb tidal current. Under natural conditions the north and northwest corners of Savave were prograding as shown on the 1943 air photo (Figure 2). At the southeast corner of Savave close to the Hospital, the dominant longshore sediment transport is southerly as shown by the shape of the coast (Figure 13) and then to the west along the south coast (Enclosure, and Figure 14).

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Figure 8. Funafuti wind data (after Carter, R. 1986).

Figure 9. Dominant sediment transport directions shown by arrows.

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Figure 10. Sand accumulation at the north corner of Savave because the concrete blocks obstruct the northwest longshore sediment transport.

Figure 11. Sand accumulation at the southwest end of the concrete blocks paved seawall because the gabion basket seawall stops the southwest longshore sediment transport northwest coast, Savave. Cf. Figure 13.

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Figure 12. Sand deposition between two seawalls at the corner, northwest coast, Savave. The short one extends to northwest and the long one to northeast, indicating the southwest longshore sediment transport. A breccia and rubble ridge is in the foreground. View to 200o.

Figure 13. The sand beach at the southwest end of Savave, showing sediment transport towards the south, and the sand accumulation area with bushes lower than old land and the top of the seawall.

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Figure 14. Prograding shore with wide sand beach and reef flat sand in front of it, at the point west of the Hospital, Savave. Bushes grow on the new land in front of coconut trees. View to 115o.

COASTAL EROSION, ACCUMULATION AND CAUSES

On the relatively untouched west and south coast of Fale, both erosion and accumulation occur, maintaining a dynamic equilibrium. Only those coasts which have been greatly influenced by human activities has severe erosion occurred.

Savave

Continuous erosion has occurred on the west coast (south of the southwest end of the long seawall), the northwest ocean coast (now behind a concrete blocks paved seawall) and the northwest part of the northeast lagoon coast (Figure 15). Before the end of the Second World War, a good sand beach was developed along the northwest ocean coast, as shown on the 1943 air photo and sand with a smooth surface was present on the reef flat in front of the beach (information provided by local elder). Now the sand beach is behind the seawall, with a 0.4-0.6 m erosion scarp at the old shoreline. The sand on the reef flat has been almost completely washed away and bare pavement extends to the base of the seawall.

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Figure 15. Summary of coastal stability of the southwest Nukufetau Atoll.

After the Second World War, especially from 1960s, the local residents built more and more houses with high (0.5-1 m) foundations. Sand, rubble and massive coral stone (for making lime) were removed from the ocean beach and reef flat west of Savave. The second reef flat ridge from south in 1984 was much smaller than in 1943 (Figures 2 and 3), because of the removal of rubble. This activity resulted in stronger wave action on the reef flat and erosion of the ocean coast. A gabion basket seawall was built on the southwest part of the ocean coast in 1978. This was subsequently replaced with the present concrete block seawall, and at the same time extended to the north corner of Savave (Figure 15).

There was no erosion on the northeast lagoon coast of Savave before 1981. An artificial channel (passage) was dredged in 1981 for small fishing boat access. It is 550 m long, 7 m wide and more than 2 m deep, extending from the lagoon reef flat edge across the reef flat to a position within 36 m of the 1981 shoreline. Even before construction was complete, the channel began

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causing severe erosion on the northwest section of the lagoon coast. Based on information from Mr Meauma Maeaga, (whose house is close by), 35 m of coastline retreat occurred on the coast facing the channel (Figure 16). A 1 m high erosion scarp is common along the shoreline and the simple sand bag seawall has been removed by people three times (Figure 17). Towards the southeast the erosion is less severe. At NFTB1, the shoreline has retreated 0.7 m and some beach sand eroded since 1989 (Appendix 1-1).

Figure 16. Historical change of the shoreline southwest of the present artificial channel (passage), lagoon coast, Savave. The shorelines before the Second World War (WW II) and in 1980 are based on the information from Mr Meauma Maeaga. The coconut trees between the shorelines before WW II and present are younger than those towards the land.

Immediately after completion of the artificial channel most of the bottom was 2 m lower than the surrounding reef flat, now it is only 0.5 m lower than reef flat. In the outer part of the channel, the depth was more than 3 m below the reef flat in 1981 and at the time of the survey was only 1-1.4 m. Sand with some rubble has been deposited in the channel; the channel has not only trapped sand transported by northwest longshore currents and waves, but also transported it into the lagoon. Sand movement in the channel is discussed below.

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Figure 17. Erosional shore with 1 m erosion scarp, close to the southwest end of the artificial channel, Savave, View to northwest.

During the Second World War, the southeast corner of Savave was very sharp (Figure 2). This extended sharp point was cut by waves during Cyclone Bebe in 1972 (Figure 3). After the artificial channel was built in 1981, the erosion along the southeast section of the lagoon coast became severe. A seawall was built 15-30 m in front of the old shoreline with an erosion scarp in 1989. Since there was an opening at the southeast between the seawall and the old shoreline, sand was transported in and deposited. Bushes and a few very young coconut trees have grown there, even though this area is lower than the surrounding areas (Figure 13).

The fact that middle part of the lagoon coast appears to be stable might be due to sediment being supplied from both the northwest and southeast. The distance between the classrooms at the Primary School and the present coastline is 9.4 m, which is almost the same as the distance measured from the 1984 air photo. Only at NHTB2 (a beach profile base station northwest of the classroom), after the coast was eroded and curved landward, has the coastline prograded 3.8 m since 1989 (Appendix 1-2). Though the middle part of the lagoon coast appears to have been stable in recent years, the reef flat sand zone is narrower than that before 1981 (based on the

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local resident information). The influence of the artificial channel is very evident in that the reef flat sand close to the southwest end of the channel is narrower (Figure 18).

Erosion has also occurred on some sections of the south and west coast of Savave. This might have been caused by beach mining. Since the reef flat between Savave and Fale is higher than the adjacent lagoon and ocean reef flats, the beach sediment thickness is less than 1 m. Therefore a little sediment removal can result in shoreline retreat; for example, the coastline has retreated 6 m on the coast southwest of the school.

Prograding coasts are present at the point west of the Hospital (Figures 2, 3 and 14) and the point southwest of the Church. A low rubble mound breakwater built for protecting a rubbish dump on the south shore of Savave before the Second World War is a successful example of coastal engineering. Bananas have grown on the land reclaimed.

Savave is a very good example of an island formed by sand accumulation. Almost all of the land soil is composed of sand with a little gravel. Savave is protected by Fale to the southwest, and there are wide reef flats to the northwest and northeast. Lagoon sand especially can be moved across the reef flat on the east side. The earliest residents settled on Fale, and the history of Savave Village is only a little over one hundred years (Lafita, 1983). The formation of Savave may therefore have been later than that of Fale and it is inferred that as recently as a few several hundred years ago.

Figure 18. Sediment distribution at the southwest end of the artificial channel (passage), Savave.

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Fale

The south and west coasts of Fale are little influenced by human activities. The south beach is composed of rubble because the rubble was moved from the reef flat to the coast in recent years by natural processes (Cyclone Bebe). Half of the south coast is prograding (Figures 15). However, most of the west coast has suffered erosion (Figure 19) and gravel has been transported towards the north, forming a prograding coast on the northern part (Figure 20) and at the north corner, which has extended 42.5 m from the old coastline (Figures 21 and 22). Most parts of the east coast of Fale have prograded 3-15 m and even more at the east corner close to Teafuasaufili. Gravelly sand has accumulated at the corner even though people have mined the beaches (Figure 23). These sediments were mainly transported from the ocean reef flat in 1972 by Cyclone Bebe during which waves moved rubble onto the reef flat and which later gradually migrated to the coast. The other prograding coast is at the northeast corner of Fale, forming a gently sloping gravely sand beach. The coastline has prograded 38 m in the last ten years (Figures 24 and 25). These accumulated sediments were transported mainly from the northeast by the longshore currents and waves and partly by the ebb currents from the reef flat between Savave and Fale.

Figure 19. The 1.6 m high erosion scarp with marked undercutting, northwest ocean prograding shore of Fale. View to 125o.

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Figure 20. The west ocean prograding shore of Fale. The creepers are growing in front of coconut tree vegetation, covering a gravel beach berm. View to northeast.

Figure 21. A cross-section of the prograding coast at the north point of Fale. The coastline has prograded 42.5 m from the old coastline with coconut trees to the new berm crest. The distance was measured with tape and the slope were estimated. Cf Figure 23.

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Figure 22. Prograding shore at the north point of Fale. View to west.

Figure 23. Prograding shore at the east corner of Fale (close to Teafuasaufili Islet). View to southwest.

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Figure 24. Profile (bearing 075o) of the prograding shore at the north point of Fale. There are old coconut trees of the 0 m point. The berm (0-38 m) was formed in about the last ten years. Cf Figure 26.

The erosional sections on the northeast coast (facing Savave) have been caused by mining, the most severe effects are on the north coast. Mining gravel has caused the shoreline to retreat 16 m since 1974 (Figure 26).

Temotuloto and three islets

The lagoon coast, the north section of the west coast, and the east of the south ocean coast of Temotuloto are eroded. The ruins of a coconut trunk indicates that the shoreline has retreated 5 m on the lagoon coast. Erosion scarps 0.4-0.9 m high on the north part of the west coast are common. Erosion scarps also occur on the north and west coasts of Teafuafalenui which is

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smaller than it was in 1943. Presently Teafuasaufili is bigger than in 1943 but the north, east and south shores were eroded. No evidence of erosion was seen on Teafuatakalau. Nukufetau Council has already prohibited mining on the coast of Savave. Local residents mine sand and gravel from Temotuloto, Fale and the three islets. Mining is the main cause of erosion on Temotuloto and other small islets.

CURRENT MEASUREMENTS IN THE CHANNEL

Bottom currents were measured from 0745 h to 1800 h (11 hour and 14 minutes) on 7 May 1996 at a site 156-170 m northeast of the slipway (or 202-216 m northeast of the present shoreline) in the artificial channel which extends outward from the shore at 045o (Table 2; Figures 27 and 28). Tides at Nukufetau are similar to those at Funafuti, which are semidiurnal with two highs and two lows of approximately equal amplitude. According to the tidal tables for Funafuti, the high tides occurred at 7:26 and 20:04 and low tides occurred at 1:11 and 13:56. Thus the start of the measurements was a little after high tide and finished two hours before the next high tide. The change of current is divided into six stages.

Stage 1, from 7:45 to 11:39 was characterized by currents of less than 20 cm/s and the directions were towards the northeast and variable. During stage 2 (the latter part of the ebb tide), from 11:39 to 13:40 for 1 hour 59 minutes currents stayed northeast and velocities rapidly increased from 20 cm/s to 68 cm/s and then reduced to 20 cm/s. In this stage, from 12:15 to 13:28 for 1 hour 13 minutes the bottom currents were more than 40 cm/s and moved bottom sand towards the lagoon. Stage 3, from 13:40 to 15:15 was characterized by northeast currents of less than 20 cm/s. Stage 4, from 15:15 to 15:57 was characterized by southwest moving currents with velocities of less than 20 cm/s. During Stage 5, from 15:57 to 16:54 for 57 minutes the southwest currents increased rapidly from 20 cm/s to 66 cm/s then decreased. The currents maintained a velocity of more than 40 cm/s for about 31 minutes, when the bottom sand was clearly moved landward. During stage 6 (end of the flood), from 16:54 to 18:00 inferred to extend to 18:04 (high tide). The currents were less than 20 cm/s and most of the time less than 10 cm/s with variable current directions.

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Figure 25. Prograding shore at the northeast point of Fale. View to north.

Figure 26. Erosional shore with erosion scarp. The shoreline has retreated 16 m since 1974, based on the information of Mr Meauma Maega, who was standing on the beachrock (breccia), which marked the position of the 1974 shoreline. The remains of a coconut trunk is 8.5 m in front of the present shoreline. North shore, Fale Island, Nukufetau Atoll, view to 150o.

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Table 2. The current speed and direction in the artificial channel, tidal level, and water depths.

The currents in the artificial channel are tidal currents. The currents in stages 1-3 are ebb currents and those in stages 4-6 are flood currents. The records of current directions in the stages 1 and 6 show that they were influenced by the waves (see wind data in Table 2). It is very clear that stage 2 (1 hour 59 minutes) with more than 20 cm/s ebb currents are much longer than stage 5 (57 minutes) for similar speed flood currents. And the duration of the higher ebb currents (more than 40 cm/s for 1 hour 13 minutes) is also much longer than the duration of the higher velocity flood currents. Therefore the lagoonward sand transportation is much more than the opposite direction. As a result, it is inferred that the net sand transport is towards the lagoon.

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Figure 27. Location of current measuring site and directions of water movement on the reef flat in falling and rising tides.

This tidal current pathway is controlled by the geomorphology of the reef flat northwest of Savave where there is a series of reef flat ridges 0.5-1 m higher than the reef flat, and which are streamlined with the narrow end lagoonward (Figure 4). In addition, the reef flat between the outer parts of the ridges is higher than the inner reef flat. When the tide goes down from the high water to a little lower than half-tide level (stage 1), the water can go to the ocean and basically does not meet any large obstructions. However, while the tide is further falling (stage 2), part of the water exiting from the reef flat is taken through the artificial channel, resulting in high current speeds (Figures 27, 28, 29 and 30). After the water on the reef flat has almost gone, the tide still goes down (stage 3), and the currents become slow (Figure 31). From when the flood tide begins to the time it reaches the height of the reef flat beside the channel (stage 4) the water level rises in the channel, forming low speed flood currents. However, after the tide level rises over the reef

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flat beside the channel the sea water rapidly spreads to the reef flat west of the channel and a great deal of water is required to supply the reef flat, forming high speed flood currents. This is an opposite process to that in stage 2. If there were no waves, stage 5 should be completely opposite from stage 2 and should extend for the same amount of time. However, during stage 2 (ebb) the waves breaking along the edge of ocean reef flat obstructs water moving to the ocean and in stage 4 (flood) the breakers help water go onto the reef flat. Therefore, the stage 4 is much shorter than the stage 2. After the tide exceeds a level a little lower than half-tide level (stage 6), the water connection between the ocean and the reef flat becomes better, so the flood currents become slower.

Figure 28. The current speed direction, and water depths measured in the channel.

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Figure 29. The channel at the beginning of the stage 2 (lower than half-tidal level). Part of the water on the reef flat west of the channel flows to the lagoon via the channel and makes ebb currents in the channel faster. View to northeast.

Figure 30. The channel in the middle of stage 2. Part of the sea water on the reef flat west of the channel flows to the lagoon via the channel causing high velocity ebb currents to develop in the channel. View to northeast.

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Figure 31. The channel in the stage 3, close to lowest tide. Almost no sea water on the reef flat west of the channel flowing to lagoon via the channel and the ebb current speed was low in the channel. View to northeast.

CONCLUSIONS

· The reef flat can be divided into five areas. There are reef flat ridges on the reef flat northwest of Savave and south of Savave and Temotuloto.

· Erosion is occurring on Fale, but the coast is little influenced by human activities.

· Savave is an area of sand accumulation. It received sediment from reef flat and the lagoon and is protected by Fale to the southwest.

· Erosion on the northwest ocean coast of Savave was caused by mining of the beach and reef flat.

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· Erosion on the lagoon coast of Savave was caused by the artificial channel, which not only traps sediment but also transports sediment to the lagoon.

· The high speed lagoonward ebb currents in the artificial channel last much longer than the high flood currents in the opposite direction. Therefore the net sand transport in the channel is always towards the lagoon.

RECOMMENDATIONS FOR COASTAL MANAGEMENT

· Mine sand and gravel/rubble from the inland parts of Fale and stopping mining from all beaches.

Mining of sand and gravel from Savave is prohibited and locals mine from the beaches of Temotuloto, Fale and three islets. This has not only caused erosion of the islands but also affects the coast of Savave. To make taro pits, gravel/rubble and sand is excavated and put beside pits on Fale, and these sediments can be used for aggregate. Of course transportation of these materials is not as easy as from nearby beaches and they must be sieved to separate sand and gravel or rubble. However, this is very necessary for protecting the coastal environment. Locals should be encouraged to dig new taro pits for getting aggregate and plant taro on Fale in a planned way. In this way it might be possible to avoid removing sediment from the beach and reef flat.

· Fill part of the artificial channel (passage) and build a pier.

The artificial channel is the cause of erosion along the lagoon coast of Savave. However, it is useful for small boat navigation at low tide. In order to improve the lagoon coastal environment, it is necessary to remodel the channel. This will include filling 125-150 m of the southwest part of the artificial channel, building a small wharf for a loading and mooring area for small boats, and building a pier to connect the wharf and shore. In this way, the longshore sediment transport pathway is not interrupted, and the beach and reef flat sand can be preserved (Figure 32).

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Figure 32. Schematic diagram of the recommended wharf and pier modifications to the artificial channel.

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REFERENCES

Carter, R. 1986. Wind and Sea Analysis, Funafuti Lagoon, Tuvalu. SOPAC Technical Report 58. Kofe 1983 Lafita, N. 1983. Nukufetau. In Laracy Hugh (editor): Tuvalu, a History. Institute of Pacific Studies and Extension Services, University of the South Pacific. McLean, R.F., Holthus, P.F., Hosking, P.L., and Woodroffe, C.D. 1991. Nukufetau. Tuvalu Land Resources Survey Island Report No. 6. 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. Smith, R., Rearic, D., Saphore, E. and Seneka, F. 1990. Survey of and Nukufetau Lagoon, Tuvalu, 3 April - 5 May 1989. SOPAC Technical Report 105. 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. 1995. Coastal Sedimentation and Coastal Management of , Funafuti Atoll, Tuvalu. SOPAC Technical Report 221. Xue, C. 1996. Coastal Geology Map of Tuvalu - Southwest Nukufetau. SOPAC Coastal Series Map 8 (Copy included as an Enclosure with this report).

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APPENDIX

Beach Profile Data

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