Coastal Erosion and Management of Amatuku Island, Funafuti Atoll, Tuvalu
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COASTAL EROSION AND MANAGEMENT OF AMATUKU ISLAND, FUNAFUTI ATOLL, 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 [3] 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 [TR234 - Xue] [4] 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. [TR234 - Xue] [5] 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 0.5- 1.5 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). [TR234 - Xue] [6] 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. [TR234 - Xue] [7] 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 km2), 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