Mapping the Coastal Morphology from Salelologa to Pu'apu'a on the Island
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Mapping the Coastal Morphology from Salelologa to Pu’apu’a on the island of Savai’i, Samoa S. P. Kim and S.-R. Lee KIGAM April 1999 SOPAC Miscellaneous Report 281 Address : Petroleum and Marine Research Division Korea Institute of Geology, Mining and Materials(KIGAM), 30, Kajong-dong,, Yusung-Ku,, Taejon, 305-350 KOREA THIS STUDY WAS FUNDED BY THE MINISTRY OF SCIENCE AND TECHNOLOGY, GOVERNMENT OF KOREA TABLE OF CONTENTS ACKNOWLEDGEMENT ............................................................................................................ 4 INTRODUCTION........................................................................................................................ 5 STUDY AREA............................................................................................................................ 5 MATERIALS AND METHODS.................................................................................................. 8 RESULTS................................................................................................................................... 9 1. MAPPING THE COASTAL MORPHOLOGY............................................................................... 9 Salelologa to Safua ............................................................................................................. 9 Safua to Eve’eve ............................................................................................................... 10 Eve’eve to Fogapoa .......................................................................................................... 10 Fogapoa to Si’ufaga .......................................................................................................... 10 Si’ufaga to Sa’asa’ai .......................................................................................................... 10 Sa’asa’ai to Pu’apu’a (Lesolo Point) ................................................................................. 11 2. BEACH PROFILING............................................................................................................ 11 3. SURFACE SAMPLING ........................................................................................................ 11 DISCUSSION AND CONCLUSION ........................................................................................ 13 REFERENCES......................................................................................................................... 14 APPENDICES.......................................................................................................................... 15 APPENDIX 1: ........................................................................................................................... 15 SOPAC draft Task Profile Coastal Morphology Mapping Salelologa to Pu’apua on the island of Savai’i, Samoa. ................................................................................................... 15 APPENDIX 2: ........................................................................................................................... 17 Map of coastal morphology of Savai’i Island (from Salelologa to Pu’apu’a) .................... 17 APPENDIX 3: ........................................................................................................................... 18 List of rock specimen and surface sediment samples...................................................... 18 APPENDIX 4: ........................................................................................................................... 20 Photographs taken during survey..................................................................................... 20 LIST OF FIGURES Figure 1. Location of study area..................................................................................... 5 Figure 2. Geologic map of study area............................................................................ 6 3 Figure 3. Beach profiles and approximate slope angles measured in Faga beach ..... 11 [MR281-Kim & Lee] 4 ACKNOWLEDGEMENT This project is funded by the Ministry of Science and Technology, Government of Republic of Korea and undertaken by the Korea Institute of Geology, Mining and Materials (KIGAM) at the request of the government of Samoa through the SOPAC. The authors appreciate the cooperation of the SOPAC Technical Secretariat and the Meteorology Division of the Ministry of Agriculture, Forests, Fisheries and Meteorology of Samoa during the whole survey period. Special thanks should be given to Mr. A. Simpson, Dr. R. Howorth and Mr. R. Smith for arranging the survey in Fiji, and to Mr. L. Talia for his sincere discussion and kind guidance during the whole survey period in Savai’i Island. We would also record here our acknowledgement to Lala, Sunita, Tatia and all the other secretaries of the SOPAC office. 5 INTRODUCTION As technical assistance to the 1998 SOPAC work program, a twelve-day survey project was undertaken from 30th, July, 1998 to 10th, Aug., 1998 in the island Savai’i, Samoa. The SOPAC work program in this area is mainly focused on understanding the shoreline movements and establishing sound coastal zone management data for fundamental protection of the important existing infrastructure. The detailed objectives of the program are as follows (Dr. R. Howorth, pers. com.; See Appendix 1. SOPAC DRAFT TASK PROFILE); 1. Establishment of the features of the coastal environment and active coastal processes including historical shoreline positions, 2. Identification of the areas where infrastructure may be under threat, 3. Identification of any potential areas for future aggregate assessment, 4. Compilation of coastal morphology data of the coastline from Salelologa to Pu’apu’a in Savai’i island, Samoa, to evaluate coastal processes, and assess any potential hazards, especially erosion. This report is a preliminary result to partially fulfill the above-mentioned objectives after undertaking a ground-check and mapping the coastline features at a scale of 1:25 000 with available aerial photographs. STUDY AREA The study area is geographically located between Salelologa village and Pu’apu’a village (13° 34¢ S – 13° 44¢ S) in Savai’i Island, Samoa (Fig. 1). This area is the most populated in Savai’i (Ward and Ashcroft, 1998) and has experienced heavy damage by the cyclones Ofa and Val in Feb., 1990 and Dec., 1991, respectively (Ward and Ashcroft, 1998; Elmqvist et al., 1998). According to the geologic map of Savai’i (New Zealand Geological Survey, 1958; (Fig. 2) the whole study area is covered with middle to late Pleistocene Mulifanua volcanics except the small areas around the wharf in Salelologa and around Lesolo Point, which are composed of the Holocene Puapua volcanics. From Mt. Asi to Pu’apu’a village, Holocene Tafagamanu sand strips composed of coral sands are distributed along coastline about 1½ m above sea level. Small creeks, inland springs and lithified sands near the sea level can be noticed at several locations. [MR281-Kim & Lee] 6 7 Fig. 2. Geologic map of study area (by the courtesy of SOPAC GIS archive; modified from New Zealand Geological Survey, 1958) 8 MATERIALS AND METHODS Before starting the survey, a list of aerial photographs taken of the study area and relevant reports and information were acquired from the SOPAC archive. Geologic map (1:100 000 scale) and topographic maps (1:20 000) were also copied for using in the field. At the Department of Lands and Survey in Apia, Western Samoa, aerial photographs taken in 1990 (New Zealand Mapping; scale 1:15 000; SN 7815, Nos 311, 313, 315, 320, 322, 324, 326) were available with some transparent papers. During the whole field survey period, ground-check was performed to assure the correct interpretation of any features seen on the photographs. Any morphologic feature which indicates coastal erosion was logged on the maps at the same time. Man-made structures like sea walls, groins, bathing pools, wharves, etc., were also logged on the maps. For semi-quantitatively evaluating the difference of beach slopes across groins in Faga beach (between Cape Tuasivi and Malae village), six beach profiles were acquired with solid sticks and a tape ruler. As necessary, rock and surface sediments were sampled to collect additional data for further studies after visual examination of grain texture and mineralogical composition. Sampling locations were logged in the topographic map or measured with latitude and longitude by using a Trimble TM GPS receiver. [MR281-Kim & Lee] 9 RESULTS 1. Mapping the coastal morphology After the survey in Samoa, brief results were reported and discussed in the SOPAC office in Fiji with interpreted aerial photographs and logged data. In the office of KIGAM, interpreted aerial photographs were compiled together with the logged features, and schematically drawn with CorelDraw program (Appendix 2; CorelTM v.8.0). The topographic map (1:20 000) was also referred to when combining and positioning the mapped areas. The final output is a digitized map of 1:25 000 scale in .DXF file format for SOPAC GIS database by using AutoCADTM. The following is a detailed description of each coastal area ; Salelologa to Safua The field survey was started from Salelologa wharf. Sea walls and underlying exposed basement rocks prevail in this area. The sea walls consist mainly of basaltic rocks of up to 2.0-m diameter with slopes of mostly 30 to 35 degrees (max. 70 degrees). The rocks still look fresh and angular. Black basaltic sand and shell fragments were found around