Seabed Surveys, Vuda to Treasure Island Via Bounty Island, Multibeam
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SEABED SURVEYS VUDA TO TREASURE ISLAND VIA BOUNTY ISLAND Multibeam bathymetry, seismic profiling and sediments Robert Smith SOPAC Secretariat July 2000 SOPAC Technical Report 312 [3] TABLE OF CONTENTS SUMMARY........................................................................................................................................5 INTRODUCTION AND OBJECTIVES..............................................................................................7 EQUIPMENT AND METHODS Navigation Control .....................................................................................................................8 Compilation of Survey Base Map..............................................................................................9 Multibeam Bathymetry...............................................................................................................9 Geophysical Profiling...............................................................................................................10 SURVEY RESULTS.......................................................................................................................10 Map sectors.............................................................................................................................11 Vuda Approaches – Sheet 1.................................................................................................11 Vuda to Bounty Island – Sheets 2-10 ...................................................................................12 Notes Sheet 7 ..................................................................................................................14 Notes Sheet 10 ................................................................................................................14 Synopsis Sheets 2-10......................................................................................................15 Bounty Island Approaches – Sheets 11-13, 18-20...............................................................15 Bounty-Island Bathymetry, Sheets 12-15 and 19-21............................................................16 Bounty to Treasure Island – the Existing Pipeline Route (Sheets 25-31) ............................16 Treasure Island Approaches (Sheet 31) ..............................................................................16 Treasure Island Bathymetry, Sheets 31-37..........................................................................17 SEISMIC INTERPRETATION.........................................................................................................19 Bounty Island...........................................................................................................................21 Depth to Bedrock.............................................................................................................21 Sand Resources ..............................................................................................................22 Treasure Island Sand Resource................................................................................................................23 CONCLUSIONS .............................................................................................................................25 RECOMMENDATIONS ..................................................................................................................26 REFERENCES ..............................................................................................................................27 APPENDICES Appendix 1 Reference Station Location Map and Details Appendix 2 HYPACK navigation files Appendix 3 Multibeam configuration, calibration and processing Appendix 4 Multibeam Bathymetry Sheets 1-37 Appendix 5 Scanned Seismic sections and interpretation Bounty Is Appendix 6 Isopach, Depth to Bedrock Maps for Bounty, Treasure Appendix 7 Sediment Grain Size and Composition Analysis Appendix 8 Compact Disk Data Files [SOPAC Technical Report 312 — Smith] [4] LIST OF FIGURES Figure Page 1 Location map showing existing pipeline from Vuda to Bounty Island................................8 2 Coast and reef details digitised from Sheet L-27, 1:50, 0000 Fiji Map Grid ......................9 3 Sketch map illustrating map series with respect to this survey ......................................10 4 Sidescan image of existing pipe some 330m west of Vuda point...................................12 5 Bathymetric cross section profile along pipe axis east to west, sheet 7 ........................13 6 Side-scan image showing pipe burial..............................................................................14 7 Seabed slope on approaches to Treasure Island............................................................17 8 Seismic section showing the 5 seismic units in section based on reflection characteristics .................................................................................................................19 9 Depth to top of bedrock Bounty Island.............................................................................21 10 Isopach of sediment thickness for Bounty Island ............................................................22 11 Isopach of sediment thickness for Treasure Island.........................................................24 [SOPAC Technical Report 312 — Smith] [5] SUMMARY This report describes results of a field survey to evaluate a pipeline route from Vuda Point to Treasure Island (Luvuka) via Bounty Island (Kadavu), sediments and depth to bedrock for Bounty Island and an evaluation of sand resources for island beach reclamation. Data acquired include multibeam bathymetric data, single-channel seismic data and sediment samples. The multibeam bathymetric data were processed and depths reduced to chart datum. Results of the multibeam bathymetry for both Bounty and Treasure Island and the pipeline route from Vuda to Treasure via Bounty define the seabed morphology well. From these data a suite of 37 maps at 1:1000 scale has been compiled based on a sounding matrix of 2 m. The survey was completed by the South Pacific Applied Geoscience Commission (SOPAC) during five days in March/April 2000 The main conclusions of the field study are as follows: The pipeline route from Vuda to Bounty appears free of obstacles except for Catlow Reef and a small patch to the east of Catlow. The current pipe lies to the south of both features. Along the route from Vuda to Bounty the existing pipe was found to wander laterally within a 100 m to 120 m corridor. This implies the existence of significant cross currents, or an alternative interpretation is that navigation control during installation of the pipe was not precise. A substantial section of the present pipe between Vuda and Catlow reef is buried. This implies one of two things; that there is significant bed transport at depth, particularly more so during cyclone weather or the sediments along this section are very fine and pipe subsidence is a contributing factor. For Bounty Island the combined data sets of multibeam bathymetry and seismic data show an extensive reef platform that has developed on an erosional high or faulted block of well-bedded strata interpreted to be of the Vuda beds. Further verification by a borehole would confirm this interpretation, with the location of the hole based on the seismic data. On approaches to Bounty, coral patches dominate the route taken by the present pipeline, making manoeuvring difficult. The bathymetry results indicate that the site considered for developing docking facilities is unsuitable. Survey results indicate that at least two alternative sites with better deep-water approaches could be considered as potential sites for wharf development, located in the southwest and west sectors of Bounty Island. [SOPAC Technical Report 312 — Smith] [6] Evident from the multibeam and seismic data is a large shelf extending eastwards from Bounty which appears to be the primary sediment sink for sediment lost from the island coastal system, reflecting the dominant transport direction for the island coastal system of west to east Sand resources for beach reclamation were found to be limited, the more significant deposits being found in an area bounded by the 15 m isobath on the shelf east of Bounty Island. Construction of shore-normal channels, groynes or seawalls that obstruct east-west longshore processes during normal tradewinds should not be considered, as these may result in further erosion along the western shoreline of the island. From Bounty to Treasure the present pipe route was traced and found to be obstacle free. The present route skirts to the east of the base of a large reef patch northeast of Bounty Island. An alternative and shorter route for the pipeline from Bounty to Treasure was investigated and is worth further consideration. Bathymetry data pertaining to this alternative route are in sheets 22- 24. No suitable areas of sand resources were identified to provide material for beach reclamation projects on Treasure Island. The more significant deposits appear to be accumulating to the south of the island and appear associated with a shore normal channel feature (?). The results from this survey indicate that the scale of existing maps and charts for the subject areas are not a suitable base on which to plan infrastructure. The main recommendations concluded from the field study: · The shelf area east of Bounty is not suited for developing a wharf facility as there are numerous obstacles to navigation