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

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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 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

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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.

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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 and it is considered the primary sediment sink for the island.

· The shelf area is better suited as a marine reserve because of the prolific patch coral-reef growth in the area.

· Two alternative areas can be considered in the southwest and west quadrants of Bounty Island for wharf development with clear approaches and deep waters close inshore.

· Although sand resources for beach reclamation are present within the shelf area east of Bounty, alternative sources removed from the island eco-system would be preferable.

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INTRODUCTION AND OBJECTIVES

The primary objectives of the survey were to survey:

· Vuda Point pipeline approach – 200 m wide centred on the existing pipeline extending 500 m offshore from the reef edge. · Bounty Island Wharf site – 300 m wide by 1000 m offshore from as close to shore as possible, including the reef area. · Treasure Island pipeline approach – 200 m wide centred on the existing pipeline extending 500 m offshore from the reef edge. · Bounty Island reef drop off (edge) – 100 m wide around the entire island at the reef edge to identify extent, thickness and quality of sand deposits. · Treasure Island reef drop off (edge) – 100 m wide around the entire island at the reef edge to identify extent, thickness and quality of sand deposits. · Existing & proposed pipeline route from Vuda Point to Treasure via Bounty Island – 100-m wide swath over the full length of the existing route as well as a swath over the proposed route. · Also to monitor progressive survey results in relation to survey objectives and liaise with the Treasure Island representative to adjust the survey area if required.

This project had the support of the Mineral Resources Department. It provides an account of the activities and the results of a field survey undertaken during the period 28 March to the 1 April 2000.

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Figure 1: Location map showing existing pipeline to Kadavu (Bounty Island) from Vuda Point, west . Luvuka (Treasure Island) lies to the northwest of Kadavu. (Source map British Admiralty Chart 1670)

EQUIPMENT and METHODS

Navigation Control

Navigation control was accomplished with a Del Norte 1009+ DGPS measuring unit. Real-time differentially corrected GPS positions accurate to less than 1 sigma indicate a positional error of ±1 m. The reference station for real-time DGPS was set on T29 (CP 2) found in survey plans SO4242 known as - Controls.

Station co-ordinates for the DGPS reference station based on the Fiji Geodetic Datum of 1986 is East 1857743.34 m, North 3924458.03 m with a reduced level of 48.773 m. For differential GPS purposes these co-ordinates were converted to WGS 84 geographicals of 170 40’ 41.0357” South and 1770 24’ 33.1795” East and a height of 46.8248 m. Station details and location are provided in Appendix 1.

Onboard navigation control and vessel guidance were controlled by Hypack hydrographic survey software from Coastal Oceanographic. Track lines were pre-planned in the survey design

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package based on survey requirements. A listing of relevant files used are tabulated in Appendix 2. Based on these files, survey repeatability is easily achieved.

Compilation of Survey Base Map

The base map used was digitised from the 1:50 000 Fiji Map Grid Topographic series sheet L-27 using MapInfo (Figure 2). Plots of the base map with bathymetry were compiled with AutoCAD RL14.

Figure 2: Section of coast and reef details digitised from Sheet L-27.

Multibeam Bathymetry

High-resolution swath mapping, using multibeam echosounder, is able to map a complete underwater landscape in a fraction of the time that is currently required by a single-beam echosounder, and with greater accuracy. Computer processing of swath-mapping data can produce data visualisations that render complex three-dimensional concepts into simple, informative, colour diagrams for the lay observer.

Swath mapping of the seafloor is carried out using a sophisticated multibeam echosounder fitted to a ship or towed at depth. A computer is used to co-ordinate the large amounts of imaging information with the ship’s position and attitude at very close time intervals. With further processing, an image can be created that represents, in fine detail, the morphology of the seafloor as well as objects on the seafloor. Details of the multibeam configuration deployment and data processing are provided in Appendix 3.

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Geophysical Profiling

Continuous seismic-reflection profiling was carried out concurrently with the multibeam bathymetric profiling. Seismic reflection provides a continuous cross-section profile of the sub- bottom geology along the track of the vessel. These data can be used to approximate sediment thickness and depth and to interpret sediment-type lithology and depositional setting based on the seismic characteristics of the internal reflectors. The unit used was a Datasonics bubble-pulse SPR 1200 profiling unit with a peak frequency centred on 400 Hz. A frequency filter window of 400–2000 Hz was used. Analogue data representing the geological cross-sections of the seismic profiles were recorded on an EPC 1600 graphic recorder. Position was annotated on the graphic recorders at an interval of 30 seconds with the position programmed and logged by Hypack.

SURVEY RESULTS

The results of the survey have been compiled as a series of 37 maps in digital format for reproduction on hardcopy at 1:1000 scale. The map index for the series is illustrated in Figure 3. Discussions of the results and findings of the survey have been generalised into seven site- specific areas tailored to the objectives of the survey.

Figure 3: Sketch map illustrating map series with respect to this survey Map Sectors:

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Vuda approaches, sheet 1: details the existing bathymetry of the approaches to Vuda Point.

Vuda to Bounty Island sheets 2-10: detail the bathymetry of existing and proposed pipeline route.

Bounty Island Approaches sheets 11 through 13, 18 through 20 show in detail the bathymetry of the existing and proposed pipeline route.

Bounty Island Bathymetry sheets 12-15 and 19-21 detail the island seabed morphology to an average depth of 30 m.

Bounty Island to Treasure Island sheets 25-31 detail the bathymetry of the existing route and proposed route to Treasure Island from Bounty.

Treasure Island Approaches and surrounding bathymetry, sheets 32-37: These sheets show in detail the bathymetry of the existing and proposed pipeline approach and detail the island seabed morphology to an average depth of 30 m.

An alternative route Bounty to Treasure sheets 22-24: These sheets detail the bathymetry for a possible alternative route for the pipeline from Bounty Island to Treasure Island.

Vuda Approaches – Sheet 1 multibeam bathymetry

A 300-m wide swath centred on the existing pipeline and extending 500 m offshore was completed. Data collected include multibeam bathymetry with digital sidescan, and single- channel seismic data. The approach bathymetry is shown in Sheet 1, compiled at 1:1000 scale with a 0. 5 m contour interval. Data points for the bathymetry are based on a 2 x 2 m grid cell. In tracking the pipe-line route the pipe was first identified in the sidescan data some 330 m west of a marker float at the start of the line from the reef crest at Vuda Point (Figure 4). Track line followed was line BOU000 – line 4 in line file “Vud-bou.lnw”.

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Figure 4: Image of the exiting pipe as seen in the sidescan data for line BOU000 some 330 m west of the start of line near the reef crest at Vuda Point heading towards Bounty. The pipe is clearly visible in the starboard trace. Horizontal scale for distance from vessel track centreline shows the pipe to be some 60 m to starboard. Note also how featureless the seafloor is in the image.

Vuda to Bounty Island- Sheets 2-10 pipeline swath multibeam bathymetry

Sheets 2-10 cover the bathymetry for the existing and proposed routes for the pipeline from Vuda to Bounty Island. From the sidescan image data for lines BOU000 and BOU 047 it is evident that the pipe does not follow a direct route to Bounty, but wanders within a corridor 100 m and more wide from Vuda to Biunadoa Reef (name in topographic map series, Catlow Reef on Admiralty Chart 1670) located on sheet 7. From here the pipe then appears to track significantly northwards before being relocated in 30 m of water at the toe of the slope up to Bounty Island. The approximate position of the pipe in this instance is plotted on sheet 18.

From the bathymetry for the leg Vuda to Bounty the seabed has a very gentle gradient westwards with few to no obstacles of note. Apart from Biunadoa (Catlow) Reef in sheet 7, there is a mound or drowned reef patch some 2 m in elevation lying to the north of point 1671 in sheet 6. The present route of the pipe does cross the toe of Biunadoa (Catlow) Reef, although

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Figure 5: Bathymetric cross section profile taken along pipe axis east to west sheet 7.

the rise is subtle. A cross-section bathymetric profile east to west through sheet 7 along the pipe axis is illustrated in Figure 5 above. Note however that there is a vertical exaggeration of approximately x 266.

More importantly, as the pipe was initially laid on the seafloor, it is apparent from this survey that there are now a number of areas where the pipe has become completely buried. Buried sections were readily identified from the sidescan data. This observation was confirmed later by the local divers who routinely dive to the pipe to effect repairs.

Towards the western boundary of sheet 5 the pipeline disappears and is interpreted to be buried under sediment based on the sidescan data. From the seismic data the sediment is a sequence of fine mud and silts some 10 m in thickness. The disappearance of the pipe is illustrated in Figure 6 in a screen capture of sidescan data from line BOU000. It shows the pipe to lie 75 m to the right (starboard) of centre line, visible in the lower right hand side of sidescan display in 25 m of water before the pipe becomes buried in the sediments of the seafloor.

The pipe emerges some 850 m further and is first seen in sheet 7 some 90 m to the north of track BOU047.

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Figure 6: Sidescan image of the pipe fading due to burial, location in sheet 5.

Notes – sheet 7

Evidence for the pipe location is last seen on the route to Bounty Island on sheet 7. A marker float at the time of survey indicated where the pipe lay with respect to the survey vessel. It is interpreted that the pipe is either partially or completely buried, as there is no sidescan evidence for it on the seabed. The pipe however does appear briefly near the start of line BOU056 in sheet 18, some 1200 m offshore from Bounty Island in a water depth of 30 m. At this point it places the pipe some 250 m plus north of the original route that was being followed to Bounty Island.

Notes – sheet 10

In sheet 10 the bathymetric contours hint at the possibility of a shallow scour moat some 100 m in width at the base of the slope to Bounty Island. This feature implies that currents are active around the base of the island but the predominant direction cannot be determined from the present data set.

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Synopsis – sheets 2-10

Results of the survey for the leg Vuda to Bounty Island, sheets 2 through 10, show the seafloor to be virtually featureless and devoid of any significant obstacles apart from Biunadoa (Catlow) Reef and a small patch mapped in sheet 6 lying to the north of the existing line. However it was also found that the existing route taken by the pipeline from Vuda to Bounty Island is not direct. This may be due to the lack of suitable positional control during the deployment process or the effects of bottom currents later displacing the pipe laterally, or a combination of both. Strong bottom currents, are implied by the burial of some sections, implying that significant seabed transport of sediment does occur.

It is also concluded that the location of the pipe as drawn on British Admiralty Chart 1670 is only representative.

Bounty Island Approach – sheets 11 through 13, 18 through 20

The bathymetry for the approaches to Bounty are depicted in sheets 11 to 13 and 18 to 20. A 300- m wide swath extending 1500 m offshore was completed. Data include multibeam swath and single-channel seismic data. This data set is based on a matrix cell size of 2 m. From this, detailed bathymetry maps have been generated with a 0.50-m contour interval.

An interesting aspect of the bathymetry in this area is the existence of a well-developed shelf that progrades east of the island. The morphology of the shelf indicates it to be the primary depositional sink for sediments lost from the island coastal system and surrounding reef. This is further confirmed when examining the bathymetry of the rest of the island and from seismic data reported on in a later section, Seismic Interpretation. Best delineated by the 20-m isobath, the shelf extends some 550 m from the shoreline. From these sheets it can be seen that the area is densely populated with coral patches of varying size and elevation, rising from a shelf with an average depth of between 15 and 20 m.

The only physical references to the location of the pipe for this data include a sidescan image of the pipe at the toe of the slope in sheet 18 and a marker buoy for the location of the pipe inshore, sheet 12. This position is marked when passed during the survey. On the traverse up the island slope the pipe could not be distinguished on the seabed due to background scatter produced by the abundance of coral reef patches on the slopes and the rapid change in depth and resulting scale changes.

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Track lines for the seismic data for the proposed wharf location have also been superimposed on the contour maps.

Bounty Island Bathymetry, Sheets 12-15 and 19-21

Sheets 12-15 and 19-21 cover the bathymetry for Bounty Island. The bathymetry seen in the south, west and north quadrants of the island reflects a steep fore-reef slope from the edge of the reef to lagoon floor within a distance of 50-100 m of the edge of the reef platform. Patch reef development in these areas is notably sparse, with the top of the reef patches falling within a range of 8-10 m below sea surface.

Bounty to Treasure Island – Existing pipeline route (Sheets 25-31)

The existing pipeline is first seen in the sidescan data on line Bou058 en route to Treasure Island between fixes 1883 and1884 in a water depth of 30.5 m – sheet 19. The pipe wanders 20 to 30 metres either side of the track line plot for BOU058. Much of the pipe sits above the seafloor and is visible on the sidescan record in 42 m of water. On the existing track the pipe passes to the northeast of a large coral patch. This is the deepest section of the pipeline. Swath data north of, but parallel to the existing pipeline indicate the new proposed route to be clear of obstacles.

Treasure Island Approaches (Sheet 31)

Sheet 32 best shows approaches to Treasure. The approach is characterised by a very narrow shelf delineated by the 15 m isobath. At best the shelf is 150 m wide and populated by coral patches but not with the density seen on the shelf of Bounty Island. A number of channel-like features are evident in the bathymetry, one of which originates at the edge of the shelf. The others two appear as mid-slope channels. The presence of these channel features indicates focusing of currents by the coral patches on the slope and at the edge of the shelf. Slope gradients from shelf to lagoon floor range between 4 and 20 degrees (Figure 7).

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Figure 7: A colour-coded sketch illustrating seabed slope angles in degrees for sheet 32 approaches to Treasure Island.

As shown in Figure 7, from the bathymetric data it is possible to derive a map showing the seabed slope. The areas of high slope angle delineate the larger coral patches.

Treasure Island Bathymetry, Sheets 32-37

The circum-bathymetry for Treasure Island is shown in sheets 32 to 37. Based on the results of the survey the actual perimeter or shape of the reef appears to differ somewhat from that depicted on the 1:50 000 topographic map and chart. The bathymetry of the island can be summarised into three zones.

Zone 1 is a narrow shelf from the outer edge of the reef platform, with a moderate slope to the lagoon floor with an abundance of coral patches. Zone 1 can be seen in sheets 32 and 35 and the southeast corner of sheet 36.

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For zone 2 there is no shelf but a steep fore-reef slope from the outer edge of the reef platform to the lagoon floor. This is seen in sheets 36 and 37 and the northwestern corner of sheet 34.

In zone 3 the shelf is subtle to absent, with a moderate slope to the lagoon floor and is sparsely populated with coral patches. Zone 3 applies to sheets 34, 33 and 32. The scarcity of coral patches in this zone suggests sedimentation rates high enough to inhibit coral growth, further suggesting that this is an area of deposition. This interpretation implies that the dominant or net sediment transport direction on the reef platform is from the north to the south. It was noted that some coastal-protection measures have been implemented on the northwest shore of the island.

Along the western and northern margins of the island there are a number of large coral patches that for some reason have not kept pace with sea-level rise. With little or no vertical growth, the tops of these patches remain at depths ranging between 4 and 10 m below present sea-level. These patches are separated from the island by deep water. As severe erosion events are associated with cyclone and storm events that have their origins to the north and west of the island group, with growth encouragement these patches offshore could form natural offshore breakwaters. A rudimentary form of growth encouragement may be achieved by placing large boulders on the coral patches as a solid substrate for the corals to attach to.

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SEISMIC INTERPRETATION

Interpretation of the data was difficult in areas where an extensive reef facies was present both in the subsurface and at the seabed. Further offshore the data were clearer, and as a result interpretations in most cases could be extrapolated to the nearshore areas. Five units can be delineated in the seismic sections based on reflection characteristics. These are illustrated in Figure 8. The five units in order of youngest to the oldest are described as follows:

Figure 8: Five seismic units based on reflection characteristics.

Unit 1: “Recent and buried reef”. This unit was mapped for both Bounty and Treasure to delineate possible sand resources and to estimate volumes of sediment present. Depth to the top of this unit was defined as the top of reef and was used to generate an isopach map of sand thickness.

Unit 2: ”Modern marine sediments”. In section this unit is best described as the most recent of the marine sediments that have infilled between reef patches in the nearshore and make up the sediments that floor the lagoon. These are a sequence of soft fine sands, silt and clays in waters greater than 20 m deep and range from fine to coarse sands in waters less than 20 m. In the

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nearshore the base of this unit is controlled by reef, and offshore the base of the sequence has been defined by horizon 1, the base of unit 3.

Unit 3 appears in section as a thin sequence of high-amplitude continuous reflectors of variable thickness that define the base of the more-recent marine and reef deposits. The base of this unit has been mapped as horizon 1 to define the level at which probable solid substrate is present for the purpose of defining a bedrock horizon below reef. Without borehole data is difficult to define the nature of this horizon, except that the high reflectivity and continuity displayed in section (Figure 9) where reef blanking effects are minimal suggests a sequence of stiff clay beds. In inshore areas unit 3 appears to be conformable with unit 5. Offshore reef patches appear in section to be founded on this unit. Depth to the base of horizon 1 has been mapped and is shown in Figure 10. A more-detailed contour representation of this horizon in map form for sheets 11 and 12 is in Appendix 6, for Bounty Island.

Unit 4: Unit 4 represents a transgressive sequence, which accompanied rising sea levels during the late Pleistocene. Offshore the unit resides between horizons 1 and 2 onlaps unit 5 in the nearshore. Within this sequence many different seismic-reflection configuration patterns are evident, illustrating sedimentary processes and environmental setting. Shingled clinoforms seen in some sections can be interpreted as a depositional unit prograding into shallow water. Slumping within the unit was also noted, this represented by chaotic reflection configuration. Other configurations noted include sheets and sediment wedges and a variety of channel fill patterns.

Unit 5 has been interpreted to represent the Vuda Beds (Dickinson 1966, 1968). This unit is defined as being a bedded sequence of sandstones, siltstones and mudstones and minor intercalated conglomerate that overlies the breccia-lava sequence of the Sabeto Volcanics and below the first incoming of Ba detritus as marked by the Saru Shoshonite. The Vuda Beds are sandstone-conglomerate. In more recent mapping the Vuda sequence (Rao 1983) has been divided into the Upper and Lower Vuda Beds. The top of this unit 5 is represented by an erosional unconformity and has been mapped as horizon 2.

From the seismic data it is interpreted that Bounty Island is based on an erosional high or faulted block of the Vuda Beds, capped by reef from which the Island of Bounty developed during the mid- Holocene sea-level high stand some 2000-4000 years before present.

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Horizon 3 is interpreted as a marker horizon at depth, which displays high continuity and amplitude within most sections. This horizon also appears to mark a change in lithology within the Vuda sequence (?). This horizon was not mapped.

Bounty Island

Depth to bedrock

On approaches to Bounty, depth to “bedrock” as interpreted is represented by horizon 1 based on the interpretation for seismic line sections 1 through 9 in Appendix 4. The general configuration of this horizon for Bounty Island is illustrated in Figure 20. Depth to bedrock for the island platform as seen in Figure 10 is an extrapolation from the seismic data offshore. Detailed contour maps (Appendix 5) of depth to this horizon have been completed for sheets 11, 12 and 13 as they relate to the area designated for wharf development. Bounty Island is sited on either an erosional high or a faulted block of the Vuda Beds, Miocene-Pliocene in age, which, once submerged due to sea- level rise, has been capped by extensive reef growth that continued up through the Holocene.

Figure 9: Depth to horizon 1 (top of bedrock) for Bounty Island, based on seismic data. The island platform at the centre of the image is based on extrapolations in depth from the seismic data offshore.

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Sand Resources

To delineate likely areas for sand resources for beach reclamation, an isopach map was generated, based on sediment thickness in the nearshore between seabed and reef substrate. Jetprobing results (Appendix 7) were also incorporated into this data set. The results of this compilation show that sand suitable for beach reclamation is a limited resource but is best developed on the shelf east of the island. This is evident in Figure 10. In general, sand sheets appear to form only a veneer of variable thickness over reef substrate. However to the east of the island, thicker deposits of sand found associated with the shelf also suggest that this area is the primary sediment sink for sand lost from the coastal system of the island. This is an important observation as this shows the dominant transport direction for material eroded from the island to be west to east.

Figure 10: Isopach map of sediment thickness for Bounty Island, based on seismic and jetprobing data.

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As the shelf to the east of the island falls within the boundaries of sheet 12, a detailed isopach map for this sheet has been compiled. For an area delimited by the 20-m isobath, an area of 174 000 m2, there is an estimated 650 000 m3 of sand for an average thickness of 3.5 m. Ponded sand deposits in the shelf area are reflected in the isopach. In some aspects this is a product of the spacing for the seismic lines, but is also not unreasonable, when examining the present-day bathymetry of sheet 12 where a number of large coral patches rise to present-day sea-level from a shelf of depth range 15-20 m. With time, areas around these patches would infill, reflecting the interpretation of the seismic data. Elsewhere around Bounty Island, accumulations of sand exist at the base of the forereef slope of the island reef platform in depths greater than 20 m. It is considered unsuitable for beach reclamation because of composition and fines content. A high level of fines would generate turbidity in the nearshore through wave action and sorting, producing conditions not suited to good coral reef growth. Details of sediments and composition for the jetprobing stations are provided in Appendix 7.

Another aspect of the isopach map Figure 10 is that it reflects the nature of the seafloor substrate where the areas colour coded blue reflect reef-dominant substrate or reef with a thin veneer of sand.

Treasure Island

Sand resources

Areas likely to contain sand resources for beach reclamation were mapped using seismic and jetprobing data. The results of the survey are shown in Figure11. The thicker accumulations of sand fall within sheets 32 and 33 which cover the southern portion of the island. Elsewhere, accumulations occur at the base of the island slope. Typically the sands are grey in colour, medium to fine, and considered not suitable for beach reclamation. Contour maps for sand thickness for sheets 32 and 33 are in Appendix 6. It should be noted that the isopach includes sediments which occur in water depths greater than 15 m. For an area of 958 943 m2 within the areas of sheet 32 and 33 there is only an estimated 160 000 m3 of sand. Jetprobe holes TR-2-1- JP, TR-3-1-JP and TR-4-1-JP were drilled in these deposits.

Some areas of Treasure Island were noted during the survey to have undergone significant erosion, forcing protection measures to be taken. This was observed along the western and northern margins of the island. Based on the isopach interpretation of the sediment accumulations offshore, material lost from the island coastal system appears to be moving anticlockwise along the southern margins of the coast. The evidence also suggests that the sand may be migrating

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offshore through a channel (?) and transported to water depths where the sand cannot return to the longshore transport system under normal tradewind conditions.

Figure 11: Isopach map of sediment thickness for Treasure Island.

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CONCLUSIONS

· The pipeline route from Vuda to Bounty appears free of obstacles except for Biunadoa (Catlow) Reef and a small patch to the east of Biunadoa. The present 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 corridor 100 m to 120 m wide. 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 Biunadoa Reef is buried. This implies one of two things: that there is significant bed transport at depth, particularly more so during cyclone weather; or that 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 through borehole data would confirm this interpretation; the locations of the holes should be based on the seismic data.

· On approaches to Bounty, coral patches dominate the route taken by the present pipeline, making manoeuvring difficult.

· The bathymetric results indicate the site considered for developing docking facilities to be 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.

· 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.

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· 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.

RECOMMENDATIONS

· The shelf area east of Bounty is not suited for developing a wharf facility as there are numerous obstacles to navigation and it is considered the primary sediment sink for the island.

· The shelf area is better suited as a marine reserve because of the prolific patch coral-reef growth in the area.

· Two alternative areas can be considered in the southwest and west quadrants of Bounty Island for wharf development with clear approaches and deep waters close inshore.

· Although sand resources for beach reclamation are present within the shelf area east of Bounty, alternative sources removed from the island eco-system would be preferable.

[SOPAC Technical Report 312 — Smith] [28]

REFERENCES

Dickinson, W.R. 1966. Problems of stratigraphic nomenclature in Fiji. Fiji Geological Survey Note 9/66.

------1968. Sedimentation of volcaniclastic strata of the Pliocene Koroimavua Group in northwest Viti Levu, Fiji. American Journal of Science 266 (6): 440-453.

Rao, Bhaskar in prep. Geology of the Lautoka Area. Fiji Mineral Resources Department Bulletin 5.

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APPENDIX 1

Reference Station Location Map and Details

Geodetic conversion details WGS 84 – Fiji MAP Grid Co-ordinates

A seven-parameter conversion was used with the following specifications

Ellipsoid = WGS-72 Delta scale = 1.0

Delta X = 0 m DeltarX = 0

Delta Y = 0 m Deltary = 0

Delta z =4.5 m Deltarz = 0.554

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APPENDIX 2

HYPACK Navigation Files

Hypack lnw files for navigation control Used for

Bou2trea Alternative pipe route Bou-app Lines surveyed for approaches to Bounty Bounty-seis Radial seismic lines for Bounty island Bou-patch Multibeam patch test lines Jetty seis Seismic lines for proposed jetty location Tre2Bou Existing pipeline route Bounty to Treasure Trea-app Lines for Treasure approaches Vud App Survey lines leading into Vuda approaches Vud-bou Existing pipeline route Vuda to Bounty Nadi.dxf Coast and reef file for survey Bou-60 Select line completed parallel slope east Bounty

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APPENDIX 3

Multibeam Configuration, Calibration and Processing

The system used is a Reson 8101 multibeam system with the following configuration of sensors:

Multibeam Echosounder: The 8101 multibeam system has 101 beams operating at a frequency of 240 khz, has a swath width at 150 degrees or 7.4 x the water depth for depths 0-70 m. Depth capability of the system is limited to 300 m. The range resolution of the system is 5 cm. The transducer head can be installed on a vessel of opportunity as an over-the-side mount on a rigged pole-and-plate assembly. The acoustic centre, that is, X, Y, Z of the subsurface unit, is used as the reference position (origin) for the survey.

Multibeam Bathymetry Collection System, the SeaBat 6042: Essentially a computer with eight serial ports, this is a dedicated data-collection system that combines the data from the onboard sensors for vessel heave, roll, pitch, heading and position, time tagging them for later processing. The 6042 records the raw data in its own format with the file extension .svy. For later processing, raw data files can be exported in a number of different formats depending on the type of multibeam software used for editing the data.

Multibeam Sidescan: From the multibeam data, sidescan imagery can be recorded and is available in an XTF format. This removes the need for a separate piece of equipment which is usually towed behind the vessel.

Navigation System: This is required to provide real-time information to the vessel helmsman for navigating along the planned track lines of the survey. This is accomplished using Hypack Hydrographic software. With HYPACK TM, the NMEA output from the mobile GPS receiver is translated into a graphical plot of the vessel movements.

Heading Sensor: A heading sensor is required to measure the orientation of the vessel. The system used is a Scan 2000 gyro compass. This provides heading data, to 0.01 of a degree, that are logged by the SeaBat 6042. The heading is output from the 6042 to the motion sensor. On installation the gyro sensor is aligned with the centre line of the survey vessel.

Motion Sensor: This is essential to correct the swath data for vessel movement, namely heave, pitch and roll. The unit used is a VRU10 motion reference unit. The sensor, once installed,

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requires that the offsets of its reference frame of origin are measured with respect to the survey origin, in this case the acoustic centre of the multibeam system, and inserted in the 6042 program setup system offsets.

Water Velocity Profiler: Sound-velocity measurements in the water column are required to correct for beam refraction as the sound passes through the water column. Sound-velocity profiles in the survey area are measured using a Seabird CTD, computed for every 0.5 m and applied during the processing and editing phase. During data collection a constant velocity of 1540 m/s was set in the SeaBat processor.

Multibeam Bathymetry Data Editor: Multibeam data, once collected, require editing and cleaning before presentation of data can be considered. This is accomplished using HYSWEEP software from Coastal Oceanographics Inc.

Tidal Reductions: All bathymetric data acquired during the survey are reduced to chart datum based on tidal corrections provided by the Nautical Almanac for Vuda Point. These corrections are applied during the editing and cleaning of the data.

Multibeam Data Presentation Software: Commercially available software that accepts X, Y, Z points can be used. Once the datasets have been cleaned and reduced, presentation of the data can be accomplished in software package is such as AutoCAD using QuickSURF, MapINFO using Vertical Mapper, or Surfer for that matter.

Patch Test Calibration

The patch test is a multibeam calibration procedure that is completed after installation and setup to calculate sonar roll, pitch, yaw and GPS latency errors in the multibeam data. Data for the patch test are collected under specific bottom terrain in a specific order. The roll angle test is done in an area where the bottom terrain is smooth and flat, running the same line in different directions at survey speed. Latency test follows, running a line twice in the same direction up a slope once at survey speed and once as slowly as possible. The pitch test is done running reciprocal lines with a slope at normal survey speed. The yaw test is done last by running offset lines in the same direction, approximately 2 to 4 times water depth apart. The roll test is by far the most important, because it is misalignment in the roll direction that leads to the greatest survey errors.

The data collected for the patch test are converted from the Reson 6042 .svy file format to a .hyp format used in the HYSWEEP patch-test program. Having completed the processing for the patch

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test, the computed angles and latency times are then configured in the setup system offsets for the multibeam data-collection system. An interesting and important feature of the 6042 is that multibeam data can be collected immediately or before running the patch test as the raw survey data can be reprocessed and exported in a different format leaving the raw data unchanged.

Multibeam Data Processing

Patch Test calibration:

Processing the patch files the following files were used in the final analysis.

Table 2: Patch test processing and results

Patch test Files Results ROLL Roll000 +0.5 degrees Roll001 LATENCY Roll003 -0.2 secs LAT001 PITCH Roll002 +1.0 degrees Roll003 YAW Roll002 +4.0 degrees Roll000

Results of the patch test used for reprocessing the data using the Win6042 program are shown in Table 2.

Multibeam Data Files

The original data files have the file extension *.svy and are to be archived on CD ROM. For processing the raw Bou*.svy files were reprocessed with the 6042 program using the patch test parameters and output as Bou*.hyp files. This file format is then imported into HYSWEEP the multibeam editing software. The *.hyp files will also be archived on CD-ROM.

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Once the processed *.hyp files have been generated, editing and cleaning of the multibeam data was completed, using HYSWEEP. Each file is first imported into the sweep editor along with a tidal-correction file and sound-velocity file. Tidal- and sound-velocity profile-correction files have been archived along with the .swp files in a directory called\datum. The graphical representation of all collected data, position, heave, heading and soundings makes it easier to separate good points from bad.

Once satisfied with the graphs, the Sweep Editor will convert the raw survey data into X, Y, Z depth points and redisplay them, again in a graphical format. In multibeam surveys, data spikes in the dataset occur due to fish, bubbles, hull turbulence, etc. The application of an automatic filter removes the spikes quickly but is best for flat bottom topography.

Sounding Reductions

Multibeam surveys produce a lot more data than are actually required, particularly for presentation. Sounding reductions of a multibeam dataset are done using the Mapper program in HYSWEEP. This program will load an entire survey and reduce the data to the desired density. This data reduction is accomplished through gridding. A grid is created from a matrix with rectangular cells of any size, the soundings are loaded and reduced to one per cell.

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APPENDIX 4

Multibeam Bathymetry Sheets 1-37

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APPENDIX 5

Scanned Seismic Sections and Interpretation Bounty Is

Jetty lines one through 9

Bounty Radial lines 2 through 12

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APPENDIX 6

Isopach and Depth to Bedrock Maps

Bounty Island Sheets 11, 12, Isopach maps of sediment thickness

Bounty Island Sheet 12 Depth to Bedrock - Horizon 1

Treasure Island Sheets 32, 33 Isopach maps sediment thickness

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APPENDIX 7

Sediment Grain Size and Composition of Samples Bounty and Treasure Islands

The major component, comprising the bulk of the sediment, is biogenic material. The sediment samples are mostly composed of sand (both coarse and medium/fine) and gravel, with mud (silt and clay) minor. Corals, molluscs, foraminifera, Halimeda and echinoids were also present.

Coral fragments generally constitute the bulk of the sediment sample followed by molluscs, of which the class Gastropoda is dominant. Bivalves are more common in large sieve apertures (e.g. 8 mm 2 mm). The most important biogenic component and a prominent sediment contributor in most reef sediments is Halimeda. Some Halimeda is present in the sediment samples and a clear trend was observed. Very few Halimeda were seen in the 4-mm sieve, but is abundant in the 2- and 1.4-mm sieves. This was followed by a decline from the 1.0-mm sieve and onwards.

Benthic foraminifera can be important components in the sediment. Quite a few species of forams were seen in the samples, beginning from the 4-mm sieves which contained worn benthic foraminfera. Forams (Calcarina spp., Amphistegina spp., Baculogypsina spp. and Sorites spp.) are major contributors to the sediment. Amphistegina spp. is the more dominant foram and was observed in all sieve sizes from 2 mm to 0.13 mm. The Sorities spp. were seen mostly in medium/fine sand. The Calcarina spp. and Baculogypsina spp. occurred from 0.71 mm to 0.25 mm. These sieve sizes also showed an abundance of Textularia spp.

Other material observed in the samples included crustacean tests, echinoid tests and spines and spicules from corals as well as sponges (Porifera). The echinoid spines and the spicules are very widely distributed in the sediment, ranging from 1.4-mm sieve size to 0.09 mm. Another common occurrence was that of pteropod shells (sea butterflies). These were seen mostly in the medium/ fine sand fractions.

Sample grading results

B – 1 – 1GS : 45.1 % of the sample is medium/fine sand and 42.4 % coarse sand. B – 1 – 2 – JP : Medium/fine sand is dominant (63.1 %), followed by coarse sand (34 %). B – 1 – 4 – 1JP : Medium/fine sand dominates with 41.5 %, and coarse sand (32.7 %). B – 1 – 6 – 1JP : 53 % medium/fine sand followed by coarse sand (42.9%).

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B – 1 – 7 – 1JP : Coarse sand dominates (50.7 %). Medium/fine sand makes up 44.4 % of the sediment sample. B – 2 – 1JP : Coarse sand is dominant (56.5 %), followed by medium/fine sand (30.2 %). B – 3 – 1JP : Medium/fine sand dominant (54.7 %) with 36 % coarse sand. B – 8 – 1JP : Coarse sand dominant (55.5 %), followed by gravel (35.7 %). B – 9 – 1JP : Sediment sample dominated by coarse sand (67.1 %), followed by medium/fine sand (25.1 %). TR – 1 – 1JP : Coarse sand dominant (47 %) and medium/fine sand makes up 32.3 % of the sediment sample. TR – 2 – 1JP : 53.3 % of the sample consists of coarse sand, and 35.8 % of medium/fine sand. TR – 3 – 1JP : Medium/fine sand dominates (50.9 %), followed by coarse sand (46.8 %). TR – 4 – 1JP : 68.5 % of the sample contains medium/fine sand and 24.4 % coarse sand. TR – 5 – 1JP : Medium/fine sand dominates (46.5 %), followed by coarse sand (45.7 %). TR – 6 – 1JP : The sample is dominated by medium/fine sand (59.2 %), followed by coarse sand (37.2 %). TR – 09 – 1GS : Coarse sand dominates (53.2 %) followed by medium/fine sand (34.7 %). TR – 10 – 1JP : Majority of the sample consists of coarse sand (67.3 %), followed by medium/fine sand (17.3 %). TR – 11 – 1JP : Coarse sand dominates (54.1 %) followed by medium/fine sand (35.8 %). TR – 12 – 1JP : 64.4 % of the sample consists of coarse sand, followed by medium/fine sand (30.5 %).

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APPENDIX 8

Compact Disk Data files

CD CONTENTS

CD-1 Sheets 1-37 DWG files of multibeam bathymetry

CD-2 Surface files(datapoints) Quicksurf format qsb file extension Tiff files Scanned Seismic data

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