Wave Inundation on the Coral Coast of Fiji

The roaring winds of the Southern Ocean and the Tasman Sea can generate some large swells, big enough to cause inundation on the Coral Coast in the south west of Viti Levu in Fiji, some 3000 km to the north. These inundation events are sometimes associated with tsunami-like long waves that hit the shore and inundate the coast with brute force. These are locally known as loka waves. To understand the origin of the loka waves and how they become so destructive in a fringing reef environment, this research monitored the waves and water levels, for 2 years, at 4 locations across the reef at a pilot site in Maui Bay on the Coral Coast of Fiji. In order to test the size of waves necessary to cause coastal inundation, a validated numerical model, XBeach, was used to simulate the development, propagation and dissipation of these infragravity waves using different water level scenarios. The result of this analysis is intended as a predictive tool to evaluate the risk of coastal inundation from ocean surface waves that can be used to support an early warning system and coastal management tool for both the tourism industry and coastal communities on the Coral Coast

Waves and Coasts in the Pacific - Cost Analysis of Wave Energy in the Pacific

Ocean waves are often cited as an appealing source of renewable energy in the Pacific but the cost effectiveness of wave energy converters (WECs) is deemed unproven and the technology is rarely considered as a reliable renewable energy resource in Pacific Island countries. However, single/stand-alone WECs could be a competitive option against fossil fuel generators because of the high cost of imported fuel. This study analyses the wave energy resource in the Pacific and calculates the potential cost and power generation of a benchmark WEC in Pacific Island countries. The type of WEC chosen depends largely on the environmental and geophysical characteristics of the wave energy site where it is to be deployed. The aim of this study was not to report on the best device for each site but rather to give advice about the islands that could benefit most from wave energy. Therefore, the cost analysis is based on a single WEC – the Pelamis device. The Pelamis device cost presented here serves as a benchmark for comparison with other WECs in different locations. Due to uncertainties and variations in potential costs across the region, the study evaluated the range of costs applicable to the whole region. The costs of the WEC, transport, installation, operation and management, refit and decommissioning are included. Site-specific potential power generation was calculated, based on a realistic power output dependent on the wave conditions. The study found that Pacific islands south of latitude 20oS receive a substantial amount of wave energy with a mean available wave resource above 20 kilowatts per metre (kW/m) and that many other islands also have potential for wave energy extraction with a mean wave resource above 7 kW/m. This study found that a Pelamis device in the Pacific could cost between USD 6,318,000 and USD 14,104,000 to install and can operate for 25 years. The energy produced by such a device could be up to 1200 megawatt hours (MWh) per year for sites exposed to large swells. Using these values, the range of the total lifetime cost of power generation was calculated to be between USD 200/MWh for exposed sites and USD 1800/MWh for more sheltered sites. The corresponding operation and maintenance generation cost are between USD 40/MWh and USD 900/MWh. These costs are on a par with the cost of generation of other renewable energies, such as wind and solar, and, for exposed sites, on a par with the cost of diesel generation. These findings suggest that wave energy is a genuine contender for the development of renewable energy in the Pacific and should no longer be ignored when planning such development; a concerted effort from all stakeholders should be made in order to benefit from this technology. Further deployment in wave technology will reduce the cost of single wave energy devices, and most small Pacific Islands would not need to deploy large-scale wave farms of ten or more devices, as power production would greatly exceed the demand. With expected rises in fuel prices in the next decades, it would be wise to investigate further the potential of wave energy technology. The deployment of WECs in the Pacific could provide an opportunity for the technology to prove itself in the region and attract the attention of investors, policy makers and decision makers to invest in wave energy development in the Pacific . Page | 2 Waves and Coasts in the Pacific Other recommendations are listed below. 1. French Polynesia, the Austral Islands in particular, should investigate potential wave energy sites. On these islands, wave energy generation could become a major renewable energy resource with a relatively low cost that could even compete with fossil fuel. 2. Tonga, Cook Islands and New Caledonia should also investigate wave energy sites and suitable wave energy devices. Wave energy has a great potential for helping these countries reach their renewable energy targets and supply energy more cheaply than other renewable energy resources. 3. Countries with a mean wave energy flux above 7 kW/m should also investigate wave energy hotspots and wave energy device options, especially in exposed locations. There, wave energy may be able to supply a significant amount of renewable energy and help these countries meet their renewable energy targets. However, wave energy in these locations may be more expensive than other types of renewable energy. 4. Countries with a mean wave energy flux of less than 7 kW/m, such as Papua New Guinea and Solomon islands, are unlikely to benefit from wave energy unless a major technological breakthrough makes wave energy devices much more efficient. These countries should therefore not consider wave energy as a significant renewable energy resource. The WACOP project has provided calculations similar to those presented in this study for more than 200 Pacific locations in wave climate reports that should be consulted as an initial assessment of the wave energy resource available.1 The WACOP project also provides a detailed wave climate analysis for Samoa, Rarotonga in Cook Islands, Tongatapu and 'Eua in Tonga, southern Viti Levu in Fiji, Efate in Vanuatu, and Funafuti in Tuvalu. These analyses include wave energy and cost calculations based on the calculations presented in this report

The influence of reef topography on storm-driven sand flux

Natural formations of rock and coral can support geologically controlled beaches, where the beach dynamics are significantly influenced by these structures. However, little is known about how alongshore variations in geological controls influence beach morphodynamics. Therefore, in this study we focus on the storm response of a beach (Yanchep in south Western Australia) that has strong alongshore variation in the level of geological control because of the heterogeneous calcarenite limestone reef. We used a modified version of XBeach to simulate the beach morphodynamics during a significant winter storm event. We find that the longshore variation in topography of the reef resulted in: (1) strong spatial difference in current distribution, including areas with strong currents jets; and (2) significant alongshore differences in sand flux, with larger fluxes in areas strongly geologically controlled by reefs. In particular, this resulted in enhanced beach erosion at the boundary of the reef where strong currents jet-exited the nearshore

The influence of coastal reefs on spatial variability in seasonal sand fluxes

The effect of coastal reefs on seasonal erosion and accretion was investigated on 2 km of sandy coast. The focus was on how reef topography drives alongshore variation in the mode and magnitude of seasonal beach erosion and accretion; and the effect of intra- and inter-annual variability in metocean conditions on seasonal sediment fluxes. This involved using monthly and 6-monthly surveys of the beach and coastal zone, and comparison with a range of metocean conditions including mean sea level, storm surges, wind, and wave power. Alongshore ‘zones’ were revealed with alternating modes of sediment transport in spring and summer compared to autumn and winter. Zone boundaries were determined by rock headlands and reefs interrupting littoral drift; the seasonal build up of sand over the reef in the south zone; and current jets generated by wave set-up over reefs. In spring and summer, constant sand resuspension and northerly littoral drift due to sea breezes allowed a sand ramp to form in the South Zone so that sand overtopped the reef to infill the lagoon. This blocked the main pathway for sand supply to downdrift zones which subsequently eroded. In autumn and winter, with the dominance of northwesterly storms and reversal in the direction of littoral drift, the South Zone eroded and sand travelled through the lagoon in the current jet to nourish the northern beaches. Inter-annual and seasonal variation in sea level, storm frequency and intensity, together with pulsational effects of local sand fluxes at Yanchep due to inter-seasonal switching in the direction of littoral drift determined marked differences in the volumes of seasonal sand transport. These seasonal ‘sediment zones’ highlighted interesting and unexplored parallels between coasts fronted seaward by coral reefs and rock formations

Inter-annual variability and longer-term changes in the wave climate of Western Australia between 1970 and 2009

Quantifying the long-term variability in wave conditions incident on a coastline is critical for predicting its resilience to future changes in the wave climate. In this study, a 40-year wave hindcast of the southern Indian Ocean has been created to assess the inter-annual variability and longer-term changes in the wave climate around Western Australia (WA) between 1970 and 2009. The model was validated against measurements from five wave buoys located along the WA coast. Changes in the mean annual significant wave height, 90th percentile wave height, peak period and mean wave direction were assessed, and the tracks of all wave events generating wave heights above 7 m were digitised and analysed for significant changes. Results show strong annual and inter-annual variability in the mean significant wave height, the 90th percentile wave height and the number of large events (wave height?>?7 m) that impact the WA coastline. A significant positive trend in annual mean wave height was found in the southwest region of WA over the 40-year simulation. This appears to be due to an increase in intensity of the storm belt in the Southern Ocean which is associated with an increasing positive polarity in the Southern Annular Mode. However, no significant trends were found in the 90th percentile wave height or the number of large wave events impacting Western Australia. Although the number of large wave events in the southern Indian Ocean have increased, their potential to impact the coastal regions of Western Australia are reduced due to storm tracks being located further south, therefore balancing the number of large wave events reaching the WA coast

Rock topography causes spatial variation in the wave, current and beach response to sea breeze activity

We hypothesized that beach profiles that are perched on natural rock structures would be better protected from waves and currents than profiles that are not fronted by rock. In southwest Western Australia many beaches, such as at Yanchep, are perched on Quaternary limestone. Yanchep Lagoon is fronted by a low-crested limestone reef that partially encloses a coastal lagoon. The spatial variation of waves and currents around the rock structures were quantified during the sea breeze cycle at locations: (1) offshore; (2) 20 m seaward of the reef; (3) inside the lagoon; and (4) in the surf zone. The spatial variation in the beach profile response was measured at two beach profiles: (1) the Exposed Profile that was not fronted directly seaward by outcropping limestone; and (2) the Sheltered Profile which was fronted seaward by submerged limestone at 2 m water depth and that was near the lagoon exit at the end of the limestone reef. The Sheltered Profile had greater volume changes during the cycle of the sea breeze whilst the Exposed Profile recovered more by overnight accretion when wind decreased. The lagoonal current drove the strong response of the Sheltered Profile and may have contributed to the lack of overnight recovery of the beach together with the seaward rock formation impeding onshore sediment transport. The different direction and speed responses of bottom-currents in the surf zone fronting the two profiles reflected the local variation in geology, the influence of the jet exiting the lagoon, and wave refraction around the reef that was measured with GPS drifters and wave-ray tracing using XBeach. Major spatial variation in waves, currents and beachface behavior at this perched beach shows the importance of the local geological setting

A preliminary exploration of the physical properties of seagrass wrack that affect its offshore transport, deposition and retention on a beach

The transport, deposition, and decomposition of seagrass wrack facilitate significant marine subsidies of material, energy, and organisms to the terrestrial environment. Over the past decade we have improved our understanding of the on‐beach decomposition of seagrass wrack and its impact on beach and island communities; however, there is a paucity of research on the transport processes that supply wrack to the beaches. The physical properties of wrack affect its buoyancy and therefore transport, but these properties vary with species, the condition of the wrack when it was formed, the time since the wrack was generated and its ambient environment in the sediment, the water column, at the water surface or on the beach. Understanding how wrack physical properties vary under a range of conditions is needed to predict wrack transport, yet these properties have not previously been reported. We modified classical parameterizations of particle transport to identify knowledge and data gaps for wrack transport processes. We present a preliminary exploration, for Posidonia sinuosa leaves and Amphibolis antarctica stems and leaves, of settling velocities of wrack fragments, critical shear stresses required for their resuspension, bulk physical characteristics of wrack accumulations on beaches (e.g., bulk density, porosity), and physical properties of key wrack components (e.g., tissue density, tensile strength). We also determined how these properties change with drying, aging, and subsequent rewetting

Swept away: ocean currents and seascape features influence genetic structure across the 18,000 Km Indo-Pacific distribution of a marine invertebrate, the black-lip pearl oyster Pinctada margaritifera

Abstract Background Genetic structure in many widely-distributed broadcast spawning marine invertebrates remains poorly understood, posing substantial challenges for their fishery management, conservation and aquaculture. Under the Core-Periphery Hypothesis (CPH), genetic diversity is expected to be highest at the centre of a species\u2019 distribution, progressively decreasing with increased differentiation towards outer range limits, as populations become increasingly isolated, fragmented and locally adapted. The unique life history characteristics of many marine invertebrates such as high dispersal rates, stochastic survival and variable recruitment are also likely to influence how populations are organised. To examine the microevolutionary forces influencing population structure, connectivity and adaptive variation in a highly-dispersive bivalve, populations of the black-lip pearl oyster Pinctada margaritifera were examined across its ~18,000\ua0km Indo-Pacific distribution. Results Analyses utilising 9,624 genome-wide SNPs and 580 oysters, discovered differing patterns of significant and substantial broad-scale genetic structure between the Indian and Pacific Ocean basins. Indian Ocean populations were markedly divergent ( F st \u2009=\u20090.2534\u20130.4177, p \u2009<\u20090.001), compared to Pacific Ocean oysters, where basin-wide gene flow was much higher ( F st \u2009=\u20090.0007\u20130.1090, p \u2009<\u20090.001). Partitioning of genetic diversity (hierarchical AMOVA) attributed 18.1% of variance between ocean basins, whereas greater proportions were resolved within samples and populations (45.8% and 35.7% respectively). Visualisation of population structure at selectively neutral loci resolved three and five discrete genetic clusters for the Indian and Pacific Oceans respectively. Evaluation of genetic structure at adaptive loci for Pacific populations (89 SNPs under directional selection; F st \u2009=\u20090.1012\u20130.4371, FDR\u2009=\u20090.05), revealed five clusters identical to those detected at neutral SNPs, suggesting environmental heterogeneity within the Pacific. Patterns of structure and connectivity were supported by Mantel tests of isolation by distance (IBD) and independent hydrodynamic particle dispersal simulations. Conclusions ..

Understanding marine larval dispersal in a broadcast-spawning invertebrate: A dispersal modelling approach for optimising spat collection of the Fijian black-lip pearl oyster Pinctada margaritifera.

Fisheries and aquaculture industries worldwide remain reliant on seed supply from wild populations, with their success and sustainability dependent on consistent larval recruitment. Larval dispersal and recruitment in the marine environment are complex processes, influenced by a multitude of physical and biological factors. Biophysical modelling has increasingly been used to investigate dispersal and recruitment dynamics, for optimising management of fisheries and aquaculture resources. In the Fiji Islands, culture of the black-lip pearl oyster (Pinctada margaritifera) is almost exclusively reliant on wild-caught juvenile oysters (spat), through a national spat collection programme. This study used a simple Lagrangian particle dispersal model to investigate current-driven larval dispersal patterns, identify potential larval settlement areas and compare simulated with physical spat-fall, to inform targeted spat collection efforts. Simulations successfully identified country-wide patterns of potential larval dispersal and settlement from 2012-2015, with east-west variations between bi-annual spawning peaks and circulation associated with El Niño Southern Oscillation. Localised regions of larval aggregation were also identified and compared to physical spat-fall recorded at 28 spat collector deployment locations. Significant and positive correlations at these sites across three separate spawning seasons (r(26) = 0.435; r(26) = 0.438; r(26) = 0.428 respectively, p = 0.02), suggest high utility of the model despite its simplicity, for informing future spat collector deployment. Simulation results will further optimise black-lip pearl oyster spat collection activity in Fiji by informing targeted collector deployments, while the model provides a versatile and highly informative toolset for the fishery management and aquaculture of other marine taxa with similar life histories

The Influence of limestone reefs on storm erosion and recovery of a perched beach

Mechanisms through which naturally-occurring hard landforms, such as rock and coral reefs, influence coastal sediment transport are still poorly understood. Therefore, field investigations were undertaken during storm conditions on the sandy beaches of Yanchep Lagoon in southwestern Australia, which are perched on Quaternary limestone reefs. During two consecutive winter storms, the response of three subaerial beach profiles were quantified at: (a) an Exposed Profile which was fronted to seaward by a predominantly sandy substrate; (b) a Reef Profile that was fronted directly seaward by limestone outcrops submerged below mean sea level; and (c) a Bluff Profile where the dry beach was perched on a limestone bluff that reached above mean sea level and that contained a shallow coastal lagoon. The subaerial beach response to the storms had considerable spatial variation alongshore and was strongly dependent on the local rock topography. The Exposed Profile eroded most with a 2m-high scarp cut into the dune while the dunes at the Reef and Bluff Profiles were stable. The Bluff Profile also eroded considerably and the coastal lagoon widened and deepened. The Reef Profile was the most stable overall because erosion was balanced by short periods of accretion during the storm period which was partly due to sediment supplied by longshore transport through the coastal lagoon from the Bluff Profile. During the month after the storms wave energy was relatively low and the beach at the Exposed Profile accreted almost to the pre-storm volume, although the scarp in the dune was still present. The Reef Profile accreted most in the month after the storms while recovery at the Bluff Profile was low. It appeared that the bluff inhibited onshore sediment transport during and after the storms and in addition, strong currents in the lagoon transported sediment alongshore to supply the other beach profiles. These observations indicated that rock topography, especially elevation relative to sea level determined if beach erosion was reduced during storms and whether accretion was dampened in the post-storm recovery phase.12 page(s