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Incident wave energy flux is responsible for sediment transport and coastal erosion in wave-dominated regions such as the southwestern Australian (SWA) coastal zone. To evaluate future wave climates under increased greenhouse gas concentration scenarios, past studies have forced global wave simulations with wind data sourced from global climate model (GCM) simulations. However, due to the generally coarse spatial resolution of global climate and wave simulations, the effects of changing offshore wave conditions and sea level rise on the nearshore wave climate are still relatively unknown. To address this gap of knowledge, we investigated the projected SWA offshore, shelf, and nearshore wave climate under two potential future greenhouse gas concentration trajectories (representative concentration pathways RCP4.5 and RCP8.5). This was achieved by downscaling an ensemble of global wave simulations, forced with winds from GCMs participating in the Coupled Model Inter-comparison Project (CMIP5), into two regional domains, using the Simulating WAves Nearshore (SWAN) wave model. The wave climate is modeled for a historical 20-year time slice (1986–2005) and a projected future 20-year time-slice (2081–2100) for both scenarios. Furthermore, we compare these scenarios to the effects of considering sea-level rise (SLR) alone (stationary wave climate), and to the effects of combined SLR and projected wind-wave change. Results indicated that the SWA shelf and nearshore wave climate is more sensitive to changes in offshore mean wave direction than offshore wave heights. Nearshore, wave energy flux was projected to increase by ∼10% in exposed areas and decrease by ∼10% in sheltered areas under both climate scenarios due to a change in wave directions, compared to an overall increase of 2–4% in offshore wave heights. With SLR, the annual mean wave energy flux was projected to increase by up to 20% in shallow water (< 30 m) as a result of decreased wave dissipation. In winter months, the longshore wave energy flux, which is responsible for littoral drift, is expected to increase by up to 39% (62%) under the RCP4.5 (RCP8.5) greenhouse gas concentration pathway with SLR. The study highlights the importance of using high-resolution wave simulations to evaluate future regional wave climates, since the coastal wave climate is more responsive to changes in wave direction and sea level than offshore wave heights. [ABSTRACT FROM AUTHOR]

Infragravity (IG) period oscillations in harbours and marinas can often lead to interruption in harbour operations due to excessive vessel movements. Field measurements in Two Rocks Marina in south-west Australia have shown that IG period oscillations were always present and the amplitude of the oscillations was related to incident swell climate and was enhanced during storm events. The marina is fronted by two shallow, shore-parallel, reef systems located ~ 3.2 and ~ 4.7 km from the shoreline. The area experiences continuous swell and frequent storm systems, particularly during winter months. This paper describes the application of a Boussinesq wave model, validated using field data, to examine: (1) source of the IG waves incident on the marina; and (2) modal characteristics of the IG period oscillations inside the marina. The cross-shore evolution of the IG wave energy was examined using simulations with different contrasting incident wave conditions, which included measured and idealised wave spectra. The model results indicated that free IG waves were generated as the wind/swell waves propagated over the offshore reef systems independent of the external forcing. During stormy sea condition, the IG energy over the primary and secondary reefs increased by a factor ~ 10 and ~ 8 respectively, compared to the IG energy at offshore. The IG wave spectrum near the marina entrance did not contain any major energy peaks, and has an almost constant energy distribution across the IG wave frequencies. However, the frequencies similar to the marina's natural oscillation periods were excited within the marina. The predicted energy distribution maps and water level snapshots inside the marina identified different oscillation modes, which included mode 1 and mode 2 oscillations corresponding to a partially enclosed water body and, zeroth mode corresponding to an open-ended water body. This study showed that in coastal regions characterised by complex offshore topography, IG waves are generated independent of offshore wave conditions, and harbours located in such environment are at risk of IG period oscillations, depending on their geometry. [ABSTRACT FROM AUTHOR]

Abstract: Surf zone drifters and a current meter were used to study the nearshore circulation patterns in the lee of groynes at Cottesloe Beach and City Beach in Western Australia. The circulation patterns revealed that a persistent re-circulation cell was present in the lee of the groyne which was driven by changes in wave set-up resulting from lower wave heights in the lee of the groyne. The re-circulation consisted of a longshore current directed towards the groyne which was deflected offshore due to groyne resulting in a rip current along the groyne face. The offshore-flowing rip current and the incoming waves converged at the offshore extent of this circulation cell, with the deflection of the rip current parallel to the shoreline and then completing the recirculation through an onshore component. The Eulerian measurements revealed that 55% of the currents on the lee side of the groyne were directed offshore and that these currents had a maximum speed of 2ms−1. Spectral analysis of the wave heights and the currents revealed several corresponding peaks in the measured spectral densities with timescales between 12s and 50min. Numerical simulations of an idealised beach with a shore-normal groyne were conducted using a circulation model driven by waves, and confirmed the formation of a persistent eddy in the lee of the groyne. Sensitivity studies indicated that the incident wave angle, wave period, and especially the wave height controlled the circulation. The eddy vorticity, a measure of an eddy''s strength, increased roughly proportional to an increase in the incident wave energy flux. [Copyright &y& Elsevier]

Abstract: The water mass characteristics of the eastern Indian Ocean margin between latitudes 21° and 35°S, adjacent to the Western Australian coast, are described using field measurements. The results indicated the presence of five different water masses as interleaving layers of salinity and dissolved oxygen concentrations in the upper 1000m of the ocean. These water masses included (i) lower salinity tropical surface water (TSW), (ii) higher salinity south Indian central water (SICW), (iii) higher dissolved oxygen subantarctic mode water (SAMW), (iv) lower salinity Antarctic intermediate water (AAIW), and (v) lower oxygen north-west Indian intermediate (NWII) water. Data collected in 2000 and 2003 were compared with historical data (1987), and interannual variability in the tropical surface water and subantarctic mode water masses were identified and linked to El Niño–southern oscillation (ENSO) events. In the study region, the circulation pattern, known as the Leeuwin current system, consists of three major currents: the Leeuwin current (LC), the Leeuwin undercurrent (LU), and shelf current systems consisting of the Capes and Ningaloo currents. In the study area''s northern region, geopotential gradients were found driving the Leeuwin current (LC) and Leeuwin undercurrent (LU), with a (negative) sea surface slope of 4×10−7 driving the LC poleward, and a (positive) slope of 1×10−7 beneath the LC driving the LU equatorward. Cross-shelf geopotential anomalies revealed the surface layer (in the upper 300m) sloped seaward at a gradient of 1.7×10−6, and the subsurface layer (between depths of 300 and 730m) sloped coastward at a gradient of 6.3×10−7. This arrangement of geopotential slopes, together with the positions of the LU relative to the LC, indicated a dynamical relationship between the LU and the LC. The data indicated the LU''s core was located at a depth of 400m and it transported subantarctic mode water (SAMW) equatorward. The presence of the Leeuwin current and Leeuwin undercurrent at the continental shelf break and slope, respectively, influenced the water mass distribution at the continental margin. Leeuwin Current induced downwelling caused the surface and subsurface water mass (SICW) and the SAMW''s upper edge to slope downward toward the shelf break while subsurface upwelling beneath the LU moved the Antarctic intermediate water (AAIW) and the SAMW''s bottom edge higher in the water column. [Copyright &y& Elsevier]

Abstract: Wave-induced sediment resuspension in nearshore regions has been observed occurring in an event-like manner and associated with the passage of wave groups. This paper describes field measurements of turbulent velocities obtained simultaneously with suspended sediment concentration and water surface elevation from Floreat Beach, Perth, Western Australia. The data were used to study the relationship between turbulent kinetic energy (TKE) on suspension events caused by wave groups and the intermittent nature of bottom turbulence production and sediment suspension. The field measurements showed the high TKE events occurred under wave crests, and sometimes under wave toughs, when the wave heights were increasing during the passage of a wave group; the TKE decreased after the maximum wave in the wave group had passed over the measurement location. High suspended sediment concentrations (ssc) and the intermittent high TKE events were not related rather the higher ssc events were associated with a secondary peak in the surface elevation, close to the maxima in the offshore velocity, and “burst” events in the Reynolds stress. [Copyright &y& Elsevier]

Abstract: The oceanic circulation of the region between Carnarvon (25°S) and Jurien Bay (31°S) was examined using observational and remotely sensed data in conjunction with a detailed numerical modelling study. The model was validated using in situ ADCP and CTD data, and the horizontal eddy viscosity parameterization was tested against field observations. The results generated using the Regional Ocean Model (ROMS v1.8) simulated the observed mean surface flow''s spatial and temporal scales, particularly those of the eddy features. The modelled results also provided insight into the Leeuwin Current (LC)''s annually averaged alongshore transport and the Leeuwin Undercurrent (LUC)''s mean flow characteristics. Analysis of the model results, between 26.8°S and 30.8°S, revealed the annually averaged alongshore transport across the LC''s southern boundary was 5.7Sv, of which the northern boundary contributed 3.60Sv or 63%. The inflow of water through the western boundary due to geostrophic balance was estimated at 2.1Sv or 37%, and found to vary meridionally. A geopotential gradient, estimated at 1.9×10−7 and acting towards the equator, drives the LUC. Located at depths between 250 and 600m, the modelled undercurrent is characterized as an energetic flow with mesoscale variability, that is strongly influenced by the more energetic surface current. Coupling was evident in the region adjacent to the Abrolhos Islands’ western boundary (29°S) where the offshore movement of LC waters by a surface meander caused the vertical scale of the undercurrent to increase, inducing an anticlockwise vorticity response at a depth of ∼180m. The resulting subsurface eddy grew preferentially downwards before a surface response was observed. The model showed that the eddy increased its vertical scale before splitting into two forming a surface and subsurface eddy with a vertical scale characteristic of the two currents. The surface eddy intensified, detached, and moved offshore, while the subsurface eddy remained attached to the topography. [Copyright &y& Elsevier]

Abstract: The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21° and 28°S. This paper presents CTD and ADCP data collected in November 2000 together with concurrent wind and satellite imagery, to provide a description of the summer surface circulation pattern along the Gascoyne continental shelf and slope. It is shown that the region comprises a complex system of currents that are influenced by offshore eddies, wind stress, varying shelf-widths, coastal topography and outflow from the hypersaline Shark Bay. Four different water types and current systems were identified from the field measurements. The Leeuwin Current (LC) is the major current flowing through the region. It transports lower salinity, warmer water along the 200m isobath, poleward. The signature of the LC gradually transformed from a warm (24.7°C), lower salinity (34.6) water in the north to a cooler (21.9°C), more saline (35.2) water in the south resulting from geostrophic inflow of offshore waters. The width and depth of the current also changed continuously responding to the changing bottom topography and the orientation of the coastline: in the northern section under the influence of the narrow shelf and steep slope, the current was strong (∼0.75ms−1) and extended deeper into the water column. In contrast, the current decelerated (to ∼0.2–0.4ms−1) when flowing past the wider continental shelf offshore of Shark Bay and then accelerated along the southern section along the steep continental slope. Downwelling events were persistently associated with the current. The Ningaloo Current (NC) was confined to the northern Gascoyne shelf within 35km of the coast. Although upwelling was detected along the northern section of the study region, adjacent to the Ningaloo coral reef, water properties suggest a re-circulation of LC water from the south. Changes in the shelf width at Point Cloates have a significant influence on the NC resulting in bifurcation of the northward current. The higher salinity outflow from Shark Bay influences the continental shelf region immediately offshore of the main entrances to the Bay through the mixing of the higher salinity outflow water with the shelf waters. The Capes Current, a wind-driven current originating to the south of the study region was identified as a cooler, more saline water mass flowing northward. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Western Australia is susceptible to tsunamis from seismic sources that originate from distant sources including the Sunda Arc. Many surface and subsurface topographic ocean features are located between the Australian continent and locations where tsunamigenic earthquakes occur. These include the Venin Meinesz Seamounts (including Christmas Island) and Horizon Ridge, Exmouth, Zenith and Cuvier Plateaus. Numerical simulations of idealised tsunamigenic earthquakes along the Sunda Arc revealed that these topographic features have a large influence on the distribution of tsunami heights, propagating speeds and energy distribution. The interaction between tsunami waves and Venin Meinesz Seamounts and Horizon Ridge, located close to the earthquake locations, scatter the tsunami energy into several beams. Exmouth Plateau acts as a focusing feature to increase wave heights between North West Cape and Barrow Island whilst Cuvier Plateau deflects energy towards Shark Bay. Although Zenith Plateau has a local effect, it does not influence tsunami waves along the coast. Southwest Australia is "sheltered" from the direct effect of tsunami waves from Sunda Arc due to the combined effects of the Seamounts and Cuvier Plateau in the scattering and refraction of tsunami waves. [ABSTRACT FROM AUTHOR]

Laboratory and field experiments have suggested that near-bed sediment resuspension is an intermittent process influenced by turbulent coherent structures. This is in contrast to many sediment transport theories that relate sediment transport rates to mean near bed flow parameters. This study presents observations of turbulent bursting events obtained from two contrasting environments dominated by unidirectional and oscillatory flow regimes with mean currents higher than the critical value for sand transport. In agreement with previous laboratory and field experiments the data investigated in this study (both in unidirectional current and oscillatory flow environments) indicated that sediment resuspension was an event based system with sediment flux controlled by ejections and sweeps. Both the magnitude of the Reynolds stress and duration of the events contributed to sediment resuspension. Wavelet analysis confirmed that over time turbulence occurred in slowly evolving clusters that were closely followed by periods of strong resuspension near the bed, evolving from the primary leading scales at low frequencies, and decaying in time after the termination of the turbulent agitation. The results highlight the necessity of considering the instantaneous Reynolds shear stress effects in the existing sediment transport equations which are developed based on a time-averaged flow parameters and mean a critical velocity. Field observations presented in this study highlight the importance of considering turbulence events along with the concept of impulse as an essential parameter to be considered into future modelling of sediment movement under both unidirectional and oscillatory flows. • In near-shore regions, sediment transport is an event based system. • High variability strong turbulence occurs in slowly evolving clusters. • Reynolds shear stress has significance into future sediment transport modelling. [ABSTRACT FROM AUTHOR]

Abstract: The effect of coastal reefs on seasonal erosion and accretion was investigated on 2km 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Deep chlorophyll maxima (DCM) have the capacity to fuel substantial fractions of total water column production. The ecological importance of a ubiquitous DCM layer ranging from 50 to 120m deep within Leeuwin Current (LC) and offshore waters of Western Australia is addressed here using data from a regional oceanographic field study conducted during the austral summer of 2000. Phytoplankton communities from surface and DCM layers were compared by examining pigments (chlorophyll a), phytoplankton carbon, photosynthetic characteristics and productivity rates estimated using 14C-based photosynthesis versus irradiance relationships. In the DCM layer, both extracted pigments (up to 0.83mg m−3) and phytoplankton carbon (6.4–54.4mg Cm−3) were maximal, and were on average 6 and 5 times larger than in the surface layer, respectively. Sensitivity analyses were performed on production estimates using regionally relevant ranges of light attenuation (K d=0.050–0.066m−1) and photoinhibition (β*=0.00–0.01mg C (mg chl a)−1 h−1 [μmolm−2 s−1]−1). These analyses provide upper and lower limits on previously reported estimates of primary production for the region, and show that small differences in light attenuation and photoinhibition can significantly affect computations of primary production and cause a shift from surface-dominated to DCM-dominated production scenarios. The contribution of the DCM layer to total water-column production ranged from a maximum of 30–70% under the scenarios examined. A regional overview of nitrate and stratification conditions in relation to the depth of the phytoplankton biomass maximum indicated that the critical balance between light and nutrients was a key factor driving DCM structure. We show that changing oceanographic conditions in both the along-shore and cross-shore directions, which included latitudinal variation in the strength of the LC, are accompanied by changes in the depth (and in turn production) of the DCM. The previously unrecognized significance of these DCM layers in the coastal eastern Indian Ocean has important implications for satellite-based estimates of production within the region. [Copyright &y& Elsevier]