Charitha Pattiaratchi

Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia

Abstract Seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology. The coastline of Perth, Western Australia, is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology. However, these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions. In summer, when northward sediment transport prevails due to sea breeze activity, beaches located south of coastal structures, headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle. These beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south. In contrast, beaches located north of obstacles will become narrower during summer and wider during winter. The usefulness of the dimensionless fall velocity Ω=H b /w s T (where Hb is the breaker height, ws is the sediment fall velocity and T is the wave period) as a predictor of presence/absence of bar morphology and beach type was investigated. It was found that Ω fluctuates around the threshold of bar formation (Ω≈1.5–2) over a variety of time scales (daily, weekly, and seasonally). These temporal variations in Ω in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions. As a result, bar occurrence and beach type can not be readily predicted using Ω along the Perth coast.

The Capes Current: a summer countercurrent flowing past Cape Leeuwin and Cape Naturaliste, Western Australia

Abstract Although the dominant boundary current off Western Australia is the poleward-flowing Leeuwin Current, satellite imagery shows that there is a cool equatorward coastal countercurrent running close inshore in the extreme southwest during the summer months. This seasonal current has been named the Capes Current as it appears to be strongest between Cape Leeuwin (34°20′S) and Cape Naturaliste (33°30′S), and it is probably linked with the general northward shelf current which has been observed previously along most of the Western Australian coastline further north. Strong northwards wind stresses between November and March slow the Leeuwin Current (which moves offshore) and drive the Capes Current, and there may be localised upwelling as well (Gersbach et al., Continental Shelf Research, 1998). It has important implications for the salmon fishery as it may affect the migration of adult salmon around Cape Leeuwin at this time of year.

Marine Plastic Pollution in Waters around Australia: Characteristics, Concentrations, and Pathways

Plastics represent the vast majority of human-made debris present in the oceans. However, their characteristics, accumulation zones, and transport pathways remain poorly assessed. We characterised and estimated the concentration of marine plastics in waters around Australia using surface net tows, and inferred their potential pathways using particle-tracking models and real drifter trajectories. The 839 marine plastics recorded were predominantly small fragments (“microplastics”, median length = 2.8 mm, mean length = 4.9 mm) resulting from the breakdown of larger objects made of polyethylene and polypropylene (e.g. packaging and fishing items). Mean sea surface plastic concentration was 4256.4 pieces km−2, and after incorporating the effect of vertical wind mixing, this value increased to 8966.3 pieces km−2. These plastics appear to be associated with a wide range of ocean currents that connect the sampled sites to their international and domestic sources, including populated areas of Australias east coast. This study shows that plastic contamination levels in surface waters of Australia are similar to those in the Caribbean Sea and Gulf of Maine, but considerably lower than those found in the subtropical gyres and Mediterranean Sea. Microplastics such as the ones described here have the potential to affect organisms ranging from megafauna to small fish and zooplankton.

Impact of sea-breeze activity on nearshore and foreshore processes in southwestern Australia

Abstract In coastal regions sheltered from the direct impact of swell- and storm-wave activity, locally generated wind waves, particularly those associated with strong sea-breeze activity, play a dominant role in controlling nearshore and foreshore processes. Field data collected from the Perth Metropolitan Coast (western Australia) during a typical summer sea-breeze cycle, are presented. It is demonstrated that the nearshore environment responds rapidly to an increase in wind speed (up to 12 m s −1 ) during the sea breeze, resulting in considerable changes to the nearshore hydrodynamics and morphology. Incident wave energy increased during the sea breeze and was associated with development of a wind-wave field with significant wave heights up to 0.9 m. Nearshore currents responded to this change in wave climate with the development of net offshore near-bed currents and a rapid increase in the mean longshore current from −1 to 1.0 m s −1 A 10-fold increase in suspended sediment concentration and a 100-fold increase in the longshore sand transport resulted from the effects of the sea-breeze system. Erosion of the beachface was coincident with the development of the wind-wave field. Sea breeze wave-driven water circulation also completely eroded beach cusps (wavelength 20–30 m), overwhelmed the rip current system associated with the beach cusps and suppressed the infra-gravity wave frequencies in the incident wave and swash record. The beach cusps reformed after the cessation of the sea breeze. It is demonstrated that the beachface is in a constant stage of adjustment to the incident wave energy through the diurnal sea-breeze cycle alternating between dissipative and reflective morphodynamic regimes. The results may be used to determine the impact of a medium-sized storm on the beachface. It is clear that the sea-breeze system plays a major role in controlling the nearshore and foreshore processes not only in this region, but also on other geographic locations where strong sea breezes are present.

Numerical modelling of tide-induced beach water table fluctuations

Field studies have shown that the elevation of the beach groundwater table varies with the tide and such variations affect significantly beach erosion or accretion. In this paper, we present a BEM (Boundary Element Method) model for simulating the tidal fluctuation of the beach groundwater table. The model solves the two-dimensional flow equation subject to free and moving boundary conditions, including the seepage dynamics at the beach face. The simulated seepage faces were found to agree with the predictions of a simple model (Turner, 1993). The advantage of the present model is, however, that it can be used with little modification to simulate more complicated cases, e.g., surface recharge from rainfall and drainage in the aquifer may be included (the latter is related to beach dewatering technique). The model also simulated well the field data of Nielsen (1990). In particular, the model replicated three distinct features of local water table fluctuations: steep rising phase versus flat falling phase, amplitude attenuation and phase lagging.

Beach water table fluctuations due to wave run‐up: Capillarity effects

High-frequency beach water table fluctuations due to wave run-up and run-down have been observed in the field [Waddell, 1976]. Such fluctuations affect the infiltration/exfiltration process across the beach face and the interstitial oxygenation process in the beach ecosystem. Accurate representation of high-frequency watertable fluctuations is of importance in the modeling of (1) the interaction between seawater and groundwater, more important, the effects on swash sediment transport and (2) the biological activities in the beach ecosystem. Capillarity effects provide a mechanism for high- frequency water table fluctuations. Previous modeling approaches adopted the assumption of saturated flow only and failed to predict the propagation of high-frequency fluctuations in the aquifer. In this paper we develop a modified kinematic boundary condition (kbc) for the water table which incorporates capillarity effects. The application of this kbc in a boundary element model enables the simulation of high-frequency water table fluctuations due to wave run-up. Numerical tests were carried out for a rectangular domain with small-amplitude oscillations; the behavior of water table responses was found to be similar to that predicted by an analytical solution based on the one-dimensional Boussinesq equation. The model was also applied to simulate the water table response to wave run- up on a sloping beach. The results showed similar features of water table fluctuations observed in the field. In particular, these fluctuations are standing wave-like with the amplitude becoming increasingly damped inland. We conclude that the modified kbc presented here is a reasonable approximation of capillarity effects on beach water table fluctuations. However, further model validation is necessary before the model can confidently be used to simulate high-frequency water table fluctuations due to wave run-up.

Millimeter-sized marine plastics: a new pelagic habitat for microorganisms and invertebrates.

Millimeter-sized plastics are abundant in most marine surface waters, and known to carry fouling organisms that potentially play key roles in the fate and ecological impacts of plastic pollution. In this study we used scanning electron microscopy to characterize biodiversity of organisms on the surface of 68 small floating plastics (length range = 1.7–24.3 mm, median = 3.2 mm) from Australia-wide coastal and oceanic, tropical to temperate sample collections. Diatoms were the most diverse group of plastic colonizers, represented by 14 genera. We also recorded ‘epiplastic’ coccolithophores (7 genera), bryozoans, barnacles (Lepas spp.), a dinoflagellate (Ceratium), an isopod (Asellota), a marine worm, marine insect eggs (Halobates sp.), as well as rounded, elongated, and spiral cells putatively identified as bacteria, cyanobacteria, and fungi. Furthermore, we observed a variety of plastic surface microtextures, including pits and grooves conforming to the shape of microorganisms, suggesting that biota may play an important role in plastic degradation. This study highlights how anthropogenic millimeter-sized polymers have created a new pelagic habitat for microorganisms and invertebrates. The ecological ramifications of this phenomenon for marine organism dispersal, ocean productivity, and biotransfer of plastic-associated pollutants, remains to be elucidated.

Timescales for detecting a significant acceleration in sea level rise

There is observational evidence that global sea level is rising and there is concern that the rate of rise will increase, significantly threatening coastal communities. However, considerable debate remains as to whether the rate of sea level rise is currently increasing and, if so, by how much. Here we provide new insights into sea level accelerations by applying the main methods that have been used previously to search for accelerations in historical data, to identify the timings (with uncertainties) at which accelerations might first be recognized in a statistically significant manner (if not apparent already) in sea level records that we have artificially extended to 2100. We find that the most important approach to earliest possible detection of a significant sea level acceleration lies in improved understanding (and subsequent removal) of interannual to multidecadal variability in sea level records.

Upwelling on the south-west coast of Australia—source of the Capes Current?

A series of temperature—salinity and CTD sections o⁄ the south-west coast of Western Australia show the presence of cooler water on the upper ((50 m) continental shelf during the summer months, indicative of upwelling in this region. These data are used to investigate the local dynamics in the region and to determine the source of the relatively cool water of the Capes Current. In particular, a CTD transect o⁄ Cape Mentelle during early February 1987, in conjunction with CTD transects taken earlier (late January 1987) o⁄ Point D’Entrecasteaux and Albany, show that cooler water o⁄ Cape Mentelle is generated by localised upwelling during the summer period, forming the source of the Capes Current. A comprehensive data set in December 1994 revealed upwelling over the inner shelf at Cape Mentelle, coincident with the cool, northerly flow of the Capes Current. Estimates of Ekman driven transports show that flushing of the upper continental shelf can occur between 5 and 9 times per summer. ( 1999 Elsevier Science Ltd. All rights reserved.

A morphodynamic model to simulate the seasonal closure of tidal inlets

Abstract Seasonally open tidal inlets usually occur in microtidal, wave-dominated coastal environments where strong seasonal variations of streamflow and wave climate are experienced. These inlets are closed to the ocean for a number of months every year due to the formation of sand bars across their entrances. The annual closure of these inlets inhibits ocean access for boats and could also cause deterioration of water quality in the estuary/lagoon connected to the inlet. As these estuaries/lagoons are commonly used as harbours or recreational facilities there is increased interest in keeping the inlets permanently open. A process-based numerical model capable of simulating inlet closure is invaluable in terms of identifying the natural processes governing inlet closure. As a further step, this type of model could also be used to determine the effect of any proposed engineering solutions to keep the inlet open on the adjacent beaches. A morphodynamic model capable of simulating the seasonal closure of inlets, which includes both longshore (LST) and cross-shore transport (CST) processes, was developed in this study. Application of the model to two idealised scenarios indicated that cross-shore processes govern inlet behaviour when LST rates were low. The Deans criterion [Dean, R.G., 1973. Heuristic models of sand transport in the surf zone. Proc. Conf. on Eng. Dynamics in the Surf Zone, Sydney, pp. 208–214.] for on–offshore transport was employed to show that, for small offshore wave incidence angles, onshore transport aided inlet closure when the offshore wave steepness ( H o / L o ) was less than the critical wave steepness ( H o / L o ) crit , while offshore transport helped to keep the inlet open when ( H o / L o ) was greater than ( H o / L o ) crit . LST was found to be the dominant process leading to inlet closure when ( H o / L o ) was much larger than ( H o / L o ) crit or when the offshore wave incidence angle was large.