Ian Eliot

'Low energy' sandy beaches in marine and estuarine environments: a review

This review was undertaken to identify locations where low energy beaches may occur and their diagnostic forms and process controls, including waves, tides and water levels. Examples are drawn from the sheltered coastline of Western Australia near Perth and fetch-limited estuarine environments on the northeast coast of the United States. We suggest that the term low energy be used in locations where: (1) non-storm significant wave heights are minimal (e.g. <0.25 m); (2) significant wave heights during strong onshore winds are low (e.g. <0.50 m); (3) beachface widths are narrow (e.g. <20 m in microtidal environments); and (4) morphologic features include those inherited from higher energy events. Micro-topographic features can persist in the swash zone of low energy beaches under non-storm wave conditions. There is little evidence of cyclic cross-shore sediment exchange. Bars, excepting transverse forms, located seaward of low still-water level do not appear to be part of the sediment exchange system with the foreshore. Developing a better definition of the term low energy requires understanding the occurrence and duration of morphological characteristics and the type, magnitude and frequency of hydrodynamic controls that are responsible for these characteristics. Efforts also should be directed toward: (1) discriminating between processes generated within basins (in true fetch-limited environments) and processes generated outside basins (that affect sheltered environments); (2) identifying the relative contributions of tide- and surge-related water level fluctuations on low energy beach shape; and (3) estimating thresholds for beach change.

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.

Predicted climate change, sea-level rise and wetland management in the Australian wet-dry tropics

The vulnerability of coastal areas in the Alligator Rivers Region (northern Australia) to predicted climate change and potential sea level rise was assessed as part of a national study. The coastal area is composed of a number of estuarine and freshwater habitats that are intricately interlinked and can not be effectively managed in isolation of each other. The outcomes of the assessment focused on the floodplain environments of the region, but are also applicable to the broader wetland environments that occur across the northern Australian wet-dry tropics. The management regime in the region is based on traditional Aboriginal ownership of much of the land, which is leased to the federal government as a national park. Scientific research has been intensive; however, important questions have been raised about the collation and effective use of this information. The vulnerability assessment framework required effective use of this information and cooperation with the management authority to identify change scenarios and management and research responses. A climate change scenario was established as the basis for predicting biophysical change in the coastal and wetland environments. The predictions suggest that large-scale change will occur and many of the existing values derived from these areas (i.e., usage by traditional Aboriginal occupants, and nature conservation) could be degraded or even lost. Recommended management responses include the initiation of specific monitoring, empowerment of local bodies to take active management steps, and to increase awareness of the likely consequences of change. Further data coordination and review are needed to ascertain the validity of the predictions and the concomitant management responses.

A user-friendly quantitative approach to classifying nearshore marine habitats along a heterogeneous coast

A scheme, which can be readily used by fisheries and environmental managers and ecologists, has been developed for quantitatively classifying the different habitats found in nearshore marine waters along the heterogeneous lower west coast of Australia. Initially, 25 beach sites, representing a wide range of nearshore environments, were separated into six a priori habitat types on the basis of characteristics that could readily be observed and were likely to influence the extent to which a particular (fish) species occupies a particular habitat. Focus was thus placed on such features as the degree of exposure to wave activity and whether or not seagrass and/or reefs were present in the nearshore vicinity. Subsequently, quantitative data for 27 environmental variables, considered likely to characterise the six habitat types, were obtained for each of the 25 sites from readily accessible sources. When the latter data were subjected to multidimensional scaling (MDS) ordination, the points for the sites representing only three of those six habitat types formed discrete groups. The bvstep routine in the Primer v5.0 statistical package (Clarke & Gorley, Primer v5.0: User Manual/Tutorial, Primer-E Ltd, Plymouth, 2001) was thus used to select a subset of the 27 environmental variables that would provide a better resolution of the six a priori habitat types. This process involved matching the distance matrix constructed from the quantitative environmental data with a matrix constructed from scored data that reflected the criteria for the initial a priori classification scheme. A subset of seven environmental variables gave the best correlation between the two matrices (ρ=0.823), and thus provided the optimal set of quantitative data for discriminating between the six a priori habitat types. These variables comprised both the direct and north-westerly fetches, the minimum distance from the shoreline to the 2 m depth contour, the distance from the shoreline to the first offshore reef chain along a south-westerly transect, and the relative contributions of bare sand, subtidal reef and seagrass. Data for these characteristics at any nearshore site along the coastline can readily be recorded by managers and ecologists and subjected to the ‘nearest-replicate’ classification procedure developed in this study to ascertain the habitat type to which that site should be assigned. Current work is using MDS ordination, in conjunction with associated statistical tests and the bvstep routine, to elucidate the extent to which the compositions of assemblages of fish, benthic macroinvertebrates, meiofauna and zooplankton in nearshore waters along the lower west coast of Australia are related to habitat type(s).

Water filtration through reflective microtidal beaches and shallow sublittoral sands and its implications for an inshore ecosystem in Western Australia

Abstract Volumes of seawater filtered through the intertidal zone were measured on three modally reflective microtidal beaches in Western Australia. The filtered volumes were large, 19 m 3 m −1 day −1 and 73 m 3 m −1 day −1 on two ‘clean’ beaches but only 0·4 m 3 m −1 per tidal cycle on a beach covered in kelp and seagrass wrack. The mean residence times of this water in the interstitial system and its percolation paths were both short, 1–7 h and 2–5 m respectively. Water input was greater across a beach cusp horn than across a cusp embayment. Most input occurred in the upper swash zone where the water table was less than 20 cm deep. Tidal variations in input volumes were evident even with tide ranges of only 20 cm. The inshore zone off these beaches filters on average 0·07 m 3 m −2 day −1 at an average depth of 5·5 m under 0·4 m waves of 6·5 s duration. The importance of these procedures in the mineralization of organic materials and the regeneration of nutrients for an inshore ‘lagoon ecosystem’ is estimated and discused.

Minor storm impact on the beachface of a sheltered sandy beach

Abstract During a squall at Sorrento, Western Australia, winds exceeded 50 km h −1 for 6.5 h and gusted up to 80 km h −1 at the storm peak. Profiles of adjacent cusp-horn and cusp-embayment locations were surveyed half hourly, for over 40 h spanning onset and passage of the storm, in conjunction with measurement of water table fluctuation and swash ranging. The sequence of beach changes followed the storm structure, although with lag effects related to tide, groundwater and swash interaction with the local morphology. Four phases of beachface adjustment were apparent. These were: (1) a period of adjustment to rising tide and energy conditions, during which the cusp-horn was eroded and sediment was deposited in the embayment; (2) beachface erosion as storm, tide and watertable approached their peak; (3) a period of adjustment to declining energy conditions, falling tide and watertable levels that began immediately after the storm peak on the cusp-horn and at mid-ebb tide in the embayment; and (4) steady beachface accretion through mid-to-low tide conditions in the latter part of storm passage. Beach responses on the cusp-horn and embayment hence were dissimilar, with net erosion resulting on one and net deposition on the other.

Regional variation of coastal morphology in southwestern Australia: a synthesis

Abstract The morphology of landforms on the south coast of Western Australia is determined predominantly by wave refraction around discrete headlands and islands. Wherever offshore structures protect the shore from the direct effects of swell, sheltered sandy beaches have developed in association with cuspate forelands and tombolos. In contrast to this open coast setting, the nearshore waters of the west coast are protected by semi-continuous reef systems, which significantly modify the morphology of large-scale accretionary landforms, beaches and foredune sequences. On the south coast, foredune plains occur primarily as fill in sheltered embayments and storm built ridges do not occur. Foredunes on the west-coast include washover ridges and low aeolian dunes on the backshore of embayment and inset beaches. The form of high wave-energy beaches of the south coast fluctuates between reflective and dissipative morphodynamic states, and most commonly between transitional and dissipative states. Sediments are mainly fine grained siliceous sands. In contrast, the low-energy west-coast beaches are composed of medium to coarse grained, calcareous sands. The beaches are planar in section, characterised by lines of debris deposited by tidal and longer-term fluctuations in sea-level and their form does not alter with short-term changes in the wave regime. Despite the very low energy micro-tidal conditions experienced by the coasts of southwestern Australia, systematic variation in the morphology of coastal landforms does occur. As protection to the coast increases from the open-fetch south-coast environment to the reef-protected west-coast setting, swell energy decreases, there is an increase in the relative importance of locally generated wind waves, wave set-up and tidal forcing of currents, and forelands become increasingly asymmetric due to the strength of longshore sediment transport.

Mean sea-level and beach-width variation at Scarborough, Western Australia

Abstract A sixteen-year record of beach-width change at six profile stations on Scarborough Beach, in the Perth Metropolitan Area of Western Australia, was analysed by Eliot and colleagues. Their results described long-term beach progradation, measured over the sixteen years, and identified cyclic beach changes with periods of 0.5, 1.0, 3.5 and 7.0 yrs, and a long-term (secular) progradational trend. In this paper, the beach-width measurements from Scarborough are compared with twelve years of mean sea-level record from Fremantle, Western Australia, approximately 18 km south of Scarborough Beach. The annual cycles of each respond in an inverse manner such that a 1-cm rise in mean sea level corresponds with a 1-m decrease in the beach width. The sea-level rise leads the beach width retreat by approximately one-fifth of a cycle. A long-period oscillation of 3.5 yrs in the mean sea level shows a similar 100:1 ratio for beach gradation, with a lead of approximately one-tenth of a cycle. However, the secular trend in beach width change was not evident in the mean sea-level record, indicating that the progradation is tied to other processes, such as a northerly sediment drift related to prevailing SW swell and associated current activity.

Low-frequency changes of sediment volume on the beachface at Warilla Beach, New South Wales, 1975-1985

Abstract Characteristic patterns of shore-parallel and shore-normal movements of sediment on a sandy beach are described in this study. The sediment movements are apparent as low-frequency variations in the volume of sediment stored above mean low water spring tide (MLWST) level on Warilla Beach, N.S.W. They were determined by an empirical orthogonal-function (EOF) analysis of beach profile records. The profiles were surveyed from eighteen stations along a 2 km long beach at least monthly, generally fortnightly, for over 10 years, from July 1975 until August 1985. Results from the EOF analysis confirm and substantially extend an earlier study of Warilla Beach when the first 5 years of the beach survey record was examined ∗ . The importance of low-frequency beachface change identified in the earlier, shorter study was confirmed from the extended data set; zones of maximum variability, linked with rip-current activity, are consistent in both studies, and similar patterns of alongshore sediment movement in the swash zone were determined from both analyses. The principal differences between the two sets of results highlight beachface responses to processes operating in different time scales. Amplitude spectra from time series describing the characteristic modes of sediment movement along and across the beachface have periodicities of 2.0, 1.7 and 1.0 years. Beach changes with periods close to 3 years are apparent in the record, but cannot be resolved by standard spectral analysis techniques. Each period is associated with a characteristic pattern of beach change; the triennial and 1.7 year cycles with alongshore sediment transport, and the biennial cycle with onshore-offshore sediment exchange. Eigenvectors from the EOF analysis identify the characteristic patterns of beach change from survey data specified to highlight the shore-parallel sediment movement along 0.5 m deep, horizontal segments of beach, and shore-normal sediment movement along the beach profiles. In the analysis of shore-parallel variation in beach profile configuration, the fundamental eigenvectors explain between 70 and 80% of beachface variance in each instance, and different patterns of sediment movement were identified for each horizontal slice. On the upper part of the beachface, all eigenvectors identified a zone of low variability along the shoreline skirting a rockwall. Below berm crest, in the swash and intertidal zones of the beach, and away from the rockwall, the northern half of the beach was highly variable. Second mode eigenvectors (cellular-flux modes) explain a further 5 to 14% of the variance and describe sediment exchanges between discrete zones in each horizontal segment of the beachface. Zones of greatest variability occurred landward of places where rip currents frequently develop. Conversely, zones of low variability are coincident with areas of beachface in the lee of places where inshore bars frequently form. Analysis of sediment movement along each profile is consistent with the beach changes described. The fundamental eigenvectors account for 91 to 99% of the variance on each profile. They describe an onshore-offshore movement of sediment through the intertidal zone of the beach.

Variation in subaerial beach sediment volume on a small sandy beach over a monthly lunar tidal cycle

Abstract Three major patterns of sediment movement along 0.5 m deep horizontal segments of beachface on a small sandy beach at Coledale, New South Wales, were determined by empirical orthogonal function (EOF) analysis of a 36 day record of beach change. The fundamental beach response is identified by the first eigenfunction mode. The mode describes sediment transfers occurring over the beachface, between the low-tide zone and the berm crest as well as sediment movement along the beach. The alongshore translocations occurred about a nodal point located approximately 0.75 of distance along the beach, from its southern end. Transfers across the beach were established by comparing the time series associated with the eigenfunctions describing alongshore transfers of sediment in different segments. The slice-to-slice comparisons showed an increasing phase down the beachface from the berm crest segment to the mid-tidal zone. The phase difference was approximately 2.5 days per 0.5 m segment. The fundamental beach response and secondary sediment transfers typical of the supratidal zone occur with an apparent 28–30 day lunar tide cycle. They were unrelated to short-term variations in the incident wave amplitude. Lower down the beachface the time series associated with the eigenfunction describing sediment translocations in the mid-tidal zone is aperiodic. The irregularities can be linked to particular process events. However, similar beach changes were not always related to similar process conditions and they were not simply related to wave-regime fluctuations. The patterns of sediment movement along Coledale Beach were consistent with similar patterns reported from elsewhere by Clarke and Eliot (1982, 1983a) in that they were closely related to variation in the nearshore water circulation system. In particular, nodal points identified by the EOF analysis commonly occurred on the beachface landward of inshore regions where bars most frequently developed. The antinodal positions were similarly associated with regions where rip channels are frequently formed.