Doctor of Philosophy
School of Earth and Environmental Sciences - Faculty of Science
Samosorn, Bongkoch, Morphological changes of a small reef island on a platform reef, PhD thesis, School of Earth and Environmental Sciences, University of Wollongong, 2006. http://ro.uow.edu.au/theses/590
This study examines: i) the morphological evolution of a small sand cay on a platform reef over past millennia; ii) changes in its shoreline over past decades; and ii) contemporary characteristics of waves on the reef platform. The study site is Warraber Island located on a platform reef in Torres Strait, Australia. The study is based upon: i) detailed radiocarbon dating; ii) 3-D morphological reconstruction using a DTM; and iii) field measurement of island topography, waves and wind.
Temporal patterns of evolution were assessed based on radiocarbon dating on both bulk samples and on specific components. Bulk radiocarbon ages are indicated to be unreliable for the determination of time of sand deposition. However, ages from shells provide a more appropriate indication of time of deposition of sand in this reef setting. Shell ages imply the continual accretion of the island over the past 3,000 years with a long-term rate of accumulation of approximately 900 m3/y. Successive stages of accretion, and episodic progradation are indicated by prominent beach ridges.
Variations in island shore position over shorter time scales were examined based on island shape over the past 40 years (1966-2004). Significant change of island shape has occurred only on the southwestern and northeastern ends, rotating in a clockwise direction; the island appears to have undergone net accretion on the northeastern end and erosion on the southwestern end.
Wind climate and medium-term wind patterns were reconstructed. The E-SSE winds, especially ESE and SE winds, have been dominant with winds from the WNNW sector subordinate. The influence of sunspot periodicity and ENSO on the wind patterns is not discernible but the initiation of solar magnetic cycle was found to coincide with times of the low magnitude of wind effect.
Wave conditions on the reef platform were examined based on the spectra of waves that were measured at a location close to the windward reef rim and five locations around the island. Development of spectral components and saturating conditions were also investigated. Wave characteristics were normally tidally modulated and related to the local wind system. In addition to incident wind wave components generated off the reef, incident short-period wave and infragravity wave components generated on the reef were prominent around the island. Combinations of these three components generate higher and shorter waves on the windward side of the island, and smaller and longer waves on the leeward side of the island. These wave characteristics can initiate movement of sediment around the island and on the island beach.
Probable wave conditions in relation to topographical development over the period of island evolution are discussed based on the contemporary wave conditions on the reef platform. Wind climate and associated patterns of waves and sediment transport are major influences on island shape in response to seasonal influence. Changing wind patterns and wave conditions in relation to nearby reefs, and human activities on the island were examined to identify probable causes of change in island shape over the past 40 years. Over the coming decades, the island is likely to be maintained by the transport of sediment already on the reef flat to the island. Further adjustments of the island morphology are anticipated due to changes in climate and sea level. However, even if there is a gradual rise in sea level, sediment supply to the island is still anticipated because of the greater capacity of waves to entrain and transport sediment, presently abundant on the reef flat.
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