Doctor of Philosophy
School of Earth & Environmental Sciences - Faculty of Science
Barry, Stephen J, A morphodynamic model of atoll-islet evolution, PhD thesis, School of Earth and Environmental Sciences, University of Wollongong, 2008. http://ro.uow.edu.au/theses/378
A morphodynamic model of atoll-islet evolution, the Sediment Allocation Model (SAM), was developed based on the assumption that islets are equilibrium landforms. The assumption that islets are equilibrium landforms implies that the volume of sediment sequestered in an islet reaches stability when considered over the time-scale of islet evolution. Stability of the islet implies that the maximum volume of an islet (accommodation) can be quantified. Morphodynamic feedback between the islet and sediment transport processes is manifest in the rate of sediment sequestration by the islet. Initial increase of islet volume is rapid, sediment sequestration slows as islet volume increases, and stops when accommodation is full. The rate of islet development is constrained by antecedent morphology, sediment supply, and sediment transport. The constraints and accommodation for islet evolution are difficult to measure directly. Estimates of the accommodation, sediment supply, the effect of antecedent morphology, and the morphodynamic feedback between islet volume and sediment sequestration are generated using measurements of islet volume in an inversion algorithm. Experimentation carried out using a hypothetical test-case indicated that errors in the measurements of islet volume used in the inversion algorithm would result in a damping of the morphodynamic feedback and overestimation of both sediment supply and accommodation. The SAM was implemented using published radiocarbon-dated samples for surveyed islets transects. A data-model template was developed to incorporate the published data into the SAM. In the data-model template an atoll-islet was represented by a xvii combination of morphological sub-units with sub-unit sequestration dominated by either lagoon processes or ocean processes. The input to the SAM was a series of sediment volumes representing islet evolution. Sediment volumes were calculated using isochron patterns that represent a hybrid of lateral accretion and vertical accretion, recognized modes of sediment accumulation on atoll-islets. Sediment volume measurements calculated using the hybrid-accretion patterns were consistent with the assumption of morphodynamic feedback between islet morphology and sediment accretion processes. Comparative metrics are required to test and refine models of islet evolution, however, suitable comparative data are not currently available. The hybrid-accretion pattern of islet evolution formed the basis for the development of a metric to determine islet location on the reef platform. Islet location was measured on four atolls as the distance from the reef crest to the oceanward beach-toe. The results of the measurement procedure were evaluated in the context of three factors that influence islet evolution: energy-exposure, sediment supply, and antecedent morphology. The results indicate that the distance from the reef crest to the oceanward beach-toe is a measurable response to the influence of the three factors.