Shoreline Salients, Cuspate Forelands and Tombolos on the Coast of Western Australia

Peta G. Sanderson, Ian Eliot

Abstract


Shoreline salients, cuspate forelands and tombolos on the Coast of Western Australia were examined photogrametrically to determine the suite of landforms that comprised the structures and whether there were regional differences in their geometries. Aerial photographs from three regions were examined, including the South Coast, from Cape Arid to Esperance; the Central West Coast, between Guilderton and Dongara; and the Ningaloo Coast, from Point Cloates to Exmouth.

The landforms were broadly classified according to morphological and genetic criteria, such as growth mechanism, degree of mobility, contemporary activity/processes, main processes involved in the supply of material, sources of supply, conditions of wave regime and stage of development. Second, morphometric information was taken directly from the aerial photographs. Following work reported by SILVESTER and HSU (1993), this was analysed to determine the constant and exponential values in the curvilinear relationship between the length of the offshore structure relative to its distance offshore and the difference between offshore distance and the ratio of salient protrusion to island length. Separate graphs have been compiled for all observations from Western Australia as well as for each region to indicate geographic differences. An examination of the geometric differences between each coastal region was undertaken using analysis of variance techniques. At the 5% significance level, the results indicated that, there are significant differences between the coastal regions for most ratios when tombolos are excluded from the data sets. Most of the South Coast forelands and tombolos are formed by the deposition of sediment resulting from the convergence of swell behind an obstacle, whereas on the Central West and Ningaloo Coasts the submerged reef provide an offshore barrier. Swell is complexly diffracted and refracted by the reefs and sediment is deposited in a less predictable manner. Flushing of lagoonal waters behind the reef either by longshore currents or offshore movement of water through breaks in the reefs appears to impede the formation of tombolos and provides an explanation for travelling versus stationary forms in the respective environments

It has been shown that the development of these forms cannot be attributable to the length of the offshore obstacle or the distance of the obstacle offshore. Explanation of this requires further investigation of the combined oceanographic processes occurring leeward of the reef chains on the Central West Coast and on the Ningaloo Coast.


Keywords


salient; cuspate foreland; tombolo; morphology; geometry; regional variation; Western Australia

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