Sheltered Sandy Beaches of Southwestern Australia

Abstract

Morphodynamic classifications of sandy beaches have been established for open-ocean, wave-dominated environments. However, many natural sandy beaches exist in embayments, or are landward of protective reefs, where they are sheltered from the full effects of ocean waves. It is therefore appropriate to question whether such low-energy beaches can be related to conceptual models of beach hierarchies, and to examine whether they have identifiable morphodynamic signatures.

Surveys were conducted of the nearshore morphology and dynamics on over fifty beaches on the microtidal coast of Southwestern Australia, between Cape Arid on the South and Geraldton on the West Coast. In most instances, surveys were conducted on beaches that were sheltered by their aspect and/or the presence of offshore reefs. The remaining surveys were conducted on wave-dominated beaches in order to provide a link to the existing morphodynamic models.

Descriptions of beach morphology, determined from the surveys, were subjected to a cluster analysis to establish groupings of similar morphologic types. This analysis provided a six-fold classification of beach morphologies and indicated a clear separation between the low- and high-energy beach morphologies on the basis of the overall scalp of the nearshore profiles. Four low-energy morphotypes were distinguished. These are essentially planar and characterized by the absence of either nearshore bars or other rhythmic features. However, the low-energy morphotypes may be discriminated by variations in beach slope and curvature.

Canonical variate analysis was conducted to examine the discrimination of the six morphotypes on the basis of their sedimentary and dynamic characteristics. This analysis indicated consistent sedimentologic differences between the morphotypes, despite moderate overlapping between several of the beach forms. The variation accords with expectations that flatter beaches tend to have finer sediments. Discrimination between the morphotypes on the basis of their dynamic variables was less revealing. This raises questions of misfitting between form and process during the surveys and may indicate the importance of storm events in the formation of these low-energy morphotypes.