Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia

Seasonal change in beach morphology is traditionally ascribed to a variatio n in the incident wave energy level with calm conditions in summer resultin g in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology. The coastli ne of Perth, Western Australia, is characterised by a large seasonal variat ion in the incident wave height and local beaches exhibit a distinct season al change in morphology. However, these morphological changes are better ex plained by a seasonal reversal in the littoral drift direction than by vari ations: in the incident wave energy conditions. In summer, when northward s ediment transport prevails due to sea breeze activity, beaches located sout h of coastal structures, headlands or rocky outcrops become wider due to th e accumulation of sediment against the obstacle. These beaches will subsequ ently erode in winter during storms when the longshore sediment transport i s toward the south. In contrast, beaches located north of obstacles will be come narrower during summer and wider during winter. The usefulness of the dimensionless fall velocity Omega = H-b/w(s)T (where H-b is the breaker hei ght, w(s) is the sediment fall velocity and T is the wave period) as a pred ictor of presence/absence of bar morphology and beach type was investigated . It was found that Omega fluctuates around the threshold of bar formation (Omega approximate to 1.5-2) over a variety of time scales (daily, weekly, and seasonally). These temporal variations in Omega in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with t he hydrodynamic conditions. As a result, bar occurrence and beach type can not be readily predicted using Omega along the Perth coast. (C) 2001 Elsevi er Science B.V. All rights reserved.