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

Seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology. The coastline of Perth, Western Australia, is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology. However, these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations: in the incident wave energy conditions. In summer, when northward sediment transport prevails due to sea breeze activity, beaches located south of coastal structures, headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle. These beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south. In contrast, beaches located north of obstacles will become 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 height, w(s) is the sediment fall velocity and T is the wave period) as a predictor 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 the hydrodynamic conditions. As a result, bar occurrence and beach type can not be readily predicted using Omega along the Perth coast. (C) 2001 Elsevier Science B.V. All rights reserved.