Hydrodynamic and morphological response of a perched beach during sea breeze activity

Beaches associated with geological or engineered structures, recognised as perched beaches, are commonplace on many coastlines around the world and especially so in South West Western Australia (SWWA). Although it is accepted that hard coastal structures will affect beach behaviour, little is known about the mechanisms through which this occurs. The microtidal Perth coast in SWWA is influenced by one of the strongest and most persistent sea breeze cycles in the world. This, together with offshore limestone reefs attenuating swell means that locally generated sea breeze waves and currents dominate the hydrodynamics for half of the year. Field measurements were made of wave, current and beach morphology changes due to strong sea breeze forcing at the perched Yanchep Beach and Lagoon on the Perth Metropolitan coast. Spatial and temporal variation of waves and currents at the beach and in the lagoon were monitored throughout several sea breeze cycles and changes in beachface morphology surveyed at two beach profiles. A 50% reduction in sea breeze wind speed was found to weaken the lagoonal currents by 50% due to less wave overtopping of the limestone reef. Results show the influence of the limestone formations on waves and currents which affects the beachface response to sea breeze. Both beach profiles showed a clear cycle of erosion and accretion to sea breeze, with differences between the profiles even though they were spaced just 120 m apart. These results provide insights into the role of geological formations on the behaviour of a perched beach.