SEMINAR: Hydrodynamics and sand transport on perched beaches in Western Australia

Beach morphology is the result of complex interactions between sand transport, mean sea level, wind, surface gravity waves, and currents, all of which act over a range of temporal and spatial scales. Interactions with rocky landforms add another level of complexity to the mechanisms of beach variability and change. At Yanchep Lagoon in south-western Australia, the sandy beaches are perched on Quaternary limestone reefs. Sand transport at Yanchep Lagoon varies over a wide range of temporal scales and is strongly modified by interactions with the topographically-complex reefs. To better understand how sand transport is affected by the reefs, a suite of numerical models that covered a cascade of spatial scales from the ocean to the beach was used.

First, the wave climate and variability of the southern Indian Ocean was hindcast from 1970 to 2009 using the WAVEWATCH III ® model validated with data around the WA coastline. There was a significant positive trend in annual mean wave height which appeared to be due to an increase in intensity of the storm belt in the Southern Ocean. The hindcast of offshore wave conditions were then used in a regional scale model (SWAN), where the nearshore transformation of the largest wave event on record was simulated. Shallow limestone reefs on the inner-shelf efficiently dissipated 70 to 80% of the wave energy. At the beach, waves breaking on nearshore reefs generated strong, complex currents. Simulations of beach morphology, using XBeach_gpu indicated that the nearshore reefs and resulting currents affect the beach morphology not only in the lee of the reef but also along the adjacent beaches to the north. The evolution ofperched beaches under climate change and sea level rise can be predicted using a better understanding of how rocky landforms influence beach morphology; and of the down-scaling of larger scale processes from the ocean to the beach.