Wave climate and coastal change in southeast Australia

Wave-driven currents are the principle mechanism for sand transport on the southeast Australian inner continental shelf and surf zone. An understanding of changes to wave climate is thus imperative for projecting and managing coastal change in the coming century. Two important climate change signatures for southeast Australia are tropical expansion and the changing behaviour of El Niño Southern Oscillation (ENSO). Results suggest that tropical expansion will lead to longer periods of more easterly modal wave conditions along the southeast Australia shelf (SEAS) than at present, punctuated by less frequent but higher magnitude storm events from the east to north-east. This will lead to more (less) cross- (along-) shore sediment transport. Parabolic bay-beaches in the lee of southern headlands are most vulnerable to these changes. On longer timescales, changes to wave-induced cross-shelf transport with tropical expansion may facilitate coastal evolution with sea-level rise. There is considerable uncertainty in the prediction of future ENSO behaviour, and the coastal response to different central Pacific (CP) versus eastern Pacific (EP) flavours of ENSO is thus far unknown. Results show that CP ENSO produces significantly different wave climates to EP ENSO along the SEAS, by the modulation of trade-wind wave generation. Results also show that a) the shoreline response to ENSO in a headland-bay beach is more complex than the existing paradigm that (anti-) clockwise rotation occurs during El Niño (La Niña), and b) coastal change between ENSO phases cannot be inferred from shifts in the deepwater wave climate, as previously assumed.Morphodynamic modelling indicates that CP ENSO leads to higher coastal vulnerability than EP ENSO during an El Niño/La Niña cycle. A new link between surf zone morphology and the shoreline equilibrium profile of headland-bay beaches has also been made, which allows the Parabolic Bay Shape Equation to be applied to real-world wave conditions with a greater level of objectivity.