by Thomas R. Mortlock, Ian D. Goodwin, John K. McAneney and Kevin Roche.

In June 2016, an unusual East Coast Low storm affected some 2000 km of the eastern
seaboard of Australia bringing heavy rain, strong winds and powerful wave conditions. While wave
heights offshore of Sydney were not exceptional, nearshore wave conditions were such that
beaches experienced some of the worst erosion in 40 years. Hydrodynamic modelling of wave
and current behaviour as well as contemporaneous sand transport shows the east to north-east
storm wave direction to be the major determinant of erosion magnitude. This arises because of
reduced energy attenuation across the continental shelf and the focussing of wave energy on coastal sections not equilibrated with such wave exposure under the prevailing south-easterly wave climate. Narrabeen–Collaroy, a well-known erosion hot spot on Sydney’s Northern Beaches, is shown to be particularly vulnerable to storms from this direction because the destructive erosion potential is amplified by the influence of the local embayment geometry. We demonstrate the magnified erosion response that occurs when there is bi-directionality between an extreme wave event and preceding modal conditions and the importance of considering wave direction in extreme value analyses. Looking at cases such as this help to analyse coastal risks.

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