EAST COAST LOWS

Mermaid Beach on the Gold Coast after 1967

Way back in the early 70s I started to take an interest in East Coast Lows (ECLs). My curiosity was heightened by reading about the impact of the 1967 impact of a cluster of storms, both tropical cyclones and ECLs, that ravaged the Gold Coast and parts of Sydney. We were living in Canada in that momentous year thus missing the erosion of dunes and destruction of property. I recall comments from my old friend , the late Peter McKenzie, who died while we were away. He had studied the impact of the 1950 storm year showing me the eroded scarp of that event at Boat Harbour, near Anna Bay. He also drew my attention to the incredible 1916 paper by E.C. Andrews in the Proceedings of the Royal Society of NSW on shoreline studies in Botany Bay. This is a must read for all who want to learn something about storm events (download here). Andrews went back to great storms in the 19^thcentury although his focus was on what happened in July 1912 when an ECL  eroded the foredunes of Botany Bay and dumped the massive “Mermaid” rock on the platform at Ben Buckler, Bondi.

History can teach us a lot about coastal erosion. I produced a tentative chronology of events that hit southern Queensland and NSW in 1974 (Search, vol. 5). Since then many have provided much more detail on the storm history of the east coast and looked at links to atmospheric phenomena such as ENSO, PDO, etc. Individuals and organisations such as Ian Goodwin, Peter Helman, Andy Short, David Chapman and reports by the PWD and the BPA in Qld. have all contributed to our understanding of ECLs and the impact on coastal features. Angus Gordon in his 1987 study on volumes of storm-induced beach erosion  defined likely storm demand for these great events ( 200-250m³  per metre  of shoreline for exposed beaches) after recording and then reviewing data from the impact of the 1974-1978 storm period.

The year 1974 was momentous in so many ways for Australia. Heavy rainfall occurred over much of the eastern half of the continent. On the east coast we experienced the southern penetration of a tropical cyclone and several ECLs. The first in April was minor in comparison with what was to come, but along with the January cyclone it helped move some sand seaward priming  beaches for the great May then June ECLs. Roger McLean and I had the good fortune of starting a survey of the beach north of the Moruya Airport in 1972. Here the foredune was sliced in 1974 by the massive and persistently high waves with run-up measured up to 5m. More storms through 1975 to 1978 continued to cut back the dunes  leading to a landward migration of around 80m or so of shoreline with  similar volume losses to that noted by Angus. Subsequent surveys by Roger and his team from ADFA to the present day (as discussed at the NSW Coastal Conference in 2014) showed the recovery of the foredune and the impact of other less severe ECLs on beach and dune morphology. Andy Short, Ian Turner and others have also documented these post 1974 events at places like Narrabeen. We now have very sophisticated techniques for measuring storm demand such as that experienced in Sydney in April.

What we would like to know is whether these severe eroding systems are going to be more intense and/or more frequent as the globe warms and especially as the East Australian Current (EAC) continues to get warmer. The jury is out on this as far as I am aware. But the possibility remains. So what triggers ECLs and how will the atmosphere respond to projected conditions of the next 50 to 100 years? Unlike tropical cyclones I understand that an ECL requires the interaction of a cold pool of air in the upper atmosphere and the warm  EAC. This can occur in autumn and extend into winter (on odd occasions as in 1967 to September). I was alerted to possible dynamics by the late David Wright of the Bureau of Meteorology in the mid-70s. He was then a long-range weather forecaster and relied a lot on trends in ocean surface temperatures. He alerted me to an incredible Bureau paper by S. Karelsky published in 1954 (another must read?). Karelsky studied hand drawn copies of thousands of met charts and plotted the axis of anti-cyclones in winter. What he discovered was the anomalous  course of the belt in 1950 dipping south facilitating a region of enhanced cyclogenesis in the Tasman Sea in proximity to areas of high sea surface temperature. A blocking high pressure was also involved allowing onshore streaming of moist air towards the east coast. This work prompted me to write a paper in 1978 (a chapter of a book in honour of Joe Jennings)  hypothesising on the dynamics of the ocean and atmosphere and how such conditions could promote a clustering of storms leading to what I termed a “culminating event” such as the June 1974 storm.

Sydney super storm aftermath at Bondi Beach, May 2015 (source: Daily Telegraph)

I am sure much more is now known about these processes. It is important that coastal science continues to examine the dynamics of ECLs and their link to shoreline change and climate change. We know that many beaches under current conditions will recover from storm wave impact; history and good monitoring has taught us that lesson. They are resilient beach-dune systems. But we now recognise that sediment deficits in some compartments are leading to long-term coastal recession. The big unknown is what will be the trigger for the more resilient beaches and dunes to switch from oscillating around a mean position to recession. Will it be a change in the magnitude/frequency of ECL stimulated by increased sea surface temperatures under pools of cold air, or will a level of sea level rise be needed? I suspect that beaches will not behave uniformly and that is one of the many challenges facing coastal managers at the local level in eastern Australia.

– words by Prof. Bruce Thom