May-June Storms 1974: Part 3 – In Retrospect
Looking back over the years we should be well aware of storm impacts along the east coast. There are now many records of such events. I was once shown a scarp at Boat Harbour near Port Stephens that was cut in the great 1950 storm and never reached again. Manly was hit hard in that storm (see image above) and there are many graphic images of the September 1967 storm on Sydney beaches. But it is Plate V (Fig. 2) in the study by E.C. Andrews at Lady Robinson’s Beach (Botany Bay/Kamay) that really resonates with me showing the 1912 erosion scarp seaward of a much more prominent 1889 scarp:
“Lady Robinson’s Beach has been visited by several great gales during the past 60 years. The greatest of these, considered from the point of view of action in this bay, was the Dunbar Storm in 1857. The next in point of severity was the Dandenong Storm in September 1876, the next again was the great storm of May 1889, while the storm of July 1912 was far less severe than any of those just mentioned although the waves in the bay were greater during 1912 than they had been since 1889 … According to the reliable testimony of old residents, the waves, during the 1857 and 1876 storms, sent drenching clouds of spray well inland of the sand dunes, a condition of things which was only reproduced to a slight extent during the great storm of 1889” (“Shoreline Studies at Botany Bay”, Jour. Proc. Royal Soc. NSW, 1916, vol. 50, 170-171).
However, the May-June 1974 storms are different in so far as they yield measurements of forces and impacts at different places along the coast (see Blog No. 260). That does not mean they were more or less “severe” as meteorological and oceanic events compared to those noted by Andrews and others. But they do provide a range of lessons from which we can better understand scientific, engineering and coastal planning implications. The following is based on personal reflections supported by discussions with Angus Gordon and Mitch Harley.
- These two storms provided quantitative measures of “storm demand”. Information on this was assessed by Gordon in his 1989 paper “Beach fluctuations and shoreline change—NSW” (8th Australasian Conference on Coastal and Ocean Engineering, Launceston, see Fig. 5.2, p.106). Important to consider here are local demands made during beach/foredune erosion at rip heads.
- Impacts of these two storms should not be viewed in isolation; the Bengello/Moruya surveys recorded an erosional trend starting in mid-1973 following a period of accretion; two storms earlier in 1974 set the scene for May-June impacts along the entire embayment; post 1974 erosion continued during events until 1978; in effect there was during the 1970s a “clustering” of storm events.
- Somewhat different meteorological conditions produced these two storms with the one in May coinciding with a very high astronomical tide. Both storms illuminated our understanding of “east coast lows”. The May storm was linked to the migration of an intense low- and high-pressure system leading to the rapid formation of a secondary low south of Sydney. This intensified as it moved north towards Newcastle (Sygna shipwreck). In the June storm a much larger depression emerged across the Tasman Sea producing long period waves for a longer duration than that in May, leading to widespread impacts along the entire coast. As previously noted this was a period of relatively warm SST.
- Measurements of wave height, period, setup and runup were made as a result of work conducted by Angus and others following the May-June event. Surging into lagoons and into Sydney Harbour was very evident at this time leading to elevated oceanic water levels that flooded and destroyed properties. The largest tidal anomaly on record at Fort Denison in Sydney Harbour of 59cm occurred on 26 May 1974. Lessons were learnt here on how such storms impact on structures inside estuaries and bays including the construction of the Banksmeadow revetment at Botany Bay (a paper on the history of this structure is under preparation by Angus Gordon outlining changes made following this event).
- The May-June storms triggered an ongoing discussion of climate circumstances in which storms of the magnitude of those in 1974 occur. This discussion extends way beyond my postulations of linkage to warmer Tasman SST in the 1970s with more global, regional and local information coming from recent storms such as those in 2007, 2016 and 2022 (see Harley et al., 2017, Scientific Reports; Vos et al., 2023, Nature Geoscience; Oliver et al., 2024, Coastal Futures). This work has involved consideration of patterns of shoreline erosion influenced by El Niño/Southern Oscillation (ENSO). The 1974 events were linked to a La Niña phase.
- Several studies in recent years have used fieldwork techniques (e.g. GPR, OSL dating and morphostratigraphy mapping) to develop high resolution records of storm impacts on established foredunes. The 1974 event is used as a reference condition. Ian Goodwin has led a team examining responses to clusters of extreme east coast lows over the past 500 years (Goodwin et al., 2020, “Coastal sediment compartments, wave climate and centennial-scale sediment budget”, in Jackson and Short (eds.), Sandy Beach Morphodynamics, Elsevier, Chapter 25, see Fig. 25.7 defining the “Ultimate Erosion Limit” equated to a 1 in 500 year or greater event—1974 event seen as 1:100 event). Tom Oliver and colleagues using similar techniques have also examined the recurrence of extreme events beyond historical timescales (e.g. in The Holocene, 2015, v. 25, 536-546; and Geophysical Research Letters, 2019, v. 46, 4705-4714).
- Satellite imagery is increasingly being used to measure shoreline change (e.g. Bishop-Taylor et al., 2021, “Mapping Australia’s dynamic coastline at mean sea level using three decades of Landsat imagery”). These measurements are seen as useful in assessing risk to erosion and recession. However, the records mostly start in the early 1980s during what we now know as a period of accretion on the NSW coast. They miss the impact of the 1970s erosion period as shown in surveyed records (e.g. Bengello) and thus may give a misleading impression of the scale of future change. In 50 years we have only monitored one complete massive erosion cycle, but historical and geohistorical studies show the scale of impacts in the 1970s is a recurrent phenomenon. There is much value in such studies.
- In the NSW Coastal Management Act 2016 provision was made for consideration of “local and regional scale effects of coastal processes and the inherently ambulatory and dynamic nature of the shoreline” (object 3 g). The term beach fluctuation zone (BFZ) was introduced to express “the range of natural locations a beach profile occupies from its fully accreted condition to its fully eroded condition”. The intent was to recognise the scale of change observed during the 1970s storm period (1974-78) giving details (s4) on both landward and seaward limits of the BFZ. Impacts of 1974 storms were recognised with the landward limit being defined by “the escarpment resulting from the erosion associated with a 1% storm event or a more extreme event of record, whichever is the greater landward limit”.
The scale of the May-June 1974 storms provided an incredible opportunity to understand first hand a really major extreme coastal event. But they were not “unprecedented” nor should they be seen in isolation. Without doubt such erosional events will recur, in future exacerbated by climate change along with higher sea levels. These storms taught us lessons that we should never forget.
Bruce Thom
Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2024. For correspondence about this blog post please email admin@australiancoastalsociety.org.au
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MAY-JUNE STORMS 1974 – PART 2