Coastal erosion and accretion beyond the historical timescale in NSW

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A paper has recently been published in Geophysical Research Letters on the use of beach morphostratigraphic records to decipher past extreme coast erosion (Timura, T. et al., GRL, 7 May 2019). The paper excited me as it is linked to four decades of beach monitoring north of Moruya Airport (Bengello Beach) that I commenced in 1972 and has been continued with great dedication by Roger McLean to this day. Timura and others in discussing the various papers that have emerged from this monitoring, note how short is this record when assessing the frequency of extreme storms that constitute a major risk to coastal settlements and infrastructure. They make two introductory comments; one, that it is not clear whether observations over this four decade period are adequate for understanding recurrence of “infrequent extraordinary events”; and two, there is still a gap in understanding impacts of projected sea level rise and possible intensification of storms due to climate change. Their paper uses data from the profiles at Bengello Beach on impacts of the severe East Coast Lows (ECLs) storms of 1974-1978 and post-storm recovery information as a reference for analysis of earlier events.

The approach used by Timura et al. involved comparison of the beach monitoring record with longer-term beach changes using ground-penetrating radar (GPR) and optically stimulated luminescence (OSL) dating. This established the morphostratigraphic signature of the 1974-78 storm events that enabled evidence for earlier comparable events to be inferred. Working in such a location has many advantages as is the site of extensive geomorphic and stratigraphic investigation that commenced in the 1970s and more recently elaborated upon by Tom Oliver and others in 2015 (see The Holocene, 25, 536-546).

The recent paper interprets past extreme erosion events from the dune-beach deposits older than that formed since 1978 with depositional units showing how the beach recovered. Of interest was the identification of beach retreat in the period 1650-1700 with a resultant beach scarp. They point to the lack of preserved recovery during this period which could indicate unfavourable conditions for beach recovery due to a temporary negative budget of beach sand and/or erosion during subsequent storms. But what followed in the mid nineteenth century was massive accretion of aeolian sand which resulted in the high foredune that stands in contrast with the all strand plain ridges to landward. This dune overlies accreted beach deposits that extends landward of the 1970s scarp (see their Fig.4). Episodes of erosion and recovery could have occurred during what they term “a gradual phase of beach progradation”. They conclude that the approach used in this study can be applied to other sediment compartments to “improve our understanding of the long to middle-term coastal dynamics that are fundamental for sustainable development of coastal areas”.

Other work has been undertaken along the NSW coast that also contributes to this understanding. Amy Dougherty (2014), and more recently with others (2018), has used similar techniques to that discussed above (see references cited in Timura et al., 2019). However, a more regional approach has been applied by Ian Goodwin and colleagues from Macquarie University. Their work also seeks to provide information for coastal planning. It was undertaken as part of their contribution to AdaptNSW (Project 4) and can be accessed online: Timura et al. (2019) do not reference the work of Goodwin et al. reported in 2015.

Goodwin focusses on the long-term history of East Coast Lows (ECLs) and their impact on the NSW coast. The object is to see how the study of past ECLs helps understand behaviour and impacts of future ECLs. They use palaeo-climate records to see how clusters of ECLs at different time scales may have operated over the past 1200 years. One conclusion is that looking back “we see the period from 1600-1900 was much stormier than now. During these centuries there are many more decades of high and persistent storm activity than we see today”. They examined LiDAR images combined with other data from accretionary coastal sequences and identify the “ultimate storm scarp”. This could be the maximum geohistorical position due to extreme long-term wave climate variability. Data from surveys such as those by McLean and others at Moruya help guide this analysis. There is general agreement on shoreline impacts and timing between the regional morphological/palaeo-climate conclusions of Goodwin and colleagues with the more recent site-specific morphostratigraphic interpretations.

We may agree with Goodwin and his team that the period between 1600 and sometime in the mid to late 1800s as being much stormier than now along the NSW coast. In some parts of the world this is regarded as the “Little Ice Age” (LIA). Looking back at a paper in the book written in honour of the great Australian geomorphologist, Joe Jennings, I speculated that the LIA “may well correspond to a period of increased storminess along the NSW coast which triggered off the most recent wave of transgressive dunes” (1978, in Davies and Williams, eds., Landform Evolution in Australia, Ch. 9). Of course the more sophisticated application of methods by Tamura, Oliver, Dougherty, Goodwin and others in recent years has given more evidence to support the possibility that this inferred stormy period had left a significant morphological imprint on accretionary deposits along the NSW coast.

It is still quite clear that extreme events continue to occur under current climate conditions, the impacts of which are well-documented by coastal engineers such as Angus Gordon and geomorphologists following the example of Andy Short. Yet it appears that before 1900 there was a somewhat different climate state when there were many more decades of high and persistent storm activity than we see today. Sand mobilisation by wave and wind forces were more dynamic than in the last 100 plus years. However, in framing the concept of Beach Fluctuation Zone (BFZ) for purposes of vulnerability assessment in the new NSW Coastal Management Act, the decision was made to relate the scale of erosion and accretion to historic events such as those during 1974-78 than go back beyond the historical record. This appears to be the most sensible way to advise planners and managers at this stage. The future under climate change may require revision, but work reported here indicates we are not currently encountering shoreline changes of the scale seen several centuries ago.

Bruce Thom

I would like to thank Mike Kinsela in the preparation of this blog; the views expressed are mine.

Words by Prof Bruce Thom. Please respect the author's thoughts and reference appropriately: (c) ACS, 2019, for correspondence about this blog post please email austcoastsoc@gmail.com

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