SHORELINE RESPONSES TO SEA-LEVEL RISE

The topic of shoreline behaviour in relation to sea-level rise has long attracted debate. This is very evident in a response by Cooper et al. in Nature Climate Change ( “Sandy beaches can survive sea-level rise”, Vol 10, November, 2020, 993-995) to a paper earlier this year by Vousdoukas et al. on “Sandy coastlines under threat of erosion” (Nature Climate Change, 2020, 10, 260-263). Cooper et al. expressed concern at a conclusion in Vousdoukas et al. that under certain circumstances beaches may be near extinct by 2100. As may be expected, a reply to Cooper et al. followed (Vousdoukas et al., Nature Climate Change, 10, 996-997, 2020).

What attracted the attention of Cooper and colleagues was the generalisation that global sea-level rise (SLR) poses a threat to the existence of beaches. In particular they expressed dismay at the application of the Bruun rule to quantify shoreline retreat. Without going into specifics they note that the underlying premise of Vousdoukas et al. is a model in which SLR promotes offshore sediment transport. While agreeing that such might happen in cases of very steep topography, “in most cases sediment transport is onshore during SLR”. They go on to elaborate on their theme that “sandy beaches are highly variable in form and setting, and it is widely accepted that there is no single response to SLR”. Their collective experience in beach geomorphic and geologic research informs them just how highly site-specific and temporally variable are sandy beach responses to SLR. Furthermore, it is important not to be influenced by what they term “incorrect model outputs” that not just create alarm but “could prompt policy responses”.

In reply, Vousdoukas et al. note they had indicated limitations to the Bruun rule in their initial paper. They also incorporated certain assumptions and correction factors so their “governing equation is applicable worldwide and is independent of the type of coastline and captures the other aspects raised by Cooper et al. (such as sediment budget, source and sinks, large-scale, long-term longshore processes)”.  In this way they argue that “these improvements make our approach vary from a straight-forward application of the Bruun rule”; from their perspective “there is at present no better way to compute future shoreline change at the global scale and none of the methods proposed by Cooper et al. can be applied at such a scale due to lack of site specific data”. It is strange that in being critical of Cooper et al., they cite concern over erosion at Narrabeen and Wamberal this year as beaches under threat when they also agree with them on the point “that the biggest threat to the continued existence of beaches is coastal defence structures that limit their ability to migrate”. Surely what is happening in these two closed sediment compartments, where at present post-storm beach recovery occurs, is that there are active attempts to build such defence structures.

Both papers agree on the need for detailed local studies to improve understanding of how shorelines will behave in response to SLR. One such study has just been published by Fruergaard et al. ( Geology, 2020, v 49) I have greatly admired the detailed morphostratigraphic work by this team on the barrier-chain, Wadden Sea, Denmark. This paper looks at the coupling between sediment availability and SLR in the late Holocene. A period of progradation of sand barrier islands accompanying a slow rate of SLR was interrupted by a period of widespread barrier deterioration because of a decrease in littoral drift “triggered by regional-scale coastal reconfiguration”. The threshold for stability of the barrier chain in relation to rate of SLR was changed. They conclude that in this region barrier islands undergo transitions from barrier progradation to transgression in response to reduced marine sediment availability even with a modest rate of SLR. From their perspective there is a need “to understand large-scale geomorphological feedbacks affecting longshore sediment transport processes in order to more accurately forecast future coastal barrier evolution”. We reached a similar conclusion in our paper on the Holocene evolution of the Ninety Mile Beach sand barrier in Gippsland, Victoria (Kennedy et al., Marine Geology, 2020).

It is apparent that along many coastlines the preferred pathway to inform coastal management decisions is to undertake detailed site-specific studies incorporating onshore and offshore data to better inform modelling required to forecast future shoreline change. Marine sediment sources must be mapped and their transport behaviour understood as far as possible. Early work by Angus Gordon and his team off Sydney beaches in the 1980s showed what can be done. More recently Linklater et al. in their study of seabed morphology on the NSW continental shelf gave us further insights using present-day technologies of what can be provided to inform regional risk assessment of shoreline change (Linklater et al. Geosciences,2019). A just completed field study of the embayed Moruya coastline has also highlighted the importance of understanding long-term variability in sediment budget and embayment interconnectivity in determining shoreline response to changing boundary conditions such as sea level and wave climate as well as contemporary anthropogenic influences ( Oliver et al., Quaternary Science Reviews, 2020,.

Global modelling may offer a “first-pass global assessment” as shown by the Vousdoukas et al. study. However, products of such work could be misleading for those seeking to understand the magnitude, frequency, and direction of shoreline change. Even our very “first-pass” review of coastal sediment compartments around the Australian coast undertaken as a project for NCCARF showed the enormous variability in shoreline characteristics and potential behaviour to climate change including SLR (Thom et al., Ocean Coast.Manag.,2018, 154, 103-120). There are locations where sediment availability is deficient pointing to early responders to SLR compared to others which may not reach a threshold for recession until a more advanced rate of SLR is reached.

Bruce Thom

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2020. For correspondence about this blog post please email austcoastsoc@gmail.com

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