Mapping the Shoreface of Coastal Sediment Compartments to Improve Shoreline Change Forecasts in New South Wales, Australia

Abstract

The potential response of shoreface depositional environments to sea level rise over the present century and beyond remains poorly understood. The shoreface is shaped by wave action across a sedimentary seabed and may aggrade or deflate depending on the balance between time-averaged wave energy and the availability and character of sediment, within the context of the inherited geological control. For embayed and accommodation-dominated coastal settings, where shoreline change is particularly sensitive to cross-shore sediment transport, whether the shoreface is a source or sink for coastal sediment during rising sea level may be a crucial determinant of future shoreline change. While simple equilibrium-based models (e.g. the Bruun Rule) are widely used in coastal risk planning practice to predict shoreline change due to sea level rise, the relevance of fundamental model assumptions to the shoreface depositional setting is often overlooked due to limited knowledge about the geomorphology of the nearshore seabed. We present high-resolution mapping of the shoreface-inner shelf in southeastern Australia from airborne lidar and vessel-based multibeam echosounder surveys, which reveals a more complex seabed than was previously known. The mapping data are used to interpret the extent, depositional character and morphodynamic state of the shoreface, by comparing the observed geomorphology to theoretical predictions from wave-driven sediment transport theory. The benefits of highresolution seabed mapping for improving shoreline change predictions in practice are explored by comparing idealised shoreline change modelling based on our understanding of shoreface geomorphology and morphodynamics before and after the mapping exercise.

The turnaround from transgression to regression of Holocene barrier systems in south‐eastern Australia: Geomorphology, geological framework and geochronology

“…The second barrier system at Bengello Beach, about 250 km south of Sydney, comprises a strandplain to the north of the Moruya (Deua) River, a smaller catchment (1550 km 2 ) with a smaller sediment supply. Although dominated by onshore sediment supply from the shoreface, Bengello Beach experiences limited net longshore sediment transport to the north (Kinsela et al, 2016(Kinsela et al, , 2020Short, 2020). In addition, Bengello Beach has been an important site, both because a program of radiocarbon dating there has provided detailed evidence of coastal behaviour over the past six millennia (Thom et al, 1981b) and because of a long-term record of beach profiling since 1972 that has provided an archive of storm erosion and subsequent beach recovery over the past four decades (Thom & Hall, 1991;McLean & Shen, 2006).…”

“…The third barrier system is a smaller strandplain landward of Pedro Beach, about 5 km south of the Moruya River mouth. Although dominated by onshore sediment supply from the inner shelf, Pedro Beach experiences limited net longshore sediment transport in a northerly direction (Kinsela et al, 2016(Kinsela et al, , 2020Short, 2020). In addition, OSL dating across this series of ridges reveals that, unlike at Bengello Beach to the north, the shoreline at Pedro Beach prograded between ca 7200 and ca 4000 years ago, and that since then it has undergone little change in position (Oliver et al, 2019).…”

“…At the Seven Mile Beach and Pedro Beach barrier systems, the washover deposits form a unit 1.0 to 2.5 m thick that directly overlies the pre-Holocene sediments. Aggradation of washover deposits likely occurred in response to storm impacts as the shoreline stabilized at these sites, which was then followed by onshore sediment supply from the upper shoreface to the beach (see Kinsela et al, 2016Kinsela et al, , 2020, resulting in beachface progradation that produces the seaward-dipping clinoforms typical of beach regression in sandy wave-dominated settings (see van Heteren et al, 1998;Neal, 2004).…”

“…south of Sydney, with an area of 7326 km 2 . It appears that the ongoing supply of marine sand from the shoreface has contributed to the formation of the successive ridges that compose the Seven Mile Beach strandplain (see Kinsela et al, 2016Kinsela et al, , 2020. This has been augmented by fluvially derived sands, since ca 2400 years ago, delivered by the river since the infill of most of the extensive plains that now flank the lower river course (Wright, 1970;Umitsu et al, 2001;Carvalho, 2018;Carvalho et al, 2019).…”

“…Unlike coastal settings in glacial forebulge regions, Australia has numerous Holocene prograded barrier systems that are dominated by net regressive sedimentary successions (Thom, 1983;Searle et al, 1988;Short et al, 1989;Roy et al, 1994;Murray-Wallace et al, 2002;Brooke et al, 2008a,b;Gontz et al, 2014;Oliver et al, 2015Oliver et al, , 2017aOliver et al, ,b,c, 2019Oliver et al, , 2020Dougherty et al, 2016Dougherty et al, , 2019aKinsela et al, 2016Kinsela et al, , 2020Tamura et al, 2018;Carvalho et al, 2019). Similar broad prograded barriers composed of a series of palaeoshoreline ridges are also common elsewhere on far-field coasts (see Scheffers et al, 2012); for example, along coasts in Brazil (Dillenburg & Hesp, 2009;Barboza et al, 2011;Dillenburg et al, 2017), Asia (Tamura et al, 2007;Brill et al, 2015) and Russia (Sander et al, 2019), as well as coasts experiencing isostatic rebound such as Denmark (Nielsen et al, 2006).…”