Wave climate, sand budget and shoreline alignment evolution of the Iluka–Woody Bay sand barrier, northern New South Wales, Australia, since 3000 yr BP
Introduction
A significant proportion of the world's coastlines has experienced persistent erosion and shoreline recession over the past few decades (Bird, 1985). Bruun (1962) argued that the long-term recession of sandy beaches was due to rising sea level. Cowell et al. (1995) established in coastline modelling that a tiny deficit of 1% in the longshore transport sand supply can cause recession equivalent to a 0.5 m rise in relative sea level (RSL). Modern coastline alignments have been observed to rotate in response to interannual to decadal mean wave direction changes and their effects on longshore sediment transport direction (Ranasinghe et al., 2004). Long-term sediment budget deficits on geological time-scales can be the underlying cause of modern shoreline recession. Clearly, all these processes have a potential influence on regional coastal behaviour, with varying degrees of impact.
Whilst previous research has provided a framework for understanding morphological differences between coastal compartments and their evolution on geological time scales, coastal behaviour on engineering time scales (large-scale coastal behaviour (LSCB)) is more complex, and linked to high frequency climate fluctuations. How does recent coastal behaviour relate to LSCB during the Holocene? Are climate, sea level and sedimentary forcing factors different in the past century when compared to the past few millennia? Have climatic fluctuations caused rotation in the regional coastal alignment over centennial to millennial time scales, and can these explain part of the 20th century pattern of coastal recession? Hence, the primary focus of this study was to investigate evidence for quasi-centennial scale variability in shoreline rotation, and gross changes in coastal alignment, due to variations in regional wave climate (principally, modal wave direction) and the resulting sediment transport direction, rates and alongshore gradients. This study provides an insight into the processes operating on both the geological and engineering time scales since the mid Holocene, and investigates LSCB on a coastline that has exhibited recession, particularly at the updrift ends of coastal compartments. The New South Wales (NSW) coast is a typical, high-energy, wave-dominated sedimentary coast (Roy, 1997) and is highly suited to this study. The regional coastline trends obliquely to the modal southeast inner-shelf wave direction and north of 33°S (Fig. 1), the coastline becomes progressively more oblique to the modal wave direction, and is characterised by longshore drift-aligned (sub parallel to the modal wave direction) coastal plain-coasts, with long, open barrier beaches, with zeta-form alignments, and inner-shelf sand sheets (Roy and Thom, 1981).
Previous research on the NSW coast has lead to the development of conceptual models for both sand transport and barrier morphology on wave-dominated coasts and inner continental shelves, and the coastal response to Pleistocene–Holocene sea-level change (Roy et al., 1994, Cowell et al., 1995). Much of this prior research has focused on progradational Holocene barriers that contain intact, relic foredune ridge sedimentary sequences (strandplains), and preserve a sequence of shoreline changes. One of the most extensive Holocene strandplains on the far north coast NSW (Roy, 1982), is located on the Clarence coastline, from Iluka to Woody Bay, north of Yamba, in far northern NSW (Fig. 1, Fig. 2).
This paper describes a detailed study of the Iluka to Woody Bay barrier and strandplain morphology, and provides an interpretation of the associated depositional history, with respect to the role of modal wave direction, on shoreline evolution, sand transport paths, sand budgets, headland sand bypassing, and alongshore gradients in longshore sand fluxes.
Section snippets
Terrestrial morphology and sediments
The study area comprises a prograded sand barrier (strandplain) located at the mouth of the Clarence River system (Fig. 2). The barrier is comprised of reworked Pleistocene marine sands, with the inclusion of Holocene biogenic carbonate (Walsh and Roy, 1983). The sand barrier extends from Iluka northwards to Woody Bay over a distance of ∼ 7 km, and is up to 2.5 km wide, with a strandplain comprising relic foredune ridges up to 6 m above mean sea level (MSL). The arcuate foredune ridges depict
Former shoreline mapping
The strandplain morphology was mapped at 1:25 000 scale, using black and white, vertical aerial photographs flown in 1942 (Maclean SVY 255, 257, 259, 260, 1:18 600 scale), and colour vertical aerial photographs flown in 2000 (NSW coastal surveillance photography, Tweed Heads – Sandon River, Yamba – South Ten Mile Beach NSW 4518, M2186, Runs 3 and 4, 1:10 000). The 1942 photography provided the best definition of the barrier surface and the relic foredune ridge plain, due to the sparse vegetation
Barrier progradation and shoreline alignment evolution
The development of barrier morphology, shoreline configuration and depositional history is discussed separately for the 3 sub-sections; the Iluka section (Fig. 4, Fig. 5), the Second Bluff–Saltwater Inlet section (Fig. 4), and the Woody Bay section (Fig. 6).
Headland sand bypassing
Previous authors have doubted that sub-aqueous sand bypassing around Woody Head occurs based on inspection of the bathymetry. Walsh and Roy (1983) and subsequent reports drawing on their results (Lord and Edwards, 2000) have suggested that the bedrock reef extending in a northerly direction offshore from Woody Head acts as a barrier to the net northerly longshore sand transport to Woody Bay. The reef extends offshore to the north of Woody Bay and hence, the shoreface sand supply is minimal.
Conclusions
The successive accretion of foredune–shoreface profiles, comprising the Iluka to Woody Bay barrier, is punctuated by multi-centennial fluctuations in wave climate, resulting in episodes of recession, realignment, and changing sediment supply rates. Between 3000 and 1500 yr BP, shoreline progradation occurred under a more southerly modal wave climate, than during 1400 to 1000 yr BP. After ∼ 1500 yr BP, the shoreline was realigned in equilibrium to a more east–southeasterly wave climate. From
Acknowledgements
This research was supported by small grants from the Research Grants Committee, University of Newcastle. The authors thank Mr Chris Dever, Mr Peter Loughnan and Mr Jason Roberts (School of Environmental and Life Science, University of Newcastle) for assistance in the field. The radiocarbon age dating was conducted at ANSTO by Dr Ugo Zoppi, under an AINSE grant no. 01/059 to IG. The authors thank Jacqui Olley and Ken McMillan for OSL sample preparation, and Harald Alksnis and Chris Leslie for
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2020, GeomorphologyCitation Excerpt :An alternative hypothesis is that the longer a foredune ridge remains active at the rear of the beach, the larger the ridge on average, since the sequence of storm wave scarping, scarp fill, aeolian sand delivery up the scarp fill, crestal deposition and foredune growth and potentially translation leads to larger more complex foredunes over time (Hesp, 1988b; Davidson-Arnott et al., 2018). A large complex foredune development post-500 yrs ago is also consistent with Goodwin et al. (2006, 2014) indication of greater storminess and foredune scarping and transgression as noted above. The initial development of The Granites barrier system began as an aggradational barrier comprising a single foredune which underwent at least two phases of stabilization and reactivation/further building from 6.7 ka BP to 5.6 ka to 4.3 ka.