Punctuated progradation of the Seven Mile Beach Holocene barrier system, southeastern Tasmania
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Progradation
Overwash
Foredune
Deglaciation
Thermoluminescence dating
Beach ridge
Abstract The Neudarss cuspate foreland is one of the largest beach ridge plains in Europe. It comprises about 140 beach ridges which can be divided into eight ridge sets from morphological criteria, trends in course and advance as well as erosional discontinuities. The reconstruction of the foreland's evolution is based on the dating of 33 samples from 14 sites by optically stimulated luminescence (OSL) and three additional datings on the basis of historical maps and aerial images. The derived progradation model shows variations in the progradation rate which are consistent with past climate and sea‐level changes. Cool (warm) periods correlate with decreased (increased) progradation rates. Both area and volume growth of the ridge sets vary in the same direction. We conclude that the progradation rate is predominantly controlled by the sediment supply which in turn depends on sea‐level variations and wind‐driven wave action. The development of the plain's relief during the last 1000 years is compared with detailed climate parameter reconstructions. The Medieval Warm Period and several phases of the Little Ice Age can clearly be traced in the morphology, thus allowing conclusions concerning predominant wind direction and aeolian activity also on smaller time scales. © 2018 John Wiley & Sons, Ltd.
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Abstract At Pedro Beach on the southeastern coast of Australia a series of foredune ridges provides an opportunity to explore the morphodynamic paradigm as it applies to coastal barrier systems using optically stimulated luminescence (OSL) dating, ground penetrating radar (GPR) and airborne LiDAR topography. A series of sandy dune‐capped ridges, increasing in height seawards, formed from c . 7000 years ago to c . 3900 years ago. During this time the shoreline straightened as the embayment filled and accommodation space for Holocene sediments diminished. Calculation of Holocene sediment accumulation above mean sea level utilising airborne LiDAR topography shows a decline in average sediment supply over this time period coupled with a decrease in shoreline progradation rate from 1.2 m/yr to 0.38 m/yr. The average ridge ‘exposure lifetime’ during this period increases resulting in higher ridges as dune‐forming processes have longer to operate. Increasing exposure to wave and wind energy also appears to have resulted in higher ridges as the sheltering effect of marginal headlands was diminished. An inherited disequilibrium shoreface profile will drive onshore accumulation of sandy sediments forming a prograded barrier; however, if there is no longer ‘accommodation space’ for sediment, this will be an overriding factor causing the cessation of progradation, as occurred c . 3900 years ago at Pedro Beach. Excess sediment in the nearshore zone after 3900 years ago may have been moved northward to nourish downdrift beaches in the compartment. A high outer foredune has formed through vertical accretion after 500 years ago, evidenced by GPR subsurface structures and OSL ages, with a distinct period of vertical and lee slope accretion and dated to the period 1890–1930 AD. The increased dune sediment transport resulting in foredune building is attributed to recent human disturbance. © 2018 John Wiley & Sons, Ltd.
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ABSTRACT Coastal ridge plains represent a valuable record of past shoreline deposition. However, there remain questions regarding shoreline behavior on intermediate timescales (sub-centennial), the impact of storms, and process of ridge genesis. We address these questions through high-resolution reconstruction of the sandy-beach progradation at Boydtown Beach in Twofold Bay, southeastern Australia, over the past 1000 years using ground-penetrating radar (GPR) and optically stimulated luminescence (OSL) dating. GPR profiles are dominated by seaward-dipping reflections that result from beach and dune progradation. Prominent reflections with heavy-mineral concentrations are also preserved resulting from storm erosion. OSL ages reveal alternative phases of steady and episodic accretion, rather than a constant progradation. We hypothesize that steady phases may result from moderate storm events where each successive storm only partially erodes the recovery of the previous event. This results in incremental seaward accretion of the active beach. Phases of episodic accretion could be the result of larger storm events or storm clusters when large post-storm recovery rapidly shifts the active shoreline seaward. The two modes of shoreline progradation (steady and episodic) appear broadly associated with a change in ridge-and-swale morphology whereby subdued ridge swale topography is associated with steady or incremental progradation and higher, better-defined ridges with episodic accretion. These results suggest that a single coastal ridge plain experiences variable intermediate-scale shoreline behavior in response to storm events which then lead to multiple modes of ridge genesis.
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Sand dunes play a significant role during coastal storms by absorbing the impacts of storm surge and high waves. Therefore, rapid profile changes and destruction of sand dunes, which may be caused by wave-induced overwash, lead to an increased flood risk landward of dunes. The effects of vegetation on dune erosion and overwash during storm events, however, have never been studied. This study is based on a laboratory experiment investigating the effects of woody plants on dune erosion and overwash of high and low dunes. During the five tests conducted foredune scarping was observed for the three high dune tests but did not occur for the two low dune tests. A narrow vegetation placed on the steep backdune of the high dune did not reduce wave overtopping and sand overwash. However, the wide vegetation figuration, which covered the backdune and foredune, reduced foredune scarping, prevented wave overtopping initially and reduced sand overwash after the initiation of wave overtopping for the high
dune. It also slowed down the erosion process of the low dune significantly by retarding wave uprush and reducing wave overtopping and overwash.
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Globally, beach- and foredune- ridge plains may archive coastal evolutionary processes, autogenic shoreline behavior, sea-level change, human coastal impacts, changing allogenic climate forcings, and time-varying sediment fluxes. The focus of this study is on the latter record. We pair coastal mapping (e.g., stratigraphic architecture, sedimentology, and morphology) and shoreline geochronology of beach and foredune ridges on Assateague Island, Chincoteague Island, and northernmost Wallops Island (ACW barriers) to reconstruct the late-Holocene evolution of a coastal sediment sink on the Virginia coast. The ACW barriers trapped 216 million cubic meters of sand through the growth of beach and foredune ridges over the last ~2000 years. From 360 to 190 years ago, sand sequestration in updrift flood-tidal deltas reduced sediment fluxes to the ACW barriers, halting island progradation and resulting in the formation of an 8+ m tall foredune ridge on Assateague Island. Progradation/elongation of southern Assateague Island resumed ~190 years ago and has since trapped an of annual average of 681,000 cubic meters of sand. Fluxes of sand into northern Wallops and Assateague islands are at least 60% of estimated regional longshore transport rates. We propose that the development of the ACW barrier system longshore sediment trap and associated wave refraction are two important drivers of downdrift barrier island change on the Virginia coast. This study emphasizes the important controls of tidal inlet sand sequestration and sediment trapping through barrier island progradation on longshore sediment fluxes along barrier coasts.
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