Observations of offshore bar decay: Sediment budgets and the role of lower shoreface processes |
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| Authors: | Troels Aagaard Aart Kroon Brian Greenwood Michael G Hughes |
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| Institution: | 1. Institute of Geography and Geology, University of Copenhagen, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark;2. Department of Physical and Environmental Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario, Canada M1C 1A4;3. School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia |
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| Abstract: | Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55 m a−1. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget. |
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| Keywords: | Nearshore processes Sediment transport Longshore bars Sediment budgets Longshore currents PC-ADP |
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