Abstract
In the course of two regional side-scan sonar surveys on the continental shelf off southern Vietnam after the winter monsoon seasons of 2003 and 2004, and covering a total distance of over 1,000 km, the widespread occurrence of large and very large subaqueous dunes was discovered. On the basis of their size, shape, depth of occurrence and orientation, the dunes were grouped into five spatially distinct regions. In each case, a different height/wavelength relationship is observed. With the exception of region no. 3 where dune dimensions follow the mean global trend, the dimensions in the other regions lie below the mean global trend. The most plausible explanation for this is sediment starvation and/or insufficient time for the larger dunes to fully adjust to changing flow conditions. A good correlation is observed between average dune height in each region and water depth, although this is not the case for dune length. The orientation of the dunes corresponds to the direction of the current pattern induced by the regional winter monsoon winds (NE to SW and S). The generally well-developed asymmetrical shapes and the large size of the dunes suggest that the wind-induced currents are strong enough to reactivate most of the dunes during the winter monsoon season, a conclusion supported also by theoretical calculations of critical current velocities. The largest dunes, which seem to have reached their maximum sizes for the local water depths, may not be reactivated regularly but rather only by exceptionally strong episodic flows.
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Acknowledgements
This work was funded by the Deutsche Forschungsgemeinschaft, grant code STA401/10-1,2. The author wishes to thank the captain and crew of the RV Nghien Cuu Bien as well as the shipboard scientific party and the ‘irreplaceable’ technicians Helmut Beese and Eric Steen. Chief scientists Karl Statteger and Nguyen Tien Hai are heartily acknowledged for making seemingly impossible arrangements work. Hartmut Hein, Robert Jagodzinski, Alex Schimanski, Witold Szczucinski, Jort Wilkens, Christian Winter and, above all, Klaus Schwarzer are deeply thanked for valuable critical remarks on earlier versions of the manuscript. Comments from two anonymous reviewers greatly improved the quality of this paper. Last but not least, Patrycja Czerniak and Bui Viet Dung are thanked for their enormous help in a multilingual literature search.
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Kubicki, A. Large and very large subaqueous dunes on the continental shelf off southern Vietnam, South China Sea. Geo-Mar Lett 28, 229–238 (2008). https://doi.org/10.1007/s00367-008-0103-9
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DOI: https://doi.org/10.1007/s00367-008-0103-9