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On the behaviour of hydrodynamic processes due to the presence of submarine sand waves
Hennings, I.; Lurin, B.; Vernemmen, C.; Vanhessche, U. (2000). On the behaviour of hydrodynamic processes due to the presence of submarine sand waves, in: Trentesaux, A. et al. Marine Sandwave Dynamics, International Workshop, March 23-24 2000, University of Lille 1, France. Proceedings.
In: Trentesaux, A.; Garlan, T. (Ed.) (2000). Marine Sandwave Dynamics, International Workshop, March 23-24 2000, University of Lille 1, France. Proceedings. Université de Lille 1: Lille. ISBN 2-11-088263-8. 240 pp., meer
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Documenttype: Congresbijdrage
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  • Hennings, I.
  • Lurin, B.
  • Vernemmen, C., meer
  • Vanhessche, U.
Abstract
    Radar signatures of the sea bed in coastal waters show that submarine sand waves superimposed on sandbanks or tidal current ridges change their orientation and character abruptly at the crest of the ridge. These observations were made when studying air- and spaceborne radar images of the southern North Sea (McLeish et al., 1981). Similar phenomena were already reported by analysing side-scan sonar records from the large sandbanks in the North Sea (Houbolt, 1968). Such observations could be evidence for changes of tidal current direction as the tidal flow approaches the crest of the sand ridge. Sand waves are flow-transverse bedforms which are oriented more or less perpendicular to the maximum tidal current velocity. Furthermore, several side-scan sonar records reveal megaripples on both sides of sand waves. The ripple height generally increases in the direction of the sand wave crest. Side-scan sonar records also reveal that the crestlines of megaripples in the troughs as well as on both sides of the slopes of the sand waves form a maximum angle of 450 with the crestline of the sand waves itself. The existence of such an angle between the crestline of megaripples and sand waves indicates that changes of the tidal current direction across sand waves can be expected. Direct evidence that such tidal current direction changes occur above sand waves is derived from measurements of the Air-Sea Interaction Drift Buoy (ASIB) system at the sea surface and from side-scan sonar records of the sea bed. Both systems were operated from on board research vessels during two C-STAR field experiments in the Hoek van Holland study area off the Dutch coast in April 1996 and in April 1997. Measurements performed by electrical resistance wires on board the ASIB system indicate that variations of the short and moderate wave direction and the wave directional spread are associated with changes of the tidal current direction at the sea surface. According to the results of the measurements derived from the ASIB system and the side-scan sonar records it can be summarized that the variation of the direction of short and moderate period water waves as well as changes of the tidal current direction across large sand waves must be considered for correct modelling of the wave-current interaction mechanism.
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