Summary
Measurements of the residual tidal currents in the vicinity of a large sand bank indicate secondary circulation effects which might explain the permanence of such features in the marine environment. Strong veering in the mean circulation is observed and this is shown to be associated with flow convergence and divergence effects in a large eddy centred on the sand bank. This eddy is maintained by vorticity generated in the coastal boundary layer as a result of tidal stream curvature and horizontal shear in the vicinity of a headland. Systematic veering in tidal current directions indicates that in the bay adjacent to the headland, flow curvature leads to secondary currents which are about an order of magnitude lower than the primary tidal flows.
The directions of residual sediment movement have been calculated from measured near-bottom currents and it is shown that due to the highly non-linear nature of sediment transport processes these can be in markedly different directions to those of the residual water movements. However, the overall movements are, in general, such as to transport sediment towards the bank.
Zusammenfassung
Messungen der Restgezeitenströme in der Nähe einer großen Sandbank deuten auf die Wirkung sekundärer Zirkulationen hin, die die Beständigkeit solcher Merkmale in der Meeresumwelt erklären könnte. Ein starkes Drehen in der mittleren Zirkulation wird beobachtet. Dies hängt mit der Konvergenz und Divergenz der Strömung in einem Wirbel über der Sandbank zusammen. Dieser Wirbel wird aufrechterhalten durch die Vorticity, die in der küstennahen Grenzschicht erzeugt wird als Resultat der Krümmung des Gezeitenstroms und horizontaler Scherungen in unmittelbarer Nähe einer Landzunge. Eine systematische Richtungsdrehung des Gezeitenstroms deutet darauf hin, daß in der Bucht, die an der Landzunge angrenzt, die Strömungskrümmung zu sekundären Strömungen führt, die etwa eine Größenordnung geringer sind als die primären Gezeitenströme.
Die Richtungen der Sedimentverlagerungen sind aus den gemessenen Strömungen dicht über dem Meeresboden berechnet worden. Es wird gezeigt, daß infolge der stark nicht-linearen Natur der Sediment-Transportprozesse die Richtungen der Sedimentverfrachtung erheblich von den Richtungen der mittleren Strömungen abweichen können. Im allgemeinen jedoch sorgt die Gesamtströmung für den Sedimenttransport zur Sandbank.
Résumé
Des mesures de courants de marée résiduels à proximité d'un grand banc de sable signalent des effets de circulation secondaires susceptibles d'expliquer la permanence de tels traits en environnement maritime. L'observation indique une rotation importante de la circulation moyenne et on montre que ceci est associé avec des effets de convergence et de divergence du courant dans un grand tourbillon centré sur le banc de sable. Ce tourbillon est maintenu par mouvement giratoire engendré dans la couche limite côtière par une incurvation du courant de marée et une coupure horizontale à proximité d'un cap. La rotation systematique des courants de marée indique que dans la baie adjacente du cap l'incurvation du courant induit des courants secondaires d'environ un ordre de grandeur moindre que les courants de marée primaires.
Les directions de déplacement des sédiments résiduels ont été calculées à partir des courants mesurés près du fond et on montre que du fait de la nature extrêmement non-linéaire des processus de transport de sédiments, ceux-ci peuvent avoir lieu dans des directions très différentes de celles des mouvements d'eau résiduels. Cependant, les mouvements d'ensemble causent en général un transport de sédiment vers le banc.
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Heathershaw, A.D., Hammond, F.D.C. Secondary circulations near sand banks and in coastal embayments. Deutsche Hydrographische Zeitschrift 33, 135–151 (1980). https://doi.org/10.1007/BF02310322
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DOI: https://doi.org/10.1007/BF02310322