Shallow sea tidal friction and sediment transport

Venn, J. F. (1989) Shallow sea tidal friction and sediment transport. Doctoral thesis, City of London Polytechnic.


The dissertation addresses one aspect of the hydrodynamic and bottom topography interactions within a shallow sea. We investigate causes for the surprising lack of overall motion of the linear sand ridges that lie within the energetic wind-wave and tidal regime of the Southern Bight.

There are many problems associated with the accurate estimation of ocean fluid velocity. However, it was found that this was the only significant parameter that could be collected and analysed with the accuracy required for a scientific investigation. No detailed measurements were taken of the sea-bed features or of the suspended and bed-load sediment concentrations.

The particular linear sand ridge investigated was the South Falls bank. The tidal flow was measured using current meter rigs on and off the bank. Analytical and numerical modelling of the hydrodynamic equations was compared with these current meter values for the fluid velocity field. It was found that a simple two-dimensional model that agreed well with observations of fluid velocity implies that, locally the tidal friction is insensitive to change of depth.

A Seasat synthetic aperture radar image of the Southern Bight showed grey-scale variation strongly related to the bottom topography, with increased brightness on the western flank of the sandbanks and sand waves of the region. A tidal surface velocity field was generated, for the time of the image, by reference to the measured velocities over the bank. It was estimated that the wave-current interaction, due to radiation stress effects, would have produced changes in the surface roughness over the bank that relate well to the image brightness variation.

The flow over a rough strip of the sea-bed within a constant depth region or a core of relict beach material are each sufficient for bank generation. The relation of the present flow regime over South Falls bank to that of the flow over a rough strip suggests that the cross-sectional area of the central portion of the bank is stable. The South Falls cross-section shape may be a result of wave blocking rather than a westward sediment transport. Secondary flow acts to give a frictional bottom flow everywhere within the Southern Bight. This must change at the bank and may integrate to helical vortices.

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