Dispersion of material by wind and tide in shallow seas

Abstract

Mixing processes in shallow seas are of particular importance in the dispersion of pollution (e.g. oil slicks) as well as in the distribution of sediment and biological material (e.g. phytoplankton and fish larvae). This study aims to further investigate the processes of wind-driven Langmuir circulation and bottom-generated turbulence and to quantify the effects they have on the dispersion of buoyant material. A numerical and analytical study is made of the dispersion of a plume of floating particles under the combined action of Langmuir circulation and a mean current. The numerical simulations show the general patterns the plume will form, with initial meanders leading to the formation of distinct windrows. The analytical model shows that the lateral diffusivity, the rate of across-current dispersion of the particles, is greatest when the angle between the direction of the wind and current is between 30° and 120°, and when the wind speed is large in comparison to the current speed. Further, it is found that the effects of Langmuir circulation will dominate the dispersion process when the wind speed is greater than about 10 times the current speed. Acoustic and visual observations of the surface effects of bottom-generated turbulence in a tidally-influenced and well-mixed region of the North Sea are presented. Although the sea bed in the area is flat, it is found that at any one time 20-30% of the water surface is affected by boils - circular regions of local upwelling - of diameter 0.9±0.2 times the water depth. The signature of individual boils persists for at least 7 minutes and, in accordance with laboratory and numerical studies, shows the appearance of eddies. Again, analytical and numerical models are constructed to estimate the effects of the boils on the dispersion of floating particles. The numerical simulations provide good qualitative agreement with the patterns observed in visual images of the dispersion of oil. Further, the horizontal diffusivities calculated by both them and the analytical methods show that the presence of boils can account for the dispersion measured by experimental methods in shallow tidal seas. In summarising and discussing the main findings of this work, particular emphasisis placed on the need for further observations of these two processes, in particular the boils and eddies and their interaction with the Langmuir circulation, to further improve the estimates of dispersion.

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