Transport Mechanisms in Water

The transport mechanisms of substances in natural water bodies are considered. Dissolved substances can be regarded as passive in that they do not affect the velocity field of the water bodies. The particles of substances can be regarded as either passive or active. Transport of substances is caused by convection, and/or molecular and turbulent diffusion. The buoyancy forces thus have some influence on the transport of substances vertically. As a rule the vertical transport of substances is several orders of magnitude less than the horizontal. One can usually neglect molecular diffusion. Convection transports the substances as solid body. The substance spreads over the whole water body by diffusion; thus the plume edges are relatively sharp. Horizontal diffusion depends mainly on the scale of the various processes and vertical diffusion on scale and fluid stratification. In stable stratified water bodies, vertical diffusion is much reduced and sometimes becomes of the same order as molecular diffusion. The horizontal and vertical diffusions are of great importance in rivers and in the upper and bottom Ekman layers of oceans, seas and lakes. The thermocline and halocline are practically impenetrable for vertical turbulent transport. Vertical flows through them appear to be due to double-diffusivity convection due to the overturning of internal waves and due to upwelling water in the coastal areas.