Turbulent diffusion in Lake Huron

Abstract

The diffusion of small floating objects and of fluorescent dyehas been studied at Douglas Point, Lake Huron. The dispersal of floating objects was complicated by surface confluences, slicks and windrows, which under certain circumstances could completely reverse the diffusion process. In the absence of such disturbing effects, however, the dispersal of floating objects exhibited an increase in rate of growth with the size of the diffusing cloud, characteristic of relative turbulent diffusion. A similar conclusion holds with regard to the diffusion of dye perpendicular to the mean current. Along the direction of the current the diffusion of dye was accelerated by the changes of current velocity with depth, much as in a pipe or channel. In the vertical, diffusion was very slow, the effective diffusivity being barely 10 times the molecular constant. Even in the horizontal the effective diffusivities describing relative diffusion were much less than values typical of the lower atmosphere or of the ocean (only of order 103 times the molecular diffusivity). A study of the meandering of the dye plume showed that this was a more important agency than relative diffusion in dispersing the dye over a large area.