Some effects of Langmuir circulation on suspended particles in lakes and reservoirs.

The quantitative extent to which the large-scale organised water motion in the surface waters of lakes and reservoirs, known as Langmuir circulation, affects the distribution and settling of suspended particles, especially the algae, is not known and has been ignored in the conventional modelling of water quality. Since the settling of these particles is an important process in determining water quality, the present study investigates the Langmuir circulation effect by means of a mathematical model, based on the two-dimensional advection-diffusion mass transport equation describing the temporal and spatial distribution of suspended particles in a typical Langmuir cell. The Langmuir circulation flow field and turbulent diffusion coefficients are empirically modelled by relating these variables to the environmental parameters. It has been shown that Langmuir circulation does affect particle distribution and settling. For particles with small sinking speeds, the circulation causes intense mixing, resulting in essentially uniform distribution of particles over the Langmuir cell. For particles with high sinking velocities, aggregation of particles can occur, giving rise to considerable reduction in sinking losses. Two preliminary laboratory experiments have been performed. The wind-wave tank experiment suggests that the Langmuir circulation scale of motion is dependent on the significant height of the surface waves, thus providing an empirical means of determining the size of Langmuir cells from environmental variables. The particle-settling tank experiment holds promise as a means of studying the effect of circulating flows on the distribution and settling of particles.