The effect of Langmuir circulation on the distribution and settling of algae and suspended particles

Abstract

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 algae and other suspended particles is not known and has thus been ignored in conventionally used water quality models. Since the distribution and settling of these particles is important in determining water quality, this study set out to investigate these effects using 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 them to environmental parameters (Buranathanitt et al., in press).

The results show that Langmuir circulation does affect particle distribution and settling. For particles with small sinking speeds, such as the lighter algae, the circulation causes intense mixing, resulting in essentially uniform distribution of particles over the cell (as assumed in the ‘well-mixed compartment model’). For particles with high sinking velocities, however, aggregation can occur, giving rise to significant reduction in sinking loss. For diatoms, effective sinking speeds are as much as 6% less than when cells are not operating while for coarse silt particles in a cell of large width to depth ratio a reduction of more than 60% is possible.

Two preliminary laboratory experiments have also been performed in order to obtain further information on both the physical characteristics of Langmuir cells and their effects on suspended particles. The results of a wind-wave tank experiment suggest that the Langmuir circulation scale of motion is related to the height of the surface waves, thus providing an empirical means of determining the size of Langmuir cells from environmental variables. A particle settling tank experiment confirmed, in a qualitative way, the results obtained in the analytical study, with both the uniformly mixed and aggregation phenomena being reproduced (Buranathanitti et al., submitted).

Full details of the work described in this abstract will be published elsewhere (Buranathanitt et al., in press; Buranathanitt et al., submitted).)