The Calculation of Transverse Eddy Diffusivity Using Turbulence Data

The rate of transverse exchange of momentum and entrained contaminants in a fluid by turbulent eddies, as defined by the transverse diffusion coefficient, is important in modeling contaminant dispersion and lateral suspended sediment fluxes. Point source effluent dispersal and overbank sediment deposition on floodplains are two significant areas of application. Traditionally, the transverse diffusion coefficient has been calculated by examining the rate of change of the variance of concentration profiles taken downstream of a dye release point. Using this method, estimates of the transverse eddy diffusivity have been calculated in sediment laden flows in natural rivers and in clear water flows in laboratory flumes. In the present study, turbulent velocity fluctuations and transverse velocity gradients derived from velocity profiles at several locations over half the width of a laboratory flume were used to calculate the transverse diffusion coefficient in both clear water and sediment laden flows. It was found that values for the transverse dispersion coefficient were comparable to those measured in natural rivers and in previous flume studies using the dye injection technique. The technique presented here provides an alternative method, using transverse Reynolds stresses and velocity gradients, to determine the transverse diffusion coefficient. The agreement between the two methods shows that it may be valid to use an approach based on turbulence quantities.