Importance of turbulent diffusion in transverse mixing in rivers

ABSTRACT An expression for the transverse mixing coefficient in a rectangular channel is derived to evaluate the importance of transverse turbulent diffusion. Although some formulas for the transverse mixing coefficient account for both turbulent diffusion and shear dispersion, most calculations of the transverse dispersion coefficient do not include the effect of transverse turbulent diffusion on shear dispersion. Both vertical and transverse turbulent diffusion contribute appreciably to transverse shear dispersion, and the direct contribution of transverse turbulent diffusion to transverse mixing is important for a range of conditions commonly observed in natural channels. A comparison of predicted and measured transverse mixing coefficients supports including turbulent diffusion in analyses of transverse mixing – both in its effect on shear dispersion and as a direct mechanism of transport.