Transverse Dispersion in Rectangular Channels

Dimensional analysis shows that the commonly used dimensionless transverse dispersion coefficient, e_z/uda*h, should depend on both the friction factor and the width-to-depth ratio of the flow. Results for e_z lead to the conclusion that secondary circulation, and not turbulent diffusion, is the dominant transport mechanism. These results help to explain the failure to find consistent trends in the variation of e_z/u*h and suggest that u*h is not a good representation for e_z. Two alternative dimensionless dispersion coefficients, e_zuW and e_z/uda*W, are introduced. Both appear to account for secondary circulation effects more properly. The dimensionless parameter e_z/uda*W is a function of the width-to-depth ratio and appears to be affected only slightly by variations in the friction factor. The parameter e_z/uW shows distinct dependence on both friction factor and width-to-depth ratio. Both of these are better dimensionless transverse dispersion coefficients to use than e_z/uda*h.