Transverse Dispersion in Rectangular Channels

Abstract

Dimensional analysis shows that the commonly used dimensionless transverse dispersion coefficient, e sub z uda h, should depend on both the friction factor and the width-to-depth ratio of the flow. Results for e sub 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 sub z/uh and suggest that uh is not a good representation for e sub z. Two alternative dimensionless dispersion coefficients, e sub z and e sub z/uda W, are introduced. Both appear to account for secondary circulation effects more properly. The dimensionless parameter e sub 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 sub z/uW shows distinct dependence on both friction factor and width-to-depth ratio. Both of these are better dimenstionless transverse dispersion coefficients to use than e sub z/udah. /ASCE/