Longitudinal Dispersion in Open Channels

The effects of cross-sectional geometry and velocity distribution on the dispersion process in straight, uniform open channels with large width-to-depth ratio are investigated. The method of investigation follows Taylor's approach and thus the results apply after the elapse of an initial period during which a constant dispersion coefficient has developed. The results of the analysis show that under the assumed conditions the longitudinal dispersion coefficient can be described by the general expresion K = CRu_*; in which R = the hydraulic radius, u_* = the shear velocity, and C = a factor. The variation of C with basic geometric and dynamic parameters is presented in the form of equations and diagrams for two velocity profiles (logarithmic and power law), and two cross-sectional shaped (triangular and circular arc). It is found that C varies significantly with cross-sectional shape, width-to-hydraulic radius ratio, and the Reynolds number. The calculated values of C and its pattern of variation are compared to available data for open channels with irregular cross section.