Abstract
This chapter describes the Taylors dispersion theory and the application of the dispersion equation to natural river system. Taylor's theory is based upon the assumptions that turbulent mixing processes occur in a similar fashion as molecular diffusion, and it is valid only away from the injection point: i.e. downstream of the initial zone. Turbulent shear dispersion is also called longitudinal dispersion. The rate of longitudinal dispersion reflects the balance between velocity shear which acts to spread tracer along the channel and transverse mixing which promotes uniform concentrations across the channel and hence counteracts the effects of velocity shear. It is observed that after a contaminant is well mixed across the entire cross-section, the final stage of the mixing process is the reduction of longitudinal gradients by dispersion. It is also observed that if the effluent discharge is constant, there is no need to be concerned by dispersion. Longitudinal dispersion is important in applications characterized by non-constant effluent releases. The dispersion coefficient in natural rivers is elaborated in the chapter.
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