7 - Turbulent dispersion and mixing: 1. Vertical and transverse mixing

Abstract

This chapter examines the basic equation of advective diffusion that is applied to vertical and transverse mixing in natural river systems. Natural rivers are characterized by turbulent flows. Turbulence is generated by boundary friction and it increases significantly the rate of mixing. It is found that when a tracer is injected into a homogeneous stream flow, the advective transport is divided into three stages which include (1) mixing near the outlet driven by initial momentum and buoyancy, (2) transverse mixing of the effluent by turbulent transport, and (3) longitudinal shear flow dispersion. In natural systems, estimates of mixing and dispersion coefficients rely heavily upon accurate estimate of the hydraulic properties of the river flow. Vertical mixing is induced by bottom friction and transverse mixing is generated by bank roughness. The transverse mixing downstream of a continuous point source is elaborated. It is found that for a channel of finite width, the principle of superposition and method of images give the downstream concentration distribution.