Two‐Dimensional Mixing in Rivers with Unsteady Pollutant Source

Abstract

A numerical model, the mixing analysis based on the Concept of Stream Tubes (MABOCOST), has been developed for the analysis of two‐dimensional, transient mixing of nonconservative substances in natural streams. The model can be applied to steady flows in sinuous, nonprismatic channels. A curvilinear coordinate system is used to account for variations in velocity, depth, and channel curvature. By dividing the stream tubes into variable length elements so that the Courant number for the grid space is always equal to unity, the problem of numerical diffusion and dispersion in the computation of streamwise advection is avoided. The model has been verified with analytical solutions for the cases of simple advection, continuous line source and instantaneous injection. Dispersion experiments were carried out in a sinuous channel with irregular bottom topography, using both slug injection and a variable rate injection of tracer. Measured time‐concentration data agree quite well with predictions using MABOCOST.