Modeling Transverse Mixing in Natural Streams

A model of transverse mixing in natural streams has been tested using dye data from a field experiment and has been proven successful. The model uses a natural coordinate system which follows the curvature of the stream and is based on the cumulative discharge concept. In the numerical solutions, different assumptions are used for the factor of diffusion, (uh²m_xe_z). The best accuracy is found when local values of (uh²m_x) are used together with a dispersion coefficient e_z which can vary in the downstream direction. The changes in the size of the dye plume can be predicted fairly well, even with an analytical solution. An analysis of available field values for e_z suggests that the dimensionless dispersion coefficient e_z/U*W can be represented as a function of the width-depth ratio and a variable which characterizes the curvature of the stream. The sinuosity is a possible variable to use. Most of the data are distributed fairly well along one curve which can be used to obtain estimates of e_z for streams with small curvatures.