Field Tests of Transverse Mixing in Rivers

Five tests were made on transverse mixing in rivers. The transverse turbulent diffusion coefficient, averaged over long flow distances, was as much as three times larger than the value for an equivalent straight rectangular flume. This increase is due to the additional mixing associated with the protective groins which were present and with the helical motion resulting from channel bends. Net transverse velocities exist in rivers and can cause the local rates of transverse spreading to be significantly different from the rate associated with the average diffusion coefficient. Recommendations are given for conditions to be met in order to obtain meaningful data from field tests on rates of transverse diffusion in rivers.