Transverse mixing coefficients of density current in compound channel

The Takase River estuary in Aomori Prefecture in Japan is a tidal river with the compound channel. The object of study is to understand mixing characteristics of salt water in compound channel reaches of the river estuary. This paper describes the comparison of the transverse mixing coefficients in compound channel with those in single section channel, and the consideration about the coefficient calculated from the field measurement data. Laboratory experiments and field measurements were carried out for this river. In the compound channel, the transverse velocity shear is produced because of the water depth difference between the main channel and the flood channel. The transverse mixing coefficients were obtained from these results using diffusion equation. In the experiments, the values of k(y)/(u.h) in compound channel were about 10 times of those in single section channel (k(y): diffusion coefficient, u.: shear velocity, h: water depth). It was found that values of k(y)/(u.h) required from field measurement data were larger than those got from experimental data. This result agreed with the conventional researches. By this study, it was found that the mixing coefficient widely varied its values depended on channel cross-sectional shape, furthermore, k(y)/(u.h) of density current in compound channel was related to the product lambda and d/H (lambda: relative water level difference of fresh water and salt water, d: water depth of flood channel, H. water depth of main channel).