## 4 Discussion
### Storage Zone Effects on Transverse Mixing  
The recirculation zone at a confluence traps the solute cloud and extends the retention time of the solute, as shown in Figure 10. However, the parameters that represent the storage zone characteristics are difficult to obtain from field measurements. In this study, the parameters for the storage zone model were estimated based on simulation results, and the relationships with transverse mixing in confluences were analyzed.  
The main parameters for the storage zone model are the residence time (\(T\)), area of the developed recirculation zone (\(A_{s}\)), and relative percentage of the zone area among the total area (\(\varepsilon\)). Table 3 lists the estimated storage zone model parameters, which were obtained from the recirculation zone in the depth-averaged flow field. \(T\) was calculated using Equation (13) based on the averaged concentration-time curves in the recirculation zone. \(A_{s}\) was determined as the area of the recirculation zone, and \(\varepsilon\) was calculated from the area of the horizontal plane containing the recirculation zone. These results indicate that all storage zone parameters decrease as the junction angle decreases as a result of a weakened recirculation zone, as shown in Figure 6. When the junction angle decreases from 90\({}^{\circ}\) to 45\({}^{\circ}\), \(T\) gradually decreases, but the decrease is substantial at 30\({}^{\circ}\). These results indicate that the solute storage effect of the recirculation zone is greatly reduced when the relative area of the storage zone (\(\varepsilon\)) reaches approximately 11%.  
Figure 16: Relationship between the transverse dispersion coefficient and flow characteristics in confluences: (**a**) Intensity of helical motion; (**b**) Change in length; (**c**) Change in width.  
In natural streams containing storage zones, 1D longitudinal mixing is achieved by shear flow dispersion and mixing in the storage zone, as shown in Equation (10). Regarding the relationship between shear flow dispersion and storage zone effects, [PERSON] et al. [56] demonstrated a negative correlation between the mass exchange rate (\(\varepsilon/T\)) and longitudinal dispersion coefficient in the flow zone (\(K_{f}\)). This inverse relationship can be said to reduce the shear flow dispersion, as mixing by the storage zone is strengthened. Additionally, this study revealed that the existence of a recirculation zone affects transverse mixing. Therefore, to discuss the storage effects on transverse mixing due to a recirculation zone, the relationship between \(\varepsilon/T\) and \(D_{T}/hu^{*}\) is plotted in Figure 17. Similar to the relationship between \(\varepsilon/T\) and \(K_{f}\), \(D_{T}/hu^{*}\) also has a negative correlation with \(\varepsilon/T\). As in [PERSON] et al. [40], the longitudinal shear dispersion has an inverse relationship with the transverse dispersion coefficient; in the present study, the decrease in \(D_{T}/hu^{*}\) leads to an increase in longitudinal shear dispersion. Therefore, \(\varepsilon/T\) and \(D_{T}/hu^{*}\) simultaneously reduce the longitudinal shear dispersion, and transverse mixing is replaced by mixing in the storage zone when the recirculation zone is strongly developed.