## 1 Introduction  
The prediction of mixing characteristics of solute inputs in rivers affects decision-making management in the operation of hydraulic structures for the safety of local residents. Solute mixing caused by shear flow is commonly analyzed using the advection-dispersion equation based on [PERSON]'s law [1]. However, the complex topography of rivers and hydraulic structures such as dams and weirs generates storage zones that induce non-Fickian mixing [2]. Storage zones in rivers mainly consist of surface transient storage in recirculation zones and hyporheic transient storage [3]. Previously, many researchers have used a one-dimensional (1D) storage model to simulate the mixing phenomena in these types of storage zones [4; 5; 6]. To increase the accuracy of these storage zone models, determination of the model parameters is important. Therefore, many previous studies have conducted parameter calibrations using concentration curves obtained from tracer experiments [7; 8; 9; 10]. However, these calibrated parameters are associated with high uncertainties because the three-dimensional (3D) characteristics of solute mixing in actual tracer tests are translated into results for 1D storage model parameter values. The inflow of tributaries into main channels is one of the main sources of solute transport. Therefore, analysis of tributary mixing characteristics is important for water quality management. [PERSON] et al. [11] conducted field studies on river confluences to prove that the discharge ratio of the main and tributary channels changes the secondary current intensity, which affects solute transverse mixing. This demonstrates the relevance of the flow characteristics between tributaries and main channels to near-field mixing analysis in confluences. Figure 1 presents a conceptual image of the flow and solute mixing characteristics near a channel confluence. In the channel confluence, a shear layer is formed and a recirculation zone is developed by the decrease in pressure on the inner side of the confluence [12], as shown in Figure 1. A circular motion is formed as a result of discharge from the tributary channel at the outer side of the confluence. The complex characteristics of the flow in the confluence change with the junction angle, discharge ratio between the main and tributary flows, discordance of the channel bed at the entrance to the confluence, geometric symmetry, density contrasts, and potentially, the width-depth ratio [13; 14; 15; 16]. Research on the analysis of flow characteristics in confluences has been conducted based on field measurements, experimental research, and numerical simulations. [PERSON] et al. [17] demonstrated that turbulent kinetic energy increased with an increase in the tributary discharge in the mixing layer developed in the recirculation zone. [PERSON] et al. [18] reported that helical motion became stronger and elongated when the tributary discharge was greater than the discharge of the main channel. [PERSON] et al. [19] demonstrated that the momentum transfers and the mixing interface width in the confluence hydrodynamic zone depend on the flow conditions. [PERSON] et al. [20] conducted analysis of the helical motion in confluences and found that flow structures originating from circular cells affect sediment transport. Other researchers have conducted numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) model to analyze 3D flow characteristics based on field measurements and laboratory experiments [21; 22; 23; 24; 25]. Additionally, detailed turbulence structures in the mixing interfaces of confluences have been analyzed through detached eddy simulations using the RANS model [26; 27; 28; 29]. The junction angle influences near-field mixing by creating a recirculation zone and helical cells downstream of the confluence. Near-field mixing at confluences can be divided into rapid mixing, where transverse mixing is completed within a short distance, and slow mixing, where transverse mixing is maintained further downstream [30]. [PERSON] et al.