## 2 Materials and Methods  
### Experimental Method  
An experiment was performed in a 50 m long circulating channel in the Andong River Experiment Center (REC) of the Korea Institute of Civil Engineering and Building Technology (KICT) located in Andong, Gyeongsangbuk-do, Korea. Water flows in the direction from A to B in the channel, and the water at the end of the channel is circulated through a pump and supplied again to the channel (Figure 1). The channel has a width of 2 m and a height of 1.25 m. The necessary measurements were acquired in a section where temperature glass was installed, such that the equipment and experimental environment could be directly examined from the outside. The suspended sediment mixture was injected at the 22 m point of the circulating channel, and the measurement instruments for turbidity and concentration were located to take measurements at the 32 m point. The flow velocity and water depth were measured using a side-looking acoustic Doppler current profiler (ADCP-SL) and ADCP-M9 at 1 m upstream and downstream positions that were relative to the turbidity and SSC measurement position. Finally, two GoPro 9 units were installed upstream and downstream of the point where the sediment supply was performed (18 m, Video 1 and 31.8 m, Video 2). Red clay powder with a particle diameter smaller than 0.044 mm was used to examine the color change of water, according to the SSC. The powder and water were sufficiently mixed with 200 L of water using a 250 L mixer, and the mixture was injected for a certain period at the following three concentrations: (1) 1 st Injection: 25 g/L (14:38:30-14:40:00); (2) 2 nd Injection: 50 g/L (14:47:00-14:48:25); and (3) 3 rd Injection: 75 g/L (15:04:00-15:05:05) (Figure 2). The turbidity was measured at the center of the channel at a height of 38 cm from the floor using a multi-item water quality meter (WQC-30), and the SSC was measured using LISST-200X at the same point (Figure 3). In the case of turbidity and concentration measurements, there were unmeasured sections between the injections of the suspended sediment mixture of each condition. In the case of turbidity measurement using WQC-30, the 90\({}^{\circ}\) scattered light method, which measures the amount of light scattered at 90\({}^{\circ}\) by particles in water was used. The instrument was calibrated using the formazine standard  
Figure 1: Experiment site: 50 m long circulating channel at the Andong River Experiment Center (REC). Figure 2: (**a**) Injection of red clay mixture using a 250 L mixer; (**b**) mixture injection method for Injection 3.  
solution, and measurements were performed every 2 s in the nephelometric turbidity units (NTU) [19]. LISST-200X measures the volumetric concentration based on particle size as the light source is scattered and calculates the laser light attenuation by suspended particles based on the transmittance of the unscattered laser light source. The particles in the 1.00-500 \(\upmu\)m range could be measured, and measurements were performed every second for the total concentration of suspended sediment, particle size distribution, optical transmission, water depth, and water temperature [20]. A total of 1642 SSCs and 828 turbidity measurements were captured; Figure 4 shows the turbidity, SSC, and transmission values that were measured throughout the experiment. Figure 4: _Cont._  
Figure 3: Measuring instruments used in the experiment: (**a**) WQC-30, LISST-200x, and GoPro 9 (Video-2 at 31.8 m), and (**b**) ADCP. The instrument presents the transmission value as the risk of the ratio of multiple scattering for the evaluation the measurement results' reliability (Figure 4c). Specifically, caution needs to be exercised against data quality degradation caused by many fine particles when the transmission value is less than 0.3; in fact, data may be neglected when the value is less than 0.1 [20]. The results indicated that the measured SSC during injection were not reliable.