## 2 Materials and Methods
### Validation Data Measurement Method  
In this study, an acoustic Doppler current profiler (ADCP, River Surveyor M9, Sontek, San Diego, CA, USA) and LISST were used for the hydraulic survey and SSC measurements of the target measuring points. The instruments were attached to the side of the boat, and the boat moved in a direction perpendicular to the flow at a speed of \(\leq\)1 m/s to perform the measurement (Figure 4).  
Regarding the measurement items, ADCP was used to measure the water depth and three-dimensional water velocity (Figure 5a). Continuous measurements of vertical SSC were conducted at a rate of one per second by fixing the LISST to a wire reel at the measuring points (3-5 points) designated in the river width direction for each section (Figure 5b). The location information of the corresponding measuring points was acquired using the GPS of  
Figure 4: (**a**) Field measurements and (**b**) measurement equipment (LISST-100X was attached to the boat).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multirow{2}{*}{**Submodel**} & \multicolumn{2}{c}{**Conditions**} & \multirow{2}{*}{**Equations**} \\ \cline{3-3} \cline{5-5}  & & \(V\) **(m/s)** & \(W\) **(m)** & \\ \hline \multirow{3}{*}{M8} & E1 & \(\leq 0.96\) & - & \(10883.47\times h^{1.6921}\times S^{0.1987}\times W^{0.0036}\times V^{2.0957}\) \\ \cline{2-5}  & E2 & & \(\leq 278.38\) & \(4208.66\times h^{1.4047}\times S^{0.9002}\times W^{-0.1067}\times V^{1.9567}\) \\ \cline{1-1} \cline{2-5}  & E3 & \(>0.96\) & \(>278.38\) & \(0.12\times h^{0.77149}\times S^{0.0951}\times W^{0.0888}\times V^{1.5412}\) \\ \hline \hline \end{tabular}
\end{table}
Table 8: Application conditions and determination coefficients of Model 8 (M8).  
ADCP. ADCP has the advantage of efficient and quick measurements of the water depth and velocity compared with the existing point-based velocity measurement method, but there are unmeasured areas near the water surface, near the riverbed, and at the edges of the river. The values for these areas are estimated using the measured data through indirect extrapolation. In the LISST-100X, which was used to obtain the SSC, volume concentration was measured according to the particle size based on light scattering, and the beam attenuation by the suspended particles was calculated through the transmittance of the unscattered light. In addition, the measurable range of the suspended sediment particle size ranged from 1.25 to 250 \(\upmu\)m, and measurements of the SSC, particle size distribution (PSD), beam attenuation, water depth, and water temperature can be performed. To estimate the total suspended sediment load of the river section from the measured results, the following two methods were used: (1) the equal-discharge-increment method (EDI) and (2) the equal-width-increment method (EWI) (Figure 2c). The EDI method requires the SSC at the center of the EDI section, and the EWI method requires the SSC at each measuring line separated by equal intervals. Grids (10 \(\times\) 1 m) were generated that subdivided the section in the direction of river width and water depth, and the measured water velocity and SSC were interpolated to obtain the distribution (Figure 5a,b).  
Figure 5: _Cont._  
The average SSC of the EDI section (Figure 5c) and the EWI section were calculated from the generated two-dimensional (2D) section to calculate the total suspended sediment load for each section. In the EDI method, the accumulated discharge in each section is the same, but the width is different. In the case of the EDI method, it is possible to analyze the SSCs for each section, and consideration of outliers is possible. In the applicable measuring points, the EDI method provides more accurate results compared with the EWI method. Therefore, in this study, the total suspended sediment load estimated based on the EDI method was used [28].