## 2 Materials and Methods  
### Experimental Method  
The primary objective of this study was to develop an imaging-based method for measuring sediment concentrations; this development required experimental investigations of color changes in underwater images in response to changes in suspended-sediment concentrations. For this, a 1.2 m \(\times\) 1.8 m water tank was created (Figure 1a).  
Five types of red clay (classified by sifting with sieves with mesh sizes of 200, 325, 700, 1000, and 2000) were considered, with each being tested on a different day. In all of the experiments, a water level of 0.695 m (1.5 m\({}^{3}\) in volume) was maintained. After 100 g of clay was introduced into the tank, a mixer was used for mixing, thereby simulating the tidal effect in estuaries and coasts. The measurement process was continued while such fluctuations within the water tank were maintained. During the analysis, turbidity and water quality factors were analyzed using a multiparameter water-analysis device--the EXO2 Multiparameter Sonde (EXO2)--and the suspended-sediment concentration and particle size distribution were obtained using the LISST-200x. In this study, the concentration of suspended sediment measured by the LISST device was considered the true concentration and used as the calibration value for developing our model, as it is unaffected by the particle size distribution [17]. Moreover, to ensure accurate measurements and eliminate discrepancies, both devices were installed at the same height (a 0.3 m water depth), and the bed condition in the water tank remained constant throughout the measurement period, effectively minimizing any potential impact of sensor alignment or changes in the bed condition on the measurement results [24; 25].  
Generally, when using the indirect method of concentration estimation using turbidity, sufficient consideration of salinity, mineral composition, nitrogen or phosphorus species, and conductivity is required [21; 22]. Notably, in this study, relatively clean water was used to maintain water quality conditions, and EXO2 was used to determine the turbidity and other parameters, such as the electronic conductivity, salinity, and potential of hydrogen (pH) every second to verify the water quality. Note that an LISST-200x sediment sensor is capable of measuring the volume concentration and suspended-sediment particle size (1.00-5000 microns) every second and assessing the reliability of the obtained results based on transmission values, which represent the probability of multiple scattering. If these measured transmission values are 0.1 or less after the red clay addition, the experiment is terminated as reliability can no longer be ensured. To account for the variation in lighting  
Figure 1: (**a**) Experimental water tank and measurement devices and (**b**) mixing after addition of suspended sediment.  
with time and water depth, a Go-Pro 9 camera and underwater light source were installed in the camera box to continuously record underwater images under constant illumination from a light source (Figure 1). During the experiment, the camera box was carefully installed parallel to the other equipment, ensuring proper alignment, and was positioned at the same height. Images were captured every second. From other underwater images, the dependence of the red/green/blue (RGB) and gray band values on the suspended-sediment concentration was identified.