Applicability of Satellite-Based Turbidity Monitoring of Nearshore Sea Surface to Coastal Sediment Management

The goal of this study is to develop a monitoring technique of nearshore water surface turbidity using Moderate-Resolution Imaging Spectroradiometer (MODIS) satellite data and to investigate its applicability to coastal sediment management. Suruga Bay and Enshu-Nada coast, Japan, were selected as a case study site along which seven major rivers have their outflows. Since MODIS L1B reflectance data is known to have high correlation with in-situ measured turbidity, this study used the L1B reflectance for monitoring of water surface turbidity. Using MODIS, we extracted spatiotemporal distributions of reflectance around the coast over 20 years from 2001 to 2020. In addition, regression analysis of obtained reflectance was conducted against river discharge and wave height, respectively. The observed reflectance around the rived mouth showed strong positive correlation with the river discharge. Near the Tenryu River mouth, the reflectance at specified discharge obtained from the regression line at each year showed clear decreasing trend, indicating decreasing trend of sediment discharge in recent 20 years. Similarly, the spatial distribution of the reflectance at specified wave height, H = 3 m, was obtained from the regression line. The spatiotemporal variation of this wave-associated turbidity showed clear eastward migration along the shore from the Tenryu River mouth. The estimated speed of migration was around 2.5 km per year. This eastward migration was consistent with the previous study, which showed eastward movement of blackish sand dominantly discharged from the Tenryu River after the construction of Sakuma Dam.