DETERMINING SUBSURFACE SUSPENDED SEDIMENT MECHANISMS THROUGH SURFICIAL REMOTE SENSING TECHNIQUES, SOUTH KOREA
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Satellite imagery can provide information on the spatial distribution of surficial suspended sediment over broad scales in coastal environments. An outstanding challenge is to determine the extent to which surficial sediment distributions can be linked to sediment processes occurring near the seabed. Recent research indicates that dense suspended sediment suspensions at the bottom of tidal channels off the southwest coast of South Korea limit upward turbulent mixing of sediment to the sea surface. The goal of this project is to determine whether this subsurface sediment process is detectable with reflectance at the sea surface measured by the Landsat 8 satellite. The project’s hypothesis is that the magnitude and variance of surficial reflectance will be lower over channels than over ridges, due to the presence of dense suspensions in channels that limit vertical mixing. On the ridges this process would not occur, because dense suspensions would flow into adjacent channels under the influence of gravity. As a result, reflectance would be higher and more variable at shallower depths. To assess this hypothesis, Acolite processing software was used to perform atmospheric corrections on Landsat 8 images, and sea surface reflectance at 655 nm and 865 nm were used as proxies for suspended sediment concentration in a total of 15 cloud-free images collected over the years 2013-2018. Reflectance in both bands was extracted over a tidal channel and over an adjacent tidal ridge using SeaDAS. The reflectance from these two points were assessed for statistical correlation with depth and other environmental variables, including sea level, wind speed, and recent precipitation levels. Results indicate that depth exerts primary control on mean and standard deviation reflectance, consistent with the project’s hypothesis. Secondary controls on reflectance are wind speed and sea level. I propose that higher windspeeds are associated with larger reflectance due to resuspension over fringing tidal flats and that the correlation of higher water levels with reduced reflectance is caused by sediment supply limitation. In order to see if these methods can identify other areas with similar suspended sediment processes, two secondary sites – located in Gyeonngi Bay, South Korea and the Gulf of Khambhat in India have been analyzed. Preliminary results show similar primary and secondary reflectance patterns. While further field work is needed to study the effects of tidally-dependant suspended sediment stratification on surface sediment concentration and associated reflectance, results indicate that remote sensing may be applicable to detection of subsurface suspended sediment processes.