Tidal flats are associated with complicated depositional and ecological environments, and have changed considerably as a result of the erosion and sedimentation caused by tidal energy; consequently, the surface sediment distribution in tidal flats must be constantly monitored and mapped. Although several studies have been conducted with the aim of classifying intertidal surface sediments using various remote sensing methods combined with field survey, most of these studies were unable to consider various sediment types, due to the low spatial resolution of remotely sensed data. Therefore, previous studies were unable to efficiently describe precise surface sediment distribution maps. In the present study, unmanned aerial vehicle (UAV) red, green, blue (RGB) orthoimagery was used in combination with a field survey (232 samples) to produce a large-scale classification map for surface sediment distribution, in accordance with sedimentology standards, using an object-based method. The object-based method is an effective technique that can classify surface sediment distribution by analyzing its correlations with spectral reflectance, grain size, and tidal channels. Therefore, we distinguished six sediment types based on their spectral reflectance and sediment properties, such as grain composition and statistical parameters. The accuracy assessment of the surface sediment classification based on these six types indicated an overall accuracy of 72.8%, with a kappa coefficient of 0.62 and 5-m error range related to the Global Positioning System (GPS) device. We found that 11 samples were misclassified due to the effects of sun glint and cloud caused by the UAV system and shellfish beds, while 14 misclassified samples were influenced by surface water related to the elevation, tidal channels, and sediment properties. These results indicate that large-scale classification of surface sediment with high accuracy is possible using UAV RGB orthoimagery.

The Hwangdo tidal flat has been subjected to local hydrodynamic changes caused by the construction of seawalls and bridges, which are changing the distribution of sediments and topographic characteristics. These changes would affect the sedimentary environments. It is essential to establish sedimentary environments dataset such as sediment distributions and elevation in order to understand the sedimentary environments and the pattern of change in the tidal flat. Therefore, between 2004 and 2013, data on sedimentary environmental factors such as surface sedimentary facies and elevation were obtained through the field survey, and soil moisture content of each sediment was measured to analysis the correlation between seawater and optical reflectance in the tidal flat. As a result, 12 sedimentary facies were distributed in the Hwangdo tidal flat, and the sand content and elevation gradually increased between 2004 and 2013. It was also shown that the amount of seawater present in the surface decreased as elevation and grain size increased. These data will be useful for understanding the changes in the sedimentary environments and for establishing plans for change and conservation management in Hwangdo tidal flat.