Jinah Eom

A Study of Decadal Sedimentation Trend Changes by Waterline Comparisons within the Ganghwa Tidal Flats Initiated by Human Activities

Abstract This study presents results of a human impact investigation of large-scale construction projects on sedimentation trend and morphologic changes in the Ganghwa tidal flats over 10 years using time-series waterlines from Landsat thematic mapper/enhanced thematic mapper. Waterlines were extracted from remotely sensed images. These images are useful for studying changes in coastlines and tidal flat topography. A reference digital elevation model (DEM) was constructed; then actual waterlines from satellite images were compared with modeled waterlines from the reference DEM. Systematic comparison with respect to horizontal differences between the two waterlines provided information on local conditions of sediment trends within the study area. Seaward or landward migration of waterlines was a clear indication of the change in sedimentation pattern. Deposition has been dominant at the main channel between the southern Ganghwa and the Yeongjong tidal flats, whereas erosion has been dominant at the eastern lower tidal flat. These sedimentation patterns complied with field observations along two survey lines and with the result from depth-sounding data. Deposition narrowed the channel and created a slightly more meandering shape. A hydrodynamic model anticipated a significant change of ocean current as a result of the coastal construction projects, and the modeling result matched well in terms of current conditions with the present sedimentation pattern analyzed by waterline comparison. A series of coastal construction projects in this area clearly affected the local hydrodynamics of tide and currents to a large extent and resulted in the changes of sedimentation trends during a decade, and it is necessary to keep the monitoring area to understand long-term impacts initiated by human activities within a relatively short period. Although it was not possible to estimate the total volume of sediment transportation with this method, trends of sedimentation processes within tidal flats can be effectively deduced.

Tidal channel distribution in relation to surface sedimentary facies based on remotely sensed data

Tidal channel networks are important factors influencing the morphodynamics of tidal flats and surface sedimentary facies. Here we investigate the relationship between channel distribution and sedimentary facies in Geunso Bay tidal flat, Korea. The tidal channel networks were extracted from a high spatial resolution aerial photograph, and for each sedimentary facies the pattern of tidal channel distribution was compared in terms of fractal analysis, channel density, and distance from the channel. The tidal channels in each sediment facies had relatively constant meandering patterns, but the density and complexity were distinguishable. The second fractal dimension was 1.87 in the mud flat, 1.41 in the mixed flat, and about 1.30 in the sand flat. The channel density was 0.036–0.06 m/m2 in the mud flat and 0–0.024 m/m2 in the mixed and sand flat areas. This implies that the tidal channels in the mud flat area represent a complex and dendritic pattern with high density compared to those in the mixed or sand flat areas. The results were used to test the applicability of adjusting sedimentary facies classification generated from interpolation of survey data. We quantitatively estimated the pattern of tidal channel distribution for each sedimentary facies based on a high spatial resolution aerial photograph. We suggest that tidal channel network features can be useful to surface sedimentary facies classification in tidal flats.

Spatiotemporal Variation in Suspended Sediment Concentrations and Related Factors of Coastal Waters Based on Multispatial Satellite Data in Gyeonggi Bay, Korea

ABSTRACT Eom, J.; Choi, J.-K.; Won, J.-S.; Ryu, J.-H.; Doxaran, D.; Ruddick, K., and Lee, S., 2017. Spatiotemporal variation in suspended sediment concentrations and related factors of coastal waters based on multispatial satellite data in Gyeonggi Bay, Korea. The variations in suspended sediment concentration (SSC) in turbid coastal waters around Gyeonggi Bay, Korea were analyzed using multiresolution ocean-color satellite imagery. Geostationary Ocean Color Imager (GOCI) and Land Satellite 7 (Landsat-7) Enhanced Thematic Mapper Plus (ETM+) images were atmospherically corrected, and an empirical algorithm was employed to generate maps of SSCs in the study area and investigate daily and annual variabilities in coastal water turbidity. SSC values were highest around 6 hours before high tide and around low tide, and the maximum values had strong positive relations with the tidal range near the sand ridge and channel (R2 values of 0.74 and 0.72, respectively), which implies that the main driver of the diurnal variability in SSC is resuspension of bottom sediment by tides in areas of shallow water. Annually, the maximum SSC value near the sand ridge was about 400 g m−3, showing remarkable variation over tidal cycles, whereas it was about 10 g m−3 in the open sea, with little variation. The SSC around the sand ridge was higher in winter than in summer, mainly because of stronger resuspension resulting from winds during the NW monsoon in winter. The SSC around the Han River estuary was higher in summer than in winter because of the river discharge, which indicates that suspended sediments supplied by the Han River do not significantly affect SSC variation in the open ocean. This study revealed that application of the high temporal resolution of GOCI, combined with the high spatial resolution of Landsat-7 ETM+, can be useful for monitoring short- and long-term variations in SSC in Korean coastal waters.

Analysis on the seasonal variations of microphytobenthos distribution in a tidal flat using remotely sensed data

The applicability of remotely sensed data to the detection and monitoring of the seasonal variation of microphytobenthos distribution in a tidal flat was examined for the Geunso-bay tidal flat in the west coast of Korean peninsula. The biomass of diatom within the surface sediments was estimated through field campaigns and the seasonal change in the spectral reflectance of the remotely sensed data was investigated. Field spectrum data were acquired monthly at the fixed locations for monitoring the microphytobenthos blooming and comparing with the spectral reflectance of satellite images. Sediments facies was also analyzed along with the spectral reflectance based on the in situ data, and the spectral characteristics of the area where microphytobenthos occupied was examined. A medium to low spatial resolution of satellite image was not suitable for the detection of the surface sediments changes in the study area due to its ambiguity of sediments facies boundary, but the seasonal changes of benthic distribution could be obviously detected. From this, we suggest that the study on the distribution of surface sedimentary facies and detailed ecological mapping in a tidal flat based upon the remote sensing images should consider the seasonal variations of microphytobenthos distribution which would be included in the spectral characteristics of the satellite images.

Standardization of sedimentary facies and topography based on the tidal channel type in Western coastal area, Korea

ABSTRACT Eom, J., Choi, J.K.., Lee, Y.K., Ryu, J.H., Won, J.S., 2013. Standardization of sedimentary facies and topography based on the tidal channel type in Western coastal area, Korea The tidal flat area along the west coast of the Korean Peninsula is among the largest in the world. The Ganghwa-do tidal flat, which is an open type, the Geunso-bay tidal flat, which is a semi-closed bay, and the Hwang-do tidal flat in Cheonsu Bay have different benthic environments, formed by different surface sedimentary facies and topographic features. These benthic environments in each tidal flat are influenced mainly by the tidal channel network. In other words, knowledge of the tidal channel distribution pattern is a key for understanding the geophysical and surface-sedimentary facies. From a quantitative estimate of the tidal channel distribution pattern, it is possible to estimate the surface sedimentary facies and topographical features. In this study, we standardized the surface sedimentary facies and topography by developing a tidal flat index in terms of the tidal channel distribution pattern. To extract tidal channel data in the study area, we used remotely sensed data, such as those from the Korea Multi-Purpose Satellite-2 (KOMPSAT-2), Landsat Enhanced Thematic Mapper Plus (ETM+), and aerial photographs. In addition, we generated a digital elevation model (DEM) using the waterline method to standardize the topographic features. Surface sedimentary facies maps were generated based on field data using an interpolation method. From these data, we calculated the tidal channel index by a fractal method. We also analyzed the spatial correlation of the tidal channel index with surface sedimentary facies and topography, respectively, using a GIS tool. Low values of the tidal channel index, which indicated a simple pattern of tidal channel distribution, were identified at areas having low elevation and coarse-grained sediments. By contrast, high values of the tidal channel index, which indicated a dendritic pattern of tidal channel distribution, were identified at areas having high elevation and fine-grained sediments.