Study on Topographic Changes in Tidal Flats of the West Coast of South Korea between 1980s and 2010s Using Remote Sensing Data

Abstract

Morphologic and topologic changes of tidal flats areas have attracted world-wide interest due to their function and importance. In recent times, diverse researches were performed to detect and estimate the temporal topographic changes of tidal flat using various remote sensing data. This study aims to estimate and evaluate topographic changes in Gomso and Hampyeong Bay of South Korea using waterline method and SAR data. First of all, the seasonal topographic variation in the tidal flat were estimated using the waterline method. A total of 18 scenes of Landsat 7 data acquired during the period from 2003 and 2004 provided individual waterlines for various tidal heights and their corresponding tide gauge observation data defines the reference height. Results showed that summer deposition is a main dominant effect in tidal flats in Gomso Bay with an average overall seasonal topographic change of 12.8 cm. In contrast, summer erosion is dominant with -5.0 cm topographic variation in Hampyeong Bay. Although both Gomso and Hampyeong Bay are classified as semi-enclosed coast tidal flat, the sedimentary facies caused by formation geometry and sediment type led to different topographic changes. Secondly, topographic changes were estimated from DEMs generated for the years 1980s and 2010s using the waterline method, Seasonal topographic changes was taken into consideration to estimate the minimum topographic changes of each bay in the study area. As a result, during1980s and 2010s Gomso Bay showed the predominant deposition regardless of the season. In the winter, the amount of deposition was about 12.4 cm

in the summer, the deposition was about 31.9 cm. For Hampyeong Bay, erosion was dominant in overall with the amount of 29.6 cm and 41.2 cm in the summer and winter, respectively, in the past 30 years. The waterline method is unique way of extract the topographic information in the past few decades. Furthermore, the temporal topographic changes during 1980s to 2010s have been analyzed. A total of 82 scenes of Landsat 2, 3, 4, 5, 7 and 8 images were used to extract the waterline and generate the DEM with winter and summer during the past 30 years (1980-2010) in the Gomso and Hampyeong Bay. From the results, the temporal topographic changes have shown clear seasonal characteristics in the winter and summer during the past 30 years, in the Gomso and Hapyeong Bay. As the 2000s the topographic was not regularly deposed and eroded, in the Gomso and Hampyeong Bay. It might be also effect on constructed big Saemangum sea dyke. Finally, the feasibility of SAR interferometry technique for monitoring topographic variation in tidal flats has been evaluated. The topography of tidal flats in Gomso Bay derived from TanDEM-X bi-static interferometry and SRTM mission data are compared with those from the waterline method. Although the temporal difference between the datasets prevented from direct topographic comparison, the spatial trends of deposition and erosion were similar to those from the waterline method. It is highly expected that monitoring topographic variation of tidal flats using SAR interferometry will become practically possible if more single-pass interferometry SAR data are collected in time series. This research is the first approach of detecting seasonal topographic variations by the waterline method. Consideration of the seasonal topographic changes is essentially important in detecting more accurate long term topographic changes in the tidal flat. By using the waterline method, it is more easy to restore the past DEM. From the effective analysis and understanding of the long term topographic changes, we can able to respond more rapidly to current and future coastal environmental changes.