Variations of sedimentary characteristics on the Hwangdo tidal flat, Cheonsu Bay at the west coast of Korea

Abstract

The western tidal flats of Korean Peninsula are well developed along the coast with gentle slopes with 4 to 10 meters of tidal ranges. Recently, rapid expansion of developed coastal area consequently changes the coastal environments. Changes of physical energy due to artificial structures have been effect on ecosystem and sedimentary environments in tidal flats. Cheonsu Bay in the mid-west coast of Korea was reduced total area since seawalls were constructed for land reclamations in 1980s. The Hwangdo tidal flat are formed inside Cheonsu Bay. The tidal flat is characterized by complex channels in its center. Tides are typically semi-diurnal with a mean tidal range of 4.59 m. In this study, we attempted to understand variations of sediment characteristics through comparing distributions of surface sediments and elevations on the tidal flat.The surface sediments for sedimentary analysis were sampled at the tidal flat in March 2004, May 2010 and June 2013. The grain-sized distributions were determined by using standard sieving and Sedigraph 5100. The inclusive graphic method was used to determine sediment type, mean, sorting, skewness and kurtosis. The elevation was measured at one transect line by using a RTK-GPS in May 2010 and June 2013. The short-term sedimentation rates were determined at one transect line by burying a plate at sub-bottom depth and periodically measuring the changes in sediment depth from February 2013 to February 2014.The digital elevation model (DEM) showed that the central tidal flat was at high elevation. The elevations were significant gradients in the west and had relatively low relief in the east. The surface sediments in the tidal flat were generally classified into three sedimentary facies. In March 2004, the muddy sand sediments dominated southeastward tidal flat. The sandy sediments dominated to eastern part and southward tidal flat and the sandy mud sediments mostly distributed to western part of the tidal flat. The area of muddy sand sediments extended to southward tidal flat in May 2010. The area of sandy sediments diminished, but that of sandy mud sediments increased eastward tidal flat. In June 2013, the muddy sand sediments were predominant in the middle to eastern part of the tidal flat. The topographic changes were investigated in the transect line in May 2010 and June 2013. The deposition occurred with net deposition of 13.36 cm during three years. The short-term sedimentation rates from February 2013 to February 2014 showed that an annual sedimentation rates were ranged -30.2 to 12.35 mm/year with the net erosion rate of -4.20 mm/year. Seasonally, the deposition occurred on western part in summer and eastern part of the tidal flat in winter with an inflow of muddy sediments.The constructions of island-connecting bridge and dock had been carried out near the Hwangdo tidal flat since 2000s. The construction of Hwangdo bridge after demolition of the dyke resulted in circulation of tidal currents at western parts of the tidal flat since 2012. In comparison of sedimentary facies between 2004 and 2013, the area of muddy sand sediments had increased, but that of sandy sediments decreased for 10 years. The topographic changes on the tidal flat between 2010 and 2013 showed that the deposition occurred with an inflow of muddy sediments by circulation of tidal currents. These changes should be effected the local hydrodynamic changes by demolition of the dyke near the tidal flat.Acknowledgements: This study was a part of the “Satellite Application Techniques for Coastal Ocean Environmental Monitoring” program funded by KIOST.