EVALUATING SEABED EROSION BY SIDE-SCAN SONAR SURVEY IN MUDDY COAST OF CHAO PHRAYA RIVER MOUTH, THAILAND

Abstract

The muddy coasts of the Upper Gulf of Thailand (UGOT) are very sensitive to coastal erosion. These areas have the most severe erosion rate in Thailand. Moreover, this erosion has also been enhanced by land subsidence in Bangkok area and vicinity as well as the sea level rise. This study aims to assess the seafloor erosion in muddy coasts of the UGOT near the Chao Phraya River mouth. The survey was done from the coast to 12 km offshore in three survey lines, Ban Khok Kam, Ban Khun Samut Chin, and Ban Klong Tamru survey lines. Side-scan sonar was used as a tool to investigate the seafloor morphology and single-beam echo sounder was used as a tool to determine the bathymetry. The bathymetric profiles surveyed on 2012 by this study were also compared with those surveyed by Hydrographic Department, Royal Thai Navy (HDRTN) from 1960 onwards. Furthermore, the sediment samples were collected to determine the water content, the laser diffraction method was applied to investigate the particle-size distribution, and the erosion of the seafloor was verified by Cesium-137 (Cs-137) concentration. The comparison among the bathymetric profiles shows an extreme change on the seafloor in the long time scale, approximately 10 years. However, in the short time scale or no more than 4 years, the change cannot be seen obviously. The results of sonographs suggest that most of the sediments on the seafloor are soft materials. The results of water content in the sediments are lower in the deeper layers but higher in the farther offshore areas. The results of particle-size distribution reveal that most of sediments in Ban Khok Kam and Ban Khun Samut Chin are clay and silt while most of that in Ban Klong Tamru is sand. The Cs-137 concentrations show low value in the sediments collected near the coast while the sediments collected far away from the coast have higher values. In conclusions, it is suggested that the sediments are eroded near the coast and redeposited in the offshore area.