BED FORMS ON THE NORTHERN SHELF AND SLOPE OF THE SOUTH CHINA SEA

Bed forms are widely distributed in the northern part of the South China Sea covering the area from the Zhujiang estuary to southern Taiwan Island and the east sea area off the Dongsha islands. The combination of dynamic conditions, such as the topographic relief, intensive bottom flow, and ample sand source, caused the formation and development ofthe sand waves.The area could be divided into three subareas undersea. The subarea A is the area on the inner shelf off the ZhuJiang estuary in a water depth ranging from 80 m to 200~250 m. Sand waves are active there at present and formed under the influence of the shelf underflow (tide,wave and current).They are tens of contimeters to 2~3 m in height with strong asymmetry. The subarea B is located on the upper shelf at a water depth of 200~600 m, covered by widely distributed sand waves,which are several meters up to tens of meters highwith relatively short wave length. They are mainly deposited on the platforms between slopes. Some scholars believe that theyare formed by the sudden release of energy from the internal waves between water layers with different densities. The subarea C is on the lower slope at water depth of 3 200~3 400 m, where several channels join together.Silt sediment waves are developed on the southwest of the channel with a height oftens of meters, and length oftens of kilometers. They are deduced as a specific sand wave originated by deep water fine grained turbidity current. All the sand waves mentioned above may render threats to the stability of submarine pipelines and platforms for oil and gas exploration.