TIDAL CURRENT MOVEMENT AND ITS BEARING ON SEDIMENT TRANSPORTATION ON LIAODONG SHOAL

Parallel linear sand ridges and troughs are widely distributed on the Liaodong Shoal in the eastern Bohai Sea.Based on the distribution data of surface sediments,the simultaneous observation of sand ridges, current movement and suspended sediment transportation as well as the bathymetric survey on the Liaodong Shoal,the distribution pattern of surface sediments on sand ridges and in troughs are studied together with the movement of the tidal currents and suspended sediments.The transport trend of sediments, the response of sand ridges and troughs distribution to tidal current are discussed.The results show that coarse sands occur mainly on sand ridges,while sandy silt and silty sand dominate troughs. Controlled by submarine topography,the current speed upon sand ridges is lower than that in troughs on the surface layer,but opposite near the bottom,and the critical depth is about 0.7 H (H is water depth). The sediments migrate on sand ridges along the direction of NNE.In a tidal cycle,the surface sediments on sand ridges may be kept in moving for 8.5~10.5 hours,while in troughs the sediments can only move for 1hour during the flood and ebb tide when the currents speed is high.Under the long-term cyclic actions of tidal current,sand ridges will be gradually eroded,while sedimentation happens in troughs,resulting in degeneration of sand ridges,and the height difference between sand ridges and troughs decreases so that the seabed terrain become flat eventually.However,this process takes time and the current geomorphic pattern of the Liaodong Shoal will remain stable for a long period.