Sand Waves Near The Shelf Break Of Northern South China Sea:Morphology,mobility And Mechanism

Submarine sand waves are a common seabed landform,the formation and evolution of sand waves are closely related to the nature of seabed sediments,regional topography and hydrodynamic environment.Therefore,the study of seabed sand waves is helpful for us to understand the evolution of sedimentary environment,the changes of sea level and the evolution of hydrodynamic environment in the study area,which is of great theoretical significance.Rapidly moving sand waves often threaten the safety of submarine pipelines and submarine cables.In addition,submarine sand deposits generally contain many metallic elements and minerals.Therefore,the study of submarine sand waves not only has theoretical significance,but also has very important engineering significance and economic value.Various types of submarine sand waves are widely distributed near the shelf break of northern South China Sea.The existence and movement of these sand waves pose potential risks for submarine artificial facilities.For example,some gas pipelines of PanYu oil and gas fields are in the sand wave distribution area,and there are suspension phenomena caused by sand wave activities.Some researchers have done some studies on sand waves near the shelf break of northern South China Sea.May be due to lacking of high-precision bathymetric data and other data,researchers have a lot of debate about the age of formation of sand waves and whether the sand waves can move or not,and where the power source of motion comes from.Some researchers believe that the sand waves are residual bedforms and will not move.Some researchers believe that sand waves are formed and migrating under modern tidal current.Some researchers believe that sand waves are moving under the influence of storm surge.In this paper,the surface sediments obtained near the shelf break of northern South China Sea were used to grain size measurement and AMS¹⁴C dating.The distribution characteristics and transport path of surface sediments are also analyzed.The morphological parameters of sand waves are extracted basing on the multi-beam bathymetry data collected for many years.The migrating distance and direction are obtained by cross-section comparison in different years.The mechanism of the formation and evolution of sand waves is discussed by analyzing the bottom current data.The results show that the surface sediments are mainly composed of sandy gravel components,while the silt and clay components are relatively small.The sand waves in the study area are modern sand waves.The sediments are mixed with glacial materials,ice-eliminating materials and Holocene materials.The ages of most sediment span a wide range showing a character of mixing old and modern sediments.The sediment in the study area are basically transported to the sea and the land.At the same time,there are also a small amount of transportation along the shelf break and across the shelf break.According to the distribution range,morphological parameters and migration of sand waves,there are five main distribution areas of sand waves in the study area.The sand waves in study area can be divided into two classes.Class 1 sand waves are distributed in deep water areas with a water depth of more than 145 m.And the slope of the seabed where sand waves are distributed is greater than 0.3°.The crest lines of sand waves are parallel to the isobath of the study area.All the class 1 sand waves incline to deep water of the SE direction and migrates to SE direction.There is a strong exponential relationship between the wavelength and the wave height of sand waves,H₁=0.0597?^0.92?R=0.67?,which is above the mean Flemming line.Class 2 sand waves are distributed in shallow waters with a water depth of less than 145 m,and the slope of the seabed is less than 0.5°.The crest lines of sand waves are intersect with the isobath,and the sand wave incline to shallow water of the NW direction and migrate to the upstream.The exponential relationship between the wavelength and wave height of sand waves is below the Flemming line,which is H₂=0.0677?^1.34?R=0.83?.The sand waves in two sub-regions of the study area have the mixed characteristics of these two classes of sand waves,which are the mixed distribution areas of sand waves.The internal solitary wave near the shelf break of northern South China Sea originates from the Luzon Strait,or is generated by the internal tidal excitation of the northern South China Sea.The internal solitary waves originating from the Luzon Strait transport across the South China Sea basin and shallow to the continental shelf and continental slope of northern South China Sea.They collide with the Dongsha Islands and are divided into two parts,and then intersect behind the Dongsha Islands.The internal solitary wave will cause a strong bottom current during the shoaling process,and only the strong current can move the sand waves in the study area.During the shoaling process,the internal solitary wave will undergo polarity conversion near the shelf break of northern South China Sea.It means the internal solitary wave will be transformed from the depression internal solitary wave to the elevation internal solitary wave.The depression internal solitary waves are mainly distributed in the deep waters of the continental slope,which cause the strong current on the seabed opposing to its propagation direction,flowing to the downstream,and the velocity is relatively large.The influence area of the depression internal solitary wave corresponds to the distribution area of class 1 sand waves.The elevation internal solitary waves are mainly distributed in the relatively shallow area of the shelf,which cause strong current on the seabed in keeping with its propagation direction,flowing to upstream,and the velocity is small.The affected region by the elevation internal solitary wave is consistent with the distribution region of class 2 sand waves.Therefore,internal waves are the main reason for the differential distribution and migration of submarine sand waves near the shelf break of the northern South China Sea.This study shows that internal waves are the main driving force for the formation and evolution of deep-water sand waves,which enriches the theory of sand wave formation and evolution,and has important guiding significance for the study of deep-water sand waves in the future.