| Sandwaves on seafloor is a kind of characteristic forms where sediments are deformed by current shear function and seabed is regularly transformed correspondingly. They are generally found in the region where there are powerful dynamic conditions and ample sediments and represent a typical character that sediments and liquid adjust one another.A large area of sandwave geomorphology has been found on the seafloor of the outer continental shelf and upper slope within a water depth of 100～250 meters in the northern South China Sea. Wave height of the sandwaves is 0.3～3m and wave length is a few meters long. They usually have anisomerous waveform with gentle slope to NW and steep slope to SE and the wave ridge line is in the direction of extending NE～SW. Second-clas sandwaves and ripples with a small geometry are developed in the two wings and wave hollow of medium to large sandwaves, and comprise duplicate seafloor sandwaves with medium to large sandwaves.Plenty of petroleum and natural gas have been found in the outer continental shelf. Recent years, with the development of marine resources, a lot of pipelines and seabed cable have been laid. At the same time, the study area is tropical marine monsoon climate with typhoon-prone region, incidents such as typhoon and tsunami will have an important effect on sandwaves motion and adjust and cause seabed to change greatly. Consequently, particular study on motion of sandwaves, mechanism of formation and conditions for growth, on one hand, makes study on sandwaves more abundant, on the other hand, have practical meaning for security of manufacture on the sea and integrity of marine establishment.Approximate calculation for bedform stability parameter and incipient motion of sediment combined with actual configuration of seafloor microtopography are adopted to study the mechanism of formation. Mathematical analysis is adopted to study motion velocity and direction of sandwaves so as to analyse stability of sandwaves.
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