Sand waves near the shelf break of the northern South China Sea: morphology and recent mobility

Abstract

Sand waves near the shelf break in the northern South China Sea have not previously been described in detail due to the lack of high-resolution bathymetric data. Their occurrence, however, is important for understanding sand transport and dispersal in the northern South China Sea. Here, using recent bathymetric data, side-scan image, bottom current measurements, and sediment samples collected in 2010 and 2016, we document the distribution, geometry, and mobility of sand waves in five regions along the shelf break of the northern South China Sea. Results show that the sand waves have distinctly different distribution patterns, geometries, and migration behaviors above and below the 145 m isobath. Sand waves at depths greater than 145 m are larger and steeper, and mostly occur on slopes steeper than 0.5°. Their crest lines are aligned parallel to the isobaths and their migration direction is toward the southeast (seaward). In comparison, the sand waves on the gentler sloping seabed at water depths < 145 m migrate toward the northwest with crest lines mostly intersecting the depth contours. Neither the heights nor the wavelengths of the sand waves correlate with water depth, although the two classes of sand wave are restricted to distinct depth intervals. The observed bottom currents, which are characterized by strong velocity pulses, probably relate to internal solitary waves which have the capacity of moving the sediment in the sand wave fields. The height-wavelength relationships of the sand waves correspond to those of subaqueous dunes. The diverging migration directions of the sand waves are interpreted as reflecting the polarity conversion of the internal solitary waves in the northern South China Sea during their propagation onto the shelf. The volume changes of the sand waves from 2010 to 2016 indicate both sediment loss and sediment accretion in various areas, suggesting complex sediment dispersal patterns near the shelf break.