Field observations of internal waves triggered sediment movements at the northern South China Sea continental slope

Abstract

Internal ocean waves, such as internal tides and internal solitary waves (ISWs), have strong horizontal and vertical velocities and play an important role in ocean energy and material exchange. Field observations indicated that internal waves had a significant effect on hydrodynamics and sediment transport on the continental shelf. However, there remained a lack of field observations on how internal waves affect sediment transport on continental slopes. In this study, a seabed observation system was set up on the continental slope of the South China Sea for 13 days, which measured the high spatiotemporal resolution velocity and mixing of the internal tides and ISWs and sediment movements in the bottom boundary layer. The results showed that the internal tides and ISWs caused strong flows of more than 0.20 m/s near the seabed of the slope, which was much higher than the local sub-inertial currents (0.03 m/s) and barotropic tides (0.03 m/s). The velocity enhancement caused by internal tides and ISWs increased the bottom shear stress from 0.01 to 0.08 Pa. When the internal tidal velocity increased, the suspended sediment concentration (SSC) increased substantially from 5 mg/L to more than 40 mg/L. When the strongest ISWs passed through the observation site, the SSC increased several times, reaching 19 mg/L. Both internal tides and ISWs agitated seabed sediment and transported it downwards along the slope with sediment transport flux of 0.26 × 10-3 and 0.08 × 10-3 kg m−2 s−1, respectively.