Hydrodynamic Processes and Bedload Transport Associated with Large-Scale Sandwaves in the Taiwan Strait

Hydrodynamic processes and bedload transport over a large-scale sandwave field were analyzed along three transects in Taiwan Strait based on measurements of water depth, current velocities within the water column, and the physical properties of the sediment at the seabed. The sandwaves in consideration have steep crests and flat troughs, with a width of 14.4-20.3 m and a length of 325-820 m. Generally, the mean H/L ratio falls in the range of large-scale sandwaves, but the ratio is larger than the average, deviating from the global trend. The H/d ratio is much higher than the global average. The current-velocity vectors at the sandwave crests, troughs, stoss sides, and lee sides turned in a clockwise manner within a tidal cycle; the current speed above the crest was relatively larger than above the trough. During the flood tide, the current speed reached a maximum in the layer 9-11 m below the water surface. During the measurement period, there was net accumulation of sediment over the study area. The migration of the sandwaves caused by bedload transport varied widely at different transects: for the three transects in consideration, migration in the flood direction occurred at two transects, with different migrating speeds; and ebb-dominated migration occurred at the third transect. The migration rate predicted by the bedload transport calculations, based on tidal current measurements, is lower than long-term observations by remote sensing, which may be due to the fact that this region is frequently influenced by storm events, and the long-term migration patterns are related to the enhanced currents and waves during storms. Further, time-velocity asymmetry of the tidal currents, as observed in the present study, may also favor the enhanced migration rate during storm events.