Giant sand waves on the Taiwan Banks, southern Taiwan Strait: Distribution, morphometric relationships, and hydrologic influence factors in a tide-dominated environment

摘要

Giant sand waves (heights > 10 m) are widely developed on the Taiwan Banks, the distribution pattern, morphotypes and morphometric relationships of which have not yet been fully clarified. Based on multi-beam bathymetric swath data covering 780 line km of survey track, giant sand waves with heights up to 22.5 m and wavelengths up to 2115 m have been recorded. The relationship between height (H) and wavelength (L) of the complete data set follows the regression trend of H-mean = 0.0625 L-0.(8152) (r = 0.94), which is in good general agreement with a previously determined average global trend of H-mean = 0.0677 L-0.(8098). This global trend did not include any data from the China seas. The giant sand waves have been grouped into four morphotypes, namely double-crested, symmetric, symmetric-asymmetric and asymmetric forms. The individual bedform morphotypes and their spatial distribution were investigated in relation to water depth, textural characteristics of the sediment, and the hydrodynamic conditions recorded by several 25-hour repeated ship-based current observations. The relationship between mean water depth (D) and mean sand wave height (H) and mean wavelength (L) on the Taiwan Banks are H-mean = 0.29D (r = 0.35) and L-mean = 36D (r = 0.42), respectively. With increasing water depth, the shape of the sand wave becomes gentler. It is found that the coarser sediment (median grain size of 0.51-0.54 mm) and stronger flow in the present study may contribute to the larger H/D and L/D ratios at the given water depths than other sand wave fields. And flows of varying from equal to unequal northerly and southerly components correspond with sand waves from symmetrical to asymmetrical forms. A decrease in velocity and a veering in direction of flows have been observed over the giant sand waves, indicating the interaction between large bedforms and the near-bed water layer in the study area.