台湾浅滩大型沙波、潮流结构和推移质输运特征

Characteristics of the large-scale sandwaves,tidal flow structure and bedload transport over the Taiwan Bank in southern China

对2006年8月所获台湾浅滩的底质、水深、多层位流速、流向以及CTD观测数据进行了分析，探讨了台湾浅滩大型沙波区水流的时空分布特征、沉积物输运过程及影响因素，结果表明，台湾浅滩动力作用较强，底质为粗砂，其上发育的沙波波长为325-822 m，波高为14.4-20.3 m，较接近由前人总结的沙波波高、波长的关系式计算出的波高和波长。沙波波峰、波谷、迎水面和背水面流速以12.5 h为周期沿顺时针方向旋转；底部最大流速出现在半日潮周期的同一时段，对应的方向均为偏南方向，且沙波迎水面的流速大于背水面的。在两个半日潮过程中研究区域的沉积物输入量大于输出量，处于淤积状态。在潮流作用下沉积物输运量较少，沙波脊线移动较小，而台湾浅滩有较大规模的沙波运动，故认为改造沙波的主要作用力应该是风暴流。在观测的断面中涨潮推移质输运量大于落潮的，且第一个半日潮涨落潮的输运能力均较第二个半日潮的大，这可能与潮流的流速-时间不对称特征及潮能变化形式有关。

On the basis of the measurement of current velocity within the water column, water depth and physical properties of seawater over the Taiwan Bank in August 2006, the spatial and temporal characteristics of current velocities over large-scale sandwaves, together with the process of sediment transport, are analyzed. The results show that the sandwaves, 14.4-20.3 m in height and 325.4-821.8 m in length, are formed under the conditions of strong current that is acting upon the bed consisting of coarse-grained sediments, with a height-length relationship similar to those identified elsewhere in previous studies. Currents at the sandwave crest, trough, stoss-side and lee-side turn in a clockwise manner with a period of 12.5 h. In the bottom layer of the water column, the largest velocity occurred at the same phase of each semidiurnal cycle, which was directed to the south; furthermore, the velocities on the stoss-side are larger than on the lee-side. During the two semidiurnal tidal cycles, the amount of sediment imported into the study area was larger than output, indicating that accretion was taking place. The sediment transported by tidal current was relatively weak, which could cause only slow migration of sandwaves. However, according to satellite observations, the sandwave migration can be high over the Taiwan Bank. This phenomenon may be caused by the strong currents associated with storm events. At the cross-section studied, sediment transport was more intensive during the flood than during the ebb, and the net fluxes varied for different semidiurnal tidal cycles, indicating the effects of the time-velocity asymmetry and the tide phase.