Estimation of Tidal Current Asymmetry in an Archipelagic Region: The Zhoushan Islands

Abstract

Tidal current asymmetry (TCA) often occurs in coastal regions. It can significantly influence bedload sediment transport. Recently, the statistical skewness of the tidal current velocity was calculated to represent the TCA. In archipelagic region, the tidal current directions vary temporally and spatially from channel to channel. This creates complexity in finding the flood–ebb axis about which to discuss the axial dissymmetry of tidal currents. In the present work, a method that involves taking the main flood direction (MFD) as the axis to split the tidal current was suggested. The MFD is the most probable direction of the strongest flood flow during each tidal cycle. The method was applied in an archipelagic region: The Zhoushan Islands. The results show that the calculated skewness well represented the TCA in waters around islands, and the degree of the TCA was mainly determined by the residual current. When the direction of the residual current was the same as the MFD, the skewness was positive, which indicated flood dominance. On the contrary, when the direction of the residual current was opposite to the MFD, the skewness was negative, which indicated ebb dominance. The stronger a residual current is, the more significant the TCA will be. Islands play an important role in forming residual circulations. Large ones force flows to move offshore around headlands or along curved channels, because of centrifugal forcing, while small ones often cause segregated flood/ebb conduits and form residual circulations. In the waterways between the Zhoushan Islands, the ebb current generally carries more sediment than the flood current. Therefore, ebb dominance always means sediment is more likely to be deposited, and vice versa. Further research into sediment transport modeling is suggested.