On the Mechanism behind the Variation of the Tidal Current Asymmetry in Response to Reclamations in Lingding Bay, China

Abstract

Tidal current asymmetry, an important factor for waterborne substance transport, is strongly affected by the changing shoreline. Lingding Bay (LDB), located in the center of the Guangdong-Hong Kong-Macao Greater Bay Area, has suffered great shoreline changes since the 1970s. A well-validated numerical model is built to investigate how the changing shoreline impacts tidal current asymmetry, which is quantified by the skewness parameter. Two types of tidal current asymmetries, the flow velocity asymmetry (FVA) and the flow duration asymmetry (FDA), are considered here. Despite their different dominant combinations, the two asymmetries display a similar tendency from the mouth to the head of the LDB. When the shoreline changes due to successive land reclamation are taken into consideration, the FVA and the FDA alter correspondingly. The results suggest that the shoreline change shifts the FVA in Deep Bay (a side embayment of LDB) from ebb dominance to flood dominance, because of the changes of its dominant constituent combinations (K1/O1/M2 and the residual current). As the shoreline propagates seaward, the increasing contributions of M2/M4 and M2/S2/MS4 enlarge the water area of positive FDA (i.e., shorter duration of low water slack) in LDB. Analyses of tidal current asymmetries in Lingding Bay show that tidal current asymmetry has been enhanced by sustained land reclamation and will further impact sediment transport in the bay.