Influence of Topographic and Shoreline Changes on Tidal Current Asymmetry in Lingding Bay

[Objective] Topographic and shoreline changes caused by human activities such as tidal flat reclamation, channel deepening by dredging, and seabed sand mining further interfere with tidal wave propagation in estuarine and coastal areas. These changes have significant impacts on key environmental and engineering issues, including channel maintenance and management, the safety and stability of coastal hydraulic structures, the transport patterns of sediment and pollutants, and saltwater intrusion. Previous studies on tidal asymmetry in the Pearl River Estuary have mostly focused on tidal asymmetry itself; however, tidal current asymmetry induced by topographic and shoreline changes exerts a greater influence on material transport. To systematically evaluate the disturbance mechanisms of such human activities on the tidal dynamic system, this study investigates the tidal current asymmetry in the Lingding Bay area caused by topographic and shoreline changes using the skewness method. [Methods] A high-resolution hydrodynamic numerical model covering the river network, estuary, and adjacent sea areas of the Pearl River Delta was established based on Delft3D-Flow Flexible Mesh. The model was rigorously calibrated and validated using observed water levels and current velocity data from multiple stations. The simulated results agreed well with the observations, indicating that the model had reliable predictive capability. On this basis, sensitivity experiments with different shoreline and topographic configurations were conducted. Using the T-Tide harmonic analysis tool, the variations in amplitude and phase of the main astronomical tidal constituents O1, K1, M2, and S2 and the shallow-water constituents M4 and MS4 between the 1970s and the 2010s were analyzed and summarized. The skewness method was adopted to calculate the flow velocity asymmetry (FVA) and flow duration asymmetry (FDA), to reveal the spatial distribution patterns of FVA and FDA in different decades, and to explore the contributions of major tidal constituent combinations to FVA and FDA. [Results and Conclusion] (1) The tidal dynamics in the Lingding Bay area exhibited a distinct tidal current asymmetry. Shoreline extension caused by reclamation and topographic incision resulting from channel dredging led to a general increase in the amplitudes of major shallow-water constituents and a northward shift of tidal wave phases, while the phase difference between the bay mouth and bay head decreased. These changes collectively weakened the ebb dominance and shortened the ebb duration, thereby enhancing the overall tidal current asymmetry in this region. (2) For different types of tidal current asymmetry, FVA was mainly controlled by topographic changes, with the combinations of K1/O1/M2 tidal constituents and the residual current term playing a dominant role. The FVA generally exhibited ebb dominance, favoring seaward sediment transport. The skewness values of FVA within Lingding Bay still showed a spatially decreasing trend along the bay. (3) In contrast, FDA was more sensitive to shoreline changes. Its variation was mainly controlled by the combinations of tidal constituents such as M2/M4 and M2/S2/MS4. Topographic and shoreline changes induced by human activities further enhanced the contribution of these tidal constituent combinations to FDA. Overall, the FDA exhibited a shorter ebb duration, with its asymmetry increasing from the bay mouth toward the bay head.