Asymmetric Tidal Dynamics in the Macro-tidal Hangzhou Bay, China

Abstract

Asymmetric tidal dynamics are of great significance for material transport and morphological evolution in estuaries. The tidal dynamics of the macro-tidal Hangzhou Bay (HZB) are characterized by flood-ebb asymmetries, spring-neap asymmetries, surface-bottom asymmetries, and up-downstream asymmetries. The mechanisms of spatio-temporal asymmetric tides and lateral flows in HZB were studied through a fully calibrated three-dimensional numerical model. The results show that tidal tides, tidal currents, and tidal asymmetry in HZB varied specially and temporally. In general, the bay was mostly flood-dominant. Temporally, tidal duration asymmetry in the bay fluctuated between spring and neap tides, with larger skewness during spring tides and smaller skewness during neap tides. The locally produced overtides are the primary sources of shallow-water tides in the bay, and the interaction between the lunar semi-diurnal tide M₂ and the solar semi-diurnal tide S₂ generates shallow-water overtides and deforms tidal asymmetries. The dissipated tidal energy may consumed by the bottom friction, with less passed to the generated shallow water overtides M₄ and M₆ tides (A_M4 = 12.07 cm, A_M6 = 3.91 cm) when comparing with the experiments that open boundary is purely forced by M₂ tide (A_M4 = 13.63 cm, A_M6 = 6.31 cm). The increased depth reduces the bottom friction and the convergence of volume, resulting in skewness values close to zero (γ_TDA = 0.220, γ_M2-M4 = 0.141, γ_M2-M4-M6 = −0.002, γ_M2-S2-MS4 = 0.105). The changes of tidal duration asymmetry caused by the increased channel convergence, reduced bay width, and reclaimed intertidal zone spatially vary in different parts. The bottom friction contributed to the generation of the shallow-water tides and asymmetries in the bay (the RVRs for M₄ and M₆ are −73.5% and −92.5%), while the Coriolis force (the RVRs for M₄ and M₆ are 4.8% and 8.9%) and nonlinear advection (the RVRs for M₄ and M₆ are −17.3% and − 21.8%) are minor factors. The findings of the study provide hydrodynamic foundations for the research of sediment transport and estuarine evolution in similar macro-tidal turbid estuaries worldwide.