Sediment fluxes and mechanisms in a macro-tidal turbid estuary during typhoon events

Abstract

In turbid macro-tidal estuaries (MTE), typhoons interact with tides and high turbidity, significantly impacting the natural and social environments of the coastal zones. A tide-wave-sediment numerical model was built and validated for Hangzhou Bay (HZB), using reconstructed near-shore typhoon wind fields. The model incorporates wave-current coupling and water-sediment density coupling and considers fine sediment flocculation and fluid mud bottom boundary layer. Results show that the typhoon remarkably enhances wave-induced bottom stress but slightly increases current-induced bottom stress. Increased wave-induced bottom stress led to intensified sediment resuspension, subsequently increasing suspended sediment concentration (SSC) and SSF during the typhoon. Typhoon wind fields primarily impact sediment transport, with large wave impacts on both the south and north banks, while the effect of air pressure was negligible. The tide-induced net SSF was eastward. Residual currents magnitudes and SSC were positively correlated with typhoon intensity. Tide-surge nonlinear effects decrease the SSC. By decomposing the net SSF, the Euler transport term (T₁) and the tidal trapping effect (T₄) were the main terms among 7 components. The winds promoted sediment transport mainly through T₁ and T₄. For wave-induced sediment transport, T₄ was the main term. The tide-surge nonlinear interaction suppressed surge-induced sediment transport through T₁ and tide-induced sediment transport through T₄. The findings reveal the mechanism of sediment transport during extreme weather conditions in macro-tidal turbid estuaries.