Sediment dynamics in Hangzhou Bay during Typhoon Mitag

The hydrodynamics and sediment characteristics of muddy estuaries and coasts during typhoons are closely related to geomorphic evolution, ecological environment, and economic development of coastal zones. Taking the macro-tidal turbid Hangzhou Bay (HZB) as an example, the sediment characteristics and the effects of wave-current interactions on sediment dynamics during Typhoon Mitag were studied using a well-calibrated numerical model. The model considers tides-wave-sediment interactions and the reconstructed typhoon wind field. The net sediment fluxes were controlled by residual currents and SSC. The waves and winds dominated the sediment resuspension. The combined interactions of currents and waves led to high SSC during the typhoon. In calm conditions, the impact of wave-current interactions was small except for the combined bottom stresses. The combined bottom stress was the primary wave-current interaction changing the sediment resuspension and increasing SSC, especially in shallow waters or during storms. The advection term, which played an essential role in reducing the SSC in HZB, mainly affected the SSC by enlarging the velocity. The wave dissipation term enhanced the vertical mixing, which involved the vertical exchange of suspended sediment and the currents. In the shallow waters of the southern bay, the wave dissipation term mostly led to the decrease of bottom stresses, the increase of currents, and the decrease of SSC, while the rise of SSC in the deep waters. The effects of the form drag, wave radiation stress, and refraction terms on suspended sediment dynamics were relatively small. The findings provide the theoretical foundations for the study of dynamics geomorphology in macro-tidal estuaries.