Suspended Sediment Transport And Its Dynamic Mechanism In The Hangzhou Bay

The estuaries and bays are the confluence zones of sea and land,where suspended sediments are transported and deposited by tide,river discharge,wind,waves,topography,and other dynamic factors,which have an important impact on coastline evolution and seabed erosion-deposition.Suspended sediment is also the carrier of nutrients,organic matter,and pollutants,which can weaken the transmission of light,reduce photosynthesis,and thereby reduce primary productivity,affecting the marine ecological environment processes.The study of the dynamic process and mechanism of suspended sediment transport in the estuary bay has great scientific importance and application value for the geomorphological evolution,ecological and environmental processes,and the sustainable use of tidal flat resources in the bay.Based on the improved ECOM-si model,this paper studied the suspended sediment transport process and its dynamic mechanism in the Hangzhou Bay and has obtained the following main conclusions and innovative achievements.1.The quantitative analysis of suspended sediment transport processes during spring-neap tides in the Hangzhou Bay in summer and winter under climatic state was presented,and the differences of suspended sediment transport processes during spring tide and neap tide were expounded.The results show that suspended sediment transport pattern"north-in and south-out"is formed in the entire sea area of Hangzhou Bay during spring tide in summer;under this pattern,suspended sediment is transported westward along the north coast and eastward along the south coast,with the residual unit width sediment flux（RUSF）values of about 1.30 kg·s^-1 and 1.12 kg·s^-1,respectively.During neap tide in summer,only the west of Hangzhou Bay forms a pattern of"north-in and south-out",and the values of RUSF transported westward and eastward are about 0.03kg·s^-1 and 0.16 kg·s^-1,respectively.Most of the suspended sediment in the bay mouth（the eastern part of Hangzhou Bay）is transported from south to north into the Changjiang Estuary with a RUSF value of approximately 0.30 kg·s^-1.The suspended sediment transport process in the Hangzhou Bay during spring tide in winter is similar to that in summer,while there are some differences during neap tide.During neap tide in winter,the suspended sediment transport in the southern part of the bay mouth is divided into two branches,one of which crosses the south side of Nanhui Cape into the Changjiang Estuary,and the other one is transported towards the northern coast of Hangzhou Bay,with RUSF values of about 0.19 kg·s^-1 and 0.17 kg·s^-1,respectively.Numerical sensitivity experiments show that suspended sediment transport in the Hangzhou Bay is mainly controlled by tidal currents,and the sediment-induced stratification significantly inhibits vertical mixing in the water column and sediment resuspension.The mechanism decomposition of the residual unit width sediment flux indicates that the Lagrangian transport term and the tidal pump transport term mainly control the"north-in and south-out"pattern in the Hangzhou Bay during spring tide,while the Lagrangian transport term and the vertical shear transport term are mainly dominated during neap tide,and the tidal pump transport term is smaller.2.It is found that suspended sediment is transported from the Changjiang Estuary to the Hangzhou Bay during spring tide,and suspended sediment is transported from the Hangzhou Bay to the Changjiang Estuary during neap tide,and the fluxes of suspended sediment exchange between the Hangzhou Bay and the Changjiang Estuary are given quantitatively,revealing the underlying dynamic mechanism.Numerical experiments results show that the net transect sediment flux（NTSF）across the cross-section from Nanhui Cape to Qiqu Archipelago in summer and winter is-1.55×10⁴kg·s^-1 and-1.62×10⁴ kg·s^-1 during spring tide,respectively（positive value:flow into the Changjiang Estuary;negative value:flow into the Hangzhou Bay）,and is 0.91×10⁴kg·s^-1 and 0.34×10⁴ kg·s^-1 during neap tide,respectively,indicating that suspended sediment is transported from the Changjiang Estuary to the Hangzhou Bay during spring tide in summer and winter,and the opposite is true during neap tide.The dominant mechanism of the difference in the suspended sediment exchange process between the Hangzhou Bay and the Changjiang Estuary during spring tide and neap tide was expounded by momentum analysis and numerical sensitivity experiments.The results show that the barotropic pressure gradient force caused by the mean sea surface slope during spring tide is the main force driving the suspended sediment from the Changjiang Estuary into the Hangzhou Bay,while the baroclinic pressure gradient force caused by the salinity gradient during neap tide is the main force driving the suspended sediment from the Hangzhou Bay into the Changjiang Estuary.This paper further calculates the NTSF across the cross-section from Nanhui Cape to Qiqu Archipelago in the summer representative month（August）and winter representative month（February）,which are-0.17×10⁴ kg·s^-1 and-0.80×10⁴ kg·s^-1,respectively,indicating that there is net sediment transport from the Changjiang Estuary into the Hangzhou Bay in both summer and winter,and it is stronger in winter than in summer.3.Taking typhoon Rumbia in 2018 as an example,the difference in the effects of waves on the vertical mixing in the Hangzhou Bay before and during the typhoon was discussed.During Typhoon Rumbia,the NTSF across the cross-section from Nanhui Cape to Qiqu Archipelago increased by 281.08%compared with that before the typhoon,indicating that more suspended sediment was transported to the Hangzhou Bay during the typhoon,significantly affecting the suspended sediment exchange process between the Hangzhou Bay and the Changjiang Estuary.Before the impact of typhoon Rumbia,waves promoted the vertical mixing in the water column,while wave-induced bottom shear stress during Typhoon Rumbia greatly promoted sediment resuspension,indirectly enhancing sediment-induced stratification（mainly in the bottom layer）and inhibiting the vertical mixing in the water column.The stratification effect induced by sediment resuspension in high turbidity estuaries during typhoons has a dampening effect on the vertical mixing.4.In recent decades,the coastline of Hangzhou Bay has been continuously changed.The Andong tidal flat area on the south coast of Hangzhou Bay was selected as a typical area to analyze the transport process of local suspended sediment and sediment deposition intensity in different ages.Comparative numerical experiments were selected for 1959,1982,and 2018,respectively,and the results showed that the suspended sediment transport to the upstream along the south coast of Hangzhou Bay decreased,and the transport to the downstream along the south coast increased.This paper further quantitatively calculated and analyzed the sediment deposition and erosion intensity in the Andong tidal flat.The results demonstrated that the sediment deposition intensity in the area above the-5 m isobath in the Andong tidal flat weakened after the full implementation of the reclamation project on the south coast of Hangzhou Bay without considering artificial siltation promotion projects,which was mainly related to the decrease in the convergence intensity of horizontal suspended sediment transport and the enhancement of vertical upward transport intensity of suspended sediment.