The Multi-scale Variations Of Suspended Sediment Dynamics In Hangzhou Bay And Its Interaction With Tidal Flat Variations

Estuaries connect lands and oceans and are highly sensitive to terrestrial and oceanic conditions.Suspended sediment concentration(SSC)and sediment transport play a key role in ecological environment,geomorphological evolution and tidal flat variations,and vice versa.In this study,we used remote sensing data and numericalmodeling to examine the the characteristics and the multiple temporal-spatial scales variations of the sediment dynamics in Hangzhou Bay(HZB),China.The results of this study can help harbor management and estuarine protection.SSC retrieved from the satellite data was used to study the multidecadal(1984-2015)dynamics of the surface turbidity maximum zone(TMZ)in HZB.The averaged deviation of the retrieved SSC from the field data was 15.6%.A sediment numerical model(ESed)was established and coupled with the Finite Volume Coastal Ocean Model(FVCOM).The sediment model considered two-way coupling of SSC and water density,sediment-induced stratification on bottom drag coefficient,and fine sediment flocculation are incorporated in the ESed numerical model.The modeled tidal level,currents and SSC agreed well with observation and remote sensing data in Hangzhou Bay.The above methods are used to study the multi-decadel variation of the surface TMZ,the characteristics of suspended sediment dynamics and the interaction of tidal flats variation and sediment dynamics in HZB.The conclusion of the thesis are as follows.1.Multi-decadel variation of the surface TMZ and SSC in HZBThe Landsat and GOCI images of HZB,covering the period from 1984 to 2015,were collected.The surface SSC was retrieved from the remote images by inversed model.The impacts of tidal amplitude and phase on remote sensing data were specifically considered and accurately removed.Five periods were organized to analyze the multidecadal variability of surface SSC:(a)1984-1988,(b)1990-1994,(c)1997-2001,(d)2002-2006,and(e)2013-2015.The area of TMZ(>700 mg/L)decreased significantly from 2205 km2 in period a to 647 km2 in period e.The annual trend ratio of TMZ area is-4.57%.The SSC values have a more asymmetric distribution pattern from 1984 to 2015,with low turbid area increased,as shown by the coefficient of skewness.Tidal flat reclamation around HZB contributed to the multidecadal reduction of the surface SSC from 1984 to 2015,through changing the hydrodynamics and reducing sediment source.The decreased sediment load from the Changjiang River also contributed to the decline in the surface SSC in HZB,by reducing the sediment source.Flooding of the Qiantang River during wet seasons played an important role in increasing the surface SSC at the bay head,as the deposited sediment during wet seasons reduced the water depth and increased the resuspension.2.Characteristics of tidal flats and suspended sediment dynamics in HZBThe suspended sediment spatial and temporal distribution patterns,longitudinal/lateral sediment transport mechanism,and the dynamics of sediment transport in south bank tidal flat were investigated via the ESed model.Results showed that the distribution of SSC in HZB had large spatial and temporal variations.The SSC are smaller near the north bank than near the south bank due to difference of depth.And the SSC between surface and bottom differentiates greatly(10 times)due to stratification.The maximum SSC during the flooding is about 1.2 times of the ebbing,and the SSC between spring and neap tide differentiates greatly(2 times),as a result of current change.Sediment moves into the bay along the north bank and out of the bay along the south bank.Tidal pump is the main mechanism of sediment transport from outside the bay to HZB.Lateral sediment fluxes generally are southward at the bottom level and northward at the surface level in HZB induced by the lateral circulation.Sediment tends to be deposited at the south bank and eroded at the north bank.Lateral momentum analysis showed that nonlinear-advection and friction contributes 60?70%to lateral circulations at three cross-sections.Coriolis contributes 11%to 26%from the estuary head to mouth.Eddies in anticlockwise direction are formed near the submarine fringe of the Andong tidal flat,due to the effects of the bathymetry and the flood-ebb tidal cycles.The erosion and deposition magnitudes were amplified,when ignoring the Coriolis force.The dominant factor deciding the dynamics of suspended sediment transport in Andong tidal flat is water depth and topography.3.Interaction of tidal flats variation and sediment dynamics in HZBThe area of the tidal flat in Hangzhou Bay(HZB)would be reduced by 1290 km2 from 1974 to 2020,which covered 25.8%of the HZB.And the coastline has large shrinks.Tidal flat reduction in the bay decreases/increases SSC values in the upstream/downstream of Zhapu through changing tidal currents and bed shear stress.The tidal flat reduction from 1974 to 2003 amplified the net sediment flux at Ganpu cross-section,while dampened that at Jinshan and Luchaogang cross-section,by change the tidal pumping effect.The deposition thus decreased subsequently in the inner bay.The net sediment flux after the year of 2003 has a reversed change.Sediment fluxes increased southward at cross-section due to enhances of lateral circulation from 1974 to 2020.Land reclamation enhances the tidal flat deposition.Local resuspension of the reduced tidal flat contributes a small part to the variations of the SSC and net sediment flux in the bay.Location of the reclamation affects SSC and net sediment fluxes.Tidal flat reduction at the head of the bay has larger impacts on tidal level,currents and suspended sediment dynamics than those reclamation at the side banks,and the magnitudes of the impacts gradually decrease from the head to the mouth of the bay.The influence of reclamation processes in different locations on the hydrological and sediment feature has a superimposed effect in HZB.