Study On Suspended Sediment Sorting Transport In Turbidity Maximum Zone Of Yangtze River Estuary

Estuarine turbidity maximum（ETM）is the dynamic turbid water areas within limits in an estuary,and is an important part of estuarine systems,the sediment concentration of which is significantly higher than in the upstream river and the downstream sea.As an important basic substance of the estuary,the characteristics of sediment have an important influence on the evolution of riverbed and shoal in the ETM.What’s more,the differences in the distribution of sediment grain size in the estuary on spatial and temporal scales are also important characterization of size-selective transporting.Studying the characteristic and mechanisms of size-selective transporting can help deepen the understanding of sediment movement in estuaries,provide scientific and technological support for riverbed development and channel maintenance,and have important theoretical and practical significance for the production and construction of estuary area.This thesis focuses on suspended sediment sorting transport of wet season in the ETM of Yangtze River.Firstly,the spatial-temporal distributions of median grain size and compositions of suspended sediment in the ETM during wet season in 2020 were analyzed.Secondly,based on hydrodynamic and sediment concentration data,this thesis analyzed the spatial and temporal distribution characteristics of the transport of suspended sediment particles at different particle sizes,as well as the transport mechanism of each component.Finally,a machine learning approach was employed to establish a model for predicting the percentage content of suspended sediment in each particle size fraction.The primary findings are summarized as follows:（1）In terms of spatial changes in the net transport of suspended sediment particles at different particle sizes in the ETM,the main transporting component in the core area is silt,accounting for 63.2%of the net sediment transport.The main components in the transition area from ETM to submarine delta front（transition area）are clay and silt,accounting for 40.9%and 43.2%of the net sediment transport,respectively.In the core area,the net transport of the three main components of sediment particles in the North Passage（NP）is 27.9%,65.0%,and7.1%for clay,silt,and sand,respectively;in the North Channel（NC）,it is 29.2%,62.7%,and 8.1%;in the South Passage（SP）,it is 26.9%,66.2%,and 6.9%.The ratios of each components of net transport flux in the core area are similar,with higher transport of silt in the SP and higher transport of clay in the NP.In the transition area,the net transport ratio of the three components in the NC is 34.4%,50.5%,and 15.1%,respectively;in the NP,it is 37.6%,57.1%,and 5.3%;in the SP,it is 29.8%,69.9%,and 0.3%,while clay and silt towards the landward side and sand towards the sea.The advection transport term is the main term towards the sea,of which the T1 accounts for more than 50%of the absolute value of the single-wide sediment transport rate.In the core area,the coarser sediment particles transported towards land by tidal pumping has a larger proportion in the transport rate.In spring tide,the contribution of tidal pumping term in the sediment transport rate of silt and sand is larger than neap tide.（2）Shed light on the temporal variation of the net flux of three components sediment in the ETM.In the core area,the ratio of silt and sands transported in spring tide is higher than that in neap tide,and the clay is lower than that of neap tide,which is most significant in the SP.In the transition area,the ratio of silt is higher than that of neap tide,and the ratios of clay and sand are lower than that of neap tide.The results of comparing the spring tide with the neap tide in the core area show that:the transport proportion of silt and sand in the NC is 7.8%higher,and the proportions of clay and sand are 3.8%and 4.0%lower,respectively;the proportion of clay in the NP is 4.2%lower,and the proportions of silt and sands are 1.1%and 3.1%higher,respectively;the proportion of clay in the SP is 18.0%lower,and the proportions of silt and sands are 15.5%and 2.5%higher,respectively.The results of the transition area show that:the transport proportion of silt in the NC is 7.1%higher,and the proportions of clay and sand are 5.6%and 1.5%lower,respectively;the proportion of silt in the NP is 11.0%higher,and the proportions of clay and sand are 7.0%and 4.0%lower,respectively;the proportion of silt in the SP is 57.0%higher,and the proportions of clay and sand are 49.9%and 7.1%lower.The results of the net flux of suspended sediment transported by grain level at flood and ebb tides show that during the spring tide in the three main slots of the core area,the proportion of clay at ebb tide is smaller than that at flood tide,and the proportion of silt and sand is larger than that at flood tide;during the neap tide in the NC and NP of the transition area,a higher proportion of silt is transported out of the mouth,and during the spring tide in the SP,a higher proportion of silt is transported towards landward.（3）Clarify the vertical variation of the net transport of three components of sediment in the ETM.In the core area,both NC and NP show a decrease in the proportion of clay from the surface to the bottom,and an increase of sand.In SP,the proportions of clay and sand decrease,while silt increases,and it is transported towards the land in the bottom layer.In the transition area,the proportion of clay decreases and the proportion of sand increases in NC.The proportion of sand decreases and the proportion of clay increases in SP,and both the middle and bottom layers are transported towards the land.In the vertical distribution of each component of sediment in the core area,the following observations were made:In NC,the surface（0.0H）sediment transport flux is 19.1 t/m.The total sediment transport flux in the middle layers（0.2～0.8H）is 164.9 t/m.And the sediment transport flux in the bottom layer（1.0H）is 21.0 t/m.In NP,the sediment transport flux on surface,middle and bottom layer are 30.8 t/m,361.7 t/m,and 44.7t/m,respectively.In SP,the sediment transport flux on surface,middle and bottom layer are 5.9 t/m、60.2 t/m、10.1 t/m,respectively,with sediment transport towards land in bottom layer.In the transitional area,the vertical variation of three components in transport rate is revealed as follows:In NC,the sediment transport flux on surface,middle and bottom layer are 9.1 t/m,29.1 t/m,0.4 t/m,respectively.In NP,the sediment transport flux on surface,middle and bottom layer are 3.0 t/m,50.0 t/m,3.0 t/m,respectively.In SP,the sediment transport flux on surface,middle and bottom layer are 1.2 t/m、26.5 t/m、14.1 t/m,respectively,and each components in middle and bottom layer transporte towards land.（4）Machine learning algorithms were applied to construct an inversion estimation model for suspended sediment components in the ETM.This model demonstrated good predictive performance for the suspended sediment component withbetween 8-32μm.The optimal RF model was obtained by selecting the water and sediment property characteristics from 1 hour prior as additional input parameters.When all feature fields were selected as inputs,the overall prediction performance of the random forest（RF）model was better than that of the support vector machine（SVM）model.By comparing different field combinations,the best performing RF model used features other than the component content from 1 hour prior as inputs.The proportion of test samplesset with±2 error in the three particle size components were 31.4%,45.6%,and 32.6%,respectively,with corresponding values of 0.50,0.72,and 0.48,and the mean absolute errors（MAE）were 5.65,3.36,and 3.36,respectively.