| The dynamic conditions of the Yangtze Estuary are very complex with significant differences between the flood and the dry season,which makes it difficult for people to understand the characteristics of flow and sediment transport and the dynamic mechanism of sediment deposition of the Yangtze Estuary especially with the intervention of human activities such as engineering construction.Based on the two-dimensional flow and sediment mathematical model developed by the river sediment Laboratory of Tianjin University,this paper was divided into three parts.First,five kinds of estuarine silting mechanism were simulated and analyzed based on the theory of the idealized estuary;Second,the two-dimensional hydrodynamic mathematical model was established to study the hydrodynamic characteristics of the Yangtze Estuary;Finally,both the temporal and spatial distribution and dynamic characteristics of sediment of the Yangtze Estuary were analyzed based on remote sensing images.The study contents and conclusions are as follows:1.Five hydrodynamic mechanisms of estuarine sedimentation were summarized and proposed: 1)River section broaden silting mechanism,which referred to the sediment siltation due to the decrease of the velocity caused by the widening of river section;2)Tide and runoff jacking silting mechanism,which referred to the sediment siltation caused by stagnating flow area when the upward tidal jacked to the downward runoff;3)River regime unsmooth silting mechanism,which referred the sediment siltation due to the blocking of the flow when the angle between the hydrodynamic axis and the mainstream line or the angle of the between ebb tidal mainstream path and the flood tidal mainstream path was too large;4)Local circulation silting mechanism,which referred to the sediment siltation consist of the falling suspended sediment caused by the action of local circulation in the dam field;5)Density current plunging silting mechanism,which referred to the sediment siltation caused by the density current which was made up the hyper-concentrated flow near the bottom while slipping into channel.2.Based on the general morphology of estuarine and the characteristics of flow and sediment,the two-dimensional flow and sediment mathematical model of the idealized estuary was established.Simulations of the hydrodynamics processes,sediment movement and scouring and sedimentation of the idealized estuarine were run corresponding to the five estuary silting mechanisms above,and the results could explain the causes of five estuarine silting mechanisms well.3.The two-dimensional depth integrating mathematical model in the Yangtze Estuary was established and was verified by the measured tidal level and velocity data during the flood season in 2012,the results showed that the model could reflect the law in the hydrodynamic changes of the Yangtze Estuary well.4.Based on the five kinds of silting mechanism and the numerical simulation results of the Yangtze Estuary,the probable causes of siltation in different regions of the Yangtze Estuary were obtained by analyzing the causes of siltation in different regions especially the Deepwater Channel of the North Channel.5.There were significant differences in the hydrodynamic characteristics of the Yangtze Estuary between the flood and the dry season,which changed with spring-neap tide,flood tide and ebb tide.The upper section of the Yangtze Estuary was mainly affected by the runoff while the lower section was mainly affected by the tide,and the influence of flood played an important role in the cause of riverbed revulsion of the Yangtze Estuary.6.The temporal and spatial distribution and the dynamic characteristics of sediment in the Yangtze Estuary were studied to obtain the quantitative information of suspended sediment concentration by using remote sensing data.The analysis showed that the suspended sediment concentration in the Yangtze Estuary increased from nearshore to offshore while greatly affected by the tidal condition and the runoff. |
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