Recent Exchange Between Suspended Sediment And Bedload And Sediment Transport In Typical Branches Of The Yangtze Estuary

Sediment transport in large estuaries is an important topic in the study of dynamic sedimentation,in which the exchange of suspended sediment and bed load process is the core scientific problem.Therefore,the research of the estuarine sediment transport and suspended sediment and bed load exchange process can greatly enrich the understanding of estuary sediment movement process,meanwhile it provides necessary theoretical basis for estuarine tidal flats resource utilization,waterway regulation and maintenance and port site selection and construction.It is well known that numerous reservoirs and pumping water diversion projects were built in the upstream of the Yangtze River recently,especially water storage and silt detention caused by the three gorges reservoir,sediment discharged into Yangtze Estuary decreases dramatically.In the new river water and sediment conditions,however,the responses of sediment transport process in different regions of the Yangtze Estuary were significantly different in processes,methods and results.Based on the relative strength between river and ocean hydrodynamic interaction,as well as river channel landform type,Therefore,three typical areas are chosen as the main study area of this paper,which are the upper and middle reach of North Channel,the middle reach of North Passage and adjacent sea areas outside the river mouth,to represent typical hydrodynamic and landscape of the Yangtze Estuarine.Based numerous in site data and topographic data,using conventional statistics method combining with modified particle size spectrum calculation,the response of sediment transport and exchange between suspended sediment and bed load in typical channels and adjacent sea areas to the reduction of sediment discharge and estuarine projects has been studied.The results reveal that the response mechanism in different estuarine area to human activities.In order to get quantitative understanding of the mechanism of tidal exchange between suspended sediment and bed load in different typical channels and adjacent sea areas of the Yangtze Estuary under the influence of human interfere.The responses of the exchange of suspended sediment and bed load process in different typical channels and adjacent sea areas of the Yangtze Estuary have mechanism commonalities and quantitative differences.Recently large upstream reservoir engineering intercept large portion of sediment,the sediment discharge into the Yangtze Estuary reduced by 70%causing the sediment concentration in the flow is unsaturated.The sediment concentration in the upper and middle reach of North Channel reduced from 1.08 kg/m3 before 2003 to recent 0.53 kg/m3.The sediment carrying capacity increases remarkably and sediment resuspension is significantly enhanced.The portion of re-suspended sediment in the flow is around 30%.Sediment size distribution in the 0.2H can be influenced by coarse sediment resuspension.The portion of re-suspended sediment in the flow during flood tide in the Turbidity Maximum reaches 98.63%and 51.51%.The sediment concentration in the adjacent sea area outside the river mouth reduced from 0.89 kg/m3 before 2003 to recent 0.24 kg/m3.Sediment resuspension from the sea floor caused slight terrain erosion and formed high sediment concentration layer near the bottom.The topographic slope increased from 4.87×10-4 before 2003 to recent 5.42×10-4.In view of the new balance of sediment and water conditions,Yangtze Estuary has entered a new era that suspended sediment and bed load exchanges more frequently.However,due to the differences of dynamic conditions,sediment composition and the influences of estuarine engineering,the responses of the exchange between suspended sediment and bed load in different typical channels and adjacent sea areas of the Yangtze Estuary are different.First of all,because of the difference of dynamic conditions in different typical channels and sea areas in Yangtze Estuary,the response of re-suspended sediment transport to the reduction of sediment discharge is different.Ebb tide flow is stronger than flood tide flow in the upper and middle reach of North Channel.The sea floor is eroded by the increased sediment carrying capacity due to the reduction of sediment discharge and the influence of tidal wave deformation.The eroded sediment is transported seaward.The middle reach of North Passage is where the river and ocean force interaction take place.It is also the transition interface of flood-ebb dominate current.The relative strength between river and ocean force varied during the tide cycle causing the asymmetric transport of re-suspended sediment which leads to seaward sediment transport of 1.37×105 kg/m during neap and moderate tide and landward transport of 1.05×105 kg/m.It indicates that the re-suspended sediment transport during a tide cycle is in a relative steady state which is the material base of high sediment concentration in the middle reach of North Passage.The lower part of estuary is the transition area from ebb-dominated current to flood-dominated current.Particularly during the dry season,near-bottom flood tide current is stronger than ebb tide current.The reduction of sediment discharge causes the erosion of sea floor and generates large amount of re-suspended sediment which is transported to the Turbidity Maximum becoming an extra sediment sourceSecond,due to the difference of sediment composition in different typical channels and adjacent sea areas of the Yangtze Estuary,the responses of the exchange between suspended sediment and bed load in different typical channels and sea areas in Yangtze Estuary are different.The sea-bed sediment is coarse in the upper and middle reach of North Channel which requires high level of flow strength.Therefore,sediment resuspension during ebb tide is intense when the flow velocity is stronger while during flood tide part of suspended sediment settling on the sea-bed.Intense sediment resuspension causes coarsening processes of suspended and sea-bed sediment.The D50 is over 100 ?m.Coarse sea-bed sediment which is usually in bedload transport motion is re-suspended becoming suspended sediment.The shoal area is eroded.Silt and mud is the main components of sediment in the middle reach of North Passage with an average median diameter of 16 ?m.Fine-grain size suspended sediment favors the flocculation process caused by mixture of fresh and salt water during slack water.Long-axis diameter of floc is over 300 ?m forming a high sediment concentration layer near the bottom.The maximum near bottom sediment concentration reaches 2.61 kg/m3.However,these fine flocculated sediments are fragile to strong flow and easy to be re-suspended during ebb and flood tide which increases the vertical sediment concentration.The median particle diameter of sediment in the outside river mouth sea area is generally less than 20 ?m which is easy to be re-suspended.But due to the recent deduction of sediment discharge,the sediment concentration in this area is relatively low enhancing the sediment carrying capacity of flow and improving resuspension of sea-bed sediment.Intense resuspension of sea-bed sediment causes the erosion of sea floor and increase the near-bottom sediment concentration.Third,because of the difference of influences of estuarine engineering in different typical channels and adjacent sea areas of the Yangtze Estuary,the responses of sediment resuspension process to the reduction of sediment discharge are different.There are several major regulating projects in North Channel including corral of Zhongyang shoal,shoal protection of New Liuhe shoal,South Shatou passage submerged jetty,Qingcaosha reservoir and Shanghai Changjiang Bridge.The water and sediment transport and exchange between suspended sediment and bed load is significantly influenced by reduction of sediment discharge and water-related project.The near-shore area of spur dikes is eroded indicating recent intense water and sediment exchange in this area.In the meantime,the diffluence pass,favors the stability of the split ratio of sediment and water in North and South Channel.Channels are adapting to the influence of human activities and making self-adjustment to a stable river regime.Recently,due to the influence of diffluence pass,deep-water channel and the split ratio of sediment and water decreases.Silting phenomena has occurred in the entrance region,middle reach and spur dike field of North Passage.Because of the massive sediment supply from sea-bed sediment resuspension,sediment concentration remains relatively high and its moderate and spring tide average values are over 1.00 kg/m3.This indicates that due to the influence of deep-water channel project,the channel-fill volume of North passage changes and the topographic erosion and siltation is uneven in this area.The exchange of between suspended sediment and bed load is more frequent which spur the North Passage to maintaining the foundation characteristic of sediment transport of Turbidity Maximum.Influenced by diffluence pass of South Channel,diffluence pass regulation project of North and South Passage and deep-water channel project,the split ratio of sediment and water decreases in North Passage which increases in the South Passage causing changes of sediment distribution in the adjacent sea areas and spatial differences of exchange between suspended sediment and bed load.The high concentration area is moving southward.Impacted by the reduction of sediment discharge directly,the topographic slope increase rate in the northern part increases from 4.8×10-6/a in last century to recent 12.72×10-6/a when the Three Gorges Reservoir starts to work.On the other hand,the topographic slope of southern part fluctuates annually due to the influences of reduction of sediment discharge from basin and estuarine engineering.