The Interaction Machanism Of Tide And Runoff And Its Influence On Suspended Sediment Transportation In Yangtze Estuary

Estuaries are often semi-closed coastal waters and are the collection places of water and sediment from river into sea.In estuaries,the interaction of strong and weak alternation of runoff forces and sea forces causes unique characteristics.Most estuaries in the world are also important ports and waterways.Their evolution characteristics and trends have always been attended by majority of scholars and engineers.Combing the variation trends and reasons of the interaction of runoff and tide,the key issues of researching fluvial process and sediment movement in estuaries include analyzing the influences and effects of the interaction of runoff and tide on suspended sediment characteristics and landform shaping.Yangtze is the third longest river in the world,and the first longest river in China.Yangtze estuary is an important area of Yangtze water and sediment into East China Sea.Broadly,the range of Yangtze estuary is from Datong County in Anhui Province to Luhuashan tide station out of Yangtze estuary.Yangtze estuary is gradually relaxed flared from west to east.The research of the interaction of runoff and tide occupies a fundamental position on water,sediment,salt,landform,and so on.The impoundment of the Three Gorges Reservoir causes the variation of the process of runoff into sea,flood discharge,and flow flattening.The action of retaining sediment causes riverbed cut.The interaction of runoff and tide will cause new adjustments and changes.It also causes new influence on sediment movement,the variation of salt,and landform shaping in estuaries.Existing researches have concerned the variation of water,sediment and salt by the change of runoff and tidal forces.At the same time,they have also sensed that they have the balance interface of runoff and tide under the impact mechanism.However,the factors and mechanisms of the interaction of runoff and tide have not systematically analyzed.The defining methods of the critical interface and threshold about transforming runoff controlling river to tide controlling river by the interaction of runoff and tide are divergent and inconclusive.On this basis,it can further research the coupling relation between the interaction of runoff and tide and sediment movement,landform shaping.Thus,combing hydrological and geomorphological data nearly 60 years from Datong to downstream river,it reveals the variation process and cause of the interaction of runoff and tide in Yangtze estuary.Combing the laws of sediment movement and the change process of landform shape,it analyses the interactional mechanism of runoff and tide.It can be as a reference for the response mechanism of sediment movement and the variation laws of estuary fluvial processes under the change water and sediment conditions.It also can provide some technical support for estuary channel regulation and water intake projects.It mainly discussed the following questions:(1)The impact factors and the mechanism of the interaction of runoff and tide.Based on the measured data of mean tidal level,average monthly tidal range,runoff discharge and MIKE 21 model,the impact factors of interaction of runoff and tide and its mechanism is studied.The runoff discharge,tide from the sea and the morphological evolution are the main fators of the interation of runoff and tide.The formula between tidal range and tidal range of the sea,the runoff discharge is established with good validation.Based on the decrease trend of tidal range as the criteria when the runoff discharge of Datong increased,the eastuary is divided into runoff controlled reach and tide controlled reach.In the upstream reached of Xuliujing,when the terrain eroding,the hydrodynamic of tide increases and the monthly average tidal range increases.The average mean tidal level decreases in runoff controlled reach and increases in the tide controlled reach.In the downstream reaches of Xuliujing,the shrink of river width of North Branch increased the tidal range and the mean tidal level.The erosion of South Branch increased the tidal power in the estuary,but the average tidal level and tidal range decreased in South Branch.The erosion of mouth bar increased the tidal power in the estuary and the tidal range increased,the tidal level decreased.It establishes the determine index of the critical interface from runoff controlling river to tidal controlling river.The judgment standard is that tidal range has or not a decreasing trend with the increasing runoff discharge.It divides the runoff controlling river and tidal controlling river.It establishes the index of tide to runoff ratio,and using M-K method distinguishes the critical interface from runoff controlling river to tidal controlling river.The parameter exists difference in flood and dry seasons.In flood seasons,when the tide to runoff ratio is less than 1.31,the critical interface moves down to Jiangyin.When the tide to runoff ratio is less than 1.20,the critical interface moves down to Tianshenggang.When the tide to runoff ratio is less than 0.89,the critical interface moves down to Xuliujing.In dry seasons,when the tide to runoff ratio is less than 2.23,the critical interface moves down to Jiangyin.When the tide to runoff ratio is less than 2.15,the critical interface moves down to Tianshenggang.The critical is not down to Xuliujing.(2)Suspended sediment transportation laws and relationship between them and seawards sediment and interaction of runoff and tide.Based on the variation laws of suspended sediment particles each station along middle and lower Yangtze,it analyses the variation of sediment characteristics.Before the Three Gorges Reservoir impounding,the sediment(d<125?m)into sea from Yichang exceeds 70%,while it from bed,lake and tributary inflow is less than 30%.After the Three Gorges Reservoir impounding,the sediment into sea from Yichang is decreasing,and it from bed is increasing.Until 2009,the sediment(d<31 ?m)into sea from bed is about 40%,and it from lake is about 30%.the sediment(d>31?m)into sea from bed exceeds 70%,and it from Yichang is less than 30%.Combining the variation laws of sediment into sea and the interactional mechanism of runoff and tide,it analyses sediment transport laws in estuary area.As the sediment into sea sharply reducing,the sediment into sea larger than 63 ?m almost deposits between Datong and Jiangyin.The coarse sediment near Xuliujing is mainly from offshore by rising flow carrying.The characteristics of sediment particles nearby Jiangyin to Xuliujing show obvious differences between flood and dry seasons.They are influenced by sediment into sea in flood seasons and by tide in dry seasons.As the southward trend of ebb flow,branch channels exist sediment sorting.The mainly features include coarse sediment southward,and fine sediment northward.The researches show that the variation of sediment particles property is also an important factor causing erosion and deposition laws adjusted.Suspended sediment thinning in North Passage is a reason of the increasing amount of back silting.(3)Variation of landform characteristics and relationship between them and interaction of runoff and tide.From the whole variation trends since 1950,the Yangtze area experienced the process from deposition to scour in Yangtze estuary area.With the sediment into sea further reducing,the main deposition zones only include nearby entrance bar and North Branch after 2009.This change causes tidal power weakening,runoff force enhancing,and critical converted interface tracing on the interaction of runoff and tide.It selects landform sharping process of South Branch as the main object.The results show the evolution characteristics in South Branch are from channel scouring and beaches depositing to channel depositing and beaches scouring.The researches reveal the main reasons influencing erosion and deposition laws in South Branch,establish the relationship of shoal area and sediment into sea,and calculate the critical amount of sediment in the process of erosion and deposition alternately changing in South Branch.Additionally,combining the interaction of runoff and tide,it establishes the relationship of tide to runoff ratio and the laws of beaches upstream and downstream.