Numerical Experiments On Effects Of River Flow And Sediment Feedings,Channel Forms And Other Factors Upon Estuarine Channel Avulsing Processes

Research on estuarine channel avulsions and shfting processes,which are commonly-seen morphological processes on the deltas and their coasts of the Yellow River and the Mississippi River and other large alluvial rivers,is important in terms of academic significance and practices in sloving the real-world problems.The main objective of this research is to reproduce numcally the estuarine channel avulsing processes by using a depth-integrated two-dimensional morndohydrodynamic and morphodynamic model.First,tests were done of the sand transport(ST)and mixed-particles-sizes sediment transport modules(MT),which claims to be able to do modelings of mud transport and a fraction of sand transport,using the field data about the narrow channels of the Lower part of the Lower Yellow River and its estuary.It is found that ST module is good enough to reproduce the significant flushing processes during the rising flood and accreting during residng flood,therefore taken as the modeling tools in the research.Second,ST was used to simulate the effects of different flow and sediment riverine feedings,channel widths,and tide levels at the outlet upon the morphological processes such as flusing and accreting processes,longitudinal profile changing processes and others.It is found that lower flow rates with higher sediment concentrations through a broader channel tends to cause more severe deposition or less erosion amountsl.Finally,ST module was used to simulate morpogolical changes in a typical scenario where a high-concebntrated flow through a channel linked to a deltaic land which is of a uniform longitudinal slope with zero-degree lateral slopes of 25 km width distributed on the both sides.It is found that ST is successful in numerically reproducing the typical morphological processes through the Yellow River estuarine course usually evoves,which are embody ed by gradual three-phased changes for channel avulsion,i.e.the first-phase changes where flow tends to get scattered,with too many small intertangledly and complicatedly linked channels formed on the delta after the start,and then disappear gradually in the downstream direction,ending up with a single-thread channel,which marks the start of the second phase,the second phase changes where a single-thread large channel,after the first phase,is formed on the delta where the singlethread channel develops gradually into one of a terraced longitudinal slope and a planeform pattern consisting of a meandering reach on the upstream linked to a downstream relative straight reach as usual,and final phase changes where the channel starts to avulse at the interjunction between the upstream curved and straight channels,i.e.a large change in the longitudinal slope.The modeling also reproduce the triggering processes of the channl avulsion,which is found that main current going sttaightly against the concaved bank,consequently causing the bank to collapse completel,leading to a channel avulsion at last.