Abstract:

Since 1998，the construction of Vshaped dike on the bifurcation of North and South Passage has led to sustained and serious sediment accumulation in the entrance area of North Passage.It subsequently causes rapid growing of the head of Jiuduansha shoal towards to northwest.Long time observations indicate that the siltation only happened on the north side of submerge breakwater,while the south side riverbed suffered severe erosion.To gain fundamental understanding and basic mechanisms of newly found morphological evolution phases,this paper analyzed the data of both different years and locations measured at bifurcation area of North and South Passage in the Yangtze estuary,especially those in the mouth of North Passage.With mathematic statistic method,the mechanism of suspended sediment transport and siltation were discussed.A dimensionless parameter was introduced to scale the relative importance of different mechanism constituents.Equations describing sediment transport mechanisms in entrance area of North Passage were presented in this paper.Sediment transport was divided into four individual contributions induced by (Lagrangian) residual flow,tidal pumping,circulation and asymmetry diffusion respectively.Lastly,the cause of continuously silting at entrance area of North Passage was investigated.The results are as follows.(1) All contributions were divided into three different orders,and these contributions varied on distinct sections.Suspended sediment transport was dominated by Lagrangian residual flow both at near entrance section and inner entrance section.At the inner part,tidal pumping and circulating were the second order sediment transport mechanics with diffusing effect followed.Lagrangian advection and tidal pumping effect controlled the middle entrance section simultaneously.(2) Sediment silting at entrance area of North Passage originated from upper reaches of the estuary.Sediment was partly intercepted by submerged breakwater and accumulated at the area of slack water,when it transported down into the channel.At the same time,sediment transported to the South Passage together with over dike flow was eroded away by strong overdike flow itself.(3) At uppermouth area,the main transport contribution,Lagrangian transport,pushed sediment seaward into North Passage,whereas,at the middle entrance section,sediment was driven by two dominated contributions simultaneously on contrary direction.Lagrangian residual flow tried to transport sediment south into the channel of North Passage,but was compensated by tidal pumping.The angle between these two components was about 165 degrees.Due to this combined interaction,net sediment finally transported from along channel to across channel.Moreover,the net transport was weaker at this area than that the downstream,and large amount of sediment silted in the entrance section of North Passage when it crossed the channel