Recent Flow And Sediment Transport In Bifurcation Area Of North And South Passage Of The Yangtze Estuary And Its Response To Regulation Works

Since1998years, with the construction of Deep-water Channel Regulation Project, flow and sediment transport in North Passage and morphological evolution of shoals and river bed under new circumstance has aroused increasing attentions, especially the problems that siltation in the channel turns out to be much larger than expected and concentrate in some particular area recently. According to the accumulation and erosion distribution in North Passage, sediment shows a tendency of silting in two locations:the middle section of the channel and the upper entrance. Compared to the closely watched to the former part, study on sediment transport and trapping on upper entrance, the impact of bifurcation project to local river bed morphological evolution are rare. Hereby, this research focused on the South and North Passage bifurcation, gained detailed morphological process of the bed and head of Jiuduansha shoal and deep insight of mechanisms of water and suspended sediment transport. Furthermore, the relation between water/sediment transport, river bed changes and engineering structure of V-shaped dike was explored with the employ of numerical model MIKE-21FM.Research indicated the head of Jiuduansha shoal stays stable under the diversion works after1998, its seaward movement trend has been eased and silted landward again since2000. Till2007, it grown2.1km up to South Channel with the direction of312～314°, which is7～9°north to the run of its downward movement period. Secondly, it showed a pattern of "Silted south-Eroded north" which most of the North Passage area was under the siltation during the last10years with the South Passage entrance, however, showed constant erosion simultaneously. Moreover, flow and sediment ratio between North and South Passage has twisted since the2000with constantly less water and sediment enter the North Passage. Our study revealed it is related with, except for the spur-dike-induced decline of the volume of North Passage, the run of V-shaped dike and decrease of flow section area of channel entrance due to the siltation.As the result of shape of down reaches of South Channel, there is about60°angle between (Eulerian) residual flow of South Channel and inner North Passage. With the construction of bifurcation project in this area of landforrm jump, the seaward flow in entrance area of the North Passage is affected significantly. Sediment of this zone mainly originate from the upper reaches of the estuary, those being transported into North Passage has been partly intercepted by the dike and silted in the front of Jiuduansha shoal, where also the major source of sediment transported at the local area. It also showed varies of sediment transport mechanisms in different sections of the bifurcation, in which the near entrance part and inner entrance part is dominated by (Lagrangian) residual flow, whereas the middle part in between is controlled both by the former one and tidal pumping effect. Moreover, the results also showed the existence of cross channel sediment transport at the middle part, but with the net transport ability weaker than there other two areas which means sediment from upper reaches silt in this area during its cross channel transport and leads to the decline of flow section area of the North Passage in the end.Flow run off over the dike from north to south was observed during the ebb and the other way around in flood. Numerical simulating confirmed the "S" structure of water and sediment transport in "South Channel-North Passage" section of the bifurcation, which is not related to the regulation works but the river bed topography. However, it strengthens the erosion and deeper the channel at the South Passage entrance, push more ebb run off the South Passage. Comparing with which this works didn't show any clear increase of net sediment transport intensity at entrance of the North Passage. Consider the compared weak sediment transport capacity at the north side, of which the shallower tendency at the entrance put more weight on flow and sediment ratio decrease of the North Passage, more water and sediment are forced into the South Passage.Readjusting the V-shaped dike redistributed the sediment transport intensity at two sides. However, this impact to the flow of bifurcation area based on the linear dike is limited, only those near the works affected to a certain extent. Moreover, our numerical model results also showed that all the readjusting solutions failed in promoting flow ratio of North Passage.