Hydraulic Geometry Of Yangtze Estuary And Equilibrium Depth Of Deepwater Channel

Yangtze Estuary is an estuary with abundant fresh water and sediment discharge, as well as three-order bifurcations and four outlets into the ocean. Located at the key junction between Yangtze waterway and the sea, the estuary occupies an important strategic position. With three phases of deep-water channel regulation in Yangtze Estuary, the navigable depth in North Passage gradually increased to 12.5m. Nevertheless, the amount of deposition in deep-water channel is still large while sediment coming from upstream has been obviously declining for years. The internal reason is that the maximum equilibrium depth can hardly meet the requirement of design depth. Therefore, hydraulic geometry of Yangtze Estuary and maximum equilibrium depth of North Passage can not only enrich the estuarine dynamics, but also have important implications for revealing the mechanism of morphological evolution and guiding the regulation of deep-water channel.A new relation for the hydraulic geometry of branching estuary was established based on time-dependent sediment transport capacity formula, ID 2D continuity equation, resistance equation and dimensionless width-depth ratio relation. The mean-depth ratio of a distributary channel to the main stream is a power function of discharge ratio with an exponent of 2/7, while the exponent will be 6/7 when it comes to the ratio of wetted cross-sectional area.Additionally, a hydraulic geometry relation of tide-dominant channel was developed by taking the linear combination of sediment transport capacity during flood with that during ebb into account, in order to suit for the middle and lower reach of North Passage. Furthermore, the relation among the cross-sectional area of main channel, width of regulation line and estuarine hydraulic parameters was derived. The relation could be used to the prediction of the equilibrium depth in North Passage after these hydraulic parameters were obtained from the Delft3D model.The maximum equilibrium depth at the top of mouth bar in North Passage before the deep-water channel regulation was calculated as 6.1 m by the proposed hydraulic geometry relation of branching estuary. The result agrees well with the actual long-term depth of about 6-7m, which proves the reasonability of the new relation.Based on the hydraulic geometry relation of branching estuary, the maximum equilibrium depth was calculated as 11.43m on average at the upper reach of North Passage after the deep-water channel regulation. According to the hydraulic geometry relation of tide-control channel, meanwhile, the maximum equilibrium depth was calculated as 9.46m on average with the shallowest of 9.31m at the middle reach and 10.27m on average at the lower reach. In general, the equilibrium depth is the largest at the upper reach and the shallowest at the middle reach.On one hand, the regulation obviously increases the maximum equilibrium depth of North Passage owing to the scouring effect of convergent flow. On the other hand, the groins are not conducive to the maintenance of navigable depth because of increase in resistance of flow and decrease in tidal prism. Consequently, the effect of regulation depends on the comprehensive action of these two aspects.Differences between the maximum equilibrium depths along North Passage and the design depth determine to a great extent the amount of deposition, the amount increases with the differences. Therefore, it is key to the deep-water channel regulation that we find a proper method for changing hydraulic conditions to adjust the cross-sectional geometry so that the maximum equilibrium depth can increase.