Influence of Qiantang River Jiuxi embankment optimization on flood control and tide protection
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Abstract:
Southwest drainage tunnel of Hangzhou City is a comprehensive water conservancy project to solve the problem of flood control and drainage in west Hangzhou.The exit of the project is to be located near the Yanshanhe sluice in Jiuxi section of Qiantang River.The Jiuxi section is located at the end of the estuary of the Qiantang River .Due to interaction of runoff and tide,the riverbed erosion and deposition of the section are relatively complex.However,after long-term regulation,the width of the narrowest section of the section has been stabilized at about 1 km.This section of the river is relatively open,the north side of the flood embankment line can be appropriately optimized,combined with the implemented Hangzhou southwest drainage channel project,outlet drainage pumping station can get a comprehensive benefit. The two dimensional mathematical model of tidal current and sediment is used to calculate the influence of density pressure caused by sediment concentration on the flow movement.The flow control equations include the continuity equation and the momentum equation.Only suspended sediment is considered in the sediment field,and the governing equation is the convection-diffusion equation.The flow control equations are solved by the finite volume-KFVS scheme,and the sediment transport equations are solved by the finite volumeaccurate Riemann method.In addition to the requirements of fast convergence,good stability,and high accuracy of the discrete scheme,the establishment of the mathematical model mainly involves the rational determination of some important parameters,which can meet the verification requirements of the measured data under the conditions of large and small tides,flood and dry discharge and so on.To fully reflect the dynamic characteristics of the combined action of tidal flow and runoff in the engineering reach,the upper boundary of the numerical simulation study area is arranged at the Fuchunjiang power station,the lower boundary is arranged at the Ganpu section,and the grid of the Jiuxi reach is partially densified,the minimum grid size is about 2 m,and the tributaries are considered by side inflow.The tidal level of the model is verified by the Hydrologic Survey data in the engineering reach during the spring tide period in June 2018,and the flood level is verified by the flood data in June 2011.After verification,the model simulates the two schemes of the levee moving out and compares the backwater height and velocity variation of the levee in Qiantang River before and after the implementation of the project. The model results show that the backwater height in front of the levee is more obvious when the levee meets the high tide in Qiantang River once in a century,the maximum is 0.05 m,but the backwater height on the opposite bank is less than 0.01 m,the velocity of the opposite bank increases about 2%,but the velocity of the water in front of the bank does not change much.When encountering Qiantang River′s flood,the local water level of the project reach rises by 0.01 m,the velocity of flow in front of the embankment increases by 1% to 2%,and the velocity of flow along the bank of the irrigation and drainage station increases by 2% to 3%,the influence of flow velocity in front of the embankment of Shanhusha Reservoir is small. The moving of the levee line is not only beneficial to the layout of the pump and sluice project,but also to the release of a large enough water area for pressure regulation before the pump,a partially narrowed trumpetshaped body of water was reconstructed to reproduce the tidal bore.On the whole,the optimization of the embankment of Jiuxi Bank has little influence on flood control and tide resistance of Qiantang River and does not influence the water-related projects such as metro line 6,Qiantang River Bridge in the region.
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