杭州城西南排通道工程出口拟布置在钱塘江九溪岸段，为研究九溪岸段堤线优化对钱塘江防洪、御潮等功能的影响，采用平面二维数学模型，模拟工程实施后钱塘江百年一遇大潮、百年一遇洪水期间沿线的壅水高度、流速变化等情况。数值计算结果表明:当钱塘江遭遇百年一遇大潮时，工程区北岸堤前水位壅高相对明显，最大可达0.05 m，南岸的水位壅高影响小于0.01 m,南岸流速增加2%左右，上下游岸段堤前水域流速变化不大;当钱塘江遭遇百年一遇洪水时，工程河段局部范围内水位壅高0.02 m左右,下游相邻岸段堤前流速增加1%～2%，南岸浦沿排灌站岸段流速增加2%～3%，珊瑚沙水库围堤前沿流速影响较小。总体上，九溪岸段堤线优化对钱塘江防洪、御潮影响较小，对区域内地铁6号线、之江大桥、钱塘江大桥等涉水工程基本无影响。

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 volumeaccurate 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 trumpetshaped 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.