In response to the potential spatial transfer of flood risks caused by the improvement of urban drainage capacity and the adverse impact on flood control safety of external drainage channels, a refined hydrological and hydrodynamic model was constructed in the central urban area of Wuxi City and adjacent Jiangnan Canal as the study area. The scenario analysis method was used to compare the differential effects of three drainage modes, namely natural gravity drainage, current pump station drainage, and increased pump station capacity drainage, on flood risks inside and outside the city. The results show that the use of pumping stations can significantly reduce the peak water level in urban areas and shorten the duration of exceeding the warning water level. However, under the existing scheduling rules, the water level improvement effect brought by further improving the pumping station capacity is no longer significant. The momentum exchange effect generated by the concentrated strong discharge at the mouth of the discharge channel in Wuxi City forms a hydraulic barrier along the way, leading to flow reversal and water level rise in the upstream section, while the downstream section is less affected. When the base flow rate of the discharge channel is low, forced lateral inflow is more likely to change the original flow state of the channel, showing obvious low flow sensitivity characteristics.

A method of standard syntaxis of the flood control and drainage based on the theory of water level safety margin is constructed, and the northern part of the Taihu Lake Basin is used as the study area to analyze the cohesion between the sub-basin flood and urban drainage standards. Through the trial calculation of different simulation schemes, the effect of changes of sub-basin flood and urban drainage standards on the inflow and outflow volume in key sections is analyzed, and the design combination of sub-basin flood and urban drainage standards is proposed based on the evaluation method of water level safety margin. The results indicate that the current sub-basin flood and urban drainage standards of the northern part of the Taihu Lake Basin basically can resist the flood with a return period of 50-year. However, the capacity and demand of flood control and drainage at the sub-basin and urban are incohesion, which shows that the water level in the urban is far lower than the flood control design water level, while the sub-basin water level is close to or even higher than the flood control design water level and even slightly higher than its flood control capacity. There are significant differences in the characteristics of changes in the flood discharge pattern caused by the improvement of flood control standards in different sub-basins. The flood control capacity of Wuchengxiyu District and Yangchengdianmao District is improved by expanding the flood discharge to the Yangtze River, while the Huxi District is profited by expanding flood discharge to the Taihu. Limiting the discharge volume of polder of urban has limited effect on alleviating the flood control pressure in the external area of urban. Finally, the sub-basin flood standard with a return period of 50-year and the urban drainage standard with a return period of 20-year in the north of Taihu Lake Basin are the design combination of standard syntaxis.