Abstract: This study develops a two-dimensional hydrodynamic model covering a river section roughly 207 km long using Anlan Simulation System to characterize its flood-risk responses and quantify variations in its inundation metrics and land-use exposure under the peak runoff discharge. The model integrates a digital elevation model, the cross-sections surveyed, the 1950-2022 discharge records observed, and certain land-use data in a case study of the Qingtongxia-Shizuishan reach of the Yellow River in Ningxia. Using the return periods of 5-100 years, we simulate the historical typical floods and design floods to assess flood inundation, comprehensive risk, and response sensitivity. Simulations show that the inundated area was in a range of 265.1-319.2 km2 under the historical-flood scenarios, and a range of 458.8-480.8 km2 under the design-flood scenarios, with high-risk zones accounting for more than 80% of the inundated area under the latter scenarios. We conduct a sensitivity analysis and find that in the case of the design flood increasing from the 5-year to 100-year, its peak discharge increases by 82.75%, whereas the inundated area, mean water depth, and maximum flood velocity increase by less than 5%. These results indicate that the levees and floodplain-channel topography constrain the inundation and weaken the responses of major hydrodynamic metrics to an increasing peak discharge. The increase in flood risk often takes the forms of longer inundation duration and earlier flood arrival. The findings are very useful for flood-control planning, floodplain land-use optimization, and emergency risk management in the study reach.

Key words: flood simulation, inundation analysis, two-dimensional model, flood response sensitivity analysis, risk assessment, Qingtongxia Reservoir