Discussion on the determination method of comprehensive risk degree in flood risk zoning

When conducting a flood and drought disaster risk survey, it is necessary to divide the flood risk areas based on the comprehensive risk level. However, there are multiple challenges in the census process: for drainage areas where multiple rivers coexist, it is difficult to independently calculate the flood risk factors of each river; The lack of river topographic maps and cross-sectional data hinders hydraulic calculations based on the 12.5 m DEM (Digital Elevation Model) and the determination of parameters for the runoff coefficient method. Hubei Province has explored the method of determining the comprehensive risk level: ① For plain areas with unclear confluence boundaries and relying on pumping stations for drainage, based on the water system structure and runoff characteristics, the area is divided into multiple confluence units, and the comprehensive risk level is determined by calculating the risk factors of floods with different recurrence periods. ② In mountainous flood threat areas lacking terrain data, by collecting river measurement data, small and medium-sized river management data, mountain flood disaster assessment data, or conducting river cross-section measurements, the flood peak flow water level extension method is used to calculate the inundation depth and range of floods of different frequencies, and the maximum inundation depth is used as the equivalent depth to calculate the comprehensive risk level. ③ When using the runoff coefficient method to calculate the comprehensive risk level in the slope area of mountainous flood threat zones and local flood threat area, the correction factor should be determined based on the terrain and the susceptibility to flood disasters. For areas with frequent mountain flood disasters, it is recommended to choose a larger value within a reasonable range. The runoff coefficient should take into account the worst-case scenario of rainfall runoff coefficient in disaster prone areas, and combine it with the investigation and evaluation of mountain flood disasters and defense plans to analyze the critical rainfall and design value rainfall when mountain floods are prone to occur. These achievements have enriched the methodological system of flood and drought disaster census and promoted the smooth progress of the census work.