## 3 Results and Discussions
### The Role of Geological Methods in Urban Flood Control
If there is enough storage space--that is, if the thickness of the vadose zone soil layer is sufficient--it is an ideal stratum for building a sponge city. The water storage capacity of the vadose zone refers to the amount of rainwater that can be stored in the effective pores of the soil layer. The porosity, natural saturation and maximum saturation obtained by engineering geological drilling, geotechnical testing and other methods, are used to calculate the effective influent porosity Ne of each soil layer by Equation (1) (as shown in Table 2). And then, the thickness \(\rm H_{t}\) of the vadose zone soil layer required to store rainfall of different intensities was calculated by Equation (2) (as shown in Table 3). The thickness of this soil layer is an indicator of underground water storage capacity. \[\rm N_{e}=(S_{max}-S_{0})\times n, \tag{1}\]  
where \(\rm S_{max}\) is the maximum saturation, \(S_{0}\) is the natural saturation, and \(n\) is the porosity. \[\rm H_{t}=(q\times S_{t}\times N_{e})/S_{t}=q\times N_{e} \tag{2}\]  
where q is the upper limit value for different rainfall intensities, and \(\rm S_{t}\) is both the rainfall area and the infiltration area of the vadose zone. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soil Mass Types** & **Seepage Velocity (mm/d)** & **Permeability Coefficient (cm/s)** & **Rainfall Intensity That Can Be Absorbed** & **Rainfall (mm/d)** \\ \hline Silty clay & 6.2 & \(7.1\times 10^{-6}\)-\(6.4\times 10^{-5}\) & light rain & \(<\)10 \\ Silt & 56.75 & \(6.6\times 10^{-5}\)-\(2.1\times 10^{-4}\) & heavy rain & 25-50 \\ Loess-like soils & 62.374 & \(5.4\times 10^{-5}\)-\(9.7\times 10^{-5}\) & heavy rain & 25-50 \\ Silty sand or & 122.60 & \(1.8\times 10^{-4}\)-\(7.2\times 10^{-4}\) & rainstorm & 50–100 \\ silty-fine sand & & & & extremely heavy \\ Medium sand & 2583.2 & \(5.0\times 10^{-3}\)-\(1.4\times 10^{-2}\) & rainstorm & \(>\)250 \\ Coarse sand & 7251.2 & \(1.8\times 10^{-2}\)-\(6.8\times 10^{-2}\) & extremely heavy & \(>\)250 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Permeability parameters and corresponding rainfall of different soil layers in the vadose zone (not considering surface runoff). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soil Mass Types** & **Porosity (\%)** & **Natural Saturation (\%)** & **Maximum Saturation (\%)** & **The Effective Influent Porosity (\%)** \\ \hline silty clay & 44.32 & 76.36 & 92.34 & 7.08 \\ silt or loess-like soils & 40.76 & 59.51 & 91.73 & 13.13 \\ silty-fine sand & 42.23 & 62.11 & 90.03 & 11.79 \\ medium sand & 36.14 & 61.19 & 94.72 & 12.12 \\ coarse sand & 35.66 & 56.29 & 93.24 & 13.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The effective influent porosity of different vadose zone soil. The area of the key study area is 1945 km\({}^{2}\), and its vadose zone is composed of mixed sands and clay. Fine silt makes up 47.65%, and 18.72% is composed of medium-coarse sand. Silt makes up 31.22%, and the rest, 2.41%, is silty clay. For the water storage capacity of a heavy rainstorm, the required thickness of soil layers such as silt, silty sand and medium-coarse sand is 1.88 to 2.12 m, while the required thickness of silty clay is 3.53 m. The rainfall of the \"720\" torrential rain that lasted for three days in Zhengzhou was 617.1 mm. Based on this calculation, the thickness of soil layers such as silt, silty sand, and medium-coarse sand to accommodate the rainfall was no more than 5.23 m. The required thickness of the clay is 8.72 m. Judging from the depth distribution of the groundwater level in study area, if the rainwater can seep into the ground, most of the vadose soil layers within the study area have the ability to store the \"720\" torrential rain.