## 3 Results and Discussions
### The Role of Geological Methods in Urban Flood Control  
3.3.1. The Role of the Geological Method-Integrating Sponge City Construction with Hydrogeology in the Prevention and Control of Urban Flood Disasters  
The concept of \"sponge city\" refers to the method of \"strengthening the management of urban planning and construction, paying full attention to the absorption, storage, slow-release and drainage of rainwater by ecosystems such as buildings, roads, green spaces, and water systems, to effectively control rainwater runoff and realize natural accumulation, penetration, and purification of the urban environment\". The concept regards cities as living organisms and uses the local conditions to improve drainage and waterlogging,with the goal of controlling urban floods [30]. Obviously, the construction of \"sponge city\" has become one of the important means of checking urban waterlogging in China. The main purpose of a sponge city in solving urban flood disasters is to improve the function of \"water absorption, storage and seepage\". Its construction guide proposes more than 10 engineering measures for the development of the sponge city (see Figure 8 for details). However, it must be regarded that each city is built under specific geological conditions and has its own unique physical and hydrogeological conditions, such as topography, lithology, rock and soil thickness of vadose zone, rainfall infiltration capacity, underground water storage performance, etc. (Figure 8). These hydrogeological conditions play an extremely important role in the \"water absorption, storage and seepage\" of urban rainwater. In the planning and construction of a sponge city, the \"engineering measures\" are organically integrated with the natural \"hydrogeological conditions\". The goal is to form a whole \"sponge\" of the city, which can reduce the construction and make the city functionally more efficient. This is the concept of the integration of engineering measures with hydrogeology, as illustrated in Figure 8.  
For urban flood control, the construction of sponge cities not only needs to consider whether rainwater can infiltrate into the ground, but also whether there is space underground to store rainwater, which means that both the infiltration capacity and water storage capacity of the vadose zone should be considered. (1) Rainfall infiltration capacity of the soil layer in the vadose zone  
The infiltration capacity of the soil layer in the vadose zone in the absence of surface runoff is an important parameter to be evaluated. This paper suggests several water seepage, infiltration and geotechnical experiments to evaluate the infiltration capacity of the vadose zone soil layers in response to different intensities of rainfall. These experiments are based on previously collected hydrogeological data by water seepage, infiltration and geotechnical experiments to obtain parameters such as the infiltration rate, the permeability coefficient, and the void ratio of the soil layer in the vadose zone. Infiltration in the various  
Figure 8: Engineering–hydrogeology integration concept for sponge city construction. soil layers of the vadose zone corresponding to the intensity of rainfall, calculated when there is no surface runoff, is shown in Table 1.  
As shown in Table 1, the infiltration capacity of silty clay can meet the requirements for most rainfalls of light rain intensity. The infiltration capacity of silt or loess can meet the requirements for heavy rain intensity. The infiltration capacity of silty sand or silty fine sand can meet the infiltration requirements for a rainstorm. The infiltration capacity of medium sand and coarse sand can meet the infiltration requirements for an extremely heavy rainstorm. Therefore, when the vadose zone is composed of fine-grained soils such as silty clay, silt, and loess, its ability to infiltrate rainfall is low, and it is not an ideal stratum for the construction of the sponge city. Coarse-grained soils such as medium sand and coarse sand have a strong rainwater infiltration capacity.