## 3 Results and Discussions  
### Analysis on the Causes of Flood Disasters in the Study Area  
Generally speaking, the urban rain-flood is caused mainly by the rainfall and the river water flowing through the urban area from the upstream rivers. As shown in Figure 7, the drainage paths of precipitation mainly include the surface runoff generated by the  
Figure 6: Buried depth map of groundwater level in the study area. rainwater. After falling to the ground, the rainwater follows multiple paths. It flows into the underground drainage pipe network, surface rivers, ditches, lakes, and wetlands. The precipitation also infiltrates the vadose zone soil layer or the urban sponge. Some rainwater will enter the sewage treatment plant designed for treatment and reuse. Thus, the stormwater will eventually enter the rivers, except for the loss through evaporation and ecological and human consumption. If the urban rainwater drainage is not smooth, the situation may lead to urban flood disasters. For the study area, the mechanism of flood disasters is controlled by the following aspects of the rainfall as well as the ground conditions:  
1. Because of the abnormal superposition of the temperate monsoon climate and global climate change, the rainfall gets concentrated from June to September. During this period, it is easy to form heavy or extremely heavy rainstorms, which is the direct cause of flood disasters in the study area. 2. The water and soil erosion in the upper reaches and source of the Jialu River Basin is quite intense. By the end of 2015, there was still a total area of 881 km\({}^{2}\) of soil erosion, and the total annual soil loss was 2.16 million tons [28]. Soil erosion upstream leads to siltation, and the flood interception capacity of the reservoirs is reduced substantially. The sediment in the middle and lower reaches can easily cause silting, which greatly reduces the flood-carrying capacity of the rivers. 3. With rapid urbanization, the urban area has increased from 65 km\({}^{2}\) in 1981 to 1181.51 km\({}^{2}\) in 2019, an increase of 18 times. As a result, the impervious underlying layers are being rapidly exposed, altering the original geological conditions. Subsequently, the ability of the rainwater to penetrate the ground is reduced, and the surface runoff coefficient is increased from 0.25-0.45 to 0.65-0.85 [29]. An increase in surface runoff by about 60% results in poor drainage conditions, leading to flood disasters. 4. During the construction of water ecological projects or the sponge city, the hydrogeological conditions are not fully utilized. For example, during the construction of projects such as wetlands, river treatment and other aquatic ecological projects, anti-seepage treatment is implemented at the bottom, which reduces the infiltration capacity. During the construction of sponge city, engineering measures such as pervious pavements are emphasized, but the natural rainwater absorption capacity of the  
Figure 7: Pathways of rainwater and flood discharge after building sponge city. aeration zone soil layer is not fully considered. This situation, which lacks integration between the two important hydrologic parameters, reduces the sponge city's rainwater and flood absorption capacity. 5. In the process of urban construction, the natural law of the urban hydrological cycle is not fully understood or paid attention to. As the flood plains are blindly developed and the river course is occupied, the drainage ditches are either buried or filled, and the open spaces are channelized. Thus, during urban construction on both riverbanks, the space for discharge space is not fully conserved. This misappropriation of the flood plain discharge space results in a significant decline in the flood discharge capacity of the river. 6. The main rainwater drainage facilities, such as the underground drainage pipe network, roads, and river systems, have not been considered as a whole, and some urban areas have no drainage facilities, resulting in a substantial decline in urban drainage capacity.