## 3 Results and Discussion
### Influence of Different Facility Combinations on Runoff Pollution Control Effect  
One of the obstacles that bother engineering designers in the process of sponge city construction is the selection of facility types and the appropriate combination of multiple facilities. By using model simulation, several studies have examined how to obtain the most construction benefit, economic benefit, and ecological advantage from a mix of diverse facility types [34]. Bioretention, permeable pavement, and rain barrels are still selected as three typical facilities in this section of the study, and they are mixed in different proportions, with the model to be used to simulate rainfall events under various rainfall conditions of return periods. The comprehensive design storage volume required by the plot to achieve the total runoff control rate in a specific year is calculated using the volume methods and then assigned to each facility using the facility ratio, with the floor space required by each facility evaluated separately. As shown in Figure 7, the parameters are set up in the SWMM model, and the simulation results are processed using the dimensionless constant curve. As shown in Figure 7, when the design rainfall return period is 10 years, the Dc value where the ratio of bioretention to the permeable pavement to rain barrel is 1:2:1 has the highest performance, that is the best runoff pollution control effect. Under the same design scale, the Dc value is the worst when the design rainfall return period is 0.5 years and the bioretention: permeable pavement: rainwater bucket is 1:1:2. According to the results of rainfall simulations, the combination can achieve higher Dc values when the percentage of facilities with higher Dc values of individual facilities is greater. The Dc value of the overall facility combination is lower when the fraction of facilities having a lower Dc value is greater. In order to achieve initial runoff volume management, the combination of LID facilities with infiltration function should be chosen as much as possible. This section of the research method helps engineering designers how to analyze. This approach may be used  
Figure 7: Dc value change curve of different facility combinations. to find the best combination scheme of different facilities in advance of the actual planning and design procedures. Effects of Different Underlying Surface Types on Runoff Pollution Control in the Same Catchment Zone  
In the same catchment zone, different underlying surface types cause runoff pollutants in diverse situations. In comparison, runoff pollution is more serious when the underlying surface is roads [35]. Therefore, it is very necessary to study the influence of source facilities on runoff pollution control under different types of underlying surfaces. The typical types of underlying surfaces include roads, green spaces, and buildings. As a consequence, the types of underlying surfaces are simplified in this section, and the intended land use classifications are no longer rigorously followed. Green space, roads, and buildings are all utilized in the same way. Simultaneously, the model's parameters relating to the land use type of the study area are updated in response to changes in the proportion of green space, roadways, and building area, and related research shows that the proportion of road area in typical blocks in China is between 20% and 40% [36]. In this part of the study, the proportion of roads was set to 20%, 30%, and 40%, and the proportion of green space was set to 10%, 30%, and 50%, respectively, and the remaining part was the building area, with a total of nine combination scenarios. Due to the change in the proportion of land use types, the comprehensive runoff coefficient will change accordingly, so it is necessary to recalculate the corresponding volume control facility scale in each scenario and update it to the model.