## 4 Conclusions  
In this paper, based on fluid dynamics equations and numerical analysis equations, a numerical simulation model of a LID facility, the concave herbaceous field, is constructed. The precipitation processes of light rain, moderate rain, and heavy rain with recurrence periods of 1a, 5a, and 10a were designed based on the rainfall intensity formula of Tianjin. A new multi-objective benefit evaluation and optimization model of LID facilities is constructed: Three-dimensional numerical simulation involving Morris parameter sensitivity analysis and TOPSIS optimal solution sequencing analysis was used to fully simulate and statistically analyze the benefit laws of flood peak reduction rate, flood peak delay rate, and total overflow reduction rate under different parameters. The analysis obtained the most significant structural parameters and revealed the optimal parameter size, and also provided a new method framework for the optimization of structural parameters and the improvement of runoff control benefits. The main conclusions are as follows:  
1. The rainwater runoff control hydraulic process of the concave herbaceous field is mainly affected by its structural parameters and precipitation recurrence period. Morris sensitivity analysis and the TOPSIS method were used to analyze the sensitivity of the model parameters, and the results showed that factors b (planting soil thickness) and c (planting soil slope) had the most significant influence on the operation effect of the concave herbaceous field.  
Figure 6: The optimal flow rate of the concave green space and the corresponding hardened surface in different recurrence periods. (**a**) 1a recurrence period real-time traffic; (**b**) 5a recurrence period real-time traffic; (**c**) 10a recurrence period real-time traffic; (**d**) Total overflow during the 1a recurrence period; (**e**) Total overflow during the 5a recurrence period; (**f**) Total overflow during the 10a recurrence period.  
According to the TOPSIS optimal solution analysis results, the variation range of the optimal runoff control benefit corresponding to the dimension parameter of the concave green space is \(-\)10-10%. The optimal structural parameters were an aquifer height of 200 mm, a planting soil thickness of 600 mm, a planting soil slope of 1.5%, a planting soil porosity of 0.45, and an overflow pipeline porosity of 0.3.
* Under the precipitation event with a recurrence period of 5a, the flood peak reduction rate, delay rate, and total runoff control rate were 88.93%, 51.11%, and 78.76%, respectively, and the optimal application condition of this optimal design scheme is the precipitation scenario where the return period is less than 10a.  
Q.Z., writing--original draft; [PERSON]. ([PERSON]), writing--review and editing; [PERSON], writing--review and editing; [PERSON], data curation; Y.Y., software; [PERSON]. ([PERSON]), resources. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the National Natural Science Foundation of China (NO. 41907149), and the China Postdoctoral Science Foundation (NO. 2018M631732).  
Not applicable.  
All the data in this manuscript are derived from the field surveys, reports, and references.  
The authors declare no conflicts of interest.