## 3 Results
### Analysis on Drainage Duration of Empirical Formula  
By substituting the drainage coefficient calculated by the empirical formula into Formula (3), the drainage duration required when the drainage flow calculated by the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Year**} & \multicolumn{2}{c}{**10-Year**} & \multicolumn{2}{c}{**5-Year**} & \multicolumn{2}{c}{**3-Year**} \\ \cline{2-7}  & Q\({}_{E}\) & Q\({}_{A}\) & Q\({}_{E}\) & Q\({}_{A}\) & Q\({}_{E}\) & Q\({}_{A}\) \\ \hline
1991 & 10.78 & 8.31 & 8.06 & 5.39 & 5.75 & 3.39 \\
1995 & 11.04 & 8.52 & 8.25 & 5.53 & 5.88 & 3.48 \\
2009 & 10.69 & 8.24 & 7.89 & 5.27 & 5.51 & 3.18 \\
2010 & 10.68 & 8.23 & 7.88 & 5.26 & 5.49 & 3.17 \\
2014 & 10.35 & 7.96 & 7.53 & 4.93 & 5.13 & 2.74 \\
2015 & 10.27 & 7.89 & 7.44 & 4.87 & 5.03 & 2.63 \\
2016 & 10.28 & 7.90 & 7.40 & 4.84 & 4.93 & 2.52 \\
2018 & 10.44 & 8.03 & 7.42 & 4.77 & 4.83 & 2.47 \\ Annual change rate & \(-1.25\%\) & \(-1.04\%\) & \(-2.39\%\) & \(-2.29\%\) & \(-3.39\%\) & \(-3.42\%\) \\ \hline \end{tabular}
\end{table}
Table 4: Drainage coefficient under discharging 3-day rainfall in 4 days with different return periods (mm/d).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Year**} & \multicolumn{2}{c}{**10-Year**} & \multicolumn{2}{c}{**5-Year**} & \multicolumn{2}{c}{**3-Year**} \\ \cline{2-7}  & Q\({}_{E}\) & Q\({}_{A}\) & Q\({}_{E}\) & Q\({}_{A}\) & Q\({}_{E}\) & Q\({}_{A}\) \\ \hline
1991 & 7.53 & 10.34 & 4.52 & 4.78 & 3.41 & 3.12 \\
1995 & 7.72 & 10.62 & 4.63 & 4.  
empirical formula was discharged at an average rate can be obtained. The results are shown in Table 5.  
As shown in Table 5, \(T_{E}\) varied with rainfall and increased with the increase in rainfall duration and return period. According to the average value, under 1-day rainfall duration, \(T_{E}\) of the 5-year return period was 15.14% higher than that of the 3-year return period, and \(T_{E}\) of the 10-year return period was 30.05% higher than that of the 5-year return period. Under 3-day rainfall duration, \(T_{\rm E}\) of the 5-year return period was 19.46% higher than that of the 3-year return period, and \(T_{\rm E}\) of the 10-year return period was 16.29% higher than that of the 5-year return period. Under 1-day rainfall duration, \(T_{E}\) increased with time and the annual change of \(T_{E}\) was small. Under 3-day rainfall duration, \(T_{E}\) decreased with time, and its annual change trend increased with the decrease in return period.  
When the design drainage duration was equal to \(T_{E}\), \(Q_{A}\) was equal to \(Q_{E}\). T\({}_{\rm E}\) ranged from 2.11 d to 2.82 d when the 1-day rainfall greater than the 3-year return period was discharged in 2 days, \(T_{E}\) was greater than the design drainage duration, and \(Q_{A}\) was larger than \(Q_{E}\), so it was safer to use the average draining formula to calculate the drainage coefficient during the construction of drainage system. \(T_{E}\) ranged from 1.83 d to 1.87 d when discharging 1-day rainfall in 2 days with 3-year return period and from 2.04d to 3.09 d when discharging 3-day rainfall in 4 days. In the above two cases, the design drainage durations were greater than the \(T_{E}\), respectively, so \(Q_{E}\) was greater than \(Q_{A}\), and it was safer to apply the empirical formula to calculate the drainage coefficient.  
The results of the 1-day rainfall duration with the 3-year return period are different from the existing literature, but the other results are the same as those in the existing literature [24]. The reason might be that the existing literature only considers the influence of the design drainage duration on \(T_{E}\), and the effects of rainfall duration and return period are also considered in our research.  
Therefore, the longer drainage duration can be used for places with good regulating effects. In order to avoid idle pumps and waste human resources and funds, the empirical formula can be selected for engineering design; at the same time, the economic feasibility of engineering also needs to be considered.