*3.2. Analysis of Drainage System Load Conditions*

The stormwater pipe network in the study area is mainly located in the paving of main and secondary roads such as the North Third Ring Road, Garden Road, Jing San Road, Zhong Zhou Avenue, Sha Men Road, East Huang He Road, and East Dong Feng Road. Stormwater is discharged into the stormwater pipe network through the catchment area and eventually discharged into the Dongfeng Drain after collection by the stormwater pipe network.

The drainage system load conditions were analysed in terms of both the nodal overflow conditions and drainage network load conditions. Figure 8 presents the distribution of the node overflows in the study area simulated under the different scenarios. It can be seen that most of the nodes showed varying degrees of overflow, which is relatively obvious. According to the statistical results in Table 4, the percentage of overflow at the nodes with a rainfall duration of 1 h increased from 79.01% to 87.36%, whereas the percentage of overflow at the nodes with a rainfall duration of 2 h increased from 79.43% to 87.36% under

the conditions of 5 a, 20 a, and 50 a rainfall recurrence periods. Under the same recurrence periods, the proportion of nodal overflows with a 2 h rainfall ephemeris increased by 0.42%, 0.26%, and 1.58%, respectively, compared with those of nodal overflows with a 1h rainfall ephemeris. The overall pattern was that the proportion of nodal overflows increased with the increases in the rainfall return period and rainfall ephemeris.

**Figure 8.** Simulated node overflow condition diagram.


**Table 4.** Overflow statistics for different scenario nodes.

In this study, the overload state of the pipe network was judged according to its water flow state. According to the simulation results, when the overload state was less than 1, it meant that the pipe network was in a non-full gravity flow state, which means that the pipe network was in a normal state. When the overload state was equal to 1, the pipe was in a pressure flow state and the hydraulic gradient < pipe slope. The main reason for this state is the insufficient overflow capacity of the downstream pipeline, which causes the pipeline to be in an overload condition. When the overload state was equal to 2, the pipe was in a pressure flow state and the hydraulic slope > pipeline slope; the pipe was overloaded due to its own drainage capacity being insufficient. The pipe network was generally considered to be overloaded in the case of overload states 1 and 2. As can be seen in Figure 9, the pipe networks in the study area were generally in an overload condition, with the flow overload being the dominant overload pipe network. The length of the overloaded pipe network increased with increasing precipitation return periods.

**Figure 9.** Simulated pipe network load conditions.

According to the overload statistics of the pipe network under rainfall return periods of 5 a, 20 a, and 50 a, as seen in Table 5, the overload water depth lengths of the pipe network with a rainfall duration of 1h were 49.07 km, 42.38 km, and 38.81 km, and the corresponding overload flow lengths of the pipe network were 241.33 km, 249.56 km, and 253.87 km. The overload flow lengths of the pipe network corresponding to 2 h were 235.16 km, 239.91 km, and 245.25 km. The above results show that the length of the overloaded pipe network increased with the increase in the rainfall return time, whereas the length of the overloaded pipe network decreased with the increase in the rainfall recurrence period.

**Table 5.** Statistics for overloading of the pipe network for different scenarios.

