**4. Conclusions**

The first result is the difference between the current and new operation of the CR. The current operation is controlled by CR level alone, while the new operation is more flexible and controlled by sewer conduit monitoring node levels. If the levels are high at the first and maximum flooding nodes, drainage pumps in the pump station are operated early. This process was applied to the rainfall events of 21 September 2010 and 27 July 2011. The best result was achieved when early operation occurred at a monitoring node level of 0.8 m. CR operation can improve the drainage effectiveness of CR in most cases.

The second notable result is the difference between the current and new DR operation. The current operation is controlled by DR level. The new DR operation is flexibly controlled through the sewer conduit monitoring node levels. When the level is low, discharge from the DR ensures sufficient space for additional inflow. When the level at monitoring nodes is high, drainage pumps in the DR are stopped. However, when the DR level is the same as the current operation, discharge by drainage pumps starts again. When monitoring node levels are around 1.1 to 1.5 m, the new DR operation shows better results than the current operation. DR operation can increase the detention effectiveness of DR in most cases. In some cases, DR operation and CR operation show worse results than the current operation, respectively. To improve flood mitigation effectiveness, cooperative operation between CR and DR is simulated and checked.

The third notable result is the difference between the current operation and the cooperative operation of urban drainage facilities (CR + DR operation). The current operation is performed depending on CR or DR level, but the condition of sewer conduits is not considered. The newly proposed cooperative operation determines storage or discharge at each urban drainage facility, depending on sewer conduit levels (determined at the monitoring nodes). When monitoring node levels are high, storage in the CR prevents flooding associated with backwater effects, and discharge from the DR prevents flooding by obtaining additional capacity. When monitoring node levels are low, the CR operates normally and the DR ensures additional space by discharge, depending on monitoring node levels. The results of cooperative operation for urban drainage facilities are better than those of the current operation. For the 2010 and 2011 events, the best results were observed when the monitoring node level was 1.4 m. Cooperative operation between CR and DR is superior to the current operation in all cases because cooperative operation has the advantages of both CR and DR operation.

Finally, we analyzed resilience as a new index for evaluating urban drainage systems. We calculated the resilience of the entire system using drainage volume and flood volume based on unit rainfall, time of concentration to target basin, and unit basin area. We assessed the resilience results for two significant rainfall events in 2010 and 2011, and found that cooperative operation of urban drainage facilities showed superior performance compared to the current operation.

All urban drainage systems are operated in drainage areas, but this study has shown that it is necessary to control CR drainage pump operation by taking into account urban stream levels. Moreover, it is necessary to conduct further studies on DR inlets, in order to allow the effective control of inflow volume.

**Acknowledgments:** This research was supported by a gran<sup>t</sup> (13AWMP-B066744-01) from the Advanced Water Management Research Program funded by the Ministry of Land, Infrastructure, and Transport of the Korean government.

**Author Contributions:** Eui Hoon Lee and Yong Sik Lee carried out the survey of previous studies. Eui Hoon Lee and Donghwi Jung wrote the draft of the manuscript and revised the draft to the final manuscript. Eui Hoon Lee simulated CR operation, DR operation and CR/DR operation. Eui Hoon Lee, Joong Hoon Kim, Donghwi Jung and Jin Gul Joo conceived the original idea of the proposed method.

**Conflicts of Interest:** The authors declare no conflict of interest.
