A study on inflow control methods for deep stormwater tunnel

Recent climate change has led to increasingly severe short-duration heavy rainfall events, resulting in stormwater volumes that exceed the capacity of existing drainage systems in Seoul. In response, the Seoul Metropolitan Government is planning to construct additional deep stormwater storage and drainage tunnels to mitigate flooding in densely populated urban areas. This study examines effective inflow-control strategies for a planned deep drainage tunnel in the Sadangcheon basin, aiming to reduce urban flooding during extreme rainfall.

The XP-SWMM hydrological and hydraulic modeling software was used to simulate flood scenarios and assess the impact of inflow control on inundation. A flood-analysis model was constructed to reflect current watershed conditions and to simulate one-dimensional sewer flow and two-dimensional surface inundation simultaneously. Using this model, the design inflow to the stormwater storage tunnel and the rainfall duration corresponding to maximum storage utilization were estimated. Optimal inflow-control conditions were derived by adjusting the operating water level of the vertical shaft gate to regulate the inflow initiation time.

Under the fixed water-level control scenario, applying inflow control delayed the time required to reach maximum storage by approximately 20 minutes compared with the uncontrolled inflow condition. The effectiveness of inflow regulation was evaluated through changes in surface inundation area and inundation volume. The results showed a reduction of approximately 34.2% in inundation area and 33.9% in inundation volume. These findings indicate that regulating inflow at the tunnel entrance allows more efficient use of limited storage capacity and helps adjust the time gap between peak flood discharge and the moment when the tunnel reaches full storage. This contributes to the stable operation of deep underground stormwater storage and drainage tunnels during extreme rainfall events.

In addition, variable water-level control conditions were applied to evaluate the tunnel’s operational flexibility under smaller-scale rainfall events. The analysis suggests that adopting adaptive inflow-control strategies can enhance the tunnel’s ability to manage a wider range of hydrologic conditions and improve overall flood-mitigation performance. Based on these results, an efficient operational approach for the planned stormwater storage and drainage tunnel is proposed.

These outcomes collectively demonstrate that inflow-control strategies can significantly improve the performance of deep stormwater storage tunnels by delaying maximum storage time, reducing inundation, and enhancing operational stability during consecutive or extreme rainfall events. The results provide practical guidance for the planning and operation of large-scale urban flood-control infrastructure under changing climate conditions.

 

Acknowledgements

This work was supported by Korea Environment Industry & Technology Institute(KEITI) through Technology development project to optimize planning, operation, and maintenance of urban flood control facilities, funded by Korea Ministry of Climate, Energy, Environment(MCEE)(RS-2024-00398012)