Analysis of River Water Level Variability Based on Discharge Rates from Flood Mitigation Facilities Using SWMM+RAS for Preventing Internal and External Flooding in Urban Watersheds

Urbanization has increased impervious surfaces, intensifying disaster risks in urban areas, particularly with climate change. Rainwater pumping stations and stormwater drainage systems are crucial for mitigating internal flooding, but their independent operation limits overall effectiveness. While integrated management of drainage facilities, including pumping stations, has been suggested, the development of systems to maximize existing technologies remains insufficient. Additionally, discharging stormwater into rivers to mitigate internal flooding can exacerbate external flooding by raising river water levels.

This study aims to integrate XP-SWMM and HEC-RAS to simulate internal runoff and surface flooding, model flood mitigation facility operations, and analyze river water level changes caused by lateral inflows in urban watersheds. The XP-SWMM model enables comprehensive runoff and 2D flood analyses, accounting for stormwater networks and hydraulic structures. Calibration and validation using water level data from Sebyeong Bridge in the Oncheoncheon watershed demonstrated good agreement between observed and simulated levels.

Lateral inflows for HEC-RAS were estimated by dividing the Oncheoncheon watershed using the Euclidean Allocation method. Initially divided into 133 sub-watersheds based on pipe connections, refinements led to 37 sub-watersheds by considering tributaries and further to 26 using contour data. Runoff hydrographs for the 26 sub-watersheds were generated using a 100-year return period 1-hour rainfall distributed via Huff's third quartile distribution.

These hydrographs were applied to HEC-RAS as lateral inflows. Terrain data were based on the ⌜Oncheoncheon River Master Plan (Busan Metropolitan City, 2017)⌟. Due to instability in upstream steep slope areas, these sections were excluded from simulations, focusing on watersheds with stormwater runoff reduction facilities. The upstream boundary condition utilized runoff from watershed 1, and the downstream boundary condition applied the 100-year base flood level. Stability was verified by assessing bridge cross-section water levels and time-step consistency.

Through HEC-RAS simulations incorporating lateral inflows, this study provides insights into optimizing operational rules for flood mitigation facilities, aiming to reduce both internal and external flooding via SWMM and HEC-RAS integration.