Assessment of Model Applicability Based on Node Ponding Conditions of the Hyoja Drainage Basin (Gwanghwamun area)

A flood analysis of the Hyoja drainage basin (Gwanghwamun area) in Seoul was conducted using the XP-SWMM model, which includes 315 pipeline datas and 293 subbasins. The model was created by considering topographical factors (e.g., pipeline slope and buildings). curve number (CN) values were determined for each subbasin based on land use and detailed soil maps for estimating infiltration. Additionally, a digital terrain model (DTM) was generated using a 1:5,000 digital topographical map and survey data.

In the XP-SWMM model, the evaluation of ponding varies depending on the ponding conditions (e.g., Ponding Allowed(PA), Link Spill Crest to 2D(2D), None Poning(NP)), leading to considerable differences in the calculation of hydraulic heads at the nodes. The Hyoja drainage basin, the focus of this study, features a steep slope in the upstream area and hilly terrain in the downstream, indicating potential limitations in accurately modeling flood volume.

To overcome the limitations of the XP-SWMM model, the surface slope and lowlands were incorporated when establishing node conditions. Areas with a slope of less than 5% were assigned the “2D” option, whereas areas with a slope greater than 5% were assigned the “NP” condition. In addition, supplementary modeling was conducted by combining certain subbasins to assess the impact of runoff from upstream areas on downstream flooding.

The flood volume analysis based on the ponding conditions under 95 mm/hr rainfall revealed that the “NP” condition tended to underestimate flood volume, while the “2D” condition tended to overestimate it by assuming ponding was present even in sloped areas. As a result, the “2D” condition was applied to basins with a surface slope of 5% or less, while the “NP” condition was applied to areas that were not lowlands. Lowlands were designated as areas with an elevation below 40.0 m, accounting for approximately 20% of the total area.

When comparing the flood volume before and after merging the subbasins, the flood volume in upstream areas with a slope of 5% or more decreased, whereas the volume in the downstream Gwanghwamun area increased. This situation was attributed to runoff from the upstream basin, which added to the increase in the downstream flood volume.

The study findings showed that by setting node ponding conditions considering basin slope and lowland conditions, simulating flooding events that closely resemble actual occurrences is possible. Thus, limitations of the XP-SWMM model can be overcome. Furthermore, merging subbasins has proven to be an effective approach for analyzing the interactions between upstream and downstream regions and assessing the impacts of critical flood zones. Technical decisions should reflect consideration for characteristics of the basin and topographical factors when designing a model

Acknowledgements

This work was supported by Korea Planning & Evaluation Institute of Industrial Technology funded by the Ministry of the Interior and Safety (MOIS, Korea). [Development and Application of Advanced Technologies for Urban Runoff Storage Capability to Reduce the Urban Flood Damage / RS-2024-00415937]