Analysis of the Characteristics of Urban Flood Inundation Modeling in GUDS (Grid-Based Urban Drainage System Analysis Model) Using Idealized Topography

Recent climate change and urbanization have increased localized heavy rainfall, leading to repeated urban inundation events and a growing need for two-dimensional inundation models that can quickly and accurately simulate various rainfall scenarios. This study compares the inundation behavior of a Grid-Based Urban Drainage System Analysis Model (GUDS), a grid-based urban drainage model which has a WCA2D-based surface module, with that of the shallow-water-equation-based commercial model XP-SWMM. Three idealized catchments—inclined, single-grid inclined, and funnel-type—were configured, and both models were run under identical inflow conditions. Model performance was evaluated using pointwise flow velocity and water depth, inundation propagation, and the occurrence of numerical instabilities. In the longitudinal single-grid case, the differences in maximum velocity and water depth were within approximately 10%, confirming that the GUDS can reproduce one-dimensional flow behavior. In the inclined and funnel-type cases, XP-SWMM produced locally faster and deeper flows, whereas GUDS yielded smoother fields. Differences in maximum water depth and inundation area were 13.3% and 14.6% in the inclined case and 9.8% and 6.7% in the funnel case, respectively. Additionally, the XP-SWMM showed numerical instabilities in steep-slope sections, while the GUDS remained stable. However, only idealized topographies and simple inflow conditions were considered, and future work should verify the reliability and generality of the GUDS for real urban catchments.