Development of Complex Underground Space Inundation Model based on Laboratory Scale Experimental Data

Urban underground spaces are highly vulnerable to flooding due to their complex, multi-level configurations and limited drainage capacity. Although numerous numerical models have been proposed to simulate inundation processes in underground environments, their reliable development and application remain constrained by the lack of reference data for validating model structure and reproducibility. In particular, few studies have systematically investigated inundation behavior within the same underground space under varying inflow locations, numbers of inlets, and discharge conditions. This study presents the development of a two-dimensional surface–underground integrated flood model based on the shallow water equations, with the aim of reproducing inundation processes in complex underground spaces. The model was formulated to represent inflow, internal propagation, and drainage processes within underground spaces in a unified computational framework. Unlike conventional approaches that treat underground spaces as lumped storage elements or simplified links, the proposed model resolves underground spaces as two-dimensional hydraulic domains, allowing lateral flow propagation and spatial water-depth distributions to be explicitly simulated. Physical hydraulic experiments were employed to support model verification and to provide controlled reference conditions for quantitative evaluation. Model validation was conducted using laboratory-scale hydraulic experiments performed with the Kyoto Oike underground space facility (1/30 scale) at the Disaster Prevention Research Institute, Kyoto University. Steady-state inflow conditions were considered at 16 surface–underground connection points, including both single-inlet and sequential multi-inlet configurations. A total of 93 experimental cases were designed by applying constant inflow rates of 100, 200, and 400 mL/s while progressively increasing the number of active inlets. Final water depths, inundation extents, and dominant outflow pathways were measured after steady conditions were reached and were directly compared with numerical results under identical geometrical and boundary conditions. The comparison results demonstrate that the developed model can reasonably reproduce inundation propagation and drainage behavior within complex underground spaces under varying inflow locations, inlet numbers, and discharge levels. Through this study, a numerical model for analyzing inundation processes in complex underground spaces was developed, and the proposed model is expected to support future underground flood risk assessment and evacuation planning in urban environments.