Mountainous urban development unintentionally reshapes spatial patterns of flood disaster risk

In mountainous regions, rigid topography constrains urbanization and often drives unregulated expansion into vulnerable floodplains. Understanding how this spatial shift alters disaster exposure and amplifies flood hazards is crucial for disaster risk reduction. This study investigates the dynamic evolution of urban flood vulnerability under the dual pressures of topographic constraints and rapid urbanization. By integrating field surveys, Telemac-2D hydrodynamic modeling, and LightGBM-SHAP machine learning, we quantified the spatiotemporal contributions of topography, vegetation, and drainage density to flood inundation from 2000 to 2020. Our analysis reveals that developing mountainous cities have inadvertently reshaped the spatial pattern of disaster risk. Topographic constraints force the dense expansion of impervious surfaces into low-lying areas, which drastically increases the exposure of high-value assets and populations to extreme flood events. Furthermore, we identified an inflection effect driven by the saturation of grey infrastructure capacity. Beyond this inflection point, engineered grey infrastructure fails to mitigate extreme floods and systematically amplifies urban vulnerability compared to natural land cover. Compounding these vulnerabilities, severe geomorphological constraints, specifically downstream channel constriction, significantly impede flood discharge and trap water within the densely populated city. We emphasized that simply densifying drainage networks cannot overcome rigid topographic bottlenecks. To systematically reduce disaster risk, mountainous cities must halt unregulated impervious expansion into floodplains and strictly protect riparian buffers to restore natural flood storage capacity.