Assessing the compound impacts of urbanization and climate change on flood hazards in rapid urbanized basin: A case study of Chebei River Basin, South China

The Chebei River Basin, a rapid urbanized region in South China. Rapid urbanization and intensified rainfall under climate change pose escalating flood hazard to cities worldwide. However, the complex, non-linear interactions between these drivers often remain poorly quantified. This study investigates the impact of urbanization and climate change on flood hazards in the Chebei River Basin during 1980–2025 considering future climate change and land-use change scenarios. A coupled one-dimensional and two-dimensional (1D–2D) hydrodynamic model was employed to simulate flood inundation, and Random Forest together with SHAP methods were applied to achieve in-depth attribution analysis for the simulation results, quantitatively revealing the driving mechanisms behind flood intensification. Urban expansion and forest degradation are identified as the two most dominant land-use drivers of flood intensification. SHAP analysis uncovered a critical nonlinear mechanism, with an urbanization rate of approximately 65 % serving as a "threshold". Below this threshold, the flood-aggravating effect of urbanization is more pronounced under moderately intense rainfall. Once exceeding the threshold, the system shifts to a runoff-dominated regime, where extreme rainfall interacts strongly with urban expansion, significantly amplifying flood hazard. This finding mechanistically explains historical flood patterns and projects future compound hazard, offering crucial scientific guidance for developing forward-looking, "threshold"-aware urban planning. • An interpretable AI framework quantifies drivers of urban flood aggravation. • A hydrological regime shift is identified at a ∼65 % urbanization tipping point. • Urban expansion and forest loss are top drivers of increased flood hazard. • Extreme rainfall synergistically amplifies the flood impact of urbanization.