Spatiotemporal coupling effects of rainfall and underlying surface on urban flood characteristics

Urban flood poses a serious threat to human life and property safety. Existing research has largely been limited to single-factor analyses under fixed scenarios, with insufficient attention paid to the coupling effects of rainfall and underlying surface conditions. To address this research gap, this study takes a certain area of Zhengzhou City as the study area and constructs a two-dimensional hydrodynamic model based on InfoWorks ICM. By combining various rainfall conditions (rainfall (RF), rainfall duration (DUR), peak timing ratio (PR), and rainfall center (RC)) and underlying surface conditions (impervious surface ratio (ISR), fixed runoff coefficient (FRC), and manning's roughness (n)), the flood characteristics of ponding areas and inundation points were obtained. A quantitative analysis method based on the generalized additive model (GAM) is proposed for inundation points. By introducing the total dominant contribution index (TDCI), the partial dependence of the flood characteristics of inundation points on the combinations of conditions is captured, thereby assessing the intensity relationships of various rainfall and underlying surface conditions. The results indicate that at the macroscopic scale, rainfall and underlying surface conditions have distinct emphases in their respective mechanisms influencing flood characteristics. The deterioration of both rainfall and underlying surface conditions exacerbates flood characteristics and significantly increases the number of ponding areas. However, the influence of the underlying surface on the extreme values of flood characteristics is weaker than that of rainfall. At the microscopic scale, the relative influence of rainfall and underlying surface conditions on peak depth follows the order: RF (TDCI:30) ≈ RC (30) > FRC (20) > DUR (12) > PR (-12) > n (-28). For start of exceedance, the order of relative influence is as follows: DUR (TDCI: 39) ≈ PR (39) > RF (30) > RC (18) > FRC (-4) > n (-40); For unit discharge, the order of relative influence is as follows: n (TDCI: 40) > RF (6) > FRC (2) > DUR (0) ≈ RC (0) > PR (-39). Inundation duration and start of exceedance are more strongly dependent on DUR and PR, while unit discharge is most sensitive to n.