Flood Response to Complex Underlying Surface Changes in the Urbanized Plain River Network Regions

ABSTRACT Floods, one of the most frequent natural disasters in urbanized plain river network regions, have been significantly altered by complex underlying surface changes. However, quantitative assessments of flood responses to various underlying surface factors at the event scale remain limited. This study examines the Changzhou Plain as a case study, analyzing changes in underlying surfaces (land use, river systems, and hydraulic structures) over the past 40 years and evaluating their impacts on flood processes using a coupled flood model. Both the coefficient of determination ( R 2 ) and the Nash–Sutcliffe efficiency (NSE) exceeded 0.90 during calibration and validation for the coupled flood model. Significant underlying surface changes, mainly including a 172.07% increase in urban land, a 67.63% decrease in agricultural land, a 7.55% reduction in river density ( R d ), and an increase in hydraulic structures to 917. Hydraulic structure operations had a more substantial influence on both the magnitude and spatial distribution of floods compared to land use and river system changes, with their effects intensifying as flood magnitudes increased. These findings enhance the understanding of the impact mechanisms of complex underlying surface changes on flood processes and provide valuable guidance for future flood management.