Heavy rain is becoming more frequent and damages bridges. We aimed to make road networks more resilient against natural disasters and present a new type of road risk assessment. It is essential to explore how river floods impact bridges.Here, we studied how high river flows destroy bridges, with a focus on the impact of “flood drag” on bridge girders. The risk of severe damage increased when girders were installed below the virtual line connecting the upstream and downstream river levee, during the flood event beyond the designed water level.Furthermore, a trial calculation of the flood drag acting on the superstructure revealed that differences in the bridge material, width, river slope and water depth affect the bridge risk.
