|This study is conducted to discuss the characteristics of active sediment transportprocesses in extreme flood hazards and investigate methodologies for numerical simulationof such hazards. Two flood disasters that recently occurred in Japan are investigated.They are characterized as a huge amount of sediment production from the mountains due tolandslides and debris flows which carried a large amount of sediment to the downstream areaswith the flood flow. They are also characterized by obvious sediment sorting within a shortdistance. In order to simulate these phenomena numerically, methodologies are proposed tocompute bedload transport using a bedload formula with a sediment transport rate proportionalto the 2.5th power of the non-dimensional tractive force. The numerical simulationfound that the conditions of sediment supply from the upstream reaches need to be carefullydetermined to evaluate extreme flood hazards such as the ones investigated in this study.