The present study aims to reveal the characteristics of flood flow with a large amount of fine sediment supply in the Gofukuya river in the disaster caused by Typhoon No.19, 2019. Since numerous landslides and debris flows occurred in the mountainous area of the Gofukuya river, a large amount of fine sediment was supplied and deposited at the plane area of the basin, that exacerbated the flood disaster. The present study proposes a method to evaluate the fine sediment runoff from the basin by considering the sediment erosion of the deposited sediment. The computed result of flood flow with fine sediment supply reproduces the flood hazards in terms of the thickness of sediment deposition and the sediment size distribution including the obvious longitudinal sediment sorting.
