This study reveals the characteristics of flood flow with active sediment transport that caused the Sozu River flood disaster in July 2018. The results of field surveys revealed that the basin contains steep mountains and flat areas, facilitating the rapid transport of sediment produced in the mountains to downstream areas during the disaster. Two-dimensional numerical simulations of flood flow with sediment transport are conducted to realize the flood flow characteristics. As a result of the simulation, a huge amount of sediment deposition in the channel near the top of the alluvial fan increases the inundation damage at downstream areas. The results indicate that within two flood peaks during the disaster, the inundation in the second flood peak is more severe than that in the first peak, whereas the discharge of the second peak is much less than that in the first peak, caused by the reduction in the capacity of the original channel owing to the sand deposition during the first and second flood peaks.

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