Debris flow causes flooding and sediment deposition when it reaches alluvial fans. Many houses have been constructed on alluvial fans, and this can affect debris flow flooding and deposition. In this study, we first conducted a field survey on recent debris flow disasters in Japan; one such disaster, the Izu Oshima sediment disaster, occurred in October 2013. Houses located upstream, in lower areas, and those facing small bridges and crossroads suffered greater damage than those located in other areas. Secondly, we performed numerical simulations using debris flow simulators (KANAKO 2D and Hyper KANAKO) to determine the effect of houses on debris flow flooding and deposition. For the simulations, grid points of the locations of houses were set taking into consideration the height of the houses from the ground elevation. We simulated typical debris flow condition and real disaster cases. The simulation results showed that when houses are present, the spread of debris flow is wide in cross-direction upstream of the houses. Houses also affect the deposition area. The presence of houses increased flooding and deposition damage in some areas, whereas it reduced damage in others. When setting the houses, the areas between the houses were set lower than the houses located at the grid points. Such areas were designated as roads, and the results showed that the flow occurred along the roads, as in real disasters.
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