It is widely recognized that the effects of a phase shift of fine sediment in large-scale debris flows are likely to be large. Therefore, in numerical simulations, it is essential to describe fine sediments in the fluid phase, and not in the solid phase. Recently, the "Kanako" numerical simulator has been widely used for a variety of objectives, particularly because it has a graphical user interface. However, to date, there is no widely available numerical simulation model for large-scale debris flows that includes the effects of phase shifts. Here, we present a modified version of Kanako to describe this phase shift for fine sediment. In the new numerical simulator, which we refer to as Kanako-LS, we assume that sediments can be classified into two groups in terms of sediment diameter (fine and coarse), and define the critical diameter of the sediment (Dc) as the smallest diameter at which sediments behave as a solid phase. Then, we test the applicability of Kanako-LS using an example of debris flows triggered by a deep-seated rapid (catastrophic) landslide in Japan. Our results suggest that Kanako-LS may be useful for a variety of types of large-scale debris flow, particularly if the amount of fine sediment and the magnitude of the interstitial fluid turbulence are sufficient.

Mitigation works are very essential for mitigation of debris-flow hazards in mountainous areas. Usually, it is difficult to assess the effectiveness of existing mitigation works in a catchment. This paper presented a method for quantitative assessment of debris flow mitigation measures by using Kanako system, a user-friendly GUI-equipped debris flow simulator that allows good visualization and easy interpretation. Kanako 2D (Ver. 2.04) was applied to a case study at Caijia Gully, Sichuan Province, China. Mitigation works including check dams, drainage channel, and deposition basin were constructed in the gully in 2001 and 2006. Kanako 2D can simulate debris flow from steep area to alluvial fan. 1D simulation was applied for assessing the effect of the check dams at the lower part of the gully, and 2D simulation was applied for the effect of the drainage channel and deposition basin on the alluvial fan. The simulation results indicate that debris flow will cause great damage to residential area on the alluvial fan if mitigation measures were not implemented in the gully. For old dams which have been filled up with deposits of previous debris flows, the results show that they still have the function for controlling debris flow due to the gradient reduction of the channel bed in front of the dams by the trapped debris flow deposition. After the comprehensive control of debris flow including trapping, drainage, and deposition in the gully, the simulation results indicate that the risk on the alluvial fan can be reduced to an acceptable level.

Debris flows often cause substantial losses of human life as well as economic losses. Damage can be estimated using numerical simulation models that describe the debris flow process. Some models can be used to determine the possible effects of sabo dams and have been practically employed to plan sabo dam arrangement. However, the existing simulation systems currently do not have efficient user interfaces, making it difficult for non-experts in debris flow simulations to run simulations without the aid of specialists. We developed a system that produces one-and two-dimensional debris flow simulations and is equipped with a graphical user interface. The system is based on an integration model and employs one-dimensional simulations of gully areas and two-dimensional simulations of alluvial fan areas; it then considers mutual influences in boundary areas between gullies and alluvial fans. Data can be input using a mouse and viewed on the monitor, and users can see real-time visualized images of a debris flow during a simulation. The interface enables users to run a debris-flow simulation without expert knowledge of the model, enabling better solutions for sabo engineering.

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Debris flows often cause substantial losses to human life and the economy. Damage can be effectively reduced using numerical simulation models, which can describe the debris flow process and determine possible effects of sabo dams, or erosion and sediment control dams. However, non-experts find it very difficult to run simulations independently, because the systems do not currently have an efficient user interface. We developed a system that produces one-and two-dimensional debris flow simulations and is equipped with a graphical user interface (GUI). The system is based on an integration model and employs onedimensional simulations for gully areas and two-dimensional simulations for alluvial fan areas, and then considers their mutual influence in boundary areas between gullies and alluvial fans. The system was developed with "MS Visual Basic.NET." Data can be input using a mouse and be checked on the monitor, users can see real-time visualized images of the debris flow during the simulation. The interface enables non-expert users to run the debris-flow simulation independently, enabling better solutions for sabo engineering.

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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Debris flows form deposits when they reach an alluvial fan until they eventually stop. However, houses located in the alluvial fan might affect the debris flow flooding and deposition processes. Few previous studies have considered the effects of houses on debris flow flooding and deposition. This study conducted model experiments and numerical simulations using the Kanako2D debris flow simulator to determine the influence of houses on debris flow flooding and deposition. The model experiments showed that when houses are present, the debris flow spreads widely in the cross direction immediately upstream of the houses, especially when the flow discharge is large or the grain size is small. Houses located in the alluvial fan also influence the deposition area. The presence of houses led to flooding and deposition damage in some places and reduced the damage in others. The simulation also demonstrated the influence of houses. Both the model experiment and the simulation showed that houses change the flooding and deposition areas.