Kana Nakatani

Effect assessment of debris flow mitigation works based on numerical simulation by using Kanako 2D

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.

Hazard mitigation planning for debris flow based on numerical simulation using Kanako simulator

Debris flow often causes enormous loss to life and property, especially on alluvial fans. Engineering structures such as retention check dams are essential to reduce the damage. In hazard mitigation evaluation and planning it is of significance to determine the location, size and type of dam and the effects of damage mitigation. We present a numerical simulation method using Kanako simulator for hazard mitigation planning of debris flow disaster in Tanjutani Gully, Kyoto City, Japan. The simulations were carried out for three situations: 1) the simulations of erosion, deposition, hydrograph changing and inundation when there were no mitigation measures; 2) the simulations of check dams in four locations (470 m, 810 m, 1,210 m and 1,610 m from the upstream end) to identify the best location; 3) the simulations of check dams of three types (closed, slit and grid) to analyze their effects on sediment trapping and discharge reduction. Based on the simulations, it was concluded that two closed check dams (located at 470 m and 1,610 m from the upstream end) in the channel and a drainage channel on the alluvial fan can reduce the risk on the alluvial fan to an acceptable level.

Influence of housing and urban development on debris flow flooding and deposition

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.

Case study of debris flow disaster scenario caused by torrential rain on Kiyomizu-dera, Kyoto, Japan - using Hyper KANAKO system

This paper presents debris-flow numerical simulations using the Hyper KANAKO system, developed by the authors. The system uses the debris flow simulator KANAKO 2D equipped with a graphical user interface (GUI); hence, a user can easily produce appropriate landform data for simulations using standard laser profiler data, and visualize the results using a GIS. Hyper KANAKO was applied to the streams around Kiyomizu-dera in Kyoto, Japan. Kiyomizu-dera is a famous temple in Japan which is visited by numerous tourists throughout the year. We simulated a disaster scenario of debris flow caused by torrential rain. We set the hydrograph using rainfall intensity data, and set the landform data using information from the Geospatial Information Authority of Japan (GSI) and a digital elevation model (DEM). We evaluated different mesh sizes and also used a digital surface model (DSM) to consider the building heights. The simulation results showed that with small mesh size, the debris flow moved through the roads, which seems realistic for a disaster situation. When buildings were considered, the flow direction changed, and a 1-m flow depth, which was deeper than in other cases, appeared in the flow path. This may pose a dangerous situation for evacuations.