This paper discusses the design load modeling of the debris flow which acts on the steel frame check dam structure from the view point of the redundancy of the design, and proposes a revised design load model in which an uncertain offset and concentration effect of debris flow action could be taken into account. The actual damaged case of the steel frame check dam is treated with a theme of the discussion. The damaged case is analyzed step by step under the several assumption of the load model and structural model, in order to clarify the ununiform load effect existed in the most probable load condition on the damaged steel frame structure. First, based on 2D analysis, the structure is safe enough against the conventional design load condition given by design manual. Second, based on 3D analysis, the structure is also evaluated to the almost equivalent safety by using uniformly distributed load following the conventional design manual concept. Third, enlarging the action area of the debris flow load till the top of the structure following the uniform load concept, the safety of the structure is evaluated as smaller than that conventional load model. But the evaluated failure mode is not similar with the actual damage. Fourth, based on a kind of random load simulation in which the debris flow load acts randomly in the space, the most probable load condition shifting the load concentration center to the right side of river. Finally, in order to evaluate such uncertain load condition at the design stage conveniently, a modified debris flow design load model is proposed. The proposed model shows preferable prediction of actual failure of the structure.

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We need to predict the timing, location, and volume of landslides for the better disaster mitigation and the better estimation of the bed deformation of mountainous rivers. However it is not so easy because a number of soil parameters, e.g., the angle of internal friction, cohesion, and porosity, are needed to predict landslide occurrence, and they have significant spatial variability. Therefore, it is important to make clear how the parameters contribute to overall slope stability and relate to each other. Few studies evaluated the contribution of each parameter to slope stability based on the slope stability analyses. However, they could not give us how landslides would be subject to the set of the parameters. In this study, we tried to understand it. We, therefore, normalized the safety factor equation for infinite slope, and introduced laminar soil layer structure into the equation. Then, we obtained new non-dimensional representation for the effects of underground water table and cohesion of soil. They are evaluated relatively with comparison to the stability of dry non-ohesive soil. And we also obtained that the effect of cohesion of soil should be evaluated from the comparison with the maximum effect of underground water table to the stability. From these explanations, we deduced the thickness of landslide, the immunity to the occurrence of landslide and the periodicity of landslide occurrence.

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Debris flow was induced by a deep-seated landslide on July, 2007 in the Funaishi River basin, Kagoshima Prefecture in Japan. We made field surveys at this site and estimated flow velocity and flow depth of the debris flow based on field observation, the stability analysis of steel cell of right wing section, marks of steel pipe generated by collision of gravel, and marks of superelevation of debris flow. It can be estimated that the flow depth was 3.3～5.0 m and the flow velocity is around 6～8 m/s in Sabo dam site and 2.1～14.3 m/s in flow down section. The flow depth equal to approximately half of slope failure depth. The peak discharge of debris flow was estimated at the Sabo dam site and flow down section.
We also compiled the flow depth and flow velocity by using the investigational data of debris flow induced by a deepseated landslide and observational data of the mountain stream where the debris flow occur frequently. It was found that the flow velocity and flow depth of debris flow induced by a deep-seated landslide is larger than that of observed debris flow induced by removal of unstable sediment on stream bed. However, flow velocity of debris flow in the Funaishi River basin was an exception. Peak discharge of debris flow induced by a deep-seated landslide is about from 10 to 100 times as large as that of peak discharge of debris flow induced by removal of unstable sediment on stream bed.

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In Japan, many of the recent check dam constructions have been of the open-type in consideration of the continuity of sediment routing from upstream to downstream reach in a river basin. Over the past 20 to 25 years, much experimental and numerical research has been conducted on sediment control using open-type check dams. Plans for size and location have been drawn up for grid-type check dams with heights of around 20 meters in river basins with huge sediment deposition volume in numerous torrents. Hydraulic flume tests are conducted using a straight channel to determine the grid size of grid-type check dam and to confirm the sediment control function. Subsequently, several runs of flume test are conducted to test three types of debris flow regimes such as rapid flows due to a natural landslide dam break, steady and uniform debris flows, and quasi-steady debris flows.
In the present study, the sediment control functions of the ‘grid-type high dam’, which is defined as a grid-type check dam with a height over 15 m, were examined using several experimental data sets, such as dimensionless sediment runoff volume, temporal changes of mean diameter and sediment concentration and sediment discharge rate passing through the check dam.

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Much of the experimental and numerical research on sediment control by open-type check dams tries to take into account the continuity of sediment routing from upstream to downstream reaches of the river basin. For example, the minimum grid size of the grid-type check dam, which is a type of open-type check dam, can be experimentally set to d95of sediment, which is based on the size distribution of bed material and flume data as determined for a straight, open channel.
In the case of a high check dam constructed in a curved ravine, the effects of curved flow and bar formation on sediment deposition in the storage area are supposed to be significant. A plan for constructing grid-type check dam with heights around 20 m can be newly proposed in a basin. In the main river with a reservoir in a basin at the downstream reach, previous heavy rainfall events may result in a huge amount of sediment deposited in the upstream reach through numerous torrents. Sediment transportation should be controlled to prevent huge amounts of sediment from moving to downstream reaches. Additionally, the main flow has many curved reaches. Therefore, sediment load passing through check dams in flash floods should be reduced as much as possible and, however, higher amounts of sediment should be transported in small sized floods.
Present study discusses preliminarily the effects of bed configuration on sediment control function in a check dam using experimental data obtained by hydraulic model tests such as sediment runoff rate from check dam, bars induced channel shifting, longitudinal and cross sectional bed profiles, temporal changes of mean diameter and sediment concentration passing through the check dam.

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A slush avalanche occurred on Mt. Fuji on March 25, 2007, causing damage including disruption of the Fuji Skyline and collapse of tourist facilities. Large-scale slush avalanches sometimes reach as far as downstream areas, causing flood damage. This kind of avalanche, called “Yukishiro” in this area, has been known since ancient times. In 1834, a severe slush avalanche occurred on Mt. Fuji, which was recorded in many historic documents.
Similar slush avalanche could be recurred on the mountain flank in the future. To prevent and mitigate damage, it is important to ascertain the actual conditions that occur during such disaster. Therefore, to investigate the development of the phenomenon, I analyzed historical records depicting the 1834 slush avalanche and also conducted field surveys at the damage sites.
It was found that the 1834 slush avalanche had been triggered by a severe collapse at the head stream area of the Yumisawa River. Diverging into the Kazamatsuri River, the avalanche flowed down to the stream area, engulfing sediment and trees that was flooding over the lava area along the way.

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Abandonment of forest plantations after clear-cutting has increased in the southern part of Kyushu, SW Japan. This abandonment of forests after clear-cutting hinders sustainable forest management and recovery of forest resources, and poses a periodic threat to soil and water resources. We investigated the current status of slope failures in and around a large-scale abandoned forest (95.6 ha) after clear-cutting, located near Kumamura Village, southern Kumamoto Prefecture. Dominant geology in the investigated area is alternating beds of sandstone and mudstone, dipping to the northwest, within the Shimanto Belt (an accretionary prism). Geologic characteristics affect the distribution of slope failures in this region. We divided the slope failures into three types : shallow road-cut failures on NW-N facing dip slopes along harvesting roads, slumps in road prism and fill, and deep-seated landslides on SE-S facing opposite dip slopes along the Butsuzo tectonic line (the boundary between the Chichibu and Shimanto terrains). Although slope failures along harvesting roads are affected by geologic characteristics, they are primarily related to inadequate road construction. In contrast, deep-seated landslides along the Butsuzo tectonic line are predominantly influenced by geologic structure in this region, independent of logging and road construction. This study provides a rough guide for planning the design and construction standards of forestry related roads and associated drainage systems to minimize landslide hazards.

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