The mechanism of landslide movement and the two-dimensional numerical simulation method are discussed based on a continuous model of solid-liquid phase flow proposed by Egashira et al.
Egashira et al obtained a constitutive equation based on the considerations of kinematic energy balance in the shear flow of solid-liquid mixture of debris flow, and found out that the quasi-static inter-granular friction stress dominates the flow characteristics. In case of debris flow on the movable bed, the quasi-static inter-granular friction stress should be balanced with the external shear stress at the bed, and the concentration of solid particles depends on only the gradient of the slope. This leads that the termination of the debris flow appears as the sedimentation of solid particles.
In case of movement of landslide body, the concentration of solid particles cannot change. Therefore the quasi-static inter-granular friction stress should not be in balance with the external shear stress at the bed. This causes the stop of the landslide body as the termination of the movement. And as no erosion and sedimentation of solid particles occurs because of no change in the concentration, the movement of landslide body can be treated as the movement of solid-liquid mixture of constant solid concentration on the fixed bed.
The quasi-static inter-granular friction stress changes discontinuously, in the direction and the magnitude of it, before and after the stop of movement. Since the stop appear within the finite differential time, it is important to find the time of the stop of the movement and to change the evaluation of the stress. The stop condition and the different evaluation of the quasi-static friction stress by the state of landslide mass, in moving or stopping, are introduced in the two-dimensional numerical simulation program. It is pointed out as the result that the numerical simulation method can explain the characteristics of the landslide movement well.

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Soil pipes are known to have significant impacts on the effective hydraulic conductivity and the storm runoff generation in slopes. Thus, pipeflow must strongly contribute to sediment movement on slopes and subsequent shallow landslide initiation. However, the effects of pipeflow on shallow landslide initiation have not been understood sufficiently because of the limited information about the mechanism of runoff and erosion by pipeflow. Hydrological observations and investigations of soil pipe morphology were conducted in the Toinotani, Kyoto University Forest in Ashiu. Based on the results of field measurements, the pipeflow agent, which affected the changes in safety factor on slopes, was evaluated by numerical calculation combined with the well-drainage theory. The calculation showed that once a collapse of the pipe walls causes clogging of a soil pipe, the value of the safety factors of the slopes with lower hydraulic conductivity soils promptly decreased with reaching below 1.0 within 200 seconds after the clogging of the soil pipe. Here we show that the clogging of a pipe outlet can play an important role in landslides initiation, which occurred at the time of the highest rainfall intensity.

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Governing equations for debris flow are composed of mass conservation equations for the flow body and bed sediment, and equation of motion. The erosion rate equation is usually added to close the governing equation system. The shear stress formula in equation of motion is derived based on uniform flow condition and has two terms; Coulomb shear stress and dynamic shear stress. In this equation set, sediment concentration of the flow is determined so that Coulomb shear stress accords with external force at the theoretical bed. Consequently, Coulomb shear stress, dynamic shear stress and sediment concentration correlate with each other. However, numerical, unreasonable disturbances take place because of the dependency of dynamic shear stress on Coulomb shear stress, when we apply this equation set to strongly unsteady debris flows. In order to solve these problems, we derived a new dynamic shear stress equation which doesn't depending on Coulomb shear stress. Numerical simulations of debris flow for erosion and deposition processes were performed. The results show that the behavior of the debris flow can be clearly explained.

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Miyakejima Volcano has resumed its eruptive activities since June 26, 2000. The series of eruptions thickly deposited volcanic ash on slope of the volcano. As a result, sediment began to discharge seriously from almost all torrents of the volcano. Previous studies on sediment discharge from newly erupted volcanoes have shown that condition of slopes changes significantly in the first year after eruptions. To know the change in the characteristics of the ash deposits that would drastically alter the characteristics of sediment discharge, the authors did field investigations in Aug. 2000 and Aug, 2001. As a result of the investigations, no significant change was found in grain size distribution, bulk density, and final infiltration capacity. On the other hand, topographic surveys showed gullies increased rapidly and ratio of the area occupied by the gullies have increased to about 10%. The gully formation results in exposure of permeable under-layer. It is considered to be possible that sediment discharge from the volcanically disturbed watersheds decreases without increase in the infiltration capacity of the volcanic-ash deposit in case that the original surface exposed by the gully formation is permeable enough to exhaust water discharged from the ash-covered source area.

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The disaster of the debris flows and flood due to the localy heavy rainfall of typhoon 16_th occurred in the area of the Shigenobu River basin, Ehime Pref. on Sept. 15th, 1999. Especially, the Omotegawa River, the left branch of the Shigenobu River, was badly damaged by the hillside failures and the debris flows. However, the Sabo facilities effectively minimized the disaster.
Here, we discussed the effects of the Sabo facilities against the disaster, using the numerical calculation.

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To make a contribution to the sustainable hillside greening, the multi types of public works should be arranged for the slope stability with ecosystem and amenity creation. From the point of view on the environmental effects, a new ideology of the environmental greening was suggested by the field studies, and it was pointed that the primary planting of the native nursery stock was the best method supported by a safety growth system. Consequently, the three blocks system composed of the foot block, planting block and water supply block was developed for a new technology. In this system, the planting block had a enough space for the nursery stock to grow up without tending because the water supply was carried out by the permeable rope using the surface tension, and this could support the nursery stocks to be alive even in a hot summer season. Furthermore, the fundamental structure of this system was the unit type and easily to be piled up, namely this could be constructed at any sites such as the slopes and riversides to re-create the ecosystem. Therefore, this block system can achieve the new reasonable greening works with many benefits.

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