After volcanic eruptions, mudflows are reportedly triggered even by small rainfalls, which have never caused mudflows before the eruptions. This study investigated the influence of volcanic ash deposits on the mudflow occurrence by the saturated hydraulic conductivity test, the water sprinkling test and the viscosity test. Laboratory experiments were carried out on volcanic ash deposits collected from Miyakejima, Mt. Usu and Sakurajima. Measured saturated hydraulic conductivity values were 0.61, 20.5 and 172.4mm /h for the ash deposits in Miyakejima, Mt. Usu and Sakurajima, respectively. Results of the water sprinkling test showed that the infiltration rate of the volcanic ash deposits in Miyakejima and Mt. Usu immediately decreased to the final infiltration rate of about 1 and 41mm h, respectively. On the other hand, the infiltration rate of the ash deposit in Sakurajima did not decrease below 160mm/ h. The viscosity test revealed that the ash deposit in Mt. Usu had much higher yield-viscosity and kinetic-viscosity values than the ash deposits in Miyakejima and Sakurajima. Based on the measured physical properties of the ash deposits, we inferred the possibility of the mudflow in each region. A larger number of mudflows are expected at Miyakejima because of the low permeability and the small viscosity of the ash deposit. At Mt. Usu, occurrence of the surface runoff is expected under heavy storm events because of the relatively low permeability. However, the mudflow may seldom occur since the viscosity of the ash deposit is high. In stead of the relatively high permeability of the ash deposits in Sakurajima, the small viscosity of the ash deposit can trigger the mudflow. It was found that these presumptions corresponded well with the reported mudflow occurrences in Miyakejima, Mt. Usu and Sakurajima.

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A large scale landslide occurred in the Chu-fen-erh-shan area at about the same time as the 921 Chi-Chi earthquake, 1999. The landslide had a width of about l km, length of about 2km, and volume of about 35 million m³. The failed slope was made of a dip slope of shale bed 22-27 degrees forword southeast. The laying shale bed was of clay 3- to 4- cm thick on the slip surface consisted of calcite educed on the clay surface. As the slope profile before the landslide showed a linear concave surface, this profile may be formed by the previous landslide. The previous landslide may also have been caused by an earthquake since the whole site of the landslide showed a convex profile.

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The aim of this study is to clearly identify the characteristics of suspended sediment load and bed load occurrences in relation to the changes of catchment scales, water discharge and rainfall characteristics in the source area, which plays an important role to plan management strategies in mountainous catchments. Three catchments (i.e. Catchments A, B, and C) with different catchment areas were chosen to investigate the volumetric changes in suspended sediment load and bed load. The result shows that suspended sediment load is significantly related with water discharge rather than rainfall. On the other hand, it is proven that bed load is more closely related with rainfall rather than water discharge. Moreover, in the power function regression (Y_s=a·Q^b) of the relationship between cumulative water discharge and suspended sediment load, the coefficient a and the exponent b show the inversely proportional relation. Especially, decreasing rate of b against a in the catchments smaller than 10 km² in area is higher than that in the catchments larger than 10km². From this result, it is considered that volumetric changes in suspended sediment load become higher with decreasing catchment area in mountainous catchments. In addition, suspended sediment load is account for 40-55 per cent of total load. It means even in mountainous catchments, a large part of total load consists of suspended sediment load.

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Run off process in watershed has to be discussed in both of water and sediment to manage and preserve the watershed land and environment. We proposed a watershed runoff model using triangle elements in this paper. The watershed is covered by triangles which form a point-polygon system. The points, a set of three points, compose a triangle, and the gravity centers of triangles around a point compose a polygon. The topography and strata structure are explained by the elevations of ground surface and boundary of each stratum at each point. The depth of the flow is defined at each point, and the flux is defined at the gravity center of each triangle, which is a vertex of a polygon. Based on the arrangement of variables, two-dimensional runoff analysis can be done using kinematic wave theory and also calculate sediment yield and the runoff process. We also demonstrate an example of calculation in this paper.

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Numerical simulation and hydraulic model test are conducted to estimate and evaluate the phenomena of debris flows such as inundation process and the effect of countermeasure facilities. These two methods are based on different principle. Therefore we wonder that they may not give same results. This leads us to consider the similarity of numerical simulation results and Froude similarity of debris flow. We discuss the similarity of debris flow based on a constitutive equation for a debris flow simulation, and examine Froude similarity of typical examples of numerical simulation results in this paper. We found, as results, 1) The Reynolds similarity of the debris flow is explained as same as Froude similarity. 2) When we scale the finite differential distance so as to match the model scale and give the finite differential time according to Froude similarity, Courant Number in a calculation at each step is the same as one in another scale calculation at the same step.

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On devastated hills in the Tanakami Mountains of Shiga Prefecture, rill formation is considered as an important factor that controls the sediment discharge (Suzuki and Fukushima, 1989). Despite of the importance, the processes of rill system development and runoff generation during the heaviest rainfall have not been clearly evaluated on devastated hills in the Tanakami Mountains, mainly due to the infrequency of rainstorm that produces remarkable water and sediment discharges. Because of the difficulty in the observation during the heaviest rainfall, we carried out the slopescale artificial rainfall experiments on a bare slope in the Tanakami Mountains. The temporal variations of rainfall, runoff, sediment discharge, and pore-water pressure were measured, and the processes of a rill system development were monitored by video tape recording during the artificial rainfalls. The artificial rainfall experiments confirmed that : (1) a highly permeable soil surface contributed to a large amount of rainfall infiltration, leading to the generation of subsurface flow, (2) a shallow soil depth allowed the soil layer to readily become a saturation condition, and a saturation-excess overland flow was partially generated on the slope with a shallow soil layer, (3) sediment flushing due to the concentrations of a subsurface flow and a saturation-excess flow occurred at the bottom of the valley, causing a rill initiation, (4) mass failures along the rill sidewalls and headwalls allowed the rill channel widened and extended upstream, resulting in the generation of small-scale debris flows. As a consequence of the generation of small-scale debris flows following the rill system development ; (5) the sediment concentration at the outlet of the slope was increased by 1 to 3 orders of magnitude, even though the intensities of rainfall and runoff were almost steady. These results suggest that individual soil movement driven by raindrop impacts and Hortonian overland flow is relatively minor, and a massive soil movement due to a subsurface flow and a saturation-excess overland flow is the dominant processes of sediment transport on a bare slope in the Tanakami Mountains.

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The 15th typhoon named Rusa hit Korea during in August 31 to September 1. It caused serious damages particularly in the eastern region where the heavy rainfall concentrated, triggered debris flows, slope collapses and flood water inundations in major rivers. Many surface failures and debris flows occurred especially on the burned slope by forest fire, in addition, tremendous volume of sediment flowed into rivers and caused dangerous aggradations. Accordingly, it is imperative to control the discharged sediment by installing sabo dams.

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