In this study, we evaluated the effect of spatial resolution of topography data on the accuracy of shallow landslide analysis using two different landslide models. First, we used a simple hydrological model combined with an infinite slope stability model to calculate the critical steady-state rainfall intensity required to cause slope failure. In the second model, we used a physical model that linked hydrological processes using a finite element method with an analysis of infinite slope stability. The two models were applied to steep forested hillslope in the Tanakami Mountains of central Japan. We used digital elevation models (DEMs) with different mesh sizes (1, 2, 5, and 10 m) and different locations of calculation points of the same mesh size DEMs for this analysis. When the mesh sizes differed, the two models calculated different areas of instability. The differences in the results were attributed to the difference in reappearance of topography and the number and location of calculation points. For the model of critical steady-state rainfall intensity, when the mesh sizes were small, a detailed topography of the slopes was available ; however, even for microdepressions on the hillslopes, the calculated contribution area was relatively large, resulting in an unstable slope area. With the combined model of infiltration and stability analysis, for the larger mesh sizes, the calculated value of the safety factor at each point affected a broad area because of the low density of calculation points in the watershed. In addition, with larger meshes (>5 m), the location of calculation points were important for the prediction of unstable areas. These differences led to under-and over-estimates of the unstable areas on the slopes.
Deep catastrophic landsides have triggered large-scale debris flows that have had serious impacts on humans. Therefore, it is important to predict the run-out process of debris flows and to identify debris flow hazard areas. Here we developed a new technique for simulation of large-scale stony debris flows. Debris flows have been modeled by mixture of solid and fluid phases. In this study, we assumed that fine sediment in debris flows behaves as fluid phase rather than solid phase. Based on this hypothesis, we developed new methods to evaluate key parameters to simulate largescale debris flows, such as sediment concentration, fluid density, and representative particle diameter and modified the continuity equation for sediment. We also proposed a new process-based method for determination of hydrographs at the lower end of the landslide scar. We conducted detailed field surveys of the past debris flow in Atsumari river in Minamata city and used topographic data from LiDAR imagery, porosity measurements of soil and weathered bedrock and the grain size distribution of the debris flow sediments to test our model. Using these new data and methods, we conducted numerical simulations of the past debris flow, which reproduced well the observed erosional and depositional pattern.
Earthquake acceleration is the main factor of slope failure triggered by an earthquake. This study investigated the relationship between the area ratio of slope failure and slope angle as well as geological properties in a 294 km2 rectangular compartment in a seismic zone off the coast of Kurosaki in Noto Kongo-kita, which was directly above the epicenter of the 2007 Noto-Hantou Earthquake. Because sufficient research has not been conducted to evaluate the area ratio of slope failure using the individual factors of soil strength, earthquake acceleration, and slope angle, we propose employing the slope unstability index (Skf), which is comprised of a slope angle factor, geological properties (cohesion and failure weight), and earthquake acceleration. GIS analysis revealed that geology determines the relation between slope angle and area ratio of slope failure (Sa) ; specifically, the soil strength of the slope is an important factor for slope failures triggered by an earthquake. Hence, the area ratio of slope failure is strongly governed by the slope unstability index (Skf).
Sediment supply from lateral sources influences on the distribution of sediment along a main stream. And the estimation of the longitudinal distribution is very important for sediment surveys at the catchment scale. A sediment pulse, composed of coarse sediment derived from lateral inputs, was observed during a storm in the Ribira Creek, Atsubetsu River catchment, Southern Hokkaido, Japan in 2003. The sediment volume supplied from tributary channels was measured using aerial photography and aerial laser scanner data. The deposited sediment volume was measured directly from digital elevation models. The initial sediment pulse along the Ribira creek was estimated by fitting a curve to the distribution of sediment storage volume with longitudinal distance. Auto-correlation and cross-correlation analysis was employed to examine the distribution of sediment mass observed along the 12 km channel course. The fitted curve had a spatial periodicity and the peak intervals were 1,500 m and 3,600 m, while the phase lags by distance against the supplied sediment distribution ranged between 100 - 300 m, 1,600 - 2,600 m, 3,600 - 4,200 m respectively. As a result, the distribution of sediment mass induced by the multiple lateral supplies, changes cyclically with distance and includes longer travel distance components than would result from a single lateral input.
Most shallow landslides occur from the middle and upper parts of slopes, although slope stability analyses for shallow landslides show landslides occur from the lower parts of slopes. Flume experiments were carried out on the movement of collapsed mass, preparing soil mass at the upper part of the flume. The movement was different by the water content of the soil mass. The soil mass with higher water content traveled longer. The dry soil was wholly transformed and the soil containing water moved keeping original shape in some cases as shown in many actual examples of shallow landslides. In order to develop simulation method applicable to the moving and transforming process of shallow landslides, which occurred from the middle and upper parts of slopes and moved incompletely fluidized, a distinct element method（DEM）was applied to the movement of the soil mass aiming at moving process of keeping original shape. Combined particles were introduced when applying DEM. Pore-springs were considered in addition to rebounding-springs. This is called an extended distinct element method（EDEM）. The method explained well the transforming process of the soil mass observed in the experiment.
Shinmoedake in Kirishima volcano erupted violently on January 26, 2011 and spewed a large amount of ash over the surrounding area, especially eastward and southeastward, in the following several days. From the viewpoint of a debrisflow hazard we examined thickness and grain size of ash deposits, infiltration properties of tephra-covered slopes, and sediment movement in the upper Takasaki River basin and the southern area of Takachihonomine after the eruption. Onsite tests for infiltration capacity of the slopes revealed a lower rate in the upper Takasaki River basin than that of the southern area of Takachihonomine, reflecting thicker deposits of fine-grained ash in the former basin than that of the latter area. This is probably the reason why sediment movement in the upper Takasaki River basin is more active than that of the southern area of Takachihonomine.