The Paleogene Kobe Group is composed of sedimentary rocks formed in reticulate-river and lacustrine environments, and often contains tuff. Many landslides are recognized on the gentle slopes of hills underlain by the Kobe Group in the western part of Sanda City, Hyogo Prefecture, and their slip surfaces are mostly tuffs. The landslide treaded in this study has different characteristics from other slides, namely, having a landslide width with twice longer than the length, a well-developed separating scarp, and linear depressions on the landslide blocks. Borehole investigations revealed that the bedding planes are inclined 5 to 6° to the river direction, and tuffaceous mudstone in the lowermost horizon of the slide blocks was fractured by the landslide movement. The tuffaceous mudstone shows deformation structures by both brittle and plastic deformations by the shearing. The thickness of the tuffaceous mudstone layer is thinner in the landslide body, and the block of the sliding mass are settled in the tuffaceous mudstone. These features correspond to those of spread-type landslide in the classification of landslides by Cruden and Varnes (1996) and others. It is considered that the strong downward erosion of the river along the foot of the landslide body is responsible for the occurrence of the spread.

  We proposed a model to estimate slope storage depth more accurately, by faithfully mirroring the water circulation process, in order to use for index in predicting sediment-related disasters. The accuracy of the analysis model was then verified for the gently undulating hillslopes in snowy areas. As a result, it is found that the slope storage depth can be calculated accurately and that the calculation accuracy of the slope storage depth in summer can be heighten by considering the evapotranspiration process. It is furthermore suggested that the evapotranspiration process has a great influence on sediment-related disasters in summer, since it acts as a resistance factor to sediment-related disasters.