Debris flow experiments under various riverbed roughness conditions confirmed that debris flows are affected by riverbed roughness. To investigate the influence of riverbed roughness on debris flows, the coefficient f′ was defined by modifying the coefficient of resistance, f, which is the ratio of the energy gradient to the friction loss. The value of f′ calculated using constitutive debris flow equations is constant for the same sediment concentration, c. Experimental results indicate that f′ increases as riverbed roughness increases with larger riverbed particles : f′ increases when c is dense or the relative flow depth, h/ks, is small. The latter is determined by dividing the flow depth, h, by the roughness height, ks. That is, the flow resistance appears to be stronger than estimated by existing constitutive equations. This suggests that existing constitutive equations do not simulate debris flow in mountain rivers well, because the beds of such rivers are covered with large particles.

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Flow behavior of debris flows in the upper stream on mountainous debris torrent is described. Data are presented from a measurement and monitoring study at the sites in Ichinosawa upper stream of the Ohya-Kuzure, the initiation area of debris flow, Shizuoka prefecture, between 1997 and 2002. Debris flow was monitored using video cameras with the specification of interval shooting, and the surface level of debris flows measured using a sonic sensor, the water level using the capacitive water depth probe, the water pressure using semiconductor pressure sensor. As the result of observation, debris flows in the study area can be classified into two types. One consists mainly of cobbles and boulders of 20cm and larger, and the mud water do not appear in the matrix of the debris flow surface. The other consists of muddy turbulent water with a little coarse debris, sometimes including few cobbles or boulders. The upper part of the former debris flow is under unsaturated conditions. The appearance of such unsaturated part is supported by the analysis based on the past studies. The latter debris flow is saturated by muddy water as a whole. In addition, both erosion and deposition are observed alternately during the event of a debris flow on the steep torrent, having 28 to 30 degrees in gradient.

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An earthquake having 7.0 Magnitude (M) occurred at 18:24 on June 26 2003 in the sea offshore Miyagi. The earthquake caused damages in huge area of Tohoku district to houses, public facilities, farmland etc, and triggered many slope failures in Sanriku and Kitakami district. Particularly there were much damage in Iwate and Miyagi prefecture. However, the magnitude of earthquake is large, the damages are comparatively small. This may be considered that the period characteristic of this earthquake was relatively short. And as a thing peculiar to sedimentrelated disaster, a slope failure due to liquefaction occurred in the gentle slope of Tsukidate town. This area was artificial filled land developed for farmland, and the failure occurred at the filling land in a valley. An incline of the failed land was gentle, in less than 10 degrees. We suppose that liquefaction-phenomenon occurred by this earthquake, so the slope failure occurred in the gentle hills. This paper summarized the investigations related to the property to the earthquake damages, focused on slope failures, rock falls, cracks.

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