Numerous studies have analyzed the relationship between seismic motions and slope stability. However, few dynamic analyses have been performed to study how differences of frequency characteristics of seismic waves affect the stability of natural slopes. In this study, we analyzed the effects of frequency by focusing on the two mechanisms : (1) the dependency of dynamic responses of natural slopes on the correspondence between the frequency range which intensify the dynamic response and the frequency characteristics of seismic waves ; (2) the dependency of indicative of slope stability on the amplitudes and frequency of responses (acceleration : A and velocity : V ). The dynamic analyses with sinusoidal wave, revealed that V correlates well with δ_r for the same A but different frequency. Furthermore, dynamic analyses with measured seismic wave suggest that if the spectrum is strong in the low frequency range, δ_r and shear stresses are greater than in the case with spectrum with strong component in the high-frequency range. This mechanism is consistent with the previous research which indicates that acceleration response spectrums, in the cases of high number of slope failures, show stronger components in the frequency range below 1.0 Hz than those of waves in the cases of low number of failures. Hence, the seismic wave with its strong component in the low frequency range (above 0.5 Hz) tend to induce large δ_r and shear stresses, then more likely to destabilize slopes, i.e., a higher risk of sediment disaster. Finally, we propose that frequency characteristics of seismic waves is one of the key factors in evaluating the hazard of slope destabilization.

Regarding the runoff analysis for sediment and flood damages (sediment laden floods), a method of identifying the parameters of the storage function method with the relationship between the runoff height (q) and the rate of decrease of runoff height (-dq/dt) was investigated. In this study, we assumed that when the rainfall amount is larger than a certain level, the entire basin becomes a contributing area for runoff, and the falling limb of the hydrograph after the end of rainfall follows a site-specific time-discharge relationship that is independent of the rainfall pattern. Moreover, we assumed that even if the rainfall amount is smaller than the certain level, the falling limb of the hydrograph can be described by the site-specific time-discharge relationship and the ratio of contributing area to the whole catchment area. The purpose of this study is to find out whether the relationship between q during recession and -dq/dt will be site-specific if the role of contributing area could be considered even in a basin with a scale of several tens of km², and to clarify whether it is possible to estimate the discharge during large-scale floods using this site-specific q～-dq/dt relationship. We analyzed observed discharge data in Uchikawa, Miyagi, which has an area of 87 km². We found once we tuned the contributing area ratio, there are single q～-dq/dt relationship for eight falling limb of hydrograph, supporting to our hypothesis. Moreover, we confirmed that the discharge during large-scale floods can be predicted using the parameter determined based on the q～-dq/dt relationship. In this analysis, it became clear that since the data in low flow periods are much larger in that of high flow, the parameters were easily influenced by low flow date. Moreover, the date in low flow showed large variation. To solve these problems, we proposed a method to extract data used for identification according to constant rate of decrease in runoff height. Compared to the conventional q-S graph identification, this method can solve the problem how to determine the initial value of the storage height, and can determine parameters directly from the observed flood level. The relationship between the spatial distribution of rainfall and the contribution area ratio was discussed.

For this study, we investigated slope stability and tree shape in two study plots of Japanese cedar plantation forest in an area with numerous collapses, in order to clarify the relationship between tree shape type and slope stability. To assess the tree shape objectively, we used 3D point cloud data obtained using ground laser scanning. Slope stability was assessed using the safety factor formula. Results show that, on a plot where the overall safety factor is low, trees with leaning stems can be found in the low safety factor area. Particularly, trees with bent stems were concentrated near the collapse. On another plot, which is a slope that includes the valley head, even though the overall safety factor is high, some leaning stem trees exist in areas where the valley head and numerous collapses occur. These findings suggest that slope stability characteristics can be inferred from differences in the distributions of standing trees classified by tree shape.

Various applications of bedload-surrogate observations, e.g., Japanese pipe hydrophones, have been proposed, including their use to verify calculations of riverbed deformation and active sediment transport after catastrophic sediment production events or during high probability flood events. Notably, bedload-surrogate data are expected to be used soon to verify sabo (erosion control) master plan. For this purpose, cross-sectional averages of bedload flux must be obtained ; however, no established technology for the conversion of information obtained from a small part of a cross section to the whole cross section is available. In this study, we attempted to estimate the cross-sectional averaged bedload flux based on the transverse distribution of shear stress obtained through the two-dimensional open-channel flow simulation. We demonstrate that the results of such analysis can be used to obtain cross-sectional averages of bedload flux.