With the object of investigating influences of earthquake motion upon landslide, the observation of earthquakes and microtremors was enforced at the landslide area of Yui in the Sizuoka Prefecture. The ratio of spectra was gotten from the data of two sets of seismographs which had been placed so that a surface layer might be put between them, and under the ratio the density, rigidity and Q value of the surface layer were presumed by the aid of the multiple reflection theory. Next, by the aid of their constants and the multiple reflection theory, acceleration and shearing stress which occur in the surface layer were calculated when a certain seismic wave enters from the base layer to the surface layer, and the shearing stress was compared with shearing strength that was gotten by soill tests, Further, by introducing acceleration and shearing stress, which were gotten according to the above-stated technique, to the calculation for stability of some sections in the Yui landslide area, influences of seismic force upon safety factor were examined.
Debris flow occures and causes serious damages, losses of human lives in paticular somewhere in Japan almost every year. Wall type sabo dams or check dams have been adopted to prevent debris flow disasters. Though, their functions have not been made clear quantitatively. Some experiments were carried out to evaluate the functions of sabo dams abainst debris flow. Results are as follows; (1) A sabo dam reduces the peak sediment concentration, makes the time of peak discharge behind and flatten sediment discharge. These functions are expected more as the capacity of dam is larger. (2) Coarse grains concentrated at the front of debris flow can be trapPed by a sabo dam. (3) Even if a sabo dam has been filled with sediment, it is effective to some extent. (4) The gradient of deposition changes as tlme passes. Depositional surface once rises up to the gradient parallel to the channel gradient when a debris flow reaches there, then drops because the supplied sediment is limited and the sediment concentration decreases gradually. (5) If the gradient below a dam is steeper than 15 degrees, the water squeezed out from the debris flow deposited at the sabo dam could generate another debris flow below the dam.
The relation between the peak discharge and the propagation speed of floods were investigated in Takaragawa Honryu basin (1905.66 ha) by the records of the three water-stage recorders arranged from the upper stream to the lower stream and also in Shozawa basin (117.90ha) by the records of the two water-stage recorders similarly arranged in the upper stream and the lower stream. As the result, constant values a & b were obtained for each observation section of the water-stage gauging stations along the following equation: t=L/a×60 Qa-b where t (min) is propagation time of flood, L(m)is the distance between the two gauging stations, Qa (m3/s) is the average of the peak discharge in the two gauging stations. At the same time, it is confirmed that the propagation time of floods reduces as the peak discharge increases. Simultaneously, if we put the mutual relation between the average of the peak discharge Qa and the average propagation speed ωa as following equation, ωa=L/t=aQab where a takes the value 0.65-0.89 and b takes the value 0.35-0.40 through Takaragawa Honryu basin and Shozawa basin and make very similar relations. Synthetically, the relation between Qa and ωa through Takaragawa basins is judged as follows. ωa=0.73Qa0.37