It was tried in th1S paper to settle the problems what would do the role of a trigger to the occurence of a mud flow in a devastated stream. For this reason, the following supposition was induced;that is to say, slide will occur in a sand layer of a channel when a ground water gets to acritical depth in which a shearing force is larger than a registance force. Forms of longitudinal and cross sections of outcropped rocks in some stream beds at Sakurajima Island were classified into the following four types;the uniformity type, the terrace type, the narrow and upheaval type, and the narrow and V shape type, .as to the sectionof the surface of the impermeable layer in a stream bed. With such models of these impermeable layers, differences among the formes of these four types were investigated as the differences of the incipient motion times of sand for flow. As the result, it was showed that the incipient motion time of sand for flow had a tendency to get short like as the narrow and V shape type>the narrow and upheaval type>the terrace type>the uniformity type. Secondly, it was showed that the increasing rate of stress, occurring in the sand layer, to the time and that of pore pressure would be of the same character, and the ratio of Δp/Δt of the terrace type, the narrow and upheaval type, the narrow and V shape type to that of the uniformity type were generally larger than unity, there was a tendency that the increase of the pore pressure in the terrace type, the narrow and upheaval type, the narrow and V shape type would be more rapid than that of the uniformity type.
In this study, the author considers on the depth of scour caused by flow downstream of an overfall from Sabo-dam. It is confirmed by experiment of scour that the jet by overfalling nappe impinges slant on the bottom boundary of scour and the successive flow produces the boundary-layer along the bottom boundary. The maximum velocity outside of the boundary-layer, u0max, is defined to be a power function of the ratio (D/η), in which D is the thickness of the jet, η is the distance of the mixing zone. The coefficient of its exponent, p, is obtained as p=3/4 by the experlment. On the other hand, non-dimentional function defining the terminal depth of scour is derived by using the Karman integral relation for the boundary-layer. α and β used as the powers in this function are expressed as α=2p-1 and β=1/(2p+γ). The special device of jet flow is made for purpose of this experiment of scour. From the experimental results of scour with this device, the value of α and β are obtained as α=1/2, β=1/2 for D/d≥1, in which d is diameter of grain. Substituting the value of α and β into the foregoing equations yeilds further p=3/4, γ=1/2. This value of p agrees with the value of previous power (p) on the u0max, and the value of γ also satisfies approximately the assumption used in the theory of boundary-layer.
The purpose of this study is to forecast the area of landslide occured by heavy storm and the author consider the mechanism of landslide as follow. Phenomena raised by the stored water infiltrating through rainfall such as the increase of soil weight, decline of soil strength and porepressure are some kind of shocks acting sloped. When many various shocks are accumulated in the solpe and mechanical change reaches up to the critical stage, landslide will break out. If the shocks happen inn proportion with precipitation, at random in watershed composed with the unit slopes that are independent each other, the state of shock occuring in the unit slope would be approximated by the Poisson Process in the probability theory. Consequently, the probability F that an unit slope may slide under total precipitation r, is equal that shock number X(r)=j reaches upto the critical value C, and given by the next gamma distribution model. F(X≥C, r)=1-∑C-1j=0(λr)f/j!e-λr where λ is the shock rate 0.01(mm-1) and C is the resistance index characteristic to region. Such mechanism acts in every slope, therefore this equation means the relation between total precipitation r, resistance index C and the theoretical landslide area ratio F of watershed. This relation was confirmed in results of surveying landslide area by using aero-photographs which scale ratio is 1:10, 000 at Fujiaka, Niyodo, Minami Izu and Nozoki region suffered from heavy storm recently. In the case of forecasting landslide area, next equation based upon the idea that many slopes are adapted to the regional peculiar storm in each watershed, is useful to estimate the resistance index C, C=0.0433J where J is the mean monthly precipitation in July.