This paper describes the mechanism of increase of water depth caused by accumulation of drifting woods at the strcade, as the third stage of a series of study on the check. As a result of theoretical and experimental research, it makes clear the following points; (a) the phenomena of diving woods at the initial stage of accumulation process is the direct cause of increase of water. (b) maximum values of backwater depth are 1-2.4 times as much as the velocity head at no accumulation. (c) the experimental result coincides approximately the calculated curve by the suggested model on backwater profile

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If a big earthquake like kanto Earthquake should hit a mountainous area, it will cause many landslides at a time. It is necessary that the amount of debris from them are ahead to estimate for erosion control. The purpose of this paper is to forecast the number and area of landslides caused by a big earthquake. The method how to estimate is as follows:
1. The four models of big Earthquake are presumed; their epicenters are at the mouth of Fuji River and Suruga Bay on Suruga Trough and each magnitude 8.0, 8.5.
2. The minimum and the maximum distance from epicenter to the equallines are measured on the distribution maps of the seismic intensity of the earthquakes that hit Shizuoka-the Chubu district. These data yield the relation between magnitude, seismic intensity and distances from epicenter. The relation is shown graphcally by the diagram, from which the distances at magnitude 8.0 and 8.5 are read.
3. The circles are drawn wirh the radius of these distances at the presumed epicenters. The mountainous area above 200 meter high where the seismic intensity is stronger than 4 is measured on the map by the auto area calculation system composed of the digigramer and electronic conputer.
4. The landslide ratio p% and density ρ per 1km² are expressed by the next empirical equations that data are quoted from the literatures,
p=7.03×10^-3exp^1.01M and ρ=1.86×10^-9α^3.5
where M is magnitude α the accelaration equivalent to the seismic intensity.
5. The total numbers and area of landslides are given by the following formulas,
N=ρ∑⁷_i=5S_i and A=∑⁷_i=5p_iS_i
where p_i and S_i are landslide ratio and the area of seismic intensity i stated above 4 and 3 respectively.
As the landslides by earthquake are apt to occure at some equilibrium slopes or ridges which weathered soil is shallow, it is consider that the depth of landslides would be about 1 meter. After all, the amount of debris from landslides by big Earthquake is estimated at 3×10⁷-2×10⁹m³ and this is 6×10³-4×10⁴m³/km².

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The state of equilibrium of scour at the stream bottom may be produced when the frictional force of sand grain and the shearing force on the scoured bottom which is given by the boundary-layer at the bottom boundary keep balance. The boundary-layer is caused by the tractive force which the velocity in potential flow (the velocity outside the boundary-layer) produces in the neighbourhood of the bottom boundary in the scoured hole. On the other hand, the jet by overfall nappe impinges slant on the bottom boundary of scour and flow forwards as a two-dimensional wall jet. Therefore, the calculation of velocity in potential flow in the neighbourhood of the scoured bottom as a wall jet may be important to discuss the depth of scour by the overfall nappe at the downstream side of Sabo-dam. In this study, the stream line of the impinging jet flow which is produced by slanting submerget jet is estimated by using stream function, and it is shown as curved lines. The equation of velocity in potential flow in the neighbourhood of scoured bottom is derived by using the potential theory. This equation is finally written as:
u₀/v_0b=Aξ/(ξ²+Bξ+C)
where, u₀ is the velocity of potential flow in the neighbourhood of the bottom, v_0b(=u_{0 max}) in the hypothetical impinging velocity of the submerged jet flow on the bottom, and ξ is the nondimensional length along the bottom boundary. Two kinds of models of bottom boundary, one of which is plain surface and the other is curved surface with the shape of scoured bottom, are prepared in this experiment. To those surfaces, grains of sand are glued as closely to each other as possible. The jet flow in this experiment is given in two-dimensional flow by the aerated free falling nappe from weir into the pool. The results of the experiments by the method described above show that the plots of measured values agree with the theoretical curve expressed by the above equation. The coefficients of the curve which are calculated from the measured data are discussed connecting with the hydraulic condition of flow.

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