There is a general tendency for many shallow landslides caused by heavy rainfall to occur at less than 1. 5 meters on forest slopes. Because of this, it is thought that the initial confining stress before the event happens is the range of 0. 2 to 0. 25kgf/cm2. The stability analysis based on Coulomb's straight criteria is frequently used to evaluate surface instability such as mentioned above at low confining stresses. However, this usually results in an overestimation of the safety factor because the experimental evidence, for example results of shear tests on sites and in the laboratory, clearly show a nonlinear decrease of the shear strength with a decrease of normal stresses and the failure envelope is curved for almost all types of subsurface soils in forest slopes. The empirical strength criteria of soil, in the from of a power relationship, has been put forward based on tests of a range of low confining stresses. In this paper, a stability analysis (based on the Fellenius Method of the nonlinear envelope) has been developed and the effect on the safety factors on a model slope of simple geometry with uniform soils and actual surface failures caused by rainfall is discussed. The value of the safety factor calculated with Coulomb's criteria is about 20 to 30% larger than that of the nonlinear figure using the model slope. Also in actual surface failures, (the difference between the Coulomb's criteria and the nonlinear one ranges from 6 to 8%) the former is larger. However, the safety factors calculated with the two methods in the submerged condition have a smaller variance than in the natural soil-water one.
One of the most general used method of counteracting debris flow using structures is to catch all the sediment transported by the debris flow. It has been pointed out that current the closed-type dams lose this ability to catch debris flow actually occurs due to a gradual decrease in the dam's sediment trap capacity. Open-type dams have been designed and constructed to let the unharmed sediment passing through freely while catching the harmful sediment downstream. It has been commonly reported that the ability to catch sediment with a grid is influenced by the ratio of the minimum grid distance (Lmin) to the maximum grain diameter (dmax). This parameter, however, dose not sufficiently explain the ability of the grid to catch the head of the debris flow. The concentration of sediment (C) and the flow velocity could also act as parameters for this purpose. In this report, grid-type dam models having a wide variety of grid distances are experimented with at different flume slopes; 10 degree, 15 degree and 20 degree. The experimental results are then analyzed using the previously mentioned three parameters. Since flow velocity does not influence the rate of reduction of peak sediment discharge (P), an experimental equation is deduced as follows: P=1-0. 11·(Lmin/dmax-1)0.36·C-0.93 This equation makes it possible to predict the effectiveness of controlling transported sediment with open-type dams on bed slopes where debris flow and flash floods occurr.
A basic analysis on the characteristics of time series fluctuation and spatial distribution of precipitation at Mt. Merapi in Indonesia is presented in this paper. In the torrid zone, precipitation is much more variable over time and space than in the temperate zone. It is important to know the relation between the deviation power and spatial scale for estimating hydrologic quantities. First, we made a comparison between rainfall characteristics of Mt. Merapi and those of Mt. Fuji in the temperate zone, with the result that the characteristics of convective rain became clear. Next, we interpolated spatial rainfall data between five rainfall points on Mt. Merapi, and combined the spatial rainfall data obtained by the radar rain gauge system to examine the interaction between the time scale and spatial scale of precipitation. As a result, it became clear that varying the duration from 10 minutes to 1 month enhanced the spatial scale that could be represented by point rainfall data. In addition, the spatial fluctuation of precipitation for 1 month at Mt. Merapi was found to be equivalent to 1 hour at Mt. Fuji.
Unprecedented heavy rainfall between 19 and 21 July 1993 caused destructive disasters in Nepal damaging major infrastructures and taking about 1, 500 lives. Debris flows were the major cause of catastrophe in the mountain areas especially in the central region. Authors executed a preliminary study on the physical characteristics of debris flow such as geology of the catchment, boulder size and its shape of deposited materials, sediment volume, bed gradient and rainfall through field survey as well as aerial photo interpretation in seven rivers. The survey result shows that the types of debris flow can be divided into three mainly based on the geology of the catchment. The first type is in the granite area of which boulder size (diameter) of deposited materials is a few meters with round shape. The second type belongs to the schist and quartzite area of the Bhimphedi Group of which boulder size is several 10s cm with rectangular shape. The last one is in the Upper Siwaliks (conglomerate) of which boulder size is about 10cm with round shape.
On May 24, 1926, Mt. Tokachi Volcano in Hokkaido erupted violently, thereby triggering in mudflow. A major mudflow destroyed Kami-Furano town at the foot of Mt. Tokachi, killing 137 people. We have surveyed the mudflow according to interviews with the mudflow witnesses and investigated the sites of witnesses observation. We have estimated the state of mudflow descent at each area based on the results of the survey. The state of mudflow can be divided into: I) a flow whose velocity of more than 40-60km/h and wave height of 5-7m can cause a destruction of a house at the foundation, II) a flow whose velocity of 20-30km/h and wave height of 1-3.5m can cause a collapse of a house, and III) a flow whose velocity and wave height is lower than II) can not destroy a house.
To prevent the debris flow disasters around the active volcanos, Sabo Master Plan for disaster protection is made. In case of making the plan, it is very important to estimate the run-off sediment volume by debris flows. Especially conditions of planning such as deposit volume of pyroclastic flow, critical rainfall for the occurrence of debris flow and so on change by volcanic activity. Moreover, time series variation of conditions on sediment movement will occur in connection with volcanic activity, and will influence the occurrence of debris flow, run-off sediment volume by debris flows. I propose the necessity of basin condition investigation based on time series and showed 4 periods of volcanic activity in the case study of Mt Unzen. Especially in case that large sediment volume of pyroclastic flow are existing at greater part of basin area, run-off sediment volume by debris flow is shown by the f ollwing equation. Qs=(2.79×R)-112.92 where, Qs: run-off sediment volume (×103m3), R: accumulated rainfall (mm) .