A huge-scale mudflow occurred in 1783 was triggered by the 1783-eruption of Mt. Asama. The mudflow caused the largest disasters along to the Agatsuma River. Several researchers investigated the flood marks, duration time of the mudflow etc. not only in the field but the interpretation of ancient manuscripts. However, there were a few researchers who have carried out about the dynamic process of the flow, such as the occurrence, the growing and propagation processes.
In this study, one-dimensional unsteady flow calculations are conducted in relation to the total volume and the peak discharge at the upper boundary in order to understand the characteristics of the flow. The comparison of the calculated results with the previous field and literature studies show that the volume, the discharge at the upper boundary, and the duration of the mudflow were 1× 10⁸m³-4 × 10⁸m³, 15×10³-20 × 10³m³/s, and 10-30 minutes at longest, respectively.

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Slope failures occur frequently in many parts of Japan in rainy seasons every year. It is necessary for disaster prevention, to predict the occurrence of slope failure. Recently, a calculation speed of calculator have made rapid progress and improved. Therefore, numerical simulation model is used frequently to predict slope failure. However, much time and labor become necessary in order to set input data to these models. In this study, easy set up method of input data to slope failure-predicting model from a handy dynamic cone penetrometer, was proposed. Following matters were found through this study. Bulk density and porosity show dependable logarithmic correlation to the magnitude of Nc value. Still more, saturated hydraulic conductivity shows dependable powers correlation to the magnitude of Nc value. By using these approximate expressions, soil characteristic coefficients to slope failure-predicting model can be decided without soil test in sandstone region. Further, in order to estimate thickness of surface soil layer without detailed investigation, boundary Nc value-predicting model was presented. Applying this model to this study area, boundary Nc value was understood to be 9 in sandstone region. Taking these results into consideration, it was clarified that the magnitude of Nc value would be a parameter indicating soil physical properties and thickness of surface soil layer. Furthermore, by investigating vertical distribution of Nc value from a handy dynamic cone penetrometer test, it is possible to estimate input data to slope failure-predicting model easily by means of boundary Nc value-predicting model. Therefore, these methods proposed in this study, can become an effective measure to predict collapse quickly, by being linked with slope failure-predicting model.

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Recently, debris flows occur less frequently at the Kamikamihori Gully, Mount Yakedake, where they had been occurring frequently in 1970's. This is supposed to be due to hydrologic change at the source area. To confirm the supposition and clarify its cause, rainfall and runoff observations at the source area were carried out in 1970's and 1990's. The repeated observations show the significant decrease in surface runoff. Furthermore, a result of a runoff analysis using a physically-based surface-runoff model for the runoff events both in 1970's and 1990's shows increase in surface roughness, initial loss of rainfall and final infiltration capacity and decrease in contributional area of runoff, which are consistent with the significant revegetation in the study slope. The history of debris flow occurrences in the Kamikamihori Gully shows the increase in the minimum hourly rainfall for debris flow occurrences. It seems to be caused by the runoff decrease due to the revegetation in the source area.

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Sediment-Retarding Basin (SRB) is one of Sabo works and usually constructed on alluvial fan to control sediment discharge.
Hydraulic experiments and numerical simulations have been conducted by researchers and engineers to investigate and improve the function of SRB by sediment volume controlled by SRB or riverbed variation as indexes. But it is essentially needed to know about not only riverbed variation but also interaction between hydraulic conditions and riverbed variation for understanding the sedimentation process in SRB.
Authors have conducted hydraulic experiment on SRB and obtained data on flow depth, flow velocity and Froude number. In the experiment, the fixed-bed channel that is 6.6 m in length, 30 cm in width and 1/20 in gradient was used. The channel has widened section as SRB. At first clear water was supplied at constant rate. Then sediment was supplied at constant rate.
There were 8 cases (Run 1-Run 8) in the experiment and these cases were different on longitudinal variation of cross sectional profile of SRB. Sediment supply was continued by the end of the experiments (for 15-25 minutes), and little riverbed variation was observed at the end of the experiment in each case.
In this paper, we consider the hydraulic characteristics at which only clear water was supplied and at the end of experiments (under the condition of equilibrium on riverbed variation) and the results of comparison between them, sedimentation process and variations of the hydraulic conditions in the process. In addition, we show some notes about methods to plan and design SRB.

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In advance to this study, we conducted an impact tests of fill-up concrete blocks using for sabo dam in order to investigate the process of detachment and failure of the blocks. In the tests a weight was dropped from various heights (H) onto test specimens composed of a layer of concrete blocks and a layer of concrete representing sabo dam, and then the concrete blocks were inspected for detachment and failure caused by the impact load. In this study we conducted finite-element analysis with respect to the test in order to presume the stress distributions and the deformation. This paper discusses the failue process from the view point of the relationship between the test results and the vertical distribution of the maximum vertical tensile stress σ_zz.

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A short description of the nature-conservation orientated control work of the SABO-Garden in Miyajima island near Hiroshima is given. The principle of self-organization in a torrent (“boulder river”) is developed and defined. The rivers are also considered as an energetic system, frequency of the hazardous events and the stability of self-organizing structures are depicted and the destruction mechanism of the structures is described. Long-term considerations of superposition of different processes are shown and proposals for designing control works are given.

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A typical erosional process on forested slopes of low relief mountains in Japan is surface soil slides, which frequently has brought about sediment disasters during the past half century. The process of occurrence and recovery of surface soil slide scars are investigated on granite slopes of the Kiso mountains. The following is obtained.
1) The occurrence is greatly affected by forest conditions, such as clear cutting, thinning, young forests, natural old forests, etc. The relative value of safty factors of potential slide slopes of various forest conditions are estimated with the density of soil slides of the respective forest condition. 2) As the recovery of vegetation proceeds, the number and the area of surface soil slides decrease exponentially with the lapse of years. 3) The expansion of slidescars during their life times is important in high altitude mountains above 1000m, which have freezing-thawing climates and frequent heavy storms.

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Devastation process of forested slopes in low relief mountains by human impacts is investigated. The impact is classified into three categories by pressure types on forests: accumulating type, shifting type and joint-working type. The pressing position on which the pressure works is also divided into three types: forest (cutting), soil surface (raking and cutting) and soil (digging). Devastation process of forested mountains is expressed with a schematic table which represents a spatial shift of human pressures and a spatial variation of devastated forests. The historical change of devastated areas is estimated with various references.

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