Over the past three years, the authors observed the rainfall, runoff and sediment yield of devastated small forest basins located in the Jianxi province, in the southern part of China. The experimental basins are in a hilly mountainous area on weathered granite ground. The area has many bare slopes and ruined forests. The devastation resulted from the moving of ground-plants, and removal of fallen leaves for use as fuel. The authors analyzed the observed data consisting primarily of rainfall, runoff, and sediment yield data. Through the observations, the authors noted that, (1) Precipitation in this area is concentrated during the rainy season from March to August, the most rainfall is considered to be discharged by overland flow. (2) In the first half of the rainy season, there is much unstable sediment on the surface due mainly to “frost heaving”. And the most sediment is discharged by overland flow, which is indicated by the ratio of the peak discharge to the peak rainfall intensity. During this season, the ratio effects the sediment discharge, (3) In the latter half of the rainy season, there is less unstable sediment yield on the surface and the discharge depends on the maximum rainfall intensity.
I studied behavior of flow type avalanche to collide on a plane. I considered an avalanche to be aggregate of ice particles and analyzed collision to a plane and led velocity after collision theoretically and evaluated the theory by experiment. As a result, it was predicted that stagnation of particles was formed by arc shape around the collision point. I suggested expression method of lateral spread angle by collision and showed calculation method of jumping height of avalanche. I did model experiment using the proposed calculation method and expressed effect of small avalanche deflector quantitatively. I showed that a jumping direction of avalanche to get over a structure was an important factor in order to make effect of small deflector big. In addition, deflector can be improved by installing overhanging members on the top of the structure to turn the avalanche back.
In order to evaluate the influence of a root system on soil reinforcement, we performed direct shear tests with soil water suction control. The suction in the tests ranged from 0 to 50cm H2O, and the normal stress ranged from 42 to 140gf/cm2. Toyoura standard sand was used for the tests. Two kinds of material simulating the root system were inserted into a shear box. The first was a bamboo stick (0.27cm in diameter, 10cm long); the second was a nylon ner (0.05cm thick, 10cm wide, 15cm long). The former simulates a thick root system; the latter, a fine root system. The experiment results are as follows. With respect to the relation between the suction and the increase of shear strength for the same normal stress, we found peak points of shear strength at 30cm H2O of suction for the bamboo stick and at 40cm H2O for the nylon net. The effect of root systems on soil reinforcement was observed in both apparent cohesion and internal friction angle. Suction values greatly affected the increase of apparent cohesion, contrary to the internal friction angle which increased independently with suction. These results suggest that the soil reinforcement depends not only on the resistance force of the root system itself, but also on the interaction between the resistance of the root system and the shear strength which is affected by both normal stress and soil moisture conditions.
It is shown possible in experiential and analytic studies that sleeve of sabo dam is damaged and destroyed by debris flow. A Technical Standard of Countermeasures against Debris Flow (Draft) disputes that the dam sleeve requires to be reinforced with iron bar and shaped steel if necessary. The standard, however, doesn't disputes a concrete design method considering impact response properties of reinforced concrete. In Austria, most sabo dams with 5 to 20 meter height are made of reinforced concrete (hereinafter referred as RC sabo dam). The RC sabo dam is evaluated to be economical comparing with the gravity concrete dam, and to give flexibitiy to design of open type sabo dams. As sabo dams to check debris flow and open type sabo dams such as slit dam have been increasing in recent years, it is necessary to study design method of RC sabo dam and reinforcing the dam sleeve. Subsequently, reinforcing with iron bar makes sabo dam ductile and strong in anti-earthquake. As the RC sabo dam is more slender compering with concrete gravity sabo dam, it seems to tend unsafe against impact force by the debris flow. Although there are a few analytic and experimental studies on the impact response properties of RC sabo dam, no synthetic study of the design method is reported. Therefore, this paper aims to present a synthetic design method of RC sabo dam and reinforcement of dam sleeve considering ultimate-limit energy capacity.
The earthquakes in March and May 1997 triggered many slope failures in the northwestern part of Kagoshima. This study describes the spatial and temporal distribution and topographical and geological characteristics of the slope failures on the basis of the field surveys and the interpretation of aerial photographs. More than 2, 600 slope failures distribute on the mountainous terrains underlain mainly by sedimentary and granitic rocks around the epicenter. About 90% of those have been identified within the distance of about 8km from the earthquake source fault in March 1997. The hillslopes underlain by granitic rocks are largely susceptible to slope failures in comparison with those of sedimentary rocks. The density of slope failures, number of slope failures per 1km2, is 30.2 in the area underlain by granitic rocks and 7.6 in the area underlain by sedimentary rocks. The density of slope failures increases with the height above sea level and the angle of slopes. In the area underlain by granitic rocks, however, many slope failures occurred at the lower height area and the gentle slopes. In the area underlain by sedimentary rocks, the orientation of failed slopes depends on the direction of vibration. On the other hand, this relation is not noted in the area underlain by deep weathered granitic rocks.
Actual state of a large scale debris flow that occurred on July 10, 1997 in the Harihara River Basin in Izumi City in Kagoshima Prefecture was studied. Two principle debris flows seemed to have flowed between about 0:45a.m and 1:00a.m. The first debris flow is characterized by the property in which the deposit consists partially of nonshear deformation soil mass block. The deposit of the second debris flow which is seemed to have occurred immediately after the first flow is, overall, a disturbed, and it is hypothesized that the debris flow, which had reached shear deformation, resulted in overall mobilization. Muddy water in a fruit farm pond or the erosion control dam was abruptly discharged downstream by the first debris flow, then it overflowed the dam and flowed down into the Harihara residential area where it washed out some of the residents. The second debris flow impacted the right wing of the erosion control dam at a speed of 10m/sec at least, destroying it, flooding and settling in the Harihara residential area. This was the result of most of the damage caused by this debris flow.