Role of the sabo facilities to the sediment discharge in the small basins, especially on the sediment gradient at a sabo-dam

It is important that both of the hard and soft counterplans function well for the prevention of sediment disaster. However, the hard counterplans, for example, sabo-dams are not exactly understood about their role and effect. The author tried to make clear the role, the effect and the limit of sabo-dam for the prevention of disaster with the manual made by the Ministry of Construction and some case studies. From the case studies, the followings became clear. In the northwestern district of Hiroshima Prefecture, where the author had investigated and collected the data, the catchment area of torrent, where debris flow had occurred, and the sediment discharge of unit area were common. However, the proportion of the sediment gradient to the original bed gradient was smaller than the standard arranged by the Ministry of Construction. Though the sabo-dams functioned according to the plan, the number and the scale of the sabo-dams were not enough for the prevention of sediment disaster

Relationship between slope failure occurrence and precipitation in the areas affected by a strong motion of earthquake

The number of collapses of slope induced by the Geiyo-earthquake occurred in March, 2001 wasoverwhelmingly small to compare with the number by other same degree earthquakes before. However, collapsesof slope are prone to occur relatively in a smaller amount of precipitation after the earthquake. The effectof seismic motion in the Geiyo-earthquake in 2001 was firstly evaluated by the maximum seismic acceleration,duration time of strong seismic motion and the direction of both strong motion and collapses of slope. And then,the amount of precipitation that was related to collapses of slope before and after the Geiyo-earthquake byexamining hourly rainfall and working rainfall (half-value period 72 hours) was considered. As a result, it wasfound that 60 to 80 percentage of precipitation, in which collapses of slope had been prone to occur before theearthquake, would be related to the phenomena such as slope failure and/or stone wall destruction, and landslidesmight occur to all direction regardless of the direction of strong motion over 250 gal in seismic acceleration

Study on the Relationship between the Particle Size of Soil and the Angle of Shear Resistance under the Low Normal Stress

It has been occasionally explained that the angle of shear resistance depends on the soil-particle size. However, the qualitative relationship between the friction coefficient and the particle size is still controversial. To elucidate the effect due to particle size, the samples with some kinds of particle sizes should be prepared by sieving. It is not ideal to get samples by crushing, because the shape of the grain becomes quite different from the original one. Also, the shear experiments should be done under the low stress level, where the particles are not crushed. To obtain the relationship between the particle size of soil and the friction coefficient, the direct shear tests were performed under the low constant normal stress less than ca. 230 gf/cm^2 with some uniform particles of three sizes. As the result, it is found that the friction coefficient increases monotonously with the increase of the particle size. This concludes that the friction coefficient (tan φ) can be expressed by only two factors; mean diameter (d_) and relative density (Dr)

土石流の流動を抑制した自然的要因についての考察

It is very important for the disaster prevention to analyze the factors which influenced on the fluidityof debris flow. We investigated such factors in debris flows occurred around Higashi-Hiroshima in 1999. Weadopted equivalent coefficient of friction and dynamic coefficient of friction to evaluate the fluidity of thedebris flows. As the result of consideration with obtained data, the following things were elucidated: (1) Debrisflow which occurred in the torrent with larger catchment area tends to have large fluidity. (2) In case of eachtorrent, as debris flow moved downwards, dynamic coefficient of friction became smaller. (3) Debris flowstended to lose their fluidity at the places where there were trees, agricultural ponds and tortuosity of the channeletc. (4) The proportion of area occupied by trees is more suitable than the number density of trees as factorsinfluenced on the fluidity of the debris flow. When the proportion of area occupied by trees would becomegreater than some per mills, debris flows at such site could be controlled by the trees

Topographic characteristics to cause landslides and fluidization of their materials : From the debris flow disasters in the northwestern part of Hiroshima Prefecture in 1988

A large number of debris flows occurred in the northwestern part of Hiroshima Prefecture on July 21st in 1988. On the basis of the survey data and the result of analysis on the type of valley in topographic maps and comparison between the contour lines in topographic maps and summit level maps, we explained that the landslides on concave landform occurred easily on steep and undissected valleys and the failures of side slope and waterside tended to occur on dissected valleys. Moreover the courses of debris flows agree well with those that we estimated by the topographic analysis. The method with the topographic analysis in this report could be utilized to make a hazard map for the prevention of debris flow disasters

Study on Distinctive Landslides Induced by an Earthquake

The Kumamoto earthquake of 2016 triggered numerous landslides. One feature of these landslides was fluidic movement of soil mass, which occurred despite little rain having fallen for a long time before the earthquake. To understand this distinctive phenomenon, we analyzed the seismic and soil characteristics of the area and evaluated their association with landslides. The results showed landslides to be concentrated in areas where seismic acceleration exceeded 800 gal. Through soil tests, we found that a mass of clay with abundant Halloysite easily becomes fluid. Therefore we inferred that some landslides were induced by fluidization of such clay layers.本研究の一部は、国土交通省河川砂防技術開発公募の砂防技術分野（平成29～31年度）「大規模地震とそれに伴う地盤の劣化に起因する連鎖複合型土砂災害の発生機構と対策」（研究代表者：丸谷知己北海道大学大学院特任教授）から支援を受けて行うことができました

Analysis of the Landslide Behavior with the Total Station Survey in Nishi-Ikawa, Tokushima Prefecture, Japan

Total station surveying has been done to find out spatially the landslide behavior at the Nishi-Ikawa landslide area in the Tokushima Prefecture. Firstly, errors which were able to enter this surveying have been checked and evaluated in order to grasp correctly the landslide behavior by this surveying method, if human errors could be kept minimum. However, the authors found that the surveying result involved errors even in the each careful set up of the instrument and that obtained 3-dimensional coordinates could be not directly utilized to analyze the landslide behavior. Also, the surveying could be made only in the season from the late autumn to early spring due to the vegetation obstruction. However, it was clarified that the change of the distance between the instrument site and each observed point and the change of the distance among the instrument sites in each surveying could be used to grasp the landslide behavior. From the analytical consideration of the surveying data, the following things were clearly pointed out:(1)There was a compressive tendency on the total area of the Nishi-Ikawa landslide slope.(2)The landslide area consisted of five blocks.(3)The uppermost block moved downwards onto the lower block, and the three middle blocks seemed to be pushed to the outer sides.(4)The road construction with the cutting and filling in 1999 seemed to be not effective to the recent landslide behavior(5)The landslide behavior around the foot part of the lowest slope could not be prevented by the countermeasure with the existing retaining wall only

Community contribution research in Idebara area in Kita-Hiroshima Town, Hiroshima Prefecture : Field investigation of sediment-related disaster and trial rainfallobservation for disaster prevention by residents themselves <Articles>

Sediment-related disasters occurred two times in Idebara area in Kita-Hiroshima Town, Hiroshima Prefecture. This area is covered by weathered granite and rhyolite, and located around the watershed of Idebara-river, which is one tributary of the most upper region of Gonokawa-river. Since the disaster in 2006, some investigation were executed to become clear a danger degree of sediment-related disasters, and to make up some countermeasures for disaster prevention and mitigation. According to this investigation, debris-flows occurred repeatedly in a few sub-watersheds and a very dangerous sub-watershed, where a new debris-flow would occur in near future, was detected. There are a lot of debris-materials in the area, which were composed of some sediment transported by past slope-failures and debris-flows. The surfacesoil is very wet and trees on it are disorderly inclined. These conditions of the investigated area and the importance of rainfall information were explained to the residents in Idebara. After the explanation meeting, a trial rainfall observation by residents themselves was started. It was verifi ed that this system was very functional and useful for people there through the torrential rainfall in July, 2010