Recently in Japan, bamboo species have been expanding into neighboring forests. This is cause for concern because such changes in vegetation can lead to destabilization of steep slopes. However, the relationship between the expansion of bamboo forests and the change in slope stability has not been fully clarified. Therefore in this study, we attempted to develop a new method to evaluate the effect of bamboo rhizomes on slope stability. This paper reports the characteristics and suitability of this method. First, we surveyed a slope, that had failed due to heavy rainfall in August 2014 and had been dominated by the Moso bamboo (Phyllostachys pubescens). Bamboo rhizomes were distributed in a shallow layer on the ground ; their underground stems were cut when the slope failed along its edge. Considering these characteristics of bamboo rhizomes, we developed a new model for the analysis of slope stability. Subsequently, we investigated the spatial distribution and ultimate tensile strength of bamboo rhizomes on steep slopes. To confirm the validity of the developed model, the stability of the failed slope was analyzed, taking the results of field investigations into consideration. Based on the results of the analysis, an appropriate safety factor was obtained and it can be concluded that our model was valid for evaluating the effects of bamboo rhizomes on slope stability.
Numerous deep-seated landslides have occurred in southern Wakayama and Nara prefectures, with over 1,000 mm of rainfall caused by typhoon no. 12 in 2011, which resulted in a great deal of damage. Understanding the occurrence and non-occurrence of deep-seated landslide conditions, it may be helpful in risk assessment and soft measures in the region, laborsaving field survey. As important factors that influence the occurrence or non-occurrence of deep-seated landslide, it may be slope direction, number of hits micro-topography, probable excess total rainfall, difference in height in the watershed, Difference of summit level and the lowest elevation, Distance from the fault. So, we studies to understand the relationship between incidence and non-incidence factors to be considered in the present study is closely related to deep-seated landslide. In addition, we extract the important factor in considering the occurrence or non-occurrence of deep-seated landslide using rough set. We have investigated the non-occurrence rules of deep-seated landslide occurrence rule. The results of the analysis by rough set, the number of points that can be described in a single rule, but there are few on the occurrence of deep-seated landslide, is that the description of about 1/3 of the total number of basin generation is possible and fit some rules All right. Further, as the non-generation rules, the rules to be little nonoccurrence condition is extracted. Was applied for the first time to the occurrence of deep-seated landslide rough set was used to analyze the conditions of occurrence and non-occurrence of shallow landslides and debris flow. We were found to be useful in the understanding of the conditions and rules extraction of the occurrence of deep-seated landslide objective.
This study conducted a detailed analysis of the failure process and sediment yield of a freeze-thaw induced landslide. As most of the slopes where freeze-thaw failure occurs are located in inaccessible areas, in mountainous environments and under severe meteorological conditions, a safe and high-precision measurement method has not been established. The study site is a slope along the Yotagiri River. In order to obtain high-density and high-precision data, terrestrial laser scanning was applied. Between 2010 and 2013, surveys were conducted four times before freezing in November and December, and three times after thawing in June of the following year. In all, seven surveys were conducted. The landslide volumes were calculated using DEMs (Digital Elevation Models), by subtraction within the landslide-affected area. By the analysis of erosion and deposition of the slope over time, it became clear that a large amount of sediment from the freeze-thaw failure has been supplied to the stream. The analysis also revealed the erosion volume per unit area, estimated at 42,100 m3/km3/year in the study area.
To understand effects of topography, such as longitudinal gradient, on erosion and deposition patterns of debris flow, we quantified topographic changes due to debris flow using aerial LiDAR datasets for recent debris flow in Japan. In larger catchments, the debris flow fan already formed and the top of debris flow fan located upper stream from the exit of the catchment. While, the top of debris flow fan was the downstream from the exit of the catchment in small catchments. We confirmed that the longitudinal gradient of the top of debris flow fan was around 10 degree, regardless of catchment size. This difference gave an impact on the deposition pattern of debris flow. The distance from the point where longitudinal slope angle was 10 degree to the lower end of deposited area was linearly increased with increase of the stream length. While, the distance from the exit of the catchment to the lower end of deposited area was not related to the stream length where catchment scale was large. Further, we found that the gradients at the position changed from erosion to deposition were highly varied, although studied catchments were located in the same region and debris flows were triggered by the same rainfall event.
This report proposes a method for measuring free-falling nappe distance and depth of debris flow in order to estimate more accurate debris flow velocity. The velocity was calculated according to a semi-theoretical equation. Nappe distance and depth were obtained using a laser range finder installed into Arimura River in Sakurajima, Japan. In the debris flow that occurred on June 21, 2014, the nappe distance（L）and depth（he）of debris flow was obtained, and the nappe distance was approximated by L＝10.1 he－0.17, where observed maximum he is 0.7m. A comparison of the velocity estimated from the semi-theoretical equation and collected using the ultrasonic velocity sensor showed that the calculated velocity is valid. The proposed method is capable of measurement even during bad weather, and it is possible to estimate the debris flow velocity at a high time resolution.