In Japan, the number of deaths caused by landslide disasters has consistently decreased for this 70 years. To maintain this situation, it is important to pay attention to risks for occurring deaths by landslide disasters. Although the trend in the number of landslide disasters would be affected by the trend in precipitation, no studies examined the trend in the number of landslide disasters and deaths to consider the trend in precipitation. This study examined the trend in the number of deaths by landslide disasters to consider the trend in precipitation. We used survey data from 1983 to 2013 in Japan. The number of landslide disasters in each year was significantly correlated to the precipitation amount averaged over northern, eastern, and western Japan between May and October (p<0.05 ; R=0.78). The relationship in the first 16 years (1983-1998) was similar to that in the last 15 years (1999-2013). The death toll by landslide disasters was correlated to the number of landslides. The slope in the first period was steeper than that in the second period, indicating a decreasing trend in the ratio of deaths to landslides. We also found a decreasing trend in the number of houses involved in landslide disasters during the analysis period. However, there was no trend in the ratio of deaths to that of completely collapsed houses. Our results suggest that the decrease in the number of houses involved in landslide disasters is the dominant factor in the reduced death toll by landslide disasters.
The prediction of sediment production by freeze-thaw action is important for preventing sediment-related disasters and managing river environments. This study conducted a freeze-thaw experiment to investigate the destruction mode of various types of weathered bedrock (representing some of the many types of bedrock in Japan) and to develop destruction models for estimating sediment production. The experiment was conducted on bedrock samples of weathered granite (WGr), weathered granite porphyry (WGp), weathered shale (WS), weathered sandstone (WSa), and weathered rhyolite (WR). Freeze-thaw cycles caused the bedrock samples to increase in porosity and decrease in weight. When the porosities of WGr, WGp, and WS became greater than 0.43, 0.1, and 0.27, respectively, small pieces started to detach from the main sample bodies. Freezing and thawing caused only slight increases in the porosities of WSa and WR and in the production of fine sediment from their surfaces. During one freeze-thaw cycle, WSa and WR produced sediment amounts equivalent to depths of less than 1.0 and 0.2 mm, respectively. It is assumed that these depths are equivalent to the rock particle size. Using the experimental results, two simple destruction models were developed. A model was applied to WGr, WGp and WS. In this model, it was assumed that the porosity increased at a certain rate for each freeze-thaw cycle. When the porosity was greater than the threshold value, the weathered bedrock decomposed to sediment. An alternative model was applied to WSa and WR. In this case it was assumed that sand particles －the size of which was determined by the constituents of the weathered bedrock －were produced with each freeze-thaw cycle. Using this model, we performed a numerical simulation, and estimated the rate of sediment production. The simulation results were in good agreement with measured data.
To clarify sediment dynamics in mountain rivers during the heaviest rainstorms, we observed river water levels and riverbed level at Harukigawa, Yamanashi prefecture. Due the large-scale landslide occurred in more than a hundred years ago, large steep devastated area is located in the upper end of Harukigawa watershed. Here we successfully described detailed sediment dynamics in three large rainstorms due to typhoons in 2011, and, several results were found. About sediment movement of mountain stream with a landslide, we found the following things. At steep mountain river, whether or not to occur riverbed variations is determined by the water level conditions. In addition, when the continuous rainfall exceeds the number 100 mm, sediment discharge will be frequent. After that, the high concentration sediment discharge with riverbed rise occurs more than once, and the more the water level is high, a large erosion and deposition occurs. During floods, the duration of the high concentration sediment discharge with riverbed rise was about up to about one hour. Therefore, temporal change of sediment discharge and river bed variation has been found that not determined by only the hydraulic conditions. The timing of the riverbed change to 800 m downstream from the confluence of the tributary is affected by the sediment discharge waveform from the tributary. If the scale of the sediment discharge is small, the impact distance will be smaller.
In order to detect the point of a deep-seated landslide, detection by a sentinel vibration sensor is an effective system for finding the occurrence point of a landslide, the scale of collapse and the occurrence time. The observation points from the sentinel sensing system must be selected carefully, because a high sensitivity sensor needs to sense a low noise level. We must select a location where the ground noise is relatively small. Thus, microtremor measurement is carried out from a carefully selected observation site. The sensor is required to be installed underground because of its sensitivity to other outside noise. We investigated the limitations of the vibration sensors that are capable of detecting the landslide volume, and relationship between epicentral distance and observe vibration of Short Time Average level (STA level) of the landslides. At Goyo site, Noise level (night time) estimated 4.7 μkine. At this site, the trigger level was 2.5 ( : STA/LTA). Thus, this system was able to exceed the 0.42 million m3 of collapse volume. When the site distance is 20 km, this system detect the two million m3 of landslides volume, though this system is not able to detect under one million m3 of landslides volume.
Present study describes formation and developing processes of debris flow resulting from landslides and slope failures, supposing that a soil block released by slope failure is saturated by water. Stability and mobility analyses of the soil block such as an equilibrium bed slope suggest that the soil block is fluidized resulting into debris flow, and that the resulted debris flow is able to reach a milder area than expected for the debris flow composed only of coarse sediment, supposing that a phase sifting takes place from solid phase to liquid one for the fine material composed of the soil block. In addition, the results show that the debris flow size such as a peak flow discharge is determined mainly by sediment erosion along the torrent bed except in the sediment depositional region. Such results provide a key for designing sediment control facilities, which is very different from a conventional method.
Pipe hydrophones have been used widely to monitor the mountain streams bed load in Japan. They consist of a microphone in a steel pipe and measure bed load indirectly using the sound of collisions. Pipe hydrophones can measure small particle bed loads more sensitively than the plate geophones used widely in Switzerland. However, in several places where hydrophones have been installed, the measurements are negatively affected because the pipes are dented by collisions with large and many sediment particles. One way to solve this problem is to increase the thickness of the pipes. However, this changing might reduce the sensitivity of the hydrophone. In this study, we conducted experiments to examine effects of the pipe thickness on measurements by hydrophone. The amount of dents due to stone collisions were also calculated using the Ellinas equation. The experiments and calculations showed that the sensitivity of thicker pipes dulled slightly, although the dents were much smaller compared with 3 mm thickness pipe, adopted generally in Japan. Therefore, hydrophones with thicker pipes are recommended to measure in streams where large particles or high sediment discharge is expected.