This paper describes the mechanism of four deep-seated catastrophic landslides (Kuridaira, Nagatono, Akatani, Tsubonouchi) caused by heavy rainfall during Typhoon Talas (T 1112) in Kii Peninsula, Southwest Japan. We conducted a field survey, and examined the physical and mechanical properties of the sliding layers. The survey revealed some common phenomena among the four landslides, such as the strata of the deep-seated catastrophic landslides consisting of heavily weathered cracked bedrock. The crossed two joint planes formed wedge-shaped blocks, such as the upper slope block and lower slope block. This allowed the ground water to deeply infiltrate the planes. The angle of the shearing resistance of the sliding layers were measured to be between 28 and 40°. Meanwhile, we recognized the occurrence of excess pore water pressure under shear deformation by the stress-controlled ring shear test. This indicates that the upper slope block started sliding and rode on the lower slope block where compression deformation was produced. Consequently, the positive excess pore water pressure occurred in the rock mass, and the deep-seated catastrophic landslide started in a body. In addition, as the excess pore water pressure increased under the shearing, the sliding soil mass gained high speed, and eventually traveled for a long distance. This indicates that excess pore water pressure is an important factor to expound the mechanism of deep-seated catastrophic landslides.
Understanding the flow characteristics of a debris-flow front is essential for evaluating the impact force and predicting the attainable time of the debris flow. However, the fluidity of the front has not been examined experimentally for either mono- or multi-granular debris flows. This study conducted flume tests of debris flows consisting of sediment particles that were either monogranular or a mix of two diameters and compared the results with numerical simulations. Particlesize segregation was examined in the mixed-diameter experiments using a high-speed video camera and five debris-flow samples obtained from the front edge of the debris flow at ca.0.5-second intervals. The experimental and calculated results showed good agreement for the monogranular debris flows in terms of flow depth, front velocity, and fluxes, whereas disagreement was seen in the mixed-particle-size debris flows. The disagreement was thought to arise from the dominant shear stress resulting from the concentration of smaller sediment particles in the lower layer of the flow due to inverse grading detected in the body of the debris flows. Additionally, the calculated velocity of the front was underestimated compared with the actual velocity, as the particle-size segregation at the front of multi-granular debris flows did not affect the fluidity in the initial and developing phases.
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. We need to consider future action for deep-seated landslides, and contemplate evacuation for the occurrence of deep-seated landslides as they have recently increased in incidence. Deep-seated landslides and the rainfall index are dominated by long-term rather than short-term rainfall indicators, but few concrete case studies are available. We studied C-band rainfall mesh data in the southern part of Nara Prefecture after typhoon no.12 to elucidate the rainfall index relevant to deep-seated landslides. We found that a deep-seated landslide happened around the end of the rainfall event after typhoon no.12, and that a deep-seated landslide occurred after 700-800 mm of rainfall fell in 48 hours with at least 900-1,000 mm of total rainfall. However, a clear relationship could not be found for rainfall totals of ≥ 1,400 mm. We did not find a clear relationship between the rainfall index and deep-seated landslides, although a correlation between a higher tendency for a large-scale landslide and a greater number of deep-seated landslides was confirmed in the medium-to long-term rainfall index. We found that the excess probability of rainfall is highly correlated with deep-seated landslide.
We tried to establish the detection methodology by viewing single high resolution synthetic aperture radar (SAR) satellite images which are now available. At first, a check list was made to qualitatively evaluate if it is or is not possible to detect landslide dams. By using that check list, high resolution SAR satellite images (TerraSAR-X, COSMO-SkyMed, RADARSAT-2) of Talas disaster in the Kii Peninsula have been evaluated. We confirmed conditions suitable for satellite as follows. All direction radar can detect landslide dams. The radar angle range suitable for obtaining images is from 35 to 50 degrees. Suitable image resolution is 3m. All sensors and bands are capable of detecting landslide dams.
Relationship of grain size distribution of artificial spawning grounds and survival of eggs of white-spotted charr (Salvelinus leucomaenis) was examined. Rates of very fine particles finer than 0.074 mm in three artificial spawning grounds were 0.8-1.4% by weight. The rates were much higher than those of stream beds in general mountain streams. It was feared that the extremely high rates of the very fine materials would prevent stream water with oxygen from reaching at laid eggs and cause low survival rate. The rate of eyed eggs was 90.0% for the laid eggs, and the rate was higher than those of other artificial spawning channels. Therefore, the high rates of very fine particles did not damage the eggs in this case. However, the results should not be generalized but a reference for the development of the artificial spawning ground technology.
In accordance with a revision of the Sediment-related Disaster Prevention Act, central or local prefectural governments are responsible for providing information related to possible disaster-affected area/time so that related municipalities can make a proper decision to give evacuation instruction in case some kind of a large-scale sediment-related disaster is imminent. It is therefore necessary that the Ministry of Land, Infrastructure, Transport and Tourism makes further enhancement of observation monitoring system so as to understand rainfall and ash fall situation over in particular active volcanoes promptly and accurately. Because of this situation, this study evaluates the ability of X-band MultiParameter radar, which was newly developed and installed at the Sakurajima (one of the most active volcanoes in Japan) area, for detecting volcanic ash from Sakurajima by comparing with the existing C-band radar observation. In this study, we also verify usefulness of various parameters obtained from dual polarization observation by using the X-band MultiParameter radar for detecting volcanic ash, as well as consider possible utilization of the parameters for monitoring volcanic ash and challenges to be solved.