It has been thought that cultivated terrace paddy fields on the landslide clod or the slope in the hilly mountainous area play the role of land conservation, such as the retention of the rain outflow. However, a detail of such a role is not yet clear. Since the abandoned terrace paddy fields increase due to a progress of recent depopulation, it is desirable to evaluate such a role in consideration of land use diversification. However, there exist very few studies which investigated and analyzed the changes of physical property and the water balance due to cultivation abandonment of terrace paddy fields from the viewpoint of infiltration. Therefore, the authors investigated physical property and water balance of cultivated and abandoned terrace paddy fields at the irrigation and non-irrigation periods, and at the heavy rainfall, for the purpose of revealing the effect of cultivated terrace paddy fields on the land conservation. As a result, it was comprehended that surface soil of abandoned terrace paddy field increases in permeability and decreases in water retentivity slightly, while the grain size distribution is unchanged. And it was also turned out that the maintenance of terrace paddy fields as cultivated land has a possibility of temporal suppress of the groundwater level rising at the time of heavy rainfall, although it could lead the rise of groundwater level in the long-run.
This study compared the resistance of tree root systems to shallow landslides using an in-situ direct shear test with the pullout resistance of tree roots. We established three study sites in 40-45-year-old alder or pine forests in Bibai, Hokkaido. We cut the trunks near the ground surface and molded soil blocks containing the root systems by excavating around the soil blocks. We put a shear box on the soil blocks and pulled the shear box horizontally, manually, using a chain block, and measured the shear force using a load cell. We also prepared soil blocks without roots and measured them in a similar manner to obtain the soil yield criterion for each study site. We regarded deviation between each measured point with roots and the yield criterion line on the normal stress-shear force plane as the reinforced shear strength due to the roots. We measured the pullout resistance of roots of trees growing near the site of the direct shear test and obtained root diameter-pullout resistance curves. The reinforced shear strength of roots in the direct shear test increased with the cross-sectional area of the roots in the shear plane. We compared the reinforced shear strength of roots using a model that infers it from the pullout resistance, angle of root deformation, and angle of internal friction with the reinforced shear strength of roots determined from the direct shear test. The strength with the direct shear test was only half or so of the strength with the model. We believe that the model overestimated the reinforced shear strength due to the roots.
Bedrock groundwater is reported to be one of the main factors governing occurrences of deep landslides. Hence, predictions of bedrock groundwater level (BGL) changes caused by rainwater infiltration are essential for assessments of the landslide vulnerability as well as for establishments of evacuation systems. This study proposed four simple functional models which correlate antecedent precipitation indices (APIs) to BGLs. Li1 and Li2 models assume linear relationships between BGLs and APIs, and Pw1 and Pw2 models employ power functions. While Li1 and Pw1 models use an API with a single half-life time (HLT), Li2 and Pw2 models use two APIs with two different HLTs. Performances of the models were examined by using BGLs observed at 8 boreholes excavated at a steep hillslope underlain by weathered granitic bedrocks. Results showed that Pw2 model was the best for reproducing BGLs observed at the 5 boreholes. Pw1 and Li1 models were selected to be the best for 2 and 1 boreholes, respectively. Optimized HLTs, which were much longer than the ordinary values used for predictions of shallow landslides and debris flows, properly represented groundwater dynamics of the studied hillslope.
During 3 to 4 September in 2011, Typhoon 1112 passed through the western part of Japan and caused heavy rainfall across the Mt.Rokko. Amount of rainfall which Typhoon 1112 brought is comparable to the heavy rainfall occurred in July 1967 which is one of the biggest disaster of the Mt.Rokko. In this study, we analyzed the characteristic of rainfall around the Mt.Rokko using the telemeter rain gauge system and the X-band multi-parameter radar observation, and we studied the effect of Mt.Rokko to the occurrence of heavy rainfall. By the three-dimensional observations of the X-band multi-parameter radar, we found clearly that the number of rain clouds generated and developed on the southern area of the Mt.Rokko, then they are advected to the north and caused heavy rainfall on the Mt.Rokko. Moreover we suggested the rainfall amount was larger on the lee side of the Mt.Rokko than front side, and such orographical feature of rainfall was well simulated by meteorological model WRF.
The authors apply Differential Interferometric Synthetic Aperture Radar (DInSAR) technique using the images of ALOS /PALSAR to estimate an area and a thickness of the volcanic ash yielded by the Mt. Kirishima (Shinmoe-dake) eruptions in 2011. The results are compared to volcanic ash distribution map in the document and the thickness by the field survey conducted by the authors. The following conclusions are obtained : 1) in terms of SAR coherence, the area between 50 mm and 100 mm volcanic ash thickness is obtained as the lower coherence area ; 2) in terms of SAR interferogram, the area with clear fringe pattern is in good agreement with the range between 50 kg/m2 and 100 kg/m2 of the volcanic ash distribution map. These results indicate that the technique of DInSAR using ALOS/PALSAR can be one of useful tools in detecting an area and a thickness of volcanic ash.
On July 28, 2013, the districts of eastern part of Yamaguchi Pref. and of western part of Shimane Pref. have been seriously damaged by a heavy rain-storm in only one day. Especially, a lot of slope failures and debris flows occurred intensely around Susa Town, Ato Town, and Tsuwano Town in both Prefectures. The areal rate of slope failures of these districts exceeded 1.0% in a dense area. The probabilities of the rainfall as a cause of the disaster were less than 1/1,000 years and were extremely rare cases. However, according to the field investigation, most of slope failures have been recognized as shallow failures and following sediment movements were not deep. Moreover, a lot of small holes, through which ground-water sprang out, and traces of surface-flows have been found out not only on the slope-failure sites, but also on the forest slopes. These phenomena can be regarded as typical features of an extreme torrential rainfall.