In 1926, due to snow melting caused by the eruption of Mount Tokachi, volcanic mudflow flowed down to the Furano River and Biei River. In Biei, within the area of volcanic mudflow, two areas were found. 1) Forest area which was not washed away by the volcanic mud flow. 2) Forest area where trees were washed away by the volcanic mudflow. It is important that a buffer zone be establish of mudflow with forest and to evaluate the capture effect of volcanic mudflow sedimentation and the depressor effect of woody debris of vegetation. In this study, It was analyzed that the actual situation of volcanic mudflow sedimentation and trees survival/loss in Biei River through deciphering aerial photographic and field survey. Based on aerial photograph, volcanic mudflow down/sedimentation area and tree survival/loss area were interpreted. As a result, it is determined while many trees were washed away in the area of volcanic mudflow flowed down, trees survived in some area of volcanic mudflow sedimentation. Moreover, existence and nonexistence of sediments were surveyed by pit-investigation and the age of trees was estimated by the result of diameter at breast height survey and tree-trunk graph analysis. Based on these two results, the volcanic mudflow and the mudflow sedimentation range were clarified in detail by interpreting the distribution of the space of original trees and after flow event trees.
The Kumamoto earthquake in Japan on April 16, 2016, triggered numerous landslides on the hillslopes of the post-caldera central cones of Aso Volcano in Kyushu, Japan. The hillslopes are mantled by ≥ 10-m thick multiple tephra layers produced by repeated eruptions. In this study, we examined the tephra stratigraphy and measured the soil strength of the tephra layers around a sliding surface in two selected landslides: the Takanodai landslide, with a depth of 10 m and that traveled a long distance, killing five people, and the San'oudani landslide, with a depth of 10 m and caused a debris flow running downstream along the San'oudani Creek. The stratigraphic surveys of these landslides demonstrated that the sliding surfaces were formed in a layer of weathered pumice that turned into soft and easily crushable grains. We identified the pumice as the Kusasenrigahama fallout pumice, approximately 30,000 years BP. The weathered pumice layer where the sliding surface was formed showed extremely high water content. We measured the soil hardness using a Yamanaka-type tester, the soil strength by cone penetration tests, and the soil cohesion and internal friction angle by vane cone shear tests. All the properties measured of the weathered pumice layer were lower than those of the upper and lower layers, suggesting that the weathered pumice layer is remarkably weak in all the tephra layers of the soil profile. A probable initiation mechanism of the landslides is as follows: first, shear failure occurs within the weathered pumice layer because of earthquake motion; next, soft pumice grains are crushed along the sliding surface as the shear displacement increases, resulting in a reduction in volume because of earth pressure; and finally, an excess pore water pressure is generated under an undrained condition, and soil shear resistance rapidly decreases.
In recent years, debris flows occurred almost annually at Tansan-dani stream in Mt. Unzen-Fugen-Dake. In a previous study, the authors proposed a debris-flow trigger model hypothesis. The hypothesis essentially assumed the increase in surface water near the exposure of pre-eruptive ground surface with a low infiltration rate. This study seeks to verify this assumption by evaluating the rainfall-runoff response through on-site hydrological observations. Firstly, images captured by time-lapse cameras were examined to identify when surface water appeared. Next, in accordance with the observed timing of the emergence of surface water, critical success index (CSI) was applied to optimize the half-life time of antecedent precipitation index and time-window of accumulative rainfall. Based on these optimized figures, runoff response was examined as follows : (i) At observation site A located on pyroclastic-flow deposits 20 meters upstream of exposed pre-eruptive ground, runoff was observed to respond sensitively to rainfall intensity. (ii) At observation site B located on exposed pre-eruptive ground, runoff with a small flow rate continued for more than a month. In addition, a relatively sensitive response to rainfall intensity was observed with increases in the flow rate.