阿蘇カルデラ壁で発生した山腹崩壊に伴う土砂と流木の到達範囲とそれに基づく危険区域設定の提案

摘要

The northeastern caldera walls of Aso Volcano in Kumamoto, Japan have a relief of approximately 300 m. The walls form long steep hillslopes, in which the upper part has a slope of more than 30 degrees and the lower part has a slope of 20 to 25 degrees. During an extreme rainstorm in July 2012， many landslides were initiated from the upper parts of the hillslopes and the hillslopes collapsed over a long distance. Eroded sediment and large woody debris reached approximately 200 m from the toes of the hillslopes, causing serious damage in residential areas. The present criterion for designating landslide hazard areas does not include the hillslopes in which the horizontal distance between the upper steeply inclined segment (more than 30 degrees in slope) and the footslope residential areas exceeds 50 m. A new criterion applicable to long-runout landslides in this kind of hillslope is required. We researched the distribution of sediment and woody debris in depositional areas of five selected landslides using LiDAR DEMs and aerial photos, analyzed sediment depth and wood size that can affect damage to houses, and discussed a new idea for designating the hazard area and for partitioning the area based on the hazard ranks of house damage. The landslide hazard area should be designated as the area within a distance of 1.5 to 3 times the hillslope height from a concave break line in slope. On this occasion, it is important to designate the hazard area regardless of the maximum distance 50 m in the present criterion even if the height of the hillslope is large. The hazard area can be partitioned into three hazard ranks based on the depth of sediment deposited and the amount of large wood transported.%平成24年7月九州北部豪雨では阿蘇カルデラ壁の長 大斜面や0次谷で多数の崩壊が発生し，激甚な土砂災害 となった。崩壊が斜面の高い位置で発生したことにより， 土砂は遠方まで到達した。一方，斜面は中腹の傾斜30 度以上の急傾斜部分と麓の居住区域との間に，傾斜30 度未満の長い区間を挟むため，土砂災害警戒区域が指定 され難い課題を有していた。このことから，長大斜面や 0次谷では山腹崩壊土砂が遠方まで到達することを考慮 して危険区域を設定する必要がある。本研究では，家屋 被害の発生した5つの崩壊地で土砂と流木の氾濫実態を 調べ，その結果に基づき危険区域の設定手法を提案し, その内部の危険度評価について考察した。以下に結論を 再揭する。