花崗岩山地における土石流発生流域の地形的特徴 : 2009年7月防府市・山口市豪雨災害の事例

概要

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Many debris flows and slope failures resulting in heavy damage to people and property were caused by a heavy rainfall on July 2009 in Hofu and Yamaguchi cities. The worst sediment disaster occurred in an area underlain by the Hofu granites, which mainly consist of three rock types in lithology: medium- to coarse-grained biotite granite (Gr), medium- to coarse-grained hornblende biotite granodiorite (Gd), and fine-grained biotite granite and granodiorite (Gf). To clarify the topographic features that contributed to the disaster, the relationship among the drainage area, relief ratio, and the type of sediment movement was investigated, and the effect of geological difference on sediment movement was examined. The results showed that there were thresholds determining the lower limit related to the debris flow discharge outside the basins. The probability of slope failures and debris flows which occurred within the drainage basins became higher with an increase in the drainage area, whereas the further increase in the drainage area brought about the probability rise in relation to debris flow discharge outside basins. Debris flow discharge outside the basins can occur in the basins of the Gr area which showed smaller drainage area or lower relief ratio than those of the Gd area.
2010-10-25