斜面水文学の視点から見た2013年10月の台風26号による伊豆大島の斜面崩壊

Abstract

Typhoon 26th attacked Izu-Oshima Island, 120 km south west of central Tokyo, on 15 to 16 October, 2013. A heavy rainfall of which the total amount for 24 hrs was 824 mm (but effective rainfall for the landslide occurrence was about 400 to 500 mm) was supplied to slopes composed of volcanic deposits. Specifically at the slopes located at 2 km east of Motomachi town (Kandachi area), shallow landslides which apparently showed a particular feature, about 1 m deep but a few hundred meter wide, were caused during this heavy rainfall. The sliding surfaces were located just on an impermeable loess layer which is at about 1 m deep in soils and deposited between the Y1 (in 1777) and Y2 (in 1684) eruptions. Sandy-muddy debris flows subsequent to the landslides killed 36 peoples and three have been missing yet. In the presented paper, to understand roughly the cause of the landslides, we show the real features of the landslides and examine the hydrological conditions at the slopes, which linked directly to the landslides induced by this heavy rainfall.