Abstract
Numerous deep-seated landslides have occurred in southern Wakayama and Nara prefectures, with over 1,000 mm of rainfall caused by typhoon no.12 in 2011, which resulted in a great deal of damage. We need to consider future action for deep-seated landslides, and contemplate evacuation for the occurrence of deep-seated landslides as they have recently increased in incidence. Deep-seated landslides and the rainfall index are dominated by long-term rather than short-term rainfall indicators, but few concrete case studies are available.
We studied C-band rainfall mesh data in the southern part of Nara Prefecture after typhoon no.12 to elucidate the rainfall index relevant to deep-seated landslides.
We found that a deep-seated landslide happened around the end of the rainfall event after typhoon no.12, and that a deep-seated landslide occurred after 700-800 mm of rainfall fell in 48 hours with at least 900-1,000 mm of total rainfall. However, a clear relationship could not be found for rainfall totals of ≥ 1,400 mm. We did not find a clear relationship between the rainfall index and deep-seated landslides, although a correlation between a higher tendency for a large-scale landslide and a greater number of deep-seated landslides was confirmed in the medium-to long-term rainfall index. We found that the excess probability of rainfall is highly correlated with deep-seated landslide.
