Landslide disasters in Ehime Prefecture resulting from the July 2018 heavy rain event in Japan

Abstract

In early July 2018, record heavy rainfalls caused enormous damage from western Japan to the Tokai region. In particular, Hiroshima, Okayama, and Ehime Prefectures suffered extensive human casualties and tremendous damage to housing units and the infrastructure due to landslides and river flooding. The authors conducted a series of reconnaissance operations and analyzed various kinds of data, including rainfall records, aerial photographs, official statistics of the disasters, and topographical and geological maps of Ehime Prefecture. In this report, the authors focus on the rainfall characteristics, geological background, and landslide statistics and distributions associated with the July 2018 events, and present examples of typical damage by geological belt. Based on this, they discuss several characteristics of the landslides, including the rainfall thresholds by geological belt, the characteristics of the runout distance of the debris flows, and the rainfall thresholds for landslide initiation in terms of the Soil Water Index (SWI). Based on an official database of a total of 413 landslides occurring in Ehime Prefecture and the authors’ newly developed database of 883 landslides in the southern part of the prefecture, several findings were obtained, for example, that slopes in the Shimanto Belt had a lower rainfall threshold for landslide initiation than slopes in the other two geological belts, namely, Sambagawa and Chichibu Belts.