[H-DS09] Human environment and disaster risk

[HDS09-06] Landslides on slopes of constructed agricultural land in Omishima, Geiyo Islands due to heavy rain in July 2018 and their causal factors

*Takashi Kimura¹, Go Sato² (1.Graduate School of Agriculture, Ehime University, 2.Graduate School of Environmental Informations, Teikyo Heisei University)

Keywords:rainfall-induced landslide, constructed agricultural land , embankment slope, land-use history

In 2018, it was a record-breaking heavy rain over a wide area in the Chugoku and Shikoku regions due to the effects of the typhoon passage and the development of the Baiu front, from June 28 to July 8. As sediment disasters caused by this heavy rain, the damage around Kure City, Hiroshima Prefecture was particularly severe, but many other disasters have simultaneously occurred in the Seto Inland Sea Islands. We report rainfall-induced landslides in Omishima, Geiyo Islands, as an example of such disasters. Several hundreds of landslides occurred on Omishima during this heavy rain, while large-scale landslides transformed into debris flows and traveled long distances concentrated in the Inokuchi and Amazaki districts of Kamiura Town in the central part of the island. Agricultural land was developed in these two districts around the early 1970s to expand the orchard areas. Many artificial slopes were constructed by modifying the topography of head hollows in the upstream regions, which were not previously used for agricultural purposes. Scarps of the landslides are located on the embankments of these artificial slopes; thus, the destabilization of embankment materials is likely to be the primary cause of landslide initiation. Because similar artificial slopes are distributed around landslide slopes, it is necessary to investigate the geometrical properties of those artificial slopes, including the thickness and areal extent of the embankment, in detail to assess the risk of future sediment disasters. Notably, agricultural lands in hilly regions such as Omishima have been rapidly abandoned in the last few decades. Such a situation could further obscure the distribution of embankment slopes and overlook the risk of future sediment disasters. Efforts are needed to visualize land-use histories using archived geographic data such as old maps and aerial photographs.