Landslide Susceptibility in a Highly Cultivated Hilly Region: Artificial Slope Construction in 1963–1979 and the Subsequent 2018 Landslide Event in Omishima, Western Japan

Agricultural land use can change hydro-geomorphic conditions and slope stability through vegetation clearance and the modification of slope topography and foundations. To better understand the effect of agricultural land use on landslide susceptibility, it is crucial to examine the spatio-temporal relationships between landslide occurrence and land-use history. This study investigates a recent landslide event in Omishima Island, western Japan. We mapped the distribution of landslides induced by the July 2018 heavy rainfall and land cover types to identify landslides that occurred on artificial slopes. 512 landslides were identified; 282 and 207 landslides occurred in the forest (35.6 km2) and the agricultural land (22.0 km2) areas, respectively. Large-scale landslides (> 1000 m2) were concentrated in agricultural land within low-relief hillslopes. The land cover maps revealed that the agricultural land area rapidly expanded around the early 1970s in this part. Many artificial slopes were constructed by modifying the topography of head hollows in the upstream regions. Large-scale landslides were located on embankments of these artificial slopes; thus, the destabilization of embankment materials is likely to be the primary cause of the landslides. Topographical analysis showed that the gradient at which the landslide occurred (S, m/m) becomes gentler as the upstream drainage area behind the embankment (A, m2) increases. This indicates that the flowing water from upstream significantly decreased the lower limiting angle for landslide initiation. The threshold condition in the form of S = α A−β was estimated using the quantile regression method: S = 0.698 A−0.132 (2nd percentile regression of 19 embankments collapsed due to the landslides). This threshold suggests that the existing embankments include 39 slopes with a higher potential of landslides. Future research should investigate the stability of those slopes in detail, both in terms of drainage and strength of embankment materials.