Distribution of Landslides Triggered by the Heavy Rainfall Event July 2018 in Uwajima City, Japan

In July 2018, many landslides were occurred by heavy rainfall in wide areas of Western Japan. Our objective was to examine factors determining the distribution of landslides in a part of Uwajima city, Ehime prefecture where was one of the greatest numbers of landslides. We used the landslide distribution map provided by Geospatial Information Authority of Japan. We also developed the distribution map of causal factors such as the rainfall characteristics, geology, slope gradient, land-use, and Normalized Difference Vegetation Index (NDVI). The rainfall distribution in each 10-min was obtained from the X band MP radar network. Based on the data, we developed the maps of the 1-h, 3-h, 12-h, and 24-h maximums and total rainfall. The geology and land-use map were obtained from National Land Numerical Information by Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT). We classified the land as forest, the other agricultural land (i.e., mostly tangerine orchards), and the others. The slope gradient was calculated based on Digital Elevation Model (DEM) with the resolution of 30 m. NDVI was calculated based on the Landsat images from United States Geological Surveys (USGS) in April 2018. These distributions were compared with the distribution of landslides using the Geographic Information System (GIS). There were 162 landslides with the total area of 0.16 km2 in the study area of 30.29 km2. The landslide density in gentle slopes (<16°) was larger than that in steep slopes (>25°). The landslide density in small NDVI was lower than that in large NDVI. There were no considerable differences between the landslide density in forests and that in the other agricultural lands. The distribution of landslides was close to the distribution of the 12-h and 24-h maximum rainfalls. We will examine the significance of each factor using multivariate analyzes like the logistic regressions for the next stage. Keywords: causal factors, distribution of landslides, rainfall characteristics