Record-breaking heavy rainfall in July 2018 caused an extremely large number of slope movements over a broad area of western Japan. We mapped the distribution of slope movements in the southern part of Hiroshima Prefecture through an interpretation of aerial photographs that were acquired after the rainfall by the Geospatial Information Authority of Japan, and counted a total of 8,497 slope-movement starting points. The widespread distribution of slope movements – from Etajima City of Hiroshima Prefecture to Kasaoka City of Okayama Prefecture – suggests that the heavy rain affected a very large area. The starting points of debris flow during this disaster were commonly close to the crest of mountain ranges. We compared the distribution of slope movements to the 24-hr rainfall accumulation during the heaviest rainfall event to clarify the factors that caused regional difference in slope-movement distribution. We found the area of highest density of the slope movements was consistent with the area receiving a cumulative rainfall of >250 mm. This observation indicated that the position of slope-movement starting points was not related to differences in geology.

1. [1] Japan Meteorological Agency (JMA), “The Prompt Report of Heavy Rain in July 2018 (‘The Heavy Rain Event of July 2018’),” 2018, https://www.data.jma.go.jp/obd/stats/data/bosai/report/2018/20180713/20180713.html (in Japanese. The title is translated by the author) [accessed July 25, 2019]
2. [2] Cabinet Office, Government of Japan, “Damage condition of heavy rain in July 2018 (9 October, 2018) on Cabinet Office,” http://www.bousai.go.jp/updates/h30typhoon7/pdf/301009_1700_h30typhoon7_01.pdf (in Japanese. The title is translated by the author) [accessed July 25, 2019]
3. [3] Committee for examining the flooding and the sediment disasters caused by the heavy rainfall in July 2018 of Hiroshima prefecturaral government office, “Special Paper for the First Meeting (10 September, 2018) of the Erosion Control Section,” 2018, https://www.pref.hiroshima.lg.jp/uploaded/attachment/330358.pdf (in Japanese. The title is translated by the author) [accessed July 25, 2019]
4. [4] T. Danjo, T. Ishizawa, and T. Kimura, “Spatial Analysis of the Landslide Characteristics Caused by Heavy Rainfall in the Northern Kyushu District in July, 2017 Using Topography, Geology, and Rainfall Levels,” J. Disaster Res., Vol.13, No.5, pp. 832-845, 2018.
5. [5] Japan Academic Network for Disaster Reduction, “Emergency Message for Residence in the Affected Area of West Japan Heavy Rain,” 2018, http://janet-dr.com/070_seimei/071_seimei.html (in Japanese. The title is translated by the author) [accessed July 25, 2019]
6. [6] http://www.gsi.go.jp/BOUSAI/H30.taihuu7gou.html [accessed December 27, 2018]
7. [7] The committee for disaster of the Association of Japanese Geographers, http://ajg-disaster.blogspot.com (in Japanese. The title is translated by the author) [accessed July 25, 2019]
8. [8] T. Uchiogi, “Landslides due to One Continual Rainfall,” Shin Sabo, Vol.23, Issue 4, pp. 21-34, 1971 (in Japanese).
9. [9] Disaster prevention information in Hiroshima Prefecture, http://www.bousai.pref.hiroshima.jp/?p=observation%2Frainfall (in Japanese. The title is translated by the author) [accessed July 25, 2019]
10. [10] Committee for examining the flooding and the sediment disasters caused by the heavy rainfall in July 2018 of Hiroshima prefecturaral government office, “Special Paper for the First Meeting (15 September, 2018) of the River and Dam Section,” 2018, https://www.pref.hiroshima.lg.jp/uploaded/attachment/325922.pdf (in Japanese. The title is translated by the author) [accessed June 22, 2019].
11. [11] Advanced Industrial Science and Technology (AIST), “Seamless Digital Geological Map of Japan (1:200,000),” https://gbank.gsj.jp/seamless/index_en.html? [accessed July 25, 2019]
12. [12] T. Yamada, T. Hibino, T. Araki, and M. Nakatsugawa, “Statistical Characteristics of Rainfall in Mountainous Basins,” J. of Hydraulic, Coastal and Environmental Engineering, Vol.527, II-33, pp. 1-13, 1995 (in Japanese).