Sediment disasters in Shikoku region in July, 2018

Katsuo SASAHARA,Takahiro IKEDA, Yuya IWAI, Koshi KAKUTA, Akito KANAZAWA, Yutaka GONDA, Yuki SAITOU, Yasuhiro SHUIN, Satoshi TAGATA, Masaharu FUJITA, Shusuke MIYATA, Hiroshi MIWA, Ikuo MURATA Kazuki YAMANOI and Takashi WADA


A heavy rainfall event in July 2018, attributable to the influence of the baiu front activated by Typhoon Prapiroon, which was named the “Heavy Rain Event of July 2018″ by the Japan Meteorological Agency, caused numerous disasters. Therefore, we conducted a field survey of sediment‐related disasters around Uwajima City and Seiyo City in Ehime Prefecture and Otoyo‐cho in Kochi Prefecture. The total rainfall amounts from July 5―9 observed at the Uwa AMeDAS (Seiyo City) and Uwajima AMeDAS stations were 539.5 mm and 381.5 mm, respectively. In the vicinity of Uwajima City and Seiyo City, many slope failures and debris flows occurred at dawn on July 7, 2018. Many of these slope failures/debris flows occurred on the south side of the mountain range forming the boundary area between Uwajima City and Seiyo City. This boundary area roughly overlaps the Butsuzo tectonic line, which geologically separates the Chichibu zone and Shimanto zone. Although many of the slope failures were shallow landslides with failure depths of about 1―5 m, relatively large slope failures also occurred in the area where the record for cumulative rainfall was updated by a large amount. In Otoyo‐cho in Kochi Prefecture, a deep‐seated landslide occurred at dawn on July 7, 2018. The total rainfall from July 5―8 observed in the proximity of the Tachikawa rain gauge station was about1200mm, and the maximum hourly rainfall, which occurred at 19:00 on July 6, was 111mm/h. This deep‐seated landslide in Otoyo‐cho was topographically located in a landslide area, and scars of previous landslide movements were confirmed from a topographical survey performed using airborne LiDAR.

Key words

heavy rainfall, slope failure, debris flow, deep seated landslide, Shikoku region