[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-P02] Crevasse-splay deposits of the August 2022 flooding of Nakasato River in Tsugaru Plain

*Koji Umeda¹, Rina Okada¹, Yuto Kobayashi³, Ching-Ying Tsou², Sa'Aya Iguchi⁴ (1.Graduate School of Science and Technology, Hirosaki University, 2.Faculty of Agriculture and Life Science, Hirosaki University, 3.Graduate School of Sustainable Community Studies, Hirosaki University, 4.Niigata Prefectural Government)

Keywords:crevasse-splay deposits, Heavy Rain of August 2022, Nakasato River, Nakasato River

Heavy rainfall triggered by a quasi-stationary front occurred on 8 - 13 August 2022, leading to flooding of the Nakasato River in Tsugaru Plain, northern east Japan. Dike breaks along the river resulted in the deposition of a large amount of sediment, so-called crevasse-splay deposits, on the plain. Crevasse splay deposits are mainly composed of fine to medium sand with gravels and can be divided into the following four units: Unit 1, gravel layer covering artificial soil layer; Unit 2, blue-grey fine sand; Unit 3 light brown fine to medium sand and mud mixtures with mud clasts and gravel, indicating traction-carpet stratification; Unit 4 light yellow medium sand including erosional basal contact. The difference between four units correspond to the changes in flow condition during the flooding.