2016 Volume 56 Issue 2 Pages 37-50
The 2015 Kanto-Tohoku Torrential Rain that hit the Kanto and Tohoku regions of eastern Japan from September 9 to September 12, 2015, resulted in a major riverine and slope disaster. Overbanking and breaching occurred along the central part of the Kinu River and formed crevasse splays on the flood plain around Kami-Misaka, Joso City, Ibaraki Prefecture. We clarified the distribution of crevasse splay deposits based on an interpretation of aerial photographs taken just after the disaster and a field survey. In addition, we performed an excavation survey and grain size analysis of the crevasse splay deposit. These resulted in the following findings.
1) Crevasse splay deposits mainly comprise fine to medium sand and can be divided into the following three units : Unit A, sandy silt to fine sand layer showing upward coarsening ; Unit B, fine to medium sand layer showing upward fining ; and Unit C, fine to medium sand layer with parallel lamination and low-angle cross-stratification.
2) The difference between these units probably corresponded to the changes in flow conditions during the flooding. Unit A, deposited at the early period of flooding, was derived from heavy rain and overbank of the river water ; Unit B, deposited during hydraulic bore, was generated by the breach of the river bank, flowing down the flood plain ; Unit C was formed by the flooding flow from the Kinu River via the breach point during the flood.
3) Typical stratigraphy of crevasse splay deposits in this study is characterized by the upward-fining unit (Unit B), which shows a difference from the known flood deposits.
