Large‐scale erosion and overbank deposition caused by the July 2013 flood of the Abu River, Yamaguchi City, Japan

Abstract

en

This paper reports on the erosion, transport, and deposition processes associated with an overbank deposit formed by the flooding of the Abu River on July 28, 2013, in Yamaguchi City, Japan. At the study site, river flows overtopped the levee revetment upstream of a meander bend cutting it off and flowing back into the main channel downstream. In this sequential process, it deposited large amounts of sediments, ranging from mud to cobbles, on the floodplain. The surface of paddy fields adjacent to a railway line, located at the center of the affected floodplain, was severely eroded by the flood flows. Overbank deposits composed of both upstream finer sediments and eroded coarser terrestrial sediments are laid down in the affected area. Large amounts of pebbles and cobbles originating from the eroded terrestrial area formed a gravelly pile on top of the sand and gravel sediments derived from the river. This finding indicates that sands and gravels were deposited prior to the formation of the gravelly pile, probably before and during peak flood flows. An inverse grading structure is evident in the lower to middle part of these comparatively thick deposits, most likely due to differences in transport pattern between entrained terrestrial gravels and upstream finer sediments.

Abstract

ja

本研究では, 2013年7月に発生した阿武川洪水によって形成された堆積物を形成直後に調査し, 得られた試料の粒度分析を行うことで, 河川氾濫に伴う侵食・運搬・堆積過程を復元した. 氾濫堆積物は氾濫原の広範囲に分布しており, 上流から運搬されてきた細粒物だけでなく, 氾濫原上で侵食された粗粒な砂礫も含んでいた. 層厚は最大約60 cmで, 比較的厚い地点では堆積物の下部から中央部にかけての逆級化構造と中央部から上部にかけての級化構造が確認された. 氾濫堆積物の形成過程で, 上流から運搬されてきた細粒物から氾濫原上で侵食された粗粒物へと供給される粒子が遷移したことが逆級化構造を形成した要因であると推測される. また, 本調査地域の氾濫原上に存在した線路などの高まりが大規模な侵食を発生させ, 巨視的な氾濫堆積物の特徴と形成プロセスを複雑にしたと考えらえる.