鬼怒川中流域，茨城県常総市上三坂地区における平成27年9月関東·東北豪雨の破堤堆積物

摘要

平成27年9月関東·東北豪雨により鬼怒川中流域の常総市上三坂地区で生じた人工堤防の決壊による堆積物について，トレンチ掘削や粒度分析を行い，その堆積学的特徴を明らかにした．破堤堆積物は全体的に細粒砂～中粒砂を主体とし，下位から順に，上方粗粒化を示す細粒砂層(ユニットA),上方細粒化を示す細粒砂～中粒砂層(ユニットB),低角斜交層理や平行葉理の発達する細粒～中粒砂層(ユニットC)に細分される．各堆積ユニットは，鬼怒川の越水，破堤，さらに河川水が氾濫原に流入した後，氾濫が収束するという一連の堆積プロセスを反映していると推定される．ユニットAは越水による氾濫堆積の特徴としてよく知られているものである．今回の破堤堆積物では，その上に上方細粒化するユニットBが重なることが特徴である．%The 2015 Kanto-Tohoku Torrential Rain that hit the Kan to and Tohoku regions of eastern Japan from September 9 to September 12,2015,resulted in a major riverine and slope disaster.Over-banking 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.