Impact of extreme typhoon events on the fluvial discharge of particulate radiocesium in Fukushima Prefecture

台風イベントに伴う河川からの粒子態Cs流出量の評価

中西 貴宏   ; 大山 卓也 ; 萩原 大樹  ; 佐久間 一幸   

Nakanishi, Takahiro; Oyama, Takuya; Hagiwara, Hiroki; Sakuma, Kazuyuki

福島県請戸川の観測結果に基づき、2019年10月の台風イベントに伴う河川からの粒子態Cs流出量を評価した。台風イベントによる土砂粒子の流出量は年間の90%を占め、それまで福島原子力発電所事故後最大だった2015年9月出水イベントの約2倍だった。しかし、期間中の粒子態Cs濃度低下により、粒子態Cs流出量は2015年の約2/3であった。また、2019年10月の粒子態Cs流出量は流域の沈着量のわずか0.1%であり、沿岸のCs蓄積量への影響は非常に限られたものだった。

The two huge typhoons in 2019, Hagibis and Bualoi, caused enormous flood damage to Fukushima. On the basis of field observations over 6 years in Ukedo River near the Fukushima Nuclear Power Plant, sediment and Cs discharges from the river catchment were quantitatively evaluated. Approximately 90% of annual sediment and Cs discharges in 2019 was occupied during the typhoons Hagibis and Bualoi events. This sediment discharge was almost twice than the discharge during the largest ever flood event since the Fukushima nuclear accident, caused by typhoon Etau in September 2015. However, Cs discharge during Hagibis and Bualoi events was two-thirds that of Etau event, because the particulate Cs concentration in river water decreased during the observation period. Moreover, Cs discharge during two typhoon events in 2019 accounted for only 0.1% of the catchment Cs deposition and the impact of radiocesium to the coastal area was extremely limited.