Numerical demonstration of massive sediment transport and cs recontamination by river flooding in Fukushima costal area

福島沿岸域での河川洪水による大規模な土砂輸送とセシウム再汚染の数値シミュレーション

町田 昌彦  ; 山田 進  ; 板倉 充洋  ; 奥村 雅彦   ; 北村 哲浩   

Machida, Masahiko; Yamada, Susumu; Itakura, Mitsuhiro; Okumura, Masahiko; Kitamura, Akihiro

河川河床土への堆積による放射性セシウムによる再汚染は、福島第一原子力発電所事故以降、福島県での環境修復にかかる中心的課題である。実際、遠隔操作ヘリによるモニタリングや直接の土壌直上での観測によって、河川敷での近くの線量と比べて増加している様子が確かめられており、かつ、福島沿岸域での2, 3の河川においては、上流域が高汚染地帯であることから、その増加傾向は顕著である。この状況が発生する機構を明らかにするため、2次元の河川シミュレーションコードiRICを用いて、土砂の堆積シミュレーションを行った。シミュレーションでは、洪水時に水位が上昇し、河川敷に達することから、洪水時の増水時間プロファイルを仮定し、土砂の堆積状況を解析した。発表では、どのようなメカニズムで河川敷堆積が起こるか、そして、河川敷での植生の影響等を明らかにする。

Radioactive Cs recontamination brought about by deposition of silt and clay on river beds has been a central issue of environmental recovery problems in Fukushima prefecture after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. In fact, the river-side sediment monitored by using remote controlled helicopters and direct sampling measurements has been confirmed to be highly contaminated compared to the other areas, which just naturally decay. Such contamination transportation is especially remarkable in a few rivers in coastal areas of Fukushima prefecture, because their water and sediment are supplied from the highly contaminated area along the northwest direction from FDNPPs. Thus, we numerically study the sediment transportation in rivers by using 2D river simulation framework named iRIC developed by Shimizu et al. Consequently, we find that flood brought about by typhoon is mainly required for the massive transport and the sediment deposition in the flood plain is efficiently promoted by plants naturally grown on the plain. In this presentation, we reveal when and where the sediment deposition occurs in the event of floods through direct numerical simulations. We believe that the results are suggestive for the next planning issue related with decontamination in highly-contaminated evacuated districts.