Sediment and Cs transport and accumulation in the Ogaki Dam of eastern Fukushima

福島県東部の大柿ダム内の土砂とセシウム137の移動と堆積

山田 進  ; 北村 哲浩   ; 操上 広志  ; 山口 正秋 ; Malins, A.  ; 町田 昌彦  

Yamada, Susumu; Kitamura, Akihiro; Kurikami, Hiroshi; Yamaguchi, Masaaki; Malins, A.; Machida, Masahiko

本論文は福島長期環境動態研究(F-TRACE)の一環として実施した2次元河川シミュレーションコードNays2Dによる福島県の大柿ダムでの放射性セシウムの流動及び堆積分布予測に関する研究についての報告である。現状のシミュレーションコードでは放射性物質の移動は計算できないが、放射性セシウムは粘土のような粒径の小さい土砂に吸着しやすいことに着目し、土砂の移動を計算しそれを放射性セシウムの移動とみなすことにした。実際に、2013年9月の洪水時や平均的な洪水時の流量等を境界条件として用いて、大柿ダム内の土砂の流れのシミュレーションを原子力機構のBX900で実施し、土砂の移動を計算した。特筆すべき成果は、粒径の小さいシルト及び粘土の振る舞いはダムの排水口の高さに大きく依存しており、排水口の位置を高くすることでダム内にシルト及び粘土が多くとどまるようになることを示したことである。この結果は、ダムの水位を適切に調整することで下流への土砂に付着したセシウムの移動を防げる可能性があることを示しており、住民の被ばくの低減に資する成果である。

The Ogaki Dam Reservoir is one of the principal irrigation dam reservoirs in the Fukushima Prefecture and its upstream river basin was heavily contaminated by radioactivity from the Fukushima Daiichi Nuclear Power Plant accident. For the purpose of environmental assessment, it is important to determine the present condition of the water in the reservoir and to understand the behavior of sediment-sorbed radioactive cesium under different modes of operation of the dam. This paper addresses this issue with numerical simulations of fluvial processes in the reservoir using the 2D simulation code Nays2D. We present results for sediment deposition on the reservoir bed and the discharge via the dam under typical yearly flood conditions. The simulations show that almost all the sand and silt that enter into the reservoir deposit onto the reservoir bed. However, the locations where they tend to deposit differ, with sand tending to deposit close to the entrance of the reservoir, whereas silt deposits throughout the reservoir. Both sand and silt settle within a few hours of entering the reservoir. In contrast, clay remains suspended in the reservoir water for a period as long as several days, thus increasing the amount that is discharged downstream from the reservoir. Under the current operating mode of the dam, about three-quarters of clay that enters the reservoir during the flood is discharged downstream. By raising the height of the dam exit, the amount of clay exiting the reservoir can be reduced by a factor of three. The results indicate that the dam can be operated to buffer radioactive cesium and limit the contamination spreading into lowland areas of the Ukedo River basin. These results should be a factor in considerations for the future operation of the Ogaki Dam, and will be of interest for other operators of dam reservoirs in areas contaminated by radioactive fallout.