Sediment and (137)Cs behaviors in the Ogaki Dam Reservoir during a heavy rainfall event.

We performed a simulation of sediment and (137)Cs behaviors in the Ogaki Dam Reservoir, one of the main irrigation reservoirs in the Fukushima prefecture, Japan, during a heavy rainfall event occurred in 2013. The one-dimensional river and reservoir simulation scheme TODAM, Time-dependent One-dimensional Degradation and Migration, was applied for calculating the time dependent migration of sediment and (137)Cs in dissolved and sediment-sorbed forms in the reservoir. Continuous observational data achieved in the upper rivers were used as the input boundary conditions for the simulation. The simulation results were compared with the continuous data achieved in the lower river and we confirmed the predicted values of sediment and (137)Cs in sediment-sorbed form at the exit of reservoir satisfactorily reproduced the observational data. We also performed sediment and (137)Cs behavioral simulation by changing the water level of the reservoir, because such a dam operation could control the quantities of sediment and (137)Cs discharge from and/or deposition in the reservoir. The simulation clarified that the reservoir played an important role to delay and buffer the movement of radioactive cesium in heavy rainfall events and the buffer effect of the reservoir depended on particle sizes of suspended sediment and the water level. It was also understood that silt deposition was the main source of the bed contamination (except for the initial fallout impact), while clay was the main carrier of (137)Cs to the lower river at a later stage of rainfall events.