Using the Ukedo basin as the sample, we estimated the river’s response to various rainfall patterns at its outlet works and headworks (turbidity and radioactive cesium concentration in water) by a calculation model called GETFLOWS (Fig. 1).

Fig.1　Calculation area and examples of rainfall patterns assumed for the calculation

Calculation results indicate that the higher the suspended solids concentration is, the higher the cesium concentration is, as shown in Figure 2.

This suggests the possibility that water intake decisions at the headworks can be made from the suspended solids concentration (turbidity), which is easy to measure.
However, the plots of the suspended solids concentration and the cesium concentration also show that there is a wide variation depending on the rainfall pattern and the location of the headworks.

Fig.2　Correlation between the concentrations of suspended solids and ¹³⁷Cs at headworks estimated by calculation

Figure 3 shows some chronological changes in the concentrations of suspended solids and cesium after rains at outlet works and headworks.
It indicates that although rainfalls result in an increase in the concentration of both suspended solids and cesium, the trend of increase varies by cesium distribution in the river basin and by the presence or absence of a dam. River water may remain turbid for a few hours to a few days.

Fig.3　Example of response analysis results for outlet works and headworks

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