大戸川における濁りの季節変化

Abstract

In order to clarify the seasonal change of water turbidity, the sediment concentration of washload and the water discharge have been observed once or twice a day in the River Daido sinceApril of 1975. On the basis of the results of the observation in seven years, we discuss relationsbetween water discharge and wash load, the statistical properties of concentration distributionfor each water stage and effects of seasonal change and flow stage on wash load.From the percentage of C>C+σ and C<C-σ for each month, in which C and σ are mean valueand standard deviation of the concentration C, respectively, it appears that the water turbidity is lowin October and November, and high in February, March, June and August. As one of the reasons, the seasonal changes of grain-size distribution of bare soil places in the river basin were observed, andthe surface material at the hill side was shown to be coarser from spring to autumn. Furthermore, it was pointed out that the concentration of wash load is higher at rising and peak flow stages, and fora flood after longer duration of low flow stage.