大戸川におけるウォシュ・ロードの年間変化

Abstract

In order to verify the applicability of a hydraulic model on the transport process ofwash load developed in the previous study1), we have been observing not only theconcentration of suspended load and water stage during a flood, but also the concentra-tion of suspended load twice a day in the drainage basin of the River Dido since Aprilof 1975.The concentration of wash load is dependent on whether the water discharge belongsto the period of rising stage or recession stage in a hydrograph, even if the water dis-charge is kept constant. The concentration in the period of rising stage is higher than thatof the recession stage. This may be due that the ratio of the discharge of sub-surfaceflow to the total discharge in the period of rising stage is less than that of the period ofrecession stage.It often appears that the concentration of wash load observed from April to July isgreater than one observed from August to November to the constant water discharge.This is related to seasonal variations of the concentration of wash load.Moreover, it was clarified that the concentration of wash load is affected by theduration time of low flow stage.These three elements which affect the concentration of wash load are discussed quali-tatively on the basis of observed results.