Studies on Controlling Suspended Sediment Discharge in a Small River Basin that is Covered with Volcanic Ash Soil in a Snowy and Cold Region. II. Characteristics of Suspended Sediment Discharge from Basins with Open Upland Field.:II. Characteristics of Su

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We report on the characteristics of suspended sediment discharge in three small basins covered with volcanic ash soil, where upland farming is widespread. Despite a small runoff ratio in summer when the ground is permeable, high suspended sediment discharge is measured because of high erodibility of soil. On the other hand, high suspended sediment discharge is measured also during snowmelt because of impermeability of the frozen subsurface. The field measurements introduced the relationship between the flow rate (Q m3/s) and the suspended load (Qs m3/s), giving the magnitude of three or four orders larger that by the empirical equation; Qs=(4×10-8∼6×10-6) Q2. Furthermore, the concentration of suspended sediment in snowmelt season is higher than that in summer. It is suggested that the origin of suspended sediment is upland fields in the small basins, because the particle size distributions are similar. The suspended sediment includes much fine silt less than 0.1 mm in diameter and flows out in a way of wash load. The control for soil erosion is effective if the buffer zone is constructed between the upland field and the river, because suspended sediment is fine and does not settle easily on the river bed.
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