Establishment of watershed conservation methodology based on the prediction of biosphere damage due to fine sediment discharge

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Concentration of solids and organic fine solids in the stream water were measured in a forested catchments where selective-cutting had been carried out. Total fine sediment and organic fine solids concentrations from selective cutting catchments were significantly higher after cutting as compared with those before cutting. The processes underlying land and riparian degradation by quantifying soil erosion and gully growth in slopes adjacent to cropland were examined. The concentration of suspended sediments was consistently higher in cultivated catchments where gully expansion causes 2-3 times more landslides than occur in forested catchments.On the basis of these soil erosion and fine sediment discharge situation, erosion velocity equation for cohesive soil was proposed by the newly developed surface shear test equipment and hydraulic model tests.Relation between soil surface cohesion of over-saturated layer and water contents at 5 mm depth revealed that as water content increased, soil surface cohesion fell exponentially. Meanwhile, erosion velocity decreased when cohesion increased, however it was increased sharply as the friction velocity increased.Sediment discharge prediction method were newly developed on the base of this equation and physical methods of restriction and control of fine sediment discharge for biosphere conservation in watershed were proposed.