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

在森林集水区进行选择性采伐，对溪水中固体和有机细颗粒的浓度进行了测定。择伐集水区采伐后的细颗粒泥沙总量和有机细固体含量均显著高于采伐前。通过量化农田附近坡地的土壤侵蚀和沟壑生长，研究了土地和河岸退化的过程。在此基础上，利用新研制的地表剪切试验装置和水工模型试验，建立了粘性土的侵蚀速度方程。过饱和层土壤表面粘聚力与5 mm深度含水率之间的关系表明，随着土壤含水率的增加，土壤表层粘聚力呈指数下降。同时，侵蚀速度随凝聚力的增大而减小，随摩擦速度的增大而急剧增大。在此基础上，发展了泥沙流量预报方法，提出了流域生物圈保护中细颗粒泥沙流量约束与控制的物理方法。

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.