Research on Free Meandering and Channel Width Formation in Channels with Mixture Grain Size Bed Material

从观测和理论两方面对1998年阿布库马河洪水期间滩地的细沙沉积进行了研究。观测发现，大部分淤积是由直径约0.1 mm的细沙组成的，这通常指的是不包括在底质中的冲沙。观测资料表明，沿主槽与滩地交界处，滩地内侧约10m处淤积最大。悬沙输运理论很好地解释了这一现象。利用另一站测得的悬沙浓度，预报的泥沙淤积量与实测值吻合较好。采用适用于动边界条件的通用非正交坐标系，数值计算了流场、河床变形、岸滩冲刷和航道迁移。流场由二维动量方程和连续性方程计算。根据推移质输运的连续性方程计算了床面变形。根据岸坡泥沙的临界坡度和河床淤积产生的床面来估算岸坡的冲刷和淤积。通过这些过程的迭代过程，计算了渠道几何形状随时间的变化。计算结果与动床和岸坡试验结果进行了比较，验证了新模型的准确性。

Fine sand deposition on a floodplain during the Abukuma River flood in 1998 is studied both observationally and theoretically. It is found from the observation that most of the deposition consists of fine sand of about 0.1 mm in diameter, which is commonly refer to washload that is not included in bed material. The observed data shows that the deposition is maximized around 10 m inside of the floodplain along the boundary between the main channel and the floodplain. This phenomenon is found to be well-explained by the theory of suspended sediment transport. The predicted value of the magnitude of the deposition agrees well with the observed value with the use of suspended sediment concentration obtained in another observation.A numerical model is proposed to investigate free meandering of river. Flow field, bed deformation, bank erosion and channel migration are calculated numerically with a general non-orthogonal coordinate system which can be applied to moving boundary conditions. Flow field is calculated from two dimensional momentum equations and continuity equation. Bed deformation is calculated from continuity equation of bed load transport. Bank erosion and deposition is estimated from critical slope of bank material and emergence of bed due to the bed deposition. Time dependent change of channel geometry is calculated with an iteration process of these procedures. Calculated results are favorably compared with experimental results with movable bed and bank, and thus the accuracy of new model is verified.