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

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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.