Bagnold Approach and Bed-Form Development

Abstract

The results of a laboratory investigation of the flow over a narrowly graded 0.49 mm sand are examined within the framework of Bagnold’s Theory for the Flow of Cohesionless Grains in Fluid and general agreement is found between the experimental and predicted behavior. Bagnold’s primary and secondary bed features are shown to correspond to ripple type and dune type bed-forms respectively. The subdivision of the secondary features between intermediate, flattened dunes, and developing dunes is seen to occur at a particular ratio of the bed load to the associated intergranular fluid stress at each of the four depths of flow investigated between 0.25 ft (0.08 m) and 1.5 ft (0.45 m). Peak dune development is suggested to represent the end of wholly viscous grain shearing in the moving grain dispersion, and consideration is given to the conditions leading to the disappearance of bed features.