Cohesionless Fine‐Sediment Bed Forms in Shallow Flows

Bed‐form data from laboratory experiments using cohesionless, fine silica solids in shallow water flows are analyzed to find dimenionless relationships for geometry and friction factor. With increasing flow discharge over a stationary flat bed, the bed‐form sequence is primary ripples, barchanoid and linguoid ripples (secondary ripples), ripples on dunes (for silt grades only), and a moving flat bed. The primaries develop from the incipient transport stress, otherwise called the primary threshold. By extrapolating dimensionless bedform length and height data to this threshold, it is shown that the geometry is proportional to the depth of the viscous sublayer thickness. The secondaries develop from a secondary threshold, which is an approximate constant stress condition at which incipient suspension also begins. The ripples on dune form is associated with a tertiary threshold for which the dimensionless stress ratio Y/Yc is about 20. And the moving flat bed form occurs at a transition threshold for which Y/Yc appears to be a natural constant at about 15. A single friction factor curve is demonstrated for all these data.