Bed form initiation from a flat sand bed

Abstract

Bed form initiation in unidirectional flow is examined on a flat bed composed of a homogeneous 0.5 mm sand. Velocity profiles taken prior to bed form development indicate that the examined flows are typical of fully turbulent, uniform, open channel flows. Under these conditions, two separate modes of bed form initiation are observed: defect and instantaneous initiation. Defect initiation occurs at lower flow stages, where sediment transport is sporadic and patchy, and is characterized by defect propagation associated with flow separation. Instantaneous initiation occurs at larger flow strengths, where sediment transport is general and widespread. This form of bed form initiation begins with the imprinting of a cross‐hatch pattern on the flat sediment bed, which leads to chevron‐shaped forms that migrate independently of the initial pattern. The chevrons eventually align to form incipient crest lines. This mode of bed form initiation does not appear to be linked to turbulent structures, but integral scales derived from velocity measurements prior to bed form development are similar to the initial bed form length scales.