Parallel lamination developed from upper-stage plane beds: A model based on the larger coherent structures of the turbulent boundary layer

Abstract

The larger coherent flow structures of free-surface turbulent currents scale on flow thickness and mean velocity, and are probably upstream-sloping horse-shoe vortices. Coupled to them is a spatial pattern of bed shear stress which, as it moves with the downstream-convected structures, seems capable of generating travelling bed waves on a deformable upper-stage plane sand bed. The wave height is on the order of a few millimetres, but the horizontal dimensions can measure decimetres or metres, increasing with flow thickness. As the bed shear stress is a maximum in the wave troughs, the sand preserved within the body of each wave is expected to be normally graded texturally. The body of each wave constitutes a graded lamina, a portion of which has some chance of permanent preservation under conditions of long-term net deposition. The scale and grading of these hypothetical laminae are consistent with the textural features and dimensions of parallel laminations associated with upper-stage plane beds observed from the field. The model is not applicable to wave swash-backwash laminations, and does not describe the case where slowly moving, steep-faced flat bars generate laminae.