Abstract

In a flume 4.3 m x 15.2 cm and with sands of mean size from 0.2 to 0.8 mm, thin, near-horizontal laminae developed under shallow flow conditions as the result of (a) the migration of bedforms which had prominent lateral segregation of coarser and finer grains and (b) by gradually raising the water surface during deposition. The laminae formed during the downstream migration of low-relief (2 to 8 mm high) ripples that were out of phase with the water surface and had variable spacings (15 to 40 cm), and by downstream migration of waves (2 to 8 mm high) with regular spacings of 3 to 6 cm that were in phase with the water surface. Laminae from 0.2 to 4 mm thick and up to 3 m long formed when coarser grains in the lee side of the bedforms migrated downstream and buried finer grains on the stoss side of the bedforms; the lack of avalanche faces prevented cross-laminae from developing. Gradually varied flow was maintained by raising the flume tailgate from 2 to 4 mm/minute, and this permitted burial and preservation of the laminae in the backwater area. The shallow depths of flow (all < 5 cm) were a major factor in the development of the low-relief bedforms. Contrary to expectations, laminae were not produced during plane-bed conditions at upper-regime flow. Because no segregation of grains took place on the plane bed, no laminae were formed. The question arises whether sequences of laminae can form in sand in the absence of bedforms which induce a downstream segregation of coarser and finer grains.

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