Abstract

The geometry and dynamics of low-relief bed forms (pebble clusters, bed-load sheets, and bars) developed in a mixed-size and mixed-density sediment (sandy gravel) were studied in a laboratory channel. In most runs, equilibrium uniform flow was established with discharge 0.05 m 3 /s, mean flow depth 0.1 m, mean flow velocity 0.82-0.87 m/s, mean bed shear stress 1.6-2.2 Pa, dimensionless bed shear stress 0.04-0.07, and mean bed-load transport rate 0.009-0.013 kg/m-s. Low-relief bed forms were observed directly and in bed-height records and bed profiles measured with a high-resolution, ultrasonic bed profiler. Pebble clusters are groups of relatively large, temporarily immobile clasts with lee side and stoss side accumulations of finer sediment, and with adjacent scoured zones with heavy-mineral concentrations. Pebble clusters are concentrated in relatively coarse areas of the bed such as in the troughs of bed-load sheets and bars. Asymmetric, sinuous-crested bed-load sheets ranged from one to two grain diameters up to 12 mm in height and 0.6-1.2 m in length. Migration rates determined from bed-form spacings and periods were typically 1-1.5 mm/s. These bed forms had relatively fine-grained crests and coarse-grained troughs and a well-mixed grain texture along the stoss side. Bed-load sheets had little associated form drag. Low-relief bars were 10-17 mm high and 1.7-2.4 m long, and typical migration rates were 0.4 mm/s. The migration of these bars was associated with fluctuations in the water-surface slope and bed-load transport rate with periods of about two hours. Low-relief bed forms in heterogeneous sediments appear to be ubiquitous features of lower-stage "plane" beds and those transitional to dune-covered beds where the bed is not completely mobile.

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