Structural Changes of Mobile Gravel Bed Surface for Increasing Flow Intensity

Abstract

The structure of a mobile bed in a laboratory channel composed of sand and gravel (D50=8 mm) was characterized over a series of experiments with steady flows from a low flow, where only the finer fractions of the bed material were in motion to flows in which most bed material grain sizes were in motion. In each experiment, sediment transport rates were observed to initially be greater than the long-term mean rates, and fluctuations in transport rate decreased in period as bed shear stresses were increased. The bed surface median grain size increased with bed shear stress, while the sand fraction of the bed material organized into longitudinally extended corridors, which persisted as flow and transport rates were increased. The presence of the sand corridors was reflected by changes in the probability density function of the bed-surface elevation standard deviation evaluated at the grain-scale. The formation and organization of these corridors may have a strong influence on sand and gravel transport in channels with mixed sand and gravel bed material.