Submitted to: International Conference on Fluvial Sedimentology
Publication Type: Abstract Only
Publication Acceptance Date: 10/15/1997
Publication Date: N/A
Technical Abstract: The dispersion of sediment in the lee of bedforms or sudden expansions in the bed topography determines local patterns of sediment transport, the nature of leeside avalanching/bedform migration and, at higher sediment transport rates, the morphology of the bedform. Results from experiments conducted over a series of fixed negative steps, with a leeside angle of 35 degrees, under both clear-water flow conditions and with two imposed sediment transport rates will be presented. Flow over the negative step is dominated by generation of a leeside flow separation zone and associated free shear layer and Kelvin-Helmholtz instabilities. The presence of sediment at these low concentrations does not appreciably alter the characteristics of the fluid phase. Slight increases in the intensity of the vertical component of turbulence in the leeside of the step may be attributed to the increased mixing generated by sediment settling. Differentiation of the fluid and sediment signal reveals the markedly different trajectories in the leeside: the sediment grains quickly plummet towards the bed, they possess higher downward velocities than the fluid and they display little recirculation whilst in suspension within the separation cell. Records also reveal the size differentiation of sediment in the leeside, with the coarser sediment setting first in the immediate leeside and the suspension of larger grains and higher sediment concentrations being linked to the separation zone free-shear layer. These results will be discussed in relation to the sediment and fluid dynamics of ripples and dunes and the suspension of sediment associated with bedforms.