Sediment suspension in oscillatory flow: measurements of instantaneous concentration at high shear

Abstract

Different syphon type suspended load probes were used together with a specially developed “carousel” sampler for measurements of the instantaneous sediment concentration in turbulent oscillatory flow over a sand bed. Shields parameters were well above the ripple/flat bed transition region, resulting in intense sediment transport over a flat bed. The measurements were performed at different levels in a large oscillating water tunnel. They showed some characteristic features of the temporal concentration variation at fixed levels, including a pronounced effect of the orientation of the suction tube relative to the flow. The variation with height of the average concentration is reasonably well described assuming a constant turbulent diffusivity, and the magnitude of this can be predicted relatively well using simple turbulence arguments. Two empirical formulae for the extrapolated bed concentration are also presented. The analysis is partly based on Wilson's finding that in sheet flow the equivalent bed roughness is nearly proportional with the Shields parameter, resulting in a roughness/grain diameter ratio much larger than 2.5, the figure often adopted for a flat bed, but without sheet flow.