Well-sorted and graded sands in oscillatory sheet-flow
Much research effort is focused on the development of reliable empirical and numerical models for the prediction of sand transport. Confidence in these models depends on good agreement between predicted and measured transport rates for controlled conditions and, in the case of process-driven numerical models, good agreement between measured and predicted "sub-processes", such as time-dependent concentration and velocity profiles. The purpose of this project was to conduct experiments that measure these transport "sub-processes" in full-scale sinusoidal and asymmetric oscillatory sheet-flow conditions for well-sorted and graded sands. Detailed measurements have been obtained of concentrations, velocities, total and fractional transport rates and particles sizes in bed samples and in suspended and transported sands. The range and level of detail in the new concentration measurements makes it possible to interrogate concentration behaviour much more rigorously than previously possible. A new equation is presented which characterises time-dependent concentration profiles in the sheet-flow layer. The equation is based on time-dependent erosion depth and reference concentration. Analysis of the dependence of these parameters on flow and bed conditions is presented. The new velocity measurements extend "deeper" into the oscillatory boundary layer than previously possible and the results show classic features of oscillatory boundary layer flow. The product of the measured velocity and concentration data gives time-dependent sediment flux profiles. Analysis of the flux profiles reveals the detailed transport processes. The effects of unsteady behaviour and the effects of interactions between different size fractions in graded beds are evident in the sediment transport results. Unsteady effects act to reduce net transport and result in a strong offshore-directed transport in the case of fine sand. There is strong interaction between size fractions in graded beds. The mobility of the finer fractions is suppressed by the presence of coarser sands whilst the mobility of the coarser fractions is increased by the presence of finer sands.