Observations of velocities, sand concentrations, and fluxes under velocity-asymmetric oscillatory flows

Abstract

U-tube measurements of instantaneous velocities, concentrations, and fluxes for a well-sorted, medium-sized sand in oscillatory sheet flow are analyzed. The experiments involved two velocity-asymmetric flows, the same two flows with an opposing current of 0.4 m/s, and a mixed skewed-asymmetric flow, all with a velocity amplitude of 1.2 m/s and flow period of 7 s. We find that the net positive transport rate beneath velocity-asymmetric oscillatory flow results from large, but opposing sand fluxes during the positive and negative flow phase. With an increase in velocity asymmetry and, in particular, velocity skewness, the difference in the magnitude of the fluxes in the two half cycles increases, leading to larger net transport rates. This trend is consistent with the observed increase in skewness of the oscillatory bed shear stress. Phase-lag effects, whereby sand stirred during the negative flow phase has not settled by the time of the negative-to-positive flow reversal and is subsequently transported during the positive flow phase, are notable but of minor importance to the net transport rate compared to earlier experiments with finer sands. In the vertical, the oscillatory flux is positive above the no-flow bed. Within the sheet flow pick-up layer, the oscillatory flux is negative and similar in magnitude to the positive flux induced by the residual flow. The 0.4 m/s opposing current causes more sand to be picked up during the negative than during the positive flow phase. Above the no-flow bed the resulting negative oscillatory flux is comparable in magnitude to the current-related flux.