Sediment Suspension and Turbulence in an Oscillating Flume

An experimental study measured suspended-sediment concentrations and turbulence above the bottom of a specially designed oscillating flume. A total of 73 concentration distributions was measured for a single fixed-bottom roughness and the same specific gravity (1.25) of sediment. Three different sediment sizes were used, 65 experiments with the same size. These experiment show a simple exponential distribution, except near the bottom, as previously found by other investigators. The slope of the concentration distribution is in the range of -5 to -15 per foot (-16 to -50 per meter) for the experiments. For the limited data on other sizes, the slope of the concentration distribution becomes more negative as fall velocity increases. Turbulent velocity fluctuations measured with a hot-film anemometer are normally distributed with mean zero for measurements at two elevations above the bed, well outside the viscous boundary layer. The root mean square of the velocity fluctuations decreases exponentially with distance above the bed, and at the bed, increases approximately linearly with increase in flume velocity. When extrapolated to typical field conditions seaward of the breaker, these experiments demonstrate the importance of fall velocity, maximum wave-induced bottom velocity, and turbulent velocity fluctuations in controlling sediment suspension by shoaling waves. However, comparisons of data obtained with the lightweight sediment in these experiments and the probable motion of quartz sand in the field suggest that sediment suspensions caused by shoaling waves offshore the breaker are likely to be limited. (AUthor)