Cross-shore variation of intermittent sediment suspension and turbulence induced by depth-limited wave breaking

Abstract

Intermittent features of turbulence and sediment suspension were investigated in a large-scale laboratory flume, under erosive and accretive conditions with measured free surface elevation, fluid velocity, and sediment concentration across the surf zone. Intermittent events of turbulence and sediment suspension occurred for a small portion of the time series but contained a significant amount of motions in these events. Comparison of intermittency statistics with previous studies conducted under different experimental conditions showed similar results, indicating that intermittency is a general aspect of turbulence and sediment suspension in the surf zone, including the bar crest, the bar trough, and the inner surf zone. The relationship of these intermittent turbulence and suspension events was examined with conditional probabilities. Here, it was found that only 20–35% of the turbulent events were associated with sediment suspension events, implying that much of the intermittent turbulent motion may act to dissipate wave energy rather than suspend sediments. On the other hand, 50–65% of the sediment suspension events were associated with turbulent events, implying that intermittent turbulent motion is one of the fundamental mechanisms for the initiation of sediment suspension in the surf zone. The sediment suspension events that were uncorrelated with turbulent events were mostly induced by strong offshore low-frequency motion, suggesting that advection plays a key role. The vertical structures of intermittency showed that the fraction of intermittent events for both turbulence and sediment concentration were vertically uniform and quantitatively similar each other, although their thresholds varied vertically. The motion contained in the intermittent suspension events increased toward the bed, while it was vertically uniform in the intermittent turbulent events. Finally, it was found that the intermittent sediment suspension events play an important role in each bathymetric change.